Hypersensitivity pneumonitis.

Hypersensitivity pneumonitis (HP) is a group of immunologically mediated lung diseases caused by the inhalation of environmental agents in susceptible individuals. Most HP patients are non-smokers and have been exposed to organic dusts from vegetable or animal products. Some HP cases are associated with exposures to relatively simple chemical compounds. HP may present as an acute, subacute, or chronic disease and may follow various clinical courses. The type of exposure is thought to be more important in the clinical outcome than the nature of the antigen. A diagnosis of HP is often considered on the basis of clinical history of exposure with resulting respiratory symptoms, but the definitive diagnosis requires a constellation of clinical, radiologic, laboratory, and pathologic findings. The characteristic histologic triad in HP includes bronchiolitis, interstitial lymphocytic infiltration, and granulomas; however, biopsy in HP cases may lack the diagnostic triad and manifest as nonspecific interstitial pneumonia (NSIP). Avoiding exposure to the offending antigen(s) is usually sufficient to resolve symptoms and physiological abnormalities. Pulmonary fibrosis and physiological abnormalities occurring in chronic HP may be irreversible. Steroid therapy is helpful for symptomatic relief, but probably does not affect the long-term prognosis. Type III and type IV hypersensitivity reactions are involved in the pathogenesis; alveolar macrophages and T cells (Th-1 type) play a central role in the immune responses after antigen exposure via their increased interaction and secretion of regulatory mediators.


I. INTRODUCTION
Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA), is not a uniform disease but a group of diseases caused by repeated inhalation of antigenic environmental agents. 1 Best known examples of HP are farmer's lung (FL) and pigeon breeder's or bird-fancier's lung (PBL or BFL), but there are many other types of HP under a variety of names. Various animal and vegetable products, chemicals, and some uncertain antigenic products have been implicated in the pathogenesis of HP, and their putative causative antigenic agents are listed in Table 1. Most HP cases are work related, although some types of HP 582   584 occur in persons who simply stay or live in a building contaminated by a bacterialaden closed ventilating system or wood structure. 2 The list of potential causative agents for HP is ever increasing with the changing environment. Some areas, such as New York City in the U.S., might face a surge of HP cases associated with yet unknown agents generated in the huge rubble of the collapsed twin towers.
The number of individuals who develop HP is still relatively low in the general population; HP often goes unrecognized and is misdiagnosed as viral pneumonia or idiopathic interstitial lung disease unless a careful history is taken. It has been estimated that between 5 and 15% of an exposed population will develop HP, suggesting a role for yet unclear host factors in the development of clinical disease. [3][4] A condition described recently as hot tub lung has an ambiguous nature showing features of both infection and HP, posing a diagnostic challenge and therapeutic dilemma. [5][6][7][8][9] This article reviews the epidemiology, spectrum of clinical and pathologic manifestations of HP, and the approach to accurate diagnosis and proper management of HP. Well-known prototypes of HP (e.g., FL, PBL), more newly described types of HP (e.g., metal worker's HP), and controversial entities (e.g., hot tub lung) are described separately for a better understanding of the disease. Current concepts on the pathogenesis of HP are also discussed based on recent findings in both clinical and basic research.

II. EPIDEMIOLOGY
Because HP is often work related, the profile of affected individuals varies considerably in different types of HP. For example, FL is much more frequent in men, and nearly all cases are in adults. 10 In general, however, HP appears to have no gender predilection. Many types of HP cases involve patients in their fourth to sixth decade of life, 11 although there is still a wide age range with some welldocumented pediatric HP cases and even cases occurring in infants. [12][13][14] Comprehensive epidemiology of HP (even in the well-studied type of HP such as FL) is still elusive due to the difficulty in study design and in making a definite diagnosis of HP. Most studies used cross-sectional surveys; only a few cohort studies have been published in the literature. [15][16][17] Large-scale surveys often depend on questionnaires plus testing for serum-precipitating antibodies against suspected antigens. [18][19][20] However, the sensitivity and specificity of questionnaires to detect past or active cases of HP is limited, and the serum-precipitating antibodies correlate poorly with a more extensive clinical evaluation. 21 The prevalence of HP in exposed workers has been difficult to estimate because of methodological problems in disease recognition as mentioned above, but appears to be widely variable depending on the circumstances. Studies of acute outbreaks among office workers with the same exposure to bacterial antigens via a closed ventilation system showed that up to 70% of those exposed developed the disease. 22 In other circumstances the prevalence of HP is much lower, as in a population of Wisconsin dairy farmers with a prevalence of 42 per 100,000 (using strict criteria for the diagnosis). 23 In general, only a small percentage of the exposed population actually develops the disease; thus, a combination of both host and environmental factors probably play a role in the manifestation of HP. The importance of genetic predisposition has been reported in an epidemiologic study that demonstrated major histocompatibiltiy complex (MHC) and tumor necrosis factoralpha (TNFα) polymorphisms among PBL patients. [24][25] Many previous studies have documented that most (80 to 95%) patients are nonsmokers in all types of HP, which is a substantially higher proportion than in a similarly exposed, but not affected population. 4,[26][27][28] The mechanism for such a remarkable predilection to HP in non-smokers is not known, but it is speculated that smoking may alter the defense mechanism or immunologic reactivity of the lung, probably exerting a mild immunosuppressive effect. 29 A previous study reported that alveolar macrophages in the smokers' lungs had decreased co-stimulatory molecules on cell surfaces that are involved in T cell signaling after antigen exposure. 30 The authors postulated that downregulation of T cell signaling due to altered alveolar macrophage phenotype in smokers may be partly responsible for the decreased incidence of HP among smokers. 30 Thus, a clinical history of heavy, active smoking is a substantial indicator against the diagnosis of HP. It is interesting to note that some interstitial lung diseases, such as eosinophilic granuloma and desquamative interstitial pneumonia (DIP), occur predominantly in smokers.

III. CLINICAL PRESENTATIONS
Although the causative antigens differ widely, the clinical syndromes are remarkably similar. The clinical presentation of HP has been divided into acute, subacute, and chronic forms on the basis of clinical features. 1 The acute form is the 586 most frequent presenting form. Symptoms of fever, chills, cough, dyspnea, chest tightness, and malaise occur about 4 to 8 h after exposure to antigens and resolve usually within a few days if there is no further exposure. The subacute form often appears insidiously over several weeks or months with marked progressive pulmonary manifestations, including cough and dyspnea. The transformation to a chronic form of the disease may occur in patients with continued low-dose exposure, and this chronic form is probably the long-term sequel of one or both of the other forms. However, not all chronic HP patients have had prior episodes of acute or subacute HP. Detailed clinical features in important types of HP are discussed separately in a later section.

A. Plain Chest Film
The chest radiographic findings are significantly different in the acute and chronic HP (Figures 1 and 2). In acute HP, chest X-rays show diffuse ill-defined airspace consolidation, which corresponds to airspace inflammatory infiltrates in the biopsy. 31 Fine nodular opacities replace the consolidation a few days later and reflect peribronchiolar inflammation and granulation tissue formation in the tissue. [31][32] The radiologic features in chronic HP are characterized by the evidence of fibrosis, including intralobular interstitial thickening, traction bronchiectasis, and honeycombing in severe cases. [31][32] The radiologic changes in acute HP tend to be more prominent in the lower lobes and spare the apices. In contrast, linear infiltrates in chronic HP tend to show upper lobe predominance with relative sparing of the bases in many cases. The nodular and ground glass densities in acute HP disappear after cessation of exposure, while the diffuse linear infiltrates in chronic HP may be irreversible. [31][32] There is usually little or no evidence of pleural effusions, pleural thickening, calcification, cavitation, atelectasis, localized radiodensities (coin lesions or masses), or intrathoracic lymphadenopathy in the acute phase of HP. Pleural effusions and thickening are also very unusual even in chronic HP cases. 1 Subcutaneous emphysema has been reported presumably as a consequence of pleural rupture due to bronchiolitis and lobular overinflation. 1

B. Computed Tomography (CT) and High Resolution Chest Tomography (HRCT)
The CT features of acute and chronic HP have been well described. [33][34][35][36] The most helpful features for making the diagnosis of acute or chronic HP are groundglass attenuation and micronodules ( Figure 3). 36 Some of the CT findings in chronic HP, particularly linear densities, ground-glass attenuation, and honey-587 FIGURE 1. Chest radiographs of an acute HP case showing hazy or ground-glass pattern with more infiltration at the right lung base.

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combing, overlap with those of usual interstitial pneumonia (UIP) and collagen vascular diseases. [33][34][35][36] Other CT findings in HP include a mosaic pattern, air trapping, mixed opacities, and fibrotic changes. [33][34][35][36] HRCT is more sensitive than plain chest films or conventional CT in detecting abnormalities in HP. [37][38] HRCT is also superior for assessing the type and extent of abnormalities. [37][38] The different patterns on HRCT do correlate with the stage of disease, histologic findings, and pulmonary function test abnormalities in some cases, [37][38] but there has been a case report showing normal-appearing HRCT in a biopsy-proven, symptomatic HP case. 39 The HRCT features of subacute HP consist of varying proportions of groundglass opacification, poorly defined centrilobular nodules, and areas of decreased attenuation ( Figure 4). 38 There is a strong correlation between the extent of the areas of decreased attenuation (i.e., less gray density and a component of the mosaic attenuation pattern) on HRCT and pulmonary function indices of airtrapping. 38 The air-trapping, graphically shown on expiratory CT, is present in most patients with subacute disease and reflects the underlying component of cellular bronchiolitis that can cause luminal narrowing and air-trapping. 38 Even in cases with chronic fibrotic disease, expiratory CT may show lobular air-trapping in the background with a reticular pattern representing underlying fibrosis. 38 A comparative radiologic study on the CT findings of idiopathic pulmonary fibrosis (IPF) and HP concluded that CT scan can be used to distinguish IPF from HP in most, but not all cases. 36 This study showed that CT scan can be used to distinguish between IPF and HP in about 90% of cases when the CT diagnosis is made with a high level of confidence. When the CT features do not allow a confident diagnosis, the accuracy of diagnosis decreases to around 60%. The presence of micronodules strongly favors HP, whereas honeycombing and lower zone or peripheral predominance favors IPF. Desquamative interstitial pneumonia (DIP) could not be reliably distinguished from acute or subacute HP in this study. 36 Chronic HP may have findings identical to those of usual interstitial pneumonia (UIP). It was concluded that lung biopsy should still be considered the gold standard for diagnosis of interstitial lung disease. 36

V. PULMONARY FUNCTION AND BLOOD GAS STUDIES
The lung function changes in HP are nonspecific and similar to other diseases, including sarcoidosis, UIP, collagen vascular diseases, drug-induced lung diseases, and other interstitial lung diseases. 40 In general, HP patients show the combination 590 of a restrictive and obstructive pattern in pulmonary function studies. Pulmonary function and blood gas studies in all forms of HP typically show a restrictive pattern with loss of lung volumes, impaired diffusion capacity of carbon monoxide (DLco), decreased compliance, and an exercise-induced hypoxemia. 41 A resting hypoxemia may also be found. Forced expiratory volume in 1-s (FEV1) to forced vital capacity (FVC) ratio usually remains normal, but maximum mid-flow rates and forced expiratory flow, mid-expiratory phase (FEF25-75%) may be decreased. These abnormalities in airflow are probably caused by bronchiolitis occurring in HP. The pressure volume curve is usually shifted down and to the right, resulting from the patchy pneumonitis. Arterial blood gas abnormalities include hypoxemia, hypocapnia, and respiratory alkalosis. 40 In chronic HP, the pattern of lung function impairment differs from the acute HP. These changes reflect the extensive bronchiolitis and fibrosis found in the chronic HP. The gas exchange impairment is predominant in a small number of the patients. [40][41] Airway obstruction, however, is a dominant feature in many cases. The flow rates are universally reduced with an additional element of increased lung compliance (reduced recoil). The latter observation is consistent with the pathological changes of emphysema described in these patients. In the chronic stage of the disease, the correlation between clinical, radiographic, and pulmonary function abnormalities is rather poor. Pulmonary function tests may return to normal after the resolution of acute disease, but in chronically ill patients impaired lung functions are expected to remain. Many patients have increased bronchial reactivity. [42][43] Many patients with HP (20 to 40%) exhibit increased nonspecific airway reactivity, which may be related to increased mast cells in the lung, or to bronchial epithelial damage. [44][45] Some patients (up to 5 to 10%) may also develop clinical asthma. 46 The increased airways reactivity and asthma tend to diminish after cessation of exposure.

A. BAL Fluid
One of the most striking laboratory findings of HP is a remarkable lymphocytosis in the bronchoalveolar lavage (BAL) fluid, often greater than 60%, whereas there is usually less than 10% in normal individuals. 40 The proportion of different cell types in the BAL fluids of HP are compared with various other diseases (Table 2). 40 Increased lymphocytes in the BAL fluid of HP patients are predominantly composed of CD8-positive cytotoxic T cells; therefore, the CD4/CD8 ratio drops well below 1.0 (typically around 0.5). This is a reversal from the normal status of 1.8. [47][48][49][50][51] Although CD8-positive T cells dominate the BAL fluid in most HP cases, some patients with a very recent exposure to an antigen may predominantly demonstrate 591 CD4-positive T cells in the BAL fluid. 1 In chronic FL and PBL, the ratio of CD4 to CD8 in BAL fluid lymphocytes may be high. 52 Due to the increase in absolute cell numbers in the BAL fluid, the absolute count of CD4-positive T cells in HP is still higher than normal. [40][41] Likewise, despite the decreased ratio of alveolar macrophage and lymphocytes in BAL fluid, absolute numbers of alveolar macrophages are actually increased in HP. [40][41] Gamma/ delta T cells are also increased in BAL fluid and in peripheral blood in HP patients. 53 It has been reported that the increase in mast cells in BAL fluid can be implicated in the pathogenesis of HP. 54 The number of lymphocytes recovered by BAL correlate with those seen in histological sections of HP. 55 Thus, BAL lymphocyte counts may be a reasonable surrogate test for a biopsy in order to assess the degree of alveolitis in active diseases.
In acute HP within 24 h of exposure, there is an initial increase of neutrophils followed by a subsequent lymphocytosis in BAL fluid. 40 This pattern of cell kinetics has also been demonstrated in an experimental model of HP 56 ( Figure 5). The neutrophil count decreases as the number of lymphocytes increases after the acute stage. However, a moderate but significant increase of the percentage of neutrophils has been found in the BAL fluid of HP patients after a recent or continuing exposure to antigen. 41 A previous study demonstrated the correlation between the lung neutrophil percentage and the percentage of lung fibrosis. 57 This study suggested that the persistent traffic of neutrophils in HP may play a role in the lung damage and fibrosis via neutrophilic proteases, including gelatinase B and collagenase 2. 57 Despite a strong association with HP, significant lymphocytic alveolitis in BAL fluid may be seen in asymptomatic patients with or without serum precipitins after an antigen exposure. 58 One clinical study concluded that the BAL lymphocytosis in asymptomatic patients did not have any clinical significance. 59 Thus, the lymphocytosis with low or reversed CD4/CD8 ratio in BAL fluid is a helpful finding but not always specific for HP.

B. Skin Testing
Skin testing has limited diagnostic value in HP. In 1965, Pepys and Jenkins observed that moldy hay antigen did not incite any cutaneous reaction in their nonatopic patients. 60 Freedman et al. obtained similar results and concluded that skin testing with moldy hay antigen was not helpful in differentiating the patients with FL from asymptomatic contacts. 61

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68% showed a late response, and the delayed reaction occurred in only 4% of individuals. Thus, these authors concluded that the intradermal injection of moldy hay was an effective way of diagnosing FL. Skin testing may be helpful in PBL, but, unfortunately, many asymptomatic pigeon breeders also have positive skin test. 40

C. Serum Precipitins
Specific serum precipitating antibodies (IgG) are found in almost all patients with HP as well as approximately 40% of asymptomatic individuals exposed to the same antigens (i.e., highly sensitive, but not specific). 40 Therefore, the absence of precipitating antibodies as determined by a reliable technique, probably excludes the diagnosis. However, the adequacy of testing serum precipitins may be a problem in diagnostic sensitivity. Krasnick et al. reported that there are more cases of undiagnosed HP recently and attributed it to the fact that physicians may rely on false-negative "HP panel" results reported by commercial laboratories. 63 These false-negative tests may have been due to improper quality controls, insensitive techniques, use of the wrong antigens, or underconcentrated serum. 63

D. Inhalation Challenge
Theoretically, the best way to establish the diagnosis of HP is perhaps to reproduce the clinical syndrome by subjecting the patient to an inhalation challenge with the putative offending antigen. However, it should be performed under appropriate medical supervision because of safety concerns. A recent study demonstrated the validity and safety of inhalation provocation testing for individuals with chronic BFL. 64 In this study, sensitivity of the test was shown to increase by adding the presence of neutrophilia in BAL fluids after an inhalation challenge to the positive criteria. 64 This test could become a valuable adjunct to current diagnostic criteria and may become very useful in differentiating chronic HP from IPF. 64

VII. PATHOLOGIC FEATURES
HP is one of the interstitial lung diseases that can be suggested on the basis of the histological findings alone. [65][66][67] The triad of histologic changes in HP is comprised of diffuse interstitial lymphocytic infiltrates, scattered small poorly formed granulomas, and cellular bronchiolitis; this triad is seen in about 50 to 75% of HP cases. However, it is important to note that this histological triad is neither pathognomonic nor specific for HP.
In typical cases of acute HP, the clinical history and the presence of serum precipitins are sufficient for the diagnosis. Thus, biopsy is rarely performed in cases of acute HP and subsequently pathologic features are poorly defined in the literature. 11 The gross pathologic findings of acute HP have not been well described and probably are not particularly distinctive. There may be consolidation with a vague nodularity corresponding to peribronchial inflammation. Because biopsy is rarely performed in the acute phase, a large series describing the histopathology has not been published.
The pathologist is far more likely to get biopsies from the subacute or chronic forms of HP. Biopsies on HP are usually performed for one of the following reasons: (1) HP is not suspected until the histological findings of the biopsy suggest the diagnosis of HP and a subsequent detailed clinical history reveals a causative exposure (even though up to two-thirds of such cases may never elicit definite exposure history); [68][69] (2) biopsy is done to confirm the diagnosis of a clinically apparent exposure history and/or to assess the degree of fibrosis; (3) biopsy may be performed to rule out an infection or some other disease process.
Subacute and chronic HP manifests itself histologically as chronic interstitial pneumonia. 11,[68][69] Classic cases of subacute HP show a triad of diffuse chronic interstitial inflammatory infiltrates, cellular bronchiolitis, and small poorly formed, nonnecrotizing granulomas or sometimes just individual giant cells without granuloma formation (Figures 6 and 7). The relative prominence of each of these

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features varies from case to case, and sometimes not all of the features are present. Interstitial fibrosis in a typical case of subacute HP tends to be overshadowed by the interstitial inflammatory infiltrate, while the opposite is true for UIP cases. When fibrosis is present, it is mostly in peribronchiolar regions in HP as opposed to subpleural and paraseptal areas in UIP. In advanced chronic HP, the lungs may show extensive fibrosis and honeycomb changes indistinguishable from UIP. Depending on how secure is the clinical diagnosis of HP, histological criteria for the morphological diagnosis of "consistent with HP" may be adjusted in individual cases. 11 The interstitial infiltrate in HP is mostly lymphocytic and relatively uniform throughout the lung parenchyma. This infiltrate varies from slight to massive and can mimic the diffuse form of lymphocytic interstitial pneumonia (LIP). Despite a high cell count on examination of BAL fluid, the intraalveolar inflammation in histologic sections is usually prominent only in the peribronchiolar regions. A previous study investigated the diagnostic accuracy of transbronchial biopsy in acute FL cases using three pathologic criteria: (1) diffuse lymphocytic infiltration, (2) focal lymphocytic infiltration, and (3) granulomas. 70 It was concluded that diffuse lymphocytic infiltrate differentiates FL from control samples and should be specifically sought. 70 FIGURE 7. Some areas in a HP case show lesser interstitial infiltration and relatively empty alveolar spaces.

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The granulomas in HP are noncaseating, nonnecrotizing and sometimes manifest as only tiny clusters of epithelioid histiocytes without giant cells (Figures 8 and 9). Giant cells may be seen individually and/or as small clusters (Figures 8 and 9). The granulomas are scattered throughout the parenchyma and may be seen interstitially or within airspaces. The giant cells in HP frequently contain inclusions: cholesterol clefts, bluish concentrically laminated calcified Schaumann bodies, or lucent birefringent oxalate crystals. These structures should not be mistaken as foreign bodies because they are the result of endogenous metabolic products. Rarely, the causative agent may be found in the granulomas such as in maple bark disease.
The degree of cellular bronchiolitis varies from case to case and may be marked or nearly absent. A morphologic study of the bronchiolitis in chronic PBL patients in Mexico reported that bronchiolar changes were proportional in type and severity to the parenchymal damage. 71 Bronchiolitis obliterans with intraluminal polyps and organizing pneumonia with airspace organization in alveolar ducts and more distal parenchyma (i.e., BOOP-like reaction) are seen quite often, especially in active cases. Foamy macrophages within alveolar spaces are common in the peribronchiolar regions, but need not be present for the diagnosis ( Figure 10). In general, the airspaces are not too hypercellular except in peribronchiolar regions,  but some cases show increased numbers of alveolar macrophages resulting in a DIP-like pattern. 11 Histologic features such as heavy eosinophilic inflammation and vasculitis are very unusual for HP. The name of HP (or its synonym EAA) may mistakenly suggest a type I hypersensitivity reaction with eosinophils as effector cells, but eosinophilic infiltration is conspicuously lacking in HP. An alternative diagnosis should be considered when there is significant eosinophilic inflammation on biopsy. Vasculitis is essentially absent in HP cases, unlike sarcoidosis, which shows frequent vascular involvement up to 60 to 70% on extensive sampling. 72 Type II pneumocyte hyperplasia is not usually a prominent feature. Pleural fibrosis may be occasionally seen, but active pleuritis would be very unusual. 11 Chronic HP may show various degrees of interstitial fibrosis and honeycombing depending on the severity. When a distribution is discernible, the scarring tends to be peribronchiolar. The development of bronchus-associated lymphoid tissue (BALT) was reported in three out of five chronic HP patients. 73 Chronic antigenic stimulation and/or inflammation in chronic HP may have caused BALT development, which, in turn, is likely to play an important role in the mucosal immune response of the disease. 73 Ultrastructural changes in HP have been reported in recent studies. [74][75] Early ultrastructural abnormalities include swelling and partial or complete detachment of type I cells, which results in denudation of alveolar epithelial basement membranes, endothelial bleb formation, and cellular swelling with luminal narrowing of capillaries. [74][75] HP can cause replication of endothelial basement membranes, which results in three or more layers and rarefaction. These changes may be also present in sarcoidosis, although with much less intensity than in HP, which may be attributable to less prominent alveolitis. 74 On the other hand, these lesions can be seen even in normal appearing areas by light microscopy in HP cases, suggesting their early occurrence in the course of HP. 74 Severe alveolitis in HP is accompanied by the damage to type I cells and their basement membranes, which results in intraalveolar fibrous plug formation (organizing pneumonia), probably via extension of interstitial myofibroblasts and fibroblasts into the alveolar spaces. In sarcoidosis, intraalveolar fibrosis is uncommon, probably due to its less severe alveolitis and alveolar epithelial damage. 74

VIII. DIAGNOSTIC APPROACH AND HISTOPATHOLOGICAL DIFFERENTIAL DIAGNOSIS
A timely diagnosis is very important so that further exposure to antigen can be avoided before irreversible lung injury occurs. A set of diagnostic criteria and supportive findings have been proposed for the diagnosis of FL, which can also be applied for other types of HP 21 ( Table 3). The most important step in the diagnosis is still the recognition of occupational and environmental exposure. 40 Physicians 599 must think of HP and recognize the temporal relationship between the appearance of symptoms and occupational or environmental exposure. In the acute form, symptoms typically occur 4 to 8 h after the antigenic challenge. Symptoms during this stage may be indistinguishable from viral, mycoplasmal pneumonia, or Legionnaire's disease. Chest radiographs, pulmonary function tests, and routine laboratory tests are not specific in HP. Serum precipitins and skin tests may be helpful, but reliable standardized antigens are not always available. Lung biopsy for tissue diagnosis is not always required, especially in the acute stage. Practically, CD8-positive T cell predominant lymphocytosis in BAL fluid is a very helpful laboratory finding, although not always specific.
The histological differential diagnosis of HP is broad and includes a large number of lesions that can show bronchiolitis, interstitial infiltrates, or granulomas. 11 UIP tends to demonstrate extensive scarring and honeycombing at the time of biopsy; the fibrosis overshadows the interstitial infiltrate and often is in a subpleural or paraseptal distribution. In idiopathic BOOP or secondary conditions with a BOOP pattern, the intraluminal organization in bronchiolar and alveolar lumens is more prominent than diffuse interstitial inflammation. The opposite is true in HP. There is, however, considerable overlap in the two conditions, and any case showing a BOOP pattern with occasional small granulomas should include HP in the diagnostic consideration. Sarcoidosis and berylliosis tend to show wellformed granulomas that are confluent and distributed as conglomerates along lymphatic routes (including bronchovascular bundles, pleura, and septa). Granulomas dominate interstitial infiltrates in sarcoidosis and berylliosis, while the reverse is true in HP. Granulomatous infections typically show necrosis within the granulomas; even in cases without necrosis, the granulomas in infectious diseases tend to be more sarcoid-like in character and also to overshadow the interstitial inflammatory infiltrates. Aspiration with foreign body or cholesterol granulomas may also mimic HP. Important diseases included in the differential diagnosis are summarized in Table 4.
Some interstitial lung diseases that are mostly seen in smokers may rarely mimic HP histologically, but a heavy smoking history would be very unusual for HP. 11 Those diseases include DIP, eosinophilic granuloma (EG), and respiratory bronchiolitis interstitial lung diseases (RBILD). Histologic findings may also be helpful in differentiating HP from those diseases. For example, RBILD shows pathologic changes restricted to the bronchioles, abundant alveolar macrophages containing tan pigments (so-called smoker's macrophages), and a lack of granulomas, all of which should lead to the correct diagnosis without too much difficulty.
Drug reactions may be histologically indistinguishable from HP, but cellular bronchiolitis is not a prominent feature of most drug reactions. Of note, there have been some case reports of HP caused by drugs ingested orally or intravenously. [76][77][78] However, these drug-induced HP-like conditions are caused by a systemically introduced antigen to the lung, thus they should be distinguished from HP caused by inhaled antigen. Some cases of HP with dense interstitial lymphoid infiltrates are indistinguishable from the diffuse form of LIP. Germinal centers are not usually a prominent feature in HP, and cases are rarely confused with diffuse lymphoid hyperplasia. Viral pneumonia may also show diffuse interstitial inflammation as well as cellular bronchiolitis, but the lack of small granulomas and the presence of viral inclusions help to differentiate this condition from HP.
Because there are cases of HP that do not show the classic histological triad, there remains an appreciable number of unclassifiable interstitial pneumonias that may be associated with HP. Nonspecific interstitial pneumonia (NSIP) is a relatively new term 79-84 that was first introduced by Katzenstein and Fiorelli in 1994 to include the cases of interstitial pneumonia that do not fit histologically with UIP, DIP, or AIP. 79 They clearly have indicated that NSIP does not represent a specific disease entity. 79 Rather, NSIP probably encompasses a spectrum of clinical associations, including connective tissue diseases, HP, slowly resolving acute lung injury, nonrepresentative biopsy of BOOP, and undiagnosable UIP, although the etiology is unknown for most examples of NSIP. 79 This original series reported that 17% of NSIP cases had histories of exposure to possible causative substances either at home or at work. Only one case was documented as having a definite relationship with a pet bird (parrot) and positive serology, thus the diagnosis was accordingly changed to HP. Other exposures in those cases included a canary, a wood stove, grain dust, coal and ash, and unknown agents in the setting of the garment industry. Despite the lack of a proven etiologic link between these exposures and NSIP in these remaining cases, it is very likely that some HP cases are diagnosed as NSIP due to insufficient information on possible exposure history and/or other supportive laboratory data.

IX. TREATMENT AND PROGNOSIS
In most cases of HP, adequate treatment consists of the patient's avoidance of the antigen, minor symptomatic therapy with antiinflammatory drugs, and, if necessary, bronchodilators. A partial prevention may be accomplished by wearing a face mask during exposure. 21 In severe cases, administration of steroids for 2 to 4 weeks produces a resolution of clinical, functional, and radiological findings. In chronic forms of the disease, steroid therapy may delay the onset of further lung damage. 40 Ideally, the treatment of choice would be complete avoidance of the offending antigen and oral steroids for symptomatic relief, although the optimum treatment in any given patient may be difficult to choose. 2 The only randomized, double-blind, placebo-controlled trial in the literature 85 supported previous general observations [86][87][88] in that steroid therapy hastened the recovery from the acute stage of FL but without a beneficial effect on the long-term prognosis. The treatment for chronic or residual disease is supportive.
The prognosis of HP is also somewhat controversial, and diseases caused by different antigens may have different outcomes. However, the type of exposure seems to be more important than the nature of the antigen itself. For example, patients with low-level exposure to pigeon antigen in a domestic environment (that is typical in Mexico) are worse off than the patients in the U.S. or in Europe who have more intermittent short-term exposure outside of home. 1,71 Many cases show an excellent response to corticosteroids, and dramatic improvements in pulmonary function after steroid treatment even when there is definite fibrosis on the biopsy in some cases. 2 In general, however, fibrosis implies an irreversible element and a worse prognosis. It is still disputed whether workers who have no other possible occupation can be treated with steroids (or even not be treated at all) and maintain their occupation without further functional loss. 21

A. Farmer's Lung
Farmer's lung (FL) refers to HP in agricultural workers, but this term has also been associated with conditions that are not HP. Thus, some authors proposed to call this condition as farmer's HP to differentiate from other types of farmerrelated pulmonary diseases, such as organic dust toxic syndrome, chronic bronchitis, and asthma. 21 FL in a narrow sense (i.e., Farmer's HP) is one of the first described and most prevalent form of HP. FL is caused by Thermoactinomycetes or fungi found in moldy hay, straw, and grain dust. 21 There is a direct relationship between intensity of contact and the risk of developing disease. 21 FL is more prevalent in cold, wet climates, and the number of cases is influenced by the climatic conditions of the preceding summer within a given region.
A lung disease seen in farmers exposed to grain dust was described in 1705, and a more modern description was reported in 1932. 21 Initially, it was felt that the disease was an infectious process caused by a fungus, but also implicated as an allergy or an irritation. It is now generally accepted that the microorganism is a bacterium designated by its current name Saccharopolyspora rectivirgula (SR), formerly known as micropolyspora faeni or Faeni rectivirgula. However, SR may not act alone and is certainly not the only microorganism in moldy hay or straw responsible for FL. It is quite possible that the initial sensitization requires a cofactor, a hypothesis supported by the observation that challenges with extracts of total hay dust induce a greater response than simple SR extract alone. Other microorganisms, including T. vulgaris and Aspergillus, are also thought to be responsible for FL, and they probably also require cofactors. Absidia corymbifera has been implicated as an etiologic agent in eastern France based on a prospective case-control study. 89 FL patients may present with a wide variety of clinical and functional abnormalities, and it is still unclear if the different presentations represent truly different forms of the disease or simply different stages. 21 FL has been classically divided into three groups: acute, subacute, and chronic. The acute form is the most frequent presenting form and the easiest to characterize. The patient with acute FL has acute systemic symptoms such as fever and chills, as well as pulmonary manifestations, including chest tightness, dyspnea, cough, etc. within a few hours of exposure to moldy hay or straw. The fever usually subsides within 24 h while less severe dyspnea may persist after 24 h. With recurrences, weight loss may ensue and dyspnea becomes progressively more continuous. Subacute is used to describe a form of FL with a more insidious onset. Dyspnea presents gradually over several weeks or months. Variations in body temperature have not been well documented in the literature about the subacute form of FL. It is possible that the acute and subacute forms of FL represent different degrees or stages of the disease with the subacute form being a variant having markedly lower levels of fever than in typical acute HP patients.
The chronic form of FL is probably the long-term sequela of the acute and/or subacute forms. A chronic cough and productive sputum are common in this phase, although transient mild bronchial hyperactivity and bronchospasm have been reported. The disease is usually not associated with asthma-like symptoms, although concomitant occupational asthma is not uncommon and FL patients may 605 have a higher risk of developing asthma. Chronic is sometimes used to designate residual lung involvement as a consequence of acute FL. Some restrict the designations to acute and chronic only and the subacute or lingering residual diseases as chronic. In one view, the chronic form may follow a succession of acute attacks or a single severe attack.
Diagnostic criteria for FL have not been firmly established due to the nonspecific nature of many clinical parameters used in the diagnosis. A clinical diagnosis of FL is generally bound to a history of exposure to a potential causative agent, symptoms, physical findings, and laboratory data. Required diagnostic criteria for FL are listed in Table 3. 21 Clinical evaluation is mainly based on the information gained by a careful history and physical examination. Physical signs may be unremarkable except inspiratory crackles on auscultation, with some patients demonstrating digital clubbing. The measurement of pulmonary flows and volumes may be useful in following disease progression.
Chest radiographs are required to rule out pneumonia, sarcoidosis, and other disorders. The distribution of infiltrates in the acute disease is diffuse but infiltrates often spare the bases in the subacute form. 21 Infiltrations may be micronodular or patchy and confluent. Hapke and collegues proposed a classification of chest X-ray findings as 0 = normal, A = miliary appearance (1-minimal, 2-definite, 3-marked), B = fibrotic changes (1-fine diffuse, 2-slightly deforming, 3grossly deforming). 90 Others have used a scoring system based on the intensity and distribution to give a score from 0 to 24. 91 A meta-analysis of available reports suggests that 80% of individuals with acute FL have abnormal chest X-rays. 92 HRCT may be useful in the cases showing normal chest X-ray, but there are only a few studies reporting the HRCT findings in FL and more studies are needed.
In measuring antibody levels, it should be remembered that there are many false positives and false negatives. A double diffusion technique is probably the most relevant compared with other methods that are more sensitive but nonspecific. 93 BAL fluid can be useful to support a diagnosis of FL. A normal lymphocyte count in BAL fluid essentially rules out FL except for inactive residual disease. 21 The diagnostic value of skin testing for FL antigen has been currently disregarded. 40 Histopathologic confirmation is not required in establishing the diagnosis of typical FL cases. Specific provocation with barn dust SR antigen or workplace exposure have been used in the past, but the test is not standardized and potentially dangerous unless proper medical back up is provided.
The only reliable medical treatment for FL known to date is systemic corticosteroids, but a randomized, double-blind, placebo-controlled trial demonstrated that there is no long-term beneficial effect despite a proven effect on the recovery from the acute stage of FL. 85 The treatment for chronic or residual disease is supportive therapy.
The real picture of the natural history and prognosis of FL is still elusive due to the inconsistency of data collection and difficulty of follow up in a cohort of patients, which may lead to the misleading conclusion based on spurious data.

606
The most salient observations from the previous studies have been summarized as following: 21 (1) at least a third of FL patients have progression of persistent symptoms, (2) most patients, even those who remain asymptomatic, have some physiologic long-term sequela, (3) residual disease may include interstitial fibrosis and/or emphysema following longstanding FL, (4) recurrences of the disease are important determinants of the ultimate clinical outcome, (5) FL can be fatal if unrecognized, (6) precipitating antibody reaction to FL antigens has no prognostic significance for the development or progression of FL, (7) steroids are unlikely to positively affect a long-term abnormality, (8) emphysema probably represents a more important outcome than previously recognized, (9) BAL cellularity is of no prognostic value, (10) new technologies and adjustments of agricultural workers' farming activities are important for the control of the disease.

B. Bird-Fancier's or Pigeon-Breeder's Lung
Bird fancier's lung (BFL) or pigeon-breeder's lung (PBL) is usually caused by exposure to the antigens of pigeons or budgerigars. [94][95] Other birds reported as the cause of bird-related HP include canaries, parrots, doves, chickens, turkeys, ducks, and geese. 96 There are also a few cases of HP caused by contact with native birds mainly owls. 96 A case report indicated that HP due to native birds in a bird ringer showed a severe HP with a very insidious onset. 96 The disease was first recognized in the mid-1960s as a recurrent interstitial pneumonitis induced by exposure to pigeon-derived material and is closely related to FL. 95 PBL is not a single uniform disease but rather a dynamic heterogeneous clinical syndrome that varies in its initial presentation and subsequent clinical course. The acute form of the disease is characterized by recurrent episodes of fevers, chills, myalgia, cough, and breathlessness occurring 4 to 8 h after an exposure. The chronic form is characterized by the insidious development of chronic breathlessness and lung fibrosis. The disease does not necessarily progress from the acute to the chronic form. Some patients present with established lung fibrosis without having experienced acute symptoms, whereas others continue to have intermittent acute symptoms for many years without progressing to permanent lung damage. 94 Fanciers who develop the disease have often remained in a state of equilibrium with the antigen for many years before the onset of symptoms, and in some patients established disease may regress despite continued antigen exposure. 1 In the chronic form of the disease, even a complete avoidance of antigen contact may fail to reverse the disease. 1 The reasons for the different clinical presentations (i.e., acute and chronic) are not clear, but are probably due to the intensity and duration of exposure (i.e., low-intensity long-duration exposure to cause chronic HP; high-intensity short-duration exposure tending to cause acute HP). 71 PBL has different presentations in different geographic areas, manifesting as an acute HP in some and chronic HP in others. Intermittent exposure of pigeon breeders to large amounts of pigeon antigens in the United States and Europe is associated with acute disease and a good prognosis, whereas chronic exposure to a few household pigeons in Mexico is associated with chronic disease and a much poorer prognosis. 71 In the U.S. and Europe, pigeon breeders keep their birds in an enclosure separate from their living areas, which they visit periodically so that exposure is intermittent. In Mexico, birds are kept in living quarters so that exposure is constant. It is of interest that bird antigens can persist in a room for substantial lengths of time (more than 18 months) after removal of the birds. 1,71 Because the relevant antigens are similar in these two examples of bird-related HP, it is likely that the type of exposure, but not the antigen characteristics, determines clinical presentation and prognosis. A histopathologic study on these Mexican PBL patients demonstrated marked small airways disease with chronic inflammation and mural fibrosis that were generally proportional to alveolar parenchymal damage. 71 However, proliferative bronchiolitis obliterans was not a prominent finding in these Mexican patients. 71 Poorly understood, constitutional factors may govern individual susceptibility and responsiveness, because only minority of fanciers develop the disease. Additional factors can also influence the disease process. For example, it is well established that cigarette smoking is associated with reduced antibody responses to pigeon antigens and a reduced incidence to PBL. 94 However, some studies suggested that the disease take a more chronic form in smokers. 94

C. Japanese Summer-Type HP and Other HP Associated with Contaminated Buildings
Recently, there has been an increasing concern over health hazards caused by indoor molds, to the degree of "public hysteria" according to some in the insurance industry, although moldy homes causing health problems have been around since biblical times. There are thousands of varieties of molds that are found in every region of the U.S., including the markedly different climates of Alaska and Hawaii. Building-related HP is more difficult to predict or prevent than HP in an occupational setting. A number of factors (e.g., an airtight house using new materials such as aluminum sashes, mattresses, cellulose-based wallboard, carpeting, etc.) may have aggravated the problem of indoor molds or other antigens. It has been generally considered that the disease would occur preferentially in old wooden houses; however, the disease occurs even in concrete houses that are only several years old. A new kind of HP named humidifier lung has been reported that was caused by the contaminated water in humidifiers and postulated to be mediated by antibodies to Flavobacterium and endotoxin. 97 Summer-type HP is peculiar to Japan and caused by inhalation of seasonal molds that contaminate home environments during the hot and humid summer season. 98 Trichosporon cutaneum, an imperfect yeast, has been implicated as a causative agent for the disease. 98 Since the initial report, a nationwide survey in Japan has been carried out covering the 10-year period from 1980 to 1989. 99 Of the 835 HP cases identified in Japan through nationwide questionnaire, 621 (74.4%) were summer-type HP. The number of cases diagnosed doubled in the latter half of the 10-year period in this study. The residence of the patients was mostly in the western and southern parts of the country, and the northernmost incidence was in Akita Prefecture at the latitude of 49 o N. Eighty-six percent of the patients experienced initial symptoms from June to September with a peak in July. Most of the involved houses were wooden (89.5%) and the mean age of the buildings was 20.5 + 1.5 years for wooden houses, and 12.3 + 1.2 years for non-wooden houses. More than half of these houses had unsanitary conditions and had shady (41.7%), damp (75.1%), or poorly ventilated (33.9%) rooms.
There were twice as many female patients as male patients, and 39.8% of female patients were homemakers without an outside occupation. The peak age incidence was in the forties, but the age range was wide (2 to 86 years of age). 99 The frequency of family occurrence was 23.8% (137 out of total 576 cases). The smoking rate was 61.4% in adult males and 5.3% in adult females. The smoking rates of normal Japanese population during the same time period averaged 65.4% in males and 13.8% in females. 99 Thus, the prevalence of cigarette smoking is obviously lower in female patients than in the general population, but not in male patients (with no clear explanation), which needs a further case-control study. House pets were reported by 28.8% of the patients. 99 Serum anti-T. cutaneum antibody activities were positive in 260 (99.2%) of the 262 cases examined at a titer of 1:32 or higher. Glucocorticoids were used in 47.9% of the patients. 99 Recently, there have been case reports that closely resemble Japanese summertype HP in other countries. [100][101] These cases also showed symptoms when at home in the summer, with recurrence in the following summer and recurrence in the same family. Although an assay of anti-T. cutaneum antibody was not done, it is very possible that the causative agent of these cases was also T. cutaneum, which is distributed worldwide. 99 In northern latitudes, symptoms produced by senstization to molds in the warmer months typically improve or disappear completely in the winter when molds are less prevalent. However, the symptoms become much more severe in the warmer months, usually between the spring and fall. This classic pattern prevails worldwide wherever temperatures vary sufficiently to influence seasonal mold contamination. However, in more southern or tropical regions, seasonal symptom variations may be less evident or even completely absent. In fact, some cases of summer-type HP in tropical regions may develop in December or January in winter, which should be remembered for recognizing the HP cases in tropical regions. 99 The winter of 1997 to 1998 has received considerable worldwide attention because of the weather phenomenon known as El Nino. This unusual weather pattern resulted in significant flooding in western states of the U.S. and caused great property damage and personal loss. In the Santa Ynez Mountain Range of Southern California, total precipitation reached an unprecedented level. Not surprisingly, there has been some speculation on the health effects of this weather pattern. El Nino has led to an unusual illness for the area of Santa Barbara and Southern California. A case of HP secondary to the excessive precipitation and flood damage to the patient's home from El Nino ("El Nino lung") was reported in association with a growth of an unusual fungus Pezizia domiciliana. 102 HP as a sentinel event implies a remediable exposure and an exposed cohort requiring evaluation. 103 A patient with HP related to her building (i.e., an index HP case) led to a questionnaire survey, which revealed that co-workers in the same building also had substantially higher symptom rates than the controls in five other buildings, although no further HP cases were identified. It is likely that moisture sources in the building included an oversized cooling system. However, the building had met all applicable regulations and standards. Screening investigations for disease are not mandated by law and are often not conducted, in part because the party responsible for the cost is unclear. The absence of regulatory or professional standards that adequately address moisture in the built environment forces occupational health professionals to rely on disease documentation strategies to justify intervention.

D. Metal Worker's HP
Metal working remains common in the U.S. as well as in the rest of the industrialized world, with 1,200,000 exposed workers in the U.S. alone. 104 Several previous cross-sectional studies on respiratory diseases among metal workers suggested that acute lung function decrements are at least weakly associated with the thoracic fraction of bacteria, total particulate matter, and possibly endotoxin in the air. 104 However, HP was not identified or sought in these studies.
In 1994, possible cases of HP among automobile production workers were first noted in southeastern Michigan. 105 Subsequently, CDC's National Institute for Occupational Safety and Health (NIOSH) evaluated potential occupational exposures associated with these illnesses. NIOSH reported preliminary findings on a total of seven biopsy-proven HP cases among automobile production workers who had jobs requiring frequent exposure to metalworking fluids (MWFs) in three different plants in the southeastern Michigan area. 105 The findings suggested the need for further evaluation of a possible association of occupational exposure to MWFs with HP.
The HP case among metal workers was first documented in 1995 and was associated with significant antibody responses to several common environmental microorganisms, including Pseudomonas fluorescens cultured from the exposure fluid. 106 Other microorganism such as Mycobacterium chelonae was also isolated from the exposure fluid but could not be definitely implicated as a possible causative organism, because worker's sera tested negative for antibodies to this organism. A second outbreak has been recorded and attributed to Acinetobacters, another common environmental Gram-negative bacteria causing occasional opportunistic infection in long-term hospital patients, and also implicated in an occupational outbreak occurring in a foundry. 104 Both Pseudomonas and Acinetobacter are ubiquitous, water-borne, nonfermentative, Gram-negative bacteria capable of utilizing diverse organic chemical substrates as energy sources. 104 Additional outbreaks and case reports have been published. A recent report on an outbreak of lung disease among workers in a metal-working plant documented 16 biopsy-confirmed cases of HP. 104 In addition, patients with asthma, bronchiolitis, and emphysema, UIP and sarcoidosis were also found. 104 Study design included a clinical examination of patients and a cross-sectional questionnaire survey of the outbreak plant and control plants. The control plants consisted of two separate facilities, one using and one not using MWF. They also performed an industrial hygiene survey with laboratory characterization of microbial flora and immunological investigation. 104 Three sources of water-based aerosols were identified that grew similar microbial flora. Although machining increased air-borne bacterial levels, the increase was not related to the concentration of viable bacteria in the sumps (MWF reservoir). Antibody testing did not identify a specific single organism. Endotoxin levels were similar in case and MWF control plant.
There was a strong association between case definitions for HP and asthma. In general, studies of MWF-exposed working populations have examined only the presence of one of the other lung diseases and most are based on measures that identify only airways disease. The patients in this study often appeared to have evidence of both HP and asthma by questionnaire. 104 Two subjects with a prior clinical diagnosis of asthma actually had evidence of interstitial disease consistent with HP on biopsy. In addition, there was at least one patient with UIP and several with other forms of lung disease. In a separate investigation, the authors identified a case of sarcoidosis in the same plant whose disease resolved promptly after leaving employment in the plant.
A recently reported outbreak in a fiber-manufacturing plant documented that several different forms of disease were encountered in that same plant, identified as asthma, humidifier fever, and HP. [107][108] Hoffman et al. identified asthma, HP, and UIP in a wet office building. 109 This suggests that increases in several different kinds of lung diseases may be identified with proper examination of individuals in a defined population with a range of appropriate techniques. Others have suggested that the spectrum of interstitial lung disease represents a genetically defined differential response to a common set of exposures. 110 Physicians should no longer assume that the presence of a second or third form of lung disease in an outbreak represents an unrelated event. Whether the commonly used questionnaires in respiratory epidemiological studies are valid may need to be reevaluated.
Lessons should be learned from the previous studies about the diagnostic approach in this setting. Physicians should have a far higher index of suspicion for HP based on the potential for bioaerosols exposures around water-based processes. Because the specific etiologic agents remain unclear for almost all cases of bioaerosols-associated disease, airborne sampling is of questionable practical utility despite its great scientific interest. In parallel, it is important to screen for the range of diseases associated with bioaerosols and moisture after sentinel cases, even if only a single form of disease is initially recognized.
The long-term consequences of HP are unclear. There is widespread agreement that the progression of clinically identified diseases often resolves as exposure is controlled. Nevertheless, low-grade disease may commonly remain unidentified. Lung function in asymptomatic pigeon breeders with ongoing exposure may continue to decline at an advanced rate. Recent case-control studies of interstitial lung diseases and sarcoidosis have consistently identified exposure to metal, farming, or other bioaerosols as a risk factor for UIP. 104 Both sarcoidosis and UIP were encountered in the plant with MWF. The age distributions of UIP and the granulomatous disorders (including sarcoidosis and HP) are different with the former occurring later in life and the latter a disease of middle age. Symptoms and acute lung function decrements are common among metal workers and are associated with exposure to respiratory bacteria, which are often interpreted as airways disease rather than HP. 104 The strong associations between HP and asthma questionnaire responses in this outbreak suggest that future work should attempt to explore and define overlaps between asthma, HP, and end-stage fibrosis more clearly, particularly with regard to early identification and prevention of disease progression and disability. Such an overlap was recognized in at least another occupational disorder such as cobalt-associated hard metal disease. 104 Adhering to standards as promulgated by the Occupational Safety and Health Administration (OSHA) does not prevent all disease, because the plant that had the index case of MWF-related HP met the OSHA criteria.

E. Hot Tub Lung
Hot tub lung refers to diffuse lung disease occurring in otherwise healthy, immunocompetent hosts without bronchiectasis and is thought to be a less wellrecognized clinicopathologic manifestation of mycobacterium avium complex (MAC) infection linked to hot tub exposures. A case report described an immunocompetent young adult who had MAC infection associated with hot tub use and was treated successfully with a four-drug regimen for MAC. 7 Clinically, radiologically, and pathologically, noninfectious diseases such as HP and sarcoidosis have been considered in the differential diagnosis. Moreover, another previous study concluded that their hot tub lung cases were more suggestive of HP than infection, based on the clinical course with spontaneous recovery and lung biopsy findings. 5 However, no serologic result was provided to support such a conclusion in any of their five hot tub lung cases included in the report. 5 Recently, a clinicopathologic study on hot tub lung was reported based on 10 cases of diffuse infiltrative lung disease caused by atypical mycobacteria, primarily MAC, in otherwise healthy people who had used a hot tub. 8 The most common symptoms were dyspnea, cough, hypoxia, and fever. Histologic examination revealed exuberant non-necrotizing, frequently bronchiolocentric granulomatous inflammation in all cases (Figures 11 and 12). The inflammation often was also associated with patchy chronic interstitial pneumonia and organization. Cultures revealed nontuberculous mycobacteria (NTM) in all cases, but staining for acid fast bacilli was positive in only one case. Four patients received corticostroids alone for presumed HP, four were treated with antimycobacterial therapy, and two received both. All patients demonstrated significant improvement at the time of follow-up. Although the histopathologic features appear to be distinct from HP, the pathogenesis of hot tub lung may involve infection, hypersensitivity, or both.

F. Others
Isolated cases of HP have been reported in the literature in association with a variety of agents, including proteolytic enzyme in a cleaner, 111 intranasal heroin use, 112 cat hair, 14 hard metal alveolitis accompanied by rheumatoid arthritis, 113 chronic ILD in nylon flocking industry workers, 114 a case of lymphocytic alveolitis, myositis, and arthritis associated with exposure to Aspergillus niger. 115 A high prevalence of mollusc shell HP in nacre factory workers has been reported. 116 HP among workers cultivating Tricholoma conglobatum (shimeji) has been also noted. 117

G. Questionable Entities
HP is defined as an immunologically mediated lung disease induced by inhaled antigen. Some lung diseases induced by systemic antigen may manifest similar to HP clinically and pathologically. For example, interstitial lung disease induced by endogenous Candida albicans reaching the lungs from the intestinal tract via the blood stream has been reported as having clinicopathologic features of HP. 118 Other examples include HP associated with oral medications such as Paclitaxel (taxol, an anticancer agent), fluoxetine hydrochrloride (prozac, an antidepressant), sotalol (a beta-blocker), and hydroxyurea (an anticancer agent). [76][77][78]119

XI. PATHOGENESIS
HP is caused by repeated inhalation of an antigen that provokes an array of humoral and cellular immune responses in the distal bronchioles and alveoli of susceptible individuals. Despite the seemingly apparent immunological nature of the disease, a fundamental difficulty in understanding the immunopathogenesis of 613 FIGURE 11. Histologic findings of a hot tub lung case are similar to those in HP cases. HP is that many of these immunologic responses are found in both symptomatic and asymptomatic individuals who have been exposed. Poorly understood host factors appear to be crucial because usually fewer than 10% of exposed individuals develop clinical disease. Pertinent issues about hypersensitivity reactions, roles of alveolar macrophages, and potential effects of viral infection in HP are discussed in this section. Various experimental models of HP and putative mediators with roles of Th1 and Th2 cytokines in HP are also covered.

A. Mechanisms of Hypersensitivity Reactions
The immune reaction in HP does not fit clearly into any one of the four classic types of hypersensitivity reactions defined by Gell and Coombs. More than 30 years ago Pepys et al. proposed that HP resulted from the immune complex deposition in the lung, that is, type III hypersensitivity reaction. 94 The acute symptoms of the disease are likely to be due to immune complexes deposited within the lung soon after inhalation of antigen. 94 These immune complexes initiate inflammation by activating complement through the classic pathway and/or by directly stimulating alveolar macrophages to secrete inflammatory mediators. 94 The time course for development of symptoms, the early neutrophilic alveolitis, the high antibody titers and the evidence for complement activation within the lung are all consistent with this proposal. 94 However, a number of observations in HP are not explained by immune complex-mediated type III reaction alone: the presence of granulomas, the fact that many individuals who develop precipitins remain asymptomatic, and the suggestion that the disease may sometimes occur in individuals who lack antibodies. 120 Prominent lymphocytosis in the BAL fluid as well as granuloma formation support the role of a type IV hypersensitivity reaction in HP. However, it should be noted that persistent immune complexes can induce granulomatous inflammation (especially large complex carbohydrate-containing antigens such as mucin), despite the general notion that granulomas are the evidence for a T cell-mediated immunopathology. 94 Antigen-specific T cells in the lung persist for some time after the acute symptoms have disappeared and would be activated after reexposure to antigen; however, it is thought to be unlikely that this occurs quickly enough to account for acute symptoms of HP. 94 In chronic HP, a cell-mediated type IV reaction may dominate the pathology, especially in patients who have not experienced symptoms of acute disease. 94 HP patients who lack serum antibodies might also be explained by a dominant type IV immunopathology. 94 An immunofluorescence study to detect immunecomplex deposition in the lung biopsies from PBL patients has been noncontributory, which decreases the importance of type III immune complex-mediated mechanism, and supports the role of type IV reaction, at least in subacute and chronic forms of HP. 121 Precipitins may not be involved in the pathogenesis of HP. [122][123] In a farming population, the presence or absence of precipitins did not correlate with chest symptoms. 124 Subjects with disease, however, tend to have higher titers of specific antibodies than asymptomatic exposed individuals, and some patients have antigen-antibody complexes in their lungs. 125 Serum precipitins are associated with BAL lymphocytosis, 59 but this may reflect only the intensity of exposure. In animals, sensitization or susceptibility to SR cannot be affected by transfusing specific antibodies. 126 It has been shown that in rabbits, serum antibodies remain high as long as the animal continues to receive the antigen, even when the lung response progressively decreases over time. There has been a report of a case who simultaneously had hypogammaglobulinemia and HP, suggesting that the humoral immunity may not be required in the pathogenesis of HP and cellular immunity probably plays a central role in developing HP. 127 Although there has been a study reporting an involvement of mast cell and histamine in FL patients, an IgEmediated type I hypersensitivity reaction is probably not a major pathway in HP. 128 It is still not clear whether there are any special properties in the antigens causing HP. A direct role of an antigen or other substances within the inhaled dust in facilitating the inflammatory events has been suggested. 21,94 Inhaled antigenic organic dusts may act as an adjuvant to enhance the immunologic reaction as well as exert other effects, including activation of pulmonary alveolar macrophages, direct toxic effect with their intrinsic enzyme activity, production of nonspecific precipitins, and histamine release. 21 In summary, susceptible individuals develop HP after repeated inhalations of an antigen via both type III and IV hypersensitivity reactions, which are probably influenced by a variety of factors related to the inciting antigen itself and regulatory mediators derived from activated T cells and macrophages.

B. Roles of Alveolar Macrophages in HP
Alveolar marcrophages (AMs) comprise a heterogeneous population of mononuclear phagocytes reflecting different stages of activation and differentiation. [129][130] These cells are able to change their phenotypic pattern and functional properties in response to exogenous stimuli. The modulation of membrane antigens probably helps AMs to effectively regulate their scavenger activity in the lung microenvironment. Because a significant portion of inhaled organic antigens reaching the lungs is first introduced to AMs due to their strategic anatomic location, the antigen-presenting capacity of AMs is probably very important in the pathogenesis of HP.
Although generally known as poor antigen-presenting cells, AMs constitutively express class II major histocompatibility complex HLA-DR and are crucial cells for the initiation and maintenance of local immune responses. [129][130] A histopathological study showed that activated AMs in HP express ICAM-1, which 616 probably enhances the antigen-presenting capacity of AMs by increasing interaction with T cells via its ligand LFA-1 on the T cell surfaces. 131 In contrast, AMs in normal lungs, UIP, experimentally induced Goodpasture's syndrome or pulmonary angiitis exhibit little or no ICAM-1 expression on their cell surface. [132][133] Another study of ICAM expression on AMs that used flowcytometry and immunocytochemical methods on BAL fluids reported that HP patients had a higher percentage of ICAM-1-positive alveolar macrophage (75% vs. 30%) and that the antigen density was more than sixfold higher when compared with control donors. 134 In this study, the levels of soluble serum ICAM-1 were also increased in HP patients, suggesting that antigen contact up-regulates ICAM-1 expression on AMs in HP, which probably resulted in shedding of some soluble ICAM-1 (sICAM-1) into circulation and a subsequent increase of sICAM-1 in serum. 134 A blockade of LFA-1, the ligand for ICAM-1, has been shown to abrogate the fibrosis and tissuedamaging reaction, but not the alveolitis, in a murine model of HP. 135 Other signs of AM activation include pronounced expression of HLA class II molecules and transferrin receptor on the cell surface. 134 Prominent changes of the alveolar marcrophage phenotype were also found in another granulomatous disease sarcoidosis with more frequent expression of ICAM-1 (CD54), KiM2, CD71, CD11b, and RFD9 than in normal lungs. This study showed that the phenotype of AMs is related to the type and intensity of the immunopathological reaction in the lung and correlates with the proportions of bronchoalveolar cells. 136 T cell activation generally requires two distinct signals. [137][138] The first signal is antigen-specific and delivered through the engagement of T-cell receptors (TCRs). The second signal is mediated by the interaction of co-stimulatory molecules present on antigen-presenting cells with their ligand on T cells. The B7/ CD28/CTLA4 is a major pathway that provides these potent co-stimulatory signals crucial for complete T cell activation. Both CD28 and CTLA4 are ligands for B7, but have different avidities in that CTLA4 binding is 20-to 100-fold higher than CD28. 138 This difference in avidity has been exploited to block B7/CD28 interactions by the use of CTLA4-Ig, a soluble fusion protein made from the extracellular portion of CTLA4 linked to the Fc portion of IgG. 139 This decoy soluble protein can decrease or block B7/CD28 costimulatory pathway by masking available B7 molecule with its higher avidity, which results in downregulation of B7/CD28induced T-cell activation and differentiation as well as in influencing Th1 and Th2 cytokine production. 138 In normal lungs, AMs have a low expression of B7 molecules and a poor capacity or function as APCs. 30,140 Cormier et al. 30 sought to study whether AMs of HP patients increase their antigen-presenting capacity by upregulating the expression of B7 costimulatory molecules (CD80, CD86). They also studied whether the stimulatory effect of B7 expression is enhanced by viral infection and abrogated by smoking. 30 The results of this study suggested the following: (1) an upregulation of B7 molecule expression is involved in the lymphocytic alveolitis of HP; (2) a viral infection could enhance HP by increasing B7 expression; (3) the protective effect of cigarette smoking in HP may be due to the low level of expression of costimulatory molecules on AMs from smokers and to their resistance to further upregulation.
In a subsequent study, the same group of authors investigated the effect of CTLA4-Ig, a CD28/B7 antagonist, on the lung inflammation in the mice inoculated with S rectivirgula (SR), a major causative agent of HP. 138 The result showed that blockade of CD28/B7 interactions by CTLA4-Ig inhibits SR-induced lung inflammation and immune response to SR Ag in mice and may provide a novel approach in the treatment of HP. 138 Although the hallmark of HP is a BAL lymphocytosis, the central event in the pathogenesis of HP is possibly the activation of the alveolar macrohages. 21 The activation of alveolar macrophages results in the production of a range of mostly proinflammatory cytokines. These may act to reinforce the alveolitis, probably leading to the recruitment of additional cell types. Cytokine production by macrophages may also account for many of the systemic symptoms. Repeated activation of macrophages may ultimately lead to development of granulomas within the lung tissue. 30 Macrophage activation could be mediated directly by immune complexes and/or incoming T-cells. 30 Subjects with acute HP were found to have alveolar macrophages that release high levels of MIP-1 alpha and IL-8. 141 IL-8 is secreted principally by cells of the macrophage/monocyte series, and it is a strong neutrophil chemoattractant in vivo and in vitro. 142 MIP-1 alpha is a heparin binding protein that has significant proinflammatory properties; it has pyrogenic activity and modulates macrophage monokine relaease. 141 In addition, MIP-1 alpha appears to attract CD8-positive T lymphocytes strongly. [143][144] Both IL-8 and MIP-1 alpha have been shown to be released at high levels in lung pathologies such as sarcoidosis and idiopathic pulmonary fibrosis. 141

C. Viral Infection
Many patients who develop HP report a recent viral respiratory infection and often complain of "flu-like" symptoms. A previous study reported the common presence of influenza A viruses in the lower airways of patients with acute HP. 145 The presence of viruses was documented by using a reverse transcription and polymerase chain reaction (RT-PCR) panel for 10 common respiratory viruses in BAL cells that were obtained from acute HP patients, along with the BAL cells from unexposed healthy volunteers as controls. 145 Immunocytochemistry was subsequently used to localize viral proteins within BAL cells as well. The results of PCR showed that influenza A virus was the most frequently detected virus in the BAL cells of this study patients and control subjects. Influenza A proteins were detected within alveolar macrophages in 9 of 13 patients and 2 of 6 control 618 subjects. This report documented the presence of viruses in the lower airways of patients with acute HP. The findings may imply a potential role of this virus in the modulation of HP during antigen exposure. 145 A murine model of HP was studied to determine whether viral infection modulates expression of HP. 146 Some mice were infected with respiratory syncytial virus (RSV) 2 weeks prior to SR exposures in this study. Control groups received saline or heat-inactivated RSV in lieu of RSV before SR exposures. SR-treated mice developed a typical, early neutrophil response and a late granulomatous inflammatory response. Upregulation of IFN-gamma and IL-2 gene expression was also found during the late response. These responses were augmented by preceding RSV infection, but not by heat-inactivated RSV or saline. Mice with RSV infection also had a greater early neutrophil response to SR, with increased macrophage inflammatory protein-2 (MIP-2, murine homologue of IL-8) release in BAL fluid. These findings suggested that viral infection can augment both the early and late inflammatory responses in HP. 146 As mentioned earlier, viral infection has been shown to enhance antigeninduced expression of costimulatory molecule B7 on the surface of AM. 30 The activation of B7-mediated T cell signaling pathway by viruses probably contributes to T cell activation and differentiation and ultimately results in pronounced lymphocytic alveolitis of HP.

D. Experimental Models of HP
There are essentially three categories of HP animal models that have been described in the literature: (1) repeated nasal or intratracheal (i.t) injection of various antigens, recapitulating the natural way of repeated antigen inhalations occurring in human HP, (2) initial sensitization via intraperitoneal (i.p.) or intravenous (i.v.) injection of antigens followed by later i.t. challenge, a modification of the first category, and (3) adoptive transfer of sensitized immune cells from one animal to another naive animal followed by a later intratracheal challenge. Mice, rats, and guinea pigs were the most frequently used animals. Several sensitive or resistant strains of mice have been recognized and used to study various mediators involved in the pathogenesis. 147 Also, some knockout mice lacking genes coding cytokines such as IFN-gamma, IL-10, and IL-12 have been made for studying the crucial roles of these cytokines. [147][148][149] The most commonly used method for repeated inhalation HP model applies lyophilized, endotoxin-free S. rectirvigula (SR) on a SR-sensitive strain of mice such as C57BL/mice. This model has been well described and often used to study immune mechanisms that cause HP. [150][151][152][153][154][155][156] A typical schedule of antigen exposure consists of installing SR antigen via intranasal canulation under light anesthesia for 3 consecutive days per week for 3 weeks. In this model, mice exposed to SR via nasal inhalation develop diffuse bronchoalveolitis and form granulomas, which closely simulate the antigen exposure and histopathology of HP. Knock-out mice lacking expression of gene coding for IFN gamma, IL-10, or IL-12 have been developed in order to study the roles of these cytokines by comparing the responses with their wild-type normal littermates using this model system. [147][148][149] Other antigens used for repeated inhalation to induce HP include Rhizopus nigricans, which is one of the most common members of the mucorales and has been implicated as a causative antigen for malt-workers lung and wood-trimmer's disease. 157 The second type of HP model eliminates the inhalation stage for sensitization, by introducing antigen systemically via intravenous (i.v.) or intraperitoneal (i.p.) injection, followed by a single i.t. challenge of the antigen 2 to 3 weeks after the systemic sensitization. 56,158 This method can avoid repeated inhalations in the first type, thus decrease the variables involved in multiple manipulations or procedures for experiments. A rodent model of HP using highly antigenic Corynebacterium parvum (C. parvum) has been described. 56 In this study, sensitized rats receiving i.t. challenge of the antigen were sacrificed at different time points to examine the kinetics of alveolar inflammation (BAL fluid inflammatory cell absolute counts with differential and proinflammatory cytokine protein levels) and histopathology as well as mRNA and protein expression of CC chemokine, CXC chemokine, and adhesion molecule in the whole lung tissue. 56 The histologic and other changes in this model closely simulated HP. In the same study, the above parameters were examined in the rats that were challenged via i.v. injection in lieu of i.t. challenge. The results showed that the i.t. or i.v. challenge with C. parvum in sensitized rats caused pulmonary granulomatous inflammation that was histologically similar to human HP and sarcoidosis, respectively. 56 Although the soluble and cellular mediators of granulomatous inflammation were qualitatively similar in both disease models, the differing anatomic source of the same antigenic challenge was responsible for the differing clinicopathological presentations. 56 An adoptive transfer model for HP uses cells from animals sensitized to SR that are cultured in the presence of SR and then injected into a naive recipient. [159][160][161] The recipient animals exhibit an exaggerated pulmonary histologic response to subsequent i.t. challenge with SR. [159][160][161] This phenomenon is dependent on the sensitization of the donors, culture of the cells with antigen, concentration of antigen, and the number of transferred cells. The cells responsible for transfer of adoptive HP have phenotypes of T cells that are Thy1.2-positive, CD4-positive, CD8-negative, and Ia-negative. 162 This model system is convenient for manipulating the subsets of T cells, and thus is very useful in dissecting the role of cellmediated immunopathogenesis in HP. A study exploring the effect of CD4 cell depletion concluded that the adoptive HP is dependent on the CD4-positive cells. 163 This study also showed that the acute inflammatory response to SR was not CD4positive cell dependent. 163 A previous study investigated the influence of Th1 or Th2 bias by using different stains of mice C3H/HeJ and BALB/c mice for Th1 and Th2 bias, respectively. 29 This study concluded that HP induced by SR can occur in animals with either a Th1 or a Th2 bias and is not altered by treatment with IL-620 4 and anti-IFN gamma. 29 It suggested that attributes of the antigen and not genetic background or cytokine environment at the site of initial sensitization determines the results of exposure to SR. 29,164

E. Mediators of HP
It has become apparent that T-cell responses are frequently polarized toward different patterns of cytokine production. In mice, CD4-positive T helper cells are classified into Th1 and Th2 types; Th1 cells make IFN-gamma but not IL-4, and the opposite is true for Th2 cells. 165 This distinction is less clear in human T helper cells and T cells producing only Th1 or Th2 cytokines do not exist in humans. IFN gamma and IL-2 are major Th1 cytokines; IL-4, IL-5, and IL-10 are Th2 cytokines. 47 In general, Th1 cytokines are present in cell-mediated immune responses and appear to be involved in chronic inflammatory conditions, whereas Th2 cytokines up-regulating IgE production are prominent in the pathogenesis of allergic diseases such as asthma. 47 Animal models suggest that HP is facilitated by the overproduction of IFN gamma and that IL-10 ameliorates severity of the disease, indicating a Th1-type response. [148][149] A study using human HP cases concluded that clinical HP is also characterized by a predominance of IFN-gamma-producing T cells, thus polarized Th1 cytokine profile in BAL T cells in humans. 47 However, a previous study suggested the functional predominance of Th2 lymphocytes in BAL fluid from HP patients. 166 Up-regulation of CC (MCP-1 and MIP-1 alpha) and CXC (MIP-2, KC, and IP-10) chemokines, inflammatory cytokines IL-1 alpha, TNF, and IL-6, and Th1-differentiating cytokine IL-12 has been reported in experimental models of HP. 56,167 Chemokines and cytokines induced by intratracheal adminstration of SR precede BALF neutrophilia and lymphocytosis and may cause differentiation of Th1 cells. 167 IFN gamma is essential for the expression of HP, because knock-out mice lacking IFN gamma gene develop minimal inflammation without granulomas after antigen exposure, which was reversed by the replacement of IFN gamma as was in wild-type mice. 148 In a murine model of HP, the role of IL-10 has been shown to modulate the effect of proinflammatory cytokines, including IFN gamma, IL-1, and TNF. 149 Mice that could not produce IL-10 had more severe pulmonary inflammatory changes than wild-type littermates, and they showed increased Aginduced expression of the IFN gamma, IL-1, and TNF genes. When IL-10 is replaced, IL-10 knock-out mice resembled their wild-type littermates. 149 IL-12 also has been shown to modulate expression of HP in the same model. 147 A study examined the role of IL-2 in the development and persistence of lymphocytic alveolitis in cases of FL. 168 This study showed that acute FL patients and ex-FL patients had more lymphocytes and higher levels of IL-2 in their BAL fluid than asymptomatic farmers. BAL T cells from acute and ex-FL patients released con-siderable amounts of IL-2 after stimulation with concanavalin A and showed dosedependent proliferative responses to IL-2. IL-2 production was decreased after treatment with prednisone. Acute FL, but not ex-FL patients, had higher levels of soluble CD25 (IL-2 receptor) in their serum than asymptomatic subjects. These results suggest that IL-2 may play a role in farmer's lung by providing a stimulus not only for the accumulation of lymphocytes but also for their persistence at the site of the hypersensitivity reaction, and that the lung is a likely source of this cytokine in vivo. 168 Other regulatory molecules have been implicated in the regulatory mechanisms of HP. A study postulated that the stress response with heat shock protein (HSP) could play a role in the pathogenesis of HP. 169 HSP response in HP may be inadequate to control the release of damaging cytokines such as IL-1, IL-8, and TNF alpha that are normally downregulated by HSP. 169 The data in this study supported the hypothesis that inadequate HSP response to further stress could contribute to the production and the effect of proinflammatory cytokines. However, HSPs are probably are not solely responsible for the development of HP.
Hara et al. reported apoptosis and FAS/Fas ligand mRNA expression in acute immune complex alveolitis in mice. 170 They also reported in another study that the up-regulation of p53, p21, and Fas in bronchiolar and alveolar epithelial cells associated with DNA damage may precipitate the process of pulmonary fibrosis in interstitial pneumonia associated with collagen vascular diseases and chronic HP. 171 The pulmonary surfactant system may also be involved in the manifestation of HP; moderate changes in biochemical and physical surfactant properties were found in HP as well as in sarcoidosis. 172 Proteoglycans are a group of glycoproteins that each consist of a core protein covalently linked to a variable number of negatively charged glycosaminoglycan chains. 173 In nongranulomatous fibrotic lung conditions, early collagen synthesis by myofibroblasts occurs in an extracellular matrix rich in the proteoglycan versican. It has been hypothesized that versican is associated with the process of collagen synthesis resulting from chronic inflammation. This study examined the localization of proteoglycans and collagen in the granulomatous lung conditions, sarcoidosis, HP, and TB. 173 Proteoglycans have a number of biological functions in the ECM, including influencing tissue mechanics, tissue water homeostasis, cell adhesion, cell migration, and the biological activities of several growth factors. A study reported the association between proteoglycans and collagen synthesis in granulomatous lung diseases, including HP. This study concluded that the deposition of versican is specific to the early remodeling process in both granulomaous and nongranulomatous lung disease. In both forms of lung fibrosis, regardless of the nature of the driving inflammatory process, collagen sysnthesis takes place in a versican-rich provisional matrix. These results suggest that versican may influence the progression of the repair process following many different types of lung injury. 173 Free radicals have been suggested to play an important role in the pathogenesis of interstitial lung diseases. Because manganese superoxide dismutase (MnSOD) and catalase are two important intracellular antioxidant enzymes that probably play a central role in lung defenses, the localization and intensity of these two enzymes were studied by an immunohistochemical method. 174 Manganese superoxide dismutase and catalase have been shown to be coordinately expressed in the alveolar region in chronic interstitial pneumonias and granulomatous diseases of the lung, including HP. 174 This simultaneous expression of mnSOD and catalase in the alveolar region suggests their protective role against the progression of lung disease.