Asthma and Covid-19: the Eosinophilic Link

The Covid-19 disease caused by the coronavirus SARS-Cov2 has reached pandemic proportions in an extremely short period of time. It happened so fast that, at this point, with more than 200.000 deaths worldwide, and despite the overwhelming efforts enlisted by the global scientific community, we still don't know much about a number of key issues, such as the way the virus causes such a damage in the lungs of patients at risk, how the infection raises the immune response (if any) against it, or what is the long term outcome of patients surviving the devastating complications of the disease. Among the questions that don't seem to gather as much interest, however, one has attired our attention. We are especially keen to find epidemiological factors that could protect people from having the disease, and, at any rate, from presenting the lethal complication thereof: the SARS or Severe Acute Respiratory Syndrome. On this route, after going through the published patient series, we have begun to work on the hypothesis that asthma could be a protective factor in the context of Covid-19. CONCLUSION: In this paper we analyzed different aspects of previous research on asthma, asthma immunology, SARS, and the innate immune system and have come to a starting model of how the asthmatic immune signature (the activation of the Th2 lymphocyte pathway and its main effector cell type, the eosinophil cell) could influence and modulate the response to the Covid-19 infection. In this model, both the Th2 program, with the eosinophil cell at its core, is posited as a natural counterbalance of the Th proinflammatory response (Th1 and Th17). The model proposes a pivotal place for the innate immunity, in which the Pattern Recognition Receptors (particularly the ssRNA recognizer, TLR7) would play a leading role in the rampant inflammatory response to the virus (Th1/Th17 driven) that underlies the development of SARS. IMPORTANCE: Although not presented as such in the literature, we put forward a vision in which innate immune receptors such as TLR7 and the TH2 programs could be the antagonic forces deciding the fate of lung diseases such as ARDS, asthma or COPD. The careful mining of this model as we describe it herein, could hopefully lead to new urgently needed therapies for the Covid-19 disease.

asthma, asthma immunology, SARS, and the innate immune system and have come to a starting model of how the asthmatic immune signature (the activation of the T h2 lymphocyte pathway and its main effector cell type, the eosinophil cell) could influence and modulate the response to the Covid-19 infection. In this model, both the T h2 program, with the eosinophil cell at its core, is posited as a natural counterbalance of the T h proinflammatory response (T h1 and T h17). T he model proposes a pivotal place for the innate immunity, in which the Pattern Recognition Receptors (particularly the ssRNA recognizer, T LR7) would play a leading role in the rampant inflammatory response to the virus (T h1/T h17 driven) that underlies the development of SARS.
IMPORT ANCE: Although not presented as such in the literature, we put forward a vision in which innate immune receptors such as T LR7 and the T H2 programs could be the antagonic forces deciding the fate of lung diseases such as ARDS, asthma or COPD. T he careful mining of this model as we describe it herein, could hopefully lead to new urgently needed therapies for the Covid-19 disease.
I n t r od u ct i on I n t r od u ct i on In December 2019, pneumonia cases caused by a new coronavirus (SARS-cov2) were starting to be diagnosed in Wuhan, China [1] [2] [3] . Since then, the virus has spread to mainland China and all over the world, reaching pandemic proportions.
T he SARS-Cov2 virus is a close relative of the SARS-cov1 virus that caused the SARS epidemic in 2003. It belongs to the family of beta-coronavirus, single stranded +RNA viruses, that includes the SARS-Cov1 and the NL63, the latter being a frequent cause of seasonal common cold. Among the viral proteins, the spike (S1) protein is essential for host recognition and viral entry (infection) into the cells. It has been recognized that the cell surface receptor for the S1 protein of SARS-Cov2 is the same as for the SARS-cov1, the virus that caused the 2003 SARS pandemic [4] .
In the majority of infected people, the SARS-cov2 virus causes a flu-like disease that resolves spontaneously (the so-called covid-19 disease). In a percentage of cases, however (between 10-20%, the subgroup of patients at risk of developing lethal complications), after the flu like syndrome, the disease can progress into a Severe Respiratory Distress Syndrome (SARS), which is the cause of hospitalization and eventually of death. Patients at risk of developing SARS after infection with the SARS-Cov2 virus include older patients (> 65 years old), people with one or more comorbidities (such as cardiovascular diseases, high blood pressure, or diabetes) and people institutionalized in nursing homes [5] .
AN EXCESSIVE IN FLAM M AT ORY REACT ION IN T H E AIRWAYS IS T H E CAUSE OF  AN EXCESSIVE IN FLAM M AT ORY REACT ION IN T H E AIRWAYS IS T H E CAUSE OF   DEAT H in COVID-19 PAT IEN T S.  DEAT H in COVID-19 PAT IEN T S. SARS occurs in the subgroup of patients at risk, where a hyper-inflammatory reaction within the lung airway and in the lung interstitial milieu occurs during the course of the viral infection. T his aberrant inflammatory response leads to tissue injury, leaking of fluid into the alveolar cavity, impaired gas exchange, and, eventually, death by respiratory failure.
In a previous paper, we have postulated that the common theme in patients at risk of developing SARS is the background pre-activation of the Renin Angiotensin System (RAS).
In these patients, an already turned-on RAS will act as a jump-start for the magnified response of the innate immune system to the SARS-Cov2 viral infection that is witnessed in cases with a severe disease presentation. Based on this premise, we have proposed a treatment plan for the consideration of the scientific and medical community. T his treatment is, with small modifications, basically the same as in the so-called SMART asthma protocol that uses a combination of an inhaled glucocorticoid (preferentially budesonide) and an inhaled long acting beta-2 adrenergic agonist (LABA) (preferentially formoterol). T he main indication of this protocol during asthma attacks is as a bronchodilator, but as we reviewed the literature, we discovered that the combination of Budesonide/formoterol is a potent local immunosuppressant, that is predicted to precisely counter the cytokine/inflammatory cell storm that underlines the first stages of SARS. In this plan, we recommend to begin treating the patients at risk early on in the disease (at the time of presentation of the first flu-like symptoms), thus emphasizing the need to begin treatment much before the appearance of dyspnea during the second week of the disease. T he sudden presentation of dyspnea is usually the confirmation that SARS is progressing and, oftentimes, too late to take effective therapeutic action against the disease [6] .

AST H M A POST ULAT ES IT SELF AS A POSSIB LE IN DEPEN DEN T PROT ECT IVE AST H M A POST ULAT ES IT SELF AS A POSSIB LE IN DEPEN DEN T PROT ECT IVE
FACT OR FOR COVID-19 FACT OR FOR COVID-19 From the study of all the major multi-center patient series of covid-19 patients (from China [7] [8] , or the USA [9] ), a few points stand out as interesting possible therapeutic leading points: 1) Among the patients with a severe clinical presentation, there is an overrepresentation of older people as well as of a number of comorbidities (cardiovascular other common related diseases such as allergic rhinitis or atopia [10] . Even more telling to the latter point, a recent study from Italy in 23.188 patients dying from Covid-19 disease, among the 16 possible comorbidities cited in the paper, there is no mention of asthma or allergic rhinitis, in spite of the high prevalence of those conditions in the general population [11] .T his raises the issue of whether suffering from asthma is in itself a protective factor against developing a symptomatic Covid -19 disease and, more especially, against presenting the lethal complications of the disease. T his alone goes beyond asthma protecting patients from developing a symptomatic covid-19 disease but, more importantly, from dying from the infection.

AST H M A AN D SARS: T H E EOSIN OPH IL CON N ECT ION AST H M A AN D SARS: T H E EOSIN OPH IL CON N ECT ION
Bronchial asthma is a chronic inflammatory disorder of the airways that affects 300 million people worldwide. It is characterized by bronchial hyperreactivity (BHR), mucus overproduction, and airway remodeling and reversible airway obstruction.
Eosinophils are special polymorphonuclear leukocytes. T hey develop in the bone marrow and migrate into blood, making up about 1-6% of white blood cells. T hey were first observed by Wharton Jones in 1846 and named by Paul Ehrlich in 1879, because they contain intracellular granules that are intensely stained by the acidophilic dye eosin.
T he presence of large specific granules, also known as secondary granules, is a characteristic feature that distinguishes eosinophils from other granulocytes (neutrophils and basophils). T he predominant substances in these granules are four proteins: the antiparasitic (helmintho-toxic) major basic protein (MBP), the bactericidal eosinophil peroxidase (EPO), the eosinophil cationic protein (ECP), and the eosinophil-derived neurotoxin (EDN) [14] . T he latter two proteins are members of the emerging RNase A gene superfamily, and are capable of degrading single-stranded RNA (ssRNA) substrates with high efficiency [15] . T he fact that two of the four most abundant eosinophilic specific proteins are ssRNAses has a central role in the physiopathogenesis of asthma but also, as we will see later on, it could have an important bearing in the possible physiopathological mechanisms underlying the protective role of asthma against SARS [16] .
T his should not come as a surprise, since SARS is a super-inflammatory disease characterized by a massive migration of inflammatory cells and the release of cytokines in the lung. Indeed, a decrease of the eosinophil blood count has been consistently observed in a number of different disease settings, all of them with a common theme: inflammation. T hus, a rapid and persistent decrease in the numbers of circulating eosinophils is a distinctive aspect of the physiological response to acute inflammation [19] .
Also, eosinopenia (low eosinophil blood count) is an independent predictor of death in patients with pneumonia but not with chronic respiratory disease [20] [21] . In fact, in another study, a cutoff of <220 eosinophils/mm3 was 100% sensitive for sepsis prediction in critically ill patients [22] .
Although important, eosinopenia would only be a good marker of disease severity in processes underlying an inflammatory activity. However, we think there is much more to it than a simple correlation with disease severity in inflammation. We believe that the peripheral blood low count (sometimes straight absence) of eosinophils reflects something very relevant to the physiopathology of inflammation at large, and more concretely to the ARDS that occurs during the course of Covid 19. T o put this in the right context, we should recall a few relevant observations from the literature.

Samples from lung biopsies or bronchoalveolar aspirates of covid-19 SARS patients show a massive infiltration of inflammatory cells (notably FCN1 macrophages) in a proteinaceous magma, but there is a striking absence of eosinophils in SARS-Cov or in
Covid-19 samples [23] .

T he analysis of both the immune cell and cytokine profile in the lungs of SARS indicates
a very strong skewing of the immune response towards a T h1 phenotype of inflammation [24] [25] . 3. Both asthma and chronic obstructive pulmonary disease (COPD) are characterized by an inflammatory process. However, in asthma, the most striking feature is the eosinophilic infiltration, whereas, in COPD, it is the CD8 T -lymphocytic infiltration of the airway wall [26] . 4. In patients with Chronic Obstructive Pulmonary Disease (COPD), a predominant T h1 inflammatory condition, a number of studies have shown that the higher the blood eosinophil count, the greater the positive response to inhaled corticosteroids in the form of reduced exacerbation [27] [28] . suggesting either a mere correlation or a causal effect of the eosinophil rise on the clinical improvement in those patients. 5. Eosinophilia in the airways is a feature of rhinitic patients (even without asthma) [29] and allergic rhinitis is also underrepresented in patients with covid-19. So far, we have shown that the massive immune response that is behind SARS is predominantly of the proinflammatory type (mainly T h1 but possible others like T h17 too), while signs of even a minimal activation of the T h2 type seem to be absent in SARS.
T his includes, the lack of presence of the major cellular effector T h2 activation: the eosinophil. It is thus reasonable that both the absence of eosinophils in lung histological samples of covid-19 patients, and the low eosinophil counts in peripheral blood, together with the massive presence of eosinophils in "low risk for SARS" conditions such as asthma and rhinitis, could suggest a protective role of these cells against the covid-19 disease and its complications. In addition, we could hypothesize that activated eosinophils as present in asthma, at least, through the massive production of ssRNA ribonucleases could hamper viral infectivity of SARS-cov2, whose genome, as any of the members of the coronavirus family, consists of a single chain of ssRNA. Although not yet thoroughly studied in the context of the SARS-Cov2 pandemic, different previous observations point to a key role of T LR7 in SARS-Cov2. First, T LR7 appears to be the key receptor for the first line response to RNA viruses [36] . More to the point, SARS-CoV ssRNAs had powerful immunostimulatory activities to induce considerable level of pro-inflammatory cytokine T NF-a, IL-6 and IL-12 release via the T LR7 and T LR8, almost 2-fold higher than the strong stimulatory ssRNA40 that was found previously from other virus [37] . Moreover, SARS-CoV ssRNA was able to cause acute lung injury in mice with a high mortality rate in vivo, suggesting that SARS-CoV specific GU-rich ssRNA plays a very important role in the cytokine storm associated with a dysregulation of the innate immunity and could open a new therapeutic strategy [38] . Finally, ssRNA40 induces cell death in mice through binding to T LR7 and activation of downstream signaling molecules, while T LR7−/− mice are entirely protected against ssRNA40 induced cell death [39] .

T LR7 ACT IVAT ION CAN CELS OUT T H E T H 2 PH EN OT YPE T LR7 ACT IVAT ION CAN CELS OUT T H E T H 2 PH EN OT YPE
T here are good reasons to view the T LR7 receptor as a counterbalance of the T h2 axis.
First, T LR7 activation prevents ovalbumin-induced airway hyperreactivity, eosinophilic inflammation, goblet cell hyperplasia and airway remodeling in murine models of asthma.
Second, T LR7 activation also inhibits viral replication in the lung and prevents virusinduced airway hyperreactivity. Furthermore, it has recently been shown that stimulating T LR7 rapidly relaxes airway smooth muscle, dilating the airways [40] . T he abrogation of the T h2 pathway by T LR7 is so relevant that it has recently been exploited to the design of novel anti-asthmatic compounds with T LR7 agonistic activity [41] .
By contrast, T h2-inducing pathological conditions such as parasitic diseases alter the response to viral infections through yet unclear mechanisms. On the one hand, IL-4, the mastermind of theT h2 response, inhibits the secretion of antiviral type I interferons and, on the other hand, it suppresses T LR7-induced production of pro-inflammatory cytokines such as T NFα, IL-12p70 and IL-6 by inhibiting ɣIFN-dependent and NFκB-dependent responses [42] T aken together this shows that the signaling pathway activated by the putative pattern recognition receptor of SARS-Cov2, the T LR7, and the T h2 pathway that is activated in asthma are, in fact, antagonic. We think that the existence of this counterregulatory loop could provide for a mechanistic hypothesis for the physiopathology of the first steps in the covid-19 disease and, central to our thesis, an answer to the observation that people with asthma and other related diseases could be protected from the dire complications  [44] . Besides the different patient series that unambiguously show the low prevalence of current smokers among patients with severe forms of SARS, evidence from the asthma literature also establishes a common thread between cigarette smoke and our thesis. In fact, it has been shown that short cigarette smoke exposure is sufficient to facilitate allergic sensitisation and the development of low-dose HDM-induced allergic asthma, possibly by affecting dendritic cell function and skewed T h2 lymphocyte differentiation [45] . T his alone could suffice to reinforce the argument in favor of the hypothetical protective effect of cigarette smoking could be mediated by the activation of the T h2-eosinophil axis. Contrary to the opinions suggesting that we should not vent this kind of speculations, we think that, as scientists, we should just confront the facts, check them up for scientific truthfulness, and try to draw conclusions from them. In this particular case, it would be interesting to analyse, as has already been proposed, what would be the role of nicotine or any other ingredient in cigarette smoke mediating this effect. T his could lead to much needed novel therapies for the disease.
Finally, we could also speculate that the high prevalence of parasitic diseases and therefore the pervasiveness of (blood or tissue) eosinophilia and the relatively low incidence of the pandemic in areas like T ropical Africa [46] or the Indian subcontinent [47] [48] [49] , could be somehow linked. Only time and further research will tell us if there is a relevant connection here.
At this point in time, because of the massive amount of information and scientific endeavour being deployed in search for answers and treatments, we cannot afford to close any open question, as long as there is a minimal likelihood that it could lead us to