key: cord-0843050-k96sb9i2
authors: Ahlawat, Shruti; Asha; Sharma, Krishna Kant
title: Immunological co-ordination between gut and lungs in SARS-CoV-2 infection
date: 2020-07-24
journal: Virus Res
DOI: 10.1016/j.virusres.2020.198103
sha: dbf30f572e42d13ad752bfa4852d1c44b16ed141
doc_id: 843050
cord_uid: k96sb9i2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has evolved into a major pandemic called coronavirus disease 2019 (COVID-19) that has created unprecedented global health emergencies and emerged as a serious threat due to its strong ability for human-to-human transmission. The reports indicate the ability of SARS-CoV-2 to affect almost any organ due to the presence of a receptor known as angiotensin converting enzyme 2 (ACE2) across the body. ACE2 receptor is majorly expressed in brush border of gut enterocytes along with the ciliated cells and alveolar epithelial type II cells in lungs. The amino acid transport function of ACE2 has been linked to gut microbial ecology in gastrointestinal (GI) tract, thereby suggesting that COVID-19 may, to some level, be linked to the enteric microbiota. The significant number of COVID-19 patients shows extra-pulmonary symptoms in the GI tract. Many subsequent studies revealed viral RNA of SARS-CoV-2 in fecal samples of COVID-19 patients. This presents a new challenge in the diagnosis and control of COVID-19 infection with a caution for proper sanitation and hygiene. Here, we aim to discuss the immunological co-ordination between gut and lungs that facilitates SARS-CoV-2 to infect and multiply in the inflammatory bowel disease (IBD) and non-IBD patients.

The human gut is an ecological niche for a huge population of enteric microbiota, majorly dominated by Bacteroidetes and Firmicutes (Foster and Neufeld 2013) that produces several metabolites to maintain the gut homeostasis (Carabotti et al., 2015; Ahlawat et al., 2020) . The J o u r n a l P r e -p r o o f gut microbiota plays important roles such as vitamin synthesis (Rowland et al., 2017) , protection against pathogens (Hillman et al., 2017) , development and maturation of host immune system (Proctor 2019) , intestinal angiogenesis (Baumgart and Carding 2007) , and differentiation and proliferation of intestinal epithelium (O'Hara and Shanahan 2006) . The gut microbial profile of each individual differs from another with alike relative abundance and dispersal among the healthy individuals. Also, the gut microbiota of a person keeps on changing throughout the life (Carabotti et al., 2015) and is most stable during adulthood (Nicholson et al., 2012) . Such that, any deviation from normal gut microbial composition is defined as "microbial dysbiosis" that is characterized by the bloom in pathobionts and instability or reduction in the populations of 'keystone' taxa like Bacteroidetes and Firmicutes (Ahlawat et al., 2020; Duboc et al., 2013) .

Purposefully, the concept of gut microbiota dysbiosis has been discussed in a mini-review published by (Brüssow 2020) .

Besides, the lungs of healthy people harbour Fusobacterium, Haemophilus, Prevotella, Streptococcus, and Veillonella as main genera, which are relatively small in size when compared to the enteric microbiota (He et al., 2017) . The emergence and maintenance of lung microbiota is governed by the equilibrium between microbial migration from the upper respiratory tract and microbial removal by the host defense systems, with small contribution from the multiplication of native microbes. Even in small concentrations, the airway microbiome is crucial to the host immunity such that an imbalance between the microbial immigration and removal predisposes its host towards the progression and exacerbations of respiratory diseases (He et al., 2017; Wypych et al., 2019) . For instance, the patients with cystic fibrosis have heightened bacterial burden in their lower airways with species like Burkholderia spp., Pseudomonas aeruginosa, and Staphylococcus aureus. Asthma is another example of a multifactorial respiratory disease where J o u r n a l P r e -p r o o f the patients have greater diversity of Actinobacteria, Firmicutes, and Proteobacteria (Marri et al., 2013) . There is a recent report of a possible link of gut microbiota with coronavirus disease (Dhar and Mohanty, 2020) ; however, an updated appraisal is still needed due to the rapid generation of clinical data on gastrointestinal (GI) symptoms. Therefore, the present review summarizes the most updated evidences that suggest the existence of an immunological coordination between two vital organs, i.e., the gut and lungs, throughout the course of infection.

The coronavirus disease is an ongoing pandemic with number of confirmed cases approaches 10.5 million and deaths surpasses 600,000 globally. The disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a positive-sense, single-stranded RNA virus of family Coronaviridae and genus Betacoronavirus (Wong et al., 2020) . The proteome of SARS-CoV-2 consists of 4 structural proteins (membrane (M), envelope (E), nucleocapsid (N), and spike (S)) , 15 mature non-structural proteins (nsp1-10 and nsp12-16), and 9 accessory proteins (Prates et al., 2020) . In general, coronaviruses are enveloped, positive-sense, non-segmented, and single-strand RNA viruses with six known species to cause human disease. SARS-CoV-2 has emerged as seventh species known to infect humans. Majority of them mostly cause mild respiratory disease. However, fatal coronaviruses have appeared sporadically in the past decades, such as the severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 and Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, which also belongs to genus Betacoronavirus (Zaki et al., 2012) . Very recently in December 2019, the cases of pneumonia with unfamiliar etiology were diagnosed in Wuhan, Hubei province of China. Later, a new coronavirus, that is, SARS-CoV-2 was obtained from the samples of lower respiratory J o u r n a l P r e -p r o o f tract of various patients (Repici et al., 2020) . The disease was noted to be like influenza, with symptoms ranging from mild respiratory to drastic lung injury, multiple organ failure driven by hyper-inflammation and "cytokine storm" syndrome (Neurath 2020), and death (Lamers et al., 2020) . Fortunately, SARS-CoV-2 has lower (~4%) mortality rate compared to other zoonotics like Ebola, SARS, and MARS, which have higher mortality rate ranging from 15-90%. However, it's unfortunate that SARS-CoV-2 could not been contained like other coronaviruses, may be due to its higher rates of asymptomatic transmission. Further, comparative genome studies have found variations in the small fragment composed of 380 amino acids across various SARS-like coronaviruses and SARS-CoV-2. The reported variations have been contemplated to be important for determining the pathogenic divergence of COVID-19 (Prates et al., 2020) .

Additionally, co-morbidities like respiratory diseases, cardiovascular diseases, hypertension, diabetes, and patient age may worsen the COVID-19 manifestations. Aging is linked to the impairment of acquired immune system, characterized by immune senescence and inflammaging or the slow occurrence of the chronic sub-clinical inflammation. Thus, it is proposed that SARS-CoV-2 infection in older men with unregulated hyper-inflammation, drastically reduced B lymphocyte-driven acquired immunity, impaired plasmacytoid dendritic cells (DCs) type I interferon (IFN) pathway, and reduced ACE2 expression, induces high mortality (Gubernatorova et al., 2020) (Figure 1 ) and also opens a Pandora's box of disease etiology.

Currently, the pathologists and clinicians are trying really hard to figure out the damage made by SARS-CoV-2 as it spread through the human body. They have discovered that even if our lungs are at prime risk, the virus can surprisingly move to other organs like the kidneys, heart and blood vessels, brain, and gut with destructive motives (Wadman et al., 2020) .

When novel coronavirus enters the nose and throat through the inhalation of virus-laden-droplets expelled from an infected person, it gets unprecedented welcome by the lining of the nose due to the presence of a receptor known as angiotensin converting enzyme 2 (ACE2). The receptor is present all over the body to assist in regulating the host blood pressure ( Figure 2 ). However, in the case of COVID-19 infection the host tissue becomes potentially accessible to the infection as the virus needs the receptors to enter the cell. Thereafter, the virus takes over the cell's machinery to replicate and invade the new cells. During this early stage, if the immune system doesn't resist SARS-CoV-2, the virus then moves down to invade the lungs, where it can grow to fatal level. The battling immune system with the invader disrupts the oxygen transfer from the air sacs to rest of the body parts. Further, the fore-front warriors, that is, white blood cells (WBCs) release chemokines that consequently directs more immune cells to target and kill the virusinfected cells, thereby leaving behind the basic pathology of pneumonia, i.e., the fluid-filled sacs with the dead cells and pus. Autopsies showed the fluid-stuffed alveoli with WBCs, mucus, and detritus of damaged lung cells (Wadman et al., 2020) . Some patients also contract secondary bacterial and fungal airway infections (Tay et al., 2020) . However, the suspected driving force in severely-ill patients is a devastating over-reaction of the human immune system called "cytokine storm", where the levels of specific cytokines rise far beyond what's generally required by the body, such that the immune cells start attacking the host's healthy tissues.

Studies have reported the increased concentrations of these cytokines in blood of hospitalized COVID-19 patients (Wadman et al., 2020; Lin et al., 2020) . The majority of serious COVID-19 cases were linked to high systemic levels of TNF, IL-1b, and IL-6 (Gubernatorova et al., 2020) , where C-reactive protein (CRP), d-dimers, ferritin (Merad and Martin 2020) , and IL-6 were found as the most significant clinical predictors of the COVID-19-associated mortality. IL-6 is a J o u r n a l P r e -p r o o f main pro-inflammatory cytokine at mucosal site during infection onset that performs various functions such as hematopoiesis, regulation to inflammation, auto-immunity, acute-phase response, and modulates host defense via several immune-stimulating mechanisms (Gubernatorova et al., 2020) . Its up-regulation is a typical feature of aging (especially in men) and continuous elevation of IL-6 is known to foster the viral replication and promote inflammation and injury of the lung tissues. Thus, the chronic elevation of IL-6, especially in older men, pre-disposes them towards high SARS-CoV-2-associated mortality (Bonafè et al., 2020) . Further, a negative correlation between cytokines concentrations and T-cell (CD4+ and CD8+) counts suggests that the "cytokine storm" actually dampens the host adaptive immunity against infection (Gubernatorova et al., 2020) . In support, the recent analysis of multi-omics data suggests a functional immune deficiency syndrome due to the repression or destruction of specific cells of the host immune system in lungs (Prates et al., 2020) . In addition, reduced IFN-γ production by CD4+ T cells (Chen et al., 2020b) , increased neutrophils, elevated naïve/memory T-cells ratio (Neurath 2020) , enhanced inflammatory monocyte-derived macrophages, and depleted tissue-resident alveolar macrophages were also observed in severe COVID-19 patients (Merad and Martin 2020) . Altogether, many studies have indicated elevated neutrophil/lymphocyte ratio as an independent major risk factor for severe COVID-19 cases (Liu et al., 2020a; Kuri-Cervantes et al., 2020; Zhang et al., 2020) . In this regard, clinical trials integrating IL-6 receptor and IL-1b blockade in COVID-19 patients have been initiated with the early encouraging results ( Figure 3 ). Further, determining the prevalence and severity of COVID-19 in patients with immune-modulatory biologics due to other co-morbidities, will provide additional insights on COVID-19 pathophysiology. It may be explored and used as a potential candidate to block a specific immune pathway to control the disease severity (Merad J o u r n a l P r e -p r o o f and Martin 2020). Very recently, the Drugs Controller General of India (DGCI) has granted an emergency approval to Itolizumab for treating COVID-19 patients with moderate-to-severe acute respiratory distress (https://www.hindustantimes.com/india-news/dcgi-approves-limited-use-ofpsoriasis-injection-for-covid/story-bkVPzdJ7Y9oaCiX2NJkypO.html).

Itolizumab is a humanized recombinant anti-CD6 monoclonal antibody, generally used in treatment of patients with chronic plaque psoriasis by inhibiting T-cell proliferation and pro-inflammatory cytokines production (https://www.biocon.com/biocon_products_bio_BF_alzumab_pa.asp).

An alteration in gut microbiota is associated to a bi-directional deviation in the relationship between the gut and several vital human organs that eventually cause severe diseases symptoms.

Recently, our gut microbiome group has reviewed the bi-directional communication network between the gut microbes and vital human organs, except the lungs (Ahlawat et al., 2020) .

Interestingly, the alterations in the lungs microbial community including airways also affect the (Hanada et al., 2018) . This suggests that the lung and the gut are closely linked organs that affect each other's homeostasis via an immunological co-ordination between them ( Figure 2 ).

Indispensably, microbes (among environmental factors) have central role in shaping the normal and pathologic immune responses in both the lung and gut (He et al., 2017) .

Similar cross-talk between gut and lungs occurs in COVID-19 cases. A study with small sample size of eight patients found that lung microbial composition in broncho-alveolar lavage fluid samples of COVID-19 patients is dominated by bacteria generally found in oral and upper respiratory tract. This was similar to patients with the community-acquired pneumonia. The microbial signatures in lung may predict the acute respiratory distress syndrome (ARDS), most common complication from COVID-19, and the long-term outcomes of the outbreak. Emerging data identifies the role of gut microbiota in improving the antiviral immunity .

Such that, various reports suggest the importance of gut microbiota modulation in reducing enteritis, ventilator-associated pneumonia, and reversing the side effects of antibiotics in order to avoid the replication of influenza virus in lung epithelium. However, at current there is no clinical evidence of gut microbiota modulation as therapeutic for treating COVID-19, but few emerging reports speculate the role of targeting the gut microbiota as a new therapeutic choice or adjuvant therapeutic option. In this way, probiotics can be used to alter the GI symptoms favourably by sustaining the balance of gut microecology and protect the respiratory system by preventing the secondary bacterial infections; thus, suggesting the crucial role of gut microbiota in ongoing COVID-19 disease Tiwari et. al., 2020) .

The According to a preliminary data, the S-protein of SARS-CoV-2 can bind another surface molecule, i.e., CD147, and is considered as an alternative pathway for virus entry inside host cells. CD147 is mainly found on hematopoietic cells including red blood cells (RBCs), neuronal, and epithelial cells. It is also linked to SARS-CoV infection, where SARS-CoV protein binds to CD147 in association with cyclophilin A and the N-protein of SARS-CoV binds to cyclophilin A in ACE2-expressing infected-host cells. Further, they suggested that the in vitro inoculation of human lung epithelial cells with SARS-CoV-2 produces cytopathic effects and ceases the cilium beating of the epithelial cells (Gubernatorova et al., 2020) . In this regard, one strategy for developing the therapeutics against the SARS-CoV-2 is by blocking the ACE2 or TMPRSS2 using compounds like baricitinib and ruxolitinib for ACE2 and camostat mesylate and nafamostat mesylate for TMPRSS2 that have been clinically approved for other co-morbidities inflammation. Therefore, this suggests a probable role of intestinal microbiota in the progression and severity of COVID-19 (Gou et al., 2020) .

Emerging data have identified the viral RNA of SARS-CoV-2 in fecal specimens and anal/rectal swabs of the COVID-19 patients even after the clearance of virus from the upper respiratory tract (Wong et al., 2020) . About 53% of sampled patient's reported viral RNA in their stool samples and up to half of patients suffer diarrhea. A recent study identified the SARS-CoV-2 protein localized in duodenal, rectal, and gastric cells in biopsies from a COVID-19 patient. Moreover, the accumulating data with the evolution and expansion of the pandemic suggests that the digestive symptoms are significantly common in COVID-19 patients (Pan et al., 2020) . Numerous reports across the globe suggest the occurrence of GI symptoms in COVID-19 patients (Table 1) and surprisingly, a subset of patients initially presented with only GI symptoms were also reported . there are no reported evidences of fecal transmission and it's still not evident that whether feces contain intact or infectious virus or just the viral RNA and proteins (Wadman et al., 2020) . Also, it's unknown whether the virus in feces is acquired from the cellular fragments of the respiratory tract or consists of the replicates from the GI tract. Anyway, it's logical to take precautionary steps to prevent the fecal-oral transmission .

There is a further need to raise awareness regarding the management of patients with pre-existing digestive diseases such as IBD (Queiroz et al., 2020) , which is characterized by the periods of remission and relapse that are triggered by an inappropriate chronic immune response, whose therapeutics involves immunosuppressive medications. Some COVID-19 manifestations may mimic an IBD exacerbation; therefore, it is suggested to test IBD patients for SARS-CoV-2 before assuming flare diagnosis as currently there is no evidence in support of SARS-CoV-2 infection as a cause of IBD flare. The immunosuppressed patients often represent atypical presentations of viral diseases (Estevinho and Magro, 2020) . However, data reported so far did not recognize the use of immunomodulator as a risk factor for COVID-19 severity (Queiroz et al., 2020) despite the theoretical fact that decreased host immune surveillance increases virus burden (Estevinho and Magro, 2020) . It was assumed that these patients are at higher risk of developing COVID-19 complications (Queiroz et al., 2020) (Aziz et al., 2020) . Therefore, priority must be given to the maintenance of IBD remission, as risk of flares outweighs the probability of contracting SARS-CoV-2 infection.

Additionally, considering the possibility of transmission via fecal-oral route, all invasive procedures like surgeries and elective endoscopies must be delayed; however, in emergent

procedures the patients should be tested for SARS-CoV-2 (Estevinho and Magro, 2020) .

Conclusively, the probable "gut-lung axis" in SARS-CoV-2 caused COVID-19 disease involves the inhalation of SARS-CoV-2-laden-droplets expelled from an infected person. This leads to the binding of SARS-CoV-2 to ACE2 and other receptors for entry inside host cells. As a result, the hyperactive immune system releases inflammatory mediators that cause lung hyper-permeability such that the virus along with inflammatory mediators via circulation migrates to intestine and binds highly expressed ACE2 receptors on enterocytes. This disrupts the intestinal permeability leading to the leakage of gut microbes and associated metabolites into circulation. The leaked microbes and products via circulation migrate to organs including lungs and cause damage.

Therefore, "Microbial dysbiosis" is also suspected due to the observation of diarrhea as a main GI symptom in patients with the COVID-19 disease (Figure 2 ). 

The authors declare they have no conflict of interest. 

Gut-organ axis: A microbial outreach and networking

Clinical Characteristics of COVID-19 Patients With Gastrointestinal Symptoms: An Analysis of Seven Patients in China

Diet, microbiota and gut-lung connection

The Incidence and Outcomes of COVID-19 in IBD Patients: A Rapid Review and Meta-analysis

Inflammatory bowel disease: cause and immunobiology

Abdominal imaging findings in COVID-19: preliminary observations

Inflammaging: why older men are the most susceptible to SARS-CoV-2 complicated outcomes

Problems with the concept of gut microbiota dysbiosis

Clinical and immunological features of severe and moderate coronavirus disease 2019

Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study

The presence of SARS-CoV-2 RNA in the feces of COVID-19 patients

Multisystem Inflammatory Syndrome Related to COVID-19 in Previously

High prevalence of concurrent gastrointestinal manifestations in patients with SARS-CoV-2: Early experience from California

ACE2 and microbiota: emerging targets for cardiopulmonary disease therapy

Diarrhea during COVID-19 infection: pathogenesis, epidemiology, prevention and management

Gut microbiota and Covid-19-possible link and implications

Connecting dysbiosis, bile-acid dysmetabolism and gut inflammation in inflammatory bowel diseases

Digestive Manifestations in Patients Hospitalized with COVID-19

The Impact of SARS-CoV-2 on Inflammatory Bowel Disease. GE-Port

Gut-brain axis: how the microbiome influences anxiety and depression

Manifestations of digestive system in hospitalized patients with novel coronavirus pneumonia in Wuhan, China: a single-center

Impact of the gut microbiota on intestinal immunity mediated by tryptophan metabolism

Gut microbiota may underlie the predisposition of healthy individuals to COVID-19

Review of Societal Recommendations Regarding Management of Patients With Inflammatory Bowel Disease During the SARS-CoV-2 Pandemic

IL-6: relevance for immunopathology of SARS-CoV-2

Digestive symptoms in COVID-19 patients with mild disease severity: clinical presentation, stool viral RNA testing, and outcomes

Respiratory viral infection-induced Microbiome alterations and secondary bacterial pneumonia

ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation

Microbiota might play a role in SARS-CoV-2 infection. Front Microbiol. 11, 1302

Gut-lung axis: the microbial contributions and clinical implications

Microbial ecology along the gastrointestinal tract

Clinical features of patients infected with 2019 novel coronavirus in Wuhan

Epidemiological, clinical and virological characteristics of 74 cases of coronavirus-infected disease 2019 (COVID-19) with gastrointestinal symptoms

Don't overlook digestive symptoms in patients with 2019 novel coronavirus disease (COVID-19)

Asthma-associated differences in microbial composition of induced sputum

Pathological inflammation in patients with COVID-19: a key role for monocytes and macrophages

Are patients with inflammatory bowel disease at increased risk for Covid-19 infection

Covid-19 and immunomodulation in IBD

Host-gut microbiota metabolic interactions

Gastrointestinal Symptoms and COVID-19: Case-Control Study from the United States

The epidemiological characteristics of an outbreak of 2019 novel coronavirus diseases (COVID-19) in China

The gut flora as a forgotten organ

Clinical characteristics of COVID-19 patients with digestive symptoms in Hubei, China: a descriptive, cross-sectional, multicenter study

ACE2-From the renin-angiotensin system to gut microbiota and malnutrition

Abdominal pain: a real challenge in novel COVID-19 infection

Functional Immune Deficiency Syndrome via Intestinal Infection in COVID-19

What's next for the human microbiome?

Management of inflammatory bowel disease patients in the COVID-19 pandemic era: a Brazilian tertiary referral center guidance

Gastrointestinal Symptoms and outcomes in hospitalized COVID-19 patients

Prevalence and Characteristics of Gastrointestinal Symptoms in Patients with SARS-CoV-2 Infection in the United States: A Multicenter Cohort Study

Coronavirus (COVID-19) outbreak: what the department of endoscopy should know

Gut microbiota functions: metabolism of nutrients and other food components

SARS-CoV-2 induced diarrhoea as onset symptom in patient with COVID-19

The trinity of COVID-19: immunity, inflammation and intervention

Probiotics at war against viruses: What is missing from the picture?

A rampage through the body

Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan

The infectious bronchitis coronavirus envelope protein alters Golgi pH to protect the spike protein and promote the release of infectious virus

Children With a Pediatric Inflammatory Multisystem Syndrome Temporally Associated With SARS-CoV-2

Covid-19 and the digestive system

The influence of the microbiome on respiratory health

Evidence for gastrointestinal infection of SARS-CoV-2

Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a single-centered, retrospective, observational study

Immune phenotyping based on neutrophil-to-lymphocyte ratio and IgG predicts disease severity and outcome for patients with COVID-19

Isolation of a novel coronavirus from a man with pneumonia in Saudi Arabia

Clinical characteristics and outcomes of COVID-19 patients with gastrointestinal symptoms admitted to Jianghan Fangcang Shelter Hospital in Wuhan

GI symptoms in females were higher than in males (62.8% v/s 37.2%); ↓ Hemoglobin, ↑ CRP, ↑ alanine aminotransferase in GI symptom group than in non-GI symptom group