Mucosal immune responses in COVID19 - a living review

Abstract

COVID-19 was initially characterised as a disease primarily of the lungs, but it is becoming increasingly clear that the SARS-CoV2 virus is able to infect many organs and cause a broad pathological response. The primary infection site is likely to be a mucosal surface, mainly the lungs or the intestine, where epithelial cells can be infected with virus. Whilst it is clear that virus within the lungs can cause severe pathology, driven by an exaggerated immune response, infection within the intestine generally seems to cause minor or no symptoms. In this review we compare the disease processes between the lungs and gastrointestinal tract, and what might drive these different responses. As the microbiome is a key part of mucosal barrier sites, we also consider the effect that microbial species may play on infection and the subsequent immune responses. Due to difficulties obtaining tissue samples there are currently few studies focused on the local mucosal response rather than the systemic response, but understanding the local immune response will become increasingly important for understanding the mechanisms of disease in order to develop better treatments.

Does Fecal-Oral Transmission of SARS-CoV-2 Due to Low Sanitation Conditions Contribute to Low Mortality Rates From COVID-19

“…This hypothesis is supported by the following finding: SARS-CoV-2 can cause severe pathology in the lungs driven by an exaggerated immune response, whereas infection in the intestine generally seems to cause minor or no symptoms [5,[18][19]. Nonetheless, the results of a meta-analysis of the prevalence and mortality of COVID-19 patients with gastrointestinal symptoms indicate that the mortality among patients with gastrointestinal symptoms is similar to the overall mortality [20].…”

“…Determining the SARS-CoV-2 viral RNA sequence in these countries for individuals suffering solely from gastrointestinal symptoms will allow clarification of whether there is a mutation that improves the effectiveness of the fecal-oral infection pathway. Because the microbiome is a key part of the intestinal mucosal barrier sites and plays a major role in shaping the immune system, it may also play an important role in infection by SARS-CoV-2 and subsequent immune responses [ 5 , 13 , 28 ]. It is therefore suggested that the intestinal microbiome of asymptomatic and symptomatic individuals should be analyzed and compared.…”

“…Respiratory symptoms, including common cold, cough, shortness of breath, runny nose and sore throat, are the most common. However, nearly half of the patients who were confirmed as having COVID-19 exhibited detectable SARS-CoV-2 RNA in their fecal samples [ 5 ]. The characteristics of gastrointestinal symptoms in COVID-19, such as lack of appetite, diarrhea, vomiting, and abdominal pain, are more insidious than the respiratory symptoms and are easily overlooked.…”

Gastrointestinal mucosal immunity and COVID-19

“…They comprise innate-like T cells ( e.g ., invariant natural killer T, innate lymphoid cells, and γδ T cells) involved in pulmonary mucosal antiviral immunity and tissue protection and repair after resolving the infection[ 43 ]. Pearson et al [ 44 ] focused on local mucosal responses during viral infection, particularly with SARS-CoV-2 in both lungs and gut. They found that IL-33 and IL-8 were increased in fecal samples of COVID-19 patients due to intestinal involvement[ 44 , 45 ].…”

“…Pearson et al [ 44 ] focused on local mucosal responses during viral infection, particularly with SARS-CoV-2 in both lungs and gut. They found that IL-33 and IL-8 were increased in fecal samples of COVID-19 patients due to intestinal involvement[ 44 , 45 ]. Simultaneously, cytokines such as IL-1b, tumor necrosis factor alpha, and IL-6 were found to decrease.…”