As the gastrointestinal tract may also be a crucial entry or interaction site of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the role of the gut mucosal immune system as a first-line physical and immunological defense is critical. Furthermore, gastrointestinal involvement and symptoms in coronavirus disease 2019 (COVID-19) patients have been linked to worse clinical outcomes. This review discusses recent data on the interactions between the virus and the immune cells and molecules in the mucosa during the infection. By carrying out appropriate investigations, the mucosal immune system role in SARS-CoV-2 infection in therapy and prevention can be established. In line with this, COVID-19 vaccines that stimulate mucosal immunity against the virus may have more advantages than the others.

Introduction: Recent studies have shown that the intestinal microbiota can modulate certain systemic metabolic and immune responses, including liver graft function and the development of complications in patients after liver transplantation (LT). Akkermansia muciniphila (AKM) and Faecalibacterium prausnitzii (FAEP) are two of the most abundant gut commensal bacteria, with mucosa-protective and anti-inflammatory effects that are important for maintaining normal intestinal homeostasis and gut barrier function. Our objective was to quantify levels of Akkermansia muciniphila and Faecalibacterium prausnitzii in immunosuppressed patients with LT. Materials and methods: Fecal samples from 23 liver transplant patients (15 adults and 8 children) and 9 non-LT controls were examined. Bacterial DNA was isolated from the samples using the stool DNA isolation kit and the obtained DNA was analyzed with commercially available qPCR kit for AKM and FAEP. Results: We found a statistically significant decrease in the amount of AKM and FAEP compared to the control group. The median values were: for AKM 8.75 for patients and 10.25 for the control group (p = 0.030), and for FAEP 9.72 and 10.47, p = 0.003, respectively. In children after LT, this difference was also statistically significant: AKM (p = 0.051) and FAEP (p = 0.014). In contrast no statistically significant differences were found between adult patients and controls, AKM (p = 0.283) and FAEP (p = 0.056), although the amount of both bacteria showed tendency for reduction. Conclusions: In this pilot study, we found a reduction in the total amount of the two studied bacteria in transplanted patients compared to the control healthy group.