Introduction Human gut microbiota plays a crucial role in providing protective responses against pathogens, particularly by regulating immune system homeostasis. There is a reciprocal interaction between the gut and lung microbiota, called the gut-lung axis (GLA). Any alteration in the gut microbiota or their metabolites can cause immune dysregulation, which can impair the antiviral activity of the immune system against respiratory viruses such as severe acute respiratory syndrome coronavirus (SARS-CoV) and SARS-CoV-2. Areas covered This narrative review mainly outlines emerging data on the mechanisms underlying the interactions between the immune system and intestinal microbial dysbiosis, which is caused by an imbalance in the levels of essential metabolites. The authors will also discuss the role of probiotics in restoring the balance of the gut microbiota and modulation of cytokine storm. Expert opinion Microbiota-derived signals regulate the immune system and protect different tissues during severe viral respiratory infections. The GLA’s equilibration could help manage the mortality and morbidity rates associated with SARS-CoV-2 infection.

In inflammatory bowel diseases (IBD), the therapeutic benefit and mucosal healing from specific probiotics may relate to the modulation of dendritic cells (DCs). Herein, we assessed the immunomodulatory effects of four probiotic strains including Lactobacillus salivarius, Bifidobacterium bifidum, Bacillus coagulans and Bacillus subtilis natto on the expression of co-stimulatory molecules, cytokine production and gene expression of signal-transducing receptors in DCs from IBD patients. Human monocyte-derived DCs from IBD patients and healthy controls were exposed to four probiotic strains. The expression of co-stimulatory molecules was assessed and supernatants were analyzed for anti-inflammatory cytokines. The gene expression of toll-like receptors (TLRs), IL-12p40 and integrin αvβ8 were also analyzed. CD80 and CD86 were induced by most probiotic strains in ulcerative colitis (UC) patients whereas only B. bifidum induced CD80 and CD86 expression in Crohn’s disease (CD) patients. IL-10 and TGF-β production was increased in a dose-independent manner while TLR expression was decreased by all probiotic bacteria except B. bifidum in DCs from UC patients. TLR-4 and TLR-9 expression was significantly downregulated while integrin ß8 was significantly increased in the DCs from CD patients. IL-12p40 expression was only significantly downregulated in DCs from CD patients. Our findings point to the general beneficial effects of probiotics in DC immunomodulation and indicate that probiotic bacteria favorably modulate the expression of co-stimulatory molecules, proinflammatory cytokines and TLRs in DCs from IBD patients.

Inflammatory bowel disease (IBD) is a disorder, which involves the gastrointestinal (GI) tract consisting Crohn's disease (CD) and ulcerative colitis (UC). The etiology of this disease is not yet clear and, hence, there are numerous medications and treatments for patients with IBD, although a definite and permanent treatment is still missing. Therefore, finding novel therapeutic approaches are vital for curing patients with IBD. In the GI tract, there are various lineages of cells with different roles that their existence is necessary for the barrier function of intestinal epithelial cells (IECs). Therefore, signaling pathways, which manage the hemostasis of cell lineages in intestine, such as Wnt, Notch, and Hippo, could have crucial roles in regulation of barrier function in the intestine. Additionally, these signaling pathways function as a governor of cell growth, tissue homeostasis, and organ size. In patients with IBD, recent studies have revealed that these signaling pathways are dysregulated that it could result in depletion or excess of a cell lineage in the intestine. Moreover, dysregulation of these signaling pathways in different cell lineages of the immune system could lead to dysregulation of the immune system's responses in IBD. In this article, we summarized the components and signaling of Wnt, Notch, and Hippo pathways and their role in the intestine and immune system. Furthermore, we reviewed latest scientific literature on the crosstalk among these three signaling pathways in IBD. An overview of these three signaling pathways and their interactions in IBD could provide a novel insight for prospective study directions into finding efficient medications or treatments.

Inflammatory bowel diseases (IBDs) are immune-mediated, chronic relapsing diseases with a rising prevalence worldwide in both adult and pediatric populations. Treatment options for immune-mediated diseases, including IBDs, are traditional steroids, immunomodulators, and biologics, none of which are capable of inducing long-lasting remission in all patients. Dendritic cells (DCs) play a fundamental role in inducing tolerance and regulating T cells and their tolerogenic functions. Hence, modulation of intestinal mucosal immunity by DCs could provide a novel, additional tool for the treatment of IBD. Recent evidence indicates that probiotic bacteria might impact immunomodulation both in vitro and in vivo by regulating DCs’ maturation and producing tolerogenic DCs (tolDCs) which, in turn, might dampen inflammation. In this review, we will discuss this evidence and the mechanisms of action of probiotics and their metabolites in inducing tolDCs in IBDs and some conditions associated with them.

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