As crucial interface organs gut and skin have much in common. Therefore it is unsurprising that several gut pathologies have skin co-morbidities. Nevertheless, the reason for this remains ill explored, and neither mainstream gastroenterology nor dermatology research have systematically investigated the 'gut-skin axis'. Here, in reviewing the field, we propose several mechanistic levels on which gut and skin may interact under physiological and pathological circumstances. We focus on the gut microbiota, with its huge metabolic capacity, and the role of dietary components as potential principle actors along the gut-skin axis. We suggest that metabolites from either the diet or the microbiota are skin accessible. After defining open key questions around the nature of these metabolites, how they are sensed, and which cutaneous changes they can induce, we propose that understanding of these pathways will lead to novel therapeutic strategies based on targeting one organ to improve the health of the other.

High expression of IL-21 and/or IL-21R has been described in T cell-mediated inflammatory diseases characterized by defects of counterregulatory mechanisms. CD4+CD25+ regulatory T cells (Treg) are a T cell subset involved in the control of the immune responses. A diminished ability of these cells to inhibit T cell activation has been documented in immune-inflammatory diseases, raising the possibility that inflammatory stimuli can block the regulatory properties of Treg. We therefore examined whether IL-21 controls CD4+CD25+ T cell function. We demonstrate in this study that IL-21 markedly enhances the proliferation of human CD4+CD25− T cells and counteracts the suppressive activities of CD4+CD25+ T cells on CD4+CD25− T cells without affecting the percentage of Foxp3+ cells or survival of Treg. Additionally, CD4+CD25+ T cells induced in the presence of IL-21 maintain the ability to suppress alloresponses. Notably, IL-21 enhances the growth of CD8+CD25− T cells but does not revert the CD4+CD25+ T cell-mediated suppression of this cell type, indicating that IL-21 makes CD4+ T cells resistant to suppression rather than inhibiting CD4+CD25+ T cell activity. Finally, we show that IL-2, IL-7, and IL-15, but not IL-21, reverse the anergic phenotype of CD4+CD25+ T cells. Data indicate that IL-21 renders human CD4+CD25− T cells resistant to Treg-mediated suppression and suggest a novel mechanism by which IL-21 could augment T cell-activated responses in human immune-inflammatory diseases.

Helicobacter pylori (Hp)-associated gastritis is characterized by an increased number of acute and chronic inflammatory cells secreting cytokines that contribute to maintain and expand the local inflammation. Locally induced IL-8 is believed to play a major role in the Hp-associated acute inflammatory response. Factors/mechanisms that regulate IL-8 induction are, however, not fully understood. In the present study we investigated whether Hp infection is associated with an increased production of IL-17, a T cell-derived cytokine capable of modulating IL-8 gene expression. We showed that both IL-17 RNA transcripts and protein were expressed at a higher level in the whole gastric mucosal and lamina propria mononuclear cell samples from Hp-infected patients than in those from uninfected subjects. Hp eradication was associated with a marked down-regulation of IL-17 expression. The addition of a neutralizing anti-IL-17 Ab to the gastric lamina propria mononuclear cell cultures resulted in a significant inhibition of IL-8 secretion, indicating that IL-17 contributes to enhance IL-8 in the Hp-colonized gastric mucosa. Consistently, stimulation of MKN 28 cells, a gastric epithelial cell line, with IL-17 increased IL-8 secretion. Finally, conditioned medium from the IL-17-stimulated MKN 28 cell cultures promoted the in vitro polymorphonuclear leukocyte migration. This effect was inhibitable by a neutralizing IL-8 but not IL-17 Ab. Together, these data indicate that biologically active IL-17 production is increased during Hp infection, suggesting the possibility that this cytokine may play an important role in the inflammatory response to the Hp colonization.

Crohn's disease [CD] and ulcerative colitis [UC], the main inflammatory bowel diseases [IBD] in humans, are chronic, immune-inflammatory diseases, the pathogenesis of which suggests a complex interaction between environmental factors and genetic susceptibility. These disabling conditions affect millions of individuals and, together with the drugs used to treat them, can put patients at risk of developing complications and other conditions. This is particularly relevant today, as coronavirus disease has rapidly spread from China to countries where IBD are more prevalent, and there is convincing evidence that Covid-19-mediated morbidity and mortality are higher in subjects with comorbidities. The primary objectives of this Viewpoint are to provide a focused overview of the factors and mechanisms by which the novel severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2] infects cells and to illustrate the link between such determinants and intestinal inflammation. We also provide clues about the reasons why the overall IBD population might have no increased risk of developing SARS-CoV-2 infection and highlight the potential of cytokine blockers, used to treat IBD patients, to prevent Covid-driven pneumonia.

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