The finding that AMPs were upregulated in colonic ECs in pIgR KO mice suggests that epithelial sensing of bacteria through microbe-associated molecular patterns are increased in mice lacking Metformin SIgs compared with WT animals, perhaps because live bacteria or microbe-associated molecular patterns can more readily reach the epithelium. This is in agreement with the observation of enhanced epithelial invasion by Salmonella typhimurium in naïve pIgR KO mice [30]. Alternatively, the altered composition of

the intestinal microbiota in pIgR KO mice could provide qualitatively different signals to the epithelium. We found 208 genes that were differentially regulated in colonic ECs of pIgR KO and WT mice when both strains had conventional intestinal microbiota. However, when

both genotypes were treated with antibiotics, this number was reduced to 27, suggesting that most of the observed CH5424802 in vivo differences in untreated mice were driven by the endogenous microbiota. Furthermore, we identified 296 genes with more than twofold differential expression between antibiotic-treated and untreated pIgR KO mice (Fig. 1). The same comparison in WT mice revealed a substantially fewer 106 genes altered [17]. Thus, a considerably higher number of genes were regulated by the commensal microbiota in pIgR KO mice than in WT mice, suggesting that the commensals drive epithelial activation in the absence of SIg. This finding is in agreement with a recent study of jejunal responses in B cell and IgA-deficient mice as well as immunocompromised humans [31]. In the absence of B cells or IgA, ECs mounted a commensal microbiota-driven immune response at the cost of reduced metabolic function. We

observed a similar microbiota-driven enhancement of epithelial immune responses in the colon of pIgR KO mice. However, there was little overlap of genes differentially expressed in jejunum of B-cell KO mice compared with WT mice [31] with genes differentially expressed in colonic epithelium of untreated pIgR KO mice compared with WT PLEKHM2 mice (this study). These differences are probably due to differences in anatomy and physiological function of the two intestinal sites. We found that several xenobiotic-metabolizing enzymes were downregulated in pIgR KO mice, in agreement with published reports that these enzymes are downregulated by the presence of intestinal bacteria [32]. We conclude that although the biological principle of enhanced epithelial defense in the absence of IgA is conserved between small and large intestine, the host expressed molecules mediating this defense differ. The fine-tuned balance between beneficial intestinal bacteria and the host is important for maintaining a healthy gut [33, 34]. Underlying causes of a perturbed host–microbiota relationship are complex.