Paneth cell defensins and the regulation of the microbiome: détente at mucosal surfaces.

Recently, our laboratory demonstrated that Paneth cell defensins, innate antimicrobial peptides that contribute to mucosal host defense, are able to regulate the composition of the intestinal bacterial microbiome. Using complementary mouse models of defensin deficiency (MMP7(-/-)) and surplus (HD5(+/+)), we noted defensin-dependent reciprocal shifts in the dominant bacterial species of the small intestine, without changes in total bacterial numbers. In addition, mice that expressed HD5 showed a significant loss of segemented filamentous bacteria (SFB), resulting in reduced numbers of Th17 cells in the lamina propria. This data showed a novel role for PC defensins in intestinal homeostasis, by regulation of the small intestinal microbiome. The microbiome plays an essential role in mediating host physiology, metabolism and immune response. The ability of PC defensins to regulate the composition of the biome suggests a much broader importance of these innate immune effectors than previously considered. In this addendum, the role of PC defensins in the regulation of the intestinal microbiome is reviewed, and discussed in the context of recent evidence that highlights the important role of PCs and defensins in the pathophysiology of inflammatory bowel disease.