Antimicrobial peptides are important effectors of innate immunity throughout the plant and animal kingdoms. In the mammalian small intestine, Paneth cell α-defensins are antimicrobial peptides that contribute to host defense against enteric pathogens. To determine if α-defensins also govern intestinal microbial ecology, we analyzed the intestinal microbiota in mice expressing a human α-defensin (DEFA5) and in mice lacking an enzyme required for processing of murine α-defensins. We detected significant α-defensin-dependent changes in microbiota composition, but not in total bacterial numbers, in these complementary models. Furthermore, DEFA5-expressing mice had striking losses of Segmented Filamentous Bacteria and fewer interleukin 17-producing lamina propria T cells. These data ascribe a new homeostatic role for α-defensins in regulating the makeup of the commensal microbiota.

Although the development of regulatory T cells (T(reg) cells) in the thymus is defined by expression of the lineage marker Foxp3, the precise function of Foxp3 in T(reg) cell lineage commitment is unknown. Here we examined T(reg) cell development and function in mice with a Foxp3 allele that directs expression of a nonfunctional fusion protein of Foxp3 and enhanced green fluorescent protein (Foxp3DeltaEGFP). Thymocyte development in Foxp3DeltaEGFP male mice and Foxp3DeltaEGFP/+ female mice recapitulated that of wild-type mice. Although mature EGFP(+) CD4(+) T cells from Foxp3DeltaEGFP mice lacked suppressor function, they maintained the characteristic T(reg) cell 'genetic signature' and failed to develop from EGFP(-) CD4(+) T cells when transferred into lymphopenic hosts, indicative of their common ontogeny with T(reg) cells. Our results indicate that T(reg) cell effector function but not lineage commitment requires the expression of functional Foxp3 protein.

In addition to thymus-derived or natural T regulatory (nTreg) cells, a second subset of induced T regulatory (iTreg) cells arises de novo from conventional CD4+ T cells in the periphery. The function of iTreg cells in tolerance was examined in a CD45RBhighCD4+ T cell transfer model of colitis. In situ-generated iTreg cells were similar to nTreg cells in their capacity to suppress T cell proliferation in vitro and their absence in vivo accelerated bowel disease. Treatment with nTreg cells resolved the colitis, but only when iTreg cells were also present. Although iTreg cells required Foxp3 for suppressive activity and phenotypic stability, their gene expression profile was distinct from the established nTreg “genetic signature,” indicative of developmental and possibly mechanistic differences. These results identified a functional role for iTreg cells in vivo and demonstrated that both iTreg and nTreg cells can act in concert to maintain tolerance.