How specific components of the microbiota, such as Clostridium and SFB, modulate the homeostasis of regulatory T cells and the TH17 subset of helper T cells remains unknown. In Immunity, Ivanov and colleagues show that SFB induce TH17 cells in a manner dependent on SFB-derived antigens and that intestinal CD11c+ dendritic cells (DCs) are necessary and sufficient for this process. The induction of TH17 cells by SFB in the lamina propria requires the expression of major histocompatibility complex class II. An SFB-conditioned intestinal environment does not induce IL-17 expression in non–SFB-specific transgenic T cells. Lamina propria TH17 cells from SFB-colonized mice, but not those from SFB-negative mice, respond strongly and specifically to SFB-derived antigens, whereas non-TH17 cells do not. The response is diverse and polyclonal. Peyer's patches or mesenteric lymph nodes are not required for the induction of TH17 cells by SFB, which suggests that DCs prime the CD4+ T cells locally in the lamina propria.

Immunity 40, 594–607 (2014)