In mammals, the development of the intestinal immune system is initiated before birth by a genetic programme. Notably, its full development and maturation only occur after birth on the influence of the huge number of bacteria which colonize the intestine at this period of time and compose the intestinal microbiota. By using original in vivo models of germ-free mice and gnotobiotic mice colonized by individual bacteria or more complex microbiota, we have shown the central role of the segmented filamentous bacterium (SFB) in inducing the coordinated maturation of intestinal adaptive immune responses. One distinctive characteristic of SFB is a host-specific adherence to the ileal mucosa and notably to the epithelium covering the Peyer's patches. Dissecting the immunostimulatory properties of SFB, we have shown that Peyer's patches are the major initiation sites of IgA responses to the microbiota. Strikingly, SFB displayed the original feature to stimulate the formation of tertiary lymphoid tissue, which provide additional sites for initiation of IgA responses but also sustain SFB-specific IL-17 response. Altogether, these data suggest that adaptive immune responses induced by SFB depend on multiple and redundant pathways, which may account for the immunostimulatory properties of SFB
