Cell surface polysaccharides of Bifidobacterium bifidum induce the generation of Foxp3+ regulatory T cells

Dysregulation of intestinal microflora is linked to inflammatory disorders associated with compromised immunosuppressive functions of Foxp3(+) T regulatory (T-r(eg)) cells. Although mucosa-associated commensal microbiota has been implicated in T-reg generation, molecular identities of the effector components controlling this process remain largely unknown. Here, we have defined Bifidobacterium bifidum as a potent inducer of Foxp3(+) T-reg cells with diverse T cell receptor specificity to dietary antigens, commensal bacteria, and B. bifidum itself. Cell surface beta-glucan/galactan (CSGG) polysaccharides of B. bifidum were identified as key components responsible for T-reg induction. CSGG efficiently recapitulated the activity of whole bacteria and acted via regulatory dendritic cells through a partially Toll-like receptor 2-mediated mechanism. T-reg cells induced by B. bifidum or purified CSGG display stable and robust suppressive capacity toward experimental colitis. By identifying CSGG as a functional component of T-reg-inducing bacteria, our studies highlight the immunomodulatory potential of CSGG and CSGG-producing microbes.