Microbiota innate stimulation is a prerequisite for T cell spontaneous proliferation and induction of experimental colitis

Little is known about how the microbiota regulates T cell proliferation and whether spontaneous T cell proliferation is involved in the pathogenesis of inflammatory bowel disease. In this study, we show that stimulation of innate pathways by microbiota-derived ligands and antigen-specific T cell stimulation are both required for intestinal inflammation. Microbiota-derived ligands promoted spontaneous T cell proliferation by activating dendritic cells (DCs) to produce IL-6 via Myd88, as shown by the spontaneous proliferation of T cells adoptively transferred into specific pathogen–free (SPF) RAG(−/−) mice, but not in germfree RAG(−/−) mice. Reconstitution of germfree RAG(−/−) mice with cecal bacterial lysate–pulsed DCs, but not with IL-6(−/−) or Myd88(−/−) DCs, restored spontaneous T cell proliferation. CBir1 TCR transgenic (CBir1 Tg) T cells, which are specific for an immunodominant microbiota antigen, induced colitis in SPF RAG(−/−) mice. Blocking the spontaneous proliferation of CBir1 Tg T cells by co-transferring bulk OT II CD4(+) T cells abrogated colitis development. Although transferred OT II T cells underwent spontaneous proliferation in RAG(−/−) mice, the recipients failed to develop colitis because of the lack of cognate antigen in the intestinal lumen. Collectively, our data demonstrate that induction of colitis requires both spontaneous proliferation of T cells driven by microbiota-derived innate signals and antigen-specific T cell proliferation.