We have recently demonstrated that microbiota stimulation of innate immune pathways is required for T cell spontaneous proliferation and chronic intestinal inflammation. Microbiota promoted spontaneous proliferation of T cells by activating dendritic cells to produce interleukin (IL)-6 via a TLR/ MyD88-dependent pathway. Although both CBir1-specific TCR transgenic (CBir1 Tg) T cells, which are specific for an immunodominant microbiota antigen and OT-II T cells, which are specific for the model antigen ovalbumin, underwent spontaneous proliferation, only CBir1 Tg T cells but not OT-II T cells induced colitis in specific pathogen-free RAG-/- mice. Blockade of IL-6 or IL-6-mediated spontaneous proliferation of CBir1 Tg T cells abrogated colitis induction in this adoptive transfer model. Our data reveal that microbiota serves as a natural adjuvant for T cell spontaneous proliferation and development of chronic intestinal inflammation and that both microbiota stimulation of innate immune cells with subsequent T cell spontaneous proliferation and microbiota antigen activation of antigen-specific TCR are required for the induction of experimental colitis. © 2010 Landes Bioscience.