Whether tolerance to commensals is lost during gastrointestinal infection is unclear. In Science, Belkaid and colleagues use a model of infection with Toxoplasma gondii to assess the effector and memory T cell responses generated against commensals during the course of a primary immune response to a pathogen. During infection with T. gondii, the ignorance of CD4+ T cells to commensal antigens is lost and microbiota-specific T cells are activated and differentiate into T helper type 1 cells but not into cells of the TH17 subset of helper T cells or regulatory T cells. Commensal-specific T cells activated during infection survive long term and persist in extralymphoid tissue in a manner similar to that of pathogen-specific memory cells, and a fraction of them become activated during secondary infection. As bacteria colonize not just the gut but also the skin and the lung, immunity at these barrier sites can be influenced by responses to commensals as well.

Science (23 August 2012) doi:10.1126/science.1220961