Credit: Stephanie Schuller / Photo Researchers, Inc.

T cells specific for commensals expand and differentiate into memory cells as a result of a parasitic infection in the gastrointestinal tract, according to a new study (Science doi:10.1126/science.1220961).

Immune tolerance to commensals in the gut is kept in check by numerous adaptive and innate immune mechanisms, preventing unwanted proinflammatory responses and mucosal damage. Whether these immune mechanisms are breached by a pathogenic infection remains unclear.

Yasmine Belkaid and her colleagues found that infection with the parasite Toxoplasma gondii induced the expansion of CD4+ T cells specific to commensal-derived flagellin. These T cells adopted a T helper type 1 (TH1) phenotype and secreted interferon-γ. Although this pool of T cells contracted over time, a small population formed memory T cells that persisted up to 240 days after infection and could be activated upon infectious challenge. Although it remains unclear whether these commensal-specific memory T cells directly contribute to epithelial damage and susceptibility to inflammatory bowel disease, these findings suggest that tolerance to commensal bacteria can be impaired by gastrointestinal infection.