Regulatory roles of IL-10–producing human follicular T cells

Mucosal lymphoid tissues such as human tonsil are colonized by bacteria and exposed to ingested and inhaled antigens, requiring tight regulation of immune responses. Antibody responses are regulated by follicular helper T (T(FH)) cells and FOXP3(+) follicular regulatory T (T(FR)) cells. Here we describe a subset of human tonsillar follicular T cells identified by expression of T(FH) markers and CD25 that are the main source of follicular T (T(F)) cell–derived IL-10. Despite lack of FOXP3 expression, CD25(+) T(F) cells resemble T reg cells in high CTLA4 expression, low IL-2 production, and their ability to repress T cell proliferation. CD25(+) T(F) cell–derived IL-10 dampens induction of B cell class-switching to IgE. In children, circulating total IgE titers were inversely correlated with the frequencies of tonsil CD25(+) T(F) cells and IL-10–producing T(F) cells but not with total T reg cells, T(FR), or IL-10–producing T cells. Thus, CD25(+) T(F) cells emerge as a subset with unique T and B cell regulatory activities that may help prevent atopy.