TH3 cells are involved in mucosal immunity and protecting mucosal surfaces in the gut from nonpathogenic non-self antigens. They mediate this non-inflammatory environment by secreting TGF-beta and IL-10. TGF-beta promotes the class switch to low concentrations of IgA which is noninflammatory. IgA does not usually activate the complement system and is not involved with phagocytosis. TH3 inhibits TH1 and TH2 cells.
Findings suggest that Th3 cells are a different linage from naturally arising CD25+ CD4+ Treg cells, but it is still unclear whether Th3 cells are the same as induced Treg cells because of the lack of a specific marker for Th3 cells. It was previously shown that TGF-β was produced by intestinal dendritic cells, which has been considered to be the source of cytokines for the induction of Th3 cells in the intestine. Additionally, since TGF-β production was induced by cytotoxic T-lymphocyte antigen 4 (CTLA-4), which is constitutively expressed on naturally arising Treg cells, it is possible that TGF-β production from Treg cells through CTLA-4–mediated signaling may stimulate the differentiation of both induced Treg cells and Th3 cells.