CTLA-4 and CD4+CD25+ regulatory T cells inhibit protective immunity to Filarial parasites in vivo

The T cell coinhibitory receptor CTLA-4 has been implicated in the down-regulation of T cell function that is a quintessential feature of chronic human filarial infections. In a laboratory model of filariasis, Litomosoides siginodontis infection of susceptible BALB/c mice, we have previously shown that susceptibility is linked both to a CD4(+)CD25(+) regulatory T (Treg) cell response, and to the development of hyporesponsive CD4(+) T cells at the infection site, the pleural cavity. We now provide evidence that L sigmodontis infection drives the proliferation and activation of CD4(+)Foxp3(+) Treg cells in vivo, demonstrated by increased uptake of BrdU and increased expression of CTLA-4, Foxp3, GITR, and CD25 compared with naive controls. The greatest increases in CTLA-4 expression were, however, seen in the CD4(+)Foxp3(-) effector T cell population which contained 78% of all CD4(+)CTLA-4(+) cells in the pleural cavity. Depletion of CD25(+) cells from the pleural CD4(+) T cell population did not increase their Ag-specific proliferative response in vitro, suggesting that their hyporesponsive phenotype is not directly mediated by CD4(+)CD25(+) Treg cells. Once infection had established, killing of adult parasites could be enhanced by neutralization of CTLA-4 in vivo, but only if performed in combination with the depletion of CD25(+) Treg cells. This work suggests that during filarial infection CTLA-4 coinhibition and CD4(+)CD25(+) Treg cells form complementary components of immune regulation that inhibit protective immunity in vivo.