| Summary: | CD4+ T cells are the major effector T cells against blood-stage Plasmodium yoelii infection. On the other hand, the lethal strain of P. yoelii (PyL) has acquired an escape mechanism from host T cell immunity by activating CD4+CD25+ regulatory T cells (Treg). Although the activation of Treg during PyL infection precludes the clearance of PyL from mice, it remains unclear whether activation of Treg is attributable to a specific response against PyL infection. Thus, we examined here whether Treg proliferate in an antigen-dependent manner during PyL infection. We also investigated the effector and regulatory mechanisms of Treg. Infection with PyL increased the number of CD4+CD25+ T cells, in which expression of Foxp3 mRNA is up-regulated. The Treg that were transferred into mice infected with PyL, but not with a non-lethal strain of P. yoelii (PyNL), proliferated during the initial 5 days following infection. The Treg from PyL-infected mice showed strong suppression compared with those from naive or PyNL-infected mice, and could suppress T cell activation by recognizing PyL- but not PyNL-derived antigens. Furthermore, the suppressive function of Treg activated in PyL-infected but not in naive mice could not be inhibited by treatment with an anti-glucocorticoid-induced TNFR family-related protein (GITR) mAb. These findings indicate that PyL infection specifically activates Treg that are specific for PyL-derived antigens. The infection also induces resistance for Treg to GITR signaling, and this eventually contributes to the escape of parasites from host T cell immunity.
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