Costimulation Through GITR Increases Follicular Helper T Cell Formation and Leads To Control Of A Chronic Viral Infection

The glucocorticoid-induced TNF receptor family-related protein (GITR) is an important costimulatory receptor on T cells. We have previously shown that enhanced costimulation through GITR increases the formation of both effector and regulatory CD4⁺ T cells. Here we explored whether it could also affect humoral immunity and T cell help to B cells. Although development of mature B cells was not affected in GITRL transgenic (tg) mice, we found that the number of follicular helper T cells (CXCR5⁺ PD1⁺ CD4⁺ T cells, Tfh) was significantly increased, including the absolute number of Tfh-B cell conjugates, as revealed by ImageStream analysis. Tfh from GITRL tg mice had normal expression levels of ICOS, SLAM and CD44 and slightly lower levels of CD62L and CCR7 compared to wild-type (WT) littermates. Interestingly, Tfh from GITRL tg mice produced more IFN-g and IL-10, which was accompanied by a biased antibody repertoire (decreased IgG3 and increased IgA, IgG2a and IgG2b). Since Tfh have been implicated in the late control of viral replication, we infected WT and GITRL tg mice with LCMV Clone 13. Surprisingly, at day 30 after infection, we could not detect viral genome in spleen and liver from GITRL tg mice, while WT mice were still infected. Also, PD-1 expression was strongly decreased on virus-specific CD8⁺ T cells, which correlated with faster viral clearance. All in all, these results indicate that GITR-mediated costimulation enhances the control of chronic viral infections, by boosting and modulating Tfh cell responses.