CD4 and viral infectious disease: Although many studies have focused on the effect of 4-1BB on CD8 T cells,62,63 systemic ligation of 4-1BB has been shown to predominantly reprogram CD4 T cells in a virus infection.49 It is unlikely that 4-1BB stimulation on CD8 T cells may play a significant role in our studies, as CD8 T cell depletion in 3H3-treated mice still disrupted the effector GC response and resulted in a substantial loss of parasite control, similar to control mice (Calôba and Vijay unpublished data).