CD4 and infection: Blocking the VIP receptor and genetic deletion of VIP/PHI led to decreased PD-1 expression on effector CD8+ T-cells and increased ICOS-L expression on CD4+ T-cells as well as augmentation of expression of Th1 cytokines (IFN-γ and TNF-α) on day 3 post-infection in CD8+ T-cells and on day 10 in CD4+ T-cells.