CD4 and infection: Taken together, our results show that MCMV.env immunization leads to strong protection of highly susceptible mice from FV infection, which relies on a strong anamnestic neutralizing antibody response that fully expands only after FV infection and is not sufficient to prevent infection at challenge, therefore, vaccine-induced CD4+ T cells as well as intrinsic CD8+ T cells are equally required for control of FV challenge.