CD4 and infection: However, DENV1 antigens could stimulate the proliferation and activation of splenocytes harvested from the YFV-immune mice, and the numbers of activated CD4+ and CD8+ effector T cells significantly increased after sequential infection of DENV1 [38], demonstrating that cross-reactive effector T cells induced by YFV-17D immunization were likely to be recalled by DENV1 infection.