It is interesting to note that the CD8+ T-cell response against F-MuLV Leader-Gag and influenza NP in C57BL/6 mice and against HBsAg in CB6F1 mice tended to be higher after co-immunization with Env than without co-immunization, which may be attributed to the presence of CD4+ T helper cell epitopes in Env (39) and to thereby improved CD4+ T cell help for the developing CD8+ T-cell response. Here, CD4 is linked to influenza.