The invention features recombinant viral vectors that take advantage of the activity of picornaviral protein 3A in modulating cytokine secretion and antigen presentation on MHC Class I (MHC I), which in turn provides for modulation of a Th1-mediated immune response to the transfected host cell. Specifically, a recombinant viral vector comprising a sequence encoding picornaviral protein 3A provides for decreased antigen presentation on MHC I and a decreased incidence of Th1-mediated immune response, while a recombinant picornaviral vector that is deficient in protein 3A production provides for increased cytokine secretion, increased antigen presentation on MHC I and increased Th1-mediated immune response towards the transfected host cell. The invention also features methods of inducing humoral or cellular immunity using the vectors.