Immunization with recombinant ALVAC/gp120 alum vaccine provided modest protection from human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) acquisition in humans and macaques. Vaccine-mediated protection was associated with the elicitation of IgG against the envelope V2 loop and of envelope-specific CD4+ T cell responses. We hypothesized that the simultaneous expression of the costimulatory molecule CD40L (CD154) by the ALVAC-HIV vector could increase both protective humoral and cellular responses. We engineered an ALVAC-SIV coexpressing CD40L with SIVmac251 (ALVAC-SIV/CD40L) gag, pol, and env genes. We compared its immunogenicity in macaques with that of a canonical ALVAC-SIV, with both given as a vector-prime/gp120 in alum boost strategy. The ALVAC-SIV/CD40L was superior to the ALVAC-SIV regimen in inducing binding and tier 1 neutralizing antibodies against the gp120. The increase in humoral responses was associated with the expression of the membrane-bound form of the CD40L by CD4+ T cells in lymph nodes. Unexpectedly, the ALVAC-SIV/CD40L vector had a blunting effect on CD4+ Th1 helper responses and instead favored the induction of myeloid-derived suppressor cells, the immune-suppressive interleukin-10 (IL-10) cytokine, and the down-modulatory tryptophan catabolism. Ultimately, this strategy failed to protect macaques from SIV acquisition. Taken together, these results underlie the importance of balanced vaccine-induced activating versus suppressive immune responses in affording protection from HIV.IMPORTANCE CD40-CD40 ligand (CD40L) interaction is crucial for inducing effective cytotoxic and humoral responses against pathogens. Because of its immunomodulatory function, CD40L has been used to enhance immune responses to vaccines, including candidate vaccines for HIV. The only successful vaccine ever tested in humans utilized a strategy combining canarypox virus-based vector (ALVAC) together with an envelope protein (gp120) adjuvanted in alum. This strategy showed limited efficacy in preventing HIV-1/SIV acquisition in humans and macaques. In both species, protection was associated with vaccine-induced antibodies against the HIV envelope and CD4+ T cell responses, including type 1 antiviral responses. In this study, we tested whether augmenting CD40L expression by coexpressing it with the ALVAC vector could increase the protective immune responses. Although coexpression of CD40L did increase humoral responses, it blunted type 1 CD4+ T cell responses against the SIV envelope protein and failed to protect macaques from viral infection.