There is a growing consensus that an HIV vaccine must elicit sterilizing immunity in order to be effective against the HIV pandemic. The development of such a vaccine requires HIV envelope-based immunogens that raise multiple modes of immunity including broadly neutralizing antibody responses. Towards this end, we have developed analogues of the gp120-CD4 complex that contain HIV(BaL) gp120 and CD4 D1D2 sequences within a single chimeric polypeptide. In a recent pilot study, we evaluated the immunogenicity a single chain complex containing rhesus macaque CD4 sequences (designated rhFLSC) in rhesus macaques. This study showed that: 1) rhFLSC elicited broadly neutralizing antibodies in three of four vaccinated animals. Thus, a single chain complex is capable of eliciting broadly neutralizing antibodies when the CD4 moiety is autologous with respect to the vaccinated subject. 2) Vaccination with rhFLSC afforded nonsterilizing "protection" against rectal challenge with a heterologous R5 SHIV (SHIV(162P3)). Specifically, immune responses raised by four immunizations with 300 ug of rhFLSC mediated accelerated clearance of post-acute plasma viremia and sustained suppression of plasma and tissue viremia compared to naive controls. This protection appeared to be associated with multiple modes of immunity raised by the rhFLSC immunogen. 3) Challenge outcome in a control group of animals immunized with soluble CD4 alone was no different from the naive control group. Based on these findings, we conclude that the protection we observed in rhFLSC-immunized macaques was due to cross-reactive anti-envelope responses. Accordingly, our hypothesis is that the amplification of these responses by a modified vaccination protocol will lead to sterilizing immunity against heterologous mucosal challenge. The attainment of such protection would represent an important advance in the development of HIV subunit vaccines, particularly if immune correlates of the protection were also established. Our hypothesis will be tested in two Specific Aims. Aim 1 will be to vaccinate animals with high doses (>300 ug) of rhFLSC that are expected to boost responses above what were obtained in the pilot study. Vaccinated animals will then receive a rectal challenge with the R5 SHIV162P3 to assess protection. Aim 2 will be to characterize cellular and humoral anti-envelope responses in the animals in order to identify correlates of protection.