DEVELOPMENT OF AN OPTIMAL LIVE ATTENUATED SIV VACCINE

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An attenuated monocyte-tropic clone of SIV (SIV/17E-C1) has properties which are unique to a live attenuated SIV vaccine. Infection with SIV/17E-C1 induces an early vigorous type-specific neutralizing antibody response which broadens during the first 7 months postinfection to include activity against SIV/DeltaB670, a heterologous primary isolate. The induction of protective immune responses coincides with the switch from type-specific to group-specific neutralizing antibody, and, at least in part, appears mediated by antibody since sera from protected monkeys passively protected 2 of 4 recipients. A nef-deleted variant of this monocyte-tropic clone, but not the lymphocyte-tropic parent SIVmac239deltanef, also induced detectable class I restricted CTL during the first 6 months postinfection. These unique properties are likely due to a unique gp120 conformation imposed by sequences which enable it to replicate in macrophages, and/or selective presentation to the immune system by the infected macrophage. We propose to further characterize the protective responses induced by SIV/17E-C1 infection, and to utilize this information to design a subunit vaccine that can induce the protective responses observed with the attenuated virus. The protection observed by intravenous challenge with cell-free virus of SIV/e-C1- infected monkeys will be extended to protection against infected cells, delivered both intravenously and at the intestinal mucosal surface. The role of antibody in passive protection will be confirmed by passive transfer of purified immunoglobulin. Antibodies binding to native versus denatured gp120 will be affinity purified, characterized in vitro and similarly analyzed to define the specificity of protective responses. Passive transfer studies will also be performed with monoclonal antibodies derived from SIV/17E-C1-infected monkeys to identify the specific epitope(s) required for protection. The contribution of cellular immunity will be determined by determining the kinetics of class I restricted CTL induction in infected monkeys, and correlating these responses to protection. The efficacy of a purified recombinant gp120 vaccine comprised of SIV/17E-C1 sequences will be evaluated to determine whether these sequences per se uniquely induce protective immunity. The ability of DNA vaccines comprised of SIV/17E sequences, alone or in combination with cytokine expression vectors, to elicit protective responses will be assessed using both direct injection of DNA and the particle delivery system developed by Agracetus. The optimal HIV vaccine strategy is one which is efficacious in both SIV (disease) and HIV (infection) model systems. Thus, the ability of SIV/HIV recombinant viruses to serve as live attenuated vaccines will be determined to extend the observations gained from SIV to HIV glycoproteins. A model for assessing effective SIV subunit vaccines, as well as heterologous (inter- clade) protection, will be established using SHIV constructs.