Interactions between antibody and Fc? receptors (Fc?Rs) mediate many biological activities of antibody. Fc?RIIIa, found on cells targeted by HIV-1, is encoded by a gene with a polymorphism affecting IgG binding affinity. This polymorphism results in either a phenylalanine (F) or a valine (V) in a part of the receptor that binds to IgG; there are three possible genotypes: FF, FV, or VV. Participants in a large HIV vaccine trial (Vax004) who had the VV genotype were more likely to become infected if they received vaccine than if they received a placebo. This finding indicated that the vaccine increased the risk of infection, probably due to antibody-dependent enhancement (ADE) of infection. ADE presents a major potential obstacle to the development of vaccines that depend wholly or partly on HIV-specific antibody responses. In the proposed research, assays will be developed to test the mechanisms responsible for the findings in the Vax004 trial. The long-term goals are to understand early events in HIV infection and develop vaccination strategies that take advantage of Fc?R biology and avoid future harm. In the R21 phase, the following hypothesis will be tested: the relationship between Fc?RIIIa genotype and HIV infection risk observed in Vax004 can be modeled in vitro. The SPECIFIC AIMS are: 1) develop an in vitro model of Fc?RIIIa genotype-dependent differences in HIV-1 replication based on direct infection of Fc?RIIIa-bearing cells. Natural killer cells (NKs), macrophages (M?s) and dendritic cells (DCs) expressing the different forms of Fc?RIIIa will be used as target cells for antibody-coated HIV and; 2) develop in vitro models of Fc?RIIIa genotype-dependent differences in HIV replication based on indirect effects of Fc?RIIIa-bearing cells. Assays will measure HIV replication in CD4+ lymphocytes after transfer from Fc?RIIIa-bearing cells or after cross-linking Fc?RIIIa. If successful in developing such assays, mechanisms that account for the increased infection risk after vaccination in VV individuals will be investigated in the R33 phase. The SPECIFIC AIMS in the R33 phase are: 1) determine the intracellular fate of opsonized virus in M?s or DCs with high and low affinity Fc?RIIIa forms; 2) determine if the efficiency of virus transfer to lymphocytes and if the production of pro-viral and anti-viral cytokines/chemokines are regulated in an Fc?RIIIa-dependent manner and; 3) develop reproducible assays to test multiple virus strains and sera for Fc?RIIIa genotype-associated enhancement. Information gained from this research will be crucial to understanding early events in HIV infection and provide direction for the development of effective HIV vaccines. [unreadable] [unreadable] [unreadable]