The identification of chemokine receptors, in addition to CD4, as coreceptors for HIV entry, not only has contributed to the understanding on viral tropism but has provided an additional target for therapeutic intervention for HIV disease. Several chemokine receptors have been shown to function as coreceptors for HIV-1 entry. The main ones are CXCR4 (for T-cell line tropic viruses) and CCR5 (for macrophage-tropic viruses). Because of the capacity of HIV to adapt when selective pressures are imposed, it is likely that any drug designed to block the interaction of HIV with, say, CCR5 will force the virus to use additional coreceptors. Thus, the determination of the complete coreceptor repertoire will be necessary. Because STRL33 is expressed in all lymphoid tissues, in collaboration with Dr J. Farber, NIAID, we tested it for coreceptor activity with HIV. We demonstrated that the expression of STRL33 in Jurkat cells conferred increased permissivity to infection by the ELI1 isolate of HIV-1. Thus, STRL33 can act as an HIV-1 coreceptor in vitro. As well as testing the coreceptor activity of STRL33 with a number of HIV-1 strains of different phenotypes, we have begun studies with HIV-2 and SIV. We have shown, in an infectivity assay, that the MAL strain of HIV-1 and the mac239 isolate of SIV use STRL33 but not as well as they use CCR5. The appearance of virus only after about 30 days in culture is indicative of adaptation. To confirm this, virus emerging after about 35 days was used to infect fresh Jurkat-STRL33 as well as the parent Jurkat cells. In this second passage, virus production was seen after about 12 days, thus demonstrating that both SIVmac239 and HIV-1 MAL had adapted to use STRL33 more efficiently. Importantly, these passaged viruses were still unable to infect Jurkat cells. That the passaged virus had adapted to use STRL33 was demonstrated by the fact that an antibody raised to STRL33 inhibited virus infection. So far, we have cloned a single envelope gene from the adapted MAL virus and shown that this Env can confer an increased capacity to use STRL33. Additional env genes will be obtained, as well as ones from SIVmac239 adapted to use STRL33. We have identified a novel coreceptor for certain SIV strains. We predicted the existence of this coreceptor from the cellular tropism of HIV-1 and SIV strains, and, using an RNase protection assay with different orphan receptor genes, were able to identify the gene and clone it. So far this coreceptor has been shown to be active with SIVagm isolates. We plan to expand the study on adaptation to additional SIV isolates and strains and to HIV-2, as well as looking at addtional coreceptors such as APJ, CX3CR1, and our novel coreceptor. Our aim is to obtain adapted SIVs that can use different coreceptors for entry. Once these variants have been isolated and characterized, they could be used for in vivo studies to determine which coreceptors are used for transmission via different routes of infection.