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Human immunodeficiency virus type 1 (HIV-1) strains isolated from the central nervous system (CNS) may represent a subgroup that displays a host cell tropism different from those isolated from peripheral blood and lymph nodes. One CNS-derived isolate, HIV-lSFl28A, which can be propagated efficiently in primary macrophage culture but not in any T-cell lines, was molecularly cloned and characterized. Recombinant viruses between HIV-lSF128A and the peripheral blood isolate HIV-lSF2 were generated in order to map the viral gene(s) responsible for the macrophage tropism. The env gene sequences of the two isolates are about 91.1% homologous, with variations scattered mainly in the hypervariable regions of gpl20. Recombinant viruses that have acquired the HIV-lSF128A env gene display HIV-lSFl28A tropism for macrophages. Furthermore, the gpl20 variable domains, V1, V2, V4, and V5, the CD4-binding domain, and the gp4l fusion domain are not directly involved in determining macrophage tropism.