Alkhatib G, Broder CC, Feng Y, Berger EA LVD, NAID, NIH, Bethesda, MD.
HIV-1 fusion processes are critically dependent on features of the CD4+ target cell. First, CD4 must be expressed on a human cell type to support infection and cell fusion, due to the requirement of an unidentified human-specific accessory fusion factor(s). Second, individual HIV-1 isolates have markedly distinct tropisms for infection of different CD4+ human cell types; some replicate in primaty macrophages but not in CD4+ T-cell lines whereas others show the opposite selectivity. The viral determinants for this tropism reside in gp120. We recently presented evidence that tropism of an HIV-1 isolate is determined primarily by fusion select-ivity of the corresponding env. A working model is that fusion selectivity is due to preferential interaction of different envs with distinct accessory fusion factors of different CD4+target cell types. We performed cell hybrid experiments, using CD4- expressing murine 3T3 cells which are nonpermissive for fusion with envs from either
HIV-1 class. Hybrids formed with HeLa cells acquired fusion permissiveness for T-cell line-tropic but not microphage-tropic envs; hybrids formed with macrophages acquired the opposite fusion selectivity. We also examined human myelocyte or monocyte/microphage cell lines; such cells can be infected by T-cell line-tropic but not microphage-tropic HIV-1 isolates, but can be rendered susceptible to microphage-tropic isolates by treatment with retinoic acid. We observed parallel changes in fusion specificity: untreated cells supported fusion with T-cell line-tropic but not microphage-tropic envs, whereas retinoic acid-treated cells acquired permissiveness for fusion with microphage-tropic envs. Thus in the untreated cells, the infection block with the microphage-tropic isolates is at the level of fusion and can be overcome by retinoic acid. Our results support the model that HIV-1 tropism is due to the presence on different CD4+ cell types of distinct accessory factors that preferentially
mediate fusion by envs from the two HIV-1 classes. Additional findings did not support suggestions by others that CD44 or CD26 function directly in entry of microphage-tropic isolates. We are developing vaccinia-based expression cloning strategies to isolate cDNAs encoding the accessory fusion factors.
