Abstract
The emergence of T cell-tropic, syncytium-inducing (T-tropic/SI) HIV-1 variants from the background of macrophage-tropic, non-syncytium-inducing (M-tropic/NSI) strains is associated with disease progression in infected individuals. HIV89.6 is a primary isolate with a transitional phenotype: like M-tropic strains it replicates in primary macrophages and lymphocytes but not in most transformed cells, yet it is also syncytium inducing. We have shown that HIV89.6 can utilize both the M-tropic and T-tropic cofactors CCR-5 and CXCR-4, respectively, in conjunction with CD4 for fusion and entry into otherwise nonpermissive nonhuman cells. To better understand the nature of restricted HIV89.6 infection of transformed cells, we analyzed its interaction with CD4-expressing transformed human HeLaCD4-LTR//β-Gal cells, which contain the β-galactosidase gene linked to the HIV-1 LTR. Here we show that HIV89.6 enters these cells and undergoes reverse transcription and integration. Furthermore, HIV89.6 induces LTR-driven β-galactosidase expression, indicating Tat-dependent trans-activation, in a similar number of cells as the permissive T-tropic/SI isolate HIVHXB. Acute infection with HIV89.6, however, produces markedly lower levels of p24 antigen and infectious virus per trans-activation-positive cell than HIVHXB. In contrast, transfection results in high levels of expression for both viruses but HlV89.6 still fails to establish spreading infection. HIV89.6 is also blocked after entry in two other nonpermissive cell lines, SUP-T1 and U937. HIV89.6 arrest in HeLaCD4-LTR/β-Gal cells at a stage subsequent to entry, reverse transcription, integration, and Tat expression is a novel level at which HIV-1 strainand cell-specific restrictions define host cell tropism. These studies emphasize that complex patterns of tropism are determined by the interplay of permissive or restricted virus-cell interactions at multiple steps in the replication cycle.
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