Background: Although HIV primarily infect CD4⁺ cells using CD4 receptors, it is well established that other CD4^- cells in the body can also be infected by HIV. Primary HIV-2 and SIV as well as laboratory strains of HIV-1 have been isolated that can enter CD4^- cells using different pathways. We have recently isolated HIV-1 variants from an AIDS patient (pt) that infected CD8⁺ cells independent of CD4 using CD8 as a receptor. We sought to examine the presence of CD4-independent viruses in other HIV-1 infected pts.

Methods: The viral quasispecies of a HIV-1 (clade D)-infected African pt (92 UG046; obtained through the NIH AIDS Research and Reagents Program) were tested for evidence of CD4-independent viruses. CD4-independent HIV-1 variant (92 UG046-T8) was isolated from the quasispecies by stringent selection of infected CD8⁺/CD4^- cells and growing the viruses in CD4-depleted PBMC. The 92 UG046-T8 isolate was tested for CD4-independent infection by its ability 1) to replicate in primary CD8⁺ and B cells; 2) to infect CD8⁺ T- and B-cell line (LCL) that were negative for CD4; 3) to infect in the presence of blocking anti-CD4 antibodies; 4) to infect other CD4^- cell lines; and 5) to infect cells with pseudotropic viruses containing only the cloned envelope from 92 UG046-T8. The envelope of 92 UG046-T8 was sequenced and compared with the major viral quasispecies (CD4-tropic) present in the same pt.

Results: The 92 UG046-T8, but not the parental viruses (92 UG046), were able to infect and replicate in CD4-depleted PBMC as well as in CD8⁺ T- and B-cell lines that had no trace of CD4 mRNA. Blocking anti-CD4 antibodies were not able to prevent infection of these cells. However, 92 UG046-T8 maintained unchanged ability to infect and replicate in CD4⁺ cells as well. Remarkably, infection of CD8⁺ cells and B-cells induced striking syncitia and produced acute cell death. Although 92 UG046-T8 was X4-tropic, infection and induction of syncitia in CD8⁺ or B-cells were independent of CXCR4. This variant was able to infect other CD4^- non-lymphoid cell lines including U87 cells that did not express most of the known HIV co-receptors. Pseudotropic HIV-1 containing only the 92 UG046-T8 envelope maintained unchanged phenotype. Interestingly, unlike the parental viruses, 92 UG046-T8 was undetectable by AMPLICOR. Several sequence changes were observed in the envelope of 92 UG046-T8 when compared to its parental viruses. The most remarkable of these changes was a single point mutation in the gp41 region that resulted in a severely truncated cytoplasmic tail.

Conclusions: These data demonstrate the presence of an almost untraceable CD4-independent HIV-1 variant with a remarkably different phenotype and genotype in the viral quasispecies in a pt. Evolution of CD4-independent HIV-1 variants with a broad tropism may play a crucial role in AIDS pathogenesis.