Background: HIV-1 infects macrophages and microglia in the brain and causes neurological disorders in infected patients. We and others have shown that brain-derived envelopes have lower CD4 dependence and higher avidity for CD4 than those from peripheral isolates, and we have also observed reduced sensitivity to the fusion inhibitor T-1249 and to the gp120-targeted small molecule BMS-378806.

Methods: Since there are genetic differences between brain and spleen envelopes throughout gp120 and in the HR2 region in the gp41 ectodomain, we investigated the viral determinants for the phenotypic differences by constructing chimeric and mutant envelopes and using them in cell-to-cell fusion assays and single-round pseudotype infections.

Results: Chimeric envelopes showed that brain gp120 V1/V2-C2-V3 region determines the low CD4 dependence and high avidity for CD4, as well as macrophage tropism and reduced sensitivity to BMS-378806. In addition, the HR2 region in brain gp41 seemed to contribute to the reduced sensitivity to T-1249 of brain envelopes since a modified T-1249-containing specific residues found in brain, but not in spleen HR2 regions, was a slightly more potent inhibitor of brain envelope-mediated cell-to-cell fusion. However, the increased fusogenicity and reduced T-1249 sensitivity of brain and certain chimeric envelopes correlated with the low CD4 dependence and high avidity for CD4 determined by brain gp120's V1-V3 region. Asparagine in position 283 (N283) within the C2 region of gp120 has been previously associated with macrophage tropism, brain infection, lower CD4 dependence, and higher CD4 affinity. When we introduced N283T in a brain envelope and in the envelope of a brain-derived isolate, and T283N in a spleen-derived counterpart, they did not alter the phenotype of the parental envelopes for CD4 dependence and avidity, macrophage tropism, fusogenicity or sensitivity to inhibitors.

Conclusions: Viral determinants of phenotypic changes in brain envelopes are complex and context dependent, but interaction with CD4 seems to determine not only macrophage tropism, but also fusogenicity and sensitivity to fusion and other entry inhibitors.