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Macrophage tropism of human immunodeficiency virus type 1 (HIV-1) is distinct from coreceptor specificity of the viral envelope glycoproteins (Env), but the virus-cell interactions that contribute to efficient HIV-1 entry into macrophages, particularly via CXCR4, are not well understood. Here, we characterized a panel of HIV-1 Envs that use CCR5 ( n = 14) or CXCR4 ( n = 6) to enter monocyte-derived macrophages (MDM) with various degrees of efficiency. Our results show that efficient CCR5-mediated MDM entry by Env-pseudotyped reporter viruses is associated with increased tolerance of several mutations within the CCR5 N terminus. In contrast, efficient CXCR4-mediated MDM entry was associated with reduced tolerance of a large deletion within the CXCR4 N terminus. Env sequence analysis and structural modeling identified amino acid variants at positions 261 and 263 within the gp41-interactive region of gp120 and a variant at position 326 within the gp120 V3 loop that were associated with efficient CXCR4-mediated MDM entry. Mutagenesis studies showed that the gp41 interaction domain variants exert a significant but strain-specific influence on CXCR4-mediated MDM entry, suggesting that the structural integrity of the gp120-gp41 interface is important for efficient CXCR4-mediated MDM entry of certain HIV-1 strains. However, the presence of Ile326 in the gp120 V3 loop stem, which we show by molecular modeling is located at the gp120-coreceptor interface and predicted to interact with the CXCR4 N terminus, was found to be critical for efficient CXCR4-mediated MDM entry of divergent CXCR4-using Envs. Together, the results of our study provide novel insights into alternative mechanisms of Env-coreceptor engagement that are associated with efficient CCR5- and CXCR4-mediated HIV-1 entry into macrophages.

We characterized human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins (Env) isolated from two HIV-1-infected CCR5Δ32 homozygotes. Envs from both subjects used CCR5 and CXCR4 for entry into transfected cells. Most R5X4 Envs were lymphocyte-tropic and used CXCR4 exclusively for entry into peripheral blood mononuclear cells (PBMC), but a subset was dually lymphocyte- and macrophage-tropic and used either CCR5 or CXCR4 for entry into PBMC and monocyte-derived macrophages. The persistence of CCR5-using HIV-1 in two CCR5Δ32 homozygotes suggests the conserved CCR5 binding domain of Env is highly stable and provides new mechanistic insights important for HIV-1 transmission and persistence.