Role of CXCR4 in cell-cell fusion and infection of monocyte-derived macrophages by primary human immunodeficiency virus type 1 (HIV-1) strains: Two distinct mechanisms of HIV-1 dual tropism

Dual-tropic human immunodeficiency virus type 1 (HIV-1) strains infect both primary macrophages and transformed T-cell lines. Prototype T-cell line-tr opic (T-tropic) st:rains use CXCR4 as their principal entry coreceptor (X4 strains), while macrophagetropic (M-tropic) strains use CCR5 (R5 strains). Prototype dual tropic strains use both coreceptors (R5X4 strains). Recently , CXCR4 expressed on macrophages was found to support infection by certain HIV-1 isolates, including the dual-tropic R5X4 strain 89.6, but not by TT-t ropic X4 prototypes like 3B. To better understand the cellular basis for du al tropism, we analyzed the macrophage coreceptors used for Env-mediated ce ll-cell fusion as well as infection by several dual-tropic HIV-1 isolates. Like 89.6, the R5X4 strain DH12 fused with and infected both wild-type and CCR5-negative macrophages. The CXCR4-specific inhibitor AMD3100 blocked DH1 2 fusion and infection in macrophages that lacked CCR5 but not in wild-type macrophages. This finding indicates two independent entry pathways in macr ophages for DH12, CCR5 and CXCR4. Three primary isolates that use CXCR4 but not CCR5 (tybe, UG021, and UG024) replicated efficiently in macrophages re gardless of whether CCR5 was present, and AMD3100 blocking of CXCR4 prevent ed infection in both CCR5 negative and wild-type macrophages. Fusion mediat ed by UG021 and UG024 Envs in both wild-type and CCR5-deficient macrophages was also blocked by AMD3100. Therefore, these isolates use CXCR4 exclusive ly for entry into macrophages. These results confirm that macrophage CXCR4 can be used for fusion and infection by primary HIV-1 isolates and indicate that CXCR4 may be the sole macrophage coreceptor for some strains. Thus, d ual tropism can result from two distinct mechanisms: utilization of both CC R5 and CXCR4 on macrophages and T-cell lines, respectively (dual-tropic R5X 4), or the ability to efficiently utilize CXCR4 on both macrophages and T-c ell lines (dual-tropic X4).