Background: Expression of murine or human CXCR4 with human CD4 renders mouse fibroblasts susceptible to transduction by X4-tropic HIV-1. In contrast, murine T-cells expressing the same factors are refractory to infection by X4-tropic HIV-1. No such infection block is observed for R5-tropic HIV-1 isolates in murine T-cells expressing human CD4 and CCR5. Surprisingly, MLV pseudotyped with X4-tropic HIV-1 Env has been reported to efficiently transduce murine T-cells expressing HIV-1 receptor molecules. Based on this observation, we investigated what unique properties of X4-tropic MLV versus X4-tropic HIV-1 permitted infection. We also further investigated the nature of the X4-tropic HIV-1 infection block.

Methods: Single-round infection assays were performed using retroviral vectors carrying reporter genes to monitor successful infection events. Viral entry in different targe T-cells was also examined via real-time PCR for early stage reverse transcriptase products. Cell-cell fusions were observed via light microscopy, FACS analysis, and enzyme diffusion assays.

Results: MLV-pseudotypes with cytoplasmic truncations of X4-tropic HIV-1 Env were able to transduce human CD4/CXCR4 expressing murine T-cells. In order to determine if relief of the infection block was due to 1) truncations in HIV-1 Env or 2) MLV core elements, we first tested HIV-1 vectors pseudotyped with truncated HIV envelope variants in challenges of murine T-cells. HIV vectors bearing wild-type or truncated Env were inefficient in transduction. However, MLV vectors, particularly those displaying truncated HIV-1 Env, efficiently transduced the T-cells. Co-culture assays using human cells expressing wild-type and truncated X4-tropic HIV-1 Env were competent to fuse with the murine T-cells.

Conclusions: In contrast to CCR5, CXCR4 function as an HIV-1 co-receptor appears to be cell-type dependent. MLV vectors do not manifest the same restrictions for reasons which are unclear. Blocks in HIV-1 infection in murine T-cells with X4-tropic HIV-1 may be at a post-entry step, although no such block is observed with R5-tropic HIV-1. This would suggest a role for CXCR4 in the post-fusion trafficking of HIV-1 in targe T-cells as well as differences of these routes in CXCR4 and CCR5 mediated infections.