In addition to CD4+ T lymphocytes, macrophages are increasingly recognized as HIV-1 target cells involved in the pathogenesis and persistence of infection including sexual transmission and early virus dissemination in both lymphoid and nonlymphoid tissues where they can constitute persistent virus reservoirs. Paradoxically, in vitro infection assays suggest that virus isolates are predominantly T-tropic (or non-M-tropic) and rarely macrophage-tropic (M-tropic). The latter are assumed to emerge under CD4+ T-cell paucity in tissues such as the brain or at late stage when the CD4 T-cell count declines. Assays to qualify HIV-1 tropism using cell-free viral strains rely on the use of free virus particles that may not fully reflect the conditions of in vivo macrophage infection through cell-to-cell viral transfer. Besides, sexual transmission makes semen as primary vector for viral dissemination. Despite the presence of HIV-1 viral variants with mixed tropism in the infected donor, almost all sexually transmitted viruses use CCR5 as an entry co-receptor suggesting that seminal fluid components may be responsible for the restriction of sexual transmission of of CXCR4-using viruses. The main objective of our work was to study and compare the ability of viruses expressing primary envelope glycoproteins (Envs) with CCR5 and/or CXCR4 usage from different stages of infection, including transmitted /founder Envs, to infect macrophage by a cell-free mode and through cell-to-cell transfer from infected CD4+ T cells. Our results show that most viruses were unable to enter macrophage as cell-free particles. In contrast, all viruses could be effectively cell-to-cell transferred to macrophage from infected CD4+ T cells. We further showed that viral transfer proceeded through Env-dependent cell-cell fusion of infected T cells with macrophage targets, leading to the formation of productively infected multinucleated giant cells. Compared to cell-free infection, infected T-cell/macrophage contacts showed enhanced interactions of R5 M- and non-M-tropic Envs with CD4 and CCR5, resulting in a reduced dependence on receptor expression levels on macrophage for viral entry. At concentrations present in semen, polyamines, including spermine and spermidine, are able to binds CXCR4 co-receptor and selectively inhibits cell-free and cell-associated X4 HIV-1 infection of cell lines and primary target cells, while having no effect on R5 viruses. Our findings suggest that seminal polyamines constitute one of the long-sought gate keepers that restricts sexual CXCR4-using HIV-1 transmission. Together, our findings likely impact different stages of the pathophysiology of HIV-1 infection regarding viral sexual transmission through genital and rectal mucosa, viral spreading in target cells and in lymphoid and non-lymphoid tissues, as well as establishment of the viral tissue reservoirs.

Keywords: HIV-1,  macrophage, cell-cell fusion, transmission,  co-receptor, viral tropic, cellular-tropism, spermine