HIV-1 cell-to-cell spread overcomes the virus entry block of non-macrophage-tropic strains in macrophages

Macrophages (M{Phi}) are increasingly recognized as HIV-1 target cells involved in the pathogenesis and persistence of infection. Paradoxically, in vitro infection assays suggest that virus isolates are mostly T-cell-tropic and rarely M{Phi}-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. However, assays to qualify HIV-1 tropism use cell-free viral particles and may not fully reflect the conditions of in vivo M{Phi} infection through cell-to-cell viral transfer. Here, we investigated the capacity of viruses expressing primary envelope glycoproteins (Envs) with CCR5 and/or CXCR4 usage from different stages of infection, including transmitted/founder Envs, to infect M{Phi} by a cell-free mode and through cell-to-cell transfer from infected CD4+ T cells. The results show that most viruses were unable to enter M{Phi} as cell-free particles, in agreement with the current view that non-M-tropic viruses inefficiently use CD4 and/or CCR5 or CXCR4 entry receptors on M{Phi}. In contrast, all viruses could be effectively cell-to-cell transferred to M{Phi} from infected CD4+ T cells. We further showed that viral transfer proceeded through Env-dependent cell-cell fusion of infected T cells with M{Phi} targets, leading to the formation of productively infected multinucleated giant cells. Compared to cell-free infection, infected T-cell/M{Phi} 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 M{Phi} for viral entry. Altogether, our results show that virus cell-to-cell transfer overcomes the entry block of isolates initially defined as non-macrophage-tropic, indicating that HIV-1 has a more prevalent tropism for M{Phi} than initially suggested. This sheds light into the role of this route of virus cell-to-cell transfer to M{Phi} in CD4+ T cell rich tissues for HIV-1 transmission, dissemination and formation of tissue viral reservoirs. Author SummaryUnderstanding how HIV-1 hijacks the functions of immune cells to promote viral spreading remains a challenge in the fight against infection. M{Phi} are ubiquitous tissue-resident cells, involved in tissue homeostasis and immunity. In HIV-1 infection, along with CD4+ T lymphocytes, M{Phi} serve as vectors for virus dissemination and as viral reservoirs, impeding HIV-1 eradication. However, the mechanisms of their infection remain incompletely understood. A paradox is that infected M{Phi} are found in a large range of tissues whereas in vitro cellular tropism assays indicate that only a limited number of HIV-1 isolates can enter M{Phi}. We hypothesized that these assays, which evaluate infection using cell-free viruses, might not fully reflect the modes of M{Phi} infection in patients. We report here that virus cell-to-cell transfer through cell-cell fusion with infected CD4+ T cells is a very effective means of infecting M{Phi}, even with virus isolates characterized as non-macrophage tropic in cell-free infection. This intercellular viral transfer is facilitated by enhanced interactions between the HIV-1 envelope glycoproteins and cellular entry receptors. We propose that M{Phi} infection through viral transfer from infected CD4+ T cells impacts different aspects of the pathophysiology of HIV-1 infection, renewing our understanding of the role of M{Phi} in HIV-1 pathogenesis and persistence.