Barriers that limit cell-free HIV-1 entry into macrophages are overcome after cell-cell fusion with infected T cells

BACKGROUND: It is increasingly recognized that macrophages (MΦ) are involved in the dissemination and persistence of HIV-1. In persons living with HIV-1, infected MΦ are found in a wide range of lymphoid and non-lymphoid tissues. Paradoxically, cellular tropism assays indicated that HIV-1 isolates are T-tropic and only rarely M-tropic, most often due to inefficient viral entry into MΦ. However, these tropism assays, because they use cell-free viral particles, might not reflect all modes of MΦ infection in vivo, in particular through cell-to-cell viral transfer. Here, we investigated whether virus isolates, previously characterized as non-M-tropic viruses in cell-free infection assays, could efficiently infect MΦ through cell-cell fusion with infected CD4+ T cells, a new mode of infection of myeloid cells we recently identified.
METHODS: We investigated the capacity of a panel of CCR5- and/or CXCR4-using Env-pseudotyped viruses and infectious molecular clones to infect MΦ by a cell-free mode or through viral transfer from infected CD4+ Jurkat T cells. Envs representative of the different stages of HIV-1 infection were used, including transmitted/founder Envs known to be non-M-tropic.
RESULTS: Single-round infection and virus-cell fusion assays showed that most of these viruses in the form of cell-free particles are inefficient to enter MΦ, whereas they can effectively infect primary CD4+ T cells. In contrast, all viruses were efficiently transferred to MΦ after Env-dependent cell-cell fusion of MΦ with infected CD4+ T cells, ultimately leading in most cases to productively infected multinucleated giant cells. These results suggested that MΦ infection through cell-cell fusion with infected T cells overcome barriers that normally restrict entry of cell-free viruses into MΦ. In line with this, formation of infected T cell/MΦ conjugates enhanced interactions between Env and CD4 and CCR5, rendering viral entry into MΦ less dependent on expression levels of those receptors.
CONCLUSIONS: These data suggest that M-tropism of HIV-1 is more widespread than initially thought based on cellular tropism assays. They also suggest that MΦ infection may be facilitated in CD4+ T cell rich tissues. These data renew our understanding of the role of MΦ in HIV-1 transmission and pathogenesis and the formation of tissue reservoirs.

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