Abstract:

HIV has caused a global pandemic, affecting millions of people worldwide. Despite significant advancements in antiretroviral therapy (cART) and prevention strategies, the establishment and persistence of viral reservoirs in the tissues of people living with HIV (PLHIV) prevents the complete eradication of the virus. Macrophages, as target cells of HIV-1, could participate in viral transmission, dissemination, and the establishment of tissue virus reservoirs. Although macrophages are poorly permissive to cell-free infection in vitro due to high expression of host cell restriction factors, we have identified an efficient mechanism for HIV-1 cell-to-cell spread in macrophages through fusion with infected T cells, leading to the formation of highly virus-productive multinucleated giant cells (MGCs). Here, we described the mechanisms regulating this cell-to-cell transfer of HIV-1 in macrophages, as well as the cellular and virological factors involved.

Our findings indicate that cell-cell fusion is the most effective and rapid route of HIV-1 transfer between infected T cells and target macrophages in vitro. This heterotypic fusion allows effective HIV-1 transfer in various tissue macrophage populations, including synovial, placental, lung alveolar, and tonsil macrophages. However, the efficiency of this transfer depends on T cell viability and the activation and polarization status of macrophages, with enhanced cell-cell fusion occurring between living infected T cells and anti-inflammatory macrophages.

Additionally, the initial interaction between infected T cells and target macrophages leads to the formation of adhesion structures, characterized by the accumulation of β2 integrins and polymerized actin at the site of contact. We identified the CD81 tetraspanin as a key inhibitor of the reorganization of the actin cytoskeletton and thus of cell-cell fusion. CD81 activates the RhoA/ROCK pathway in macrophages, which triggers myosin II phosphorylation, promoting actomyosin contractility, and preventing fusion with infected T cells.

To further elucidate the dynamics of HIV-1 cell-cell fusion between infected T cells and macrophages, we explored the virological and cellular factors regulating HIV-1 transfer. Our results report that viral auxiliary proteins (Nef, Vif, Vpr, and Vpu), well-known for counteracting host cell restriction factors and essential for cell-free infection of macrophages, do not play a significant role in HIV-1 cell-to-cell spread in macrophages. Moreover, type I IFN, which typically provides a strong immune response against HIV-1, does not effectively prevent cell-cell fusion between infected T cells and macrophages. In agreement, this process of cell-cell fusion and the formation of HIV-1 MGCs bypasses restrictions imposed by IFN-stimulated APOBEC3G deaminases and the restriction factor SERINC5. Additionally, this HIV-1 cell-to-cell spreading in macrophages is resistant to several cART drugs, including inhibitors targeting HIV-1 capsid, nuclear translocation, reverse transcription, and integration.

To understand the disruption of these pathways, we investigated the nuclei present in MGCs. MGCs contain at least one T cell nucleus from the initial fusion event between an infected CD4 T cell and a macrophage. Our results indicate that these T cell nuclei remain transcriptionally active in MGCs long after fusion, sustaining the HIV-1 replication cycle. Surprisingly, HIV-1 DNA was also found in most macrophage nuclei, suggesting that it originates from preintegration complexes formed in infected CD4 T cells before fusion. These findings reveal a novel mechanism of HIV-1 cell-to-cell transfer in macrophages and provide insights into the establishment and maintenance of MGCs in viral reservoirs in PLHIV.

Keywords: HIV-1, macrophage, myeloid cells, viral transfer, cell-cell fusion, restriction factors, viral proteins, interferons, anti-retroviral treatment