T Cell-Macrophage Fusion Triggers Multinucleated Giant Cell Formation for HIV-1 Spreading

HIV-1-infected macrophages participate in virus dissemination and establishment of virus reservoirs in host tissues, but the mechanisms for virus cell-to-cell transfer to macrophages remain unknown. Here, we reveal the mechanisms for cell-to-cell transfer from infected T cells to macrophages and virus spreading between macrophages. We show that contacts between infected T lymphocytes and macrophages lead to cell fusion for the fast and massive transfer of CCR5-tropic viruses to macrophages. Through the merge of viral material between T cells and macrophages, these newly formed lymphocyte-macrophage fused cells acquire the ability to fuse with neighboring noninfected macrophages. Together, these two-step envelope-dependent cell fusion processes lead to the formation of highly virus-productive multinucleated giant cells reminiscent of the infected multinucleated giant macrophages detected in HIV-1-infected patients and simian immunodeficiency virus-infected macaques. These mechanisms represent an original mode of virus transmission for viral spreading and a new model for the formation of macrophage virus reservoirs during infection. IMPORTANCE We reveal a very efficient mechanism involved in cell-to-cell transfer from infected T cells to macrophages and subsequent virus spreading between macrophages by a two-step cell fusion process. Infected T cells first establish contacts and fuse with macrophage targets. The newly formed lymphocyte-macrophage fused cells then acquire the ability to fuse with surrounding uninfected macrophages, leading to the formation of infected multinucleated giant cells that can survive for a long time, as evidencedin vivo in lymphoid organs and the central nervous system. This route of infection may be a major determinant for virus dissemination and the formation of macrophage virus reservoirs in host tissues during HIV-1 infection.