Tunneling nanotubes (TNTs), the long membrane extensions connecting distant cells, have emerged as a novel form of cell-to-cell communication. However, it is not fully understood how and to what extent TNTs contribute to intercellular spread of pathogens including HIV-1. In this study, we show that HIV-1 promotes TNT formation per se via its protein Nef and a cellular protein M-Sec, which appears to mediate approximately half of viral spread among monocyte-derived macrophages (MDMs). A small compound that inhibits M-Sec-induced TNT formation reduced HIV-1 production by almost half in MDMs. Such inhibition was not observed with Nef-deficient mutant HIV-1 that fails to promote TNT formation and replicates less efficiently than the wild-type HIV-1 in MDMs. The TNT inhibitor-sensitive/Nef-promoting viral production was also observed in a T cell line ectopically expressing M-Sec, but not in another M-Sec2 T cell line. Our results suggest the importance of TNTs in HIV-1 spread among MDMs and might answer the long-standing question how Nef promotes HIV-1 production in a cell type-specific manner.
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