Unprotected sexual intercourse with HIV-infected men is the major cause of new infections. HIV virions are released into semen by various cells of the male genital tract as well as by infected monocytes and lymphocytes present in semen. Some of these virions may attach to the surface of cells, infected or uninfected. We investigated whether cells carrying attached HIV on their surface can transmit infection. We addressed this question in a model system of human tissue ex vivo exposed to monocytes and lymphocytes carrying HIV on their surfaces. We gamma-irradiated these cells to prevent their productive infection. In spite of comparable amounts of HIV attached to monocytes and lymphocytes, only monocytes were capable of transmitting infection and triggering productive infection in tissue. This HIV-1 transmission was mediated by cell-cell contacts. Our experiments suggest that in vivo, HIV attached to infected or uninfected monocytes, which far outnumber lymphocytes in HIV-infected semen, may contribute to sexual transmission of HIV from men to their partners.
The vast majority of new HIV infections occur through sexual transmission in which HIV is transferred from semen of an infected male to an uninfected partner. In semen, HIV-1 particles may exist as free floating virions, inside infected cells or attached to the surface of cells, whether they are infected or not. Here we investigated whether HIV attached at the surface of monocytes or lymphocytes could transmit infection to human tissue. Incubation of human tissue with monocyte-attached HIV resulted in tissue productive infection. In contrast, there was no infection of tissues when they were incubated with lymphocyte-attached HIV-1. Our results highlight the important role that seminal monocytes may play in HIV transmission in vivo, especially since monocytes far outnumber lymphocytes in semen of HIV-infected individuals.