Cell-to-cell spread of HIV-1 via virological synapses
Human Immunodeficiency Virus, type 1 (HIV-1) primarily infects CD4+ T cells and macrophages, which leads to an acquired immunodeficiency syndrome (AIDS). HIV-1 can infect cells through cell-free or cell-associated routes. While infection by cell-free virus has been studied extensively, cell-to-cell infection has only recently been appreciated between CD4+ T cells. Here it is shown that HIV-1 spreads from cell-to-cell from macrophages to CD4+ T cells, and that cell-to-cell infection can result in a multi-copy infection. In order to study cell-to-cell infection, the laboratory of Benjamin Chen developed an infectious molecular clone of HIV-1 that labels the Gag protein internally with the green fluorescent protein (GFP), called HIV Gag-iGFP. Labeling Gag in this manner recapitulates intracellular Gag localization in mammalian cells by immunofluorescence. Using this strategy to track HIV-1 assembly and the virus particles themselves, macrophages were found to form virological synapses with autologous CD4+ T cells and transferred HIV-1 particles to them. The multiplicity of infection in CD4+ T cells by either a cell-free or a cell-to-cell route was evaluated as well as how the co-infection frequency changes as the overall infection frequency changes by both routes. In contrast to cell-free infection, which followed a random, Poisson distribution, cell-to-cell infection produced multi-copy infected cells at a frequency proportional to the infection frequency. Direct fluorescence in situ hybridization revealed more proviral DNA copies in the nuclei of infected cells following a cell-to-cell infection than a cell-free infection. Altogether, these data indicate that cell-to-cell infection occurs between different types of cells in the immune system and that cell-to-cell infection allows for multi-copy infection, which may act to increase viral diversity by buffering less-fit mutants through complementation.