Unidirectional budding of HIV-1 at the site of cell-to-cell contact is associated with co-polarization of intercellular adhesion molecules and HIV-1 viral matrix protein.

OBJECTIVES: To explore the possibility that HIV-1 budding and cellular adhesion molecules co-polarize at cell-to-cell contact sites. To investigate the incorporation of host-cell-derived adhesion molecules into HIV-1.
METHODS: The cellular sites involved in HIV-1 budding were examined by transmission electron microscopy. Single and double immunocytochemistry staining was used to evaluate the cellular distribution of the viral matrix protein and adhesion molecules. Quantitative flow cytometry was used to measure the cellular expression of adhesion molecules. An immunocapture technique was used to measure the presence of cell-derived proteins on HIV-1. The captured virus was measured by a p24 antigen assay. The infectivity of virus captured by monoclonal antibodies was tested by measuring the virus antigen yield in supernatants after the addition of sensitive cells.
RESULTS: Released and budding HIV-1 was mainly localized at the cell-to-cell contact regions. This feature was consistent with a polarized staining for the virus matrix protein p18 at cell-to-cell contact regions. Intercellular adhesion molecules (ICAM)-1 in HIV-1-infected cells were polarized on both isolated cells and syncytia, co-localizing with HIV-1 matrix protein. HIV-1 incorporated all the adhesion molecules expressed by the host cells, although without quantitative correlation with their cellular expression.
CONCLUSIONS: HIV-1 is released at cell-to-cell membrane contact sites. Both ICAM-1 and virus matrix protein co-polarized on isolated cells and syncytia at the sites involved in the recruitment of uninfected cells. The impressive concentration of ICAM at cell sites where most virions are released may account for the acquisition of these membrane proteins by the HIV-1 progeny, and may be important for the cell-mediated spread.