Flow cytometry analysis of cell population dynamics and cell cycle during HIV-1 envelope-mediated formation of syncytia in vitro

Cell fusion occurs in physiological and pathological conditions and plays a role in regulation of cell fate. The analysis of cell population dynamics and cell cycle in cell–cell fusion experiments is necessary to determine changes in the quantitative equilibrium of cell populations and to identify potential bystander effects. Here, using cocultures of Jurkat HIV-1 envelope expressing cells and CD4 + cells as a model system and flow cytometry for the analysis, the number, viability, and cell cycle status of the populations participating in fusion were determined. In 3-day cocultures, a sustained reduction of the number of CD4 + cells was observed while they showed high viability and normal cell cycle progression; fusion, but not inhibition of proliferation or death, accounted for their decrease. In contrast, the number of Env + cells decreased in cocultures due to fusion, death, and an inherent arrest at G1. Most of syncytia formed in the first 6 h of coculture showed DNA synthesis activity, indicating that the efficient recruitment of proliferating cells contributed to amplify the removal of CD4 + cells by syncytia formation. Late in cocultures, approximately 50% of syncytia were viable and a subpopulation still underwent DNA synthesis, even when the recruitment of additional cells was prevented by the addition of the fusion inhibitor T-20, indicating that a population of syncytia may progress into the cell cycle. These results show that the quantitative analysis of cellular outcomes of cell–cell fusion can be performed by flow cytometry.