The human immunodeficiency virus, HIV, induces syncytium formation in cultures of many T cell lines. These syncytia have previously been viewed as disorganized fusion products in the throes of death. Evidence is presented that in HIV1-infected SupT1 cultures, syncytia five times to over one hundred times larger than single cells organize their many nuclei into blastula-like balls, reorganize their cytoskeleton to mimic that of a single cell, and extend single, giant pseudopods in a polar fashion. Medium-sized syncytia are capable of translocation through extension of these giant pseudopods. The rate of translocation of syncytia is comparable to that of single cells. Single cell motility, syncytium motility and pseudopod extension also appear to play roles in the recruitment of cells into syncytia. Finally, condensation of F-actin at cell-syncytium and syncytium-syncytium adhesion sites suggests the involvement of the cytoskeleton in the adhesion and/or subsequent fusion event. These results suggest that the fusion events involved in HIV-induced syncytia formation involve both cell motility and reorganization of the cytoskeleton, and demonstrate that syncytia are highly organized, motile entities.

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