TEMPERATURE-DEPENDENCE OF CELL-CELL FUSION INDUCED BY THE ENVELOPE GLYCOPROTEIN OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1

We investigated cell-cell fusion induced by the envelope glycoprotein of human immunodeficiency virus type 1 strain IIIB expressed on the su rface of CHO cells. These cells formed syncytia when incubated togethe r with CD4-positive human lymphoblastoid SupT1 cells or HeLa-CD4 cells but not when incubated with CD4-negative cell lines. A new assay for binding and fusion was developed by using fluorescent phospholipid ana logs that were produced in SupT1 cells by metabolic incorporation of B ODIPY-labeled fatty acids. Fusion occurred as early as 10 min after mi xing of labeled SupT1 cells with unlabeled CHO-gp160 cells at 37 degre es C. When both the fluorescence assay and formation of syncytia were used, fusion of SupT1 and HeLa-CD4 cells with CHO-gp160 tells was obse rved only at temperatures above 25 degrees C, confirming recent observ ations (Y.-K. Fu, T. K. Hart, Z. L. Jonak, and P. J. Bugelski, J. Viro l. 67:3818-3825, 1993). This temperature dependence was not observed w ith influenza virus-induced cell-cell fusion, which was quantitatively similar at both 20 and 37 degrees C, indicating that cell-cell fusion in general is not temperature dependent in this range. gp120-CD4-spec ific cell-cell binding was found over the entire 0 to 37 degrees C ran ge but increased markedly above 25 degrees C. The enhanced binding and fusion were reduced by cytochalasins B and D. Binding of soluble gp12 0 to CD4-expressing cells was equivalent at 37 and 16 degrees C. Toget her, these data indicate that during gp120-gp41-induced syncytium form ation, initial cell-cell binding is followed by a cytoskeleton-depende nt increase in the number of gp120-CD4 complexes, leading to an increa se in the avidity of cell-cell binding. The increased number of gp120- CD4 complexes is required for fusion, which suggests that the formatio n of a fusion complex consisting of multiple CD4 and gp120-gp41 molecu les is a step in the fusion mechanism.