Characterization of new syncytium-inhibiting monoclonal antibodies implicates lipid rafts in human T-cell leukemia virus type 1 syncytium formation

We have previously shown that erythroleukemia cells (K562) transfected with vascular adhesion molecule I (VCAM-1) are susceptible to human T-cell leuk emia virus type 1 (HTLV-1)-induced syncytium formation. Since expression of VCAM-1 alone is not sufficient to render cells susceptible to HTLV-1 fusio n, K562 cells appear to express a second molecule critical for HTLV-induced syncytium formation. By immunizing mice with K562 cells, we have isolated four monoclonal antibodies (MAbs), K5.M1, K5.M2, K5.M3, and K5.M4, that inh ibit HTLV-induced syncytium formation between infected MT2 cells and suscep tible K562/VCATM1 cells. These MAbs recognize distinct proteins on the surf ace of cells as determined by cell phenotyping, immunoprecipitation, and We stern blot analysis. Since three of the proteins recognized by the MAbs app ear to be GPI linked, we isolated lipid rafts and determined by immunoblot analysis that all four MAbs recognize proteins that sort entirely or in lar ge part to lipid rafts. Dispersion of lipid rafts on the cells by cholester ol depletion with beta -cyclodextrin resulted in inhibition of syncytium fo rmation, and this effect was not seen when the beta -cyclodextrin was prelo aded with cholesterol before treating the cells. The results of these studi es suggest that lipid rafts may play an important role in HTLV-1 syncytium formation.