Fusion efficiency of CHO cells expressing HLA-C and HIV-1 Env. Panel A: Syncytia formation after co-cultivation of effector CHO cells expressing gp120/gp41 and HLA-C, or CHO cells expressing only gp120/gp41, with target CHO-CD4-CCR5 cells. The number and the extent of syncytia is significantly higher (p < 0.05) when effector cells express HLA-C. Panel B: ELISA analysis of Env expression. CHO, negative control; CHO-gp120, cells stably expressing the Env gene of the R5 tropic HIV-1 isolate 91US005; CHO-gp120-HLA-C: CHO-gp120 cells stably expressing HLA-Cw4; gp120: positive control, consisting of a mixture of five different gp120s. The higher fusion efficiency of CHO-gp120-HLA-C cells is not due to an increased level of Env expression, since they express 27% less gp120 than CHO-gp120 cells.