Comparison of the fusion efficiency of HLA-C silenced HeLa-Env cells with 3T3.T4.CCR5 and 3T3.T4.CXCR4 cells. HLA-C silenced (+, grey bars) and non-silenced (-, black bars) HeLa cells expressing gp120/gp41 of different HIV-1 isolates (ADA, LAI, NDK) co-cultivated with NIH 3T3.T4.CXCR4 and NIH 3T3.T4.CCR5 cells. Fusion efficiency of X4 tropic gp120 LAI is significantly lower (p < 0.01) in HLA-C silenced cells when fusing with CXCR4 target cells. Similarly, fusion efficiency of the R5 tropic gp120 ADA is lower (p < 0.01) in HLA-C silenced cells when fusing with CCR5 target cells. The fusion of ADA gp120 in HLA-C silenced cells with cells expressing CXCR4 is significantly (p < 0.01) less efficient, while that of LAI gp120 with cells expressing CCR5 is similar, irrespective of HLA-C silencing. The NDK gp120 is HLA-C insensitive, when using either the CXCR4 or the CCR5 co-receptor.