CXCR4 and HIV-1 infection: In order to evaluate a potential role of immune activation and co-receptor expression in the susceptibility to HIV-1 infection, we examined lymphocyte subsets, expression of activation markers (HLA-DR and CD38) and HIV-1 co-receptors (CXCR4 and CCR5) on T cells in fresh whole blood from 25 EUs and 24 UCs.