T cell subsets differ in susceptibility to DC-mediated transmission of R5 and X4 HIV-1. (A) DC were incubated with R5 or X4 HIV-1, or mock treated, followed by extensive washing to remove unbound virus. DC were subsequently co-cultured with CD4+ naïve T cells (TN), central memory T cells (TCM) or effector memory T cells (TEM) in the presence of saquinavir to prevent spreading infection (single-cycle transmission assay). Two days after transmission, T cells were harvested and stained for CD3 and intracellular CA-p24 to determine the percentage HIV+ T cells. Representative FACS plots are shown. (B) Summary of one representative experiment. Error bars represent standard deviations. * p < 0.05 ; ** p < 0.01; *** p < 0.001.