ICAM-1 expression is required for MDC-mediated transmission. Differently matured MDCs as indicated on the left were analyzed by FACS for the expression of ICAM-1 (A), ICAM-2 (B), ICAM-3 (C), and LFA-1 (D). The mean fluorescence intensity is indicated. Open histograms represent isotype controls. (E) Blocking antibodies against ICAM-1 and -3 were preincubated with HIV-1–loaded, R-848–matured MDCs before coculture with LuSIV cells. Error bars represent standard deviations. *P < .01 compared with control antibody.