Titration of X4 and R5 pseudotyped viruses. (A) Serial dilutions of X4 and R5 pseudotyped viruses were used to infect CD4+ T cells isolated from a healthy donor. CD4+ T cells (n = 100,000) from a randomly selected subset of uninfected donors (triangles), ES (diamonds), and viremic patients (circles) were infected with 1.68, 0.96, or 0.48 ng p24 equivalents of X4 virus (B) or 2.7 ng p24 equivalents of R5 virus (C). Each symbol represents cells from an individual patient that were infected with different concentrations of virus.