Full-length transmitted/founder viruses use low levels of the cells surface CD4 inefficiently. (A) A representative flow cytometry experiment showing CD4 expression levels on minimally (low) and maximally (high) induced 293-Affinofile cells in comparison with CD4+ primary T cells. (B) Equal amounts of p24 of YU-2 and clinically derived transmitted viruses were used to infect 293-Affinofile cells, which were maximally induced to express high levels of CCR5 and high (black bars) or low (white bars) levels of CD4. Cells were washed, fixed, permeabilized with saponin and stained with anti-HIV-1 p24 FITC-conjugated monoclonal antibody 48 h post-infection. Percentage infection was determined by flow cytometry. Data are representative of at least two independent experiments and error bars represent the standard deviation. (C) 293-Affinofile cells were induced to express different cell surface levels of CD4 and CCR5. Cells were infected with 50 ng of p24 of YU-2 and ZM247F and percentage of infection determined by flow cytometry. Data shown are representative example of three independent experiments.