Susceptibility of peripheral blood mononuclear cells to in vitro HIV-1 infection. PBMCs (105 cells/well in 96-well microplates) from 45 EUs and 44 UCs were infected with HIV-1 NL-4.3 (A) or with HIV-1 BaL (B) in quadruplicate. Infections were performed either 24 h before PHA activation of PBMC (BA), or after 3-days PHA activation (AA). Standard PBMC were infected in parallel in each assay. HIV-1 replication was monitored by p24 antigen measure in culture supernatants. The p24 values of each individual PBMC infection were compared to the mean p24 value of standard PBMC. Results are expressed as the percent of the standard PBMC supernatant p24 at the peak of infection. Box-plots represent the 25th and the 75th percentiles and bars indicate the lowest and the highest values. The horizontal line in the boxes indicates the median. Comparisons between the groups were made using the non parametric Mann-Whitney U test.