Discordant replication of R5 and X4 viruses in CD4⁺ T-cell lines established from children and adults. (A) Pediatric and adult cells were infected with 2 different MOI (0.1 and 1) of the laboratory-adapted R5 HIV-1_BaL and X4 HIV-1_IIIB (top panels) and isogenic viruses NL-AD8 (R5) and NL4-3 (X4; bottom panels). (Left panels) The results from a single infection representative of 6 independent experiments. (Right panels) A single infection of cell lines obtained from an adult, representative of 5 independent experiments. All experiments were performed in quintuplicate replicas. (B) Means ± SEM of the peak levels of replication observed in all aforementioned infections. R5 and X4 viruses showed comparable efficiency of replication in adult cell lines, but only R5 viruses spread in child cells. (C) Equal susceptibility of PHA blasts established from children's PBMCs (N = 7) to R5 (BaL) and X4 (LAI/IIIB) productive HIV-1 infection. (D) PBMCs from an independent child were infected by R5 or X4 HIV-1 as whole T-cell blasts (left panel), after enrichment in CD4⁺ T cells (middle panel), or after expansion and passage onto feeder cells (right panel). Both viruses replicated efficiently in the first 2 conditions, whereas only R5, but not X4, HIV-1 efficiently spread in CD4⁺ T cells cocultured for 3 days onto feeder cells.