Proliferative responses of HIV-1 rg120 W61D vaccinee PBMC to HIV-1 env gp120 peptide pools. Proliferation was measured by flow cytometry using a CFSE dye dilution assay reporting the percentage of cells that have undergone at least two cell divisions. Assays were performed in triplicate and the mean response ± SEM are shown. Responses to the vaccine isolate W61D peptide pools were compared to heterologous IIIB and SF33 isolates. Panel A shows the response to peptide pools covering variable regions V1/V2, V3 and V4. The C1 pool contains peptides with homology across all three isolates. The C2 pool contains peptides with homology between W61D and IIIB. The C3 pool contains peptides with homology between W61D and SF33. Panel B compares the mean responses of the 4 peptides that comprise the C2 pool, with homology between W61D and IIIB, with the corresponding non-homologous SF33 peptides, individually. A strong response to the W61D peptide ARP7035.19 is common to both G19 and G21which is lost when the corresponding SF33 peptide ARP7117.18 is substituted.