Detection of Id antigen–specific CD4⁺ T cells in postvaccination PBMCs (day +180). (A) Schematic showing the experimental design for detection of antigen-specific T-cell responses after 2 rounds of stimulation with autologous protein antigen-loaded immortalized B cells as antigen-presenting cells, as described in “Methods.” Mean GM-CSF concentration in culture supernatants after coculture of Id protein– or KLH (internal control)–loaded immortalized autologous B cells (or SEB; data not shown) by in vitro–expanded peripheral blood CD4⁺ T cells (B) or therapeutic activated T-cell products (C). Numbers refer to individual patients who had received Id-KLH immunization. Corresponding Id proteins from patients 263 and 255 served as specificity controls for other patients. Differences in cytokine concentrations were analyzed using a 2-tailed unpaired Student t test. ELISA, enzyme-linked immunosorbent assay.