Peptide immunotherapy in cat allergic asthmatic subjects induces linked epitope suppression. PBMCs were isolated before and after a randomized, double-blind, placebo-controlled trial of peptide immunotherapy and frozen. Cells were thawed and cultured with individual peptides and with cat allergen extract for 6 d before quantification of proliferation and cytokine release. Assays were performed once on both pre- and posttreatment samples. Peptides shown by number on the x axis correspond to amino acid sequences in chain 1 (peptides 1–7) and chain 2 (peptides 8–16) of Fel d 1 sequence as follows: 1:1–17, 2:12–28, 3:23–38, 4:29–45, 5:39–55, 6:48–63, 7:54–69, 8:1–16, 9:7–23, 10:20–35, 11:29–44, 12:40–55, 13:48–63, 14:56–71, 15:67–82, 16:77–92. (a) Proliferation data were normalized by conversion to stimulation index (SI; cpm for peptide/allergen culture divided by cpm of medium alone culture). Peptide immunotherapy resulted in statistically significant reductions in proliferative responses to cat allergen extract and to treatment peptides and nontreatment peptides (peptide number shown in shaded boxes). The response to each peptide is represented in paired bars. Pretreatment, open bars; posttreatment, filled bars. IL-4 (b) and IL-13 (c) production to individual peptides and cat allergen extract was reduced as indicated by statistical significance. No significant changes in proliferative or cytokine responses to the control antigen–purified protein derivative (PPD) of Mycobacterium tuberculosis were observed. Data are median, range, and interquartiles (box-and-whisker plot). *, P < 0.05; **, P < 0.01; ***, P < 0.001.