Human proteinase 3 (PRTN3) is a leukemia-associated antigen specifically recognized by CD8+ cytotoxic T-lymphocytes (CTL). PRTN3 also has been shown to elicit both antibody responses and T-cell proliferation in patients with Wegener's granulomatosis. In order to improve current vaccines that aim to stimulate CTL without inducing harmful autoimmune disease, it is necessary to study the role of PRTN3-specific CD4+ T-helper (TH) and CD4+ T-regulatory (Treg) cells. Since both TH and Treg cells recognize antigens in the context of HLA-class-II-molecules, identification of HLA-class-II-associated peptide-epitopes from self-antigens such as PRTN3 is required. Here, we analyzed T-cell responses against proteinase 3 using synthetic peptides predicted to serve as HLA-DR-restricted epitopes. We first screened a panel of ten epitope peptide candidates selected with the TEPITOPE program and found that nine out of ten peptides induced PRTN3 peptide-specific proliferation of T-cells with precursor frequencies of 0-1.1 x 10(-6). For one peptide-epitope, PRTN3(235), T-cell-clones were demonstrated to be capable of recognizing naturally processed protein antigen in a HLA-DR-restricted fashion. PRTN3(235)-specific T-cells could be stimulated from the blood of healthy individuals with multiple HLA-DR-genotypes. In summary, the identified PRTN3(235)-epitope can be used to study the role of CD4+ TH- and Treg-cells in immune responses against PRTN3 in leukemia patients and patients with Wegener's disease.