Abstract
The SSX2 gene encodes the tumor‐specific antigen HOM‐MEL‐40/SSX2 expressed in a broad spectrum of tumors of different origin, against which humoral and CD8^+^ T‐cell‐mediated MHC‐I‐restricted responses have been demonstrated. Searching for promiscuous MHC‐II‐restricted peptides that might be suitable as a CD4^+^ stimulating vaccine for many patients, we used the SYFPEITHI algorithm and identified a HOM‐MEL‐40/SSX2‐derived pentadecamer epitope (p45–59) that induced specific CD4^+^ T‐cell responses restricted by the HLA‐DRB1 subtypes *0701, *1101 and *1302 that have a cumulative prevalence of ∼ 25% in the Caucasian population. The CD4^+^‐mediated response against p45–59 and its DR restriction was demonstrated by inhibition with anti‐CD4 and HLA‐DR antibodies, respectively, and by blocking experiments using HLA‐specific antibodies. The natural processing and presentation of p45–59 was demonstrated by recognition of the SSX2^+^ melanoma cell line Me 275 as well as autologous and allogeneic dendritic cells pulsed with whole‐protein SSX2 by T cells with specificity for p45–59. p45–59 was able to induce responses in 3/6 breast cancer patients and 1/5 healthy controls. No correlation was found between CD4^+^ T‐cell responses against p45–59 reactivity and anti‐SSX2 antibody titers in the serum of patients, suggesting that CD4^+^ and B‐cell responses are regulated independently. p45–59 holds promise as a broadly applicable peptide vaccine for patients with SSX2‐positive neoplasms. © 2004 Wiley‐Liss, Inc.