Purpose: ESO is a tumor-specific antigen with wide expression in human tumors of different histologic types and remarkable spontaneous immunogenicity. We have previously shown that specific T_H1 and antibody responses can be elicited in patients with no detectable preexisting immune responses by vaccination with rESO administered with Montanide ISA-51 and CpG ODN 7909. The purpose of the present study was to characterize vaccine-induced ESO-specific CD4⁺ T cell responses.

Experimental Design: We generated CD4⁺ T cell clones from patient C2, who had the highest CD4⁺ T cell response to the vaccine, and analyzed their fine specificity and HLA class II restriction to determine the recognized epitope. We then assessed the response to the identified epitope in all vaccinated patients expressing the corresponding HLA class II allele.

Results: We found that ESO-specific CD4⁺ T cell clones from patient C2 recognize peptide ESO_119-143 (core region 123-137) presented by HLA-DR52b (HLA-DRB3*0202), a MHC class II allele expressed by about half of Caucasians. Importantly, following vaccination, all patients expressing DR52b developed significant responses to the identified epitope, accounting for, on average, half of the total CD4⁺ T cell responses to the 119-143 immunodominant region. In addition, analysis of ESO-specific DR52b-restricted CD4⁺ T cells at the clonal level revealed significant conservation of T cell receptor usage among different individuals.

Conclusions: The identification of a DR52b-restricted epitope from ESO that is immunodominant in the context of vaccine-elicited immune responses is instrumental for the immunologic monitoring of vaccination trials targeting this important tumor antigen.