Data from The CD4<sup>+</sup> T-Cell Response of Melanoma Patients to a MAGE-A3 Peptide Vaccine Involves Potential Regulatory T Cells

Melanoma patients were injected with various vaccines containing a MAGE-A3 peptide presented by HLA-DP4. Anti–MAGE-A3.DP4 T cells were not detectable in the blood before vaccination, but their frequencies after vaccination ranged from 2 × 10⁻⁶ to 2 × 10⁻³ among the CD4⁺ blood T lymphocytes of the patients. The CD4⁺ blood T lymphocytes that stained ex vivo with HLA-DP4 tetramers folded with the MAGE-A3 peptide were selected by flow cytometry and amplified under clonal conditions. About 5% of the CD4⁺ T-cell clones that recognized the MAGE-A3.DP4 antigen had a CD25⁺ phenotype in the resting state. These CD25⁺ clones had a high capacity to suppress the proliferation of another T-cell clone after peptide stimulation in vitro. Most of them had high FOXP3 expression in the resting state and an unmethylated FOXP3 intron 1. They produced active transforming growth factor-β but none of cytokines IFN-γ, interleukin-2 (IL-2), IL-4, IL-5, and IL-10. About 20% of CD25⁻ clones had a significant but lower suppressive activity. Most of the CD25⁻ clonal populations contained cells that expressed FOXP3 in the resting state, but FOXP3 demethylation was not observed. We conclude that MAGE-A3.DP4 vaccination can produce CD4⁺ T cells that may exert regulatory T-cell function in vivo. [Cancer Res 2009;69(10):4335–45]