The question of whether T cell responses to SEREX-defined tumor antigens are under regulation of naturally occurring CD4+CD25+ regulatory T cells (nTreg cells) has not been answered. To address this issue, we first identified an HLA-A2.1-restricted T cell antigen epitope of SEREX-identified tumor antigen CML66L, 66Pa. The HLA-A2.1/66Pa peptide complex in vitro stimulated the in vivo-primed T cells as shown by increased T cell proliferation, higher secretion of the T cell cytokine interferon-gamma (IFN-gamma), increased production of intracellular IFN-gamma in CD8+ T cells, and higher T cell-mediated cytotoxicities of CML66L+ human tumor cells. This suggests that CML66L elicits T cell immune responses. We also developed a novel internal reference epitope for identification of T cell epitopes by construction of chimeric CML66L containing myeloid antigen proteinase 3 epitope Pr1 as a control. Finally, we found that nTreg cells regulates T cell responses to 66Pa, and that depletion of nTreg cells via a pro-apoptotic protein Bax-dependent mechanism enhances polyclonal T cell responses to 66Pa. These findings provide new insights into the T cell participation in SEREX-defined anti-tumor immune responses and novel direction in enhancement of anti-leukemia immunotherapy by modulation of homeostasis of nTreg cells.