Specific targets of cellular immunity in human premalignancy are largely unknown. Monoclonal gammopathy of undetermined significance (MGUS) represents a precursor lesion to myeloma (MM). We show that antigenic targets of spontaneous immunity in MGUS differ from MM. MGUS patients frequently mount a humoral and cellular immune response against SOX2, a gene critical for self-renewal in embryonal stem cells. Intranuclear expression of SOX2 marks the clonogenic CD138⁻ compartment in MGUS. SOX2 expression is also detected in a proportion of CD138⁺ cells in MM patients. However, these patients lack anti-SOX2 immunity. Cellular immunity to SOX2 inhibits the clonogenic growth of MGUS cells in vitro. Detection of anti-SOX2 T cells predicts favorable clinical outcome in patients with asymptomatic plasmaproliferative disorders. Harnessing immunity to antigens expressed by tumor progenitor cells may be critical for prevention and therapy of human cancer.

NY-ESO-1 is a member of the cancer-testis family of tumor antigens that elicits strong humoral and cellular immune responses in patients with NY-ESO-1–expressing cancers. Since CD4⁺ T lymphocytes play a critical role in generating antigen-specific cytotoxic T lymphocyte and antibody responses, we searched for NY-ESO-1 epitopes presented by histocompatibility leukocyte antigen (HLA) class II molecules. Autologous monocyte-derived dendritic cells of cancer patients were incubated with recombinant NY-ESO-1 protein and used in enzyme-linked immunospot (ELISPOT) assays to detect NY-ESO-1–specific CD4⁺ T lymphocyte responses. To identify possible epitopes presented by distinct HLA class II alleles, overlapping 18-mer peptides derived from NY-ESO-1 were synthetized and tested for recognition by CD4⁺ T lymphocytes in autologous settings. We identified three NY-ESO-1–derived peptides presented by DRB4*0101–0103 and recognized by CD4⁺ T lymphocytes of two melanoma patients sharing these HLA class II alleles. Specificity of recognition was confirmed by proliferation assays. The characterization of HLA class II–restricted epitopes will be useful for the assessment of spontaneous and vaccine-induced immune responses of cancer patients against defined tumor antigens. Further, the therapeutic efficacy of active immunization using antigenic HLA class I–restricted peptides may be improved by adding HLA class II–presented epitopes.