Expression of synovial sarcoma X (SSX) antigens in epithelial ovarian cancer and identification of SSX-4 epitopes recognized by CD4⁺ T cells

Purpose: Synovial sarcoma X (SSX) breakpoint genes are expressed in a variety of cancers but not in normal tissues, except for testis, and are potential targets for immunotherapy. The aims of this study were to determine the expression and immunogenicity of these antigens in patients with epithelial ovarian cancer (EOC). Experimental Design: SSX-1-, SSX-2-, and SSX-4-specific reverse transcription-PCR were done on a panel of EOC specimens. Sera from a subgroup of the patients were tested for SSX-2 and SSX-4 antibody by ELISA and recombinant antigen expression on yeast surface (RAYS), in vitro stimulation of peripheral blood mononuclear cells from a patient bearing SSX-4-expressing tumor with a pool of long peptides spanning the protein sequence was used for assessment of SSX-4-specific CD4⁺ T cells recognizing distinct antigenic sequences restricted by HLA class II alleles. Results: Our results indicate expression of SSX-1, SSX-2, and SSX-4 in 2.5%, 10%, and 16% of 120 EOC specimens, respectively. When all three SSX antigens are considered, aberrant expression was found in 26% of ovarian tumors. Antibodies to SSX-2 and SSX-4 were detectable by ELISA and RAYS in two patients. SSX-4-specific CD4 ⁺ T cells recognizing two previously undescribed SSX-4-derived T-cell epitopes in association with HLA-DR (SSX-4: 51-70 and SSX-4: 61-180) were identified. Conclusions: Our study shows aberrant expression of SSX antigens in a proportion of patients with EOC. The evidence of humoral immunity to SSX-2 and SSX-4, and SSX-4-specific CD4⁺ T cells among circulating lymphocytes in patients with antigen expressing EOC suggest that these antigens are attractive targets for specific immunotherapy in EOC.