Identification of a common HLA-A*0201-restricted epitope among SSX family members by mimicking altered peptide ligands strategy.

The synovial sarcoma X breakpoint (SSX) gene family contains nine members. The SSX proteins are CT (cancer/testis) antigens and can be expressed in many tumor types. T cell immune response against SSX protein can be detected in tumor patients and mice expressing any SSX. Screening predominant protective epitopes might improve the low immunogenicity against these “self” CT antigens. Herein, we predicted HLA-A*0201-restricted epitopes for all nine SSX family members, followed by validation with epitope molecular modeling, peptide/HLA-A*0201 affinity, and binding stability assays. We obtained four highly homologous candidate epitopes with the high immunogenicity scores designated P1, P4, P5 and P6, from the nine SSX members. Each of the four candidates could elicit strong epitope-specific CTL immune responses, but P4 could evoke more interferon γ (IFN-γ)-producing T cells and more potent CTLs that could lyse more target cells. Importantly, almost all of the four epitopes induced CTLs could cross-lyse the mutual targets both in vitro in human PBMCs and HLA-A2.1/Kb transgenic mice, but P4 showed superiority to other epitopes in term of cross-cytolysis. All of these results demonstrate that P4 can induce anti-tumor immunity in a fashion superior to other candidates, and may be the “common” CTL epitope among all SSX-expressing tumors. Due to its documented responses herein, P4 has potential application in peptide-mediated immunotherapy.