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CTL Activation Using the Natural Low-Affinity Epitope 222-229 from Tyrosinase-Related Protein 1 Leads to Tumor Rejection

Department of Immunology, College of Medicine, Mayo Clinic, Rochester, Minnesota

Requests for reprints: Larry R. Pease, Department of Immunology, College of Medicine, Mayo Clinic, 200 First Street Southwest, Rochester, MN 55905. Phone: 507-284-8177; Fax: 507-266-0981; E-mail: pease.larry{at}mayo.edu.

Key Words: TRP1 • TRP2 • B7-DCXAb • B16 • CTL

Vaccine strategies for cancer immunotherapy have focused on peptide ligands with high affinity for MHC class I. Largely, these vaccines have not been therapeutic. We have examined the peptide specificity of a strongly protective T-cell response that eradicates established B16 melanoma and find that the recognized epitope is generated by a low-affinity MHC class I ligand from tyrosinase-related protein 1 (TRP1). Cytotoxic T-cell responses are induced against TRP1_222-229 by several vaccination schemes using a Toll-like receptor agonist, T regulatory cell depletion, or the immune modulator B7-DCXAb to drive immunity. TRP1₂₂₂ CTL are generated from multiple antigen sources, including antigens expressed by tumors growing in situ, tumor cell lysates, and peptide vaccines. The key finding in this study is that protection from freshly implanted or established B16 tumors is primarily mediated by TRP1₂₂₂-specific CTL and not by CTL specific for more traditional melanoma antigens such as TRP2 or gp100. This finding challenges the assumption that the optimal peptide antigens for cancer vaccines are high-affinity MHC ligands. We propose that when administered appropriately, native low-affinity MHC ligands are optimal inducers of immunotherapeutic CTL. [Cancer Res 2009;69(7):3114–20]