Purpose: Wilms' tumor 1 protein (WT1), a transcription factor overexpressed in malignant mesothelioma, leukemias, and other solid tumors, is an ideal target for immunotherapy. WT1 class I peptide epitopes that were identified and shown to stimulate CD8(+) T cells are being tested as vaccine candidates in several clinical trials. The induction and maintenance of a robust memory CD8(+) cytotoxic T-cell response requires CD4(+) T-cell help.
Experimental design: Three HLA class II peptide epitopes of WT1 with high predictive affinities to multiple HLA-DRB1 molecules were identified using the SYFPEITHI algorithm. Due to the highly polymorphic nature of the HLA class II alleles, such reactivity is critical in the development of a broadly useful therapeutic. One of the WT1 CD4(+) peptide epitopes, 122-140, comprises a previously identified CD8(+) peptide epitope (126-134). By mutating residue 126 from an arginine to a tyrosine, we embedded a synthetic immunogenic analogue CD8(+) epitope (126-134) inside the longer peptide (122-140). This analogue was previously designed to improve immunogenicity and induce a potent CD8(+) response.
Results: WT1 peptides 328-349 and 423-441 are able to stimulate a peptide-specific CD4(+) response that can recognize WT1(+) tumor cells in multiple HLA-DRB1 settings as determined by IFN-gamma enzyme-linked immunospot assays. The mutated WT1 peptide epitope 122-140 is able to induce CD4(+) and cytotoxic CD8(+) WT1-specific T-cell responses that can recognize the native WT1 epitopes on the surface of human WT1(+) cancer cells. Cross-priming experiments showed that antigen-presenting cells pulsed with either mesothelioma or leukemia tumor lysates can process and present each of the CD4(+) peptides identified.
Conclusions: These studies provide the rationale for using the WT1 CD4(+) peptides in conjunction with CD8(+) peptide epitopes to vaccinate patients with WT1-expressing cancers.