Alternative BCR/ABL splice variants in Philadelphia cromosome positive leukemias result in novel tumor specific fusion proteins that may represent potential targets for immunotherapic approaches

New, potentially tumor-specific antigens have been described in Bcr/Abl positive leukemias. Besides the main BCR/ABL hybrid fusion transcripts, a small number of transcripts derived from alternative splicing between BCR exons 1, 13, and 14 with ABL exons 4 and 5 have been identified. These variants are expressed in chronic myelogenous leukemia and acute lymphocytic leukemia patients. The transcriptional products were characterized at their C-terminus by a large amino acid portion derived from out of frame (OOF) reading of the ABL gene. This OOF peptide is expressed only in leukemic cells and has no homology with known human proteins. In order to study an in vivo model, three 39-amino acid peptides, each corresponding to a third of the whole human OOF peptide sequence, were tested for their capacity to elicit specific immune responses in HLA A2.1 transgenic mice. Peptides A and B, but not C, induced the production of specific antisera, while only A and C induced the generation of specific cytotoxic T lymphocytes which were able to kill the target cells in vitro in a class I HLA A2.1 restricted manner. In this study the target cells were human chronic myelogenous leukemia cell line K562, transfected with HLA A2.1 and positive for Bcr/Abl OOF fusion protein. This data, together with flow cytofluorimetric (FACS) analysis of sera from peptide-immune mice, confirmed that Bcr/Abl OOF proteins derived from BCR/ABL alternative splicing may be processed and presented in association with HLA A2.1 molecules on the surface of leukemic cells