Optimizing immunity against BCR-ABL positive leukemia

Abstract

Chronic myelogenous leukemia (CML) is a clonal disorder characterized by proliferation of cells expressing BCR-ABL fusion proteins. The BCR-ABL fusion proteins are tumor-specific antigens and represent reasonable targets for immunologic approaches against CML. We have utilized a free-solution-isoelectric focusing technique (FS-IEF) to obtain chaperone rich cell lysates (CRCL) from tumors. We found that CRCL derived from 12B1, an aggressive bcr-abl⁺ murine tumor activated dendritic cells (DCs) by upregulating CD40 and MHC-II on their cell surface and stimulating them to produce interleukin-12 (IL-12). Vaccination of mice with 12B1 CRCL pulsed DCs generated potent, tumor specific, long lasting, CD4⁺ and CD8⁺ T cell dependent immune responses. We have further demonstrated that immunization with 12B1 CRCL induced BCR-ABL specific cytotoxic T lymphocytes, indicating that BCR-ABL peptides are chaperoned by CRCL and are cross-presented to CD8⁺ T cells. Moreover, other antigenic peptides may also be present in the antigen repertoire of CRCL and contribute to the superior immunogenicity of 12B1-derived CRCL. To better optimize immunity against CML, we investigated the effects of combining imatinib mesylate with 12B1 CRCL. Imatinib mesylate specifically inhibits BCR-ABL kinase activity and has been very successful in treating patients with CML. However, the development of drug resistance has led to drug combination approaches. We have shown that the combination of imatinib with DCs loaded with 12B1-derived CRCL yielded potent anti-tumor activity. In addition to tumor-derived heat shock proteins, we have also studied the immunogenicity of apoptotic leukemic cells. We have previously reported that vaccination of mice with stressed apoptotic leukemic cells elicited potent anti-tumor immunity. We have found that stressed apoptotic leukemic cells, compared with non-stressed apoptotic ones, have higher capacity to upregulate CD40, CD80, and CD86 on the surface of DCs, to stimulate DCs to secrete IL-12, and to enhance their immunostimulatory functions in a mixed lymphocyte reaction (MLR). These findings demonstrate that apoptotic tumor cells can be immunogenic when stressed, and that DCs play a key role in determining whether a T cell response will be generated.