The Immunogenicity of BCR/ABL⁺ Cells Is Dominated by BCR/ABL-Regulated Antigens and Is Impaired by the Inhibition of BCR/ABL Kinase Activity.

The inhibition of BCR/ABL kinase activity by imatinib mesylate (IM, STI571, Gleevec^®) is the standard therapy for patients with Philadelphia chromosome⁺ (Ph⁺) chronic myeloid leukemia (CML). However, the long term treatment with IM or other BCR/ABL kinase inhibitors may be limited due to the development of resistant disease and accumulating side effects. Immunotherapeutical approaches directed against Ph⁺ CML may overcome these problems. So far, the research to develop an immunotherapy against Ph⁺ leukemia focused on the BCR-ABL fusion protein, giving the promising opportunity to stimulate cytotoxic T lymphocytes against the joining region segment of p210 BCR/ABL. However, only a limited number of peptides spanning the fusion region is endogenously processed and presented in context with HLA class I molecules. In contrast, the constitutively active BCR/ABL kinase leads to the upregulation and activation of multiple proteins, which may subsequently result in the expression of different, probably leukemia-associated antigens on the cell surface of BCR/ABL-positive malignant cells. In this study, we investigated the immunogenicity of antigens upregulated by BCR-ABL kinase activity, and verified the capacity of these antigens to induce an anti-leukemia T cell response in vitro. We performed CD8⁺ T-cell stimulations with dendritic cells transfected with mRNA coding BCR/ABL wild-type (WT). Following two stimulations, the proliferating T-cell populations were analyzed for cytokine secretion (IFN-γ) in response to target cells expressing either BCR/ABL WT or a kinase deficient (KD, K1172R) variant of BCR/ABL. With this experimental setting it was possible to compare the immunogenic potential of antigens upregulated by BCR/ABL with the immunogenicity of the BCR/ABL protein itself. We were not able to activate T-cell populations directed against either the breakpoint-region of BCR/ABL or the BCR- or ABL part of the fusion protein. In contrast, we show here that the constitutively active kinase domain of BCR/ABL has a key role in enhancing the immunogenicity of BCR/ABL⁺ cells. A broad, HLA-dependent T-cell immune response specifically directed against BCR/ABL regulated antigens was detected. The inhibition of BCR/ABL kinase activity by IM significantly impaired the immunogenicity of BCR/ABL⁺ cells. We found the same T-cell reactivity pattern in several healthy donors and a CML patient. This is the first study demonstrating the major contribution of the BCR/ABL kinase domain to the immunogenicity of BCR/ABL⁺ cells as T-cell responses against these cells are dominated by BCR/ABL regulated antigens, and not by BCR/ABL itself. These results may contribute to the design of clinical vaccination trials for the treatment CML patients with minimal residual disease, e.g. following successful induction therapy with BCR/ABL inhibitors.