Robust Anti-Leukemia CD4+ and CD8+ T Cell Responses in CML Patients Treated with Imatinib Mesylate.

Imatinib mesylate, a selective inhibitor of the bcr/abl tyrosine kinase, has revolutionized the treatment of CML and is the first-line therapy for most patients. Most CML patients in chronic phase achieve hematologic remission under treatment with imatinib, with up to 70% also achieving cytogenetic remission. However, imatinib therapy is not curative, as patients who discontinue the drug invariably relapse. Thus, the need to find alternate, potentially curative therapies remains. Low levels of CD8+ T cell responses to certain HLA restricted peptides have been detected in CML patients after successful treatment. To determine, if CML patients in remission on imatinib develop and sustain anti-leukemia CD4+ or CD8+ T cell responses, blood samples from patients before and several time points after treatment were collected and analyzed. Pre-treatment samples were utilized as sources of autologous leukemic cells to detect anti-leukemia T cell responses in post-treatment remission samples. Autologous leukemic samples alone and remission samples alone served as controls. In 7 of the 14 patients investigated, significant IFN-γ responses (p<0.01) in ELISPOT assay were detectable when patients achieved cytogenetic remission, and peaked at 10–15 months (median 36 SFCs, range 26–71 SFCs), before they slowly decreased (up to 46 months post-treatment) to levels similar to those from leukemic samples alone and remission samples alone (6 SFCs, range 0–13 SFCs). These results correlated with T cell responses in cytokine flow cytometry (TNF-α 1.4–40%, IFN-γ 1.0–6.6%, p<0.05), and showed a predominance for CD4+ T cells. Furthermore, TNF-α and IFN-γ production (p<0.05) was confirmed in CD4+ and CD8+ T cell clones generated in a remission sample from one patient using multiplex cytokine assay: 6.43–37.90 pg/ml for TNF-α, 14.90–110.42 pg/ml for IFN-γ. CD8+ T cell clones (HLA-A0201+) were tested for reactivity against HLA-A0201 restricted peptides derived from leukemia associated antigens proteinase 3, Wilms tumor 1 and bcr/abl, as well as tumor associated antigens p53, p68, and human telomerase (hTERT) in IFN-γ ELISPOT assays. No IFN-γ responses were detected in the CD8+ T cell clones when stimulated with the peptides compared to stimulation with an irrelevant peptide (0–15 SFCs, p=0.55), suggesting that these clones react to as yet to be defined leukemia associated antigens. Together, these data show that CD4+ T cells play an important role in anti-leukemia immune responses in patients in remission (sustained over time) and might contribute to killing of leukemic cells, possibly via TNF-α. Banked autologous leukemic cells could be used for vaccination of CML patients in order to enhance anti-leukemia T cell responses and may, in combination with imatinib, lead to eradication of residual leukemic cells with a durable cure.