Functional Alterations of NKT Cells in Chronic Myeloid Leukemia Patients Can Be Reversed by Imatinib Mesylate In Vitro.

Chronic myeloid leukemia (CML) is a myeloproliferative disorder characterized by the presence of the pathognomonic Philadelphia chromosome and the chimeric BCR-ABL oncoprotein with deregulated tyrosine kinase activity. It has been shown previously that T cell immunity contributes to the control of CML, and several arguments suggest an implication of NKT cells in this anti-tumoral immunity. We thus compared frequency, phenotype and functions of blood NKT cells (defined by the CD1d tetramer⁺ Vα24⁺ staining) in healthy subjects and patients with CML. Three groups of patients were studied, including

1. Patients in chronic phase (CP) (either at diagnosis or unresponsive to treatment)

2. patients in major/complete cytogenetic remission induced by interferon-alpha (IFN-α) or

3. patients in major/complete cytogenetic remission induced by imatinib mesylate (IM, a specific inhibitor of the BCR-ABL tyrosine kinase).

Our results showed that blood NKT cells frequency was not significantly different between healthy donors (n = 17), CP patients (n = 14) and IM-treated patients (n = 16) (0.062 % versus 0.079 % versus 0.041 % respectively). On the other hand, this frequency defined as above was found to be dramatically decreased in patients in complete remission after IFN-α therapy ( 0.01 %, n = 15 patients). We have then analyzed from the phenotypic point of view NKT cells from these three groups. This ex vivo phenotypic study showed that NKT phenotype (expression of CCR7 and CD161) was clearly modified in the IFN-treated group as compared to IM-treated or CP patients and healthy donors, with a clear enrichment in CD161^-CCR7⁺ NKT cells (49% versus 26%, 22% and18% respectively). This CD161^-CCR7⁺ phenotype has been described as the central memory T cell phenotype, with increased lymph-node homing and antigen-presenting cell-stimulating capacities. We have then performed functional studies of NKT cells measuring their proliferative response to α-galactosylceramide (αGC) as a specific triggering antigen. NKT proliferative response to α-GC was abolished in CP patients (2-fold expansion versus 83-fold in healthy donors). This functional impairment was found to be restored in patients treated with IM and in patients treated with IFN-α (106-fold and 20-fold expansion respectively), although this latter group had a strongly depleted NKT compartment. More interestingly, the incubation of CP CML cells in the presence of IM (0.5 and 1 micromolar, n = 5) led to the partial restoration of the NKT cell reactivity to α-GC (29-fold expansion versus α-GC alone). Thus, our results suggest that IFN-α therapy leads to the generation of "central memory-like phenotype" NKT cells, which could play an important role in the long-term remissions observed in these patients. Moreover, our results strongly suggest that IM is able to partially restore the antigenic-response of CML NKT cells in vitro and in vivo, suggesting a role of BCR-ABL in the anergic state of these cells as this was observed at diagnosis. The IM-induced restoration of NKT cell proliferation defect in CP patients suggest that the antileukemic effect of IM could also be partially due to this action in vivo. Cellular mechanisms involved in this phenomenon are currently under study.