Activation Levels of Natural Killer Cells and CD8⁺ T Cells Correlate Highly with Sustained Complete Molecular Response After Discontinuation of Imatinib in Chronic Myeloid Leukemia Patients

Recently, it has been recognized that some chronic myeloid leukemia (CML) patients with a complete molecular response (CMR) are able to maintain the CMR after discontinuation of imatinib. Mahon et al. reported that among patients with a CMR lasting at least 2 years, CMR was sustained in 41% after discontinuation of imatinib (Lancet Oncol. 2010;11:1029). Similarly, Takahashi et al. reported that 47% of Japanese CML patients with CMR maintained CMR after imatinib discontinuation (Haematologica 2012;97:903). Moreover, Ohyashiki et al. have recently demonstrated that higher peripheral natural killer (NK) cell counts are associated with a reduced risk of relapse after halting imatinib (Br. J. Haematol. 2012;157:254). These findings suggest that, although CML stem cells may remain, even if CMR status is attained, some CML patients could discontinue imatinib therapy and maintain a stable condition, possibly owing to immune surveillance.

To more precisely identify these patients who can safely discontinue imatinib, we characterized the immunophenotype profiles of CML patients. We compared the profiles among CML patients who received imatinib with CMR for more than 2 consecutive years (CMR group), those who could not sustain CMR but maintained a major molecular response (fluctuated CMR group), those who sustained CMR for more than 6 months after discontinuation of imatinib (STIM group), those who relapsed after discontinuation of imatinib (relapse group), and healthy volunteers (control group).

Peripheral blood mononuclear cells (PBMCs) from CML patients and healthy volunteers were separated using a Ficoll density gradient, and immunophenotyping analysis was performed with a 5-color flow cytometry panel, including antibodies against the following cell surface antigens and effector molecules: CD3, CD8, CD45RO, CD56, CCR7, IFN-g, granzyme B, and perforin. After PBMCs were stimulated with phorbol 12-myristate 13-acetate and ionomycin for 4 h in the presence of monensin, cell surface antigens were stained, fixed, and permeabilized. Resultant cells were then intracellularly stained and analyzed with the FACSCanto II flow cytometer.

The percentage of effector populations of NK cells, such as interferon (IFN)-g⁺CD3⁻CD56⁺ cells, was significantly higher in the CMR and STIM groups than in the control group. In contrast, the percentage of effector populations of CD8⁺ T cells, such as IFN-g⁺CD8⁺ T cells, was significantly higher in the STIM and control groups than in the CMR group. Moreover, the percentage of effector populations of NK cells, but not CD8⁺ T cells, was significantly higher in the CMR group than in the fluctuated CMR group. On the other hand, CML patients with a lower percentage of effector populations of NK cells and CD8⁺T cells, and those with a higher percentage of these effector populations, but whose percentage had decreased after imatinib cessation, had a tendency to relapse after discontinuation of imatinib.

The percentage of effector populations of NK cells and CD8⁺ T cells, such as those expressing IFN-g, correlated highly with sustained CMR after discontinuation of imatinib. Moreover, the activation level of NK cells, but not CD8⁺ T cells, inversely correlated with the BCR-ABL1 transcript level. Taken together, these results suggest that whether imatinib treatment can be safely discontinued may depend greatly on the immunological activation status of both NK cells and CD8⁺ T cells in CML patients.

No relevant conflicts of interest to declare.