Mono/Oligoclonal T and NK Cells Are Common in Philadelphia Chromosome Positive (Ph+) Leukemia Patients at Diagnosis and Expand During Successful Tyrosine Kinase Inhibitor Therapy.

Central to current treatment of Ph+ leukemia patients are tyrosine kinase inhibitors (TKIs), which predominantly target the BCR-ABL1 kinase in malignant cells. However, broader-spectrum 2^nd generation TKIs, such as bosutinib, dasatinib and nilotinib, also inhibit off-target kinases with important physiological functions. Several in vitro studies have implied that TKIs may have immunosuppressive effects by suppressing activation and proliferation of effector lymphocytes. In contrast, we recently observed immunostimulation during dasatinib therapy in the form of marked expansion of clonal cytotoxic lymphocytes (T- and NK cells) resulting in chronic LGL-type lymphocytosis in peripheral blood (PB). The prevalence, detailed molecular background and clinical implications of clonal lymphocytes during TKI therapy are currently unknown. The aim of this study was to comprehensively analyze clonality and evolution of lymphocyte clones during TKI therapy.

The study population included patients with Ph+ leukemia, both CML (n=28) and ALL patients (n=4) on dasatinib (n=23) and imatinib (n=9) therapies. In addition, samples from 12 healthy controls and diagnostic samples from the nine imatinib treated patients were analyzed. Lymphocyte clonality was determined by analysis of PB mononuclear cells (MNC) for clonal T cell receptor (TCR) γ and δ gene rearrangements by 18 primer pairs covering most known clonal TCR γ and δ rearrangements. Upon positive reaction in heteroduplex analysis, the purified PCR products were sequenced. If clonal rearrangement was observed, allele-spesific PCR primers were designed to allow for quantitative follow-up of lymphocyte clones in each patient.

Sequencing-confirmed clonal TCR γ rearrangement was observed only in 1 of 12 healthy controls and no TCR δ gene rearrangements were found in this group. Surprisingly, 7 of 9 (78%) CML patients showed clonal TCR rearrangements at diagnosis. In 3 patients the clonal rearrangement was detected in the TCR δ genes, in 7 patients in the TCR γ genes and 3 patients had rearrangemens both in TCR δ and γ genes. After one year of imatinib treatment the same clones could be detected in 5 of the 7 patients (71%). Although clonal cells were observed, none of the imatinib patients had signs of a concomitant lymphoproliferative disorder and the distribution of lymphocyte subclasses was normal. Next, 23 patients treated with dasatinib were studied, 10 without (LGL^neg) and 13 with PB LGL lymphocytosis (LGL^pos) including T- or NK-cell expansions. In all LGL^pos dasatinib patients (including patients with a CD3^neg NK-cell expansion) clonal TCR γ or δ rearrangements were found. In LGL^neg dasatinib patients the prevalence of TCR rearrangements was 80%. LGL^pos patients had more often clonal rearrangements in TCR δ genes (62%) than LGL^neg patients (10%). No differences in clonal rearranged TCR γ genes (77% vs. 80%) were detected. Most patients displayed more than one clonal TCR rearrangement. Quantitative follow-up of LGL^pos patients revealed that the expansion of a single predominant lymphocyte clone accounted for LGL lymphocytosis. Intriguingly, quantitative follow-up of lymphocyte clones by PCR showed that the observed clones existed at low levels already before start of dasatinib therapy during imatinib treatment, but no lymphocyte expansions were then seen. Sorting of lymphocytes showed that clonal cells resided in the CD8⁺ and CD4⁺ T-cell populations and, strikingly, also among CD16/56⁺CD3^neg NK cells. All dasatinib patients with NK cell expansions (n=3) showed TCR δ rearrangements in their NK cells.

Clonal lymphocytes could rarely be found in healthy controls. In contrast, they were frequently present in CML patients at diagnosis and persisted during TKI therapy. In a distinct subgroup of dasatinib treated patients, clonal cells massively expanded during successful therapy. Clonal TCR rearrangements were detected in CD4⁺, CD8⁺ and, unexpectedly, also in NK cells. The epitopes and function of clonal, CML-associated lymphocytes are under investigation. Previous studies showed that clonal expansions during dasatinib were associated with excellent, long-lasting therapy responses in advanced leukemia. We therefore hypothesize, that the clonal lymphocytes present at CML diagnosis may be anergic anti-leukemic cells and part of the immune escape mechanisms inherent to leukemogenesis and that dasatinib therapy can reverse this anergy.

Ekblom:BMS: Honoraria. Seggewiss:BMS: Honoraria. Porkka:BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding. Mustjoki:BMS: Honoraria.