Disruption of the Peripheral Lymphoid Niche Contributes to Lymphopenia in CML Patients Undergoing Imatinib Treatments

PURPOSE: Chronic myelogenous leukemia (CML) is a disorder affecting early hematopoietic stem cells (HSC) and is characterized by excessive proliferation and accumulation of myeloid progenitors and progeny in the periphery. During the chronic phase of the disease, CML patients are normally at low risk of developing infections but such complications tend to rise during the progression of the disease. Gleevec (imatinib mesylate) is currently administered as first line therapy for patients with Philadelphia chromosome-positive CML. Despite the relative high specificity of tyrosine kinase inhibitor (TKI) treatment towards the BCR-ABL fusion protein, off-target multikinase inhibitory effects occur and can interfere with normal hematopoiesis. This study was conducted to evaluate how myeloid and lymphoid immune homeostasis are affected by Imatinib mesylate.

METHODS: Healthy volunteer donors (n=25) and CML patients were recruited during their first visit at our CML clinic. Seven CML patients were treated with Imatinib (400mg). The median time of Gleevec treatment was 2.9 years (range: 0.5-10.9). The median time of remission post TKI was 1.1 years (range: 0.3-3). Phenotypic analysis of dendritic cell (DCs) subsets: plasmacytoid (pDCs) and myeloid type 1, 2 and 3 (mDC1, mDC2, mDC3) were evaluated by flow cytometry. Percentage and absolute numbers of naive and memory CD4⁺ and CD8⁺ T cells, NK cells and B cells were also evaluated. DCs were differentiated from purified CD34⁺ cells culturedwith GM-CSF (800 U/ml) or Flt3-L (50ng/ml), IL-4 (10 U/ml) and TNFa (50 U/ml), in the presence of varying concentrations of Imatinib mesylate (0 to 5µM/mL). TCR and IL-7 signaling were evaluated based on ERK-phosphorylation (-p) and STAT5-p after incubation with 3µM of Imatinib.

RESULTS AND CONCLUSION: At diagnosis, several CML patients have a deficit in DCs resulting from a severe skewing affecting BM progenitor cells. After initiating Gleevec therapy, normalization of stem cell progenitors occurs but DC counts remain well below normal levels in all CML patients. We demonstrated a direct and dose dependent interference of Imatinib on GM-CSF and Flt3-L pathways for DC differentiation from CD34⁺ stem cells. For T lymphocytes, Imatinib interfered with TCR and IL-7 signaling through the inhibition of ERK and STAT5 phosphorylation respectively. The failure to maintain adequate numbers of DCs combined to diminished homeostatic response to cytokines and TCR stimuli explains T cell lymphopenia in these patients. Such immune dysfunction is at least in part responsible for infectious complications that are often increased in patients treated with Imatinib.