Effect of Tyrosine Kinase Inhibitors on Myeloid Derived Suppressor Cells in Chronic Myeloid Leukemia Patients

Myeloid-derived suppressor cells utilize different mechanisms to block both innate and adaptive arms of anti-tumour immunity, mostly through inhibition of T cell activation and expansion . Human monocytic myeloid derived suppressor cells are mostly identified as CD14+ cells with negative or low expression of HLADR. And also express high levels of CD11b and CD33 antigen . Human granulocytic myeloid derived suppressor cells are usually defined as CD66b+ CD11b+ CD15+ HLADR- cells and display an intermediate expression of CD33 and a variable expression of CD11b, depending on their maturation stage .

Chronic myeloid leukemia (CML) is a hematological cancer, characterized by a reciprocal chromosomal translocation between chromosomes 9 and 22 [t(9;22)], producing the Bcr-Abl oncogene. Tyrosine kinase inhibitors represent the standard of care for CML patients and exert a dual mode of action: direct oncokinase inhibition and restoration of effector-mediated immune surveillance, which is rendered dysfunctional in CML patients at diagnosis, prior to TKI therapy. TKIs such as imatinib, and more potent second-generation nilotinib and dasatinib induce a high rate of deep molecular response (DMR, BCR-ABL1 ≤ 0.01%) in CML patients. As a result, the more recent goal of therapy in CML treatment is to induce a durable deep molecular response as a prelude to successful treatment-free remission .

Accumulation of both Gr-MDSCs and Mo-MDSCs cells has been found in the peripheral blood of chronic myeloid leukemia (CML) patients. They are part of the tumor clone showing BCR/ABL expression.

BCR-ABL tyrosine kinase inhibitors (TKI) are able to induce remission in CML patients but not to eliminate leukemia stem cells , which can regenerate leukemia on drug discontinuation .

Unfortunately, molecular relapse is observed after cessation of tyrosine kinase inhibitors in 61-66% of CML patients, previously in complete molecular response (presumably due to the reactivation of dormant CML LSCs that are resistant to TKI-induced leukemic cell ablation. Thus, current research efforts aim to develop additional therapies to target these TKI-refractory CML LSCs .