Myeloid Derived Suppressor Cells and Chronic Myeloid Leukemia

Overview

The suppression of the immune system creates a permissive environment for development and progression of cancer. One population of immunosuppressive cells that have become the focus of intense study is myeloid derived suppressor cells , immature myeloid cells able to induce immune-escape, angiogenesis, and tumor progression. Two different subpopulations have been identified and studied: granulocytic and monocytic myeloid derived suppressor cells with a different immunophenotype and immunosuppressive properties

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 . With the aim of increasing cure rates and make it possible for patients to discontinue treatment, TKI therapies are currently evaluated in combination with immune modulators .

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