Phenotypic and functional changes of NK cells in patients with myelodysplastic syndrome and acute myeloid leukemia treated with hypomethylating drugs

Natural killer cells (NK cells) are cytotoxic lymphocytes that play a pivotal role in maintaining immunological surveillance and in developing an innate immune response. Since the discovery of NK cells in 1973, the mechanisms of their functioning have been studied in details, and there is currently no doubt that they play a special role in the process of recognition and destruction of transformed and malignant cells. Understanding the role of NK cells in antitumor immunity, on the one hand, leads to emergence of new immunotherapeutic strategies and, on the other hand, allows to adjust the existing treatment regimens for tumor diseases, in accordance with the principle of primum non nocere. Optimization of cancer therapy protocols executed in order to protect immune cells from death and functional impairment is an important problem that cannot be successfully resolved without regular aggregation of the results from disparate studies and critical analysis of the all accumulated data.The objective of this review is to create a relevant and holistic picture of changes in the phenotypic and functional characteristics of NK cells in patients with two related hematological diseases – myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). For the treatment of both illnesses, drugs from the group of hypomethylating agents are used, the acting mechanism of which, unlike classical cytostatic agents, is based on modulation of the tumor cell genes expression. All the cells of the body are being affected, including NK cells, since these drugs act nonspecifically. Such an interaction leads to a hypomethylation of NK cell DNA and changes the expression of functional receptors, which, in turn, provide the development of antitumor NK cell immune response.Of course, just the fact of changing gene expression in certain cells does not allow us to fully judge the drug’s impact on the state of immune system. Meanwhile, the origin of this change and its role are important in the context of the disease pathogenesis. Ultimately, a simple description of an increase or decrease in a single receptor expression is not illustrative, since it can lead to uncertain consequences. For this reason, the current review, in addition to describing the existing data on the changes of NK cell receptors expression under the influence of hypomethylating drugs, gives a special attention to critical analysis of functional characteristics of NK cells, including their cytotoxic activity aimed at malignant blast cells, being a determinant of clinical course in the described diseases.