Myeloid Derived Suppressor Cells (MDSCs) Are Increased and Exert Immunosuppressive Activity In CML Patients At Diagnosis

In some solid tumors it has been demonstrated that a subpopulation of myeloid cells, defined as “myeloid-derived suppressor cells” (MDSCs), plays an important role in inducing T cell tolerance by production of arginase 1 (arg1) that depletes microenvironment of arginine, an essential aminoacid for T cell function. Since chronic myeloid leukemia (CML) patients have high levels of immature myeloid cells it is of interest to investigate if these cells have MDSCs phenotype and activity. The aim of this study was to analyze MDSCs and investigate their activity in CML patients.

MDSCs were analyzed in peripheral blood (PB) of 20 healthy donors (HD) and 30 CML patients at diagnosis. In 21 patients MDSCs were also measured during TKI treatment. Granulocytic MDSCs (G-MDSCs) were identified as CD11b+CD33+CD14-HLADR- cells, while the monocytic MDSCs (Mo-MDSCs) as CD14+HLADR by cytofluorimetric analysis. Arg1 expression was assessed using real time PCR and Western Blot. Arg activity was measured in granulocyte lysates using a colorimetric test after enzymatic activation and arginine hydrolysis. Microvesicles (MV) were isolated from CML serum at diagnosis (n=5) by sequential ultracentrifugation.

CML patients showed high levels of Mo- and G-MDSCs at diagnosis in comparison to HD (41±8 and 82,5±12,2% respectively for CML vs 9±2,1 and 55±5,3% for HD; p<0.001), while after TKIs therapy both subpopulations decreased, returning to normal values. T-reg (CD4+ CD25^high Foxp3+ cells) were also significantly increased in CML patients at diagnosis in respect to HD (9±2% vs 6,1±0,8%, p<0.001) with a significant correlation with the percentage of Gr-MDSCs (r=0,6254; p<0.001). Both in PB and purified granulocytic cells, Arg1 expression showed a 30 fold increase in CML at diagnosis compared to HD (p<0.001) and decreased after therapy. The same data were confirmed by Western Blot analysis. Arg enzymatic activity in granulocytes resulted also increased in CML (n=10) compared to HD (n=10) (p<0.001). The suppressive function of CML G-MDSCs was demonstrated by their ability to inhibit the proliferation of CFSE+ HD T cells (p<0.001). In addition, an increase of Mo-MDSCs in vitro was observed after incubation of HD monocytes with CML sera (29±13%; p<0.0001) or MV (8±2,8%; p<0.05).

MDSCs are increased in CML patients at diagnosis and decrease during TKIs treatment. CML granulocytes have high arg1 activity and immunosuppressive activity. Moreover, CML serum as well as CML microvesicles increase the percentage of HD Mo-MDSCs.

No relevant conflicts of interest to declare.