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Natural Killer (NK) cells are critical in host defense against malignant transformation and are potent antileukemic cytotoxic effectors. In the present study, we investigated the peripheral NK function in patients with myelodysplastic syndromes (MDS). We demonstrated that the peripheral NK cell population was quantitatively normal in MDS patients. Furthermore, NK cells displayed an expression of the activating natural cytotoxicity receptors (NCR) NKp46 and NKp30 as well as NKG2D similar to that observed in donors, but exert a highly decreased constitutive cytolytic activity compared to resting normal NK cells. Although activation with IL-2 resulted in the upregulation of NKp46 expression by MDS-NK cells, their cytolytic function remained deeply altered as compared to activated donor NK cells. In addition, MDS NK cells did not proliferate in vitro, and displayed an increased rate of apoptosis in response to IL-2 stimulation although the spontaneous apoptosis was not significantly increased. Interestingly, a proportion of peripheral MDS-NK cells were derived from the MDS clone as the cytogenetic anomaly found in bone marrow karyotype was also detected in 20-50% of circulating NK cells. In conclusion, NK cells' cytolytic function and proliferative capacities in response to activation by cytokines are profoundly altered in MDS.

Thrombocytopenia is generally of central origin in MDS, but can be due to peripheral platelet destruction in some cases. We studied platelet lifespan in 61 MDS cases with platelets < 70,000/mm3 and marrow blasts < 10%. Nine of them (15%) had a major platelet lifespan reduction (< 3.5 days), and were considered for splenectomy. Three of them were not splenectomized due to rapid death, patient refusal and older age plus liver predominance of platelet sequestration, respectively. The remaining six patients (two females and four males, median age 50 years, range 32 to 65) were splenectomized 3 to 21 months after diagnosis. Before splenectomy, five of them had RA and one had CMML. Platelets counts ranged from 5000 to 30,000/mm3 and did not durably respond to other treatments. Three of the patients has a relapse of platelet counts, concomitantly required platelet transfusion due to recurrent blending, whereas three had anemia (two required erythrocyte transfusion) and four had neutropenia. Three months after surgery, platelet counts ranged from 55,000 to 160,000/mm3 (> 100,000/mm3 in four cases), no patient required platelet or erythrocyte transfusion, but there was no effect on neutrophil counts. Three patients had a relapse of platelet counts, concomitant with progression to AML in two of them, whereas the third relapsing case achieved normal platelet counts with further danazol. One patient died with normal platelet counts 12 months after splenectomy (from sepsis, probably related to neutropenia rather than splenectomy). Two patients remained with normal platelet counts 10 and 52 months after surgery. Our findings suggest that the mechanism of thrombocytopenia should be studied more often in 'low risk' MDS (i.e. with low bone marrow blast counts) with thrombocytopenia, as about 15% of them appear to have peripheral platelet destruction. Some of those patients may benefit from splenectomy.