Tumor-Derived Microvesicles From Sera of Patients with Acute Myeloid Leukemia Suppresses NK Cell Cytotoxicity Via Transforming Growth Factor Beta-1.

Abstract 2678

Poster Board II-654

Natural killer (NK) cell cytotoxicity in patients with acute myeloid leukemia (AML) is significantly decreased relative to that in normal controls (NC). However, the mechanisms responsible for low NK cell activity in AML are not known. We considered the possibility that tumor-cell-derived microvesicles (MV) mediate suppression of NK cells. MV originate from the endosomal compartment of activated normal and neoplastic cells. Evidence suggests that tumor-derived MV exert detrimental effects on cells of the immune system and may play a role in tumor progression. To determine their contribution to immune suppression in AML, MV were isolated from sera of patients newly diagnosed with AML prior to any treatment and used to evaluate MV-mediated NK cell suppression. The protein content of MV isolated using exclusion chromatography and ultracentrifugation from sera of 19 AML patients was significantly higher than that of MV isolated from sera of 25 NC (75μg±12/mL vs 1.2μg±0.4/mL, p<0.001 ). MV from AML patients were positive for membrane-associated TGFb-1 and FasL in Western blots, whereas no TGFb-1 or FasL was detected in MV from NC. For functional assays, NK cells sorted from peripheral blood of NC were cultured with MV isolated from sera of the AML patients. A significant decrease in NK cell cytotoxicity was observed after co-incubation with MV (2412 LU before vs 1640 LU after, p<0.002). Concomitantly, a decrease in the expression of the NK cell activating receptor, NKG2D, was observed (57% before vs 38% after, p<0.001). The addition of TGFb1-neutralizing antibody abrogated the effects of MV on the NK cell cytotoxicity and receptor expression. The increased levels in sera of AML patients of MV mediating potent NK cell suppression is likely to compromise anti-tumor immune responses. Therefore, modulation of the levels and functions of MV might provide new immunotherapeutic approaches in AML.