Hanna BS, McClanahan F, Yazdanparast H, Zaborsky N, Kalter V, Rossner PM, Benner A, Durr C, Egle A, Gribben JG, Lichter P, Seiffert M. 4). B. shows total ROS production in presence and absence of OFA (10g/ml) and OFA-derived F(ab’)2 fragments (10g/ml). C., D. Total ROS production in presence and absence of the ROS formation inhibitors histamine dihydrochloride (HDC;100M) and diphenylene iodonium chloride (DPI; 3M) (= 4). Statistical significance for all those Apoptosis Inhibitor (M50054) figures was determined by one-way ANOVAs and the Bonferroni post test. (RLU;relative light Apoptosis Inhibitor (M50054) units) * 0.05, ** 0.01, *** 0.001. We next determined whether the presence of monocytes interfered with NK cell-mediated killing of autologous CLL cells. In accordance with earlier reports [22, 23], NK cells isolated from CLL patients induced significant CLL cell death in the presence of RTX with only minor cytotoxicity in the absence of a linking antibody. Monocytes failed to exert substantial RTX-dependent cytotoxicity against CLL cells. Instead, NK cell ADCC was strongly reduced in the presence of monocytes (Physique 2A-2B). HDC and the ROS-degrading enzyme catalase both partially restored the diminished ADCC of NK cells. Neither HDC nor catalase affected CLL cell viability or ADCC by NK cells in the absence of monocytes (data not shown). The NK cell-activating cytokine IL-2 augmented RTX-mediated ADCC by NK cells but did not rescue NK cells from ROS-induced inhibition (Physique ?(Figure2B).2B). Comparable results were obtained using OFA in ADCC assays (data not shown). Open in a separate window Physique 2 Monocytes restricted NK cell ADCC against autologous leukemic cells by production of ROSA., B. NK cells and CFSE-labeled CLL cells were co-cultured for four hours in the presence or absence of autologous monocytes at an NK:Mo:CLL-ratio of 2:2:1 and IL-2 (500IU/ml), rituximab (10g/ml), HDC (100M), ranitidine (Ran; 100M) or catalase (Cat; 200IU/ml). ADCC was inhibited by the presence of monocytes, but largely restored by anti-oxidative brokers HDC or catalase. (= 5-7). C. Representative dot-plot depicting the read-out for lysed leukemic cells of panels Apoptosis Inhibitor (M50054) A and B. Percentages denote the proportion of lysed leukemic cells, thus staining positive for the Live/Dead stain. D. Monocytes were found to decrease the density of surface-bound rituximab on CLL cells, a mechanism referred to as trogocytosis. E. NK cell-mediated ADCC of CLL cells previously exposed to monocytes, and thus allowing for antigen removal by trogocytosis, was lowered in 7 out of 8 performed experiments. F. Monocyte-mediated trogocytosis was unaffected by addition of anti-oxidative substances (= 4). * 0.05, ** 0.01, *** 0.001. The incomplete restoration of cytotoxicity by anti-oxidative compounds suggested that additional mechanisms might have contributed to the observed inhibition of ADCC by monocytes. Previous studies have show that monocytes upon conversation with CD20 mAb-opsonized CLL cells may shave off or extract the antibody-antigen complex from the CLL cells, a mechanism known as trogocytosis, thus Apoptosis Inhibitor (M50054) reducing the amount of antibody bound to the CLL cells and limiting NK cell-mediated ADCC [17, 18]. To address the impact of this inhibitory mechanism, we exposed CD20 mAb-opsonized CLL cells to monocytes and ITGAV decided the level of bound antibody on CLL cells after 45 minutes of incubation. As shown in Physique ?Determine2D,2D, monocytes reduced the amount of RTX bound to CLL cells. To investigate whether this reduction of bound antibody could explain the incomplete restoration of ADCC by antioxidative brokers, we removed monocytes from the CLL cells using anti-CD14 beads, re-introduced RTX (10g/ml) and decided the CLL susceptibility to ADCC. As shown in Physique ?Determine2E,2E, monocyte-induced trogocytosis of bound mAbs and antigens caused a slight, reduction of ADCC in 7 out of 8 experiments, though the observed reduction was not statistically significant. The addition of HDC, catalase or DPI did not affect the ability of monocytes to reduce RTX/OFA binding to CLL cells (Physique ?(Figure2F).2F). The restoration of ADCC observed in the presence of anti-oxidative substances Apoptosis Inhibitor (M50054) was not due to inhibition of trogocytosis, but it is possible that trogocytosis, at least in part, could explain the incomplete restoration of ADCC by anti-oxidative reagents. It was previously exhibited that ROS produced from normal and leukemic myeloid cells trigger programmed cell death in NK cells [7,.