Abstract 3247: Granulocytic MDSCs (CD11b+CD15+CD14-HLADR-) in peripheral blood of mesothelioma patients are elevated and suppress T cell proliferation and function via reactive oxygen species

Abstract

Abstract The role of myeloid derived suppressor cells (MDSCs) in mediating tumor immunosuppression in patients with malignant mesothelioma has not been well characterized. The goal of our study was to analyze for the presence of both monocytic (Mo) and granulocytic (Gr) MDSCs in blood of mesothelioma patients, and characterize their mechanism of suppression of T cells responses. We evaluated the peripheral blood of patients (n = 23) for the presence of Gr-MDSC (CD11b+CD15+CD14-HLADR-) and Mo-MDSC (CD11b+CD14+HLADR-). Gr-MDSC (62.3±13.9% vs. 47.8±17.8%; p = 0.005) as well as monocytic MDSC (0.2±0.4% vs. 0.1±0.3%; p = 0.01) were significantly elevated in peripheral blood of mesothelioma patients as compared to healthy donors (n = 21). The frequency of Gr-MDSC was higher than Mo-MDSC. Thus, Gr-MDSCs appeared to be the main immunosuppressive population and were investigated further.Gr-MDSC from mesothelioma patients were found to inhibit the proliferation of both autologous CD4 and CD8 T cells (mean inhibition of proliferation of 61.9% for CD4 and 75.5% for CD8 at 1:2 T: Gr-MDSC ratio) and was accompanied by a decrease in IFN-γ released by T cells in the co-culture supernatants (92.4 pg/mL at 1:2 ratio). To determine the mechanism by which Gr-MDSC inhibit T cells, we tested the levels of reactive oxygen species (ROS), and nitrite oxide species and arginase activity in Gr-MDSC sorted from peripheral blood. Both arginase (0.3±0.3 vs. 0.2±0.3 μM) and nitrite levels (1.8±2.5 vs. 0 μM) in Gr-MDSC were below detection limit in both patients and donors and were marginally affected by the addition of their respective inhibitors (nor-NOHA and L-NMMA). However, the levels of ROS in Gr-MDSCs derived from mesothelioma patients were on average 3 folds higher than the Gr-myeloid cells from healthy donors (average MFI ∼15000 vs. ∼5000). In summary our results show that the major immunosuppressive cell population in mesothelioma patients are Gr-MDSCs and they suppress T cell proliferation and activation through release of reactive oxygen species. Citation Format: Swati Khanna, Francis Mussai, Anish Thomas, Gary Middleton, Constance Yuan, Betsy Morrow, Jingli Zhang, Ira Pastan, Maryalice Stetler-Stevenson, Raffit Hassan. Granulocytic MDSCs (CD11b+CD15+CD14-HLADR-) in peripheral blood of mesothelioma patients are elevated and suppress T cell proliferation and function via reactive oxygen species. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3247.