Adults with indolent systemic mastocytosis have elevated peripheral blood myeloid-derived suppressor cells compared to chronic urticaria and healthy controls. (HUM7P.320)

MDSCs are a heterogeneous population of immature myeloid cells shown to be immunomodulatory in many diseases. In humans, they are identified by surface expression of CD33 and CD11b and lack of HLA DR. MDSCs are further divided into monocytic (M-MDSCs, CD14+) and granulocytic (G-MDSCs, CD15+) subsets. Our previous data showed that the prevalence and suppressive activity of murine MDSCs was augmented by the presence of mast cells (MCs). Indolent systemic mastocytosis (ISM) is a human disease caused by the clonal expansion of MCs. We propose that increased MCs and MC mediators frequently associated with mastocytosis produce an increase in peripheral blood MDSCs. Peripheral blood mononuclear cells (PBMCs) were isolated from blood obtained from adults with ISM and from chronic urticaria (CU, drug control), and healthy (H) age-matched controls. PBMCs, labeled with antibodies against HLA-DR, CD33, CD11b, CD15, and CD14, were analyzed by flow cytometry. MDSC concentrations in blood (x 106 cells/mL, mean ± standard error) were calculated. MDSC concentration was elevated (p