Abstract 453: Myeloid regulatory cells in cancer patients

Abstract

Abstract Adaptive immunotherapy can augment host immunity, although the outcomes to date in cancer patients have been limited, due in part, to increased numbers of myeloid derived suppressor cells (MDSCs) in the peripheral blood. MDSCs are key cellular regulators that can limit T-cell and dendritic cell (DC) number and function. They are believed to be myeloid precursors that are increased as part of the hematopoietic response to myelosuppression, stress, chemotherapy, and tumor growth. Recently, we found that mononuclear cell apheresis of some cancer patients (melanoma, leukemia and lymphoma), but not normal donors were enriched for hypodense myeloid cells as confirmed by elutriation and Ficoll-Hypaque gradients. These cells are also found in the peripheral blood, although their frequency is enriched by mononuclear cell apheresis. Despite their low density, cytologically they appear as granulocytes containing < 1% bands or metamyelocytes. Phenotypic analysis by flow cytometry revealed that >98% of the granulocytes (FSxSS), were MDSCs, defined as linage−(CD3, CD19 and CD56)DR−CD14dullCD13dull CD11b+CD33+. Consistent with an immature granulocyte phenotype > 50% of the granulocytes were CD16dull/− as compared to <3% of the granulocytes in the PB or apheresis products of normal donors. The frequency of these cells in an apheresis product or the peripheral blood from cancer patients can be as high as 60% of total nucleated cells and following mononuclear cell elutriation >95% of nucleated cells. It is emphasized that this is not observed in all patients and in our limited experience occurs in 20% or less of patients with advanced disease. Enrichment of hypodense MDSCs by Miltenyi magnetic bead selection of CD11b+ cells following subtractive depletion of DR+ cells provided a 60-85% pure population of MDSCs that can suppress an allogeneic T-cell response and by quantitative TaqMan analysis have significantly increased levels of NOS2, IL10, LTB4R1, MPO, MMP9 and VEGFα. We conclude that the majority of MDSCs in cancer patients are immature granulocytes; rather than myeloid precursors, as evidenced by their CD16dull phenotype, a lack of CD34 expression and segmented granulocyte morphology. Further, the presence of hypodense MDSCs has a significant impact on monocyte enrichment (apheresis, elutriation and Ficoll Hypaque separation) for maturation of monocytes into dendritic cells and use as a vaccine. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 453. doi:10.1158/1538-7445.AM2011-453