Abstract 3647: Characterization of the relationship between mature neutrophils and tumor associated granulocytic myeloid-derived suppressor cells

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Abstract Myeloid-derived suppressor cells (MDSC) is a group of cells with potent immune suppressive activity that accumulate in many pathologic conditions. In cancer, most of MDSC are represented by cells with granulocytic phenotype, CD11b+Ly6ClowLy6G+, and morphology and are thus called granulocytic MDSC (G-MDSC). This phenotype and morphology are also shared by mature neutrophils (Neu) in naive mice. The relationship, if any, between G-MDSC and Neu remains unclear. In this study, we compared tumor associated G-MDSC and mature Neu. G-MDSC were freshly isolated from spleen of tumor-bearing mice. Because there are no known specific markers that differentiate between Neu and G-MDSC in tumor being mice or Neu and immature myeloid cells in naïve mice; Neu were isolated from naive mice by mobilizing them to the peritoneum (pNeu). We found that in contrast to pNeu, a substantial proportion of G-MDSC express the M-CSF receptor, CD115, CXCR4 and the CD244 molecule, also known as 2B4. pNeu have high amounts of lysosomes, endosomes and proteasomes, which was validated using immunofluorescence microscopy to measure the expression of associated proteins, LAMP2, EEA1, PSMA5, respectively. G-MDSC had significantly lower expression of LAMP2, EEA1, PSMA5. Also, G-MDSC, compared to pNeu, had significantly lower phagocytic activity, as measured by the uptake of latex beads in vitro. On the other hand, G-MDSC, but not pNeu, suppressed T-cell IFN-γ production upon antigen-specific stimulation. Interestingly, pNeu had higher arginase activity and nitric oxide production than G-MDSC, while G-MDSC had a significantly higher increase in reactive oxygen species levels upon PMA stimulation in vitro. This may explain the differences in suppressive activity. G-MDSC survived 3 days in culture, in the presence of GM-CSF. Within 24 hours in culture with GM-CSF, they became phenotypically and functionally similar to pNeu, with increased phagocytic capacity and LAMP2 expression and decreased expression of CD115, CXCR4, and CD244. These data suggest that, in cancer, G-MDSC may represent pathologically activated precursors of neutrophils. 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 3647. doi:10.1158/1538-7445.AM2011-3647