Abstract 3616: Hypoxia determines the fate of myeloid cell differentiation by controlling STAT3 activation in tumor site

Abstract

Abstract The abnormal differentiation of myeloid cells is the key element of altered immune response in cancer. Myeloid derived suppressor cells (MDSC) are a major component of immune suppressive network. MDSC accumulate in large numbers in peripheral lymphoid organs and inside tumors. Recent data indicated that the fate of MDSC depends on the localization of these cells. In contrast to peripheral lymphoid organs, inside tumors, MDSC rapidly differentiate to tumor associated macrophages with potent immune suppressive activity. Hypoxia is intricate part of tumor microenvironment. We have found that hypoxia plays a major role in regulation of MDSC differentiation to macrophages in tumor site. Hypoxia caused dramatic reduction in phosphorylated STAT3 in MDSC. These results were unexpected, since high level of STAT3 activity is a hallmark of MDSC present in blood and lymphoid organs. In our experiments spleen MDSC in tumor-bearing mice had much higher level of STAT3 phosphorylation than their counterparts in tumor site from the same mice. In cancer patients MDSC in tumor site also had substantially lower level of pSTAT3 than MDSC in peripheral blood. In vitro culture of spleen MDSC in hypoxia led to the suppression of STAT3 activation. Furthermore, mice with constitutively active STAT3 had shown the dramatic expansion of MDSC and down-regulation of macrophage population in tumor microenvironment, suggesting that STAT3 is a downstream target for hypoxia driven myeloid cells differentiation in cancer. We found that CD45 protein tyrosine phosphatase was a molecule that regulated STAT3 activation in hypoxia. The activity but not expression of CD45 phosphatase was up-regulated in tumor MDSC as compared to MDSC in spleens. The exposure of spleen MDSC to hypoxia led to the significant increase in CD45 activity. The siRNA mediated silencing of CD45 abrogated the effect of hypoxia on the down-regulation of STAT3 activity in MDSC. This suggests that hypoxia driven up-regulation of CD45 phosphatase activity suppresses STAT3 activation, which in turn plays a key role in differentiation of myeloid cells in tumor site. These unexpected findings may provide rationale for re-evaluation of therapeutic strategies targeting myeloid cells in cancer. Citation Format: Vinit Kumar. Hypoxia determines the fate of myeloid cell differentiation by controlling STAT3 activation in tumor site. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3616. doi:10.1158/1538-7445.AM2014-3616