Abstract

Abstract S100A9 is a pro-inflammatory molecule, secreted by myeloid cells, which has been reported to recruit leukocytes to the sites of inflammation or tumors. We have previously found that up-regulation of S100A9 expression in myeloid progenitor cells in cancer resulted in inhibition of DC differentiation and promotion of expansion of myeloid-derived suppressor cells (MDSC). It remained unclear how S100A9 could regulate MDSC expansion and function. In this study we evaluated the effects of secreted (extracellular) vs intracellular S100A9 on MDSC. Recombinant S100A9 or S100A8 proteins separately or as heterodimers did not promote MDSC expansion from hematopoietic progenitor cells (HPC), or affect their suppressive activity. However, recombinant proteins had a strong chemotactic effect on MDSC. Interestingly, this effect on immature myeloid cells isolated from naïve tumor-free mice was substantially lower. To study intracellular activity of S100A9 we used S100A9 transgenic (Tg) and knockout (KO) mice. Mice were injected with several different weakly and strongly immunogeneic tumors. As expected Tg mice had increased expansion of MDSC and KO mice had substantially reduced number of these cells. Different levels of MDSC did not affect the growth of weakly immunogeneic tumors. In contrast, overexpression of S100A9 in mice lead to significantly accelerated growth of immunogenic tumors. Lack of S100A9 resulted in fast rejection of immunogeneic tumors. Our data indicate that extracellular and intracellular S100A9 had different signalling in myeloid cells and apparently affect different aspects of their function. S100A9 over-expression, in vivo, inhibits the immune response, most likely by promoting MDSC accumulation and/or suppressive activity. Our data suggest that targeting of S100A9 in cancer can be an attractive therapeutic option. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5399. doi:1538-7445.AM2012-5399

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