Microenvironment Induced Myelodysplastic Syndrome (MDS) in S100A9 Transgenic Mice Caused by Myeloid-Derived Suppressor Cells (MDSC)

Abstract 788 Understanding the pathophysiology of myelodysplastic syndrome (MDS) is limited by a complex molecular mechanism and lack of an adequate animal model that recapitulates the role of inflammation in the abnormal hematopoiesis. We recently showed that patients with MDS have expansion of inflammation-related hematopoietic suppressive cells called immature myeloid-derived suppressor cells (MDSC) that display direct cytotoxic and suppressive effects on autologous hematopoietic progenitor cells (HPCs). Expansion of bone marrow (BM) MDSCs contributed to the production of inflammatory cytokines and reduced HPC survival underlying BM failure in lower risk patients. Here we provide evidence that MDSC activation, expansion and development is driven by overexpression of inflammatory-related signaling molecules, myeloid-related protein 8 (MRP8, encoded by S100A8) and MRP14 (encoded by S100A9). Both MRP proteins serve as the native endogenous ligands for Toll-like receptor 4 (TLR4), which is an important damage-associated molecular pattern (DAMP) mediating inflammatory response. We found higher expression of MRP8 and MRP14 in BM mononuclear cells from MDS patients compared to healthy donors, in whom these proteins were not detectable. High surface expression of both TLR2 and TLR4 in MDS MDSCs compared to healthy donor MDSCs confirmed that this signaling pathway is activated in MDS. Inhibition of MRP8/MRP14 proteins in MDSCs using specific shRNAs dramatically attenuated IL-10 and TGF-β production and rescued BFU-E and CFU-GM colony formation of autologous bone marrow progenitors. These data show that inflammation-associated MRP8/MRP14 expression plays a critical role in the suppressive activities of MDS MDSCs. We therefore generated S100A9 transgenic mice (S100A9Tg) overexpressing the murine MRP14 homologue and investigated the role of this protein in bone marrow failure. Significant MDSC accumulation was evident in the BM of S100A9Tg mice by 6 weeks, but not in S100 knockout (KO) or wild type (WT) mice. Similar to human MDS, MDSCs from S100A9Tg mice, but not S100KO or WT mice, significantly inhibited BFU-E colony formation. Depletion of MDSCs in vitro rescued BM colony formation in the S100A9Tg mice indicating that the BM suppression is mediated by MDSC cells. TGF-β and IL-10 secretion was significantly increased in S100A9Tg mice, substantiating the role of S100A9 as an essential inflammatory factor that regulating MDSC suppressive activity. Analogous to human MDS, 6-month old S100A9Tg mice developed ineffective hematopoiesis with severe anemia, leukopenia, and thrombocytopenia accompanied by MDS-like morphological features. BM aspirates and core biopsies from S100A9Tg mice were hypercellular with trilineage cytological dysplasia characteristic of MDS. Treatment with ATRA, which induced the differentiation of MDSCs rescued hematopoiesis in S100A9Tg mice. Our findings indicate that primary BM expansion of MDSC is sufficient to perturb hematopoiesis and result in the development of MDS, supporting the notion of microenvironment-conducive oncogenesis. S100A9Tg transgenic mice provide a novel in vivo model of human MDS for target discovery and testing of novel therapeutics. Disclosures: No relevant conflicts of interest to declare.