Myeloid-Derived Suppressor Cells (MDSC) Are Effectors of Bone Marrow Suppression in Lower Risk Myelodysplastic Syndromes (MDS).

Abstract 597 The acquisition of genetic abnormalities that lead to ineffective hematopoiesis is a characteristic of MDS. This event is mediated in part by an interaction of inflammatory intermediates with the bone marrow (BM) microenvironment; however, the mechanism by which chromosomal damage occurs to the stem/progenitor compartment (HSC/HPC) is unknown. We have identified a unique population of myeloid derived suppressor cells (MDSC) bearing a LIN-HLA-DR-CD33+ surface phenotype. These MDSC elicit hematopoietic suppression through the elaboration of nitric oxide, arginase, and inflammatory cytokines. This study found that MDSCs accumulated in excess in the BM of patients with lower risk MDS (mean, 32.32% ± 13.3; n=12) compared to BM from healthy age matched controls (mean 2.1%±0.9; n=8) and non-MDS cancer patients (n=8 mean 2.06%±1.5 p<0.0001). Fluorescence in situ hybridization (FISH) was performed to determine whether MDSCs represent a distinct cell population from the abnormal MDS clone. MDSCs from BM of 5 patients having chromosomal abnormality were separated by FACS sorting based on LIN-HLA-DR-CD33+ phenotype and the presence of chromosomal abnormalities was determined in this population and compared to non-MDSCs. Monosomy 7 and deletion of 5q chromosomal abnormalities resided within the non-MDSC hematopoietic compartment. This indicates that MDSCs in low risk MDS patients may represent a unique cell population from the HPCs with clonal potential. Furthermore, the key cytokines involved in MDSC suppressive function, TGFβ, VEGF and IL-10, were higher in MDSCs isolated from MDS patients compared to controls. Using four-color immunostaining, we discovered that MDSCs are capable of direct cytotoxicity against autologous erythroid precursors (CD71+ and CD235a+), as evidenced by increased polarized granule mobilization toward the site of cellular contact. Moreover, it was observed that the accumulation of MDSC in the BM from MDS patients has an impact on hematopoietic differentiation. In the presence of MDSCs, the formation of BFU-E (burst forming unit erythroid) was significantly suppressed in all patients tested (n=6). In contrast depletion of the MDSC by FACS sorting reconstituted the formation of BFU-E (21± 4.2 in MDSC depleted bone marrow verse 0.8± 0.6 in unsorted BM cells, respectively). These data strongly suggest that the presence of MDSCs in the BM microenvironment of MDS patients contribute to suppression of HPC development. Based on these findings, a novel form of adaptive immunotherapy based on the induction of MDSC maturation can be envisioned. DAP12 (DNAX-activating protein of 12kDa) is an adaptor protein that mediates signaling of dentritic cell and monocyte maturation. In order to determine if DAP12 signaling induces MDSC maturation and reversal of suppressive function, a genetically modified, constitutively activated form was introduced into BM mononuclear cells (BM-MNC) using both recombinant adenoviral and lentiviral gene transduction. Infection of BM-MNC from MDS patients with constitutively active DAP12 increased expression of maturation surface markers CD14, CD15 and HLA-DR and increased BFU-E colony formation (31± 2.1 verse control 7±0.7) after 14 days. These results suggest that active signaling through DAP12 has potential therapeutic implications in MDS by driving maturation of immature myeloid cells reversing the inflammatory changes that contribute to malignant transformation. In conclusion, our previously unknown findings have begun to elucidate whether the presence of MDSCs in the BM microenvironment of MDS patients contributes to the pathogenesis of MDS by providing a suppressive microenvironment and repress BM maturation associated with impaired HSC/HPC development. Additionally, DAP12 may be a potential therapeutic pathway in which to stimulate MDSC maturation, reversing the suppressive effects on HSC/HPC development. Disclosures: No relevant conflicts of interest to declare.