Novel Therapeutic Approach to Improve Hematopoiesis By Targeting Myeloid Derived Suppressor Cells with a Humanized Anti-CD33 Antibody

Myelodysplastic syndrome (MDS) is one of the major types of acquired bone marrow failure characterized by impaired peripheral blood cell production (anemia and cytopenias). Although much has been learned regarding the molecular genetic events involved in the pathogenesis of the MDS hematopoietic stem/progenitor cell compartment (HSC/HPC), investigations of the environmental pressures underlying disease initiation have been limited. Prior investigations have shown that inflammation in the bone marrow microenvironment contributes to hematopoietic impairment, with inflammatory molecules providing regulatory cues driving the proliferation and apoptotic death of HSC/HPC. We have reported that, the accumulation of myeloid-derived suppressor cells (MDSCs) in the local inflammatory bone marrow microenvironment plays a major role in the direct pathogenesis of MDS. MDSCs function by producing mediators and inflammatory cytokines capable of suppressing hematopoiesis and are defined by the lack of all lineage markers and only one key surface receptor, CD33 (CD33⁺HLA-DR⁻Lin⁻). We recently uncovered that this receptor is greatly expressed in MDSC isolated from patients with MDS and plays an important role in MDSC-mediated hematopoietic suppressive function. We also found that this ITIM (immune-receptor tyrosine-based inhibitor motif) containing molecule could induce suppressive cytokines after its engagement with its newly identified ligand S100A9. Therefore, we tested the hypothesis that with a fully human IgG1 monoclonal antibody against CD33 (mAb33.1) we could prevent the engagement of this ligand/receptor pair as well as induce ADCC of pathogenic MDSC through the reduction in the accumulation of CD33⁺ MDSC, the immune suppression and the restoration of hematopoiesis in MDS bone marrow specimens. After testing primary specimens with mAb33.1 we saw a significant increase in ADCC activity via NK as evidenced by significant reduction in the proportion of MDSC in culture as compared to isotype control treated cells. This decrease correlated with an increase in CD107a granule mobilization as well as an increased cytotoxicity in a killing assay without changing the proportion of NK cells in the culture. Functionally, the concentration of secreted IL-10, TGFb and VEGF were decreased, as was the gene expression of these suppressive cytokines after treatment with mAb33.1 but was restored when the antibody was cross-linked with an anti-human IgG antibody demonstrating the blocking ability of the antibody in preventing CD33 downstream signaling. Importantly, all of these observations correlated with the restoration of hematopoiesis, as there was a significant increase in the formation of CFU-GM and BFU-E colonies on a methylcellulose assay (n=9). In addition mAb33.1 also displayed a protective effect on HSC by blocking ROS production and reducing DNA damage, as demonstrated by comet and H2AX assays. This work provides the ground for the development of a novel group of therapies directly aimed at the suppressive MDSC and blockage of their signaling, rather than directly targeting the malignant clone, with the long term goal of improving the local microenvironment. This strategy will provide the background to assess its clinical potential to serve as a therapeutic target in MDS and AML.