Myeloid Derived Suppressor Cells and Their Potential Impact on Minimal Residual Disease Status and Disease Response in a Cohort of Patients with Plasma Cell Dyscrasias

Myeloid Derived Suppressor Cells (MDSCs) are known to promote immune-suppressive and tumorigenic effects in plasma cell dyscrasias (PCDs), including multiple myeloma, and may play a role in treatment response and minimal residual disease MRD status in patients undergoing cancer-directed therapies. MDSCs are classified by their granulocyte-like effects (g-MDSCs) or monocyte-like effects (m-MDSCs). To our knowledge, there are no published studies that correlate minimal residual disease (MRD) with the presence of MDSCs and immune effector-cell populations in the blood and bone marrow of patients with PCDs. In 14 patients with PCDs, we defined immune-cell populations in the blood and marrow and correlated them with MRD status, treatment regimens, disease status (stable vs. progressive) and autologous stem cell transplantation status. In the entire cohort, there were a higher proportion of m-MDSCs in blood (blood 5.56%, aspirate 2.47%). There were a higher proportion of g-MDSCs in aspirate (blood 0.04%, aspirate 0.16%). Interestingly, in patients who remained MRD positive (3/13), the proportions were reversed, and we identified more m-MDSC in aspirate than blood, as well as a low proportion of g-MDSCs in both the aspirate and bone marrow. In patients achieving undetectable MRD (10/13) there were higher proportions of CD8 and CD4 T-cells, with no differences in MDSC populations. Not surprisingly, in patients treated with the anti-CD38 antibody daratumumab (5/14), fewer m-MDSCs were isolated in the blood and aspirate, relative to daratumumab-untreated patients (9/14).Daratumumab treatment decreased activated CD4 and CD8 T cells (HLA-DR+/CD38-, HLA-DR-/CD38+ or HLA-DR+/CD38+ CD4 and CD8 T cells) and increased the proportion of inactive HLA-DR-/CD38- CD4 and CD8 T-cells. In patients with progressive disease (3/14), there was a paucity of all immune effector-cells, including MDSCs, T-cells and monocytes with the exception of neutrophils. MDSC populations were no different in pre-transplant patients (3/14) compared to post-transplant (11/14). In this preliminary study, we have enumerated the presence of MDSCs and their potential impact in patients with PCDs undergoing cancer-directed therapy.