Expanded tumor-associated polymorphonuclear myeloid-derived suppressor cells in Waldenstrom macroglobulinemia display immune suppressive activity

The role of the bone marrow (BM) microenvironment in regulating the antitumor immune response in Waldenstrom macroglobulinemia (WM) remains poorly understood. Here we transcriptionally and phenotypically profiled non-malignant (CD19^- CD138^-) BM cells from WM patients with a focus on myeloid derived suppressive cells (MDSCs) to provide a deeper understanding of their role in WM. We found that HLA-DR^lowCD11b⁺CD33⁺ MDSCs were significantly increased in WM patients as compared to normal controls, with an expansion of predominantly polymorphonuclear (PMN)-MDSCs. Single-cell immunogenomic profiling of WM MDSCs identified an immune-suppressive gene signature with upregulated inflammatory pathways associated with interferon and tumor necrosis factor (TNF) signaling. Gene signatures associated with an inflammatory and immune suppressive environment were predominately expressed in PMN-MDSCs. In vitro, WM PMN-MDSCs demonstrated robust T-cell suppression and their viability and expansion was notably enhanced by granulocyte colony stimulating factor (G-CSF) and TNFα. Furthermore, BM malignant B-cells attracted PMN-MDSCs to a greater degree than monocytic MDSCs. Collectively, these data suggest that malignant WM B cells actively recruit PMN-MDSCs which promote an immunosuppressive BM microenvironment through a direct T cell inhibition, while release of G-CSF/TNFα in the microenvironment further promotes PMN-MDSC expansion and in turn immune suppression. Targeting PMN-MDSCs may therefore represent a potential therapeutic strategy in patients with WM.