Enhanced sialylation of bone marrow mesenchymal stromal cells contributes to immune remodeling through macrophage polarization in multiple myeloma patients.

INTRODUCTION: In multiple myeloma (MM), the immune system is disrupted, partly due to tumor-modified bone marrow mesenchymal stromal cells (BM-MSCs). These MSCs show altered glycosylation, including increased sialylation, which may promote immune evasion through Siglec interactions. METHODS: We quantified sialic acids and Siglec ligands on healthy donors-, MM-, T-MM-(treated multiple myeloma) and MGUS (monoclonal gammopathy of undetermined significance)-MSCs, and examined how different culture conditions (myeloma coculture, conditioned medium, hypoxia, inflammation, 3D culture) affect MSC glycosylation. Expression of 84 glycosylation-related genes was screened by qPCR. Macrophage polarization was then assessed after modulating MSC sialylation using inhibitors or an activator. RESULTS: MM-MSCs exhibited significantly higher surface sialylation than HD-, T-MM-, and MGUS-MSCs. MM-MSC sialylation was positively correlated with the percentage of plasma cells and the LDH levels of MM patients. Direct MSC-plasma cell cocultures further enhanced hypersialylation compared with conditioned medium or transwell coculture, indicating a contact-dependent mechanism. qPCR screening identified two dysregulated genes, ST6Gal1 and NEU2, implicated in the sialylation pathway. Inflammation and hypoxia appeared as key players in the expression of Siglec-7L, Siglec-9L and ST6Gal1 by MSCs. Functionally, we showed that increased sialylation promoted an M2-like phenotype as measured by CD206, whereas inhibition shifted macrophages toward an M1-like profile as measured by CD40 and HLA-DR. We also showed that when MSC sialylation is inhibited, their ability to induce an M2 phenotype is reduced, as evidenced by the lower production of M2-associated cytokines. CONCLUSION: Our findings suggest that BM-MSC hypersialylation may contribute to immune remodeling in MM.