B cells suppress medullary granulopoiesis by an extracellular glycosylation-dependent mechanism.

The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection， B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models， ST6GAL1 from B cells alters the sialylation profile of bone marrow populations， and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma， ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production， but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.