Sugar tags control immune cell exit from bone marrow

White blood cells called monocytes circulate in the blood to patrol the body and respond to infection and inflammation, but they must first leave the bone marrow. How monocytes exit the marrow is tightly controlled and important for immune surveillance, yet parts of that control system remain mysterious. Polysialylation — the addition of long chains of a sugar called polysialic acid to proteins — is well known for shaping brain development, but its role in the immune system has not been well explored. Guided by publicly available human genomic data that linked natural variants in the gene ST8SIA4 to lower circulating monocyte and lymphocyte counts and greater risk of SLE, researchers led by corresponding author Yoann Rombouts set out to test whether ST8SIA4 matters for monocyte biology. They studied genetically modified mice that lack St8sia4 and followed monocyte behavior at steady state and during infection with Mycobacterium tuberculosis, aiming to understand whether and how ST8SIA4-dependent polysialylation controls monocyte movement out of the bone marrow.

The team used St8sia4 -/- mice to test the role of ST8SIA4 and found these animals had monocytopenia — fewer circulating monocytes — caused by a failure of inflammatory monocytes to exit the bone marrow. This defect was linked to impaired CCR2-mediated egress: CCR2 is the chemokine receptor that normally guides monocytes in response to its ligands. The researchers identified NCAM1 as the principal polysialylated protein on inflammatory monocytes and showed that Ncam1 -/- mice phenocopied the monocytopenia seen in St8sia4 -/- mice. Mechanistic experiments revealed that loss of ST8SIA4-dependent polysialylation reduced engagement and internalization of the CCR2 ligands CCL2 and CCL7, disrupted CCR2 surface organization, and produced extensive cytoskeletal remodeling in monocytes. Importantly, the data indicated that polysialic acid itself does not act as a co-receptor for these chemokines, pointing to a different way that polysialylation influences chemokine receptor function.

These findings reveal a previously unrecognized regulator of monocyte mobilization: ST8SIA4-mediated polysialylation of NCAM1 tunes how CCR2 functions and thus whether inflammatory monocytes are released from the bone marrow. By linking human ST8SIA4 variants to altered blood cell counts and increased risk of SLE, and by demonstrating parallel defects in St8sia4 -/- and Ncam1 -/- mice, the study establishes polysialylation as a critical modifier of immune cell trafficking. The work reshapes our understanding of how sugar modifications on cell-surface proteins control chemokine responses rather than serving as simple co-receptors, and it has broad implications for immune surveillance and inflammatory responses, including during Mycobacterium tuberculosis infection. As a basic science advance, the study opens new avenues to think about targeting polysialylation, ST8SIA4, NCAM1, or CCR2 organization to influence monocyte behavior in disease.