Proinflammatory Macrophage Activation by the Polysialic Acid-Siglec-16 Axis Is Linked to Increased Survival of Patients with Glioblastoma

Interactions with tumor-associated microglia and macrophages (TAM) are critical for glioblastoma progression. Polysialic acid (polySia) is a tumor-associated glycan, but its frequency of occurrence and its prognostic value in glioblastoma are disputed. Through interactions with the opposing immune receptors Siglec-11 and Siglec-16, polySia is implicated in the regulation of microglia and macrophage activity. However, due to a nonfunctional SIGLEC16P allele, SIGLEC16 penetrance is less than 40%. Here, we explored possible consequences of SIGLEC16 status and tumor cell-associated polySia on glioblastoma outcome.Formalin-fixed paraffin-embedded specimens of two independent cohorts with 70 and 100 patients with newly diagnosed glioblastoma were retrospectively analyzed for SIGLEC16 and polySia status in relation to overall survival. Inflammatory TAM activation was assessed in tumors, in heterotypic tumor spheroids consisting of polySia-positive glioblastoma cells and Siglec-16-positive or Siglec-16-negative macrophages, and by exposing Siglec-16-positive or Siglec-16-negative macrophages to glioblastoma cell-derived membrane fractions.Overall survival of SIGLEC16 carriers with polySia-positive tumors was increased. Consistent with proinflammatory Siglec-16 signaling, levels of TAM positive for the M2 marker CD163 were reduced, whereas the M1 marker CD74 and TNF expression were increased, and CD8+ T cells enhanced in SIGLEC16/polySia double-positive tumors. Correspondingly, TNF production was elevated in heterotypic spheroid cultures with Siglec-16-expressing macrophages. Furthermore, a higher, mainly M1-like cytokine release and activating immune signaling was observed in SIGLEC16-positive as compared with SIGLEC16-negative macrophages confronted with glioblastoma cell-derived membranes.Collectively, these results strongly suggest that proinflammatory TAM activation causes the better outcome in patients with glioblastoma with a functional polySia-Siglec-16 axis.