The perinecrotic niche of glioblastoma drives tumor-associated macrophage polarization and immunosuppression via podoplanin-mediated CLEC5A activation.

Glioblastoma (GBM), isocitrate dehydrogenase-WT (IDH-WT) (WHO grade 4) is the most common malignant glioma in adults and is characterized by a hypoxic and immunosuppressive tumor microenvironment (TME). Bone marrow-derived tumor-associated macrophages (TAMs) dominate the immune landscape in GBM and are recruited to the perinecrotic niche following the onset of necrosis. C-type lectin domain-containing 5A (CLEC5A) has the strongest association with poor clinical outcomes among immune-related genes in GBM and is preferentially expressed in hypoxic, perinecrotic TAMs. CLEC5A overexpression promotes TAM polarization toward an immunosuppressive phenotype and secretion of immunoregulatory cytokines. Using the replication-competent avian sarcoma-leukosis virus long terminal repeat with a splice acceptor (RCAS)/tumor virus A (tv-a) system GBM model with bone marrow transplantation from Clec5a-/- donor mice, we demonstrated that CLEC5A loss prolonged survival, delayed tumor progression, and attenuated TME immunosuppression. Mechanistically, podoplanin (PDPN) expressed on glioma cells directly engaged CLEC5A and triggered downstream Syk/JAK/STAT3 signaling in TAMs. Pharmacologic Syk inhibition suppressed glioma growth, diminished TAM infiltration and polarization, reversed the immunosuppressive TME, and prolonged survival in vivo. Collectively, our findings indicate that the PDPN/CLEC5A/Syk/STAT3 axis orchestrates TAM polarization and TME immunosuppression in the perinecrotic niche of GBM, highlighting CLEC5A/Syk as a promising therapeutic target for reversing the immunosuppressive TME and improving outcomes.