IMMU-20. Platelets drive immune suppression and glioblastoma growth in a sex-dependent manner via PAR4 signaling

Abstract Glioblastoma (GBM) is the most common primary malignant brain tumor, and current therapies provide only palliation, not a cure. GBM is difficult to treat due to its complex, multi-faceted development involving various cell types in the tumor microenvironment (TME). Recent observations highlight sex differences in these interactions, influenced by biological, hormonal, and immune factors affecting disease progression, treatment responses, survival, and the TME. Platelets, key regulators of immune responses and tumor progression, may contribute to these sex-based differences by altering TME dynamics, though precise molecular mechanisms remain unclear. Our previous work showed that thrombin, a major platelet activator via protease-activated receptor 1 (PAR1) and protease-activated receptor 4 (PAR4), is secreted by cancer stem cells into the tumor microenvironment. We now demonstrate that GBM patients exhibit heightened platelet reactivity driven by PAR4 signaling. In murine GBM models, targeting PAR4 with BMS986120 and genetic inhibition of PAR4 prolong survival in females but not males. This survival advantage is estrogen-dependent and TME-specific, driven by enhanced CD8+ T cell infiltration regulated by an estrogen receptor response element on the PAR4 promoter. Inhibiting platelet PAR4 signaling decreases alpha-granule secretion in female tumor-bearing mice while enriching alternative exocytosis pathways, thereby influencing CD8+ T cell activity. PAR4-activated platelets within the TME suppress CD8+ T cell function, and CD8+ T cell depletion eliminates tumor-induced platelet reactivity and the survival benefit of PAR4 inhibition. These findings demonstrate how activated platelets interact and regulate immune cell populations in GBM. These results suggest that the hyper-thrombotic state seen in many GBM patients, which serves as a major risk of death, simultaneously contributes to the immunosuppressive TME. In addition, these results identify therapeutic strategies to leverage the platelet hyperactivity seen in GBM by hyperactive thrombin-PAR4 for sex-dependent therapeutic purposes.