303 - Blockade of PD-1/PD-L1 Pathway Regulates Microglia Activation and Glycolipid Metabolism Via GPD1 to Ameliorate Radiation-Induced Brain Injury

Purpose/Objective(s): PD-1 inhibitors are standard in oncology, yet their impact on radiation-induced brain injury (RIBI) and microglial immunometabolism remains unclear. This study elucidates the mechanism by which PD-1 blockade modulates microglial metabolic reprogramming to alleviate RIBI.

Materials/Methods: C57BL/6J mice received 20 Gy whole-brain irradiation followed by anti-PD-1 therapy. Cognitive function (Morris water maze) and histology were assessed to establish therapeutic efficacy. To explore the mechanism, hippocampal microglia underwent scRNA-seq and LC-MS/GC-MS metabolomics. Finally, to validate causality, a GPD1-overexpressing (OE-GPD1) lentiviral model was established in irradiated BV2 microglia (10 Gy) treated with anti-PD-1, followed by Seahorse XF and $^{13}$C-glucose flux analysis.

Results: In vivo, radiation induced significant spatial memory deficits and hippocampal neuronal apoptosis, which were effectively reversed by anti-PD-1 therapy ($P$<0.05) . scRNA-seq revealed that radiation depleted homeostatic Cst3+ microglia while enriching pro-inflammatory Ccl3+ clusters. This polarization coincided with a metabolic bottleneck characterized by upregulated Gpd1 ($P$=0.002) and accumulation of the lipid precursor DHAP, indicating a shunting of glucose carbon toward long-chain triacylglycerol (TAG) synthesis rather than the TCA cycle. PD-1 blockade normalized GPD1 expression and restored mitochondrial flux. Crucially, lentiviral GPD1 overexpression abolished these therapeutic benefits. Even in the presence of PD-1 blockade, OE-GPD1 cells exhibited suppressed mitochondrial respiration, reinstated lipid droplet accumulation ($P$<0.0001), and reverted to an iNOS+ M1 phenotype (Table 1), confirming GPD1 as the critical mediato.

Conclusion: PD-1 blockade confers neuroprotection against RIBI by suppressing GPD1 expression, thereby correcting the pathogenic shunting of glucose into toxic lipid synthesis pathways. These findings define a novel "immunometabolic checkpoint" in microglia and provide a mechanistic rationale for combining immunotherapy with cranial radiotherapy to mitigate radiation-induced toxicity.