AhR activation mitigates graft-versus-host disease of the central nervous system by reducing microglial NF-κB signaling

• Antibiotic treatment in GVHD promotes microglial activation by interfering with AhR signaling.

• AhR-ligand treatment ameliorates CNS-GVHD by reducing NF-κB signaling in microglia thereby providing a novel therapeutic target.

Acute Graft-versus-Host Disease (GVHD) that occurs after allogeneic hematopoietic cell transplantation (allo-HCT) can affect the central nervous system (CNS). The majority of allo-HCT patients receive antibiotic treatment, which alters the microbiome and essential microbiome-derived metabolites. We investigated the impact of microbiome modifications on CNS-GVHD and therapeutic strategies to overcome the microbiome-derived metabolite depletion. Antibiotic treatment of mice undergoing allo-HCT increased microglia numbers in the brain, indicating increased inflammation. In addition, microglia morphology shifted towards a highly branched phenotype. Consistent with a pro-inflammatory phenotype microglia exhibited increased NF-κB and Src activity. Antibiotic treatment caused the depletion of the bacteria-derived arylhydrocarbon receptor (AhR) ligand indole-3-acetate in the brain. Conversely, treatment of primary microglia with the AhR-ligand- 6-formylindolo (3, 2-b) carbazole (FICZ) reduced NF-κB activity and phagocytic potential. Microglia expansion and morphological changes were reversed by AhR-ligand-FICZ-treatment. Moreover, the AhR-ligand indole-3-acetate was also reduced in the CNS of patients that developed acute GVHD concomitant with increased microglial NF-κB expression. In summary, we demonstrated that antibiotic treatment and a subsequent decrease of AhR-ligands resulted in increased microglia activation during CNS-GVHD. FICZ-treatment hampered CNS inflammation by inhibiting NF-κB activity, thereby providing a metabolic modifier to interfere with pathogenic microglia signaling and CNS-GVHD in vivo.