A GITRL-mTORC1-GM-CSF positive loop promotes pathogenic Th17 response in exacerbating Primary Sjögren's Syndrome.

OBJECTIVES: Glucocorticoid-induced Tumor-Necrosis-Factor-Receptor Family-related Protein (GITR), with its ligand (GITRL), plays an important role in CD4+ T-cell-mediated autoimmunity. This study aimed to investigate the underlying mechanisms of GITRL in primary Sjögren's Syndrome (pSS).

METHODS: pSS patients and healthy controls were recruited. Serum GITRL and Th17-related cytokines were determinated. RNA-Sequencing was performed to decipher key signal pathways. Non Obese Diabetes (NOD) mice was adopted as experimental Sjögren models and recombinant adeno-associated virus (rAAV) transduction was conducted to verify the therapeutic potentials of targeting GITRL in vivo.

RESULTS: Serum GITRL was significantly higher in pSS patients and showed a positive correlation with leukopenia, thrombocytopenia, autoantibodies, lung involvement and disease activity. Serum GITRL was correlated with Th17-related cytokines. GITRL promoted expansion of Th17 and Th17.1 cells. Expansion of granulocyte-macrophage-colony-stimulating-factor (GM-CSF+ )CD4+ T cells induced by GITRL could be inhibited by blockade of GITRL. Moreover, GM-CSF could stimulate GITRL expression on monocytes. RNA-Sequencing revealed mammalian target of rapamycin complexes 1 (mTORC1) might be the key modulator. The increased phosphorylation of S6 and STAT3 and expansion of Th17 and Th17.1 cells induced by GITRL were effectively inhibited by rapamycin, suggesting a GITRL/mTORC1/GM-CSF positive loop in pathogenic Th17 response in pSS. Administration of rAAV vector expressing shRNA targeting GITRL alleviated disease progression in NOD mice.

CONCLUSIONS: Our results identified the pathogenic role of GITRL in exacerbating disease activity and promoting pathogenic Th17 response in pSS through a GITRL/mTORC1/GM-CSF positive loop. These findings suggest GITRL might be a promising therapeutic target in the treatment of pSS.