A GITRL‐mTORC1‐GM‐CSF Positive Loop Promotes Pathogenic Th17 Response in Primary Sjögren Syndrome.

Objective: Glucocorticoid‐induced tumor necrosis factor receptor superfamily–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 syndrome (pSS). Methods: Patients with pSS and healthy controls were recruited. Serum GITRL and Th17‐related cytokines were determined. RNA sequencing was performed to decipher key signal pathways. Nonobese diabetes (NOD) mice were 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 patients with pSS and showed a positive correlation with leukopenia, thrombocytopenia, autoantibodies, lung involvement, and disease activity. Serum GITRL was correlated with Th17‐related cytokines. GITRL promoted the expansion of Th17 and Th17.1 cells. Expansion of granulocyte‐macrophage colony‐stimulating factor positive (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 the 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 an rAAV vector expressing short hairpin RNA targeting GITRL alleviated disease progression in NOD mice. Conclusion: 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 loop. These findings suggest GITRL might be a promising therapeutic target in the treatment of pSS. [ABSTRACT FROM AUTHOR]