Melatonin relieves Th17/CD4-CD8- T cells inflammatory responses via nuclear-receptor dependent manner in peripheral blood of primary Sjögren's syndrome

Objectives: Primary Sjögren's syndrome (pSS) is a complex systemic autoimmune disease whose clinical spectrum extends from sicca syndrome to systemic involvement. T helper 17 cells (Th17) and CD4^-CD8^- (double negative, DN) T cells are actively involved in the pathogenesis of pSS. Melatonin shows important immunoregulatory functions in multiple T cell-mediated autoimmune diseases. However, the effects of melatonin on the immune cells of pSS patients are unclear. Hence, this study was aimed to evaluate the effects of melatonin on the immune responses of peripheral pathogenic Th17 and DN T cells from pSS patients, and explore the underlying receptor-related mechanism.

Methods: The concentration of serum and saliva melatonin of pSS patients and healthy controls (HCs) were detected using Enzyme-linked immunosorbent assays (ELISA). Expression of arylalkylamine N-acetyltransferase (AANAT) and hydroxyindole O-methyltransferase (HIOMT) were conducted in labial glands samples by immunohistochemistry. The mechanism underlying the effects of melatonin on Th17 and DN T cells responses in peripheral blood from pSS was investigated by quantitative real-time polymerase chain reaction (RT-PCR), flow cytometry, ELISA, cell viability, and proliferation assays.

Results: Serum and saliva melatonin levels were lower in pSS patients than in HCs, which were negatively correlated with disease activity. The expression levels of melatonin's biosynthetic enzymes (AANAT, HIOMT) and nuclear receptors (RORα, RORγ) were significantly increased in peripheral blood mononuclear cells (PBMCs) from pSS patients. Furthermore, in vitro melatonin administration decreased the expression of melatonin effector/receptor system in peripheral blood of pSS patients. More importantly, Melatonin inhibited pathogenic responses of peripheral Th17 and DN T cells in PBMCs from pSS, which was independent of melatonin membrane receptors. However, melatonin nuclear receptor antagonist SR1001 enhanced the inhibitory ability of melatonin on Th17 and DN T cells production, and agonist SR1078 weakened the effects of melatonin. Additionally, overexpression of the melatonin effector/receptor system in pSS patients appeared to be involved in the disease, due to that melatonin effector/receptor system expression was correlated with the frequency of Th17 or DN T cells.

Conclusion: Melatonin relieved the inflammatory responses of Th17 and DN T cells in PBMCs from pSS patients in a nuclear receptors-dependent manner,suggesting that melatonin might be beneficial to pSS.