Primary Sjogren's syndrome (pSS) is a slowly progressing systemic autoimmune disorder characterized by lymphocytic infiltration into moisture secreting glands, especially salivary and lacrimal glands, leading to the development of xerostomia and keratoconjunctivitis sicca (Brito-Zeron et al., 2016). Multiple studies have provided evidence that vigorous inflammatory responses, excessive oxidative stress and over-activation of apoptosis are interdependent and interactional events, which play an essential role in the destruction of glandular tissue and the loss of secretory function (Jimenez et al., 2002, Ryo et al., 2006, Bharaj et al., 2021). Management of pSS is primarily based on symptomatic treatment to relieve dry mouth and dry eyes (Brito-Zeron et al., 2016). However, there is no therapeutic intervention available that can reverse glandular dysfunction and thereby well improve sicca symptoms. Consequently, it is of great importance to search a novel, safe and efficient therapeutic option to provide excellent relief of glandular dysfunction in pSS.

Melatonin is a natural indolamine with pleiotropic biological and pharmacological effects, including anti-inflammatory, antioxidant, and anti-apoptotic activities (Reiter et al., 2014). These biological properties are characteristics closely related to the pathogenesis and treatment of glandular dysfunction in pSS. A previous study by our group revealed that pSS patients showed lower serum and saliva melatonin levels, which were negatively associated with the severity of the disease (Liu et al., 2022a). Concomitantly, saliva melatonin level was inversely correlated with unstimulated saliva flow rate (Liu et al., 2022a). Moreover, pSS-like animals showed lower melatonin concentration in serum and salivary gland homogenates, compared with control animals (Liu et al., 2022b). It was reasonable to speculate that the decreased levels of melatonin in saliva and serum from pSS could be the results of using melatonin to scavenge excessive inflammatory mediates, oxidized as well as apoptotic products (Liu et al., 2022a). Accordingly, melatonin may be a promising endogenous protective molecule for the improvement of SG function in pSS. It is now well-established that a number of critical physiological functions of melatonin are mediated via its receptors (Reiter et al., 2014). Typically, the melatonin receptor family consists of two main types, one is membrane receptors, including melatonin receptor 1 A (MTNR1A, MT1) and melatonin receptor 1B (MTNR1B, MT2), and another is specific high-affinity nuclear receptors, named retinoid-related orphan nuclear hormone receptor (RORs) and retinoid Z receptors (RZRs) (Reiter et al., 2014). Interestingly, pSS patients and animal models displayed higher RORα and RORγ expression in SGs, which tended to increase with disease stage/focus score (Weng et al., 2018). Hence, it is significant to explore the potential mechanisms of melatonin and its receptors in the improvement of glandular function in pSS.

Interleukin-6 (IL-6), as a typical proinflammatory cytokine, forms a complex with IL-6 receptor (IL-6R) and coreceptor glycoprotein 130 (gp130), which in turn initiates a cascade reaction including Janus kinase (JAK) activation, phosphorylation of Signal transducer and activator of transcription 3 (STAT3), and subsequent dimer formation, nuclear translocation, and gene transcription (Mohr et al., 2012). IL-6/STAT3 signaling pathway plays an important role in regulating inflammation, oxidative stress, and cell apoptosis (Charras et al., 2019, Huo et al., 2021, Latourte et al., 2017). Hyper-activation of STAT3 signaling is involved in the development of a variety of inflammatory diseases (Lee et al., 2017, Kwok et al., 2012a, Chang et al., 2019). Importantly, experiments on cells in vitro or on animal models in vivo have confirmed that IL-6/STAT3 could serve as an important potential therapeutic target for treatment or alleviation of inflammatory disorders, including uveitis, systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), autoimmune myocarditis, autoimmune arthritis and pSS (Charras et al., 2019, Lee et al., 2017, Chang et al., 2019, Yu et al., 2012, Park et al., 2014, Edwards et al., 2015). Intriguingly, recent researches have discovered that melatonin exerts physiological or pharmacological functions via a STAT3-dependent mechanism in apoptosis, oxidative stress and inflammation (Zhen et al., 2020, Gao et al., 2021). It seems meaningful to unravel the functional link between melatonin and IL-6/STAT3 in improving glandular function in pSS. Therefore, the objectives of current study were to evaluate the protective effects of melatonin on SGs in pSS and the role of melatonin receptors and IL-6/STAT3 signaling pathway in this process.