Primary Sjögren's syndrome (pSS) is an autoimmune disease characterized by dry eye and dry mouth resulting from pro-inflammatory lymphocytic infiltration of lacrimal and salivary glands [1]. This disease affects 4 million Americans, mainly female [2]. Clinically, primary SS is diagnosed when only lacrimal and salivary glands are affected [1], while dry eye is the major negative impact on the quality of life of patients with both pSS and secondary SS [3]. The current management is limited to symptomatic control and anti-inflammatory medications [4] due to lack of knowledge about disease mechanisms.

An important clinical diagnostic marker for pSS is the presence of autoantibodies against components of the cell nucleus, such as antinuclear antibodies (ANA), anti-Ro/SSA, and anti-La/SSB [5]. In addition, high levels of cell-free DNA were observed in the serum of pSS patients [6]. High levels of type I interferons (IFN), which are synthesized by cells upon nucleic acid stimulation, were observed in the bloodstream of patients with pSS [7,8]. Collectively, these clinical findings indicate that pathogenic DNA sensing pathways might play a key role in autoimmune diseases including pSS.

When DNA enters the cytoplasm, a number of cytosolic pattern recognition receptors (PRRs) can be activated, the major ones are toll-like receptor 9 (TLR9) [9], absent in melanoma 2 (AIM2) [10], and cyclic GMP–AMP synthase (cGAS) [11]. Among these major pathways, self-pathogenic DNA has been reported to activate AIM2 and cGAS due to their non-selective binding activity to double-stranded (ds) DNA [12], whereas TLR9 is more selective for the detection of viral DNA [9].

AIM2 is composed of 2 domains: hematopoietic expression, interferon-inducible nature, and nuclear localization (HIN) domain and pyrin domain (PYD) [13]. The HIN domain non-selectively binds dsDNA [13], which allows AIM2 to be activated by a variety of types of cytoplasmic DNA, including foreign DNA from virus/bacteria, or self-DNA from either exosomes or herniation from a leaky nuclear envelope [14]. The PYD domain helps in recruiting Apoptosis-associated speck-like protein containing a CARD (ASC) in inflammasome assembly and activation. The macromolecular signaling platform caused by ASC oligomerization is known as the ASC “speck” (Fig. 1A). Upon activation, inflammasomes recruit caspase-1 and cleaves immature pro-IL-1β to mature IL-1β that is secreted and contributes to the inflammatory process [15]. When cGAS binds to DNA, the cGAS produces Cyclic-GMP-AMP (cGAMP). This molecule, then, directly interacts with STING leading to its activation and production of type I IFN [15,16]. Upon activation, STING monomers are transported to the endoplasmic reticulum (ER) to form dimers or oligomers, where they activate the tumor necrosis factor receptor-associated factor (TRAF) family member-associated NF-kappa-B activator (TANK)-binding kinase 1 (TBK1), leading to inflammation and eventually cell death [12,16]. Although these innate immune mechanisms protect the body from pathogens, desynchronized activation may result in chronic inflammation and tissue damage. High levels of cGAMP were observed in the serum of SLE patients [17]; Moreover, mutations in STING1 gene (also known as transmembrane protein 173, TMEM173) can directly lead to a severe autoimmune disease named STING-Associated Vasculopathy with onset in Infancy (SAVI) [18]. Despite being widely discussed in SLE, the role of the cGAS-STING axis, along with the AIM2 inflammasome, remains unclear in pSS.

As a major organ being affected by pSS, the lacrimal gland is a tubuloacinar exocrine gland responsible for secreting the aqueous layer of the tear film that consists of proteins, electrolytes, and water [19]. Lacrimal gland epithelium is composed of three major cell types: ductal, acinar, and myoepithelial cells (MECs). MECs cells surround the acinar cells that are the primary glandular secretory epithelium. They express α-smooth muscle actin (αSMA) and can contract thereby regulating fluid secretion by the lacrimal gland. Besides contraction, MECs facilitate several critical functions of the lacrimal gland including secreting extracellular matrix to form the basal membrane of acini, exchange of signals between acini and stroma, and keeping a high level of plasticity to differentiate during tissue damage [20]. We reported altered AIM2 gene expression in MECs cultured from the lacrimal gland in a mouse model of pSS [20,21]. In this study, we investigated the effect of self-genomic DNA (gDNA) in primary culture of lacrimal gland MECs and evaluated the activation of both AIM2 and cGAS-STING DNA sensing pathways, cell fate, and cell function.