Wakayama symposium: interface between innate and adaptive immunity in dry eye disease

In DED, a chronic inflammatory reaction is generated at the ocular surface, accompanied by the destruction of epithelial tight junctions and finally sloughing of the surface epithelia [6]. The epithelium is part of the innate immune response, playing a crucial role in preventing the invasion of ocular tissue by foreign bodies or microorganisms [7]. Eyelid blinking, the barrier posed by the epithelium, secretory proteins such as lysozyme, and conjunctival mucous are additional aspects of the innate immune response at the ocular surface [8].

Corneal and conjunctival epithelial and epithelial-associated Langerhans cells are known to express a range of both toll-like receptors (TLR) and NOD-like receptors (NLR). The TLR and NLR pathways are the primary route by which host cells detect the presence of foreign invaders. The responses triggered include the production of cytokines, chemokines, and antimicrobial peptides. Increasing the expression of TLR or NLR may prevent the risk of infection but may also lead to inflammation. There is some evidence that TLR expression is modulated at the ocular surface in DED. In an experimental DED model, TLR2-4 and TLR9 expression was increased at the ocular surface and lacrimal glands [9, 10].

The rate of epithelial turnover is increased in DED, with viable corneal surface cells shed by classical apotosis [11, 12]. Dead cells can release endogenous extracellular DNA (eDNA), a type of damage-associated molecular pattern (DAMP) that activates innate immunity [13]. Sonawane et al. found that eDNA and neutrophils were present on the ocular surface in DED patients and suggested that eDNA production and clearance mechanisms are dysregulated in DED [14]. The authors suggested that accumulated eDNA and neutrophil extracellular trap (NET) in the pre-corneal tear film would result in the inflammation characteristic of DED.

Lactoferrin, which is secreted by acinar cells of the lacrimal glands, has anti-inflammatory, anti-cancer, and immune-modulating properties. Lysozyme, another secretory protein from acinar cells in the main lacrimal gland and conjunctival accessory lacrimal glands, attacks the cell walls of bacteria. The levels of lactoferrin and lysozyme are reported to be decreased in the tear fluid of DED patients [15, 16]. The level of secretory IgA (sIgA), a key factor that protects against microbes in the mucosa, is decreased in DED patients [16]. Thus, decreased sIgA levels inhibit proper modulation of the ocular surface flora, which in turn increases the risk of microbial infection. The expression of secretory phospholipase A2 (sPLA2) is increased in the tears of DED patients, which suggests that sPLA2 plays a major role in preventing microbial infection at the ocular surface in DED patients [17]. The altered expression of molecules involved in the innate immune response in dry eye may have other effects on the innate immune system as well as trigger an adaptive response if not properly managed.

Mucins keep the ocular surface moist and protect it from external stimuli, and the induction of mucin from the ocular surface may facilitate the stability of the tear film [18]. Mucins are high-molecular weight glycoproteins characterized by their extensive O-glycosylation [19]. Major mucins expressed by the ocular surface epithelia include cell surface-associated mucins MUC1, MUC4, MUC16, and the gel-forming mucin MUC5AC. Recent advances using functional assays have allowed the examination of their roles in the protection of corneal and conjunctival epithelia. Among these, MUC1 genotype polymorphism has been shown in DED [20]. Differences in MUC1 genotypes between healthy controls and DED patients could explain the loss of ocular surface integrity. The loss of mucins at the ocular surface may result in an increased risk of infection and epithelial stress, which could trigger the adaptive immune response.

Very recently, Desiccating stress-induced chemokine expression in the epithelium was shown to be dependent on upregulation of NKG2D/RAE-1 and release of IFN-γ in experimental DED [21]. Upregulation of CXCL9, CXCL10, and CXCL11 expression was notedto be T cell–independent, requiring IFN-γ–producing NKG2D⁺ NK cells that are activated in response to DS-induced stress signals. This suggest that the triggering immune response in DED pathology.

Environmental triggers may also contribute to the initiation and perpetuation of the disease. The activation of innate immunity results in antigen presentation, T-cell activation, and the release of pro-inflammatory cytokines [22]. These changes activate the adaptive immune system and auto-antibodies. No known antibodies are known to be involved in DED; however, there is mounting evidence that DED is a localized self-antigen–driven autoimmune-based inflammatory disease [4].

Immune-mediated inflammatory disease (IMID) is associated with various conditions. While the exact etiology is undefined, both genetic and environmental factors play an important role in its pathogenesis. Recently, genome-wide association studies (GWASs) using millions of nucleotide polymorphisms as genomic markers and non-synonymous single nucleotide polymorphisms (nsSNP) have identified loci involved in IMID susceptibility [23]. Murakami et al. also reported that IL1B polymorphisms (genotypes CC, TT, and AA in positions -511, -31, and 3877, respectively) were significantly less frequent in Sjogren’s syndrome patients than controls or patients with SLE [24]. We identified certain genetic variations by screening for pro-inflammatory cytokine genes in Korean non-Sjogren’s DED patients. Among the screened genes, rs1143634 of interleukin-1 beta (IL1B) and rs8192284 of interleukin-6 receptor (IL6R) were linked to susceptibility among Korean non-Sjogren’s DED patients [25]. IL1B is a pro-inflammatory cytokine that is considered to be important in the pathogenesis of DED [26]. We also observed that the genotypic and allelic distributions of rs8192284 in the IL6R gene differed between DED patients and controls. The exact role of IL6 is unclear, and both its pro- and anti-inflammatory properties require further investigation. The balance between pro- and anti-inflammatory effects of IL6 was previously suggested to favor pro-inflammatory molecules under various environmental conditions [27].

The inflammatory response can resolve insults to the tissue and restore homeostasis [5]. However, inappropriate control of the natural defense mechanism results in chronic inflammation and disease progression. The resolution of inflammation is critical to the transition from innate to adaptive immunity [5, 28]. The inflammatory IL6 and soluble IL6 receptors are known to direct the transition from innate to adaptive immunity [28]. Traditionally, IL6 was regarded as an activator of acute-phase responses; recent findings have shown a series of IL6-mediated events are critical for resolving the innate immune response [29]. Levels of IL6, which may contribute to DED pathogenesis, are reported to be increased in the tears and conjunctival epithelium of DED patients [30]. The expression of soluble IL6 receptor (sIL-6R) in tears is upregulated in chronic inflammatory conditions affecting the ocular surface [31, 32]. Previously, we demonstrated that the levels of IL-1B, IL-6 and their soluble receptors sIL-1R1 and sIL-6R were significantly elevated in the tears of patients with DED [33]. We assumed that the IL-6/sIL-6R complex mediates the pathogenesis of DED and that the marked elevation of its natural antagonist, soluble glycoprotein 130 (Sgp130), may act to preserve ocular homeostasis in response to local inflammation. Interestingly, we also demonstrated decreased levels of IL-17A and interferon gamma (INF-r) in the tears of DED patients. We interpreted the results as evidence that the suppressor T-cell, which is a natural inhibitor of self-reactive Th1 (INF-r), Th2 (IL4+), and Th17(IL17A) cells, may function to some extent in the mild and early stages of DED [34]. In the clinical setting, it is rare to see advanced DED unaccompanied by systemic autoimmune disease. Moreover, patching the eyelid to protect against external stimuli and supplying artificial tears without any anti-inflammatory agent can resolve the symptoms and signs of mild DED. This phenomenon suggests that normal tissue architecture is preserved despite local insult and inflammation. However, when some mediators such as sIL-6 act negatively, various cytokines and chemokines act on both arms of the immunologic response. The interface between the innate and adaptive immune responses, and triggering factors initiating adaptive immune responses in DED remains to be elucidated by future clinical trials and research approaches.