**3.3 The epithelial barrier dysfunction: leaky gut as a third element of pathogenesis**

The epithelium of the gastrointestinal tract or commonly known as the epithelial barrier is the largest mucosal lining that forms an interface between a mammalian host and the external environment [30]. Protecting the body from pathogens and foreign substances is the primary function of the epithelial barrier [31]. It is through the anatomical structure of the GI that processes such as digestion, absorption, and neuroendocrine network, as well as immune function balance, take place. Trillions of microbial inhabitants inhabit the lumen of the gut. These microbes play an important role in digestion and modulate the immune system [32]. The regulation of molecular trafficking between the intestinal lumen and the submucosa via the paracellular space maintains the capability of the intestinal permeability which interacts continuously with various bodies such as foodborne pathogens and antigens. Paracellular space is estimated to measure between 10 and 15 Å. Physiologically, solutes with a molecular radius of over 15 Å (~3.5 kDa) are not susceptible to uptake through this pathway [33]. The intestinal permeability is the property that allows solutes and fluids to pass between the lumen and tissues. Additionally, intestinal barrier function is determined by how well mucus and other extracellular components, such as mucus, prevent this exchange [32].

Transfers of macromolecules are largely affected by the paracellular permeability of epithelium, which is influenced by the intercellular tight junctions (TJs) [34]. The TJs are highly dynamic structures that serve a variety of functions both physiologically and pathologically in the intestinal epithelium [35]. The Zonulin protein appears to modulate intercellular TJs, and it has been shown that Zonulin expression is elevated in conditions associated with dysfunction of TJs, such as celiac disease [36–40]. An impaired epithelial barrier is associated with a wide range of chronic diseases, including allergies, autoimmune diseases, and metabolic disorders [41]. Changes in the permeability of the GI tract's epithelial lining facilitate a passage for commensal bacteria and their products from the lumen into the bloodstream creating what is known as a "leaky gut". There has been a growing interest over the past decade in the role of leaky gut's association with autoimmune diseases. There have been some suggestions that the leakage of pathogens into the body system results in autoimmunity making the leaky gut a third source of pathogenesis besides environmental triggers and genetic predisposition [42]. A dysbiosis which is a perturbation of the structural dynamics of the microbial community in the intestinal tract causes leaky gut condition and it is closely entangled with autoimmune diseases. As discussed here, the microbiota and particularly the intestinal microbes are important in the immune system and their disturbance can be associated with autoimmune diseases.

There are various immune cells such as T and B cells as well as macrophages and dendritic cells which are found beneath the layer of lamina propria of the intestinal epithelium. These cells are crucial for the maintenance of hemostasis in the intestinal epithelium. Epithelial cells suppress inflammation by generating regulatory dendritic cells, regulatory T and B cells, as well as anti-inflammatory cytokines [43]. In the event of a leaky gut and damage to the epithelial barrier, some pathogens such as *Staphylococcus aureus* may colonize areas such as leaky barrier areas [41]. In turn, dysbiotic microbiota moves to the interepithelial and subepithelial spaces, activating a local or systemic inflammatory response suspected to contribute to many immune-mediated diseases. There then follows a series of events that lead to chronic periepithelial inflammation with leaky epithelial barriers. It is not understood that

the autoimmune response occurs before the epithelial barrier insult or post the insult. The causes of the epithelial barrier's insults could be variable and include but are not limited to genetic predisposition such as filaggrins and TJ polymorphisms, environmental factors such as microplastics and food emulsifiers [44, 45], allergens such as house dust [46], microbiota's flora, surfactants, and dietary factors [47]. For a detailed review, we refer the reader to [41].

The intestinal commensal is exposed to the host's immune system in various organs due to epithelial intestinal barrier leakage and autoimmune diseases. It has been observed that few of the GI intestinal epithelial cells (IECs) are essential for maintaining intestinal homeostasis and in the function of the intestinal epithelium, as well as participating in IBD pathogenesis [48]. There is collective evidence about the role of the epithelial barrier in EoE. It is reported that EoE–linked calpain 14 is an IL-13–induced protease that mediates esophageal epithelial barrier impairment [49]. There is also a reported role of TGF-β1 in the alterial esophageal epithelial barrier function by attenuation of claudin-7 in EoE [50]. The role of epithelial barrier dysfunction is well established in EoE and we refer the reader for in-depth scope review [51]. In ADs, there is an association between leaky gut and the development of AD. For instance, in CD, an increase in the number of apoptotic IEC in the peritoneal mucosa is reported as well as impaired epithelial barrier function [52]. It is reported that epithelial barrier is dysfunctional through TJs defects [52]. This is a growing area of research and by shedding the light more on the relationship between epithelial barrier dysfunction and what is known as leaky gut syndrome, the association between it and GI autoimmune diseases if confirmed can provide a therapeutic route in the treatment and prevention of GI autoimmune diseases.
