Interdisciplinary Aspects of Primary Sjogren's Syndrome

**3**

**Chapter 1**

Cholangitis

*Maria Maślińska*

**1. Introduction**

Introductory Chapter:

Autoimmune Epithelitis -

Discussion about Sjögren's

course depends on particular genetic and epigenetic conditions.

immunological and main clinical features of pSS and PBC.

Syndrome and Primary Biliary

Epithelial tissue constitutes a barrier between organs and the environment. The epithelium lines external surfaces of internal organs and inner surfaces of the walls of blood vessels. It is also a tissue that exocrine and endocrine glands consist of. As they separate the organism from the outer environment, the epithelial structures form the first line of defense against external factors but, at the same time, an entry gate for them influencing the development of the body's microbiome and autoimmune diseases, which are associated with the disorders of microbiome composition (dysbiosis) [1, 2]. The epithelium, also as a target for viruses, interacts with the invading pathogens and is actively involved in immune response, whose

Epithelial cells are often subject to apoptosis, which makes them an important source of autoantigens. Moreover, in many autoimmune diseases, epithelial cells are damaged, which leads to further release and exposition of autoantigens, with the epithelium being subject to the immune response. For example, thyrocytes are responsible for providing the main immunogens (e.g., thyroglobulin, thyroid peroxidase, TSH receptor) in autoimmune thyroiditis [3], synoviocytes are a source of cyclic citrullinated peptides in rheumatoid arthritis, and oligodendrocytes in multiple sclerosis or pancreatic endocrine glandular epithelium (beta-cells) in type 1 diabetes are a source among others of proinsulin or glutamic acid decarboxylase [4]. Hence the suggestion puts forward that autoimmune diseases could be otherwise classified as the autoimmune inflammation of the epithelium [5]. However, there are autoimmune diseases which, due to their effect on the exocrine glands, are particularly associated with epithelial damage and an autoimmune process, the primary Sjögren's syndrome (pSS) among them. The primary Sjögren's syndrome is an autoimmune disease in which the exocrine glandular epithelium is a main source of autoantigens—such as Ro/SS-A and La/SS-B ribonucleoproteins [6]. Quite often pSS may coexist with another autoimmune disorder—a primary biliary cholangitis (PBC). In PBC the epithelium (biliary epithelial cells of small bile duct) is the starting point of the autoimmune process [7]. The pathogenesis of both diseases is similar, with the significant role of epithelial cell apoptosis. **Table 1** presents the
