Oral mucosal pellicle as an immune protection against micro-organisms in patients with recurrent aphthous stomatitis: A hypothesis

Abstract Recurrent Aphthous Stomatitis (RAS) is the most common ulcerative diseases of oral mucosa affecting an estimate of 20% of the world’s population. Majority of the people affected by RAS are under 30 years of age. RAS is located on the lining (non-keratinized) oral mucosa, i.e. buccal mucosa, lateral side of the tongue, soft palate, lip mucosa, or the floor of mouth. An aphthous ulcer develops when lymphocytic cells infiltrate into the epithelium and cause an edema due to transient inflammatory stimuli. Bacteria, viruses and fungi have been suggested to cause aphthous lesions, but findings regarding oral pathogens are conflicting. Prior consensus has been that RAS is a multifactorial condition, with microbes, allergies, nutritional deficiencies, genetic factors, certain illnesses, immunodeficiency, hormonal changes, trauma and stress among others, contributing to the condition. In spite of many suggestions and investigations, the etiology and pathophysiology of RAS remains uncertain. Our hypothesis focuses on mucin proteins that have been shown to play a role in the formation of protective mucosal pellicle, which serves as the first line of defense between oral epithelium and pathogens within the oral cavity. Mucins, including transmembrane mucin 1 (MUC1), and salivary mucins MUC5B and MUC7 form a protein network that is strongly retained to oral epithelium. The role of the mucosal pellicle in pathophysiology of RAS is unknown. Structural variations have been found in the salivary MUC7 terminal end oligosaccharides in RAS patients, rendering the protein unable to agglutinate pathogens. Furthermore, low levels of MUC1 fail to provide a scaffold for assembly of salivary mucins. We introduce a new hypothesis, the alterations in the structure of these glycoproteins could have a profound impact on the oral mucosal barrier function. On the other hand, micro-organisms secreting their mucolytic enzymes destroy the mucosal pellicle causing oral ulcers.