Metagenomic Analysis of the Oral Microbiome in Patients With Oral Lichen Planus: A Pilot Study

Lichen planus is a chronic dermatological disease. It is characterized by papules that are localized in areas exposed to mechanical irritation, mostly in areas of friction from clothing, on the inner and back parts of the legs, on the inner parts of the forearms, and on the wrists. It usually manifests itself as itching on the skin. Lichen planus can be observed in 40% of cases in both the skin and oral mucosa, 35% in the skin, and 25% only in the oral mucosa. In oral lichen planus (OLP), unlike in the skin, itching does not occur. Lesions in the oral mucosa appear as white, non-exfoliating, keratinized epithelial changes consisting of fine lines resembling a net or leaf veins, surrounded by erythema. Lesions are generally asymptomatic, but different types may present with symptoms such as pain or burning.

Andreassen (21) classified the clinical types of OLP as follows:

1. Reticular: Characterized by small gray-white papules that do not separate from the mucosa when wiped. They are typically located on the cheek mucosa and are symmetrical. These papules may have a thin erythematous border, and these formations often resemble a net or leaf veins (Wickham's lines).
2. Papular: Fine, linear extensions are seen around white hyperkeratotic formations that are 0.5-1 mm in diameter and raised above the surface.
3. Hypertrophic: These are dense white formations that are raised above the surface, particularly seen in smokers. Clinically, this type of OLP is difficult to distinguish from leukoplakia. Wickham's lines around the lesion may aid in establishing a preliminary diagnosis of lichen planus.
4. Atrophic: In this type, erythema is prominent and there is thinning of the epithelium. Those localized in the gingiva may transform into pemphigoid gingivitis. It may occur in conjunction with erosive and bullous OLP. This type may show a possibility of malignant formation. e. Erosive: This is the most common clinical type of OLP, characterized by erosions covered with dense fibrin and shallow ulcers. Erosions cause a burning sensation in OLP patients. This complaint may increase with mechanical, chemical, and thermal irritation associated with food intake.

Despite significant efforts over the past two decades to determine the etiology of OLP, the exact mechanism remains unclear. Previous studies have shown that T cells are involved in the pathogenesis of the disease, but their specific roles in this process remain unclear. Generally, an exogenous antigen, autoantigen, or superantigen triggers an inflammatory response leading to apoptosis of basal epithelial keratinocytes in the oral epithelium, resulting in chronic inflammation. A more plausible theory suggests that multiple antigens, combined with various environmental factors, trigger an inflammatory process. Therefore, the etiology is expected to be multifactorial, as in many other diseases, rather than a single causal factor, which is the approach most researchers use to study OLP.

Studies specific to OLP have shown that rigorous oral hygiene and plaque control can improve gingival lichen planus lesions, suggesting that the oral microbiome may play a role in the onset or exacerbation of OLP. The role of the oral microbiome has also been examined in other inflammatory oral mucosal disorders, such as recurrent aphthous stomatitis and geographic tongue, as well as in other autoimmune diseases, including Sjögren's syndrome, rheumatoid arthritis, and systemic lupus erythematosus. Additionally, recent studies on burning mouth syndrome have also shown that the oral microbiome is effective. Although the exact role of the oral microbiome in the pathogenesis of these conditions remains unclear, it can be suggested that it may also play a role in OLP.