Cause of Lichen Planus
Lichen planus is a chronic T-cell mediated autoimmune inflammatory disease of unknown etiology, though multiple triggers and associations have been identified including immune dysregulation, genetic factors, infections (particularly hepatitis C virus in certain geographic regions), drugs, stress, and contact allergens.
Pathophysiology
The disease fundamentally involves T-cell autoimmunity targeting keratinocytes as the primary mechanism 1. The inflammatory response is triggered when altered self-antigens on basal keratinocyte surfaces—potentially modified by viruses, drugs, or other factors—become targets of the T-cell response 1. Recent research demonstrates that the cGAS-STING-TBK1 pathway activation in γδ T cells, triggered by aberrant accumulation of cell-free DNA, promotes inflammatory responses in oral lichen planus 2.
Primary Triggers and Associations
Autoimmune Mechanisms
- Immune dysregulation is the most accepted theory, with organ-specific antibodies found in affected patients 3
- Increased incidence of other autoimmune conditions (diabetes, vitiligo, alopecia, thyroid disease) occurs in 22-60% of patients 3
- Genetic associations with HLA class II antigens are documented 4
Infectious Triggers
Hepatitis C virus (HCV) shows the strongest infectious association, particularly for oral lichen planus, though this varies significantly by geographic region 5, 6, 7. The association results from T-cell mediated autoimmune reactions 5. However, type-1 interferons are major drivers of lichenoid inflammation, which explains why interferon-based antiviral therapy can worsen or trigger lichen planus 5.
Other investigated but less established infectious agents include:
- Human papillomavirus (controversial, with conflicting evidence) 3
- Borrelia burgdorferi (association remains controversial, particularly outside Europe) 4, 3
- Intratissue bacteria have been detected within oral lichen planus lesions and infiltrated T cells, with decreased Streptococcus and increased gingivitis/periodontitis-associated bacteria 8
Drug-Induced Lichenoid Reactions
Numerous medications can trigger lichenoid reactions that are clinically and histologically indistinguishable from idiopathic lichen planus 1. Immune checkpoint inhibitors (ICIs) represent an emerging and well-documented cause, with up to 6% of ICI-treated patients developing lichen planus or lichenoid disease, typically 6-12 weeks after treatment initiation 9.
Physical and Environmental Triggers
- Trauma and Koebnerization: Lichen planus can appear at sites of trauma, surgical wounds, following radiotherapy, and after sunburn 4, 3
- Contact allergens: Dental amalgam and other contact allergens may cause lichenoid reactions 1
- Emotional stress: Reported as a trigger by 77% of patients with oral lichen planus 10
Dietary and Allergic Triggers
Food allergens can trigger flares in oral lichen planus, with approximately 51% of patients reporting triggers overlapping with oral allergy syndrome 10, 11. Trigger avoidance resulted in symptom improvement in 89% and decreased flare frequency in 70% of patients who actively avoided known triggers 10.
Important Clinical Caveats
- Sexual abuse may trigger lichen sclerosus (a different condition often confused with lichen planus) through trauma-induced Koebnerization, though the two conditions should not be conflated 3, 12
- The exact etiology remains unknown despite these identified associations 3, 7
- Hormonal influences have been postulated but not definitively proven, with no clear association with pregnancy, contraceptive use, or hormone replacement 3
- A family history is reported in 12% of patients, suggesting genetic predisposition 4
Practical Implications
When evaluating patients with lichen planus:
- Screen for hepatitis C, particularly in endemic areas
- Review medication history for potential drug-induced causes
- Assess for other autoimmune conditions, especially thyroid disease
- Consider recent trauma or stress as potential triggers
- In oral lichen planus, evaluate for food allergen triggers and consider trial of trigger avoidance 10, 11
- For ICI-related cases, onset typically occurs 6-12 weeks post-treatment initiation 9