Regulation of Immune Cells in Oral Lichen Planus

Oral mucosal lichen planus (OMLP) is a common immunological disease with significant morbidity. Objectives: The aims of this study were: to compare the numbers and distribution of the recently described T-helper (Th) lymphocyte subsets, T regulatory cells (Tregs, FoxP3⁺) and Th-17 cells (IL-17⁺), known to control some immune reactions in OMLP, and to determine which cell types expressed FoxP3 and/or IL-17. The hypothesis was that FoxP3⁺ and IL-17⁺ cells regulate the immune response in OMLP. Methods: Ethical approval was obtained. Immunohistochemistry was used to investigate the presence of FoxP3⁺ or IL-17⁺ cells in 12 inflammatory control sections and 17 sections of OMLP. Double-labelling immunofluorescence (IF) was used to determine the type of cell expressing FoxP3/IL-17. Student’s t test was used to determine the statistical significance of the quantitative results, with p<0.05 considered statistically significant. Intra-and inter-observer differences were assessed and these results were compared using Pearson’s chi-squared test. Results: OMLP displayed significantly more FoxP3⁺ cells (mean 79.3 vs. 20.6 cells/area, p<0.05) and fewer IL-17⁺ cells (mean 1.05 vs. 3.30 cells/area, p<0.05) than non-specific inflammatory cases. The majority of FoxP3⁺ cells were in the sub-epithelial infiltrate, while IL-17⁺ cells were deeper in the stromal tissues. IF showed that FoxP3⁺ cells co-localised with T cells, while the IL-17⁺ cells did not, rather they morphologically resembled mast cells. All IL-17⁺ cells were positive for mast cell tryptase but not all cells with the appearance of mast cells were positive for IL-17.  Conclusions: The higher proportion of FoxP3⁺ cells in OMLP supports the hypothesis that these cells are involved in the immune response of the disease, while the down-regulation of IL-17⁺ cells could also be important in the immune control of OMLP. The identification of IL-17⁺ cells as mast cells in OMLP rather than T lymphocytes as previously considered is also of relevance.