Identifying the potential origin of mucin in primary cutaneous mucinoses-A retrospective study and analysis using histopathology and multiplex fluorescence staining.

Background: Primary cutaneous mucinoses (PCM) are rare diseases characterized by dermal or follicular mucin deposits.
Objectives: A retrospective study characterizing PCM to compare dermal with follicular mucin to identify its potential origin on a single-cell level.
Material and Methods: Patients diagnosed with PCM between 2010 and 2020 at our department were included in this study. Biopsy specimens were stained using conventional mucin stains (Alcian blue, PAS) and MUC1 immunohistochemical staining. Multiplex fluorescence staining (MFS) was used to investigate which cells were associated with MUC1 expression in select cases.
Results: Thirty-one patients with PCM were included, 14 with follicular mucinosis (FM), 8 with reticular erythematous mucinosis, 2 with scleredema, 6 with pretibial myxedema and one patient with lichen myxedematosus. In all 31 specimens, mucin stained positive for Alcian blue and negative for PAS. In FM, mucin deposition was exclusively found in hair follicles and sebaceous glands. None of the other entities showed mucin deposits in follicular epithelial structures. Using MFS, all cases showed CD4+ and CD8+ T cells, tissue histiocytes, fibroblasts and pan-cytokeratin+ cells. These cells expressed MUC1 at different intensities. MUC1 expression in tissue histiocytes, fibroblasts, CD4+ and CD8+ T cells, and follicular epithelial cells of FM was significantly higher than the same cell types in the dermal mucinoses (p < 0.001). CD8+ T cells were significantly more involved in expression of MUC1 than all other analysed cell types in FM. This finding was also significant in comparison with dermal mucinoses.
Conclusion: Various cell types seem to contribute to mucin production in PCM. Using MFS, we showed that CD8+ T cells seem to be more involved in the production of mucin in FM than in dermal mucinoses, which could indicate that mucin in dermal and follicular epithelial mucinoses have different origins.
(© 2023 The Authors. Journal of the European Academy of Dermatology and Venereology published by John Wiley & Sons Ltd on behalf of European Academy of Dermatology and Venereology.)