Supplementary Material for: Folliculitis decalvans has a heterogeneous microbiological signature and impaired immunological response.

Background: Folliculitis decalvans (FD) is a rare primary neutrophilic scarring alopecia whose etiology has not been completely elucidated yet. Objective: To determine if the follicular microbiota residing in FD-affected hair follicles had a distinct microbiological signature and if an aberrant immune response was present in the pathogenesis of FD. Methods: We conducted a cross-sectional study of ten patients affected by FD. Trichoscopy-guided follicular biopsies were taken from affected and healthy scalp to identify the follicular microbiome using next-generation sequencing. We searched for microbiological biomarkers of FD-affected follicles using the linear discriminant analysis (LDA) effect size (LEfSe) tool. Additionally, peripheral blood mononuclear cells were obtained, and their cytokine production was quantified after incubation with pathogen-associated molecular patterns isolated from patients’ biopsies and compared with healthy controls. Results: β-diversity analysis showed statistically significant differences regarding bacteria comparing follicular microbiota of healthy and FD-affected hairs. Ruminococcaceae, Agathobacter sp., Tyzzerella sp. and Bacteriodales vadin HA21 family were good predictors of disease status. IL-10, TNF-α and IL-6 levels were significantly decreased in patients after incubation with various strains of bacteria compared with controls. Conclusion: FD hair follicles have a specific heterogenous follicular bacterial microbiota signature. Additionally, these patients seem to have an impaired immunological response.