Engineering microbial symbiosis and dysbiosis reveals a new AhR-mediated mechanism underlying dandruff pathogenesis

BACKGROUND: The skin microbiome plays a pivotal role in regulating epidermal barrier integrity and immune homeostasis. However, the molecular mechanisms through which microbial dysbiosis drives dermatological disease and in particular, the pathways by which alterations in the scalp microbiome give rise to the pathological features of dandruff are not fully understood. OBJECTIVES: This study aimed to establish and validate microbially colonised, full-thickness human skin equivalents (HSEs) that incorporate scalp-relevant bacterial and fungal microbiome species, to dissect the molecular pathways linking microbiome composition to epidermal morphology, barrier function and skin homeostasis. METHODS: We engineered HSEs colonised with microbial consortia representing healthy (5M) and dandruff-associated (5MP) scalp microbiomes. Morphological and histological analyses were used to assess epidermal architecture and barrier integrity. Expression of key barrier proteins and enzymes involved in corneodesmosome hydrolysis was quantified. Bulk RNA-sequencing was performed to identify differentially regulated signalling pathways, followed by protein validation using immunofluorescence analysis. Key findings were further corroborated with human scalp biopsy specimens from individuals with and without dandruff. RESULTS: HSEs colonised with the 5M microbiome maintained normal epidermal morphology and expression of barrier-associated proteins. In contrast, HSEs colonised with the 5MP microbiome developed hallmark dandruff-like phenotypes, including altered epidermal morphology, reduced barrier protein expression, and abnormal corneodesmosome degradation. Transcriptomic analysis and protein validation revealed significant attenuation of the aryl hydrocarbon receptor (AhR) signalling pathway in 5MP-colonised HSEs. Consistent downregulation of AhR and associated proteins was observed in dandruff patient samples, confirming the clinical relevance. CONCLUSIONS: Microbial dysbiosis on the scalp can compromise AhR signalling. This study provides mechanistic evidence linking microbiome composition to pathological epidermal changes. The developed microbially colonised HSE model provides a versatile and clinically relevant tool for advancing our understanding of microbiome-driven skin pathology and translating mechanistic insights into precision interventions.