Culture supernatants from human-derived commensal bacteria alleviate DNCB-induced atopic dermatitis through modulation of inflammatory and barrier-associated pathways

Introduction: Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterized by immune dysregulation, impaired epidermal barrier function, and recurrent episodes of itching and inflammation. Emerging evidence suggests that skin-resident microbiota influence host immune responses and may modulate AD pathogenesis. Here, we investigated the anti-inflammatory, barrier-restoring, and neuro-supportive effects of culture supernatants (CSs) derived from skin-resident bacteria. Methods: Human keratinocytes (HaCaT) stimulated with tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) were treated with CSs from various isolates. For in vivo evaluation, a 2,4-dinitrochlorobenzene (DNCB)-induced AD-like mouse model was utilized, receiving topical applications of the CSs. Furthermore, differentiated SH-SY5Y neuronal cells were treated with keratinocyte- or fibroblast-conditioned media, prepared after stimulation with bacterial CSs, to evaluate their neurotrophic potential. Results: CSs from Brachybacterium paraconglomeratum and Brevibacterium casei significantly suppressed interleukin-6 (IL-6) and C-C motif chemokine ligand 17 (CCL17) while restoring filaggrin expression. In keratinocytes and human dermal fibroblasts, these CSs increased brain-derived neurotrophic factor (BDNF) expression. In the DNCB-induced AD-like mouse model, topical application of B. paraconglomeratum and B. casei CSs reduced epidermal hyperplasia and immune cell infiltration, downregulated tyrosine hydroxylase (TH), and restored cutaneous BDNF, glial cell line-derived neurotrophic factor (GDNF), and filaggrin (FLG) expression. In differentiated SH-SY5Y neuronal cells, the conditioned media treatments markedly upregulated BDNF, GDNF, and nerve growth factor (NGF). Mechanistically, CS treatment inhibited p38 MAPK and JAK-STAT signaling. Discussion: Collectively, these findings demonstrate that specific skin-derived bacterial metabolites exert coordinated anti-inflammatory, barrier-reinforcing, and neurotrophic activities, thereby promoting associated changes in neurotrophic markers. Such microbial products may serve as promising biologic candidates for managing atopic dermatitis.