High-throughput proteomics uncovers molecular clusters and biomarkers of severity in bullous pemphigoid

Background: Bullous pemphigoid is a chronic autoimmune blistering disease characterized by subepidermal blisters and caused by autoantibodies against BP180 and BP230. Although type 2 inflammation is considered relevant to its pathogenesis, the full spectrum of immune dysregulation in BP remains incompletely defined. Objectives: To comprehensively profile local and systemic inflammatory responses in BP and explore their potential associations with disease severity, clinical features and treatment outcomes. Methods: A total of 29 skin biopsies and 27 matched serum samples from patients with bullous pemphigoid, along with samples from 14 healthy controls, were analysed using the Olink Target 96 Inflammation panel. Analyses included principal component analysis, unsupervised clustering, differential expression, gene set enrichment and k-means clustering to define molecular subgroups. Proteomic data were further compared with a publicly available single-cell transcriptomic dataset from bullous pemphigoid skin for external validation. Results: Proteomic analysis revealed distinct inflammatory signatures in bullous pemphigoid skin and sera compared to controls, with 22 and 16 differentially expressed proteins, respectively. CCL13, IL-6, OSM, TNFSF14 and CCL19 were significantly elevated in both compartments. Cutaneous CCL13, expressed mainly by keratinocytes, strongly correlated with the bullous pemphigoid disease activity index score and autoantibody titres. Two molecular skin clusters were identified, with one showing broader, predominantly type 2 inflammatory activation and significantly higher titres of BP180-IgG antibodies. CCL13 was a major driver of cluster separation. Higher baseline serum IL22RA1 levels were observed in non-responder patients at 1-year follow-up. Proteomic findings showed high concordance with transcriptomic data derived from publicly available single-cell RNA sequencing of bullous pemphigoid lesional skin. Conclusions: CCL13 was among the most prominent inflammatory proteins identified in bullous pemphigoid. The emergence of two distinct skin proteomic clusters highlights a biological heterogeneity of bullous pemphigoid, indicating the need for endotype-driven, personalized treatment strategies.