Complement profiling and neutrophil single-cell analysis reveal inflammation via the classical complement pathway in Behçet’s syndrome

Behçet’s syndrome (BS) is a systemic immune-mediated disorder characterised by recurrent mucocutaneous, skin, and ocular inflammation, with severe neurological, vascular, or gastrointestinal involvement in some individuals. Although neutrophilic inflammation is considered central to BS pathogenesis, the mechanisms linking immune activation, complement pathways, and tissue inflammation remain unclear. Serum immune profiles from individuals with BS and healthy donors were assessed by measuring complement-related components, cytokines, chemokines, and inflammatory markers. Single-cell RNA sequencing was performed on fresh white blood cells containing neutrophils and cerebrospinal fluid (CSF) cells. Acutely inflamed skin tissue was examined using histological and immunohistochemical analyses. Comprehensive serum profiling revealed increased levels of complement activation products in BS, particularly during active disease. Active C1s—a marker of classical complement pathway activation—was elevated in BS sera and correlated with C-reactive protein levels. Single-cell transcriptomic analyses identified expansion of a neutrophil subset enriched for type II interferon-stimulated gene signatures. Monocyte analyses revealed increased complement receptor expression in an EGR1-high CD14 monocyte subset and elevated C1q-related gene expression in HLA-DR-expressing monocytes with strong type II interferon signatures. CSF macrophages also exhibited complement-related transcriptional features, including increased C1q and complement receptor expression. Immunohistochemical analysis of BS skin tissue showed C1q and C3 deposition around small vessels with neutrophilic infiltration. These findings suggest that activation of the classical complement pathway is linked to neutrophilic inflammation in BS and may promote tissue inflammation through interactions among inflammatory cytokine signalling, monocyte activation, and complement-related pathways. This integrated multimodal analysis clarifies the cellular context of complement activation in BS and supports further investigation of complement-targeted therapeutic strategies.