Redistribution of peripheral blood immune cell subsets and remodeling of intercellular communication in rare Behçet's disease: a single-cell transcriptomics study.

BACKGROUND: Behçet's disease (BD) is a rare chronic systemic vasculitis of unknown etiology. The heterogeneity of peripheral blood immune cells and the dysregulation of intercellular communication in BD remain insufficiently characterized. The present study employs single-cell transcriptomics to comprehensively delineate the peripheral blood immune cell atlas of BD, identify candidate diagnostic biomarkers, and dissect the remodeling patterns of intercellular communication networks. METHODS: Single-cell RNA sequencing data of peripheral blood mononuclear cells (PBMCs) from 8 BD patients and 8 healthy controls (HCs) (GSE198616) were analyzed using Seurat v5, clusterProfiler, LASSO regression, and CellChat v1.6. Differential expression analysis at both single-cell and pseudobulk levels, GSEA, and ssGSEA were performed, with external validation carried out using two independent transcriptomic datasets (GSE61399 and GSE205867). RESULTS: Nine major immune cell types were identified, and monocytes were further subdivided into three subsets. BD patients exhibited subset-specific monocyte redistribution at the group level: CD14+ classical monocytes showed a significantly higher proportion (8.54% vs. 1.51%, p < 0.001), whereas FCGR3A+ non-classical monocytes were significantly reduced (3.04% vs. 7.23%, p < 0.001), accompanied by elevated proportions of CD8+ T cells, CD4+ T cells, plasma cells, and proliferating cells. Differential expression analysis identified 86 upregulated and 84 downregulated genes, enriched in interferon and inflammatory pathways. LASSO regression selected nine feature genes; IFI44L demonstrated moderate diagnostic potential (AUC = 0.734, 95% CI: 0.449-1, p = 0.1304). Pseudobulk validation at the individual level confirmed consistent upregulation of the core genes IFI44L, GBP1, and FCGR1A in BD. Intercellular communication analysis revealed attenuated costimulatory signaling (CD40, ICAM) in BD, whereas the ANNEXIN pathway was significantly enhanced in CD8+ T cells, plasma cells, and proliferating cells, and PECAM1 was increased in plasma cells and proliferating cells but reduced in monocytes. External validation confirmed consistent upregulation of LASSO-selected core genes in CD14+ monocytes, along with robust activation of type I interferon signaling in both CD14+ monocytes (NES = +3.04, FDR < 0.001) and CD4+ T cells (NES = +2.96, FDR < 0.001). Notably, monocytes displayed overall communication suppression despite strong transcriptional activation of interferon signaling, suggesting a subtype-specific regulatory state of being transcriptionally active yet communication-suppressed. CONCLUSION: This study delineates the peripheral blood immune landscape of BD, revealing subset-specific monocyte redistribution and subtype-specific remodeling of intercellular communication. These findings generate new hypotheses for diagnostic biomarker development and communication-targeted therapeutic strategies in BD, warranting further experimental validation in well-phenotyped cohorts.