Epidermal HLA-DQB1 exerts negative regulation of mast cell activation through the TSLP-driven SPP1-expressing macrophage axis.

BACKGROUND: Keratinocytes(KCs) and the major histocompatibility complex class II (MHC-II) are implicated in inflammatory skin diseases. However, the precise expression profiles and causal role of keratinocyte -intrinsic MHC-II in urticaria remain largely undefined. OBJECTIVE: To investigate the causal relationship and functional mechanism of epidermal MHC-II in IgE-mediated skin inflammation. METHODS: Mendelian randomization (MR) and immunohistochemistry were employed to evaluate the genetic and clinical relevance of MHC-II in urticaria. A keratinocyte--specific H2-Ab1 knockout mouse model(K14Cre/+H2-Ab1fl/f) was generated, and IgE induced passive cutaneous anaphylaxis(PCA) was assessed. Functional validation was performed using pharmacologic activation/inhibition. Immune responses and identify downstream signaling pathways were characterized using flow cytometry, single-cell RNA sequencing(scRNA-seq), chromatin immunoprecipitation (ChIP) assay, RNA interference, luciferase reporter gene assays, and western blotting. RESULTS: MR analysis identified a protective causal effect of skin HLA-DQB1 expression on urticaria risk. Epidermal MHC-II levels negatively correlated with disease severity and positively correlated with disease control scores in patients. Epidermal H2-Ab1 knockout exacerbated PCA responses in mice. Mechanistically, deletion of HLA-DQB1/H2-Ab1 in KCs reduced repressive H3K9me3 histone methylation at the TSLP promoter via downregulation of Suv39h1, leading to TSLP overexpression. TSLP promoted STAT5-dependent secreted phosphoprotein 1(SPP1) expression in macrophages, polarizing a pro-inflammatory SPP1+ subset. SPP1 directly activated mast cells, enhancing degranulation and inflammatory mediator release. This KC-macrophage-MC axis was validated in co-culture systems. CONCLUSION: Epidermal HLA-DQB1/H2-Ab1 negatively regulates IgE-mediated skin inflammation by epigenetically suppressing TSLP expression, thereby restraining a TSLP-SPP1+ macrophage-MC activation cascade. These findings reveal a novel immunoregulatory function of keratinocyte MHC-II and highlight the HLA-DQB1-TSLP-SPP1+ macrophage axis as a potential therapeutic target in urticaria.