Mendelian Randomization and Experimental Validation Identify Key Immune Signatures in Bullous Pemphigoid.

Objective: This study aims to investigate the potential causal relationships between genetically determined peripheral immunophenotypes and bullous pemphigoid (BP). Methods: Leveraging publicly available genetic data from GWAS of 731 immunophenotypes and BP, we assessed potential associations between immunophenotypes and BP risk using Mendelian randomization (MR) and scRNA-seq of eQTLs analysis. The IVW method with FDR-adjusted q-values was the primary tool for estimating causal effects, supplemented by MR-Egger, weighted mode, weighted median, and simple mode for further investigation. LOO analysis, MR-PRESSO, and MR pleiotropy residual sum were used to ensure result robustness, exclude horizontal pleiotropy outliers, and assess heterogeneity. Single-cell sequencing data were used to explore transcriptional correlates of the MR-identified signatures, and flow cytometry was used to examine their changes in the circulation of BP-like mice. Results: Two-sample MR identified 52 potential immune-related phenotypes associated with BP, of which monocyte-related signatures remained significant after false discovery rate (FDR) correction. Monocytic myeloid-derived suppressor cells (M-MDSCs), CD16 (FcγRIII) on CD14-CD16 + monocytes, and CD62L on monocytes were identified as having a potential causal association with BP, (M-MDSCs: OR=1.69, 95% CI=1.35-2.13, q=0.002; CD16: OR=0.83, 95% CI =0.75-0.92, q=0.038; CD62L: OR=0.53, 95% CI=0.39-0.72, q=0.011). scRNA-seq revealed that CD16 and CD62L expression was significantly upregulated in neutrophils and dendritic cells of BP patients; furthermore, Functional enrichment analysis showed these immune cells are involved in pathogen recognition and inflammatory response. In BP-like mice, flow cytometry detected an increase in M-MDSCs and a decrease in CD16 (FcγRIII) on CD14-CD16+ monocytes in the circulation; CD62L was not assessed in the murine model. Conclusion: These findings suggest that M-MDSC absolute count is positively associated with BP risk, while CD62L expression on monocytes and CD16 expression on CD14-CD16+ monocytes are negatively associated. These immune signatures are potential contributors to BP susceptibility that require further mechanistic validation. However, the BP case cohort included in the GWAS dataset had a limited sample size; future mechanistic studies are needed to elucidate their specific underlying functional mechanisms.