D1.424 - Single-cell transcriptomic profiling of allergen-restimulated PBMCs reveals innate immune activation in equine insect bite hypersensitivity

Insect bite hypersensitivity (IBH) is a common, seasonal, IgE-mediated allergic dermatitis in horses triggered by bites from Culicoides midges. Previous studies using allergen-restimulated peripheral blood mononuclear cells (PBMC) demonstrated an imbalance in the Th2/Treg immune response in IBH-affected horses. However, the broader PBMC response to allergen restimulation has not yet been comprehensively characterized in IBH. The objective was to investigate peripheral immune responses to Culicoides allergens in healthy and IBH-affected horses using a non-hypothesis-driven single-cell RNA sequencing approach.

PBMC from six healthy and six IBH-affected Icelandic horses were restimulated for 18 hours with a mixture of nine major recombinant Culicoides allergens or cultured in medium. Cells were sequenced by 10x Chromium scRNA-seq, processed with Seurat, manually annotated. Differential gene expression was assessed using edgeR. 

Following allergen stimulation, IBH-affected horses exhibited a distinct innate immune activation profile compared to healthy controls. M2-polarized macrophages, cDC2, and monocyte-derived dendritic cells showed enhanced pro-inflammatory activity, increased signaling associated with Th2 recruitment, and transcriptional features linked to tissue remodeling. These macrophages also demonstrate altered lipid handling, suggesting changes in phagocytic and metabolic functions. Within the adaptive compartment, T cells from IBH-affected horses displayed a more activated helper-type phenotype characterized by recruitment and polarization functions, whereas T cells from healthy horses retained a more cytotoxic response profile.

 Our findings reveal an early and coordinated activation of M2 macrophages and innate immune pathways in IBH, promoting Th2-skewed inflammation. These results highlight the critical interplay between innate and adaptive immunity in the pathogenesis of equine IBH.

This study was supported by the Swiss National Science Foundation grant no. 310030_208152