Equine keratinocytes in the pathogenesis of insect bite hypersensitivity: Just another brick in the wall?

Abstract

Equine insect bite hypersensitivity (IBH) is the most common skin disease affecting horses. It is described as an IgE-mediated, Type I hypersensitivity reaction to salivary gland proteins of Culicoides insects. Together with Th2 cells, epithelial barrier cells play an important role in development of Type I hypersensitivities. In order to elucidate the role of equine keratinocytes in development of IBH, we stimulated keratinocytes derived from IBH-affected (IBH-KER) (n = 9) and healthy horses (H-KER) (n = 9) with Culicoides recombinant allergens and extract, allergic cytokine milieu (ACM) and a Toll like receptor ligand 1/2 (TLR-1/2-L) and investigated their transcriptomes. Stimulation of keratinocytes with Culicoides allergens did not induce transcriptional changes. However, when stimulated with allergic cytokine milieu, their gene expression significantly changed. We found upregulation of genes encoding for CCL5, -11, -20, -27 and interleukins such as IL31. We also found a strong downregulation of genes such as SCEL and KRT16 involved in the formation of epithelial barrier. Following stimulation with TLR-1/2-L, keratinocytes significantly upregulated expression of genes affecting Toll like receptor and NOD-receptor signaling pathway as well as NF-kappa B signaling pathway, among others. The transcriptomes of IBH-KER and H-KER were very similar: without stimulations they only differed in one gene (CTSL); following stimulation with allergic cytokine milieu we found only 23 differentially expressed genes (e.g. CXCL10 and 11) and following stimulation with TLR-1/2-L they only differed by expression of seven genes. Our data suggests that keratinocytes contribute to the innate immune response and are able to elicit responses to different stimuli, possibly playing a role in the pathogenesis of IBH.

Cytokines and chemokines skin gene expression in correlation with immune cells in blood and severity in equine insect bite hypersensitivity

“…Indeed, TSLP and IL-33 were both expressed in lesional IBH biopsies and levels correlated with disease severity with high sensitivity and specificity. Also, TSLP has been confirmed in earlier studies to play an important role in IBH by mRNA quantification either in lesional IBH biopsies ( 17 ) or in culicoides allergen-stimulated equine primary keratinocytes ( 37 , 76 , 77 ). The scratching of insect bites seemed to cause early skin barrier defects and related TSLP upregulation, which may be important in priming a type 2 response.…”

“…CCR5 is expressed on Th1-type lymphocytes, monocytes/macrophages, basophils, eosinophils, and dendritic cells (DCs) when they are in the immature stage ( 36 ) and may play a role in innate immune cell migration, particularly in the migration of eosinophils. CXCL10 and CCR5 have been described earlier in the context of IBH in a study comparing RNAseq data in IL-4/-5/-13 and TNFα-stimulated equine keratinocytes derived from IBH and healthy horses ( 37 ). Finally, eotaxins have also been described in the context of IBH and, as human analogs are responsible for the recruitment, accumulation, and activation of eosinophils in the skin ( 38 , 39 ), with CCL26 potentially being involved in the transepithelial migration of eosinophils ( 40 ).…”

Insect Bite Hypersensitivity in Horses: Causes, Diagnosis, Scoring and New Therapies

“…Horses with IBH have been found to have impairment of the epithelial barrier, which can predispose them to the development of IBH [26,27]. Keratinocytes also contribute to the innate immune response, which may also play a role in the pathogenesis of IBH [27].…”