Seasonal differences in cytokine expression in the skin of Shetland ponies suffering from insect bite hypersensitivity.

Insect bite hypersensitivity (IBH) in horses is a seasonal, IgE-mediated, pruritic skin disorder primarily caused by Culicoides spp. We hypothesize that a mixed Th2/Th1-type immune status, off season, alters into Th2-dominated immune reactivity in the skin of IBH-affected ponies in the IBH season. To study these immune response patterns Culicoides-specific IgE levels, skin histopathology and cytokine and transcription factor mRNA expression (IL4, IL10, IL13, IFNγ, FoxP3 and CD3(ζ)) in lesional and non-lesional skin of ponies affected by IBH in the IBH season were compared with those of the same animals off season and those in skin of healthy ponies in both seasons. The present study revealed a significantly higher histopathology score in lesional skin of affected ponies than in non-lesional skin and skin of healthy ponies in the IBH season. Culicoides obsoletus-specific IgE serum levels of ponies with IBH were significantly higher than those in healthy ponies in both seasons. Interestingly, C. obsoletus-specific IgE serum levels within each group were the same in the IBH season and off season. The expression of IL4, IL13 and IFNγ mRNA in skin biopsies in the IBH season showed a significant increase compared to off season in both skin derived from healthy control ponies (n=14) as well as in lesional and in non-lesional skin from IBH-affected animals (n=17). This apparently general up-regulation of cytokine expression during the IBH season directly correlated with an increased CD3(ζ) mRNA expression in the skin, indicating an overall increased T cell influx during the summer months. The only significant difference observed between lesional skin from IBH-affected animals as compared to skin from healthy control animals in the IBH season was a lower expression of IL13/CD3(ζ) in the affected animals. FoxP3 and IL10 levels were unaffected, except for a lower expression of FoxP3 in healthy control skin in the IBH season as compared to off season, In addition, the increased level of C. obsoletus-specific IgE did not correlate with higher histological scores in LE skin. In summary, our data indicate a general immune activation in the skin of both healthy and IBH-affected ponies during the IBH season that potentially obscures the Culicoides-specific immune reaction pattern, even in lesional skin of IBH-affected animals.