The immunological mechanisms explaining development of an allergy in some individuals
and not in others remain incompletely understood. Insect bite hypersensitivity (IBH) is a
common, seasonal, IgE-mediated, pruritic skin disorder that affects considerable proportions
of horses of different breeds, which is caused by bites of the insect Culicoides obsoletus
(C. obsoletus).We investigated the allergen-specific immune status of individual horses
that had either been diagnosed to be healthy or to suffer of IBH. Following intradermal allergen
injection, skin biopsies were taken of IBH-affected and healthy ponies and cytokine expression
was determined by RT-PCR. In addition, allergen-specific antibody titers were
measured and cytokine expression of in vitro stimulated, allergen-specific CD4 T-cells was
determined. 24 hrs after allergen injection, a significant increase in mRNA expression of the
type-2 cytokine IL-4 was observed in the skin of IBH-affected Shetland ponies. In the skin of
healthy ponies, however, an increase in IFNγ mRNA expression was found. Analysis of allergen-
specific antibody titers revealed that all animals produced allergen-specific antibodies,
and allergen-specific stimulation of CD4 T-cells revealed a significant higher
percentage of IFNγ-expressing CD4 T-cells in healthy ponies compared to IBH-affected ponies.
These data indicate that horses not affected by IBH, in contrast to the so far established
dogma, are not immunologically ignorant but have a Th1-skewed allergen-specific
immune response that appears to protect against IBH-associated symptoms. To our knowledge
this is the first demonstration of a natural situation, in which an allergen-specific immune
skewing is protective in an allergic disorder.
S1 Table. Primers and probes as well as the PCR conditions used for qRT-PCR .
S2 Table. Histological scores from healthy and IBH-affected ponies at different time points
after allergen injection. Paraffin-embedded biopsies were cut into sections and stained with
either haematoxylin-eosin (HE). Sections were graded according to a semi-quantitative grading
system (0 = absent, 1 = minimal, 2 = mild, 3 = moderate, 4 = severe) as previously described 12;
average and range between brackets.