Increased IL-4 and decreased regulatory cytokine production following relocation of Icelandic horses from a high to low endoparasite environment.

This study examines the immune response of Icelandic horses when relocated to areas where they are exposed to a certain type of parasitic insect that their immune systems are not accustomed to. The researchers found that the horses, while carrying a high parasite load, appeared to have a low amount of a specific immune cell (IL-4) which was suppressed by other regulatory elements of the immune response, potentially due to their exposure to a high level of helminth (worm) infections in their native environment.

Understanding the Research

• The study investigates Insect bite hypersensitivity (IBH), an allergic reaction in horses that is triggered by the bites of a particular species of midge, belonging to the Culicoides group. This allergic reaction is known to be virtually absent in Icelandic horses when they are in their native country, presumably due to the absence of Culicoides there.
• The phenomenon of interest is that when these horses are moved to parts of continental Europe where the midge does exist, more than 50% of first-generation relocated horses exhibit IBH, while less than 10% of their second-generation offspring (born in Europe) show the symptoms, suggesting some form of adaptation or altered immune response over generations.
• Upon examining the immune responses in the horses’ blood, it was found that the first-generation horses were producing a higher concentrate of IL-4. This is a type of cell-signalling protein, known as a cytokine, often produced during allergies and infections from parasites.

Horses in Iceland and Switzerland: A Comparison

• The study sought to investigate the reasons behind this by comparing parasite loads and T-cell responses between Icelandic horses living in their native country and those living in Switzerland.
• The horses in Iceland showed higher overall parasite loads (including tapeworms), indicated by faecal egg counts and blood serum IgE levels, yet displayed a low proportion of IL-4 production. This contrasted with previous findings suggesting allergies and parasite infections often induce increased IL-4 production.
• No IL-4-producing cells were identified in the horses’ blood cells when stimulated with Culicoides extract. By adding anti-inflammatory cells (IL-10 and TGF-beta1) to the cultures of horse cells, the production of IL-4 increased after exposure to polyclonal or parasite antigens, but not to Culicoides extracts — these regulatory cells were detected in high numbers in horses living in Iceland.

Conclusions and Implications

• This research adds to the understanding of the complex immune reactions in horses, particularly in the context of movement between different environments.
• The findings suggest that the heavy helminth infections experienced by horses living in Iceland may suppress IL-4 production, mediated by the regulatory cells IL-10 and TGF-beta1. This may in turn affect the capacity of horses to respond to new parasites like Culicoides, and increase their susceptibility to IBH when moved to new environments.