Characterisation of lymphocyte subpopulations in the skin and circulation of horses with sweet itch (Culicoides hypersensitivity)

Circulating lymphocyte numbers are elevated in horses with the allergic ski n disease sweet itch and skin lesions are typified by an infiltrate of eosi nophils and mononuclear cells, the latter of which have not been fully char acterised, The aim of the present study was to characterise the lymphocyte subpopulations in the circulation and skin of ponies with sweet itch by flo w cytometry and a newly developed modified alkaline phosphatase immunohisto chemical technique, Sweet itch ponies were found to have significantly grea ter numbers of circulating CD5+ and CD4+ T-lymphocytes than normal animals. Increased numbers of CD3+ T-lymphocytes, most of which were CD4+, and eosi nophils were present in the skin of these animals following intradermal inj ection of a Culicoides antigen extract (97 +/- 21 vs. 449 +/- 49 CD3+ T-lym phocytes/mm(2) in deep dermis of vehicle vs. antigen injected sites; 83 +/- 8% CD4+ T-lymphocytes at antigen injected site), T-lymphocytes, which are thought to be important in the pathogenesis of human allergic skin disease, may therefore contribute to the development of sweet itch lesions via the release of cytokines which can cause eosinophil accumulation and activation . An understanding of the pathology of this disease may lead to a more rati onal approach to therapy.