Evaluation of delayed hypersensitivity responses in normal horses and immunodeficient foals

Delayed hypersensitivity (DH) responses of normal and immunodeficient horses were evaluated with antigens [dinitrochlorobenzene (DNCB), keyhole limpet hemocyanin (KLH)] and phytolectins [phytohemagglutinin (PHA), concanavalin A (Con A)]. Immunologically normal horses sensitized with 5 daily applications of 2 mg of DNCB developed positive skin reactions upon challenge with 0.4 mg of DNCB. The delayed onset of the reaction and the predominately mononuclear cell infiltration at the test site indicated these were DH reactions. Normal horses sensitized with 500 microgram of KLH and challenged with 100 microgram of KLH developed skin reactions appearing earlier and composed of neutrophils and mononuclear cells, suggesting involvement of both humor and cellular mechanisms. Arabian foals with combined immunodeficiency failed to respond visibly to either antigen. Intradermal injection of 50 microgram of PHA induced visible reactions in all normal horses tested. The slow development and the predominately mononuclear cell infiltrate of PHA reactions resembled DH responses to antigen. Reactions were induced by Con A in 15 of 16 normal adult horses and 7 of 14 normal foals. The reactions were rapid in onset and contained numerous eosinophils. All 6 foals with combined immunodeficiency failed to respond to PHA injection, whereas 5 of 6 did not respond to Con A. Based on the results of these studies, it appears that in vivo skin tests with PHA and DNCB can be used to screen horses for deficient T-lymphocyte responses.