Induction of tissue eosinophilia by platelet-activating factor in Merino sheep

A study was undertaken on the capacity of platelet-activating factor (PAF) to induce eosinophil accumulation in the mammary glands of non-lactating sheep. Platelet-activating factor induced dose-dependent accumulation of eosinophils in mammary exudates 24 h after infusion. Infection, by intraruminal injection of 20,000 infective Trichostrongylus colubriformis larvae, did not modify the responsiveness of outbred sheep to intramammary infusion of PAF. Mature ewes from high and low responder lines of a flock of sheep, selected on the basis of their responses to vaccination and experimental challenge with T. colubriformis as lambs, did not differ in the magnitude of the eosinophil responses to doses of PAF from 5 x 10(-13) to 5 x 10(-7) mol per gland. Intramammary infusion of an extract from third stage larvae of Haemonchus contortus elicited inflammatory exudates containing five- to ten-fold more eosinophils than that elicited by the highest dose of PAF tested. The experiments indicate that the eosinophil chemotactic agonist PAF can induce tissue eosinophilia in sheep and thus may play a role in directing the accumulation of eosinophils in tissues during disease states such as gastrointestinal parasitism.