Eosinophil-mediated killing of Haemonchus contortus larvae: effect of eosinophil activation and role of antibody, complement and interleukin-5

Eosinophils have previously been shown to accumulate around the tissue inva sive (L3) stage of sheep gastrointestinal parasites in vivo. In this study, eosinophils obtained from mammary washes of sheep, were shown to immobiliz e and kill H. contortus larvae in vitro in the presence of antibody specifi c against a defined L3 surface antigen. Eosinophils obtained from sheep pri med by repeated infusion of H.contortus larvae were more effective than eos inophils obtained after a single infusion of parasite extract in Fasciola h epatica infected ewes suggesting the former were activated in vivo. The lev el of larval immobilization in the presence of antibody was significantly i ncreased when complement was added to cultures containing activated eosinop hils. The addition of interleukin-5 to larval cultures containing antibody and complement resulted in a significant increase in larval immobilization with unactivated eosinophils suggesting that eosinophil effector function i s enhanced following priming with this cytokine. Ultrastructural analysis o f the eosinophil/larvae interaction at 6h of incubation revealed degranulat ion of adhering eosinophils onto the surface of larvae. By 24h of incubatio n, many larvae showed signs of damage and most eosinophils had degenerated. These results suggest that eosinophil-mediated killing may be an effector mechanism for the elimination of L3 H, contortus larvae in immune sheep.