HISTAMINE-INDUCED ADHERENCE AND MIGRATION OF EQUINE EOSINOPHILS

Objectives-To examine effects of histamine on equine eosinophil adhere nce in vitro and to determine the histamine receptor subtype(s) and ce ll surface adhesion molecules that mediate this response. In addition, to determine the receptor subtypes involved in histamine-induced eosi nophil migration. Animals-8 healthy ponies. Procedure-Effects of hista mine on equine eosinophil adherence to serum- or fibronectin-coated pl astic, and migration in a microchemotaxis assay were examined. In some experiments, eosinophils were pretreated with histamine receptor anta gonists or monoclonal antibodies raised against cell adhesion molecule s. For comparison, the effect of histamine on equine neutrophil adhere nce and migration was studied. Results-Histamine induced adherence of equine eosinophils, but not neutrophils, to serum- and fibronectin-coa ted plastic (P < 0.01). Histamine also caused migration of equine eosi nophils, but not neutrophils (P < 0.01). Histamine-induced adherence a nd migration of equine eosinophils were inhibited by histamine, (H-1)- receptor antagonists chlorpheniramine and mepyramine (P.< 0.01), but n ot H-2- or H-3-receptor antagonists cimetidine and thioperamide. Monoc lonal antibodies raised against CD18, but not very late antigen 4, red uced histamine-induced equine eosinophil adherence to serum- and fibro nectin-coated plastic (P < 0.01). Conclusions-When released from mast cells or basophils, histamine could stimulate adherence and migration of equine eosinophils via H-1 receptor activation and induce adherence of equine eosinophils to opsonized surfaces or dermal connective tiss ue matrix proteins via CD18 activation. Clinical Relevance-Histamine m ay have a part in regulating equine eosinophil function during parasit ic killing or antigen-induced responses in horses with insect hypersen sitivity.