Objectives: To examine effects of histamine on equine eosinophil adherence in vitro and to determine the histamine receptor subtype(s) and cell surface adhesion molecules that mediate this response. In addition, to determine the receptor subtypes involved in histamine-induced eosinophil migration.
Animals: 8 healthy ponies.
Procedure: Effects of histamine on equine eosinophil adherence to serum- or fibronectin-coated plastic, and migration in a microchemotaxis assay were examined. In some experiments, eosinophils were pretreated with histamine receptor antagonists or monoclonal antibodies raised against cell adhesion molecules. For comparison, the effect of histamine on equine neutrophil adherence and migration was studied.
Results: Histamine induced adherence of equine eosinophils, but not neutrophils, to serum- and fibronectin-coated plastic (P < 0.01). Histamine also caused migration of equine eosinophils, but not neutrophils (P < 0.01). Histamine-induced adherence and migration of equine eosinophils were inhibited by histamine, (H,)-receptor antagonists chlorpheniramine and mepyramine (P < 0.01), but not H2- or H3-receptor antagonists cimetidine and thioperamide. Monoclonal antibodies raised against CD18, but not very late antigen 4, reduced histamine-induced equine eosinophil adherence to serum- and fibronectin-coated plastic (P < 0.01).
Conclusions: When released from mast cells or basophils, histamine could stimulate adherence and migration of equine eosinophils via H, receptor activation and induce adherence of equine eosinophils to opsonized surfaces or dermal connective tissue matrix proteins via CD18 activation.
Clinical relevance: Histamine may have a part in regulating equine eosinophil function during parasitic killing or antigen-induced responses in horses with insect hypersensitivity.