Histamine induces cytoskeletal changes in human eosinophils via the H4 receptor

1 Histamine (0.004-2 muM) induced a concentration-dependent shape change of human eosinophils, but not of neutrophils or basophils, detected as an increase in forward scatter (FSC) in the gated autofluorescence/forward scatter (GAFS) assay. 2 The histamine-induced eosinophil shape change was completely abolished by thioperamide (10 muM), an H-3/H-4 receptor antagonist, but was not inhibited by pyrilamine or cimetidine (10 muM), H-1 and H-2 receptor antagonists, respectively. The H-4 receptor agonists, clobenpropit and clozapine (0.004-2 muM), which are also H-3 receptor antagonists, both induced eosinophil shape change, which was inhibited by thioperamide (10 muM). The H-3/H-4 receptor agonists, imetit, R-alpha-methyl histamine and N-alpha-methyl histamine (0.004-2 muM) also induced eosinophil shape change. 3 Histamine induced actin polymerisation (0.015-10 muM), intracellular calcium mobilisation (10100 muM) and a significant upregulation of expression of the cell adhesion molecule CD11b (0.004-10 muM) in eosinophils, all of which were inhibited by thioperamide (10-100 muM). In addition, the H-4 receptor agonist/H-3 receptor antagonist clozapine (20 muM) stimulated a rise in intracellular calcium in eosinophils. 4 Activation of H-4 receptors by histamine (1 muM) primed eosinophils for increased chemotactic responses to eotaxin, but histamine (0.1-10 muM) did not directly induce chemotaxis of eosinophils. 5 Pertussis toxin (1 mug ml(-1)) inhibited shape change and actin polymerisation responses induced by histamine showing that these effects are mediated by coupling to a Galpha(i/o) G-protein. 6 This study demonstrates that human eosinophils express functional H-4 receptors and may provide a novel target for allergic disease therapy.