Incomplete activation of human eosinophils via the histamine H4-receptor: Evidence for ligand-specific receptor conformations

Eosinophils play a crucial role in the pathogenesis of allergic diseases. Histamine activates eosinophils via the H4-receptor (H4R). However, pharmacological analysis of the H4R in eosinophils is still incomplete, and cell purity is a problem. The H4R antagonist 1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methylpiperazine (JNJ7777120) has recently been reported to exhibit paradoxical stimulatory effects in some systems. Therefore, the first aim of our study was to pharmacologically re-examine HxR subtypes on human eosinophils using a highly purified preparation (97 ± 2%). The second aim was to compare the effects of histamine with those induced by well-known activators of eosinophil functions, i.e. eotaxin-1 and formyl peptides. Histamine and the H4R-selective agonist 2-cyano-1-[4-(1H-imidazol-4-yl)butyl]-3-[(2-phenylthio)ethyl]guanidine (UR-PI376) increased intracellular calcium concentration ([Ca2+]i) and activated chemotaxis. JNJ7777120 per se exhibited no stimulatory effects but inhibited stimulation by histamine and UR-PI376. Blockade of the H2R by famotidine enhanced histamine-induced chemotaxis but not rises in [Ca2+]i. Compared to eotaxin and formyl peptides, the effect of histamine on eosinophil chemotaxis was only small. Formyl peptides but not histamine activated reactive oxygen species formation and release of eosinophil peroxidase. In conclusion, histamine is only an incomplete eosinophil activator with the H2R blunting the small chemotactic response to H4R activation. We also noted several differences in potencies of histamine, UR-PI376 and JNJ7777120 in calcium and chemotaxis assays and when compared to results in the literature. This indicates functional selectivity of H4R ligands, thus ligand-specific stabilization of distinct receptor conformations, inducing distinct biological responses.

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