IDENTIFICATION OF A HISTAMINE H₄ RECEPTOR ON HUMAN EOSINOPHILS—ROLE IN EOSINOPHIL CHEMOTAXIS

ABSTRACT

Eosinophils are recruited to sites of inflammation via the action of a number of chemical mediators, including PAF, leukotrienes, eotaxins, ECF-A and histamine. Although many of the cell-surface receptors for these mediators have been identified, histamine-driven chemotaxis has not been conclusively attributed to any of the three known histamine receptor subtypes, suggesting the possibility of a 4th histamine-responsive receptor on eosinophils. We have identified and cloned a novel G protein-coupled receptor (GPCR), termed Pfi-013, from an IL-5 stimulated eosinophil cDNA library which is homologous to the human histamine H₃ receptor, both at the sequence and gene structure level. Expression data indicates that Pfi-013 is predominantly expressed in peripheral blood leukocytes, with lower expression levels in spleen, testis and colon. Ligand-binding studies using Pfi-013 expressed in HEK-293Gα15 cells, demonstrates specific binding to histamine with a K_d of 3.28 ± 0.76 nM and possesses a unique rank order of potency against known histaminergic compounds in a competitive ligand-binding assay (histamine < clobenpropit < iodophenpropit < thioperamide < R-α-methylhistamine < cimetidine < pyrilamine). We have therefore termed this receptor human histamine H₄. Chemotaxis studies on isolated human eosinophils have confirmed that histamine is chemotactic and that agonists of the known histamine receptors (H₁, H₂, and H₃) do not induce such a response. Furthermore, studies employing histamine-receptor antagonists have shown an inhibition of chemotaxis only by the H₃ antagonists clobenpropit and thioperamide. Since these compounds are also antagonists of hH4 we postulate that the receptor mediating histaminergic chemotaxis is this novel histamine H₄ receptor.

Key Words: :

• G protein-coupled receptor
• Histamine
• Eosinophil
• Chemotaxis
• Human