Histamine downregulates monocyte CCL2 production through the histamine H4 receptor

Background: The expression of the recently cloned histamine H-4 receptor (H4R) by leukocytes suggests a role in immunomodulation. Objective: The expression and function of the H4R on human monocytes obtained from peripheral blood was investigated. Methods: H4R expression was studied by using How cytometry. Effects of H4R stimulation on Ca2+ mobilization was determined fluorometrically, CCL2 production was determined by means of ELISA, intracellular CCL2 staining was measured with flow cytometry, and CCL2 mRNA was measured by using real-time quantitative LightCycler PCR. The relevance of CCL2 production was determined in chemotaxis transmigration assays. Results: H4R protein was expressed by monocytes and upregulated by IFN-gamma. H4R agonists (clobenpropit and 4-methylhistamine) induce a Ca2+ mobilization in monocytes, which could be blocked with the selective H4R antagonist JNJ7777120. Furthermore, H4R agonists downregulated CCL2 protein production. This effect could also be blocked by JNJ7777120. Supernatants of H4R agonist-stimulated monocytes attracted less monocytes in transmigration assays. The downregulation of CCL2 production was regulated at different levels. First, the synthesis of CCL2 mRNA was significantly decreased. Second, intracellular staining suggested an inhibition of CCL2 secretion after stimulation with H4R agonists. Conclusion: Human monocytes express the H4R, and its stimulation leads to a Ca2+ influx and an inhibition of CCL2 production, resulting in a reduction of monocyte recruitment. Clinical implications: The H4R could represent an important anti-inflammatory receptor on monocytes and could be an interesting target for drug development.