Background: The expression of the recently cloned histamine H(4) receptor (H(4)R) by leukocytes suggests a role in immunomodulation.
Objective: The expression and function of the H(4)R on human monocytes obtained from peripheral blood was investigated.
Methods: H(4)R expression was studied by using flow cytometry. Effects of H(4)R stimulation on Ca(2+) 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: H(4)R protein was expressed by monocytes and upregulated by IFN-gamma. H(4)R agonists (clobenpropit and 4-methylhistamine) induce a Ca(2+) mobilization in monocytes, which could be blocked with the selective H(4)R antagonist JNJ7,777,120. Furthermore, H(4)R agonists downregulated CCL2 protein production. This effect could also be blocked by JNJ7,777,120. Supernatants of H(4)R 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 H(4)R agonists.
Conclusion: Human monocytes express the H(4)R, and its stimulation leads to a Ca(2+) influx and an inhibition of CCL2 production, resulting in a reduction of monocyte recruitment.
Clinical implications: The H(4)R could represent an important anti-inflammatory receptor on monocytes and could be an interesting target for drug development.