Serotonin activates murine alveolar macrophages through 5-HT2C receptors.

Serotonin (5-HT), known as neuromodulator, regulates immune responses and inflammatory cascades. The expression and function of 5-HT receptors on alveolar macrophages (AM), which are the major fraction of pulmonary immune cells, remain elusive. Therefore, we determined the expression of 5-HT type 2 receptors and investigated the effects evoked by stimulation with 5-HT in AM compared with alveolar epithelial cells (AEC). Quantitative PCR (qPCR) analysis revealed expression of the receptors 5-HT(2A) and 5-HT(2B) in AEC and of 5-HT(2C) in AM. In AM, 5-HT (10(-5) M) induced a rise in intracellular calcium concentration ([Ca(2+)](i)) that was initiated by release of Ca(2+) from intracellular stores and depended on extracellular Ca(2+) in a sustained phase. This 5-HT-induced increase in [Ca(2+)](i) was not observed in AM treated with the 5-HT(2C)-selective inhibitor RS-102221 and in AM derived from 5-HT(2C)-deficient mice. AM stimulated with 5-HT (10(-5) M) showed increased expression of CCL2 (MCP-1) mRNA as assayed by qPCR at 4 h and augmented production of CCL2 protein as determined by dot-blot assay and ELISA at 24 h. Notably, in 5-HT(2C)-deficient AM, CCL2 production was not induced by 5-HT treatment. Moreover, transcriptional responses to 5-HT exposure assayed by microarray experiments were only observed in AM from wild-type animals and not in AM derived from 5-HT(2C)-deficient mice. Taken together, these data demonstrate the presence of functional 5-HT(2C) receptors on AM and suggest a role of 5-HT as novel modulator of AM function. These effects are exclusively driven by the 5-HT(2C) receptor, thereby providing the potential for selective intervention.