Serotonin [5-hydroxytryptamine (5-HT)] plays an important role in many organs as a peripheral hormone. Most of the body’s serotonin is circulating in the bloodstream, transported by blood platelets and is released upon activation. The functions of serotonin are mediated by members of the 7 known mammalian serotonin receptor subtype classes (15 known subtypes), the serotonin transporter (SERT), and by covalent binding of serotonin to different effector proteins. Almost all immune cells express at least one serotonin component. In recent years, a number of immunoregulatory functions have been ascribed to serotonin. In monocytes/macrophages, for example, serotonin modulates cytokine secretion. Serotonin can also suppress the release of tumor necrosis factor-α and interleukin-1β by activating serotonin receptors. Furthermore, neutrophil recruitment and T-cell activation can both be mediated by serotonin. These are only a few of the known immunomodulatory roles of serotonin that we will review here.

Key Points• During inflammation, serotonin released by platelets activates vessel wall promoting leukocyte adhesion and recruitment.• Absence of platelet serotonin improves survival after lipopolysaccharide-induced endotoxic shock.The majority of peripheral serotonin is stored in platelets, which secrete it on activation. Serotonin releases Weibel-Palade bodies (WPBs) and we asked whether absence of platelet serotonin affects neutrophil recruitment in inflammatory responses. Tryptophan hydroxylase (Tph)1-deficient mice, lacking non-neuronal serotonin, showed mild leukocytosis compared with wild-type (WT), primarily driven by an elevated neutrophil count. Despite this, 50% fewer leukocytes rolled on unstimulated mesenteric venous endothelium of Tph1 ؊/؊ mice. The velocity of rolling leukocytes was higher in Tph1 ؊/؊ mice, indicating fewer selectin-mediated interactions with endothelium. Stimulation of endothelium with histamine, a secretagogue of WPBs, or injection of serotonin normalized the rolling in Tph1 ؊/؊ mice. Diminished rolling in Tph1 ؊/؊ mice resulted in reduced firm adhesion of leukocytes after lipopolysaccharide treatment. Blocking platelet serotonin uptake with fluoxetine in WT mice reduced serum serotonin by > 80% and similarly reduced leukocyte rolling and adhesion. Four hours after inflammatory stimulation, neutrophil extravasation into lung, peritoneum, and skin wounds was reduced in Tph1 ؊/؊ mice, whereas in vitro neutrophil chemotaxis was independent of serotonin. Survival of lipopolysaccharide-induced endotoxic shock was improved in Tph1 ؊/؊ mice. In conclusion, platelet serotonin promotes the recruitment of neutrophils in acute inflammation, supporting an important role for platelet serotonin in innate immunity. (Blood. 2013;121(6):1008-1015) IntroductionPlatelets store serotonin in their dense granules at millimolar concentration and secrete it when they become activated. 1,2 This requires a complex mechanism of uptake, storage, and targeted release that is similar to that in neurons, with the exception that platelets are not stationary but circulate in high numbers throughout the vasculature. Platelets do not synthesize serotonin but incorporate and store serotonin that is synthesized in duodenal enterochromaffin cells and secreted into blood. Several different effects of non-neuronal serotonin have been unraveled in the past, including prohemostatic (on platelets and vascular smooth muscle cells), 3,4 mitogenic (on hepatocytes and pulmonary smooth muscle cells), 5,6 and immunomodulatory (on lymphocytes, monocytes, and smooth muscle cells) 7-9 functions. In vitro studies have shown that serotonin also activates the release of Weibel-Palade bodies (WPBs) from endothelial cells, which would promote leukocyte rolling via the WPB constituent P-selectin. 10,11 However, it is not clear whether serotonin influences neutrophil-endothelial interactions, a central step in early innate immune responses.We chose 2 approaches to study serotonin effects on leukocyte rolling and recruitment: genetic defi...

Rationale CD40L figures prominently in chronic inflammatory diseases such as atherosclerosis. However, since CD40L potently regulates immune function and haemostasis by interaction with CD40 receptor and the platelet integrin GPIIb/IIIa, its global inhibition compromises host defense and generated thromboembolic complications in clinical trials. We recently reported that CD40L mediates atherogenesis independently of CD40 and proposed Mac-1 as an alternate receptor. Objective Here, we molecularly characterized the CD40L-Mac-1 interaction and tested whether its selective inhibition by a small peptide modulates inflammation and atherogenesis in vivo. Methods and Results CD40L concentration-dependently bound to Mac-1 I-domain in solid phase binding assays, and a high affinity interaction was revealed by surface-plasmon-resonance analysis. We identified the motif EQLKKSKTL, an exposed loop between the α1 helix and the β-sheet B, on Mac-1 as binding site for CD40L. A linear peptide mimicking this sequence, M7, specifically inhibited the interaction of CD40L and Mac-1. cM7, a cyclisized version optimized for in vivo use, decreased peritoneal inflammation and inflammatory cell recruitment in vivo. Finally, LDLr-/- mice treated with intraperitoneal injections of cM7 developed smaller, less inflamed atherosclerotic lesions featuring characteristics of stability. However, cM7 did not interfere with CD40L-CD40 binding in vitro and CD40L-GPIIb/IIIa-mediated thrombus formation in vivo. Conclusions We present the novel finding that CD40L binds to the EQLKKSKTL motif on Mac-1 mediating leukocyte recruitment and atherogenesis. Specific inhibition of CD40L-Mac-1 binding may represent an attractive anti-inflammatory treatment strategy for atherosclerosis and other inflammatory conditions, potentially avoiding the unwanted immunologic and thrombotic effects of global inhibition of CD40L.