Platelets circulate in an activated state in patients with inflammatory bowel disease (IBD), but their role in the pathogenesis of IBD is unclear. The recent demonstration that activated platelets express CD40 ligand (L) provides a mechanism of interaction with CD40-positive endothelial cells, inducing them to produce proinflammatory mediators. We investigated whether platelets from patients with IBD express enhanced levels of CD40L and induce human intestinal microvascular endothelial cells (HIMEC) to up-regulate cell adhesion molecule (CAM) expression and secrete chemokines.

CD40L expression was assessed in resting and thrombin-activated platelets by flow cytometry and in mucosal microthrombi by confocal microscopy. Platelet-HIMEC cocultures were used to study CAM up-regulation, and interleukin (IL)-8 and RANTES production by HIMEC.

IBD platelets expressed significantly higher CD40L levels than those of healthy subjects, and CD40L-positive platelets were detected in IBD-involved mucosa. Activated platelets up-regulated expression of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 as well as production of interleukin 8 by HIMEC in a CD40-dependent fashion. High levels of RANTES were present in platelet-HIMEC cocultures and platelets were identified as the source of this chemokine, which mediated T-cell adhesion to HIMEC.

These results show that platelets can actively contribute to mucosal inflammation and represent a previously unrecognized component of IBD pathogenesis.