Activated platelets promote increased monocyte expression of CXCR5 through prostaglandin E2-related mechanisms and enhance the anti-inflammatory effects of CXCL13

Background: We have previously shown that the homeostatic chemokine CXCL13 is up-regulated in monocytes in atherosclerosis, mediating anti- apoptotic and anti-inflammatory effects. Objective: To investigate the regulation of CXCL13s receptor, CXCR5. Methods/patients: In vitro studies in THP-1 and primary monocytes and studies of CXCR5 expression in thrombus material obtained at the site of plaque rupture during myocardial infarction (MI). Results: Our major findings were: (i) toll-like receptor agonists and particularly beta-adrenergic receptor activation and releasate from thrombin-activated platelets increased CXCR5 mRNA levels in monocytes. (ii) The platelet-mediated induction of CXCR5 involved prostaglandin E-2/cAMP/protein kinase A-dependent as well as RANTES-dependent pathways with NF kappa B activation as a potential common down-stream mediator. (iii) Releasate from thrombin-activated platelets augmented the anti-inflammatory effects of CXCL13 in monocytes at least partly by enhancing the effects of CXCL13 on CXCR5 expression. (iv) We found strong immunostaining of CXCR5 in thrombus material obtained at the site of plaque rupture in patients with ST elevation MI (STEMI) and in unstable carotid lesions, co-localized with platelets. Conclusion: Our findings suggest that platelet-mediated signaling through CXCR5 may be active in vivo during plaque destabilization, potentially representing a counteracting mechanism to inflammation.