CD40-CD40L and platelet function: beyond hemostasis.

Traditionally, platelets have been thought of as anuclear, subcellular fragments derived from megakaryocytes circulating in blood as small discs and mitigating hemorrhage. This hemostatic process requires platelet activation, a complex chain of events involving rapid structural changes that activate adhesion receptors, remodel the cytoskeleton, and lead to the eventual synthesis and secretion of various platelet-derived factors. The result of this cascade is the formation of platelet-dependent thrombosis that primarily attenuates bleeding but, in pathophysiological settings, contributes to occlusive vascular events. These clinical realities have led to the bias that platelets are primarily involved in thrombosis and hemostasis. This bias has been reflected in the predominant use of platelet aggregometry as the standard method for quantifying platelet activation ex vivo. This technique, however, only identifies platelet-platelet binding (homotypic aggregates) and, in clinical settings, has promoted a relatively simplistic vision of platelet function. The focus on platelet-dependent thrombosis has made the platelet aggregate a common therapeutic target in syndromes involving vascular occlusion. However, merely preventing the process of platelet-platelet binding may not always translate into clinical efficacy as seen in the recent disappointing results associated with the use of the oral IIb/IIIa inhibitors.1 Recently, studies have suggested that platelets also participate in inflammatory reactions. Platelets are known to produce inflammatory mediators including platelet-derived growth factor, platelet factor 4, and transforming growth factor. Platelets are also known to bind, via P-selectin (CD62P) expressed on the surface of activated platelets to the leukocyte receptor, P-selectin glycoprotein ligand-1 (PSGL1). The relevance of this binding is supported by a study demonstrating that the infusion of recombinant soluble human form of P-selectin glycoprotein ligand-1 in an animal model of vascular injury reduced myocardial reperfusion injury and preserved vascular endothelial function.2 The clinical implications of heterotypic aggregates are shown by studies demonstrating that after acute myocardial infarction, circulating monocyte-platelet aggregates are both increased and are a more sensitive marker of in vivo platelet activation than platelet surface P-selectin.3 Plaque rupture promotes activation of the inflammatory responses, and the consistent finding of heterotypic aggregates highlights the close interaction between inflammation and thrombosis in vascular disease. Contributing to our understanding of the role of platelets in inflammation are CD40-CD40 ligand (CD40L) interactions. CD40 is a membrane glycoprotein belonging to the tumor necrosis family receptor superfamily and its ligand CD40L (CD154) is a glycoprotein from the tumor necrosis factor family. CD40-CD40L interactions are central in immune responses and inflammation. Ligation of CD40 on various vascular cells contributes to the pathogenesis of atherosclerotic, thrombotic, and inflammatory processes.4,5 On endothelial cells or monocytes, the engagement of CD40 leads to the synthesis of adhesion molecules, chemokines, and tissue factor and causes the activation of matrix metalloproteinases. CD40L has also been detected in platelets4 where, after stimulation, it is translocated to the platelet surface (Figure). The surface-expressed CD40 ligand is then cleaved from the platelet over a period of minutes to hours subsequently generating a soluble fragment (soluble CD40 ligand or sCD40L).6 It is estimated that 95% of the circulating sCD40L is derived from platelets.6 Found to be increased on platelets in fresh thrombus,4 sCD40L has been shown to be elevated in cardiovascular disease7,8 and associated with increased cardiovascular risk in apparently healthy women.9 Although sCD40L has been characterized as a marker of thrombotic diseases, much less is known about its direct role in platelet function. It has been suggested that CD40L is an IIb 3 ligand, a platelet agonist, and contributes to stability of arterial thrombi.6 In this issue of Circulation Research, Inwald and colleagues10 add to the limited information concerning the effects of sCD40L on platelet function and demonstrate that CD40 ligation may be a mechanism for platelet activation. After incubation of platelets with the trimeric form of sCD40L (sCD40LT), they showed enhanced CD62P expression as well as dense and -granule release. Interestingly, the presence of a glycoprotein IIb/IIIa blocking agent did not alter sCD40LT-induced CD62P expression. In addition, -thromboglobulin and [C]-5-HT were shown to be present after ligation reflecting and dense granule release, respectively. An intriguing part of this study is the confirmatory data using subjects with X-linked hyper-IgM syndrome and known absence of CD40L expression. The ability to utilize these subjects is particularly interesting in platelet-dependent studies as genetic manipulation in cell culture is not possible. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Whitaker Cardiovascular Institute and Evans Department of Medicine, Boston University School of Medicine, Boston, Mass. Correspondence to Jane E. Freedman, MD, Boston University School of Medicine, 715 Albany St, W507, Boston, MA 02118. E-mail [email protected] (Circ Res. 2003;92:944-946.) © 2003 American Heart Association, Inc.