Background-Peripheral homing of progenitor cells in areas of diseased organs is critical for tissue regeneration. The chemokine stromal cell-derived factor-1 (SDF-1) regulates homing of CD34 ϩ stem cells. We evaluated the role of platelet-derived SDF-1 in adhesion and differentiation of human CD34 ϩ cells into endothelial progenitor cells. Methods and Results-Adherent platelets express substantial amounts of SDF-1 and recruit CD34 ϩ cells in vitro and in vivo. A monoclonal antibody to SDF-1 or to its counterreceptor, CXCR4, inhibits stem cell adhesion on adherent platelets under high arterial shear in vitro and after carotid ligation in mice, as determined by intravital fluorescence microscopy. Platelets that adhere to human arterial endothelial cells enhance the adhesion of CD34 ϩ cells on endothelium under flow conditions, a process that is inhibited by anti-SDF-1. During intestinal ischemia/reperfusion in mice, anti-SDF-1 and anti-CXCR4, but not isotype control antibodies, abolish the recruitment of CD34 ϩ cells in microcirculation. Moreover, platelet-derived SDF-1 binding to CXCR4 receptor promotes platelet-induced differentiation of CD34 ϩ cells into endothelial progenitor cells, as verified by colony-forming assays in vitro. Conclusions-These

Recruitment of human CD34+ progenitor cells toward vascular lesions and differentiation into vascular cells has been regarded as a critical initial step in atherosclerosis. Previously we found that adherent platelets represent potential mediators of progenitor cell homing besides their role in thrombus formation. On the other hand, foam cell formation represents a key process in atherosclerotic plaque formation. To investigate whether platelets are involved in progenitor cell recruitment and differentiation into endothelial cells and foam cells, we examined the interactions of platelets and CD34+ progenitor cells. Cocultivation experiments showed that human platelets recruit CD34+ progenitor cells via the specific adhesion receptors P-selectin/PSGL-1 and beta1- and beta2-integrins. Furthermore, platelets were found to induce differentiation of CD34+ progenitor cells into mature foam cells and endothelial cells. Platelet-induced foam cell generation could be prevented partially by HMG coenzyme A reductase inhibitors via reduction of matrix metalloproteinase-9 (MMP-9) secretion. Finally, agonists of peroxisome proliferator-activated receptor-alpha and -gamma attenuated platelet-induced foam cell generation and production of MMP-9. The present study describes a potentially important mechanism of platelet-induced foam cell formation and generation of endothelium in atherogenesis and atheroprogression. The understanding and modulation of these mechanisms may offer new treatment strategies for patients at high risk for atherosclerotic diseases.

Human β-defensins (hBD) are antimicrobial peptides that curb microbial activity. Although hBD's are primarily expressed by epithelial cells, we show that human platelets express hBD-1 that has both predicted and novel antibacterial activities. We observed that activated platelets surround Staphylococcus aureus (S. aureus), forcing the pathogens into clusters that have a reduced growth rate compared to S. aureus alone. Given the microbicidal activity of β-defensins, we determined whether hBD family members were present in platelets and found mRNA and protein for hBD-1. We also established that hBD-1 protein resided in extragranular cytoplasmic compartments of platelets. Consistent with this localization pattern, agonists that elicit granular secretion by platelets did not readily induce hBD-1 release. Nevertheless, platelets released hBD-1 when they were stimulated by α-toxin, a S. aureus product that permeabilizes target cells. Platelet-derived hBD-1 significantly impaired the growth of clinical strains of S. aureus. hBD-1 also induced robust neutrophil extracellular trap (NET) formation by target polymorphonuclear leukocytes (PMNs), which is a novel antimicrobial function of β-defensins that was not previously identified. Taken together, these data demonstrate that hBD-1 is a previously-unrecognized component of platelets that displays classic antimicrobial activity and, in addition, signals PMNs to extrude DNA lattices that capture and kill bacteria.

Summary.  An expanding body of evidence continues to build on the role of platelets as initial actors in the development of atherosclerotic lesions. Platelets bind to leukocytes and endothelial cells, and initiate monocyte transformation into macrophages. Platelets internalize oxidized phospholipids and promote foam cell formation. Platelets also recruit progenitor cells to the scene that are able to differentiate into foam cells or endothelial cells depending on conditions. Platelets tip the scales in the initiation, development and total extent of atherosclerotic lesions.

Platelets contribute to processes beyond thrombus formation and may play a so far underestimated role as an immune cell in various circumstances. This review outlines immune functions of platelets in host defense, but also how they may contribute to mechanisms of infectious diseases. A particular emphasis is placed on the interaction of platelets with other immune cells. Furthermore, this article outlines the features of atherosclerosis as an inflammatory vascular disease highlighting the role of platelet crosstalk with cellular and soluble factors involved in atheroprogression. Understanding, how platelets influence these processes of vascular remodeling will shed light on their role for tissue homeostasis beyond intravascular thrombosis. Finally, translational implications of platelet-mediated inflammation in atherosclerosis are discussed.

Abstract-Atherosclerosis is an inflammatory disease. Platelets can "inflame" the vascular wall by various mechanisms and thereby initiate and support the development of atherosclerosis. Platelet interaction with leukocytes, endothelial cells, and circulating progenitor cells triggers autocrine and paracrine activation processes, leading to inflammatory and atherogenic cascades at the vascular wall. This review highlights the molecular key components and pathways used by platelets to trigger and accelerate inflammation at the vascular wall and, thereby, atherosclerosis. A therosclerosis is a chronic inflammatory disease. 1 However, the contribution of platelets to the process of atherosclerosis was unclear until this millennium. Recently, we and others could provide conclusive evidence that platelets are crucially involved in atherogenesis. 2,3 In apoEdeficient mice, platelets were found to adhere to the vascular endothelium of the carotid artery even before leukocyte invasion and before the development of manifest atherosclerotic lesions. 2 Platelet adhesion was found to be mainly mediated by both platelet glycoproteins (GP) Ib␣ and ␣ IIb ␤ 3 and coincided with inflammatory gene expression. 2 Consequently, prolonged antibody blockade of platelet GP Ib␣ prohibited leukocyte accumulation in the vascular wall and attenuated atherosclerotic lesion formation. Further, infused activated wild-type (but not P-selectin-deficient) platelets were found to promote the formation of atherosclerotic lesions in wild-type mice. 3 In the meantime, we know that the interruption of platelet interaction with the vascular wall by any intervention, such as antibody inhibition or knockout of platelet adhesion receptors (eg, GP IIb, GP Ib␣, or P-selectin), substantially reduces the formation of atherosclerotic lesions in different mouse models. [3][4][5] Interestingly, effective inhibition of downstream activation cascades (eg, CD40/CD40L) can also inhibit atherosclerosis. Although the specific platelet contribution has not been proved in this context, disruption of CD40-CD40L in mouse models of atherosclerosis could attenuate plaque formation 6,7 and could even stabilize and halt the progression of established lesions. 8

Rationale: Macrophage migration inhibitory factor (MIF) is released on platelet activation. Circulating MIF could potentially regulate platelets and thereby platelet-mediated inflammatory and regenerative mechanisms. However, the effect of MIF on platelets is unknown. Objective: The present study evaluated MIF in regulating platelet survival and thrombotic potential. Methods and Results: MIF interacted with CXCR4-CXCR7 on platelets, defining CXCR7 as a hitherto unrecognized receptor for MIF on platelets. MIF internalized CXCR4, but unlike CXCL12 (SDF-1α), it did not phosphorylate Erk1/2 after CXCR4 ligation because of the lack of CD74 and failed in subsequent CXCR7 externalization. MIF did not alter the activation status of platelets. However, MIF rescued platelets from activation and BH3 mimetic ABT-737–induced apoptosis in vitro via CXCR7 and enhanced circulating platelet survival when administered in vivo. The antiapoptotic effect of MIF was absent in Cxcr7 −/− murine embryonic cells but pronounced in CXCR7-transfected Madin–Darby canine kidney cells. This prosurvival effect was attributed to the MIF–CXCR7–initiated PI3K-Akt pathway. MIF induced CXCR7-Akt–dependent phosphorylation of BCL-2 antagonist of cell death (BAD) both in vitro and in vivo. Consequentially, MIF failed to rescue Akt −/− platelets from thrombin-induced apoptosis when challenged ex vivo, also in prolonging platelet survival and in inducing BAD phosphorylation among Akt −/− mice in vivo. MIF reduced thrombus formation under arterial flow conditions in vitro and retarded thrombotic occlusion after FeCl 3 -induced arterial injury in vivo, an effect mediated through CXCR7. Conclusion: MIF interaction with CXCR7 modulates platelet survival and thrombotic potential both in vitro and in vivo and thus could regulate thrombosis and inflammation.

Objective-Thrombotic events and immunoinflammatory processes take place next to each other during vascular remodeling in atherosclerotic lesions. In this study we investigated the interaction of platelets with dendritic cells (DCs). Methods and Results-The rolling of DCs on platelets was mediated by PSGL-1. Firm adhesion of DCs was mediated through integrin ␣ M ␤ 2 (Mac-1). In vivo, adhesion of DCs to injured carotid arteries in mice was mediated by platelets. Pretreatment with soluble GPVI, which inhibits platelet adhesion to collagen, substantially reduced recruitment of DCs to the injured vessel wall. In addition, preincubation of DCs with sJAM-C significantly reduced their adhesion to platelets. Coincubation of DCs with platelets induced maturation of DCs, as shown by enhanced expression of CD83. In the presence of platelets, DC-induced lymphocyte proliferation was significantly enhanced. Moreover, coincubation of DCs with platelets resulted in platelet phagocytosis by DCs, as verified by different cell phagocytosis assays. Finally, platelet/DC interaction resulted in apoptosis of DCs mediated by a JAM-Cdependent mechanism. Conclusions-Recruitment of DCs by platelets, which is mediated via CD11b/CD18 (Mac-1) and platelet JAM-C, leads to DC activation and platelet phagocytosis. This process may be of importance for progression of atherosclerotic lesions. 3 In atherosclerotic plaques, the number of DCs is substantially enhanced and DCs preferentially accumulate at rupture-prone regions. 4,5 Recently, DCs were shown to accumulate in the intima of atherosclerosis-predisposed regions of the aorta of C57BL/6 mice. 6 However, the mechanisms involved in the recruitment of circulating DCs at site of vascular lesions are poorly understood so far.It is well recognized that platelets rapidly adhere to the extracellular matrix of the subendothelium at sites of vascular lesions. If this process is controlled, platelets passivate vascular injury and initiate the healing process. 1 However, uncontrolled platelet-mediated thrombus formation leads to acute thrombotic occlusion or plaque progression resulting in, eg, acute coronary syndrome. 7 Platelet-mediated cell recruitment to the atherosclerotic plaque plays a central role for vascular repair mechanisms. DCs participate both in the innate and adaptive immune system and represent highly specialized antigen-presenting cells. 8 Thereby, they are capable of stimulating naive, memory, and effector T-cells, as well as activating natural killer cells. 8 Proteins are internalized by phagocytosis, degraded into short peptides, and presented via the MHC II receptors. 9 During maturation, DCs express various adhesion receptors, which enable DCs to interact with other cell types and mediate homing of DCs to target tissues. 4,8 Original The present study evaluates the role of platelets for DC adhesion to vascular lesions and shows that platelets play a critical role for the recruitment and function of DCs. Materials and MethodsDCs were generated from buffy coats derived fr...