Elisa A. Liehn

SDF-1α/CXCR4 Axis Is Instrumental in Neointimal Hyperplasia and Recruitment of Smooth Muscle Progenitor Cells

Recent evidence infers a contribution of smooth muscle cell (SMC) progenitors and stromal cell-derived factor (SDF)-1&agr; to neointima formation after arterial injury. Inhibition of plaque area and SMC content in apolipoprotein E-deficient mice repopulated with LacZ+ or CXCR4−/− BM or lentiviral transfer of an antagonist reveals a crucial involvement of local SDF-1&agr; and its receptor CXCR4 in neointimal hyperplasia via recruitment of BM-derived SMC progenitors. After arterial injury, SDF-1&; expression in medial SMCs is preceded by apoptosis and inhibited by blocking caspase-dependent apoptosis. SDF-1&agr; binds to platelets at the site of injury, triggers CXCR4- and P-selectin-dependent arrest of progenitor cells on injured arteries or matrix-adherent platelets, preferentially mobilizes and recruits c-kit−/platelet–derived growth factor receptor (PDGFR)-&bgr;+/lineage−/sca-1+ progenitors for neointimal SMCs without being required for their differentiation. Hence, the SDF-1&agr;/CXCR4 axis is pivotal for vascular remodeling by recruiting a subset of SMC progenitors in response to apoptosis and in concert with platelets, epitomizing its importance for tissue repair and identifying a prime target to limit lesion development.

Disrupting functional interactions between platelet chemokines inhibits atherosclerosis in hyperlipidemic mice

Atherosclerosis is characterized by chronic inflammation of the arterial wall due to chemokine-driven mononuclear cell recruitment. Activated platelets can synergize with chemokines to exacerbate atherogenesis; for example, by deposition of the chemokines platelet factor-4 (PF4, also known as CXCL4) and RANTES (CCL5), triggering monocyte arrest on inflamed endothelium. Homo-oligomerization is required for the recruitment functions of CCL5, and chemokine heteromerization has more recently emerged as an additional regulatory mechanism, as evidenced by a mutual modulation of CXCL8 and CXCL4 activities and by enhanced monocyte arrest resulting from CCL5-CXCL4 interactions. The CCL5 antagonist Met-RANTES reduces diet-induced atherosclerosis; however, CCL5 antagonism may not be therapeutically feasible, as suggested by studies using Ccl5-deficient mice which imply that direct CCL5 blockade would severely compromise systemic immune responses, delay macrophage-mediated viral clearance and impair normal T cell functions. Here we determined structural features of CCL5-CXCL4 heteromers and designed stable peptide inhibitors that specifically disrupt proinflammatory CCL5-CXCL4 interactions, thereby attenuating monocyte recruitment and reducing atherosclerosis without the aforementioned side effects. These results establish the in vivo relevance of chemokine heteromers and show the potential of targeting heteromer formation to achieve therapeutic effects.

Statin Treatment After Onset of Sepsis in a Murine Model Improves Survival

Background—HMG-CoA-reductase inhibitors have been shown to exhibit pronounced immunomodulatory effects independent of lipid lowering. We have recently demonstrated that pretreatment with simvastatin profoundly improves survival in a cecal ligation and perforation (CLP) model of sepsis. Here, we studied whether treatment with simvastatin after onset of sepsis-induced hemodynamic alterations is beneficial and whether prolonged survival can also be achieved with other statins. Methods and Results—Mice were rendered septic by CLP. At 6 hours after sepsis induction, when profound hemodynamic alterations were manifest, treatment with atorvastatin, fluvastatin, pravastatin, simvastatin, or placebo was initiated. Except for fluvastatin (27±2.3 hours), survival time was extended from 23±1.2 hours for placebo-treated mice to 37±3.6 hours for simvastatin-treated, to 40±4.2 hours for atorvastatin-treated, and to 39±3.9 hours for pravastatin-treated mice. This profound improvement is based on the preservation of cardiac function and hemodynamic status in statin-treated animals, both of which are severely impaired in untreated CLP mice. As underlying mechanisms, improved susceptibility to endothelial nitric oxide synthase stimulation and reduced endothelial adhesion of leukocytes could be demonstrated after statin treatment. Conclusions—Well established in the treatment of lipid disorders and coronary artery disease, statins harbor the additional and novel potential of effective sepsis treatment. This benefit extends to several but not all statins tested.

HMG-CoA Reductase Inhibitor Simvastatin Profoundly Improves Survival in a Murine Model of Sepsis

Background—HMG-CoA reductase inhibitors, such as simvastatin, have been shown to exhibit pronounced immunomodulatory effects independent of lipid lowering but to date have not been used to treat severe inflammatory disease such as sepsis. We thus approached the question of whether treatment with simvastatin might improve cardiovascular function and survival in sepsis. Methods and Results—Mice treated with simvastatin and rendered septic by cecal ligation and perforation (CLP) show a mean survival time close to 4 times the value found in untreated mice. This dramatic improvement is based on a complete preservation of cardiac function and hemodynamic status, which are severely impaired in untreated CLP mice [eg, 20 hours after CLP, cardiac output declined from 1.24±0.09 to 0.87±0.11 mL · min−1 · g−1 in untreated mice (P <0.005; n=12), while remaining unaltered (1.21±0.08 mL · min−1 · g−1 at baseline and 1.15±0.1 mL · min−1 · g−1 20 hours after CLP, P =NS, n=12) in CLP mice treated with simvastatin]. Untreated CLP mice remained refractory to β-stimulation, whereas the responsiveness to dobutamine was restored by treatment with simvastatin. Susceptibility of coronary flow to endothelial nitric oxide synthase (eNOS) stimulation by bradykinin was close to 3 times as pronounced in untreated CLP mice as in untreated sham-operated mice, indicating a high level of eNOS activation secondary to sepsis. In addition, treatment with simvastatin reversed inflammatory alterations in CLP mice, namely, increased monocyte adhesion to endothelium. Conclusions—Simvastatin, which is well established in the treatment of lipid disorders and coronary artery disease, might have the additional potential of being an effective agent in sepsis treatment.

Ccr5 But Not Ccr1 Deficiency Reduces Development of Diet-Induced Atherosclerosis in Mice

Objective—Chemokines and their receptors are crucially involved in the development of atherosclerotic lesions by directing monocyte and T cell recruitment. The CC-chemokine receptors 1 (CCR1) and 5 (CCR5) expressed on these cells bind chemokines implicated in atherosclerosis, namely CCL5/RANTES. Although general blockade of CCL5 receptors reduces atherosclerosis, specific roles of CCR1 and CCR5 have not been unequivocally determined. Methods and Results—We provide two independent lines of investigation to dissect the effects of Ccr1 and Ccr5 deletion in apolipoprotein E–deficient (ApoE−/−) mice in a collaboration between Aachen/Germany and Geneva/Switzerland. Different strains of ApoE−/−Ccr5−/− mice, ApoE−/−Ccr1−/− mice or respective littermates, were fed a high-fat diet for 10 to 12 weeks. Plaque areas were quantified in the aortic roots and thoracoabdominal aortas. Concordantly, both laboratories found that lesion formation was reduced in ApoE−/−Ccr5−/− mice. Plaque quality and immune cells were assessed by immunohistochemistry or mRNA analysis. Whereas lesional macrophage content, aortic CD4, and Th1-related Tim3 expression were reduced, smooth muscle cell (SMC) content and expression of interleukin-10 in plaques, lesional SMCs, and splenocytes were elevated. Protection against lesion formation by Ccr5 deficiency was sustained over 22 weeks of high-fat diet or over 26 weeks of chow diet. Conversely, plaque area, T cell, and interferon-&ggr; content were increased in ApoE−/−Ccr1−/− mice. Conclusion—Genetic deletion of Ccr5 but not Ccr1 in ApoE−/− mice protects from diet-induced atherosclerosis, associated with a more stable plaque phenotype, reduced mononuclear cell infiltration, Th1-type immune responses, and increased interleukin-10 expression. This corroborates CCR5 as a promising therapeutic target.

Importance of CXC Chemokine Receptor 2 in the Homing of Human Peripheral Blood Endothelial Progenitor Cells to Sites of Arterial Injury

Circulating endothelial progenitor cells (EPCs) may contribute to endothelial regeneration; however, the exact mechanisms of their arterial homing remain elusive. We examined the role of the angiogenic chemokine receptor CXCR2 in the homing of human EPCs. Isolated EPCs expressed CXCR2 together with kinase insert domain–containing receptor, CD31, vascular endothelial cadherin, and CXCR4. Adhesion assays under flow conditions showed that EPCs preferentially adhered to &bgr;2-integrin ligands, that firm arrest on fibronectin or fibrinogen was enhanced by the CXCR2 ligands CXCL1 or CXCL7, and that blockade of CXCR2 significantly reduced EPC adhesion on platelet-coated endothelial matrix. This was corroborated by the involvement of CXCR2 in EPC recruitment to denuded areas of murine carotid arteries ex vivo and in vivo. Notably, blocking CXCR2 inhibited the incorporation of human EPCs expressing CXCR2 at sites of arterial injury in athymic nude mice. Immunoreactivity for the &bgr;-thromboglobulin isoform CXCL7 was observed in murine platelets and denuded smooth muscle cells (SMCs) early after wire injury, and transcripts for CXCL7 and CXCL1 were detected in isolated human arterial SMCs. Human KDR+CXCR2+ cells showed better in situ adhesion to injured murine carotid arteries than KDR+CXCR2− cells, were predominantly CD14+, and improved CXCR2-dependent endothelial recovery after injury in nude mice. In conclusion, our data clearly demonstrate the importance of CXCR2 for the homing of circulating EPCs to sites of arterial injury and for endothelial recovery in vivo.

Platelet Microparticles Enhance the Vasoregenerative Potential of Angiogenic Early Outgrowth Cells After Vascular Injury

Background— Angiogenic early outgrowth cells (EOCs) have been reported to contribute to endothelial regeneration and to limit neointima formation after vascular injury. Vascular pathologies comprise platelet activation and concomitant generation of platelet microparticles (PMPs). We hypothesized that PMPs may interact with EOCs in the context of vascular injury and modulate their regenerative potential. Methods and Results— Using flow cytometry, confocal microscopy, and scanning electron microscopy, we demonstrated the binding of thrombin/collagen-induced PMPs to EOCs with subsequent membrane assimilation and incorporation. This interaction promoted phenotypic alterations of EOCs with increased expression of endothelial cell markers and transfer of the chemokine receptor CXCR4 to EOCs with enhanced responsiveness to its ligand CXCL12/SDF-1&agr;. In addition, PMPs augmented the adhesion of EOCs to extracellular matrix components and to the injured vessel wall and accelerated cytoskeletal reorganization and migration of EOCs. PMPs induced changes in the EOC secretome toward a more proangiogenic profile and amplified the EOC-mediated induction of proliferation, migration, and capillary tube formation by mature endothelial cells. Compared with untreated EOCs, the injection of PMP-treated EOCs resulted in accelerated reendothelialization after arterial denudation injury in athymic nude mice, whereas the EOC-mediated reduction of neointima formation remained unchanged. Conclusions— Our data provide evidence that PMPs can boost the potential of EOCs to restore endothelial integrity after vascular injury. Major mechanisms involve the enhancement of EOC recruitment, migration, differentiation, and release of proangiogenic factors.

Neointimal Smooth Muscle Cells Display a Proinflammatory Phenotype Resulting in Increased Leukocyte Recruitment Mediated by P-Selectin and Chemokines

Abstract— Leukocyte recruitment is crucial for the response to vascular injury in spontaneous and accelerated atherosclerosis. Whereas the mechanisms of leukocyte adhesion to endothelium or matrix-bound platelets have been characterized, less is known about the proadhesive role of smooth muscle cells (SMCs) exposed after endothelial denudation. In laminar flow assays, neointimal rat SMCs (niSMCs) supported a 2.5-fold higher arrest of monocytes and “memory” T lymphocytes than medial SMCs, which was dependent on both P-selectin and VLA-4, as demonstrated by blocking antibodies. The increase in monocyte arrest on niSMCs was triggered by the CXC chemokine GRO-&agr; and fractalkine, whereas “memory” T cell arrest was mediated by stromal cell–derived factor (SDF)-1&agr;. This functional phenotype was paralleled by a constitutively increased mRNA and surface expression of P-selectin and of relevant chemokines in niSMCs, as assessed by real-time PCR and flow cytometry. The increased expression of P-selectin in niSMCs versus medial SMCs was associated with enhanced NF-&kgr;B activity, as revealed by immunofluorescence staining for nuclear p65 in vitro. Inhibition of NF-&kgr;B by adenoviral I&kgr;B&agr; in niSMCs resulted in a marked reduction of increased leukocyte arrest in flow. Furthermore, P-selectin expression by niSMCs in vivo was confirmed in a hypercholesterolemic mouse model of vascular injury by double immunofluorescence and by RT-PCR after laser microdissection. In conclusion, we have identified a NF-&kgr;B–mediated proinflammatory phenotype of niSMCs that is characterized by increased P-selectin and chemokine expression and thereby effectively supports leukocyte recruitment.

CD73/Ecto-5′-Nucleotidase Protects Against Vascular Inflammation and Neointima Formation

Background— Although CD73/ecto-5-nucleotidase has been implicated in maintaining vasoprotection, its role in regulating endothelial adhesion molecule or inflammatory monocyte recruitment (eg, in the context of vascular injury) remains to be defined. Methods and Results— Compared with wild-type mice, CD73-deficient (CD73−/−) mice exhibit increased luminal staining and protein and transcript expression for vascular cell adhesion molecule (VCAM)-1 in carotid arteries. In vitro, aortic endothelial cells (ECs) from CD73−/− mice display an upregulation of mRNA and protein expression of VCAM-1, associated with increased nuclear factor (NF)-&kgr;B activity, as determined by chromatin cross-linking and immunoprecipitation or quantitative p65 binding assays. CD73−/− ECs and carotid arteries perfused ex vivo supported increased monocyte arrest under flow conditions, which was mediated by &agr;4&bgr;1 integrin. After wire injury of carotid arteries, CD73 expression and activity were upregulated in wild-type mice, whereas neointimal plaque formation and macrophage content were increased in CD73−/− mice versus wild-type mice, concomitant with elevated NF-&kgr;B activation, luminal VCAM-1 expression, and soluble VCAM-1 concentrations. In contrast, reconstitution of wild-type mice with CD73−/− versus CD73+/+ BM did not significantly exacerbate neointima formation. Treatment with the specific A2A receptor agonist ATL-146e reversed the increased VCAM-1 transcript and protein expression in CD73−/− ECs and inhibited monocyte arrest on CD73−/− ECs. Continuous infusion of ATL-146e prevented neointima formation in CD73−/− mice. Conclusions— Our data epitomize the importance of vascular CD73 in limiting endothelial activation and monocyte recruitment via generation of adenosine acting through the A2A receptor, providing a molecular basis for therapeutic protection against vascular inflammation and neointimal hyperplasia.

Crucial Role of the CCL2/CCR2 Axis in Neointimal Hyperplasia After Arterial Injury in Hyperlipidemic Mice Involves Early Monocyte Recruitment and CCL2 Presentation on Platelets

Monocyte chemoattractant protein-1 (also known as CC chemokine ligand 2 [CCL2]) and its receptor CC chemokine receptor 2 (CCR2) play a central role in the inflammatory response and neointimal formation after vascular injury. In the context of hyperlipidemia, this appears to involve neointimal monocyte infiltration. Hence, we investigated the function of the CCL2/CCR2 axis in early monocyte recruitment to injured arteries. Wire-induced injury of the carotid artery in apoE−/− mice caused a rapid increase of JE/CCL2 protein in the vessel wall peaking at 24 hours after injury, whereas serum JE/CCL2 was increased solely at 6 hours and blood cell-associated levels were unaltered, as demonstrated by enzyme-linked immunosorbent assay. Immunohistochemistry revealed intense staining for JE/CCL2 in smooth muscle cells (SMCs) and in association with platelets adherent to the denuded vessel wall 24 hours after injury. In vitro, exogenous or SMC-derived JE/CCL2 binds to the platelet surface and triggers monocyte arrest on adherent platelets but not on SMCs in flow assays. Accordingly, monocyte arrest in ex vivo perfused apoE−/− carotid arteries isolated 24 hours after injury was profoundly inhibited by pretreatment with a JE/CCL2 antibody. In CCR2−/−/apoE−/− mice, neointimal plaque area was reduced by 47% compared with CCR2+/+/apoE−/− mice. Moreover, CCR2 deletion markedly decreased neointimal macrophage content while expanding SMC content. Vascular JE/CCL2 expressed by SMCs and immobilized by adherent platelets after endothelial denudation is crucial for mediating early monocyte recruitment to injured arteries in hyperlipidemic mice. This mechanism may explain reduced neointimal macrophage infiltration and lesion formation in CCR2-deficient apoE−/− mice.