Many diseases have an inflammatory component, where neutrophil interactions with the vascular endothelium lead to barrier dysfunction and increased permeability. Neutrophils increase permeability through secreted products like the chemokines CXCL1, 2, 3 and 8, through adhesion-dependent processes like β2 integrins interacting with endothelial ICAM-1, and combinations, where β2 integrin engagement leads to degranulation and secretion of heparin-binding protein (HBP), which in turn increases permeability. Some neutrophil products like arachidonic acid or leukotriene (LT)A4 are further processed by endothelial enzymes through transcellular metabolism before the resulting products thromboxane A2, LTB4 or LTC4 can activate their cognate receptors. Neutrophils also generate reactive oxygen species that induce vascular leakage. This review focuses on the mechanisms of neutrophil-mediated leakage.

Peripheral arterial disease is a major health problem and there is a significant need to develop therapies to prevent its progression to claudication and critical limb ischemia. Promising results in rodent models of arterial occlusion have generally failed to predict clinical success and led to questions of their relevance. While sub-optimal models may have contributed to the lack of progress, we suggest that advancement has also been hindered by misconceptions of the human capacity for compensation and the specific vessels which are of primary importance. We present and summarize new and existing data from humans, Ossabaw miniature pigs, and rodents which provide compelling evidence that natural compensation to occlusion of a major artery (i) may completely restore perfusion, (ii) occurs in specific pre-existing small arteries, rather than the distal vasculature, via mechanisms involving flow-mediated dilation and remodeling (iii) is impaired by cardiovascular risk factors which suppress the flow-mediated mechanisms and (iv) can be restored by reversal of endothelial dysfunction. We propose that restoration of the capacity for flow-mediated dilation and remodeling in small arteries represents a largely unexplored potential therapeutic opportunity to enhance compensation for major arterial occlusion and prevent the progression to critical limb ischemia in the peripheral circulation.

arteriogenesis; hindlimb ischemia; necrosis; regeneration; NADPH oxidase 2 IN THE PERIPHERAL CIRCULATION, the dilation and enlargement of preexisting vessels that form collateral pathways subsequent to arterial occlusion are the primary vascular compensations that preserve tissue viability and maintain function. These vessels dilate within seconds (25,39,45,70) and undergo expansion for weeks (20,25,45,69). In various species, the preexisting vessels are the size of the smallest arteries, and they enlarge ϳ100% (14,20,35,45,56). Available clinical studies have indicated that subsequent to arterial occlusion in the peripheral circulation, the primary vessels that enlarge as collaterals are preexisting arteries (4,29,57). The largest of the preexisting vessels are typically those that become the dominant collaterals (35,45,51). Combined anatomic and modeling studies have predicted that hindlimb flow subsequent to femoral artery occlusion is primarily determined by these collaterals (22,56). This is consistent with studies of segmental resistances that demonstrated that compensation in the collateral vessels is of significantly greater hemodynamic importance than adaptations in the distal microvasculature (43,71,75). Nevertheless, few studies investigating the mechanisms of vascular compensation subsequent to arterial occlusion in mice have specifically identified and studied these preexisting vessels that form the primary collateral pathways. Such investigations are needed because angiogenesis and collateral growth are initiated by different stimuli, and differences exist in the molecules and mechanisms that mediate these important processes (7, 11).Leukocytes, especially lymphocytes (63, 74) and macrophages (2,33,37), have been shown to have an important role in vascular compensation to hindlimb ischemia. Recent studies by Tojo et al. (66) and Urao et al. (72) have established that NADPH oxidase 2 (Nox2)-derived ROS from bone marrowderived cells (BMDCs) have an important role in neovascularization in the ischemic mouse hindlimb. The initial report (66) concluded from microsphere data that collateral growth was impaired, but did not specifically identify collateral bypass vessels or measure their diameters.To investigate the hypothesis that Nox2-derived NAD(P)H oxidase mediates primary collateral growth subsequent to arterial occlusion, the present study used Rac2-null (Rac2 Ϫ/Ϫ ) and Nox2-null (Nox2 Ϫ/Ϫ ) mice and a novel method of identifying primary hindlimb collaterals. Rac2 is expressed primarily, if not exclusively, in hematopoietic cells (38, 52) and binds to and activates Nox2-containing NAD(P)H oxidase (24,40). In addition, leukocytes from Rac2 Ϫ/Ϫ and Nox2 Ϫ/Ϫ mice have impaired function related to reduced ROS production (47,66,72). We present a method to identify the dominant or primary collateral that should be the major collateral supplying flow to

Acute lung injury (ALI) is a prevalent disease associated with high mortality. 12/15-lipoxygenase (12/15-LO) is an enzyme producing 12-hydroxyeicosatetraenoic acid (HETE) and 15-HETE from arachidonic acid. To test whether 12/15-LO is involved in increasing vascular permeability in the lung, we investigated the role of 12/15-LO in murine models of LPS-induced pulmonary inflammation and clinically relevant acid-induced ALI. The vascular permeability increase upon LPS inhalation was abolished in Alox15−/− mice lacking 12/15-LO and in wild-type mice after pharmacological blockade of 12/15-LO. Alox15−/− mice also showed improved gas exchange, reduced permeability increase, and prolonged survival in the acid-induced ALI model. Bone marrow chimeras and reconstitution experiments revealed that 12-HETE produced by hematopoietic cells regulates vascular permeability through a CXCR2-dependent mechanism. Our findings suggest that 12/15-LO-derived 12-HETE is a key mediator of vascular permeability in acute lung injury.

Mutations in the NF1 tumor suppressor gene cause Neurofibromatosis type 1 (NF1). Neurofibromin, the protein product of NF1, functions as a negative regulator of Ras activity. Some NF1 patients develop cardiovascular disease, which represents an underrecognized disease complication and contributes to excess morbidity and mortality. Specifically, NF1 patients develop arterial occlusion resulting in tissue ischemia and sudden death. Murine studies demonstrate that heterozygous inactivation of Nf1 (Nf1(+/-)) in bone marrow cells enhances neointima formation following arterial injury. Macrophages infiltrate Nf1(+/-) neointimas, and NF1 patients have increased circulating inflammatory monocytes in their peripheral blood. Therefore, we tested the hypothesis that heterozygous inactivation of Nf1 in myeloid cells is sufficient for neointima formation. Specific ablation of a single copy of the Nf1 gene in myeloid cells alone mobilizes a discrete pro-inflammatory murine monocyte population via a cell autonomous and gene-dosage dependent mechanism. Furthermore, lineage-restricted heterozygous inactivation of Nf1 in myeloid cells is sufficient to reproduce the enhanced neointima formation observed in Nf1(+/-) mice when compared with wild-type controls, and homozygous inactivation of Nf1 in myeloid cells amplified the degree of arterial stenosis after arterial injury. Treatment of Nf1(+/-) mice with rosuvastatin, a stain with anti-inflammatory properties, significantly reduced neointima formation when compared with control. These studies identify neurofibromin-deficient myeloid cells as critical cellular effectors of Nf1(+/-) neointima formation and propose a potential therapeutic for NF1 cardiovascular disease.