Hypoxic injury to human alveolar macrophages accelerates release of previously bound neutrophil elastase. Implications for lung connective tissue injury including pulmonary emphysema

Intraluminal thrombus: Innocent bystander or factor in abdominal aortic aneurysm pathogenesis?

BACKGROUND

Abdominal aortic aneurysms (AAAs) represent a complex multifactorial hemodynamic, thrombotic, and inflammatory process that can ultimately result in aortic rupture and death. Despite improved screening and surgical management of AAAs, the mortality rates have remained high after rupture, and little progress has occurred in the development of nonoperative treatments. Intraluminal thrombus (ILT) is present in most AAAs and might be involved in AAA pathogenesis. The present review examined the latest clinical and experimental evidence for possible involvement of the ILT in AAA growth and rupture.

METHODS

A literature review was performed after a search of the PubMed database from 2012 to June 2020 using the terms "abdominal aortic aneurysm" and "intraluminal thrombus."

RESULTS

The structure, composition, and hemodynamics of ILT formation and propagation were reviewed in relation to the hemostatic and proteolytic factors favoring ILT deposition. The potential effects of the ILT on AAA wall degeneration and rupture, including a review of the current controversies regarding the position, thickness, and composition of ILT, are presented. Although initially potentially protective against increased wall stress, increasing evidence has shown that an increased volume and greater age of the ILT have direct detrimental effects on aortic wall integrity, which might predispose to an increased rupture risk.

CONCLUSIONS

ILT does not appear to be an innocent bystander in AAA pathophysiology. However, its exact role remains elusive and controversial. Despite computational evidence of a possible protective role of the ILT in reducing wall stress, increasing evidence has shown that the ILT promotes AAA wall degeneration in humans and in animal models. Further research, with large animal models and with more chronic ILT is crucial for a better understanding of the role of the ILT in AAAs and for the potential development of targeted therapies to slow or halt AAA progression.

The Role of Neutrophils and Neutrophil Elastase in Pulmonary Arterial Hypertension

Pulmonary arterial hypertension (PAH) is a severe vasculopathy characterized by the presence of fibrotic lesions in the arterial wall and the loss of small distal pulmonary arteries. The vasculopathy is accompanied by perivascular inflammation and increased protease levels, with neutrophil elastase notably implicated in aberrant vascular remodeling. However, the source of elevated elastase levels in PAH remains unclear. A major source of neutrophil elastase is the neutrophil, an understudied cell population in PAH. The principal function of neutrophils is to destroy invading pathogens by means of phagocytosis and NET formation, but proteases, chemokines, and cytokines implicated in PAH can be released by and/or prime and activate neutrophils. This review focuses on the contribution of inflammation to the development and progression of the disease, highlighting studies implicating neutrophils, neutrophil elastase, and other neutrophil proteases in PAH. The roles of cytokines, chemokines, and neutrophil elastase in the disease are discussed and we describe new insight into the role neutrophils potentially play in the pathogenesis of PAH.

Local Diameter, Wall Stress, and Thrombus Thickness Influence the Local Growth of Abdominal Aortic Aneurysms

Purpose: To investigate the influence of the local diameter, the intraluminal thrombus (ILT) thickness, and wall stress on the local growth rate of abdominal aortic aneurysms. Methods: The infrarenal aortas of 90 asymptomatic abdominal aortic aneurysm (AAA) patients (mean age 70 years; 77 men) were retrospectively reconstructed from at least 2 computed tomography angiography scans (median follow-up of 1 year) and biomechanically analyzed with the finite element method. Each individual AAA model was automatically sliced orthogonally to the lumen centerline and represented by 100 cross sections with corresponding diameters, ILT thicknesses, and wall stresses. The data were grouped according to these parameters for comparison of differences among the variables. Results: Diameter growth was continuously distributed over the entire aneurysm sac, reaching absolute and relative median peaks of 3.06 mm/y and 7.3%/y, respectively. The local growth rate was dependent on the local baseline diameter, the local ILT thickness, and for wall segments not covered by ILT, also on the local wall stress level (all p<0.001). For wall segments that were covered by a thick ILT layer, wall stress did not affect the growth rate (p=0.08). Conclusion: Diameter is not only a strong global predictor but also a local predictor of aneurysm growth. In addition, and independent of the diameter, the ILT thickness and wall stress (for the ILT-free wall) also influence the local growth rate. The high stress sensitivity of nondilated aortic walls suggests that wall stress peaks could initiate AAA formation. In contrast, local diameters and ILT thicknesses determine AAA growth for dilated and ILT-covered aortic walls.

Local Quantification of Wall Thickness and Intraluminal Thrombus Offer Insight into the Mechanical Properties of the Aneurysmal Aorta

Wall stress is a powerful tool to assist clinical decisions in rupture risk assessment of abdominal aortic aneurysms. Key modeling assumptions that influence wall stress magnitude and distribution are the inclusion or exclusion of the intraluminal thrombus in the model and the assumption of a uniform wall thickness. We employed a combined numerical-experimental approach to test the hypothesis that abdominal aortic aneurysm (AAA) wall tissues with different thickness as well as wall tissues covered by different thrombus thickness, exhibit differences in the mechanical behavior. Ultimate tissue strength was measured from in vitro tensile testing of AAA specimens and material properties of the wall were estimated by fitting the results of the tensile tests to a histo-mechanical constitutive model. Results showed a decrease in tissue strength and collagen stiffness with increasing wall thickness, supporting the hypothesis of wall thickening being mediated by accumulation of non load-bearing components. Additionally, an increase in thrombus deposition resulted in a reduction of elastin content, collagen stiffness and tissue strength. Local wall thickness and thrombus coverage may be used as surrogate measures of local mechanical properties of the tissue, and therefore, are possible candidates to improve the specificity of AAA wall stress and rupture risk evaluations.

Intraluminal abdominal aortic aneurysm thrombus is associated with disruption of wall integrity

OBJECTIVE

An association of intraluminal thrombus (ILT) with abdominal aortic aneurysm (AAA) growth has been suggested. Previous in vitro experiments have demonstrated that aneurysm-associated thrombus may secrete proteolytic enzymes and may develop local hypoxia that might lead to the formation of tissue-damaging reactive oxygen species. In this study, we assessed the hypothesis that ventral ILT thickness is associated with markers of proteolysis and with lipid oxidation in the underlying AAA vessel wall.

METHODS

Ventral AAA tissue was collected from asymptomatic patients at the site of maximal diameter during open aneurysm repair. Segments were divided, one part for biochemical measurements and one for histologic analyses. We measured total cathepsin B, cathepsin S levels, and matrix metalloproteinase (MMP)-2 and MMP-9 activity. Myeloperoxidase and thiobarbituric acid reactive substances were determined as measures of lipid oxidation. Histologic segments were analyzed semiquantitatively for the presence of collagen, elastin, vascular smooth muscle cells (VSMCs), and inflammatory cells. Preoperative computed tomography angiography scans of 83 consecutive patients were analyzed. A three-dimensional reconstruction was obtained, and a center lumen line of the aorta was constructed. Ventral ILT thickness was measured in the anteroposterior direction at the level of maximal aneurysm diameter on the orthogonal slices.

RESULTS

Ventral ILT thickness was positively correlated with aortic diameter (r=0.25; P=.02) and with MMP-2 levels (r=0.27; P=.02). No biochemical correlations were observed with MMP-9 activity or cathepsin B and S expression. No correlation between ventral ILT thickness and myeloperoxidase or thiobarbituric acid reactive substances was observed. Ventral ILT thickness was negatively correlated with VSMCs (no staining, 18.5 [interquartile range, 12.0-25.5] mm; minor, 17.6 [10.7-22.1] mm; moderate, 14.5 [4.6-21.7] mm; and heavy, 8.0 [0.0-12.3] mm, respectively; P=.01) and the amount of elastin (no staining, 18.6 [12.2-30.0] mm; minor, 16.5 [9.0-22.1] mm; moderate, 11.7 [2.5-15.3] mm; and heavy 7.7 [0.0-7.7] mm, respectively; P=.01) in the medial aortic layer.

CONCLUSIONS

ILT thickness appeared to be associated with VSMCs apoptosis and elastin degradation and was positively associated with MMP-2 concentrations in the underlying wall. This suggests that ILT thickness affects AAA wall stability and might contribute to AAA growth and rupture. ILT thickness was not correlated with markers of lipid oxidation.

Vascular remodeling and extracellular matrix breakdown in the uterine spiral arteries during pregnancy

During pregnancy, trophoblasts invade and transform the maternal spiral arteries that supply blood to the placenta. Recent work has revealed that this process occurs in several stages, and details of the molecular and cellular mechanisms are beginning to emerge, including changes that precede or accompany trophoblastic colonization of the vascular media. Disruption and eventual loss of smooth muscle cells and their associated extracellular matrix are central to physiological transformation. Advances in understanding will lead to the identification of the causative factors involved in failure of remodeling in pathological pregnancies and suggest novel diagnostic and therapeutic avenues.

Biomechanical determinants of abdominal aortic aneurysm rupture

Rupture of abdominal aortic aneurysm (AAA) represents a significant clinical event, having a mortality rate of 90% and being currently ranked as the 13th leading cause of death in the US. The ability to reliably evaluate the susceptibility of a particular AAA to rupture on a case-specific basis could vastly improve the clinical management of these patients. Because AAA rupture represents a mechanical failure of the degenerated aortic wall, biomechanical considerations are important to understand this process and to improve our predictions of its occurrence. Presented here is an overview of research to date related to the biomechanics of AAA rupture. This includes a summary of results related to ex vivo and in vivo mechanical testing, noninvasive AAA wall stress estimations, and potential mechanisms of AAA wall weakening. We conclude with a demonstration of a biomechanics-based approach to predicting AAA rupture on a patient-specific basis, which may ultimately prove to be superior to the widely and currently used maximum diameter criterion.

A Review of Biological Factors Implicated in Abdominal Aortic Aneurysm Rupture

Abdominal aortic aneurysm (AAA) rupture is the 13th commonest cause of death in the Western World. Although considerable research has been applied to the aetiology and mechanism of aneurysm expansion, little is known about the mechanism of rupture. Aneurysm rupture was historically considered to be a simple physical process that occurred when the aortic wall could no longer contain the haemodynamic stress of the circulation. However, AAAs do not conform to the law of Laplace and there is growing evidence that aneurysm rupture involves a complex series of biological changes in the aortic wall. This paper reviews the available data on patient variables associated with aneurysm rupture and presents the evidence implicating biological factors in AAA rupture.

Association of intraluminal thrombus in abdominal aortic aneurysm with local hypoxia and wall weakening

PURPOSE

Our previous computer models suggested that intraluminal thrombus (ILT) within an abdominal aortic aneurysm (AAA) attenuates oxygen diffusion to the AAA wall, possibly causing localized hypoxia and contributing to wall weakening. The purpose of this work was to investigate this possibility.

METHODS

In one arm of this study, patients with AAA were placed in one of two groups: (1) those with an ILT of 4-mm or greater thickness on the anterior surface or (2) those with little (< 4 mm) or no ILT at this site. During surgical resection but before aortic cross-clamping, a needle-type polarographic partial pressure of oxygen (PO2) electrode was inserted into the wall of the exposed AAA, and the PO2 was measured. The probe was advanced, and measurements were made midway through the thrombus and in the lumen. Mural and mid-ILT PO2 measurements were normalized by the intraluminal PO2 measurement to account for patient variability. In the second arm of this study, two AAA wall specimens were obtained from two different sites of the same aneurysm at the time of surgical resection: group I specimens had thick adherent ILT, and group II specimens had thinner or no adherent ILT. Nonaneurysmal tissue was also obtained from the infrarenal aorta of organ donors. Specimens were subjected to histologic, immunohistochemical, and tensile strength analyses to provide data on degree of inflammation (% area inflammatory cells), neovascularization (number of capillaries per high-power field), and tensile strength (peak attainable load). Additional specimens were subjected to Western blotting and immunohistochemistry for qualitative evaluation of expression of the cellular hypoxia marker oxygen-regulated protein.

RESULTS

The PO2 measured within the AAA wall in group I (n = 4) and group II (n = 7) patients was 18% +/- 9% luminal value versus 60% +/- 6% (mean +/- SEM; P <.01). The normalized PO2 within the ILT of group I patients was 39% +/- 10% (P =.08 with respect to the group I wall value). Group I tissue specimens showed greater inflammation (P <.05) compared with both group II specimens and nonaneurysmal tissue: 2.9% +/- 0.6% area (n = 7) versus 1.7% +/- 0.3% area (n = 7) versus 0.2% +/- 0.1% area (n = 3), respectively. We found similar differences for neovascularization (number of vessels/high-power field), but only group I versus control was significantly different (P <.05): 16.9 +/- 1.6 (n = 7) vs 13.0 +/- 2.3 (n = 7) vs 8.7 +/- 2.0 (n = 3), respectively. Both Western blotting and immunohistochemistry results suggest that oxygen-regulated protein is more abundantly expressed in group I versus group II specimens. Tensile strength of group I specimens was significantly less (P <.05) than that for group II specimens: 138 +/- 19 N/cm2 (n = 7) versus 216 +/- 34 N/cm2 (n = 7), respectively.

CONCLUSION

Our results suggest that localized hypoxia occurs in regions of thicker ILT in AAA. This may lead to increased, localized mural neovascularization and inflammation, as well as regional wall weakening. We conclude that ILT may play an important role in the pathology and natural history of AAA.

Endogenous lipid pneumonia- and alveolar proteinosis-type changes in the vicinity of non-small cell lung cancer: histiopathologic, immunohistochemical, and ultrastructural evaluation

Endogenous lipid pneumonia (ELP) and pulmonary alveolar proteinosis (AP) are the most common pathological conditions of lung tissue in which there is an increased accumulation of surfactant elements in pulmonary alveoli. This paper evaluates the morphology, morphogenesis, and pathogenesis of ELP- and AP-type changes in the immediate vicinity of non-small cell lung cancer (NSCLC). The analyses used lung tissue obtained from the vicinity of 56 NSCLC. ELP- or/and AP-type changes in histopathological studies were found in lung tissue surrounding 35 NSCLC. Ultrastructural analyses by transmission electronmicroscopy identified 15 cases with advanced ELP and confirmed the possibility of coexistence of ELP- and AP-type changes in the vicinity of NSCLC and the evolution of ELP into AP. Most cases analyzed showed morphological features characteristic of both AP and ELP, the intensity and degree of these changes being considerably differentiated. Type II pneumocytes, macrophages, and neutrophilic granulocytes play a significant part in the morphogenesis of ELP and AP.

Disodium cromoglycate in the treatment of bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) has become the most common form of chronic lung disease in the neonate. Recently, we have experienced a severe case of BPD and examined the effect of disodium cromoglycate (DSCG) on BPD. The gestational age and birthweight of the patient were 27 weeks and 1,000 g, respectively. Although RDS subsided after surfactant replacement therapy, the arterial-alveolar oxygen tension ratio (a/APO2) gradually decreased and FiO2 increased with age, respectively, and pure oxygen supplementation was eventually required after 67 days of life. The DSCG treatment was commenced at 80 days of life. After 6 days of the inhalation therapy, a/APO2 gradually increased. After 10 days of the treatment, the baby was extubated. While the baby was intubated, intratracheal lavage fluid samples were obtained. Eosinophilic cationic protein (ECP) and polymorphonuclear (PMN) elastase concentrations were determined. ECP and PMN elastase concentrations of intratracheal lavage fluids gradually decreased with the DSCG treatment. These results may indicate that DSCG has led to an improvement of pulmonary function and facilitated weaning from mechanical ventilation in an infant with BPD.

Neutrophil accumulation in the lung in alpha 1-antitrypsin deficiency. Spontaneous release of leukotriene B4 by alveolar macrophages

The emphysema of alpha 1-antitrypsin (alpha 1AT) deficiency is conceptualized to result from insufficient alpha 1AT allowing neutrophil elastase to destroy lung parenchyma. In addition to the deficiency of alpha 1AT in these individuals resulting from mutations in the alpha 1AT gene, it is recognized that, for unknown reasons, there are also increased numbers of neutrophils in their lungs compared with normal individuals. With the knowledge that alveolar macrophages have surface receptors for neutrophil elastase, we hypothesized that the neutrophil accumulation in the lower respiratory tract in alpha 1AT deficiency may result, in part, from release of neutrophil chemotactic activity by alveolar macrophages as they bind uninhibited neutrophil elastase. Consistent with this hypothesis, alpha 1AT-deficient alveolar macrophages spontaneously released nearly threefold more neutrophil chemotactic activity than normal alveolar macrophages. Analysis of alpha 1AT-deficient macrophage supernates by reverse-phase HPLC, molecular sieve chromatography, radioimmunoassay, and absorption with anti-LTB4 antibody revealed that the majority of the chemotactic activity was leukotriene B4 (LTB4), a mediator absent from normal macrophage supernates. Consistent with this hypothesis, incubation of normal macrophages with human neutrophil elastase resulted in the release of the same neutrophil chemotactic mediator. Furthermore, purified human alpha 1AT was able to prevent the neutrophil elastase from stimulating the macrophages to release the chemotactic factor. Together, these findings suggest that the absence of a normal antineutrophil elastase screen in the lower respiratory tract permits free neutrophil elastase to bind to alveolar macrophages, resulting in the release of LTB4, a process which attracts neutrophils to the alveoli of alpha 1AT deficient individuals, thus accelerating the lung destruction that characterizes this disorder.

Uptake of extracellular enzyme by a novel pathway is a major determinant of cathepsin L levels in human macrophages

The phorbol myristate acetate (PMA)-differentiated myelomonocytic cell line, THP-1, and human alveolar macrophages contain the cysteine proteinase cathepsin L. This enzyme is synthesized as a 43-kD proenzyme and processed to the active 25-kD form. Differentiation of THP-1 cells in the presence of human serum resulted in an increase in the size of the vacuolar compartment and the accumulation of more 25-kD cathepsin L antigen, as compared with THP-1 cells differentiated in the presence of fetal calf serum. Cells cultured in both types of sera have equivalent levels of cathepsin L mRNA. Metabolic labeling experiments demonstrated equivalent rates of synthesis, processing to the active form, and persistence in both culture conditions. An extracellular source of enzyme was documented by immunoblotting human serum which demonstrated 25-kD cathepsin L antigen; furthermore, we demonstrated that both THP-1 cells, differentiated in human serum, and human alveolar macrophages take up the 43-kD proenzyme and process it to the 25-kD form. Thus, human serum contains a factor(s) that induces both a marked increase in the size of the vacuolar compartment in differentiated THP-1 cells and a novel pathway that is responsible for the uptake and processing of extracellular cathepsin L. The activity of this inducible pathway is a major determinant of levels of intracellular cathepsin L. Cathepsin L is a potent elastase and the regulation of its uptake and processing may play a role in the pathogenesis of disease processes characterized by the destruction of elastin, such as pulmonary emphysema.

Control of exogenous proteinases and their inhibitors at the macrophage cell surface

The actions and availability of human neutrophil elastase and its protein inhibitor, Eglin, when co-incubated with macrophages were investigated. Eglin did not induce radical production by mouse peritoneal macrophages; nor were specific binding sites for Eglin detected on these cells. Mouse peritoneal macrophages could inactivate both elastase and Eglin extensively, when these targets were used at concentrations appropriate to the extravascular fluids. Two methods were used for assessing such inactivation: one, as in previous literature, only took account of molecules remaining in the supernatant after interaction with the cells; the other (lacking from most previous studies) took into account all target molecules, including those associated with the cells. From an analysis of both types of experiment, it was shown that the cell-derived inactivators were stable products, whose quantity was not significantly influenced by the induction of a macrophage oxidative burst and its associated free radicals. They were probably mainly proteinases and proteinase inhibitors. Thus, mouse peritoneal macrophages restrict the activity of proteinases and inhibitors by means of stable molecules, such as proteins. Other mononuclear phagocytes may use free radicals and oxidants more extensively in this respect.

The effect of activation of macrophages on the advancement of experimental lung emphysema

Experiments were carried out on 68 Wistar rats of both sexes. Five s.c. injections of BCG vaccine (once a week) were applied as macrophage mobilizing and activating agent. Papain or physiological saline was given once intratracheally after the 3rd dose of BCG. 4 weeks after the latter substances had been administered the animals were sacrificed. The lungs and the pulmonary alveolar macrophages (PAM) were examined using histological and morphometric techniques; SEM and TEM were applied for assessing ultrastructure of PAM and lung sections. In rats given BCG the number of PAM was increased, and their morphological features showed enhanced activity. In these animals the aggravated emphysematous changes induced by papain should be ascribed to the cumulation of PAM and their increased activity in the lungs.

Age-dependent variations of intralysosomal enzyme release from human PMN leukocytes under various stimuli

The intralysosomal beta glucuronidase and elastase release from polymorphonuclear leukocytes (PMNL) of young and aged male subjects were determined after 60-min incubation with 10 micrograms/ml Cytochalasin B (CB), 10(-6) M of Ca ionophore A 23187 and various concentrations of human low density lipoprotein (LDL). The beta glucoronidase secretion was triggered by both A 23187 and LDL; however, no significant differences were found between the enzyme release from PMNLs of young and aged subjects. In contrast, a marked elastase release was triggered in the young group only by LDL, whereas in the aged group, all of the applied drugs induced a significant elastase release. LDL caused the most dramatic enzyme release from PMNLs of aged males. It was concluded that the release of PMNL-elastase after LDL incorporation as well as by CB and Ca ionophore stimulation may be an age-related process.

Absorption of elastase through the jejunal mucosa of the rat. An immunocytochemical study

In order to reveal the absorption process of elastase from the intestine, hog pancreatic elastase was injected into the ligated jejunum lumen of the rat, and the tissues were cytochemically observed at various times after injection. The peroxidase anti-peroxidase (PAP) method using anti-hog-elastase rabbit antibody was used for light microscopy, and the anti-elastase Fab'-peroxidase conjugate was used for electron microscopy. The tissues stained by the PAP method exhibited a dense deposition of reaction products on the luminal surface of epithelial cells and a moderate deposition in the blood and lymph capillaries of the intestinal villi. Immunoelectron microscopy revealed that the reaction product was deposited on the surface of the microvilli and in their pocketing; some was found in the pinocytotic vesicles in the terminal-web area and on the inner surface of the enlarged smooth endoplasmic reticulum. Round droplets which gave a positive reaction were found in the widened intercellular cleft and the thick basement membrane lining the blood capillaries and lymphatics. The jejunum retained its normal ultrastructure. The results indicate that the elastase molecules, which were introduced into the rat jejunum lumen, were absorbed without being decomposed through healthy intestinal epithelial cells by pinocytosis and translocated into blood and lymph capillaries.