Immunoreactivity of adventitial matrix fibrils of normal and aneurysmal abdominal aorta with antibodies against vitronectin and fibrinogen

Autoimmunity in the Abdominal Aortic Aneurysm and its Association with Smoking

Smoking increases the risk of abdominal aortic aneurysm (AAA) in both humans and mice, although the underlying mechanisms are not completely understood. An adventitial aortic antigen, AAAP-40, has been partially sequenced. It has motifs with similarities to all three fibrinogen chains and appears to be connected in evolution to a large family of proteins called fibrinogen-related proteins. Fibrinogen may undergo non-enzymatic nitration, which may result from exposure to nitric oxide in cigarette smoke. Nitration of proteins renders them more immunogenic. It has recently been reported that anti-fibrinogen antibody promotes AAA development in mice. Also, anti-fibrinogen antibodies are present in patients with AAA. These matters are reviewed in the overall context of autoimmunity in AAA. The evidence suggests that smoking amplifies an auto-immune reaction that is critical to the pathogenesis of AAA.

Aortic aneurysm association with SLE - a case-control study

OBJECTIVES

Aortic aneurysm is a life threatening cardiovascular complication in patients with systemic lupus erythematosus (SLE).The purpose of this study was to investigate the association between SLE and occurrence of aortic aneurysms.

METHODS

Patients with SLE were compared with age- and sex-matched controls regarding the proportion of aortic aneurysm in a case-control study. Chi-square and t-tests were used for univariate analysis and a logistic regression model was used for multivariate analysis. The study was performed utilizing the medical database of Clalit Health Services.

RESULTS

The study included 5018 patients with SLE and 25,090 age- and sex-matched controls. The proportion of aortic aneurysm in patients with SLE was increased compared with the proportion in controls (0.6% and 0.1%, respectively, p < 0.001). In a multivariate analysis SLE was associated with the coexistence of aortic aneurysms (odds ratio 2.06, 95% confidence interval 1.21-3.51).

CONCLUSIONS

Patients with SLE have a higher proportion of aortic aneurysms as compared with matched controls. Therefore, physicians treating patients with SLE should be aware of this life threatening association.

Exploring autoimmunity in the pathogenesis of abdominal aortic aneurysms

The abdominal aortic aneurysm (AAA) is a disease process that carries significant morbidity and mortality in the absence of early identification and treatment. While current management includes surveillance and surgical treatment of low- and high-risk aneurysms, respectively, our narrow understanding of the pathophysiology of AAAs limits our ability to more effectively manage and perhaps even prevent the occurrence of this highly morbid disease. Over the past couple of decades, there has been considerable interest in exploring the role of autoimmunity as an etiological component of AAA. This review covers the current literature pertaining to this immunological process, focusing on research that highlights the local and systemic immune components found in both human patients and murine models. A better understanding of the autoimmune mechanisms in the pathogenesis of AAAs can pave the way to novel and improved treatment strategies in this patient population.

Fibrinogen-specific antibody induces abdominal aortic aneurysm in mice through complement lectin pathway activation

Abdominal aortic aneurysm (AAA) is a common vascular disease associated with high mortality rate due to progressive enlargement and eventual rupture. There is currently no established therapy known to alter the rate of aneurysmal expansion. Thus, understanding the processes that initiate and sustain aneurysmal growth is pivotal for the development of medical therapies aimed at halting disease progression. Using an elastase-induced AAA mouse model that recapitulates key features of human AAA, we previously reported that a natural IgG antibody directs alternative pathway complement activation and initiates the inflammatory process that culminates in aneurysmal development. The target of this natural antibody, however, was unknown. Herein we identify a natural IgG that binds to fibrinogen deposited in elastase-perfused aortic tissues, activates the complement lectin pathway (LP), and induces AAA. Moreover, we establish that alterations in the glycosylation patterns of this antibody critically affect its ability to activate the LP in vivo. We find that LP activation precedes the alternative pathway and absence of the LP complement protein mannan-binding lectin abrogates elastase-induced AAA. In human AAA tissues the mouse anti-fibrinogen antibody recognizes epitopes that localize to the same areas that stain positively for mannan-binding lectin, which suggests that the complement LP is engaged in humans as well. Lastly, we demonstrate that circulating antibodies in a subset of AAA patients react against fibrinogen or fibrinogen-associated epitopes in human aneurysmal tissues. Our findings support the concept that an autoimmune process directed at aortic wall self-antigens may play a central role in the immunopathogenesis of AAA.

Antibody directs properdin-dependent activation of the complement alternative pathway in a mouse model of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a complex inflammatory vascular disease. There are currently limited treatment options for AAA when surgery is inapplicable. Therefore, insights into molecular mechanisms underlying AAA pathogenesis may reveal therapeutic targets that could be manipulated pharmacologically or biologically to halt disease progression. Using an elastase-induced AAA mouse model, we previously established that the complement alternative pathway (AP) plays a critical role in the development of AAA. However, the mechanism by which complement AP is initiated remains undefined. The complement protein properdin, traditionally viewed as a positive regulator of the AP, may also initiate complement activation by binding directly to target surfaces. In this study, we sought to determine whether properdin serves as a focal point for the initiation of the AP complement activation in AAA. Using a properdin loss of function mutation in mice and a mutant form of the complement factor B protein that produces a stable, properdin-free AP C3 convertase, we show that properdin is required for the development of elastase-induced AAA in its primary role as a convertase stabilizer. Unexpectedly, we find that, in AAA, natural IgG antibodies direct AP-mediated complement activation. The absence of IgG abrogates C3 deposition in elastase-perfused aortic wall and protects animals from AAA development. We also determine that blockade of properdin activity prevents aneurysm formation. These results indicate that an innate immune response to self-antigens activates the complement system and initiates the inflammatory cascade in AAA. Moreover, the study suggests that properdin-targeting strategies may halt aneurysmal growth.

Differential gene expression of 36-kDa microfibril-associated glycoprotein (MAGP-36/MFAP4) in rat organs

By using quantitative Western blot analysis and the real time polymerase chain reaction technique, we investigated the differential gene expression of microfibril-associated glycoprotein (MAGP-36) in rat organs. The gene was expressed highly in sites rich in elastic fibers, such as aorta, skin, and esophagus. However, MAGP-36 was also expressed highly in some other sites containing no elastic fibers. In lung and trachea, the expression levels of MAGP-36 mRNA were about seven times higher than those in other elastic tissues, although the protein abundances were almost at the same levels as other elastic tissues. MAGP-36 seemed to be secreted outside these organs. In brain, kidney, and spleen, although the expression levels of MAGP-36 mRNA were low, substantial amounts of MAGP-36 protein were detected. An immunohistochemical study revealed that MAGP-36 was present at the brush border of the S3 segment of proximal tubules in kidney. Since MAGP-36 is known to bind to mannan, MAGP-36 might be involved in mannose transport in the S3 segment. Thus, MAGP-36 might be multifunctional and present in a wide variety of sites in various organs.

36-kDa microfibril-associated glycoprotein (MAGP-36) is an elastin-binding protein increased in chick aortae during development and growth

MAGP-36 was discovered in porcine aorta in 1989 and is thought to be one of the microfibril-associated proteins. MAGP-36 has been localized on the surface of elastic fibers or laminae in immunohistochemical studies. However, its functional role in the aorta is obscure. Herein, we report on the binding activity of MAGP-36 to components of the aortic wall and its accumulation pattern in the aorta during development and growth. In vitro, MAGP-36 bound to elastin and collagen in a Ca(2+)-dependent manner, and mediated the adhesion of human aortic smooth muscle cells. This cell adhesion mostly depended on the RGD-containing domain of MAGP-36. We examined the accumulation of MAGP-36 with quantitative Western blot analysis and immunoelectron microscopy in chick aortae during development and growth. The amount of MAGP-36 increased on the surface of elastic fibers or laminae between days 14 and 34 after the start of incubation, and reached a plateau at about 53 days. This accumulation of MAGP-36 roughly correlated with an increase in blood pressure for this period. Thus, MAGP-36 might be a bridging protein that connects elastin to other components of the aortic wall and might play a role in maintaining the integrity of the aortic structure under arterial pressure.

The role of a putative microfibrillar protein (80 kDa) in abdominal aortic aneurysm disease

BACKGROUND

We previously have reported the partial amino acid sequence of a putative aortic autoantigen in patients with abdominal aortic aneurysm (AAA) disease that has homologies with an elastin-associated microfibrillar protein found in aorta of pigs. This study was conducted to further define the role that microfibrillar proteins may play as autoantigens in AAA disease.

MATERIALS AND METHODS

An extraction procedure was performed on AAA tissue using high concentrations of guanidinium hydrochloride (GuHCl) under reducing conditions. The microfibrillar extract was then probed with immunoglobulin (Ig) G isolated with Protein A from phosphate-buffered saline (PBS) extracts of 10 AAA specimens and 6 atherosclerotic, nonaneurysmal aortas. Immunoblotting was also performed with serum IgG from 9 AAA patients and 9 normal control patients. Immunohistochemistry using goat anti-human IgG (Fc-specific) on AAA tissue and AAA wall IgG on normal aorta were also performed.

RESULTS

Eight of 10 AAA wall IgG reacted with an 80-kDa protein from the aortic microfibrillar extract, compared to 0 out of 6 atherosclerotic wall IgG (P = 0.0035, Fischer's Exact Test). Staining of the 80-kDa band appeared to increase with progressive additions of GuHCl, up to extract SKGCGC. Immunoblotting using serum IgG from 9 AAA patients and 9 normal control patients on the aneurysm microfibrillar extracts revealed no reactive bands. Immunohistochemistry using IgG from AAA wall showed the localization of the antibodies to the adventitial connective tissue matrix, mainly collagen fibers.

CONCLUSIONS

These observations suggest that a collagen-associated protein, extractable by a microfibrillar extraction procedure from aortic aneurysm tissue, may be among the targets of an autoimmune response in AAA disease.

Abdominal aortic aneurysm as an autoimmune disease

The pathogenesis of abdominal aortic aneurysms (AAAs) is unknown. We hypothesize that the autoimmune disease process plays a key role in the development of AAAs. Both cellular and humoral immunity is involved in the pathogenesis of AAAs. Triggers of autoimmunity are multifactorial. Certain HLA typing is closely related to AAAs, and a certain viral infection may have a potential role in the etiology of AAA via a molecular mimicry mechanism. The autoantigen is located in the microfibrillar compartment of the aortic wall as a normal structure. Patients with AAA are immunoreactive with this novel structural protein. If in the future the autoantigen is fully elucidated, serum testing to detect antibody against the autoantigen can be performed.

Initial steps in the unifying theory of the pathogenesis of artery aneurysms

OBJECTIVES

The objective of this study was to investigate the changes in histology and proteolysis and the role of apoptosis in the development of peripheral artery aneurysms.

METHODS

Eighteen popliteal, 6 abdominal aortic, 10 iliac, 2 carotid, and 6 femoral artery aneurysm specimens were obtained from patients undergoing elective surgical repair. All were males with ages 48 to 93 (mean 71 years). Normal controls were obtained from patients matched for age, sex, and major risk factors. Vascular smooth muscle cells (VSMC), macrophages, T lymphocytes, and apoptosis-regulating molecules were detected immunohistochemically. Detection of apoptosis was by TUNEL assay. Proteolytic activity was determined by 10% gelatin gel zymography.

RESULTS

Paucity of VSMCs, increased amount of inflammatory infiltrate, and fragmentation of elastic lamellae were observed in aneurysmal tissues as compared to normal arteries (P < 0.02). There is increased gelatinolytic activity at 92, 84, 72, and 67 kDa in the aneurysmal tissues. There are fewer CD68+ macrophages and T cells in the media of controls than in the aneurysms (P = 0.01). Apoptosis is significantly high in aneurysm tissues (P < 0.01) and the degree of apoptosis was in the order AAA>FAA>PAA>IAA>CAA. There is increased expression of Bax, CPP-32, Fas, and p53 in PAA specimens as compared to normal popliteal arteries (P < 0.05).

CONCLUSIONS

These data confirm the apparent architectural disruption and loss of VSMCs that are hallmarks of aneurysm development, in peripheral artery aneurysms. Apoptosis and signaling molecules capable of initiating cell death may play a significant role in the pathogenesis of all aneurysms. Our data suggest a common etiology between the various types of aneurysms.