Analysis of the T cell receptor V beta repertoire in human aortic aneurysms

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.

Immune responses elicited by apoB-100-derived peptides in mice

Peptides derived from apolipoprotein B (apoB)-100 have been previously used in vaccine preparations to treat atherosclerosis. Such vaccines have been shown to reduce atherosclerotic plaque development by 50 % in experimental animals, and this effect is associated with induction of T helper (Th)2 immune responses. In this study we immunised apolipoprotein E-deficient (apoE(-/-)) mice with apoB-100-derived peptides P2, P45 and P210. Animals received BSA-conjugated peptides or peptide-loaded bone marrow-derived dendritic cells (DCs). We used enzyme-linked immunosorbent assays to assess the synthesis of anti-peptide-specific IgG1 and IgG2a as well as the levels of interleukin (IL-)10 and interferon gamma (IFN-γ) in plasma of immunised animals. We also measured the effect of immunisation on the number of spleen-derived CD4(+) and CD8(+) regulatory T cells (Tregs) in these animals. Peptide and peptide-loaded DC immunisation significantly increased the levels of peptide-specific immunoglobulins and the number of Tregs in apoE(-/-) mice. This was accompanied by a significant increase in the secretion of IL-10 with no effect on IFN-γ levels. The results also show that the peptides can modulate the homing properties of DCs. Altogether, this study provides novel evidence for the immune mechanisms excerpted by apoB-100-derived peptides and their effect on Tregs and DCs relevant to their use in vaccine preparations.

Syndecan-1 displays a protective role in aortic aneurysm formation by modulating T cell-mediated responses

OBJECTIVE

Chronic inflammation drives progressive and pathological remodeling inherent to formation of abdominal aortic aneurysm (AAA). Syndecan-1 (Sdc-1) is a cell surface heparan sulfate proteoglycan that displays the capacity to modulate inflammatory processes within the vascular wall. In the current investigation, the role of Sdc-1 in AAA formation was examined using 2 models of experimental aneurysm induction, angiotensin II infusion and elastase perfusion.

METHODS AND RESULTS

Sdc-1 deficiency exacerbated AAA formation in both experimental models and was associated with increased degradation of elastin, greater protease activity, and enhanced inflammatory cell recruitment into the aortic wall. Bone marrow transplantation studies indicated that deficiency of Sdc-1 in marrow-derived cells significantly contributed to AAA severity. Immunostaining revealed augmented Sdc-1 expression in a subset of AAA localized macrophages. We specifically characterized a higher percentage of CD4(+) T cells in Sdc-1-deficient AAA, and antibody depletion studies established the active role of T cells in aneurysmal dilatation. Finally, we confirmed the ability of Sdc-1 macrophage to modulate the inflammatory chemokine environment.

CONCLUSIONS

These investigations identify cross-talk between Sdc-1-expressing macrophages and AAA-localized CD4(+) T cells, with Sdc-1 providing an important counterbalance to T-cell-driven inflammation in the vascular wall.

The role of human leukocyte antigen genes in the formation of abdominal aortic aneurysms

BACKGROUND

Increasing evidence suggests an autoimmune component to abdominal aortic aneurysm (AAA) formation. This study was conducted to determine if a difference exists in human leukocyte antigen (HLA) allele distribution between patients with AAA and population controls, and between patients with small and large AAA.

METHODS

Patients with known AAA attending the vascular unit were consented for recruitment. HLA-A, HLA-B and HLA-DR was determined by polymerase chain reaction and sequence-specific oligonucleotide probes. The distribution of these alleles in the Northern Ireland general population was obtained from the histocompatibility and immunogenetics database. The chi(2) test was used for statistical analysis with Bonferroni correction.

RESULTS

A total of 241 AAA patients were recruited, with a wide range of aneurysm size. In class I, the most frequent allele families were HLA-A*02 and *01 and HLA-B*07, *08, and *44. In class II, HLA-DRB1*03, *04, *07, and *15 were the most frequent. HLA-A*11 was lower in AAA cases (10.4% vs 15.0%; P = .08), whereas HLA-B*08 was lower in the controls (29.8% vs 36.5%; P = .05) and HLA-DRB1*11 was lower in cases (4.2% vs 8.1%; P = .05). After Bonferroni correction, however, the proportion of allele families was not significantly different in AAA patients compared with the proportion seen in controls. HLA-DRB1*11 and *14 had a lower prevalence in large AAAs (0.9% vs 6.7% [P = .05]; 0.0% vs 5.9% [P = .03]). HLA-A*68 was also lower in large AAA (1.9% vs 11.9%; P = .0075). After Bonferroni correction, however, no difference was demonstrated between small and large aneurysms.

CONCLUSION

This study provides more definitive results on this important subject and has failed to demonstrate the risk association between AAA and these alleles as reported by others. Therefore, the role of these particular genes and the autoimmune component in AAA etiology does not appear to be as crucial as previously proposed.

Topological determinants and consequences of adventitial responses to arterial wall injury

Arteries are composed of 3 concentric tissue layers which exhibit different structures and properties. Because arterial injury is generally initiated at the interface with circulating blood, most studies performed to unravel the mechanisms involved in injury-induced arterial responses have focused on the innermost layer (intima) rather than on the outermost adventitial layer. In the present review, we focus on the involvement of the adventitia in response to various types of arterial injury leading to vascular remodeling. Physiologically, soluble vascular mediators are centrifugally conveyed by mass transport toward the adventitia. Moreover, in pathological conditions, neomediators and antigens can be generated within the arterial wall, whose outward conveyance triggers different patterns of local adventitial response. Adventitial angiogenesis, immunoinflammation, and fibrosis sequentially interact and their net balance defines the participation of the adventitial response in arterial pathology. In the present review we discuss 4 pathological entities in which the adventitial response to arterial wall injury participates in arterial wall remodeling. Hence, the adventitial adaptive immune response predominates in chronic rejection. Inflammatory phagocytic cell recruitment and initiation of a shift from innate to adaptive immunity characterize the adventitial response to products of proteolysis in abdominal aortic aneurysm. Adventitial sprouting of neovessels, leading to intraplaque hemorrhages, predominates in atherothrombosis. Adventitial fibrosis characterizes the response to mechanical stress and is responsible for the constrictive remodeling of arterial segments and initiating interstitial fibrosis in perivascular tissues. These adventitial events, therefore, have an impact not only on the vessel wall biology but also on the surrounding tissue.

Pathogenesis of thoracic and abdominal aortic aneurysms

The major disease processes affecting the aorta are aortic aneurysms and dissections. Aneurysms are usually described in terms of their anatomic location, with thoracic aortic aneurysms (TAAs) involving the ascending and descending aorta in the thoracic cavity and abdominal aortic aneurysms (AAAs) involving the infrarenal abdominal aorta. Both thoracic and abdominal aortas are elastic arteries, and share similarities in their physical structures and cellular components. However, thoracic and abdominal aortas differ in their biochemical properties and the origin of their vascular smooth muscle cells (VSMCs). These similarities and differences between thoracic and abdominal aortas provide the basis for the various pathologic mechanisms observed in this disease. This review focuses on the comparison of the pathologic mechanisms involved in TAA and AAA.

Chronic Inflammation, Immune Response, and Infection in Abdominal Aortic Aneurysms

Abdominal aortic aneurysms (AAA) are associated with atherosclerosis, transmural degenerative processes, neovascularization, decrease in content of vascular smooth muscle cells, and a chronic infiltration, mainly located in the outer aortic wall. The chronic infiltration consists mainly of macrophages, lymphocytes, and plasma cells. The dominant cells are Th2 restricted CD3+ lymphocytes expressing interleukine 4, 5, 8, and 10, and tumor necrosis factor-alpha for regulation of the local immune response. They also produce interferon-gamma and CD40 ligand to stimulate surrounding cells to produce matrix metalloproteases and cysteine proteases for aortic matrix remodeling. The lymphocyte activation may be mediated by microorganisms as well as autoantigens generated from vascular structural proteins, perhaps through molecular mimicry. As in autoimmune diseases, the risk of AAA is increased by certain genotypes concerning human leucocyte antigen class II. These types are also associated with increased aneurysmal inflammation indicating a genetic susceptibility to aortic inflammation. Chlamydia pneumoniae is often detected in AAA but the validity of the methods can be questioned, and two small antibiotic trials have been disappointing. However, serum antibodies against C. pneumoniae have been associated with AAA growth and cross-react with AAA wall proteins. Thus, immune responses mediated by microorganisms and autoantigens may play a pivotal role in AAA pathogenesis.

Adaptive cellular immunity in aortic aneurysms: cause, consequence, or context?

Abdominal aortic aneurysms are common and life threatening. Although CD4(+) T cells are abundant in aneurysm tissue, their role in disease progression remains unclear. A new study shows that mouse aortic allografts placed in animals lacking IFN-gamma receptors develop a Th2 inflammatory response with aortic aneurysms, whereas Th1 responses promote intimal hyperplasia. It is expected that these surprising findings will stimulate further efforts to clarify whether adaptive cellular immunity in aneurysm disease is detrimental or potentially beneficial.

T(H)2 predominant immune responses prevail in human abdominal aortic aneurysm

T lymphocytes localize within lesions of two diametrically opposed expressions of atherosclerosis: stenosis-producing plaques and ectasia-producing abdominal aortic aneurysm (AAA). T(H)1 immune responses appear to predominate in human stenotic lesions. However, little information exists regarding the nature of the T-cell infiltrate in AAAs. We demonstrate here that AAAs predominantly express T(H)2-associated cytokines and correspondingly lack mediators associated with the T(H)1 response as determined by Western blot and immunohistochemical analysis. In particular, aneurysmal tissue expressed interleukin (IL)-4, IL-5, and IL-10, cytokines not or only faintly detected in nondiseased tissue or stenotic atheroma. In contrast, AAAs contained low levels of the T(H)1 characteristic cytokines IL-2 and IL-15, which are amply expressed in stenotic lesions. Notably, stenotic lesions, but not AAAs, contained mature forms of the interferon-gamma-inducing cytokines IL-12 and IL-18 as well as the IL-18-processing enzyme caspase-1. Moreover, aneurysmal tissue lacked the receptor for interferon-gamma, although both types of lesions contained this T(H)1-promoting cytokine. These findings suggest that the functional repertoire of T cells differs in stenotic and aneurysmal lesions, and provide a novel framework for understanding the mechanisms of these diametrically opposite expressions of atherosclerosis.

Vascular-associated lymphoid tissue (VALT) involvement in aortic aneurysm

Vascular-associated lymphoid tissue (VALT) consisting of accumulations of immunocompetent and antigen presenting cells has recently been recognised in the arterial wall. In this study, we investigated the involvement of VALT in immune responses in abdominal aortic aneurysms (AAAs). Tissue samples were collected during operations from 31 patients with atherosclerotic infrarenal abdominal aortic aneurysms ranging in diameters from 5 to 8 cm. The specimens were immediately frozen and examined using single and double immunohistochemical staining. T-cell subpopulations, B-cells, dendritic cells and macrophages were identified using cell type specific antibodies. Cell contacts were examined by electron microscopy. Most inflammatory infiltrates were found in the adventitia. T-cells were the predominant cell type in a majority of inflammatory infiltrates but in seventeen cases, typical lymphoid follicles with B-cells forming germinative centres were also observed. In eight cases, the lymphoid follicles aggregated in lymph node-like structures. Dendritic cells were present within all inflammatory infiltrates and contacted lymphocytes. The present observations show that in aortic aneurysm, VALT is involved in immune responses and its activation mostly occurs in the adventitia. The formation of lymphoid follicles and lymph node-like structures in the adventitia suggests that VALT might locally serve the entire complex of both cellular and humoral immune responses in the aneurysmal wall.

Regulation of lysyl oxidase by interferon-gamma in rat aortic smooth muscle cells

Lysyl oxidase is an essential catalyst for the cross-linking of extracellular collagen and elastin. Abnormalities in lysyl oxidase activity may contribute to the pathogenesis of arterial diseases characterized by abnormal matrix remodeling. This study tested the hypothesis that interferon (IFN)-gamma, a proinflammatory cytokine present in aortic aneurysm and arteriosclerotic plaque rupture, downregulates lysyl oxidase gene expression in rat aortic smooth muscle cells. Steady-state lysyl oxidase mRNA levels decreased in a concentration- and time-dependent manner to 30% of control levels after 24 hours of treatment with IFN-gamma. Cell layer lysyl oxidase activity decreased in parallel with the observed changes in steady-state mRNA. Nuclear runoff studies suggested that transcriptional regulation was responsible for at least 40% of the observed downregulation. mRNA decay studies suggested that IFN-gamma also decreased lysyl oxidase mRNA half-life from 9 to 6 hours. Downregulation of lysyl oxidase by IFN-gamma did not appear to require new protein synthesis. This study documents that IFN-gamma downregulates lysyl oxidase gene expression in rat aortic smooth muscle cells by transcriptional and posttranscriptional mechanisms. If similar regulation occurs in vivo, it is possible that IFN-gamma-mediated changes in lysyl oxidase may contribute to arterial diseases characterized by abnormal extracellular matrix.