Chronic inflammation of the arterial wall has been implicated in the development of abdominal aortic aneurysm (AAA). However, the detailed molecular mechanism(s) by which inflammatory cells contributes to AAA pathogenesis remains largely unclear. In their article in Clinical Science, Krishna et al. have reported that depletion of CD11c+ dendritic cells inhibited experimental AAA formation in mice. The authors also demonstrated a decrease in CD4 and CD8 positive T cells in the circulation, lower plasma neutrophil elastase activity, and aortic matrix remodeling. These novel findings will help clarify the underlying mechanisms of AAA progression and may provide a new target for future therapeutic research in AAA formation.
Abdominal aortic aneurysm (AAA) is a cardiovascular disease associated with several risk factors such as age, male gender, a positive family history and smoking . Surgical repair is the only effective treatment for AAA upon development and no pharmacological therapy has been established to slow AAA or prevent AAA rupture in humans . It is a general consensus that human AAAs have multifactorial aspects including genetic predisposition, environmental and mechanical stimulation, matrix metalloprotease (MMP)-mediated structural damage, and complex inflammatory processes [1,3,4]. However, detailed molecular mechanism by which certain inflammatory cell type(s) specifically contribute to the AAA etiology remains largely elusive .
Dendritic cells (DCs) are responsible for uptake of antigen, processing, and antigen presentation to T cells and B cells . Upon detection of external pathogens, proinflammatory DCs signal to peripheral immune tissues via the blood and lymphoid systems, elevate the expression of major histocompatibility complex (MHC) class II, and stimulate adaptive immune responses to produce B cells and plasma cells [7,8]. However, various cells with the characteristics of DCs are also found in tissues with low grade chronic inflammation [9,10]. The characteristic and fate of DCs is determined by a surrounding environment. Maturing DCs in the periphery stimulate immune response by linking the innate and adaptive immune system, whereas immature DCs maintain the immunological homeostasis by acting as regulatory cells that induce immune tolerance [10,11].
Upon AAA progression, a variety of immune cells including lymphocytes, macrophages, mast cells, neutrophils, and natural killer cells infiltrate aortic wall and contributes to the sequence of inflammatory response [12–15]. Toll-like receptors (TLRs) involved in the innate immune system are the first line of host defense mechanism in mammals. TLRs are expressed on many cell types including macrophages and DCs. The recognition of invading microbes by TLRs on DCs induces proinflammatory cytokine production and enhanced antigen presentation to naive T cells, and finally activates antigen-specific adaptive immune responses. However, chronic activation of TLRs, which are activated not only by microbial endotoxins but also by damage-associated molecular pattern molecules (DAMPs), have been implicated in vascular inflammation . In the vasculature, TLRs are expressed in inflammatory cells, endothelial cells, and smooth muscle cells, and among them, TLR2 and TLR4 have been implicated in the inflammatory process of AAA . These TLRs probably contribute to AAA progression by increasing proinflammatory chemokines such as interleukin-6 (IL-6) and monocyte chemotactic protein-1, stimulating c-Jun NH2-terminal protein kinase pathway, and up-regulating the expression of MMP-2 and MMP-9 [18,19]. Upon induction, these MMPs destroy aortic medial elastic collagen and induce apoptosis and disrupt aortic wall stability, thus causing degradation of smooth muscle cells and loss of extracellular matrix in an environment promoting AAA development and rupture . Taken together, TLR-expressing DCs probably play a role in AAA development; however, their roles in AAA pathogenesis remains understudied compared with those in atherosclerosis. Figure 1.
CD11c.DOG.ApoE−/− and angiotensin II infusion model for CD11Chi dendritic cell depletion leading to reduced abdominal aortic aneurysm (AAA) pathology through reduced circulating B and T cells and aortic elastin degradation
Upon TLR activation, DCs change in the mature state in the atherosclerotic plaques with region progression . In the early stage of plaque formation, monocyte derived DCs are generated and CD11c+ antigen presenting cells contribute to foam cell formation. In addition, matured DCs in the region are associated with secretion of pro-inflammatory cytokines and activation of naïve T cells through antigen-specific pathway, both of which promote atherosclerosis [22,23].
To study the role of DCs in AAA development, the authors used an AAA model in apolipoprotein E (apoE) -/- mice infused with angiotensin II (Ang II) . Systemic CD11c+ cell depletion was then achieved by Diphtheria toxin (D.tox) injections. Depletion of CD11c+ cells significantly attenuated the maximum supra-renal aorta diameter compared with vehicle control. This article also demonstrated that limited growth of AAA in the CD11c+ depleted group was associated with a decrease in circulating T cells and B cells. CD4+ T cells contribute to the progression of AAA formation since depletion of CD4+ T cell proportions has been shown previously to decrease cytokines IL-2 and IFN-γ, and in turn reduced inflammation and suppress AAA progression . IgE activation on CD4+ T cells is also involved in AAA formation in the AngII model . In contrast, a shift toward T helper type 2 cells causes a dysfunctional IL-4 expression, resulting in reduced AAA formation .
Importantly, not only T cell but B cell is crucial in pathogenesis of AAA. B-cell depletion is also effective to prevent AAA in apoE-/- mice induced by AngII. It was described that B-cell depletion by anti-CD20 antibody resulted in reduced AAA dilation, elastase activity, and increased immunomodulatory enzyme indole 2,3-dioxygenase through plasmacytoid dendritic cells (pDCs), which is a part of the DC subset. Interestingly, these pDCs appeared in the aorta following B-cell depletion and led to an increase in regulatory T (Treg) cell infiltration, which suggests B-cell depletion leads to an immunosuppressive environment in order to reduce AAA progression . The contribution of B cells in AAA with inflammation and tissue destruction has been confirmed in a mouse AAA model induced with CaCl2 . B cells can be divided into two groups: B1 and B2 cells. B1 cells are an important role in the process of innate immunity, whereas B2 cells are component of adaptive immune system by secreting antibody. It is interesting to note that adoptive transfer of B2 cells suppressed AAA formation in an elastase model . In addition, one study demonstrates that mice lacking mature T and B lymphocytes still develop AAA in apoE-/- background with AngII infusion despite a protection against atherosclerosis . However, the findings could suggest a yin-yang balance among overall T- and B-cell subpopulations proceeding AAA, thus the removal of overall balance may not affect the pathology.
Neutrophils have been recognized as initial contributors in adventitial inflammation in reference to AAA formation. Thus, depletion of neutrophils prevented AAA in the elastase model . Once neutrophil granulocytes are activated, neutrophil elastase is released and enhances adventitial inflammation and degeneration of extracellular matrix . The contribution of neutrophil in AAA also involves extracellular trap release and activation of pDCs . This study further demonstrated that depletion of pDCs reduced T cell recruitment to aortic wall, limited MMP activity and decreased AAA promotion in the elastase model. However, Krishna et al. demonstrated that depletion of CD11c+ DCs led to significantly lower plasma neutrophil elastase levels  suggesting a collaborative communication between DCs and neutrophil in AAA. A recent study demonstrated that depletion of IL-27, which is a member of IL-6/IL-12 cytokine superfamily that regulates various hematopoietic cells in autoimmunity, led lower level of neutrophil and CD11c+ cells in AAA regions with suppression of adventitial inflammation and MMP activity, which suggest the role of IL-27 in such communication . In terms of the limitation of this study, it is not clear that whether the deletion of CD11c+ cell by D. tox injection suppressed the effect of Ang II or it suppressed the progression of AAA induced by Ang II because the authors did not utilize Ang II-independent AAA model. Given the conflicts in the contribution of T cell-driven cytokines such as IFN-γ in preclinical AAA models [34,35], lack of cytokine assessments is another shortcoming of this study, which would have had translational impact.
Clinically, pDCs appear increased in human AAA tissue . It was also shown in human AAA that elastin fragments generate an inflammatory response in the form of self-recognizing elastin fragments antibodies that may contribute to AAA development . Therefore, Krishna et al’s impactful finding of elevated neutrophil elastase activity in their model proposes a translational target for AAA. Recent human tissue analyses further confirmed the involvement of CD4+ and CD8+ T cells in AAA pathology , thus highlighting the findings by the authors demonstrating the inhibition of T-cell activation by targeting DCs in mouse AAAs. While these findings suggest a potential for immunosuppressive therapy against AAA, both preclinical and clinical supports are still limited [2,38]. Most recently, the ARREST Clinical Trial, started in 2017, will assess the efficacy of mesenchymal stem cells in the reduction of peripheral blood cytokines, inflammation and AAA pathology through manipulation of T-cell populations . In conclusion, the authors’ study lends further support to the relevance of DC contribution to AAA induced by Ang II infusion via several immune mechanisms with multiple immune cell types. Further research with multiple AAA models and humans is thus necessary to seek an immunotherapy to target DCs and or T cells in human AAA.
The authors declare that there are no competing interests associated with the manuscript.