Trapping of misdirected dendritic cells in the granulomatous lesions of giant cell arteritis

Vascular disease persistence in giant cell arteritis: are stromal cells neglected?

Giant cell arteritis (GCA), the most common systemic vasculitis, is characterised by aberrant interactions between infiltrating and resident cells of the vessel wall. Ageing and breach of tolerance are prerequisites for GCA development, resulting in dendritic and T-cell dysfunction. Inflammatory cytokines polarise T-cells, activate resident macrophages and synergistically enhance vascular inflammation, providing a loop of autoreactivity. These events originate in the adventitia, commonly regarded as the biological epicentre of the vessel wall, with additional recruitment of cells that infiltrate and migrate towards the intima. Thus, GCA-vessels exhibit infiltrates across the vascular layers, with various cytokines and growth factors amplifying the pathogenic process. These events activate ineffective repair mechanisms, where dysfunctional vascular smooth muscle cells and fibroblasts phenotypically shift along their lineage and colonise the intima. While high-dose glucocorticoids broadly suppress these inflammatory events, they cause well known deleterious effects. Despite the emerging targeted therapeutics, disease relapse remains common, affecting >50% of patients. This may reflect a discrepancy between systemic and local mediators of inflammation. Indeed, temporal arteries and aortas of GCA-patients can show immune-mediated abnormalities, despite the treatment induced clinical remission. The mechanisms of persistence of vascular disease in GCA remain elusive. Studies in other chronic inflammatory diseases point to the fibroblasts (and their lineage cells including myofibroblasts) as possible orchestrators or even effectors of disease chronicity through interactions with immune cells. Here, we critically review the contribution of immune and stromal cells to GCA pathogenesis and analyse the molecular mechanisms by which these would underpin the persistence of vascular disease.

Current Insights into Tissue Injury of Giant Cell Arteritis: From Acute Inflammatory Responses towards Inappropriate Tissue Remodeling

Giant cell arteritis (GCA) is an autoimmune disease affecting large vessels in patients over 50 years old. It is an exemplary model of a classic inflammatory disorder with IL-6 playing the leading role. The main comorbidities that may appear acutely or chronically are vascular occlusion leading to blindness and thoracic aorta aneurysm formation, respectively. The tissue inflammatory bulk is expressed as acute or chronic delayed-type hypersensitivity reactions, the latter being apparent by giant cell formation. The activated monocytes/macrophages are associated with pronounced Th1 and Th17 responses. B-cells and neutrophils also participate in the inflammatory lesion. However, the exact order of appearance and mechanistic interactions between cells are hindered by the lack of cellular and molecular information from early disease stages and accurate experimental models. Recently, senescent cells and neutrophil extracellular traps have been described in tissue lesions. These structures can remain in tissues for a prolonged period, potentially favoring inflammatory responses and tissue remodeling. In this review, current advances in GCA pathogenesis are discussed in different inflammatory phases. Through the description of these-often overlapping-phases, cells, molecules, and small lipid mediators with pathogenetic potential are described.

The Evolving Role of Dendritic Cells in Atherosclerosis

Atherosclerosis, a major contributor to cardiovascular morbidity and mortality, is characterized by chronic inflammation of the arterial wall. This inflammatory process is initiated and maintained by both innate and adaptive immunity. Dendritic cells (DCs), which are antigen-presenting cells, play a crucial role in the development of atherosclerosis and consist of various subtypes with distinct functional abilities. Following the recognition and binding of antigens, DCs become potent activators of cellular responses, bridging the innate and adaptive immune systems. The modulation of specific DC subpopulations can have either pro-atherogenic or atheroprotective effects, highlighting the dual pro-inflammatory or tolerogenic roles of DCs. In this work, we provide a comprehensive overview of the evolving roles of DCs and their subtypes in the promotion or limitation of atherosclerosis development. Additionally, we explore antigen pulsing and pharmacological approaches to modulate the function of DCs in the context of atherosclerosis.

Giant Cell Arteritis: Advances in Understanding Pathogenesis and Implications for Clinical Practice

Giant cell arteritis (GCA) is a noninfectious granulomatous vasculitis of unknown etiology affecting individuals older than 50 years. Two forms of GCA have been identified: a cranial form involving the medium-caliber temporal artery causing temporal arteritis (TA) and an extracranial form involving the large vessels, mainly the thoracic aorta and its branches. GCA generally affects individuals with a genetic predisposition, but several epigenetic (micro)environmental factors are often critical for the onset of this vasculitis. A key role in the pathogenesis of GCA is played by cells of both the innate and adaptive immune systems, which contribute to the formation of granulomas that may include giant cells, a hallmark of the disease, and arterial tertiary follicular organs. Cells of the vessel wall cells, including vascular smooth muscle cells (VSMCs) and endothelial cells, actively contribute to vascular remodeling responsible for vascular stenosis and ischemic complications. This review will discuss new insights into the molecular and cellular pathogenetic mechanisms of GCA, as well as the implications of these findings for the development of new diagnostic biomarkers and targeted drugs that could hopefully replace glucocorticoids (GCs), still the backbone of therapy for this vasculitis.

Aberrant phenotype of circulating antigen presenting cells in giant cell arteritis and polymyalgia rheumatica

Background

Giant Cell Arteritis (GCA) and Polymyalgia Rheumatica (PMR) are overlapping inflammatory diseases. Antigen-presenting cells (APCs), including monocytes and dendritic cells (DCs), are main contributors to the immunopathology of GCA and PMR. However, little is known about APC phenotypes in the peripheral blood at the time of GCA/PMR diagnosis.

Methods

APCs among peripheral blood mononuclear cells (PBMCs) of treatment-naive GCA and PMR patients were compared to those in age- and sex-matched healthy controls (HCs) using flow cytometry (n=15 in each group). We identified three monocyte subsets, and three DC subsets: plasmacytoid DCs (pDCs), CD141+ conventional DCs (cDC1) and CD1c+ conventional DCs (cDC2). Each of these subsets was analyzed for expression of pattern recognition receptors (TLR2, TLR4), immune checkpoints (CD86, PDL1, CD40) and activation markers (HLA-DR, CD11c).

Results

t-SNE plots revealed a differential clustering of APCs between GCA/PMR and HCs. Further analyses showed shifts in monocyte subsets and a lower proportion of the small population of cDC1 cells in GCA/PMR, whereas cDC2 proportions correlated negatively with CRP (r=-0.52). Classical monocytes of GCA/PMR patients show reduced expression of TLR2, HLA-DR, CD11c, which was in contrast to non-classical monocytes that showed higher marker expression. Additionally, single cell RNA sequencing in GCA patients identified a number of differentially expressed genes related to inflammation and metabolism in APCs.

Conclusion

Circulating non-classical monocytes display an activated phenotype in GCA/PMR patients at diagnosis, whereas classical monocytes show reduced expression of activation markers. Whether these findings reflect APC migration patterns or the effects of long-term inflammation remains to be investigated.

Vasculitogenic T Cells in Large Vessel Vasculitis

Vasculitis is an autoimmune disease of unknown etiology that causes inflammation of the blood vessels. Large vessel vasculitis is classified as either giant cell arteritis (GCA), which occurs exclusively in the elderly, or Takayasu arteritis (TAK), which mainly affects young women. Various cell types are involved in the pathogenesis of large vessel vasculitis. Among these, dendritic cells located between the adventitia and the media initiate the inflammatory cascade as antigen-presenting cells, followed by activation of macrophages and T cells contributing to vessel wall destruction. In both diseases, naive CD4⁺ T cells are polarized to differentiate into Th1 or Th17 cells, whereas differentiation into regulatory T cells, which suppress vascular inflammation, is inhibited. Skewed T cell differentiation is the result of aberrant intracellular signaling, such as the mechanistic target of rapamycin (mTOR) or the Janus kinase signal transducer and activator of transcription (JAK-STAT) pathways. It has also become clear that tissue niches in the vasculature fuel activated T cells and maintain tissue-resident memory T cells. In this review, we outline the most recent understanding of the pathophysiology of large vessel vasculitis. Then, we provide a summary of skewed T cell differentiation in the vasculature and peripheral blood. Finally, new therapeutic strategies for correcting skewed T cell differentiation as well as aberrant intracellular signaling are discussed.

Novel PET Imaging of Inflammatory Targets and Cells for the Diagnosis and Monitoring of Giant Cell Arteritis and Polymyalgia Rheumatica

Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two interrelated inflammatory diseases affecting patients above 50 years of age. Patients with GCA suffer from granulomatous inflammation of medium- to large-sized arteries. This inflammation can lead to severe ischemic complications (e.g., irreversible vision loss and stroke) and aneurysm-related complications (such as aortic dissection). On the other hand, patients suffering from PMR present with proximal stiffness and pain due to inflammation of the shoulder and pelvic girdles. PMR is observed in 40-60% of patients with GCA, while up to 21% of patients suffering from PMR are also affected by GCA. Due to the risk of ischemic complications, GCA has to be promptly treated upon clinical suspicion. The treatment of both GCA and PMR still heavily relies on glucocorticoids (GCs), although novel targeted therapies are emerging. Imaging has a central position in the diagnosis of GCA and PMR. While [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) has proven to be a valuable tool for diagnosis of GCA and PMR, it possesses major drawbacks such as unspecific uptake in cells with high glucose metabolism, high background activity in several non-target organs and a decrease of diagnostic accuracy already after a short course of GC treatment. In recent years, our understanding of the immunopathogenesis of GCA and, to some extent, PMR has advanced. In this review, we summarize the current knowledge on the cellular heterogeneity in the immunopathology of GCA/PMR and discuss how recent advances in specific tissue infiltrating leukocyte and stromal cell profiles may be exploited as a source of novel targets for imaging. Finally, we discuss prospective novel PET radiotracers that may be useful for the diagnosis and treatment monitoring in GCA and PMR.

New Insights into the Pathogenesis of Giant Cell Arteritis: Mechanisms Involved in Maintaining Vascular Inflammation

The giant cell arteritis (GCA) pathophysiology is complex and multifactorial, involving a predisposing genetic background, the role of immune aging and the activation of vascular dendritic cells by an unknown trigger. Once activated, dendritic cells recruit CD4 T cells and induce their activation, proliferation and polarization into Th1 and Th17, which produce interferon-gamma (IFN-γ) and interleukin-17 (IL-17), respectively. IFN-γ triggers the production of chemokines by vascular smooth muscle cells, which leads to the recruitment of additional CD4 and CD8 T cells and also monocytes that differentiate into macrophages. Recent data have shown that IL-17, IFN-γ and GM-CSF induce the differentiation of macrophage subpopulations, which play a role in the destruction of the arterial wall, in neoangiogenesis or intimal hyperplasia. Under the influence of different mediators, mainly endothelin-1 and PDGF, vascular smooth muscle cells migrate to the intima, proliferate and change their phenotype to become myofibroblasts that further proliferate and produce extracellular matrix proteins, increasing the vascular stenosis. In addition, several defects in the immune regulatory mechanisms probably contribute to chronic vascular inflammation in GCA: a defect in the PD-1/PD-L1 pathway, a quantitative and qualitative Treg deficiency, the implication of resident cells, the role of GM-CSF and IL-6, the implication of the NOTCH pathway and the role of mucosal‑associated invariant T cells and tissue‑resident memory T cells.

Self-Tolerance of Vascular Tissues Is Broken Down by Vascular Dendritic Cells in Response to Systemic Inflammation to Initiate Regional Autoinflammation

The correlation of infections with vascular autoinflammatory diseases such as vasculitis and atherosclerosis has been long recognized, and progressive inflammation with the formation of tertiary lymphoid organs in arterial adventitia intensively studied, the immunological basis of the nondiseased vasculatures that predispose to subsequent vascular autoimmunity during inflammation, however, is not well characterized. Here, we investigated the vascular immunity in situ of steady-state C57BL/6 mice and found that healthy vascular tissues contained a comprehensive set of immune cells with relatively higher proportion of innate components than lymphoid organs. Notably, a complete set of dendritic cell (DC) subsets was observed with monocyte-derived DCs (moDCs) constituting a major proportion; this is in contrast to moDCs being considered rare in the steady state. Interestingly, these vascular DCs constitutively expressed more suppressive factors with cDC1 for PD-L1 and moDCs for IL-10; this is concordant with the inhibitive phenotype of T cells in normal vascular tissues. The immunotolerant state of the vascular tissues, however, was readily eroded by systemic inflammation, demonstrated by the upregulation of proinflammatory cytokines and enhanced antigen presentation by vascular DCs to activate both cellular and humoral immunity in situ, which ultimately led to vascular destruction. Different vascular DC subsets elicited selective effects: moDCs were potent cytokine producers and B-cell activators, whereas cDCs, particularly, cDC1, were efficient at presenting antigens to stimulate T cells. Together, we unveil regional immunological features of vascular tissues to explain their dual facets under physiological versus pathological conditions for the better understanding and treatment of cardiovascular autoinflammation.

Age as a risk factor in vasculitis

Two vasculitides, giant cell arteritis (GCA) and Takayasu arteritis (TAK), are recognized as autoimmune and autoinflammatory diseases that manifest exclusively within the aorta and its large branches. In both entities, the age of the affected host is a critical risk factor. TAK manifests during the 2nd-4th decade of life, occurring while the immune system is at its height of performance. GCA is a disease of older individuals, with infrequent cases during the 6th decade and peak incidence during the 8th decade of life. In both vasculitides, macrophages and T cells infiltrate into the adventitia and media of affected vessels, induce granulomatous inflammation, cause vessel wall destruction, and reprogram vascular cells to drive adventitial and neointimal expansion. In GCA, abnormal immunity originates in an aged immune system and evolves within the aged vascular microenvironment. One hallmark of the aging immune system is the preferential loss of CD8⁺ T cell function. Accordingly, in GCA but not in TAK, CD8⁺ effector T cells play a negligible role and anti-inflammatory CD8⁺ T regulatory cells are selectively impaired. Here, we review current evidence of how the process of immunosenescence impacts the risk for GCA and how fundamental differences in the age of the immune system translate into differences in the granulomatous immunopathology of TAK versus GCA.

Glucocorticoid Effects on Tissue Residing Immune Cells in Giant Cell Arteritis: Importance of GM-CSF

Giant cell arteritis (GCA) is a systemic granulomatous vasculitis clinically characterized by a prompt response to glucocorticoid therapy. Dendritic cells (DCs) play a central role in the pathogenesis of the disease and are increased in temporal arteries from GCA patients. The aim of this study was to determine the effects of glucocorticoid therapy on granulomatous infiltrates and on peripheral DCs of GCA patients. Immunohistochemical staining of temporal artery specimens from 41 GCA patients revealed a rapid reduction of the number of DCs after initiation of glucocorticoid treatment. TUNEL staining was performed to quantify apoptotic S100+ DC, CD3+ T cells, and CD68+ macrophages in the granulomatous infiltrates. An increase of apoptotic cells up to 9 ± 2% after 4–5 days of glucocorticoid therapy and up to 27 ± 5% (p < 0.001, compared to earlier timepoints) after 6–10 days was detected. A decrease of CCL19 and CCL21 expression was observed after starting glucocorticoid therapy. Granulocyte-macrophage colony-stimulating factor (GM-CSF) expression also significantly decreased under glucocorticoid therapy. No GM-CSF expression was detected in the control specimens. Glucocorticoid therapy leads to a rapid, time-dependent reduction of DCs in temporal arteries from GCA patients and reduction of mediators for cell migration. Our data suggest GM-CSF as a novel therapeutic target of GCA.

The Immunopathology of Giant Cell Arteritis Across Disease Spectra

Giant cell arteritis (GCA) is a granulomatous systemic vasculitis of large- and medium-sized arteries that affects the elderly. In recent years, advances in diagnostic imaging have revealed a greater degree of large vessel involvement than previously recognized, distinguishing classical cranial- from large vessel (LV)- GCA. GCA often co-occurs with the poorly understood inflammatory arthritis/bursitis condition polymyalgia rheumatica (PMR) and has overlapping features with other non-infectious granulomatous vasculitides that affect the aorta, namely Takayasu Arteritis (TAK) and the more recently described clinically isolated aortitis (CIA). Here, we review the literature focused on the immunopathology of GCA on the background of the three settings in which comparisons are informative: LV and cranial variants of GCA; PMR and GCA; the three granulomatous vasculitides (GCA, TAK, and CIA). We discuss overlapping and unique features between these conditions across clinical presentation, epidemiology, imaging, and conventional histology. We propose a model of GCA where abnormally activated circulating cells, especially monocytes and CD4⁺ T cells, enter arteries after an unknown stimulus and cooperate to destroy it and review the evidence for how this mechanistically occurs in active disease and improves with treatment.

Innate and Adaptive Immunity in Giant Cell Arteritis

Autoimmune diseases can afflict every organ system, including blood vessels that are critically important for host survival. The most frequent autoimmune vasculitis is giant cell arteritis (GCA), which causes aggressive wall inflammation in medium and large arteries and results in vaso-occlusive wall remodeling. GCA shares with other autoimmune diseases that it occurs in genetically predisposed individuals, that females are at higher risk, and that environmental triggers are suspected to beget the loss of immunological tolerance. GCA has features that distinguish it from other autoimmune diseases and predict the need for tailored diagnostic and therapeutic approaches. At the core of GCA pathology are CD4⁺ T cells that gain access to the protected tissue niche of the vessel wall, differentiate into cytokine producers, attain tissue residency, and enforce macrophages differentiation into tissue-destructive effector cells. Several signaling pathways have been implicated in initiating and sustaining pathogenic CD4⁺ T cell function, including the NOTCH1-Jagged1 pathway, the CD28 co-stimulatory pathway, the PD-1/PD-L1 co-inhibitory pathway, and the JAK/STAT signaling pathway. Inadequacy of mechanisms that normally dampen immune responses, such as defective expression of the PD-L1 ligand and malfunction of immunosuppressive CD8⁺ T regulatory cells are a common theme in GCA immunopathology. Recent studies are providing a string of novel mechanisms that will permit more precise pathogenic modeling and therapeutic targeting in GCA and will fundamentally inform how abnormal immune responses in blood vessels lead to disease.

Notch Signaling Regulation in Autoinflammatory Diseases

Notch pathway is a highly conserved intracellular signaling route that modulates a vast variety of cellular processes including proliferation, differentiation, migration, cell fate and death. Recently, the presence of a strict crosstalk between Notch signaling and inflammation has been described, although the precise molecular mechanisms underlying this interplay have not yet been fully unravelled. Disruptions in Notch cascade, due both to direct mutations and/or to an altered regulation in the core components of Notch signaling, might lead to hypo- or hyperactivation of Notch target genes and signaling molecules, ultimately contributing to the onset of autoinflammatory diseases. To date, alterations in Notch signaling have been reported as associated with three autoinflammatory disorders, therefore, suggesting a possible role of Notch in the pathogenesis of the following diseases: hidradenitis suppurativa (HS), Behçet disease (BD), and giant cell arteritis (GCA). In this review, we aim at better characterizing the interplay between Notch and autoinflammatory diseases, trying to identify the role of this signaling route in the context of these disorders.

Treatment of giant-cell arteritis: from broad spectrum immunosuppressive agents to targeted therapies

For decades, the treatment of GCA has relied on glucocorticoids. Work over the past two decades has supported a modest efficacy of MTX but no clear benefit from anti-TNF-based therapies. More recently, the therapeutic armamentarium for GCA has expanded. The availability of agents targeting specific cytokines, cytokine receptors or signalling pathways, along with a better, although still limited, understanding of the immunopathology of GCA, are opening further therapeutic possibilities. Blocking IL-6 receptor with tocilizumab has been effective in maintaining remission and reducing glucocorticoid exposure and tocilizumab has been approved for the treatment of GCA. However, nearly half of the patients do not benefit from tocilizumab and additional options need to be investigated. This review focuses on standard therapeutic approaches and on targeted therapies that have been or are currently under investigation.

[Physiopathology of giant cell arteritis: From inflammation to vascular remodeling]

Giant cell arteritis (GCA) is a large-vessel vasculitis involving the aorta and its main branches, especially supra aortic branches. Although much progress has been made, the pathophysiology remains incompletely understood. An initial trigger, suspected of infectious origin, lead to the maturation and recruitment of dendritic cells (DC). The lack of migration of these DC allows the local recruitment of T-lymphocytes (LT). These LT- CD4⁺ polarize in Type 1 helper (Th1), Th17 but also Th9. A qualitative and quantitative deficit in regulatory T cells (Treg) is observed under the influence of IL-21 overproduction. In addition, an imbalance in the Th17/Treg balance is favored by IL-6. The secretion of IFN-γ, IL-17, IL-6, IL-33 is responsible for a sustained local inflammatory reaction that is organized around tertiary lymphoid follicles. Locally recruited macrophages secrete reactive forms of oxygen together with VEGF and PDGF. These growth factors, together with neurotrophins and endothelin contribute to increase the proliferation of vascular smooth muscle cells (VSMCs). The imbalance between matrix metalloproteases (MMP)-2, MMP-9 and MMP-14 and tissue inhibitors of metalloproteases (TIMP)-1 and TIMP-2 also contribute to the remodeling process occurring in the vessel wall. Finally, arterial neovascularization contribute to the perpetuation of lymphocyte recruitment. This persistent remodeling is sometimes complicated by ischemic events responsible for the initial severity of the disease.

Chlamydia and Lipids Engage a Common Signaling Pathway That Promotes Atherogenesis

BACKGROUND

Recent studies indicate that Toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88) signaling promote the development of high fat diet-induced atherosclerosis in hypercholesterolemic mice.

OBJECTIVES

The authors investigated the role of TLR4/MyD88 signaling in hematopoietic and stromal cells in the development and infection-mediated acceleration of atherosclerosis.

METHODS

The authors generated bone marrow chimeras between wild-type and Tlr4^-/- mice, as well as wild-type and Myd88^-/- mice. All mice were on the Apoe^-/- background and fed high fat diet. The authors infected the chimeric mice with C. pneumoniae (CP) and fed them high fat diet.

RESULTS

Aortic sinus plaques and lipid content were significantly reduced in Apoe^-/- mice that received Tlr4^-/-or Myd88^-/- bone marrow compared with control animals despite similar cholesterol levels. Similarly, Tlr4 or Myd88 deficiency in stromal cells also led to a reduction in the lesion area and lipid in aortic sinus plaques. MyD88 expression only in CD11c+ dendritic cells (myeloid cells) in cells was sufficient in otherwise MyD88-deficient mice to induce CP infection-mediated acceleration of atherosclerosis, underlining the key role of MyD88 in CD11c+ dendritic cells (myeloid cells). Whereas CP infection markedly accelerated atherosclerosis in TLR4- or MyD88-positive chimeras, CP infection had a minimal effect on atherosclerosis in TLR4- or MyD88-deficient mice (either in the hematopoietic or stromal cell compartments).

CONCLUSIONS

The authors show that both CP infection and metabolic stress associated with dyslipidemia use the same innate immune response pathway, utilizing TLR4/MyD88 signaling, with similar relative contributions in bone marrow-derived hematopoietic cells and in stromal cells. Further studies are required to understand this intricate and complex cross talk among innate and adaptive immune systems in various conditions to more effectively design dendritic cell-mediated atheroprotective vaccines and other therapeutic strategies.

Reviewing the Pathophysiology Behind the Advances in the Management of Giant Cell Arteritis

Improving understanding of the underlying pathophysiology of giant cell arteritis (GCA) is transforming clinical management by identifying novel avenues for targeted therapies. One key area of concern for both clinicians and patients with GCA is glucocorticoid (GC) morbidity. The first randomised controlled trials of targeted treatment to reduce cumulative GC use in GCA have been published, with tocilizumab, an interleukin (IL)-6 receptor inhibitor, now the first ever licensed treatment for GCA. Further potential therapies are emerging owing to our enhanced understanding of the pathophysiology of the disease. Other improvements in the care of our patients are rapid access pathways and imaging techniques, such as ultrasound, which are becoming part of modern rheumatology practice in the UK, Europe and beyond. These have been highlighted in the literature to reduce delay in diagnosis and improve long-term outcomes for those investigated for GCA.

Pathogenesis of large vessel vasculitis: Implications for disease classification and future therapies

Despite being recognised over a century ago, the aetiology and pathogenesis of large vessel vasculitis (LVV) still remains elusive. Takayasu’s arteritis (TA) and giant cell arteritis (GCA) represent the two major categories of LVV, each with distinctive clinical features. Over the last 10 years an increased understanding of the immunopathogenesis of the inflammatory cascade within the aortic wall has revived the view that LVVs may represent subtypes of the same pathological process, with implications in the treatment of this disease. In this review, the histological, genetic and immunopathological features of TA and GCA will be discussed and the evidence for a common underlying disease mechanism examined. Novel markers of disease activity and therapies based on advances in our understanding of the immunopathogenesis of these conditions will also be discussed.

Pathogenesis of giant-cell arteritis: how targeted therapies are influencing our understanding of the mechanisms involved

GCA is a chronic granulomatous vasculitis that affects large- and medium-sized vessels. Both the innate and the adaptive immune system are thought to play an important role in the initial events of the pathogenesis of GCA. Amplification cascades are involved in the subsequent development and progression of the disease, resulting in vascular inflammation, remodelling and occlusion. The development of large-vessel vasculitis in genetically modified mice has provided some evidence regarding potential mechanisms that lead to vascular inflammation. However, the participation of specific mechanistic pathways in GCA has not been fully established because of the paucity and limitations of functional models. Treatment of GCA is evolving, and novel therapies are being incorporated into the GCA treatment landscape. In addition, to improve the management of GCA, targeted therapies are providing functional proof of concept of the relevance of particular pathogenic mechanisms in the development of GCA and in sustaining vascular inflammation.

Expression and Function of IL12/23 Related Cytokine Subunits (p35, p40, and p19) in Giant-Cell Arteritis Lesions: Contribution of p40 to Th1- and Th17-Mediated Inflammatory Pathways

Background

Giant-cell arteritis (GCA) is considered a T helper (Th)1- and Th17-mediated disease. Interleukin (IL)-12 is a heterodimeric cytokine (p35/p40) involved in Th1 differentiation. When combining with p19 subunit, p40 compose IL-23, a powerful pro-inflammatory cytokine that maintains Th17 response.

Objectives

The aims of this study were to investigate p40, p35, and p19 subunit expression in GCA lesions and their combinations to conform different cytokines, to assess the effect of glucocorticoid treatment on subunit expression, and to explore functional roles of p40 by culturing temporal artery sections with a neutralizing anti-human IL-12/IL-23p40 antibody.

Methods and results

p40 and p19 mRNA concentrations measured by real-time RT-PCR were significantly higher in temporal arteries from 50 patients compared to 20 controls (4.35 ± 4.06 vs 0.51 ± 0.75; p < 0.0001 and 20.32 ± 21.78 vs 4.17 ± 4.43 relative units; p < 0.0001, respectively). No differences were found in constitutively expressed p35 mRNA. Contrarily, p40 and p19 mRNAs were decreased in temporal arteries from 16 treated GCA patients vs those from 34 treatment-naïve GCA patients. Accordingly, dexamethasone reduced p40 and p19 expression in cultured arteries. Subunit associations to conform IL-12 and IL-23 were confirmed by proximity-ligation assay in GCA lesions. Immunofluorescence revealed widespread p19 and p35 expression by inflammatory cells, independent from p40. Blocking IL-12/IL-23p40 tended to reduce IFNγ and IL-17 mRNA production by cultured GCA arteries and tended to increase Th17 inducers IL-1β and IL-6.

Conclusion

IL-12 and IL-23 heterodimers are increased in GCA lesions and decrease with glucocorticoid treatment. p19 and p35 subunits are much more abundant than p40, indicating an independent role for these subunits or their potential association with alternative subunits. The modest effect of IL-12/IL-23p40 neutralization may indicate compensation by redundant cytokines or cytokines resulting from alternative combinations.

The microvascular niche instructs T cells in large vessel vasculitis via the VEGF-Jagged1-Notch pathway

Microvascular networks in the adventitia of large arteries control access of inflammatory cells to the inner wall layers (media and intima) and thus protect the immune privilege of the aorta and its major branches. In autoimmune vasculitis giant cell arteritis (GCA), CD4 T helper 1 (T_H1) and T_H17 cells invade into the wall of the aorta and large elastic arteries to form tissue-destructive granulomas. Whether the disease microenvironment provides instructive cues for vasculitogenic T cells is unknown. We report that adventitial microvascular endothelial cells (mvECs) perform immunoregulatory functions by up-regulating the expression of the Notch ligand Jagged1. Vascular endothelial growth factor (VEGF), abundantly present in GCA patients' blood, induced Jagged1 expression, allowing mvECs to regulate effector T cell induction via the Notch-mTORC1 (mammalian target of rapamycin complex 1) pathway. We found that circulating CD4 T cells in GCA patients have left the quiescent state, actively signal through the Notch pathway, and differentiate into T_H1 and T_H17 effector cells. In an in vivo model of large vessel vasculitis, exogenous VEGF functioned as an effective amplifier to recruit and activate vasculitogenic T cells. Thus, systemic VEGF co-opts endothelial Jagged1 to trigger aberrant Notch signaling, biases responsiveness of CD4 T cells, and induces pathogenic effector functions. Adventitial microvascular networks function as an instructive tissue niche, which can be exploited to target vasculitogenic immunity in large vessel vasculitis.

Giant cell arteritis: pathogenic mechanisms and new potential therapeutic targets

Giant cell arteritis (GCA) is the most common idiopathic systemic vasculitis in persons aged 50 years or greater. Treatment options for GCA, to-date, have been limited and have consisted primarily of glucocorticoids. Significant advances in the understanding of the genetic and cellular mechanisms in GCA are leading to identification of potential pathogenic targets. The recent success of interleukin-6 blockade in the treatment of GCA has opened the landscape to targeted biologic therapy. T cells, particularly T helper 1 and T helper 17 cell lineages have been identified as key inflammatory cells in both active and chronic vascular inflammatory lesions. Therapeutic agents, including abatacept and ustekinumab, which can impede both vasculitogenic cell lines are of particular interest. Inhibition of signalling pathways, including the janus kinase-signal tranducers and activation of transcription (JAK-STAT) and Notch pathways are evolving options. Tocilizumab has shown clear benefit in both newly diagnosed and relapsing patients with GCA and approval of this medication for treatment of GCA has led to rapid incorporation into treatment regimens. More information is required to understand the long-term outcomes of tocilizumab and other investigational targeted therapeutics in the treatment of GCA.

Immune checkpoint dysfunction in large and medium vessel vasculitis

Giant cell arteritis (GCA) is a granulomatous vasculitis of the aorta and its medium-sized branch vessels. CD4 T cells, macrophages, and dendritic cells (DCs) build granulomatous infiltrates that injure the vessel wall and elicit a maladaptive response to injury. Pathological consequences include fragmentation of elastic membranes, destruction of the medial layer, microvascular neoangiogenesis, massive outgrowth of myofibroblasts, and lumen-occlusive intimal hyperplasia. Antigens have been suspected to drive the local activation of vasculitogenic CD4 T cells, but recent data have suggested a more generalized defect in the threshold setting of such T cells, rendering them hyperreactive. Under physiological conditions, immune checkpoints provide negative signals to curb T cell activation and prevent inflammation-associated tissue destruction. This protective mechanism is disrupted in GCA. Vessel wall DCs fail to express the immunoinhibitory ligand programmed cell death ligand-1, leaving lesional T cells unchecked. Consequently, programmed cell death protein-1-positive CD4 T cells can enter the immunoprivileged vessel wall, where they produce a broad spectrum of inflammatory cytokines (interferon-γ, IL-17, and IL-21) and have a direct role in driving intimal hyperplasia and intramural neoangiogenesis. The deficiency of the programmed cell death protein-1 immune checkpoint in GCA, promoting unopposed T cell immunity, contrasts with checkpoint hyperactivity in cancer patients in whom excessive programmed cell death ligand-1 expression paralyzes the function of antitumor T cells. Excessive checkpoint activity is the principle underlying cancer-immune evasion and is therapeutically targeted by immunotherapy with checkpoint inhibitors. Such checkpoint inhibitors, which unleash anticancer T cells and induce immune-related toxicity, may lead to drug-induced vasculitis.

Analysis of the common genetic component of large-vessel vasculitides through a meta-Immunochip strategy

Giant cell arteritis (GCA) and Takayasu’s arteritis (TAK) are major forms of large-vessel vasculitis (LVV) that share clinical features. To evaluate their genetic similarities, we analysed Immunochip genotyping data from 1,434 LVV patients and 3,814 unaffected controls. Genetic pleiotropy was also estimated. The HLA region harboured the main disease-specific associations. GCA was mostly associated with class II genes (HLA-DRB1/HLA-DQA1) whereas TAK was mostly associated with class I genes (HLA-B/MICA). Both the statistical significance and effect size of the HLA signals were considerably reduced in the cross-disease meta-analysis in comparison with the analysis of GCA and TAK separately. Consequently, no significant genetic correlation between these two diseases was observed when HLA variants were tested. Outside the HLA region, only one polymorphism located nearby the IL12B gene surpassed the study-wide significance threshold in the meta-analysis of the discovery datasets (rs755374, P = 7.54E-07; OR_GCA = 1.19, OR_TAK = 1.50). This marker was confirmed as novel GCA risk factor using four additional cohorts (P_GCA = 5.52E-04, OR_GCA = 1.16). Taken together, our results provide evidence of strong genetic differences between GCA and TAK in the HLA. Outside this region, common susceptibility factors were suggested, especially within the IL12B locus.

The role of toll like receptors in giant cell arteritis

GCA is a common primary systemic vasculitis that results in granulomatous inflammation of medium to large arteries. Both innate and adaptive immune mechanisms combine to drive intimal hyperplasia, luminal stenosis and ultimately occlusion. While the pathogenesis of GCA is incompletely understood, the activation of resident adventitial dendritic cells via toll like receptors (TLRs) appears to be a crucial inciting event. Here we explore the role of TLRs in the pathogenesis of GCA, including their effects on dendritic cell and T cell activation and recruitment, putative infectious triggers for GCA and the potential of TLR inhibition as a novel therapeutic strategy in GCA.

New developments in giant cell arteritis

Giant cell arteritis (GCA) is a medium-to-large vessel vasculitis with potentially sight- and life- threatening complications. Our understanding of the pathogenesis, diagnosis, and treatment of GCA has advanced rapidly in recent times. The validity of using the American College of Rheumatology guidelines for diagnosis of GCA in a clinical setting has been robustly challenged. Erythrocyte sedimentation rate, an important marker of inflammation, is lowered by the use of statins and nonsteroidal anti-inflammatory drugs. Conversely, it may be falsely elevated with a low hematocrit. Despite the emergence of new diagnostic modalities, temporal artery biopsy remains the gold standard. Evidence suggests that shorter biopsy lengths and biopsies done weeks to months after initiation of steroid therapy are still useful. New imaging techniques such as positron emission tomography have shown that vascular inflammation in GCA is more widespread than originally thought. GCA, Takayasu arteritis, and polymyalgia rheumatica are no longer thought to exist as distinct entities and are more likely parts of a spectrum of disease. A range of immunosuppressive drugs have been used in conjunction with corticosteroids to treat GCA. In particular, interleukin-6 inhibitors are showing promise as a therapy.

Toll-like receptors mediating vascular malfunction: Lessons from receptor subtypes

Toll-like receptors (TLR) are a subfamily of pattern recognition receptors (PRR) implicated in a variety of vascular abnormalities. However, the pathophysiological role and the interplay between different TLR-mediated innate and adaptive immune responses during the development of vascular diseases remain largely unspecified. TLR are widely distributed in both immune and nonimmune cells in the blood vessel wall. The expressions and locations of TLR are dynamically regulated in response to distinct molecular patterns derived from pathogens or damaged host cells. As a result, the outcome of TLR signaling is agonist- and cell type-dependent. A better understanding of discrete TLR signaling pathways in the vasculature will provide unprecedented opportunities for the discovery of novel therapies in many inflammatory vascular diseases. The present brief review discusses the role of individual TLR in controlling cellular functions of the vascular system, by focusing on the inflammatory responses within the blood vessel wall which contribute to the development of hypertension and atherosclerosis.

[Pathogenesis of large vessel vasculitis]

Giant cell arteritis (GCA) and Takayasu's arteritis (TA) are two granulomatous vasculitis affecting large arteries that present specific epidemiological and clinical features. Their pathogenesis is not fully understood but major advances have been obtained during the last years, thus allowing the emergence of new therapeutic strategies. GCA and TA develop on a specific genetic background but share some similarities regarding the immunological pathways involved in their pathogenesis. The trigger of these diseases is not clearly identified but it is thought that an infectious agent could activate and lead to the maturation of dendritic cells that are localized in the adventitia of arteries. Then, the cells of the adaptative immune response are recruited and activated: CD4 T cells that polarize into Th1 and Th17 cells, cytotoxic CD8 T cells and Natural Killer cells. Furthermore, the T regulatory cells (Treg) are decreased both in GCA and TA. Humoral immune response seems also to be involved, especially in TA. Then, the cytokines produced by T lymphocytes (especially IL-17 and IFN-γ) trigger the recruitment and activation of monocytes and their differentiation into macrophages and multinuclear giant cells that produce IL-1β and IL-6 that are responsible for general symptoms of GCA and TA, and cytotoxic mediators and growth factors that trigger the remodeling of the arterial wall leading to aneurysms and ischemic manifestations of GCA an TA.

Innate immune cells in the pathogenesis of primary systemic vasculitis

Innate immune system forms the first line of defense against foreign substances. Neutrophils, eosinophils, erythrocytes, platelets, monocytes, macrophages, dendritic cells, γδ T cells, natural killer and natural killer T cells comprise the innate immune system. Genetic polymorphisms influencing the activation of innate immune cells predispose to development of vasculitis and influence its severity. Abnormally activated innate immune cells cross-talk with other cells of the innate immune system, present antigens more efficiently and activate T and B lymphocytes and cause tissue destruction via cell-mediated cytotoxicity and release of pro-inflammatory cytokines. These secreted cytokines further recruit other cells to the sites of vascular injury. They are involved in both the initiation as well as the perpetuation of vasculitis. Evidences suggest reversal of aberrant activation of immune cells in response to therapy. Understanding the role of innate immune cells in vasculitis helps understand the potential of therapeutic modulation of their activation to treat vasculitis.

New insights into the pathogenesis of giant cell arteritis and hopes for the clinic

Giant cell arteritis is a complex immune-mediated disease that involves large blood vessels in individuals older than 50 years. Recent studies have confirmed a strong association of this form of vasculitis with the HLA region, particularly with HLA class II genes. However, other non-HLA loci, such as protein tyrosine phosphatase non-receptor type 22, may also account for the susceptibility to giant cell arteritis. In addition, genetic variants located in genes encoding proinflammatory cytokines seem to influence the phenotypic expression of the disease, including the risk of severe ischemic complications, the presence of polymyalgia rheumatica and the higher incidence of relapses observed in some patients. The identification of putative genetic markers of disease severity could have clear therapeutic implications, as it may allow us to identify patients who are potentially responders to specific treatments.

DNA methylation analysis of the temporal artery microenvironment in giant cell arteritis

OBJECTIVE

To investigate the inflammatory response in giant cell arteritis (GCA) by characterising the DNA methylation pattern within the temporal artery microenvironment.

METHODS

Twelve patients with non-equivocal histological evidence for GCA and 12 age-matched, sex-matched and ethnicity-matched controls with normal biopsies were studied. DNA was extracted from the affected portions of temporal artery tissue in patients with GCA and from histologically confirmed normal arteries in controls. Genome-wide DNA methylation status was evaluated using the Illumina Infinium HumanMethylation450 BeadChip Array. Differentially methylated loci between affected and unaffected arterial tissues were identified, and subsequent bioinformatic analysis performed. Immunohistochemistry was used to examine tissue expression patterns in temporal artery biopsies.

RESULTS

We identified 1555 hypomethylated CG sites (853 genes) in affected temporal artery tissue from patients with GCA compared with normal controls. Gene ontology enrichment analysis of hypomethylated genes revealed significant representation in T cell activation and differentiation pathways, including both TH1 and TH17 signatures. Our DNA methylation data suggest a role for increased activity of the calcineurin/nuclear factor of activated T cells (NFAT) signalling pathway in GCA, confirmed by immunohistochemistry showing increased expression and nuclear localisation of NFAT1. NFAT signalling downstream targets such as interleukin (IL)-21/IL-21R and CD40L were overexpressed in GCA-affected arteries. Further, proinflammatory genes including TNF, LTA, LTB, CCR7, RUNX3, CD6, CD40LG, IL2, IL6, NLRP1, IL1B, IL18, IL21, IL23R and IFNG were hypomethylated in the cellular milieu of GCA arteries.

CONCLUSIONS

We characterised the inflammatory response in GCA-affected arteries using 'epigenetic immunophenotyping' and identified molecules and pathways relevant to disease pathogenesis in GCA.

Serpin treatment suppresses inflammatory vascular lesions in temporal artery implants (TAI) from patients with giant cell arteritis

Giant cell arteritis (GCA) and Takayasu’s disease are inflammatory vasculitic syndromes (IVS) causing sudden blindness and widespread arterial obstruction and aneurysm formation. Glucocorticoids and aspirin are mainstays of treatment, predominantly targeting T cells. Serp-1, a Myxomavirus-derived serpin, blocks macrophage and T cells in a wide range of animal models. Serp-1 also reduced markers of myocardial injury in a Phase IIa clinical trial for unstable coronary disease. In recent work, we detected improved survival and decreased arterial inflammation in a mouse Herpesvirus model of IVS. Here we examine Serp-1 treatment of human temporal artery (TA) biopsies from patients with suspected TA GCA arteritis after implant (TAI) into the aorta of immunodeficient SCID (severe combined immunodeficiency) mice. TAI positive for arteritis (GCA^pos) had significantly increased inflammation and plaque when compared to negative TAI (GCA^neg). Serp-1 significantly reduced intimal inflammation and CD11b⁺ cell infiltrates in TAI, with reduced splenocyte Th1, Th17, and Treg. Splenocytes from mice with GCA^pos grafts had increased gene expression for interleukin-1beta (IL-1β), IL-17, and CD25 and decreased Factor II. Serp-1 decreased IL-1β expression. In conclusion, GCA^pos TAI xenografts in mice provide a viable disease model and have increased intimal inflammation as expected and Serp-1 significantly reduces vascular inflammatory lesions with reduced IL-1β.

T cell-macrophage interactions and granuloma formation in vasculitis

Granuloma formation, bringing into close proximity highly activated macrophages and T cells, is a typical event in inflammatory blood vessel diseases, and is noted in the name of several of the vasculitides. It is not known whether specific properties of the microenvironment in the blood vessel wall or the immediate surroundings of blood vessels contribute to granuloma formation and, in some cases, generation of multinucleated giant cells. Granulomas provide a specialized niche to optimize macrophage-T cell interactions, strongly activating both cell types. This is mirrored by the intensity of the systemic inflammation encountered in patients with vasculitis, often presenting with malaise, weight loss, fever, and strongly upregulated acute phase responses. As a sophisticated and highly organized structure, granulomas can serve as an ideal site to induce differentiation and maturation of T cells. The granulomas possibly seed aberrant Th1 and Th17 cells into the circulation, which are known to be the main pathogenic cells in vasculitis. Through the induction of memory T cells, aberrant innate immune responses can imprint the host immune system for decades to come and promote chronicity of the disease process. Improved understanding of T cell-macrophage interactions will redefine pathogenic models in the vasculitides and provide new avenues for immunomodulatory therapy.

Immune mechanisms in medium and large-vessel vasculitis

Vasculitis of the medium and large arteries, most often presenting as giant cell arteritis (GCA), is an infrequent, but potentially fatal, type of immune-mediated vascular disease. The site of the aberrant immune reaction, the mural layers of the artery, is strictly defined by vascular dendritic cells, endothelial cells, vascular smooth muscle cells and fibroblasts, which engage in an interaction with T cells and macrophages to, ultimately, cause luminal stenosis or aneurysmal wall damage of the vessel. A multitude of effector cytokines, all known as critical mediators in host-protective immunity, have been identified in vasculitic lesions. Two dominant cytokine clusters--the IL-6-IL-17 axis and the IL-12-IFN-γ axis--have been linked to disease activity. These two clusters seem to serve different roles in the vasculitic process. The IL-6-IL-17 cluster is highly responsive to standard corticosteroid therapy, whereas the IL-12-IFN-γ cluster is resistant to steroid-mediated immunosuppression. The information exchange between vascular and immune cells and stabilization of the vasculitic process involves members of the Notch receptor and ligand family. Focusing on elements in the tissue context of GCA, instead of broadly suppressing host immunity, might enable a more tailored therapeutic approach that avoids unwanted adverse effects of aggressive immunosuppression.

The immunopathology of giant cell arteritis: diagnostic and therapeutic implications

Giant cell arteritis (GCA) is an important cause of preventable blindness, most commonly due to anterior ischemic optic neuropathy. Ischemic tissue injury is the end result of a process that begins within the walls of susceptible arteries in which local dendritic cells (DCs) recruit and activate CD4 T cells that, in turn, direct the activity of effector macrophages. In response to the immune attack, the blood vessel forms lumen-stenosing intima. Multiple cascades of excessive T-cell reactivity contribute to the autoimmune features of giant cell arteritis with TH1 and TH17 immunity responsible for the early phase and TH1 immunity promoting chronic-smoldering inflammation. These cascades are only partially overlapping, supporting the concept that a multitude of instigators induce and sustain vascular inflammation. The artery actively participates in the abnormal immune response through endogenous immune sentinels, so-called vascular DCs embedded in the adventitia. Advancing age, the strongest of all risk factors for GCA, contributes to both, the dysfunction of the immune system and the vascular system. Expansion of the therapeutic armamentarium for GCA needs to focus on approaches that mitigate the impact of the aging artery and adapt to the needs of the immunosenescent host.

Role of oral cyclophosphamide in the treatment of giant cell arteritis

OBJECTIVE

Glucocorticoid (GC)-related adverse events greatly contribute to the outcome in giant cell arteritis (GCA). CYC was investigated as a steroid-sparing agent in GCA.

METHODS

Nineteen patients treated with CYC were retrospectively analysed. CYC was administered in 15 of the 19 patients after failure of high doses of GC or relapse during medium to high doses of GC, with or without MTX, while CYC was used ab initio in 4 of the 19 patients, all with type 2 diabetes. Follow-up ranged from 1 month to nearly 9 years after the end of CYC treatment.

RESULTS

The efficacy of CYC was observed in 15 of the 19 patients, and remission was still present 6-12 months after CYC suspension in 12 of the 13 patients. GCs were suspended in 6 of the 15 patients, and they were continued at a dose ≤5 mg/day of prednisone in all the remaining responders. Relapse occurred in 4 of the 15 patients, usually >12 months after CYC suspension. Suspension of GC daily dose or reduction to ≤5 mg/day of prednisone occurred within the first 6 months of follow-up after the beginning of CYC in 10 of the 15 patients. Ten adverse events were registered in nine patients, with recovery usually soon after the suspension of CYC or dose reduction. However, one death occurred due to acute hepatitis. Disappearance of the inflammatory infiltrate could be demonstrated when temporal artery biopsy was repeated 3 months after CYC in one patient.

CONCLUSION

CYC may represent a useful option for patients requiring prolonged medium- to high-dose GC therapy and at high risk of GC-related side effects.