Purpose of review
With progressive age, the immune system and the propensity for abnormal immunity change fundamentally. Individuals >50 years of age are more susceptible to infection and cancer, but also at higher risk for chronic inflammation and immune-mediated tissue damage. The process of immunosenescence is accelerated in rheumatoid arthritis.
Premature T cell senescence occurs not only in RA, it has also been involved in morbidity and mortality of chronic HIV infection. Senescent cells acquire the “senescence associated secretory phenotype (SASP)”, which promotes and sustains tissue inflammation. Molecular mechanisms underlying T cell aging are beginning to be understood. Besides the contraction of T cell diversity due to uneven clonal expansion, senescent T cells have defects in balancing cytoplasmic kinase and phosphatase activities, changing their activation thresholds. Also, leakiness in repairing DNA lesions and uncapped telomeres imposes genomic stress. Age-induced changes in the tissue microenvironment may alter T cell responses.
Gain- and loss-of-function in senescent T cells undermine protective immunity and create the conditions for chronic tissue inflammation, a combination typically encountered in RA. Genetic programs involved in T cell signaling and DNA repair are of high interest in the search for underlying molecular defects.
immune aging; DNA damage; telomere; T cell signaling; SASP
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 granulomatous inflammation, 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 jeopardize the immune privilege of the vessel wall. 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, likely contributes to 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
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, has been identified in the vasculitic lesions. Two dominant cytokine clusters, one centering on the IL-6/IL-17 axis, the other on the IL-12/IFN-γ axis, have been connected with disease activity. These two clusters appear 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, may allow for a more tailored therapeutic approach and spare patients the unwanted side-effects of aggressive immunosuppression.
T cells from RA patients are hypoglycolytic due to insufficient induction of the glycolytic activator PFKFB3, resulting in impaired autophagy and reduced ROS production.
In the HLA class II–associated autoimmune syndrome rheumatoid arthritis (RA), CD4 T cells are critical drivers of pathogenic immunity. We have explored the metabolic activity of RA T cells and its impact on cellular function and fate. Naive CD4 T cells from RA patients failed to metabolize equal amounts of glucose as age-matched control cells, generated less intracellular ATP, and were apoptosis-susceptible. The defect was attributed to insufficient induction of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3), a regulatory and rate-limiting glycolytic enzyme known to cause the Warburg effect. Forced overexpression of PFKFB3 in RA T cells restored glycolytic flux and protected cells from excessive apoptosis. Hypoglycolytic RA T cells diverted glucose toward the pentose phosphate pathway, generated more NADPH, and consumed intracellular reactive oxygen species (ROS). PFKFB3 deficiency also constrained the ability of RA T cells to resort to autophagy as an alternative means to provide energy and biosynthetic precursor molecules. PFKFB3 silencing and overexpression identified a novel extraglycolytic role of the enzyme in autophagy regulation. In essence, T cells in RA patients, even those in a naive state, are metabolically reprogrammed with insufficient up-regulation of the glycolytic activator PFKFB3, rendering them energy-deprived, ROS- and autophagy-deficient, apoptosis-sensitive, and prone to undergo senescence.
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.
macrophage; dendritic cell; T cell; granuloma; vasculitis
Giant cell arteritis (GCA) is a granulomatous vasculitis of the aorta and its branches that causes blindness, stroke and aortic aneurysm. CD4 T-cells are key pathogenic regulators, instructed by vessel wall dendritic cells (DC) to differentiate into vasculitic T cells. The unique pathways driving this DC-T-cell interaction are incompletely understood but may provide novel therapeutic targets for a disease in which the only established therapy is chronic treatment with high doses of corticosteroids.
Methods and Results
Immunohistochemical and gene expression analysis of GCA-affected temporal arteries revealed abundant expression of the NOTCH receptor and its ligands Jagged1 and Delta1. Cleavage of the NOTCH intracellular domain (NICD) in wall-infiltrating T cells indicated ongoing NOTCH pathway activation in large vessel vasculitis. NOTCH activation did not occur in small vessel vasculitis affecting branches of the vasa vasorum tree. We devised two strategies to block NOTCH pathway activation; γ-secretase inhibitor treatment, preventing nuclear translocation of the NICD, and competing for receptor-ligand interactions through excess soluble ligand, Jagged1-Fc. In humanized mice carrying human arteries NOTCH pathway disruption had strong immunosuppressive effects, inhibiting T-cell activation in the early and established phase of vascular inflammation. NOTCH inhibition was particularly effective in downregulating Th17 responses, but also markedly suppressed Th1 responses.
Blocking NOTCH signaling depleted T cells from the vascular infiltrates, implicating NOTCH-NOTCH ligand interactions in regulating T-cell retention and survival in vessel wall inflammation. Modulating the NOTCH signaling cascade emerges as a promising new strategy for immunosuppressive therapy of large vessel vasculitis.
Arteries; Inflammation; T-cell; NOTCH; Costimulation; IFN-γ; IL-17
In the older adult the benefits of vaccination to prevent infectious disease are limited, mainly due to the adaptive immune system’s inability to generate protective immunity. Age-dependent decline in immune competence, often referred to as immunosenescence, results from progressive deterioration of innate and adaptive immune responses and most of the insights into mechanisms of immune aging have derived from studies in murine models. In this Review, we explore how well these models are applicable to understand the aging process throughout the 80–100 years of human life and discuss recent advances in uncovering and characterizing the mechanisms underlying age-associated defective adaptive immunity in humans.
CD8 T cells stimulated with a suboptimal dose of anti-CD3 antibodies (100 pg/ml) in the presence of IL-15 retain a naïve phenotype with expression of CD45RA, CD28, CD27 and CCR7 but acquire new functions and differentiate into immunosuppressive T cells. CD8+CCR7+ Tregs express FOXP3 and prevent CD4 T cells from responding to T-cell receptor stimulation and entering the cell cycle. Naïve CD4 T cells are more susceptible to inhibition than memory cells. The suppressive activity of CD8+CCR7+ Tregs is not mediated by IL-10, TGF-β, CTLA-4, CCL4 or adenosine and relies on interference with very early steps of the TCR signaling cascade. Specifically, CD8+CCR7+ Tregs prevent TCR-induced phosphorylation of ZAP70 and dampen the rise of intracellular calcium in CD4 T cells. The inducibility of CD8+CCR7+ Tregs is correlated to the age of the individual with peripheral blood lymphocytes of donors older than 60 years yielding low numbers of FOXP3low CD8 Treg cells. Loss of CD8+CCR7+ Tregs in the elderly host may be of relevance in the aging immune system as immunosenescence is associated with a state of chronic smoldering inflammation.
In rheumatoid arthritis (RA), hematopoietic progenitor cells (HPC) have age-inappropriate telomeric shortening suggesting premature senescence and possible restriction of proliferative capacity. In response to hematopoietic growth factors RA-derived CD34+ HPC expanded significantly less than age-matched controls. Cell surface receptors for stem cell factor (SCF), Flt 3-Ligand, IL-3 and IL-6 were intact in RA HPC but the cells had lower transcript levels of cell cycle genes, compatible with insufficient signal strength in the ERK pathway. Cytokine-induced phosphorylation of ERK1/2 was diminished in RA HPC whereas phosphorylated STAT3 and STAT5 molecules accumulated to a similar extent as in controls. Confocal microscopy demonstrated that the membrane-proximal colocalization of K-Ras and B-Raf were less efficient in RA-derived CD34+ cells. Thus, hyporesponsiveness of RA HPC to growth factors results from dampening of the ERK signaling pathways; with a defect localized in the very early steps of the ERK signaling cascade.
rheumatoid arthritis; hematopoietic progenitor cells; CD34; ERK signaling; STAT signaling
Cancer cells have long been known to fuel their pathogenic growth habits by sustaining a high glycolytic flux, first described almost 90 years ago as the so-called Warburg effect. Immune cells utilize a similar strategy to generate the energy carriers and metabolic intermediates they need to produce biomass and inflammatory mediators. Resting lymphocytes generate energy through oxidative phosphorylation and breakdown of fatty acids, and upon activation rapidly switch to aerobic glycolysis and low tricarboxylic acid flux. T cells in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) have a disease-specific metabolic signature that may explain, at least in part, why they are dysfunctional. RA T cells are characterized by low adenosine triphosphate and lactate levels and increased availability of the cellular reductant NADPH. This anti-Warburg effect results from insufficient activity of the glycolytic enzyme phosphofructokinase and differentiates the metabolic status in RA T cells from those in cancer cells. Excess production of reactive oxygen species and a defect in lipid metabolism characterizes metabolic conditions in SLE T cells. Owing to increased production of the glycosphingolipids lactosylceramide, globotriaosylceramide and monosialotetrahexosylganglioside, SLE T cells change membrane raft formation and fail to phosphorylate pERK, yet hyperproliferate. Borrowing from cancer metabolomics, the metabolic modifications occurring in autoimmune disease are probably heterogeneous and context dependent. Variations of glucose, amino acid and lipid metabolism in different disease states may provide opportunities to develop biomarkers and exploit metabolic pathways as therapeutic targets.
Purpose of review
Granuloma formation in giant cell arteritis (GCA) emphasizes the role of the adaptive immunity and highlights the role of antigen-specific T cells. Recent data demonstrate that at least two separate lineages of CD4 T-cells participate in vascular inflammation, providing an important clue that multiple disease instigators may initiate pathogenic immunity.
IFN-γ-producing Th1 cells and IL-17-producing Th17 cells have been implicated in GCA. Patients with biopsy-positive GCA underwent two consecutive temporal artery biopsies, one prior to therapy and one while on corticosteroids. In untreated patients, Th1 and Th17 cells co-existed in the vascular lesions. Following therapy, Th17 cells were essentially lost, whereas Th1 cells persisted almost unaffected. In the peripheral blood of untreated patients Th17 frequencies were increased eightfold, but normalized with therapy. Blood Th1 cells were doubled in frequency, independent of therapy. Corticosteroids functioned by selectively suppressing IL-1β, IL-6 and IL-23-releasing antigen-presenting cells (APC), disrupting induction of Th17 cells.
At least two distinct CD4 T-cell subsets promote vascular inflammation in GCA. In early disease, APCs promote differentiation of Th17 as well as Th1 cells. Chronic disease is characterized by persistent Th1-inducing signals, independent of IL-17-mediated inflammation. More than one disease instigator may trigger APCs to induce multiple T cell lineages. Cocktails of therapies will be needed for appropriate disease control.
IL-17; IFN-γ; T cell; Antigen-presenting cell
A 79-year-old woman presents with new-onset pain in her neck and both shoulders. She takes 7.5 mg of prednisone per day for giant-cell arteritis. Occipital tenderness and diplopia developed 11 months before presentation. At that time, her erythrocyte sedimentation rate was elevated, at 78 mm per hour, and a temporal-artery biopsy revealed granulomatous arteritis. The diplopia resolved after 6 days of treatment with 60 mg of prednisone daily. Neither headache nor visual symptoms developed when the glucocorticoids were tapered. How should this patient’s care be managed?
Age is an important risk for autoimmunity, and many autoimmune diseases preferentially occur in the second half of adulthood when immune competence has declined and thymic T cell generation has ceased. Many tolerance checkpoints have to fail for an autoimmune disease to develop, and several of those are susceptible to the immune aging process. Homeostatic T cell proliferation which is mainly responsible for T cell replenishment during adulthood can lead to the selection of T cells with increased affinity to self- or neoantigens and enhanced growth and survival properties. These cells can acquire a memory-like phenotype, in particular under lymphopenic conditions. Accumulation of end-differentiated effector T cells, either specific for self-antigen or for latent viruses, have a low activation threshold due to the expression of signaling and regulatory molecules and generate an inflammatory environment with their ability to be cytotoxic and to produce excessive amounts of cytokines and thereby inducing or amplifying autoimmune responses.
Age; Autoimmunity; T cell memory; T cell homeostasis; CD45RA T effector cells
In the autoimmune syndrome rheumatoid arthritis (RA), T-cells and T-cell precursors have age-inappropriate shortening of telomeres and accumulate DNA double strand breaks. Whether damaged DNA elicits DNA repair activity and how this affects T-cell function and survival is unknown. Here, we report that naïve and resting T cells from RA patients are susceptible to undergo apoptosis. In such T cells, unrepaired DNA stimulates a p53-ATM-independent pathway involving the nonhomologous-end-joining protein DNA-PKcs. Upregulation of DNA-PKcs transcription, protein expression and phosphorylation in RA T cells co-occurs with diminished expression of the Ku70/80 heterodimer, limiting DNA repair capacity. Inhibition of DNA-PKcs kinase activity or gene silencing of DNA-PKcs protects RA T-cells from apoptosis. DNA-PKcs induces T-cell death by activating the JNK pathway and upregulating the apoptogenic BH3-only proteins Bim and Bmf. In essence, in rheumatoid arthritis, the DNA-PKcs-JNK-Bim/Bmf axis transmits genotoxic stress into shortened survival of naïve resting T cells, imposing chronic proliferative turnover of the immune system and premature immunosenescence. Therapeutic blockade of the DNA-PK-dependent cell-death machinery may rejuvenate the immune system in RA.
Apoptosis; DNA damage; DNA-PKcs; Rheumatoid Arthritis; T cell
Noninvasive diagnosis of giant cell arteritis (GCA) remains challenging, particularly with regard to evaluation of extracranial arterial disease.
The objective of the study was to retrospectively review extracranial involvement in patients with GCA and/or polymyalgia rheumatica (PMR), evaluated with magnetic resonance imaging (MRI), especially 3-dimensional contrast-enhanced magnetic resonance angiography images of the aortic arch and its branches.
Clinical information, biopsy status, and MRI examinations of 28 patients with GCA/PMR were reviewed. Patient images were mixed randomly with 20 normal control images and were independently reviewed by 2 radiologists. Interobserver agreement for detection of arterial stenosis was determined by the k coefficient.
Both readers described vascular alterations in keeping with extracranial GCA in 19 of 28 patients (67%) with good interobserver agreement (k = 0.73) and with even higher agreement on diagnosing nonocclusive versus occlusive disease (k = 1.00). The most common lesions were bilateral axillary stenosis or obstructions, observed by both readers in 8 patients (28%). Among the 19 patients with magnetic resonance angiography lesions in the subclavian/axillary arteries, 12 (75%) had biopsy-proven GCA, but only 5 (41%) of these patients had clinical features of large artery disease.
In our series review, MRI could provide accurate information on involvement of the aortic arch and its branches in extracranial GCA, depicting different degrees of stenosis. Our analysis also illustrates that occult large artery vasculitis should be considered in patients without biopsy-proven GCA, patients with classic GCA but without clinical signs of large artery disease, and in patients initially diagnosed as having PMR.
large artery vasculitis; giant cell arteritis; MRI; MRA
Immune-mediated damage to medium-sized arteries results in wall remodeling with intimal hyperplasia, luminal stenosis and tissue ischemia. In the case of the aorta, vasculitis may result in dissection, aneurysm or rupture. The response-to-injury program of the blood vessel is a concerted action between the immune system and wall-resident cells, involving the release of growth and angiogenic factors from macrophages and giant cells and the migration and hyperproliferation of vascular smooth muscle cells. Innate immune cells, specifically, dendritic cells (DC) positioned in the vessel wall, have been implicated in the earliest steps of vasculitis. Pathogen-derived molecular patterns are capable of activating vascular DC and initiating adaptive immune responses. The pattern of the emerging vessel wall inflammation is ultimately determined by the initial insult. Ligands to toll-like receptor (TLR) 4, such as lipopolysaccharides, facilitate the recruitment of CD4 T cells that invade deep into the wall and distribute in a panarteritic pattern. Conversely, ligands for TLR5 condition vascular DC to support perivasculitic infiltrates. In essence, both innate and adaptive immune reactions collaborate to render the arterial wall susceptible to inflammatory damage. Unique features of the tissue microenvironment, including specialized DC, shape the course of the inflammatory response. Differences in vascular damage pattern encountered in different patients may relate to distinct instigators of vasculitis.
A defining feature of the eukaryotic genome is the presence of linear chromosomes. This arrangement, however, poses several challenges with regard to chromosomal replication and maintenance. To prevent the loss of coding sequences and to suppress gross chromosomal rearrangements, linear chromosomes are capped by repetitive nucleoprotein structures, called telomeres. Each cell division results in a progressive shortening of telomeres that, below a certain threshold, promotes genome instability, senescence, and apoptosis. Telomeric erosion, maintenance, and repair take center stage in determining cell fate. Cells of the immune system are under enormous proliferative demand, stressing telomeric intactness. Lymphocytes are capable of upregulating telomerase, an enzyme that can elongate telomeric sequences and, thus, prolong cellular lifespan. Therefore, telomere dynamics are critical in preserving immune function and have become a focus for studies of immunosenescence and autoimmunity. In this review, we describe the role of telomeric nucleoproteins in shaping telomere architecture and in suppressing DNA damage responses. We summarize new insights into the regulation of telomerase activity, hereditary disorders associated with telomere dysfunction, the role of telomere loss in immune aging, and the impact of telomere dysfunction in chronic inflammatory disease.
Telomeres; Aging; Immunosenescence
In giant cell arteritis (GCA), vasculitic damage of the aorta and its branches is combined with a syndrome of intense systemic inflammation. Therapeutically, glucocorticoids (GC) remain the golden standard as they promptly and effectively suppress acute manifestations; however, they fail to eradicate vessel wall infiltrates. The effects of glucocorticoids on the systemic and vascular components of GCA are not understood.
Methods and Results
The immunoprofile of untreated and GC-treated GCA was examined in peripheral blood and temporal artery biopsies with protein quantification assays, flow cytometry, quantitative real-time PCR, and immunohistochemistry. Plasma IFN-γ and IL-17 and frequencies of IFN-γ-producing and IL-17-producing T cells were markedly elevated prior to therapy. GC treatment suppressed the Th17 but not the Th1 arm in the blood and the vascular lesions. Analysis of monocytes/macrophages in the circulation and in temporal arteries revealed GC-mediated suppression of Th17-promoting cytokines (IL-1β, IL-6, and IL-23), but sparing of Th1-promoting cytokines (IL-12). In human artery-SCID mouse chimeras, in which patient-derived T cells cause inflammation of engrafted human temporal arteries, glucocorticoids were similarly selective in inhibiting Th17 cells and leaving Th1 cells unaffected.
Two pathogenic pathways mediated by Th17 and Th1 cells contribute to the systemic and vascular manifestations of GCA. IL-17-producing Th17 cells are sensitive to GC-mediated suppression, but IFN-γ-producing Th1 responses persist in treated patients. Targeting steroid-resistant Th1 responses will be necessary to resolve chronic smoldering vasculitis. Monitoring Th17 and Th1 frequencies can aid in assessing disease activity in GCA.
giant cell arteritis; glucocorticoids; Th1; Th17
Large vessel vasculitides, such as Takayasu arteritis and giant cell arteritis (GCA), affect vital arteries and cause clinical complications by either luminal occlusion or vessel wall destruction. Inflammatory infiltrates, often with granulomatous arrangements, are distributed as a panarteritis throughout all of the artery’s wall layers or cluster in the adventitia as a perivasculitis. Factors determining the architecture and compartmentalization of vasculitis are unknown. Human macrovessels are populated by indigenous dendritic cells (DC) positioned in the adventitia. Herein, we report that these vascular DC sense bacterial pathogens and regulate the patterning of the emerging arteritis. In human temporal artery-SCID chimeras, lipopolysaccharides stimulating Toll-like receptor (TLR) 4 and flagellin stimulating TLR5 trigger vascular DC and induce T-cell recruitment and activation. However, the architecture of the evolving inflammation is ligand specific; TLR4 ligands cause transmural panarteritis and TLR5 ligands promote adventitial perivasculitis. Underlying mechanisms involve selective recruitment of functional T cell subsets. Specifically, TLR4-mediated DC stimulation markedly enhances production of the chemokine CCL20, biasing recruitment towards CCL20-responsive CCR6+ T cells. In adoptive transfer experiments,CCR6+ T cells produce an arteritis pattern with media-invasive T cells damaging vascular smooth muscle cells. Also, CCR6+ T cells dominate the vasculitic infiltrates in patients with panarteritic GCA. Thus, depending on the original danger signal, vascular DC edit the emerging immune response by differentially recruiting specialized T effector cells and direct the disease process toward distinct types of vasculitis.
T cell; Toll-like receptor; inflammation; vascular inflammation
In rheumatoid arthritis (RA), telomeres of lymphoid and myeloid cells are age-inappropriately shortened, suggesting excessive turnover of hematopoietic precursor cells (HPC). We have examined functional competence (proliferative capacity, maintenance of telomeric reserve) of CD34+ HPC in RA.
Frequencies of peripheral blood CD34+CD45+ HPC of 63 rheumatoid factor positive RA patients and 48 matched controls were measured by flow cytometry. Proliferative burst, cell cycle dynamics, and induction of lineage-restricted receptors were tested in purified CD34+ HPC after stimulating with early hematopoietins. Telomeric sequences were quantified by real-time PCR. HPC functions were correlated with disease duration, activity, severity, and treatment.
In healthy donors, CD34+ HPC accounted for 0.05% of nucleated cells; their numbers were strictly age-dependent and declined at a rate of 1.3%/year. In RA patients, CD34+ HPC frequencies were age-independently reduced to 0.03%. Upon growth factor stimulation, control HPC passed through 5 replication cycles over 4 days. In contrast, RA-derived HPC completed only 3 generations. Telomeres of RA CD34+ HPC were age-inappropriately shortened by 1,600 bp. All HPC defects were independent from disease duration, disease activity, and smoking status; and were present to the same degree in untreated patients.
In RA, circulating bone marrow-derived progenitor cells are diminished, with concentrations stagnated at levels typical for old control individuals. HPC from RA patients display growth factor non-responsiveness and sluggish cell cycle progression; marked telomere shortening indicates proliferative stress-induced senescence. Defective HPC function independent from disease activity markers suggests bone marrow failure as a potential pathogenic factor in RA.
In rheumatoid arthritis (RA), dysfunctional T cells sustain chronic inflammatory immune responses in the synovium. Even unprimed T cells are under excessive replication pressure, suggesting an intrinsic defect in T cell regeneration. In naive CD4 CD45RA+ T cells from RA patients, DNA damage load and apoptosis rates were markedly higher than in controls; repair of radiation-induced DNA breaks was blunted and delayed. DNA damage was highest in newly diagnosed untreated patients. RA T cells failed to produce sufficient transcripts and protein of the DNA repair kinase ataxia telangiectasia (AT) mutated (ATM). NBS1, RAD50, MRE11, and p53 were also repressed. ATM knockdown mimicked the biological effects characteristic for RA T cells. Conversely, ATM overexpression reconstituted DNA repair capabilities, response patterns to genotoxic stress, and production of MRE11 complex components and rescued RA T cells from apoptotic death. In conclusion, ATM deficiency in RA disrupts DNA repair and renders T cells sensitive to apoptosis. Apoptotic attrition of naive T cells imposes lymphopenia-induced proliferation, leading to premature immunosenescence and an autoimmune-biased T cell repertoire. Restoration of DNA repair mechanisms emerges as an important therapeutic target in RA.
Inflammatory vasculopathies, ranging from the vasculitides (Takayasu arteritis, giant cell arteritis, and polyarteritis nodosa) to atherosclerosis display remarkable target tissue tropisms for selected vascular beds. Molecular mechanisms directing wall inflammation to restricted anatomical sites within the vascular tree are not understood. We have examined the ability of 6 different human macrovessels (aorta, subclavian, carotid, mesenteric, iliac, temporal arteries) to initiate innate and adaptive immune responses by comparing pathogen-sensing and T-cell stimulatory capacities.
Methods and Results
Gene expression analysis for pathogen-sensing Toll-like receptors (TLR) 1-9 showed vessel-specific profiles with TLR2 and 4 ubiquitously present, TLR7 and 9 absent, and TLR1, 3, 5, 6 and 8 expressed in selective patterns. Experiments with vessel walls stripped of the intimal or adventitial layer identified dendritic cells (DC) at the media-adventitia junction as the dominant pathogen sensors. In human artery-SCID mouse chimeras, adoptively transferred human T cells initiated vessel wall inflammation if wall-embedded DC were conditioned with TLR ligands. Wall-infiltrating T cells displayed vessel-specific activation profiles with differential production of CD40L, lymphotoxin-α, and interferon-γ. Vascular bed-specific TLR fingerprints were functionally relevant as exemplified by differential responsiveness of iliac and subclavian vessels to TLR5 but not TLR4 ligands.
Populated by indigenous DC, medium and large human arteries have immune sensing and T-cell stimulatory functions. Each vessel in the macrovascular tree utilizes a distinct TLR profile and supports selective T-cell responses imposing vessel-specific risk for inflammatory vasculopathies.
arteries; immune system; inflammation
Dendritic cells (DCs) shape T-cell response patterns and determine early, intermediate, and late outcomes of immune recognition events. They either facilitate immunostimulation or induce tolerance, possibly determined by initial DC activation signals, such as binding Toll-like receptor (TLR) ligands. Here we report that DC stimulation through the TLR3 ligand dsRNA [poly(I:C)] limits CD4 T-cell proliferation, curtailing adaptive immune responses. CD4+ T cells instructed by either LPS or poly(I:C)-conditioned DCs promptly upregulated the activation marker CD69. Whereas LPS-pretreated DCs subsequently sustained T-cell clonal expansion, proliferation of CD4+ T cells exposed to poly(I:C)-pretreated DCs was markedly suppressed. This proliferative defect required DC-T cell contact, was independent of IFN-α, and was overcome by exogenous IL-2, indicating T-cell anergy. Coinciding with the downregulation, CD4+ T cells expressed the inhibitory receptor PD-1. Antibodies blocking the PD-1 ligand PD-L1 restored proliferation. dsRNA-stimulated DCs preferentially induced PD-L1, whereas poly(I:C) and LPS both upregulated the costimulatory molecule CD86 to a comparable extent. Poly(dA-dT), a ligand targeting the cytoplasmic RNA helicase pattern-recognition pathway, failed to selectively induce PD-L1 upregulation, assigning this effect to the TLR3 pathway. Poly(I:C)-conditioned DCs promoted accumulation of phosphorylated SHP-2, the intracellular phosphatase mediating PD-1 inhibitory effects. The ability of dsRNA to bias DC differentiation toward providing inhibitory signals to interacting CD4+ T cells may be instrumental in viral immune evasion. Conversely, TLR3 ligands may have therapeutic value in silencing pathogenic immune responses.
dendritic cell; T cell; Toll-like receptor 3; PD-L1
Acute coronary syndromes (ACS) are precipitated by a rupture of the atherosclerotic plaque, often at the site of T cell and macrophage infiltration. Here, we show that plaque-infiltrating CD4 T cells effectively kill vascular smooth muscle cells (VSMC). VSMCs sensitive to T cell–mediated killing express the death receptor DR5 (TNF-related apoptosis-inducing ligand [TRAIL] receptor 2), and anti-TRAIL and anti-DR5 antibodies block T cell–mediated apoptosis. CD4 T cells that express TRAIL upon stimulation are expanded in patients with ACS and more effectively induce VSMC apoptosis. Adoptive transfer of plaque-derived CD4 T cells into immunodeficient mice that are engrafted with human atherosclerotic plaque results in apoptosis of VSMCs, which was prevented by coadministration of anti-TRAIL antibody. These data identify that the death pathway is triggered by TRAIL-producing CD4 T cells as a direct mechanism of VSMC apoptosis, a process which may lead to plaque destabilization.
RA is a quintessential autoimmune disease with a growing number of cells, mediators, and pathways implicated in this tissue-injurious inflammation. Now Kuhn and colleagues have provided convincing evidence that autoantibodies reacting with citrullinated proteins, known for their sensitivity and specificity as biomarkers in RA, enhance tissue damage in collagen-induced arthritis (see the related article beginning on page 961). This study adds yet another soldier to the growing army of autoaggressive mechanisms that underlie RA. With great success researchers have dismantled the pathogenic subunits of RA, adding gene to gene, molecule to molecule, and pathway to pathway in an ever more complex scheme of dysfunction. The complexity of the emerging disease model leaves us speechless. It seems that with this wealth of data available, we need to develop a new theory for this disease. We may want to seek guidance from our colleagues in physics and mathematics who have successfully integrated their knowledge of elementary particles and the complexity of their interacting forces by formulating the string theory.