JOURNAL OF THE AMERICAN COLLEGE OF CARDIOLOGY
VOL. 70, NO. 11, 2017
PUBLISHED BY ELSEVIER ON BEHALF OF THE AMERICAN
COLLEGE OF CARDIOLOGY FOUNDATION
ISSN 0735-1097/$36.00
http://dx.doi.org/10.1016/j.jacc.2017.07.750
REVIEW TOPIC OF THE WEEK
Unraveling Vascular Inflammation
From Immunology to Imaging
Heather L. Teague, PHD,a Mark A. Ahlman, MD,b Abass Alavi, MD, PHD,c Denisa D. Wagner, PHD,d
Andrew H. Lichtman, MD, PHD,e Matthias Nahrendorf, MD, PHD,f Filip K. Swirski, PHD,f Frank Nestle, MD,g
Joel M. Gelfand, MD, MSCE,h Mariana J. Kaplan, MD,i Steven Grinspoon, MD,j Paul M. Ridker, MD,e
David E. Newby, DM, PHD,k Ahmed Tawakol, MD, PHD,l Zahi A. Fayad, PHD,m Nehal N. Mehta, MD, MSCEl
ABSTRACT
Inflammation is a critical factor in early atherosclerosis and its progression to myocardial infarction. The search for
valid surrogate markers of arterial vascular inflammation led to the increasing use of positron emission tomography/
computed tomography. Indeed, vascular inflammation is associated with future risk for myocardial infarction and can
be modulated with short-term therapies, such as statins, that mitigate cardiovascular risk. However, to better
understand vascular inflammation and its mechanisms, a panel was recently convened of world experts in
immunology, human translational research, and positron emission tomographic vascular imaging. This contemporary
review first strives to understand the diverse roles of immune cells implicated in atherogenesis. Next, the authors
describe human chronic inflammatory disease models that can help elucidate the pathophysiology of vascular
inflammation. Finally, the authors review positron emission tomography–based imaging techniques to characterize
the vessel wall in vivo. (J Am Coll Cardiol 2017;70:1403–12) Published by Elsevier on behalf of the American College
of Cardiology Foundation.
From the aNational Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland; bRadiology and Imaging
Sciences, Clinical Center, National Institutes of Health, Bethesda, Maryland; cUniversity of Pennsylvania, Philadelphia, Pennsylvania; dHarvard Medical School, Boston, Massachusetts; eBrigham and Women’s Hospital and Harvard Medical School, Boston,
Massachusetts; fCenter for Systems Biology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts;
g
Kings College, London, United Kingdom; hPerelman School of Medicine, Philadelphia, Pennsylvania; iNational Institute of
Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, Maryland; jProgram in Nutritional
Metabolism, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts; kBritish Heart Foundation
Centre for Cardiovascular Science, University of Edinburgh, Edinburgh, United Kingdom; lCardiovascular Research Center,
Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts; and the mIcahn School of Medicine at Mount
Sinai, New York, New York. Dr. Nestle is an employee of Sanofi. Dr. Gelfand has served as a consultant for Coherus (data and safety
monitoring board), Dermira, Janssen Biologics, Merck (data and safety monitoring board), Novartis, Regeneron, Dr. Reddy’s
Laboratories, Sanofi, and Pfizer; has received honoraria and research grants (to the Trustees of the University of Pennsylvania)
from Abbvie, Janssen, Novartis, Regeneron, Sanofi, Celgene, and Pfizer; has received payment for continuing medical education
work related to psoriasis that was supported indirectly by Eli Lilly and Abbvie; and coholds a patent on resiquimod for the
treatment of cutaneous T cell lymphoma. Dr. Grinspoon has served as a consultant for and received honoraria from Navidea
Pharmaceuticals and Theratechnologies; and has received research grants (to the Massachusetts General Hospital) from Navidea
Listen to this manuscript’s
Pharmaceuticals, KOWA Pharmaceuticals, Gilead Sciences, and Theratechnologies. Dr. Ridker has received investigator-initiated
audio summary by
research support from the National Heart, Lung, and Blood Institute, Pfizer, AstraZenenca, Novartis, and Kowa; and is listed as a
JACC Editor-in-Chief
coinventor on patents held by the Brigham and Women’s Hospital that relate to the use of inflammatory biomarkers in cardio-
Dr. Valentin Fuster.
vascular disease and diabetes that have been licensed to AstraZeneca and Siemens. Dr. Tawakol has served as a consultant for
Actelion Pharmaceuticals; and has received research grants (to the Massachusetts General Hospital) from Actelion and Genentech.
Dr. Mehta is a full-time U.S. government employee; and has received research grants from Abbvie, Janssen, Novartis, and Celgene.
All other authors have reported that they have no relationships relevant to the contents of this paper to disclose. Drs. Teague and
Ahlman contributed equally to this work. James Rudd, PhD, served as Guest Editor for this paper.
Manuscript received June 16, 2017; revised manuscript received July 20, 2017, accepted July 20, 2017.
Teague et al.
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Immunology of Vascular Inflammation
C
ABBREVIATIONS
AND ACRONYMS
CT = computed tomographic
ardiovascular disease (CVD) remains
local macrophage proliferation, rather than monocyte
the leading cause of death world-
recruitment, becomes more important in lesion
wide, highlighting the need to eluci-
growth.
date its pathogenesis. Once considered a
In addition to the accumulation of monocytes in
passive biological process, CVD is now recog-
atherosclerotic lesions, these innate immune cells
nized as an active, immune-driven process
contribute to the biological response following a
that may begin in childhood (1). Current
myocardial infarction (MI). Monocytes are both
research into the natural history of atheroma
destructive and protective, in that they give rise to
development has implicated many immune
infarct rupture and contribute to infarct healing.
cells, including phagocytes, lymphocytes,
However, an overabundance of monocytes can
MI = myocardial infarction
dendritic cells, and neutrophils (2). Because
interfere with healing, resulting in heart failure. In an
MR = magnetic resonance
these cells play a major role in initiating pla-
acute MI, an oversupply of monocytes to the aorta is
NaF =
que development and complication, leuko-
rapid, concomitant with a reduction in C-X-C motif
NET = neutrophil extracellular
cytes are promising targets for acute and
chemokine ligand 12 expression in the bone marrow.
trap
chronic atherosclerosis therapy. However,
Diminished C-X-C motif chemokine ligand 12 expres-
PAD4 = peptidylarginine
the complexity of the immune system and
sion enables myeloid cells and their progenitors to exit
its role as a defensive force against infection
the bone marrow and take up residence in the spleen,
requires novel tools to precisely identify and
where they trigger extramedullary hematopoiesis (5).
treat only the inflammatory cells or processes
Additionally, differentiated leukocytes, especially
that promote
atherosclerosis. Biomedical
monocytes and neutrophils, take up residence in end-
engineering, specifically in human imaging,
organ tissues, giving rise to plaques and inflammation.
offers unique possibilities for diagnosing
The vascular sympathetic innervation (i.e., nerve
and treating atherosclerotic plaque inflammation
fibers traveling along the aorta and arterioles) plays a
before cardiovascular (CV) events. Thus, interfacing
role in the increased emergency supply of leukocytes.
novel engineering to enhance human imaging with
In the periphery, sympathetic innervation activates
immunology will be essential to accelerate advances
endothelial cells on the luminal surface of athero-
in management of this disease.
sclerotic
CV = cardiovascular
CVD = cardiovascular disease
FDG =
18
F-fluorodeoxyglucose
HIV = human immunodeficiency
virus
IFN = interferon
18
F–sodium fluoride
deiminase–4
PET = positron emission
tomographic
SLE = systemic lupus
erythematosus
VI = vascular inflammation
plaques,
increasing
adhesion
molecule
In this review, we begin with an overview of the
expression, which augments leukocyte recruitment
emerging understanding of CVD as a systemic
(6). To directly investigate the role of the vascular
inflammatory disorder relating to monocytes, mac-
sympathetic innervation system in atherogenesis, the
rophages, neutrophils, and T cells. We then discuss
role of stress in monocyte proliferation is a topic of
specific human conditions with increased CVD risk to
ongoing investigation. Stress elevates noradrenaline
study the natural history of atherosclerosis, including
levels in the bone marrow and activates bone marrow
human immunodeficiency virus (HIV), systemic lupus
stem cells (7). Hematopoietic stem and progenitor cell
erythematosus (SLE), and psoriasis as human models
proliferation is significantly enhanced, C-X-C motif
of
chemokine ligand 12 is lowered, and monocytes enter
vascular
disease
initiation
and
progression.
Finally, we review current applications of positron
the systemic circulation
emission tomographic (PET) imaging and emerging
Collectively, these mechanisms suggest a multiorgan
PET tracer agents used in vascular characterization.
communication system that activates the bone
marrow through the sympathetic innervation system,
IMMUNOLOGY OF INFLAMMATION AS IT
increasing hematopoietic stem and progenitor cell
PERTAINS TO THE VESSEL WALL
proliferation
MONOCYTES AND MACROPHAGES. The most numerous
cells in atherosclerotic plaques are macrophages (3),
leukocytes that are central to innate immunity. In
atherosclerosis,
macrophage
accumulation
com-
mences as bone marrow–derived, Ly6Chigh monocytes
infiltrate
the
lesion.
These
in increased numbers.
Ly6Chigh
inflammatory monocytes exit the bone marrow in a C-C
and
thus
enhancing
leukocytosis.
Therefore, current studies aim to unravel the systemic
mechanisms that control the production, recruitment,
differentiation, and proliferation of monocytes and
their descendent macrophages in atherosclerosis, to
determine how these processes can be balanced to
exert the most benefit, and to elucidate specific control
points in atherogenesis.
motif chemokine receptor 2–dependent manner,
NEUTROPHILS. Neutrophils are increasingly recog-
accumulate in the vessel wall, and differentiate into
nized in the initiation of atherosclerotic plaque
macrophages, which are sustained through self-
development. Neutrophils are the initial immune cell
renewal (4). Notably, as atherosclerosis progresses,
to infiltrate inflammatory sites produced by injury or
Teague et al.
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Immunology of Vascular Inflammation
F I G U R E 1 Formation of Neutrophil Extracellular Traps Induces Vascular Damage
Neutrophil extracellular traps (NETs) in the timeline of deep vein thrombosis (DVT): a model. (A) DVT is initiated by local hypoxia and
activation of endothelial cells (ECs) as a result of flow restriction and disturbances. Activated endothelium releases ultralarge von Willebrand
factor (ULVWF) and P-selectin from Weibel-Palade bodies (WPBs), which mediate platelet and neutrophil adhesion. Activated platelets recruit
tissue factor (TF)–containing microparticles that enhance thrombin generation in the growing thrombus. (B) Activated platelets and
endothelium or other stimulus induce NET formation in adherent neutrophils. NETs provide an additional scaffold for platelet and red blood
cell (RBC) adhesion, promote fibrin formation, and exacerbate platelet and endothelial activation. (C) Plasmin, a disintegrin and
metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), and deoxyribonuclease (DNase) mediate thrombolysis by
degrading fibrin, ULVWF, and deoxyribonucleic acid, respectively. Monocytes/macrophages (Mø) release an additional source of DNase and
generate plasmin and promote restoration of blood flow (10).
infection. Although the antimicrobial action of neu-
NETosis, it is evident that NETs themselves are
trophils is indispensable to combat infection, their
proinflammatory,
mechanisms of action yield significant tissue damage
damage, and are highly prothrombotic (Figure 1) (10).
and toxic debris. A newly identified defense mecha-
Additionally, NETs provide a communication plat-
nism is the ability of neutrophils to form neutrophil
form between neutrophils and macrophages, priming
induce
endothelial
and
tissue
extracellular traps (NETs) (8); however, NET forma-
macrophages to produce pro–interleukin-1b , culmi-
tion, or “NETosis,” remains poorly understood.
nating in atherosclerotic plaque destabilization (11).
Currently,
been
NET formation is a peptidylarginine deiminase–4
described, suicidal and vital NETosis. During suicidal
2
distinct
(PAD4)–dependent mechanism induced by extracel-
NETosis, the internal membranes of the neutrophils
lular stimuli via microbes, activated platelets, cyto-
dissolve, followed by cell lysis and decondensed
kines,
chromatin release (8). During an infectious process,
cholesterol crystals. Upon activation, PAD4 converts
neutrophils directly secrete nuclear contents from the
arginine residues on histones to citrulline, triggering
cell without killing the neutrophils (9). Although
chromatin decondensation and NET release (12).
many
Consequently, PAD4 is expelled in conjunction with
questions
mechanisms
remain
have
unanswered
regarding
antibodies
to
neutrophils,
hypoxia,
and
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Immunology of Vascular Inflammation
F I G U R E 2 Vascular Images of Chronically Inflamed Human Models
Representative
18
F-fluorodeoxyglucose positron emission tomographic/computed tomographic imaging of the aorta in a healthy volunteer
(A), compared with the aortas of patients with human immunodeficiency virus (B), psoriasis (C), and systemic lupus erythematosus (D).
a milieu of toxic proteins, resulting in citrullination
T CELLS. When immune effector mechanisms are
of surrounding proteins, altering their functional
active within the blood vessel wall or endothelial
properties and producing neoantigens, which may
surface,
play a role in autoimmune diseases (13). In a murine
ischemia usually result. T cells play important roles in
model of MI, ischemia of the heart induces NETosis.
the
Furthermore, PAD4 /, a murine model incapable of
in atherosclerotic lesions. Human arteries have
vascular
promotion
dysfunction,
and
regulation
(IFN)– g –producing
thrombosis,
of
and
and
inflammation
interleukin-17–
NETosis, displays smaller infarct size and has
interferon
significantly better heart function, demonstrated by
producing CD4 þ T cells in atherosclerotic lesions, and
an elevated ejection fraction subsequent to an
mouse studies have shown that IFN-g -producing T
ischemic event (14). In aged or diabetic mice, neu-
helper 1 cells promote atheroma development (16).
trophils are primed for NETosis, producing excessive
Consistent with the finding that T cells are involved
thrombosis and inflammation (15). Myocardial NET
in plaque inflammation, murine models show that the
deposition delays wound healing, leading to fibrosis
B7-CD28 T-cell costimulatory pathway is involved in
in the cardiac pressure-overload model. Intriguingly,
promoting proatherogenic T-cell responses, as well as
spontaneous fibrosis of organs produced by aging is
atheroprotective regulatory T-cell responses (17).
greatly reduced in PAD4/ mice. Functionally,
Importantly, murine models indicate that inhibitory
PAD4/ hearts are comparable with young murine
members of the B7-CD28 family, in particular PD-L1
hearts, and their systolic and diastolic function does
(B7-H1) expressed on antigen-presenting cells, endo-
not decline with age (15). Thus, there is recent
thelium, and the cells of various tissues, and its
interest in further study of the specific roles of
receptor on T cells, PD-1, are important in suppressing
neutrophils, NETosis, and the PAD4 pathway in
T cell–driven inflammation in arteriosclerosis and
atherosclerosis.
myocytes (18). For example, mice lacking PD-L1 or
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F I G U R E 3 Fluorine-18–Sodium Fluoride Uptake in the
Coronary Arteries
Immunology of Vascular Inflammation
HUMAN DISEASE MODELS OF IMAGING TO
STUDY VASCULAR INFLAMMATION
Currently, 2 large ongoing CV trials in patients with
prior MI are testing if treatment of inflammation will
reduce a second CV event: CIRT (Cardiovascular
Inflammation Reduction Trial) and inhibition of
interleukin-1B
in
CANTOS
(Canakinumab
Anti-
Inflammatory Thrombosis Outcomes Study). There is
an initial report that CANTOS met the primary
endpoint for a reduction in recurrent major adverse CV
events. These trials will provide critical data on
whether inhibition of nonspecific T-cell inflammation
(methotrexate) or inflammasome activation (canakinumab) reduces further CV events in high-risk patients. As the results of these trials become available,
emerging data from human chronic diseases associated with high CV risk and systemic inflammation
provide models to understand vascular disease initiation and progression. Indeed,
A discrete focus of
18
F–sodium fluoride uptake overlying an
otherwise heavily calcified left anterior descending coronary
18
F-fluorodeoxyglucose
(FDG) PET/computed tomographic (CT) imaging has
been used to characterize vascular inflammation (VI)
artery, suggesting a locus of active calcification with potentially
in HIV infection, psoriasis, and SLE (Figure 2).
increased vulnerability for rupture (arrow).
HUMAN IMMUNODEFICIENCY VIRUS. HIV treatment
has become very effective over the past decade;
however, the rate of MI, stroke, and sudden cardiac
PD-1 display a marked increase in CD4þ- and CD8þ-
death remains elevated 50% to 100% in HIV infection
mediated inflammation in arterial lesions. Mice lack-
(21). Although dyslipidemia is more prevalent in HIV
ing PD-1 or PD-L1, or treated with PD-1 blockade, are
infection, traditional risks, including dyslipidemia,
more susceptible to T cell–mediated myocardial
hypertension, and diabetes, only account for 25% of
injury. Furthermore, IFN-g-induced up-regulation of
the excess risk. Using coronary CT angiography, a
PD-L1 by heart endothelial cells in vitro or in vivo
study demonstrated a high prevalence of noncalcified
protects against CD8þ cytotoxic T lymphocyte–
plaque with high-risk morphological features, asso-
mediated damage (19).
Mouse studies demonstrating the protective roles
ciated with increased immune activation indexes (22)
and
inflammation,
of the PD-1 pathway in arteries and heart highlight the
remained
elevated
possibility of increasing CV risks by targeting PD-1 or
therapy (23).
on
FDG
after
PET
effective
imaging
that
antiretroviral
PD-L1 in cancer immunotherapy. In fact, many cases
Among patients with HIV infection, coronary pla-
of acute severe lymphocytic myocarditis are now
ques are often inflamed and noncalcified (24), and
being reported in the context of checkpoint blockade
patients exhibit increased myocardial fibrosis imaged
cancer immunotherapy, including patients treated
by magnetic resonance (MR) (25). Imaging with FDG
with anti-PD-1 (20). Histopathologic analyses of
has demonstrated that HIV-infected patients with an
tissues from some of these cases reveal up-regulation
undetectable viral load have increased aortic target-
of endothelial human leukocyte antigen–antigen D
to-background ratios compared with healthy control
related, as well as myocyte and endothelial PD-L1
subjects and subjects with known CVD, indicating
associated with T-cell infiltrates, consistent with an
significant arterial inflammation, even in the context
IFN-mediated effect.
of
immune
restoration
and
viral
suppression
Notably, CVD initiation and progression involve
(Figure 2) (26). Together, coronary CT angiography
biological activity from multiple immune cells, both
and FDG PET imaging have helped identify the
innate and adaptive. Future studies deciphering the
unique pathophysiology of arterial inflammation and
interplay among these immune cells in CVD are crit-
plaque in HIV infection, proved to be a readout for the
ical for developing therapies targeting CVD initiation
efficacy of anti-inflammatory strategies now being
in order to reduce clinical CVD outcomes and
targeted in HIV infection, including newer statins.
ultimately decrease CVD prevalence.
Statins effectively decrease noncalcified plaque as
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Immunology of Vascular Inflammation
T A B L E 1 A Summary of Agents and Their Potential Mechanisms of Uptake Applicable to
Vascular Inflammation Imaging
psoriasis experience premature mortality, dying
approximately 5 years younger than their nonpsoriatic counterparts in adjusted analysis (32). Over
Agent (Ref. #)
18
11
Potential Mechanism of Uptake
10-year periods, patients with moderate to severe
F-FLT (62)
Structural analogue of thymidine, images DNA synthesis within
atheroma
C-PK11195 (63)
Affinity for translocator protein, upregulated on inflammatory
cells
major CV event, beyond the conventional Framinglational
18
F-A85380 (64)
Binds arterial nicotinic acetylcholine receptors, possibly related
to vascular damage
18
F-choline (65)
Images increased cell wall synthesis within atheroma
studies
have
demonstrated
dramatic
up-regulation of genes known to adversely affect CV
Affinity for somatostatin receptors, which are highly expressed
on macrophages
risk in the skin lesions of psoriasis. Furthermore,
Cu-ATSM (66)
Trapped within cells in hypoxic state
F-MISO (67)
Trapped within cells in hypoxic state
genetic construct is confined to the skin demonstrate
64
68
Images neoangiogenesis as a result of hypoxia or chronic
inflammation
Cu-DOTA-CANF (69)
Images neoangiogenesis via natriuretic peptide receptor
affinity
18
F-FDG
A glucose analogue imaging increased metabolic rate in the
presence of inflammation and hypoxia
18
F–sodium fluoride (59)
Images active calcification as a result of necrosis or
inflammation
64
68
transgenic mouse models of psoriasis in which the
aortic inflammation and thrombosis (34). Similarly,
Ga-NOTA-RGD (68)
18
ham risk score calculation (33). Interestingly, trans-
Ga-DOTA-octreotate (60)
68
18
psoriasis have about 6% excess risk for developing a
patients with psoriasis have an increased amount of
aortic FDG PET activity that is positively correlated
with skin disease severity (35).
A series of randomized, placebo-controlled clinical
trials are
evaluating the impact of treatments
such as adalimumab compared with phototherapy
Ga-CXCR4 (70)
Images CXCR4 receptor expressed by inflammatory cells
F-florbetapen (61)
Imaging b-amyloid plaque as a component of inflammation
11
C-PK11195 ¼ 11C-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide; A85380 ¼
3-([2S]-azetidinylmethoxy)pyridine dihydrochloride; ATSM ¼ diacetyl-bis(N-methylthiosemicarbazone;
CXCR4 ¼ C-X-C chemokine receptor type 4; DOTA-CANF ¼ 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic
acid atrial natriuretic factor; FDG ¼ fluorodeoxyglucose; FLT ¼ fluorothymidine; MISO ¼ fluoromisonidazole;
NOTA-RGD ¼ 1,4,7-triazacyclononane-N,N0 ,N00 -triacetic acid arginine-glycine-aspartate.
(NCT01553058), ustekinumab (NCT02187172), and
secukinumab (NCT02690701) on aortic inflammation
(measured by FDG PET imaging) and CV biomarkers
in psoriasis. These trials will provide greater insight
into the ability of anti-inflammatory therapies to
diminish the risk for CVD.
SYSTEMIC LUPUS ERYTHEMATOSUS. Patients with
SLE have a heightened risk for developing atherowell as low-density lipoprotein in patients with HIV
sclerosis, including MI and stroke (36), with young
infection (27). On the basis of these data, a large trial
women having up to a 50-fold increased risk for
of 6,500 patients with HIV infection and a low to
developing vascular events (37). Furthermore, even
moderate risk for CVD is now under way to test
patients with SLE with no prior CVD events have
whether pitavastatin can prevent CV events by
subclinical
reducing low-density lipoprotein and concomitantly
dysfunction, arterial stiffness, myocardial perfusion
improve inflammatory pathways of immune activa-
abnormalities, carotid plaque, and coronary calcifi-
tion
cation not fully explained by Framingham risk score
(REPRIEVE
[Randomized
Trial
to
Prevent
Vascular Events in HIV]; NCT02344290).
vascular
dysfunction,
endothelial
or by medications used to treat SLE (36).
PSORIASIS. Psoriasis is a chronic T helper 1 cell
Altered innate immune responses characteristic of
T helper 17 cell inflammatory disease that affects
SLE contribute to the development of lupus vascul-
more than 125 million people worldwide and about
opathy and atherosclerotic plaque formation (38).
2% to 4% of the adult population, with the most
Type I IFNs, a group of cytokines elevated in many
common
psoriasis.
patients with SLE, particularly during periods of flare,
The pathophysiology of the disease is localized
have pleiotropic deleterious roles in the vasculature
skin
hyperproliferation,
(39). Type I IFNs independently associated with
up-regulated T-cell and neutrophil activation, and
endothelial function, carotid plaque, and severity of
increases in C-reactive protein, serum amyloid A, and
coronary calcification in patients without histories of
intercellular adhesion molecule 1 (28). Consistent
CVD (40). Specifically, these cytokines promote an
with
large
imbalance of enhanced endothelial cell damage and
population-based epidemiological studies suggest
decreased vascular repair that may promote initiation
that patients with psoriasis, particularly moderate to
of vasculopathy and contribute to the development of
severe, have an increased risk for MI, stroke, and
foam-cell formation and platelet activation. SLE is
CV mortality independent of traditional risk factors
also characterized by the presence of a distinct subset
(29–31). Notably, studies demonstrate strong associ-
of pathogenic neutrophils, low-density granulocytes,
ations of psoriasis and CVD with increasing severity
which have an enhanced capacity to form NETs
of skin disease. Patients with moderate to severe
(38,39). NETs can amplify inflammatory responses in
type
manifesting
inflammation,
chronically
as
epidermal
inflamed
plaque
pathologies,
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Immunology of Vascular Inflammation
C ENTR AL I LL U STRA T I O N Progression of Vascular Inflammation in Human Inflammatory Models
Teague, H.L. et al. J Am Coll Cardiol. 2017;70(11):1403–12.
A gross illustration of the aortic arch has been taken in cross section to magnify the vessel wall. Neutrophil activation due to systemic inflammation leads to
the formation of neutrophil extracellular traps and may initiate damage to the endothelium. Monocytes and T cells then infiltrate the lesion. Monocytes
differentiate into macrophages, where they proliferate to sustain their population. Macrophages within the vessel wall have high glycolytic rates and take up the
18
F-fluorodeoxyglucose (FDG) tracer, which is detectable by FDG positron emission tomography in human models of inflammation. IFN ¼ interferon; IL ¼ interleukin;
TNF ¼ tumor necrosis factor.
the plaque and other tissues, including inflamma-
formation in murine models of lupus and athero-
some activation in macrophages and induction of
sclerosis (41). Overall, SLE represents a unique model
type I IFN synthesis by plasmacytoid dendritic cells
for understanding the role of the innate immune
(38,41).
system in the development of vascular disease, and
Two pathways are of potential interest in targeting
vascular risk in SLE: the inhibition of type 1 IFNs and
may allow the characterization of novel therapeutic
targets in subsets of patients.
the blockage of NETosis through PAD-4 inhibition.
Type 1 IFNs signal through the JAK/STAT pathway,
PET IMAGING OF THE ARTERY WALL
which is inhibited by tofacitinib. Tofacitinib ameliorated vascular dysfunction in a murine lupus model
Currently, FDG PET imaging is a cost-effective clinical
(42) and is being tested in humans in an ongoing
standard of care for diagnosis, staging, and moni-
clinical trial (NCT02535689). Inhibition of pathways
toring the response of many malignancies, and its
implicated in NET formation using chemical in-
role is significantly enhanced by the introduction of
hibitors of PAD-4 leads to amelioration of endothelial
PET/CT imaging and PET/MR imaging. Imaging
dysfunction, a prothrombotic phenotype, and plaque
human inflammation models with positron emission
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Immunology of Vascular Inflammation
tomography allows investigations to be conducted
there are opportunities for other imaging agents for
without expensive animal models, a step toward
atherosclerosis, such as vascular calcification imaged
clinical translation.
with
18
F–sodium fluoride (NaF). Originally approved
PET imaging of VI using FDG was first described
by the U.S. Food and Drug Administration for bone
more than 15 years ago (43,44) and has become
imaging in oncology, NaF PET imaging is an emerging
important in atherosclerosis research. FDG accumu-
technique reputed to capture a different aspect of the
lates within living cells in proportion to their glyco-
development of atheroma compared with FDG (59),
lytic rates (45). Several tissues have particularly high
related to calcification and microcalcification within
glycolytic rates and hence tend to accumulate FDG,
an atherosclerotic plaque (Figure 3). In patients with
including
and
MI and stable angina, Joshi et al. (59) demonstrated
inflammatory cells. Phagocytes have particularly high
conspicuous uptake of NaF in plaque, with histolog-
tumors,
brain
tissue,
myocytes,
glycolytic rates, especially after proinflammatory
ical evidence of macrophage infiltration, calcification,
activation (46), leading to high FDG retention. A
necrosis, and apoptosis, as well as high-risk plaque
potential advantage of PET imaging in addition to
features imaged by intravascular ultrasound. Advan-
other available anatomic imaging techniques (e.g., CT
tages of NaF over FDG include rapid background
imaging, MR, and ultrasound) is through the ability to
blood and tissue clearance and absence of activity in
administer miniscule concentrations of a substance
the myocardium, improving the potential for imaging
that can be targeted for a specific molecular process.
pathological NaF uptake in the coronary arteries (53).
PET images can be interpreted both qualitatively and
As with FDG, elements of the cell/process specificity
quantitatively to evaluate early phases of disease or
of NaF in atherosclerosis have been described, and
short-term changes in VI related to treatment that
imaging technique standardization is an active realm
may not have concordant changes in morphology.
of research.
Several
studies
show
that
FDG
uptake,
when
In parallel with further FDG and NaF work, inves-
measured in the arterial wall in vivo, reflects the level
tigation continues with other PET, CT, and MR tech-
of macrophage accumulation within the atheroma
niques to study varied mechanisms and applications
(47). Atherosclerotic lesions show higher VI by FDG
for CVD imaging. An abbreviated list of PET agents
and tend to experience greater subsequent progres-
currently under investigation is shown in Table 1.
sion (48). Moreover, the FDG measurement of VI may
Agents from the list that already have Food and Drug
provide an independent index of the risk for subse-
Administration approval for human imaging of
68
quent CV events (48–50), which is the subject of
nonvascular processes include
larger ongoing prospective studies (51) to evaluate the
and
standardization of ideal imaging procedures (52,53)
affinity for somatostatin receptors that are present in
and subsequent clinical application.
high concentration on inflammatory leukocytes (60).
18
F-florbetapen.
68
Ga-DOTA-octreotate
Ga-DOTA-octreotate has high
FDG imaging of VI is increasingly used to evaluate
Its low background activity and high signal make it a
therapeutic compounds targeting atherosclerosis.
promising candidate for plaque imaging, and its
The best therapies studied in this regard are statins,
feasibility in humans has been demonstrated in
18
which show a reduction in the arterial FDG signal
comparison with FDG (60).
consistent with their impact on CV events in ran-
agent that is used clinically to characterize b-amyloid
F-florbetapen is a PET
domized clinical trials (54,55). However, a lack of
plaque in neurodegenerative processes, such as Alz-
reduction in arterial FDG uptake associates with a
heimer’s disease. Bucerius et al. (61) demonstrated
parallel failure to reduce CV events for a number of
that higher carotid artery uptake of
experimental
Prospective
associated with male sex, independent of central
outcome data and response to novel therapies using
nervous system uptake, which potentially implicates
FDG in vascular diseases are limited. However, given
the presence of higher concentrations of inflamma-
treatments
(56–58).
18
F-florbetapen is
the general concordance between VI imaging and
tory b-amyloid within the vessel wall. Further in-
CVD outcomes, relatively small and swift FDG imag-
vestigations of the performance characteristics of
ing studies may facilitate drug discovery.
these new agents are ongoing.
Although
addresses
In summary, with a new understanding of immu-
treatment-related
nologic processes applied to the established feasibility
questions, the specificity for inflammation with this
of promising PET agents for imaging atherosclerosis
agent is not clearly defined, because of the variable
in humans, we now can administer these techniques
affinity for glucose of all cells in the body. Addition-
in newly identified human disease models to better
ally, high FDG activity in the blood pool and tissues
understand
near the vessel wall complicates quantification. Thus,
Acknowledging the great potential for pathology-
certain
FDG
imaging
pathophysiological
adequately
and
atherogenesis
(Central
Illustration).
Teague et al.
JACC VOL. 70, NO. 11, 2017
SEPTEMBER 12, 2017:1403–12
Immunology of Vascular Inflammation
specific and quantitative information that PET imag-
pathways to target for future CVD therapeutics.
ing has for atherosclerosis, an outcome-based multi-
Furthermore, the use of PET imaging to measure VI
disciplinary
investigation,
may improve our ability to identify the most prom-
integrating biology, engineering, imaging physics, and
ising therapies to take to phase III clinical trials. As
economics, will continue to help us understand how
such, advances in immunology and vascular imaging
these techniques ultimately fit into clinical care and
have the potential to accelerate discovery of new
translational research.
treatments to reduce the burden of CVD.
approach
for
further
CONCLUSIONS
ADDRESS FOR CORRESPONDENCE: Dr. Nehal N.
Inflammation has emerged as a critical factor in early
Mehta,
atherosclerosis. An understanding of the immune
National Heart, Lung, and Blood Institute, 10 Center
Cardiovascular
and
Pulmonary
Branch,
cells involved in the initiation and progression of
Drive, CRC, Room 5-5140, Bethesda, Maryland, 20892.
VI may facilitate the identification of important
E-mail: nehal.mehta@nih.gov.
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KEY WORDS cardiovascular imaging,
monocytes, neutrophils, T cells