Introduction
Myocarditis is defined by WHO as an inflammatory disease affecting the myocardium,
diagnosed by endomyocardial biopsy (EMB) using established histological, immunological,
and immunohistochemical criteria; it may be idiopathic, infectious, or autoimmune
(1–4). A wide variety of histological myocarditis patterns have been described according
to the diverse etiologies and to the stage of the disease at the time of EMB ascertainment.
Viral infections represent the most common cause in Europe and North America (5).
Different viral genomes can be detected in the myocardium of patients with myocarditis
and dilated cardiomyopathy (DCM) using molecular techniques (6–14). When no infectious
agents are identified on EMB and other known causes are excluded, autoimmune myocarditis
(AMy) is the presumed etiology (15). These so-called “autoimmune myocarditis” or “virus-negative
myocarditis” may occur as a distinct disease with exclusive cardiac involvement, or
in the context of systemic autoimmune or inflammatory disorders with extracardiac
involvement (5, 16–23).
It is likely that several etiologic types of myocarditis confluence in a common immune-mediated
pathogenic process leading to chronic inflammation and tissue damage. Irrespective
of triggering agents, acute inflammation may progress to subacute and chronic stages
and eventually result in tissue remodeling, fibrosis, contractile dysfunction, and
finally DCM (4, 6, 8, 24–29). Besides contractile dysfunction, both early myocardial
inflammation and late fibrotic changes play a critical role in the development of
the arrhythmic burden, which makes myocarditis one of the leading causes of sudden
death (19, 27–31). Clearly, treatment interventions effectively curbing the acute
inflammatory process at an early stage can prevent late cardiac remodeling and improve
patient’s outcome. Emerging evidence on the pathogenic mechanisms underlying cardiac
inflammation is paving the way to novel, promising treatment strategies for myocarditis.
Pathogenesis
Both autoimmune and innate inflammatory responses are involved in the pathogenesis
of myocarditis and its sequelae; however, the specific contribution of either mechanism
is not completely understood. As most of immune-mediated diseases, VNM is considered
a multifactorial entity, with several immunologic mechanisms contributing to disease
development and progression. Environmental triggers of myocardium damage lead to local
inflammation and exposure of cryptic self-antigens: excessive innate and myocardium-specific
immune responses ensue in genetically predisposed individuals (16, 31–35) and result
in a self-sustained autoimmune or inflammatory cycle independent of the original triggering
agent.
Traditionally, persistent autoimmune responses have been postulated to underlie the
progression from myocarditis to DCM. Histologically, a lymphocytic infiltrate characterizes
VNM, which is historically considered a CD4+ T cells mediated disease. Accordingly,
transfer of CD4+ T-cells to severe combined immunodeficient mice induced the disease,
while CD4+ T-cells depletion ameliorated experimental AMy (EAM). Additional immune
cell-types characterizing cardiac inflammation in EAM include T-helper (Th)-1 and
Th-17 cells, neutrophils, eosinophils, and monocytes/macrophages, representing a predominantly
adaptive cellularity (32, 34). The role of innate immunity and auto-inflammatory mechanisms
in the pathogenesis of myocarditis has traditionally received much less attention.
Still, activation of the innate inflammatory response is required to prime and kick-off
the adaptive immune response, and persistent inflammation is a critical cause of progressive
tissue damage.
The Auto-Inflammatory Hypothesis
As most tissues, the heart exhibits a stereotyped, highly conserved response to injuries,
characterized by an intense inflammatory reaction. This response, eventually leading
to myocardial inflammation and heart failure (HF), is mediated by the pro-inflammatory
cytokine interleukin (IL)-1, as indicated by both clinical and experimental evidence.
IL-1 is an apical pro-inflammatory cytokine: two distinct ligands (IL-1α and IL-1β)
with high sequence homology exert their biological activity by binding the IL-1 type-I
receptor (IL-1RI), which transduces pro-inflammatory signals leading to the synthesis
and expression of hundreds of secondary inflammatory mediators (35). IL-1α is produced
by most epithelial cells as a fully active pro-inflammatory mediator, which is released
upon cell death or stress in the case of tissue injury, thereby acting as an “alarmin.”
Conversely, IL-1β is mostly produced by monocyte-macrophages as an inactive precursor
and requires cleavage by an intracellular molecular complex termed “inflammasome”
in order to be secreted and exert potent pro-inflammatory effects (35–38). The same
cells that produce IL-1α or IL-1β also synthesize various regulatory molecules to
curb excessive inflammation (35–38), including the IL-1 receptor antagonist (IL-1Ra).
Competitive binding of IL-1Ra to IL-1RI prevents IL-1 signaling and inhibits IL-1-mediated
inflammation (35–37).
In myocarditis, IL-1α is released from the dying myocardium together with debris and
other inflammatory mediators, and these in turn activate the inflammasomes in nearby
cells (35–38). Runaway IL-1-mediated inflammation ensues, progressively causing apoptosis
of cardiomyocytes, loss of contractile tissue, fibrosis, cardiomyopathy, HF, and arrhythmic
outburst (39) (Figure 1). Intracellular aggregates of apoptosis-associated speck-like
protein containing CARD (ASC) and/or caspase-1, indicative of the formation of the
inflammasome, can be observed in leukocytes, cardiomyocytes, fibroblasts, and endothelial-cells
in EMB samples from patients with acute lymphocytic myocarditis or diagnosed as having
myocarditis post-mortem (39). Of note, the number of inflammasome-containing leukocytes
per-field correlated with HF severity: patients presenting with HF or with depressed
left-ventricular ejection fraction (LVEF) at admission, as well as those who did not
experience a significant 6-month LVEF recovery, exhibited a greater number of inflammasomes
and inflammasome-containing leukocytes in heart specimens. These data indicate that
a stereotyped tissue inflammatory response characterized by the formation of the inflammasome
is present and associated with disease severity in acute myocarditis, thus substantiating
the critical role of IL-1β in both myocardial inflammation and systolic impairment.
Figure 1
Pathogenesis of myocarditis: the auto-inflammatory hypothesis. In myocarditis, a detrimental
inflammatory response leads to cardiac damage, clinically manifested with contractile
dysfunction. The pro-inflammatory cytokine interleukin (IL)-1 is of pivotal importance
in the pathogenesis of myocardial inflammation. Inflammation of the heart results
in myocardial injury. IL-1α is released from the dying myocardiocytes, together with
intracellular debris and inflammatory mediators; these in turn activate a molecular
complex known as the “inflammasome,” resulting in processing and release of active
IL-1β by infiltrating inflammatory cells. Runaway cardiac inflammation ensues, leading
to apoptosis of cardiomyocytes and loss of contractile tissue, cardiomyopathy, and
heart failure. Given the dual efficacy against myocardial inflammation and contractile
dysfunction, prompt pharmacologic inhibition of IL-1 can arrest the progression of
uncontrolled inflammation, thus preventing extensive tissue damage and arrhythmic
complications and restoring cardiac function.
The mechanisms linking IL-1 to impaired contractile function include inhibition of
L-type calcium channels, uncoupling of the β-adrenergic receptor (β-AR) from the adenylyl-cyclase
(40–46), and transcriptional and posttranslational changes in phospholamban and sarcoplasmic/endoplasmic
reticulum calcium ATPase (47). IL-1 also increases nitric oxide (NO)-synthase expression
leading to an increased NO activity, which mediates disruption of calcium and β-AR
signaling and mitochondrial dysfunction (38, 48–50).
Experimental observations in animal models also confirm the role of IL-1 in inflammatory
HF: plasma from advanced HF patients induced a significant systolic and diastolic
dysfunction and reduced contractile reserve in mice following a single injection,
suggesting the presence of circulating cardiodepressant factors (51, 52). Notably,
administration of exogenous IL-1β to mice had comparable effects, while pre-blocking
of IL-1 prevented contractile dysfunction induced by HF serum, indicating that cardiodepressant
effects are IL-1-mediated (52, 53). This role of IL-1 as a cardiodepressant factor
is also described in sepsis (54).
Beyond a direct cardiodepressant activity, several lines of evidence point at a central
role of IL-1 signaling in the induction and development of the immune processes associated
with acute myocarditis. IL-1 expression is markedly upregulated in experimental models
of AMy. Coxsackievirus-induced myocarditis in mice is also characterized by heart
infiltration with inflammatory cells secreting IL-1 and TNF-α (55). Persistently elevated
IL-1β expression was noted in the chronic stage of myocarditis in a mouse model of
post-myocarditis DCM induced by the encephalomyocarditis-virus (56). Consistently,
mice deficient for IL-1RI were protected from the development of AM (57), and IL-1Ra
administration or expression had beneficial effects in experimental models of inflammatory
cardiomyopathy: specifically, delivery of human IL-1Ra via plasmid vectors into the
hearts of experimental animals with coxsackieviral or AMy decreased myocardial inflammation
and reduced mortality (58–61). These findings were paralleled by observations in humans,
as increased IL-1β mRNA levels were found in EMBs from patients with viral myocarditis
(57) and idiopathic DCM (58). Taken together, these evidences strongly suggest that
IL-1 inhibition has the potentiality to simultaneously curb the heart inflammatory
response and ameliorate myocardial contractility. Therapeutic agents specifically
blocking IL-1 are available and used to treat a broad variety of autoimmune and inflammatory
conditions (22, 62–65). Anakinra, the recombinant form of the naturally occurring
IL-1Ra, blocks the activity of both IL-1α and IL-1β and is used to treat conditions
characterized by IL-1-mediated inflammation. However, despite convincing evidence
indicating a pivotal role of IL-1 in the pathogenesis of myocardial inflammation and
myocarditis-related systolic dysfunction, the use of IL-1 blocking agents has been
only anecdotally reported in this clinical condition.
Current and Future Therapeutic Perspectives
Symptomatic treatment of HF and of arrhythmias is the mainstay of treatment in myocarditis
(66, 67). So far, conventional immunosuppressive drugs (steroids, cyclosporine, and
azathioprine), rather than targeted treatments, have been used in order to target
the pro-inflammatory mediators of the disease. Single-center randomized trials (RTs)
showed some benefits in chronic VNM/DCM (68, 69), in giant-cell myocarditis (70) and
in Amy (71). Recently, a single center controlled study demonstrated that combination
of azathioprine and steroids can be moderately effective in the treatment of VNM refractory
to standard of care (69). In the TIMIC-trial, patients with HF and biopsy-proven VNM
were randomized to receive either prednisone 1 mg/kg per day for 4 weeks, followed
by 0.33 mg/kg per day for 5-month plus azathioprine 2 mg/kg per day for 6 months,
or placebo in addition to conventional therapy for HF. Primary outcome was the 6-month
improvement in LVEF. Patients receiving prednisone plus azathioprine exhibited an
improvement of LVEF and a decrease in LV volumes compared with baseline, while no
patient in the placebo arm showed any improvement in LVEF. These results have been
subsequently reproduced by other European observational studies (72, 73). However,
a proportion of patients with myocarditis treated with current immunosuppressive strategies
showed a complete lack of response, an observation indicating that some critical mechanisms
of disease pathogenesis and inflammatory tissue damage are not susceptible to conventional
immunosuppression.
Therapeutic IL-1 Blockade
Given the pivotal role of IL-1 in cardiac inflammation, it is expected that treatment
with IL-1 blocking agents would curb inflammation and afford significant clinical
benefits in patients with myocarditis. Clinical experience with specific IL-1 inhibition
provides unambiguous confirmation to the role of IL-1 in human myocardial inflammation
(Table 1). Specifically, in different RTs of ST-elevation myocardial infarction (STEMI)
and HF, treatment with anakinra was associated with improved cardiac contractility
and function. Also of note, in all these trials, anakinra treatment was associated
with marked reductions in serum inflammatory markers. In the recent, massive CANTOS
trial of 10,061 patients with previous myocardial infarction and C-reactive protein
(CRP) > 2 mg/l, selective blockade of IL-1β with the monoclonal antibody canakinumab
conferred protection against recurrent cardiovascular events (74); secondary analyses
of results determined that more robust reductions in CRP after the first dose of canakinumab
predicted greater clinical benefits, again confirming the role of IL-1 as driving
force of cardiac and systemic inflammation (74–80).
Table 1
Clinical experience with interleukin-1 inhibition in cardiac diseases.
Study (reference) (year)
Population (n)
Study design
Results
VCU-ART (75) (2010)
a
STEMI (10)
Double-blinded, randomized vs placebo
Anakinra treatment decreased LVESVi and LVEDVi on CMR and TTE (3 months). Trend toward
decreased CRP levels correlated with the change in LVESVi and LVEDVi
AIR-HF (51) (2012)
HfrEF and elevated CRP levels (7)
Open-label, single-arm, non-randomized design. Anakinra 100 mg daily for 14 days
Change in aerobic capacity (peak VO2) and ventilatory efficiency (VE/VCO2) between
baseline and 14 days
VCUART2 (76) (2013)
a
STEMI (30)
Double-blind, randomized: vs placebo
No significant change in LVESVi, LVEDVi, and LVEF on cardiac MRI and echocardiography
(3 months). Anakinra treatment: blunted the increase in CRP levels
VCUART 3 (2014)
https://ClinicalTrials.gov
Identifier: NCT01950299
STEMI (99)
Double-blind, randomized vs placebo
Results are awaitedPrimary endpoint: CRP levels in 14 daysSecondary: change in LVESVi,
LVEF, and new-onset HF (12 months)
CANTOS (74, 80) (2012)
b
Post-myocardial infarction patients with elevated CRP (10,061)
Double-blind, randomized, multi-center, international, subcutaneous canakinumab 50,
150, or 300 mg every 3 months vs placebo. Median follow-up of 3.8 years
The 150-mg dose met the prespecified multiplicity adjusted threshold for statistical
significance for a composite end point of non-fatal myocardial infarction, non-fatal
stroke, or cardiovascular death. Nearly all of the risk reduction was observed in
non-fatal MI. No significant difference in stroke, cardiovascular mortality, or overall
mortality
Ikonomidis et al. (86) (2008)
Rheumatoid arthritis (23)
Double-blind, randomized cross-over trial. Anakinra 100 mg (single injection) vs placebo,
baseline compared to 3 h after treatment. After 2 days, the alternate treatment was
given
d
Improvement of FMD, CFR, arterial compliance, resistance, longitudinal strain, circumferential
strain, peak twisting, untwisting velocity, LVEF, apoptotic, and oxidative markers
in CAD than in non-CAD patients
Ikonomidis et al. (87) (2014)
Rheumatoid arthritis + CAD (60), rheumatoid arthritis + no CAD (20)
MRC-ILA Heart Study (77) (2014)
c
NSTEMI (182)
Double-blind, randomized vs placebo
Reduction in CRP levels (area-under-the-curve for CRP) over the first 7 days
D-HART (82) (2014)
HFpEF (12)
Double-blind, randomized, placebo-controlled, crossover trial: anakinra 100 mg daily
vs placebo for 14 days and an additional 14 days of the alternate treatment
Improvement in peak oxygen consumption and a significant reduction in plasma CRP levels
D-HART2 (78) (2014)
HFpEF and elevated CRP levels (30)
2:1 double-blind, randomized, placebo-controlled, single center trial: anakinra 100 mg
daily or placebo for 12 weeks
Results are awaited
REDHART (2014)
https://ClinicalTrials.gov
Identifier: NCT01936909
HFrEF with recently decompensated HF (60)
Double-blind, randomized, placebo-controlled: anakinra 100 mg daily for2 or 12 weeks
or placebo for 12 weeks, follow-up of 24 weeks
Study completed. Results have yet to be publishedPrimary outcome measure: aerobic
exercise capacity (peak VO2) and ventilator efficiency at 2 weeks (follow-up = 24 weeks)Secondary:
survival free of hospital admissions
ADHF (79) (2014)
HFrEF, acute decompensation, and elevated CRP levels (30)
Double-blind, randomized, placebo-controlled: anakinra 100 mg twice daily for 3 days
followed by 100 mg once daily for 11 more days
CRP reduction, greater recovery in LVEF. No difference in length of hospital stay
Cavalli et al. (83) (2017)
Fulminant viral myocarditis (1), life-threatening myocarditis in AOSD (1) and T-cell
lymphomas (1)
Case reports
Full recovery from cardiogenic shock, rapidly amelioration of cardiac function allowing
weaning from mechanical circulatory and respiratory support
Cavalli et al. (84) (2016)
Parisi et al. (85) (2017)
ARAMIS (ongoing)
https://ClinicalTrials.gov
Identifier: NCT03018834
Acute myocarditis (120)
Double-blinded, randomized clinical trial Phase IIb of superiorityANAKINRA 100 mg/daily
subcutaneously once a day vs placebo until hospital discharge, for a maximum of 14 days,
in addition to standard care: ACE and Beta-blocker for 6 months
Results are awaitedPrimary outcome measures: no. of days alive free of any myocarditis
complications defined as ventricular arrhythmias, HF, chest pain, ventricular dysfunction
defined as LVEF < 50%, within 28 days post hospitalization
n, number; AOSD, adult-onset Still disease; CAD, coronary artery disease; CFR, coronary
flow reserve; CRP, C-reactive protein serym levels; FMD, flow-mediated reserve; HF,
heart failure; HFpEF, heart failure with preserved ejection fraction; HFrEF, heart
failure with reduced ejection fraction; LVEDVi, left ventricle end-diastolic volume
index; LVEF, left ventricle ejection fraction; LVESVi, left ventricle end-systolic
volume index; CMR, cardiac magnetic resonance; NSTEMI, non-ST-segment elevation myocardial
infarction; ref, reference; STEMI, ST-segment elevation myocardial infarction; TTE,
trans-thoracic echocardiography.
aVCU-ART and VCUART2 combined showed reduction in incidence of new HF with anakinra.
bAn exploratory analysis showed a marked reduction in the incidence of lung cancer,
as well as lung cancer mortality and total cancer mortality. The treatment was associated
with a higher incidence of fatal infection than was placebo.
cHigher incidence of major adverse cardiac events at 12 months with anakinra.
dChronic: non randomized study: anakinra (150 mg, n = 23) vs prednisolone (+5 mg over
regular dose, n = 19) for 30 days.
Besides dampening of inflammation, the beneficial effects of IL-1 blockade on cardiac
function include amelioration of myocardial contractility. Early ex vivo studies with
human atrial heart strips revealed that picomolar concentrations of IL-1 suppress
contractile force (81). It was thus not unexpected that administration of anakinra
to patients with refractory HF could improve exercise tolerance, while also dampening
systemic pro-inflammatory mediators (51). IL-1 blockade with anakinra was also effective
in the management of diastolic HF in a separate double-blind, placebo controlled trial
(82). This direct beneficial effect of IL-1 inhibition on contractile function explains
in part the near-instant clinical improvement observed in life-threatening cases of
fulminant myocarditis, irrespective of the etiology (83–85). Similarly, in previous
studies on patients with rheumatoid arthritis, anakinra promptly increased myocardial
contractility within hours of a single dose administration (86, 87).
Given this dual efficacy on tissue inflammation and contractile dysfunction, anakinra
seems a particularly attractive option for patients with inflammatory HF due to myocarditis.
A double blind, randomized, phase-IIb placebo-controlled clinical trial of anakinra
in patients with acute myocarditis is ongoing (ARAMIS-trial, https://www.clinicaltrials.gov/ct2/show/NCT03018834?term=anakinra&cond=myocarditis&rank=1).
Experience is currently more limited with IL-1 blocking agents in the setting of chronic,
VNM, and no clinical trials are presently evaluating this therapeutic option. However,
VNM and DCM are histologically characterized by various degrees of inflammatory features,
which precede and accompany fibrotic changes and drive both systolic impairment and
arrhythmic complications. Prompt blockade of IL-1 can arrest the progression of uncontrolled
inflammation, thus preventing fibrotic damage, curbing the arrhythmic burden, and
restoring cardiac function. Also in light of a rapid onset of action and an excellent
safety profile, IL-1 inhibition with anakinra represents a particularly suitable treatment
option for conditions characterized by inflammatory HF, such as myocarditis with reduced
LVEF.
Conclusive Remarks
The inflammatory response in myocarditis spirals into a cycle of auto-inflammation,
as intracellular contents released from dying myocardiocytes trigger the activation
of the inflammasome and the uncontrolled release of IL-1 from neighboring cells. Selective
and prompt pharmacologic inhibition of IL-1 dampens runaway inflammation and tissue
damage, thus preventing arrhythmic complications and restoring cardiac function. Dual
effectiveness against myocardial inflammation and contractile dysfunction renders
IL-1 blockade a suitable therapeutic option for myocarditis.
Author Contributions
GDL: conceived the hypothesis, contributed to the understanding of pathogenic mechanisms,
clinical presentation and prognostic meaning of autoimmune myocarditis, and drafted
the manuscript. GC: conceived the hypothesis, contributed to the understanding of
biological effects of IL-1 and to the therapeutic usefulness of IL-1 in a broad spectrum
of rheumatic diseases blockade, and critically revised the manuscript. CC: participated
in previous studies focused on IL-1 blockade and myocarditis and critically revised
the manuscript. MT: conceived the hypothesis and critically revised the manuscript.
LD: conceived the hypothesis, critically revised the manuscript, and gave the approval
of the final version.
Conflict of Interest Statement
The authors declare that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.