When the Immune System Attacks the Heart: New Understanding of T Cells in Acute and Chronic Myocarditis

Table of Contents

• Key Points
• Background: Why This Research Matters
• The Central Idea: A Breakdown in Immune Tolerance
• How Heart-Specific T Cells Are Made — and Normally Controlled
• Molecular Mimicry: When Germ Proteins Look Like Heart Proteins
• Infection-Associated Bystander Activation
• Loss of Immune Regulation: The "Brakes Off" Mechanism
• How Myocarditis Presents and How It Progresses
• Diagnostic Workup: How Doctors Identify Myocarditis
• Echocardiography and Imaging
• Clinical Implications: What This Means for Patients
• Limitations and Knowledge Gaps
• Recommendations and Future Directions
• Frequently Asked Questions
• Source Information

Key Points

• Myocarditis is often driven by heart-specific T cells that escape normal control, causing inflammation and damage to the heart muscle.
• It affects an estimated 10.2 to 105.6 people per 100,000, or about 1.8 million new cases worldwide each year, though many cases go undiagnosed.
• Common symptoms include chest pain (82%–95%), fever (58%–65%), shortness of breath (19%–49%), and fainting (5%–7%).
• An LVEF below 50% is linked to a four-fold increased risk of serious cardiovascular events, including in-hospital death.
• Immune checkpoint inhibitor myocarditis is rare but can have a mortality rate up to 50%; early recognition and monitoring are crucial.

Background: Why This Research Matters

Myocardial inflammatory disease (inflammation of the heart muscle) happens when immune cells move into the heart. This invasion causes a wide range of symptoms and disease patterns, which doctors group together under the term "inflammatory myopericardial syndrome."

Myocarditis — the clinical result of heart muscle inflammation — can be mild or fulminant (sudden and severe) in the acute phase. In the chronic phase, it can stay stable, come back repeatedly, or steadily worsen. Both acute myocarditis and its long-term consequences, such as chronic myocarditis and inflammatory cardiomyopathy (heart muscle damage with persistent inflammation and pump dysfunction), are classic examples of what happens when immune cell activity in the heart goes too far.

New forms of myocarditis have appeared over the past decade. This trend has pushed T lymphocytes (a type of white blood cell central to targeted immune responses) into the spotlight. The most informative example is myocarditis caused by novel cancer therapies called immune checkpoint inhibitors (ICIs) — drugs that unleash the immune system against tumors.

The pattern of disease in ICI myocarditis teaches two lessons:

• Complex, multi-layered tolerance mechanisms normally stop autoimmune damage to the heart.
• When those mechanisms fail, immune cell activation and heart damage can happen quickly and extensively.

ICIs work by weakening the immune mechanisms that restrain T cell activity. Based on this, experimental models of ICI myocarditis and data from real patients both suggest that heart-specific T cells are the main drivers of tissue destruction in the heart muscle. More broadly, excessive activation of heart-specific T cells — triggered by bacterial structures that resemble heart proteins, or by the global immune activation seen during viral infection or vaccination — can boost and speed up damage to heart tissue.

This review integrates older virus- and pathogen-centered models into a single overarching framework built on T cell tolerance and activation. The authors connect current clinical practice with mechanistic insights and identify the knowledge gaps that must be filled to improve diagnosis and treatment.

The Central Idea: A Breakdown in Immune Tolerance

The immune system exists to tell the difference between non-infectious "self" and infectious "non-self." The innate immune system — the fast, first-line defense — uses germline-encoded receptors (built-in, hardwired detectors) to spot conserved patterns and structures of pathogens, such as bacterial lipopolysaccharides (molecules from bacterial cell walls).

The adaptive immune system works differently. It uses genetically modifiable receptors: antibodies made by B cells, and T cell receptors (TCRs) expressed by T cells. This lets it recognize a huge variety of antigenic structures (foreign protein fragments).

The enormous diversity of the TCR repertoire lets T cells recognize a large fraction of pathogen-derived antigens displayed by the major histocompatibility complex (MHC) — the molecular "display stand" that presents protein fragments to T cells. But this broad reactivity carries a risk. Self-antigens (the body's own proteins) and harmless antigens from commensal bacteria (the normal, beneficial microbes living on and in us) can also be recognized by T cells, which can cause autoimmune inflammatory disease.

Emerging data suggest that abnormal T cell regulation plays a critical role in myocarditis. Specifically, the loss of immune tolerance to cardiac antigens — the heart's own proteins — appears to be a defining event.

How Heart-Specific T Cells Are Made — and Normally Controlled

T cell progenitors are produced in the bone marrow and then educated in the thymus (a gland behind the breastbone). After this training, they migrate as naive T cells (inexperienced cells that have not yet met their target) into lymph nodes. There they are clonally expanded — copied into many identical cells — and programmed to become effector T cells that travel into tissues.

A key safety step happens in the thymus. Self-reactive T cell clones that recognize the body's own tissue antigens are deleted. Several cardiac antigens, such as myosin heavy chain 7 (MYH7) and cardiac troponin T (TNNT2), are expressed by medullary thymic epithelial cells in the thymus. Because these heart proteins are shown to developing T cells, reactive clones are eliminated through apoptosis (programmed cell death).

But not every heart protein is displayed in the thymus. Cardiac proteins such as MYH6 are not expressed there. This allows MYH6-specific T cells to escape the thymus and migrate to the lymph nodes that drain the heart.

Activating naive T cells and expanding them in lymph nodes requires two signals:

1. Recognition of peptide–MHC complexes on dendritic cells (immune cells that present antigens).
2. Costimulatory signals, such as CD28 on the T cell binding to CD80/CD86 molecules on the dendritic cell.

This second signal matters clinically. Mice that lack CD28 develop less experimental autoimmune myocarditis. When the same dendritic cell presents antigens on both MHC class I and class II molecules, it strengthens cross-presentation and activates CD8+ T cells, with help from interleukin-2 (IL-2) supplied by CD4+ T cells.

Once activated and expanded, heart-specific T cells migrate toward the heart. There, two distinct types of damage occur:

• CD4+ T effector cells release interferon-gamma (IFN-γ) after recognizing cardiac antigen presented by cardiac dendritic cells. This activates inflammatory macrophages (scavenger immune cells), which then cause tissue damage through tumour necrosis factor (TNF).
• CD8+ T cells damage cardiomyocytes (heart muscle cells) directly by releasing cytotoxic molecules such as granzyme B.

In short, the development and differentiation of heart-specific T cells are tightly controlled by many cellular and molecular layers. Damage to the heart muscle results either from abnormal stimulation of these cells or from the loss of specific regulatory circuits.

Molecular Mimicry: When Germ Proteins Look Like Heart Proteins

The number of microbial antigenic peptides (protein fragments from microbes) far exceeds the number of recombined TCRs. As a result, a single TCR can recognize many structurally related peptides. This principle is called molecular mimicry.

Researchers have proposed that myocardial inflammatory disease results from cross-reactivity with pathogen-derived antigen. However, how much TCR cross-reactivity against disease-causing viruses actually contributes to autoimmune disease remains unresolved.

The gut microbiome adds another layer. Commensal microbes carry peptides that mimic self-antigens. One documented example involves β-galactosidase peptides from Bacteroides species, which MYH6-specific T cells recognize. When such mimic peptides activate cross-reactive MYH6-specific CD4+ T cells, inflammatory macrophages in the heart become activated and cardiac damage worsens.

The authors call for deeper analysis to fully characterize how cross-reactive heart-specific TCRs behave and what impact they have on acute myocarditis and other inflammatory heart conditions.

Infection-Associated Bystander Activation

Many patients with acute myocarditis report a recent viral infection and infection-related symptoms. Yet endomyocardial biopsies (tissue samples taken from inside the heart) in both children and adults with myocarditis are frequently virus-negative.

Several questions remain open. It is unclear how much ubiquitous viruses found in the heart muscle of healthy people — such as Parvovirus B19 — contribute to inflammatory heart disease. The role of lymphotropic viruses (viruses that infect immune cells), such as human herpesvirus 4 and 6, in myocarditis also remains to be clarified.

Current consensus holds that the systemic spread of a virus, rather than its replication inside heart tissue, is the key trigger. In this model, systemic activation of pathogen recognition receptors — such as TLR7 (toll-like receptor 7), which detects viral RNA — switches on the innate immune system and promotes heart muscle inflammation.

Infection-triggered TLR7 stimulation of dendritic cells increases production of type I interferons (IFN-α/β) and IL-12. These signals boost the dendritic cells' ability to present antigen, including cross-presentation to CD8+ T cells. Viral infection with systemic release of inflammatory mediators is commonly called a "cytokine storm," as seen in SARS-CoV-2 and influenza infection.

Vaccination can trigger similar pathways. The ability of mRNA vaccines to induce CD8+ T cell responses in the spleen depends on type I interferon and macrophage activation. It is therefore conceivable that SARS-CoV-2 vaccination-associated myocarditis is driven by the same processes that activate heart-specific T cells during infection.

However, the incidence of vaccine-associated myocarditis is low and the course is usually mild compared with SARS-CoV-2 infection. This suggests that the relatively low level of systemic bystander activation during vaccination produces only weak activation of heart-specific T cells, unlike the strong cytokine storm of severe viral infection. The authors state clearly that the protective benefit of SARS-CoV-2 vaccination greatly outweighs the relatively small risk of developing usually mild myocarditis as a side effect.

Loss of Immune Regulation: The "Brakes Off" Mechanism

T cell activation normally triggers built-in regulatory mechanisms that prevent excessive proliferation and tissue damage. This dynamic tuning happens both in lymph nodes and during the effector phase in tissues. It involves increased expression of co-inhibitory molecules such as CTLA4 (cytotoxic T lymphocyte-associated protein 4) and control of T cell proliferation through the transcription factor Foxp3.

CD8+ T cell activity is controlled at multiple levels:

• Regulatory T cells supply immune-attenuating cytokines such as IL-10 and TGF-β (transforming growth factor beta).
• Effector T cells in the target tissue increase expression of PD-1 (programmed cell death protein 1), which promotes their exhaustion and protects the tissue from immune damage.
• Inflamed tissues react by increasing expression of PD-1 ligands (PD-L1), which dampens potentially harmful CD8+ T cell activity.

When these brakes fail, the consequences are severe. Mice that lack the co-inhibitory molecules CTLA4 and PD-1 develop multi-organ inflammation, including myocarditis.

Cancer patients treated with antibodies directed against PD-1, PD-L1, and/or CTLA4 develop a broad range of autoimmune side effects — including myocarditis. ICI myocarditis is rare but potentially severe, with a mortality rate of up to 50%. In these patients, self-reactive T cells target MYH6.

MYH6-specific T cells are also present in the circulation of healthy individuals. This finding shows that the immune system must use elaborate regulatory mechanisms to keep heart-specific T cells in check. Autoreactive CD4+ and CD8+ T cells that recognize MYH6 and other, still-unknown self-antigens are probably the cause of the rapid onset of myocarditis after ICI treatment. Yet the risk factors and immune mechanisms that predispose some cancer patients to ICI myocarditis while sparing others remain largely unknown.

Pinpointing these basic mechanisms is therefore necessary to allow patient stratification (identifying who is at highest risk) and to optimize anti-inflammatory treatment for both acute and chronic myocarditis.

How Myocarditis Presents and How It Progresses

Recognizing T cell-driven heart muscle inflammation from different triggers is the key to mechanism-based clinical phenotyping (grouping patients by the biology causing their disease). Acute myocarditis — whether it follows an infection or the start of ICI treatment — typically appears within a few weeks.

Mild cases resolve completely within 30 days. However, around 3%–9% of patients develop acute fulminant myocarditis (FM), marked by rapid onset and progression to severe complications such as cardiogenic shock (the heart cannot pump enough blood) or life-threatening arrhythmias (dangerous heart rhythm disturbances). Mortality in FM is high, especially in ICI myocarditis, where it reaches 50%.

After the initial acute episode, outcomes diverge into three paths:

• Resolution: complete recovery, including symptom relief, normalization of biomarkers, and resolution of structural abnormalities.
• Recurrent myocarditis: more than one acute episode requiring hospitalization.
• Chronic myocarditis: persistent heart inflammation lasting more than three months (the review's figure describes it as inflammation persisting beyond 30 days from initial symptom onset).

Inflammatory cardiomyopathy is better defined — chronic inflammation combined with persistent cardiac dysfunction and ventricular remodelling (changes in heart chamber shape and size, with either dilated or non-dilated morphology). Chronic myocarditis itself remains ill-defined. It is still uncertain whether chronic myocarditis is an intermediate stage between acute myocarditis and inflammatory cardiomyopathy, or a distinct disease entity marked by ongoing cardiac inflammation without cardiac dysfunction.

Notably, patients with chronic disease show responsiveness to cardiac myosin antigens. This suggests that persistent heart inflammation can be driven by autoimmune T cell-mediated processes.

How common is myocarditis?

Estimating true prevalence is hard because many cases go undiagnosed. Globally, estimates range from about 10.2 to 105.6 cases per 100,000 people. That translates to roughly 1.8 million new cases each year.

Common symptoms and who is affected

Clinical manifestations vary substantially, but common patterns are distinguishable:

• Chest pain — the most prevalent symptom, reported in approximately 82%–95% of cases (about 8 to 9 in every 10 patients).
• Fever — occurring in 58%–65% of patients (roughly 6 in 10).
• Dyspnoea (shortness of breath) — 19%–49% of patients.
• Syncope (fainting) — 5%–7% of patients (about 1 in 20 to 1 in 14).

There are sex differences in the median age of diagnosis. In men, the median age ranges from 30 to 45 years. In women, it spans 42 to 45 years. Acute myocarditis is more prevalent in males. Sex differences in ICI myocarditis are less pronounced in younger patients but may be a relevant risk factor in the elderly.

One finding carries major clinical weight: patients who present with a left ventricular ejection fraction (LVEF — the percentage of blood pumped out of the heart's main chamber with each beat) below 50% face a four-fold increased risk of adverse cardiovascular events. These include acute decompensated heart failure, ventricular arrhythmias, and in-hospital death.

Additional risk enhancers in ICI myocarditis include the presence of an active thymoma (a tumor of the thymus gland), concomitant myositis (muscle inflammation occurring at the same time), and a reduced LVEF below 50%. All three are associated with an increased risk of 30-day mortality.

In COVID-19 vaccine-related myocarditis, males aged 12–30 years have the highest risk. Symptoms typically begin 1–14 days after vaccination, following the second dose of the primary mRNA vaccine series.

This diverse presentation underscores why doctors need a high index of suspicion for myocarditis. They should consider it in patients with these symptoms and no alternative explanation — particularly when there has been recent exposure to potential triggers such as infection or immunotherapy, or when systemic signs suggest systemic immune activation. Early recognition, appropriate diagnostic evaluation, and mechanism-based classification are crucial for the best possible outcomes.

Diagnostic Workup: How Doctors Identify Myocarditis

Doctors should consider myocarditis in patients who present with compatible symptoms and history, along with distinct findings on first-line assessment. These findings include laboratory evidence of myocardial injury and inflammation, or an abnormal ECG (electrocardiogram, a recording of the heart's electrical activity).

Because symptoms overlap with other conditions, obstructive coronary artery disease (blocked heart arteries) must be excluded promptly. Novel biomarkers could eventually help with this. However, the markers used in current first-line assessment lack specificity — meaning abnormal results can have many causes, and normal values do not rule out myocarditis.

Blood test performance in the review:

• High-sensitivity troponin I or T (proteins released when heart muscle is injured) — elevated in approximately 64%–100% of myocarditis cases.
• CRP (C-reactive protein, a general marker of inflammation) — elevated in 54%–99% of cases.

In ICI myocarditis specifically, elevated troponin T levels correlate with worse outcomes and provide greater sensitivity for monitoring patients. In addition, the neutrophil-lymphocyte ratio (NLR — a comparison of two types of white blood cells) may serve as a potential biomarker, with a high NLR indicating a worse prognosis.

Viral serology (blood tests for antibodies against viruses) may indicate a recent viral illness. But it lacks sensitivity and specificity, and recent guidelines do not recommend it for diagnosing myocarditis.

Echocardiography and Imaging

Echocardiography (ultrasound imaging of the heart) is essential for evaluating risk profiles in the first-line diagnostic workup. In chronic myocardial inflammation, the sensitivity of cardiac magnetic resonance imaging (CMR), currently considered the gold standard for diagnosing myocarditis in stable patients, is 62%, which the review considers insufficient for diagnostic purposes. Diagnostic accuracy varies based on the first-line diagnostic approach used and the specific imaging technique applied.

The review points forward to newer tools — deep phenotyping, transcriptomic immunoprofiling (analyzing patterns of gene activity in immune cells), and imaging and biomarker strategies — as the next frontier for identifying which patients have T cell-driven disease.

Clinical Implications: What This Means for Patients

This framework changes how doctors think about myocarditis. Instead of treating it as a single disease with a single cause, it becomes a family of conditions united by one shared problem: heart-specific T cells that have escaped normal control.

The practical consequences are significant:

• Cancer patients on checkpoint inhibitors need vigilance. Because ICI myocarditis can strike rapidly and carries mortality up to 50%, early recognition matters enormously. Elevated troponin T and a high NLR can help identify patients at greater risk.
• Symptoms should never be dismissed. Chest pain appears in 82%–95% of cases. Fever, shortness of breath, and fainting are also common. Together with a recent infection or immunotherapy, these symptoms should prompt evaluation.
• LVEF matters as a risk marker. An LVEF below 50% quadruples the risk of serious cardiovascular events, including in-hospital death.
• Vaccination remains strongly protective. The risk of vaccine-associated myocarditis is low, usually mild, and concentrated in males aged 12–30 within 1–14 days of the second mRNA dose. The authors state the protective benefit clearly outweighs this small risk.

The review also reframes treatment strategy. Instead of broad immunosuppression, the field is moving toward "T cell-centric" targeted therapy. The authors highlight approaches including checkpoint modulation (adjusting the immune brakes rather than removing them entirely), adoptive transfer (using selected immune cells as treatment), and antigen-specific tolerance (teaching the immune system to tolerate specific heart proteins).

Limitations and Knowledge Gaps

The review is explicit about what remains unknown. These gaps limit how precisely doctors can diagnose and treat patients today.

• Cross-reactivity is not fully characterized. Scientists do not yet know how much TCR cross-reactivity against pathogenic viruses contributes to autoimmune heart disease.
• Biopsies are often virus-negative. This leaves unclear whether viruses cause damage directly in the heart or mostly act from a distance through systemic immune activation.
• Common viruses complicate the picture. Viruses such as Parvovirus B19 are found in the hearts of healthy people, so their role in disease remains uncertain. The role of human herpesvirus 4 and 6 is also unresolved.
• Who is at risk for ICI myocarditis is unknown. The risk factors and immune mechanisms that predispose some cancer patients — and spare others — remain largely elusive.
• Chronic myocarditis is ill-defined. It is still unclear whether it is an intermediate stage or a distinct disease entity.
• Prevalence numbers are estimates. Because many cases go undiagnosed, the true burden of myocarditis is difficult to measure.
• First-line markers lack specificity. Normal troponin or CRP values do not exclude myocarditis, and echocardiography reaches only 62% sensitivity in chronic inflammation.

Recommendations and Future Directions

The authors outline a clear research and clinical agenda built on the loss of T cell tolerance as the central concept guiding both diagnosis and therapy.

1. Characterize heart-specific TCR cross-reactivity. Deeper analysis is needed to understand how mimic peptides from microbes and the microbiome activate heart-reactive T cells.
2. Build patient stratification tools. Understanding the basic mechanisms of T cell-mediated myocardial inflammation would let doctors identify who needs which anti-inflammatory treatment.
3. Develop mechanism-based classification. Grouping patients by the biological cause of their disease — infection-triggered, ICI-triggered, or autoimmune — should guide treatment choices.
4. Advance T cell-centric therapies. Priorities include checkpoint modulation, adoptive transfer, and antigen-specific tolerance approaches that target the harmful T cells without broadly suppressing immunity.
5. Improve diagnostic precision. Deep phenotyping, transcriptomic immunoprofiling, advanced imaging, and better biomarkers are needed to replace nonspecific first-line tests.
6. Maintain a high index of suspicion. Clinicians should test for myocarditis in patients with compatible symptoms and recent infection, immunotherapy exposure, or signs of systemic immune activation — and should exclude coronary artery disease promptly.

For patients, the practical takeaways are straightforward. Take chest pain, unexplained shortness of breath, fainting, and fever seriously — especially if you have recently had a viral infection or started cancer immunotherapy. Ask your care team about troponin testing if you develop these symptoms on checkpoint inhibitors. And keep vaccination decisions in perspective: the authors state that the protection offered by SARS-CoV-2 vaccination greatly outweighs the small risk of usually mild myocarditis.

Frequently Asked Questions

What causes myocarditis?

Myocarditis is inflammation of the heart muscle. According to a 2026 review, it is often driven by heart-specific T cells—immune cells that normally ignore the heart but can turn against it. Two main forces trigger this: 'brakes off,' when cancer immunotherapy removes natural restraints, and 'accelerator on,' when infections or other immune triggers push these cells into action.

What are the symptoms of myocarditis?

Common symptoms include chest pain, reported in about 82%–95% of cases; fever in 58%–65%; shortness of breath in 19%–49%; and fainting in 5%–7%. Symptoms can vary, so doctors should consider myocarditis in patients with these symptoms and no other explanation, especially after a recent infection or immunotherapy.

How is myocarditis diagnosed?

Doctors use symptoms, history, blood tests, ECG, and imaging. High-sensitivity troponin is elevated in about 64%–100% of cases, and CRP in 54%–99%. Echocardiography is essential for first-line risk assessment, but its sensitivity is only 62% in chronic inflammation. Because symptoms overlap with other conditions, blocked heart arteries must be excluded promptly.

What does a low LVEF mean in myocarditis?

LVEF (left ventricular ejection fraction) is the percentage of blood pumped out of the heart's main chamber with each beat. In myocarditis, an LVEF below 50% is linked to a four-fold increased risk of adverse cardiovascular events, including acute decompensated heart failure, ventricular arrhythmias, and in-hospital death. It is an important risk marker.

What is the risk of myocarditis from cancer immunotherapy?

Immune checkpoint inhibitor (ICI) myocarditis is rare but potentially severe, with a mortality rate of up to 50%. It can strike rapidly. Risk factors include an active thymoma, concomitant myositis, and an LVEF below 50%. Elevated troponin T and a high neutrophil-lymphocyte ratio can help identify patients at greater risk. Early recognition is critical.

Is myocarditis from COVID-19 vaccination a concern?

The risk of vaccine-associated myocarditis is low and the course is usually mild. It is concentrated in males aged 12–30, typically 1–14 days after the second mRNA dose. The authors state that the protective benefit of SARS-CoV-2 vaccination greatly outweighs this small risk of usually mild myocarditis.

When should a patient with suspected or newly diagnosed T cell-driven myocarditis seek a second opinion?

Seek a second opinion when the diagnosis is uncertain or the cause is unclear. Symptoms overlap with other conditions, and first-line markers lack specificity: normal troponin or CRP values do not exclude myocarditis, and echocardiography reaches only 62% sensitivity in chronic inflammation. A second review can help distinguish infection-triggered, immune checkpoint inhibitor-triggered, or autoimmune disease, since this classification guides treatment choices. It also matters when a patient on checkpoint inhibitors develops chest pain, shortness of breath, fainting, or fever, or when LVEF is below 50%, which quadruples the risk of serious cardiovascular events. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: T cells in acute and chronic myocarditis: from diagnosis to treatment.

Authors: Song EJ, Joachimbauer A, Tasca S, Baylis R, Schmidt D, Ludewig B, Moslehi JJ.

Author affiliations: Section of Cardio-Oncology & Immunology, Cardiovascular Research Institute, University of California San Francisco; Division of Cardiology, Department of Medicine, University of California San Francisco; Institute of Immunobiology, Cantonal Hospital St. Gallen, Switzerland; Department of Cardiology, University Heart Center, University Hospital Zurich, Switzerland; and the Center for Translational and Experimental Cardiology, University Hospital Zurich, Switzerland

Publication details: State of the Art Review, European Heart Journal (2026), volume 47, pages 2255–2270. DOI: 10.1093/eurheartj/ehaf1080. Received 5 June 2025; revised 20 August 2025; accepted 17 December 2025; published online ahead of print 29 January 2026. Published by Oxford University Press on behalf of the European Society of Cardiology as an Open Access article under a Creative Commons Attribution License.

Keywords: Myocarditis; T lymphocytes; Adaptive immune response; Immunosuppression; Cardio-oncology

This patient-friendly article is based on peer-reviewed research.