Endomyocardial Biopsies in the Diagnostic Workup of Immune Checkpoint Inhibitor Associated Myocarditis

Abstract

Purpose of Review

To identify the role of endomyocardial biopsy in the diagnosis, prognostication, and future understanding of mechanisms for immune checkpoint inhibitor myocarditis.

Recent Findings

The use of endomyocardial biopsy in immune checkpoint inhibitor myocarditis has led to recognizing a spectrum of disease that correlates with mild symptoms to more fulminant presentations. The primary infiltrate observed is CD8+ T cells but more studies are suggesting that the presence of CD68+ macrophages may portend worse prognosis.

Summary

Currently, endomyocardial biopsy still has a role in diagnosing immune checkpoint inhibitor myocarditis especially when there is uncertainty from non-invasive diagnostic tools. Emerging evidence suggests endomyocardial biopsy can help with prognosis determination and potentially even guide future cancer therapy. Future research into understanding mechanisms of immune checkpoint inhibitor myocarditis will require endomyocardial biopsy for tissue acquisition.

Introduction

Immune checkpoint inhibitors (ICIs) transformed the treatment of cancer over the past decade as indications expand across multiple malignancies and at earlier stages of disease. Unfortunately, in contrast to the improvements in cancer outcomes, ICIs can cause autoimmune-like diseases affecting any organ system in the body, termed immune related adverse events (irAEs). The cardiovascular irAE of most concern is myocarditis (ICIMy) due to early reports describing mortality rates approaching 50% [1, 2]. However, as awareness, recognition, and treatment of ICIMy have improved, the mortality has decreased with some reports as low as 3% [3–7]. The incidence of ICIMy is only 1% which has hindered effective surveillance and screening for this uncommon irAE [8–11]. Additionally, the early diagnosis of ICIMy is imperative to initiate treatment and for the implications on future ICI therapy [12–15]. The association of ICIMy with major adverse cardiovascular events and mortality has prompted guidelines to recommend an aggressive immunosuppression regimen and the discontinuation of ICIs in patients with clinically significant myocarditis. The diagnosis of ICIMy remains challenging and relies on a combination of clinical syndrome, laboratory values, multimodality imaging (electrocardiogram, echocardiogram, cardiac magnetic resonance imaging (CMR), and at times endomyocardial biopsy (EMB) [16–18]. This review will focus on the role of EMB in the diagnosis of ICIMy discussing the histopathologic criteria, strengths, limitations, technical considerations, and future directions.

Myocarditis Clinical Diagnosis and the Role of EMB

The clinical presentation of ICIMy can be variable. While a subset of patients present with fulminant life-threatening arrhythmias, cardiac-specific symptoms such as palpitations, chest pain or orthopnea, are infrequently encountered. There is a spectrum of clinical presentations from mild non-specific symptoms with troponin elevation to those with fulminant disease presenting with either advanced atrioventricular block, ventricular arrhythmias, and/or cardiogenic shock [19]. Distinguishable from other forms of fulminant myocarditis, ICIMy can present with life-threatening arrhythmias and have high rates of major adverse cardiovascular events even with normal left ventricular ejection fraction [20]. Additionally, ICIMy is often more severe and has higher mortality when presenting concomitantly with other neuromuscular irAEs such as myositis and/or myasthenia gravis [21]. Early reports of ICIMy included only the most severe/fulminant cases, however as recognition of this disease has improved the wider spectrum of disease has become apparent. Mild cases of myocarditis often present with non-specific symptoms such as fatigue and dyspnea [22]. Immune checkpoint inhibitor associated myocarditis (ICIMy) can be diagnosed using established clinical diagnostic criteria. According to the European Society of Cardiology (ESC), clinically suspected myocarditis is the presence of compatible clinical manifestation along with either one of the following diagnostic results, including increased cardiac biomarkers, new abnormal electrocardiographic findings, echocardiographic abnormalities, or tissue characterization changes on the cardiac magnetic resonance image without significant coronary artery disease [23]. In the cardio-oncology arena, the standardized case definitions of myocarditis related to cancer therapeutics were proposed, where definite cases were those that are integrated between the clinical presentation, biomarker elevation, imaging, and histopathologic confirmation when available [24]. The frameworks are compatible with multimodal diagnostic model in which clinical, imaging, and pathological data are combined to enhance accuracy of diagnosis among patients undergoing immune checkpoint inhibitors.

Since the introduction of biomarker surveillance prior to ICI infusions as a form of screening for ICIMy, another frequent mode of presentation is the asymptomatic elevation of cardiac biomarkers. An electrocardiogram (ECG) and an echocardiogram are routinely performed as part of standard ICIMy workup. These tests can be performed in a timely manner, are widely available, and they are associated with a low cost. Although there aren’t any findings in these tests specific for myocarditis, they can provide significant information for the consideration of differential diagnoses. On the other hand, cardiac MRI and EMB are typically reliant on the patient’s hemodynamic stability and the institution’s resources and expertise. However, they are the diagnostic tests that can more reliably provide evidence that a patient has myocarditis. Since cardiac MRI is a non-invasive test and more widely available than EMB it is often the preferred next diagnostic step and proposed algorithms include cardiac MRI before EMB [22]. The diagnosis of ICIMy by cardiac MRI is made by the modified Lake Louise Criteria, requiring the presence myocardial edema (increased native T2 or T2 signal intensity) and non-ischemic injury pattern (increased T1, extracellular volume fraction, or late gadolinium enhancement) [23]. However, studies have shown the limitations of cardiac MRI for the diagnosis of ICIMy with the presence of myocardial edema and non-ischemic injury patterns generally being present in less than 50% of patients with ICIMy [25]. The lack of diagnostic certainty in cardiac MRI has led to use of EMB but the existing potential of a complication can still promote caution against EMB, despite available evidence to support that EMB can be performed safely and with a very low rate of complications by experienced physicians [26, 27]. For these reasons, in most centers EMB is typically performed late in the evaluation of ICIMy after all other diagnostic tests have been performed and there remains diagnostic uncertainty.

The decision to perform an EMB in any patient has two main considerations. Firstly, the anticipated yield of the procedure, including the pretest probability of the suspected diagnosis and risk of sampling error. Secondly, the expected prognostic or diagnostic value the results may provide. The American Heart Association (AHA) provided a scientific statement stating that EMB may be considered in rapidly progressive heart failure patients where the cause can be confirmed by histology only and if effective therapy is available for the given diagnosis [28]. However, the addition of an EMB to the workup of suspected ICIMy should no longer be limited to confirm or rule out the diagnosis. The 2024 ACC Expert Consensus for the Diagnosis and Management of Myocarditis recommended performing an EMB in asymptomatic patients but with clinical suspicion of myocarditis in the setting of ICI therapy [29]. This updated recommendation reflects the potential for this procedure to offer valuable prognostic value.

The diagnosis of myocarditis by histopathologic criteria has long relied on the Dallas Criteria, which require the presence of 2 features on light microscopy: (1) myocyte necrosis or degradation, or both, and (2) inflammatory infiltrate adjacent to degenerating myocytes [30, 31]. Accurate quantification of the inflammatory infiltrate cannot be done on light microscopy alone, and the Dallas panel suggested it should be semi-quantified as mild, moderate, or severe. The European Society of Cardiology (ESC) criteria introduced immunohistochemical criteria in 2013, expanding the Dallas Criteria definition, and defined an abnormal inflammatory infiltrate as ≥ 14 leucocytes/mm² including up to 4 monocytes/mm² and required the presence of CD3 + T cells > 7/mm² [32]. The new 2025 ESC guidelines for management of myocarditis continue to endorse the use of Dallas criteria, defining active lymphocytic myocarditis by the presence of myocyte necrosis in conjunction to an inflammatory infiltrate composed of CD3 + T cells > 7/mm² and CD68 + macrophages [33]. However, they also recognize the need of new quantitative criteria, as there is still a lack of consensus agreement on defining myocarditis by histopathology [33]. The Society for Cardiovascular Pathology (SCVP) and the Association for European Cardiovascular Pathology (AECVP) spent 3 years creating the “Seaport Criteria” for defining myocarditis (Table 1) [34]. These criteria offer definitions for severe, moderate or mild myocarditis and introduce the concept of scattered increased T lymphocytes of unclear significance (SITUS). No myocarditis is defined as the absence of inflammation above baseline myocardial inflammatory cells. They introduce the definition of a lymphocyte cluster, defined as a group of five or more lymphocytes situated closely together, within roughly twice the diameter of a lymphocyte. Also, the criteria define myocyte injury as the presence of single-cell hypereosinophilia, nuclear karyorrhexis/karyolysis, sarcolemmal membrane scalloping, or myocyte dropout. However, they do not require mild and moderate grades of myocarditis to have myocyte injury, which represents an important distinction from the previous Dallas Criteria.

Table 1.

Seaport criteria for myocarditis

Term	Histology grade	Myocyte injury present?	Inflammatory histopathological findings
No myocarditis	–	No	No inflammation above baseline myocardial inflammatory cell presence
SITUS	–	No	6–14 non-clustered CD3 + cells/busiest hpf
Mild myocarditis	1	Yes* or No	A single focal discrete (interstitial) cluster of ≥ 5 lymphocytes (CD3 + or H&E) A diffuse interstitial infiltrate of ≥ 15 CD3 + cells/busiest hpf
Moderate myocarditis	2	Yes* or No	Two or more clusters of lymphocytes
Severe myocardits	3	Yes	Extensive and heavy predominantly lymphocytic infiltrate with myocyte injury

Lymphocytic myocarditis criteria. SITUS scattered increased T lymphocytes of undetermined significance, hpf = 40x objective high-powered field; * = Single-cell hypereosinophilia; nuclear karyorrhexis/karyolysis; sarcolemmal membrane scalloping; myocyte dropout. Adapted from Halushka et al. [34]

Histopathologic and Immunohistochemical Features of ICI-Associated Myocarditis

The normal myocardium contains resident macrophages [35–37], and a smaller population of lymphoid cells, with an average count of 3.6/sq mm lymphocytes and a CD4+/ CD8 + ratio of approximately 1.44 [24]. The type of inflammatory infiltrate observed on immunohistochemistry of the heart samples from patients with ICIMy is similar to that observed in viral myocarditis and acute cellular rejection in heart transplants recipients [38–42]. The predominant infiltrating cells are CD8 + T cells which one study described in a 2:1 ratio with CD4 + T cells [42], although the ratio can vary across patients with the majority of studies describing a greater ratio of CD8 + T cells than CD4 + T cells [38, 43]. A significant histiocytic infiltrate that includes a monocyte/macrophage CD68 + lineage is commonly reported in addition to the T cells in ICIMy, which can occasionally supersede that of the lymphocytic infiltrate [40–42, 44] (Fig. 1). Notably, ICIMy cases show a significantly higher CD68 + to CD3 + ratio compared to acute cellular allograft rejection myocarditis [41]. Another benefit of EMB, is the ability to detect viral infections as possible etiologic agents of myocarditis. For cytomegalovirus direct investigation by light and electron microscopy can reveal intracellular virus inclusion bodies. Another technique for identifying viral genomes is the use of polymerase chain reaction (PCR) on EMB as recommended by the European Cardiovascular Pathology and the Society for Cardiovascular Pathology guidelines [45]. However, the sensitivity of detecting viral genomes by PCR may be limited due to degradation of genomes after samples are collected, therefore it is also recommended to use chemical stabilization reagents, such as RNAlater, or cryofixation to prevent degradation of viral DNA and RNA [45, 46]. Metagenomic next generation sequencing (NGS) is an emerging technology that has shown early promising results for improving viral genome detection from EMB and can even help to distinguish between latent and acute viral infections [47]. It is important to distinguish biopsy findings of ICIMy from other forms of myocarditis such as giant cell myocarditis, eosinophilic myocarditis, and sarcoidosis. Giant cell myocarditis will have widespread multinucleated giant cells, multinucleated cells formed by the fusion of macrophages, and CD4 + T cells that are not typical for ICIMy [48]. Eosinophilic myocarditis is characterized by predominant eosinophils on the EMB typical of an allergic reaction and described with use of other cancer therapeutics rather than checkpoint inhibitors [49]. Cardiac sarcoidosis will have EMB with non-necrotic granulomas and giant cells which are not observed in ICIMy [50]. Other immunohistochemistry markers have shown to be more sensitive markers for inflammation in ICIMy and may be more specific for high grade inflammation such as tenascin C, granzyme B, and HLA-DR [51]. Additional immunohistochemical stains for inflammation assessment have been described for viral myocarditis. For example, the immunohistochemical staining of CD45 + cells has been shown to be more sensitive than CD3 + when detecting myocarditis, albeit in viral myocarditis and not ICIMy [52]. Another study, evaluated use of macrophage-1 antigen (≥ 40 cells/mm²), CD45RO (≥ 21 cells/mm²), and leukocyte function antigen-1 (≥ 12.5 cells/mm²) markers, and found that 26% of patients with negative CD3 + staining who would have otherwise been determined negative for myocarditis were actually positive [53]. Further evaluation of these inflammatory markers for better detection and prognostication of ICIMy are needed. Finally, eosinophils and CD20 + B cells are not typically observed on immunohistochemical stains of patients with ICIMy [41, 42].

Fig. 1.

Representative histopathologic findings from endomyocardial biopsy for immune checkpoint inhibitor myocarditis. (A)- H&E stain showing patchy inflammatory infiltrate in area of myocyte loss consistent with definite myocarditis. (B)- Immunohistochemical staining for PD-L1 showing distribution of PD-L1 uptake around cardiomyocytes near the location of inflammatory infiltrate. (C)- Immunohistochemical staining showing CD3+ T cell infiltration. (D)- Immunohistochemical staining showing minimal CD4+ T cell infiltration.(E)- Immunohistochemical staining showing CD8+ T cell infiltration. (F)- Immunohistochemical staining showing CD68+ monocyte/macrophage lineage infiltration

Attempts have been made to grade the severity of ICIMy based on the endomyocardial biopsy findings. Champion and Stone first proposed a grading system for ICIMy in which EMB slides were categorized as high- or low- grade based on the number of CD3 + cells per high power field, with high-grade ICIMy defined by more than 50 CD3 + cells per high power field [41]. The use of C4d+ immunohistochemical stain to reveal complement-mediated myocyte necrosis demonstrated that all patients with high-grade ICIMy exhibited C4d+ myocyte injury in more than 5 of 10 high-power fields, a finding absent in all low-grade cases [41]. Additionally, high-grade specimens demonstrated a substantially greater number of CD68 + cells per high power field, with nearly a tenfold increase compared to low-grade ICIMy [41]. We also proposed a grading system for ICIMy based on the degree of inflammatory infiltrate and the presence or absence of overt cardiomyocyte damage. Grade 2 is definite ICIMy by Dallas Criteria with dense inflammatory infiltrate (> 40 inflammatory cells per high power field) and overt myocyte damage. Grade 1 is myocardial inflammation in the absence of overt myocyte damage and can be further classified into 1 A or 1B depending on the inflammatory cell count per high power field. For systematic reporting, a scoring system for histology and immunohistochemical findings is also provided (Table 2) [54]. In our cohort, we also showed that patients with Grade 2 ICIMy had greater density of CD68 + monocyte/macrophage lineage infiltrate compared to Grade 1 ICIMy. The term “myocardial inflammation” was used for Grades 1 A and 1B because without myocyte loss, the biopsy findings were not definite for myocarditis by Dallas Criteria. However, according to the new Seaport Criteria these would have been defined as mild or moderate myocarditis. Nevertheless, based on the diagnostic criteria, these patients had a clinical diagnosis of ICIMy with significant troponin elevation, albeit lower than the troponin elevation observed in the patients with Grade 2 ICIMy by EMB [55]. The presence of inflammatory infiltrate without myocyte loss, which we called “myocardial inflammation”, used to be termed “borderline myocarditis” in the literature [30, 31].

Table 2.

Pathology grading and scoring system of myocardial inflammation during immune checkpoint inhibitor therapy

Pathology grade of myocardial inflammation
Diagnosis	Grade	Description
Negative	0	Negative for inflammatory infiltrate.
Myocardial inflammation	1	Multifocal inflammatory infiltrates without overt cardiomyocyte loss by light microscopy
	1 A	Mild inflammatory cell score by immunohistochemistry (10–20 inflammatory cells/high power field)
	1B	At least moderate inflammatory cell score by immunohistochemistry (> 20 inflammatory cells/high power field)
Definite myocarditis	2	Multifocal inflammatory cell infiltrates (> 40 inflammatory cells/high power field) with overt cardiomyocyte loss by light microscopy.
Scoring system
Histopathology	0 = negative, 1 + = mild, 2 + = moderate, 3 + = marked
Immunohistochemistry	0 = 0–10 positive cells, 1 + = 10–20 positive cells, 2 + = 21–40 positive cells, 3+ = >40 positive cells per high power field.

Another important aspect when interpreting EMB H&E slides for the presence of both inflammatory infiltrate and myocyte loss is that myocyte loss needs to be in the same area as inflammatory infiltrate and cannot be explained by ischemia [30, 32–34]. This is important to determine as the population of patients being evaluated for ICIMy are typically elderly patients with many traditional cardiovascular risk factors and at times have the presence of concomitant obstructive coronary artery disease [33, 56, 57]. The European Society of Cardiology (myocarditis guidelines from 2013, prior to the recognition of ICIMy, mentioned obviating the need for EMB if a patient was found to have obstructive coronary disease [32]. However, ICIMy can occur even in individuals with chronic obstructive coronary artery disease, especially in the absence of the classic acute coronary syndrome presentation. When clinical suspicion for ICIMy is high, then EMB should remain an important component of the diagnostic workup consistent with more recent ESC guidelines [33].

Contemporary studies have shown that it is possible to perform advanced immune profiling with gene expression profiling performed on samples from EMB revealing distinct transcriptomic signatures in ICIMy. The intracardiac T cell phenotypes have shown CD8 + T cell clusters expressing markers of cytotoxicity (GZMK, CCL5, GNLY, NKG7, CST7) [58, 59]. Populations of myeloid cells (IL1B + TNF+ myeloid cells and C1Q^hiMRC1 + myeloid cells expressing FCGR3A) were found to contribute to worse clinical outcomes in patients [59]. The use of EMB has also allowed for T cell receptor sequencing analysis revealing clones of T cell in the myocardium that are targeted against α-myosin which is expressed both in the cardiac tissue and the tumor [59, 60]. Another factor that may aid in differentiation of ICIMy from other forms is the use of circulating miRNAs, such as hs-Chr8:96), which when paired with biopsy findings may aid in diagnosis of myocarditis [61, 62].

EMB Strengths and Limitations

The diagnosis of ICIMy is multifaceted and EMB is only one of the many diagnostic tools that can be utilized. As with any diagnostic test, EMB has its own strengths and limitations that should be considered prior to its use. One of the strengths of EMB is that if positive for both inflammatory infiltrate and cardiomyocyte damage the diagnosis is definitive, as suggested by the Bonaca and ICOS/ESC criteria. However, any additional diagnostic information can be reviewed on a case-by-case basis and integrated into the overall adjudication. This could result in the upgrade or downgrade of Bonaca diagnostic category for a given patient. To ensure that this diagnosis and the expected findings are being evaluated, it is important to make sure there is communication between the clinical team and the pathologist interpreting the histopathology slides. More important than even the procedural expertise to perform biopsy, is to have a cardiac pathologist interpreting the slides with expertise in either ICIMy, myocarditis in general, or transplant acute cellular rejection. When reporting histopathology findings, detailed descriptions of the light microscopy findings (inflammatory infiltrate +/- myocyte loss) and immunohistochemical stains with either quantitative or semi-quantitative analysis of the inflammatory infiltrate is optimal for the guiding decision-making by the clinical team. The second strength of EMB is the emerging role of grading severity of ICIMy by histopathology which may in the future have implications for rechallenging patients with ICI. Currently, there are no imaging techniques or laboratory findings that can distinguish those patients with inflammatory infiltrate and with myocyte loss and those without myocyte loss. Lastly, the strength of EMB is that it is a test that can be performed in acutely ill patients or patients with other implanted devices [63]. This contrasts with CMR, the best non-invasive test for cardiac tissue characterization, that requires patients to lie flat for long periods of time with breath holds and has limitations with implanted and/or cardiac support devices. Endomyocardial biopsy also has its limitations. The Dallas criteria have been criticized for their low sensitivity influenced by sampling error, and their susceptibility to interobserver variability and histopathologic interpretation [64–67]. Myocarditis is known to have a patchy distribution across the myocardium, and typically only half of the samples obtained for each patient will have histopathologic ex-vivo proven myocarditis [64, 65]. When it comes to ICIMy, this is not an exception. One study reported that only 2 of 17 (11.7%) patients that underwent EMB for ICIMy workup met Dallas criteria [44]. If a CD68 + inflammatory infiltrate > 50 cells per high-power field without cardiomyocyte injury was utilized to increase diagnostic certainty, an additional 6 (35%) biopsied cases could be upgraded (2/7 possible to probable and 4/7 probable to definite) [44]. Our study demonstrated that only 7 of 24 (29.2%) ICIMy had Grade 2 Definitive ICIMy on EMB, equivalent to the Dallas criteria. Additionally, 13 (54%) had an inflammatory infiltrate without overt cardiomyocyte damage and 4 (16.7%) patients still met diagnostic criteria with a negative EMB [54]. Thus, a limitation of EMB is that the sensitivity for capturing patchy areas of myocarditis is low especially when taking a limited number of samples. Previous studies have shown that this sampling error is reduced when > 4 samples are obtained, and heart failure society guidelines recommend taking at least 5 samples when performing EMB [68]. Despite obtaining multiple EMB samples there is still the possibility of sampling error and missing a diagnosis of ICIMy. Guidelines recommend starting high-dose pulse steroids as soon as myocarditis is considered a likely possibility, and patients can start their immunosuppressive treatment early in the presentation. Although there have not been any studies evaluating if immunosuppression interferes with EMB findings, this potentially represents a further hindrance to the sensitivity of EMB [16–18]. Another downside of EMB is that it is an invasive test that has inherent risk of complications. The most serious and feared complication is cardiac perforation, particularly with right ventricular biopsies. In experienced centers, the risk of this complication is < 1% but is still a risk that needs to be discussed and considered [26, 27]. Along with this risk, at times patients are on anticoagulation or have coagulopathies that are prohibitive to pursuing EMB. Lastly, not all centers, especially community centers, have specialized procedural expertise/capability and the pathology expertise to perform and interpret EMB. Therefore, the use of EMB is generally limited to large academic centers.

Future Directions: EMB Prognostic Value for ICIMy and Guiding Management

The term “borderline myocarditis”, equivalent to Grade 1 myocardial inflammation in our proposed histopathological grading system, has been abandoned because there was no apparent clinical significance or prognostic value for the workup of general myocarditis. However, in Champion’s and our cohort, the clinical significance of having “myocardial inflammation” or “borderline myocarditis” on EMB for patients with ICIMy was that their clinical course was less severe [41, 54]. In Champion’s cohort, eight out of ten patients met full Dallas Criteria, but only three were considered high-grade per their definition. While all the patients with low-grade myocarditis survived, all patients with high-grade myocarditis died [41]. In our cohort, four patients (36%) with myocardial inflammation and seven patients (70%) with negative biopsies never received immunosuppressive therapy. A subset of them even resumed their ICI treatment, and none of them experienced any adverse cardiovascular events [54]. This suggests that patients with myocardial inflammation may represent a lower risk cohort, potentially managed with less intensive immunosuppression and rechallenged safely with ICI. Additionally, evaluating the quantitative assessment of inflammatory infiltrate (20 cells/hpf versus 10–20 cells/hpf in our previous study) and the detection of whether viral genomes are present can impact the prognosis of patients and decision on treatment [54].

Although further research is needed to validate these findings in larger cohorts, grading the severity of EMB findings and integrating with the rest of the clinical picture and results from other ancillary tests can potentially change management While the 2021 American Society of Cancer Oncology (ASCO) guideline on the diagnosis and management of irAEs recommended upfront steroid treatment with a dose of 1–2 mg/kg/day of prednisone [18], the guidelines that have followed increased the recommendation to 500–1000 mg/day of methylprednisolone [17]. This was supported by a multi-center retrospective showing that MACE-free survival was comparatively improved with early initiation (within 24 h of presentation) of high dose (501–1000 mg/day) steroids [15]. With increasing awareness and detection of a broader spectrum of ICIMy disease, this recommendation may no longer be applicable to all suspected cases. Looking to minimize steroid-induced adverse events should also be encouraged in the management of irAEs and finding more targeted immunomodulators based on mechanisms of toxicity will be further enabled by research utilizing EMB samples. More recent guidelines have emphasized the early use of non-steroidal therapies for patients with fulminant disease or refractory disease [17]. The choice of which non-steroidal immunomodulator is still not established. One steroid adjunctive therapy that has been proposed is the use of abatacept and ruxolitinib for the treatment of ICIMy showing substantial mortality benefit in a small cohort (60% to 3%) [69]. Abatacept is a CTLA4 fusion protein that inhibits the upstream T cell activation and ruxolitinib is a JAK kinase inhibitor inhibiting the downstream production of IFN-γ. Identification of the underlying mechanisms and immune microenvironment of the heart is better understood and elucidated by use of EMB for research purposes. Using EMB for advanced immune profiling has led to identification of clonally expanded CD8 + T cells and the upregulation of IFN- γ signaling which helps to further guide and discover novel targets for treatment [58, 59].

Another potential benefit of identifying markers specific to ICIMy on EMB is the use of these markers for targeted imaging. Nuclear imaging offers a noninvasive means to visualize myocardial inflammation, and novel molecular targets are emerging beyond conventional fluorodeoxyglucose (FDG) uptake. Previous studies evaluating FDG positron emission tomography (PET) for the diagnosis of ICIMy have been hampered by feasibility issues due to the need for dietary preparation for myocardial imaging and the lack of specificity of FDG uptake for ICIMy [70, 71]. Therefore, nuclear tracers that do not require dietary preparation and that target specific molecular targets observed in ICIMy are preferable. For example, the expression of CD8 and PD-L1 are widely described on EMB from patients with ICIMy and several radiolabeled antibodies or fragments against PD L1 or CD8 may enable direct visualization of immune checkpoint pathway engagement and cytotoxic T cell infiltration, respectively. Several PD L1 and CD8 targeted nuclear tracers are in development ableit with without testing for the diagnosis of ICIMy [72, 73]. Another promising avenue is through the identification of key pathogenic cells in biopsies of patients with ICIMy such as macrophages that express C‑C chemokine receptor type 2 (CCR2) [74]. Molecular imaging has been performed to visualize CCR2 + macrophages in the myocardium but not specifically for patients with ICIMy [75]. Similarly, C‑X‑C chemokine receptor type 4 (CXCR4) is a G‑protein–coupled receptor expressed on immune cells and is a key mediator in immune cell trafficking and inflammation which has been utilized for imaging post myocardial inflammation but no for ICIMy [76].Fibroblast activation protein (FAP), which is upregulated in activated cardiac fibroblasts during inflammatory injury, has been a target with fibroblast activation protein inhibitor (FAPI) derived tracers (⁶⁸Ga FAPI) and one study showed potential for the detection of ICIMy [77]. By integrating receptors found on EMB findings that are specific to ICIMy into nuclear imaging targets, clinicians may be able to more precisely diagnosis, prognosticate, and guide immunosuppressive therapy without the need for EMB.

Conclusion

The diagnosis and treatment of ICIMy evolved rapidly over the last 15 years. The diagnosis can be challenging especially with limitations in tissue characterization with current imaging modalities. There is a role for EMB in ICIMy diagnosis and emerging literature suggests EMB can provide prognostic implications and may even guide treatment. Future research in identifying underlying mechanisms of ICIMy will require the use of EMB to acquire tissue. Ultimately, the goal will be to have a simple, non-invasive test for ICIMy, but until then EMB will continue to be utilized.

Key References

• Halushka MK, d'Amati G, Bois MC, Fallon JT, Giordano C, Klingel K, et al. Lymphocytic myocarditis: A histopathologic definition and classification from the Society for Cardiovascular Pathology and Association for European Cardiovascular Pathology. I: Endomyocardial biopsy. Cardiovasc Pathol. 2025;78:107759. 10.1016/j.carpath.2025.107759.

  • ○ This manuscript highlights a novel criteria and grading system for defining myocarditis, albeit not specific for immune checkpoint inhibitor myocarditis. It identifies low grades of myocarditis that do not have the traditional requirement from the Dallas Criteria to have myocyte loss/necrosis.
• Champion SN, Stone JR. Immune checkpoint inhibitor associated myocarditis occurs in both high-grade and low-grade forms. Mod Pathol. 2020;33(1):99-108. 10.1038/s41379-019-0363-0.

  • ○ This was one of the first studies to identify high and low grade endomyocardial biopsy pathology findings that correlated with clinical outcomes.
• Palaskas NL, Segura A, Lelenwa L, Siddiqui BA, Subudhi SK, Lopez-Mattei J, et al. Immune checkpoint inhibitor myocarditis: elucidating the spectrum of disease through endomyocardial biopsy. Eur J Heart Fail. 2021;23(10):1725-35. 10.1002/ejhf.2265.

  • ○ This was the first study to identify patients with immune checkpoint inhibitor myocarditis that only had inflammatory infiltrate on biopsy. This questioned whether these patients should truly be classified as myocarditis as a subset of these patients were able to continue immune checkpoint inhibitor therapy without any immunosuppression without any adverse cardiovascular events.

Author Contributions

KOM wrote the initial draft and contributed to editing/revisions. HE wrote the initial draft and contributed to editing/revisions. CMC contributed to editing/revisions. CI contributed to editing/revisions. NLP wrote the initial draft and contributed to editing/revisions. All authors reviewed the manuscript.

Funding

None specifically for this study.

Data Availability

No datasets were generated or analysed during the current study.

Declarations

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.

Competing Interest

The authors declare no competing interests.

Conflict of Interest

Nicolas Palaskas is a consultant for Kiniksa Pharmaceuticals. Keila C. Ostos-Mendoza, Hadi Al Etri, Carlos Manuel Martinez Cerda, and Cezar Iliescu declare that they have no conflict of interest.

Disclosures

Dr. Palaskas is supported by the Cancer Prevention & Research Institute of Texas (CPRIT) RP200670, NIH/NCI 1P01CA261669-01, FDA 1U01FD008717-01, Kiniksa Pharmaceuticals.