Immune checkpoint inhibitors (ICI) have emerged as promising treatment options for many cancers. ICIs exert their therapeutic effects by targeting immune inhibitory molecules on T-cells in adaptive immunity, such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell death 1 (PD-1) and its ligand programmed cell death ligand 1 (PD-L1). ICIs, however, can result in a wide variety of immune-related adverse events (iRAE), including myocarditis, a rare and potentially deadly complication that necessitates early diagnosis [1]. Data from VigiBase, the World Health Organization’s (WHO) global database of individual drug case safety reports, indicates myocarditis occurred 11 times more in those who received ICI than those who did not receive an ICI [2]. The reported incidence of ICI-associated myocarditis (ICI-M) varies between 0.04% to 1.14% [3]. ICI-M can occur among all age groups and usually results in an ominous outcome with a fatality rate of 50% [4], with higher mortality observed in combination of anti-CTLA-4 and anti-PD-1/PD-L1 therapy compared to monotherapy [2].

Currently, several possible mechanisms have been suggested to explain ICI-M. ICI-M can be due to the activation of self-reactive T-cells that target myocardial antigens and subsequently mediate cardiac tissue damage. T-cell mediated myocardial tissue damage can also result from antigen mimicry between tumor and cardiac antigens that in turn provokes T-cells reaction to native cardiac antigen(s) [1]. Furthermore, animal studies suggest a potential role for PD-1/PD-L1 and CTLA-4 in myocardial tissue. PD-1 deficiency in MRL mice leads to CD4⁺ and CD8⁺ lymphocyte infiltration in the myocardium and a high titer of autoantibodies against alpha myosin [5]. C57BL6/J mice with Pdcd1–/–Ctla4+/– genotype usually die prematurely due to myocarditis with predominantly CD8⁺ T-cell myocardial infiltration, similar to ICI-M patients [6]. Interestingly, PD-1 deficient mice on BALB/c genetic background, develop dilated cardiomyopathy and a high titer of antibodies against troponin I, rather than lymphocyte infiltration and myocarditis [7]. CTLA-4 deficient mice develop severe lymphoproliferative disorder with lymphocyte infiltration in almost all organs, including myocardial tissue, with a fatal outcome [8]. A better understanding of the underlying mechanisms causing ICI-M may lead  to the development of novel diagnostic assays and potential therapeutic targets.

Myocarditis is an inflammatory myocardial disease due to immune-mediated responses. A handful of conditions such as viral infections, immune diseases, pregnancy, some vaccines and medications have been shown to be associated with myocarditis [9]. Although the diagnostic strategies for ICI-M can parallel those for other myocarditis, the added challenge in ICI-M is that the patient population treated with ICIs has cancer and requires ICIs for improved morbidity and mortality. Discontinuation of these medications will greatly limit their options for life-saving cancer therapy. Nevertheless, accurate and timely diagnosis of ICI-M will result in the appropriate discontinuation of therapy and removal of the inciting agent that causes disease.

Traditionally, an endomyocardial biopsy demonstrating lymphocytic infiltration of myocardium has served as the gold standard for ICI-M diagnosis; however, this procedure is invasive and subject to sampling bias. Several complementary approaches have been introduced to better diagnose ICI-M. The electrocardiogram (ECG) is a useful, albeit non-specific tool to screen for cases of suspected ICI-M. Some ECG features that should prompt further evaluation for possible ICI-M, usually include the PR interval prolongation, atrioventricular blocks, ventricular arrhythmias, and ST-T segment changes [3]. Although serum cardiac troponin I, B-natriuretic peptide (BNP), and CK-MB have been suggested as laboratory markers for ICI-M diagnosis, any interpretation of these markers in isolation should be performed with caution. BNP, for example, can be elevated directly due to inflammatory pathways event with normal filling pressures in the absence of ventricular stress. CK-MB also can rise in the setting of myositis, which is another adverse effect of ICI therapy.  Although cardiac troponin I is the most specific marker for myocardial injury related to ICI-myocarditis and has significant prognostic value [10], it can also be elevated as a result of other myocardial insults (i.e., myocardial infarction, heart failure). Because of the nonspecific nature of the above ECG and laboratory findings, the addition of cardiac magnetic resonance (CMR), which is used to diagnose other types of myocarditis, is required for more definitive non-invasive diagnosis of ICI-M. CMR can identify the presence of inflammation, edema, and fibrosis using T1 and T2 mapping and late gadolinium enhancement as well as reductions in systolic function [3,11].

More recently, advances in immune cells profiling using time of flight mass cytometry (CyTOF), T-cell receptor (TCR) sequencing (TCR-seq), and single cell RNA sequencing (scRNA-seq) have improved our knowledge of the pathogenesis of iRAE and more specifically ICI-M, paving the way for the development of more specific diagnostic biomarkers. In a study by Lozano et al. single cell profiling was performed on peripheral blood mononuclear cells (PBMC) in 18 patients, 8 of whom experienced severe iRAE. By applying CyTOF, scRNA-seq, and TCR-seq, they showed that CD4⁺ effector memory cells subpopulation and higher TCR diversity in CD4⁺ effector memory cells were associated with severe iRAE [12]. However, they did not specify the type of iRAE observed in patients and it is unclear whether their findings can be applied to ICI-M. Another study by Zhu et al. compared PBMCs of 23 healthy patients not on ICI, 8 patients on ICI treatment without any iRAE, 13 patients with iRAE other than ICI-M, and 8 patients with ICI-M. They observed a relative decrease in circulating T-cells/ B-cells and an increase in monocytes/macrophages in ICI-treated patients compared with non-ICI patients. They also found no difference in CD4⁺ and CD8⁺ T-cell populations between non-ICI patients and ICI-treated patients with or without any kind of iRAE. However, when comparing CD8⁺ subpopulations, they found that the Temra CD8⁺ subpopulation was increased in ICI-treated patients [13]. Patients with ICI-M also had higher Temra CD8⁺ cells compared to other ICI-treated patients with or without iRAE [13]. Given that these initial studies included a relative small number of patients and identified two different subpopulations of lymphocytes that may play pathogenic roles, additional studies that profile the immune function of larger cohorts of patients is needed. Moreover, although blood is convenient surrogate, studies that confirm that the changes in blood mirror changes in the affected tissue are also required.  

In summary, the development of ICI-M remains a major barrier in the continuation of chemotherapy for patients with cancer. Early and accurate diagnosis of ICI-M remains a critical issue for oncologists and cardiologists.  For now, it is recommended that patients at high risk for myocarditis (i.e., cardiovascular co-morbidities, combination therapy with anti-CTLA-4 and anti-PD-1/PD-L1, diabetes, etc.) undergo a baseline ECG and troponin test prior to initiation of ICI therapy. If signs/symptoms occur, ECG, troponin, chest X-ray, echocardiography, and BNP should be performed. Additionally CMR, stress test, and endomyocardial biopsy should be considered [14]. While recent studies incorporating advanced immune profiling suggest that the emergence of characteristic lymphocyte subpopulations may identify patients at risk for ICI-related complications, additional studies in larger patient cohorts are needed to determine the generalizability of these findings and to determine whether these immune subpopulations mediate disease.