Myocardial infiltration is observed following infection by different coxsackieviruses via the coxsackievirus-adenovirus receptor (CAR) (51). CVB3 virus RNA is recognized by TLR3, TLR4 and melanoma differentiation-associated protein (MDA)-5 (52). Damage-associated molecular patterns (DAMPs) initiate the immune response from antigen presenting cells including mast cells, macrophages and dendritic cells. In that context, histamine, complement, and IL-1β activate infiltration of a diverse range of immune cells to the heart (6), including monocytes (6), natural killer cells (53), neutrophils (54), lymphocytes (55), and macrophages (55, 56). Macrophages are the dominant immune infiltrate in males with lower levels of CD4⁺ and CD8⁺ T cells as well as γδ T cells (56), B cells (57) and mast cells (20, 32). Females have higher levels of T and B cells as well as T_reg compared to males (58). B cells might play a role in progression to chronic disease, especially in females and autoimmune myocarditis (57), while T_reg cells might protect from progression to chronic myocarditis (53). Furthermore, Ly6C^high monocytes are detected in EMBs (59). Ly6C^high monocytes play a crucial role in early inflammation during acute myocarditis and are recognized for their ability to produce abundant levels of proinflammatory cytokines such as IL-1β (60–63). Additionally, they possess phagocytic capabilities (60–63).

Also in enterovirus myocarditis, the effect of IFNβ therapy has been explored (64). As stated earlier, IFNs including IFNβ and IFNγ reduce CVB3 replication and remodeling, which prevents progression to chronic cardiomyopathy (46, 47). Elevated IFN responses are a key reason that C57BL/6 mice do not progress from myocarditis to DCM in animal models of myocarditis (46, 47). Other studies explored pocapavir, pleconaril and IVIG in neonates with enterovirus myocarditis (65–67). Consensus statements, however, do not recommend antiviral therapies in enterovirus myocarditis as there is not sufficient evidence (3).

Soluble anti-CAR antibody reduces the incidence of acute and chronic CVB3-induced myocarditis in mice by preventing viral infection (68, 69). In another study, progression to chronic cardiomyopathy was prevented by anti-mouse IL-1β therapy (70). A primary pathway increasing acute inflammation and promoting progression to DCM in white background male mice is the TLR4-induced IL-1β response (20, 70–72). IL-1β increases IL-6 that is needed for IL-17/Th17 responses that promote remodeling and chronic cardiomyopathy (73). Male mice had increased IL-17-related responses compared to females and showed more fibrosis leading to chronic cardiomyopathy in CVB3-induced myocarditis (74). However, IL-17 does not increase acute myocarditis but is important for the progression to chronic cardiomyopathy which also predominantly occurs in males (20, 75, 76). Additionally, in acute murine CVB3 myocarditis high expression levels of osteopontin were found to be associated with the development of extensive fibrosis that can be reduced by treatment with a vitamin D analog (77). A recent study demonstrated that administering eplerenone can regulate the acute immune response and protect against myocardial remodeling in CVB3-induced myocarditis (78). Similarly, we had previously shown in an animal model of myocardial infarction that inflammatory genes activated during ischemia were downregulated through treatment with eplerenone (79). This response to treatment was much greater in females (79).

Parvovirus B19 (B19V) infects endothelial cells (80) which may lead to apoptosis of cardiomyocytes (80, 81). Only high copy numbers of B19V DNA in the myocardium were found to be associated with acute lymphocytic myocarditis (82). It is likely that the toxic, non-structural viral protein NS1 triggers the release of proinflammatory cytokines (83), although the pathogenetic role of B19V in myocarditis is still controversial (84). Immune cell infiltration in patients with Parvovirus B19V myocarditis is dominated by macrophages and lymphocytes (85). CD4⁺ T cells play a role in acute Parvovirus B19V-related myocarditis (86). In individuals, striking CD8⁺ T cell responses were observed, which were sustained or even increased over many months after the resolution of acute disease (87).

Although no guidelines currently recommend the use of IVIG for patients with severe Parvovirus B19V viremia and associated complications such as transient aplastic crisis or chronic pure red cell aplasia, some studies suggest that IVIG may be beneficial in such cases (88). Other authors showed no significant improvement in cardiac systolic function in patients undergoing IVIG, thus rendering the potential benefits of IVIG uncertain (89). No therapy is recommended when B19V copy number is low and cardiac inflammation is absent in EMB (3). When EMB is positive for inflammation although B19V copy number is low, immunosuppressive therapy may be considered (90).

New therapeutic strategies are under investigation, including the synthetic nucleotide analogues cidofovir and brincidofovir as well as flavonoid molecules and hydroxyurea (88).

HCMV is usually acquired during childhood and is known to infect several cell types, such as endothelial cells, epithelial cells, and immune cells (91, 92). Usually, HCMV infection remains asymptomatic until the occurrence of immunosuppression, thereby posing a significant concern as a complication in individuals such as organ transplant recipients or Human Immunodeficiency Virus (HIV) patients (92).

Male BALB/c mice infected with murine CMV (MCMV) develop acute and chronic inflammatory myocarditis similar to CVB3 and autoimmune models, where macrophages predominate with fewer T-, B-, and other cells during the acute phase of the disease (37, 93, 94).

A reduction of viral burden might be achieved with the anti-herpesvirus drugs ganciclovir or acyclovir, although the efficacy in HCMV-induced myocarditis has not been studied directly (95, 96). Viral infection was reduced in MCMV-induced myocarditis if ganciclovir and cidofovir were administered during the innate immune response but not if it was given during acute myocarditis (97). A controlled trial suggested that a CMV hyperimmunoglobulin treatment may be effective in HCMV myocarditis (98). Antiviral therapy is not generally recommended for patients with virus-induced myocarditis and should be reserved for individual cases (3). Consultation with an infectious disease specialist is recommended before initiating antiviral therapy (3).

EBV invades cardiac tissue by initially infecting resting human B lymphocytes (96) and eventually infiltrating both the myocardium and pericardium (99). EMB from patients with EBV myocarditis typically reveals lymphocytic infiltration, consisting of predominantly CD8⁺ T cells but also CD4⁺ T cells (99). In this case, high numbers of EBV-encoded RNA copies were demonstrated in CD8+ T lymphocytes (99).

HHV6-induced myocarditis has been associated with a myocardial infiltration of CD4⁺ and CD8⁺ T cells (100). Figure 2 illustrates EMB findings representative for HHV6-associated lymphocytic myocarditis. In a case report, Rohayem and colleagues reported a patient suffering from lethal acute B19V and HHV6 coinfection who at autopsy was found to have diffuse infiltration of the myocardium with mononuclear cells and neutrophils as well as edema, degeneration and loss of myocardial cells (101). However, low HHV6 copy numbers are a common finding and are likely to have no clinical impact (102).

Chronic HCV infection may lead to cardiomyocyte hypertrophy and therefore hypertrophic cardiomyopathy due to expression of the HCV-core protein and overactivation of transcription factor AP-1 (103). The HCV-core protein plays a significant role in the viral nucleocapsids and has an impact on the transcription of cellular protooncogenes in hepatocytes (103, 104). HCV is also recognized to contribute to the development of insulin resistance and the generation of reactive oxygen species (105). The mechanisms that cause damage in relation to HCV-associated cardiomyopathy are not well understood. Hypotheses aiming to explain myocardial damage include direct mitochondrial disruption and oxidative stress to cardiomyocytes and a chronic systemic inflammatory state, which might lead to myocardial inflammation (103, 106). Furthermore, HCV core protein may lead to the activation of profibrotic pathways eventually leading to cardiac fibrosis (103, 106). In HCV-associated myocarditis, monocytes and CD68⁺ macrophages are the dominating cells infiltrating the heart (107).

HLA-DPB1*0901 and HLA-DRB1*1201 are associated with progression to chronic cardiomyopathy and persistence of HCV as well as the development of DCM (106). Mononuclear cells (in particular monocytes and CD68⁺ macrophages) are the primary target cells for treating extrahepatic manifestations in HCV infections (107).

Even though the viral genome typically persists in the myocardium after HCV infection, immunosuppression may be beneficial for HCV myocarditis, as it was shown to improve cardiac function (108).

In some cases of HIV-associated cardiomyopathy, diffuse myocardial damage with variable degrees of hypertrophy and degenerative changes leading to hydropic changes within the cardiomyocytes has been noted (109, 110). The HIV gp120 protein mediates myocardial injury and dysfunction through a nitric oxide dependent mechanism (111). Interstitial and endocardial fiber damage is increased, leading to fibrosis (109, 110, 112). T cells dominate the inflammatory infiltrate, with a majority of CD8⁺ T cells (109).

Cytokine-mediated cardiotoxicity and autoimmune response against components of the heart are likely involved in myocarditis following influenza virus infection (113). Monocytes, dendritic cells, and macrophages dominate the myocardial immune infiltration. The latter two secrete cytokines such as IFNs and TNF-α, contributing to increased acute inflammation (114, 115).

MERS-CoV-associated cardiomyopathy may be associated with myocyte hypertrophy, moderate coronary atherosclerosis and patchy myocardial fibrosis (116). There is limited data regarding myocardial immune infiltration. Lymphocytic infiltration (117, 118) predominantly with dendritic cells (119), macrophages (120), and T cells (121) has been described in MERS-CoV-induced myocarditis.

Edema and atrophy of myocardial fibers is commonly seen in myocarditis associated with SARS-CoV infection (122). Immune infiltration primarily consists of lymphocytes. Also, monocytes and plasma cells infiltrating the endothelium have been described (122). As the ACE2 receptor is the entry point for SARS-CoV into cells, the protein might represent a therapeutic target (123).

The role of SARS-CoV-2 on myocarditis was recently reviewed (124). Briefly, autopsy and histopathological findings suggest extensive lymphocytic infiltration in the myo- and pericardium after lethal SARS-CoV-2 infection (125, 126). An inflammatory state with predominantly CD68⁺ macrophages (124, 127) as well as enhanced monocyte recruitment (128) has been reported for cardiomyopathy associated with SARS-CoV-2 infection. Furthermore, a cytokine storm, dominated by IL-1β has been shown to play a major role in the pathophysiology of severe corona virus disease 2019 (COVID)-19 (129–131). IL-6 has been associated with cardiac dysfunction, as evidenced by reduced left ventricular function obtained with speckle tracking echocardiography in patients hospitalized due to COVID-19 (124, 132).

ACE2 receptor (124, 133) and transmembrane protease serine subtype 2 (TMPRSS2) (96, 124) interaction enables SARS-CoV-2 to enter its target cells (133). The interaction with the ACE2 receptor has been suggested as a potential direct cytotoxic effect of SARS-CoV-2 (133). While the possibility of direct damage to the heart by SARS-CoV-2 has been discussed (124), the prevailing absence of the virus within cardiomyocytes of COVID-19-associated myocarditis patients (134, 135) further reinforces the concept of cytokine-induced damage to the heart.

Several antiviral therapies are currently being investigated, such as protease inhibitors (e.g., lopinavir-ritonavir, darunavir), RNA polymerase inhibitors (remdesivir) and anti-cytokine agents (e.g., IL-6 receptor antagonists) (96). In patients with SARS-CoV-2, the IL-1β antagonist canakinumab improved clinical recovery and reduced cardiac injury at 28 days post infection (136, 137).

Staphylococcus typically builds micro abscesses within the myocardium (140). Methicillin-resistant Staphylococcus aureus (MRSA)-induced myocarditis is characterized by focal myocyte necrosis (141).

The pathomechanisms of myocarditis following streptococcal upper airway infections are not well understood. Streptococcal toxins and cross-reactivity of IgG with streptococcus antigens and cardiac myosin have been postulated as possible causative mechanisms (142, 143). Mononuclear cells, especially lymphocytes (of which CD4⁺ T cells were predominating) are found in EMB (144). Neutrophil infiltration and micro abscesses containing bacteria have also been described (145).

Pneumococcus microlesions appear widely spread throughout the myocardium, but especially in the ventricles (146). Such microlesions manifest as areas within the myocardium with a reduced number of cardiomyocytes accompanied by an enlarged intercellular space filled with pneumococci (147). These lesions are further characterized by expansion of the intercellular space caused by extracellular vacuolation, the apparent loss of cardiomyocytes and the stark absence of infiltrating immune cells, as observed in both a BALB/c mouse model and human disease (147–149). Fibrosis is a common consequence of this response (148, 149).

Macrophages and neutrophils are subverted through biofilm production by pneumococcus (150).

Bacterial adhesin choline-binding protein A and its interaction with laminin receptor of vascular endothelial cells represents the cellular entry point for pneumococci (151). Also, cobinding of phosphorylcholine residues on the bacterial cell membrane to platelet-activating factor receptor is an additional activating mechanism (151). Neutralizing these interactions could be a possible pharmacological approach (151).

Meningococcus myocarditis is rare (152), but might occur during meningococcus sepsis (153, 154). There is no information regarding immune cell infiltration patterns in the heart available, but neutrophils and lymphocytes accumulate around meningeal vessels of patients suffering from meningococcal meningitis (155).

Myocarditis caused by gonococci usually occurs secondary to or during endocarditis (156, 157) leading to perivalvular abscesses (156, 158). Neutrophils are the dominant immune cells infiltrating the valves and spatially related myocardium during such an infection (158). Fibrosis is a common result of gonococcal myocarditis (158).

Following salmonella gastroenteritis, the heart and aorta may be a secondary target of inflammation (159). It remains uncertain whether this represents a sterile inflammation lacking bacterial myocardial infiltration, that occurs subsequent to the infection, or if it is a secondary manifestation directly linked to the salmonella infection (159). Non-typhoid salmonella might lead to multifocal biventricular inflammation in subepicardial and midmyocardial tissue (160). There is no data in the literature regarding the composition of the immune cell infiltrate.

Infiltration by Mycobacterium tuberculosis of the heart follows no specific pattern (161). Similar to tuberculosis of the lungs, myocardial involvement comes in 3 different types: tuberculomas, miliary tubercles or an uncommon diffuse infiltrative type (161). Commonly, the left ventricle is affected (162). Usually, a giant cell (163–165) and lymphocytic (164, 165) infiltrate accompanies the infection. Granulation (166) as well as scar tissue and fibrosis might result (163).

M. tuberculosis inactivated antigens are used as the adjuvant (mimic infection) in complete Freund's adjuvant that is required to induce disease in autoimmune models of myocarditis (29). The dominant immune response in natural infections and in experimental autoimmune myocarditis are macrophages and Th17-type responses (167–169). These findings suggest that M. tuberculosis can drive myocarditis and DCM in the context of an autoimmune response.

Myocarditis due to Mycoplasma pneumoniae infection has been described as myopericarditis (170). Fibrosis of the heart valves has been reported (171). Data are limited concerning EMB findings on cellular components of the immune infiltrate in the context of mycoplasma infection.

Brucella myocarditis is very rare (172) and tends to occur in the left ventricular myocardium, in particular subpericardial (173, 174). Following infection with Brucella, endocardial tissue might undergo calcification and become fibrotic (175). Data are limited regarding the type of immune infiltration.

Chagas disease is caused by Trypanosoma cruzi, which is a parasite that replicates within host cells, including cardiac myocytes (176). Cardiac involvement in Chagas disease typically leads to inflammation, fibrosis, diffuse ventricular wall motion abnormalities and arrhythmias especially when prolonged inflammation occurs (177, 178). The myocardium shows signs of necrosis, areas of myocellular hypertrophy and predominantly mononuclear cell infiltration (176). Macrophages, eosinophils, neutrophils and mast cells are found in myocardial tissue as well, but to a lesser extent (176). Proinflammatory cytokines such as TNFα and IFNγ promote inflammation which can further lead to autoimmunity (179). Furthermore, the persistence of the parasite is associated with high grade myocarditis (180), as tissue damage partly results directly from the pathogen itself (176).