**3.1 Acute myocarditis during the initial Virus invasion**

During the period of active viremia, cardiotropic RNA or DNA viruses interact with specific vascular endothelial cell receptors before to be translated in the endomyocardial target cells to produce viral proteins (Huber, 1993; Li et al., 2000) (Table 1). Mechanisms of viral entry into the host cell remains crucial and are of major interest because each of them is putative target for novel therapeutics. Before to interact specifically with the virus, expression of levels of the virus receptors for CVB3, PVB19, HVV6 (CAR/DAF, P-Antigen/ Alpha5-beta1 integrin and CD46, respectively) on endovascular endothelial cells of coronary vessels modulate the diffusion of these viral agents into the vessels and their subsequent translation into the endomyocardial target cells (Khül et al., 2005a; Yajima & Knowlton, 2009). Some inflammatory mechanisms as a primary persistent infection by HHV6 or PVB19 may modulate cell membrane receptors and /or immune suppression and preventing virus clearance; altogether these mechanisms could facilitate an endomyocardial super infection by a second viral agent as described in 27 % of virus positive DCM patients (Khül et al., 2005a) (Table 1).

Viral Myocarditis: Physiopathology and Diagnosis 91

cells capable to detect the viral antigen will destroy the infected cardiac cells through cytokines or perforines secretion (Kawai, 1999, Matsumori et al., 1994). In addition, some myocardic cellular antigens of the host present can share epitopic similarities (molecular mimicry) with viral antigens therefore inducing an autoimmune trait that can sustain the inflammatory response and therefore the chronic inflammation phases (Figure 1). This subacute phase is known to be linked to autoimmune responses as many patients produce autoantibodies and auto-reactive T cells against heart proteins (Domenico & Gaetano, 2006; Feldman & McNamara, 2000) and the amplification of this phenomenon can lead to the destruction of cardiomyocytes. The mechanisms linking enteroviral infection with sub-acute myocarditis, relapses and post inflammatory heart failure could be mainly mediated by dendritic cells (DCs) that are specialized in antigen processing and presentation and most important in priming of T cells within lymph nodes. The EV-induced autoimmune myocarditis may require activation of these cells via CD40 with Toll like receptors (TLRs 3, 4, 7) or RNA helicases (RIG-I, MAD-5) co-stimulation (Kramer et al., 2008). Moreover in human infections, it is clear that human enteroviruses (HEVs) can escape from immunological system by decreasing the specific functions of immunity cell system (Oldstone, 2006). Recently it has been demonstrated that Enteroviruses could infect and consequently modify the maturation process and some specialized functions of DCs (Kramer et al., 2007). Therefore, Enteroviruses or some of its components could activate or decrease some DCs functions acting as modulators of innate response system or

**3.3 Chronic myocarditis with a potential evolution to the dilated cardiomyopathy** 

Dennert & McNamara, 2009; Kawai, 2009; Khül et al., 2008a).

**4. The Virological mechanisms of cardiac infection** 

After the active virus replication resulting in acute and sub acute myocarditis phases, the pathological signs of myocarditis generally disappeared and the destroyed myocytes are replaced by diffuse fibrosis (Dec et al., 1985) (Figure 1). At this stage, a progressive heart biventricular dilatation with a cardiac failure can be observed and has been related to cardiac persistent or chronic viral replication mechanisms (Andréoletti et al., 2000; Badorff et al., 1999) (Figure 1). This persistent or chronic viral infection could be related with various cardiac cell dysfunctions as impairment of Ca+ efflux in cardiomyocytes and of loss of cell contractility, apoptosis balance deregulation, cleavage of dystrophine, modulation of cellular signalling pathways or alteration of the extracellular matrix (Andréoletti et al., 2009;

Viruses generally infect human beings by fecal-oral (enteroviruses, *parvovirus* B19, *Herpesviruses*) or respiratory routes (enteroviruses, *influenza viruses A & B, parainfluenza viruses I II III*) and they usually perform a first phase of multiplication into the airway epithelial cells of the higher respiratory tract or tonsils; this can be followed by a potential rapid invasion of the lower respiratory tract as observed during influenza virus infection. After this initial and local replication phase, the viruses can diffuse by lymphatic way to the general circulation (viremic phase) allowing them to reach the cardiac tissues. In the heart, the virus infects and replicate actively into the cardiac myocytes but also into the cardiac fibroblasts that can play the role of reservoir cells for a persistent infection (Andréoletti et

autoimmunity (Kramer et al., 2007).

**4.1 Acute viral myocarditis** 

**clinical stage** 


EM indicated electron microscopy; IHC, immunohistochemistry ; ISH, in situ hybridrization; LCM + PCR, polymerase chain reaction using tissue samples isolated with laser capture microdissection; CAR, coxsackie and adenovirus receptor; DAF, decay accelerating factor ; CD, cluster of differentiation and ND, not determined

Table. 1. Cardiovascular tissular localization of common cardiotropic viruses and local suspected cellular receptors involved in the process of infection.

At the beginning of human viral infection, the first line of immune defense consists in the innate immune response (NKT cells, monocytes and macrophages). Cell-mediated immunity also plays an important role in viral clearing. Cytotoxic (CD8+) cells recognize degraded viral protein fragments that are presented by major histocompatibility-complex class I antigens on the myocyte surface (Seko et al., 1990) (Figure 1). The Toll-like receptors located at the surface or inside of dendritic cells (more particularly the Toll-like-receptors 4, 7, and 8 in the case of virus Coxsackie B infection) could recognize certain viral proteins or genomic and then activate intracellular signals (NF-kB pathways) responsible for a fast synthesis and secretion of pro-inflammatory cytokines as interferons and chemokines and also nitric oxide (Ayach et al., 2003; Kawai, 1999; Matsumori et al., 1994) (Figure 1). The proinflammatory cytokines then attract and activate many immune system cell lines to the site of the viral infection and they are directly responsible for cardiac myocytes lysis and also for lost of cardiomyocyte contractility and apoptosis cell-dead (Feldman & McNamara, 2000; Ventéo et al., 2010) (Figure 1).

#### **3.2 Sub-acute myocarditis with an activation of the immune system and the development of autoimmunity**

The activation of the immune system at the time of the viral invasion reaches to the initiation of specific cellular immunity response and to the immune shift towards a specific immune response. The viral particles are captured by antigen processing cells (APC) and degraded within the Golgi apparatus before being presented at the cell surface by the major histocompatibility complex class I (MHC class I) to the CD8+ lymphocytes. These primed T

**Cellular Receptor / Coreceptor Involved**

> CAR / Integrins (αvβ3 and αvβ5)

Gb4Cer / Integrin (α5β1); Ku80 autoantigen

Integrins ( α2β1, α6β1, and αvβ3)

Lymphocytes ND Cioc et al., 2002

**References** 

Shi et al., 2009; Wickham et al., 1993

Shi et al., 2009; Coyne & Bergelson, 2006

Bultmann et al., 2003; Bönsch et al., 2010

Caruso et al., 2002; Santoro et al., 1999

Feire et al., 2004; Kyto et al., 2005

Chimenti et al., 2004; Takano et al., 2008

**Cardiovascular tissular Localization** 

Cardiomyocytes, fibroblasts, endothelial cells

Endothelial cells, cardiomyocytes

Infection Endothelial cells CD46

Macrophages,

EM indicated electron microscopy; IHC, immunohistochemistry ; ISH, in situ hybridrization; LCM + PCR, polymerase chain reaction using tissue samples isolated with laser capture microdissection; CAR, coxsackie and adenovirus receptor; DAF, decay accelerating factor ; CD, cluster of differentiation and

Table. 1. Cardiovascular tissular localization of common cardiotropic viruses and local

**3.2 Sub-acute myocarditis with an activation of the immune system and the** 

The activation of the immune system at the time of the viral invasion reaches to the initiation of specific cellular immunity response and to the immune shift towards a specific immune response. The viral particles are captured by antigen processing cells (APC) and degraded within the Golgi apparatus before being presented at the cell surface by the major histocompatibility complex class I (MHC class I) to the CD8+ lymphocytes. These primed T

At the beginning of human viral infection, the first line of immune defense consists in the innate immune response (NKT cells, monocytes and macrophages). Cell-mediated immunity also plays an important role in viral clearing. Cytotoxic (CD8+) cells recognize degraded viral protein fragments that are presented by major histocompatibility-complex class I antigens on the myocyte surface (Seko et al., 1990) (Figure 1). The Toll-like receptors located at the surface or inside of dendritic cells (more particularly the Toll-like-receptors 4, 7, and 8 in the case of virus Coxsackie B infection) could recognize certain viral proteins or genomic and then activate intracellular signals (NF-kB pathways) responsible for a fast synthesis and secretion of pro-inflammatory cytokines as interferons and chemokines and also nitric oxide (Ayach et al., 2003; Kawai, 1999; Matsumori et al., 1994) (Figure 1). The proinflammatory cytokines then attract and activate many immune system cell lines to the site of the viral infection and they are directly responsible for cardiac myocytes lysis and also for lost of cardiomyocyte contractility and apoptosis cell-dead (Feldman & McNamara, 2000;

Coxsackievirus B IHC, ISH Cardiomyocytes CAR / DAF

Virus ISH Lymphocytes B CD21

suspected cellular receptors involved in the process of infection.

**Common cardiotropic Viruses** 

Enterovirus/

Parvovirus B19

Human Herpes Virus 6

Epstein-Barr

ND, not determined

Influenza Virus ISH

Ventéo et al., 2010) (Figure 1).

**development of autoimmunity** 

Adenovirus EM

**Detection Methods** 

ISH, LCM + PCR

In vitro

Cytomegalovirus ISH Cardiomyocytes

cells capable to detect the viral antigen will destroy the infected cardiac cells through cytokines or perforines secretion (Kawai, 1999, Matsumori et al., 1994). In addition, some myocardic cellular antigens of the host present can share epitopic similarities (molecular mimicry) with viral antigens therefore inducing an autoimmune trait that can sustain the inflammatory response and therefore the chronic inflammation phases (Figure 1). This subacute phase is known to be linked to autoimmune responses as many patients produce autoantibodies and auto-reactive T cells against heart proteins (Domenico & Gaetano, 2006; Feldman & McNamara, 2000) and the amplification of this phenomenon can lead to the destruction of cardiomyocytes. The mechanisms linking enteroviral infection with sub-acute myocarditis, relapses and post inflammatory heart failure could be mainly mediated by dendritic cells (DCs) that are specialized in antigen processing and presentation and most important in priming of T cells within lymph nodes. The EV-induced autoimmune myocarditis may require activation of these cells via CD40 with Toll like receptors (TLRs 3, 4, 7) or RNA helicases (RIG-I, MAD-5) co-stimulation (Kramer et al., 2008). Moreover in human infections, it is clear that human enteroviruses (HEVs) can escape from immunological system by decreasing the specific functions of immunity cell system (Oldstone, 2006). Recently it has been demonstrated that Enteroviruses could infect and consequently modify the maturation process and some specialized functions of DCs (Kramer et al., 2007). Therefore, Enteroviruses or some of its components could activate or decrease some DCs functions acting as modulators of innate response system or autoimmunity (Kramer et al., 2007).

#### **3.3 Chronic myocarditis with a potential evolution to the dilated cardiomyopathy clinical stage**

After the active virus replication resulting in acute and sub acute myocarditis phases, the pathological signs of myocarditis generally disappeared and the destroyed myocytes are replaced by diffuse fibrosis (Dec et al., 1985) (Figure 1). At this stage, a progressive heart biventricular dilatation with a cardiac failure can be observed and has been related to cardiac persistent or chronic viral replication mechanisms (Andréoletti et al., 2000; Badorff et al., 1999) (Figure 1). This persistent or chronic viral infection could be related with various cardiac cell dysfunctions as impairment of Ca+ efflux in cardiomyocytes and of loss of cell contractility, apoptosis balance deregulation, cleavage of dystrophine, modulation of cellular signalling pathways or alteration of the extracellular matrix (Andréoletti et al., 2009; Dennert & McNamara, 2009; Kawai, 2009; Khül et al., 2008a).
