**1. Introduction**

216 Myocarditis

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> Myocarditis is defined as an inflammation of myocardium where the infiltrating leukocytes are intimately associated with cardiomyocyte necrosis or drop-out (Liu, 2005; Woodruff, 1980). Cardiac damage may be minimal and self-limiting or may result in chronic fibrosis and cardiac dysfunction leading to death in children and young adults (Eckart et al., 2004; Fabre, 2006; Solberg et al., 2010). As discussed in other chapters of this book, infections with a highly diverse group of viruses, bacteria, fungi, and worms have been implicated in infectious myocarditis (Friman et al., 1995). Enteroviruses and adenoviruses are usually considered as the predominant viral etiological agents, and are associated with approximately 80% of clinical myocarditis where a viral infection is documented. However, virtually any virus infection may initiate myocarditis (Bowles et al., 2003; Woodruff, 1980). While seasonal influenza virus is only a minor etiological agent in myocarditis, evidence from the most recent influenza H1N1 pandemic (Vila de Muga et al., 2010; Wiegand et al., 2010; Zheng et al., 2010) suggests a higher incidence of both mortality and morbidity, and accounts for 5% of complications in infected children (Zheng et al., 2010).

> Myocardial injury results either directly from replication and induction of death or dysfunction in infected cardiocytes, or from host responses to infection (Huber, 2010). Although anti-viral host responses (innate or adaptive) are intended to control and eliminate the infection, cytokines and by-products such as nitric oxide or oxygen free radicals may also damage adjacent uninfected cells (Szalay et al., 2006). Innate immunity is the initial host response to infection and usually occurs within hours or days of virus introduction. The major characteristic of the innate response, besides its rapidity, is that it is broadly reactive to multiple infectious agents. While it is highly unlikely that innate immunity can completely eliminate the infection, it can suppress microbial replication until the far more potent and highly specific adaptive immune response kicks in. The reason for this is quite simple, viruses replicate rapidly with, for example, one picornavirus infected cell in tissue culture producing up to a million progeny virions within 18-24 hrs. In vivo, such rapid and uncontrolled growth could result in extensive tissue injury or death of the organism prior to a useful adaptive immune response being established since during a primary immune response, production of meaningful numbers of virus-specific T cells could

Innate Lymphocyte Effectors

(Brener & Gazzinelli, 1997).

innate immunity.

**3. Natural killer T and γδ T cells** 

**3.1 CD1 molecules and regulation of their expression** 

regulation of MHC molecules (Hewitt, 2003).

(Natural Killer, Natural Killer T and γδ T Cells) in Infection and Myocarditis 219

NK inhibitory receptors recognize major histocompatibility complex class I (MHC I) molecules which represents one mechanism by which NK cells distinguish between infected and normal cells, as many viruses attempt to evade the immune system through down-

Evidence for a major role of NK cells in clinical myocarditis is rather weak. NK cells were not observed in heart tissue from 18 cases of biopsy proven myocarditis (Chow et al., 1989). Other studies have found either increased numbers of NK like cells in peripheral blood of dilated cardiomyopathy patients (Yokoyama, 1988) or diminution of NK cell activity in such patients (Maisch et al., 1985). Studies in patients with Chagas' disease find no alteration in NK cells early in the disease but an increase in these innate effectors occurs at later stages (Sathler-Avelar et al., 2003). One potential problem with clinical studies is that diagnosis of myocarditis or dilated cardiomyopathy is a relatively late event in the disease process and may be quite removed from the initiating acute infection (Woodruff, 1980). In fact, while viral genomic sequences can be detected in clinical heart biopsies for months and possibly for years, it is rare for infectious virus to be isolated from the hearts of myocarditis patients. Any role for NK cells may be over by the time human tissue is studied. The best evidence that NK cells might participate in viral myocarditis comes from mouse models. These studies indicate that NK cells are important in controlling coxsackievirus B infections in vivo (Gauntt et al., 1988; Gauntt et al., 1989; Vella & Festenstein, 1992) as depletion of these cells substantially increases virus titers in the heart or pancreas. The ability of NK cells to suppress virus infection may relate to their cytolytic activity to infected cardiocytes. Rapid elimination of infected cells before virus replication is complete would restrict the number of progeny virions produced and therefore limit the next cycle of infection. The second mechanism by which NK cells may help control virus infection is through either augmenting or accelerating the adaptive immune response to the virus. NK cells directly interact with both dendritic cells and activated T cells causing maturation of the dendritic cells and increased activation of the T cells (Zingoni et al., 2005). Interactions occur through up-regulation of OX40L on the NK cells and OX40 on activate CD4+ lymphocytes. Also, NK cells contain high concentration of pre-formed cytokines which can be rapidly released upon NK receptor engagement and these cytokines provide the environment necessary for optimal adaptive immunity development. As with the mouse model of CVB3 myocarditis, NK cells also control spread of Trypanosoma cruzi in the mouse model of Chagas' disease

The other two major innate lymphocyte populations are natural killer T (NKT) and γδ T cells. NKT cells primarily recognize lipid antigens presented by CD1d molecules. The γδ T cells represent a more diverse population and in many cases, the antigen specificity of these cells is not known. However, as discussed below, the γδ T cells known to be involved in experimental viral myocarditis are also CD1d restricted. For this reason, description of the CD1 family of molecules is provided followed by discussion of the NKT and γδ T cells in

There are five distinct CD1 molecules, CD1a, CD1b, CD1c, CD1d and CD1e (Figure 1). Although these different molecules most likely arose from a single common ancestral gene

take 7-10 days after virus inoculation. The best known innate immunity results from microbial products binding to and activating Toll-Like Receptors (TLR) or RNA helicases (RIG-I and MDA-5) which activate transcription factors (NFkB) leading to expression of cytokines (TNFα, IL-1β and IL-6) and nitric oxide (Hosoi et al., 2004; Michelsen et al., 2004); or interferon response factors (IRF3/7) leading to expression of type 1 interferons (IFNα/β) RANTES and IP-10 (O'Neill, 2004; Vogel et al., 2003). These roles for TLR are discussed elsewhere. This review will concentrate on lymphocytes belonging to the innate immune response and discuss their role in myocarditis. These lymphocytes include natural killer (NK), natural killer T (NKT) and γδ T cells.
