Unresolved issues in theories of autoimmune disease using myocarditis as a framework

Abstract

Many theories of autoimmune disease have been proposed since the discovery that the immune system can attack the body. These theories include the hidden or cryptic antigen theory, modified antigen theory, T cell bypass, T cell-B cell mismatch, epitope spread or drift, the bystander effect, molecular mimicry, anti-idiotype theory, antigenic complementarity, and dual-affinity T cell receptors. We critically review these theories and relevant mathematical models as they apply to autoimmune myocarditis. All theories share the common assumption that autoimmune diseases are triggered by environmental factors such as infections or chemical exposure. Most, but not all, theories and mathematical models are unifactorial assuming single-agent causation of disease. Experimental and clinical evidence and mathematical models exist to support some aspects of most theories, but evidence/models that support one theory almost invariably supports other theories as well. More importantly, every theory (and every model) lacks the ability to account for some key autoimmune disease phenomena such as the fundamental roles of innate immunity, sex differences in disease susceptibility, the necessity for adjuvants in experimental animal models, and the often paradoxical effect of exposure timing and dose on disease induction. We argue that a more comprehensive and integrated theory of autoimmunity associated with new mathematical models is needed and suggest specific experimental and clinical tests for each major theory that might help to clarify how they relate to clinical disease and reveal how theories are related.

Keywords: Antigenic complementarity; Autoimmune disease theories and modeling; Infection; Myocarditis; Sex differences.

Figure 1. Hidden Antigen Theory (HAT)

Left: Some antigens (black pentagons) are sequestered within cells or tissues that are inaccessible to the developing immune system so that their corresponding T cells are not deleted or tolerized. Tissue damage or infection (black dots) activates an immune response (stellated cells). Center: Cellular or tissue damage releases hidden antigens, which provoke a second immune response (antibody shapes). Right: Autoimmune attack directed at the cells harboring the hidden antigens. The tissue damage or infection that provoked the release of hidden antigen is likely to be resolved long before the autoimmune effects are observed and the initiating cause therefore remaining obscure [40, 41].

Figure 2. Anti-Idiotype Theory (AIT)

Viruses and other microbes (white stellate forms) utilize molecularly complementary cell surface receptors (black pentagons) in order to target specific cell types. Some idiotypic antibodies (Id Ab) against such microbes will be complementary to the microbial ligands used to target infection. Such idiotypic antibodies will therefore mimic the cell surface receptors. Autoimmune disease may arise if the idiotypic antibodies induce an anti-idiotypic response (Anti-Id Ab) because the anti-idiotypic response will mimic the microbial ligand, therefore attacking the cell surface receptors [74, 89].

Figure 3. Similarity between Coxsackievirus and Adenovirus Receptor (CAR) and Human Cardiac Myosin (MYH6)

A similarity search using LALIGN reveals that CAR has multiple regions mimicking cardiac myosin (two of which are shown) so that putative anti-cardiac myosin antibodies in CVB3-induced myocarditis may originate as anti-idiotypic antibodies directed at CAR, as predicted by AIT. According to the anti-idiotype theory (AIT), a coxsackievirus infection might lead to the production of anti-idiotype antibodies that mimic the receptor it uses to infect heart tissue (see Figure 2). One of these receptors is CAR. No evidence of antibodies against CAR have been reported in myocarditis, but we suggest that perhaps antibodies have been misidentified as anti-cardiac myosin antibodies since CAR shares significant similarities to cardiac myosin and anti-cardiac myosin antibodies are common in myocarditis [80, 81].

Figure 4. Molecular Mimicry (MM) Theory of Autoimmune Disease

Many microbial proteins mimic host proteins resulting in epitope mimicry. Antibodies or T cells activated against a microbial epitope may therefore share weak affinity for the corresponding host epitope so that infection may induce autoimmune disease directed at cells displaying the epitope mimic [, –102, 135].

Figure 5. Molecular (or Epitope) Mimicry of the Streptococcal M Protein for Human Cardiac Myosin

Cunningham et al. demonstrated that the M protein of group A streptococci has many significant regions of homology with human cardiac myosin (of which only one is displayed here), that this sequence mimicry translates into immunologic cross-reactivity between the epitopes, and that such epitope mimics can be used (with CFA) to induce an animal model of EAM [, –113].

Figure 6. Molecular (or Epitope) Mimicry of Coxsackievirus B3 for Human Cardiac Actin

One of many similar protein sequences shared by coxsackieviruses with human cardiac actin that may also act as epitope mimics in autoimmune myocarditis [80, 81].

Figure 7. Modified Version of Molecular Mimicry (MM) Theory

One of the difficulties faced by the theory of molecular mimicry as it is applied to autoimmune myocarditis is that cardiac myosin is effectively a hidden antigen (see HAT, Figure 1), represented here by “Self Protein 2”, which should not be “visible” to the immune system. In order for the immune system to attack a hidden antigen such as cardiac myosin, the cells harboring it must be damaged. One way to create such cellular damage is by a viral infection. Another way is if there is a second cell surface protein (“Self Protein 1”) that mimics both the microbial trigger of the disease as well as the hidden antigen. In this modified version of MM, the immune response (antibody shapes) initiated by the microbe will cross-react with the cell surface host protein (“Self Protein 1”) damaging the cell and releasing the more antigenic hidden host protein (“Self Protein 2”). Thus, molecular mimicry between GAS and the coxsackievirus and adenovirus receptor (CAR) (Figure 3) could initiate an attack on cardiomyocytes resulting in damage that releases cardiac myosin; the similarities between GAS, CAR and cardiac myosin would then drive the subsequent autoimmune disease process.

Figure 8. Dual TCR (DTCR) Theory of Autoimmunity

Some T cells display more than one T cell receptor (TCR) so that they can be activated by more than one antigen. Activation of a T cell by a microbe and subsequent production of antibodies could therefore inadvertently initiate activation of an unrelated immune response that crossreacts with host tissues [28, 135, 178].

Figure 9. Antigenic Complementarity Theory of Autoimmunity (ACT)

The antigenic complementarity theory postulates that autoimmunity results from co-infection with pairs of pathogens, at least one of which mimics a host protein [–191, 202]. On the left, Microbe 1 has antigens that are molecularly complementary to antigens on Microbe 2. The antigens on Microbe 1 induce Antibody 1 (Ab1). The antigens on Microbe 2 induce Antibody 2 (Ab2). Because of the antigenic complementarity, Ab1 will be complementary to Ab2, which is to say that these two idiotypic antibodies will act as if they are an idiotype-anti-idiotype pair (see Figure 2). Thus, at the center, Ab1 will bind to both Microbe 1 and Ab2, while Ab2 will bind to both Microbe 2 and Ab1, thus producing circulating immune ICs. If, in addition, either Microbe 1 or Microbe 2 mimics a host protein (center top and bottom), then the antibodies induced by the microbes will also target these host proteins (right), just as is the case in Molecular Mimicry theory (see Figure 4). Thus, ACT combines basic elements of AIT with MM. ACT, however, also suggests a mechanism by which tolerance is broken in autoimmunity, which is that each idiotypic immune response mimics both a host and a microbial antigen so that the immune system itself becomes “confused” as to what is “self” and “nonself” and engages in an internal immunological civil war (center). Another unique prediction of ACT is that the molecular targets of autoimmune disease will themselves be complementary (center right).

Figure 10. Antigenic Complementarity Theory Applied to T Cell-Mediated Autoimmunity

The same logic just outlined in Figure 9 with regard to antibody-mediated autoimmunity can also be applied to explaining T cell-mediated autoimmunity provoked by pairs of microbes bearing complementary antigens. The result will be to induce pairs of T cells bearing complementary T cell receptors (TCR-1 and TCR-2) that will act as if they are idiotype-anti-idiotype pairs. Such complementary idiotypic T cells will attack each other, forming perivascular cuffs or other cellular aggregates (the cellular equivalent of circulating ICs). If the antigens mimic host proteins, then these complementary T cells will also attack the host tissue. As in Figure 9, ACT predicts that autoimmune disease begins with an immunologic civil war.

Figure 11. Antigenic Complementarity Theory 2 (ACT2)

Preston and Pendergraft have proposed an alternative version of ACT in which antigenic complementarity is mediated through antisense proteins [–191]. Every genetically-encoded protein has, according to antisense protein theory, a complementary protein encoded in the complementary (non-coding) strand of DNA. If a microbe displays a protein that is an antisense protein to a genetically encoded host protein, and this microbial protein also mimics a host protein, then all of the effects outlined above in Figure 9 will follow and autoimmunity disease will be initiated against the tissue that encodes the antisense protein mimic.

Figure 12. Antigenic Complementarity in Autoimmune Myocarditis

Root-Bernstein, et al. [81] have provided experimental evidence for most of the key assumptions implicit in ACT. This figure summarizes the experimental findings. As noted in Figure 5, the M protein of group A streptococci (GAS) mimics cardiac myosin. As noted in Figure 6, proteins of coxsackieviruses (CV) mimic cardiac actin. GAS antibodies bind to CV antibodies forming immune complexes. GAS antibodies recognize cardiac myosin. CV antibodies recognize actin. Actin and cardiac myosin are molecularly complementary to each other, forming actomyosin.