A. M. Ercolini and S. D. Miller (review by Jon Trister MD)
There are many identified autoimmune diseases . Multiple factors are thought to contribute to the development of immune response to self, including genetics, age and environment. In particular, viruses, bacteria and other infectious pathogens are the major postulated environmental triggers of autoimmunity.
Multiple arms of the immune system may be involved in autoimmune pathology. Antigens are taken up by antigen- presenting cells (APCs) such as dendritic cells (DCs) and macrophages and processed into peptides which are loaded onto major histocompatibility complex (MHC) molecules for presentation to T cells via clonotypic T cell receptors (TCRs). Cytolytic T cells (Tc, activated by MHC Class I on APC) can directly lyse a target, while T helper cells (Th, activated by MHC class II) release cytokines that can have direct effects or can activate macrophages, monocytes and B cells. B cells themselves have surface receptors that can bind surface antigens. Upon receiving signals from Th cells, the B cell secretes antibodies specific for the antigens. Antibody may bind its specific target alone or may bind to and activate macrophages simultaneously via the Fc receptor.
There are multiple mechanisms by which host infection by a pathogen can lead to autoimmunity.The pathogen may carry elements that are similar enough in amino acid sequence or structure to self-antigen that the pathogen acts as a self-‘mimic’. Termed ‘molecular mimicry’, T or B cells that are activated in response to the pathogen are also cross- reactive to self and lead to direct damage and further activation of other arms of the immune system. The pathogen may also lead to disease via epitope spreading. In this model the immune response to a persisting pathogen, or direct lysis by the persisting pathogen, causes damage to self-tissue. Antigens released from damaged tissue are taken up by APCs, and this initiates a self-specific immune response. ‘Bystander activation’ describes an indirect or non-specific activation of autoimmune cells caused by the inflammatory environment present during infection. A domino effect can occur, where the non-specific activation of one arm of the immune system leads to the activation of other arms. Lastly, infection may lead autoimmunity through the processing and presentation of ‘cryptic antigens’. In contrast to dominant antigenic determinants, subdominant cryptic antigens are normally invisible to the immune system. The inflammatory environment that arises after infection can induce increased protease production and differential processing of released self-epitopes by APCs.
Coxsackievirus B (CVB) is the most common cause of infectious myocarditis. Infectious virus and viral RNA can be isolated from patients’ hearts.CVB3 can cause myocarditis in mice; in most mouse strains, the virus titre peaks at day 4 post-infection and is undetectable after 14 days. The chronic stage of the disease (day 28 onwards) is characterized by mononuclear cell infiltration into the myocardium and the production of antibodies to cardiac myosin which, because of the absence of virus, argues for autoim- munity as the pathophysiological mechanism at this stage of disease. In vitro, cardiac myocytes can be infected and lysed by the virus and CVB infection causes myocardial destruction.
This damage may lead to autoimmunity via epitope spreading. In mice, virus-specific antibodies arise soon after infection, followed by antibodies to several cardiac proteins such as myosin, tropomyosin and actin.
While these studies show that numerous autoimmune mechanisms can lead to cardiomyopathy in infected mice, it remains uncertain if autoimmunity accounts for the pathology seen in humans.
Streptococcus pyogenes: group A strepcococcus
Infection with S. pyogenes can lead to inflammation of the heart, and the involvement of lymphocytes in cardiac pathology has been suggested for some time.
Studies have shown that bacterial materials and DNA can persist in host tissue for some years after infection, so it is possible that ongoing immunity against the bacteria may lead to bystander damage to the organ. However, it is accepted most predominantly that the autoimmune reaction is caused by molecular mimicry. Myosin has been identified as the dominant autoantigen in the heart, and myosin-reactive mAb derived from patients with acute rheumatic fever were shown be cross-reactive to both M protein (the major virulence factor of group A streptococci) and the streptococcus carbohydrate epitope N-acetylglucosamine.
Although somewhat controversial , infection with S. pyogenes has also been associated with the development of movement and behavioural disorders such as Sydenham chorea, Tourette’s syndrome and obsessive–compulsive disorder . Patients with these disorders often have antibodies to the basal ganglia in the brain, and molecular mimicry between basal ganglia and S. pyogenes-derived proteins remains the major postulated mechanism of disease induction. An early paper demonstrated antibody cross-reactivity between S. pyogenes membrane and neuronal cytoplasm in patients with Sydenham chorea .
Using serum, cerebrospinal fluid (CSF) and mAb derived from Sydenham chorea patients, dual-specific antibodies were found that react with both the immune-dominant carbohydrate epitope on S. pyogenes cell wall (GlcNAc) and with lysoganglioside GM1 on the surface of neurones. The same group demonstrated that GlcNAc-reactive antibodies from the sera of patients with pediatric autoimmune neuropsychiatric disorders associated with streptococci was inhibited by lysoganglioside GM1, and that lysoganglio- side GM1-reactive mAb from Sydenham chorea patients could also react with intracellular brain protein beta-tubulin.
Chagas disease is caused by infection with the protozoan parasite T. cruzi. 10–30% of infected individuals develop the disease, which occurs in two major clinical phases, acute and chronic. The acute phase is characterized by parasitaemia, preferentially in heart muscle cells, and inflammatory infiltration of infected tissue. This is followed by an asymptomatic indeterminant phase, which can last up to 30 years. Patients who progress to the chronic phase of the disease are affected mainly by irreversible cardiomyopathy.Although it has been suggested that parasite persistence can contribute to chronic Chagas disease cardiomyopathy (CCC), T. cruzi antigens and DNA can also be detected in infected people who remain asymptomatic. This suggests that the tissue destruction that characterizes this phase may be largely autoimmune. CCC is characterized histopathologically by mononuclear cell infiltrates, with CD8+ T cells outnumbering CD4+ T cells 2:1. Local production of interferon (IFN)-g, TNF-a, IL-4 and IL-6 has been reported. In addition, real-time polymerase chain reaction (PCR) analysis showed selective up-regulation of IFN-g-inducible chemokines and chemokine receptors in CCC heart tissue. Collectively, these data suggest that bystander tissue destruction mediated by inflammatory cytokines (especially IFN-g) may play a role in CCC pathology. Although the chronic phase usually affects the heart, a subset of patients develop motor dysfunction of the gastrointestinal tract, essentially through the destruction of neurones of the enteric nervous system. It was discovered that antibodies raised in rabbit against a flagellum- associated surface protein on T. cruzi (FL-160) are cross- reactive with a 48-kDa protein found exclusively in nervous tissue . It was then found that antibodies raised against the amino terminus of FL-160 react to a different epitope on mammalian sciatic nerve than antibodies raised against the carboxyl terminus. The medical relevance of this apparent mimicry is uncertain, as the ability of human sera to react to FL-160 did not correlate with clinical disease. Other studies have also shown molecular similarity between T. cruzi antigens and antigens from mammalian nervous tissue.
In the United States, Lyme disease is caused by the tick-borne spirochete Borrelia burgdorfeii (Bb). Sixty per cent of untreated patients develop arthritis that can last for several years, mainly in large joints such as the knee. These patients have high titres of Bb-specific antibodies, and Bb DNA can be detected in the joint fluid by PCR. Treat- ment of these patients with antibiotics usually ameliorates the arthritis, which indicates that bystander inflammatory response to the spirochete is responsible for early Lyme arthritis. A subset of patients will progress from acute to chronic arthritis despite treatment with antibiotics and lack of detectable Bb DNA in synovial fluid. Antibiotic- resistant Lyme arthritis is associated with the MHC class II alleles human leucocyte antigen (HLA)-DRB1*0401, *0101 and *0404, indicating that its mechanism is T cell-mediated and distinct from acute Lyme arthritis. Cellular and humoral responses to outer surface protein A (OspA) of Bb develop in around 70% of patients with antibiotic-resistant Lyme arthritis, often at the beginning of prolonged arthritic episodes. T cell and humoral responses to OspA, but not to other spirochete antigens, were found to correlate with the presence or severity of arthritis. Synovial fluid mono- nuclear cells from patients with antibiotic-resistant arthritis produced IFN-g in response to both OspA and LFA1a, suggesting that mimicry between these two proteins may cause the inflammation associated with arthritis. LFA-1a has also been identified in the synovia of patients with antibiotic-resistant Lyme arthritis.
Neurological complications, including myelitis and peripheral neuropathy, can occur in 10–12% of untreated patients infected with Bb and can arise even after antibiotic treatment. Patients with chronic neuroborreliosis have been reported to have antibodies reactive to nerve axons in their serum, as well as antibodies and T cells specific for myelin basic protein (MBP) in spinal fluid. Patient serum that was reactive to axons and neuroblastoma cells was also cross-reactive with Bb flagellin.
Next, it was discovered that a mAb for flagellin was cross-reactive with human heat shock protein 60 and with neuroblastoma cell lines and slowed neurite outgrowth in culture. Antibody cross- reactivity has also been described between human central nervous system (CNS) proteins and Bb OspA.
Herpes simplex virus
Herpetic stromal keratitis (HSK) is caused by corneal infection by herpes simplex virus (HSV) and can lead to blindness. Whereas progression from epithelial infection to stromal keratitis is not prevented by antiviral drugs, the symptoms of HSK can be alleviated with immunosuppressive drugs such as corticosteroids, indicating that HSK is an autoimmune disease. Because of the difficulties associated with studying the disease in humans, much of the characterization of HSK has utilized murine infection with HSV-1. Within 72 h of infection proinflammatory cytokines IL-1 and IL-6 are produced, which leads to influx of neutrophils into the corneal stroma. In humans, T cells may cause pathology via bystander destruction.
Uveitis is a group of intra-ocular inflammatory diseases that are potentially blinding.
It is believed that many sub- groups of this disease are autoimmune-mediated, in part because of the strong association with certain HLA alleles. Humoral and cellular responses to the retinal antigens interphotoreceptor retinoid binding protein and S-antigen are well characterized in humans and animal models in rodents and primates (experimental autoimmune uveitis, EAU) are based on injecting these proteins in complete Freund’s adjuvant. Singh et al. identified a CD4+ T cell epitope in human S-antigen and several virus and Escherichia coli-derived peptides with sequence similarity rotavirus and bovine milk casein . In the same study, patients with uveitis were found to have an increased T cell and antibody response to S-antigen and the two identified mimics compared with healthy donors. Aside for a report of an outbreak of uveitis in children after echovirus infection, no pathogen has yet to be associated epidemiologically with uveitis.
Type I diabetes (T1D) results from autoimmune destruction of pancreatic cells by autoreactive T cells and/or inflammatory cytokines. Although there is a definite genetic component to T1D, the concordance rate in monozygotic twins is only approximately 40%, and epidemiological evidence suggests that pathogens play a role in development. Many different viruses have been associated with T1D development. Studies showed a higher incidence of T1D in people with congenital rubella and antibodies to pancreatic islet cells in rubella- infected patients. Similarly, Cytomegalovirus (CMV) was isolated from T1D patients and antibodies to pancreatic islet cells detected in CMV-infected patients. It was also noted that mumps infection often preceded the onset of T1D in children. A convincing study showed that Coxsackievirus (CVB4) isolated from the pancreas of acute-onset patients could induce diabetes upon transfer into susceptible mice. CVB4-specific IgM antibodies could be detected in children newly diagnosed with T1D. There is some evidence that CVB4 may cause T1D via molecular mimicry. T cells isolated from T1D patients reacted with both glutamic acid decarboxylase (GAD-65) (an identified autoantigen in T1D) and protein 2C in CVB4. However, another study did not observe similar T cell cross-reactivity, and yet another showed that cross-reactivity was observed in both diseased patients and healthy controls. As the virus has also been shown to replicate in exocrine pancreas, it is possible that cytokine release and HLA up-regulation following mumps virus infection may lead to autoimmunity.
Guillain–Barré syndrome (GBS) is a paralytic illness affecting both myelin and axons of the peripheral nervous system. Several studies have demonstrated anti-glycolipid antibodies in the serum of a proportion of patients .There are different clinical variants of the disease, which can correlate with the specific type of glycolipid targeted by the antibodies. Glycolipids found most commonly in neural tissues include the Gangliosides and Cerebrosides. Onset of GBS occurs days or weeks following an infection or immu- nization. Although several microorganisms have been associated with GBS development, Campylobacter jejuni is the most extensively studied pathogen as it is a common antecedent to GBS. In addition, there is mounting evidence suggesting that lipopolysaccharide (LPS) on the outer core of the bacteria can mimic host gangliosides. LPS from C. jejuni serotypes associated with GBS were shown to resemble human Several studies have shown that C. jejuni serotypes associated with GBS are more likely to contain ganglioside-like epitopes compared with serotypes isolated from C. jejuni-infected patients with gastroenteritis but no neurological symptoms, with one study linking ganglioside mimicry to specific GBS clinical subtypes.Patients infected with Mycoplasma pneumoniae prior to the development of GBS often have antibodies to galactocerebroside. These antibodies can cross-react with glycolipids on M. pneumoniae Associated antibodies to GM1 have also been reported. Similar to what occurs following C. jejuni infection, patients infected with Haemophilus influenzae can develop antibodies to bacterial LPS that are cross-reactive with ganglioside. The presence of a ganglioside-like structure on the surface of H. influenzae suggests that molecular mimicry may explain its association with GBS induction.
Multiple sclerosis (MS) is characterized by a loss of the myelin sheath surrounding axons in the CNS. Demyelination is associated with elevated levels of CD4+ T cells specific for major myelin proteins, and the disease is generally thought to be autoimmune. Although it is not known precisely what triggers the development of MS, it is well established that relapses or disease flares in patients diagnosed with the relapsing–remitting form of MS are often associated with exogenous infections, particular upper respiratory infections. In total, more than 24 viral agents have been linked to MS. Most of the associations have been circumstantial, but some studies have found evidence of specific pathogens in human tissue. Antigens from herpesvirus type 6 were found in MS plaques but not from tissues from other neurological disorders. Similarly, compared with CSF from patients with other neurological diseases, CSF from MS patients was shown to have higher levels of the bacteria Chlamydia pneumoniae. In vitro studies have also provided evidence linking MS and infectious agents. Despite the difficulty in linking MS to any one pathogen, the amount of epidemiological evidence reported over the years shows that environmental factors play a strong role in disease development, and suggests that a cumulative lifetime exposure to certain microorganisms can influence disease development. In addition, a recent study showed that the degree of concordance for monozygotic twins (generally reported at 40% or less) was influenced by environmental factors.
The immune system has evolved checks and balances to prevent the destruction of host tissue. It is perhaps not surprising that a strong immune response to an invading pathogen could disrupt this regulation and lead to autoimmunity. There is significant evidence suggesting that different classes of pathogens (bacteria, viruses and parasites) are involved in triggering or propagating self- reactive immune responses. However, the evidence for a definitive link for infection-induced autoimmunity is stronger for certain diseases than for others.
The argument for infection-induced pathology is much stronger for diseases associated with one or two specific pathogens than for diseases with multiple causal associations. For example, the fact that infection with C. jejuni is a common antecedent to GBS makes a strong argument that this disease is infection-triggered. In contrast, for diseases such as TID and MS that have been associated with dozens of pathogens, but none in particular, much more needs to be done to make a convincing case. The most compelling proof
would be the disappearance of symptoms with the clearance of the infection. This is the case in Lyme disease, where treatment with antibiotics alleviates acute arthritis. There are many ways a pathogen can cause disease even after the infection has been cleared. In these cases, epidemiological studies showing that people infected with a particular agent have an increased incidence of these diseases compared with people never infected, while not wholly definitive, would certainly strengthen the infection-induced autoimmunity argument.
In human autoimmune diseases, where direct evidence for a role for a particular pathogen is still uncertain, it is all the more important to have supporting animal models. The strongest support comes from animal models in which infection with the agent thought to induce disease in humans causes similar symptoms in animals, as exemplified by induction of heart disease in mice infected with T. cruzi and CVB and arthritis in mice infected with Bb. In other animal models, disease can be shown to be induced by priming with a pathogen-derived antigen, thus strengthening the argument for the involvement of that pathogen in the human disease. The ability to induce heart disease in rats primed with Streptococcal M protein is strong evidence that S. pyogenes causes heart disease in humans via molecular mimicry. Although the link between S. pyogenes infection and neurological disorders in humans is uncertain, at best, the fact that movement and behaviour disorders can be induced in mice primed with S. pyogenes homogenate also lends credibility to that theory. In cases where it is uncertain whether a disease pathology is actually autoimmune (such as uveitis and myocarditis following CVB infection), animal models have played a crucial role in elucidating the potential mechanisms of disease induction.
The heterogeneity of the human population, rather than the weakness of the data, may be in play in instances where the evidence linking infection and autoimmunity is tenuous or even conflicting. It is not difficult to imagine that some people may be more susceptible to developing autoimmune disease following a particular infection than others, or that mimic peptides derived from different infectious agents may be able to trigger a particular autoimmune disease depending on the ability of the infected individual to present various epitopes in the context of their various HLA molecules. Defining the genetic markers that predispose patients to different autoimmune diseases with a suspected infectious trigger would be an important contribution to defining the underlying disease pathogenesis.