Klebsiella pneumoniae and HLA B27-associated diseases of Reiter's syndrome and ankylosing spondylitis

Examining pathogenic concepts of autoimmune hepatitis for cues to future investigations and interventions

BACKGROUND

Multiple pathogenic mechanisms have been implicated in autoimmune hepatitis, but they have not fully explained susceptibility, triggering events, and maintenance or escalation of the disease. Furthermore, they have not identified a critical defect that can be targeted. The goals of this review are to examine the diverse pathogenic mechanisms that have been considered in autoimmune hepatitis, indicate investigational opportunities to validate their contribution, and suggest interventions that might evolve to modify their impact. English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed. Genetic and epigenetic factors can affect susceptibility by influencing the expression of immune regulatory genes. Thymic dysfunction, possibly related to deficient production of programmed cell death protein-1, can allow autoreactive T cells to escape deletion, and alterations in the intestinal microbiome may help overcome immune tolerance and affect gender bias. Environmental factors may trigger the disease or induce epigenetic changes in gene function. Molecular mimicry, epitope spread, bystander activation, neo-antigen production, lymphocytic polyspecificity, and disturbances in immune inhibitory mechanisms may maintain or escalate the disease. Interventions that modify epigenetic effects on gene expression, alter intestinal dysbiosis, eliminate deleterious environmental factors, and target critical pathogenic mechanisms are therapeutic possibilities that might reduce risk, individualize management, and improve outcome. In conclusion, diverse pathogenic mechanisms have been implicated in autoimmune hepatitis, and they may identify a critical factor or sequence that can be validated and used to direct future management and preventive strategies.

Molecular Mimicry, Autoimmunity, and Infection: The Cross-Reactive Antigens of Group A Streptococci and their Sequelae

The group A streptococci are associated with a group of diseases affecting the heart, brain, and joints that are collectively referred to as acute rheumatic fever. The streptococcal immune-mediated sequelae, including acute rheumatic fever, are due to antibody and cellular immune responses that target antigens in the heart and brain as well as the group A streptococcal cross-reactive antigens as reviewed in this article. The pathogenesis of acute rheumatic fever, rheumatic heart disease, Sydenham chorea, and other autoimmune sequelae is related to autoantibodies that are characteristic of autoimmune diseases and result from the immune responses against group A streptococcal infection by the host. The sharing of host and streptococcal epitopes leads to molecular mimicry between the streptococcal and host antigens that are recognized by the autoantibodies during the host response. This article elaborates on the discoveries that led to a better understanding of the pathogenesis of disease and provides an overview of the history and the most current thought about the immune responses against the host and streptococcal cross-reactive antigens in group A streptococcal sequelae.

Molecular mimicry: its evolution from concept to mechanism as a cause of autoimmune diseases

On a clonal level, certain antibodies and T cells can interact with dissimilar antigens found in microbes and in host cells. More than 5% of over 800 monoclonal antibodies derived from multiple RNA and DNA viruses, as well as from a large number of T cell clones, engage in such interactions. Several of these cross-reactions, which we termed molecular mimicry, are against unique host proteins involved in autoimmune responses and diseases. Thus, molecular mimicry initiated as a host response to a virus or a microbial infection, but alternatively cross-reacting with an appropriate host-antigen, can be a mechanism for instigating an autoimmune disease. Molecular mimicry provides an explanation for the genetic observation that identical twins rarely manifest the same autoimmune disease and the documented epidemiologic evidence that microbial and/or viral infections often precede autoimmune disorders.

Molecular mimicry and auto-immunity

The term "molecular mimicry" was coined by R. Damian in 1964, who was first to suggest that antigenic determinants of micro-organisms may resemble antigenic determinants of their host. Damian suggested that this similarity served as a defense mechanism of a microorganism from the host's immune system and prevented the development of immune response to the micro-organism, thereby protecting it from host defense. Years later, the term "molecular mimicry" was attributed a different meaning-namely, antigenic determinants of microorganisms might elicit an auto-immune response that harms the host. The concept of molecular mimicry is based on a structural similarity between a pathogen or metabolite and self-structures. The similarity could be expressed as shared amino acid sequences (linear or mimotope) or similar conformational structure between a pathogen and self-antigen. "Molecular mimicry" has become a very popular explanation for the frequent association of infection with auto-immune disease.

Molecular mimicry and immune-mediated diseases

Molecular mimicry has been proposed as a pathogenetic mechanism for autoimmune disease, as well as a probe useful in uncovering its etiologic agents. The hypothesis is based in part on the abundant epidemiological, clinical, and experimental evidence of an association of infectious agents with autoimmune disease and observed cross-reactivity of immune reagents with host 'self' antigens and microbial determinants. For our purpose, molecular mimicry is defined as similar structures shared by molecules from dissimilar genes or by their protein products. Either the molecules' linear amino acid sequences or their conformational fits may be shared, even though their origins are as separate as, for example, a virus and a normal host self determinant. An immune response against the determinant shared by the host and virus can evoke a tissue-specific immune response that is presumably capable of eliciting cell and tissue destruction. The probable mechanism is generation of cytotoxic cross-reactive effector lymphocytes or antibodies that recognize specific determinants on target cells. The induction of cross-reactivity does not require a replicating agent, and immune-mediated injury can occur after the immunogen has been removed a hit-and-run event. Hence, the viral or microbial infection that initiates the autoimmune phenomenon may not be present by the time overt disease develops. By a complementary mechanism, the microbe can induce cellular injury and release self antigens, which generate immune responses that cross-react with additional but genetically distinct self antigens. In both scenarios, analysis of the T cells or antibodies specifically engaged in the autoimmune response and disease provides a fingerprint for uncovering the initiating infectious agent.

Scleritis-associated uveitis

PURPOSE

Anterior uveitis may accompany scleritis. This study was undertaken to analyze the incidence, characteristics, and meaning of uveitis in the course of scleritis.

METHODS

Patient characteristics, scleritis type, ocular complications, and specific systemic diseases were evaluated in patients with scleritis-associated uveitis; comparisons were made between patients with scleritis-associated uveitis and patients with scleritis without uveitis.

RESULTS

Seventy three (42%) of 172 patients with scleritis had anterior uveitis. Scleritis-associated uveitis ranged from mild to moderate intensity and always was related to the presence of active scleritis. Patients with scleritis-associated uveitis had more necrotizing scleritis (37%, P = 0.0001), decrease in vision (49%, P = 0.0046), peripheral ulcerative keratitis (22%, P = 0.0095), and glaucoma (19%, P = 0.0313) than did patients with scleritis without uveitis. Patients with scleritis-associated uveitis did not have any specific associated systemic disease more often than did patients with scleritis without uveitis.

CONCLUSION

Extension of scleral inflammation to the anterior uveal tract is a consequence of a more severe disease with possible ocular complications that may cause progressive visual loss. The occurrence of anterior uveitis in the course of scleritis entails a poor ocular prognosis. The authors believe, therefore, that the anterior uveal tract should be evaluated at every follow-up visit of a patient with scleritis, so that emergence of this important prognostic condition (anterior uveitis) may be detected promptly and systemic therapy instituted appropriately.

Using transgenic mouse models to dissect the pathogenesis of virus-induced autoimmune disorders of the islets of Langerhans and the central nervous system

Viruses have often been associated with autoimmune diseases. One mechanism by which self-destruction can be triggered is molecular mimicry. Many examples of cross-reactive immune responses between pathogens and self-antigens have been described. This review presents two transgenic models of autoimmune disease induced by a virus through activation of anti-self lymphocytes. Viral antigens are expressed as transgenes either in beta-cells of the pancreas or in the oligodendrocytes of the CNS. Infection by a virus encoding the same gene activated autoreactive T cells that cleared the viral infection, and as a consequence of transgene expression resulted in organ-specific autoimmune disease. In both transgenic mouse models, autoreactive lymphocytes that escaped thymic negative selection were present in the periphery. Several factors are described that play a role in the regulation of the self-reactive process precipitated by a viral infection. These include the quantity of activated autoreactive T cells, the affinity of these T cells, the number of memory T cells generated following primary infection, costimulation by accessory molecules, and the types and locations of cytokines produced. In addition, unique barriers exist in target tissues that prevent or suppress autoreactive responses and define to a large extent the outcome of disease. Restimulation of autoreactive memory lymphocytes may be required to bypass these barriers and enhance autoimmune disease. Therapy directed at modifying these factors can reduce and even prevent autoimmune disease after it has been initiated.

Common peptide epitope in mumps virus nucleocapsid protein and MHC class II-associated invariant chain

The present study describes a 7 amino acid-long sequence (YQQQGRL) which is identical in HLA-associated invariant chain and mumps virus nucleocapsid protein and is additionally followed by one conservative amino acid pair. As such a long amino acid homology is extremely rare in two evolutionarily unrelated proteins the possibility that it could induce immunological cross-reactivity was evaluated. Several antigenicity indices suggested high antigen potential within this region. Synthetic peptides containing this sequence were reactive with 31% of monoclonal antibodies specific for mumps virus nucleocapsid protein in ELISA. High antibody levels against this epitope were found in 7% of mumps-seropositive human sera and antibody levels clearly increased after natural mumps infections and mumps vaccinations. Rabbit antibodies raised against a synthetic invariant chain peptide AYF-LYQQQGRLDKL-C reacted with corresponding nucleocapsid peptide RFAKYQQQGRLEAR-C and antibodies against the nucleocapsid peptide reacted with the invariant chain peptide. Rabbit antibodies against the invariant chain peptide also reacted with nucleocapsid molecules in formaldehyde-fixed mumps virus-infected cells, and antibodies against the nucleocapsid peptide reacted with invariant chains expressed in methanol-fixed cells. One monoclonal antibody specific for the nucleocapsid molecule also reacted with cells expressing invariant chains. In immunoprecipitation rabbit antibodies against the invariant chain peptide bound to invariant chains while antibodies against the nucleocapsid peptide did not. The results suggest that there is antigenic similarity in mumps virus nucleocapsid molecule and HLA-associated invariant chain which may cause immunological cross-reactivity between these molecules.