Rheumatic fever: a streptococcal-induced autoimmune disease?

T cell subsets: an integral component in pathogenesis of rheumatic heart disease

Acute rheumatic fever (ARF) is a consequence of pharyngeal infection of group A streptococcal (GAS) infection. Carditis is the most common manifestation of ARF which occurs in 30-45% of the susceptible individuals. Overlooked ARF cases might further progress towards rheumatic heart disease (RHD) in susceptible individuals, which ultimately leads to permanent heart valve damage. Molecular mimicry between streptococcal antigens and human proteins is the most widely accepted theory to describe the pathogenesis of RHD. In the recent past, various subsets of T cells have been reported to play an imperative role in the pathogenesis of RHD. Alterations in various T cell subsets, viz. Th1, Th2, Th17, and Treg cells, and their signature cytokines influence the immune responses and are associated with pathogenesis of RHD. Association of other T cell subsets (Th3, Th9, Th22, and T_FH) is not defined in context of RHD. Several investigations have confirmed the up-regulation of adhesion molecules and thus infiltration of T cells into the heart tissues. T cells secrete both Th type 1 and type 2 cytokines and these auto-reactive T cells play a key role in progression of heart valve damage. In this review, we are going to discuss about the role of T cell subsets and their corresponding cytokines in the pathogenesis of RHD.

Molecular mimicry, bystander activation, or viral persistence: infections and autoimmune disease

Virus infections and autoimmune disease have long been linked. These infections often precede the occurrence of inflammation in the target organ. Several mechanisms often used to explain the association of autoimmunity and virus infection are molecular mimicry, bystander activation (with or without epitope spreading), and viral persistence. These mechanisms have been used separately or in various combinations to account for the immunopathology observed at the site of infection and/or sites of autoimmune disease, such as the brain, heart, and pancreas. These mechanisms are discussed in the context of multiple sclerosis, myocarditis, and diabetes, three immune-medicated diseases often linked with virus infections.

Immunopathological mechanisms in bacterial-host interactions

The ability of bacteria to cause immunopathological damage in the host may take a variety of forms. These pathways may be conveniently grouped under three major headings: (1) organisms that can cause damage via shared antigenic determinants between host and bacterium; (2) those organisms that suppress the host’s response; and (3) organisms that release substances with specific biological properties or have receptors for specific tissue sites. The group A streptococcus is among the most versatile of these bacteria because it appears that it may use all three pathways in various streptococcal-related disease states. In rheumatic fever and chorea it appears that cross-reactive antigens play a major role in inducing immunopathological damage in that there is both a heightened humoral and cellular reaction by the host to these cross-reactive determinants. Recent evidence also indicates that rheumatic fever individuals express certain B cell antigens that may be associated with susceptibility to the disease. In the other complications of streptococcal infections, namely poststreptococcal glomerulonephritis, the bacterium uses both suppression of the host’s immune response and the excretion of a particular protein common to all nephritis-associated strains to achieve its immunopathological damage. In this context, other examples of bacterial-host interactions will be discussed as evidence for the common pathways used by microbes to cause immunopathological damage in the host.

Antibody levels and in vitro lymphoproliferative responses to Streptococcus pyogenes erythrogenic toxin A and mitogen of patients with rheumatic fever

We investigated the in vitro lymphoproliferative responses to a streptococcal mitogen and erythrogenic toxin A of children with acute rheumatic fever (ARF) and patients with chronic rheumatic heart disease (CRHD). Antibody levels to the streptococcal products were also analyzed in the sera of those with ARF or chronic rheumatic heart disease as well as in the sera of children with streptococcal pharyngitis or poststreptococcal glomerulonephritis. Our results demonstrated that the individuals had depressed lymphoproliferative responses during the active stage of rheumatic fever. The depressed responses were not found either to be induced by time-sensitive mitogen-specific suppressor cells or to be related to a dose-response phenomenon. On the other hand, antibody levels to the extracellular mitogens were significantly elevated in the sera of children with ARF compared with the levels in the rest of the groups. The hyperresponsiveness noted among children with ARF was found to be at a quantitative level and was not due to recognition of more epitopes, as determined by Western blotting (immunoblotting). The profile of immune responsiveness in children with ARF to the streptococcal extracellular mitogens is discussed in relation to the pathogenesis of disease.

Role of the immune system in the etiology of rheumatic fever

In order to summarize all the data regarding the immunological findings in RF we have elaborated the following theory about the pathogenesis of RF (fig. 1). The disease will develop only after repeated beta-hemolytic group A streptococcal throat infection in susceptible individuals. In these people, who have a specific immune response to repeated streptococcal infection due to their DR region, a decrease in suppressor activity will develop. This will lead to an increased specific humoral and cellular immune response to streptococcal antigen which cross-reacts with specific human tissue as the heart or the brain. Alternatively or concomitantly specific autoantibodies against these tissues are made. These antibodies together with the cellular response will cause the clinical manifestation seen in RF. This is only a proposed theory and still much more work has to be done to solve the enigma regarding the pathogenesis of RF.