[ Scientific Activities - Actividades Científicas ]"Immunopathogenic aspects of Chagas Heart Disease" or Understanding the Pathogenesis of Chagas Disease Cardiomyopathy Towards the end of the Millenium Dr. Edecio Cunha-Neto
Heart Institute (InCor)
School of Medicine
University of São Paulo
São Paulo, Brazil
The pathogenesis of chronic Chagasdisease cardiomyopathy (CCC) is under intense debate, and the susceptibility factors leading 30% of infected patients to undergo develop heart disease are largely unknown 1,2. Early reports of the scarcity of T. cruzi at the heart lesions have implicated the heart-inflammatory infiltrate and diffuse myocarditis as the ultimate effectors of heart tissue damage 3,4, but the identification of T. cruzi antigen and DNA in CCC hearts by immunohistochemical and PCR techniques 5,6 led some authors to postulate a direct role for heart parasitism in heart damage. Recent studies, however, showed that T. cruzi DNA could be detected in hearts of both CCC and ASY individuals 7,8. Together with the findings that low-grade T. cruzi parasitism can be found in several other organs in the presence of very sparse inflammatory foci 3 and in the absence of any major functional damage 9,10, data may suggest that the mere presence of T. cruzi in tissue may not be a sufficient stimulus for diffuse myocarditis and severe heart damage leading to dilated cardiomyopathy.
The autoimmune hypothesis of pathogenesis postulates that inflammatory T lymphocytes in the heart should recognize a heart component as a result of chronic T. cruzi infection, and mount a tissue-damaging delayed-type hypersensitivity response. In murine models of chronic Chagasdisease myocarditis, tissue lesions were induced by transfer of CD4+ T cells from infected animals 11. Given this finding and the histopathology observations, one should look for a T cell capable of recognizing a heart-specific antigen and producing inflammatory cytokines in response to it. Furthermore, such a pathogenic T cell should occur more often among CCC than ASY patients. We will review evidence for the involvement of immune mediated tissue damage and autoimmune recognition in Chagas disease cardiomyopathy.
In search of defined, heart-specific antigenic targets
An anti-heart autoimmune response could be triggered either by the display of intracellular sequestered heart proteins secondary to parasite-driven myocarditis during acute infection, or by molecular mimicry with some T. cruzi antigen bearing antigenic similarity to a given heart protein; molecular mimicry is the most established autoimmunity-inducing mechanism. Tables I, II, III, IV, V list the autoantigens and molecular mimicry systems that have been described in the study of experimental or human Chagas disease.
Molecular mimicry between cardiac myosin and T. cruzi proteins in CCC
Our group has been studying the role of cardiac myosin, the most abundant heart protein, as a target antigen in experimental and human Chagas disease and CCC. Cardiac myosin was the first defined heart-specific antigen recognized by both CD4+ T cells and autoantibodies from T. cruzi infected mice suggested a role for anti-cardiac myosin autoimmunity in CCC pathogenesis (12,13; reviewed in 1). To test whether cardiac myosin was an important target antigen in human CCC, our group studied anti-cardiac myosin antibodies in sera from CCC and ASY patients. We were able to identify crossreactive antibodies between cardiac myosin heavy chain and the recombinant, tandemly repetitive protein B13 from T. cruzi 14 in 100% of CCC sera but only 14% of ASY sera 15. Since heart-infiltrating T cells seem to be the ultimate effectors of heart tissue damage, we studied T cell clones obtained from T cruzi-free heart biopsy from a CCC patient. Such T cell clones crossreactively recognize cardiac myosin and T. cruzi antigen B13 16. Furthermore, T cells infiltrating the heart of CCC patients possess an inflammatory T1-type cytokine profile with a predominance of Interferon-g (IFN-g) and tumor necrosis factor-a (TNF-a) but no Interleukin-4 (IL-4), an anti-inflammatory T2-type cytokine 17. Together, results suggest that heart damage is secondary to inflammatory cytokines and a delayed-type hypersensitivity process started or maintained by heart-crossreactive T cells.
In order to compare antigen recognition among CCC and ASY patients, we studied the peripheral blood from patients in blood groups. Even though peripheral blood lymphocytes do not respond to cardiac myosin, sensitization of lymphocytes from a non-Chagasic individual with B13 protein elicits cardiac myosin-crossreactive T cell clones 18, suggesting that along T. cruzi infection, it is B13 sensitization that elicits cardiac myosin-responsive T cells. T cell responses to T. cruzi B13 protein were indistinguishable among CCC and ASY individuals; T cells recognize B13 protein in an MHC class II-restricted fashion. Given the fact that tandemly repetitive units of B13 protein show sequence variation19,20, we recently tested the recognition of all the variant epitopes by CCC and ASY individuals. Variant B13 epitopes recognized by CCC patients were different from those recognized by ASY 21. It is tempting to speculate that the differential anti-B13 T cell recognition repertoire of CCC patients is more prone to crossreactively recognize cardiac myosin than that of ASY patients with consequences for disease progression.
Cytokine production in human Chagasdisease
We studied whether there would be a difference in inflammatory T1-type (IFN-g) and anti-inflammatory T2-type (IL-4) cytokine production among CCC and ASY patients, as well as normal controls. Among both CCC and ASY groups, B13-induced and polyclonal mitogen (PHA)-induced cytokine responses were characterized by high levels of IFN-g and negligible IL-4 production, essentially the same picture observed among heart-infiltrating T cells 22. PBMC from normal individuals, however, respond to T. cruzi B13 antigen (probably sensitized to an environmental crossreactive antigen) in a dichotomous manner: roughly 40% produce Interleukin-4 while other 40% produce low levels of IFN-g (unpublished observations), indicating an important heterogeneity among individuals prior to T. cruzi infection which could potentially affect the control of parasitism upon early infection. Furthermore, a higher frequency of PHA-stimulated IFN-g producing peripheral T cells has been found among CCC than ASY or N individuals 21. The systemic shift to a T1-type cytokine profile in CCC and ASY patients is probably related to the Interleukin-12 inducing activity of trypomastigote mucin glycoconjugates along chronic T. cruzi infection 23. Recent results from our lab have shown that allelic forms of the TNF-a gene associated with high cytokine production frequently appear in CCC but not ASY patients 24. This may be relevant to CCC pathogenesis since it has been observed that end-stage heart failure patients of non-Chagasic aetiology display increased serum levels of TNF-a.
Data in the literature are consistent with the view that T. cruzi infection may trigger potentially pathogenic T cells, as has been postulated for other infectious agents in organ-specific autoimmune diseases. The provision of the initial antigenic stimulus (e.g. T. cruzi B13 protein) in the presence of costimulatory signals and IL-12 production, generates "experienced", HLA-restricted, B13-specific, IFN-g producing CD4+ T cells in a large majority of infected individuals. These "experienced" T cells are able to migrate to heart tissue, where myocardial protein (e.g. myosin) epitopes are constitutively presented by class II MHC molecules on heart interstitial macrophages, and potentially can start tissue damage. However, the presence of such IFN-g producing B13-specific T cells in the peripheral blood of T. cruzi infected ASY individuals without heart disease indicates that their presence may be necessary but not sufficient for the establishment of diffuse myocarditis and heart damage. Our data are consistent with a situation where the generation of a heart-damaging T cell infiltrate could only occur in a subset of T. cruzi-infected individuals whose T cell repertoire allows crossreactive recognition of heart antigen (e.g. cardiac myosin) by T. cruzi (e.g. B13)-specific T cells. The possession of such a crossreactive T cell repertoire could be the mechanism underlying differential susceptibility to CCC progression. It can be hypothesized that the generation of a pathogenic T cell infiltrate in Chagas disease cardiomyopathy is a multistep process where pre-infection immunological features of the host, like the cytokine balance or the crossreactive T cell recognition repertoire, are major determinants. Systemic, rather than local, chronic T. cruzi infection may function as a trigger/booster, generating pathogenic T cells in susceptible individuals that may subsequently migrate to the heart and induce tissue damage. Furthermore, progression to tissue damage may be dependent on the genetic profile of the individual, especially genes related to the immune system and heart function (e.g. TNF-a and other cytokines, HLA, TCR, along with gene loci whose mutational variants cause dilated cardiomyopathy.
Perspectives in therapy: "intelligent drugs" and immunomodulation
Chemotherapy with available anti-T. cruzi drugs does not attain complete parasite clearance from adult chronically infected patients. Persistence of T. cruzi in chronically infected hosts is probably important for the maintenance of the inflammatory status that is relevant to heart damage, and may be a result of escape mechanisms that allow intracellular parasites to survive immune attack., like blockade of antigen presentation or cytotoxic mechanisms, as described in viruses. The precise identification of such escape mechanisms from intracellular forms may allow subsequent drug targeting which may lead to the cure of chronic T. cruzi infection.
The identification of the relevant autoantigenic target in CCC, and the T1-type cytokine profile observed in CCC heart lesions may allow the use of antigen-specific tolerance induction/immune deviation therapy (in which cytokines produced in the the target organ switch from inflammatory, T1, to anti-inflammatory, T2) to block the heart lesion process without interfering with the protective immune response against the infectious agent. Positive results in human autoimmune diseases using oral tolerance with organ-specific antigens to induce immune deviation 25.
Hipothetical sequence of events in a T. cruzi infected individual predisposed towards developing CCC
Events that may control development of CCC among T. cruzi infected individuals
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