A Trypanosoma cruzi membrane protein shares an epitope with a lymphocyte activation antigen and induces crossreactive antibodies

Myeloid-Derived Suppressor Cells in Trypanosoma cruzi Infection

Myeloid-derived suppressor cells (MDSCs) are immature heterogeneous myeloid cells that expand in pathologic conditions as cancer, trauma, and infection. Although characterization of MDSCs is continuously revisited, the best feature is their suppressor activity. There are many markers for MDSC identification, it is distinctive that they express inducible nitric oxide synthase (iNOS) and arginase 1, which can mediate immune suppression. MDSCs can have a medullary origin as a result of emergency myelopoiesis, but also can have an extramedullary origin. Early studies on Trypanosoma cruzi infection showed severe immunosuppression, and several mechanisms involving parasite antigens and host cell mediators were described as inhibition of IL-2 and IL-2R. Another mechanism of immunosuppression involving tumor necrosis factor/interferon γ-dependent nitric oxide production by inducible nitric oxide synthase was also described. Moreover, other studies showed that nitric oxide was produced by CD11b⁺ Gr-1⁺ MDSCs in the spleen, and later iNOS and arginase 1 expressed in CD11b⁺Ly6C⁺Ly6G^lo monocytic MDSC were found in spleen and heart of T. cruzi infected mice that suppressed T cell proliferation. Uncontrolled expansion of monocytic MDSCs leads to L-arginine depletion which hinders nitric oxide production leading to death. Supplement of L-arginine partially reverts L-arginine depletion and survival, suggesting that L-arginine could be administered along with anti-parasitical drugs. On the other hand, pharmacological inhibition of MDSCs leads to death in mice, suggesting that some expansion of MDSCs is needed for an efficient immune response. The role of signaling molecules mediating immune suppression as reactive oxygen species, reactive nitrogen species, as well as prostaglandin E2, characteristics of MDSCs, in T. cruzi infection is not fully understood. We review and discuss the role of these reactive species mediators produced by MDSCs. Finally, we discuss the latest results that link the SLAMF1 immune receptor with reactive oxygen species. Interaction of the parasite with the SLAMF1 modulates parasite virulence through myeloid cell infectivity and reactive oxygen species production. We discuss the possible strategies for targeting MDSCs and SLAMF1 receptor in acute Trypanosoma cruzi infection in mice, to evaluate a possible translational application in human acute infections.

Autoimmunity in Chronic Chagas Disease: A Road of Multiple Pathways to Cardiomyopathy?

Chagas disease (CD), a neglected tropical disease caused by the protozoan Trypanosoma cruzi, affects around six million individuals in Latin America. Currently, CD occurs worldwide, becoming a significant public health concern due to its silent aspect and high morbimortality rate. T. cruzi presents different escape strategies which allow its evasion from the host immune system, enabling its persistence and the establishment of chronic infection which leads to the development of chronic Chagas cardiomyopathy (CCC). The potent immune stimuli generated by T. cruzi persistence may result in tissue damage and inflammatory response. In addition, molecular mimicry between parasites molecules and host proteins may result in cross-reaction with self-molecules and consequently in autoimmune features including autoantibodies and autoreactive cells. Although controversial, there is evidence demonstrating a role for autoimmunity in the clinical progression of CCC. Nevertheless, the exact mechanism underlying the generation of an autoimmune response in human CD progression is unknown. In this review, we summarize the recent findings and hypotheses related to the autoimmune mechanisms involved in the development and progression of CCC.

Autoimmunity

The scarcity of Trypanosoma cruzi in inflammatory lesions of chronic Chagas disease led early investigators to suggest that tissue damage had an autoimmune nature. In spite of parasite persistence in chronic Chagas disease, several reports indicate that inflammatory tissue damage may not be correlated to the local presence of T. cruzi. A significant number of reports have described autoantibodies and self-reactive T cells, often cross-reactive with T. cruzi antigens, both in patients and in animal models. Evidence for a direct pathogenetic role of autoimmunity was suggested by the development of lesions after immunization with T. cruzi antigens or passive transfer of lymphocytes from infected animals, and the amelioration of chronic myocarditis in animals made tolerant to myocardial antigens. Autoimmune and T. cruzi-specific innate or adaptative responses are not incompatible or mutually exclusive, and it is likely that a combination of both is involved in the pathogenesis of chronic Chagas disease cardiomyopathy. The association between persistent infection and autoimmune diseases-such as multiple sclerosis or diabetes mellitus-suggests that post-infectious autoimmunity may be a frequent finding. Here, we critically review evidence for autoimmune phenomena and their possible pathogenetic role in human Chagas disease and animal models, with a focus on chronic Chagas disease cardiomyopathy.

Induction of cardiac autoimmunity in Chagas heart disease: A case for molecular mimicry

Up to 18 million of individuals are infected by the protozoan parasite Trypanosoma cruzi in Latin America, one third of whom will develop chronic Chagas disease cardiomyopathy (CCC) up to 30 years after infection. Cardiomyocyte destruction is associated with a T cell-rich inflammatory infiltrate and fibrosis. The presence of such lesions in the relative scarcity of parasites in the heart, suggested that CCC might be due, in part, to a postinfectious autoimmune process. Over the last two decades, a significant amount of reports of autoimmune and molecular mimicry phenomena have been described in CCC. The authors will review the evidence in support of an autoimmune basis for CCC pathogenesis in humans and experimental animals, with a special emphasis on molecular mimicry as a fundamental mechanism of autoimmunity.

Structural basis of the cross-reaction between an antibody to the Trypanosoma cruzi ribosomal P2beta protein and the human beta1 adrenergic receptor

Antibodies from patients with Chagas heart disease and monoclonal antibodies (or mAb) to the carboxy-terminal end (B cell epitope R13) of the ribosomal P2beta protein of Trypanosoma cruzi (TcP2beta) cross-react with the beta1 adrenergic receptor (beta1-AR). Two single-chain Fv fragments (scFv) C5 and B7 derived from the variable regions of the anti-R13 mAb 17.2 were expressed. scFv C5 was a dimer and bound to TcP2beta with an affinity of K(d) = 8 nM, whereas scFv B7 was monomeric and had less affinity than scFv C5 for TcP2beta, K(d) = 46 nM. The affinity constant of scFv C5 to the second extracellular loop of the human beta1-AR was of 10 microM. Moreover, scFv C5 induced an increase in cAMP levels of CHO-K cells transfected with the human beta1-AR; scFv B7 had no effect but blocked isoproterenol stimulation. The agonist-like activity of scFv C5 and the antagonist activity of scFv B7 were both confirmed in vivo on heart beating frequency after their passive transfer to mice. Molecular modeling of the variable region of mAb 17.2 indicated which amino acids were likely to be involved in recognizing both peptide EDDDMGFGLF, derived from the R13 epitope of TcP2beta, and peptide ESDEARRCYN from the second extracellular loop of the human beta1-AR. It is plausible that the recently described cross-reaction of mAb 17.2 with rhodopsin can also be explained by this model. The physiological effects of this type of anti-T. cruzi antibodies may increase the liability of patients with Chagas disease.

Trypanosoma cruzi and mammalian heart cross-reactive antigens

Monoclonal antibodies produced against T. cruzi microsomal fraction (Mc) were used to investigate the presence of molecular mimicry between the parasite and mammalian tissues. A total of 42 cell lines secreting anti-Mc antibodies were characterized and selected by ELISA, dot blotting and Western blotting assays. Twenty seven supernatants reactive with Mc and/or parasite cytosol (CS) also reacted with human myocardial and/or skeletal muscle antigens by dot blotting assay. Twelve among those cross-reactive hybridomes, which happen to be all of the IgM isotype and to recognize structures on the surface and/or flagellum of the parasite, were selected for cell cloning. Western blotting analysis of these 12 monoclonal antibodies revealed that they mainly recognized bands of 65, 45, 34 and 27 kDa on myocardium and bands of 71, 59, 44 and 30-27 kDa on skeletal muscle. Moreover, seven among them, when assayed by immuno-histochemistry on human and hamster myocardium and skeletal muscle, recognized cytoplasmic antigens, although the monoclonal antibodies 5F2 and 5A9B11 did also bind to the vessel muscle layer. Competitive assays proved the specificity of tissue structures recognition by these monoclonal antibodies. Moreover, this reactivity resulted to be organ specific as they failed to react on lung, stomach and kidney samples. These results demonstrate the cross-reactivity of mammalian and parasite antigens, thus supporting the possibility that molecular mimicry plays a central role in the development of chagasic cardiomyopathy.

Pathogenic autoantibodies are routinely generated during the response to foreign antigen: a paradigm for autoimmune disease

The immune system's ability to distinguish self and nonself is essential for both host defense against foreign agents and protection of self-antigens from autoimmune destruction. Such discrimination is complicated by extensive structural homology shared between foreign and self antigens. One hypothesis to explain the development of an autoimmune response is that some B cells activated by foreign antigen acquire, through somatic mutation, specificity for both the eliciting foreign antigen and self antigen. If such clones arise frequently, there must be a mechanism for their elimination. We have analyzed the extent of autoreactivity arising in a nonautoimmune host during the response to a foreign antigen. To overcome the process of apoptosis in primary B cells that might routinely eliminate autoreactive clones, we generated B-cell hybridomas from spleen cells of immunized mice by using a fusion partner constitutively expressing bcl-2. Multiple lines were obtained that recognize simultaneously the hapten phosphorylcholine and the self antigen double-stranded DNA. This dual specificity was not present early but was detected by day 10 after immunization. Some of these cross-reactive antibodies deposit in kidneys in a pattern similar to what is seen in autoimmune disease. These results demonstrate that autoantibodies arise at a high frequency as part of a response to foreign antigen. It has previously been shown that autoreactivity is regulated by central deletion; these data demonstrate a need for negative selection in peripheral lymphoid organs also, to regulate autoantibodies acquiring their self-specificity by somatic mutation.

Leishmania major parasites share an epitope with the murine CD3-T cell receptor complex

After immunization of BALB/c mice with a low molecular mass fraction (FrD; < or = 31 kDa) isolated from a soluble extract of Leishmania major promastigotes, a panel of monoclonal antibodies (mAb) was obtained. One of these antibodies (mAb 9C) recognized a cytosol-associated antigen from L. major of approximately 21 kDa as shown by Western blot and immunoprecipitation. In addition, mAb 9C reacted with surface structures of murine splenic T cells and T cell clones. Reactivity was confined to murine cells, but was not strain restricted. Immunoprecipitation studies and surface-labeling experiments with CD4+ T cell clones and the T cell receptor (TCR)-CD3-T cell line TG40 transfected with V alpha/beta chains from human TCR and concomitant co-expression of murine CD3 suggested that mAb 9C binds to an epitope located within the murine CD3-TCR complex. In addition, mAb 9C induced strong T cell proliferation. We conclude that L. major parasites share an epitope with the murine CD3-TCR complex which is functionally important for T cell activation.