Publisher Summary Trypanosoma cruzi (T. cruzi), the causative agent of Chagas disease, is a digenetic trypanosomatid, which circulates in the bloodstream of the vertebrate host as trypomastigotes and has an obligatory intracellular phase in which the parasite multiplies as amastigotes, which differentiate into trypomastigotes. From an immunological point of view, therefore, the parasite presents stages that are directly exposed to the effector elements of the immune response, such as antibodies and macrophages, as well as stages that are sequestrated within host cells. T. cruzi infection is characterized by an acute phase with large numbers of parasites and by a sub-patent chronic phase in which circulating and tissue stages are difficult to detect. The mechanisms involved in the resistance to the parasite and in the control of parasitism during the chronic phase are not known. Chagas disease is life-long and spontaneous cures do not occur. There is increasing evidence that auto-immune processes participate in the pathogenesis of the cardiac and digestive forms of the disease. These many obscure aspects and challenging problems explain the increasing use of T. cruzi as a model for the study of humoral and cellular immunity in both basic and applied immunology.

Immunity to Trypanosoma cruzi.

The role of CD4+ and CD8+ T cells in the surveillance of Trypanosoma cruzi or Trypanosoma brucei brucei was studied in mice which lacked CD4 or CD8 molecules and which were generated by embryonic stem cell technology. Whereas wild-type mice infected with T. cruzi (Tulahuen strain) displayed low levels of parasitemia and no mortality, striking increases in parasite growth and mortality occurred in both CD8- and CD4- mice. On the contrary, CD8- and, to a lesser degree, CD4- mice showed enhanced resistance to T. b. brucei. T-cell-dependent immunoglobulin G-specific responses were produced in CD8- but not CD4- mice. Normal T-cell proliferative responses were measured in both CD4- and CD8- mice. Interleukin-4 production after concanavalin A or anti-CD3 monoclonal antibody stimulation was strikingly enhanced in CD8- but not CD4- spleen cells, whereas gamma interferon production was normal in both CD4- and CD8- spleen cells. Spleen and lymph node cells from CD8- (but not CD4-) mice at 20 days postinfection with T. cruzi had higher levels of interleukin-4 mRNA than the wild-type controls, as shown in a competitive polymerase chain reaction assay. On the other hand, CD4- (but not CD8-) mice at 20 days postinfection with T. cruzi had lower levels of gamma interferon mRNA than the wild-type mice.

Differential susceptibilities of mice genomically deleted of CD4 and CD8 to infections with Trypanosoma cruzi or Trypanosoma brucei.

T lymphocytes provide a major line of defence against many protozoan parasites. The aim of this work was to determine the role of T-cell helper/inducer subset (T h/i) in the resistance to Trypanosoma cruzi in a murine model. The importance of natural killer (NK) cells in the resistance to the parasite was also evaluated. BALB/c and C57BL/6 mice were injected with either monoclonal antibodies against L3T4, Thy 1.2, NK1.1, or with a polyclonal rabbit antiserum against NK cells (anti-asialo GM-1). The effect of in vivo administration of these antibodies was tested in separate functional assays. After antibody treatment, mice were infected with a low dose of T. cruzi in the bloodstream form. Mice depleted of, or reduced in T, T h/i, or NK cell activity all developed higher parasitaemia and had higher mortality than their control counterparts. Mice injected with anti-L3T4 monoclonal antibodies were unable to generate a specific antibody response to the parasite. Treatment of mice with alpha/beta interferon, which is known to boost NK cell activity, resulted in an enhanced resistance to the parasite. Our data indicate that T h/i cells as well as NK cells are of vital importance in controlling parasitaemia and reducing mortality in T. cruzi-infected mice. Finally, we also demonstrate that the production of antibodies specific for T. cruzi is strictly T helper cell-dependent.

Role of T helper/inducer cells as well as natural killer cells in resistance to Trypanosoma cruzi infection.

Previous observations demonstrated severe thymocyte depletion in mice undergoing acute Chagas' disease These data led us to investigate the status of the thymic microenvironment in these animals Young adult C57BL/6 and C3H/HeJ mice were infected ip with 10(5) blood-derived trypomastigote forms of Trypanosoma cruzi (CL strain) and killed 7-14 days after infection Sera were then analyzed for thymic hormone (thymulin) levels, and frozen thymus sections were studied by immunohistochemistry for the expression of functional antigens (thymulin and Ia), the distribution of distinct thymic epithelial cell subsets and extracellular matrix components Infected mice exhibited a transient decrease in thymulin production and those with severe thymic atrophy showed a denser Ia-bearing cellular network In addition, an abnormal localization of the TR5 and CK18 antigens restricted to the medullary and cortical TEC subsets, respectively, was observed Furthermore, an increase in the basement membrane proteins was detected within thymic lobules We suggest that the thymic microenvironment is also affected during T cruzi infection, extending the concept that the thymus should be regarded as a target in Chagas' disease

Studies on the thymus in chagas' disease

In order to characterize the role played by CD4 + T lymphocytes in the immunopathology of acute Trypanosoma cruzi infection, we compared the numbers of blood and tissue parasites and the heart inflammatory reaction in normal and anti-CD4 antibody-treated C3H mice. Treatment of mice with anti-CD4 mAb during acute infection markedly inhibited T-helper-cell-dependent activities, as measured by peritoneal macrophage activation and immunoglobulin secretion by splenic B lymphocytes. After in vivo inactivation of helper T cells, the number of blood and tissue parasites significantly increased, while the inflammatory cellular infiltrates of heart muscles diminished. Our results indicate that CD4 + T lymphocytes play a dual role in the immunopathology of acute experimental Chagas' disease.

Parasitic load increases and myocardial inflammation decreases in Trypanosoma cruzi-infected mice after inactivation of helper T cells.

Neonatally thymectomized C3H or Rockland mice infected with trypanomastigotes of Trypanosoma cruzi had higher parasitaemia and shorter survival than their non-thymectomized counterparts. The appearance of antibodies detectable by IFT, IHA or DAT was delayed in neonatally thymectomized mice infected with epimastigotes of T. cruzi. Less thymectomized than non-thymectomized mice survived infection with epimastigotes. Our findings suggest that thymus-dependent, immune mechanisms are involved in controlling infection with T. cruzi in mice.

J.F. Janovsky

Papers

The effect of immuno-depression due to neonatal thymectomy on infections with trypanosoma cruzi in mice