The importance of apoptosis for immune regulation in Chagas disease

Host cell apoptosis plays an important immune regulatory role in parasitic infections. Infection of mice with Trypanosoma cruzi, the causative agent of Chagas disease, induces lymphocyte apoptosis. In addition, phagocytosis of apoptotic cells stimulates the growth of T. cruzi inside host macrophages. In spite of progress made in this area, the importance of apoptosis in the pathogenesis of Chagas disease remains unclear. Here we review the evidence of apoptosis in mice and humans infected with T. cruzi. We also discuss the mechanisms by which apoptosis can influence underlying host responses and tissue damage during Chagas disease progression.

Inhibition of caspase-8 activity reduces IFN-gamma expression by T cells from Leishmania major infection

Following infection with Leishmania major, T cell activation and apoptosis can be detected in draining lymph nodes of C57BL/6-infected mice. We investigated the mechanisms involved in apoptosis and cytokine expression following Tcellactivation. After two weeks of infection, apoptotic T cells were not detected in draining lymph nodes but activation with anti-CD3 induced apoptosis in both CD4 and CD8 T cells. Treatment with anti-FasLigand, caspase-8 or caspase- 9 inhibitors did not block activation-induced T-cell death. We also investigated whether the blockade of caspase-8 activity would affect the expression of type-1 or type-2 cytokines. At early stages of infection, both CD4 and CD8 T cells expressed IFN-gamma upon activation. Treatment with the caspase-8 inhibitor zIETD-fmk (benzyl-oxycarbonyl-Ile- Glu(OMe)-Thr-Asp(OMe)-fluoromethyl ketone) reduced the proportion of CD8 T cells and IFN-gamma expression in both CD4 and CD8T cells. We conclude that a non apoptotic role of caspase-8 activity may be required for T cell-mediated type-1 responses during L. major infection.

A ativação e a morte por apoptose de linfócitos T foram observadas em linfonodos drenantes de camundongos C57BL/6 infectados com Leishmania major. Investigamos os mecanismos envolvidos na apoptose e na expressão de citocinas após a ativação de linfócitos T. Após duas semanas de infecção, embora as células apoptóticas ainda não sejam detectadas em linfonodos drenantes, células T CD4 e CD8 sofrem apoptose após ativação com anti-CD3. O tratamento com anticorpo antagonista anti-Ligante de Fas, ou com inibidores das caspases-8 e 9, não bloqueou a morte induzida por ativação das células T. Investigamos também se a inibição da atividade da caspase-8 poderia afetar a expressão de citocinas tipo-1 ou tipo-2. Nos estágios iniciais da infecção, células T CD4 e CD8 de animais infectados com L. major expressaram IFN-gama após ativação. O tratamento com o inibidor de caspase-8 zIETD (benzoil-oxicarbonil-Ile-Glu(OMe)-Thr-Asp(OMe)-fluorometilcetona) durante a estimulação de células T reduziu a proporção de células T CD8 e a expressão de IFN-gama por células T CD4 e CD8. Concluimos que a atividade não apoptótica de caspase-8 pode ser necessária para o estabelecimento da imunidade mediada por células T durante a infecção por L. major.

Control of differential CD4+ T lymphocyte behavior in vitro is dictated by the pattern of CD molecule involvement

T lymphocyte responses in vitro are not all-or-none choices to environmental stimulation, but follow at least three distinct patterns: full activation and expansion, anergy induction, and receptor-mediated suicide by apoptosis. In vitro model systems were devised to investigate the differential control of T cell responses by surface CD activation molecules, CD4+ T cells from T. cruzi-infected mice are severely impaired in their proliferative response to TCR stimulation. TCR stimulation leads to CD4+ T cell suicide by apoptosis, but CD3 stimulation is less efficient in this effect. Triggering of normal CD4 T cells through CD4 coincident with TCR activation, does not affect proliferative responses, but induces marked morphological changes in the T cells, which become adherent, form extended cytoplasmic projections, and acquire motile behavior. This response requires IL4 production, and can be markedly upregulated by exogenous IL4. Autoreactive CD4 T cell functioning can help syngeneic B cells to produce a TH2 pattern of immunoglobulin isotypes following stimulation by a thymus independent antigen. These results indicate that distinct patterns of functional behavior in vitro can be induced, depending both on the past experience of the T cell and on the exact array of stimulatory CD antigens engaged in the process of activation. The relevance of these constraints in generating variable behavior for immunoregulation is discussed.