Fas ligand triggers pulmonary silicosis.

We investigated the role of Fas ligand in murine silicosis. Wild-type mice instilled with silica developed severe pulmonary inflammation, with local production of tumor necrosis factor (TNF)-alpha, and interstitial neutrophil and macrophage infiltration in the lungs. Strikingly, Fas ligand-deficient generalized lymphoproliferative disease mutant (gld) mice did not develop silicosis. The gld mice had markedly reduced neutrophil extravasation into bronchoalveolar space, and did not show increased TNF-alpha production, nor pulmonary inflammation. Bone marrow chimeras and local adoptive transfer demonstrated that wild-type, but not Fas ligand-deficient lung macrophages recruit neutrophils and initiate silicosis. Silica induced Fas ligand expression in lung macrophages in vitro and in vivo, and promoted Fas ligand-dependent macrophage apoptosis. Administration of neutralizing anti-Fas ligand antibody in vivo blocked induction of silicosis. Thus, Fas ligand plays a central role in induction of pulmonary silicosis.

Cyclosporin A-treated guinea pig responder cells secrete a genetically restricted factor that suppresses the mixed leukocyte reaction.

Cyclosporin A (CY A) is a hydrophobic, undecapeptide, fungal metabolite with potent immunosuppressive effects on T lymphocyte-mediated immune responses. Suppressor T lymphocytes generated during a mixed leukocyte reaction (MLR) performed in the presence of CY A, release a factor that suppresses a primary MLR of responder T cells, which is derived from the same strain as the factor producer but lacks specificity for the stimulator cell. These results suggest a finely regulated pathway by which CY A may induce and maintain a permanent state of transplantation tolerance in vivo.

Apoptotic CD8 T-lymphocytes disable macrophage-mediated immunity to Trypanosoma cruzi infection.

Chagas disease is caused by infection with the protozoan Trypanosoma cruzi. CD8 T-lymphocytes help to control infection, but apoptosis of CD8 T cells disrupts immunity and efferocytosis can enhance parasite infection within macrophages. Here, we investigate how apoptosis of activated CD8 T cells affects M1 and M2 macrophage phenotypes. First, we found that CD8 T-lymphocytes and inflammatory monocytes/macrophages infiltrate peritoneum during acute T. cruzi infection. We show that treatment with anti-Fas ligand (FasL) prevents lymphocyte apoptosis, upregulates type-1 responses to parasite antigens, and reduces infection in macrophages cocultured with activated CD8 T cells. Anti-FasL skews mixed M1/M2 macrophage profiles into polarized M1 phenotype, both in vitro and following injection in infected mice. Moreover, inhibition of T-cell apoptosis induces a broad reprogramming of cytokine responses and improves macrophage-mediated immunity to T. cruzi. The results indicate that disposal of apoptotic CD8 T cells increases M2-macrophage differentiation and contributes to parasite persistence.

The dual role of CTLA-4 in Leishmania infection.

The role of CTLA-4 in inducing the production of transforming growth factor beta (TGF-beta) from T cells during a Leishmania infection has only recently been recognized. However, CTLA-4 and TGF-beta affect T helper cells differently, depending on the maturation. This review discusses the data obtained from different experimental models and demonstrates that CTLA-4 is a target molecule for vaccination and therapy against leishmaniasis.

Apoptosis and parasitism: from the parasite to the host immune response.

Apoptosis, a form of programmed cell death (PCD), plays a central role in normal tissue development as well as in the pathogenesis of different diseases. PCD is responsible for the non-inflammatory physiological elimination of potentially harmful or unnecessary cells during embryogenesis, and for the proper functioning of continuous cell renewal systems in adult organisms. Maturation of the immune system and the specific immune response are examples of situations where PCD plays important roles. This review discusses the importance of apoptosis in two fundamental elements of a host-parasite interaction: the parasite (Section 1), and the host's immune response (Section 2). Section 1 discusses questions raised by the description of apoptosis in unicellular eukaryotes, such as the evolutionary origin of the molecular components of PCD, its role in the emergence and maintenance of parasitism, and the constraints of a multicellular organization for the proper operation of a cell death programme. The proposal is that PCD can occur in any situation where living cells display features of an organized network which operates through interactions within themselves and/or with elements of their environment. The possibility is also discussed that evolutionary relics of a complete cell death system may operate in unicellular parasites with functions other than inducing cell death. Section 2 reviews data on the mechanisms of host-cell PCD and the consequences of this phenomenon in host defence and pathogenesis. Infectious agents, from viruses to parasites, can either delay or induce apoptosis of different types of host cells. Apoptosis following lymphocyte polyclonal activation and stimulation of peripheral T lymphocytes, as a result of the engagement of specific counter-receptor systems, is of special interest for defining host immunocompetence and mechanisms of immunopathology.

Independent and cooperative behavior of Thy1 and CD3:TcR T cell signalling pathways.

Normal resting spleen T lymphocytes from mice were stimulated in vitro by monoclonal antibodies (mAbs) against either Thy 1 or CD3:TcR surface protein molecules. Although both mAbs were mitogenic, anti-Thy 1 activation generated 5 times more IL2 secretion than anti-CD3 activation under similar conditions. Production of IL3-like activity was comparable for both Thy1 and CD3-mediated activation. In addition, non-mitogenic doses of anti-CD3 and anti-Thy1 (0.16 micrograms/ml and 0.0125% ascites, respectively) mAbs induced T cell activation when provided together. These results indicate that Thy1 signalling cooperates with the CD3:TcR pathway to activate T cells. However, the Thy1 pathway is also regulated independently since IL2 production is larger when stimulated by anti-Thy1 than anti-CD3 mAbs.

Apoptosis as a cause of T-cell unresponsiveness in experimental Chagas' disease.

A murine model of Chagas' disease induced by metacyclic forms of T. cruzi was used to evaluate T-cell function during infection. T-cell unresponsiveness to TcR;CD3 stimulation in vitro and lymphocyte activation in vivo occurred simultaneously. These paradoxical findings are discussed in the light of recent evidence that mature activated T cells become susceptible to TcR-mediated apoptosis. Activation-induced death in T cells from T. cruzi-infected mice has recently been demonstrated in this model. Evidence that TcR-induced death of activated T cells could be a cause for T-cell unresponsiveness in vitro and in vivo, as well as the possible molecular mechanisms involved, are discussed.

Apoptosis in parasites and parasite-induced apoptosis in the host immune system: a new approach to parasitic diseases.

Apoptosis, a form of programmed cell death (PCD), has been described as essential for normal organogenesis and tissue development, as well as for the proper function of cell-renewal systems in adult organisms. Apoptosis is also pivotal in the pathogenesis of several different diseases. In this paper we discuss, from two different points of view, the role of apoptosis in parasitic diseases. The description of apoptotic death in three different species of heteroxenic trypanosomatids is reviewed, and considerations on the phylogenesis of apoptosis and on the eventual role of PCD on their mechanism of pathogenesis are made. From a different perspective, an increasing body of evidence is making clear that regulation of host cell apoptosis is an important factor on the definition of a host-pathogen interaction. As an example, the molecular mechanisms by which Trypanosoma cruzi is able to induce apoptosis in immunocompetent cells, in a murine model of Chagas' disease, and the consequences of this phenomenon on the outcome of the experimental disease are discussed.

Evasion of immune responses by Trypanosoma cruzi, the etiological agent of Chagas disease.

Infection with the protozoan parasite Trypanosoma cruzi leads to Chagas disease, which affects millions of people in Latin America. Infection with T. cruzi cannot be eliminated by the immune system. A better understanding of immune evasion mechanisms is required in order to develop more effective vaccines. During the acute phase, parasites replicate extensively and release immunomodulatory molecules that delay parasite-specific responses mediated by T cells. This immune evasion allows the parasite to spread in the host. In the chronic phase, parasite evasion relies on its replication strategy of hijacking the TGF-ß signaling pathway involved in inflammation and tissue regeneration. In this article, the mechanisms of immune evasion described for T. cruzi are reviewed.

Evasion of immune responses by Trypanosoma cruzi, the etiological agent of Chagas disease

Infection with the protozoan parasite Trypanosoma cruzi leads to Chagas disease, which affects millions of people in Latin America. Infection with T. cruzi cannot be eliminated by the immune system. A better understanding of immune evasion mechanisms is required in order to develop more effective vaccines. During the acute phase, parasites replicate extensively and release immunomodulatory molecules that delay parasite-specific responses mediated by T cells. This immune evasion allows the parasite to spread in the host. In the chronic phase, parasite evasion relies on its replication strategy of hijacking the TGF-β signaling pathway involved in inflammation and tissue regeneration. In this article, the mechanisms of immune evasion described for T. cruzi are reviewed.

Neutrophils, apoptosis and phagocytic clearance: an innate sequence of cellular responses regulating intramacrophagic parasite infections.

In complex organisms, apoptosis is a constitutive cell death process that is involved in physiological regulation of cell numbers and that can also be induced in the course of inflammatory and immune responses. Neutrophils are among the first cells recruited during inflammation. Neutrophils constitutively die by apoptosis at inflamed sites, and are ingested by macrophages. Recent studies investigated how phagocytic clearance of senescent neutrophils influences the survival of intracellular protozoan parasites that have been phagocytosed by, or have invaded phagocytes. The results indicate that neutrophil clearance plays an unexpected role in regulation of intramacrophagic protozoan parasite infection.

Susceptible hosts: a resort for parasites right in the eye of the immune response.

Trypanosomatid protozoan parasites express an aggressive strategy of parasitism by infecting host macrophages and inducing extensive T-lymphocyte activation. One goal of such strategy is to drive the immune response of genetically susceptible hosts to a state of unresponsiveness regarding parasite killing. Unresponsiveness is achieved through different mechanisms, depending on the parasite species. In this brief review, recent findings on the molecular and cellular bases of the parasites' exploitation of host immune responses are discussed.

Cell-mediated immunity in experimental Trypanosoma cruzi infection.

Infection of humans with the protozoan Trypanosoma cruzi leads to Chagas disease, or American trypanosomiasis, a disease that affects nearly 20 million people, and constitutes one of the largest socioeconomic burdens in Latin America. Much of the present knowledge on pathogenic mechanisms underlying T. cruzi infection comes from experimental murine models. Here, George A. DosReis reviews recent findings about the features of host cell-mediated immunity against the parasite and possible mechanisms leading to chronic infection.

Trypanosoma cruzi-induced immunosuppression: selective triggering of CD4+ T-cell death by the T-cell receptor-CD3 pathway and not by the CD69 or Ly-6 activation pathway.

In a model of experimental Chagas' disease induced with metacyclic forms of Trypanosoma cruzi, CD4+ but not CD8+ T cells undergo T-cell receptor (TCR)-CD3-mediated activation-induced cell death (AICD) in vitro. CD4+ T cells from T. cruzi-infected mice also developed unresponsiveness in proliferative responses to TCR-CD3-mediated stimulation. A linear correlation was found between extent of proliferative unresponsiveness and loss of CD4+ T-cell viability. CD4+ T-cell activation through the CD69 or Ly-6 A/E pathway, on the other hand, did not result in proliferative unresponsiveness compared with controls. Lack of suppression in proliferation assays correlated with lack of AICD by cells stimulated through the CD69 or Ly-6 A/E pathway. Concomitant stimulation through CD69, however, did not rescue CD4+ T cells from CD3-induced death. Flow cytometry study of cells stimulated in vitro showed no defect in interleukin-2 receptor expression by CD4+ T cells from infected donors, which escaped TCR-mediated AICD. In vivo injection of anti-CD3 into acutely infected mice, but not into control mice, led to splenocyte DNA fragmentation and failed to increase splenic CD4+ T-cell numbers. These results show that TCR-CD3-mediated AICD is involved in CD4+ T-cell unresponsiveness in vitro following infection with T. cruzi. In addition, successful activation of these cells through the CD69 and Ly-6 pathways is due to differences in the inability of these stimuli to trigger AICD. Since TCR-CD3-mediated AICD can be induced in vivo in infected mice, these findings may be relevant for the onset of immunological disturbances in the host.

Analysis of autoreactive I region-restricted T cell colonies isolated from the guinea pig syngeneic mixed leukocyte reaction and from immune responses to conventional foreign antigens.

Guinea pig proliferating T cell colonies were isolated from T cell populations stimulated during the syngeneic mixed leukocyte reaction (SMLR) or following positive selection of immune T lymphocytes specific for pork insulin (PI) or the copolymer of L-glutamic acid, L-lysine (GL). SMLR-responding T cell colonies could be isolated in the absence of any extrinsic antigen and were strictly restricted to the recognition of Ia molecules on stimulator peritoneal exudate cells (PEC) and required both stimulator cells and interleukin 2-enriched fluids for optimal proliferative responses. Blocking of T cell colony proliferation with a panel of monoclonal anti-Ia antibodies showed that SMLR T cell colonies were restricted by discrete and distinct self-Ia epitopes. Analysis of individual T cell colonies generated against PI and GL revealed three types of colonies: (a) antigen specific, I region-restricted; (b) autoreactive, I region-restricted; and (c) antigen specific, but also autoreactive. These doubly reactive colonies were restricted to the same Ia epitope when stimulated with self-PEC alone or when stimulated with self-PEC in the presence of the relevant exogenous antigen. These results substantiate the hypothesis that both the SMLR and antigen-specific responses are mediated by a common set of precursor T lymphocytes and that the guinea pig SMLR is at least in part the result of the polyclonal proliferative responses of several distinct antigen-reactive T cell clones in the absence of exogenous antigen.

Naturally activated and resting T cells differ in their activation requirements for growth and secretory activities.

"Naturally activated" (NA) and "small resting" (SR) T lymphocytes were stimulated with anti-CD3 or anti-thy 1 monoclonal antibodies (mAbs). Both proliferation and secreted IL-2, IL-3/GM-CSF activities were found in NA T cell, but not in SR T cell cultures. SR T cells could be fully activated by anti-CD3 only if PMA or IL-2 was added to the cultures. NA T cell proliferation induced with anti-CD3 was blocked with anti-IL-2 or anti-IL-2R mAbs. The combination of anti-CD3 and rec IL-4 was not effective in promoting SR T cell proliferation. IL-4 plays a minor role in NA T cell activation with anti-CD3, as assayed with neutralizing anti IL-4 mAbs. No differences in the proliferative and secretory activities were found when NA or SR T cells were stimulated with Con A. Both NA and SR T cells responded when stimulated with the calcium ionophone A23187 plus PMA. Only NA T cells responded to A23187 alone. The mechanisms and the possible physiologic relevance of this differential responsiveness behavior are discussed.

The central role of Fas-ligand cell signaling in inflammatory lung diseases.

Following inflammation and injury in the lung, loss of epithelial cell precursors could determine the balance between tissue regeneration and fibrosis. This review discusses evidence that proapoptotic Fas-Fas ligand (FasL) signaling plays a central role in pulmonary inflammation, injury and fibrosis. FasL signaling induces inflammatory apoptosis in epithelial cells and alveolar macrophages, with concomitant IL-1 beta and chemokine release, leading to neutrophil infiltration. FasL signaling plays a critical role in models of acute lung injury, idiopathic pulmonary fibrosis and silicosis; blockade of Fas-FasL interactions either prevents or attenuates pulmonary inflammation and fibrosis. Serologic and immunohistochemical studies in patients support a major pathogenic role of Fas and FasL molecules in inflammatory lung diseases. Identification of the pathogenic role of FasL could facilitate the discovery of more effective treatments for currently untreatable inflammatory lung diseases.

Programmed T-cell death in experimental chagas disease.

In mature T cells, programmed cell death is thought to serve a regulatory function by limiting both the duration and amplitude of immune responses. Programmed cell death might also be involved in immuno-pathogenesis of certain infectious diseases: recent evidence suggests that programmed T-cell death plays an important role in immune suppression during viral infections. In this article, George DosReis, Maria Evangelina Fonseca and Marcela Lopes review their findings on programmed T-cell death in experimental infection induced by the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. They also discuss the differential behavior of CD4(+) and CD8(+) T-cell subsets regarding programmed cell death, and same possible pathogenic aspects of host-parasite interaction, where abnormal or exaggerated programmed T-cell death could be involved.

Purinergic modulation of T-lymphocyte activation: differential susceptibility of distinct activation steps and correlation with intracellular 3',5'-cyclic adenosine monophosphate accumulation.

The mechanism by which purinergic agonists modulate murine T-lymphocyte activation and proliferation was investigated. Adenosine and other compounds such as ATP and 2-chloroadenosine (ClAdo) were found to block T-cell mitogenesis induced by concanavalin A (Con A) in a dose-dependent fashion. The nonmetabolizable adenosine analog ClAdo was the most potent agent capable of inhibiting T-cell mitogenesis. Extracellular addition of the permeable cAMP analog dibutyryl cyclic AMP (dbcAMP) also led to a dose-dependent blockade of T-cell mitogenesis, although with less efficiency when compared to ClAdo. Addition of IL-2-enriched fluids failed to reverse blockade of T-cell mitogenesis by ClAdo or dbcAMP. ClAdo blocked T-cell enlargement induced after 20 hr of culture with Con A. We analyzed the effect of micromolar concentrations of ClAdo on interleukin-2 (IL-2) production, expression of IL-2 receptors (7D4 and 3C7 surface antigens), and induction of IL-2 responsiveness after in vitro cultivation with Con A. ClAdo inhibited both IL-2 secretion and induction of IL-2 responsiveness up to control levels in the same dose range it inhibited T-cell mitogenesis. However, cell surface expression of IL-2 receptors was not affected. Short incubations of resting splenic T cells with ClAdo led to a dose-dependent accumulation of cyclic AMP in responding cells. This effect was markedly reduced by the purinergic antagonist 3-isobutyl-1-methylxanthine (IBMX) but was not prevented by the adenosine uptake blocker dipyridamole. ClAdo elicited cAMP accumulation in the same dose range it inhibited T-cell activation events. Extracellular administration of dbcAMP to splenic T cells stimulated by Con A mimicked the effects of ClAdo on T-cell activation parameters, as revealed by a dose-dependent blockade of both IL-2 secretion and IL-2 responsiveness induction, without affecting IL-2 receptor expression. Short incubations of Con A-activated T-cell blasts with ClAdo also led to a dose-dependent accumulation of cAMP. We then analyzed the effect of purines and dbcAMP on IL-2-mediated activated T-cell growth. Purines caused a dose-dependent inhibition of IL-2-mediated T-cell proliferation and ClAdo was the most potent purinergic agonist tested. The effect of ClAdo on Con A-induced T blasts was shifted to the right, if compared to earlier T-cell activation steps.(ABSTRACT TRUNCATED AT 400 WORDS)

T-lymphocytes in experimental Leishmania amazonensis infection: comparison between immunized and naive BALB/c mice.

Highly susceptible naive BALB/c mice or mice that had previously been immunized i.v. with solubilized homologous antigen (partially resistant) were infected with Leishmania amazonensis. Histologically, the main differences between the two groups were lymphocytic infiltration and macrophage activation. Assays of T-cell function at 3 and 10 weeks after infection revealed that purified T-cells did not proliferate following treatment with leishmania antigen. A mitogenic anti-CD3 monoclonal antibody (mAb) failed to activate T-cells after 3 weeks of infection as judged by proliferation and IL-2 secretion assays. After 10 weeks of infection, anti-CD3 mAb fully activated T-cells to proliferation and IL-2 secretion. On the other hand, T-cells released IL-3 in response to leishmania antigen, anti-CD3 mAb and anti-Thy1 mAb at 3 and 10 weeks post-infection. Surprisingly, a mitogenic anti-Thy 1 mAb (G7) fully activated T-cells even at 3 weeks of infection as judged by proliferative and IL-2 secretion assays. No significant differences were found in the proliferative or interleukin secretory responses of T-cells from animals that had been infected in either the presence or the absence of prior immunization. Since the Thy1 triggering pathway has different accessory cell and cytokine requirements than does the CD3: TCR lymphocyte activation pathway, it is possible that immunization was more effective in changing the cellular interactions of the T-lymphocyte than in altering its intrinsic capabilities.