Macrophage activation and histopathological findings in Calomys callosus and Swiss mice infected with several strains of Trypanosoma cruzi

Peritoneal macrophage activation as measured by H2O2 release and histopathology was compared between Swiss mice and Calomys callosus, a wild rodent, reservoir of Trypanosoma cruzi, during the course of infection with four strains of this parasite. In mice F and Y strain infections result in high parasitemia and mortality while with silvatic strains Costalimai and M226 parasitemia is sub-patent, with very low mortality. H2O2 release peaked at 33,6 and 59 nM/2 x 10(elevado a sexta potência) cells for strains Y and F, respectively, 48 and 50 nM/2 x 10 (elevado a sexta potência) for strains Costalimai and M226, at different days after infection. Histopathological findings of myositis, myocarditis, necrotizing artheritis and abscence of macrophage parasitism were foud for strains F and Costalimai. Y strain infection presented moderate myocarditis and myositis, with parasites multiplying within macrophages. In C. callosus all four strains resulted in patent parasitemia wich was eventually overcome, with scarce mortality. H2O2 release for strains Y or F was comparable to that of mice-peaks of 27 and 53 nM/2 x 10 (elevado a sexta potência) cells, with lower values for strains Costalimai and M226 - 16.5 and 4.6 nM/2 x 10(elevado a sexta potência)cells, respectively. Histopathological lesions with Y and F strain injected animals were comparable to those of mice at the onset of infections; they subsided completely at the later stages with Y strain and partially with F strain infected C. callosus. In Costalimai infected C. callosus practically no histopathological alterations were observed.

Transmission immunity in malaria: reflections on the underlying immune mechanisms during natural infections and following artificial immunization

Malaria transmission-blocking immunity has been studied in natural malaria infections in man, during infections in animals and following artificial immunization of animals with sexual stage malaria parasites. Effective immunity, which prevents infectivity of a malarial infection to mosquitoes, has been observed under all of these circumstances. Two general types of effector mechanism have been identified. One is an antibody mediated mechanism which acts against the extracellular sexual stages of the parasite within the midgut of a blood feeding mosquito. The other is a cytokine mediated mechanism which inactivates the gametocytes of the parasites while in the circulation of the vertebrate host. Both effects have been observed during natural infections and following artificial immunization. The basis of induction of transmission-blocking immunity, including the nature of the memory for such immunity, however, may be very different in different host/parasite systems and during natural infection of following artificial immunization. Following artificial immunization a strong immune memory for transmission blocking immunity has been observed in animal systems. By contrast, following natural infections in man immune memory for transmission blocking immunity has been found to be weak and short lived if it occurs at all. It is suggested that the immunogens which induce natural transmission blocking immunity may be CD4+ independent.

Molecular mechanisms and biological importance of Plasmodium falciparum erythrocyte rosetting

Rosetting, i.e. the spontaneous binding of uninfected to malaria infected erythrocytes and endothelial cytoadherence may hinder the blood flow and lead to serve Plasmodium falciparum malaria. Falciparum isolates obtained from unconscious patients all form rosettes and/or express a significantly higher man rosetting rate than isolates from patients with uncomplicated malaria. Furthermore, sera of patients with cerebral malaria are devoid of anti-rosetting activity while sera from patients with mild disease carry high levels of anti-rosetting antibodies. The presence of anti-rosetting antibodies also seems important for the efficient interaction of rosetting infected rbc and leucocytes. Two parasite derived rosetting ligands of Mr 22k and Mr28K named "rosettins, have been found on the surface of rosetting infected erythrocytes. CD36 has in at least some strains of parasites been found to function as a rosetting receptor on the uninfectederythrocyte. Heparin disrupts rosettes of P. falciparum in vitro and inhibits the sequestration of rosetting cells ex vivo. In conclusion, rosetting seems a crucial factor in the development of cerebral malaria and treatment of patients with anti-rosetting substances might become an effectivew adjunct in the treatment of severe malaria.

Mechanisms of protective immunity against asexual blood stages of Plasmodium falciparum in the experimental host Saimiri

In the Saimiri monkey, an experimental host for human malaria, acquired protection against Plasmodium falciparum blood stages depends on the IgG antibody populations developed. In vivo protective anti-falciparum activity of IgG antibodies is correlated with the in vivo opsonizing activity promoting phagocytosis of parasited red bloood cells. In contrast, non protective antibodies inhibit this mechanism by competing at the target level. A similar phenomenon can be and human infection. Anti-cytoadherent and anti-rosette antibodies developed by Saimiri and humans prevent the development of physiopathological events like cerebral malaria which can also occur in this experimental host. Furthermore, transfer to protective human anti-falciparum IgG antibodies into infected Saimiri monkeys exerts an anti parasite activity as efficient as that observed when it is transfered into acute falciparum malaria patients, making the Saimiri an even more attractive host. Studies on the role of immunocompetent cells in the protective immune reponse are still in their infancy, however the existance of a restricted polymorphism of MHC II class molecules in the Saimiri confers additional theoretical and practical importance to this model.

Mechanisms of evasion of Schistosoma mansoni schistosomula to the lethal activity of complement

Schistosomula of Schistosoma mansoni became resistant to antibody-dependent complement damage in vitro after pre-incubation with normal human erythrocytes (NHuE) whatever the ABO or Rh blood group. Resistant parasites were shown to acquire host decay accelerating factor (DAF) , a 70 kDa glycoprotein attached to the membrane of NHue by a GPI anchor. IgG2a mAb anti-human DAF (IA10) immunoprecipitated a 70 kDa molecule from 125I-labeled schistosomula pre-incubated with NHuE and inhibited their resistance to complement-dependent killing in vtro. Incubationof schistosomula with erytrocytes from patients with paroxsimal nocturnal hemoglobinuria (PNHE) or SRBC, wich are DAF-deficient, did not protect the parasites from complement lesion. Supernatant of 100,000 x g collected from NHuE incubated for 24 h in defined medium was shown to contain a soluble form of DAF and to protect schistosomula from complement killing. Schistosomula treated with trypsin before incubation with NHuE ghosts did not become resistant to complement damage. On the other hand, pre-treatment with chymotrypsin did not interfere with the acquisition of resistance by the schistosomula. These results indicate that, in vitro, NHuE DAF can be transferred to schistosomula in a soluble form and that the binding of this molecule to the parasite surface is dependent upon trypsin-sensitive chymotrypsin-insensitive polipeptide(s) present on the surface of the worm.

Trypanosoma cruzi recognition by macrophages and muscle cells: perspectives after a 15-year study

Macrophages and muscle cells are the main targets for invasion of Trypanosoma cruzi. Ultrastructural studies of this phenomenon in vitro showed that invasion occurs by endocytosis, with attachment and internalization being mediated by different components capable of recognizing epi-or trypomastigotes (TRY). A parasitophorus vacuole was formed in both cell types, thereafter fusing with lysosomes. Then, the mechanism of T. cruzi invasion of host cells (HC) is essentially similar (during a primary infection in the abscence of a specific immune response), regardless of wether the target cell is a professional or a non-professional phagocytic cell. Using sugars, lectins, glycosidases, proteinases and proteinase inhibitors, we observed that the relative balance between exposed sialic acid and galactose/N-acetyl galactosamine (GAL) residues on the TRY surface, determines the parasite's capacity to invade HC, and that lectin-mediated phagocytosis with GAL specificity is important for internalization of T. cruzi into macrophages. On the other hand, GAL on the surface to heart muscle cells participate on TRY adhesion. TRY need to process proteolytically both the HC and their own surface, to expose the necessary ligands and receptors that allow binding to, and internalization in the host cell. The diverse range of molecular mechanisms which the parasite could use to invade the host cell may correspond to differences in the available "receptors"on the surface of each specific cell type. Acute phase components, with lectin or proteinase inhibitory activities (a-macroglobulins), may also be involved in T. cruzi-host cell interaction.

Immunoregulatory mechanisms and Chagas' disease

During the course of experimental Chagas' disease, several immune disorders occur. In the acute phase, T and B cell plyclonal activation is associated to immunossupression. At the chronic stage. T cells - of the TH2 subset - participate to the pathology characteristic of Chagas'disease. Data obtained after infection of BALB/Xid mice suggest that polyclonal activation may be dependent on B1 (CD5) cell activation. Moreover, these mice fail to develop the pathological features of the chronic infection. Control of lymphokine secretion might play a key role in the clinical status of Chagas'disease.

Mechanisms of immune protection in the asexual blood stage infection by Plasmodium falciparum: analysis by in vitro and ex-vivo assays

Mechanisms of immune protection against the asexual blood stage infection by Plasmodium falciparum are reviewed. Recent studies of two independent lines of research developed at the Institute Pasteur, in humans and primate infections clearly indicate an obligatory interaction of antibodies and effector cells to express the anti-parasitic effect.

Immune mechanisms underlying the premunition against Plasmodium falciparum malaria

The most unique characteristic of a parasite when it is in its normal host is the ability to make itself tolerated, which clearly indicates that it has sophisticated means to ensure the neutrality of its host. This is true also in the case of Plasmodium falciparum, since after numerous malaria attacks an equilibrium is reached with a chronic stage of infection, characterized by a relatively low parasitemia, and low or no disease (Sergent & Parrot 1935). We shall briefly review the main characteristics of this state of "premunition", and present data suggesting that the underlying mechanisms of defense rely on the cooperation between cell and antibodies, leading to an antibody dependent cellular inhibition of the intra-erythrocytic growth of the parasite.

Novel mechanisms of immune evasion by Schistosoma mansoni

The interaction of Schistosoma mansoni with its host's immune system is largely affected by multiple specific and non-specific evasion mechanisms employed by the parasite to reduce the host's immune reactivity. Only little is known about these mechanisms on the molecular level. The four molecules described below are intrinsic parasitic proteins recently identified and studied in our laboratory. 1. m28-A 28kDa membrane serine protease. m28 cleaves iC3b and can thus restrict attack by effector cells utilizing complement receptors (especially CR3). Treatment with protease inhibitors potentiates killing of schistosomula by complement plus neutrophils. 2. Smpi56-A 56kDa serine protease inhibitor. Smpi56 binds covalently to m28 and to neutrophil's elastase and blocks their proteolytic activity. 3. P70-A 70kDa C3b binding protein. The postulated activity of P70 includes binding to C3b and blocking of complement activation of the C3 step. 4. SCIP-1-A 94kDa schistosome complement inhibitor. SCIP-1 shows antigenic and functional similarities to the human 18kDa complement inhibitor CD59. Like CD59, SCIP-1 binds to C8 and C9 and blocks formation of the complement membrane attack complex. Antibodies directed to human CD59 bind to schistosomula and potentiate their killing by complement. The structure and function of these four proteins as well as their capacity to induce protection from infection with S. mansoni are under investigation.

Mechanisms of formation and function of eosinophil lipid bodies: inducible intracellular sites involved in arachidonic acid metabolism

Lipid bodies, inducible lipid-rich cytoplasmic inclusions, are characteristically abundant in cells associated with inflammation, including eosinophils. Here we reviewed the formation and function of lipid bodies in human eosinophils. We now have evidence that the formation of lipid bodies is not attributable to adverse mechanisms, but is centrally mediated by specific signal transduction pathways. Arachidonic acid and other cis fatty acids by an NSAID-inhibitable process, diglycerides, and PAF by a 5-lipoxygenase dependent pathway are potent stimulators of lipid body induction. Lipid body formation develops rapidly by processes that involve PKC, PLC, and de novo mRNA and protein synthesis. These structures clearly serve as repositoires of arachidonyl-phospholipids and are more than inert depots. Specific enzymes, including cytosolic phospholipase A2, MAP kinases, lipoxygenases and cyclooxygenases, associate with lipid bodies. Lipid bodies appear to be dynamic, organelle-like structures involved in intracellular pathways of lipid mobilization and metabolism. Indeed, increases in lipid body numbers correlated with enhanced production of both lipoxygenase- and cyclooxygenase-derived eicosanoids. We hypothesize that lipid bodies are distinct inducible sites for generating eicosanoids as paracrine mediators with varied activities in inflammation. The capacity of lipid body formation to be specifically and rapidly induced in leukocytes enhances eicosanoid mediator formation, and conversely pharmacologic inhibition of lipid body induction represents a potential novel and specific target for anti-inflammatory therapy.

Mechanisms of cell accumulation induced by Mycobacterium bovis BCG

Mycobacteria, specially Mycobacterium tuberculosis are among the micro-organisms that are increasing dramatically the number of infections with death, all over the world. A great number of animal experimental models have been proposed to investigate the mechanisms involved in the host response against these intracellular parasites. Studies of airway infection in guinea-pigs and rabbits, as well as, in mice intravenously infected with BCG have made an important contribution to our understanding of the virulence, pathogenesis and the immunology of mycobacterial infections. Although, there are few models to study the mechanisms of the initial inflammatory process induced by the first contact with the Mycobacteria, and the relevance of the acute generation of inflammatory mediators, cytokines and leukocyte infiltration to the development of the mycobacterial infection. In this work we reviewed our results obtained with a model of M. bovis BCG-induced pleurisy in mice, describing the mechanisms involved in the leukocyte influx induced by BCG at 24 hr. Different mechanisms appear to be related with the influx of neutrophils, eosinophils and mononuclear cells and distinct inflammatory mediators, cytokines and adhesion molecules are involved in the BCG-induced cell accumulation.

Nitric oxide mediates the microbicidal activity of eosinophils

There are several experimental evidences that nitric oxide (NO) is involved in the microbicidal activity of macrophages against a number of intracellular pathogens including Leishmania major, Trypanozoma cruzi, Toxoplasma gondii. It is also well known that eosinophils (EO) have microbicidal activity against many parasites such as Schistosoma mansoni, Trichinella spiralis, T. cruzi and L. amazonensis. The purpose of this study was to investigate if NO is involved in the microbicidal activity of EO against L. major. Eosinophils harvested from peritoneal cavity of rats released spontaneously after 24 and 48 hr a small amount of nitrite. This release was enhanced by the treatment of cells with IFN-gamma (200 IU/ml). This release was blocked by addition of the NO synthase inhibitor, L-NIO (100 mu M) into the culture. To determinate the leishmanicidal activity of eosinophils the parasites were incubated with activated eosinophils with IFN-gamma and the ability of surviving parasites to incorporate [³H]thymidine was evaluated. IFN-gamma-activated eosinophils were able to kill L. major and to release high levels of nitrite. The ability to destroy L. major and the release of NO were completely blocked by L-NIO. These results indicate that activated eosinophils release NO which is involved in the microbicidal activity of these cells against L. major.