A Toxoplasma gondii Superantigen: Biological effects and implications for the host-parasite interaction

Antiegressin acts late in the intracellular growth cycle of Trypanosoma cruzi to inhibit parasite egress

Trypanosoma cruzi is the causative agent of Chagas disease, which is characterized by acute and chronic phases. During the former, parasitemia rises dramatically, then decreases significantly during the chronic phase. Immune mechanisms responsible for the parasitemia reduction have not been thoroughly elucidated. The goal of the present study was to further characterize the immune response during chronic infection. Previously, we described antiegressin, an antibody in sera from chronically infected mice. The in vitro presence of antiegressin inhibits parasite egress from infected host cells. Antiegressin appears by day 14 of an in vivo infection and is maintained through at least day 280 postinfection. The in vitro functional activity of antiegressin is initiated late in the 4-6 days intracellular growth cycle of T. cruzi; antiegressin may be added at day 4, inhibiting parasite release at day 5. Immunocytochemical staining using antineuraminidase demonstrates the presence of mature parasites inside host BALB/c fibroblasts grown in the presence of antiegressin. These results demonstrate the ability of antiegressin to inhibit emergence of developmentally mature trypomastigotes from infected host cells late in their intracellular growth cycle. We believe this antibody plays an important and novel role in achieving the low-parasitemia characteristic of chronic Chagas disease.

Pathogenicity of selected Toxoplasma gondii isolates in young pigs

The pathogenicity in 7-week-old pigs to five different Toxoplasma gondii strains of various host species origin was compared after i.v. inoculation of 10(4) tachyzoites. Additionally, one group of pigs was inoculated i.v. with 10(6) tachyzoites of the reference strain, SSI 119. In response to the infection a significant effect of T. gondii tachyzoite inoculation dose as well as differences among strains could be observed in several parameters. The 10(6)-dose inoculated pigs showed variable degrees of clinical illness and recurrent episodes of fever 4-17 days p.i., while pigs of four of the 10(4) tachyzoite inoculated groups experienced a short-lived rise in body temperature from day 6-8 p.i. without any apparent illness or inappetence. Control pigs and pigs infected with the least pathogenic strain had normal body temperature throughout the experiment. In all inoculated pigs, T. gondii-specific IgM and IgG antibodies appeared from day 8-10 and 10-17 p.i., respectively. Serum levels of alkaline phosphatase and the acute phase protein haptoglobin were decreased or increased, respectively, in response to the infection. Differential leukocyte count on peripheral blood revealed a significant lymphocytopenia on day 6 p.i. equal to both CD4+ and CD8+ T-cells, but shifting towards a reduced ratio of CD4+/CD8+ T-cells from day 8-14 p.i. In the 10(6)-dose inoculated pigs a considerable increase in zymosan induced and spontaneous oxidative burst capacity of peripheral blood leukocytes was observed from 6 days p.i. compared with control pigs. Oxidative burst capacity was not examined for other pigs. In conclusion, several useful parameters to identify differences in T. gondii pathogenicity other than mortality were identified. Furthermore, even at low doses, significant differences between recently collected Danish T. gondii field isolates were demonstrated after i.v. inoculation in young pigs.

Control of coccidiosis: lessons from other sporozoa

Coccidiosis is the most important parasitic infection in poultry worldwide and also causes problems in cattle, sheep and goats. Control is largely limited to good husbandry and prophylactic chemotherapy using a range of drugs against which resistance is rapidly acquired. Attempts at vaccination using conventional vaccines have been disappointing and there is now a need for a new approach. Research into the immunology of coccidiosis has lagged behind that of other sporozoans and there are useful lessons that might be learned from studies on toxoplasmosis, cryptosporidiosis, theileriosis and malaria. In these infections the emphasis has turned to the cytokine network that drives the response towards protection. Central to these studies are the roles of interferon-gamma, interleukin-12 and activated macrophages with the involvement of nitric oxide in parasite killing. Cytotoxic T cells have also increasingly been implicated. Research has shown that different immune responses can be elicited by manipulating the cytokine system and these new concepts can be applied to the design of peptide or recombinant vaccines, and the possibilities of developing such vaccines against coccidiosis will be discussed.

The role of IL-4 in adult acquired and congenital toxoplasmosis

The course of Toxoplasma gondii infection was studied in IL-4-deficient mice from three genetic backgrounds and their wild-type counterparts following peroral inoculation of tissue cysts. Survival rates were significantly reduced in disease-susceptible C57 BL/6 mice and F1 (C57BL/6 x 129Sv) mice deficient in IL-4 compared with wild-type controls. In contrast, this difference was not observed in T. gondii-resistant BALB/c mice. However, brain tissue cyst burdens in IL-4-deficient mice were either equivalent to (C57BL/6 and BALB/c mice) or significantly less (B6/129 mice) than similarly infected wild-type mice. Thus strain-specific differences in the course of T. gondii were demonstrated in the absence of IL-4. The course of T. gondii infection was also compared between B6/129 IL-4-deficient mice and their wild-type counterparts following peroral challenge with 20 tissue cysts on day 12 of pregnancy. Age-matched non-pregnant IL-4-/- and IL-4+/+ mice were also infected to assess the role of IL-4 on T. gondii infection during pregnancy. Disease phenotypes, as measured by mortality, were reversed if infections were initiated during pregnancy compared with non-pregnant infection. Thus significant mortality occurred immediately post partum in IL-4+/+ mothers, while all IL-4-/- mothers survived. Cyst burdens 28 days p.i. were significantly lower in IL-4-/- mothers than IL-4+/+ mothers and both IL-4-/- and IL-4+/+ non-pregnant mice. Congenital disease transmission as measured by foetal death or vertical disease transmission was independent of the presence or absence of IL-4. These studies demonstrate a role for IL-4 in pregnancy-induced immunosuppression and the associated increased susceptibility to T. gondii infection.

Toxoplasma gondii infection in the mountain hare (Lepus timidus) and domestic rabbit (Oryctolagus cuniculus). II. Early immune reactions

As already reported, the mountain hare is much more susceptible than the domestic rabbit to oral inoculation with Toxoplasma gondii, as judged by pathological changes and dissemination of parasites within the body. In the present paper, further interspecies variations are reported. Concentrations of the acute phase reactant haptoglobin were raised in hares but not in rabbits one week post-infection (pi), probably reflecting the severe tissue damage present. No difference in the early humoral immune response of hares and rabbits was found, both species producing IgM and IgG antibodies to T. gondii one week pi. Lymphocyte stimulation tests performed before and one week after inoculation showed a high proliferative response to the parasite in blood cell cultures from rabbits but not hares. The fatal outcome of T. gondii infection in the hares is probably due, at least in part, to the lack of cellular response.

Designer vaccines for parasitic diseases

Conventional ways of developing vaccines against infections, either on pragmatic grounds or by identifying protective antigens and attempting to mimic natural immune responses, have largely been unsuccessful for parasitic-infections, mainly because of the complexity of the immunological processes involved. It is clear that a new approach is required and it is now known that the "immunological environment" in which the immune response is initiated is as, or more, important than the actual antigens used. CD4+ and CD8+ T1 cells, through the agency of IL-2 and IFN-gamma, direct the response towards cell-mediated immunity involving cytotoxicity and macrophage activation, whereas T2 cells, through the agency of IL-4 and IL-10, direct the response towards antibody production. The two poles are counter-regulatory in that IFN-gamma inhibits antibody formation and IL-4 and IL-10 inhibit macrophage activation. However, immune responses are not immutable and can be artificially driven towards one or other pole, for example IFN-gamma, IL-2 and IL-12 favour T1 responses, whereas IL-4 and IL-10 favour the T2 type. With this knowledge, it is possible to design recombinant or nucleic acid vaccines that include gene products or genes for desirable cytokines as well as the appropriate antigen. For example, in experimental leishmaniasis, protective immune responses can be induced by the incorporation of genes for IL-2 and IFN-gamma into recombinant Salmonella typhimurium vectors and nucleic acid vaccines. A similar approach might be appropriate in experimental schistosomiasis, in which exogenous IL-12 drives the immune response towards the T1 pole and ameliorates T2-mediated pathology. These approaches require novel delivery systems and these have already begun to produce encouraging results. However, simply modifying the nature and route of administration of the vaccine is not enough and attention has now turned to the effector molecules involved, for example nitric oxide, and the signaling systems that are modified by the presence of particular cytokines.