Immunopathology of toxoplasmosis

Murine spleen and lymph node cellular composition and function during cyclophosphamide and splenectomy induced resistance to Toxoplasma gondii

Murine toxoplasmosis caused by a low virulence, cyst-forming strain of Toxcoplasma gondii (Pe strain) is characterized by splenomegaly, lymphadenopathy, decreased delayed-type hypersensitivity (DTH), and the presence of toxoplasma cysts in brain tissue. Cyclophosphamide (CY) in a single dose of 100 mg/kg injected 3 days before infection, or splenectomy 3 weeks before infection, augmented DTH and decreased the number of toxoplasma brain cysts. CY-induced augmentation of resistance during the first 3 weeks of murine toxoplasmosis was associated with: (1) an increase in mononuclear phagocytes and a decrease in T lymphocytes (including Lyt2+ cells) in spleens and lymph nodes; (2) suppressed toxoplasma antigen induced proliferation of cultured spleen cells: (3) augmentation of antigen induced proliferation of cultured lymph node cells; and (4) low levels of interferon-gamma production in both spleen and lymph node cultures. The best correlate of the enhanced in-vivo effects of CY was proliferation of nylon wool-purified lymph node cells to toxoplasma antigen. The presence of Lyt2+ cells in lymph nodes of toxoplasma infected mice inhibited maximal proliferation.

Spleen and lymph node cell populations, in vitro cell proliferation and interferon-gamma production during the primary immune response to Toxoplasma gondii

An animal model for the study of transient lymphadenopathy-splenomegaly during toxoplasmosis is presented. Injection of CBA/J mice with the low virulent, cyst-forming strain of Toxoplasma gondii (Pe strain) induces a three to four fold increase in weight and cellularity of spleen and lymph nodes with peak changes at 30-50 days after infection. The spleen displays marked haemopoiesis, a 30 fold increase in mononuclear phagocytes, and a two fold increase in Lyt2+ lymphocytes. Lymph nodes show a five fold increase in mononuclear phagocytes and a four and a half fold increase in Lyt2+ T cells. The increase in mononuclear phagocytes significantly alters T cell/macrophage ratios and this is associated with decreases in in vitro cell proliferation to mitogen and toxoplasma antigen. The relationship between alterations in cell balance of mononuclear phagocytes and T cell subsets and the expression of transient immune dysfunction can now be examined by modulating changes in these cell types.

Cell-mediated killing of protozoa

Cell-mediated immunity represents an important host defence mechanism against protozoal infections. The effector cells directly involved are neutrophils, macrophages and, ultimately, activated macrophages. Within this simple scheme there are, however, considerable variations in activity. Effector cells from different animal species, and even from different strains of the same species, may be more or less effective in controlling a certain protozoal infection. Different protozoa differ in their susceptibility to cell-mediated killing according to genus, species, strain and morphological form. The most susceptible morphological form is that which occurs in the insect vector, and which has not yet adapted to protect itself from the vertebrate host. Epimastigotes of Trypanosoma and promastigotes of Leishmania are readily killed by phagocytic cells, while the corresponding trypomastigote and amastigote forms are considerably more resistant. Protozoa which live in macrophages, such as amastigotes of Leishmania, endozoites (tachyzoites) of Toxoplasma and amastigotes of reticulotropic strains of T. cruzi, have developed a remarkable resistance to the microbicidal activity of the host cell. Conversely, amastigotes of myotropic strains of T. cruzi, which live in muscle cells, have not developed this resistance to cell-mediated killing by macrophages. Readily accessible protozoa, such as T. brucei trypomastigotes and Plasmodium merozoites in the bloodstream, while they lack the marked resistance developed by reticulotropic protozoa, have a partial protection since they are attacked by phagocytic cells only when specific antibody is present. Granulocyte-mediated killing can be largely attributed to neutrophils. Eosinophils appear to play only a minor role and compete ineffectually when neutrophils are also present. The only group of protozoal species which may be significantly controlled by eosinophils are the stercorarian species of Trypanosoma. In vitro experiments show that antibody-coated trypomastigotes of T. cruzi can be killed by eosinophils, although there is little evidence that this occurs in vivo. Interestingly, this is the only species that has been reported to be susceptible to the major basic protein of eosinophils, a toxic component of the lysosomal granules which is very active against helminths. Neutrophils are not very active against endozoites of Toxoplasma gondii, Trypanosoma, trypomastigotes of salivarian Trypanosoma, free merozoites of Plasmodium, and promastigotes and amastigotes of Leishmania.(ABSTRACT TRUNCATED AT 400 WORDS)

Toxoplasmosis and the wild rabbit Oryctolagus cuniculus in Victoria, Australia with suggested mechanisms for dissemination of oocysts

Sera or serum eluates from 1697 wild rabbits [Oryctolagus cuniculus (L)], collected over the past 10 years from 24 sites in 5 geographic regions of Victoria, were examined for antibodies to Toxoplasma gondii. Sera generally were classified into three broad groups, negative (titre less than 10), weakly reactive (titre 10 to 100) and strongly positive (titre greater than 1000). Strongly positive sera came from rabbits collected in a small number of localized areas, most significantly the Melbourne and Metropolitan Board of Works (MMBW) Sewerage Farm at Werribee and Mud Island in Port Phillip Bay.

Chronological development of cellular immunity in human toxoplasmosis

A dichotomy between the resolution of clinical illness in acute toxoplasmosis and the responsiveness of B and T lymphocytes to toxoplasma antigens is described.

Histological and ultrastructural studies of the interaction of Toxoplasma gondii Tachyzoites with mouse omentum in experimental infection

Mouse omentum was studied after intraperitoneal challenge with tachyzoites of Toxoplasma gondii. Parasites inhabit omental histiocytes, fibroblasts, mesothelial cells, and free peritoneal macrophages. Recently infected cells showed enhanced metabolic and functional activity. Villous projections of the parasitophorous vacuole wall appeared, usually opposite the anterior pole of the parasite. In mesothelial cells, projections formed terminal swellings not observed in other infected cells. Activation of host cells was followed by reduction of the density of the cytoplasmic matrix, autophagosome formation, and intracellular edema, indicating the damage. The wall of the parasitophorous vacuole loses the supporting host cell endoplasmic reticulum that was attached to the vacuole just after entrance of the parasite into the cell. Then lysis of the parasitophorous vacuole and complete cell destruction occurs. The growth of parasites in undamaged cells does not coincide with the inflammatory response. Inflammation of the peritoneum develops only after the start of mass destruction of infected cells. Thus tachyzoites of Toxoplasma exert significant pathogenic effects by their ability to activate the host cell, causing lysis of the parasitophorous vacuole and subsequent destruction of the entire cell.