Effect of Eimeria falciformis infection on the development of toxoplasmosis in mice

White mice previously infected with 10(2), 10(3) or 10(4) Eimeria falciformis oocysts on days 0, 5, 10 or 30 were inoculated per os with 10(1), 10(2), 10(3) or 10(4) Toxoplasma oocysts. While the results obtained for mice with higher Toxoplasma inocula were consistent, animals with 10(1) and 10(2) oocysts previous inoculation with Eimeria showed important differences related with those infected only with Toxoplasma. For example, survival time was higher in animals infected with both parasites, especially if inoculated with Eimeria 30 days before Toxoplasma infection. Furthermore the number of T. gondii cysts found in the animals previously infected with Eimeria was lower compared with mice inoculated with Toxoplasma only. Body weight of mice infected with Toxoplasma previous infection with Eimeria was almost normal in relation to those infected only with Toxoplasma, indicating a probable pathological effect due to the parasite, more evident in "non immunized" mice.     

Toxoplasma gondii actively inhibits neuronal function in chronically infected mice

Upon infection with the obligate intracellular parasite Toxoplasma gondii, fast replicating tachyzoites infect a broad spectrum of host cells including neurons. Under the pressure of the immune response, tachyzoites convert into slow-replicating bradyzoites, which persist as cysts in neurons. Currently, it is unclear whether T. gondii alters the functional activity of neurons, which may contribute to altered behaviour of T. gondii-infected mice and men. In the present study we demonstrate that upon oral infection with T. gondii cysts, chronically infected BALB/c mice lost over time their natural fear against cat urine which was paralleled by the persistence of the parasite in brain regions affecting behaviour and odor perception. Detailed immunohistochemistry showed that in infected neurons not only parasitic cysts but also the host cell cytoplasm and some axons stained positive for Toxoplasma antigen suggesting that parasitic proteins might directly interfere with neuronal function. In fact, in vitro live cell calcium (Ca(2+)) imaging studies revealed that tachyzoites actively manipulated Ca(2+) signalling upon glutamate stimulation leading either to hyper- or hypo-responsive neurons. Experiments with the endoplasmatic reticulum Ca(2+) uptake inhibitor thapsigargin indicate that tachyzoites deplete Ca(2+) stores in the endoplasmatic reticulum. Furthermore in vivo studies revealed that the activity-dependent uptake of the potassium analogue thallium was reduced in cyst harbouring neurons indicating their functional impairment. The percentage of non-functional neurons increased over time In conclusion, both bradyzoites and tachyzoites functionally silence infected neurons, which may significantly contribute to the altered behaviour of the host.     

Kinetics of parasite burdens in blood and tissues during murine toxoplasmosis

A sensitive real-time PCR technique was used to examine the distribution of Toxoplasma gondii in the blood and tissues of mice during acute and chronic infection. Groups of Swiss Albino mice, inoculated i.p. with 10(2) or 10(6) tachyzoites of the RH strain as a typical type-1 strain, or fed 10 cysts of the Me49 strain as a typical type-2 strain, were killed at different time points post-infection (p.i.), and blood and organs including the lungs, brain and liver were harvested for DNA extraction. Toxoplasma DNA was quantified by a real-time PCR targeted at the 529bp gene fragment, with a detection limit of a single parasite per g/ml of tissue. The results showed a strain- and dose-dependent spread of Toxoplasma. In infection with type-1 parasites, in case of a high infective dose, Toxoplasma DNA was detected within 24h p.i. in all analyzed tissues including the brain. Conversely, in case of a low infective dose, parasitaemia was undetectable early p.i., at a time when Toxoplasma DNA was detected in the tissues, but reached very high levels as infection progressed. With both infective doses, pre-death parasite burdens were higher in the blood than in the tissues, whereas the same loads in the lungs suggest that reaching these Toxoplasma burdens may be critical for survival. In infection with Me49 parasites, steady high parasite burdens were noted up to the end of the experiment at d42 only in the brain, parasitaemia was low but detectable throughout, and Toxoplasma DNA was completely cleared only from the liver. These data are important to better understand the pathogenesis of toxoplasmosis, and also as baseline data for the experimental evaluation of novel chemotherapeutics.     

Mic1-3KO tachyzoite a live attenuated vaccine candidate against toxoplasmosis derived from a type I strain shows features of type II strain

Vaccination with live attenuated parasites has been shown to induce high level of protection against Toxoplasma gondii. In this study we compared the Mic1-3KO tachyzoite (a live attenuated strain) with the parental wild type (WT) tachyzoite in terms of virulence in mice in vivo, dissemination in mouse tissues and persistence in mouse brain. Survival of mice infected with the Mic1-3KO parasites correlated with reduced parasite burden in mouse tissues compared to the parental strain. Like the WT parasite, Mic1-3KO is able to form tissue cysts in vivo which are not, in our experimental conditions, infectious when given by oral route. Infection with the attenuated tachyzoite induced lower levels of cytokine and chemokine than with the parental strain. These data demonstrate that the deleted strain derived from a type I strain behaves like type II strain in outbred mice in terms of virulence, dissemination in mouse tissue and persistence in brain.     

Experimental congenital toxoplasmosis in Wistar and Holtzman rats

Congenital toxoplasmosis was evaluated in Wistar and Holtzman rats using two strains of Toxoplasma gondii isolated in Brazil. Pregnant rats were inoculated by subcutaneous or intraperitoneal routes with 10(6) or 8 x 10(6) tachyzoites of N strain (virulent for mice) and by subcutaneous or oral routes with 10(2) or 1.2 x 10(3) cysts of P strain (avirulent for mice). The tissues of rat pups born from these rats were bioassayed for T. gondii infection. T. gondii was not observed in the pups born from rats inoculated with N strain. In the animals inoculated with P strain, congenital toxoplasmosis occurred in 22.8% (Wistar rats inoculated with 10(2) cysts by the subcutaneous route), 11.4% (Wistar rats inoculated with 10(2) cysts by the oral route), 21.2% (Wistar rats inoculated with 1.2 x 10(3) cysts by the oral route) and 2.9% of fetal infection (Holtzman rats inoculated with 10(2) cysts by the oral route). None of the pups born from chronically infected mother were infected with T. gondii.     

Proliferation of Toxoplasma gondii in inflammatory macrophages in vivo is associated with diminished oxygen radical production in the host cell

While reactive oxygen species (ROS) can kill Toxoplasma gondii in vitro the role these molecules play in vivo is not known. We used a flow cytometry-based assay to investigate the relationship between intracellular infection and ROS production during acute peritoneal toxoplasmosis in mice. A distinct population of ROS(+) inflammatory macrophages, detected by the oxidation of hydroethidine, was observed to increase progressively in frequency during the course of infection, and to be inversely correlated with the degree of cell parasitization. These data imply that either intracellular parasites inhibit ROS synthesis or, alternatively, ROS-producing cells contain anti-Toxoplasma activity. The latter interpretation was supported by the finding that uninfected ROS-producing inflammatory macrophages were resistant to infection in vivo. However, in the same animals, ROS-producing macrophages that had previously been parasitized could readily be infected with additional parasites, suggesting that the difference in ROS production between highly infected and less infected cells was not due to ROS-associated killing of parasites within these cells. In addition, macrophages infected with T. gondii in vitro and then briefly transferred to acutely infected mice upregulated ROS production in a manner that was again inversely correlated with the degree of intracellular parasitization. Taken together, these findings suggest that both ROS-associated anti-Toxoplasma activity and parasite-driven inhibition of ROS production underlie the observed pattern of ROS production. ROS function and parasite evasion of this function may contribute significantly to the balance between host defense and disease progression during acute infection.     

Toxoplasma gondii: detection of MIC10 antigen in sera of experimentally infected mice

We developed a sandwich ELISA for the detection of circulating Toxoplasma gondii MIC10 antigens. In T. gondii culture supernatant, MIC10 was detected in a growth dependent manner. Mice were infected with a lethal dose of either a virulent RH strain, an avirulent Beverley strain or a sub-lethal dose of a PLK strain of T. gondii. MIC10 appeared 2 days after infection and increased gradually in the sera of RH-infected mice. A detectable but significantly lower amount of MIC10 was observed in the sera of mice infected intraperitoneally with Beverley tachyzoites. In contrast, the MIC10 antigen in mice sera following oral infection with Beverley cysts was below detectable levels during the course of the experiment. In sera of PLK-infected mice, MIC10 was predominantly observed between late acute and early chronic phase. Our data show that the kinetics of circulating MIC10 differs depending on the strain and route of infection.     

Stage conversion of Toxoplasma gondii RH parasites in mice by treatment with atovaquone and pyrrolidine dithiocarbamate

The mouse-virulent RH strain of Toxoplasma gondii is generally considered to have lost its cyst-forming capacity, and conversion of RH tachyzoites into cysts in non-immune mice has previously been shown exclusively following early treatment with sulfadiazine (SDZ). We here describe the development of tissue cysts in mice infected with RH strain parasites and treated with atovaquone (ATO) combined with pyrrolidine dithiocarbamate (PDTC). Groups of Swiss-Webster mice infected intraperitoneally (i.p.) with 10(2) RH tachyzoites were treated with 5, 25 and 100 mg of ATO/kg per day alone or combined with PDTC at 250 mg/kg per day from day 1 postinfection (p.i.) for 14 days. A total of 19 mice survived the 6-week observation period. Of these, brain cysts were recovered in nine (47%), with burdens ranging from 50 to 3120 (mean +/- S.D. = 622 +/- 963). All cyst-harboring mice had high specific IgG antibody levels (1:10,240-1:40,960, corresponding to 500-2000 IU/ml), as did one mouse in which cysts were not demonstrated, which was therefore included in the group of mice with residual infection. Bioassay performed to test the infectivity of these cysts produced acute lethal toxoplasmosis following i.p. inoculation in all instances (100%), and importantly, following peroral inoculation in four (29%). The recovered tachyzoites were highly infectious. In addition, significantly elevated interferon gamma (IFN-gamma) in the treated mice which developed residual infection compared with any group of infection-free (treated or subinoculated) mice, indicates immunological control of the parasite in the latent form. In conclusion, early treatment of mice infected with T. gondii RH tachyzoites with ATO and PDTC induces conversion into tissue cysts, thus providing a new model for studying the mechanism(s) of T. gondii stage conversion.     

Toxoplasma gondii and mucosal immunity

Toxoplasma gondii, an intracellular parasite infects the host through the oral route. Infection induces a cascade of immunological events that involve both the components of the innate and adaptative immune responses. Alteration of the homeostatic balance of infected intestine results in an acute inflammatory ileitis in certain strains of inbred mice. Both the infected enterocytes as well as the CD4 T cells from the lamina propria produce chemokines and cytokines that are necessary to clear the parasite whereas CD8 intraepithelial lymphocytes secrete transforming growth factor beta that reduces the inflammation. In this review, we describe the salient features of this complex network of interactions among the different components of the gut-associated lymphoid tissue cell population that are induced after oral infection with T. gondii.     

Influence of low-density lipoprotein (LDL) receptor on lipid composition, inflammation and parasitism during Toxoplasma gondii infection

Intracellular replication of Toxoplasma gondii requires cholesterol uptake by host cell low-density lipoprotein receptor (LDLr), a critical element in atherosclerosis. We evaluated host parasitism, inflammatory responses and development of atherosclerosis in LDLr knockout (LDLr(-/-)) and their controls C57BL/6 mice infected with T. gondii. Our results show that T. gondii cysts were reduced in LDLr(-/-) mice when compared to C57BL/6 mice. However, in presence of hypercholesterolemic diet, parasite growth in LDLr(-/-) mice was similar to that seen in infected C57BL/6 mice. In presence of a hypercholesterolemic diet, T. gondii infection leads to a 60% reduction of serum triacylglycerol, total and atherogenic lipoprotein cholesterol. When aortic valve lesion was analyzed, infected mice showed a reduction of atherosclerotic lesion area as well as CD36 expression. MCP-1, SRA-I, SRA-II, ICAM-1 and VCAM-1 mRNA expression was kept similar between infected and control groups. Thus, despite the intense inflammatory process, the drastic reduction in serum lipids seems to limit the development of atherosclerosis in LDLr(-/-) mice infected with T. gondii. In conclusion, our results indicate that T. gondii employs host LDLr to acquire cholesterol and favor its growth. However, in the presence of hypercholesterolemia, T. gondii parasites are able to acquire cholesterol-rich lipoproteins through an alternative host receptor, and overcome LDLr deficiency, favoring host parasitism and impairing lipid loading of foam cells.     

The neurotropic parasite Toxoplasma gondii increases dopamine metabolism

The highly prevalent parasite Toxoplasma gondii manipulates its host's behavior. In infected rodents, the behavioral changes increase the likelihood that the parasite will be transmitted back to its definitive cat host, an essential step in completion of the parasite's life cycle. The mechanism(s) responsible for behavioral changes in the host is unknown but two lines of published evidence suggest that the parasite alters neurotransmitter signal transduction: the disruption of the parasite-induced behavioral changes with medications used to treat psychiatric disease (specifically dopamine antagonists) and identification of a tyrosine hydroxylase encoded in the parasite genome. In this study, infection of mammalian dopaminergic cells with T. gondii enhanced the levels of K+-induced release of dopamine several-fold, with a direct correlation between the number of infected cells and the quantity of dopamine released. Immunostaining brain sections of infected mice with dopamine antibody showed intense staining of encysted parasites. Based on these analyses, T. gondii orchestrates a significant increase in dopamine metabolism in neural cells. Tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, was also found in intracellular tissue cysts in brain tissue with antibodies specific for the parasite-encoded tyrosine hydroxylase. These observations provide a mechanism for parasite-induced behavioral changes. The observed effects on dopamine metabolism could also be relevant in interpreting reports of psychobehavioral changes in toxoplasmosis-infected humans.     

The expression of lactate dehydrogenase is important for the cell cycle of Toxoplasma gondii

In Toxoplasma gondii, lactate dehydrogenase is encoded by two independent and developmentally regulated genes LDH1 and LDH2. These genes and their products have been implicated in the control of a metabolic flux during parasite differentiation. To investigate the significance of LDH1 and LDH2 in this process, we generated stable transgenic parasite lines in which the expression of these two expressed isoforms of lactate dehydrogenase was knocked down in a stage-specific manner. These LDH knockdown parasites exhibited variable growth rates in either the tachyzoite or the bradyzoite stage, as compared with the parental parasites. Their differentiation processes were impaired when the parasites were grown under in vitro conditions. In vivo studies in a murine model system revealed that tachyzoites of these parasite lines were unable to form significant numbers of tissue cysts and to establish a chronic infection. Most importantly, all mice that were initially infected with tachyzoites of either of the four LDH knockdown lines survived a subsequent challenge with tachyzoites of the parental parasites (10(4)), a dose that usually causes 100% mortality, suggesting that live vaccination of mice with the LDH knockdown tachyzoites can confer protection against T. gondii. Thus, we conclude that LDH expression is essential for parasite differentiation. The knockdown of LDH1 and LDH2 expression gave rise to virulence-attenuated parasites that were unable to exhibit a significant brain cyst burden in a murine model of chronic infection.     

The induction of acute ileitis by a single microbial antigen of Toxoplasma gondii

The role of specific microbial Ags in the induction of experimental inflammatory bowel disease is poorly understood. Oral infection of susceptible C57BL/6 mice with Toxoplasma gondii results in a lethal ileitis within 7-9 days postinfection. An immunodominant Ag of T. gondii (surface Ag 1 (SAG1)) that induces a robust B and T cell-specific response has been identified and a SAG1-deficient parasite (Deltasag1) engineered. We investigated the ability of Deltasag1 parasite to induce a lethal intestinal inflammatory response in susceptible mice. C57BL/6 mice orally infected with Deltasag1 parasites failed to develop ileitis. In vitro, the mutant parasites replicate in both enterocytes and dendritic cells. In vivo, infection with the mutant parasites was associated with a decrease in the chemokine and cytokine production within several compartments of the gut-associated cell population. RAG-deficient (RAG1(-/-)) mice are resistant to the development of the ileitis after T. gondii infection. Adoptive transfer of Ag-specific CD4(+) effector T lymphocytes isolated from C57BL/6-infected mice into RAG(-/-) mice conferred susceptibility to the development of the intestinal disease. In contrast, CD4(+) effector T lymphocytes from mice infected with the mutant Deltasag1 strain failed to transfer the pathology. In addition, resistant mice (BALB/c) that fail to develop ileitis following oral infection with T. gondii were rendered susceptible following intranasal presensitization with the SAG1 protein. This process was associated with a shift toward a Th1 response. These findings demonstrate that a single Ag (SAG1) of T. gondii can elicit a lethal inflammatory process in this experimental model of pathogen-driven ileitis.     

Reduction in adhesiveness to extracellular matrix components, modulation of adhesion molecules and in vivo migration of murine macrophages infected with Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite, able to disseminate into deep tissues and cross biological barriers, reaching immunoprivileged sites such as the brain and retina. In order to investigate whether the parasite uses leukocyte trafficking to disseminate throughout the host, the adhesive potential to extracellular matrix components, the expression of adhesion molecules and the in vivo migration of murine macrophages infected with RH strain of T. gondii were investigated. Cellular adhesion to fibronectin, laminin and collagen IV decreased after 24 h of T. gondii infection. However, the decrease in adhesion of infected macrophages observed at early infection was reversed after 48 h. Moreover, decreased adhesion was dependent on active penetration, since heat-killed parasites were unable to reproduce it. Expression of integrins alphaL, alpha4 and alpha5 chains was downmodulated early postinfection, but a progressive regain of expression was observed after 12 h of infection. Expression of beta2, alphav and alpha4 integrins by peritoneal macrophages at late infection was also gradually reestablished. The assessment of in vivo migration of infected macrophages labeled with the fluorescent dye 5-chloromethylfluorescein diacetate showed a 48-h delay in migration to cervical lymph nodes when compared to LPS pre-stimulated macrophages. Furthermore, cells that migrate to distal lymph nodes were loaded with live parasites. Taken together, these results provide insights about T. gondii escape from the host immune response, placing the macrophage as a "Trojan horse", contributing to parasite dissemination and access to immunoprivileged sites.     

Localized recrudescence of Toxoplasma infections in the central nervous system of immunocompromised mice assessed by in vivo bioluminescence imaging

Reactivation of infection in the central nervous system (CNS) with the opportunistic parasite Toxoplasma gondii is a major concern in chronically infected immunocompromised individuals. Yet, the pathophysiology associated with recrudescence of infection remains poorly characterized. The onset of acute reactivated Toxoplasma encephalitis in the murine model was assessed using bioluminescence imaging as a spatio-temporal indicator. An uneven distribution of recrudescence of infection in the CNS was found. Foci of recrudescence after immunosuppression were most commonly located in frontal and parietal cortex, whereas little infection was found in the cerebellum. Recrudescence was also more common in grey matter than in white matter. Pathology was exacerbated in mice deficient in interferon gamma receptors (IFN gamma R(-/-)) corroborating the importance of interferon gamma (IFN gamma) for control of CNS infection. Analysis of parasitic foci identified abundant leukocyte infiltration (CD45+, CD4+, CD8+, F4/80+ cells) in the vicinity of replicating parasites and microvasculature. This is the first report that addresses the suborganic localization of acute Toxoplasma encephalitis in the murine model. Collectively, the findings suggest that the localization of reactivation foci in the CNS, in conjunction with immune responses, influences the outcome of acute reactivated Toxoplasma encephalitis.     

Increased susceptibility to Toxoplasma gondii infection in SAG-1 transgenic mice

SAG-1, one of the major surface proteins of Toxoplasma gondii, has been reported to play an important role in immune and pathogenic mechanisms of the parasites but its exact function is still unclear. We investigated the time courses of T. gondii infection in B6C3F1 transgenic mice carrying the SAG-1 gene. SAG-1 transgenic mice were infected intraperitoneally with a high virulent RH strain or a low virulent Beverley strain of T. gondii. When infected with RH strain tachyzoites, no significant differences in time courses of survivals between SAG-1 transgenic and wild-type mice were observed. Both groups succumbed to an acute infection within 8 days after infection. However, a lower survival rate (20%) was observed in SAG-1 transgenic mice than in wild-type (80%), when infected with Beverley strain cysts. This result indicates that SAG-1 transgenic mice are more susceptible to T. gondii infection as compared with their wild-type counterpart. ELISA using recombinant SAG-1 protein indicates that SAG-1 transgenic mice do not produce antibodies to the SAG-1 molecule. These findings may provide a critical tool for analysing the molecular mechanisms of pathogenesis and host immune responses during toxoplasmosis.     

Aging and the immune response. I. Antibody formation and chronic infection in Toxoplasma gondii-infected mice

Studies were performed to determine the effect of aging on the antibody response and cyst formations after infection with a relatively avirulent strain of the intracellular parasite Toxoplasma gondii. When compared with young mice (4 months), aged mice showed a significant decrease in the magnitude of humoral immune response to infection. This decrease was observed at the peak of the acute infection and also during chronic infection. Evaluation of the presence of Toxoplasma cysts, a measure of the latent infection, revealed that the numbers of tissue cysts present 11 weeks after infection increased with the age of the mice at time of infection. The larger numbers of cysts in older mice which had received the same inoculum size of T. gondii as young mice, together with our previous observations of increased susceptibility of these older mice to T. gondii, suggest that an age-related decrease in the early immune response to this infection allows an increased multiplication of the organism in vivo, leading to increased cyst numbers or death.     

TLR9 is required for the gut-associated lymphoid tissue response following oral infection of Toxoplasma gondii

TLRs expressed by a variety of cells, including epithelial cells, B cells, and dendritic cells, are important initiators of the immune response following stimulation with various microbial products. Several of the TLRs require the adaptor protein, MyD88, which is an important mediator for the immune response following Toxoplasma gondii infection. Previously, TLR9-mediated innate immune responses were predominantly associated with ligation of unmethylated bacterial CpG DNA. In this study, we show that TLR9 is required for the Th1-type inflammatory response that ensues following oral infection with T. gondii. After oral infection with T. gondii, susceptible wild-type (WT; C57BL/6) but not TLR9(-/-) (B6 background) mice develop a Th1-dependent acute lethal ileitis; TLR9(-/-) mice have higher parasite burdens than control WT mice, consistent with depressed IFN-gamma-dependent parasite killing. A reduction in the total T cell and IFN-gamma-producing T cell frequencies was observed in the lamina propria of the TLR9(-/-) parasite-infected mice. TLR9 and type I IFN production was observed by cells from infected intestines in WT mice. TLR9 expression by dendritic cell populations is essential for their expansion in the mesenteric lymph nodes of infected mice. Infection of chimeric mice deleted of TLR9 in either the hemopoietic or nonhemopoietic compartments demonstrated that TLR9 expression by cells from both compartments is important for efficient T cell responses to oral infection. These observations demonstrate that TLR9 mediates the innate response to oral parasite infection and is involved in the development of an effective Th1-type immune response.     

Decreased resistance of B cell-deficient mice to infection with Toxoplasma gondii despite unimpaired expression of IFN-gamma, TNF-alpha, and inducible nitric oxide synthase

The role of B cells in resistance against Toxoplasma gondii was studied using B cell-deficient (muMT) mice. Following peroral infection with 10 cysts of the ME49 strain, all muMT mice survived the acute stage of the infection but died between 3 and 4 wk after infection. In contrast, all control mice were alive at 8 wk after infection. At the stage during which muMT animals succumbed to the infection, parasite replication and pathology were most evident in their brains; small numbers of tachyzoites were also detectable in their lungs. Significantly greater numbers of T. gondii cysts and areas of inflammation associated with tachyzoites were observed in brains of muMT than in control mice. Large areas of necrosis associated with numerous tachyzoites were observed only in brains of muMT mice. Anti-T. gondii IgG Abs were detected only in sera of control mice, whereas similar levels of IFN-gamma were detected in sera of both strains of mice. Amounts of mRNA for IFN-gamma, IL-10, and inducible NO synthase in the brain did not differ between infected muMT and control mice. Expression of mRNA for TNF-alpha was increased in brains of muMT mice. Administration of polyclonal rabbit anti-T. gondii IgG Ab prevented early mortality and pathology associated with tachyzoites in the brain in the infected muMT mice. These results indicate that B cells play an important role, most likely through their production of specific Abs, in resistance to persistent active (tachyzoite) infection with T. gondii in mice, especially in the brain and lung.     

Immune responses of different mouse strains after challenge with equivalent lethal doses of Toxoplasma gondii

Most immunological studies that utilize different strains of inbred mice following T. gondii infection fail to compensate for differences in host susceptibility to the size of the parasite innoculum. To address this concern, susceptible C57BL/6 and resistant CBA/J mice were orally infected with either an equivalent 50% lethal dose (LD50) of brain cysts of the 76K strain of T. gondii (15 cysts in C57BL/6, 400 cysts in CBA/J) or the same dose of parasites in each mouse strain. C57BL/6 mice receiving 400 cysts (LD50 of CBA/J mice) died post infection, whereas CBA/J mice that received 15 cysts (LD50 of C57BL/6 mice) survived. Parasite loads in the brains and serum Toxoplasma-specific IgG1 titers of LD50-infected C57BL/6 mice were significantly higher than those in LD50- or 15 cysts-infected CBA/J mice, whereas splenocyte proliferation to Toxoplasma antigen and the percentage of CD8 alpha+ T cells were reduced in LD50-infected C57BL/6 mice. In contrast, serum IgG2a and IgM titers, the percentage of gamma delta T cells and IFN-gamma expression of spleen of LD50-infected CBA/J mice were higher than those of either 15 cysts-infected CBA/J mice or LD50-infected C57BL/6 mice. These observations demonstrate that the immune response between LD50-infected C57BL/6 and CBA/J mice was more prominent when compared to C57BL/6 or CBA/J mice receiving the same parasite inoculum. These observations would suggest that caution must be excersized in the planning and interpretation of data when the size of the parasite inoculum has not been adjusted for mouse strain.