The protozoan parasite Toxoplasma gondii blocks the innate aversion of rats for cat urine, instead producing an attraction to the pheromone; this may increase the likelihood of a cat predating a rat. This is thought to reflect adaptive, behavioral manipulation by Toxoplasma in that the parasite, although capable of infecting rats, reproduces sexually only in the gut of the cat. The ''behavioral manipulation'' hypothesis postulates that a parasite will specifically manipulate host behaviors essential for enhancing its own transmission. However, the neural circuits implicated in innate fear, anxiety, and learned fear all overlap considerably, raising the possibility that Toxoplasma may disrupt all of these nonspecifically. We investigated these conflicting predictions. In mice and rats, latent Toxoplasma infection converted the aversion to feline odors into attraction. Such loss of fear is remarkably specific, because infection did not diminish learned fear, anxiety-like behavior, olfaction, or nonaversive learning. These effects are associated with a tendency for parasite cysts to be more abundant in amygdalar structures than those found in other regions of the brain. By closely examining other types of behavioral patterns that were predicted to be altered we show that the behavioral effect of chronic Toxoplasma infection is highly specific. Overall, this study provides a strong argument in support of the behavioral manipulation hypothesis. Proximate mechanisms of such behavioral manipulations remain unknown, although a subtle tropism on part of the parasite remains a potent possibility.behavioral manipulation ͉ fear ͉ parasites ͉ predator T he ''behavioral manipulation'' hypothesis states that a parasite can alter host behavior specifically to increase its own transmission efficiency (1, 2). After an acute infection, the protozoan parasite Toxoplasma gondii latently persists in the brain for the life of an infected host, offering an opportunity to study the behavioral manipulation hypothesis (3). Toxoplasma reproduces sexually in a two-species life cycle (4). The sexual phase of its reproduction occurs in the feline intestine, from which highly stable oocysts are excreted in the feces. Grazing animals, including rodents, can then ingest these oocysts. In these hosts, Toxoplasma forms cysts and persists in the central nervous system. The life cycle is completed when a cat eats an infected animal. Recent reports indicate that the parasite blunts the innate aversion of rats for the urine of cats, converting this aversion to an attraction (5), although it does not interfere with energetically costly behaviors related to mating success and social status (6). These findings agree with the behavioral manipulation hypothesis, which predicts that parasites will alter only behaviors that are beneficial to their transmission while leaving other behaviors intact.Several studies have investigated the innate fear of laboratory rodents toward cat odors (7-11). These studies have delineated a neuroanatomical circuit comprising ...

Toxoplasma gondii is an obligate intracellular parasite that persists for the life of a mammalian host. The parasite's ability to block the potent IFN-␥ response may be one of the key mechanisms that allow Toxoplasma to persist. Using a genome-wide microarray analysis, we show here a complete dysregulation of IFN-␥-inducible gene expression in human fibroblasts infected with Toxoplasma. Notably, 46 of the 127 IFN-␥-responsive genes were induced and 19 were suppressed in infected cells before they were exposed to IFN-␥, indicating that other stimuli produced during infection may also regulate these genes. Following IFN-␥ treatment, none of the 127 IFN-␥-responsive genes could be significantly induced in infected cells. Immunofluorescence assays showed at single-cell levels that infected cells, regardless of which Toxoplasma strain was used, could not be activated by IFN-␥ to up-regulate the expression of IFN regulatory factor 1, a transcription factor that is under the direct control of STAT1, whereas uninfected cells in the same culture expressed IFN regulatory factor 1 normally in response to IFN-␥. STAT1 trafficked to the nucleus normally and indistinguishably in all uninfected and infected cells treated with IFN-␥, indicating that the inhibitory effects of Toxoplasma infection likely occur via blocking STAT1 transcriptional activity in the nucleus. In contrast, a closely related apicomplexan, Neospora caninum, was unable to inhibit IFN-␥-induced gene expression. A differential ability to interfere with the IFN-␥ response may, in part, account for the differences in the pathogenesis seen among Toxoplasma and Neospora parasite strains.

Toxoplasma gondii is a ubiquitous parasite that persists for the life of a healthy mammalian host. A latent, chronic infection can reactivate upon immunosuppression and cause life-threatening diseases, such as encephalitis. A key to the pathogenesis is the parasite's interconversion between the tachyzoite (in acute infection) and bradyzoite (in chronic infection) stages. This developmental switch is marked by differential expression of numerous, closely related surface proteins belonging to the SRS (SAG1-related sequence) superfamily. To probe the functions of bradyzoite-specific SRSs, we created a bioluminescent strain lacking the expression of SRS9, one of the most abundant SRSs of the bradyzoite stage. Imaging of mice intraperitoneally infected with tachyzoites revealed that during an acute infection, wild-type and ⌬srs9 strains replicated at similar rates, disseminated systemically following similar kinetics, and initially yielded similar brain cyst numbers. However, during a chronic infection, ⌬srs9 cyst loads substantially decreased compared to those of the wild type, suggesting that SRS9 plays a role in maintaining parasite persistence in the brain. In oral infection with bradyzoite cysts, the ⌬srs9 strain showed oral infectivity and dissemination patterns indistinguishable from those of the wild type. When chronically infected mice were treated with the immunosuppressant dexamethasone, however, the ⌬srs9 strain reactivated in the intestinal tissue after only 8 to 9 days, versus 2 weeks for the wild-type strain. Thus, SRS9 appears to play an important role in both persistence in the brain and reactivation in the intestine. Possible mechanisms for this are discussed.Toxoplasma gondii is an intracellular protozoan parasite that can infect a wide range of mammalian hosts. The parasite switches between two different developmental forms in its intermediate hosts: the tachyzoite, which rapidly divides and disseminates during an acute infection; and the bradyzoite, which encysts and persists in tissues. One hallmark of the Toxoplasma developmental switch is the differential expression of numerous, closely related glycophosphatidylinositol-anchored surface proteins belonging to the SRS (SAG1-related sequence) superfamily (Ͼ160 putative genes) (10). Nucleotide sequence identity can be 20 to 90%, depending on subfamilies. SAG1, the most abundant SRS antigen, is specific to the tachyzoite stage and serves as a prototype (22). The majority of SRS antigen genes are ϳ1.5 kb in length, lack introns, and are found scattered throughout the genome in tandemly arrayed multigenic clusters with intergenic distances ranging typically from 1.5 to 2.5 kb (10). The transcription start site is in the same orientation for every gene within a cluster, indicating that these clusters have probably arisen through gene duplication. For the majority of SRS sequences for which multiple expressed sequence tags exist, mRNA expression is developmentally regulated such that tachyzoites and bradyzoites express largely nonoverlapping sets of mu...

Toxoplasma persists in the face of a functional immune system. This success critically depends on the ability of parasites to activate a strong adaptive immune response during acute infection with tachyzoites that eliminates most of the parasites and to undergo stage conversion to bradyzoites that encyst and persist predominantly in the brain. A dramatic change in antigenic composition occurs during stage conversion, such that tachyzoites and bradyzoites express closely related but antigenically distinct sets of surface Ags belonging to the surface Ag 1 (SAG1)-related sequence (SRS) family. To test the contribution of this antigenic switch to parasite persistence, we engineered parasites to constitutively express the normally bradyzoite-specific SRS9 (SRS9c) mutants and tachyzoite-specific SAG1 (SAG1c) mutants. SRS9c but not wild-type parasites elicited a SRS9-specific immune response marked by IFN-γ production, suggesting that stage-specificity of SRS Ags determines their immunogenicity in infection. The induction of a SRS9-specific immune response correlated with a continual decrease in the number of SRS9c cysts persisting in the brain. In contrast, SAG1c mutants produced reduced brain cyst loads early in chronic infection, but these substantially increased over time accompanying a hyperproduction of IFN-γ, TNF-α, and IL-10, and severe encephalitis. We conclude that stage-specific expression of SRS Ags is among the key mechanisms by which optimal parasite persistency is established and maintained.

Glutamic acid decarboxylase (GAD)65 is an early and important antigen in both human diabetes mellitus and the nonobese diabetic (NOD) mouse. However, the exact role of GAD65-specific T cells in diabetes pathogenesis is unclear. T cell responses to GAD65 occur early in diabetes pathogenesis, yet only one GAD65-specific T cell clone of many identified can transfer diabetes. We have generated transgenic mice on the NOD background expressing a T cell receptor (TCR)-specific for peptide epitope 286–300 (p286) of GAD65. These mice have GAD65-specific CD4+ T cells, as shown by staining with an I-Ag7(p286) tetramer reagent. Lymphocytes from these TCR transgenic mice proliferate and make interferon γ, interleukin (IL)-2, tumor necrosis factor (TNF)-α, and IL-10 when stimulated in vitro with GAD65 peptide 286–300, yet these TCR transgenic animals do not spontaneously develop diabetes, and insulitis is virtually undetectable. Furthermore, in vitro activated CD4 T cells from GAD 286 TCR transgenic mice express higher levels of CTL-associated antigen (CTLA)-4 than nontransgenic littermates. CD4+ T cells, or p286-tetramer+CD4+ Tcells, from GAD65 286–300-specific TCR transgenic mice delay diabetes induced in NOD.scid mice by diabetic NOD spleen cells. This data suggests that GAD65 peptide 286–300-specific T cells have disease protective capacity and are not pathogenic.