Toxoplasma co-opts host gene expression by injection of a polymorphic kinase homologue.

Toxoplasma gondii, an obligate intracellular parasite of the phylum Apicomplexa, can cause severe disease in humans with an immature or suppressed immune system. The outcome of Toxoplasma infection is highly dependent on the strain type, as are many of its in vitro growth properties. Here we use genetic crosses between type II and III lines to show that strain-specific differences in the modulation of host cell transcription are mediated by a putative protein kinase, ROP16. Upon invasion by the parasite, this polymorphic protein is released from the apical organelles known as rhoptries and injected into the host cell, where it ultimately affects the activation of signal transducer and activator of transcription (STAT) signalling pathways and consequent downstream effects on a key host cytokine, interleukin (IL)-12. Our findings provide a new mechanism for how an intracellular eukaryotic pathogen can interact with its host and reveal important differences in how different Toxoplasma lineages have evolved to exploit this interaction.

Polymorphic secreted kinases are key virulence factors in toxoplasmosis.

The majority of known Toxoplasma gondii isolates from Europe and North America belong to three clonal lines that differ dramatically in their virulence, depending on the host. To identify the responsible genes, we mapped virulence in F(1) progeny derived from crosses between type II and type III strains, which we introduced into mice. Five virulence (VIR) loci were thus identified, and for two of these, genetic complementation showed that a predicted protein kinase (ROP18 and ROP16, respectively) is the key molecule. Both are hypervariable rhoptry proteins that are secreted into the host cell upon invasion. These results suggest that secreted kinases unique to the Apicomplexa are crucial in the host-pathogen interaction.

Signaling pathways initiated in macrophages after engagement of type A scavenger receptors.

Scavenger receptors are macrophage cell surface molecules associated with endocytic uptake of lipoproteins and binding of microbial ligands. Macrophage class A scavenger receptors (SR-As) interact with ligands to induce cellular signaling leading to gene transcription and cytokine release. We used inhibitors of early and late signaling to block SR-A-mediated polyinosinic-polycytidilic acid (poly I:C) and lipoteichoic acid (LTA) activation of RAW 264.7 macrophages. Effects of multiple inhibitors on tumor necrosis factor (TNF)-alpha release were monitored to determine requirements for inflammatory cytokine production. Cycloheximide, monodansylcadaverine, and cytochalasin B all blocked TNF-alpha release from macrophages stimulated with LTA or poly I:C, whereas monensin only nominally reduced TNF-alpha production. Selected inhibitors of downstream signaling events reduced SR-A-dependent TNF-alpha release by >95% after stimulation with either ligand, whereas others were ineffective. The PKC inhibitor H7 reduced LTA-dependent secretion of TNF-alpha by 94% but inhibited poly I:C-dependent TNF-alpha production only by 50%. Priming of RAW 264.7 cells with interferon-gamma potentiated the response to poly I:C but did not alter inhibitor effects. These results demonstrated that for both ligands tested here, early events of receptor internalization are requisite for cellular activation. The response pattern suggests that tyrosine phosphorylation and activation of the MAP kinase pathway are key components of SR-A-mediated signal transduction cascades.

Analysis of macrophage activation in African trypanosomiasis.

African trypanosomes cause a fatal disease of man and animals that is characterized by extensive functional, histological, and pathological changes in the lymphoid tissues of infected hosts, including an increase in the numbers and activation state of macrophages. Macrophage activation during infection is the result of exposure of these cells to parasite components and host-derived IFN-gamma, produced in response to parasite antigens. The balance of these different activation signals may determine the outcome of infection. In the experiments described here, we assessed the ability of the variant surface glycoprotein (VSG) of the organism Trypanosoma brucei rhodesiense (T.b. rhodesiense) to activate macrophages directly. Our results demonstrate that macrophages bind and are activated by the VSG molecule. The resulting profile of activation differs from that stimulated by IFN-gamma. These results suggest that the interaction of host macrophages with VSG released during parasite infection may be a key component of trypanosomiasis.

Analysis of interferon-gamma-dependent and -independent pathways of macrophage activation.

Macrophages are a cellular cornerstone of the innate immune response. The outcome of macrophage activity during development of an immune response to microbes results from macrophage activation by both organism-derived and host-derived factors. In order to more fully understand the spectrum of responses expressed by macrophages when encountering these distinct stimuli, we investigated the similarities and differences between interferon-gamma receptor (IFN-gammaR)-dependent macrophage activation and stimulation of macrophages through the Type A1 scavenger receptor (SR). We observed distinct patterns of macrophage activation depending on the nature of the ligand. IFN-gamma and the SR ligand lipotechoic acid (LTA) induced largely non-overlapping sets of genes. The use of two additional SR ligands, maleylated bovine serum albumin and the polydeoxynucleotide poly dI:dC, revealed differences within SR activation-induced gene expression. We also observed that priming with IFN-gamma resulted in an enhanced response to subsequent SR-mediated activation. These results suggest that full potentiation of macrophage activity during development of an antimicrobial immune response is achieved by activation of these cells through multiple receptors.

Women's attributions of responsibility for date rape: the influence of empathy and sex-role stereotyping.

The purpose of the study presented here was to investigate the relationship among sex-role stereotyping, empathy with the victim, and subsequent blaming of the victim in response to a date-rape scenario. It was hypothesized that sex-typed (traditional) females would be less likely to perceive forced sex on a date as rape and would attribute more responsibility to the victim than would more egalitarian (nontraditional) females. It was also predicted that the enhancement of victim empathy would result in less victim blame. The subjects were 76 female undergraduates who were chosen on the basis of their extreme scores on a sex-role stereotyping scale. Vignettes describing a date rape were used to manipulate victim empathy. Findings indicated that although attributions of responsibility were influenced by the subject's sex-role stereotyping, the manipulation of empathy had no apparent influence on victim blame. Furthermore, the lack of correlation between the degree of victim empathy and the subject's own history of victimization suggests that victim empathy is not a component in victim blame.