Differential gene expression in mice infected with distinct Toxoplasma strains

Abstract

Toxoplasma gondii is the causative agent of toxoplasmosis in human and animals. In a mouse model, T. gondii strains can be divided into three groups, including the virulent, intermediately virulent, and nonvirulent. The clonal type I, II, and III T. gondii strains belong to these three groups, respectively. To better understand the basis of virulence phenotypes, we investigated mouse gene expression responses to the infection of different T. gondii strains at day 5 after intraperitoneal inoculation with 500 tachyzoites. The transcriptomes of mouse peritoneal cells showed that 1,927, 1,573, and 1,009 transcripts were altered more than 2-fold by type I, II, and III infections, respectively, and that the majority of altered transcripts were shared. Overall transcription patterns were similar in type I and type II infections, and both had greater changes than infection with type III. Quantification of parasite burden in mouse spleens showed that the burden with type I infection was 1,000 times higher than that of type II and that the type II burden was 20 times higher than that of type III. Fluorescence-activated cell sorting revealed that type I and II infections had comparable macrophage populations, and both were higher than the population with type III infection. In addition, type I infection had a higher percentage of neutrophils than type II and III infections. Taken together, these results suggested that there is a common gene expression response to T. gondii infection in mice. This response is further modified by parasite strain-specific factors that determine their distinct virulence phenotypes.

Fig 1

Hierarchical clustering analysis of gene expression in CD-1 outbred mice at day 5 postinfection with T. gondii GT1, PTG, and CTG strains. Mice that were not infected with T. gondii were included as negative controls (NEG). Red indicates high expression levels; blue indicates low expression levels. The bars on the left indicate clusters of genes with similar expression levels among different infections.

Fig 2

Venn diagram of gene expression in mice infected with GT1, PTG, and CTG strains. Genes with ≥2-fold up- or downregulation in infected mice versus uninfected (NEG) mice were included.

Fig 3

Comparison of mouse gene expression profiles for GT1, PTG, and CTG infection based on biological processes. Mouse genes that were up- or downregulated are sorted by the natural log of the fold change relative to the negative controls.

Fig 4

Parasite burdens in mice at day 5 postinfection. Parasite load was determined by qPCR. GT1 has a significantly higher parasite burden than PTG and CTG (*, P = 0.001, by ANOVA).

Fig 5

FACS analysis. Mice were infected with GT1, PTG, and CTG strains. Peritoneal cells were collected and stained for markers of dendritic cells (CD11c), macrophages (F4/80), and neutrophils (Ly-6G) and subsequently identified by FACS analysis.