Identifying how interferon-induced immune and Toxoplasma secreted factors interact to control infection in susceptible and resistant livestock species

Toxoplasma is an important foodborne parasite in terms of animal productivity, human health, and food safety. Toxoplasma is a major cause of abortions and still-births in livestock, particularly pigs and sheep. Chronic infection also increases the susceptibility of livestock to other devastating pathogens, such as porcine reproductive and respiratory syndrome virus (PRRSV) that can kill over 80% of infected pigs.

In humans, Toxoplasma is the principal cause of coma and early death in HIV/AIDS patients, ocular disease and blindness in immune-competent individuals, and neurological disorders in the developing foetus. In terms of food safety, meat products from chronically infected livestock, particularly pigs, is the main source of foodborne human Toxoplasma infections.

Except for a live attenuated vaccine for sheep, no safe anti-parasitic treatment or prophylactic approaches exists to target Toxoplasma in livestock and humans, making the development of novel tools to repel Toxoplasma an important priority for biomedical research.

Interferon gamma (IFNg), an inflammatory immune factor, is critical to successful anti-parasitic responses to Toxoplasma in almost all hosts. For example, human macrophages or mice with defects in detecting IFNg are highly susceptible to Toxoplasma. Meanwhile, Toxoplasma secretes proteins into host cells that enable it to evade IFNg-induced cellular processes to establish a safe niche to replicate.

Thus, the outcome of Toxoplasma infection, which varies significantly between host species, is a complex interplay between IFNg-induced host responses and parasite virulence mechanisms.

Among livestock species, cattle are more resistant to Toxoplasma than pigs. For instance, acute infection is often asymptomatic in cattle but can cause fever, general weakness, and abortion in pigs. In preliminary studies, we confirmed that cattle and pig macrophages, which are also the cells preferentially infected in vivo, recapitulate this difference in susceptibility to Toxoplasma.

First, we observed that IFNg-stimulated cattle macrophages are more resistant to Toxoplasma than pig macrophages. This difference was not observed in naïve macrophages, confirming an essential role for IFNg. Secondly, we observed differential expression of over 500 interferon stimulated genes (ISGs) between Toxoplasma-infected cattle and pig macrophages.

Guided by these preliminary data, we postulate that difference in susceptibility to Toxoplasma between cattle and pigs is due to differences in Toxoplasma virulence or IFNg-induced anti-parasitic mechanisms between cattle and pigs. Here, we will address this hypothesis by using a panel of molecular biology and parasitology approaches, including CRISPR/cas9 gene knockout screens and parasite growth assays, to identify how Toxoplasma genes and ISGs control Toxoplasma infection in cattle and pig macrophages.

The outputs from this work will advance our knowledge on the mechanisms that control Toxoplasma in cattle and pigs. In the long-term, information about ISGs that inhibit Toxoplasma in cattle or parasite genes that enhance parasite virulence in pigs, can be exploited to accelerate the development of tools to reduce the disease burden and zoonotic threat of Toxoplasma in pigs.

In addition, the ISGs and Toxoplasma gene knockout system that we will developed in this proposal, will be useful tools to study other pathogens controlled by IFNg in cattle, including Neospora or Toxoplasma virulence mechanisms in other susceptible hosts, including sheep.