Study shows how genetic defects in Toxoplasma are rescued by co-infection

Researchers have shown how genetic defects in Toxoplasma gondii are rescued if co-infected with normal parasites, thanks to supportive secreted proteins.

The study was published today as a Version of Record, after previously appearing as a Reviewed Preprint in eLife, is described by the editors as important work, with convincing evidence to show how Toxoplasma gondii parasites missing a protein called MYR1 can still survive and proliferate when co-infected with normal parasites, due to secreted factors. The findings also reveal a potential limitation of pooled CRISPR screens in studying parasite biology in live hosts. The study was lead by authors Francesca Torelli and Diogo M. da Fonseca, both postdoctoral researchers in Moritz Treeck’s laboratories at the Francis Crick Institute in London, UK and the Gulbenkian Institute for Molecular Medicine (GIMM), Lisbon, Portugal. 

Toxoplasma gondii is a microscopic parasite that can infect almost all warm-blooded animals, including humans. In fact, it is estimated that over a third of the world’s human population may unknowingly carry the parasite. Toxoplasma gondii is infamous for its ability to manipulate the behaviour of its host – most notably by making rodents less afraid of cats, helping the parasite complete its life cycle in feline intestines.

The protein MYR1 is a key player in helping Toxoplasma parasites secrete proteins into host cells to manipulate their function. 

“In previous work, we have shown that mice infected with Toxoplasma strains lacking MYR1 survive, underscoring the protein’s role in the parasite’s survival and proliferation”, explains Francesca Torelli. “Interestingly, MYR1 mutants appeared to show no fitness defects when co-infected with parasites with no mutations in this locus, leading us to hypothesize that these normal parasites release helpful supporting molecules – an effect known as paracrine signalling.”

In the current study, Torelli, da Fonseca and colleagues sought to validate this hypothesis. First, they tested whether MYR1 was necessary for Toxoplasma to survive in immune cells activated by interferon-gamma (IFN-γ), which enhances immune defenses. MYR1-deficient parasites performed similarly to normal parasites, suggesting that MYR1 is not critical for parasite survival in these conditions. 

Next, the team tracked the growth of MYR1-deficient parasites in live mice using bioluminescent imaging. These mutants grew more slowly than normal parasites, but they still expanded during early infection and formed a small number of cysts in the brain of surviving mice. This suggests that while MYR1 helps the parasite thrive, it is not essential for survival or cyst formation.

To test for paracrine effects, the researchers infected mice with a mix of MYR1-deficient parasites and either normal or mutant parasites. The MYR1-deficient parasites grew better when normal parasites were present, confirming that secreted factors from the normal parasites supported their growth.

“When we measured pro-inflammatory molecules (cytokines) in the infected mice, we found higher levels of IFN-γ and other cytokines when normal parasites were present. However, the supportive effect on MYR1-deficient parasites persisted, even in mice lacking T and B cells, suggesting that adaptive immunity is not essential for the paracrine rescue to occur”, says Diogo da Fonseca.

“We have shown that fitness defects in Toxoplasma gondii parasites lacking MYR1 can be rescued if co-infected with wild-type parasites, and shine light on Toxoplasma’s ability to subvert the host immune response beyond the infected cell”, says senior author Moritz Treeck. “Our findings also highlight an important limitation of pooled CRISPR screens in mice, which is also probably encountered in CRISPR screens in general where paracrine effects occur. Future applications of CRISPR screens, for example in combination with functional assays to explore immunological contexts could help understand how infected cells affect the neighbouring environment to support infection and contribute to parasite survival in the host.” 

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