Host genetics uncover IFN-γ-dependent Toxoplasma genes in in vivo CRISPR screens(Yamamoto Lab, in Cell Reports)
While the presence of harmful critters in your home may be addressed by a call to the exterminator,
harmful parasites in the body can be much harder to expel. A single-celled parasite known as
Toxoplasma gondii (hereafter Toxoplasma) commonly infects humans and other animals, and theresulting condition, known as toxoplasmosis, can cause severe health issues in people with weakened
immune systems. Recently, researchers in Japan have developed a tool to investigate how
Toxoplasma promotes its survival within a host, shedding new light on potential strategies for thetreatment and prevention of toxoplasmosis.
In a new study published in Cell Reports, researchers from Osaka University conducted a genetic
screen using CRISPR technology in mice to confirm known genes and identify new genes involved inToxoplasma fitness, i.e., the ability of the parasite to survive and thrive within a host. CRISPR genome
editing is a method in which short fragments of genetic material known as guide RNAs (gRNAs) areutilized to modify specific sequences of DNA in the genome.

Host genetics is combined with parasite genetic screening
CRISPR screens, in which many gRNAs are used to systematically modify target genes in an organism,
can be used to infer which genes are involved in a specific biological process. The research team
aimed to use a CRISPR screen in Toxoplasma-infected mice to identify virulence factors, which aretraits that help Toxoplasma evade the host immune system. While virulence factors associated withToxoplasma have been previously identified in hosts with healthy immune systems, these factors
have not been fully explored in hosts with weakened immune systems.

“When Toxoplasma infection occurs, the hostʼs immune system responds by producing a protein
known as interferon-γ (IFN-γ), which stimulates other molecules to help prevent Toxoplasma frommultiplying,” explains senior author Masahiro Yamamoto. “We performed our CRISPR screen in both
healthy mice and immunocompromised mice that lack the receptor for IFN-γ, which allowed us toevaluate the effects of both host genetics and parasite genetics on immune-related virulence factors.”
The researchers infected mice with Toxoplasma; these Toxoplasma contained gRNA libraries that
affected certain genes of interest. Later, the team extracted Toxoplasma from the mice and evaluated
the gene expression of the parasite. This analysis allowed the research team to identify which genes
seem to promote Toxoplasma fitness, and also facilitated comparison of gene expression between
healthy and immunodeficient mice.
“Our highly reproducible, in vivo screen identified both IFN-γ-dependent and IFN-γ-independent
fitness genes that are involved in the promotion of Toxoplasma virulence,” says lead author Yuta
Tachibana. “We successfully identified many virulence factors of Toxoplasma.”
These genes represent potential targets for the treatment and prevention of Toxoplasma infection.
Additionally, the research teamʼs method of in vivo genetic screening can serve as a foundation forthe development of new therapeutics and vaccines for toxoplasmosis.

Journal: Cell Reports (June 1, 2023 online)
Title: “Host genetics highlights IFN-γ-dependent Toxoplasma genes encoding secreted and nonsecreted virulence factors in in vivo CRISPR screens”
Authors: Yuta Tachibana, Emi Hashizaki, Miwa Sasai and Masahiro Yamamoto
DOI: https://doi.org/10.1016/j.celrep.2023.112592