# Restriction of Toxoplasma growth in human cells

> **NIH NIH R01** · UNIVERSITY OF CALIFORNIA AT DAVIS · 2024 · $623,349

## Abstract

Toxoplasma gondii is an obligate intracellular parasite that can cause severe disease in congenitally infected
infants and in immunosuppressed people. Toxoplasma co-opts host cells by secreting effector proteins, called
ROPs and GRAs, into the host cell. In mice, the cytokine interferon gamma (IFNγ) is essential to control
Toxoplasma by inducing a variety of parasiticidal mechanisms while in humans the cytokine tumor necrosis
factor (TNF)α can compensate for the absence of IFNγ. The IFNγ-induced mechanisms that control
Toxoplasma in rodents are largely absent in humans and most of the parasite’s ROPs and GRAs that
determine virulence in rodents play no role in counteracting the human IFNγ response. Thus, despite
Toxoplasma’s enormous health implications, there is currently a lack of mechanistic knowledge on (i) how
IFNγ- or TNFα-stimulated primary human cells inhibit Toxoplasma growth; and (ii) how Toxoplasma secreted
effectors allow parasite growth in stimulated primary human cells. Our long-term goal is to determine how
Toxoplasma secreted effectors mediate its survival in humans, even in the presence of a fully functioning
immune system, which could help the development of targeted interventions to minimize the adverse health
effects of toxoplasmosis in at-risk individuals. The overall objective for this application is to determine the
mechanisms by which activated primary human cells detect and destroy Toxoplasma and how Toxoplasma
counteracts these mechanisms. Our hypotheses are (i) that mechanisms of Toxoplasma growth inhibition in
stimulated human cells are guanylate-binding protein (GBP1)-mediated breakage of the vacuole followed by
endolysosomal fusion with the parasitophororous vacuole, which is Toxoplasma’s replication niche, and
induction of early parasite egress; and (ii) that the novel Toxoplasma GRA effectors identified from our
genome-wide parasite loss-of-function screen in IFNγ-stimulated human cells determine parasite resistance to
IFNγ by preventing early parasite egress. The hypotheses will be tested by pursuing two specific aims: 1)
Determine the mechanism by which IFNγ/TNFα-stimulated human cells inhibit Toxoplasma growth; and 2)
Determine the mechanism by which Toxoplasma effectors affect parasite fitness in IFNγ/TNFα-stimulated
human cells. The research proposed in this application is innovative because it is using results from an
unbiased innovative CRISPR/Cas9 loss-of-function screen that identified Toxoplasma genes that determine
resistance to IFNγ-mediated parasite growth inhibition in primary human cells. These results are expected to
have an important positive impact because they will provide a science-based framework for the future
development of novel targets for therapy, e.g., by stimulating host parasite killing mechanism or by inhibiting
parasite virulence determinants. Our results will also aid prediction of Toxoplasma strain virulence in humans
and identification of human toxoplasmosis susceptibility loci.

## Key facts

- **NIH application ID:** 10801127
- **Project number:** 1R01AI173803-01A1
- **Recipient organization:** UNIVERSITY OF CALIFORNIA AT DAVIS
- **Principal Investigator:** JEROEN SAEIJ
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $623,349
- **Award type:** 1
- **Project period:** 2023-11-13 → 2028-10-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10801127

## Citation

> US National Institutes of Health, RePORTER application 10801127, Restriction of Toxoplasma growth in human cells (1R01AI173803-01A1). Retrieved via AI Analytics 2026-05-26 from https://api.ai-analytics.org/grant/nih/10801127. Licensed CC0.

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