# Genetic barcoding to track Toxoplasma cyst heterogeneity during brain colonization, reactivation, and drug treatment.

> **NIH NIH R21** · UNIVERSITY OF CALIFORNIA AT DAVIS · 2023 · $189,806

## Abstract

Project Summary
Toxoplasma tissue cysts form the basis for lifelong chronic infections in humans. In AIDS patients, the immune
system can no longer control recrudescent parasites from brain cysts leading to uncontrolled parasite
replication and Toxoplasma encephalitis (TE). New drug candidates are needed to treat TE since current drugs
have toxic side effects and they do not eliminate the chronic cyst stages. It is not known why current drugs do
not eliminate all brain cysts. Because recrudescent parasites from cysts can form new cysts over time, it is
unclear if in each cyst there is a fraction of treatment-resistant parasites or if entire cysts are either drug
resistant or susceptible. Similarly, when Toxoplasma reactivates during immunosuppression and causes TE it
is not known if this is due to only some cysts reactivating or if most cysts reactivate. Because individual cysts
cannot be distinguished in a pool, current methods to assess cyst dynamics during drug treatment or
reactivation rely on the crude measure of following total brain cyst burden. There is, therefore, a critical need to
develop methodology that allows one to follow the within host-dynamics of brain colonization and brain cysts
heterogeneity during reactivation and drug treatment. Our preliminary data show that we can use genetically
barcoded parasites to follow the fate of individual parasites within a host. Our central hypothesis is that
heterogeneity of the parasites inside cysts determines heterogeneity in resistance to therapeutics or
reactivation. In our first aim, we will use barcoded parasites expressing luciferase to follow the dynamics of
cyst formation in immunocompetent mice and cyst reactivation induced by immunosuppression. This will allow
us to determine what fraction of the initial Toxoplasma inoculum disseminates into the brain and subsequent
brain cysts dynamics, on the individual cyst level, over time. This is critical information that will allow us to
determine the complexity of loss-of-function parasite libraries that can be analyzed in future pooled screens
designed to identify parasite genes that are essential for brain colonization, cyst formation, cyst persistence
and parasite recrudescence. In this aim we will also determine what fraction of cysts reactivates and
disseminates. In our second aim, we will determine if cyst resistance to drug treatment is due to a fraction of
individual cysts being resistant or to a fraction of parasites within each cyst being resistant. These results are
expected to have an important positive impact because they are expected to allow 1) a more refined
assessment of the efficacy of drugs designed to eliminate chronic tissue cysts; 2) the design of future pooled
loss-of-function screens to identify parasite genes that are essential for brain colonization, cyst formation, cyst
persistence, or parasite recrudescence.

## Key facts

- **NIH application ID:** 10664008
- **Project number:** 5R21AI170420-02
- **Recipient organization:** UNIVERSITY OF CALIFORNIA AT DAVIS
- **Principal Investigator:** JEROEN SAEIJ
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2023
- **Award amount:** $189,806
- **Award type:** 5
- **Project period:** 2022-07-12 → 2025-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10664008

## Citation

> US National Institutes of Health, RePORTER application 10664008, Genetic barcoding to track Toxoplasma cyst heterogeneity during brain colonization, reactivation, and drug treatment. (5R21AI170420-02). Retrieved via AI Analytics 2026-05-23 from https://api.ai-analytics.org/grant/nih/10664008. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*