# Spatiotemporal Diffusion, Colonization, and Antibody Responses in Susceptible C57BL/6J Mice Orally Infected with Toxoplasma gondii Cysts

## TL;DR

This study tracks how Toxoplasma gondii spreads in mice, colonizes organs, and triggers immune responses after oral infection.

## Contribution

The study provides a detailed spatiotemporal analysis of T. gondii infection dynamics and antibody responses in C57BL/6J mice.

## Key findings

- T. gondii rapidly spreads to multiple organs and breaches the blood–brain barrier within 7 days.
- Toxoplasma-specific IgG antibodies increase and remain stable for two months.
- The brain is the primary predilection site for T. gondii cysts, leading to chronic meningitis.

## Abstract

Toxoplasma gondii is an important zoonotic parasite infecting humans and the majority of other warm-blooded animals. The C57BL/6J mouse is susceptible to T. gondii infection and is considered an ideal model organism for T. gondii studies. This research explored the spatiotemporal dynamics of infection, colonization patterns, and antibody response fluctuations in C57BL/6J mice orally infected with T. gondii Type II strain cysts. The mice were orally infected with T. gondii cysts, and their clinical symptoms were monitored daily. The parasite load in various organs was assayed using qPCR targeting the T. gondii B1 gene. Serum antibody responses were assessed using ELISA. The cyst burden in the mouse brain was evaluated through histological analysis and immunofluorescence. T. gondii infection led to clinical manifestations in mice, such as fever and weight loss. The parasite rapidly invaded the animals’ small intestine, spleen, lungs, liver, and heart through the bloodstream and breached the blood–brain barrier to colonize the brain. The production of Toxoplasma-specific IgG antibodies increased and remained stable for two months (i.e., until the end of the experiment). The parasite disseminated rapidly throughout the body, infiltrating most tissues and organs and causing severe enteritis and multi-organ damage due to inflammation. Our findings offer valuable insights into spatiotemporal spread patterns, colonization, predilection sites of infection, dynamic antibody responses, and histopathological changes in C57BL/6J mice after oral infection with T. gondii cysts. These insights advance our understanding of the mechanisms underlying T. gondii pathogenesis and host–T. gondii interaction.

Toxoplasma gondii is an obligate intracellular protozoan that infects humans and other mammals. The C57BL/6J mouse strain is regarded as an ideal model organism for studying T. gondii due to its susceptibility to T. gondii infection and its other advantages over other laboratory animals. However, systematic studies on the response dynamics of the susceptible C57BL/6J mice after oral infection with T. gondii cysts are lacking. To address this research gap, we investigated the spatiotemporal dynamics of infection, colonization, and antibody fluctuations in susceptible C57BL/6J mice orally infected with Type II T. gondii ME49 strain cysts. Mice were orally challenged with T. gondii cysts to examine the infection dynamics. Daily monitoring was conducted for 60 days post-infection (dpi) to assess animals’ clinical signs and survival rates. The parasite burden in various organs was quantified using qPCR targeting the T. gondii B1 gene. The serum antibody responses were evaluated using ELISA. The cyst burden in the mouse brain was assessed via histology and immunofluorescence. T. gondii infection induced clinical symptoms in the mice, including fever and weight loss. T. gondii rapidly invaded the mice’s small intestine, spleen, lungs, liver, and heart via the bloodstream within 1–5 dpi. T. gondii had breached the blood–brain barrier and colonized the brain by 7 dpi. The levels of Toxoplasma-specific IgG antibodies increased and stabilized for two months (until the experiment ended). Systemic parasite dissemination occurred rapidly, infiltrating most tissues and organs, leading to pronounced enteritis and multi-organ damage due to inflammation. The tachyzoites differentiated into bradyzoites when T. gondii infection progressed from the acute to the chronic phase in mice, forming tissue cysts in organs, including the muscles and brain. As a result, the predilection site of T. gondii in mice is the brain, which is where the cysts persisted for the host’s lifetime and continuously induced meningitis. These findings provide valuable insights into the spatiotemporal diffusion, colonization, predilection sites, temporal antibody dynamics, pathogen detection methodologies, and histopathological changes in C57BL/6J mice following oral infection with T. gondii cysts. These insights are important for elucidating T. gondii’s pathogenesis and host–T. gondii interaction.