# A humanized IFN-γ mouse model reveals skin eschar formation, enhanced susceptibility and scrub typhus pathogenesis

## TL;DR

A new genetically modified mouse model with human IFN-γ signaling shows increased susceptibility to scrub typhus and mimics human disease features like skin eschar formation.

## Contribution

The first genetically engineered mouse model with human IFN-γ signaling that mimics human scrub typhus disease and immune responses.

## Key findings

- hIFNG/hIFNGR mice show increased susceptibility to intradermal Ot infection with greater weight loss and bacterial burden.
- These mice develop skin eschar-like lesions and systemic inflammation resembling human scrub typhus pathology.
- The model exhibits dysregulated T cell and neutrophil responses and reduced interferon-stimulated gene expression.

## Abstract

Scrub typhus, caused by Orientia tsutsugamushi (Ot) bacteria, is a serious acute febrile illness associated with significant mortality. No effective vaccine is currently available, largely due to the complex Ot strain diversity and an incomplete understanding of protective immune mechanisms. To overcome these challenges, there is a critical need for a suitable animal model that mimics human disease through the natural route of infection. Here, we report for the first time that a genetically engineered humanized mouse strain (with triple knockout/knock-in of IFN-γ and its receptors, abbreviated as hIFNG/hIFNGR), exhibits increased susceptibility to intradermal Ot infection compared to wild-type (WT) mice. This is evidenced by greater body weight loss, elevated bacterial burden, and reduced expression of interferon-stimulated genes (ISGs). hIFNG/hIFNGR mice exhibit pronounced biochemical abnormalities and tissue pathology accompanied by dysregulated T cell and neutrophil responses following infection. Notably, this novel mouse strain with human IFN-γ signaling can develop skin eschar-like lesions resembling those observed in human patients. Overall, our study introduces a promising mouse model to dissect the immunopathogenesis of scrub typhus and evaluate future vaccine candidates.

Scrub typhus is a serious disease caused by the obligately intracellular bacterium Ot that spreads to humans through the bite of larval mites called chiggers. Although it is treatable with antibiotics, delayed diagnosis and treatment can lead to severe infection and fatal outcomes. Unfortunately, we still lack a clear understanding of how this infection causes disease, in part due to the lack of laboratory models that accurately reflect human responses to infection. Our recent reports have suggested an important role of IFN-γ in host protection against Ot infection. In this study, we used a new genetically modified mouse strain that carries functionally attenuated human IFN-γ signaling in place of its mouse counterpart. We found that hIFNG/hIFNGR mice are more vulnerable to infection, develop skin eschar lesions like those in patients, and show signs of systemic inflammation and organ damage. Their immune response also resembled what has been observed in human patients. This new mouse model can help scientists better understand the mechanisms as to how this bacterial species causes severe disease outcomes in patients. Once those mechanisms are understood, this mouse model will serve a further purpose as a tool for testing new vaccines and treatments to aid humans at-risk for scrub typhus.