# STAT1-mediated interferon signaling in the hematopoietic system is essential for restricting Usutu virus infection in vivo

## TL;DR

This study shows that STAT1-mediated interferon signaling in blood cells is crucial for fighting Usutu virus infection in mice.

## Contribution

The study identifies hematopoietic cells as central to USUV pathogenesis and highlights the role of innate immunity in restricting the virus.

## Key findings

- STAT1-deficient mice showed lethal USUV infection due to systemic viral dissemination.
- Intact innate immune signaling in hematopoietic cells is essential for host defense against USUV.
- B, T, and NK cells are not necessary for restricting USUV infection.

## Abstract

Usutu virus (USUV) is an emerging mosquito-borne flavivirus known to induce neuroinvasive disease in birds, mice, and humans in European and African countries. The mechanisms of infection and dissemination remain poorly understood. Thus, elucidating how USUV spreads in a susceptible host is crucial for identifying therapeutic targets. To investigate host defenses against USUV, we generated an infectious clone of the TC508 isolate. After characterizing its replication dynamics in cultured cells from multiple species, we investigated its pathogenesis in an array of mice with genetic perturbations. Previous studies demonstrated that whole-body deletion of type I interferon (IFN) signaling led to widespread USUV infection and fatality in mice. Here, we observed the same lethal phenotype in STAT1-deficient mice and identified hematopoietic cells specifically as central to USUV pathogenesis in a mammalian host. Deletion of STAT1 in all hematopoietic subsets, but not hepatocytes, neurons, macrophages or conventional dendritic cells, was sufficient for systemic viral dissemination and ultimate fatality. Conversely, mice lacking functional B, T, and natural killer (NK) cells but with intact myeloid cells were resistant to USUV. Our findings provide new insights into the tissue-specific barriers that regulate USUV infection and underscore the importance of innate immunity in host defense for this important emerging flavivirus.

Mosquito-borne viruses are continual threats to human and animal health globally. With climate change, the geographic distribution of mosquitoes is projected to expand, leading to increased burden of their accompanying viruses. The flavivirus genus consists of well-known threats including yellow fever virus, Zika virus, dengue viruses, West Nile virus, and Japanese encephalitis virus as well as closely related, but understudied species, such as Usutu virus. Although most Usutu virus infections in humans are asymptomatic, symptoms of neuroinvasive disease in individuals have been recorded. Further, this virus is highly lethal in avian species and has caused massive die-offs of Eurasian blackbirds. Here, we investigated Usutu virus infection in various cell lines and mouse strains to gain insight into how this virus establishes infection in a susceptible host. Cell lines derived from humans, mice, chicken, and mosquitoes were all susceptible to Usutu virus. Immunocompetent mice were resistant to infection, but mice with disrupted innate immune signaling succumbed to lethal infection. Notably, intact innate immune signaling was essential in hematopoietic cells, but dispensable in liver and neuronal cells. Additionally, B, T, and natural killer cells were not necessary to restrict Usutu virus infection.