# A44 STAT1 ACTS IN INNATE IMMUNE CELLS TO PREVENT LIVER PATHOLOGY FOLLOWING ASYMPTOMATIC INTESTINAL VIRAL INFECTION

## TL;DR

This study shows that STAT1 in innate immune cells prevents severe liver damage caused by a harmless gut virus in mice.

## Contribution

The study identifies STAT1 as a critical host factor in controlling asymptomatic intestinal viral infections to prevent liver pathology.

## Key findings

- STAT1-deficient mice develop severe liver necrosis and weight loss after asymptomatic intestinal viral infection.
- Innate immune cells lacking STAT1 fail to control viral replication, leading to liver pathology.
- STAT1 expression in hematopoietic cells is necessary and sufficient to prevent disease.

## Abstract

In the mammalian gut, interactions between the host and commensal organisms must be regulated to prevent damage and derive benefits from these organisms. While this regulation has been well-studied in the context of bacteria, much less is known about how commensal-like viruses in the intestine are managed by the host.

Evidence demonstrates that viral commensals can provide benefits to the host which supplement those from bacteria. In mice, depletion of the intestinal virome enhances susceptibility to DSS-induced colitis. Contributions of the virome can also be studied using murine norovirus strain CR6 (CR6), which forms persistent asymptomatic infections in the gut; this infection protects mice from both C. rodentium- and DSS-induced colitis. Despite these benefits, host-encoded mechanisms regulating viral commensalism remain understudied.

One host-encoded mechanism is STAT1. In contrast to asymptomatic infection in wild-type mice, STAT1-deficient mice (Stat1-/-) develop severe disease characterized by liver necrosis and weight loss following CR6 infection. However, it remains unknown how the loss of STAT1 leads to CR6-induced disease.

We aimed to define STAT1-dependent mechanisms of protection from CR6-induced liver pathology.

To determine the contribution of STAT1 in protection from CR6-induced disease, Stat1-/- and STAT1-sufficient Stat1+/- littermates were infected i.v. with CR6. At days 3, 5, and 7 days post-infection, clinical disease, cell-intrinsic viral loads, pathology, and cellular infiltration were assessed. Bone marrow chimeras were used to evaluate the requirement for hematopoietic vs non-hematopoietic STAT1 in preventing disease.

By 3 days post-infection, myeloid cells infiltrate the liver of CR6-infected Stat1-/- mice. Clinical disease is associated with accumulation of myeloid cells, including macrophage and dendritic cells in the liver . Sorting of liver-infiltrating immune cell populations revealed that Stat1-/- macrophage have elevated viral loads compared to Stat1+/- littermates (Figure). Notably, CD11b+ cells were enriched within the necrotic lesions characteristic of CR6-induced disease. Consistent with a role for innate immune cells in disease, bone marrow-derived macrophage from Stat1-/- mice failed to control viral replication in vitro. Further, bone marrow chimeras revealed that STAT1 expression in hematopoietic cells is necessary and sufficient to prevent CR6-induced disease. Together, these data suggest that in the absence of STAT1, innate immune cells become heavily infected with CR6 and mediate severe liver pathology.

Our data suggest that STAT1 is critical to maintaining a commensal relationship with the intestinal virome. In the absence of STAT1, failure to restrict the replication of viral commensal CR6 in innate immune cells leads to severe liver pathology.

STAT1-deficient innate immune cells fail to control viral replication.
Stat1

-/-
and Stat1+/- mice were infected i.v. with CR6. 5 days post-infection, macrophage, dendritic cell, neutrophil, and monocyte populations were isolated from the liver by FACS. Viral genome copies were quantified by RT-qPCR. LoD = Limit of Detection. Mann-Whitney U test, ** = p ampersand:003C 0.01.

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