# Limiting viral replication in hematopoietic cells delays Rift Valley fever virus disease progression in C57BL/6 mice

## TL;DR

Limiting Rift Valley fever virus replication in blood-forming cells in mice delays disease progression, suggesting these cells play a key role in spreading the virus.

## Contribution

A recombinant virus with restricted replication in hematopoietic cells was used to demonstrate their role in RVFV pathogenesis.

## Key findings

- RVFVmiR-142 showed restricted replication in hematopoietic cells in vitro and in mice.
- Mice infected with RVFVmiR-142 had delayed disease progression compared to controls.
- Increased type I IFN response in lymph nodes correlated with reduced viral replication.

## Abstract

Rift Valley fever virus (RVFV) causes mild to severe disease in livestock and humans. It was first identified in 1931 during an epizootic in Kenya and has spread across Africa and into the Middle East. Hematopoietic cells are one of the major targets of RVFV in vivo; however, their contribution to RVFV pathogenesis remains poorly understood. To address this, we generated a recombinant miRNA-targeted virus, RVFVmiR-142, to limit viral replication in hematopoietic cells and evaluated the consequences on RVFV disease manifestations in C57BL/6 mice. MicroRNAs are evolutionarily conserved non-coding RNAs that regulate mRNA expression. RVFVmiR-142 includes an insertion of four repeated sequences targeted by hematopoietic-specific miRNA-142. RVFVmiR-MM, which contains four repeats of sequences that are not targets of any known miRNA, was generated as a control. RVFVmiR-142 showed restricted replication in vitro compared to RVFVmiR-MM. C57BL/6 mice infected with 2 TCID50 of RVFVmiR-142 had delayed disease progression vs RVFVmiR-MM-infected mice, the phenotype of which was overcome by higher infection doses. The difference in disease progression at low dose was eliminated in MiR-142 KO mice, confirming the specificity of the phenotype. A timed euthanasia study showed delayed viral replication of RVFVmiR-142 compared to RVFVmiR-MM in infected mice, most notably in tissues largely composed of hematopoietic cells. Furthermore, control of RVFVmiR-142 replication in the popliteal lymph nodes correlated with an increased type I IFN response, which was lacking in the liver tissue where RVFVmiR-142 replication continued to increase. These data suggest that RVFV replication in hematopoietic cells contributes to viral amplification and/or spread.

RVFV is a segmented, single-stranded, negative-sense RNA virus vectored by diverse genera of mosquitoes and can infect a variety of wild animals, domesticated livestock, and humans. Despite the increase in both the range and frequency of RVFV outbreaks over the years, there is currently no vaccine or treatment available for human use against RVFV. Mononuclear phagocytic cells (MPCs) are one of the major targets of RVFV in vivo; however, the contribution of RVFV replication in these cells to its pathogenesis has not been well characterized. In this study, we generated a recombinant miRNA-targeted virus with restricted replication in hematopoietic cells and examined its pathogenesis in C57BL/6 mice. This study demonstrates that hematopoietic cell infection contributes to viral pathogenesis by augmenting viral amplification and/or spread.