Receptor transporter protein 4 (RTP4)-mediated repression of hepatitis C virus replication in mouse cells.

Hepatitis C virus (HCV) exhibits a narrow species tropism, causing robust infections only in humans and experimentally inoculated chimpanzees. While many host factors and restriction factors are known, many more likely remain unknown, which has limited the development of mouse or other small animal models for HCV. One putative restriction factor, the black flying fox orthologue of receptor transporter protein 4 (RTP4), was previously shown to potently inhibit viral genome replication of several ER-replicating RNA viruses. Since the murine but not the human ortholog is a potent inhibitor of HCV, we aimed to analyze the potential role for RTP4 in restricting HCV replication in mice. We demonstrated that mouse RTP4 (mmRTP4) functions as a dominant inhibitor of HCV infection. Via interspecies domain-mapping, we identified the zinc-finger domain (ZFD) of murine RTP4 as essential for inhibiting HCV, consistent with prior work. Introducing mmRTP4 into HCV-infected Huh7 cells profoundly reduced HCV NS5A protein production and virion release, demonstrating that mmRTP4 can also disrupt already established HCV replication complexes. This inhibition of HCV was not driven by induction of interferon-stimulated genes based on bulk RNA-seq, suggesting that mmRTP4 might directly act on HCV replication. Indeed, by in situ proximity ligation, we found that mmRTP4 directly associates with the HCV NS5A protein significantly more than human RTP4 during infection. However, disrupting RTP4 expression in mice expressing humanized alleles of CD81 and occludin (OCLN) - the species specific cellular factors mediating HCV uptake - did not increase permissiveness irrespective of the immunocompetence of the mice. Collectively, our work provides detailed insights into the role of RTP4 in contributing to HCV's narrow host range and will inform downstream development of a more comprehensive small-animal model for this important pathogen.