Cross-utilization of viral polymerase: parainfluenza virus hijacks the RdRp of porcine sapelovirus to facilitate its replication during co-infection

Parainfluenza virus is a significant respiratory pathogen affecting both humans and animals, capable of causing acute respiratory tract infections in infants and immunocompromised individuals. Recently, parainfluenza virus 5 (PIV5) has been frequently detected in swine diarrheal samples. However, experimental infections revealed that PIV5 alone induces only mild clinical symptoms. Further analysis suggested that co-infection with porcine sapelovirus (PSV) may underlie the observed disease severity. This hypothesis was confirmed through animal studies. Co-infection was shown to occur intracellularly, where PSV significantly enhanced PIV5 replication. Mechanistically, this effect was primarily mediated by PSV 3D, an RNA-dependent RNA polymerase (RdRp). PSV 3D directly interacted with PIV5 nucleoprotein and genomic RNA. Mini-genome assays demonstrated that PSV 3D substantially increased PIV5 genomic activity. Using an infectious cDNA clone of PIV5 and a series of mutants, we further confirmed that PSV 3D promoted transcription of both plus- and minus-strand PIV5 RNAs. Notably, RdRp (3D) proteins from two other picornaviruses also enhanced PIV5 RNA synthesis, suggesting a conserved function across picornaviruses. In summary, this study provides the first evidence that PIV5 hijacks the RdRp of a co-infecting virus from a different viral family to support its own replication. These findings advance our understanding of virus-virus interactions and offer a novel perspective in virology. IMPORTANCE: The gastrointestinal tract harbors a vast and diverse community of microorganisms, making it an ideal environment for exploring microbial interactions. While virus-bacteria interactions have been widely studied, virus-virus interactions remain largely uncharacterized. In this study, we demonstrated that porcine sapelovirus (PSV) and parainfluenza virus 5 (PIV5) co-infect cells and directly interact within the host. Specifically, the RNA-dependent RNA polymerase protein (3D) of PSV significantly promoted PIV5 replication by interacting with key components of the PIV5 ribonucleoprotein complex and enhancing the synthesis of both plus- and minus-strand viral RNAs. Similar effects were observed with the 3D proteins from two additional picornaviruses, suggesting a shared mechanism among picornaviruses in facilitating co-infecting virus replication. This work uncovers a novel cross-family polymerase hijacking event and provides important insights into virus-virus interactions, highlighting new potential targets for the control and prevention of swine enteric diseases.