Type II grass carp reovirus utilizes autophagosomes for viroplasm formation and subclinical persistent infection

Grass carp reovirus (GCRV) is the most virulent pathogen within the genus <i>Aquareovirus</i>, belonging to the family <i>Spinareoviridae</i>. GCRV is categorized into three genotypes, with type II (GCRV-II) being the predominant strain circulating in China. Reoviruses are known to replicate and assemble in cytoplasmic inclusion bodies termed viroplasms; however, information regarding the formation of GCRV-II viroplasms and their specific roles in virus infection remains largely unknown. In this study, we investigated the formation and characteristics of viroplasms during GCRV-II infection. Immunofluorescence and confocal microscopy indicate that GCRV-II infection induces the formation of viroplasms, with the nonstructural protein NS79 being the key protein responsible for this process. Live-cell imaging and fluorescence recovery after photobleaching assays reveal that GCRV-II viroplasms lack liquid-like properties. Transmission electron microscopy confirms that GCRV-II viroplasms are membranous structures. Notably, we demonstrate that GCRV-II infection induces autophagy and the formation of autophagosomes and that GCRV-II utilizes these autophagosomes for viroplasm formation and virion assembly. Furthermore, we found that GCRV-II uses autophagosomes to evade the host immune system, establishing subclinical persistent infection. GCRV-II also employs autophagosomes for nonlytic release and viral spread. Collectively, these findings highlight distinctive characteristics of GCRV-II viroplasms compared to those of other animal reoviruses, offering valuable insights for the prevention and control of this virus.IMPORTANCEGrass carp reovirus (GCRV) is categorized into three genotypes, with GCRV-II being the most prevalent in China. Despite reoviruses being known for their replication and assembly in viroplasms, the specifics of GCRV-II viroplasm formation and its role in infection were unclear. Our study demonstrates that GCRV-II infection triggers the formation of viroplasms, primarily mediated by the nonstructural protein NS79. GCRV-II viroplasms are membranous structures that lack liquid-like properties, which are significantly different from the viroplasms of other reoviruses. Notably, our research unveils that GCRV-II infection induces autophagy and utilizes autophagosomes for viroplasm formation and virion assembly. Furthermore, we also confirm that GCRV-II utilizes autophagosomes for subclinical persistent infection, nonlytic release, and viral spread. Our results indicate that GCRV-II hijacks autophagosomes to form viroplasms and complete its life cycle. The characteristics of GCRV-II are significantly different from those of other animal reoviruses, providing important information for prevention and control of this virus.