Dynamics of the transcriptomic landscape of OsHV-1 replication in haemocytes of the Pacific oyster (Magallana gigas)

Since the 1990s, the Pacific oyster ( Magallana gigas ) has experienced repeated mortality events associated with Ostreid herpesvirus 1 (OsHV-1). Although the virus has been genomically characterised, its replication cycle and its interactions with the oyster immune system are still not well understood. In particular, little is known about the dynamics of OsHV-1 gene expression and the immune responses of haemocytes from oysters with varying susceptibility to the virus. While some studies have focused on the expression of specific viral and host genes in whole oysters, none have provided a comprehensive analysis of genome-wide expression across multiple post-infection time points in haemocytes. The lack of oyster cell lines makes studying virus–host interactions in vitro challenging. However, haemocytes, the key immune cells circulating in haemolymph, can be maintained in vitro in the short term and represent a relevant model for analysing infection dynamics. In this study, haemocytes from two M. gigas batches, one highly susceptible and one less susceptible to OsHV-1, were infected in vitro . We tracked the viral and host transcriptomes over a 24-h period post-infection using high-throughput dual transcriptomics. Our results provide a detailed overview of the OsHV-1 transcriptomic landscape in haemocytes from high- and low-susceptible M. gigas over time. In addition, weighted correlation network analysis of host gene expression provided insights into the haemocytes’ response to infection and highlighted batch-specific immune responses. This comprehensive transcriptomic study is the first to describe virus–host interactions across multiple stages of infection in haemocytes from Pacific oysters, showing contrasted survival when exposed to OsHV-1.