Autogenous vaccination protects Pacific oysters against OsHV-1 by reshaping the transcriptome.

Vaccination (immune priming) against Ostreid herpesvirus 1 (OsHV-1) is a strategy for managing Pacific Oyster Mortality Syndrome (POMS), yet the transcriptional mechanisms underlying protection remain poorly defined. We evaluated the transcriptome in Pacific oysters (Magallana gigas) administered heat-treated (HT) and live OsHV-1 vaccines at 18 °C and 22 °C. All priming strategies improved survival after challenge with virulent OsHV-1 at 22 °C, with relative percent survival of 57.5% for HT OsHV-1 administered at 18 °C, 76.2% for HT OsHV-1 at 22 °C and 85.0% for live OsHV-1 at 18 °C. Primed surviving oysters exhibited significantly lower viral loads after challenge than controls, with distinct transcriptomic responses among live and HT OsHV-1 treatments. Live OsHV-1, which provided the highest protection, triggered a rapid antiviral response one day after priming, characterised by upregulation of predicted orthologs of viral RNA sensors and interferon-stimulated-like signalling components including RLR-family genes, DHX58, IFI44L and a MAVS-like adaptor. In contrast, HT OsHV-1 induced a delayed response on day four, enriched for protein quality control, endoplasmic reticulum-associated degradation and ribosome biogenesis pathways. By day 12, gene expression of primed oysters had returned to baseline. In live OsHV-1-primed oysters, post-challenge gene modules were enriched for predicted orthologs of antiviral and immune signalling components and DNA replication regulators, indicative of a coordinated antiviral response and increased haemocyte activity. Overall, these results highlight transcriptional signatures underlying OsHV-1-based vaccination, which is developing as a strategy in integrated POMS management.