For more than a decade, Pacific oyster mortality syndrome (POMS) has severely impacted the Crassostrea gigas aquaculture industry, at times killing up to 100% of young farmed Pacific oysters, a key commercial species that is cultivated globally. These disease outbreaks have caused major financial losses for the oyster aquaculture industry.

Infectious diseases are a major constraint to the expansion of shellfish production worldwide. Pacific oyster mortality syndrome (POMS), a polymicrobial disease triggered by the Ostreid herpesvirus-1 (OsHV-1), has devastated the global Pacific oyster (Crassostrea gigas) aquaculture industry. Recent ground-breaking research revealed that C. gigas possess an immune memory, capable of adaption, which improves the immune response upon a second exposure to a pathogen. This paradigm shift opens the door for developing ‘vaccines’ to improve shellfish survival during disease outbreaks. In the present study, we developed an in-vitro assay using hemocytes – the main effectors of the C. gigas immune system – collected from juvenile oysters susceptible to OsHV-1. The potency of multiple antigen preparations (e.g., chemically and physically inactivated OsHV-1, viral DNA, and protein extracts) to stimulate an immune response in hemocytes was evaluated using flow cytometry and droplet digital PCR to measure immune-related subcellular functions and gene expression, respectively. The immune response to the different antigens was benchmarked against that of hemocytes treated with Poly (I:C). We identified 10 antigen preparations capable of inducing immune stimulation in hemocytes (ROS production and positively expressed immune- related genes) after 1 h of exposure, without causing cytotoxicity. These findings are significant, as they evidence the potential for priming the innate immunity of oysters using viral antigens, which may enable cost-effective therapeutic treatment to mitigate OsHV-1/POMS. Further testing of these antigen preparations using an in-vivo infection model is essential to validate promising candidate pseudo-vaccines.

Background:In recent years, the aquaculture industry of Crassostrea gigas has been severely impacted by geographically widespread outbreaks of a polymicrobial disease, the Pacific Oyster Mortality Syndrome (POMS). The aetiology of these recurring summer mortality events and the disease progression have notably been recently described in France, revealing a combined development of viral (namely the Ostreid Herpesvirus, OsHV-1 µVar) and bacterial infections. In New Zealand (NZ), mass mortality episodes associated with OsHV-1 have been recorded in juvenile C. gigas since 2010 and selective breeding to improve resistance to OsHV-1 has been effectively used to mitigate the impact of the disease. However, POMS infection process and the role of the host-microbiota during infection have still not been well established in NZ. Results:Using a laboratory-based experimental infection approach, we challenged ten biparental oyster families with previously established contrasted genetically based ability to survive POMS in the field. Molecular analyses (viral load quantification, viral gene expression and 16S rRNA gene sequencing) were combined with histopathological observations to describe the temporal kinetics of infection to POMS and to characterize the role of microbiota during infection. Our main findings showed (1) a delay in viral infection resulting in a late onset of mortality in oysters compared to previous observations from France, and (2) a lack of evidence of major fatal bacteraemia in infected oysters. Bacterial profiling associated the microbiota composition with mortality rate, viral load, and viral replication, allowing the identification of potentially deleterious and beneficial bacterial taxa that can influence the outcome of the disease. Mycoplasma ASV-1 was identified as a significant predictor of mortality in oysters. Conclusion:Collectively, these results could improve current disease management and aquaculture practices and help understanding the mechanisms behind genetic resistance to POMS. Ultimately, microbiome composition could be used to predict oyster mortality following exposure to OsHV-1 and be used as a complementary screening tool in selective breeding and/or a diagnostic tool to determine shellfish health.

Crassostrea gigas est la principale espèce d’huître cultivée dans le monde. Depuis 2008, de sévères épisodes de mortalités affectent les huîtres âgées de moins d’un an en Europe et en Océanie et sont associées à l’émergence de l’Ostreid herpèsvirus μVar (OsHV-1 μVar). En Europe, ces mortalités sont saisonnières et surviennent lorsque la température de l’eau de mer est comprise entre 16°C et 24°C. Dans le cadre de ce travail, l’effet des hautes températures (21°C, 26°C et 29°C) est évalué sur la sensibilité des huîtres à OsHV-1 mais aussi sur la persistance et la virulence du virus. La survie des huîtres infectées maintenues à 29°C (86%) est supérieure à la survie des huîtres placées à 21°C (52%) et à 26°C (43%).Les températures élevées (29°C) diminuent la sensibilité des huîtres à OsHV-1 sans altérer l'infectivité du virus et sa virulence. L’exposition des huîtres infectées à 29°C pourrait réduire l’expression des gènes viraux et la synthèse de virions par la réduction de l’expression de gènes hôtes codant pour des protéines impliquées dans la transcription et la traduction, la réduction de l’expression de gènes impliqués dans le catabolisme, le transport des métabolites, et synthèse de macromolécules.Finalement, l’induction conjointe de l’apoptose, des processus d’ubiquitinylation et de la réponse immunitaire, pourrait permettre l’élimination d’OsHV-1.