A Sustained Immune Response Supports Long-Term Antiviral Immune Priming in the Pacific Oyster, Crassostrea gigas

Abstract

Over the last decade, innate immune priming has been evidenced in many invertebrate phyla. If mechanistic models have been proposed, molecular studies aiming to substantiate these models have remained scarce. We reveal here the transcriptional signature associated with immune priming in the oyster Crassostrea gigas. Oysters were fully protected against Ostreid herpesvirus 1 (OsHV-1), a major oyster pathogen, after priming with poly(I·C), which mimics viral double-stranded RNA. Global analysis through RNA sequencing of oyster and viral genes after immune priming and viral infection revealed that poly(I·C) induces a strong antiviral response that impairs OsHV-1 replication. Protection is based on a sustained upregulation of immune genes, notably genes involved in the interferon pathway and apoptosis, which control subsequent viral infection. This persistent antiviral alert state remains active over 4 months and supports antiviral protection in the long term. This acquired resistance mechanism reinforces the molecular foundations of the sustained response model of immune priming. It further opens the way to applications (pseudovaccination) to cope with a recurrent disease that causes dramatic economic losses in the shellfish farming industry worldwide. IMPORTANCE In the last decade, important discoveries have shown that resistance to reinfection can be achieved without a functional adaptive immune system, introducing the concept of innate immune memory in invertebrates. However, this field has been constrained by the limited number of molecular mechanisms evidenced to support these phenomena. Taking advantage of an invertebrate species, the Pacific oyster (Crassostrea gigas), in which we evidenced one of the longest and most effective periods of protection against viral infection observed in an invertebrate, we provide the first comprehensive transcriptomic analysis of antiviral innate immune priming. We show that priming with poly(I·C) induced a massive upregulation of immune-related genes, which control subsequent viral infection, and it was maintained for over 4 months after priming. This acquired resistant mechanism reinforces the molecular foundations of the sustained response model of immune priming. It opens the way to pseudovaccination to prevent the recurrent diseases that currently afflict economically or ecologically important invertebrates.

High temperature induces transcriptomic changes inCrassostrea gigasthat hinders progress of Ostreid herpesvirus (OsHV-1) and promotes survival

“…We found that temperature modulates oyster immunity as reported in Meistertzheim et al, 2007. For instance, at 29°C, eleven functional categories related to immune response were enriched and genes coding for pattern recognition receptors were up-regulated. Temperature increases the expression of genes related to immunity such as Toll like receptors, RIG-I, and Tumor necrosis factors in healthy oysters (Meistertzheim et al, 2007;Zhang et al, 2015, Lafont et al, 2020.…”

Towards a Mechanism for Poly(I·C) Antiviral Priming in Oysters

“…Most interestingly, the majority of genes induced by poly(I•C) are also induced by OsHV-1 infection, suggesting that poly(I•C) primes an immune response similar to that of the virus, which results in a protective antiviral state. Further supporting this interpretation, Lafont et al observed only minor transcriptional changes following OsHV-1 infection in poly(I•C)-primed Commentary ® oysters (5). Strikingly, gene ontology analysis revealed a strong enrichment of immune functions among poly(I•C)-induced genes.…”

“…The work directed by Caroline Montagnani (5) sheds new light on the importance of IFN-like responses for invertebrate antiviral immunity. The authors and others had previously shown that treating oysters with dsRNA or poly(I•C) confers a long-term protection against ostreid herpesvirus 1 (OsHV-1) (6).…”

“…This virus causes Pacific oyster mortality syndrome, which represents an important economic burden for shellfish farmers. Using RNA sequencing, Lafont et al explored the genome-wide transcriptomic changes triggered by poly(I•C), a synthetic dsRNA analog that is a potent agonist of TLR3 and MDA5 in mammals and of OsHV-1 (5). Poly(I•C) treatment induced a strong transcriptional response (more than 4,000 genes differentially expressed, representing approximately 16% of the annotated transcriptome), with a majority of genes upregulated at one or several time points posttreatment.…”

“…IRFs were lost in insects (*) and nematodes. The work by Lafont et al(5) identified an IFN-like response in the mollusk Crassostrea gigas. The scale bar is in million years (MY).…”