Immune Control of Herpesvirus Infection in Molluscs

Marine Invertebrate Antimicrobial Peptides and Their Potential as Novel Peptide Antibiotics

Marine invertebrates constantly interact with a wide range of microorganisms in their aquatic environment and possess an effective defense system that has enabled their existence for millions of years. Their lack of acquired immunity sets marine invertebrates apart from other marine animals. Invertebrates could rely on their innate immunity, providing the first line of defense, survival, and thriving. The innate immune system of marine invertebrates includes various biologically active compounds, and specifically, antimicrobial peptides. Nowadays, there is a revive of interest in these peptides due to the urgent need to discover novel drugs against antibiotic-resistant bacterial strains, a pressing global concern in modern healthcare. Modern technologies offer extensive possibilities for the development of innovative drugs based on these compounds, which can act against bacteria, fungi, protozoa, and viruses. This review focuses on structural peculiarities, biological functions, gene expression, biosynthesis, mechanisms of antimicrobial action, regulatory activities, and prospects for the therapeutic use of antimicrobial peptides derived from marine invertebrates.

Inactivated ostreid herpesvirus-1 induces an innate immune response in the Pacific oyster, Crassostrea gigas, hemocytes

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.

Transcriptional expression analysis reveals multiple effects of nonylphenol exposure on scallop immune system

Nonylphenol (NP) is an endocrine disruptor and environmental hormone representing alkylphenol compounds. Marine mollusks are an important source of protein for people worldwide. Many researchers have begun to study the effect of NP on marine mollusks immune system in view of its toxicity; however, the underlying molecular mechanisms require in-depth analysis. In this study, we focused on the transcriptional expression change of immune-related genes and antioxidant enzymes activities variation after NP exposure in a marine bivalve mollusk, Chlamys farreri, to explore the immunomodulatory capacity of NP in marine mollusks. We identified MAVS (Mitochondrial antiviral signaling protein), a key adaptor molecule in the RLR (RIG-I like receptor) pathway, and studied the expression of multiple immune-related genes in response to different concentrations of NP. The key genes involved in RLR/TLR (Toll like receptor) innate immune pathway, apoptosis, and cellular antioxidation mechanism were investigated. Changes in the enzymatic activities of scallop antioxidant enzymes after NP exposure were also examined. The results revealed that the genes expression and the antioxidant enzymes activities show significant changes, thus proving that NP stimulation affects the scallop immune system. Our research results demonstrate the immunomodulatory capacity of NP in marine bivalve mollusks and lay the foundation for further in-depth analysis of the molecular mechanism of NP toxicity.

The Role of Anti-Viral Effector Molecules in Mollusc Hemolymph

Molluscs are major contributors to the international and Australian aquaculture industries, however, their immune systems remain poorly understood due to limited access to draft genomes and evidence of divergences from model organisms. As invertebrates, molluscs lack adaptive immune systems or 'memory', and rely solely on innate immunity for antimicrobial defence. Hemolymph, the circulatory fluid of invertebrates, contains hemocytes which secrete effector molecules with immune regulatory functions. Interactions between mollusc effector molecules and bacterial and fungal pathogens have been well documented, however, there is limited knowledge of their roles against viruses, which cause high mortality and significant production losses in these species. Of the major effector molecules, only the direct acting protein dicer-2 and the antimicrobial peptides (AMPs) hemocyanin and myticin-C have shown antiviral activity. A better understanding of these effector molecules may allow for the manipulation of mollusc proteomes to enhance antiviral and overall antimicrobial defence to prevent future outbreaks and minimize economic outbreaks. Moreover, effector molecule research may yield the description and production of novel antimicrobial treatments for a broad host range of animal species.

Marine mollusc extracts-Potential source of SARS-CoV-2 antivirals

Severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is a novel human coronavirus and the causative agent of coronavirus disease 2019 (Covid‐19). There is an urgent need for effective antivirals to treat current Covid‐19 cases and protect those unable to be vaccinated against SARS‐CoV‐2. Marine molluscs live in an environment containing high virus densities (>10⁷ virus particles per ml), and there are an estimated 100,000 species in the phylum Mollusca, demonstrating the success of their innate immune system. Mollusc‐derived antivirals are yet to be used clinically despite the activity of many extracts, including against human viruses, being demonstrated in vitro. Hemolymph of the Pacific oyster (Crassostrea gigas) has in vitro antiviral activity against herpes simplex virus and human adenovirus, while antiviral action against SARS‐CoV‐2 has been proposed by in silico studies. Such evidence suggests that molluscs, and in particular C. gigas hemolymph, may represent a source of antivirals for human coronaviruses.