Oral immunization of grouper with recombinant Chlamydomonas to prevent viral nervous necrosis and the effect of which on intestinal microbiota

Although recombinant subunit vaccines are among the safest and most effective currently available, their rapid progress has been hampered by their high cost and need for sophisticated infrastructure. When used as a vehicle for vaccine production, unicellular microalgae are inexpensive, easy to grow and store, and have been shown to fold a variety of vaccine antigens appropriately, allowing for oral delivery of the vaccines possible. The current study investigated the possibility of expressing the grouper nervous necrosis virus (NNV) coat protein in Chlamydomonas reinhardtii as an oral vaccine against viral nervous necrosis(VNN). The identified recombinant algal strains were mixed with commercial feeds at a 1:1 ratio and fed to grouper larva as an immunization feed. The results of the challenge experiments showed that the survival rate of grouper with oral vaccine was increased by 26.7 %-36.7 % compared with the control group, and the amount of brain cavitation was lower in the orally vaccinated grouper. In addition, immune-related genes, including immunoglobulin M(IgM), major histocompatibility complex class I (MHC-Iα), tumour necrosis factor α (TNF-α), interleukin 16 (IL-16), and interleukin 1 beta (IL-1β) were upregulated after oral vaccination. High-throughput 16S rRNA sequencing of grouper gut microbes revealed that immunofeeding altered the structure of the grouper intestinal microbiota, increasing the abundance of antimicrobial and nutrient metabolism-associated microorganisms while decreasing the abundance of pathogenesis-associated harmful microorganisms. The current research shows that recombinant C. reinhardtii producing the NNV coat protein is a promising candidate for oral vaccination against NNV disease in groupers.