Adaptive Immune Function Of Rainbow Trout Swim Bladder And Intestinal Mucosal B Cells Induced By IHNV

Fish live in a special environment,rich in pathogenic microorganisms.To prevent the pathogens invasion,fish have formed an extensive mucosal system,including skin,gills,and swim bladder(SB).Mucosal-associated lymphoid tissues(MALTs)constitute the first line of defense against pathogenic infection.Previous studies have found that parasites and pathogenic bacteria can induce a strong adaptive immune response in fish mucosa,in which B cells and immunoglobulin(Ig)play key roles.However,the roles of mucosal adaptive immune response against viruses and which Ig plays a main role are not clear.To solve these scientific problems,we carried out the following research.The SB of teleosts and the lungs of mammals are homologous organs,originating from the"proto-lungs"of primitive ray-finned fishes.To adapt to different living environments,they have evolved different functions of controlling buoyancy in water and breathing air.However,as a gas-filled organ,both have a similar mucosal epithelial structure,suggesting the same immune mechanism may exist.When the lungs of mammals are infected by influenza or SARS-Co V-2,the lung MALT can produce a large amount of secretory Ig A(s Ig A)to neutralize the virus.There is also diffused mucosal-associated lymphoid tissue(D-MALT)in fish SB mucosa(SM).Here,we studied virus-induced mucosal adaptive immune function in rainbow trout(Oncorhynchus mykiss)SB.We constructed a model of infectious hematopoietic necrosis virus(IHNV)infection.In this model,the IHNV was directly delivered to the surface of the SM for infection by way of SB injection.After IHNV infection,the SM produced a large number of Ig T~+B cells,and secreted large amount of virus-specific Ig T,which was transported into the SB mucus through polymeric immunoglobulin receptors(p Ig R)to play a neutralizing function,resisting the viral invasion.Importantly,fish that survived initial virus exposure were able to survive rechallenge with high doses of virus,suggesting some protection.However,when the Ig T~+B cells of these surviving fish were depleted,the specific Ig T in the mucus decreased significantly.They lost this protection,and the SM became highly sensitive to IHNV,and the fish mortality increased,proving the important role of Ig T in the defense against IHNV invasion.Lots of microorganisms colonized on the surface of fish mucous membranes,which interact with the body and are in a stable state.They participate in a series of physiological activities such as nutrition and metabolism of the body.When stimulated by external antigens,the dynamic balance of microorganisms is easily broken,and it will also cause health problems.However,current research on the interaction between the mucosal immune system,commensal microbiota,and viral pathogens in fish is very limited.In this study,the effect of virus infection on intestinal mucosal immunity and microbial community structure was investigated.After IHNV infection,severe tissue damage occurred in the intestinal mucosa,and the expression of innate and adaptive immune genes was significantly up-regulated.Further research found that the adaptive immune response of the intestinal mucosa was similar to that of the SM.The number of Ig T~+B cells in the intestinal mucosa increased significantly,and there were a large number of virus-specific Ig T in the intestinal mucus.Interestingly,RNA-seq analysis found that the intestinal mucosa not only stimulated antiviral immune response,but also induced antibacterial immune pathways.Through 16S r RNA sequencing,it was found that the intestinal microbial homeostasis of rainbow trout was disrupted after IHNV infection.The flora was disordered,and the probiotics(such as Faecalibacterium)decreased significantly,while the abundance of some conditional pathogens(such as Shigella)was significantly increased,inducing secondary bacterial infection.It is worth noting that the expression of immune-related genes in the intestines of fish that survived virus infection will return to the level before infection,and the microbial community composition will gradually recover.Thus,our findings suggest that viral infection not only induces a mucosal immune response,but also alters the composition of the mucosal microbial community,leading to an imbalance in microbial homeostasis.Therefore,secondary bacterial infections must also be considered when developing strategies to control viral infections.