Researchers Reveal Immune Evasion Mechanisms of Fish Virus

Aquaculture is crucial to agricultural economy, yet many aquaculture species are threatened by some serious diseases that caused by viruses, such as spring viremia of carp virus (SVCV) and grass carp reovirus (GCRV). There are few studies regarding the evasion mechanisms of SVCV/GCRV to interfere with fish immune systems.  

In recent years, the Research Group of Fish Molecular Immunology from Institute of Hydrobiology (IHB) of Chinese Academy of Sciences has been focusing on the immune evasion mechanisms of SVCV and GCRV and made several progress. 

In fish, interferon (IFN) response is a key component of innate immunity which represents the first line to defend against virus infection. Their previous studies have demonstrated that the N protein of SVCV could inhibit host IFN production by degrading MAVS in a Ub–proteasome manner (Journal of Immunology, 2016), which was also recommended by the editor of the Journal of Immunology with the topic "Fish Virus Is Unfriendly to IFN"; Then they found that SVCV P protein functions as a decoy substrate for cellular TBK1, leading to the reduction of IRF3 phosphorylation and suppression of IFN expression (Journal of Virology, 2016).  

They also revealed that GCRV VP41 prevents the fish IFN response by attenuating the phosphorylation of MITA for viral evasion (Journal of Virology, 2017). These studies confirmed that fish viruses reduce the production of host IFN for self-proliferation, which provides strong evidence for aquatic virus in immune evasion in lower vertebrates. 

Upon virus infection, the tumor suppressor p53 is a pivotal regulator of cell-cycle arrest and cellular apoptosis, which are employed by host cells to defend against viruses. Usually, cell-cycle arrest is beneficial for viral early infection to plunder a host’s resources for viral replication, and late apoptosis is preferred for virion release. Thus, viruses possess evolutionary reasons to encode p53 antagonistic proteins.  

Recently, this research group also revealed that SVCV employs two distinct manners to regulate the host key factor p53 expression, lowering p53 expression with N protein and increasing it with P protein to promote viral infection. Early in SVCV infection, significant induction was observed in the S phase and p53 was decreased in the protein level. Further experiments demonstrated that p53 interacted with SVCV N protein and was degraded by suppressing the K63-linked ubiquitination. However, the increase of p53 was observed late in the infection and experiments suggested that p53 was bound to SVCV P protein and stabilized by enhancing the K63-linked ubiquitination.  

Finally, lysine residue 358 was the key site for p53 K63-linked ubiquitination by the N and P proteins. These findings uncover the strategy for the subversion of p53-mediated host innate immune responses by aquatic viruses. The relevant research results have been published online (PLoS Pathogens, 2019). The first author of the above researches is Associate Prof. LI Shun and/or Assistant Prof. LU Long-Feng. These findings have clarified the pathogenesis of fish viruses from the perspective of virus-cell interaction. It is of great significance for the detection, prevention and elimination of fish viruses, and the development of vaccines and diagnostic reagents.