Differential expression profile of innate immune genes in the liver of olive flounder (Paralichthys olivaceus) against viral haemorrhagic septicaemia virus (VHSV) at host susceptible and non-susceptible temperatures

Viral haemorrhagic septicaemia virus (VHSV) is known to cause high mortality in olive flounder (Paralichthys olivaceus) at 15 degrees C, but no mortality at 20 degrees C. In order to understand the immune response at 15 degrees C and 20 degrees C against VHSV, differential expression kinetics of various innate immune-related genes in the kidney were analyzed in our previous study, and the current study is focused on immune responses in the liver. Fishes were intraperitoneally injected with VHSV (10(7.8)TCID(50)/fish) and reared at 15 degrees C and 20 degrees C. Viral copy number in the 15 degrees C group peaked at 1 dpi and remained high until 2 dpi; whereas in the 20 degrees C group, viral copies were much lower than in the 15 degrees C group. The expression kinetics results revealed significantly high levels of hepcidin expression in the 20 degrees C group in comparison to the 15 degrees C group at all time points. Cathepsin-L gene showed early upregulation at 12 hpi followed by a sharp decline at 1 dpi in both the groups; whereas, at 2-4 dpi, only the 20 degrees C group demonstrated subsequent upregulation of cathepsin-L transcript. The genes of complement pathways, C3 and factor-D showed prolonged downregulation trend in the 15 degrees C group against the normal expression level in the 20 degrees C group. Significant upregulation of ISG15, Mx, IL-1 beta and IL-8 genes were observed in the 20 degrees C group during 1-2 dpi in comparison with slower and lower responses in the 15 degrees C group. The MHC-II, MHC-I and CD8 gene transcripts showed significant upregulation only in the 20 degrees C group in contrast with downregulation in the 15 degrees C group, whereas, CD4 expression was very high at 12 hpi to 2 dpi in both the groups. The genes related to the apoptotic pathway, p53, HSP70, and caspase-3, also displayed consistent downregulation in the 15 degrees C group during the entire experimental period in comparison with the other group. In summary, it can be observed that most of the innate immune genes were under-expressed or have little to no response at 15 degrees C, whereas, the same genes exhibited high level of expression at 20 degrees C. Thus, it can be inferred that temperature plays a key role in facilitating the host liver to orchestrate a coordinative anti-VHSV immune response ultimately leading to host survival in the 20 degrees C group. Oppositely, the under-expression of important genes at 15 degrees C especially the genes related to the apoptotic pathway, cytotoxic T-cell pathway, and the complement system, resulted in failure of the host immune system to combat the viral proliferation.