IPNV with high and low virulence: host immune responses and viral mutations during infection

Abstract

BackgroundInfectious pancreatic necrosis virus (IPNV) is an aquatic member of the Birnaviridae family that causes widespread disease in salmonids. IPNV is represented by multiple strains with markedly different virulence. Comparison of isolates reveals hyper variable regions (HVR), which are presumably associated with pathogenicity. However little is known about the rates and modes of sequence divergence and molecular mechanisms that determine virulence. Also how the host response may influence IPNV virulence is poorly described.MethodsIn this study we compared two field isolates of IPNV (NFH-Ar and NFH-El). The sequence changes, replication and mortality were assessed following experimental challenge of Atlantic salmon. Gene expression analyses with qPCR and microarray were applied to examine the immune responses in head kidney.ResultsSignificant differences in mortality were observed between the two isolates, and viral load in the pancreas at 13 days post infection (d p.i.) was more than 4 orders of magnitude greater for NFH-Ar in comparison with NFH-El. Sequence comparison of five viral genes from the IPNV isolates revealed different mutation rates and Ka/Ks ratios. A strong tendency towards non-synonymous mutations was found in the HRV of VP2 and in VP3. All mutations in VP5 produced precocious stop codons. Prior to the challenge, NFH-Ar and NFH-El possessed high and low virulence motifs in VP2, respectively. Nucleotide substitutions were noticed already during passage of viruses in CHSE-214 cells and their accumulation continued in the challenged fish. The sequence changes were notably directed towards low virulence. Co-ordinated activation of anti-viral genes with diverse functions (IFN-a1 and c, sensors - Rig-I, MDA-5, TLR8 and 9, signal transducers - Srk2, MyD88, effectors - Mx, galectin 9, galectin binding protein, antigen presentation - b2-microglobulin) was observed at 13 d p.i. (NFH-Ar) and 29 d p.i. (both isolates).ConclusionsMortality and expression levels of the immune genes were directly related to the rate of viral replication, which was in turn associated with sequences of viral genes. Rapid changes in the viral genome that dramatically reduced virus proliferation might indicate a higher susceptibility to protective mechanism employed by the host. Disease outbreak and mortality depend on a delicate balance between host defence, regulation of signalling cascades and virus genomic properties.

Characterization of the IFN pathway in the teleost fish gonad against vertically transmitted viral nervous necrosis virus

“…This fact would suggest a high IFN or antiviral response in the sea bass gonad and a very low response in the brain, which could explain the low resistance of this fish species, but this needs to be confirmed at functional level. These results are in agreement with other studies in fish showing the upregulation of most of these genes after virus infection in several tissues or their antiviral function after cell line overexpression (Biacchesi et al, 2009;Chen et al, 2015;Feng et al, 2011;Rise et al, 2008Rise et al, , 2010Skjesol et al, 2011;Su et al, 2010;Sun et al, 2011;Xiang et al, 2011;Yang et al, 2011), and support the fact that the sequences identified in our study mediate the IFN activation cascade. In the case of tbk1, which is also activated by the TLR response, it is only upregulated in sea bass specimens infected with VNNV.…”

“…Similar upregulation has been documented in the brain of sea bass and Atlantic halibut exposed to VNNV (Dios et al, 2007;Rise et al, 2010), and this supports our data. Moreover, these sensors are also upregulated by several fish RNA viruses or polyI: C in several tissues of fish, such as spleen, head-kidney, liver or intestine, as well as in some fish cell lines, leading to an increase in the type I IFN-mediated antiviral response (Feng et al, 2011;Rise et al, 2008Rise et al, , 2010Skjesol et al, 2011;Su et al, 2010;Yang et al, 2011). Moreover, fish rig1 and mda5 transient overexpression leads to the induction of ifn expression and confers an antiviral state (Biacchesi et al, 2009;Sun et al, 2011).…”

“…This family has three members: RIG-I (also known as DDX58), melanoma differentiation-associated gene 5 (MDA5, also known as IFIN1) and Laboratory of Genetics and Physiology 2 (LGP2, also known as DHX58). These sensors are upregulated by Viral hemorrhagic septicemia virus (VHSV), Spring viremia of carp virus, Grass carp reovirus (GCRV; Aquareovius C), Viral nervous necrosis virus (VNNV; members of the genus Betanodavirus) and Infectious pancreatic necrosis virus (IPNV), as well as by polyinosinic acid (polyI: C; a synthetic analogue of viral dsRNA), leading to an increase in the IFN-mediated antiviral response (Chen et al, 2015;Feng et al, 2011;Rise et al, 2008Rise et al, , 2010Skjesol et al, 2011;Su et al, 2010;Yang et al, 2011). However, further studies are needed to definitely define their roles in the antiviral response, and to identify and characterize their mediators in the molecular pathway leading to IFN activation.…”

Molecular characterization of Tasmanian aquabirnaviruses from 1998 to 2013

“…The length of VP5 can vary dramatically in IPNV (i.e. 28 aa to 133 aa) but the functional significance of such changes remains unclear (Skjesol et al 2011). The effect of the premature stop codons detected within TABV in this study are also unknown but did not correlate with the disease events of 2011 to 2013.…”

DNA vaccine-mediated innate immune response triggered by PRRs in teleosts

“…The expression levels of the fish RLR mRNAs were increased in various tissues upon infection by several viruses, such as SVCV, grass carp Ctenopharyngodon idellus reovirus (GCRV) and IPNV [72][73][74][75][76][77][78][79].…”