The VP2 protein of infectious pancreatic necrosis virus (IPNV) modulates immunogenicity in Atlantic salmon (Salmo salar L)

Resumen del trabajo presentado en el XII Congreso Nacional de Virología, celebrado en Burgos (España), del 9 al 12 de junio de 2013IPNV is the aetiological agent of an acute and widespread disease that causes high mortality in fry and juvenile salmonid fish, as well as in smolts after their transfer to seawater. IPNV is a member of the Birnaviridae family, a group of double stranded non-enveloped RNA viruses that are 60nm in diameter. The IPNV genome is comprised of two parts, segment A, encoding the VP2, VP3, VP4 and VP5 proteins, while segment B codes for the RNA-dependent RNA polymerase, VP1. VP2 is the largest protein in this virus, to which most of the neutralizing epitopes of IPNV map. We have explored the ability of the VP2 protein of IPNV to elicit an immune response when an expression plasmid containing the gene is injected intramuscularly into salmon, analysing the transcription and kinetics of a group of selected genes in the kidney of these fish. The antiviral genes IFN-¿ and Mx, the anti-inflammatory interleukin IL10, and the inflammatory IL 8, 11 and 12 genes were considered, as well as other genes encoding specific elements of the immune response, such as interferon-¿ (a Th1 marker) and the membrane bound immunoglobulin isotype M (mIgM) as a marker of humoral activity. VP2 induced the expression of genes related to both innate and specific immunity, and their expression was modulated similarly in time course experiments. Peak expression of all the genes was reached on day 7 post vaccination and the maximum titre of anti-IPNV neutralizing antibodies was detected on day 15. The recovery and quantification of virus from VP2-stimulated and control fish provided evidence of some degree of protection. Moreover, the relevance of the VP2 gene in the virulence of IPNV was evident in experiments ¿in vitro¿ with small interfering RNAs (siRNAs). Three siRNAs were designed to silence VP2 expression by RNA interference and they were tested in BF-2 cells. Transfection of these siRNAs reduced the production of IPNV particles to a different extent, while the corresponding controls did not affect the particles recovered in supernatants from virally infected cells. Thus, these siRNAs specifically inhibited the expression of the viral target gene, reducing the corresponding viral load. In summary, the VP2-capsid plays a role in the immunogenicity and virulence of IPNV, providing a target antigen for the development of new IPNV vaccines.This work was supported by grant AGL2010-18454 from MINECO, Spain