Although there are some commercial vaccines available against infectious pancreatic necrosis virus (IPNV), the disease still continues to be a major problem for aquaculture development worldwide. In the current work, we constructed a DNA vaccine against IPNV (pIPNV-PP) by cloning the long open reading frame of the polyprotein encoded by the viral RNA segment A. In vitro, the vaccine is properly translated giving the functional IPNV polyprotein since preVP2, VP2 and VP3 proteins were detected because of the VP4-protease cleavage. EPC cells transfected with the vaccine plasmid expressed the viral proteins and induced the expression of type I interferon (IFN)-induced Mx genes. Furthermore, IPNV synthesized proteins seemed to assemble in virus-like particles as evidenced by electron microscopy. In vivo, rainbow trout specimens were intramuscularly injected with the vaccine and expression of immune-relevant genes, the presence of neutralizing antibodies and effect on viral load was determined. The pIPNV-PP vaccine was expressed at the injection site and up-regulated MHC Ialpha, MHC IIalpha, type-I interferon (IFN), Mx, CD4 and CD8alpha gene expression in the muscle, head kidney or spleen, although to a much lower extent than the up-regulations observed in response to an effective DNA vaccine against viral hemorrhagic septicaemia virus (VHSV). However, the IPNV vaccine was also very effective in terms of acquired immunity since it elicited neutralizing antibodies (in 6 out of 8 trout fingerlings) and decreased 665-fold the viral load after IPNV infection. The effectiveness of this new IPNV DNA vaccine and its possible mechanism of action are discussed and compared to other viral vaccines. Vaccine.2010 Apr 2628(19):3291-300. Epub 2010 Mar 10.Cuesta A,Chaves-Poza E,de Las Heras Al,Saint-Jean SR,Pérez-Prieto S,Tafalla C.