The ORFs 5 to 7 encode for the three major structural proteins of porcine reproductive and respiratory syndrome virus (PRRSV): the envelope glycoprotein GP5 (25-26 kDa), the non-glycosylated membrane protein M (18-19 kDa) and the nucleocapsid protein N (14-15 kDa), respectively (Mardassi et al., 1995; Meulenberg et al., 1993). The latter structural proteins of PRRSV are closely associated both in the infected-cells and in the virion, the GP5 and M proteins being associated in the form of heterodimers (Mardassi et al., 1996). The GP5, which is highly glycosylated (Mardassi et al., 1996), have been found to play an important role in the induction of a protective immune response (Pirzadeh and Dea, 1998). Immunization experiments of mice with E. coli-expressed GST-ORFS recombinant fusion protein, as well as with purified PRRSV, induced specific anti-GP5 neutra-lizing MAbs (Pirzadeh and Dea, 1997). The antibodies in PRRSV-infected pigs antisera responsible for the viral neutralisation in cell culture have been also determined to be GP5 specific (Gonin et al., 1999). In DNA immunization experiments, pigs injected with a plasmidic vector expressing the ORF5 gene not only produced neutralizing antibodies to PRRSV, but were also protected against development of clinical disease and lung lesions following an intratracheal challenge with a high infectious dose of PRRSV. On the other hand, parenteral inoculation of SPF piglets with E. coli-expressed GST-ORF5 recombinant fusion protein rather resulted in an increased severity of lung lesions, despite the development of high titers (> 2048) of non-neutralizing antibody titers to GP5 (Pirzadeh and Dea, 1998). Consequently, the use of an eucaryotic expression system should be preconized to produce large amounts of a recombinant protein that would have to preserve the major characteristics (glycosylation, conformational epitopes) of the native major envelope glycoprotein of PRRSV. Recently, a replication defective human type 5 adenovirus (Ad) has been used successfully for the eucaryotic expression and characterization of the ORF3 product of PRRSV (Mardassi et al., 1998). Previous investigators have also demonstrated that recombinant Ad (recAd) carrying the structural genes of different viruses, mainly the envelope or spike glycoproteins of murine and porcine coronaviruses, were effective for the induction of antibodies to the expressed rec glycoproteins that conferred protection (Both et al., 1993; Torres et al., 1995; Wesseling et al., 1993).KeywordsRecombinant AdenovirusNucleocapsid ProteinRespiratory Syndrome VirusMajor Structural ProteinSpike GlycoproteinThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.