Current knowledge on the structural proteins of porcine reproductive and respiratory syndrome (PRRS) virus: comparison of the North American and European isolates

Porcine reproductive and respiratory syndrome virus (PRRSV) belongs to the recently recognized Arteriviridae family within the genus Arterivirus, order Nidovirales, which also includes equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV). Mature viral particles are composed of an envelope 50–72 nm in diameter, with an isometric core about 20–30 nm enclosing a linear positive-stranded RNA genome of approximately 15 kb. The virions are assembled by the budding of preformed nucleocapsids into the lumen of the smooth endoplasmic reticulum and/or Golgi apparatus. The mature virions are then released by exocytosis. The viral genome contains eight open reading frames (ORFs) which are transcribed in cells as a nested set of subgenomic mRNAs. The ORF1a and ORF1b situated at the 5′end of the genome represent nearly 75% of the viral genome and code for proteins with apparent replicase and polymerase activities. The major structural proteins consist of a 25 kDa envelope glycoprotein (GP(5)), an 18–19 kDa unglycosylated membrane protein (M), and a 15 kDa nucleocapsid (N) protein, encoded by ORFs 5, 6 and 7, respectively. The N protein is the more abundant protein of the virion and is highly antigenic, which therefore makes it a suitable candidate for the detection of virus-specific antibodies and diagnosis of the disease. Four to five domains of antigenic importance have been identified for the N protein, a common conformational antigenic site for European and North American strains being localized in the central region of the protein. In cells and virions, both M and GP(5) occur in heterodimeric complexes linked by disulfide bonds. The expression products of ORFs 2 and 4 are also incorporated into virus particles as additional minor membrane-associated glycoproteins designated as GP(2) and GP(4), with M(r) of 29 and 31 kDa, respectively. The structural nature of the ORF3 product, a highly glycosylated protein with an apparent M(r) of 42 kDa, is still being debated, in view of the apparently conflicting data on its presence in virus particles. Nonetheless, the GP(3) of North American and European strains has been shown to be antigenic, providing protection for piglets against PRRSV infection in the absence of a noticeable neutralizing humoral response. Pigs exposed to the native form of GP(5) by means of DNA immunization develop specific neutralizing and protecting antibodies. The GP(5) is involved in antigenic variability, apoptosis, and possibly antibody-dependent enhancement phenomena. The GP(4) also possesses antigenic determinants that trigger the immune system to produce neutralizing antibodies. Each of the PRRSV structural proteins carries common and type-specific antigenic determinants that permit the ability to differentiate between European and North American strains. The potential use of the PRRSV structural proteins in subunit recombinant-type vaccines is also discussed.