Antigenic variability among North American and European strains of porcine reproductive and respiratory syndrome virus as defined by monoclonal antibodies to the matrix protein.

Two hybridoma cell lines producing monoclonal antibodies (Mabs) to the 19-kDa matrix (M) protein of porcine reproductive and respiratory syndrome virus (PRRSV) were obtained from BALB/c mice that were immunized with a reference Quebec tissue culture-adapted strain (strain IAF-Klop). The polypeptide specificities of the MAbs were determined by immunoblotting and immunoprecipitation tests with concentrated and purified preparations of the virus and by determining their reactivities with the Escherichia coli-expressed gene products of open reading frames 5 to 7. The two anti-M protein MAbs (MAbs IAFK3 and IAFK6) and another MAb (MAb IAFK8) directed to the 15-kDa nucleocapsid (N) protein were devoid of virus-neutralizing activity. A library of four anti-N protein MAbs (MAbs IAFK8, SDOW17, VO17, and EP147) and two anti-M protein MAbs (MAbs IAFK6 and IAFK3) was used to investigate, by an indirect fluorescent-antibody assay, the antigenic diversity of 15 Canadian PRRSV isolates, in comparison with those of the U.S. ATCC VR2332 attenuated vaccine strain and two reference European (Lelystad and Weybridge) PRRSV strains. The North American and European PRRSV isolates tested shared the epitopes recognized by anti-N protein MAbs IAFK8 and SDOW17, but three distinct patterns could be identified on the basis of their reactivities with the other anti-PRRSV MAbs. No reactivity to the anti-M protein MAbs was observed by either European PRRSV isolate or the attenuated U.S. vaccine strain.