Antigenic and genetic variations of the 15 kD nucleocapsid protein of porcine reproductive and respiratory syndrome virus isolates

The antigenic variability of the 15 kD nucleocapsid protein of porcine repr oductive and respiratory syndrome (PRRS) virus was characterized with a pan el of 24 monoclonal antibodies (MAbs) raised against the American PRRS viru s isolate ISU-P. Five continuous epitopes designated EpORF7-A through E wer e revealed by the reactivity pattern of these MAbs with 67 American field i solates, two modified-live vaccine viruses, and the European Lelystad virus as determined by the indirect immnofluorescence assay and Western immunobl otting and confirmed by additivity and blocking enzyme-linked immunosorbent assays. The reactivity pattern of isolates in the IFA permitted their subd ivision into 4 American antigenic groups which represented 84.1, 11.6, 2.9 and 1.4% of viruses tested. The antigenic variation among isolates was corr elated to single, group specific nucleotide substitutions and mediated by a combination of at least 4 of the 5 epitopes. EpORF7-A was conserved in all American isolates and the Lelystad virus which constituted a separate anti genic group. Consequently, monoclonal antibodies specific for EpORF7-A may prove useful as the antigenic basis for a universal diagnostic test for the PRRS virus. EpORF7-C, D and E were only present in the American isolates t ested.