J RAPPE¹, N RUGGLI<sup>1

1</sup>Institute of Virology and Immunology IVI, Sensemattstrasse 293, 3147 Mittelhäusern, Switzerland

Porcine reproductive and respiratory syndrome virus (PRRSV) is an arterivirus responsible for a widespread contagious disease of domestic pigs with high economic impact. Switzerland is one of the rare PRRSV-free countries in Europe, although sporadic outbreaks have occurred in the past. PRRSV strain IVI1173 isolated during the latest short Swiss outbreak of 2012 was entirely sequenced, and a full-length functional cDNA clone was constructed. Genomic characterization of IVI1173 revealed the importance of amino acid 90 of the nulceocapsid (N) as part of a conformational epitope. Indeed, IVI1173 was not detected by SDOW17, a commercial monoclonal antibody against PRRSV N widely used to detect PRRSV-infected cells. Substitution of alanine at position 90 – N(A90) – with a threonine present in most isolates at this position restored reactivity of the recombinant IVI1173-N(T90) to SDOW17 completely. The relevance of this amino acid for PRRSV in general was further studied by mutagenesis of N at position 90 in the backbone of the genotype II clone RVB-581 and in a panel of nucleocapsid proteins of 12 genotype I strains. N(A90) totally disrupted or severely affected the discontinuous epitope in 9 out of 10 tested strains. Due to the loss of antigenicity implied by this mutation, its prevalence among 225 genotype I strains was screened: while N(A90) is rare in the other subgroups, alanine at this position is highly frequent in the Russian subtype 1 (70%) and the subtype 2 (45%) isolates. In conclusion, this study highlights the variable antigenic features of N among genotype I PRRSV strains.