KINETICS OF HUMORAL IMMUNE-RESPONSE TO THE MAJOR STRUCTURAL PROTEINS OF THE PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS

The kinetics of appearance of antibodies directed to the major structu ral proteins N, M and E of porcine reproductive and respiratory syndro me virus (PRRSV) was followed in pigs naturally- and experimentally-ex posed to the virus. Specific IgM antibody titers were first detected b y indirect immunofluorescence (IIF) at the end of the first week of PR RSV infection, peaked by day 14 to 21 post-inoculation (p.i.), then ra pidly decreased to undetectable levels by day 35 to 42 p.i. On the oth er hand, specific IgG antibody titers peaked by day 21 to 28 p.i. and remained unchanged to the end of the 6- or 9-week observation period; in addition, a persistent viremia was observed. Virus neutralizing (VN ) antibody titers >8 were not detected until 3 to 4 weeks p.i. Taken t ogether, the results obtained by Western blotting analyses using purif ied virus and E. coli-expressed ORFs 5 to 7 gene products, suggested t hat antibodies directed against the envelope E protein appear by day 7 p.i., whereas antibodies directed against the nucleocapsid N and memb rane M proteins can only be detected by the end of the second week p.i . No correlation could be demonstrated between VN and IIF antibody tit ers, viremia, and viral protein specificities of circulating antibodie s at various times p.i.