Development of a serological marker system for epidemiological studies of porcine reproductive and respiratory syndrome virusL. Yanga, E.A. Nelsonb, T.M. Wellerb, K.-J. Yoona, W. Wub and K.B. Platta
a Department of Veterinary Microbiology and Preventive Medicine, College of Veterinary Medicine, Iowa State University, Ames, Iowa 50011, USA
b Department of Veterinary Science, South Dakota State University, Brookings, South Dakota 57007, USA
Abstract - Porcine reproductive and respiratory syndrome (PRRS) continues to be a major viral disease of swine throughout the world. Clinical signs vary between herds, indicating that viruses may differ in pathogenicity. Furthermore, effective use of vaccines and management approaches to control PRRS have not always been successful. Consequently, the objective of this study was to develop a marker system based on epitopic differences among PRRS virus isolates that could be of value in epidemiological studies and possibly allow for correlations to be made between antigenic variations, pathogenicity and effective vaccine use. Accordingly, a panel of 60 monoclonal antibodies (MAbs) representing the products of open reading frames (ORFs) 2 through 7 was used in an indirect fluorescent antibody (IFA) assay to determine the feasibility of categorizing field isolates of PRRS virus. Monoclonal antibodies used in this study were produced at Iowa State University and South Dakota State University, USA. Mice were immunized with whole intact virus particles or selected ORFs 2 and 4 subcloned into a pCDNA3.1/Zeo (+) vector. A total of 60 MAbs were identified and characterized with respect to their protein specificity and isotypes. Two, 3, 6, 3, 3 and 43 MAbs represented products of open reading frames 2 through 7, respectively. The viruses tested in the present study included 177 American field isolates, 3 modified-live vaccine viruses (PRIME PAC PRRS of Schering-Plough Animal Health, RespPRRS / Repro of Boehringer Ingelheim / NOBL Laboratories and Suvaxyn PRRS of Fort Dodge Animal Health). The Lelystad virus was used as European reference isolate. These viruses were propagated in MARC-145 cells in 96-well microtiter plates for the IFA assay. Monoclonal antibodies were categorized into 20 groups based on their seroreactivity patterns with the 177 field isolates, the 3 vaccine viruses and Lelystad virus. The number of groups representing products of ORFs 2 to 7 was 2, 2, 2, 3, 3 and 8, respectively. The seroreactivity patterns of these MAbs also permitted the categorization of the field isolates and vaccine viruses into 25 distinct groups. Group 1 included 52 (28.9%) isolates and was recognized by all 60 MAbs. Group 2 included 61 (33.9%) isolates and was recognized by all MAbs except 3 MAbs representing the ORF3 product. The number of isolates represented by the remaining 22 groups ranged from 1 to 7 isolates. Group 25 included the Lelystad virus that was only recognized by 5 MAbs that represented the ORF 4 and 7 products. The vaccine viruses, PRIME PAC PRRS, Suvaxyn PRRS and RespPRRS / Repro could be differentiated from each other. However, only PRIME PAC PRRS and Suvaxyn PRRS could be distinguished from field isolates. The preceding data further demonstrates that MAbs specific for 6 proteins can be used to categorize PRRS virus isolates and vaccine viruses based on antigenic variability. This ability to categorize PRRS viruses may prove useful in providing a virus marker system for epidemiological studies. The stability of these isolates in pig populations must still be determined before the value of a serological marker system can be fully evaluated.
Corresponding author: K.B. Platta Tel.: (1) 515 294 4940; fax: (1) 515 294 5509;
© INRA, EDP Sciences 2000