Porcine Reproductive and Respiratory Syndrome Virus isolates differ in their susceptibility to neutralization

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is highly heterogenic. This heterogeneity has an effect on antigenic composition of PRRSV and might create differences in sensitivity to neutralization between isolates. The sensitivity to neutralization could be an important feature of PRRSV isolates because it is likely that isolates resistant to neutralization pose a significant challenge for the development of vaccines that elicit broad protective immunity. Nonetheless, little information is available for understanding or categorizing the viral neutralization phenotype of PRRSV isolates. Consequently, the main purpose of this study was to determine whether PRRSV isolates differ in their susceptibility to neutralization and if they can be classified in different categories based on their neutralization phenotype. For this purpose, a panel of 39 PRRSV isolates and a set of 30 hyperimmune monospecific sera were used in cross-neutralization assays. The results of this study indicate that PRRSV isolates differ in their sensitivity to neutralization and k-means clustering system allowed classifying the isolates in four different categories according to their neutralization phenotype: highly sensitive, sensitive, moderately sensitive and resistant to neutralization. Further analyses using two additional clustering systems that considered individual data for the classification of the isolates confirmed that classification obtained by k-means is accurate in most cases and that only in a few instances classification is less stringent. Sequences of GP3, GP4 and GP5 were analyzed but no correlation could be found between the sequence of previously identified neutralizing epitopes or the number of N-linked glycosylation sites in different proteins and the neutralization phenotype of the isolates. These data provide the first systematic assessment of overall neutralization sensitivities of a panel of diverse PRRSV isolates. The classification of the isolates provides a useful tool to facilitate the systematic characterization of neutralizing antibody production elicited by new vaccine candidates.