Viral characteristics of plaque variants of porcine reproductive and respiratory syndrome virus

Plaque characteristics of porcine reproductive and respiratory syndrome (PRRS) virus isolates were examined using MARC-145 line cells. The plaque morphology of PRRS virus isolates was variable in size and heterogenic in population. Upon serial passages of the PRRS virus isolates on MARC-145 tells, heterogeneity was maintained but numbers of the large plaque size virus were increased with certain isolates. A PRRS virus isolate with variable plaque sizes was subcloned into 2 populations : small plaque ($H_S$) and large plaque ($H_L$) viruses. Growth kinetics of the subclones were then determined in MARC-145 cells, and production of the structural polypeptides was analyzed by SDS-PAGE. In a comparison of the growth kinetics, the $H_S$ virus showed higher infectivity titers during the first 48 hours but slower to reach the peak titier than $H_L$ virus did. In a nucleotide sequence comparison, differences of 4 nucleotides in open reading frames 5-6 gene were found between $H_S$ and $H_L$ viruses. Both the $H_S$ and $H_L$ clones produced 5 polypeptide bands with molecular weights of 15, 19, 26, 36 and 42 kD. The 5 bands were detected at 48 hours postinoculation (PI) with antisera to $H_L$ and another large plaque virus ($W_L$) and at 72 hours PI with $H_S$ virus antiserum. The present results demonstrate differences of biologic and molecular characteristics between the two PRRS virus plaque clones.