Adaptions of field PRRSVs in Marc-145 cells were determined by variations in the minor envelope proteins GP2a-GP3.
Zhang HL , Tang YD , Liu CX , Xiang LR , Zhang WL, Leng CL , Wang Q , An TQ , Peng JM , Tian ZJ , Cai XH .
Vet Microbiol. 2018 Aug;222:46-54. doi: 10.1016/j.vetmic.2018.06.021.

Abstract
The recent rapid evolution of PRRSVs has resulted in certain biological characteristic changes, such as the fact that an increasing number of field PRRSVs can be isolated from PAMs but not from Marc-145 cells. In this study, we first isolated Marc-145-unadaptive field PRRSV strains from PAMs; sequence analysis showed that these PRRSVs belong to the HP-PRRSV (lineage 8) branch or NADC30-Like (lineage 1) branch. We further found major variations in ORF2-4 regions. To explore the viral adaptation mechanisms in detail, we constructed a full-length cDNA clone of MY-376, a Marc-145-unadaptive PRRSV. Construction of serially chimeric viruses of HuN4-F112 (a Marc-145-adaptive strain) and MY-376 demonstrated that variation in the minor envelope protein (GP2a and GP3) complex is a main determinant of PRRSV tropism for Marc-145 cells.