Effect of the host cell line on the vaccine efficacy of an attenuated porcine reproductive and respiratory syndrome virus

The abilities of the modified-live Prime Pac (PP) strain of porcine reproductive and respiratory syndrome virus (PRRSV), propagated in either traditional simian cells (MARC-145) or in a novel porcine alveolar macrophage cell line (ZMAC), to confer pigs protection against subsequent PRRSV challenge were compared. Eight week-old pigs were injected with PP virus grown in one of the two cell types and then exposed 4 weeks later to the “atypical” PRRSV isolate NADC-20. Control animals were similarly challenged or remained PRRSV-naïve. While the average adjusted body weight (aabw) of the strict control group increased 22% by 10 days post challenge (pc), this value for the non-vaccinated, challenged group dropped 4%. In contrast, prior immunization with PP virus, regardless of its host cell source, ameliorated this effect by affording a >9% rise in aabw. Likewise, nearly equivalent protection was extended to both groups of vaccinates in regards to the temporal elimination of their pc clinical distress and viremia. However, the PP virus propagated in ZMAC cells appeared to be more efficacious since four of the six pigs receiving this biologic cleared the challenge virus from the their lungs by 10 days pc as compared to only one member of the other vaccinated group. Notably, the predominant quasispecies in the ZMAC cell-prepared PP virus stock contained a highly conserved N-glycosylation site at position 184 in its glycoprotein 2 while this entity was underrepresented in the MARC-145 cell grown biologic. Since glycoprotein 2 is involved in infectivity, such additional glycosylation may enhance virus replication in porcine alveolar macrophages.