Lack of In Vitro and In Vivo Effects of Lipopolysaccharide on Porcine Circovirus Type 2 Infection

Porcine circovirus type 2 (PCV2) is the causative agent of postweaning multisystemic wasting syndrome (PMWS). The presence of immunostimulating factors or concurrent infections seems to be crucial for PMWS development. Lipopolysaccharide (LPS) is a potent immunological activator and has recently been suggested to enhance PCV2 replication in vitro. This study was designed to evaluate the effects of different LPS products on PCV2 in vitro replication of pulmonary macrophages (PMs), and on the potential ability to trigger PMWS in cesarean-derived, colostrum-deprived (CDCD) PCV2-inoculated piglets. In vitro studies using two different PCV2 isolates (Stoon-1010 and 1452/3) showed the presence of PCV2 antigen within the cytoplasm to a variable degree; PCV2 Stoon-1010 was barely detectable (<1% of stained cells), and PCV2 1452/3 was seen in the cytoplasm of more than 85% of PMs. However, no differences were found in intracytoplasmic PCV2 signals among different LPS treatments, or between the LPS-treated and non-treated PMs. Moreover, almost no intranuclear signals for PCV2 antigen were detected in PMs. The in vivo experiment included twenty 7-day-old CDCD piglets divided into four groups: control (n = 4), control/LPS (n = 4), PCV2 (n = 6), and PCV2/LPS (n = 6). The control and control/LPS groups were inoculated intranasally with a cell culture medium (MEM), and the PCV2 and PCV2/LPS groups were inoculated with a Spanish isolate of PCV2 (Burgos). The control/LPS and PCV2/LPS groups were inoculated intraperitoneally with LPS on PCV2 inoculation day. All pigs remained clinically healthy during the entire experimental period (29 days). Animals inoculated with LPS had significant hyperthermia within the first 24 hours post-inoculation. No differences in gross or histological findings were observed among the PCV2 and PCV2/LPS inoculated pigs. All PCV2-infected piglets developed a subclinical infection with the virus. Our results showed that LPS did not increase in vitro viral replication and did not trigger PMWS in PCV2-inoculated pigs.