ABSTRACT. Pseudorabies virus (PRV) propagated in rabbit kidney-derived RK-13 cells (PRV-RK) was neutralized by serum obtained from specific pathogen-free pigs through the activation of complement. The virus-neutralizing activity of swine serum was lost after treatment with ethylene glycol-bis-aminoethylether-N,N,N',N'-tetraacetic acid (EGTA) or ethylenediaminetetraacetic acid (EDTA). Anti-C1q and anti-IgM antibodies also inhibited virus-neutralizing activity. Though IgG-depleted swine serum neutralized PRV, IgM and IgG-free swine serum lost virus-neutralizing activity. Pre-incubation of swine serum with RK-13 cells, but not with swine kidney-derived CPK cells, at 4°C eliminated the virus-neutralizing activity to PRV-RK. Results indicated that swine serum contained natural IgM against an antigen(s) on the RK-13 cell surface and that this surface antigen was integrated into the PRV envelope during the budding process. Thus the natural IgM in swine serum reacted with the RK-13 antigen on the viral envelope, activated the complement cascade and neutralized the PRV-RK. KEY WORDS: complement, natural antibodies, PRV.J. Vet. Med. Sci. 67(3): 229-234, 2005 We previously reported that pseudorabies virus (PRV) grown in rabbit kidney-derived RK-13 cells was neutralized by swine serum while the same virus grown in porcine kidney-derived CPK cells was resistant [17]. However, the virus-neutralizing activity of swine serum was lost when serum was heat-inactivated at 56°C for 30 min. We speculated that neutralization of PRV was caused by activation of complement via the alternative pathway because of the apparent absence of anti-PRV antibodies in swine serum [17]. We also found that a similar phenomenon was observed when PRV grown in several cell lines was incubated with human serum [13]. PRV grown in several cell lines also showed different resistant levels against rabbit serum [13].It has been known that human immunodeficiency virus type 1 acquires cell membrane complement regulatory factors on its envelope from cells during the budding process, thereby escaping from complement attack [18]. Therefore it was assumed that RK-13-grown PRV acquired complement regulatory factors from the host rabbit cells and these were less effective against swine complement attack. On the other hand PRV grown in CPK cells acquired swine complement regulatory factors which are more effective regulators against swine complement and thus were resistant to swine complement attack [17].However, further study revealed that RK-13 cells were lysed by swine serum via the activation of complement classical pathway (Hayashi et al., in preparation). It was found that swine serum contained natural IgM antibodies that specifically reacted with an antigen or antigens on the RK-13 cell surface. Therefore a question arose whether the anti-RK-13 natural IgM in swine serum is responsible for the neutralization of PRV grown in RK-13 cells.In this study, using swine serum and PRV propagated in RK-13 cells (PRV-RK), we investigated the mechanism for the virus-neu...
