Three isolates of rinderpest virus (RPV) with different in vivo virulence were able to infect and productively replicate in bovine monocytic cells. They differed in their kinetics of replication and the morphological changes induced in infected cultures. The highly virulent RPV-Saudi infected > 80% of cells within 6 days p.i. (m.o.i. = 0.1 TCID50 per cell). Under identical conditions, > 50% of cells were infected by the 'mild' (causes minimal mortality in vivo) isolate RPV-Egypt, whereas only 25% were infected by the avirulent RPV-RBOK. Infection by all three viruses produced infectious progeny, induced the formation of syncytia and stellate cells with long processes, and down-regulated MHC class II expression; there was no apparent effect on MHC class I nor LFA-1. RPV-Saudi was the most efficient at generating progeny virus and producing syncytia. While RPV-RBOK was the least efficient at inducing syncytia, RPV-Egypt was the least efficient for progeny virus production. In contrast, RPV-Egypt was particularly efficient at inducing stellate cell formation and down-regulating MHC class II expression. These results indicate a relationship between in vivo virulence and the characteristics of replication and induced morphological changes in monocytes/macrophages. The down-regulation of MHC class II expression would offer a means by which the virus could evade immune recognition. This would be particularly useful for the more cell-associated, but less efficient at maturing, RPV-Egypt.


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