ABSTRACT. Non-cytopathogenic (NCP) viruses of bovine viral diarrhea (BVD) virus were detected at a low ratio by the reverse plaque formation method from virus samples after several plaque clonings of cytopathogenic (CP) BVD viruses; NADL and Osloss strains. This phenomenon suggests that the NCP BVD viruses are produced at a low ratio during the propagation of CP BVD viruses in vitro. To investigate the differences between the parent CP BVD virus and the NCP BVD virus as a real progeny, the regions flanking the insertion of cellular mRNA in the p125 domain of NADL and Osloss strains were amplified by RT-PCR and cloned into pGEM 3Z plasmid vector, and then sequenced. Consequently, it was confirmed that sequences of cellular mRNA insertion of CP BVD viruses, NADL and Osloss strains, were completely and exactly deleted from the NCP BVD viruses which were real progeny of CP BVD viruses, NADL and Osloss strains. These results suggest that NCP BVD viruses may revert from CP biotype to NCP biotype by the deletion of cellular mRNA insertion in the viral genome of CP BVD viruses (NADL and Osloss strains). proposed that a NCP BVD virus mutated to a CP BVD virus by taking up cellular sequences during a recombination event in persistently infected animals [21,22]. This hypothesis, however, was challenged by observations that some CP BVD viruses lacked the insertion in their genomes [7,9,12]. Therefore, the understanding of the cytopathogenicity of the BVD virus seems far from complete. Recently, NCP BVD viruses were isolated from CP BVD virus stocks and each virus stock of CP BVD virus contained simultaneously two kinds of NCP BVD viruses showing a different manner of interference [24]. It was speculated that the mutation from the CP biotype to the NCP biotype occurred during the propagation of CP BVD viruses in vitro. This paper deals with the possibility that the reversion of BVD virus from CP biotype to NCP biotype is caused by the deletion of cellular mRNA insertion in viral genome of CP BVD viruses; NADL and Osloss strains.
MATERIALS AND METHODS
Viruses and cells:The NADL [13] and Osloss strains [26] of CP BVD virus and derivative NCP BVD viruses (NADL-END + , NADL-END -, Osloss-END + and Osloss-END -strains) newly isolated from plaque cloned NADL and Osloss strains in our laboratory were used in this study. The NADL strain originated from the American Type Culture Collection (Rockville, MD, U.S.A.), and the Osloss strain was kindly provided by Prof. B. Liess, Institute for Virology, Hannover Veterinary School, Germany. Stock viruses were propagated in bovine testicle (BT) cell cultures and stored at 80°C. The BT cells were grown in Eagle's minimum essential medium (MEM) containing 10% fetal