Transmission and propagation in cell culture of virus produced by cells transfected with an infectious molecular clone of bovine leukemia virus.

A full-length molecular clone of bovine leukemia virus (BLV) pBLV-IF with two copies of a long terminal repeat (LTR) was constructed from a previously isolated, covalently closed, circular DNA clone, pB6490, that has one copy of the LTR and the pX region split at an EcoRI site. This molecular clone directed the synthesis of viral proteins and the induction of syncytia in transiently transfected cells. In addition, virus particles were released into the culture medium. Serial passages of transient transfectants also resulted in propagation of BLV. After transfection of five cell lines with linearized pBLV-IF and a neomycin-resistance gene, BLV-producing transfectants were established in cell lines COS-1 and 23CLN that did not form syncytia upon expression of BLV. In HeLa and FLK cells, BLV produced by a stable COS-1 transfectant was transmitted by both cell-free and cell-to-cell infection. Thus, pBLV-IF encoded an infectious provirus that successfully induced primary and secondary infections. This study indicates that the infectious molecular clone and the virus-producing transfectants could be useful for further examination of the biological properties of BLV.