Spontaneous expression of an endogenous retrovirus by the equine sarcoid-derived MC-1 cell line.

A retrovirus is spontaneously released into the culture medium of the equine sarcoid-derived MC-1 cell line. The MC-1 virus did not exhibit in vitro transforming activity or replication when tested on equine fibroblasts or a variety of other mammalian cell cultures. Complementary DNA, synthesized using detergent-activated MC-1 virus RNA-dependent DNA polymerase, detected homologous sequences in the DNA of an established equine dermal cell line and in the DNA of primary equine dermal fibroblasts. Iododeoxyuridine or azacytidine induced a replication-deficient endogenous retrovirus in the normal fibroblasts and amplified the production of MC-1 virus by the tumor cells. It was concluded that the endogenous virus, repressed in normal equine cells, is spontaneously expressed by the tumor cells.