Bovine Leukosis Virus as a Model for Human Retroviruses

The most common neoplasm of the bovine species is, by far, lymphoid leukosis. Enzootic bovine leukosis (EBL) has been recognized as a neoplasm of infectious origin tor half a century. The agent was identified as a retrovirus of exogenous origin (1). This virus, named bovine leukemia virus, was found to be unrelated to any known retrovirus family, until the discovery of the human T cell lymphotropic viruses (HTLV) in 1980 (2). BLV, HTLV-I and HTLV-II share a number of biochemical, biological and immunological features which suggest that the three leukemia viruses belong to a new family of retroviruses. According to the same criteria, a more distant relationship to HTLV-III was found. Recently, primate viruses related to the HTLV viruses (STLV-I and III) were identified (3–5). HTLV-III was shown to belong to the lentivirus family by its extensive sequence homology to the ovine VISNA virus (6). BLV, the HTLV and STLV leukemia viruses, HTLV-III, STLV-III and other lentiviruses were shown to form a unique group of retroviruses characterized by the presence of a tat gene (tat = trans-acting transcriptional activation) in their genome (7,8). The tat gene product is believed to play a major role in the induction of the transformed phenotype by BLV and HTLV-I and II (7).