Tissue distribution and quantification of bovine leukemia virus proviral DNA in cows after a long-term infection with wild-type strains and the attenuated BLV vaccine strain.

Bovine leukemia virus (BLV) is a B-lymphotropic oncogenic retrovirus that establishes a persistent infection in cattle, with lymphosarcomas occurring in up to 10% of infected animals. This study aimed to investigate the in vivo distribution of BLV provirus in organs and muscle tissues of infected animals after a long-term infection and to evaluate the safety of an attenuated BLV vaccine strain. The study focused on three cows that were part of a larger trial initiated in 2010 designed to evaluate the performance of the attenuated BLV strain. After seven years of infection, one animal from each experimental group was euthanized: one naturally infected with the BLV strain circulating in Argentina (Arg), and two experimentally inoculated with either a Belgian wild-type cloned provirus (WT) or the attenuated pBLV6073DX vaccine strain (Vac). Samples from blood, organs and muscle tissues were analyzed by nested-PCR and quantitative PCR (qPCR). At euthanasia, proviral load (PVL) in blood was high in Arg and WT (452,767 and 75,151 copies/µg DNA, respectively), but extremely low in Vac (55 copies/µg DNA). BLV provirus was detected in all tissues of Arg and WT, including muscle, whereas in Vac it was only detectable at very low levels in the spleen and the precrural lymph node. These findings provide novel evidence of viral biodistribution during chronic infection and, although based on a limited number of animals, represent an essential proof of concept by providing key long-term safety data on the attenuated vaccine strain that contributed to its subsequent regulatory evaluation and deregulation.