Persistent infection of rabbits with bovine leukemia virus associated with development of immune dysfunction

Bovine leukemia virus (BLV) infection of rabbits provides a safe and relatively inexpensive in vivo mammalian system for the study of the mechanisms controlling expression of a unique group of lymphotropic retroviruses. This group of viruses, which includes C-type human T-lymphotropic virus types I and II and lentiviruslike human immunodeficiency virus type 1, possesses genes coding for "trans-activating" products. Rabbits experimentally inoculated with BLV became persistently infected, as demonstrated by a number of tests. All BLV-inoculated rabbits developed persistent serum antibody to BLV. Furthermore, all BLV-inoculated rabbits had peripheral blood mononuclear cells which, when stimulated, expressed the virus, as demonstrated by viral induction of syncytium formation in a BLV-susceptible fibroblast line. The presence of BLV in circulating cells was confirmed by using peripheral blood mononuclear cells from randomly selected BLV-inoculated rabbits, which showed the presence of viral reverse transcriptase activity, BLV transcriptional activity, or BLV proviral DNA. Additional tests showed that infected lymphocytes maintained in culture with recombinant human interleukin-2 formed multinucleated giant cells and produced virus when incubated in cytokine-containing medium. BLV-infected rabbits also showed alterations in several parameters associated with immunity, beginning 6 months after inoculation. Thirty-eight percent of infected rabbits developed abnormally low T-cell responses, as measured by phytolectin stimulation, and T-cell responses cycled between normal and abnormally low over a period of 20 to 24 months. Forty-four percent of rabbits infected for longer than 12 months suffered from recurrent conjunctivitis and rhinitis. By 24 months postinoculation, 28% of infected rabbits were dead or were killed because of poor clinical condition.