Abstract
Two groups of 3 rabbits each were immunized with either recombinant vaccinia virus, WR‐SFB5env, carrying the human T‐cell lymphotropic virus type I (HTLV‐1) env gene at the site of the hemagglutinin gene of the WR strain, or control vaccinia virus, HA‐WR, lacking the functional hemagglutinin gene. All 6 rabbits responded with anti‐vaccinia virus antibodies. WR‐SFB5env elicited anti‐HTLV‐1 env antibodies but no vesicular stomatitis virus (HTLV‐1) pseudotype neutralizing antibodies in all 3 rabbits. After 10 weeks, the animals were challenged by transfusion of blood from an HTLV‐1‐infected rabbit. Two of the 3 vaccinated rabbits and all 3 control rabbits became infected with HTLV‐1, as indicated by seroconversion and detection of HTLV‐1 proviral sequences by polymerase chain reaction. The rabbit that had been protected from initial challenge became infected with HTLV‐1 upon rechallenge 12 weeks after the first challenge. In view of the proven prophylactic effect of passive immunization against HTLV‐1, our vaccine trial failed because WR‐SFB5__env__ was incapable of inducing neutralizing antibodies against HTLV‐1 in the immunized animals. It remains to be studied whether cell‐mediated immunity such as antibody‐dependent cellular cytotoxicity was involved in the temporary protection of I vaccinated rabbit.