Immunization with viruslike particles induces long-term protection of rabbits against challenge with cottontail rabbit papillomavirus

Rabbits were immunized with recombinant baculovirus-produced virus-like particles (VLPs) of cottontail rabbit papillomavirus (CRPV) to determine whether these antigens could induce long-term protection against experimental challenge with CRPV. Infectious CRPV and human papillomavirus type II L1 VLPs were used as positive and negative control immunogens, respectively. Three groups of immunized animals were challenged with 10-fold serial dilutions of infectious CRPV at 2 weeks, 6 months, and 12 months after immunizations. Antibody liters in serum reached 1:10,000 immediately after the final booster immunization and then decayed to 1:150 at 6 months and 1:100 at 12 months in unchallenged rabbits. Serum neutralization tilers followed similar kinetics. Papillomas grew on control-immunized rabbits at sites challenged with 10^-1 (100% of sites), 10^-2 (96% of sites), 10^-3 (63% of sites), and 10^-4 (13% of sites) dilutions of virus. At 2 weeks after CRPV L1 VLP immunizations, the rabbits were completely protected against virus challenge. At both 6 and 12 months after CRPV L1 VLP immunizations, strong protection was also observed. In the last two groups, three of seven rabbits were completely protected and only 4 of 14 or 29% of sites challenged with 10^-1 dilution of virus grew papillomas. Papillomas growing at these four sites were also reduced in size (3.5 ± 0.7 mm) at 50 days postchallenge compared with sites challenged with 10^-1 dilution on control-immunized rabbits (13.2 ± 4.2 mm). The results demonstrate that strong and long-lasting protection against experimental challenge with papillomaviruses can be achieved with VLP immunogens.