Metastatic melanoma has a poor prognosis, with a median survival of 13 months after diagnosis. New Zealand has among the highest melanoma rates in the world with more than 2000 cases registered every year. Metastatic melanoma continues to be a challenging disease to treat, but recent immunotherapeutic approaches have demonstrated promising results. Our laboratory has developed a cancer vaccine using a virus-like particle (VLP) derived from Rabbit hemorrhagic disease virus (RHDV), which acts as a highly immunogenic scaffold to deliver tumour-associated antigens (TAAs) to the immune system. In vivo studies to date have shown that one or two doses of the VLP carrying gp100 (a melanoma-associated antigen) can specifically activate an anti-tumour response and trigger the formation of immunological memory against gp100 to prevent tumour recurrence. Administering multiple doses of a vaccine often achieves better responses in vivo, so the key aim of this study was to determine what effect multiple dosing of RHDV VLP coupled to gp100 would have on the anti-tumour response.
RHDV VLP was successfully synthesized in a baculovirus expression system using Sf9 insect cells and subsequently used in a series of in vivo experiments. C57BL/6 mice were used in all experiments, receiving either 1, 3 or 6 doses of gp100-carrying VLP (n = 5 per group). An in vivo cytotoxicity assay showed that 3 doses of the VLP vaccine given 5 days apart elicited the highest levels of antigen-specific lysis in a target cell population, compared to a single dose or controls. Therapeutic tumour trials also showed that multiple dosing elicited a stronger anti-tumour response – mice that were first inoculated with B16 melanoma cells and then received 3 or 6 doses of the vaccine 5 days apart had the best overall survival, compared to controls and those that received a single dose. Mice that were tumour-free for 50 days were then rechallenged with B16 cells to assess the immunological memory response, and were found to have increased overall survival, with one mouse from the 3 dose cohort remaining tumour-free. The antibody response against the VLP in these mice was also examined via indirect ELISA. It was found that high levels of antibody against the capsid protein of the VLP were produced in all treated mice, which increased with each additional dose of the vaccine administered. A VLP uptake assay identified that the presence of antibody against the VLP can enhance the early uptake of VLP by DCs, but whether this has an effect on the anti-tumour response remains unclear.
Overall, these preliminary results show that treatment involving multiple dosing of RHDV VLP coupled to the melanoma-associated antigen gp100 does somewhat enhance the anti-tumour response in vivo compared to treatment with a single dose, but the reasons for this need to be investigated further. Future work will focus on determining the role that the antibodies against the VLP play in the anti-tumour response, especially in relation to antigen-presenting cells, and further optimizing the vaccine for clinical trials.
