Immunotolerance in the laboratory opossum (Monodelphis domestica) to xenografted mouse melanoma.

Monodelphis domestica, a South American marsupial, has been developed as a laboratory animal model for allogeneic and xenogeneic cancer research by taking advantage of its underdeveloped immune system in the early days of life. The limited immunological capability during this period provides an opportunity to induce tolerance to grafted tumor tissue in juvenile and adult opossums. In this study, we injected multiple doses of mouse B16 melanoma cells into opossums at different developmental ages (i.e., suckling young, juveniles, and adults) to determine whether immunotolerance could develop as a result of repeated "desensitizing" injections. We found that establishment and growth of xenografted mouse melanoma cells could be established after full immune capability of the animals had been achieved. The tumors thus produced could sustain their growth for as long as 6 weeks before beginning to regress. Our results highlight the potential of the laboratory opossum as a natural mammalian model to study host immunotolerance to xenografted tumor cells.