Malignant melanoma in ultraviolet irradiated laboratory opossums: initiation in suckling young, metastasis in adults, and xenograft behavior in nude mice.

Litters of suckling young of the laboratory opossum (Monodelphis domestica) were irradiated with UV light from sunlamps with a spectral emission peak at 302 nm (UVB) to induce melanocytic nevi. Total doses of 0.87-5.0 kJ/m2 were divided equally among up to 14 exposures during the 19 days from birth. Of 358 sucklings exposed, 217 survived to weaning, and 22 (10%) possessed a nevus when shaved and examined at or after weaning. Affected animals were then exposed 3 times/week to 125 J/m2 of UVB for up to 45 weeks to promote progression to malignancy. Nevi of 8 of the 20 chronically-exposed animals progressed to malignant melanoma with metastases to lymph node(s). Cell cultures were prepared from affected nodes to confirm that pigmented nodal cells were metastatic melanomas. One established cell line (TD15L) contained highly pigmented, dendritic, malignant melanoma cells. These cells, injected s.c. as xenogeneic grafts into athymic nude mice, remained viable in the subcutis and were moderately tumorigenic in the dermis. UVR exposure of Monodelphis sucklings is a novel, effective, and proficient way of initiating melanocytic lesions for studies on susceptibility and progression to melanoma, and the cell lines derived from these melanomas will provide promising new reagents for chemotherapy and immunotherapy investigations.