REGRESSION BY DIFFERENTIATION IN THE SINCLAIR SWINE MODEL OF CUTANEOUS MELANOMA

The spontaneous regression of melanoma in Sinclair miniature swine inv olves the replacement of tumours by pigmented cells, hitherto interpre ted as pigment-laden macrophages (PLMs). We hypothesized that these re sidual cells are terminally differentiated melanoma cells, not monocyt e-derived macrophages. Swine melanoma explants with no regression were transplanted into severe combined immunodeficient (SCID) mice. Harves ted transplant sites were examined by routine light and electron micro scopy techniques. Paraffin sections were also stained with Hoeschst dy e and examined by fluorescence microscopy. All but one site had comple tely regressed and were replaced by PLM-like cells. Hoeschst staining indicated they were of swine, not mouse, origin. The ultrastructural f eatures of the single, partially regressed lesion demonstrated many pr emelanosomes in these cells. We conclude that tumour differentiation i s an important mechanism of regression in the Sinclair swine melanoma model.