Effect of Cathepsine L1 on Transformation of Malignant Melanoma B16 Cells into Melanocytes

We studied the effects of cathepsins produced by immunocompetent cells of intact mice, mice with B16 melanoma, mice with removed B16 melanoma, and mice with removed Ehrlich carcinoma on the growth of B16 melanoma. Incubation of B16 melanoma cells with cathepsins from immunocompetent cells of intact mice, mice with B16 melanoma, and mice with removed Ehrlich carcinoma did not affect tumor growth. Incubation of B16 melanoma cells with cathepsin from immunocompetent cells of mice with removed B16 melanoma was followed by complete loss of malignancy by these cells. Melanoma cells formed no tumor node within at least 1 year after transplantation, though exhibited high proliferative activity and formed pigmented nevi. The formation of a nevus by B16 melanoma cells is described for the first time. The existence of a mechanism regulating proliferation of malignant cells in the tumor-bearing host was hypothesized. Studies of this mechanism are expected to promote the development of new methods for the treatment of tumors.