Inhibition of tumor growth, invasion and metastasis in papain-immunized mice.

Papain-immunized mice possess serum antibodies which cross-react with cathepsin-B- and cathepsin-H-like endopeptidases isolated from B16 melanoma cells. The growth rate, invasion and metastasis of both the B16 melanoma and the Lewis lung carcinoma were inhibited in mice immunized with papain. These animals presented an increased mean survival time as compared to the tumor-bearing nonimmunized controls. Quantitative microscopy suggested that vasodilation and edema, associated with tumor invasion, are, at least partially, sustained by proteolytic enzymes, being strongly reduced when tumor cells were inoculated in papain-immunized mice.