Cell-mediated Immunity against Malignant Melanoma in Monozygous Twins

We observed congruent contemporaneous concordance of malignant melanoma in identical brothers of a set of triplets. Four and one-half years after surgery, one twin had metastatic disease and the other was apparently free of tumor. Blastoid transformation of lymphocytes measured by DNA synthesis and cytotoxicity of lymphocytes for melanoma cells measured by fluorescein diacetate loss in mixed lymphocyte-tumor cell cultures were both substantially greater with lymphocytes from the tumor-free twin. Since the brothers were monozygous, this cannot be attributed to transplantation-antigenic differences. Possible explanations include neoantigenization of the tumor and/or depletion or blocking of sensitized lymphocytes from the autochthonous host. The data extend the evidence for immunity against tumors in man and the techniques for demonstrating it.