Lymphocyte cytotoxicity reactions to leukemia‐associated antigens in identical twins

Cellular cytotoxicity reactions to human leukemia cells were measured with a sensitive, quantitative assay of cell‐mediated immunity. Freshly explanted leukemia cells, rather than tissue culture cells, were tested to ensure that the antigens detected were not acquired in vitro. Identical twins, one member of each pair having leukemia, were used to minimize genetic variables and histocompatibility differences, thereby providing comparable normal cells to control for the leukemic cells. Leukemia‐associated antigens were detected on the cells of seven of ten leukemic patients. In no instance was reactivity observed against only the cells of the normal twin. Lymphocytes from three identical twins, seven parents, two siblings and eight normal unrelated individuals were cytotoxic for cells from the leukemic patients but not for cells from these patients' normal identical twins. Lymphocytes from adults had a much higher incidence of reactivity (46%) against the leukemic cells than did the cells of children (9%). Positive lymphocyte cytotoxicity reactions to leukemia‐associated antigens indicate previous sensitization which could have resulted from infection with an environmental agent such as a virus.