Regressor serum factor-dependent nonspecific killers in tumor-bearing mice

Spleen cells obtained from C3H/He or C57BL/6 mice bearing syngeneic ascitic tumor cells in the early stage of tumor progression had activity to lyse various tumor cells in vitro when serum from MM2-regressor C3H/He was added to the reaction mixture. The serum component responsible for the reaction was of non-immunoglobulin nature. The serum factor-dependent cytotoxic cells were Thy-1-positive cells which were not adherent to nylon wool or Sephadex G-10 and were not natural killer cells. Allogeneic tumor cells and syngeneic tumor cells of near-tetraploid chromosomes were susceptible to such lysis whereas syngeneic near-diploid cells were resistant. The serum factor was absorbed by susceptible cells and not by resistant cells.