Mechanisms by which tumors avoid destruction by the immune system BCG-catalyzed increase in IgG and IgA blocking activity of lymphocyte-mediated cytotoxicity.

The immunoglobulin IgM fraction from the serum of one week sarcoma-bearing BALB/c mice increased, the IgG and IgA fractions from the same animal had no effect on the lymphocytemediated cytotoxic release of 51Cr from 51Cr-labelled sarcoma cells. These latter two immunoglobulin fractions from serum of 14 Days, or more, sarcoma-bearing mice inhibited the lymphocyte-mediated cytotoxic activity. The IgA, IgG or IgM fractions from mice inoculated with bacillus Calmette-Guerin (BCG) had no effect, but if these same animals were inoculated with viable sarcoma cells (10(2) to 10(6) cells per mouse) IgG and IgA fractions inhibited the lymphocyte-mediated cytotoxic activity. The magnitude of inhibition was greatest with sera or IgG from tumor-bearing BCG-treated animals.