Mechanisms of cellular cytotoxicity mediated by effector cells from rats with spontaneous tumors

We have previously shown that effector cells from BD X rats bearing spontaneous tumors display increased cytotoxicity towards syngeneic tumor cells compared to effector cells from untreated rats (Zöller and Matzku, 1980 a). The increased in vitro cytotoxicity of lymphoid cells from tumor-bearing (TB) animals was not T-cell mediated and the question was raised, whether it was due solely to increased natural killer (NK) cell activity, or whether humoral factors could also be involved. We now prove that in a long-term assay, the presence of B cells is indeed mainly responsible for increased TB cytotoxicity, since: (1) After depletion of surface immunoglobulin positive cells (s-Ig+), TB cytotoxicity no longer exceeds cytotoxicity of effector cells from untreated rats; (2) Mixtures of s-Ig+ cells from TB animals with Fc-receptor positive (Fc-R+) cells from untreated rats restore the increased TB cytotoxicity; (3) Addition of rabbit Fab' anti rat F(ab')2 reduces TB-effector cell cytoxicity. A minor contribution of the increased TB cytotoxicity by activation/numerical increase of NK cells will be discussed.