Evidence for two factors in sera of tumor-immunized mice which induce specific lymphoid cell-dependent cytotoxicity: IgG2 and a rapidly appearing factor not associated with IgG or IgM.

As previously shown, sera from tumor-bearing mice can induce specific antiserum-dependent lymphoid cell-mediated cytotoxicity (ADC) to syngeneic tumor cells in vitro. The ADC activity in such sera is now shown to be removed by adsorption of the sera to Sepharose-linked rabbit anti-mouse immunoglobulin or to goat anti-mouse IgG2. Immunoglobulins recovered by elution from the affinity columns mediated ADC to the specific tumor cells. In addition, the eluted immunoglobulin passively "armed" normal lymph-node cells in vivo so they were specifically cytotoxic when tested in vitro. Sera taken 48 h after inoculation of tumor or syngeneic tumor cells (before the appearance of palpable tumor) were also shown previously to induce lymphoid cell-mediated cytotoxicity in vitro. This cytotoxic activity was not removed by adsorption of the sera to either anti-IgG or anti-IgM-linked Sepharose. Thus both IgG and an early-appearing serum component which is apparently neither IgG nor IgM can induce specific cell-mediated cytotoxicity by non-immune lymphoid cells in vitro.