Studies of human sera with cytotoxic activity

42 human sera showing in vitro cytotoxic activity of restricted or broad HL-A specificities with test human lymphocytes were studied for the molecular and immunoglobulin class of cytotoxic antibody using sucrose gradient separations, DEAE-cellulose chromatography, and Sephadex G-200 gel filtration. Sera originated from patients with previous multiple pregnancies (19), multiply transfused patients (8), subacute bacterial endocarditis (4), systemic lupus (2), and human umbilical cord sera (9). In 32 of 42 instances, predominant cytotoxic activity was found in high molecular weight gradient fractions; however, DEAE chromatographic separations revealed cytotoxic activity in initial buffer fractions containing primarily gammaG globulin. Gradient separations of cytotoxic activity within initial gammaG DEAE fractions showed localization of cytotoxicity only in high molecular weight materials. Confirmation of high molecular weight gammaG cytotoxic activity was obtained by resistance to mercaptoethanol treatment, abolition of activity after absorption only with specific anti-gammaG antisera, and by the finding that high molecular weight cytotoxic activity in gradients or gel filtration run at neutral pH 7.4 became 7S when separations were rerun at an acidic pH of 4.0. Such 7S activity again became rapidly sedimenting when the same fractions were again rerun in gradients at neutral pH.19S gammaM cytotoxic activity was documented in a panel of 15 human sera containing anti-"I" cold agglutinins. In this instance the cytotoxic activity appeared to be related to the cold agglutinin antibody since it was mercaptoethanol sensitive and could be demonstrated in monoclonal antibody eluates containing primarily gammaM. No in vitro protection again cytotoxic effect was demonstrated when isolated human 19S anti-gamma-globulins were aded to the lymphocytotoxic assay system. With the exception of cold agglutinins, human cytotoxic antibodies appear to be primarily gammaG producing in vitro lymphocyte killing either as 7S gammaG globulin or as rapidly sedimenting aggregates or complexes of gammaG molecules.