Increased toxicity of diphtheria toxin for human lymphoblastoid cells following covalent linkage to anti-(human lymphocyte) globulin or its F(ab')2 fragment.

Anti-(human lymphocyte) globulin was reacted with a mixed anhydride derivative of chlorambucil to give a product which was in turn reacted with diphtheria toxin. The resulting conjugate was partially purified and was found to possess an ability similar to that of the native antibody to bind to the human lymphoblastoid cell lines, CLA4 and Daudi. Daudi cells, as had been observed previously with CLA4 cells, lacked the high sensitivity to diphtheria toxin normally characteristic of cells of human origin. Thus treatment with free toxin at a concentration of 1 microgram/ml was without effect upon their ability to incorporate [3H]leucine. By contrast, Daudi cells were highly sensitive to toxin conjugated to anti-(human lymphocyte) globulin or to its F(ab')2 fragment. Exposure for 24 h to a solution of conjugate containing toxin at a concentration of 0.5 ng/ml caused a reduction of 50% in the leucine uptake by Daudi cells. The toxicity of the conjugate could be blocked by diphtheria antitoxin or by pretreatment of the cells with non-conjugated antibody. Toxin linked to normal horse IgG or to its F(ab')2 fragment was without cytotoxic effect upon Daudi cells. Furthermore both the conjugate with anti-(human lymphocyte) globulin and that with normal IgG were approximately 100-fold less able than non-conjugated toxin to inhibit protein synthesis by a human fibroblast cell line to which the antibody showed no appreciable binding. Thus the conjugates are relatively ineffective against cells which lack an antigen to which the antibody moiety can bind. In contrast with the greatly increased toxicity of diphtheria toxin for human lymphoblastoid cells following its linkage to anti-(human lymphocyte) globulin, a conjugate of toxin linked to anti-(mouse lymphocyte) globulin was ineffective against murine spleen cells in vitro.