A ricin A chain-containing immunotoxin that kills human T lymphocytes in vitro

An immunotoxin specific for human T lymphocytes was prepared by coupling an IgG(2a) anti-CD3 murine monoclonal antibody (64.1) to purified ricin A chain (64.1-A). Treatment of blood mononuclear cells with this immunotoxin at a concentration of 1.7 x 10^-9 mol/L for two hours at 37° C in the presence of 20 mmol/L NH₄Cl decreased phytohemagglutinin-stimulated protein synthesis by 95%. In addition, a sensitive culture assay showed that fewer than 0.03% T cells remained after treatment of human bone marrow mononuclear cells with 64.1-A at a concentration of 1.7 x 10^-9 mol/L. The inhibition of protein synthesis could be prevented by preincubating cells with unconjugated 64.1 antibody but not by preincubating cells with a control IgG(2a) antibody that binds to a different T cell antigen (CD5). At concentrations up to 1 x 10^-8 mol/L, 64.1-A had little effect on blood mononuclear cells from baboons or human myeloid precursors (CFU-GM), which do not express the CD3 antigen recognized by 64.1. Taken together, these results indicate that the toxicity of 64.1-A was specific and the 64.1-A may be a useful reagent for depleting T cells from donor marrow as a means of preventing acute graft-v-host disease after allogeneic bone marrow transplantation.