Characteristics of toxin-neutralization by anti-tetanus human monoclonal antibodies directed against the three functional domains [A], [B] and [C] of the tetanus toxin molecule and a reliable method for evaluating the protective effects of monoclonal antibodies.

Five anti-tetanus human monoclonal antibodies (MAbs) produced by hybrid cell lines we established previously were characterized. Their abilities to neutralize tetanus toxin in vitro and to protect mice against challenge with toxin were studied by observing the changes in the progress of symptoms in mice. Immunostaining showed that MAbs MAb-G4 and G2 recognized the N-terminal domain, [A] and the C-terminal domain, [C] of the tetanus toxin molecule, respectively, while MAbs MAb-G1, G3 and G6 recognized its middle domain, [B]. Enzyme-linked immunosorbent assay showed that the binding affinity of MAb-G3 was 2.9 x 10(10) M-1 and those of the other MAbs were as high as approximately 10(11) M-1. In in vitro neutralization experiments, at sufficient doses all the MAbs as single reagents protected mice completely against the effect of tetanus toxin. However, at lower doses than those sufficient to rescue mice, the kinetic patterns of progress of symptoms with the individual MAbs differed with each other and, except for MAb-G4, were different from that of anti-tetanus human polyclonal antibody. They suppressed the development and/or slowed the rate of progress of symptoms for over 96 h and delayed death of the mice. We propose that the comparison of the minimum survival dose with that of human polyclonal antibody of known international units is a reliable method for estimating the actual protective activity of a MAb. Intravenous (IV) injection of doses of individual MAbs or their mixtures at over 0.03 IU per mouse protected mice from subsequent challenge with 20 MLD of tetanus toxin.(ABSTRACT TRUNCATED AT 250 WORDS)