The use of antibody Fab fragments specifically directed to two different complementary parts of the tetanus toxin molecule for studying the mode of action of the toxin.

The injection of 500 minimal lethal doses (MLDs) of tetanus toxin into mice routinely causes a flaccid-type paralysis and death within 8 h. Non-precipitating antibody fragments (Fab) directed against each of two papain cleavage products of tetanus toxin (Ibc and IIc) were used to study this botulinum toxin-type effect of tetanus toxin. Ibc (100,000 daltons) is a toxic fragment which does not bind to gangliosides but will produce a flaccid type paralysis when injected into mice. Treatment of intact tetanus toxin (500 MLDs) with Fab-Ibc prevents the flaccid type paralysis and such mice will die from a spastic paralysis after about 24 h. IIc (50,000 daltons) is an atoxic fragment of tetanus toxin which binds tightly to gangliosides. Treatment of tetanus toxin with Fab-IIc prior to intracerebral injection converts the characteristic spastic paralysis to a flaccid paralysis. It is proposed that the botulinum toxin-type effect of tetanus toxin complexed to Fab-IIc results from the inability of such complexes to be transported to the central nervous system. Moreover, the ability of Fab-Ibc to prevent flaccid paralysis, but not spastic paralysis, suggests that both types of paralysis may be mediated by the same portion of the tetanus toxin molecule.