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To investigate the feasibility of substituting human mAb (HmAb) for human polyclonal preparations in the treatment of infections, we employed anti-tetanus toxin (TT) as a model system. We established a large panel of hybridomas secreting anti-TT HmAb and compared their fine specificities and protectivity with those exhibited by tetanus immune globulin (TIG). Analysis of three different commercial TIG preparations indicated that the majority of anti-TT antibodies is directed against epitopes expressed by the A fragment, the L chain of TT. Absorption of TIG with purified A fragment completely abolished its protective capacity in mice. Absorption with C fragment, the carboxy-terminal portion of the H chain of TT, had no discernible effect, illustrating the crucial importance of anti-A fragment antibody. The vast majority of more than 100 generated TT-specific HmAb showed specificity for the A fragment. Six HmAb with significant neutralizing activity were identified and further characterized. Five of them recognized the A fragment, whereas one, ST12, bound to both the A fragment and the C fragment with equal affinity. ST12 by itself conferred long lasting protection against TT intoxication when singly administered, and the remainder mediated only a delayed death. ST12 conferred a protection of 13.2 IU/100 micrograms IgG. However, when individual HmAb were combined, synergistic effects were observed. Optimal potency (43 IU/100 micrograms IgG) was obtained with a combination of two HmAb. To obtain a 250-IU dose, only 0.7 mg of this mixture was required in contrast to 100 to 170 mg IgG for TIG.

We report the generation and the characterization of a set of human monoclonal antibodies (HmAb) specific for Gram-negative bacteria of Klebsiella pneumoniae. The eight human hybridomas secrete either IgM kappa, IgA1 kappa, or IgA2 kappa antibodies. One HmAb binds bacteria of only one serotype. Five HmAb recognize non-overlapping clusters of 2, 3, or 10 different serotypes. The remaining two HmAb both recognize three serotypes. Two serotypes are recognized by both HmAb, and in addition both HmAb bind one more nonidentical serotype. These results suggest that in man, epitopes are immunodominant, different from serotype-specific determinants detected by conventional rabbit antisera. Screening of clinical isolates revealed that the HmAb recognize not only representative typing strains but also most isolates of the corresponding serotype. In addition, most of the isolates that were non-typable by polyclonal antisera were recognized by one of the HmAb. Fine specificity analyses revealed that all HmAb are highly specific for the isolated capsular polysaccharides (CPS) of bacteria within the corresponding cluster of serotypes. However, the avidity of a HmAb for the different CPS can differ significantly. Taken together, our results suggest that the unequivocal interactions between HmAb and CPS offer the basis for an alternative, better defined classification system, and that passive immunization with a limited number of HmAb may provide a feasible strategy for the protection against the majority of fatal, nosocomial infections with multidrug-resistant strains of K. pneumoniae.