Comparison of functional activities between IgG1 and IgM class-switched human monoclonal antibodies reactive with group B streptococci or Escherichia coli K1.

The influence of valence and heavy chain on antibody activity was investigated using transfectoma-derived, class-switched IgG1 and IgM human monoclonal antibodies (MAbs) reactive with the bacterial pathogens Escherichia coli K1 and group B Streptococcus species. IgG-IgM pairs were compared in vitro for antigen binding and opsonic activities and in vivo for protective efficacy in neonatal rats. For the anti-E. coli pair, the IgM MAb was 1000-fold more potent in all assay formats. Importantly, the 50% protection dose (PD50) of the IgM MAb was 10-20 ng/rat, while 100 micrograms of the IgG MAb was only minimally protective. For the group B streptococcal MAbs, the IgM was 100- and 4500-fold more potent in binding and opsonization assays, respectively. However, while 20 micrograms of IgM protected neonatal rats, 100 micrograms of IgG MAb was partly protective. These experiments demonstrate the utility of recombinant DNA technology for creating a panel of antibodies that may aid in selecting potential immunotherapeutic candidates.