A comparative characterization of dipentameric (IgM)2 and pentameric IgM species present in preparations of a monoclonal IgM for therapeutic use

IgM aggregates in biotechnologically produced preparations have been reported, however, in vitro characteristics and in vivo activity of IgM aggregates have not been well studied. We separated two species of the human monoclonal IgM antibody KBPA-101 by size-exclusion chromatography. Molecular weight determination indicated the presence of dipentameric and pentameric forms. We present here the results of a comparative characterization of these IgM species, including in vitro and in vivo effector function against Pseudomonas aeruginosa. Dipentameric (IgM)2 species were observed to dissociate into pentameric IgM at 37 °C, suggesting a dynamic equilibrium, in which the pentameric species is the predominant form. In vitro antigen binding (P. aeruginosa LPS) and IgM-mediated complement-dependent phagocytosis of labeled bacterial cells did not differ significantly between the dipentameric (IgM)2 and pentameric IgM species. Furthermore, the in vivo efficacy of dipentameric and pentameric IgM in protecting mice from a lethal dose of P. aeruginosa through passive immunization was nearly equivalent. In conclusion, low concentrations of dipentameric (IgM)2 may contain an additional but equally active component of the principal biological form. The data presented in this work support the conclusion that the pentameric form of IgM directed against the O-polysaccharide moiety of P. aeruginosa serotype IATS-O11 and dipentameric (IgM)2 are functionally equivalent.