Use of an isogenic mutant constructed in Moraxella catarrhalis to identifya protective epitope of outer membrane protein B1 defined by monoclonal antibody 11C6

Moraxella catarrhalis-induced otitis media continues to be a significant ca use of infection in young children, prompting increased efforts at identify ing effective vaccine antigens. We have previously demonstrated that M. cat arrhalis expresses specific outer membrane proteins (OMPs) in response to i ron limitation and that this organism can utilize transferrin and lactoferr in for in vitro growth. One of these proteins, which binds human transferri n, is OMP B1. As the human host presents a naturally iron-limited environme nt, proteins, like OMP B1, which are expressed in response to this nutritio nal stress are potential vaccine antigens. In this study, we have developed monoclonal antibody (MAb) 11C6, which reacts to a surface-exposed epitope of OMP B1 expressed by M. catarrhalis 7169. This antibody was used to clone ompB1, and sequence analysis suggested that OMP B1 is the M. catarrhalis h omologue to the transferrin binding protein B described for pathogenic Neis seriaceae, Haemophilus influenzae, Actinobacillus pleuropneumoniae, and M. catarrhalis. Expression of recombinant OMP B1 on the surface of Escherichia coli confers transferrin binding activity, confirming that this protein is likely involved in iron acquisition. In addition, ompB1 was used to constr uct an isogenic mutant in M. catarrhalis 7169. This mutant, termed 7169b12, was used as the control in bactericidal assays designed to determine if OM P B1 elicits protective antibodies. In the presence of MAb 11C6 and human c omplement, wild-type 7169 demonstrated a 99% decline in viability, whereas the ompB1 isogenic mutant was resistant to this bactericidal activity. Furt her analysis with MAb 11C6 revealed the presence of this OMP B1 epitope on 31% of the clinical isolates tested. These data suggest that OMP B1 is a po tential vaccine antigen against M. catarrhalis infections.