Outer membrane protein B1, an iron-repressible protein conserved in the outer membrane of Moraxella (Branhamella) catarrhalis, binds human transferrin.

Moraxella (Branhamella) catarrhalis is a gram-negative human mucosal pathogen, which primarily causes otitis media in young children. However, this bacterium is also a common cause of lower respiratory tract infections in adults with underlying lung disease. Our previous data have shown that M. catarrhalis expresses iron-repressible outer membrane proteins in response to iron limitation. We have extended these observations to demonstrate that one of these proteins, termed outer membrane protein (OMP) B1, binds human transferrin. Using a newly developed monoclonal antibody to OMP B1, we determined that this protein is conserved in the iron-stressed outer membranes of all clinical isolates of M. catarrhalis tested to date. Furthermore, our data have confirmed that children infected with M. catarrhalis have immunoglobulin G antibodies to OMP B1 in their convalescent sera. These current data suggest that OMP B1 is immunogenic and expressed in vivo and may be involved in an iron uptake mechanism utilized by M. catarrhalis.