Moraxella (Branhamella) catarrhalis

Abstract

Outer membrane protein (OMP) CopB, an iron-repressible 81-kDa major OMP of Moraxella (Branhamella) catarrhalis has been a major focus of investigation. To assess CopB as a potential vaccine antigen, we elucidated the degree of antigenic and sequence heterogeneity in this protein among strains of M. catarrhalis. Two monoclonal antibodies, 1F5 and 2.9F, which bind to surface-exposed epitopes on CopB recognized 60 and 70% of the strains, respectively. The degree of sequence heterogeneity in CopB was assessed by cloning and sequencing the CopB gene from two different strains of M. catarrhalis and comparing with the published sequence. There was 92 to 96% homology between the sequences at the nucleotide level and 90 to 95% homology at the amino acid level. The variability in the protein sequence is confined mainly to three moderately variable regions. Restriction fragment length polymorphism (RFLP) analysis of the CopB genes obtained from 20 diverse strains by PCR was performed. Ninety percent of the potential restriction sites in the constant regions and 47% of the potential restriction sites in the variable regions were present in the 20 strains, indicating that the pattern of variable and constant areas in the CopB gene is a general pattern among strains of M. catarrhalis. We conclude that the CopB gene is largely conserved among strains of M. catarrhalis and contains discrete regions which show moderate heterogeneity among strains.Moraxella (Branhamella) catarrhalis has emerged as an important human pathogen over the last two decades. In recent studies, it is the third most common bacterial pathogen isolated from the middle ear fluid of children with otitis media (2, 7). There has been a steady increase in the rate of isolation of M. catarrhalis from the sputum of adults with acute exacerbations of chronic obstructive pulmonary disease (9,15,18,19).The basic characteristics of the outer membrane proteins (OMPs) of M. catarrhalis have been well described. The major OMPs of M. catarrhalis are 98 to 20 kDa in size and have been named A to H (3, 16). In addition, a high-molecular-weight OMP of 300 to 700 kDa has been identified (12,14).CopB, an 81-kDa major OMP, has been a major focus of investigation (1,10,11). This protein is identical to OMP B2 (16, 21). The gene encoding this OMP has been cloned and sequenced from one strain (10). Though its precise function is not known, CopB is iron repressible and is involved in iron uptake by M. catarrhalis from lactoferrin and transferrin (1, 5).Several lines of evidence suggest that CopB is a potential vaccine antigen. In a murine model, a monoclonal antibody directed towards this OMP enhances clearance of M. catarrhalis from the lung (10). An isogenic mutant which does not express CopB has diminished ability to survive in the same murine pulmonary clearance model (11). This isogenic mutant also has diminished serum resistance to normal human sera in comparison to the wild type strain (11). To further assess CopB as a potential vaccine antigen, it will be important to know ...