The Moraxella catarrhalis immunoglobulin D (IgD)-binding protein (MID) is a 200-kDa outer membrane protein displaying a unique and specific affinity for human IgD. MID is found in the majority of M. catarrhalis strains. In the present paper, we show that MID-expressing M. catarrhalis strains agglutinate human erythrocytes and bind to type II alveolar epithelial cells. In contrast, M. catarrhalis isolates with low MID expression levels and two mutants deficient in MID, but with readily detectable UspA1 expression, do not agglutinate erythrocytes and have a 50% lower adhesive capacity. To examine the adhesive part of MID, the protein was dissected into nine fragments covering the entire molecule. The truncated MID proteins were expressed in Escherichia coli, purified, and used for raising polyclonal antibodies in rabbits. Interestingly, by using recombinant fragments, we show that the hemagglutinating and adhesive part of MID is localized within the 150-amino-acid fragment MID Moraxella (Branhamella) catarrhalis is often a harmless commensal in the respiratory tract and can be detected in nasopharyngeal cultures from 66% of children during the first year of life and from approximately 4% of adults at any given time. However, the species has increasingly been recognized as an important pathogen in respiratory tract infections in both children and adults (4, 15). After Haemophilus influenzae and Streptococcus pneumoniae, M. catarrhalis is the third most common bacterial agent in acute otitis media in children. In adults and the elderly, M. catarrhalis is a common cause of lower respiratory tract infections, particularly in those with predisposing conditions such as chronic obstructive pulmonary disease. M. catarrhalis is often implicated as a cause of sinusitis in both children and adults. Furthermore, the emergence of antibiotic resistance suggests that the incidence of M. catarrhalis infections may continue to rise. More than 90% of M. catarrhalis clinical isolates are resistant to penicillin, and M. catarrhalis has developed resistance at a rate unprecedented for any bacterial species. The emergence of M. catarrhalis as a significant cause of human disease has generated much interest in the identification of potential vaccine antigens (20). M. catarrhalis vaccine development is at the antigen candidate identification stage, and researchers are searching for potential vaccine antigens that elicit antibodies with capacity to limit the bacterium's pathogenicity.Two decades ago, M. catarrhalis was shown to display a strong affinity for soluble human immunoglobulin D (IgD) (9). IgD binding at the cellular level explains the strong mitogenic effects of M. catarrhalis on human lymphocytes (3, 10). In addition, it was demonstrated that M. catarrhalis stimulates the proliferation of high-density (mature) B lymphocytes expressing a high density of IgD B-cell receptors (BCR) and that soluble nonmitogenic monoclonal antibodies (MAbs) reactive with human IgD selectively inhibit the B-lymphocyte response. Inhibition by anti-Ig...

A novel surface protein of the bacterial species Moraxella catarrhalis that displays a high affinity for IgD (MID) was solubilized in Empigen and isolated by ion exchange chromatography and gel filtration. The apparent molecular mass of monomeric MID was estimated to ∼200 kDa by SDS-PAGE. The mid gene was cloned and expressed in Escherichia coli. The complete mid nucleotide gene sequence was determined, and the deduced amino acid sequence consists of 2123 residues. The sequence of MID has no similarity to other Ig-binding proteins and differs from all previously described outer membrane proteins of M. catarrhalis. MID was found to exhibit unique Ig-binding properties. Thus, in ELISA, dot blots, and Western blots, MID bound two purified IgD myeloma proteins, four IgD myeloma sera, and finally one IgD standard serum. No binding of MID was detected to IgG, IgM, IgA, or IgE myeloma proteins. MID also bound to the surface-expressed B cell receptor IgD, but not to other membrane molecules on human PBLs. This novel Ig-binding reagent promises to be of theoretical and practical interest in immunological research.

Outer membrane vesicles (OMVs) play an important role in the persistence of Helicobacter pylori infection. Helicobacter OMVs carry a plethora of virulence factors, including catalase (KatA), an antioxidant enzyme that counteracts the host respiratory burst. We found KatA to be enriched and surface-associated in OMVs compared to bacterial cells. This conferred OMV-dependent KatA activity resulting in neutralization of H2O2 and NaClO, and rescue of surrounding bacteria from oxidative damage. The antioxidant activity of OMVs was abolished by deletion of KatA. In conclusion, enrichment of antioxidative KatA in OMVs is highly important for efficient immune evasion.

The adhesin protein E (PE) of the human respiratory pathogen nontypeable Haemophilus influenzae (NTHi) exists in all clinical isolates. In the present study, NTHi adherence to epithelial cells of various origins was further analyzed. The number of intraepithelial PE-deficient NTHi was decreased compared with PE-expressing NTHi. Interestingly, PE-expressing NTHi or Escherichia coli transformants, in addition to soluble recombinant PE22-160 without a lipid moiety, induced a proinflammatory cell response. The adhesive PE domain was defined within PE84-108, and preincubation of epithelial cells with this peptide blocked adhesion of several clinical NTHi isolates. Mice immunized with PE84-108 cleared NTHi up to 8-fold more efficiently on pulmonary challenge than did mice immunized with a control peptide. Finally, anti-PE mouse antibodies from vaccinated mice prevented NTHi adhesion. Our data suggest that the ubiquitous adhesin PE plays an important role in the pathogenesis of NTHi infection.

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