Moraxella catarrhalis ubiquitous surface protein A2 (UspA2) mediates resistance to the bactericidal activity of normal human serum. In this study, an interaction between the complement fluid phase regulator of the classical pathway, C4b binding protein (C4BP), and M. catarrhalis mutants lacking UspA1 and/or UspA2 was analyzed by flow cytometry and a RIA. Two clinical isolates of M. catarrhalis expressed UspA2 at a higher density than UspA1. The UspA1 mutants showed a decreased C4BP binding (37.6% reduction), whereas the UspA2-deficient Moraxella mutants displayed a strongly reduced (94.6%) C4BP binding compared with the wild type. In addition, experiments with recombinantly expressed UspA150–770 and UspA230–539 showed that C4BP (range, 1–1000 nM) bound to the two proteins in a dose-dependent manner. The equilibrium constants (KD) for the UspA150–770 and UspA230–539 interactions with a single subunit of C4BP were 13 μM and 1.1 μM, respectively. The main isoform of C4BP contains seven identical α-chains and one β-chain linked together with disulfide bridges, and the α-chains contain eight complement control protein (CCP) modules. The UspA1 and A2 bound to the α-chain of C4BP, and experiments with C4BP lacking CCP2, CCP5, or CCP7 showed that these three CCPs were important for the Usp binding. Importantly, C4BP bound to the surface of M. catarrhalis retained its cofactor activity as determined by analysis of C4b degradation. Taken together, M. catarrhalis interferes with the classical complement activation pathway by binding C4BP to UspA1 and UspA2.

Nontypeable Haemophilus influenzae is a significant pathogen in children, causing otitis media, sinusitis, conjunctivitis, pneumonia, and occasionally invasive infections. H. influenzae type b conjugate vaccines have no effect on infections caused by nontypeable strains because nontypeable strains are nonencapsulated. Approximately, one-third of episodes of otitis media are caused by nontypeable H. influenzae and the bacterium is the most common cause of recurrent otitis media. Recent progress in elucidating molecular mechanisms of pathogenesis, understanding the role of biofilms in otitis media and an increasing understanding of immune responses have potential for development of novel strategies to improve prevention and treatment of otitis media caused by nontypeable H. influenzae. Feasibility of vaccination for prevention of otitis media due to nontypeable H. influenzae was recently demonstrated in a clinical trial with a vaccine that included the surface virulence factor, protein D.

Introduction of a conjugated vaccine against encapsulated Haemophilus influenzae type b (Hib) has led to a dramatic reduction of invasive Hib disease. However, an increasing incidence of invasive disease by H. influenzae non-type b has recently been reported. Non-type b strains have been suggested to be opportunists in an invasive context, but information on clinical consequences and related medical conditions is scarce. In this retrospective study, all H. influenzae isolates (n = 410) from blood and cerebrospinal fluid in three metropolitan Swedish regions between 1997 and 2009 from a population of approximately 3 million individuals were identified. All available isolates were serotyped by PCR (n = 250). We observed a statistically significant increase in the incidence of invasive H. influenzae disease, ascribed to non-typeable H. influenzae (NTHi) and encapsulated strains type f (Hif) in mainly individuals >60 years of age. The medical reports from a subset of 136 cases of invasive Haemophilus disease revealed that 48% of invasive NTHi cases and 59% of invasive Hif cases, respectively, met the criteria of severe sepsis or septic shock according to the ACCP/SCCM classification of sepsis grading. One-fifth of invasive NTHi cases and more than one-third of invasive Hif cases were admitted to intensive care units. Only 37% of patients with invasive non-type b disease had evidence of immunocompromise, of which conditions related to impaired humoral immunity was the most common. The clinical burden of invasive non-type b H. influenzae disease, measured as days of hospitalization/100 000 individuals at risk and year, increased significantly throughout the study period.

Protein A from Staphylococcus aureus (SpA) is known to bind to the Fc region of most mammalian IgG classes. In the present article data are presented showing that SpA is a highly efficient mitogen for human peripheral B lymphocytes, with no detectable activity for T lymphocytes. In order to achieve optimal stimulating conditions SpA should be presented to the lymphocytes on an insoluble matrix, such as the SpA-positive bacteria themselves or SpA covalently attached to Sephadex or Sepharose beads. Using such conditions SpA is equivalent with regard to stimulatory capacity for B lymphocytes as phytohemagglutinin is for the human T lymphocytes. Specificity controls proved beyond doubt that SpA and not any other contaminating product is the B cell mitogen. It is concluded that SpA as an inducer of human B lymphocyte division might serve as a highly useful assay in the clinical assessment of B lymphocyte function. It should also be a suitable tool in the fine analysis of B lymphocyte activation via the specific interactions with surface IgG molecules.

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...