A hag mutant of Moraxella catarrhalis strain O35E is deficient in hemagglutination, autoagglutination, and immunoglobulin D-binding activities

Binding of vitronectin by the Moraxella catarrhalis UspA2 protein interferes with late stages of the complement cascade

Many Moraxella catarrhalis strains are resistant to the bactericidal activity of normal human serum (NHS). The UspA2 protein of the serum-resistant strain O35E has previously been shown to be directly involved in conferring serum resistance on this strain. Testing of 11 additional serum-resistant M. catarrhalis wild-type isolates and their uspA1 and uspA2 mutants showed that the uspA1 mutants of all 11 strains were consistently serum resistant and that the uspA2 mutants of these same 11 strains were always serum sensitive. Analysis of complement deposition on four different serum-resistant M. catarrhalis strains and their serum-sensitive uspA2 mutants showed that, for three of these four strain sets, the wild-type and mutant strains bound similar amounts of early complement components. In contrast, there was a significant reduction in the amount of the polymerized C9 on the wild-type strains relative to that on the uspA2 mutants. These same three wild-type strains bound more vitronectin than did their uspA2 mutants. UspA2 proteins from these three strains, when expressed in Haemophilus influenzae, bound vitronectin and conferred serum resistance on this organism. Furthermore, vitronectin-depleted NHS exhibited bactericidal activity against these same three serum-resistant wild-type strains; addition of purified vitronectin to this serum restored serum resistance. In contrast, binding of the complement regulator C4b-binding protein by the M. catarrhalis strains used in this study was found to be highly variable and did not appear to correlate with the serum-resistant phenotype. These results indicate that binding of vitronectin by UspA2 is involved in the serum resistance of M. catarrhalis; this represents the first example of vitronectin-mediated serum resistance on a microbe.

Biofilm formation by Moraxella catarrhalis in vitro: roles of the UspA1 adhesin and the Hag hemagglutinin

Mutant analysis was used to identify Moraxella catarrhalis gene products necessary for biofilm development in a crystal violet-based assay involving 24-well tissue culture plates. The wild-type M. catarrhalis strains that formed the most extensive biofilms in this system proved to be refractory to transposon mutagenesis, so an M. catarrhalis strain was constructed that was both able to form biofilms in vitro and amenable to transposon mutagenesis. Chromosomal DNA from the biofilm-positive strain O46E was used to transform the biofilm-negative strain O35E; transformants able to form biofilms were identified and subjected to transposon-mediated mutagenesis. Biofilm-negative mutants of these transformants were shown to have a transposon insertion in the uspA1 gene. Nucleotide sequence analysis revealed that the biofilm-positive transformant T14 contained a hybrid O46E-O35E uspA1 gene, with the N-terminal 155 amino acids being derived from the O46E UspA1 protein. Transformant T14 was also shown to be unable to express the Hag protein, which normally extends from the surface of the M. catarrhalis cell. Introduction of a wild-type O35E hag gene into T14 eliminated its ability to form a biofilm. When the hybrid O46E-O35E uspA1 gene from T14 was used to replace the uspA1 gene of O35E, this transformant strain did not form a biofilm. However, inactivation of the hag gene did allow biofilm formation by strain O35E expressing the hybrid O46E-O35E uspA1 gene product. The Hag protein was shown to have an inhibitory or negative effect on biofilm formation by these M. catarrhalis strains in the crystal violet-based assay.

Adhesion mediated by autotransporters of Gram-negative bacteria: Structural and functional features

The ability of bacterial proteins to promote adhesion to biological surfaces is a fundamental step in bacterial infections. Some bacterial adhesins belong to the family of autotransporters, which are secreted to the surface of Gram-negative bacteria by an elegantly simple mechanism. This review will summarize their functional and structural features.

Moraxella catarrhalis stimulates the release of proinflammatory cytokines and prostaglandin E from human respiratory epithelial cells and monocyte-derived macrophages

The outer membrane proteins of Moraxella catarrhalis, a bacterial pathogen which causes disease in both children and adults, play an important role in its phenotypic properties. However, their proinflammatory potential with regard to respiratory epithelium and macrophages is unclear. To this end, we examined the cytokine- and mediator-inducing capacity of a heat-killed wild-type M. catarrhalis strain and a nonautoagglutinating mutant as well as their outer membrane proteins and secretory/excretory products using the A549 respiratory epithelial cell line. The outer membrane proteins and secretory/excretory products from both isolates as well as the heat-killed bacteria all induced interleukin (IL)-6, IL-8 and prostaglandin E2, but not IL-1beta, from the A549 cell line in a dose- and time-dependent manner. Heat-killed bacteria and secretory/excretory products stimulated the release of IL-1beta, IL-6, IL-8 and prostaglandin E2 from human monocyte-derived macrophages. Both heat-killed isolates also stimulated nuclear translocation and transactivation of nuclear factor-kappaB. The heat-killed wild-type autoagglutinating isolate induced significantly greater amounts of IL-6 and IL-8 from A549 cells than the nonautoagglutinating mutant compared with the monocyte-derived macrophages but no significant differences in the amounts induced by the two strains were observed. These differences were also evident when the respiratory cell line was stimulated with outer membrane proteins as well as in the degree of nuclear factor-kappaB transactivation. There was little difference in the stimulatory activity of the secretory/excretory products. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis analyses revealed some differences in the outer membrane proteins and secretory excretory products between the two isolates. Combined, these data show that M. catarrhalis secretory excretory products and outer membrane proteins are associated with the induction of inflammatory responses in both respiratory epithelium and macrophages.

Cold shock response of the UspA1 outer membrane adhesin of Moraxella catarrhalis

Colonization of the human nasopharynx exposes Moraxella catarrhalis, a common cause of otitis media in children and exacerbations of chronic obstructive pulmonary disease in adults, to sudden downshifts in temperature, occurring when the host breathes cold air. We investigated whether in vitro cold shock influences the expressions of the outer membrane adhesins UspA1 and hemagglutinin, which are considered virulence factors, and of an M. catarrhalis homolog of recA, a housekeeping gene, which in Escherichia coli is induced by cold shock. Quantitative real-time reverse transcriptase PCR was used for measuring mRNA copy number. A screening experiment revealed that a cold shock at 26 degrees C maximally induced the copy number of uspA1. In comparison with 37 degrees C conditions, a 1-hour cold shock at 26 degrees C increased copy numbers of uspA1 and recA by 2.5-fold (11.2 +/- 1.8 versus 4.5 +/- 0.8 copies/CFU) and 2.7-fold (0.30 +/- 0.10 versus 0.11 +/- 0.06), respectively, but did not induce transcription of hag. Exposure to 26 degrees C increased surface expression of UspA1, as assessed by fluorescence-activated cell sorter analysis, and resulted in a significant increase in adherence of strain O35E to Chang human conjunctival cells (97.1% +/- 2.0% versus 48.3% +/- 9.2% at 37 degrees C; P = 0.01). Cold shock induction of uspA1 and recA was detected in strains belonging to either phylogenetic subpopulation of M. catarrhalis (16S rRNA types 1 and 2/3) and was most pronounced in type 2/3 strains (4- to 25-fold for uspA1), which do not express detectable amounts of UspA1 protein at 37 degrees C. These data indicate that cold shock at a physiologically relevant temperature of 26 degrees C induces the expression of at least one virulence factor (UspA1). To our knowledge, no similar data are available for other nasopharyngeal pathogens.

Antigenic specificity of the mucosal antibody response to Moraxella catarrhalis in chronic obstructive pulmonary disease

Moraxella catarrhalis is an important human mucosal pathogen causing otitis media in children and lower respiratory tract infection in adults with chronic obstructive pulmonary disease (COPD). Little is known about the mucosal antibody response to M. catarrhalis in adults with COPD. In this study, 10 pairs of well-characterized sputum supernatant samples from adults with COPD who had acquired and subsequently cleared M. catarrhalis from their respiratory tracts were studied in detail in an effort to begin to elucidate potentially protective immune responses. Flow cytometry analysis was used to study the distribution of immunoglobulin isotypes in paired preacquisition and postclearance sputum samples. The results showed that immunoglobulin A (IgA) is the predominant M. catarrhalis-specific immunoglobulin isotype and that the sputum IgA contains a secretory component, indicating that it is locally produced at the mucosal site. Most patients made new sputum IgA responses to the adhesins UspA1 and Hag, along with the surface protein UspA2. A smaller proportion of patients made new sputum IgA responses to the iron-regulated proteins TbpB and CopB and to lipooligosaccharide. These results have important implications in understanding the mucosal immune response to M. catarrhalis in the setting of COPD and in elucidating the elements of a protective immune response.

Ionic binding of C3 to the human pathogen Moraxella catarrhalis is a unique mechanism for combating innate immunity

Moraxella catarrhalis ubiquitous surface proteins A1 and A2 (UspA1/A2) interfere with the classical pathway of the complement system by binding C4b-binding protein. In this study we demonstrate that M. catarrhalis UspA1 and A2 noncovalently and in a dose-dependent manner bind both the third component of complement (C3) from EDTA-treated serum and methylamine-treated C3. In contrast, related Moraxella subspecies (n = 13) or other human pathogenic bacteria (n = 13) do not bind C3 or methylamine-treated C3. Experiments with recombinant proteins and M. catarrhalis mutants devoid of UspA1/A2 revealed that UspA1/A2 exert their actions by absorbing and neutralizing C3 from serum and restrain complement activation. UspA2 was responsible for most of the effect, and the Moraxella mutant lacking UspA2 was more sensitive to the lytic effect of human serum compared with the wild type. Interestingly, among the large number of bacteria analyzed, only M. catarrhalis has this unique ability to interfere with the innate immune system of complement by binding C3.

The IgD-binding domain of the Moraxella IgD-binding protein MID (MID962-1200) activates human B cells in the presence of T cell cytokines

Moraxella catarrhalis immunoglobulin D (IgD)-binding protein (MID) is an outer membrane protein with specific affinity for soluble and cell-bound human IgD. Here, we demonstrate that mutated M. catarrhalis strains devoid of MID show a 75% decreased activation of human B cells as compared with wild-type bacteria. In contrast to MID-expressing Moraxella, the MID-deficient Moraxella mutants did not bind to human CD19+ IgD+ B cells. The smallest MID fragment with preserved IgD-binding capacity comprises 238 amino acids (MID(962-1200)). To prove the specificity of MID(962-1200) for IgD, a Chinese hamster ovary (CHO) cell line expressing membrane-anchored human IgD was manufactured. MID(962-1200) bound strongly to the recombinant IgD on CHO cells. Moreover, MID(962-1200) stimulated peripheral blood lymphocyte (PBL) proliferation 5- and 15-fold at 0.1 and 1.0 microg/ml, respectively. This activation could be blocked completely by antibodies directed against the CD40 ligand (CD154). MID(962-1200) also activated purified B cells in the presence of interleukin (IL)-2 or IL-4. An increased IL-6 production was seen after stimulation with MID(962-1200), as revealed by a human cytokine protein array. MID(962-1200) fused to green fluorescent protein (GFP) bound to human B cells and activated PBL to the same degree as MID(962-1200). Taken together, MID is the only IgD-binding protein in Moraxella. Furthermore, the novel T cell-independent antigen MID(962-1200) may, together with MID(962-1200)-GFP, be considered as promising reagents in the study of IgD-dependent B cell activation.

Development of a shuttle vector for Moraxella catarrhalis

Efforts to perform genetic analysis in Moraxella catarrhalis have been hampered by the lack of a cloning vector. M. catarrhalis strain E22 was previously shown to contain plasmid pLQ510 which lacked a selectable antibiotic resistance marker. Several methods were used to eliminate unnecessary DNA from pLQ510. Then, a 1.2 kb spectinomycin resistance cartridge, a multiple cloning site, and the origin of replication from pACYC184 were cloned into this plasmid backbone to obtain the 7.2 kb plasmid pWW102B. This new plasmid could replicate in M. catarrhalis as well as in both Escherichia coli and Haemophilus influenzae. This shuttle vector was used to clone and express two different M. catarrhalis genes, respectively, encoding an adhesin and a protein involved in serum resistance. When these two plasmids were introduced into appropriate M. catarrhalis mutants, they complemented the phenotypic deficiency of each mutant. This is the first report of functional complementation in trans in this pathogen.

Hag directly mediates the adherence of Moraxella catarrhalis to human middle ear cells

Moraxella catarrhalis is a human pathogen that causes otitis media in young children and lung infections in patients with chronic obstructive pulmonary disease. In this study, the role of the surface protein Hag in the adherence of multiple M. catarrhalis strains was examined. The hag genes of four clinical isolates were disrupted with a spectinomycin resistance cassette, and the binding of isogenic mutants to primary cultures of human middle ear epithelial cells (HMEE), as well as A549 pneumocytes, was measured. These experiments revealed that the attachment of most mutants to both cell types was 10-fold less than that of their wild-type progenitors. To determine whether Hag directly mediates adherence to human cells, the hag genes from three M. catarrhalis isolates were cloned and expressed in a nonadherent Escherichia coli cloning strain. At least 17-fold more E. coli bacteria expressing Hag attached to HMEE cells than an adherence-negative control. Surprisingly, Hag expression did not increase the binding of recombinant E. coli to A549 monolayers. Our data demonstrate that the involvement of Hag in M. catarrhalis adherence to A549 and HMEE cells is conserved among isolates and that Hag directly mediates binding to HMEE cells.

Adhesion of Moraxella catarrhalis to human bronchial epithelium characterized by a novel fluorescence-based assay

Moraxella catarrhalis is a major cause of infectious exacerbations of chronic obstructive lung disease. Adhesion of this pathogen to epithelial cells is critical for its pathogenicity. Although much work has been done on identifying surface molecules of M. catarrhalis as adhesins, several adhesion assays were used in these studies which has never been validated or compared to each other. In the present study, we have examined the capacity of M. catarrhalis to adhere to different human epithelial cells. By using the two most commonly used adhesion assays based on the enumeration of colony-forming units or on the counting of adherent bacteria per epithelial cell by light microscopy, we identified significant limitations of both methods. These arose either from differences in strain-specific adhesion pattern on the epithelial cell surface or the dependence on the state of confluence of the epithelial cell layer. We developed a new fluorescence-based adhesion assay and compared our results to the two conventional methods. We demonstrated that the fluorescence-based adhesion assay offers a reliable and convenient method for the quantification of M. catarrhalis adhesion to confluent epithelial cell monolayers.

The UspA2 protein of Moraxella catarrhalis is directly involved in the expression of serum resistance

Many strains of Moraxella catarrhalis are resistant to the bactericidal activity of normal human serum. Previous studies have shown that mutations involving the insertion of an antibiotic resistance cartridge into the M. catarrhalis uspA2 gene resulted in the conversion of a serum-resistant strain to a serum-sensitive phenotype. In the present study, the deletion of the entire uspA2 gene from the serum-resistant M. catarrhalis strain O35E resulted in a serum-sensitive phenotype and did not affect either the rate of growth or the lipooligosaccharide expression profile of this mutant. Inactivation of the classical complement pathway in normal human serum with Mg2+ and EGTA resulted in the survival of this uspA2 mutant. In contrast, blocking of the alternative complement pathway did not protect this uspA2 mutant from complement-mediated killing. To determine whether the UspA2 protein is directly involved in serum resistance, transformation and allelic exchange were used to replace the uspA2 gene in the serum-resistant strain O35E with the uspA2 gene from the serum-sensitive M. catarrhalis strain MC317. The resultant O35E transformant exhibited a serum-sensitive phenotype. Similarly, when the uspA2 gene from the serum-resistant strain O35E was used to replace the uspA2 gene in the serum-sensitive strain MC317, the MC317 transformant acquired serum resistance. The use of hybrid O35E-MC317 uspA2 genes showed that the N-terminal half of the O35E protein contained a 102-amino-acid region that was involved in the expression of serum resistance. In addition, when the uspA2 genes from strains O35E and MC317 were cloned and expressed in Haemophilus influenzae DB117, only the O35E UspA2 protein caused a significant increase in the serum resistance of the H. influenzae recombinant strain. These results prove that the UspA2 protein is directly involved in the expression of serum resistance by certain M. catarrhalis strains.

Expression of type IV pili by Moraxella catarrhalis is essential for natural competence and is affected by iron limitation

Type IV pili, filamentous surface appendages primarily composed of a single protein subunit termed pilin, play a crucial role in the initiation of disease by a wide range of pathogenic bacteria. Although previous electron microscopic studies suggested that pili might be present on the surface of Moraxella catarrhalis isolates, detailed molecular and phenotypic analyses of these structures have not been reported to date. We identified and cloned the M. catarrhalis genes encoding PilA, the major pilin subunit, PilQ, the outer membrane secretin through which the pilus filament is extruded, and PilT, the NTPase that mediates pilin disassembly and retraction. To initiate investigation of the role of this surface organelle in pathogenesis, isogenic pilA, pilT, and pilQ mutants were constructed in M. catarrhalis strain 7169. Comparative analyses of the wild-type 7169 strain and three isogenic pil mutants demonstrated that M. catarrhalis expresses type IV pili that are essential for natural genetic transformation. Our studies suggest type IV pilus production by M. catarrhalis is constitutive and ubiquitous, although pilin expression was demonstrated to be iron responsive and Fur regulated. These data indicate that additional studies aimed at elucidating the prevalence and role of type IV pili in the pathogenesis and host response to M. catarrhalis infections are warranted.

Type V protein secretion pathway: the autotransporter story

Gram-negative bacteria possess an outer membrane layer which constrains uptake and secretion of solutes and polypeptides. To overcome this barrier, bacteria have developed several systems for protein secretion. The type V secretion pathway encompasses the autotransporter proteins, the two-partner secretion system, and the recently described type Vc or AT-2 family of proteins. Since its discovery in the late 1980s, this family of secreted proteins has expanded continuously, due largely to the advent of the genomic age, to become the largest group of secreted proteins in gram-negative bacteria. Several of these proteins play essential roles in the pathogenesis of bacterial infections and have been characterized in detail, demonstrating a diverse array of function including the ability to condense host cell actin and to modulate apoptosis. However, most of the autotransporter proteins remain to be characterized. In light of new discoveries and controversies in this research field, this review considers the autotransporter secretion process in the context of the more general field of bacterial protein translocation and exoprotein function.

The Emerging PathogenMoraxella catarrhalisInteracts with Complement Inhibitor C4b Binding Protein through Ubiquitous Surface Proteins A1 and A2

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 UspA1(50-770) and UspA2(30-539) showed that C4BP (range, 1-1000 nM) bound to the two proteins in a dose-dependent manner. The equilibrium constants (K(D)) for the UspA1(50-770) and UspA2(30-539) interactions with a single subunit of C4BP were 13 microM and 1.1 microM, respectively. The main isoform of C4BP contains seven identical alpha-chains and one beta-chain linked together with disulfide bridges, and the alpha-chains contain eight complement control protein (CCP) modules. The UspA1 and A2 bound to the alpha-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.

The Moraxella catarrhalis porin-like outer membrane protein CD is an adhesin for human lung cells

The outer membrane protein CD (OMPCD) of Moraxella catarrhalis is an outer membrane protein with several attributes of a potential vaccine antigen. We isolated four transposon mutants of strain O35E on the basis of their reduced binding to A549 human lung cells in microcolony formation assays, and we determined that they contain a transposon in ompCD. We also found that these transposon insertions had pleiotropic effects: mutants grew slower, became serum sensitive, bound approximately 10-fold less to A549 cells, and appeared transparent when grown on solid medium. We confirmed that these various phenotypes could be attributed solely to disruption of ompCD by constructing the isogenic strain O35E.CD1. O35E-ompCD was cloned, and recombinant Escherichia coli bacteria expressing the gene product exhibited a 10-fold increase in adherence to A549 cells. This is the first report of M. catarrhalis ompCD mutants, and our findings demonstrate that this gene product is an adhesin for human lung cells.

Salivary antibodies directed against outer membrane proteins of Moraxella catarrhalis in healthy adults

Moraxella catarrhalis is a major mucosal pathogen of the human respiratory tract, but the mucosal immune response directed against surface components of this organism has not been characterized in detail. The aim of this study was to investigate the salivary immunoglobulin A (IgA) response toward outer membrane proteins (OMP) of M. catarrhalis in healthy adults, the group of individuals least likely to be colonized and thus most likely to display mucosal immunity. Unstimulated saliva samples collected from 14 healthy adult volunteers were subjected to IgA immunoblot analysis with OMP preparations of M. catarrhalis strain O35E. Immunoblot analysis revealed a consistent pattern of IgA reactivity, with the appearance of five major bands located at >250, 200, 120, 80, and 60 kDa. Eleven (79%) of 14 saliva samples elicited reactivity to all five bands. Immunoblot analysis with a set of isogenic knockout mutants lacking the expression of individual OMP was used to determine the identities of OMP giving rise to IgA bands. Human saliva was shown consistently to exhibit IgA-binding activity for oligomeric UspA2 (>250 kDa), hemagglutinin (200 kDa), monomeric UspA1 (120 kDa), transferrin-binding protein B (TbpB), monomeric UspA2, CopB, and presumably OMP CD. TbpB, oligomeric UspA2, and CopB formed a cluster of bands at about 80 kDa. These data indicate that the human salivary IgA response is directed consistently against a small number of major OMP, some of which are presently considered vaccine candidates. The functional properties of these mucosal antibodies remain to be elucidated.

The Hag protein of Moraxella catarrhalis strain O35E is associated with adherence to human lung and middle ear cells

Previous studies have demonstrated that the Moraxella catarrhalis surface antigen UspA1 is an adhesin for Chang human conjunctival cells. The present report demonstrates that lack of UspA1 expression does not affect the adherence of strain O35E to A549 human lung cells or primary cultures of human middle ear epithelial (HMEE) cells. These results imply that another molecule mediates the adherence of M. catarrhalis to these two cell lines. To identify this adhesin, strain O35E was mutagenized with a transposon and 1,000 mutants were screened in a microcolony formation assay using A549 cells. Nine independent isolates exhibited an 8- to 19-fold reduction in adherence and contained a transposon in the same locus. Nucleotide sequence data and PCR analysis indicated that the transposons were inserted in different locations in the gene encoding the surface protein Hag. Quantitative assays using one representative transposon mutant, O35E.TN2, showed considerably decreased binding to A549 as well as HMEE cells. However, this mutant adhered at wild-type levels to Chang conjunctival cells. These findings suggest that the M. catarrhalis Hag protein is an adhesin for cell lines derived from human lung and middle ear tissues.

Identification of a Moraxella catarrhalis outer membrane protein exhibiting both adhesin and lipolytic activities

The UspA1 and Hag proteins have previously been shown to be involved in the ability of the Moraxella catarrhalis wild-type strain O35E to bind to human Chang and A549 cells, respectively. In an effort to identify novel adhesins, we generated a plasmid library of M. catarrhalis DNA fragments, which was introduced into a nonadherent Escherichia coli strain. Recombinant E. coli bacteria were subsequently enriched for clones that gained the ability to bind to Chang and A549 cells, yielding the plasmid pELFOS190. Transposon mutagenesis of this plasmid identified the potential adhesin gene mcaP (M. catarrhalis adherence protein). Sequence analysis revealed that McaP is related to autotransporter proteins and has substantial similarity with the GDSL family of lipolytic enzymes, particularly the Moraxella bovis phospholipase B. Expression of the mcaP gene product by E. coli increased adherence to Chang, A549, and 16HBE14o(-) polarized human bronchial cells 50- to 100-fold. Spectrophotometric assays with p-nitrophenol derivatives also demonstrated that McaP is an esterase. Furthermore, thin-layer chromatography revealed that McaP cleaves both phosphatidylcholine and lysophosphatidylcholine. McaP releases fatty acids and glycerophosphorylcholine upon cleavage of phosphatidylcholine, thus exhibiting phospholipase B activity. The construction and characterization of isogenic M. catarrhalis O35E mutants demonstrated that the lack of McaP expression abolishes esterase activity and considerably decreases adherence to several human cell lines.

The immunoglobulin D-binding protein MID from Moraxella catarrhalis is also an adhesin

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(764-913). In addition, antibodies against full-length MID, MID(764-913), or a 30-amino-acid consensus sequence (MID(775-804)) inhibited adhesion to alveolar epithelial cells. Antibodies against UspA1, an outer membrane protein expressed in essentially all M. catarrhalis strains, also inhibited adhesion, suggesting that both MID and UspA1 are needed for optimal attachment to epithelial cells. Taken together, in addition to MID-dependent IgD binding, we have demonstrated that the outer membrane protein MID is a novel adhesin that would be a suitable target for a future vaccine against M. catarrhalis.

The Moraxella catarrhalis immunoglobulin D-binding protein MID has conserved sequences and is regulated by a mechanism corresponding to phase variation

The prevalence of the Moraxella catarrhalis immunoglobulin D (IgD)-binding outer membrane protein MID and its gene was determined in 91 clinical isolates and in 7 culture collection strains. Eighty-four percent of the clinical Moraxella strains expressed MID-dependent IgD binding. The mid gene was detected in all strains as revealed by homology of the signal peptide sequence and a conserved area in the 3' end of the gene. When MID proteins from five different strains were compared, an identity of 65.3 to 85.0% and a similarity of 71.2 to 89.1% were detected. Gene analyses showed several amino acid repeat motifs in the open reading frames, and MID could be called a putative autotransport protein. Interestingly, homopolymeric [polyguanine [poly(G)]] tracts were detected at the 5' ends within the open reading frames. By flow cytometry, using human IgD and fluorescein isothiocyanate-conjugated anti-IgD polyclonal antibodies, most strains showed two peaks: one high- and one low-intensity peak. All isolates expressing high levels of MID had 1, 2, or 3 triplets of G's in their poly(G) tracts, while strains not expressing MID had 4, 7, 8, or 10 G's in their poly(G) tracts or point mutations causing a putative preterminated translation. Northern blot analysis revealed that the mid gene was regulated at the transcriptional level. Experiments with nonclumping variants of M. catarrhalis proved that bacteria lost their MID expression by removing a G in their poly(G) tracts. Moraxella strains isolated from the nasopharynx or from blood and sputum specimens expressed MID at approximately the same frequency. In addition, no variation was observed between strains of different geographical origins (Australia, Europe, Japan, or the United States). MID and the mid gene were found solely in M. catarrhalis, whereas related Neisseria and Moraxella species did not express MID. Taken together, MID appears to be a conserved protein that can be found in essentially all M. catarrhalis strains. Furthermore, MID is governed by poly(G) tracts when bacteria undergo phase variation.

Mucosal immune response to specific outer membrane proteins of Moraxella catarrhalis in young children

BACKGROUND

Moraxella catarrhalis is an important cause of otitis media. A number of candidate antigens for a future infant otitis media vaccine have been identified, but their mucosal immunogenicity induced by nasopharyngeal M. catarrhalis colonization has not been characterized. The aim of this study was to determine the salivary IgA response to M. catarrhalis outer membrane proteins (OMP) in young children.

METHODS

Children ages 1 to 24 months evaluated for acute respiratory tract infection were prospectively enrolled. M. catarrhalis nasopharyngeal colonization was determined by (1) selective culture and (2) detection by reverse transcription-PCR of messenger RNA specific for the OMP UspA1 and UspA2. Salivary IgA responses were detected by immunoblot analysis of M. catarrhalis OMP. Isogenic knockout mutants for UspA1, UspA2, hemagglutinin (Hag), transferrin-binding protein B (TbpB) and CopB were constructed for identification of specific target OMP.

RESULTS

Sixty-six patients were studied. The rates of M. catarrhalis colonization by culture, reverse transcription-PCR for messenger RNA and mRNA were 40, 94 and 58%, respectively. Anti-M. catarrhalis salivary IgA was detected in 62 patients (94%). IgA directed against a >250-kDa antigen (assigned to UspA1/UspA2 by mutant analysis) and a 200-kDa antigen (Hag) were detected in 65 and 70% of patients, respectively. Bands at 80 to 85 kDa (82%) consisted of IgA directed against monomeric UspA2, TbpB and CopB.

CONCLUSIONS

colonization occurring in early infancy is associated with a consistent mucosal immune response directed against the UspA proteins, Hag and other OMP. The data suggest that several M. catarrhalis OMP are immunogens of the nasopharyngeal mucosal immune system of infants.