Mining the Moraxella catarrhalis genome: identification of potential vaccine antigens expressed during human infection

Otitis media: recent advances in otitis media vaccine development and model systems

Otitis media is an inflammatory disorder of the middle ear caused by airways-associated bacterial or viral infections. It is one of the most common childhood infections as globally more than 80% of children are diagnosed with acute otitis media by 3 years of age and it is a common reason for doctor's visits, antibiotics prescriptions, and surgery among children. Otitis media is a multifactorial disease with various genetic, immunologic, infectious, and environmental factors predisposing children to develop ear infections. Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis are the most common culprits responsible for acute otitis media. Despite the massive global disease burden, the pathogenesis of otitis media is still unclear and requires extensive future research. Antibiotics are the preferred treatment to cure middle ear infections, however, the antimicrobial resistance rate of common middle ear pathogens has increased considerably over the years. At present, pneumococcal and influenza vaccines are administered as a preventive measure against otitis media, nevertheless, these vaccines are only beneficial in preventing carriage and/or disease caused by vaccine serotypes. Otitis media caused by non-vaccine serotype pneumococci, non-typeable H. influenza, and M. catarrhalis remain an important healthcare burden. The development of multi-species vaccines is an arduous process but is required to reduce the global burden of this disease. Many novel vaccines against S. pneumoniae, non-typeable H. influenza, and M. catarrhalis are in preclinical trials. It is anticipated that these vaccines will lower the disease burden and provide better protection against otitis media. To study disease pathology the rat, mouse, and chinchilla are commonly used to induce experimental acute otitis media to test new therapeutics, including antibiotics and vaccines. Each of these models has its advantages and disadvantages, yet there is still a need to develop an improved animal model providing a better correlated mechanistic understanding of human middle ear infections, thereby underpinning the development of more effective otitis media therapeutics. This review provides an updated summary of current vaccines against otitis media, various animal models of otitis media, their limitations, and some future insights in this field providing a springboard in the development of new animal models and novel vaccines for otitis media.

The characterization of Moraxella catarrhalis carried in the general population

Moraxella catarrhalis is a common cause of respiratory tract infection, particularly otitis media in children, whilst it is also associated with the onset of exacerbation in chronic obstructive pulmonary disease in adults. Despite the need for an efficacious vaccine against M. catarrhalis, no candidates have progressed to clinical trial. This study, therefore, aimed to characterize the diversity of M. catarrhalis isolated from the upper respiratory tract of healthy children and adults, to gain a better understanding of the epidemiology of M. catarrhalis and the distribution of genes associated with virulence factors, to aid vaccine efforts. Isolates were sequenced and the presence of target genes reported. Contrary to prevailing data, this study found that lipooligosaccharide (LOS) B serotypes are not exclusively associated with 16S type 1. In addition, a particularly low prevalence of LOS B and high prevalence of LOS C serotypes was observed. M. catarrhalis isolates showed low prevalence of antimicrobial resistance and a high gene prevalence for a number of the target genes investigated: ompB2 (also known as copB), ompCD, ompE, ompG1a, ompG1b, mid (also known as hag), mcaP, m35, tbpA, lbpA, tbpB, lbpB, msp22, msp75 and msp78, afeA, pilA, pilQ, pilT, mod, oppA, sbp2, mcmA and mclS.

Immunogenicity of trimeric autotransporter adhesins and their potential as vaccine targets

The current problem of increasing antibiotic resistance and the resurgence of numerous infections indicate the need for novel vaccination strategies more than ever. In vaccine development, the search for and the selection of adequate vaccine antigens is the first important step. In recent years, bacterial outer membrane proteins have become of major interest, as they are the main proteins interacting with the extracellular environment. Trimeric autotransporter adhesins (TAAs) are important virulence factors in many Gram-negative bacteria, are localised on the bacterial surface, and mediate the first adherence to host cells in the course of infection. One example is the Neisseria adhesin A (NadA), which is currently used as a subunit in a licensed vaccine against Neisseria meningitidis. Other TAAs that seem promising vaccine candidates are the Acinetobacter trimeric autotransporter (Ata), the Haemophilus influenzae adhesin (Hia), and TAAs of the genus Bartonella. Here, we review the suitability of various TAAs as vaccine candidates.

Synchrony in serum antibody response to conserved proteins of Moraxella catarrhalis in young children

Conserved Moraxella catarrhalis (Mcat) proteins, oligopeptide permease (Opp)A, hemagglutinin (Hag), outer membrane protein (OMP) CD, Pilin A clade 2 (PilA2), and Moraxella surface protein (Msp) 22 have been studied as vaccine candidates. Children who experience frequent acute otitis media (AOM) confirmed with pathogen identification by tympanocentesis are referred to as stringently-defined otitis prone (sOP). Synchrony of serum antibody responses against 5 Mcat proteins, OppA, Hag, OMP CD, PilA2, and Msp22 resulting from nasopharyngeal colonization and AOM was studied for 85 non-otitis prone (NOP) children and 34 sOP children. Changes in serum IgG were quantitated with ELISA. Serum IgG antibody levels against OppA, Hag, OMP CD, and Msp22 rose in synchrony in NOP and sOP children; that is, the proteins appeared equally and highly immunogenic in children at age 6 to 22-25 months old and then leveled off in their rise at 22-25 to 30 months old. In contrast, rises of PilA2 were slow from 6 months old and kept constant and did not level off significantly before 30 months old. OppA, Hag, OMP CD, and Msp22 elicited a synchronous acquisition of naturally-induced serum antibody in young children. A multi-valent Mcat protein vaccine combining OppA, Hag, OMP CD, and Msp22 may exhibit less antigen competition when administered as a combination vaccine in young children.

A Moraxella catarrhalis vaccine to protect against otitis media and exacerbations of COPD: An update on current progress and challenges

Moraxella catarrhalis is a major cause of morbidity and mortality worldwide, especially causing otitis media in young children and exacerbations of chronic obstructive pulmonary disease in adults. This pathogen uses several virulence mechanisms to colonize and survive in its host, including adherence and invasion of host cells, formation of polymicrobial biofilms with other bacterial pathogens, and production of β-lactamase. Given the global impact of otitis media and COPD, an effective vaccine to prevent M. catarrhalis infection would have a huge impact on the quality of life in both patient populations by preventing disease, thus reducing morbidity and health care costs. A number of promising vaccine antigens have been identified for M. catarrhalis. The development of improved animal models of M. catarrhalis disease and identification of a correlate of protection are needed to accelerate vaccine development. This review will discuss the current state of M. catarrhalis vaccine development, and the challenges that must be addressed to succeed.

Virulence determinants of Moraxella catarrhalis: distribution and considerations for vaccine development

Moraxella catarrhalis is a human-restricted opportunistic bacterial pathogen of the respiratory mucosa. It frequently colonizes the nasopharynx asymptomatically, but is also an important causative agent of otitis media (OM) in children, and plays a significant role in acute exacerbations of chronic obstructive pulmonary disease (COPD) in adults. As the current treatment options for M. catarrhalis infection in OM and exacerbations of COPD are often ineffective, the development of an efficacious vaccine is warranted. However, no vaccine candidates for M. catarrhalis have progressed to clinical trials, and information regarding the distribution of M. catarrhalis virulence factors and vaccine candidates is inconsistent in the literature. It is largely unknown if virulence is associated with particular strains or subpopulations of M. catarrhalis, or if differences in clinical manifestation can be attributed to the heterogeneous expression of specific M. catarrhalis virulence factors in the circulating population. Further investigation of the distribution of M. catarrhalis virulence factors in the context of carriage and disease is required so that vaccine development may be targeted at relevant antigens that are conserved among disease-causing strains. The challenge of determining which of the proposed M. catarrhalis virulence factors are relevant to human disease is amplified by the lack of a standardized M. catarrhalis typing system to facilitate direct comparisons of worldwide isolates. Here we summarize and evaluate proposed relationships between M. catarrhalis subpopulations and specific virulence factors in the context of colonization and disease, as well as the current methods used to infer these associations.

A Cation-Binding Surface Protein as a Vaccine Antigen To Prevent Moraxella catarrhalis Otitis Media and Infections in Chronic Obstructive Pulmonary Disease

Moraxella catarrhalis is an exclusively human respiratory tract pathogen that is a common cause of otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease. A vaccine to prevent these infections would have a major impact on reducing the substantial global morbidity and mortality in these populations. Through a genome mining approach, we identified AfeA, an ∼32-kDa substrate binding protein of an ABC transport system, as an excellent candidate vaccine antigen. Recombinant AfeA was expressed and purified and binds ferric, ferrous, manganese, and zinc ions, as demonstrated by thermal shift assays. It is a highly conserved protein that is present in all strains of M. catarrhalis Immunization with recombinant purified AfeA induces high-titer antibodies that recognize the native M. catarrhalis protein. AfeA expresses abundant epitopes on the bacterial surface and induces protective responses in the mouse pulmonary clearance model following aerosol challenge with M. catarrhalis Finally, AfeA is expressed during human respiratory tract infection of adults with chronic obstructive pulmonary disease (COPD). Based on these observations, AfeA is an excellent vaccine antigen to be included in a vaccine to prevent infections caused by M. catarrhalis.

Serum antibody response to Moraxella catarrhalis proteins in stringently defined otitis prone children

BACKGROUND

Moraxella catarrhalis (Mcat) is a frequent pathogen of acute otitis media (AOM) in young children. Here we prospectively assessed naturally-induced serum antibodies to four Mcat vaccine candidate proteins in stringently defined otitis prone (sOP) and non-otitis prone (NOP) children age 6-36months old following nasopharyngeal (NP) colonization, at onset of AOM and convalescence from AOM.

METHODS

Serum IgG and IgM antibody against recombinant Mcat proteins, oligopeptide permease A (OppA), outer membrane protein (OMP) CD, hemagglutinin (Hag), and PilA clade 2 (PilA2), were quantitated by ELISA.

RESULTS

During NP colonization by Mcat all four antigens were immunogenic in both sOP and NOP children. However, sOP children had lower antibody responses than NOP children across age 6-36months, similar to our findings for protein vaccine candidates of Streptococcus pneumoniae (Spn) and Nontypeable Haemophilus influenzae (NTHi). sOP children displayed a later and lower peak of antibody rise than NOP children for all four antigens during NP colonization of Mcat. The age-dependent increase of antibody ranked as OppA>Hag5-9>OMP CD>PilA2 in both sOP and NOP children. Lower serum antibody levels to the Mcat antigens were measured in sOP compared to NOP children at the onset of AOM. We did not find a consistent significant increase of antibody at the convalescence phase after an AOM event.

CONCLUSIONS

sOP children is a highly vulnerable population that mount lower serum antibody responses to Mcat candidate vaccine proteins compared to NOP children during asymptomatic NP carriage and at onset of AOM.

Potential impact of a Moraxella catarrhalis vaccine in COPD

Moraxella catarrhalis is the second most common cause of exacerbations in adults with COPD, resulting in enormous morbidity and mortality in this clinical setting. Vaccine development for M. catarrhalis has lagged behind the other two important causes of exacerbations in COPD, nontypeable Haemophilus influenzae and Streptococcus pneumoniae. While no licensed vaccine is currently available for M. catarrhalis, several promising candidate vaccine antigens have been identified and characterized and are close to entering clinical trials. Key steps that are required to advance vaccines for M. catarrhalis along the translational pipeline include standardization of assay systems to assess candidate antigens, identification of a reliable correlate of protection and expansion of partnerships between industry, academia and government to overcome regulatory hurdles. A vaccine to prevent M. catarrhalis infections in COPD would have a major impact in reducing morbidity, mortality and healthcare costs in COPD.

Sulfate-binding protein, CysP, is a candidate vaccine antigen of Moraxella catarrhalis

Moraxella catarrhalis causes otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease (COPD). A vaccine to prevent M. catarrhalis infections would have an enormous impact globally in preventing morbidity caused by M. catarrhalis in these populations. Using a genome mining approach we have identified a sulfate binding protein, CysP, of an ATP binding cassette (ABC) transporter system as a novel candidate vaccine antigen. CysP expresses epitopes on the bacterial surface and is highly conserved among strains. Immunization with CysP induces potentially protective immune responses in a murine pulmonary clearance model. In view of these features that indicate CysP is a promising vaccine antigen, we conducted further studies to elucidate its function. These studies demonstrated that CysP binds sulfate and thiosulfate ions, plays a nutritional role for the organism and functions in intracellular survival of M. catarrhalis in human respiratory epithelial cells. The observations that CysP has features of a vaccine antigen and also plays an important role in growth and survival of the organism indicate that CysP is an excellent candidate vaccine antigen to prevent M. catarrhalis otitis media and infections in adults with COPD.

ATP-Binding Cassette (ABC) Transporters of the Human Respiratory Tract Pathogen, Moraxella catarrhalis: Role in Virulence

Moraxella catarrhalis is a human respiratory tract pathogen that causes otitis media (middle ear infections) in children and respiratory tract infections in adults with chronic obstructive pulmonary disease. In view of the huge global burden of disease caused by M. catarrhalis, the development of vaccines to prevent these infections and better approaches to treatment have become priorities. In previous work, we used a genome mining approach that identified three substrate binding proteins (SBPs) of ATP-binding cassette (ABC) transporters as promising candidate vaccine antigens. In the present study, we performed a comprehensive assessment of 19 SBPs of 15 ABC transporter systems in the M. catarrhalis genome by engineering knockout mutants and studying their role in assays that assess mechanisms of infection. The capacity of M. catarrhalis to survive and grow in the nutrient-limited and hostile environment of the human respiratory tract, including intracellular growth, account in part for its virulence. The results show that ABC transporters that mediate uptake of peptides, amino acids, cations and anions play important roles in pathogenesis by enabling M. catarrhalis to 1) grow in nutrient-limited conditions, 2) invade and survive in human respiratory epithelial cells and 3) persist in the lungs in a murine pulmonary clearance model. The knockout mutants of SBPs and ABC transporters showed different patterns of activity in the assay systems, supporting the conclusion that different SBPs and ABC transporters function at different stages in the pathogenesis of infection. These results indicate that ABC transporters are nutritional virulence factors, functioning to enable the survival of M catarrhalis in the diverse microenvironments of the respiratory tract. Based on the role of ABC transporters as virulence factors of M. catarrhalis, these molecules represent potential drug targets to eradicate the organism from the human respiratory tract.

Immunoinformatics Identifies a Lactoferrin Binding Protein A Peptide as a Promising Vaccine With a Global Protective Prospective Against Moraxella catarrhalis

BACKGROUND

Moraxella catarrhalis is an established pathogen that is causing substantial infections to both children and adults. However, so far there is no effective vaccine to halt the spread of these infections.

METHODS

Immunoinformatics tools were used to predict M. catarrhalis epitopes that could offer immunoprotection among major proportions of human populations worldwide. Mice were immunized with the best 3 peptides and then challenged with M. catarrhalis in the pulmonary clearance model. Finally, antibodies against these epitopes were detected in humans.

RESULTS

Immunoinformatics analyses identified 44 epitopes that are predicted to be good major histocompatibility complex class II binders and at the same time show high population coverage worldwide. After intraperitoneal immunization of mice with the best 3 peptides, peptide A, derived from lactoferrin-binding protein A, showed superior activity in immunogenicity and in clearing M. catarrhalis from mouse lungs. Higher clearance was obtained by combining intraperitoneal and intranasal immunization. In the serum samples from children with otitis media infected with M. catarrhalis, antibody levels against peptide A were significantly lower than in samples from children without otitis media.

CONCLUSIONS

Peptide A is the first promising peptide-based vaccine against M. catarrhalis Immunoinformatics predicts that it should have a global protection around the world.

The Vaccine Candidate Substrate Binding Protein SBP2 Plays a Key Role in Arginine Uptake, Which Is Required for Growth of Moraxella catarrhalis

Moraxella catarrhalis is an exclusively human pathogen that is an important cause of otitis media in children and lower respiratory tract infections in adults with chronic obstructive pulmonary disease. A vaccine to prevent M. catarrhalis infections would have an enormous global impact in reducing morbidity resulting from these infections. Substrate binding protein 2 (SBP2) of an ABC transporter system has recently been identified as a promising vaccine candidate antigen on the bacterial surface of M. catarrhalis. In this study, we showed that SBP1, -2, and -3 individually bind different basic amino acids with exquisite specificity. We engineered mutants that each expressed a single SBP from this gene cluster and showed in growth experiments that SBP1, -2, and -3 serve a nutritional function through acquisition of amino acids for the bacterium. SBP2 mediates uptake of arginine, a strict growth requirement of M. catarrhalis. Adherence and invasion assays demonstrated that SBP1 and SBP3 play a role in invasion of human respiratory epithelial cells, consistent with a nutritional role in intracellular survival in the human respiratory tract. This work demonstrates that the SBPs of an ABC transporter system function in the uptake of basic amino acids to support growth of M. catarrhalis. The critical role of SBP2 in arginine uptake may contribute to its potential as a vaccine antigen.

Moraxella catarrhalis-produced nitric oxide has dual roles in pathogenicity and clearance of infection in bacterial-host cell co-cultures

In humans, the free radical nitric oxide (NO) is a concentration-dependent multifunctional signaling or toxic molecule that modulates various physiological and pathological processes, and innate immunity against bacterial infections. Because the expression of bacterial genes encoding nitrite reductase (AniA) and NO reductase (NorB) is highly upregulated in biofilms in vitro, it is important to investigate whether bacterial NO-metabolism might subvert host NO signaling and play pathogenic roles during infection. The Moraxella catarrhalis AniA and NorB directly function in production and reduction of NO. Using M. catarrhalis-human bronchial epithelial cell (HBEC) co-cultures, we recently reported AniA/nitrite-dependent cytotoxic effects on HBECs, including altered protein profiles of HBECs and induced HBEC apoptosis, suggesting bacterial nitrite reduction likely dysregulates host cell gene expression. To further clarify whether nitrite reduction-derived NO or nitrite-dependent stimulation of bacterial growth was responsible for adverse effects on HBECs, we monitored bacterial nitrite reduction, levels of NO in co-cultures and resulted dynamic effects on HBEC proliferation and bacterial viability. This study demonstrated that M. catarrhalis nitrite reduction-derived NO was responsible for observed adverse effects on HBECs at mid-to-late stages of infection. More importantly, our data showed that while nitrite promoted bacterial growth and biofilm formation at early hours of infection, nitrite reduction-derived NO was toxic towards M. catarrhalis in maturing biofilms, suggesting nitrite reduction-derived NO might be a possible dualistic mechanism by which M. catarrhalis promotes diseases and spontaneous resolutions.

Serum antibody response to Moraxella catarrhalis proteins OMP CD, OppA, Msp22, Hag, and PilA2 after nasopharyngeal colonization and acute otitis media in children

BACKGROUND

There is no licensed vaccine for Moraxella catarrhalis (Mcat), which is a prominent bacterium causing acute otitis media (AOM) in children and lower respiratory tract infections in adults. Nasopharyngeal (NP) colonization caused by respiratory bacteria results in natural immunization of the host. To identify Mcat antigens as vaccine candidates, we evaluated the development of naturally induced antibodies to 5 Mcat surface proteins in children 6-30 months of age during Mcat NP colonization and AOM.

METHODS

Human serum IgG against the recombinant Mcat proteins, outer membrane protein (OMP) CD, oligopeptide permease (Opp)A, hemagglutinin (Hag), Moraxella surface protein (Msp)22, and PilA clade 2 (PilA2) was quantitated by using an ELISA assay.

RESULTS

There were 223 Mcat NP colonization episodes documented in 111 (60%) of 184 children in the study. Thirty five Mcat AOM episodes occurred in 30 (16%) of 184 children. All 5 Mcat candidate vaccine antigens evaluated stimulated a significant rise in serum IgG levles over time from 6 to 36 months of age (P<0.001), with a rank order as follows: Msp22=OppA>OMP CD=Hag=PilA2. Children with no detectable Mcat NP colonization showed a higher serum IgG level against OppA, Hag, and Msp22 compared to those with Mcat NP colonization (P<0.05). Individual data showed that some children responded to AOM with an antibody increase to one or more of the studied Mcat proteins but some children failed to respond.

CONCLUSIONS

Serum antibody to Mcat candidate vaccine proteins OMP CD, OppA, Msp22, Hag, and PilA2 increased with age in naturally immunized children age 6-30 months following Mcat NP colonization and AOM. High antibody levels against OppA, Msp22, and Hag correlated with reduced carriage. The results support further investigation of these vaccine candidates in protecting against Mcat colonization and infection.

Vaccine targets against Moraxella catarrhalis

INTRODUCTION

Moraxella catarrhalis is a prominent pathogen that causes acute otitis media in children and lower respiratory tract infections in adults, resulting in a significant socioeconomic burden on healthcare systems globally. No vaccine is currently available for M. catarrhalis. Promising M. catarrhalis target antigens have been characterized in animal models and should soon enter human clinical trials.

AREAS COVERED

This review discusses the detailed features and research status of current candidate target antigens for an M. catarrhalis vaccine. The approaches for assessing M. catarrhalis vaccine efficacy are also discussed.

EXPERT OPINION

Targeting the key molecules contributing to serum resistance may be a viable strategy to identify effective vaccine targets among M. catarrhalis antigens. Elucidating the role and mechanisms of the serum and mucosal immune responses to M. catarrhalis is significant for vaccine target selection, testing and evaluation. Developing animal models closely simulating M. catarrhalis-caused human respiratory diseases is of great benefit in better understanding pathogenesis and evaluating vaccine efficacy. Carrying out clinical trials will be a landmark in the progress of M. catarrhalis vaccine research. Combined multicomponent vaccines will be a focus of future M. catarrhalis vaccine studies.

Expeditious screening of candidate proteins for microbial vaccines

Advancements in high-throughput "omics" technologies have revolutionized the way vaccine candidates are identified. Now every surface expressed protein that an organism produces can be identified in silico and possibly made available for the rapid development of recombinant/subunit vaccines. However, evaluating the antigenicity of a large number of candidate proteins is an immense challenge, typically requiring cloning of several hundred candidates followed by immunogenicity screening. Here we report the development of a rapid, high-throughput method for screening candidate proteins for vaccines. This method involves utilizing a coupled, cell-free transcription-translation system to screen tagged proteins that are captured at the C-termini using appropriate ligand coated wells in 96 well ELISA plates. The template DNA for the cell-free expression is generated by two sequential PCRs and includes gene coding sequences, promoter, terminator, other necessary cis-acting elements and appropriate tag sequences. The process generates expressible candidate proteins containing two different peptide tags at the N- and the C-termini of the protein molecules. Proteins are screened in parallel for their quantity and immunoreactivity with N-terminal tag antibodies and antisera raised against the pathogen of interest, respectively. Normalization against the total detectable bound protein in the control wells allows for the identification of highly immunoreactive candidates. For this study we selected 30 representatives of >300 potential candidate proteins from Mannheimia haemolytica, a bacterial agent of pneumonia in feedlot cattle for expression with N-terminal Strep-II and C-terminal His(x6)-tag and evaluated their relative immunoreactivities using Strep-tactin-HRP and rabbit antisera generated against M. haemolytica. Using this system we were able to swiftly and quantitatively analyze and rank the suitability of proteins to identify potentially viable vaccine candidates, with the majority of the high ranking candidates being associated with virulence and pathogenicity. The system is adaptable to any bacterial target and presents an alternative to conventional laborious cloning, expression and screening procedures.

Role of the oligopeptide permease ABC Transporter of Moraxella catarrhalis in nutrient acquisition and persistence in the respiratory tract

Moraxella catarrhalis is a strict human pathogen that causes otitis media in children and exacerbations of chronic obstructive pulmonary disease in adults, resulting in significant worldwide morbidity and mortality. M. catarrhalis has a growth requirement for arginine; thus, acquiring arginine is important for fitness and survival. M. catarrhalis has a putative oligopeptide permease ABC transport operon (opp) consisting of five genes (oppB, oppC, oppD, oppF, and oppA), encoding two permeases, two ATPases, and a substrate binding protein. Thermal shift assays showed that the purified recombinant substrate binding protein OppA binds to peptides 3 to 16 amino acid residues in length regardless of the amino acid composition. A mutant in which the oppBCDFA gene cluster is knocked out showed impaired growth in minimal medium where the only source of arginine came from a peptide 5 to 10 amino acid residues in length. Whether methylated arginine supports growth of M. catarrhalis is important in understanding fitness in the respiratory tract because methylated arginine is abundant in host tissues. No growth of wild-type M. catarrhalis was observed in minimal medium in which arginine was present only in methylated form, indicating that the bacterium requires l-arginine. An oppA knockout mutant showed marked impairment in its capacity to persist in the respiratory tract compared to the wild type in a mouse pulmonary clearance model. We conclude that the Opp system mediates both uptake of peptides and fitness in the respiratory tract.

Substrate binding protein SBP2 of a putative ABC transporter as a novel vaccine antigen of Moraxella catarrhalis

Moraxella catarrhalis is a common respiratory tract pathogen that causes otitis media in children and infections in adults with chronic obstructive pulmonary disease. Since the introduction of the pneumococcal conjugate vaccines with/without protein D of nontypeable Haemophilus influenzae, M. catarrhalis has become a high-priority pathogen in otitis media. For the development of antibacterial vaccines and therapies, substrate binding proteins of ATP-binding cassette transporters are important targets. In this study, we identified and characterized a substrate binding protein, SBP2, of M. catarrhalis. Among 30 clinical isolates tested, the sbp2 gene sequence was highly conserved. In 2 different analyses (whole-cell enzyme-linked immunosorbent assay and flow cytometry), polyclonal antibodies raised to recombinant SBP2 demonstrated that SBP2 expresses epitopes on the bacterial surface of the wild type but not the sbp2 mutant. Mice immunized with recombinant SBP2 showed significantly enhanced clearance of M. catarrhalis from the lung compared to that in the control group at both 25-μg and 50-μg doses (P < 0.001). We conclude that SBP2 is a novel, attractive candidate as a vaccine antigen against M. catarrhalis.

Phagocytic dysfunction of human alveolar macrophages and severity of chronic obstructive pulmonary disease

BACKGROUND

Alveolar macrophages in chronic obstructive pulmonary disease (COPD) have fundamental impairment of phagocytosis for nontypeable Haemophilus influenzae (NTHI). However, relative selectivity of dysfunctional phagocytosis among diverse respiratory pathogens: NTHI, Moraxella catarrhalis (MC), Streptococcus pneumoniae (SP), and nonbacterial particles, as well as the contribution of impaired phagocytosis to severity of COPD, has not been explored.

METHODS

Alveolar macrophages, obtained from nonsmokers (n = 20), COPD ex-smokers (n = 32), and COPD active smokers (n = 64), were incubated with labeled NTHI, MC, SP, and fluorescent microspheres. Phagocytosis was measured as intracellular percentages of each.

RESULTS

Alveolar macrophages of COPD ex-smokers and active smokers had impaired complement-independent phagocytosis of NTHI (P = .003) and MC (P = .0007) but not SP or microspheres. Nonetheless, complement-mediated phagocytosis was enhanced within each group only for SP. Defective phagocytosis was significantly greater for NTHI than for MC among COPD active smokers (P < .0001) and ex-smokers (P = .028). Moreover, severity of COPD (FEV1%predicted) correlated with impaired AM phagocytosis for NTHI (P = .0016) and MC (P = .01).

CONCLUSIONS

These studies delineate pathogen- and host-specific differences in defective alveolar macrophages phagocytosis of respiratory bacteria in COPD, further elucidating the immunologic basis for bacterial persistence in COPD and provide the first demonstration of association of impaired phagocytosis to severity of disease.

Role of the zinc uptake ABC transporter of Moraxella catarrhalis in persistence in the respiratory tract

Moraxella catarrhalis is a human respiratory tract pathogen that causes otitis media in children and lower respiratory tract infections in adults with chronic obstructive pulmonary disease. We have identified and characterized a zinc uptake ABC transporter that is present in all strains of M. catarrhalis tested. A mutant in which the znu gene cluster is knocked out shows markedly impaired growth compared to the wild type in medium that contains trace zinc; growth is restored to wild-type levels by supplementing medium with zinc but not with other divalent cations. Thermal-shift assays showed that the purified recombinant substrate binding protein ZnuA binds zinc but does not bind other divalent cations. Invasion assays with human respiratory epithelial cells demonstrated that the zinc ABC transporter of M. catarrhalis is critical for invasion of respiratory epithelial cells, an observation that is especially relevant because an intracellular reservoir of M. catarrhalis is present in the human respiratory tract and this reservoir is important for persistence. The znu knockout mutant showed marked impairment in its capacity to persist in the respiratory tract compared to the wild type in a mouse pulmonary clearance model. We conclude that the zinc uptake ABC transporter mediates uptake of zinc in environments with very low zinc concentrations and is critical for full virulence of M. catarrhalis in the respiratory tract in facilitating intracellular invasion of epithelial cells and persistence in the respiratory tract.

Panel 5: Microbiology and immunology panel

OBJECTIVE

The objective is to perform a comprehensive review of the literature from January 2007 through June 2011 on the virology, bacteriology, and immunology related to otitis media.

DATA SOURCES

PubMed database of the National Library of Medicine.

REVIEW METHODS

Three subpanels with co-chairs comprising experts in the virology, bacteriology, and immunology of otitis media were formed. Each of the panels reviewed the literature in their respective fields and wrote draft reviews. The reviews were shared with all panel members, and a second draft was created. The entire panel met at the 10th International Symposium on Recent Advances in Otitis Media in June 2011 and discussed the review and refined the content further. A final draft was created, circulated, and approved by the panel.

CONCLUSION

Excellent progress has been made in the past 4 years in advancing an understanding of the microbiology and immunology of otitis media. Advances include laboratory-based basic studies, cell-based assays, work in animal models, and clinical studies.

IMPLICATIONS FOR PRACTICE

The advances of the past 4 years formed the basis of a series of short-term and long-term research goals in an effort to guide the field. Accomplishing these goals will provide opportunities for the development of novel interventions, including new ways to better treat and prevent otitis media.

Panel 6: Vaccines

OBJECTIVE

To update progress on the effectiveness of vaccine for prevention of acute otitis media (AOM) and identification of promising candidate antigens against Streptococcus pneumoniae, nontypeable Haemophilus influenzae, and Moraxella catarrhalis.

REVIEW METHODS

Literature searches were performed in OvidSP and PubMed restricted to articles published between June 2007 and September 2011. Search terms included otitis media, vaccines, vaccine antigens, and each of the otitis pathogens and candidate antigens identified in the ninth conference report.

CONCLUSIONS

The current report provides further evidence for the effectiveness of pneumococcal conjugate vaccines (PCVs) in the prevention of otitis media. Observational studies demonstrate a greater decline in AOM episodes than reported in clinical efficacy trials. Unmet challenges include extending protection to additional serotypes and additional pathogens, the need to prevent early episodes, the development of correlates of protection for protein antigens, and the need to define where an otitis media vaccine strategy fits with priorities for child health.

IMPLICATIONS FOR PRACTICE

Acute otitis media continues to be a burden on children and families, especially those who suffer from frequent recurrences. The 7-valent PCV (PCV7) has reduced the burden of disease as well as shifted the pneumococcal serotypes and the distribution of otopathogens currently reported in children with AOM. Antibiotic resistance remains an ongoing challenge. Multiple candidate antigens have demonstrated the necessary requirements of conservation, surface exposure, immunogenicity, and protection in animal models. Further research on the role of each antigen in pathogenesis, in the development of correlates of protection in animal models, and in new adjuvants to elicit responses in the youngest infants is likely to be productive and permit more antigens to move into human clinical trials.

Comprehensive antigen screening identifies Moraxella catarrhalis proteins that induce protection in a mouse pulmonary clearance model

Moraxella catarrhalis is one of the three most common causative bacterial pathogens of otitis media, however no effective vaccine against M. catarrhalis has been developed so far. To identify M. catarrhalis vaccine candidate antigens, we used carefully selected sera from children with otitis media and healthy individuals to screen small-fragment genomic libraries that are expressed to display frame-selected peptides on a bacterial cell surface. This ANTIGENome technology led to the identification of 214 antigens, 23 of which were selected by in vitro or in vivo studies for additional characterization. Eight of the 23 candidates were tested in a Moraxella mouse pulmonary clearance model, and 3 of these antigens induced significantly faster bacterial clearance compared to adjuvant or to the previously characterized antigen OmpCD. The most significant protection data were obtained with the antigen MCR_1416 (Msp22), which was further investigated for its biological function by in vitro studies suggesting that Msp22 is a heme binding protein. This study comprises one of the most exhaustive studies to identify potential vaccine candidate antigens against the bacterial pathogen M. catarrhalis.

Moraxella catarrhalis: from interactions with the host immune system to vaccine development

Moraxella catarrhalis is a human-restricted commensal that over the last two decades has developed into an emerging respiratory tract pathogen. The bacterial species is equipped with various adhesins to facilitate its colonization. Successful evasion of the human immune system is a prerequisite for Moraxella infection. This strategy involves induction of an excessive proinflammatory response, intervention of granulocyte recruitment to the infection site, activation of selected pattern recognition receptors and cellular adhesion molecules to counteract the host bacteriolytic attack, as well as, finally, reprogramming of antigen presenting cells. Host immunomodulator molecules are also exploited by Moraxella to aid in resistance against complement killing and host bactericidal molecules. Thus, breaking the basis of Moraxella immune evasion mechanisms is fundamental for future invention of effective therapy in controlling Moraxella infection.

Bacterium-generated nitric oxide hijacks host tumor necrosis factor alpha signaling and modulates the host cell cycle in vitro

In mammalian cells, nitric oxide (NO·) is an important signal molecule with concentration-dependent and often controversial functions of promoting cell survival and inducing cell death. An inducible nitric oxide synthase (iNOS) in various mammalian cells produces higher levels of NO· from l-arginine upon infections to eliminate pathogens. In this study, we reveal novel pathogenic roles of NO· generated by bacteria in bacterium-host cell cocultures using Moraxella catarrhalis, a respiratory tract disease-causing bacterium, as a biological producer of NO·. We recently demonstrated that M. catarrhalis cells that express the nitrite reductase (AniA protein) can produce NO· by reducing nitrite. Our study suggests that, in the presence of pathophysiological levels of nitrite, this opportunistic pathogen hijacks host cell signaling and modulates host gene expression through its ability to produce NO· from nitrite. Bacterium-generated NO· significantly increases the secretion of tumor necrosis factor alpha (TNF-α) and modulates the expression of apoptotic proteins, therefore triggering host cell programmed death partially through TNF-α signaling. Furthermore, our study reveals that bacterium-generated NO· stalls host cell division and directly results in the death of dividing cells by reducing the levels of an essential regulator of cell division. This study provides unique insight into why NO· may exert more severe cytotoxic effects on fast growing cells, providing an important molecular basis for NO·-mediated pathogenesis in infections and possible therapeutic applications of NO·-releasing molecules in tumorigenesis. This study strongly suggests that bacterium-generated NO· can play important pathogenic roles during infections.

Use of the chinchilla model for nasopharyngeal colonization to study gene expression by Moraxella catarrhalis

Young adult chinchillas were atraumatically inoculated with Moraxella catarrhalis via the nasal route. Detailed histopathologic examination of nasopharyngeal tissues isolated from these M. catarrhalis-infected animals revealed the presence of significant inflammation within the epithelium. Absence of similar histopathologic findings in sham-inoculated animals confirmed that M. catarrhalis was exposed to significant host-derived factors in this environment. Twenty-four hours after inoculation, viable M. catarrhalis organisms were recovered from the nasal cavity and nasopharynx of the animals in numbers sufficient for DNA microarray analysis. More than 100 M. catarrhalis genes were upregulated in vivo, including open reading frames (ORFs) encoding proteins that are involved in a truncated denitrification pathway or in the oxidative stress response, as well as several putative transcriptional regulators. Additionally, 200 M. catarrhalis genes were found to be downregulated when this bacterium was introduced into the nasopharynx. These downregulated genes included ORFs encoding several well-characterized M. catarrhalis surface proteins including Hag, McaP, and MchA1. Real-time reverse transcriptase PCR (RT-PCR) was utilized as a stringent control to validate the results of in vivo gene expression patterns as measured by DNA microarray analysis. Inactivation of one of the genes (MC ORF 1550) that was upregulated in vivo resulted in a decrease in the ability of M. catarrhalis to survive in the chinchilla nasopharynx over a 3-day period. This is the first evaluation of global transcriptome expression by M. catarrhalis cells in vivo.

Expression of urease by Haemophilus influenzae during human respiratory tract infection and role in survival in an acid environment

Background

Nontypeable Haemophilus influenzae is a common cause of otitis media in children and lower respiratory tract infection in adults with chronic obstructive pulmonary disease (COPD). Prior studies have shown that H. influenzae expresses abundant urease during growth in the middle ear of the chinchilla and in pooled human sputum, suggesting that expression of urease is important for colonization and infection in the hostile environments of the middle ear and in the airways in adults. Virtually nothing else is known about the urease of H. influenzae, which was characterized in the present study.

Results

Analysis by reverse transcriptase PCR revealed that the ure gene cluster is expressed as a single transcript. Knockout mutants of a urease structural gene (ureC) and of the entire ure operon demonstrated no detectable urease activity indicating that this operon is the only one encoding an active urease. The ure operon is present in all strains tested, including clinical isolates from otitis media and COPD. Urease activity decreased as nitrogen availability increased. To test the hypothesis that urease is expressed during human infection, purified recombinant urease C was used in ELISA with pre acquisition and post infection serum from adults with COPD who experienced infections caused by H. influenzae. A total of 28% of patients developed new antibodies following infection indicating that H. influenzae expresses urease during airway infection. Bacterial viability assays performed at varying pH indicate that urease mediates survival of H. influenzae in an acid environment.

Conclusions

The H. influenzae genome contains a single urease operon that mediates urease expression and that is present in all clinical isolates tested. Nitrogen availability is a determinant of urease expression. H. influenzae expresses urease during human respiratory tract infection and urease is a target of the human antibody response. Expression of urease enhances viability in an acid environment. Taken together, these observations suggest that urease is important for survival and replication of H. influenzae in the human respiratory tract.

The Moraxella catarrhalis nitric oxide reductase is essential for nitric oxide detoxification

Moraxella catarrhalis is a Gram-negative obligate aerobe that is an important cause of human respiratory tract infections. The M. catarrhalis genome encodes a predicted truncated denitrification pathway that reduces nitrate to nitrous oxide. We have previously shown that expression of both the M. catarrhalis aniA (encoding a nitrite reductase) and norB (encoding a putative nitric oxide reductase) genes is repressed by the transcriptional regulator NsrR under aerobic conditions and that M. catarrhalis O35E nsrR mutants are unable to grow in the presence of low concentrations of nitrite (W. Wang, et al., J. Bacteriol. 190:7762-7772, 2008). In this study, we constructed an M. catarrhalis norB mutant and showed that planktonic growth of this mutant is inhibited by low levels of nitrite, whether or not an nsrR mutation is present. To determine the importance of NorB in this truncated denitrification pathway, we analyzed the metabolism of nitrogen oxides by norB, aniA norB, and nsrR norB mutants. We found that norB mutants are unable to reduce nitric oxide and produce little or no nitrous oxide from nitrite. Furthermore, nitric oxide produced from nitrite by the AniA protein is bactericidal for a Moraxella catarrhalis O35E norB mutant but not for wild-type O35E bacteria under aerobic growth conditions in vitro, suggesting that nitric oxide catabolism in M. catarrhalis is accomplished primarily by the norB gene product. Measurement of bacterial protein S-nitrosylation directly implicates nitrosative stress resulting from AniA-dependent nitric oxide formation as a cause of the growth inhibition of norB and nsrR mutants by nitrite.

Characterization and evaluation of the Moraxella catarrhalis oligopeptide permease A as a mucosal vaccine antigen

Moraxella catarrhalis is a common cause of otitis media in children and of lower respiratory tract infections in adults with chronic obstructive pulmonary disease; therefore, these two groups would benefit from a vaccine to prevent M. catarrhalis infections. A genome mining approach for vaccine antigens identified oligopeptide permease protein A (OppA), an oligopeptide binding protein of an apparent oligopeptide transport system. Analysis of the oppA gene by PCR and sequence analysis revealed that OppA is highly conserved among clinical isolates of M. catarrhalis. Recombinant OppA was expressed as a lipoprotein and purified, and an oppA knockout mutant was constructed. Antiserum raised to recombinant purified OppA recognized epitopes on the bacterial surface of the wild type but not the OppA knockout mutant. Antibodies raised to purified recombinant OppA recognized native OppA in multiple strains. Intranasal immunization of mice induced systemic and mucosal antibodies to OppA and resulted in enhanced clearance of M. catarrhalis in a mouse pulmonary clearance model. OppA is a highly conserved, immunogenic protein that expresses epitopes on the bacterial surface and that induces potentially protective immune responses in a mouse model. OppA should be evaluated further as a vaccine antigen for M. catarrhalis.

Colonization of healthy children by Moraxella catarrhalis is characterized by genotype heterogeneity, virulence gene diversity and co-colonization with Haemophilus influenzae

The colonization dynamics of Moraxella catarrhalis were studied in a population comprising 1079 healthy children living in Rotterdam, The Netherlands (the Generation R Focus cohort). A total of 2751 nasal swabs were obtained during four clinic visits timed to take place at 1.5, 6, 14 and 24 months of age, yielding a total of 709 M. catarrhalis and 621 Haemophilus influenzae isolates. Between January 2004 and December 2006, approximate but regular 6-monthly cycles of colonization were observed, with peak colonization incidences occurring in the autumn/winter for M. catarrhalis, and winter/spring for H. influenzae. Co-colonization was significantly more likely than single-species colonization with either M. catarrhalis or H. influenzae, with genotypic analysis revealing no clonality for co-colonizing or single colonizers of either bacterial species. This finding is especially relevant considering the recent discovery of the importance of H. influenzae-M. catarrhalis quorum sensing in biofilm formation and host clearance. Bacterial genotype heterogeneity was maintained over the 3-year period of the study, even within this relatively localized geographical region, and there was no association of genotypes with either season or year of isolation. Furthermore, chronological and genotypic diversity in three immunologically important M. catarrhalis virulence genes (uspA1, uspA2 and hag/mid) was also observed. This study indicates that genotypic variation is a key factor contributing to the success of M. catarrhalis colonization of healthy children in the first years of life. Furthermore, variation in immunologically relevant virulence genes within colonizing populations, and even within genotypically identical M. catarrhalis isolates, may be a result of immune evasion by this pathogen. Finally, the factors facilitating M. catarrhalis and H. influenzae co-colonization need to be further investigated.

Genome analysis of Moraxella catarrhalis strain BBH18, [corrected] a human respiratory tract pathogen

Moraxella catarrhalis is an emerging human-restricted respiratory tract pathogen that is a common cause of childhood otitis media and exacerbations of chronic obstructive pulmonary disease in adults. Here, we report the first completely assembled and annotated genome sequence of an isolate of M. catarrhalis, strain RH4, which originally was isolated from blood of an infected patient. The RH4 genome consists of 1,863,286 nucleotides that form 1,886 protein-encoding genes. Comparison of the RH4 genome to the ATCC 43617 contigs demonstrated that the gene content of both strains is highly conserved. In silico phylogenetic analyses based on both 16S rRNA and multilocus sequence typing revealed that RH4 belongs to the seroresistant lineage. We were able to identify almost the entire repertoire of known M. catarrhalis virulence factors and mapped the members of the biosynthetic pathways for lipooligosaccharide, peptidoglycan, and type IV pili. Reconstruction of the central metabolic pathways suggested that RH4 relies on fatty acid and acetate metabolism, as the genes encoding the enzymes required for the glyoxylate pathway, the tricarboxylic acid cycle, the gluconeogenic pathway, the nonoxidative branch of the pentose phosphate pathway, the beta-oxidation pathway of fatty acids, and acetate metabolism were present. Moreover, pathways important for survival under challenging in vivo conditions, such as the iron-acquisition pathways, nitrogen metabolism, and oxidative stress responses, were identified. Finally, we showed by microarray expression profiling that approximately 88% of the predicted coding sequences are transcribed under in vitro conditions. Overall, these results provide a foundation for future research into the mechanisms of M. catarrhalis pathogenesis and vaccine development.

Molecular aspects of Moraxella catarrhalis pathogenesis

In recent years, Moraxella catarrhalis has established its position as an important human mucosal pathogen, no longer being regarded as just a commensal bacterium. Further, current research in the field has led to a better understanding of the molecular mechanisms involved in M. catarrhalis pathogenesis, including mechanisms associated with cellular adherence, target cell invasion, modulation of the host's immune response, and metabolism. Additionally, in order to be successful in the host, M. catarrhalis has to be able to interact and compete with the commensal flora and overcome stressful environmental conditions, such as nutrient limitation. In this review, we provide a timely overview of the current understanding of the molecular mechanisms associated with M. catarrhalis virulence and pathogenesis.

Characterization of proteins Msp22 and Msp75 as vaccine antigens of Moraxella catarrhalis

Moraxella catarrhalis is a respiratory tract pathogen causing otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease. This study examined two newly identified proteins as potential vaccine antigens. Antisera raised to recombinant purified proteins Msp22 and Msp75 recognized corresponding native proteins in multiple strains of M. catarrhalis. Vaccine formulations individually administered subcutaneously and intranasally showed enhanced clearance of M. catarrhalis in a mouse pulmonary clearance model by both routes of administration. Msp22 and Msp75 are antigenically conserved proteins that induce potentially protective immune responses and should be examined further as vaccine antigens for M. catarrhalis.

Identification of a bacteriocin and its cognate immunity factor expressed by Moraxella catarrhalis

Background

Bacteriocins are antimicrobial proteins and peptides ribosomally synthesized by some bacteria which can effect both intraspecies and interspecies killing.

Results

Moraxella catarrhalis strain E22 containing plasmid pLQ510 was shown to inhibit the growth of M. catarrhalis strain O35E. Two genes (mcbA and mcbB) in pLQ510 encoded proteins predicted to be involved in the secretion of a bacteriocin. Immediately downstream from these two genes, a very short ORF (mcbC) encoded a protein which had some homology to double-glycine bacteriocins produced by other bacteria. A second very short ORF (mcbI) immediately downstream from mcbC encoded a protein which had no significant similarity to other proteins in the databases. Cloning and expression of the mcbI gene in M. catarrhalis O35E indicated that this gene encoded the cognate immunity factor. Reverse transcriptase-PCR was used to show that the mcbA, mcbB, mcbC, and mcbI ORFs were transcriptionally linked. This four-gene cluster was subsequently shown to be present in the chromosome of several M. catarrhalis strains including O12E. Inactivation of the mcbA, mcbB, or mcbC ORFs in M. catarrhalis O12E eliminated the ability of this strain to inhibit the growth of M. catarrhalis O35E. In co-culture experiments involving a M. catarrhalis strain containing the mcbABCI locus and one which lacked this locus, the former strain became the predominant member of the culture after overnight growth in broth.

Conclusion

This is the first description of a bacteriocin and its cognate immunity factor produced by M. catarrhalis. The killing activity of the McbC protein raises the possibility that it might serve to lyse other M. catarrhalis strains that lack the mcbABCI locus, thereby making their DNA available for lateral gene transfer.

Prophylactic vaccinations in chronic obstructive pulmonary disease: current status

PURPOSE OF REVIEW

Acute exacerbations of chronic obstructive pulmonary disease (COPD) are a major cause of morbidity and mortality worldwide. Most acute exacerbations are triggered by community-acquired respiratory infections. Medications to treat COPD exacerbations are limited; therefore, identifying effective ways to prevent exacerbations are needed. Influenza and pneumococcal vaccines are currently recommended for all persons with COPD. However, current immunization rates are far lower than the Healthy People 2010 Goals. The reasons for nonadherence are multifactorial and strategies for overcoming these barriers are discussed.

RECENT FINDINGS

Influenza vaccine clearly reduces the number of acute exacerbations that occur in persons with COPD. Influenza vaccine may reduce hospitalizations and mortality from COPD, but the evidence is not conclusive. Pneumococcal vaccine reduces the incidence of invasive pneumococcal disease. However, there is not enough evidence to conclude that pneumococcal vaccination in persons with COPD has a significant impact on reducing morbidity or mortality. Vaccination with both influenza and pneumococcal vaccine may produce an additive effect that reduces exacerbations more effectively than either vaccine alone. Whole genome sequencing of bacteria and genome mining may represent a powerful way to identify novel potential vaccine antigens for future vaccine development.

SUMMARY

Although clinical trial data are limited, vaccinations can prevent some of the infections that cause COPD exacerbations and should be administered to all patients with COPD. Vaccines do not cause exacerbations of COPD. Patient and physician barriers to vaccination can be overcome with targeted education and system-wide interventions. Further research efforts should focus on improving current vaccines and identifying novel targets for future vaccine development.

Bibliography. Current world literature

This bibliography is compiled by clinicians from the journals listed at the end of this publication. It is based on literature entered into our database between 1 November 2007 and 31 October 2008 (articles are generally added to the database about two and a half months after publication). In addition, the bibliography contains every paper annotated by reviewers; these references were obtained from a variety of bibliographic databases and published between the beginning of the review period and the time of going to press. The bibliography has been grouped into topics that relate to the reviews in this issue.

Use of Moraxella catarrhalis lipooligosaccharide mutants to identify specific oligosaccharide epitopes recognized by human serum antibodies

Moraxella catarrhalis is a causative agent of otitis media in children and lower respiratory tract infections in adults suffering from chronic obstructive pulmonary disease (COPD). This strict human pathogen continues to be a significant cause of disease in this broad spectrum of patients because there is no available vaccine. Although numerous putative vaccine antigens have been described, little is known about the human immune response to M. catarrhalis infection in vivo. Human serum antibodies are directed at a number of surface proteins, and lipooligosaccharides (LOS) and detoxified LOS may be an effective immunogen in mice. In this study, we used a specific LOS-based enzyme-linked immunosorbent assay (ELISA), containing the three major M. catarrhalis serotypes together with a complete series of truncated LOS mutants, to detect the development of new antibodies to specific regions of the oligosaccharide molecule. We compared serum samples from COPD patients who had recently cleared an M. catarrhalis infection to serum samples collected prior to their infection. Variability in the antibody response to LOS was observed, as some patients developed serotype-specific antibodies, others developed antibodies to the LOS of each serotype, others developed broadly cross-reactive antibodies, and some did not develop new antibodies. These newly developed human antibodies are directed at both side chains and core structures in the LOS molecule. This LOS-based ELISA can be used to dissect the human antibody response to both internal and external carbohydrate epitopes, thus providing a better understanding of the humoral immune response to M. catarrhalis LOS epitopes developed during natural infection.

Identification of domains of the Hag/MID surface protein recognized by systemic and mucosal antibodies in adults with chronic obstructive pulmonary disease following clearance of Moraxella catarrhalis

Moraxella catarrhalis is a common cause of respiratory tract infection in the setting of chronic obstructive pulmonary disease (COPD). Adults with COPD acquire and clear strains of M. catarrhalis from the respiratory tract continuously and develop strain-specific protection following clearance of a strain. In previous work, we identified Hag/MID (Moraxella immunoglobulin D-binding protein), a large multifunctional surface protein that acts as an adhesin and hemagglutinin, as a target of antibody responses in adults with COPD after clearance of M. catarrhalis. The goal of the present study was to characterize the domains of Hag/MID to which humans make antibodies, including both systemic and mucosal antibody responses. Analysis of recombinant peptide constructs, which spanned the M. catarrhalis strain O35E Hag/MID protein, with well-characterized serum and sputum samples revealed that most adults with COPD made antibodies directed toward a region of the molecule bounded by amino acids 706 to 863. Serum immunoglobulin G (IgG) and IgA purified from sputum both recognized the same domain. Some flanking sequence of this fragment was necessary for the epitope(s) in this region to maintain its conformation to bind human antibodies. These results reveal that humans consistently generate both systemic and mucosal antibody responses to an immunodominant region of the Hag/MID molecule, which was previously shown to overlap with several biologically relevant domains, including epithelial cell adherence, IgD binding, collagen binding, and hemagglutination.

Identification of a repressor of a truncated denitrification pathway in Moraxella catarrhalis

Growth of Moraxella catarrhalis in a biofilm resulted in marked upregulation of two open reading frames (ORFs), aniA and norB, predicted to encode a nitrite reductase and a nitric oxide reductase, respectively (W. Wang, L. Reitzer, D. A. Rasko, M. M. Pearson, R. J. Blick, C. Laurence, and E. J. Hansen, Infect. Immun. 75:4959-4971, 2007). An ORF designated nsrR, which was located between aniA and norB, was shown to encode a predicted transcriptional regulator. Inactivation of nsrR resulted in increased expression of aniA and norB in three different M. catarrhalis strains, as measured by both DNA microarray analysis and quantitative reverse transcriptase PCR. Provision of a wild-type nsrR gene in trans in an nsrR mutant resulted in decreased expression of the AniA protein. DNA microarray analysis revealed that two other ORFs (MC ORF 683 and MC ORF 1550) were also consistently upregulated in an nsrR mutant. Consumption of both nitrite and nitric oxide occurred more rapidly with cells of an nsrR mutant than with wild-type cells. However, growth of nsrR mutants was completely inhibited by a low level of sodium nitrite. This inhibition of growth by nitrite was significantly reversed by introduction of an aniA mutation into the nsrR mutant and was completely reversed by the presence of a wild-type nsrR gene in trans. NsrR regulation of the expression of aniA was sensitive to nitrite, whereas NsrR regulation of norB was sensitive to nitric oxide.