Complement regulator-acquiring surface protein 1 of Borrelia burgdorferi binds to human bone morphogenic protein 2, several extracellular matrix proteins, and plasminogen

Lyme disease-causing Borrelia burgdorferi spirochetes express up to 5 complement regulator-acquiring surface proteins (CRASPs). To better define how CRASP-1 contributes to infection, we aimed to identify novel CRASP-1-binding host proteins. Here, we identified a number of novel human CRASP-1-binding proteins, including bone morphogenic protein 2, collagen I, collagen III, collagen IV, fibronectin, laminin, and plasminogen. The plasminogen-binding regions were located in 2 separate regions of CRASP-1. Our results demonstrated that plasminogen-bound CRASP-1 can be converted to plasmin by the urokinase-type plasminogen activator and that proteolytically active plasmin cleaves the synthetic chromogenic substrate S-2251 and the natural substrate fibrinogen. In conclusion, CRASP-1 is a multifunctional protein of B. burgdorferi that binds to several human extracellular matrix proteins and plasminogen. These interactions may contribute to adhesion, bacterial colonization, and organ tropism and may allow dissemination of B. burgdorferi in the host.

Nontypeable Haemophilus influenzae protein E binds vitronectin and is important for serum resistance

Nontypeable Haemophilus influenzae (NTHi) commonly causes local disease in the upper and lower respiratory tract and has recently been shown to interfere with both the classical and alternative pathways of complement activation. The terminal pathway of the complement system is regulated by vitronectin that is a component of both plasma and the extracellular matrix. In this study, we identify protein E (PE; 16 kDa), which is a recently characterized ubiquitous outer membrane protein, as a vitronectin-binding protein of NTHi. A PE-deficient NTHi mutant had a markedly reduced survival in serum compared with the PE-expressing isogenic NTHi wild type. Moreover, the PE-deficient mutant showed a significantly decreased binding to both soluble and immobilized vitronectin. In parallel, PE-expressing Escherichia coli bound soluble vitronectin and adhered to immobilized vitronectin compared with controls. Surface plasmon resonance technology revealed a K(D) of 0.4 microM for the interaction between recombinant PE and immobilized vitronectin. Moreover, the PE-dependent vitronectin-binding site was located at the heparin-binding domains of vitronectin and the major vitronectin-binding domain was found in the central core of PE (aa 84-108). Importantly, vitronectin bound to the surface of NTHi 3655 reduced membrane attack complex-induced hemolysis. In contrast to incubation with normal human serum, NTHi 3655 showed a reduced survival in vitronectin-depleted human serum, thus demonstrating that vitronectin mediates a protective role at the bacterial surface. Our findings show that PE, by binding vitronectin, may play an important role in NTHi pathogenesis.

Haemophilus influenzae surface fibrils contribute to serum resistance by interacting with vitronectin

Vitronectin inhibits the membrane attack complex of the complement system and is found both in plasma and the extracellular matrix. In this study, we have identified the outer membrane protein Haemophilus surface fibrils (Hsf) as the major vitronectin-binding protein in encapsulated H. influenzae type b. A H. influenzae mutant devoid of Hsf showed a significantly decreased binding to both soluble and immobilized vitronectin as compared with the wild-type counterpart. Moreover, Escherichia coli-expressing Hsf at the surface strongly adhered to immobilized vitronectin. Importantly, the H. influenzae Hsf mutant had a markedly reduced survival as compared with the wild-type bacterium when incubated with normal human serum. A series of truncated Hsf fragments were recombinantly manufactured in E. coli. The vitronectin binding regions were located within two separate binding domains. In conclusion, Hsf interacts with vitronectin and thereby inhibits the complement-mediated bactericidal activity, and thus is a major H. influenzae virulence factor.

Haemophilus influenzae and the complement system

The respiratory tract pathogen Haemophilus influenzae is responsible for a variety of infections in humans including septicemia, bronchitis, pneumonia, and acute otitis media. The pathogenesis of H. influenzae relies on its capacity to resist human host defenses including the complement system, and thus H. influenzae has developed several efficient strategies to circumvent complement attack. In addition to attracting specific host complement regulators directly to the bacterial surface, the capsule, lipooligosaccharides, and several outer membrane proteins contribute to resistance against complement-mediated attacks and hence increased bacterial survival. Insights into the mechanisms of complement evasion by H. influenzae are important for understanding pathogenesis and for developing vaccines and new therapies aimed at patients with, for example, chronic obstructive pulmonary disease. Here we overview current knowledge on the different mechanisms by which H. influenzae evades attack by the host complement system.

Complement factor H-related proteins CFHR2 and CFHR5 represent novel ligands for the infection-associated CRASP proteins of Borrelia burgdorferi

BACKGROUND

One virulence property of Borrelia burgdorferi is its resistance to innate immunity, in particular to complement-mediated killing. Serum-resistant B. burgdorferi express up to five distinct complement regulator-acquiring surface proteins (CRASP) which interact with complement regulator factor H (CFH) and factor H-like protein 1 (FHL1) or factor H-related protein 1 (CFHR1). In the present study we elucidate the role of the infection-associated CRASP-3 and CRASP-5 protein to serve as ligands for additional complement regulatory proteins as well as for complement resistance of B. burgdorferi.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate whether CRASP-5 and CRASP-3 interact with various human proteins, both borrelial proteins were immobilized on magnetic beads. Following incubation with human serum, bound proteins were eluted and separated by Glycine-SDS-PAGE. In addition to CFH and CFHR1, complement regulators CFHR2 and CFHR5 were identified as novel ligands for both borrelial proteins by employing MALDI-TOF. To further assess the contributions of CRASP-3 and CRASP-5 to complement resistance, a serum-sensitive B. garinii strain G1 which lacks all CFH-binding proteins was used as a valuable model for functional analyses. Both CRASPs expressed on the B. garinii outer surface bound CFH as well as CFHR1 and CFHR2 in ELISA. In contrast, live B. garinii bound CFHR1, CFHR2, and CFHR5 and only miniscute amounts of CFH as demonstrated by serum adsorption assays and FACS analyses. Further functional analysis revealed that upon NHS incubation, CRASP-3 or CRASP-5 expressing borreliae were killed by complement.

CONCLUSIONS/SIGNIFICANCE

In the absence of CFH and the presence of CFHR1, CFHR2 and CFHR5, assembly and integration of the membrane attack complex was not efficiently inhibited indicating that CFH in co-operation with CFHR1, CFHR2 and CFHR5 supports complement evasion of B. burgdorferi.

The choline-binding protein PspC of Streptococcus pneumoniae interacts with the C-terminal heparin-binding domain of vitronectin

Adherence of Streptococcus pneumoniae is directly mediated by interactions of adhesins with eukaryotic cellular receptors or indirectly by exploiting matrix and serum proteins as molecular bridges. Pneumococci engage vitronectin, the human adhesive glycoprotein and complement inhibitor, to facilitate attachment to epithelial cells of the mucosal cavity, thereby modulating host cell signaling. In this study, we identified PspC as a vitronectin-binding protein interacting with the C-terminal heparin-binding domain of vitronectin. PspC is a multifunctional surface-exposed choline-binding protein displaying various adhesive properties. Vitronectin binding required the R domains in the mature PspC protein, which are also essential for the interaction with the ectodomain of the polymeric immunoglobulin receptor and secretory IgA. Consequently, secretory IgA competitively inhibited binding of vitronectin to purified PspC and to PspC-expressing pneumococci. In contrast, Factor H, which binds to the N-terminal part of mature PspC molecules, did not interfere with the PspC-vitronectin interaction. Using a series of vitronectin peptides, the C-terminal heparin-binding domain was shown to be essential for the interaction of soluble vitronectin with PspC. Binding experiments with immobilized vitronectin suggested a region N-terminal to the identified heparin-binding domain as an additional binding region for PspC, suggesting that soluble, immobilized, as well as cellularly bound vitronectin possesses different conformations. Finally, vitronectin bound to PspC was functionally active and inhibited the deposition of the terminal complement complex. In conclusion, this study identifies and characterizes (on the molecular level) the interaction between the pneumococcal adhesin PspC and the human glycoprotein vitronectin.

Interaction with C4b-binding protein contributes to nontypeable Haemophilus influenzae serum resistance

Complement evasion by various mechanisms is important for microbial virulence and survival in the host. One strategy used by some pathogenic bacteria is to bind the complement inhibitor of the classical pathway, C4b-binding protein (C4BP). In this study, we have identified a novel interaction between nontypeable Haemophilus influenzae (NTHi) and C4BP, whereas the majority of the typeable H. influenzae (a-f) tested showed no binding. One of the clinical isolates, NTHi 506, displayed a particularly high binding of C4BP and was used for detailed analysis of the interaction. Importantly, a low C4BP-binding isolate (NTHi 69) showed an increased deposition of C3b followed by reduced survival as compared with NTHi 506 when exposed to normal human serum. The main isoform of C4BP contains seven identical alpha-chains and one beta-chain linked together with disulfide bridges. Each alpha-chain is composed of eight complement control protein (CCP) modules and we have found that the NTHi 506 strain did not interact with rC4BP lacking CCP2 or CCP7 showing that these two CCPs are important for the binding. Importantly, C4BP bound to the surface of H. influenzae retained its cofactor activity as determined by analysis of C3b and C4b degradation. Taken together, NTHi interferes with the classical complement activation pathway by binding to C4BP.

Tuf of Streptococcus pneumoniae is a surface displayed human complement regulator binding protein

Streptococcus pneumoniae is a Gram-positive bacterium, causing acute sinusitis, otitis media, and severe diseases such as pneumonia, bacteraemia, meningitis and sepsis. Here we identify elongation factor Tu (Tuf) as a new Factor H binding protein of S. pneumoniae. The surface protein PspC which also binds a series of other human immune inhibitors, was the first identified pneumococcal Factor H binding protein of S. pneumoniae. Pneumococcal Tuf, a 55 kDa pneumococcal moonlighting protein which is displayed on the surface of pneumococci, is also located in the cytoplasm and is detected in the culture supernatant. Tuf binds the human complement inhibitors Factor H, FHL-1, CFHR1 and also the proenzyme plasminogen. Factor H and FHL-1 bound to Tuf, retain their complement regulatory activities. Similarly, plasminogen bound to Tuf was accessible for the activator uPA and activated plasmin cleaved the synthetic chromogenic substrate S-2251 as well as the natural substrates fibrinogen and the complement proteins C3 and C3b. Taken together, Tuf of S. pneumoniae is a new multi-functional bacterial virulence factor that helps the pathogen in complement escape and likely also in ECM degradation.

The complement fitness factor H: role in human diseases and for immune escape of pathogens, like pneumococci

Factor H is the central regulator of the alternative complement pathway and controls early activation of the complement cascade at the level of the C3 convertase. Mutations in the Factor H gene are associated with severe and diverse diseases including the rare renal disorders hemolytic uremic syndrome (HUS) and membranoproliferative glomerulonephritis (MPGN) also termed dense deposit disease (DDD), as well as the more frequent retinal disease age related macular degeneration (AMD). In addition, pathogenic microbes utilize host complement Factor H for immune evasion and these pathogens express specific surface receptors which bind host innate immune regulators. Sequence variations or mutations of one single gene, coding for the host regulator Factor H, form the basis for multiple, different disorders such as human renal and retinal diseases as well as infections. This association of Factor H but also of additional related complement components and regulators with the same diseases demonstrate an important role of complement, particularly of the alternative pathway, for tissue homeostasis. Disturbances of this central immune surveillance system lead to damage of autologous tissues and surfaces and result in autoimmune diseases. Remarkably, pathogenic microbes copy this mechanism of immune surveillance: they mimic the composition of host cell's, bind Factor H to their surface and engage acquired host Factor H for immune disguise.

Haemophilus influenzae protein E binds to the extracellular matrix by concurrently interacting with laminin and vitronectin

Nontypeable Haemophilus influenzae (NTHi) causes otitis media and is commonly found in patients with chronic obstructive pulmonary disease (COPD). Adhesins are important for bacterial attachment and colonization. Protein E (PE) is a recently characterized ubiquitous 16 kDa adhesin with vitronectin-binding capacity that results in increased survival in serum. In addition to PE, NTHi utilizes Haemophilus adhesion protein (Hap) that binds to the basement-membrane glycoprotein laminin. We show that most clinical isolates bind laminin and that both Hap and PE are crucial for the NTHi-dependent interaction with laminin as revealed with different mutants. The laminin-binding region is located at the N-terminus of PE, and PE binds to the heparin-binding C-terminal globular domain of laminin. PE simultaneously attracts vitronectin and laminin at separate binding sites, proving the multifunctional nature of the adhesin. This previously unknown PE-dependent interaction with laminin may contribute to NTHi colonization, particularly in smokers with COPD.

BGA66 and BGA71 facilitate complement resistance of Borrelia bavariensis by inhibiting assembly of the membrane attack complex

Borrelia (B.) bavariensis exhibits a marked tropism for nervous tissues and frequently causes neurological manifestations in humans. The molecular mechanism by which B. bavariensis overcomes innate immunity, in particular, complement remains elusive. In contrast to other serum-resistant spirochetes, none of the B. bavariensis isolates investigated bound complement regulators of the alternative (AP) and classical pathway (CP) or proteolytically inactivated complement components. Focusing on outer surface proteins BGA66 and BGA71, we demonstrated that both molecules either inhibit AP, CP and terminal pathway (TP) activation, or block activation of the CP and TP respectively. Both molecules bind complement components C7, C8 and C9, and thereby prevent assembly of the terminal complement complex. This inhibitory activity was confirmed by the introduction of the BGA66 and BGA71 encoding genes into a serum-sensitive B. garinii strain. Transformed spirochetes producing either BGA66 or BGA71 overcome complement-mediated killing, thus indicating that both proteins independently facilitate serum resistance of B. bavariensis. The generation of C-terminally truncated proteins as well as a chimeric BGA71 protein lead to the localization of the complement-interacting binding site within the N-terminus. Collectively, our data reveal a novel immune evasion strategy of B. bavariensis that is directed against the activation of the TP.

Yersinia enterocolitica YadA mediates complement evasion by recruitment and inactivation of C3 products

Yersinia adhesin A (YadA) is a major virulence factor of Yersinia enterocolitica. YadA mediates host cell binding and autoaggregation and protects the pathogen from killing by the complement system. Previous studies demonstrated that YadA is the most important single factor mediating serum resistance of Y. enterocolitica, presumably by binding C4b binding protein (C4BP) and factor H, which are both complement inhibitors. Factor H acts as a cofactor for factor I-mediated cleavage of C3b into the inactive form iC3b and thus prevents formation of inflammatory effector compounds and the terminal complement complex. In this study, we challenged the current direct binding model of factor H to YadA and show that Y. enterocolitica YadA recruits C3b and iC3b directly, without the need of an active complement cascade or additional serum factors. Enhanced binding of C3b does not decrease survival of YadA-expressing Yersiniae because C3b becomes readily inactivated by factor H and factor I. Binding of factor H to YadA is greatly reduced in the absence of C3. Experiments using Yersinia lacking YadA or expressing YadA with reduced trimeric stability clearly demonstrate that both the presence and full trimeric stability of YadA are essential for complement resistance. A novel mechanism of factor H binding is presented in which YadA exploits recruitment of C3b or iC3b to attract large amounts of factor H. As a consequence, formation of the terminal complement complex is limited and bacterial survival is enhanced. These findings add a new aspect of how Y. enterocolitica effectively evades the host complement system.

The glycolytic enzyme enolase represents a plasminogen-binding protein on the surface of a wide variety of medically important fungal species

Allergies are an increasing issue in human health and can, eventually, cause severe anaphylactic shock. Aspergillus fumigatus and Candida albicans are leading causes of life-threatening invasive fungal infections in immunocompromised patients, but can also cause severe allergic responses in otherwise healthy individuals. The glycolytic enzyme enolase is known as a major allergen despite its function in intracellular metabolism. Therefore, its presentation on surfaces of different fungal species was investigated by using antibodies raised against recombinant enolases from A. fumigatus and C. albicans. Examination of antibody specificity revealed cross-reactivity to cell-free extracts from Aspergillus terreus, Aspergillus flavus, Aspergillus nidulans and Candida glabrata, but not against any of the three human enolases. Antibody specificity was further confirmed by hybridization with other recombinant fungal enolases, where the antibodies recognized different subsets of fungal enolases. When surface presentation of enolase was tested on intact fungal cells, a positive staining was obtained with those antibodies that also recognized the enzyme from the respective cell-free extract. This implies a general surface presentation of this glycolytic enzyme among fungal species and provides hints for its predominant recognition as an allergen. Additionally, A. fumigatus and C. albicans enolase bound to human plasminogen, which remained accessible for the plasminogen activator uPA. This implies a potential role of enolase in the invasion and dissemination process during fungal infections.

Conserved Patterns of Microbial Immune Escape: Pathogenic Microbes of Diverse Origin Target the Human Terminal Complement Inhibitor Vitronectin via a Single Common Motif

Pathogenicity of many microbes relies on their capacity to resist innate immunity, and to survive and persist in an immunocompetent human host microbes have developed highly efficient and sophisticated complement evasion strategies. Here we show that different human pathogens including Gram-negative and Gram-positive bacteria, as well as the fungal pathogen Candida albicans, acquire the human terminal complement regulator vitronectin to their surface. By using truncated vitronectin fragments we found that all analyzed microbial pathogens (n = 13) bound human vitronectin via the same C-terminal heparin-binding domain (amino acids 352-374). This specific interaction leaves the terminal complement complex (TCC) regulatory region of vitronectin accessible, allowing inhibition of C5b-7 membrane insertion and C9 polymerization. Vitronectin complexed with the various microbes and corresponding proteins was thus functionally active and inhibited complement-mediated C5b-9 deposition. Taken together, diverse microbial pathogens expressing different structurally unrelated vitronectin-binding molecules interact with host vitronectin via the same conserved region to allow versatile control of the host innate immune response.

Identification of a Haemophilus influenzae factor H-Binding lipoprotein involved in serum resistance

Haemophilus influenzae is a Gram-negative human pathogen that resides in the upper respiratory tract. Encapsulated H. influenzae type b (Hib) and type f (Hif) are the most common serotypes associated with invasive disease. H. influenzae displays various strategies to circumvent the host innate immune response, including the bactericidal effect of the complement system. In this study, we identified an H. influenzae lipoprotein having the ability to bind factor H (FH), the major regulator of the alternative pathway of complement activation. This protein, named protein H (PH), was surface exposed and was found in all clinical Hib and Hif isolates tested. Deletion of the gene encoding for PH (lph) in Hib and Hif significantly reduced the interaction between bacteria and FH. When Hib and Hif PH variants were separately expressed in nontypeable (unencapsulated) H. influenzae, which did not bind FH, an increased FH affinity was observed. We recombinantly expressed the two PH variants in Escherichia coli, and despite sharing only 56% identical amino acids, both FH-binding Haemophilus proteins similarly interacted with the complement regulator FH short consensus repeats 7 and 18-20. Importantly, Hib and Hif resistance against the bactericidal effect of human serum was significantly reduced when bacterial mutants devoid of PH were tested. In conclusion, we have characterized a hitherto unknown bacterial protein that is crucial for mediating an interaction between the human pathogen H. influenzae and FH. This novel interaction is important for H. influenzae resistance against complement activation and will consequently promote bacterial pathogenesis.

Immune evasion of Moraxella catarrhalis involves ubiquitous surface protein A-dependent C3d binding

The complement system plays an important role in eliminating invading pathogens. Activation of complement results in C3b deposition (opsonization), phagocytosis, anaphylatoxin (C3a, C5a) release, and consequently cell lysis. Moraxella catarrhalis is a human respiratory pathogen commonly found in children with otitis media and in adults with chronic obstructive pulmonary disease. The species has evolved multiple complement evasion strategies, which among others involves the ubiquitous surface protein (Usp) family consisting of UspA1, A2, and A2 hybrid. In the present study, we found that the ability of M. catarrhalis to bind C3 correlated with UspA expression and that C3 binding contributed to serum resistance in a large number of clinical isolates. Recombinantly expressed UspA1 and A2 inhibit both the alternative and classical pathways, C3b deposition, and C3a generation when bound to the C3 molecule. We also revealed that the M. catarrhalis UspA-binding domain on C3b was located to C3d and that the major bacterial C3d-binding domains were within UspA1(299-452) and UspA2(165-318). The interaction with C3 was not species specific since UspA-expressing M. catarrhalis also bound mouse C3 that resulted in inhibition of the alternative pathway of mouse complement. Taken together, the binding of C3 to UspAs is an efficient strategy of Moraxella to block the activation of complement and to inhibit C3a-mediated inflammation.

Binding of vitronectin and Factor H to Hic contributes to immune evasion of Streptococcus pneumoniae serotype 3

Streptococcus pneumoniae serotype 3 strains are highly resistant to opsonophagocytosis due to recruitment of the complement inhibitor Factor H via Hic, a member of the pneumococcal surface protein C (PspC) family. In this study, we demonstrated that Hic also interacts with vitronectin, a fluid-phase regulator involved in haemostasis, angiogenesis, and the terminal complement cascade as well as a component of the extracellular matrix. Blocking of Hic by specific antiserum or genetic deletion significantly reduced pneumococcal binding to soluble and immobilised vitronectin and to Factor H, respectively. In parallel, ectopic expression of Hic on the surface of Lactococcus lactis conferred binding to soluble and immobilised vitronectin as well as Factor H. Molecular analyses with truncated Hic fragments narrowed down the vitronectin-binding site to the central core of Hic (aa 151-201). This vitronectin-binding region is separate from that of Factor H, which binds to the N-terminus of Hic (aa 38-92). Binding of pneumococcal Hic was localised to the C-terminal heparin-binding domain (HBD3) of vitronectin. However, an N-terminal region to HBD3 was further involved in Hic-binding to immobilised vitronectin. Finally, vitronectin bound to Hic was functionally active and inhibited formation of the terminal complement complex. In conclusion, Hic interacts with vitronectin and simultaneously with Factor H, and both human proteins may contribute to colonisation and invasive disease caused by serotype 3 pneumococci.

Vitronectin Binds to a Specific Stretch within the Head Region of Yersinia Adhesin A and Thereby Modulates Yersinia enterocolitica Host Interaction

Complement resistance is an important virulence trait of Yersinia enterocolitica (Ye). The predominant virulence factor expressed by Ye is Yersinia adhesin A (YadA), which enables bacterial attachment to host cells and extracellular matrix and additionally allows the acquisition of soluble serum factors. The serum glycoprotein vitronectin (Vn) acts as an inhibitory regulator of the terminal complement complex by inhibiting the lytic pore formation. Here, we show YadA-mediated direct interaction of Ye with Vn and investigated the role of this Vn binding during mouse infection in vivo. Using different Yersinia strains, we identified a short stretch in the YadA head domain of Ye O:9 E40, similar to the 'uptake region' of Y. pseudotuberculosis YPIII YadA, as crucial for efficient Vn binding. Using recombinant fragments of Vn, we found the C-terminal part of Vn, including heparin-binding domain 3, to be responsible for binding to YadA. Moreover, we found that Vn bound to the bacterial surface is still functionally active and thus inhibits C5b-9 formation. In a mouse infection model, we demonstrate that Vn reduces complement-mediated killing of Ye O:9 E40 and, thus, improved bacterial survival. Taken together, these findings show that YadA-mediated Vn binding influences Ye pathogenesis.

Sexuality of women in middle age: the Göteborg study

Until recently there has been a paucity of factual data on the effects of the climacteric upon women's sexuality. The basic questions here are: do changes occur in sexual behaviour, sexual interest or responsiveness during the climacteric years? If so, what are the variations and how are they caused?

Two variables used extensively as a measure of sexuality are coital frequency and orgasmic frequency. Kinsey et al. (1953) showed that the degree of sexual activity remained fairly constant in unmarried women up to 55 years of age. By contrast, sexual activity in unmarried men declined gradually from puberty onwards. For married men and women they found that the frequency of sexual outlets declined during the life span. Kinsey et al. emphasized that the decreasing frequency of sexual intercourse and orgasm in marriage does not prove that the sexual capacity of the woman is influenced by her own ageing but could instead be a result of her husband's ageing. These authors were of the opinion that there is little evidence of any ageing in the sexual capacity of the female until late in her life.

Characterization of the IgD binding site of encapsulated Haemophilus influenzae serotype b

Encapsulated Haemophilus influenzae is a causative agent of invasive disease, such as meningitis and septicemia. Several interactions exist between H. influenzae and the human host. H. influenzae has been reported to bind IgD in a nonimmune manner, but the responsible protein has not yet been identified. To define the binding site on IgD for H. influenzae, full-length IgD and four chimeric IgDs with interspersed IgG sequences and Ag specificity for dansyl chloride were expressed in stably transfected Chinese hamster ovary cells. The binding of recombinant IgD to a panel of encapsulated H. influenzae serotype b (Hib) and nontypeable strains were investigated using a whole cell ELISA and flow cytometry. IgD binding was detected in 50% of the encapsulated Hib strains examined, whereas nontypeable H. influenzae did not interact with IgD. Finally, mapping experiments using the chimeric IgD/IgG indicated that IgD CH1 aa 198-224 were involved in the interaction between IgD and H. influenzae. Thus, by using recombinant IgD and chimeras with defined Ag specificity, we have confirmed that Hib specifically binds IgD, and that this binding involves the IgD CH1 region.