Characterization of antigens from nontypable Haemophilus influenzae recognized by human bactericidal antibodies. Role of Haemophilus outer membrane proteins

Antigenic characterization of the oligosaccharide portion of the lipooligosaccharide of nontypable Haemophilus influenzae

Monoclonal antibodies (MAbs) directed against epitopes in the oligosaccharide portion of the lipooligosaccharide (LOS) of nontypable Haemophilus influenzae (NTHI) were used to characterize the LOS of this pathogen. Western blot (immunoblot) analysis with four LOS-specific MAbs and proteinase K-derived LOS preparations from 69 NTHI strains allowed the classification of these strains into nine LOS antigenic groups. The use of these MAbs in a more sensitive colony blot radioimmunoassay system together with these same NTHI strains identified 14 LOS antigenic groups. Extensive cross-reactivity was detected between the LOS epitopes of these NTHI strains and the LOS of H. influenzae type b. The epitopes recognized by these MAbs were not accessible to antibody on the surface of every strain. These LOS epitopes were also not stably expressed by NTHI growing in vitro; the observed frequency of LOS antigen variation ranged from 1 to 24% when large numbers of colonies of NTHI strains were screened for reactivity with the LOS-directed MAbs in the colony blot radioimmunoassay. This LOS antigenic variation was sometimes associated with alterations in the profile of the LOS molecule as resolved by dodecyl sulfate-polyacrylamide gradient gel electrophoresis followed by staining with silver. These data indicate that considerable antigenic diversity exists among NTHI strains with regard to the oligosaccharide epitopes in their LOS molecules.

Physical heterogeneity of neisserial lipooligosaccharides reflects oligosaccharides that differ in apparent molecular weight, chemical composition, and antigenic expression

We studied the oligosaccharides (OS) of outer membrane lipooligosaccharides (LOS) of Neisseria gonorrhoeae and Neisseria meningitidis. OS from the LOS of an individual neisserial strain always eluted from Sephadex G-50S as multiple peaks; the polyacrylamide gel elution profiles were nearly identical to the polyacrylamide gel electrophoresis profiles of the sodium dodecyl sulfate-disaggregated native LOS from which the OS were derived. Neisserial OS coeluted with Dex14 to Dex27 dextran oligomers (Mr, 2,210 to 4,320). Monosaccharide composition varied among the several OS released from the LOS of a single strain. The two OS of a gonococcal strain sensitive to normal human serum (NHS) bacteriolysis (sers) varied in their ability to inhibit the binding of NHS immunoglobulin M to their parental LOS. The OS that was rich in hexosamines inhibited NHS immune lysis of its parent strain; the OS that was poor in hexosamines did not. We conclude that structural differences in their OS account for the Mr heterogeneity of the LOS of a strain.

Specificity of antibodies against Neisseria gonorrhoeae that stimulate neutrophil chemotaxis. Role of antibodies directed against lipooligosaccharides

Five strains each of Neisseria gonorrhoeae sensitive or resistant to complement (C) dependent killing by normal human serum (NHS) were examined for their ability to stimulate chemotaxis of polymorphonuclear leukocytes (PMNs) after preincubation with NHS; or IgM or IgG derived from NHS. Serum-sensitive N. gonorrhoeae stimulated C-dependent chemotaxis when opsonized with IgM, but not IgG, however, serum-resistant strains, taken as a whole, failed to promote chemotaxis when opsonized with either isotype. IgM titers in NHS against lipooligosaccharide (LOS) antigens from individual serum-sensitive, but not serum-resistant strains, correlated with the magnitude of chemotaxis generated by the corresponding opsonized strains (r = 0.99). Western blots demonstrated that IgM and IgG from NHS recognized different antigenic determinants on LOS from serum-sensitive gonococci. IgM from NHS immunopurified against serum-sensitive LOS accounted for two-thirds of the chemotaxis promoting activity present in whole serum. IgG titers in NHS against LOS antigens from individual serum-resistant strains also correlated with magnitude of chemotaxis generated by the corresponding opsonized strains (r = 0.87), although most opsonized serum-resistant strains did not generate significantly higher magnitudes of chemotaxis than controls. In contrast, a serum-resistant isolate from a patient with disseminated gonococcal infection (DGI) stimulated chemotaxis when opsonized with IgG obtained from the patient's convalescent serum. By Western blot, convalescent IgG antibody recognized an additional determinant on serum-resistant LOS not seen by normal IgG.

Development of an immunoglobulin A-specific anti-Haemophilus influenzae antibody assay for detection of antibody in human mucosal secretions

A micro-enzyme-linked immunosorbent assay for quantitation of immunoglobulin A-specific anti-Haemophilus influenzae antibody is described and characterized. It had a sensitivity of 27 ng/ml, which is appropriate to detect antibody levels in normal saliva. Specificity for H. influenzae was achieved with the H1H2 group of antigens. Absorption studies for a range of bacteria showed little cross-reactivity, with the exception of Pseudomonas aeruginosa. Absorption studies involving various antigen preparations obtained from H. influenzae indicated that the H1H2 antigen group included significant amounts of surface antigen. An analysis of saliva from normal subjects and patients with chronic obstructive lung disease showed significant differences in levels of antibody, highlighting the potential value of the assay.

Antigenic characterization of the P6 protein of nontypable Haemophilus influenzae

The purpose of this study was to characterize the degree of antigenic heterogeneity or conservation of a 16,600-dalton outer membrane protein (P6) among strains of nontypable Haemophilus influenzae. Immunization of rabbits with P6 isolated from individual strains resulted in antibody to P6 of all 25 strains tested. The titers of antibody in the sera were similar among the strains. Whole organisms of two strains were used to immunize rabbits, and antibodies were produced to P6 of all strains tested. Monoclonal antibodies developed to P6 from mice immunized with whole cells of three different strains recognized determinants on P6 of all 25 strains tested. Finally, pooled normal human serum contained antibodies to P6 of all 25 strains assayed. These studies indicate that P6 is a highly conserved antigen on the outer membrane of nontypable H. influenzae.

Immunoglobulin G antibodies directed against protein III block killing of serum-resistant Neisseria gonorrhoeae by immune serum

Neisseria gonorrhoeae that resist complement-dependent killing by normal human serum (NHS) are sometimes killed by immune convalescent serum from patients recovering from disseminated gonococcal infection (DGI). In these studies, killing by immune serum was prevented or blocked by IgG isolated from NHS. Purified human IgG antibodies directed against gonococcal protein III, an antigenically conserved outer membrane protein, contained most of the blocking activity in IgG. Antibodies specific for gonococcal porin (protein I), the major outer membrane protein, displayed no blocking function. In separate experiments, immune convalescent DGI serum which did not exhibit bactericidal activity was restored to killing by selective depletion of protein III antibodies by immunoabsorption. These studies indicate that protein III antibodies in normal and immune human serum play a role in serum resistance of N. gonorrhoeae.

Identification of a 16,600-dalton outer membrane protein on nontypeable Haemophilus influenzae as a target for human serum bactericidal antibody

A 16,600-D outer membrane protein is present in all strains of Haemophilus influenzae and antibodies to this protein are present in human serum. This study was designed to assess the role of this outer membrane protein (P6) in nontypeable H. influenzae as a target for human serum bactericidal antibody. P6 was isolated and coupled to an affinity column. Depleting normal human serum of antibodies to P6 by affinity chromatography resulted in reduced bactericidal activity of that serum for nontypeable H. influenzae. Immunopurified antibodies to P6 from human serum were bactericidal. Finally, preincubation of bacteria with a monoclonal antibody that recognizes a surface epitope on P6, inhibited human serum bactericidal killing. Taken together, these experiments establish that P6 is a target for human bactericidal antibodies. This observation provides evidence that P6 plays a potentially important role in human immunity to infection by nontypeable H. influenzae.

Protection by serum antibodies in experimental nontypable Haemophilus influenzae otitis media

The chinchilla experimental model of otitis media was used to examine the importance of serum antibodies in protection against disease caused by nontypable Haemophilus influenzae. An immune serum pool was prepared by immunizing chinchillas with killed bacterial cells of nontypable H. influenzae 3245. Pooled preimmune or immune serum from these immunized animals was administered intravenously to a group of nonimmune chinchillas 1 day before intrabullar challenge with strain 3245. Of 5 animals receiving preimmune serum, 5 developed otitis media compared with 0 of 10 animals receiving immune serum (P = 0.008). The immune serum pool contained antibodies directed against both surface-exposed outer membrane proteins and lipopolysaccharide (LPS). The 39-kilodalton major outer membrane protein was the immunodominant surface protein. Anti-LPS antibodies were removed from the immune serum pool by affinity chromatography, and affinity-purified anti-LPS antibodies were recovered. Immune serum, immune serum absorbed of LPS antibodies, or affinity-purified LPS antibodies were then administered to another group of experimental animals 1 day before bacterial challenge. Of four animals that received the affinity-purified LPS antibodies, four developed otitis compared with zero of four animals that received the immune serum or zero of four animals that received the LPS-absorbed immune serum (P = 0.028). These studies indicate that passive immunization with immune serum is protective in experimental nontypable H. influenzae otitis media and that bacterial outer membrane proteins may be the principal targets of protective antibody.

Antigenic heterogeneity of outer membrane proteins of nontypable Haemophilus influenzae is a basis for a serotyping system

A serotyping system for nontypable Haemophilus influenzae (NTHI) was developed by using isolated outer membrane protein (OMP) preparations and rabbit antisera. OMPs of 23 strains were isolated by molecular sieve chromatography of outer membranes in 1.5% sodium deoxycholate buffer. These OMP preparations were relatively free of lipopolysaccharide as determined by silver staining of sodium dodecyl sulfate gels and by dot assay with a monoclonal antibody which is specific for the lipid A of H. influenzae. Three antisera raised to whole organisms were used to serotype 21 of 23 strains with a kinetic enzyme-linked immunosorbent assay. Digestion of OMP preparations with proteinase K removed greater than 90% of the antigenic reactivity, indicating that the system is based on OMP antigens. Marked antigenic heterogeneity of OMPs exists among strains of NTHI. By determining the pattern of serological reactivity of OMPs with the three antisera, isolates were divided into groups based on antigenic differences. Six serotypes were identified. This OMP serotyping system is based on multiple antigenic determinants. Future studies will focus on identifying serotype-specific epitopes to further refine this serological classification scheme for NTHI.