Variable region sequences and idiotypic expression of a protective human immunoglobulin M antibody to capsular polysaccharides of Neisseria meningitidis group B and Escherichia coli K1

Monoclonal antibodies to meningococcal factor H binding protein with overlapping epitopes and discordant functional activity

Background

Meningococcal factor H binding protein (fHbp) is a promising vaccine candidate. Anti-fHbp antibodies can bind to meningococci and elicit complement-mediated bactericidal activity directly. The antibodies also can block binding of the human complement down-regulator, factor H (fH). Without bound fH, the organism would be expected to have increased susceptibility to bacteriolysis. Here we describe bactericidal activity of two anti-fHbp mAbs with overlapping epitopes in relation to their different effects on fH binding and bactericidal activity.

Methods and Principal Findings

Both mAbs recognized prevalent fHbp sequence variants in variant group 1. Using yeast display and site-specific mutagenesis, binding of one of the mAbs (JAR 1, IgG3) to fHbp was eliminated by a single amino acid substitution, R204A, and was decreased by K143A but not by R204H or D142A. The JAR 1 epitope overlapped that of previously described mAb (mAb502, IgG2a) whose binding to fHbp was eliminated by R204A or R204H substitutions, and was decreased by D142A but not by K143A. Although JAR 1 and mAb502 appeared to have overlapping epitopes, only JAR 1 inhibited binding of fH to fHbp and had human complement-mediated bactericidal activity. mAb502 enhanced fH binding and lacked human complement-mediated bactericidal activity. To control for confounding effects of different mouse IgG subclasses on complement activation, we created chimeric mAbs in which the mouse mAb502 or JAR 1 paratopes were paired with human IgG1 constant regions. While both chimeric mAbs showed similar binding to fHbp, only JAR 1, which inhibited fH binding, had human complement-mediated bactericidal activity.

Conclusions

The lack of human complement-mediated bactericidal activity by anti-fHbp mAb502 appeared to result from an inability to inhibit binding of fH. These results underscore the importance of inhibition of fH binding for anti-fHbp mAb bactericidal activity.

Complement-dependent synergistic bactericidal activity of antibodies against factor H-binding protein, a sparsely distributed meningococcal vaccine antigen

BACKGROUND

Antibodies to factor H (fH)-binding protein (fHBP), a meningococcal vaccine antigen, activate classical complement pathway serum bactericidal activity (SBA) and block binding of the complement inhibitor fH.

METHODS

To understand these 2 functions in protection, we investigated the interactions of human complement and 2 anti-fHBP monoclonal antibodies (MAbs) with encapsulated Neisseria meningitidis.

RESULTS

JAR 3 (IgG3) blocks fH binding and elicits SBA against 2 strains with naturally high fHBP expression and a low-expressing strain genetically engineered to express high fHBP levels. JAR 4 (IgG2a) does not block fH binding or elicit SBA. Neither MAb alone elicits SBA against 2 other strains with low fHBP expression, but together the MAbs increase C4b binding and elicit SBA; JAR 3 alone also is bactericidal in whole blood. In nonimmune blood, fHBP knockout mutants from high-expressing stains do not survive, but mutants of low-expressing strains do.

CONCLUSIONS

Expression of fHBP is a prerequisite for bacterial survival in blood only by strains with naturally high fHBP expression. In low-expressing strains, combinations of 2 nonbactericidal anti-fHBP MAbs can bind to nonoverlapping epitopes, engage C1q, activate C4, and mediate classical complement pathway SBA. In the absence of sufficient C4b binding for SBA, an individual MAb can have opsonophagocytic bactericidal activity.

A rAb screening method for improving the probability of identifying peptide mimotopes of carbohydrate antigens

Peptide mimotopes have been investigated as surrogate antigens of carbohydrate (CHO) targets on pathogen and tumor cells in vaccine and therapeutic discovery. One of the main bottlenecks in peptide mimotope discovery is the inability of initial screening regimes to differentiate between true mimotopes and non-mimotopes. As a result, subsequent in vivo analysis of putative peptide mimotopes is often inefficient requiring the use of experimental animals during a lengthy in vivo immunization process. Here, we demonstrate a rapid preliminary screening method to identify putative mimotopes using a recombinant antibody (rAb) library, which may increase the probability of identifying peptides that will elicit a CHO-cross-reactive response in vivo. A human naïve rAb library was screened against both an established peptide mimotope and a non-mimotope of the Group B Streptococcus (GBS) type III polysaccharide to determine if selected antibodies cross-reacted with the original GBS polysaccharide. We were able to differentiate between these two peptides because peptide-binding Abs that cross-reacted to GBS was isolated only with the peptide mimotope. We discuss the feasibility of using this method to significantly increase the breadth of screening and reduce the discovery time for peptide mimotopes.

Molecular analysis of anti-N-propionyl Neisseria meningitidis group B polysaccharide monoclonal antibodies

The capsular polysaccharide of Neisseria meningitidis group B (MBPS) is a polymer of alpha (2-->8) N-acetyl neuraminic acid, which is chemically identical to polysialic acid (PSA) expressed in human tissues. Antibodies from mice immunized with a MBPS-protein conjugate vaccine in which N-acetyl groups have been replaced by propionyl groups (N-Pr MBPS) can be bactericidal and show minimal or no cross-reactivity with human PSA. To investigate the molecular basis for antigen recognition, we cloned and sequenced the variable region (V) genes of five bactericidal anti-N-Pr MBPS murine mAbs and produced computer models of the combining sites. The results were compared to those reported in the literature for two autoreactive anti-MBPS. The V region genes of the anti-N-Pr MBPS mAbs and the anti-MBPS autoreactive mAbs are derived from a limited set of germline V, J, and D genes. However, the anti-N-Pr MBPS mAbs are more mutated than the anti-MBPS mAbs and the former use V-D-J editing that introduces arginine in H-CDR3. Models of the respective combining sites indicate that the anti-MBPS or anti-N-Pr MBPS mAbs that react with host PSA have relatively wide and shallow grooves with a high overall positive charge, consistent with recognition of extended helical polysaccharide structures recognized by the autoreactive mAbs. In contrast, anti-N-Pr MBPS mAbs that do not react with host PSA contain pockets and deep clefts that are consistent with recognition of discrete structural features of individual residues.

Epitopes recognized by a nonautoreactive murine anti-N-propionyl meningococcal group B polysaccharide monoclonal antibody

The capsular polysaccharide of Neisseria meningitidis group B (MBPS) is a polymer of alpha (2-->8) N-acetyl neuraminic acid. The polysaccharide is chemically identical to an autoantigen, polysialic acid (PSA), and is a poor immunogen, even when conjugated to protein carriers. Immunization of mice with MBPS-protein conjugate vaccines, in which N-acetyl groups have been replaced by propionyl groups (N-Pr MBPS), elicits serum bactericidal antibodies. A subpopulation of these antibodies do not cross-react with human PSA. The reasons for the increased immunogenicity of N-Pr MBPS and the antigenic targets of the bactericidal nonautoreactive antibodies are unknown. In this study, we investigated the antigenic targets of a protective murine monoclonal antibody (MAb) prepared against a N-Pr MBPS-tetanus toxoid conjugate vaccine. Binding of the MAb to N-Pr MBPS (as demonstrated by an enzyme-linked immunosorbent assay) and bactericidal activity were inhibited by de-N-acetylated MBPS and re-N-acetylated MBPS, which indicate that N-propionyl groups are not obligatory determinants for binding. The results of affinity selection from a preparation of N-Pr MBPS and matrix-assisted laser desorption ionization-time of flight mass spectroscopic analysis indicated that the minimal epitope recognized by the MAb is a MBPS disaccharide containing one de-N-acetylated residue. Thus, the bacterial capsular epitope recognized by this bactericidal, nonautoreactive, anti-group-B MAb likely contains de-N-acetyl residues.

Preferential use of lambda light chains is associated with defective mouse antibody responses to the capsular polysaccharide of Neisseria meningitidis group B

The capsular polysaccharide of Neisseria meningitidis group B (CpsB) is a very poor immunogen in mammals; this has been considered to be due to the induction of tolerance to cross-reactive host glycoconjugates. It has hampered the development of an effective vaccine against this meningococcal group for many years. Syngeneic populations have a similar tolerogenic background. Thus, we used the variability in ability to mount CpsB-specific immunoglobulin (Ig) responses of individuals from these populations to reveal underlying mechanisms to tolerance contributing to the poor immunogenicity of CpsB. Here we analyze by ELISA, the individual CpsB-specific Ig response of BALB/c and other syngeneic mice to immunization with intact bacteria, using the distribution of light chains as a direct indicator of the repertoire dynamics of the response. Although approximately 96% of anti-CpsB Ig bear kappa-light chains, BALB/c mouse populations were heterogeneous in the light chain composition of their individual anti-CpsB Ig responses. The proportion of kappa and lambda-light chains used for anti-CpsB Ig was a private characteristic that remained relatively constant, for each individual, through repetitive immunizations regardless of the bacterial stimuli size. Despite the prevalence of individual use of kappa-light chains, 5% of BALB/c mice showed restricted usage of lambda-light chains in their CpsB-specific Ig responses, and an additional 11% use them significantly. The preferential use of lambda-light chains in these mice was strongly associated with defective IgM, and absent or barely detectable IgG anti-CpsB responses even after repetitive bacterial immunization. We conclude that differences in the private repertoire of specific Ig also contribute to mouse unresponsiveness to CpsB.

Molecular techniques for the investigation of meningococcal disease epidemiology

Meningococcal disease remains a major cause of childhood morbidity and mortality world wide and no comprehensive vaccine is available against the causative organism, Neisseria meningitidis. Molecular studies of the diversity of this bacterium have provided a number of key insights into its biology, which have implications for control of meningococcal disease. These have included the identification of hyperinvasive lineages and the correlation of genetic type with antigenic type and disease epidemiology. In practical terms, such studies have enabled the application of DNA-based technologies in the development of improved methods for diagnosis and epidemiological monitoring. These data are of especial importance with the current, and ongoing, development and introduction of new meningococcal vaccines.

Molecular mimetics of polysaccharide epitopes as vaccine candidates for prevention of Neisseria meningitidis serogroup B disease

Neisseria meningitidis is a major cause of meningitis and sepsis. Despite nearly 25 years of work, there is no promising vaccine candidate for prevention of disease caused by meningococcal B strains. This review summarizes newer approaches for eliciting protective meningococcal B immune responses, including the use of molecular mimetics of group B polysaccharide and conserved membrane proteins as immunogens. The capsular polysaccharide of this organism is conserved and serum antibody to this capsule confers protection against disease. However, the immunogenicity of meningococcal B polysaccharide-based vaccines is poor. Further, a portion of the antibody elicited has autoantibody activity. Recently, our laboratory produced a panel of murine monoclonal antibodies (Mabs) that react specifically with capsular polysaccharide epitopes on meningococcal B that are distinct from host polysialic acid. These Mabs elicit complement-mediated bactericidal activity and confer passive protection in animal models. The anti-capsular Mabs were used to identify molecular mimetics from phage display peptide libraries. The resulting peptides were antigenic mimetics as defined by binding to the Mabs used to select them but, to date, are poor immunogenic mimetics in failing to elicit anti-capsular antibodies.

Bactericidal Monoclonal Antibodies That Define Unique Meningococcal B Polysaccharide Epitopes That Do Not Cross-React with Human Polysialic Acid

The poor immunogenicity of the Neisseria meningitidis group B polysaccharide capsule, a homopolymer of α(2→8) sialic acid, has been attributed to immunologic tolerance induced by prenatal exposure to host polysialyated glycoproteins. Substitution of N-propionyl (N-Pr) for N-acetyl groups on the meningococcal B polysaccharide, and conjugation of the resulting polysaccharide to a protein carrier, have been reported to yield a conjugate vaccine that elicits protective Abs with minimal autoantibody activity. To characterize the protective epitopes on the derivatized polysaccharide, we isolated 30 anti-N-Pr meningococcal B polysaccharide mAbs. These Abs were heterogeneous with respect to complement-mediated bactericidal activity, fine antigenic specificity, and autoantibody activity as defined by binding to the neuroblastoma cell line, CHP-134, which expresses long-chain α(2→8)-linked polysialic acid. Eighteen of the Abs could activate complement-mediated bacteriolysis. Seven of these 18 Abs cross-reacted with N-acetyl meningococcal B polysaccharide by ELISA and had strong autoantibody activity. Thus, N-Pr meningococcal B polysaccharide conjugate vaccine has the potential to elicit autoantibodies. However, 7 of the 18 bactericidal mAbs had no detectable autoantibody activity. These Abs may be useful for the identification of molecular mimetics capable of eliciting protective Abs specific to the bacteria, without the risk of evoking autoimmune disease.

Peptide epitopes recognized by a human anti-cryptococcal glucuronoxylomannan antibody

Cryptococcus neoformans causes meningitis in 6 to 8% of individuals with AIDS. Recently, immunotherapeutic modalities including antibody therapy have been proposed for the treatment of cryptococcal meningitis in AIDS patients. This is a rational approach because existing antifungal agents fail to eradicate the infection in the setting of profound immunosuppression. Both murine and human antibodies elicited by the investigational cryptococcal capsular polysaccharide vaccine glucuronoxylomannan-tetanus toxoid (GXM-TT) have been shown to be biologically functional in different model systems. The human immunoglobulin M (lambda) GXM monoclonal antibody (MAb) 2E9 expresses idiotypes that are also found in naturally occurring anti-GXM antibodies and opsonic GXM-TT sera. However, the specificity of human anti-GXM antibodies and their possible role in protection against cryptococcosis are not known. In an effort to discover epitopes that are recognized by human anti-GXM antibodies, we screened a random decapeptide phage display library with the human anti-GXM MAb 2E9. An enzyme-linked immunosorbent assay (ELISA)-based screening method led to the selection of phages with peptide inserts that bound 2E9 and inhibited 2E9-GXM binding. Analysis of the amino acid sequences of these phages revealed an increased frequency of combinations of QTGLD residues. Inhibition ELISAs demonstrated that phages with QTG/TL/D motifs inhibited 2E9-GXM binding better than phages with different motifs. A peptide synthesized from one of the inhibitory phages, peptide 13 (GMDGT QLDRW), inhibited GXM binding to solid-phase 2E9 and 2E9 binding to solid-phase GXM. Peptide 13 also inhibited the GXM binding of GXM-TT immune sera and naturally occurring serum antibodies from human immunodeficiency virus (HIV)-negative, but not HIV-positive, individuals. Taken together, our data indicate that the peptide epitopes selected by 2E9 mimic GXM epitopes and that peptide 13 may be a mimotope of a GXM epitope that is recognized by human anti-GXM antibodies.

Expression of sialic acid and polysialic acid in serogroup B Neisseria meningitidis: divergent transcription of biosynthesis and transport operons through a common promoter region

We studied capsule-defective (Cap-) serogroup B meningococcal mutants created through Tn916 or omega-fragment mutagenesis. The Cap- phenotypes were the results of insertions in three of four linked genes (synX, synC, and synD) involved in CMP-N-acetylneuraminic acid and polysialic acid capsule biosynthesis, and in ctrA the first of four linked genes involved in capsule membrane transport. Mutations in the CMP-N-acetylneuraminic acid biosynthesis genes synX and synC caused defects in lipooligosaccharide sialylation but not mutations in the putative (alpha2 -> 8)-linked polysialyltransferase (synD) or in ctrA. Reverse transcriptase PCR studies indicated that the four biosynthesis genes (synX to -D) and the capsule transport genes (ctr to -D) were separately transcribed as operons. The operons were separated by a 134-bp intergenic region. Primer extension of synX and ctrA demonstrated that transcription of the operons was divergently initiated from adjacent start sites present in the intergenic region. Both transcriptional start sites were preceded by a perfect -10 Pribnow promoter binding region. The synX to -D, but not the ctrA to -D, transcriptional start site was preceded by a sequence bearing strong homology to the consensus sigma 70 -35 promoter binding sequence. Both promoters showed transcriptional activity when cloned behind a lacZ reporter gene in Escherichia coli. Our results confirm the intrinsic relationship between polysialic acid capsule biosynthesis and lipooligosaccharide sialylation pathways in group B Neisseria meningitidis. Our study also suggests that the intergenic region separating the synX to -D and ctrA to -D operons is an important control point for the regulation of group B capsule expression through coordinated transcriptional regulation of the synX to -D and drA to -D promoters.

Antibody responses to the capsular polysaccharide of Neisseria meningitidis serogroup B in patients with meningococcal disease

We measured antibody responses to meningococcal serogroup B (MenB) polysaccharide (PS) by enzyme-linked immunosorbent assay (ELISA) in sera from 94 patients from The Netherlands with disease caused by Neisseria meningitidis group B. The patients ranged in age from 3 to 73 years (mean age, 18.8 years). In initial studies we showed that the binding of a panel of MenB PS-reactive human immunoglobulin M (IgM) paraproteins to biotinylated MenB PS bound to avidin-coated microtiter wells was inhibited > 90% by the addition of soluble MenB PS or encapsulated group B meningococci. In contrast, inhibition of IgM anti-MenB PS antibody-binding activity in many of the patient sera was less than 50% (range, 20 to 94%). These data suggested a high frequency of nonspecific binding in the patient sera. Therefore, all serum samples were assayed in replicate in the presence or absence of soluble MenB PS, and only the inhibitable fraction of the binding signal was used to calculate the anti-MenB PS antibody concentrations. In 17 control patients with meningococcal disease caused by serogroup A or C strains, there was no significant difference in the respective IgM or IgG anti-MenB PS antibody concentrations in paired acute- and convalescent-phase sera. In contrast, in patients with MenB disease, the geometric mean IgM anti-MenB PS antibody concentration increased from 3.9 units/ml in acute-phase serum to 10.5 units/ml in convalescent-phase serum (P < 0.001). The corresponding geometric mean IgG anti-MenB PS antibody titers were 1:27 and 1:36 (P < 0.05). There was only a weak relationship between age and the magnitude of the logarithm of the antibody concentrations in convalescent-phase sera (for IgM, r2 = 0.06 and P < 0.05; for IgG, r2 = 0.08 and P < 0.01). Our data indicate that precautions are needed to avoid nonspecificity in measuring serum antibody responses to MenB PS by ELISA. Furthermore, although this PS is thought to be a poor immunogen, patients as young as 3 years of age recovering from MenB disease demonstrate both ImG and IgG antibody responses in serum.

Human immunoglobulin M paraproteins cross-reactive with Neisseria meningitidis group B polysaccharide and fetal brain

Three hundred fifty-nine serum samples from patients with immunoglobulin M (IgM) or IgG monoclonal gammopathies were tested for binding to the capsular polysaccharide (PS) of Neisseria meningitidis group B (MenB PS, poly-alpha[2-->8]-N-acetylneuraminic acid). Of 159 IgM paraproteins, 7 (4.4%) were positive, compared with 0 of 200 IgG paraproteins (P < 0.05). Since MenB PS reactivity was limited to the IgM paraproteins, the 159 IgM paraproteins were tested by enzyme-linked immunosorbent assay (ELISA) for reactivity with seven other bacterial PSs. None reacted with meningococcal A or C, Haemophilus influenzae type b, or Streptococcus pneumoniae type 3, 6, 14, or 23 PS. The specificity of the MenB PS-reactive antibodies was confirmed by demonstration of binding to N. meningitidis group B cells but not to a capsular PS-deficient mutant and by specific inhibition of binding to solid-phase MenB PS by soluble MenB PS in an ELISA. Five of five antibodies tested protected infant rats from bacteremia caused by Escherichia coli K1, an organism with a PS capsule that also is composed of poly-alpha[2-->8]-N-acetylneuraminic acid. Each of the seven MenB PS-reactive paraproteins had autoantibody activity as defined by binding to homogenates of calf brain in a radioimmunoassay. For six of the seven antibodies, binding to calf brain was inhibited by the addition of soluble MenB PS. Thus, approximately 4% of human IgM paraproteins have autoantibody activity to poly-alpha[2-->8]-N-acetylneuraminic acid, an antigen expressed in fetal brain and cross-reactive with the MenB capsular PS. The reason for this skewing of the IgM paraprotein repertoire toward reactivity with poly-alpha[2-->8]-N-acetylneuraminic acid antigenic determinants is unknown.