Epitope mapping of polyclonal clotting factor VIII-inhibitory antibodies using phage display

Predicting inhibitor development using a random peptide phage-display library approach in the SIPPET cohort

ABSTRACT

Inhibitor development is the most severe complication of hemophilia A (HA) care and is associated with increased morbidity and mortality. This study aimed to use a novel immunoglobulin G epitope mapping method to explore the factor VIII (FVIII)-specific epitope profile in the SIPPET cohort population and to develop an epitope mapping-based inhibitor prediction model. The population consisted of 122 previously untreated patients with severe HA who were followed up for 50 days of exposure to FVIII or 3 years, whichever occurred first. Sampling was performed before FVIII treatment and at the end of the follow-up. The outcome was inhibitor development. The FVIII epitope repertoire was assessed by means of a novel random peptide phage-display assay. A least absolute shrinkage and selection operator (LASSO) regression model and a random forest model were fitted on posttreatment sample data and validated in pretreatment sample data. The predictive performance of these models was assessed by the C-statistic and a calibration plot. We identified 27 775 peptides putatively directed against FVIII, which were used as input for the statistical models. The C-statistic of the LASSO and random forest models were good at 0.78 (95% confidence interval [CI], 0.69-0.86) and 0.80 (95% CI, 0.72-0.89). Model calibration of both models was moderately good. Two statistical models, developed on data from a novel random peptide phage display assay, were used to predict inhibitor development before exposure to exogenous FVIII. These models can be used to set up diagnostic tests that predict the risk of inhibitor development before starting treatment with FVIII.

Epitope mapping via selection of anti-FVIII antibody-specific phagepresented peptide ligands that mimic the antibody binding sites

The most serious complication in today’s treatment of congenital haemophilia A is the development of neutralising antibodies (inhibitors) against factor VIII (FVIII). Although FVIII inhibitors can be eliminated by immune tolerance induction (ITI) based on repeated administration of high doses of FVIII, 20–30% of patients fail to become tolerant. Persistence of FVIII inhibitors is associated with increased morbidity and mortality. Data from recent studies provide evidence for a potential association between ITI outcome and epitope specificity of FVIII inhibitors. Nevertheless the determination of epitopes and their clinical relevance has not yet been established. In this study a general strategy for the identification of anti-FVIII antibody epitopes in haemophilia A patient plasma was to be demonstrated. Phage-displayed peptide libraries were screened against anti-FVIII antibodies to isolate specific peptides. Peptide specificity was confirmed by FVIII-sensitive ELISA binding. Peptide residues essential for antibody binding were identified by mutational analysis and epitopes were predicted via FVIII homology search. The proposed mapping strategy was validated for the monoclonal murine antibody (mAb) 2–76. Binding studies with FVIII variants confirmed the location of the predicted epitope at the level of individual amino acids. In addition, anti-FVIII antibody-specific peptide ligands were selected for 10 haemophilia A patients with FVIII inhibitors. Detailed epitope mapping for three of them showed binding sites on the A2, A3 and C2 domains. Precise epitope mapping of anti-FVIII antibodies using antibody-specific peptide ligands can be a useful approach to identify antigenic sites on FVIII.

Correlation between antigenicity and variability in the vls antigenic variation system of Borrelia burgdorferi

Many parasites have evolved antigenic variation systems that alter surface proteins in order to evade recognition by presently expressed antibodies and subsequent death. Although the amino acid positions in antigens to which antibodies most commonly target are expected to be the most variable, this assumption has not been investigated. Using the vls antigenic variation system of Borrelia burgdorferi as a model, we first investigated this assumption computationally and then developed a sensitive immunoassay to experimentally validate the computational results. There was a strong correlation between variability at an amino acid position and each of the computational metrics associated with antibody reactivity. However, empirical measures of antibody reactivity were not consistently greater at the variable amino acid positions than at the invariant amino acid positions. The inconsistent experimental support for this hypothesis suggests that the biological effect of variability at an amino acid position is obfuscated by other factors.

A possible mechanism for Inv22-related F8 large deletions in severe hemophilia A patients with high responding factor VIII inhibitors

BACKGROUND

Intron 22 inversion (Inv22) of the coagulation factor (F)VIII gene (F8) is a frequent cause of severe hemophilia A. In addition to Inv22, a variety of F8 mutations (1492 unique mutations) causing hemophilia A have been reported, of which 171 involve deletions of over 50 bp (HAMSTeRs database; http://hadb.org.uk/). However, only 10% of these large deletions have been fully characterized at the nucleotide level.

PATIENTS AND METHODS

We investigated gene abnormalities in three unrelated severe hemophilia A patients with high titer FVIII inhibitors. They had previously been shown to carry large deletions of the F8, but the precise gene abnormalities remain to be elucidated.

RESULTS

Inverse shifting-PCR (IS-PCR) Inv22 diagnostic tests revealed that these patients carried either type I or II Inv22. However, they showed a wild-type (WT) pattern in the IS-PCR Inv22 complementary tests. We further analyzed their X chromosomes to account for the puzzling results, and found that they had different centromeric breakpoints in the Inv22 X chromosomes, adjacent to the palindromic regions containing int22h-2 or -3, and their spacer region, respectively. The connections appeared to be shifted towards the telomere of the WT F8 Xq28, resulting in a new telomere with an additional intact int22h copy.

CONCLUSIONS

These gene rearrangements might result from double-strand breaks in the most distal regions of the long arms of the Inv22 X chromosomes, followed by DNA restorations using the WT F8 Xq28 by non-homologous end joining or break-induced replication; thus leading to large F8 deletions in severe hemophilia A patients.

Multimerization of peptide mimotopes for blocking of factor VIII neutralizing antibodies

About 30 % of patients with severe hemophilia A develop neutralizing antibodies (inhibitors) to coagulation factor VIII (FVIII) upon treatment with exogenous factor preparations. Two peptides, C6 (NPVENMMDRDSQ) and H10 (QSPWQTWFTRAL), that mimic putative inhibitor epitopes (mimotopes), were previously selected by phage display screening of plasma samples from patients with inhibitors. Synthetic peptide mimotopes inhibited IgG binding to FVIII (IC(50): 30-50 microM). This effect was increased by an equimolar combination of both mimotopes. Mimotopes were fused to the C-terminal multimerization domain of the C4bp alpha-chain and expressed as multimers in 293T cells. Multimerized mimotopes showed improved binding to anti-FVIII IgG and prolonged in vitro half-life relative to synthetic peptides. The two mimotopes were combined in heteromultimers by co-transfection of 293T cells with respective vectors, resulting in bi-specific molecules that almost completely blocked polyclonal antibody binding to FVIII (IC(50): 2-3 microM). This strategy is capable of functionally improving synthetic peptides by multimerization and could provide a basis for novel therapeutic approaches for patients with hemophilia A and inhibitors.

Purification of polyclonal anti-conformational antibodies for use in affinity selection from random peptide phage display libraries: a study using the hydatid vaccine EG95

The use of polyclonal antibodies to screen random peptide phage display libraries often results in the recognition of a large number of peptides that mimic linear epitopes on various proteins. There appears to be a bias in the use of this technology toward the selection of peptides that mimic linear epitopes. In many circumstances the correct folding of a protein immunogen is required for conferring protection. The use of random peptide phage display libraries to identify peptide mimics of conformational epitopes in these cases requires a strategy for overcoming this bias. Conformational epitopes on the hydatid vaccine EG95 have been shown to result in protective immunity in sheep, whereas linear epitopes are not protective. In this paper we describe a strategy that results in the purification of polyclonal antibodies directed against conformational epitopes while eliminating antibodies directed against linear epitopes. These affinity purified antibodies were then used to select a peptide from a random peptide phage display library that has the capacity to mimic conformational epitopes on EG95. This peptide was subsequently used to affinity purify monospecific antibodies against EG95.

Screening of ACE-inhibitory peptides from a random peptide-displayed phage library using ACE-coupled liposomes

Angiotensin I converting enzyme (ACE)-inhibitory peptides were screened from a random peptide-displayed phage library using ACE-coupled liposomes. Among four kinds of inhibitory peptides selected by biopanning with two different elution strategies, a peptide (LSTLRSFCA) showed the highest inhibitory activity with an IC(50) value of 3microM. By measuring inhibitory activities of fragments of the peptide, it was found that the RSFCA region was a functional site to inhibit strongly the ACE catalytic activity, and particularly both Arg and Cys residues were essential for the strong inhibitory activity. The inhibitory activity of RRFCA was slightly increased, while that of the RSFRA, in which the Cys residue was replaced by Arg, was decreased to greater extent in comparison with the inhibitory activity of RSFCA. Taking into account the results obtained from the SPOT analysis, it was suggested that the Arg and Phe residues in RSFCA were important for a specific interaction with ACE, and the Cys residue inhibited the ACE activity. The cystein-based ACE-inhibitory peptides have not been isolated from processed food materials. These findings suggested that the biopanning method utilizing protein-coupled liposomes and random peptide libraries might have a possibility to screen new functional peptides that are not found in processed food materials.

The protein structure and effect of factor VIII

Factor VIII (FVIII) is a key component of the fluid phase of the blood coagulation system. The proteases efficiently cleave FVIII at three sites, two within the heavy and one within the light chain resulting in alteration of its covalent structure and conformation and yielding the active cofactor, FVIIIa. FVIIIa is a trimer composed of A1, A2 and A3-C1-C2 subunits. The role of FVIIIa is to markedly increase the catalytic efficiency of factor IXa in the activation of factor X. Variants of these factors frequently also lead to severe bleeding disorders.

The most polymorphic residue on Plasmodium falciparum apical membrane antigen 1 determines binding of an invasion-inhibitory antibody

Apical membrane antigen 1 (AMA1) is currently one of the leading malarial vaccine candidates. Anti-AMA1 antibodies can inhibit the invasion of erythrocytes by Plasmodium merozoites and prevent the multiplication of blood-stage parasites. Here we describe an anti-AMA1 monoclonal antibody (MAb 1F9) that inhibits the invasion of Plasmodium falciparum parasites in vitro. We show that both reactivity of MAb 1F9 with AMA1 and MAb 1F9-mediated invasion inhibition were strain specific. Site-directed mutagenesis of a fragment of AMA1 displayed on M13 bacteriophage identified a single polymorphic residue in domain I of AMA1 that is critical for MAb 1F9 binding. The identities of all other polymorphic residues investigated in this domain had little effect on the binding of the antibody. Examination of the P. falciparum AMA1 crystal structure localized this residue to a surface-exposed alpha-helix at the apex of the polypeptide. This description of a polymorphic inhibitory epitope on AMA1 adds supporting evidence to the hypothesis that immune pressure is responsible for the polymorphisms seen in this molecule.

Identification of the insulin-like growth factor II receptor as a novel receptor for binding and invasion by Listeria monocytogenes

The gram-positive bacterium Listeria monocytogenes causes a life-threatening disease known as listeriosis. The mechanism by which L. monocytogenes invades mammalian cells is not fully understood, but the processes involved may provide targets to prevent and treat listeriosis. Here, for the first time, we have identified the insulin-like growth factor II receptor (IGFIIR; also known as the cation-independent mannose 6-phosphate receptor (CI)M6PR or CD222) as a novel receptor for binding and invasion of Listeria species. Random peptide phage display was employed to select a peptide sequence by panning with immobilized L. monocytogenes cells; this peptide sequence corresponds to a sequence within the mannose 6-phosphate binding site of the IGFIIR. All Listeria spp. specifically bound the labeled peptide but not a control peptide, which was demonstrated using fluorescence spectrophotometry and fluorescence-activated cell sorting. Further evidence for binding of the receptor by L. monocytogenes and L. innocua was provided by affinity purification of the bovine IGFIIR from fetal calf serum by use of magnetic beads coated with cell preparations of Listeria spp. as affinity matrices. Adherence to and invasion of mammalian cells by L. monocytogenes was significantly inhibited by both the synthetic peptide and mannose 6-phosphate but not by appropriate controls. These observations indicate a role for the IGFIIR in the adherence and invasion of L. monocytogenes of mammalian cells, perhaps in combination with known mechanisms. Ligation of IGFIIR by L. monocytogenes may be a novel mechanism that contributes to the regulation of infectivity, possibly in combination with other mechanisms.

Mapping of FVIII inhibitor epitopes using cellulose-bound synthetic peptide arrays

Epitope mapping using antibodies against factor VIII (FVIII) has been performed using blotting techniques with truncated and/or digested FVIII molecules. Here, we focused on the precise mapping of affinity purified IgG from patients with an immune response against blood clotting FVIII using synthetic peptide arrays on cellulose membranes comprising the entire sequence of FVIII. The aim was to elucidate the epitope profile from different inhibitors and possibly detect new epitopes, which have not been described before. The epitope patterns from five patients showed reactivity with all domains in the FVIII molecule, but were different between various patients. These results included epitopes usually buried within the folded protein. However, in competition assays using FVIII as competitive agent in a mixture with inhibitor IgG, the most immunogenic regions were located in the FVIII light chain. Our results show that the C1 domain was the region with highest immunogenicity in all patients. Here, we demonstrate that the SPOT method is very well suited for the precise location of epitopes in the core of the protein, which usually cannot be detected by other methods.

Peptide inhibitor of pancreatic lipase selected by phage display using different elution strategies

Interference with fat hydrolysis results in the reduced use of ingested lipids. Inhibition of pancreatic lipase reduces the efficiency of fat absorption in the small intestine and thereby initiates modest long-term reduction in body weight. In an attempt to select peptides with affinity for the surface of pancreatic lipase and potential inhibitory activity, a random, cyclic heptapeptide phage-displayed library was used. Five independent selections, differing in elution step, were performed. In three selection protocols, a sequential elution strategy was applied in anticipation of improving the selection of high-affinity clones. Four heptapeptides with the highest affinity, seemingly for pancreatic lipase, were selected, synthesized, and characterized for their capacity to inhibit enzyme function. Although no clear consensus among the sequenced peptides was found, one of the selected peptides inhibited pancreatic lipase with an apparent inhibition constant of 16 muM.