Reactivity profile of anti-factor VIII antibodies with designed synthetic peptides mimicking epitopes of the C2 and a1 domains

In Silico Study of Correlation between Missense Variations of F8 Gene and Inhibitor Formation in Severe Hemophilia A

Objective:

Deleterious substitutions of the F8 gene are responsible for causing hemophilia A, which is an inherited bleeding disorder resulting from reduced or absent activity of the coagulant protein factor VIII (FVIII). The most important complication in treatment is inhibitor development toward therapeutic factor VIII. In this study, we aimed to analyze the effects of deleterious substitutions in the F8 gene upon protein structure and function.

Materials and Methods:

All tests were conducted by computational methods from the CHAMP (CDC Hemophilia A Mutation Project) database. We performed an in silico analysis of deleterious variations using five software programs, Sift, PolyPhen-2, Align-GVGD, KD4v, and MutationTaster, in order to analyze the correlation between variation and the disease. We also studied the correlation between these variations and inhibitor formation.

Results:

Our analysis showed that these in silico tools are coherent and that there are more variations in the A than the C domains. Moreover, we noticed that there are more deleterious variations than neutral variations in each of the A and C domains. We also found that 13.51% of the patients suffered from a severe form of hemophilia A and that carriers of missense variations developed inhibitors. Also, for the first time, we determined that variation nature is not associated with inhibitor formation. Furthermore, this analysis showed that the risk of developing inhibitors increases when the variation causes a change of amino acid class.

Conclusion:

This study will help to correctly associate variations with inhibitor development and aid in early characterization of novel variants.

Benchmarking the PEPOP methods for mimicking discontinuous epitopes

Background

Computational methods provide approaches to identify epitopes in protein Ags to help characterizing potential biomarkers identified by high-throughput genomic or proteomic experiments. PEPOP version 1.0 was developed as an antigenic or immunogenic peptide prediction tool. We have now improved this tool by implementing 32 new methods (PEPOP version 2.0) to guide the choice of peptides that mimic discontinuous epitopes and thus potentially able to replace the cognate protein Ag in its interaction with an Ab. In the present work, we describe these new methods and the benchmarking of their performances.

Results

Benchmarking was carried out by comparing the peptides predicted by the different methods and the corresponding epitopes determined by X-ray crystallography in a dataset of 75 Ag-Ab complexes. The Sensitivity (Se) and Positive Predictive Value (PPV) parameters were used to assess the performance of these methods. The results were compared to that of peptides obtained either by chance or by using the SUPERFICIAL tool, the only available comparable method.

Conclusion

The PEPOP methods were more efficient than, or as much as chance, and 33 of the 34 PEPOP methods performed better than SUPERFICIAL. Overall, “optimized” methods (tools that use the traveling salesman problem approach to design peptides) can predict peptides that best match true epitopes in most cases.

Computer-predicted peptides that mimic discontinuous epitopes on the A2 domain of factor VIII

Development of antibodies (Abs) against factor VIII (FVIII) is a severe complication of haemophilia A treatment. Recent publications suggest that domain specificity of anti-FVIII antibodies, particularly during immune tolerance induction (ITI), might be related to the outcome of the treatment. Obtaining suitable tools for a fine mapping of discontinuous epitopes could thus be helpful. The aim of this study was to map discontinuous epitopes on FVIII A2 domain using a new epitope prediction functionality of the PEPOP bioinformatics tool and a peptide inhibition assay based on the Luminex technology. We predicted, selected and synthesized 40 peptides mimicking discontinuous epitopes on the A2 domain of FVIII. A new inhibition assays using Luminex technology was performed to identify peptides able to inhibit the binding of anti-A2 Abs to A2 domain. We identified two peptides (IFKKLYHVWTKEVG and LYSRRLPKGVKHFD) able to block the binding of anti-A2 allo-antibodies to this domain. The three-dimensional representation of these two peptides on the A2 domain revealed that they are localized on a limited region of A2. We also confirmed that residues 484-508 of the A2 domain define an antigenic site. We suggest that dissection of the antibody response during ITI using synthetic peptide epitopes could provide important information for the management of patients with inhibitors.

Discontinuous epitopes on the C2 domain of coagulation Factor VIII mapped by computer-designed synthetic peptides

The occurrence of alloantibodies against Factor VIII (FVIII) is the main iatrogenic complication in haemophilia A (HA). Anti-FVIII autoantibodies may also spontaneously appear in non-HA patients, leading to acquired haemophilia A. In both contexts, the antibody response against FVIII is complex and difficult to analyse due to the lack of suitable tools. Our purpose was to comprehensively map, at the amino acid level, discontinuous epitopes of the C2 domain of FVIII targeted by patients' antibodies. We synthesized 33 synthetic peptides, which were predicted by the bioinformatic algorithm PEPOP to mimic C2 domain discontinuous epitopes. Using an inhibition assay based on the x-MAP technology, we evaluated their ability to block the binding to the C2 domain of anti-C2 domain antibodies from pooled plasma samples. Nine peptides were thus selected and tested again in individual plasma samples. Our results support the view that C2 domain epitopes are organized as an epitopic mosaic distributed around the molecule, showed that each patient displayed a specific anti-C2 epitopic profile, and confirmed the complexity and variability of the immune response against the C2 domain of FVIII. This ability to finely map epitopes could be further used to follow the antibody specificity modifications over time.

A shift towards a T cell cytokine deficiency along with an anti-inflammatory/regulatory microenvironment may enable the synthesis of anti-FVIII inhibitors in haemophilia A patients

Despite the clinical relevance of anti-factor VIII (FVIII) antibodies (anti-FVIII inhibitors) impairing haemostatic activity of haemophilia A (HA) patients, the immunological mechanisms underlying their production are unknown. Aiming to understand more clearly the immune response in patients with [HAα-FVIII(+)] and without [HAα-FVIII(-)] anti-FVIII inhibitors, we have characterized the cytokine pattern of peripheral blood leucocytes, using an in vitro stimulation of whole blood samples with plasma-derived (pFVIII) or recombinant FVIII (rFVIII). The results highlighted decreased levels of tumour necrosis factor (TNF)-α(+) neutrophils with higher interleukin (IL)-5/TNF-α ratio in HAα-FVIII(+). All HA samples displayed decreased levels of IL-10(+) monocytes when compared to the blood donor (BD) samples. HAα-FVIII(+) showed lower levels of TNF-α(+) monocytes and increased IL-10/TNF-α ratio. Analysis of adaptive immunity revealed increased levels of interferon (IFN)-γ(+) , TNF-α(+) and IL-4(+) T-cells, from both CD4(+) and CD8(+) T cells, in HAα-FVIII(-) when compared to BD. Moreover, increased frequency of IL-10(+) B cells and higher levels of α-FVIII IgG1 were observed in HAα-FVIII(-). Basal levels of cytokine(+) B-cells, similar to BD, and higher levels of α-FVIII IgG4 are major features in HAα-FVIII(+). The global cytokine profile demonstrated a major anti-inflammatory/regulatory pattern in HAα-FVIII(+), confirmed by the in vitro stimuli with pFVIII or rFVIII. The polarized anti-inflammatory/regulatory immune response in HAα-FVIII(+) and the mixed pattern with a bias towards an inflammatory cytokine profile, modulated by IL-4 in HAα-FVIII(-), may be the key element to drive the development of distinct subclasses of anti-FVIII antibodies. These finding have implications for the design of safe and effective therapeutic protocols to control inhibitors synthesis in HA patients.

Quantitation of anti-factor VIII antibodies in human plasma

The presence of antibodies (Abs) in hemophilia A patients can potentially influence the therapeutic qualities of factor VIII (fVIII) administration. Much work has been focused on the presence of inhibitory antibodies, whereas the quantitation of noninhibitory anti-fVIII antibodies has been largely undetermined. Our objective was to develop a sensitive and specific fluorescence-based immunoassay (FLI) for the quantitation of anti-fVIIIAbs in human plasma. Affinity-purified human anti-fVIIIAb, isolated from a hemophilia A subject, was used as a calibrator with a detectability limit of 40 (+/-1.5) pM. The calibrator and the human plasma anti-fVIIIAb were captured on recombinant fVIII (rfVIII)- coupled microspheres and probed with mouse anti-human Ig-R-phycoerythrin. Plasma samples from 150 healthy donors and 39 inhibitor-negative hemophilia A subjects were compared with 4 inhibitor-positive hemophilia A plasma samples with inhibitor titers of 1 BU/mL (94.6 +/- 0.8 nM), 11 BU/mL (214.3 +/- 7.1 nM), 106 BU/mL (2209.4 +/- 84.9 nM), 140 BU/mL (2417.7 +/- 3.8 nM) as measured by the Nijmegen method. We also describe the validation of a mouse anti-human fVIIIAb as a surrogate calibrator. Four healthy individuals (3%) showed detectable anti-fVIIIAb in the range of 0.6 to 6.2 nM, whereas 13 (33%) of the 39 inhibitor-free hemophilia A subjects were positive for anti-fVIIIAb in the range of 0.5 to 20 nM. The method may be useful for therapeutic management of hemophilia A patients.