Identification of novel factor VIII inhibitor epitopes using synthetic peptide arrays

Molecular Mechanisms of Inhibitor Development in Hemophilia

The development of neutralizing antibodies in hemophilia is a serious complication of factor replacement therapy. These antibodies, also known as “inhibitors”, significantly increase morbidity within the hemophilia population and lower the quality of life for these patients. People with severe hemophilia A have an overall 25–40% lifetime risk of inhibitor development, compared to that of 5–15% lifetime risk in those with moderate/mild hemophilia A. The risk is lower in hemophilia B population (about 1–5%) and occurrence of inhibitors is almost only seen in patients with severe hemophilia B. The understanding of the pathophysiological mechanism leading to the development of inhibitors in patients with hemophilia has improved considerably over the last 2 decades. Identification of early biomarkers which predict inhibitor development in previously untreated patients with hemophilia will assist in risk identification and possible early intervention strategies. In this review, we aim to summarize the molecular mechanisms of inhibitor development in hemophilia and to identify potential areas in need of further investigation.

FVIII inhibitors: pathogenesis and avoidance

The pathogenesis of inhibitory antibodies has been the focus of major scientific interest over the last decades, and several studies on underlying immune mechanisms and risk factors for formation of these antibodies have been performed with the aim of improving the ability to both predict and prevent their appearance. It seems clear that the decisive factors for the immune response to the deficient factor are multiple and involve components of both a constitutional and therapy-related nature. A scientific concern and obstacle for research in the area of hemophilia is the relatively small cohorts available for studies and the resulting risk of confounded and biased results. Careful interpretation of data is recommended to avoid treatment decisions based on a weak scientific platform. This review will summarize current concepts of the underlying immunological mechanisms and risk factors for development of inhibitory antibodies in patients with hemophilia A and discuss how these findings may be interpreted and influence our clinical management of patients.

Multisite haplotype on cattle chromosome 3 is associated with quantitative trait locus effects on lactation traits

The goal of this study was to identify candidate genes and DNA polymorphisms for quantitative trait loci (QTL) affecting milk yield (MY), fat yield (FY), and protein yield (PY) previously mapped to bovine chromosome 3 (BTA3). To accomplish this, 373 half-siblings sired by three bulls previously shown to be segregating for lactation trait QTL, and 263 additional sires in the U.S. Dairy Bull DNA Repository (DBDR) were genotyped for 2,500 SNPs within a 16.3 Mbp QTL critical region on BTA3. Targeted resequencing of ∼1.8 Mbp within the QTL critical region of one of the QTL heterozygous sires identified additional polymorphisms useful for association studies. Twenty-three single nucleotide polymorphisms (SNPs) within a fine-mapped region were associated with effects on breeding values for MY, FY, or PY in DBDR sires, of which five SNPs were in strong linkage disequilibrium in the population. This multisite haplotype included SNPs located within exons or promoters of four tightly linked genes: RAP1A, ADORA3, OVGP1, and C3H1orf88. An SNP within RAP1A showed strong evidence of a recent selective sweep based on integrated haplotype score and was also associated with breeding value for PY. Because of its known function in alveolar lumen formation in the mammary gland, RAP1A is thus a strong candidate gene for QTL effects on lactation traits. Our results provide a detailed assessment of a QTL region that will be a useful guide for complex traits analysis in humans and other noninbred species.

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

Antibodies (Abs) that block factor VIII (FVIII) activity occur in hemophilia A patients treated with FVIII replacement therapy and severely impair treatment. In this work, we designed and synthesized ten peptides whose sequences are found in putative epitopes at the surface of a1 and C2 domains of the FVIII molecule. These peptides were screened for their ability to inhibit the binding of anti-FVIII Abs from plasmas of hemophilia A patients to FVIII. All peptides were efficient in inhibiting anti-FVIII Abs in plasma from patients with inhibitors, with however different efficiencies. It was found that each tested patient's plasma had a different profile of reactivity with peptides, consistent with an individual anti-FVIII Ab specificity. The profile of recognized peptides was also changing during the treatment of the patients. Three peptides were used in an affinity chromatography assay to attempt to remove anti-FVIII Abs from patients' plasma. Anti-FVIII IgGs were significantly captured by the peptide-Sepharose affinity matrixes as assessed by enzyme-linked immunosorbent assay. However, due to the low level of Abs in the plasma samples, other methods (Chromogenic and Bethesda assays) were not sensitive enough to properly detect the reduction of inhibitors.

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.

Epitope specificity of anti-FVIII antibodies during immune tolerance therapy with factor VIII preparation containing von Willebrand factor

The study aimed at characterizing the putative changes in the epitope specificity of anti-FVIII antibodies during a successful immune tolerance treatment of the haemophilia A patient with the factor VIII (FVIII) preparation containing the von Willebrand factor (VWF). At the beginning of treatment, anti-FVIII inhibitory antibodies recognizing predominantly the light chain of FVIII were prevalent and persisted throughout the treatment. More detailed characterization of the FVIII antibody epitope specificity by using GST-fusion proteins corresponding to different FVIII domains revealed the prevalence of C1-domain-specific antibodies, while a remarkably lower amount of antibodies were targeted at the C2 and the a3 domains of the FVIII light chain and towards the A2 and the A1 domain of the FVIII heavy chain. The epitope specificity of antibodies remained rather unchanged throughout treatment except the elevated level of C2-domain-specific FVIII antibodies after a temporary interruption of treatment. The patient's antibodies were unable to interfere with the FVIII binding to VWF or to phospholipids, but inhibited FXa generation and the binding of FX to FVIII on the phospholipid monolayer. Thus, a unique pattern of the epitope specificity of FVIII antibodies and the mechanism to inhibit FVIII:C activity by FVIII-light-chain-specific antibodies were characterized.

Neoantigens and antibodies to factor VIII

It is well known that 10-20% of severe haemophiliacs are likely to develop inhibitors to factor VIII, usually soon after the commencement of therapy. Two recent inhibitor outbreaks have occurred in patients treated for a number of years on switching to a product subjected to additional virus inactivation. Hence the incidence of inhibitor formation may be affected by the type of product used for treatment and the potential for processing to result in 'neoantigens'. Examination of the parts of factor VIII interacting with inhibiting antibodies, and the effect of various therapies on these, can teach us something about the mechanisms involved in antibody formation. However, the development of pre-clinical assays to assess products and processes for neoantigen formation should allow the prevention of inhibitor outbreaks. This review summarizes current in vitro and in vivo approaches to this problem, concluding that most available assays are inadequate for this purpose, with competitive immunoassay and phospholipid binding providing the most hopeful route forward.