Uptake of blood coagulation factor VIII by dendritic cells is mediated via its C1 domain

Structural basis for inhibition of coagulation factor VIII reveals a shared antigenic hotspot on the C1 domain

BACKGROUND

Hemophilia A arises from dysfunctional or deficient coagulation factor (F)VIII and leads to inefficient fibrin clot formation and uncontrolled bleeding events. The development of antibody inhibitors is a clinical complication in hemophilia A patients receiving FVIII replacement therapy. LE2E9 is an anti-C1 domain inhibitor previously isolated from a mild/moderate hemophilia A patient and disrupts FVIII interactions with von Willebrand factor and FIXa, though the intermolecular contacts that underpin LE2E9-mediated FVIII neutralization are undefined.

OBJECTIVES

To determine the structure of the complex between FVIII and LE2E9 and characterize its mechanism of inhibition.

METHODS

FVIII was bound to the antigen binding fragment (Fab) of NB2E9, a recombinant construct of LE2E9, and its structure was determined by cryogenic electron microscopy.

RESULTS

This report communicates the 3.46 Å structure of FVIII bound to NB2E9, with its epitope comprising FVIII residues S2040 to Y2043, K2065 to W2070, and R2150 to H2155. Structural analysis reveals that the LE2E9 epitope overlaps with portions of the epitope for 2A9, a murine-derived inhibitor, suggesting that these residues represent a shared antigenic region on the C1 domain between FVIII-/- mice and hemophilia A patients. Furthermore, the FVIII:NB2E9 structure elucidates the orientation of the LE2E9 glycan, illustrating how the glycan sterically blocks interactions between the FVIII C1 domain and the von Willebrand factor D' domain. A putative model of the FVIIIa:FIXa complex suggests potential clashing between the NB2E9 glycan and FIXa light chain.

CONCLUSION

These results describe an antigenic "hotspot" on the FVIII C1 domain and provide a structural basis for engineering FVIII replacement therapeutics with reduced antigenicity.

Factor VIII antibody immune complexes modulate the humoral response to factor VIII in an epitope-dependent manner

Introduction

Soluble antigens complexed with immunoglobulin G (IgG) antibodies can induce robust adaptive immune responses in vitro and in animal models of disease. Factor VIII immune complexes (FVIII-ICs) have been detected in individuals with hemophilia A and severe von Willebrand disease following FVIII infusions. Yet, it is unclear if and how FVIII-ICs affect antibody development over time.

Methods

In this study, we analyzed internalization of FVIII complexed with epitope-mapped FVIII-specific IgG monoclonal antibodies (MAbs) by murine bone marrow-derived dendritic cells (BMDCs) in vitro and antibody development in hemophilia A (FVIII-/-) mice injected with FVIII-IC over time.

Results

FVIII complexed with 2-116 (A1 domain MAb), 2-113 (A3 domain MAb), and I55 (C2 domain MAb) significantly increased FVIII uptake by BMDC but only FVIII/2-116 enhanced antibody titers in FVIII-/- mice compared to FVIII alone. FVIII/4A4 (A2 domain MAb) showed similar FVIII uptake by BMDC to that of isolated FVIII yet significantly increased antibody titers when injected in FVIII-/- mice. Enhanced antibody responses observed with FVIII/2-116 and FVIII/4A4 complexes in vivo were abrogated in the absence of the FVIII carrier protein von Willebrand factor.

Conclusion

These findings suggest that a subset of FVIII-IC modulates the humoral response to FVIII in an epitope-dependent manner, which may provide insight into the antibody response observed in some patients with hemophilia A.

Hemophilia a patients with inhibitors: Mechanistic insights and novel therapeutic implications

The development of coagulation factor VIII (FVIII) inhibitory antibodies is a serious complication in hemophilia A (HA) patients after FVIII replacement therapy. Inhibitors render regular prophylaxis ineffective and increase the risk of morbidity and mortality. Immune tolerance induction (ITI) regimens have become the only clinically proven therapy for eradicating these inhibitors. However, this is a lengthy and costly strategy. For HA patients with high titer inhibitors, bypassing or new hemostatic agents must be used in clinical prophylaxis due to the ineffective ITI regimens. Since multiple genetic and environmental factors are involved in the pathogenesis of inhibitor generation, understanding the mechanisms by which inhibitors develop could help identify critical targets that can be exploited to prevent or eradicate inhibitors. In this review, we provide a comprehensive overview of the recent advances related to mechanistic insights into anti-FVIII antibody development and discuss novel therapeutic approaches for HA patients with inhibitors.

Modulating the microenvironment during FVIII uptake influences the nature of FVIII-peptides presented by antigen-presenting cells

Background and aims

Hemophilia A is a severe bleeding disorder caused by the deficiency of functionally active coagulation factor VIII (FVIII). The induction of neutralizing anti-drug antibodies is a major complication in the treatment of hemophilia A patients with FVIII replacement therapies. Why some patients develop neutralizing antibodies (FVIII inhibitors) while others do not is not well understood. Previous studies indicated that the induction of FVIII inhibitors requires cognate interactions between FVIII-specific B cells and FVIII-specific CD4+ T cells in germinal center reactions. In this study, we investigated the FVIII peptide repertoire presented by antigen-presenting cells (APCs) under different microenvironment conditions that are expected to alter the uptake of FVIII by APCs. The aim of this study was to better understand the association between different microenvironment conditions during FVIII uptake and the FVIII peptide patterns presented by APCs.

Methods

We used a FVIII-specific CD4+ T cell hybridoma library derived from humanized HLA-DRB1*1501 (human MHC class II) hemophilic mice that were treated with human FVIII. APCs obtained from the same mouse strain were preincubated with FVIII under different conditions which are expected to alter the uptake of FVIII by APCs. Subsequently, these preincubated APCs were used to stimulate the FVIII-specific CD4+ T cell hybridoma library. Stimulation of peptide-specific CD4+ T-cell hybridoma clones was assessed by analyzing the IL-2 release into cell culture supernatants.

Results

The results of this study indicate that the specific microenvironment conditions during FVIII uptake by APCs determine the peptide specificities of subsequently activated FVIII-specific CD4+ T cell hybridoma clones. Incubation of APCs with FVIII complexed with von Willebrand Factor, FVIII activated by thrombin or FVIII combined with a blockade of receptors on APCs previously associated with FVIII uptake and clearance, resulted in distinct peptide repertoires of subsequently activated hybridoma clones.

Conclusion

Based on our data we conclude that the specific microenvironment during FVIII uptake by APCs determines the FVIII peptide repertoire presented on MHC class II expressed by APCs and the peptide specificity of subsequently activated FVIII-specific CD4+ T cell hybridoma clones.

Influence of N-glycosylation in the A and C domains on the immunogenicity of factor VIII

The most significant complication in hemophilia A treatment is the formation of inhibitors against factor VIII (FVIII) protein. Glycans and glycan-binding proteins are central to a properly functioning immune system. This study focuses on whether glycosylation of FVIII plays an important role in induction and regulation of anti-FVIII immune responses. We investigated the potential roles of 4 N-glycosylation sites, including N41 and N239 in the A1 domain, N1810 in the A3 domain, and N2118 in the C1 domain of FVIII, in moderating its immunogenicity. Glycomics analysis of plasma-derived FVIII revealed that sites N41, N239, and N1810 contain mostly sialylated complex glycoforms, while high mannose glycans dominate at site N2118. A missense variant that substitutes asparagine (N) to glutamine (Q) was introduced to eliminate glycosylation on each of these sites. Following gene transfer of plasmids encoding B domain deleted FVIII (BDD-FVIII) and each of these 4 FVIII variants, it was found that specific activity of FVIII in plasma remained similar among all treatment groups. Slightly increased or comparable immune responses in N41Q, N239Q, and N1810Q FVIII variant plasmid-treated mice and significantly decreased immune responses in N2118Q FVIII plasmid-treated mice were observed when compared with BDD-FVIII plasmid-treated mice. The reduction of inhibitor response by N2118Q FVIII variant was also demonstrated in AAV-mediated gene transfer experiments. Furthermore, a specific glycopeptide epitope surrounding the N2118 glycosylation site was identified and characterized to activate T cells in an FVIII-specific proliferation assay. These results indicate that N-glycosylation of FVIII can have significant impact on its immunogenicity.

Isolated Variable Domains of an Antibody Can Assemble on Blood Coagulation Factor VIII into a Functional Fv-like Complex

Single-chain variable fragments (scFv) are antigen-recognizing variable fragments of antibodies (FV) where both subunits (V_L and V_H) are connected via an artificial linker. One particular scFv, iKM33, directed against blood coagulation factor VIII (FVIII) was shown to inhibit major FVIII functions and is useful in FVIII research. We aimed to investigate the properties of iKM33 enabled with protease-dependent disintegration. Three variants of iKM33 bearing thrombin cleavage sites within the linker were expressed using a baculovirus system and purified by two-step chromatography. All proteins retained strong binding to FVIII by surface plasmon resonance, and upon thrombin cleavage, dissociated into V_L and V_H as shown by size-exclusion chromatography. However, in FVIII activity and low-density lipoprotein receptor-related protein 1 binding assays, the thrombin-cleaved iKM33 variants were still inhibitory. In a pull-down assay using an FVIII-affinity sorbent, the isolated V_H, a mixture of V_L and V_H, and intact iKM33 were carried over via FVIII analyzed by electrophoresis. We concluded that the isolated V_L and V_H assembled into scFv-like heterodimer on FVIII, and the isolated V_H alone also bound FVIII. We discuss the potential use of both protease-cleavable scFvs and isolated Fv subunits retaining high affinity to the antigens in various practical applications such as therapeutics, diagnostics, and research.

Removal of single-site N-linked glycans on factor VIII alters binding of domain-specific monoclonal antibodies

BACKGROUND

A portion of individuals with hemophilia A develop neutralizing antibodies called inhibitors to glycoprotein factor VIII (FVIII). There are multiple risk factors that contribute to the risk of inhibitor formation. However, knowledge of the role of FVIII asparagine (N)-linked glycosylation in FVIII immunity is limited.

OBJECTIVE

To evaluate the effect of site-specific N-linked glycan removal on FVIII biochemical properties, endocytosis by murine bone marrow-derived dendritic cells (BMDCs), and antibody responses.

METHODS

Four recombinant B domain-deleted (BDD) FVIII variants with single-site amino acid substitutions to remove N-linked glycans were produced for experimental assays.

RESULTS

BDD FVIII-N41G, FVIII-N239A, FVIII-N1810A, and FVIII-N2118A with confirmed removal of N-linked glycans and similar glycosylation profiles to BDD FVIII were produced. There were no differences in thrombin activation or von Willebrand factor binding of FVIII variants compared with BDD FVIII; however, reduced FVIII expression, activity, and specific activity was observed with all variants. BDD FVIII-N41G and FVIII-N1810A had reduced uptake by BMDCs, but there were no differences in antibody development in immunized hemophilia A mice compared with BDD FVIII. Half of a repertoire of 12 domain-specific FVIII MAbs had significantly reduced binding to ≥1 FVIII variant with a 50% decrease in A1 domain MAb 2-116 binding to FVIII-N239A.

CONCLUSIONS

Modifications of FVIII N-linked glycans reduced FVIII endocytosis by BMDCs and binding of domain-specific FVIII MAbs, but did not alter de novo antibody production in hemophilia A mice, suggesting that N-glycans do not significantly contribute to inhibitor formation.

B cell-activating factor modulates the factor VIII immune response in hemophilia A

Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell development could help identify therapeutic targets. The B cell-activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody-mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI.

Investigating Optimal Autologous Cellular Platforms for Prenatal or Perinatal Factor VIII Delivery to Treat Hemophilia A

Patients with the severe form of hemophilia A (HA) present with a severe phenotype, and can suffer from life-threatening, spontaneous hemorrhaging. While prophylactic FVIII infusions have revolutionized the clinical management of HA, this treatment is short-lived, expensive, and it is not available to many A patients worldwide. In the present study, we evaluated a panel of readily available cell types for their suitability as cellular vehicles to deliver long-lasting FVIII replacement following transduction with a retroviral vector encoding a B domain-deleted human F8 transgene. Given the immune hurdles that currently plague factor replacement therapy, we focused our investigation on cell types that we deemed to be most relevant to either prenatal or very early postnatal treatment and that could, ideally, be autologously derived. Our findings identify several promising candidates for use as cell-based FVIII delivery vehicles and lay the groundwork for future mechanistic studies to delineate bottlenecks to efficient production and secretion of FVIII following genetic-modification.

Iron-Driven Alterations on Red Blood Cell-Derived Microvesicles Amplify Coagulation during Hemolysis via the Intrinsic Tenase Complex

Hemolytic disorders characterized by complement-mediated intravascular hemolysis, such as autoimmune hemolytic anemia and paroxysmal nocturnal hemoglobinuria, are often complicated by life-threatening thromboembolic complications. Severe hemolytic episodes result in the release of red blood cell (RBC)-derived proinflammatory and oxidatively reactive mediators (e.g., extracellular hemoglobin, heme, and iron) into plasma. Here, we studied the role of these hemolytic mediators in coagulation activation by measuring factor Xa (FXa) and thrombin generation in the presence of RBC lysates. Our results show that hemolytic microvesicles (HMVs) formed during hemolysis stimulate thrombin generation through a mechanism involving FVIII and FIX, the so-called intrinsic tenase complex. Iron scavenging during hemolysis using deferoxamine decreased the ability of the HMVs to enhance thrombin generation. Furthermore, the addition of ferric chloride (FeCl₃) to plasma propagated thrombin generation in a FVIII- and FIX-dependent manner suggesting that iron positively affects blood coagulation. Phosphatidylserine (PS) blockade using lactadherin and iron chelation using deferoxamine reduced intrinsic tenase activity in a purified system containing HMVs as source of phospholipids confirming that both PS and iron ions contribute to the procoagulant effect of the HMVs. Finally, the effects of FeCl₃ and HMVs decreased in the presence of ascorbate and glutathione indicating that oxidative stress plays a role in hypercoagulability. Overall, our results provide evidence for the contribution of iron ions derived from hemolytic RBCs to thrombin generation. These findings add to our understanding of the pathogenesis of thrombosis in hemolytic diseases.

The molecular mechanism of LRP1 in physiological vascular homeostasis and signal transduction pathways

Interactions between vascular smooth muscle cells (VSMCs), endothelial cells (ECs), pericytes (PCs) and macrophages (MФ), the major components of blood vessels, play a crucial role in maintaining vascular structural and functional homeostasis. Low-density lipoprotein (LDL) receptor-related protein-1 (LRP1), a transmembrane receptor protein belonging to the LDL receptor family, plays multifunctional roles in maintaining endocytosis, homeostasis, and signal transduction. Accumulating evidence suggests that LRP1 modulates vascular homeostasis mainly by regulating vasoactive substances and specific intracellular signaling pathways, including the plasminogen activator inhibitor 1 (PAI-1) signaling pathway, platelet-derived growth factor (PDGF) signaling pathway, transforming growth factor-β (TGF-β) signaling pathway and vascular endothelial growth factor (VEGF) signaling pathway. The aim of the present review is to focus on recent advances in the discovery and mechanism of vascular homeostasis regulated by LRP1-dependent signaling pathways. These recent discoveries expand our understanding of the mechanisms controlling LRP1 as a target for studies on vascular complications.

Iron-driven alterations on red blood cell-derived microvesicles amplify coagulation during hemolysis via the intrinsic tenase complex

Hemolytic disorders characterized by complement-mediated intravascular hemolysis, such as autoimmune hemolytic anemia and paroxysmal nocturnal hemoglobinuria, are often complicated by life-threatening thromboembolic complications. Severe hemolytic episodes result in the release of red blood cell (RBC)-derived pro-inflammatory and oxidatively reactive mediators (e.g. extracellular hemoglobin, heme and iron) into plasma. Here, we studied the role of these hemolytic mediators in coagulation activation by measuring FXa and thrombin generation in the presence of RBC lysates. Our results show that hemolytic microvesicles (HMVs) formed during hemolysis stimulate thrombin generation through a mechanism involving FVIII and FIX, the so-called intrinsic tenase complex. Iron scavenging during hemolysis using deferoxamine decreased the ability of the HMVs to enhance thrombin generation. Furthermore, the addition of ferric chloride (FeCl3) to plasma propagated thrombin generation in a FVIII and FIX-dependent manner suggesting that iron positively affects blood coagulation. Phosphatidylserine (PS) blockade using lactadherin and iron chelation using deferoxamine reduced intrinsic tenase activity in a purified system containing HMVs as source of phospholipids confirming that both PS and iron ions contribute to the procoagulant effect of the HMVs. Finally, the effects of FeCl3 and HMVs decreased in the presence of ascorbate and glutathione indicating that oxidative stress plays a role in hypercoagulability. Overall, our results provide evidence for the contribution of iron ions derived from hemolytic RBCs to thrombin generation. These findings add to our understanding of the pathogenesis of thrombosis in hemolytic diseases.

Molecular determinants of the factor VIII/von Willebrand factor complex revealed by BIVV001 cryo-electron microscopy

Interaction of factor VIII (FVIII) with von Willebrand factor (VWF) is mediated by the VWF D'D3 domains and thrombin-mediated release is essential for hemostasis after vascular injury. VWF-D'D3 mutations resulting in loss of FVIII binding are the underlying cause of von Willebrand disease (VWD) type 2N. Furthermore, the FVIII-VWF interaction has significant implications for the development of therapeutics for bleeding disorders, particularly hemophilia A, in which endogenous VWF clearance imposes a half-life ceiling on replacement FVIII therapy. To understand the structural basis of FVIII engagement by VWF, we solved the structure of BIVV001 by cryo-electron microscopy to 2.9 Å resolution. BIVV001 is a bioengineered clinical-stage FVIII molecule for the treatment of hemophilia A. In BIVV001, VWF-D'D3 is covalently linked to an Fc domain of a B domain-deleted recombinant FVIII (rFVIII) Fc fusion protein, resulting in a stabilized rFVIII/VWF-D'D3 complex. Our rFVIII/VWF structure resolves BIVV001 architecture and provides a detailed spatial understanding of previous biochemical and clinical observations related to FVIII-VWF engagement. Notably, the FVIII acidic a3 peptide region (FVIII-a3), established as a critical determinant of FVIII/VWF complex formation, inserts into a basic groove formed at the VWF-D'/rFVIII interface. Our structure shows direct interaction of sulfated Y1680 in FVIII-a3 and VWF-R816 that, when mutated, leads to severe hemophilia A or VWD type 2N, respectively. These results provide insight on this key coagulation complex, explain the structural basis of many hemophilia A and VWD type 2N mutations, and inform studies to further elucidate how VWF dissociates rapidly from FVIII upon activation.

Immune Responses to Plasma-Derived Versus Recombinant FVIII Products

The most severe side effect of hemophilia treatment is the inhibitor development occurring in 30% of patients, during the earliest stages of treatment with factor (F)VIII concentrates. These catastrophic immune responses rapidly inactivate the infused FVIII, rendering the treatment ineffective. This complication is associated with a substantial morbidity and mortality. The risk factors involved in the onset of the inhibitors are both genetic and environmental. The source of FVIII products, i.e. plasma-derived or recombinant FVIII products, is considered one of the most relevant factors for inhibitor development. Numerous studies in the literature report conflicting data on the different immunogenicity of the products. The SIPPET randomized trial showed an increased in the inhibitor rate in patients using recombinant FVIII products than those receiving plasma-derived products in the first exposure days. The SIPPET randomized trial showed an increase in the inhibitor rate in patients using recombinant FVIII products compared to those treated with plasma-derived products in the first days of exposure. The potential increase in the immunogenicity of recombinant products can be attributed to several factors such as: the different post-translational modification in different cell lines, the presence of protein aggregates, and the role played by the chaperon protein of FVIII, the von Willebrand factor, which modulates the uptake of FVIII by antigen presenting cells (APCs). Furthermore, the presence of non-neutralizing antibodies against FVIII has shown to be in increased inhibitor development as demonstrated in a sub-analysis of the SIPPET study. In addition, the presence of the specific subclasses of the immunoglobulins may also be an important biomarker to indicate whether the inhibitor will evolve into a persistent neutralizing antibody or a transient one that would disappear without any specific treatment. Recently, the availability of novel non-replacement therapies as well as emicizumab, administered by weekly subcutaneous infusion, have significantly changed the quality of life of patients with inhibitors showing a considerable reduction of the annual bleeding rate and in most patients the absence of bleeding. Although, these novel drugs improve patients' quality of life, they do not abolish the need to infuse FVIII during acute bleeding or surgery. Therefore, the issue of immunogenicity against FVIII still remains an important side effect of hemophilia treatment.

Hemophilia A Inhibitor Subjects Show Unique PBMC Gene Expression Profiles That Include Up-Regulated Innate Immune Modulators

Formation of pathological anti-FVIII antibodies, or "inhibitors," is the most serious complication of therapeutic FVIII infusions, affecting up to 1/3 of severe Hemophilia A (HA) patients. Inhibitor formation is a classical T-cell dependent adaptive immune response. As such, it requires help from the innate immune system. However, the roles of innate immune cells and mechanisms of inhibitor development vs. immune tolerance, achieved with or without Immune Tolerance Induction (ITI) therapy, are not well-understood. To address these questions, temporal transcriptomics profiling of FVIII-stimulated peripheral blood mononuclear cells (PBMCs) was carried out for HA subjects with and without a current or historic inhibitor using RNA-Seq. PBMCs were isolated from 40 subjects in the following groups: HA with an inhibitor that resolved either following ITI or spontaneously; HA with a current inhibitor; HA with no inhibitor history and non-HA controls. PBMCs were stimulated with 5 nM FVIII and RNA was isolated 4, 16, 24, and 48 h following stimulation. Time-series differential expression analysis was performed and distinct transcriptional signatures were identified for each group, providing clues as to cellular mechanisms leading to or accompanying their disparate anti-FVIII antibody responses. Subjects with a current inhibitor showed differential expression of 56 genes and a clustering analysis identified three major temporal profiles. Interestingly, gene ontology enrichments featured innate immune modulators, including NLRP3, TLR8, IL32, CLEC10A, and COLEC12. NLRP3 and TLR8 are associated with enhanced secretion of the pro-inflammatory cytokines IL-1β and TNFα, while IL32, which has several isoforms, has been associated with both inflammatory and regulatory immune processes. RNA-Seq results were validated by RT-qPCR, ELISAs, multiplex cytokine analysis, and flow cytometry. The inflammatory status of HA patients suffering from an ongoing inhibitor includes up-regulated innate immune modulators, which may act as ongoing danger signals that influence the responses to, and eventual outcomes of, ITI therapy.

Unexpected enhancement of FVIII immunogenicity by endothelial expression in lentivirus-transduced and transgenic mice

Factor VIII (FVIII) replacement therapy for hemophilia A is complicated by development of inhibitory antibodies (inhibitors) in ∼30% of patients. Because endothelial cells (ECs) are the primary physiologic expression site, we probed the therapeutic potential of genetically restoring FVIII expression selectively in ECs in hemophilia A mice (FVIIInull). Expression of FVIII was driven by the Tie2 promoter in the context of lentivirus (LV)-mediated in situ transduction (T2F8LV) or embryonic stem cell-mediated transgenesis (T2F8Tg). Both endothelial expression approaches were associated with a strikingly robust immune response. Following in situ T2F8LV-mediated EC transduction, all FVIIInull mice developed inhibitors but had no detectable plasma FVIII. In the transgenic approach, the T2F8Tg mice had normalized plasma FVIII levels, but showed strong sensitivity to developing an FVIII immune response upon FVIII immunization. A single injection of FVIII with incomplete Freund adjuvant led to high titers of inhibitors and reduction of plasma FVIII to undetectable levels. Because ECs are putative major histocompatibility complex class II (MHCII)-expressing nonhematopoietic, "semiprofessional" antigen-presenting cells (APCs), we asked whether they might directly influence the FVIII immune responses. Imaging and flow cytometric studies confirmed that both murine and human ECs express MHCII and efficiently bind and take up FVIII protein in vitro. Moreover, microvascular ECs preconditioned ex vivo with inflammatory cytokines could functionally present exogenously taken-up FVIII to previously primed CD4+/CXCR5+ T follicular helper (Tfh) cells to drive FVIII-specific proliferation. Our results show an unanticipated immunogenicity of EC-expressed FVIII and suggest a context-dependent role for ECs in the regulation of inhibitors as auxiliary APCs for Tfh cells.

Removal of Mannose-Ending Glycan at Asn2118 Abrogates FVIII Presentation by Human Monocyte-Derived Dendritic Cells

The development of an immune response against therapeutic factor VIII is the major complication in hemophilia A patients. Oligomannose carbohydrates at N239 and/or N2118 on factor VIII allow its binding to the macrophage mannose receptor expressed on human dendritic cells, thereby leading to factor VIII endocytosis and presentation to CD4+ T lymphocytes. Here, we investigated whether altering the interaction of factor VIII with mannose-sensitive receptors on antigen-presenting cells may be a strategy to reduce factor VIII immunogenicity. Gene transfer experiments in factor VIII-deficient mice indicated that N239Q and/or N2118Q factor VIII mutants have similar specific activities as compared to non-mutated factor VIII; N239Q/N2118Q mutant corrected blood loss upon tail clip. Production of the corresponding recombinant FVIII mutants or light chains indicated that removal of the N-linked glycosylation site at N2118 is sufficient to abrogate in vitro the activation of FVIII-specific CD4+ T cells by human monocyte-derived dendritic cells. However, removal of mannose-ending glycans at N2118 did not alter factor VIII endocytosis and presentation to CD4+ T cells by mouse antigen-presenting cells. In agreement with this, the N2118Q mutation did not reduce factor VIII immunogenicity in factor VIII-deficient mice. Our results highlight differences in the endocytic pathways between human and mouse dendritic cell subsets, and dissimilarities in tissue distribution and function of endocytic receptors such as CD206 in both species. Further investigations in preclinical models of hemophilia A closer to humans are needed to decipher the exact role of mannose-ending glycans in factor VIII immunogenicity.

Tolerating Factor VIII: Recent Progress

Development of neutralizing antibodies against biotherapeutic agents administered to prevent or treat various clinical conditions is a longstanding and growing problem faced by patients, medical providers and pharmaceutical companies. The hemophilia A community has deep experience with attempting to manage such deleterious immune responses, as the lifesaving protein drug factor VIII (FVIII) has been in use for decades. Hemophilia A is a bleeding disorder caused by genetic mutations that result in absent or dysfunctional FVIII. Prophylactic treatment consists of regular intravenous FVIII infusions. Unfortunately, 1/4 to 1/3 of patients develop neutralizing anti-FVIII antibodies, referred to clinically as “inhibitors,” which result in a serious bleeding diathesis. Until recently, the only therapeutic option for these patients was “Immune Tolerance Induction,” consisting of intensive FVIII administration, which is extraordinarily expensive and fails in ~30% of cases. There has been tremendous recent progress in developing novel potential clinical alternatives for the treatment of hemophilia A, ranging from encouraging results of gene therapy trials, to use of other hemostatic agents (either promoting coagulation or slowing down anti-coagulant or fibrinolytic pathways) to “bypass” the need for FVIII or supplement FVIII replacement therapy. Although these approaches are promising, there is widespread agreement that preventing or reversing inhibitors remains a high priority. Risk profiles of novel therapies are still unknown or incomplete, and FVIII will likely continue to be considered the optimal hemostatic agent to support surgery and manage trauma, or to combine with other therapies. We describe here recent exciting studies, most still pre-clinical, that address FVIII immunogenicity and suggest novel interventions to prevent or reverse inhibitor development. Studies of FVIII uptake, processing and presentation on antigen-presenting cells, epitope mapping, and the roles of complement, heme, von Willebrand factor, glycans, and the microbiome in FVIII immunogenicity are elucidating mechanisms of primary and secondary immune responses and suggesting additional novel targets. Promising tolerogenic therapies include development of FVIII-Fc fusion proteins, nanoparticle-based therapies, oral tolerance, and engineering of regulatory or cytotoxic T cells to render them FVIII-specific. Importantly, these studies are highly applicable to other scenarios where establishing immune tolerance to a defined antigen is a clinical priority.

Factor VIII Fc Fusion Protein but not FVIII Drives Human Monocyte-Derived Dendritic Cell Activation via FcγRIIa

This study compares the effect of recombinant Factor VIII Fc fusion protein (rFVIII-Fc) with recombinant FVIII (rFVIII) on monocyte-derived dendritic cells (moDC's). Cells treated with rFVIII-Fc showed morphological changes typical for cell activation, had a significant up-regulation of cell activation markers and produced higher levels of pro-inflammatory cytokines. Even after stimulation with Lipopolysaccharides, the addition of rFVIII-Fc led to increased expression of activation markers, indicating that rFVIII-Fc is capable of amplifying the maturation signal. On the contrary, cultivation of moDC's with rFVIII did not alter cell morphology or increase surface activation marker expression and pro-inflammatory cytokine production. The binding of the Fc domain to the activating Fcγ receptor IIa (FcγRIIa) can cause cell activation. Therefore, the effect of rFVIII-Fc on FcγRIIa was analyzed in detail. Cultivation of moDC's with rFVIII-Fc led to increased phosphorylation of FcγRIIa, which was not detected for rFVIII. Blocking FcγRIIa prior to the cultivation with rFVIII-Fc significantly reduced the activating effect of rFVIII-Fc, indicating that rFVIII-Fc-induced moDC activation was caused by FcγRIIa. Moreover, rFVIII-Fc bound to FCGR2A-transfected human embryonic kidney 293 cells. Taken together, our data present a new mechanism of moDC activation by rFVIII-Fc via FcγRIIa.

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.

The endothelial lectin clearance receptor CLEC4M binds and internalizes factor VIII in a VWF-dependent and independent manner

Essentials CLEC4M is an endocytic receptor for factor FVIII. CLEC4M interacts with FVIII in a VWF-dependent and independent manner. CLEC4M binds to mannose-containing glycans on FVIII. CLEC4M internalization of FVIII involves clathrin coated pits. SUMMARY: Background von Willebrand factor (VWF) and factor VIII (FVIII) circulate in the plasma as a non-covalent complex, and the majority of FVIII is likely to be cleared by VWF-dependent pathways. Clearance of VWF-free FVIII is rapid and underlies the pathological basis of some quantitative FVIII deficiencies. The receptor pathways that regulate the clearance of VWF-bound and VWF-free FVIII are incompletely uncharacterized. The human liver-expressed endothelial lectin CLEC4M has been previously characterized as a clearance receptor for VWF, although its influence on FVIII is unknown. Objective The interaction between FVIII and CLEC4M was characterized in the presence or absence of VWF. Methods FVIII interactions with CLEC4M were evaluated by in vitro cell-based and solid phase binding assays. Interactions between FVIII and CLEC4M or liver sinusoidal endothelial cells were evaluated in vivo by immunohistochemistry. Results CLEC4M-expressing HEK 293 cells bound and internalized recombinant and plasma-derived FVIII through VWF-dependent and independent mechanisms. CLEC4M binding to recombinant FVIII was dependent on mannose-exposed N-linked glycans. CLEC4M mediated FVIII internalization via a clathrin-coated pit-dependent mechanism, resulting in transport of FVIII from early and late endosomes for catabolism by lysosomes. In vivo hepatic expression of CLEC4M after hydrodynamic liver transfer was associated with a decrease in plasma levels of endogenous murine FVIII:C in normal mice, whereas infused recombinant human FVIII was associated with sinusoidal endothelial cells in the presence or absence of VWF. Conclusions These findings suggest that CLEC4M is a novel clearance receptor that interacts with mannose-exposed glycans on FVIII in the presence or absence of VWF.

Reprogrammed CD4+ T Cells That Express FoxP3+ Control Inhibitory Antibody Formation in Hemophilia A Mice

Coagulation Factor VIII (FVIII) replacement therapy in hemophilia A patients is complicated by the development of inhibitory antibodies, which often render the treatment ineffective. Previous studies demonstrated a strong correlation between induction of regulatory T cells (Treg) and tolerance to the therapeutic protein. We, therefore, set out to evaluate whether the adoptive transfer of FVIII-specific CD4⁺ Treg cells prevents inhibitor response to FVIII protein therapy. To this end, we first retrovirally transduced FoxP3⁺ into FVIII-specific CD4⁺ cells, which resulted in cells that stably express FoxP3, are phenotypically similar to peripherally induced Tregs and are antigen specific suppressors, as judged by in vitro assays. Upon transfer of the FVIII-specific CD4⁺ FoxP3⁺ cells into hemophilia A mice, development of inhibitory antibodies in response to administering FVIII protein was completely suppressed. Suppression was extended for 2 months, even after transferred cells were no longer detectable in the secondary lymphoid organs of recipient animals. Upon co-transfer of FoxP3⁺-transduced cells with the B cell depleting anti-CD20 into mice with pre-existing inhibitory antibodies to FVIII, the escalation of inhibitory antibody titers in response to subsequent FVIII protein therapy was dramatically reduced. We conclude that reprogramed FoxP3 expressing cells are capable of inducing the in vivo conversion of endogenous FVIII peripheral Tregs, which results in sustained suppression of FVIII inhibitors caused by replacement therapy in recipient hemophilia A animals.

Pathogenic immune response to therapeutic factor VIII: exacerbated response or failed induction of tolerance?

Therapeutic factor VIII is highly immunogenic. Despite intensive research in the last decades, the reasons why 5-30% of patients with hemophilia A (of all severities) develop inhibitory anti-factor VIII antibodies (inhibitors) following replacement therapy remain an enigma. Under physiological conditions, endogenous factor VIII is recognized by the immune system. Likewise, numerous observations indicate that, in hemophilia A patients without inhibitors, exogenous therapeutic factor VIII is immunologically assessed and tolerated. A large part of the research on the immunogenicity of therapeutic factor VIII is attempting to identify the ‘danger signals’ that act as adjuvants to the deleterious anti-factor VIII immune responses. However, several of the inflammatory assaults concomitant to factor VIII administration initially hypothesized as potential sources of danger signals (e.g., bleeding, infection, and vaccination) have been disproved to be such signals. Conversely, recent evidence suggests that cells from inhibitor-negative patients are able to activate anti-inflammatory and tolerogenic mechanisms required to suppress deleterious immune responses, while cells from inhibitor-positive patients are not. Based on the available observations, we propose a model in which all hemophilia A patients develop anti-factor VIII immune responses during replacement therapy irrespective of associated danger signals. We further postulate that the onset of clinically relevant factor VIII inhibitors results from an inability to develop counteractive tolerogenic responses to exogenous factor VIII rather than from an exacerbated activation of the immune system at the time of factor VIII administration. A better understanding of the pathogenesis of neutralizing anti-factor VIII antibodies will have repercussions on the clinical management of patients and highlight new strategies to achieve active immune tolerance to therapeutic factor VIII.

N-linked glycosylation modulates the immunogenicity of recombinant human factor VIII in hemophilia A mice

Immune responses to factor VIII remain the greatest complication in the treatment of severe hemophilia A. Recent epidemiological evidence has highlighted that recombinant factor VIII produced in baby hamster kidney cells is more immunogenic than factor VIII produced in Chinese hamster ovary cells. Glycosylation differences have been hypothesized to influence the immunogenicity of these synthetic concentrates. In two hemophilia A mouse models, baby hamster kidney cell-derived factor VIII elicited a stronger immune response compared to Chinese hamster ovary cell-derived factor VIII. Furthermore, factor VIII produced in baby hamster kidney cells exhibited accelerated clearance from circulation independent of von Willebrand factor. Lectin and mass spectrometry analysis of total N-linked glycans revealed differences in high-mannose glycans, sialylation, and the occupancy of glycan sites. Factor VIII desialylation did not influence binding to murine splenocytes or dendritic cells, nor surface co-stimulatory molecule expression. We did, however, observe increased levels of immunoglobulin M specific to baby hamster kidney-derived factor VIII in naïve hemophilia A mice. De-N-glycosylation enhanced immunoglobulin M binding, suggesting that N-glycan occupancy masks epitopes. Elevated levels of immunoglobulin M and immunoglobulin G specific to baby hamster kidney-derived factor VIII were also observed in healthy individuals, and de-N-glycosylation increased immunoglobulin G binding. Collectively, our data suggest that factor VIII produced in baby hamster kidney cells is more immunogenic than that produced in Chinese hamster ovary cells, and that incomplete occupancy of N-linked glycosylation sites leads to the formation of immunoglobulin M- and immunoglobulin G-factor VIII immune complexes that contribute to the enhanced clearance and immunogenicity in these mouse models of hemophilia A.

Biochemical characterization and immunogenicity of Neureight, a recombinant full-length factor VIII produced by fed-batch process in disposable bioreactors

Hemophilia A is a X-linked recessive bleeding disorder consecutive to the lack of circulating pro-coagulant factor VIII (FVIII). The most efficient strategy to treat or prevent bleeding in patients with hemophilia A relies on replacement therapy using exogenous FVIII. Commercially available recombinant FVIII are produced using an expensive perfusion technology in stainless steel fermenters. A fed-batch fermentation technology was recently developed to produce 'Neureight', a full-length recombinant human FVIII, in Chinese hamster ovary (CHO) cells. Here, we investigated the structural and functional integrity and lack of increased immunogenicity of Neureight, as compared to two commercially available full-length FVIII products, Helixate and Advate, produced in baby hamster kidney or CHO cells, respectively. Our results demonstrate the purity, stability and functional integrity of Neureight with a standard specific activity of 4235 ± 556 IU/mg. The glycosylation and sulfation profiles of Neureight were similar to that of Advate, with the absence of the antigenic carbohydrate epitopes α-Gal and Neu5Gc, and with sulfation of Y1680, that is critical for FVIII binding to von Willebrand factor (VWF). The endocytosis of Neureight by human immature dendritic cells was inhibited by VWF, and its half-life in FVIII-deficient mice was similar to that of Advate, confirming unaltered binding to VWF. In vitro and in vivo assays indicated a similar immunogenicity for Neureight, Advate and Helixate. In conclusion, the production of full-length FVIII in a fed-batch fermentation mode generates a product that presents similar biochemical, functional and immunogenic properties as products developed using the classical perfusion technology.

Early cellular interactions and immune transcriptome profiles in human factor VIII-exposed hemophilia A mice

Essentials Initial immune cell interactions leading to factor (F) VIII immunity are not well characterized. We assessed cellular interactions and expression profiles in hemophilia A mice. MARCO+, followed by SIGLEC1+ and SIGNR1+ macrophages co-localize most with human FVIII. The splenic transcriptome highlights potential therapeutic targets to prevent inhibitors.

SUMMARY

Background Developing factor VIII (FVIII) inhibitory antibodies is the most serious complication in hemophilia A treatment, representing a significant health and economic burden. A better understanding of the early events in an immune response leading to this outcome may provide insight into inhibitor development. Objective To identify early mediators of FVIII immunity and to detail immune expression profiles in the spleen and liver. Methods C57Bl/6 F8 E16 knockout mice were infused with 5-20 μg (2000-8000 IU kg-1 ) of recombinant FVIII. Spleens were frozen at various time-points post-infusion and stained for FVIII and cellular markers. Splenic and liver RNA expression analysis was performed 3 h post-infusion of 0.6 μg (240 IU kg-1 ) FVIII by nCounter technology using a 561-gene immunology panel. Results FVIII localization in the spleen did not change over 2.5 h. We observed significantly higher co-localization of FVIII with MARCO+ cells compared with SIGLEC1+ and SIGNR1+ in the splenic marginal zone. FVIII exhibited little co-localization with CD11c+ dendritic cells and the macrophage mannose receptor, CD206. Following FVIII infusion, the splenic mRNA profiling identified genes such as Tnfaip6 and Il23r, which are implicated in chemotaxis and a proinflammatory Th17 response, respectively. In contrast, an upregulation of Gfi1 in the liver suggests an anti-inflammatory environment. Conclusions FVIII co-localizes predominantly with marginal zone macrophages (MARCO+ ) in the murine spleen following intravenous infusion. Targeting pathways that are implicated in the early FVIII innate immune response in the spleen may lead to therapeutic interventions to prevent inhibitor formation.

Danger signal-dependent activation of human dendritic cells by plasma-derived factor VIII products

Treatment of haemophilia A by infusions of the clotting factor VIII (FVIII) results in the development of inhibitors/anti-drug antibodies in up to 25 % of patients. Mechanisms leading to immunogenicity of FVIII products are not yet fully understood. Amongst other factors, danger signals as elicited upon infection or surgery have been proposed to play a role. In the present study, we focused on effects of danger signals on maturation and activation of dendritic cells (DC) in the context of FVIII application. Human monocyte-derived DC were treated with FVIII alone, with a danger signal alone or a combination of both. By testing more than 60 different healthy donors, we show that FVIII and the bacterial danger signal lipopolysaccharide synergise in increasing DC activation, as characterised by increased expression of co-stimulatory molecules and secretion of pro-inflammatory cytokines. The degree and frequency of this synergistic activation correlate with CD86 expression levels on immature DC prior to stimulation. In our assay system, plasma-derived but not recombinant FVIII products activate human DC in a danger signal-dependent manner. Further tested danger signals, such as R848 also induced DC activation in combination with FVIII, albeit not in every tested donor. In our hands, human DC but not human B cells or macrophages could be activated by FVIII in a danger signal-dependent manner. Our results suggest that immunogenicity of FVIII is a result of multiple factors including the presence of danger, predisposition of the patient, and the choice of a FVIII product for treatment.

Complement C3 is a novel modulator of the anti-factor VIII immune response

Development of neutralizing antibodies against therapeutic Factor VIII (FVIII) is the most serious complication of the treatment of hemophilia A. There is growing evidence to show the multifactorial origin of the anti-FVIII immune response, combining both genetic and environmental factors. While a role for the complement system on innate as well as adaptive immunity has been documented, the implication of complement activation on the onset of the anti-FVIII immune response is unknown. Here, using in vitro assays for FVIII endocytosis by human monocyte-derived dendritic cells and presentation to T cells, as well as in vivo complement depletion in FVIII-deficient mice, we show a novel role for complement C3 in enhancing the immune response against therapeutic FVIII. In vitro, complement C3 and its cleavage product C3b enhanced FVIII endocytosis by dendritic cells and presentation to a FVIII-specific CD4⁺ T-cell hybridoma. The C1 domain of FVIII had previously been shown to play an important role in FVIII endocytosis, and alanine substitutions of the K2092, F2093 and R2090 C1 residues drastically reduce FVIII uptake in vitro. Interestingly, complement activation rescued the endocytosis of the FVIII C1 domain triple mutant. In a mouse model of severe hemophilia A, transient complement C3 depletion by humanized cobra venom factor, which does not generate anaphylatoxin C5a, significantly reduced the primary anti-FVIII immune response, but did not affect anti-FVIII recall immune responses. Taken together, our results suggest an important adjuvant role for the complement cascade in the initiation of the immune response to therapeutic FVIII.

To serve and protect: The modulatory role of von Willebrand factor on factor VIII immunogenicity

Hemophilia A is a bleeding disorder characterized by the absence or dysfunction of blood coagulation factor VIII (FVIII). Patients are treated with regular infusions of FVIII concentrate. In response to treatment, approximately 30% of patients with severe hemophilia A develop inhibitory antibodies targeting FVIII. Both patient and treatment related risk factors for inhibitor development have been described. Multiple studies comparing the immunogenicity of recombinant and plasma-derived FVIII have yielded conflicting results. The randomized controlled SIPPET (Survey of Inhibitors in Plasma-Product Exposed Toddlers) trial demonstrated an increased risk of inhibitor development of recombinant FVIII when compared to von Willebrand factor (VWF)-containing plasma-derived FVIII. Presently, it is unclear which mechanism underlies the reduced immunogenicity of plasma-derived FVIII. In this review we address the potential role of VWF on FVIII immunogenicity and we discuss how VWF affects the immune recognition, processing and presentation of FVIII. We also briefly discuss the potential impact of glycan-composition on FVIII immunogenicity. It is well established that VWF shields the uptake of FVIII by antigen presenting cells. We have recently shown that VWF binds to the surface of dendritic cells. Here, we present a novel model in which surface bound FVIII-VWF complexes regulate the internalization of FVIII. Binding of FVIII to VWF is critically dependent on sulfation of Tyr1699 (HVGS numbering) in the light chain of FVIII. Incomplete sulfation of Tyr1699 has been suggested to occur in several recombinant FVIII products resulting in a loss of VWF binding. We hypothesize that this results in alternative pathways of FVIII internalization by antigen presenting cells which are not regulated by VWF. This hypothetical mechanism may explain the reduced immunogenicity of VWF containing plasma-derived FVIII concentrates as found in the SIPPET study.

Innovating immune tolerance induction for haemophilia

INTRODUCTION

Haemophilia A is an X-linked bleeding disorder characterized by a deficiency of coagulation protein factor VIII (FVIII). A challenging complication of therapeutic FVIII infusions is the formation of neutralizing alloantibodies against the FVIII protein defined as inhibitors. The development of FVIII inhibitors drastically alters the quality of life of the patients and is associated with tremendous increases in morbidity as well as treatment costs.

AIM

Current clinical immune tolerance induction protocols to reverse inhibitors are lengthy, costly and not effective in all patients. Prophylactic protocols to prevent inhibitor formation have not yet been developed in the clinical setting. However, there has been ample progress towards this goal in recent years in preclinical studies using animal models of haemophilia.

METHODS

Here, we review the mechanisms that lead to inhibitor formation against FVIII and two promising new strategies for antigen-specific tolerance induction.

RESULTS

CD4+ T cells play an important role in the FVIII-specific B cell response. Immune tolerance can be induced based on transplacental delivery of FVIII domains fused to Fc or on oral delivery of leaf cells from chloroplast transgenic crop plants.

CONCLUSIONS

Recent literature suggests that prophylactic tolerance induction protocols for FVIII may be feasible in haemophilia A patients.

The C1 and C2 domains of blood coagulation factor VIII mediate its endocytosis by dendritic cells

The development of inhibitory antibodies to therapeutic factor VIII is the major complication of replacement therapy in patients with hemophilia A. The first step in the initiation of the anti-factor VIII immune response is factor VIII interaction with receptor(s) on antigen-presenting cells, followed by endocytosis and presentation to naïve CD4⁺ T cells. Recent studies indicate a role for the C1 domain in factor VIII uptake. We investigated whether charged residues in the C2 domain participate in immunogenic factor VIII uptake. Co-incubation of factor VIII with BO2C11, a monoclonal C2-specific immunoglobulin G, reduced factor VIII endocytosis by dendritic cells and presentation to CD4⁺ T cells, and diminished factor VIII immunogenicity in factor VIII-deficient mice. The mutation of basic residues within the BO2C11 epitope of C2 replicated reduced in vitro immunogenic uptake, but failed to prevent factor VIII immunogenicity in mice. BO2C11 prevents factor VIII binding to von Willebrand factor, thus potentially biasing factor VIII immunogenicity by perturbing its half-life. Interestingly, a factor VIII^Y1680C mutant, that does not bind von Willebrand factor, demonstrated unaltered endocytosis by dendritic cells as well as immunogenicity in factor VIII-deficient mice. Co-incubation of factor VIII^Y1680C with BO2C11, however, resulted in decreased factor VIII immunogenicity in vivo In addition, a previously described triple C1 mutant showed decreased uptake in vitro, and reduced immunogenicity in vivo, but only in the absence of endogenous von Willebrand factor. Taken together, the results indicate that residues in the C1 and/or C2 domains of factor VIII are implicated in immunogenic factor VIII uptake, at least in vitro Conversely, in vivo, the binding to endogenous von Willebrand factor masks the reducing effect of mutations in the C domains on factor VIII immunogenicity.

Key insights to understand the immunogenicity of FVIII products

The treatment of haemophilia has made significant progress in recent decades, and patients are now being treated safely with great clotting products. However, inhibitor development remains the largest problem, particularly in children. Consequently, the haemostasis that was obtained with traditional clotting factors is not being achieved. Moreover, inhibitor complications translate into adult life and there are an increasing number of situations where adult patients with an inhibitor require major surgery but the clinician is faced with the knowledge that required haemostasis levels are difficult to achieve. Therefore, it is of upmost importance to consider factors relating to inhibitor development, and to determine how inhibitors can be prevented and/or eliminated. Of the various factors at play with regard to inhibitor development, it is important to consider the immunogenicity of factor VIII (FVIII) products, and this topic is the focus of the current paper.

High-affinity, noninhibitory pathogenic C1 domain antibodies are present in patients with hemophilia A and inhibitors

Inhibitor formation in hemophilia A is the most feared treatment-related complication of factor VIII (fVIII) therapy. Most inhibitor patients with hemophilia A develop antibodies against the fVIII A2 and C2 domains. Recent evidence demonstrates that the C1 domain contributes to the inhibitor response. Inhibitory anti-C1 monoclonal antibodies (mAbs) have been identified that bind to putative phospholipid and von Willebrand factor (VWF) binding epitopes and block endocytosis of fVIII by antigen presenting cells. We now demonstrate by competitive enzyme-linked immunosorbent assay and hydrogen-deuterium exchange mass spectrometry that 7 of 9 anti-human C1 mAbs tested recognize an epitope distinct from the C1 phospholipid binding site. These mAbs, designated group A, display high binding affinities for fVIII, weakly inhibit fVIII procoagulant activity, poorly inhibit fVIII binding to phospholipid, and exhibit heterogeneity with respect to blocking fVIII binding to VWF. Another mAb, designated group B, inhibits fVIII procoagulant activity, fVIII binding to VWF and phospholipid, fVIIIa incorporation into the intrinsic Xase complex, thrombin generation in plasma, and fVIII uptake by dendritic cells. Group A and B epitopes are distinct from the epitope recognized by the canonical, human-derived inhibitory anti-C1 mAb, KM33, whose epitope overlaps both groups A and B. Antibodies recognizing group A and B epitopes are present in inhibitor plasmas from patients with hemophilia A. Additionally, group A and B mAbs increase fVIII clearance and are pathogenic in a hemophilia A mouse tail snip bleeding model. Group A anti-C1 mAbs represent the first identification of pathogenic, weakly inhibitory antibodies that increase fVIII clearance.

von Willebrand factor binds to the surface of dendritic cells and modulates peptide presentation of factor VIII

It has been proposed that von Willebrand factor might affect factor VIII immunogenicity by reducing factor VIII uptake by antigen presenting cells. Here we investigate the interaction of recombinant von Willebrand factor with immature monocyte-derived dendritic cells using flow cytometry and confocal microscopy. Surprisingly, von Willebrand factor was not internalized by immature dendritic cells, but remained bound to the cell surface. As von Willebrand factor reduces the uptake of factor VIII, we investigated the repertoire of factor VIII presented peptides when in complex with von Willebrand factor. Interestingly, factor VIII-derived peptides were still abundantly presented on major histocompatibility complex class II molecules, even though a reduction of factor VIII uptake by immature dendritic cells was observed. Inspection of peptide profiles from 5 different donors showed that different core factor VIII peptide sequences were presented upon incubation with factor VIII/von Willebrand factor complex when compared to factor VIII alone. No von Willebrand factor peptides were detected when immature dendritic cells were pulsed with different concentrations of von Willebrand factor, confirming lack of von Willebrand factor endocytosis. Several von Willebrand factor derived peptides were recovered when cells were pulsed with von Willebrand factor/factor VIII complex, suggesting that factor VIII promotes endocytosis of small amounts of von Willebrand factor by immature dendritic cells. Taken together, our results establish that von Willebrand factor is poorly internalized by immature dendritic cells. We also show that von Willebrand factor modulates the internalization and presentation of factor VIII-derived peptides on major histocompatibility complex class II.

To clear or to fear: An innate perspective on factor VIII immunity

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FVIII inhibitor development involves a combination of innate immune modulators.

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Clearance and immunity is influenced at 3 levels: the protein, cell, and location.

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Cells associated with FVIII half-life may influence the immune response against FVIII.

The enigma that is factor VIII immunogenicity remains ever pertinent in the treatment of hemophilia A. Development of neutralizing antibodies against the therapeutic protein in 25–30% of patients likely depends on the appropriate activation of the innate immune response shortly following antigen encounter. Our understanding of this important immunological synapse remains ill-defined. In this review, we examine the three distinct factors contributing to the fate of factor VIII almost immediately after infusion: the characteristics of the protein, the cell, and the microenvironment. We propose a continuum between clearance and antigen presentation that facilitates removal of FVIII from circulation leading to either tolerance or immunity.

Alloantibodies to therapeutic factor VIII in hemophilia A: the role of von Willebrand factor in regulating factor VIII immunogenicity

The rising incidence of neutralizing antibodies (inhibitors) against therapeutic factor VIII prompted the conduct of studies to answer the question as to whether this rise is related to the introduction of recombinant factor VIII products. The present article summarizes current opinions and results of non-clinical and clinical studies on the immunogenic potential of recombinant compared to plasma-derived factor VIII concentrates. Numerous studies provided circumstantial evidence that von Willebrand factor, the natural chaperone protein present in plasma-derived factor VIII products, plays an important role in protecting exogenous factor VIII from uptake by antigen presenting cells and from recognition by immune effectors. However, the definite contribution of von Willebrand factor in reducing the inhibitor risk and in the achievement of immune tolerance is still under debate.

Intramedullary megakaryocytes internalize released platelet factor 4 and store it in alpha granules

BACKGROUND

Megakaryocytes express and store platelet factor 4 (PF4) in alpha granules. In vivo, PF4 is a clinically relevant, negative regulator of megakaryopoiesis and hematopoietic stem cell replication. These findings would suggest a regulated source of free intramedullary PF4.

OBJECTIVES

Define the source of free intramedullary PF4 and its intramedullary life cycle.

METHODS

We interrogated both murine and human bone marrow-derived cells during megakaryopoiesis in vitro by using confocal microscopy and enzyme-linked immunosorbent assay. With immunohistochemistry, we examined in vivo free PF4 in murine bone marrow before and after radiation injury and in the setting of megakaryocytopenia and thrombocytopenia.

RESULTS

Exogenously added human PF4 is internalized by murine megakaryocytes. Human megakaryocytes similarly take up murine PF4 but not the related chemokine, platelet basic protein. Confocal microscopy shows that internalized PF4 colocalizes with endogenous PF4 in alpha granules and is available for release on thrombin stimulation. Immunohistochemistry shows free PF4 in the marrow, but not another alphagranule protein, von Willebrand factor. Free PF4 increases with radiation injury and decreases with megakaryocytopenia. Consistent with the known role of low-density lipoprotein receptor-related protein 1 in the negative paracrine effect of PF4 on megakaryopoiesis, PF4 internalization is at least partially low-density lipoprotein receptor-related protein 1 dependent.

CONCLUSIONS

PF4 has a complex intramedullary life cycle with important implications in megakaryopoiesis and hematopoietic stem cell replication not seen with other tested alpha granule proteins.

Factor VIII Interacts with the Endocytic Receptor Low-density Lipoprotein Receptor-related Protein 1 via an Extended Surface Comprising "Hot-Spot" Lysine Residues

Lysine residues are implicated in driving the ligand binding to the LDL receptor family. However, it has remained unclear how specificity is regulated. Using coagulation factor VIII as a model ligand, we now study the contribution of individual lysine residues in the interaction with the largest member of the LDL receptor family, low-density lipoprotein receptor-related protein (LRP1). Using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and SPR interaction analysis on a library of lysine replacement variants as two independent approaches, we demonstrate that the interaction between factor VIII (FVIII) and LRP1 occurs over an extended surface containing multiple lysine residues. None of the individual lysine residues account completely for LRP1 binding, suggesting an additive binding model. Together with structural docking studies, our data suggest that FVIII interacts with LRP1 via an extended surface of multiple lysine residues that starts at the bottom of the C1 domain and winds around the FVIII molecule.

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.

New predictive approaches for ITI treatment

Immune tolerance induction (ITI) therapy in patients with haemophilia A and inhibitors constitutes a huge burden for affected patients and families and poses a large economic burden for a chronic disease. Concerted research efforts are attempting to optimize the therapeutic approach to the prevention and eradication of inhibitors. The Italian ITI Registry has provided data on 110 patients who completed ITI therapy as at July 2013. Analysis of independent predictors of success showed that, together with previously recognized factors - namely inhibitor titre prior to ITI, historical peak titre and peak titre on ITI - the type of causative FVIII gene mutation also contributes to the identification of patients with good prognosis and may be useful to optimize candidate selection and treatment regimens. Numerous studies have demonstrated that inhibitor reactivity against different FVIII products varies and is lower against concentrates containing von Willebrand factor (VWF). An Italian study compared inhibitor titres against a panel of FVIII concentrates in vitro and correlated titres with the capacity to inhibit maximum thrombin generation as measured by the thrombin generation assay (TGA). Observations led to the design of the PredictTGA study which aims to correlate TGA results with epitope specificity, inhibitor reactivity against different FVIII concentrates and clinical data in inhibitor patients receiving FVIII in the context of ITI or as prophylactic/on demand treatment. At the immunological level, it is known that T cells drive inhibitor development and that B cells secrete FVIII-specific antibodies. As understanding increases about the immunological response in ITI, it is becoming apparent that modulation of T-cell- and B-cell-mediated responses offers a range of potential new and specific approaches to prevent and eliminate inhibitors as well as individualize ITI therapy.

Limited promiscuity of HLA-DRB1 presented peptides derived of blood coagulation factor VIII

The formation of inhibitory antibodies directed against coagulation factor VIII (FVIII) is a severe complication in the treatment of hemophilia A patients. The induction of anti-FVIII antibodies is a CD4⁺ T cell-dependent process. Activation of FVIII-specific CD4⁺ T cells is dependent on the presentation of FVIII-derived peptides on MHC class II by antigen-presenting cells. Previously, we have shown that FVIII-pulsed human monocyte-derived dendritic cells can present peptides from several FVIII domains. In this study we show that FVIII peptides are presented on immature as well as mature dendritic cells. In immature dendritic cells half of the FVIII-loaded MHC class II molecules are retained within the cell, whereas in LPS-matured dendritic cells the majority of MHC class II/peptide complexes is present on the plasma membrane. Time-course studies revealed that presentation of FVIII-derived peptides was optimal between 12 and 24 hours after maturation but persisted for at least 96 hours. We also show that macrophages are able to internalize FVIII as efficiently as dendritic cells, however FVIII was presented on MHC class II with a lower efficiency and with different epitopes compared to dendritic cells. In total, 48 FVIII core-peptides were identified using a DCs derived of 8 different donors. Five HLA-promiscuous FVIII peptide regions were found – these were presented by at least 4 out of 8 donors. The remaining 42 peptide core regions in FVIII were presented by DCs derived from a single (30 peptides) or two to three donors (12 peptides). Overall, our findings show that a broad repertoire of FVIII peptides can be presented on HLA-DR.

Factor VIII C1 domain spikes 2092-2093 and 2158-2159 comprise regions that modulate cofactor function and cellular uptake

The C1 domain of factor VIII (FVIII) has been implicated in binding to multiple constituents, including phospholipids, von Willebrand factor, and low-density lipoprotein receptor-related protein (LRP). We have previously described a human monoclonal antibody called KM33 that blocks these interactions as well as cellular uptake by LRP-expressing cells. To unambiguously identify the apparent "hot spot" on FVIII to which this antibody binds, we have employed hydrogen-deuterium exchange mass spectrometry. The results showed that KM33 protects FVIII regions 2091-2104 and 2157-2162 from hydrogen-deuterium exchange. These comprise the two C1 domain spikes 2092-2093 and 2158-2159. Spike 2092-2093 has been demonstrated recently to contribute to assembly with lipid membranes with low phosphatidylserine (PS) content. Therefore, spike 2158-2159 might serve a similar role. This was assessed by replacement of Arg-2159 for Asn, which introduces a motif for N-linked glycosylation. Binding studies revealed that the purified, glycosylated R2159N variant had lost its interaction with antibody KM33 but retained substantial binding to von Willebrand factor and LRP. Cellular uptake of the R2159N variant was reduced both by LRP-expressing U87-MG cells and by human monocyte-derived dendritic cells. FVIII activity was virtually normal on membranes containing 15% PS but reduced at low PS content. These findings suggest that the C1 domain spikes 2092-2093 and 2158-2159 together modulate FVIII membrane assembly by a subtle, PS-dependent mechanism. These findings contribute evidence in favor of an increasingly important role of the C1 domain in FVIII biology.