Induction of partial immune tolerance to factor VIII through prior mucosal exposure to the factor VIII C2 domain

Escape or Fight: Inhibitors in Hemophilia A

Replacement therapy with coagulation factor VIII (FVIII) represents the current clinical treatment for patients affected by hemophilia A (HA). This treatment while effective is, however, hampered by the formation of antibodies which inhibit the activity of infused FVIII in up to 30% of treated patients. Immune tolerance induction (ITI) protocols, which envisage frequent infusions of high doses of FVIII to confront this side effect, dramatically increase the already high costs associated to a patient's therapy and are not always effective in all treated patients. Therefore, there are clear unmet needs that must be addressed in order to improve the outcome of these treatments for HA patients. Taking advantage of preclinical mouse models of hemophilia, several strategies have been proposed in recent years to prevent inhibitor formation and eradicate the pre-existing immunity to FVIII inhibitor positive patients. Herein, we will review some of the most promising strategies developed to avoid and eradicate inhibitors, including the use of immunomodulatory drugs or molecules, oral or transplacental delivery as well as cell and gene therapy approaches. The goal is to improve and potentiate the current ITI protocols and eventually make them obsolete.

Innovative Approaches for Immune Tolerance to Factor VIII in the Treatment of Hemophilia A

Hemophilia A (coagulation factor VIII deficiency) is a debilitating genetic disorder that is primarily treated with intravenous replacement therapy. Despite a variety of factor VIII protein formulations available, the risk of developing anti-dug antibodies (“inhibitors”) remains. Overall, 20–30% of patients with severe disease develop inhibitors. Current clinical immune tolerance induction protocols to eliminate inhibitors are not effective in all patients, and there are no prophylactic protocols to prevent the immune response. New experimental therapies, such as gene and cell therapies, show promising results in pre-clinical studies in animal models of hemophilia. Examples include hepatic gene transfer with viral vectors, genetically engineered regulatory T cells (Treg), in vivo Treg induction using immune modulatory drugs, and maternal antigen transfer. Furthermore, an oral tolerance protocol is being developed based on transgenic lettuce plants, which suppressed inhibitor formation in hemophilic mice and dogs. Hopefully, some of these innovative approaches will reduce the risk of and/or more effectively eliminate inhibitor formation in future treatment of hemophilia A.

Oral Tolerance Induction in Hemophilia B Dogs Fed with Transplastomic Lettuce

Anti-drug antibodies in hemophilia patients substantially complicate treatment. Their elimination through immune tolerance induction (ITI) protocols poses enormous costs, and ITI is often ineffective for factor IX (FIX) inhibitors. Moreover, there is no prophylactic ITI protocol to prevent anti-drug antibody (ADA) formation. Using general immune suppression is problematic. To address this urgent unmet medical need, we delivered antigen bioencapsulated in plant cells to hemophilia B dogs. Commercial-scale production of CTB-FIX fusion expressed in lettuce chloroplasts was done in a hydroponic facility. CTB-FIX (∼1 mg/g) in lyophilized cells was stable with proper folding, disulfide bonds, and pentamer assembly after 30-month storage at ambient temperature. Robust suppression of immunoglobulin G (IgG)/inhibitor and IgE formation against intravenous FIX was observed in three of four hemophilia B dogs fed with lyophilized lettuce cells expressing CTB-FIX. No side effects were detected after feeding CTB-FIX-lyophilized plant cells for >300 days. Coagulation times were markedly shortened by intravenous FIX in orally tolerized treated dogs, in contrast to control dogs that formed high-titer antibodies to FIX. Commercial-scale production, stability, prolonged storage of lyophilized cells, and efficacy in tolerance induction in a large, non-rodent model of human disease offer a novel concept for oral tolerance and low-cost production and delivery of biopharmaceuticals.

Recombinant factor VIII Fc (rFVIIIFc) fusion protein reduces immunogenicity and induces tolerance in hemophilia A mice

Anti-factor VIII (FVIII) antibodies is a major complication of FVIII replacement therapy for hemophilia A. We investigated the immune response to recombinant human factor VIII Fc (rFVIIIFc) in comparison to BDD-rFVIII and full-length rFVIII (FL-rFVIII) in hemophilia A mice. Repeated administration of therapeutically relevant doses of rFVIIIFc in these mice resulted in significantly lower antibody responses to rFVIII compared to BDD-rFVIII and FL-rFVIII and reduced antibody production upon subsequent challenge with high doses of rFVIIIFc. The induction of a tolerogenic response by rFVIIIFc was associated with higher percentage of regulatory T-cells, a lower percentage of pro-inflammatory splenic T-cells, and up-regulation of tolerogenic cytokines and markers. Disruption of Fc interactions with either FcRn or Fcγ receptors diminished tolerance induction, suggesting the involvement of these pathways. These results indicate that rFVIIIFc reduces immunogenicity and imparts tolerance to rFVIII demonstrating that recombinant therapeutic proteins may be modified to influence immunogenicity and facilitate tolerance.

Plant-based oral tolerance to hemophilia therapy employs a complex immune regulatory response including LAP+CD4+ T cells

Coagulation factor replacement therapy for the X-linked bleeding disorder hemophilia is severely complicated by antibody ("inhibitor") formation. We previously found that oral delivery to hemophilic mice of cholera toxin B subunit-coagulation factor fusion proteins expressed in chloroplasts of transgenic plants suppressed inhibitor formation directed against factors VIII and IX and anaphylaxis against factor IX (FIX). This observation and the relatively high concentration of antigen in the chloroplasts prompted us to evaluate the underlying tolerance mechanisms. The combination of oral delivery of bioencapsulated FIX and intravenous replacement therapy induced a complex, interleukin-10 (IL-10)-dependent, antigen-specific systemic immune suppression of pathogenic antibody formation (immunoglobulin [Ig] 1/inhibitors, IgE) in hemophilia B mice. Tolerance induction was also successful in preimmune mice but required prolonged oral delivery once replacement therapy was resumed. Orally delivered antigen, initially targeted to epithelial cells, was taken up by dendritic cells throughout the small intestine and additionally by F4/80(+) cells in the duodenum. Consistent with the immunomodulatory responses, frequencies of tolerogenic CD103(+) and plasmacytoid dendritic cells were increased. Ultimately, latency-associated peptide expressing CD4(+) regulatory T cells (CD4(+)CD25(-)LAP(+) cells with upregulated IL-10 and transforming growth factor-β (TGF-β) expression) as well as conventional CD4(+)CD25(+) regulatory T cells systemically suppressed anti-FIX responses.

Inhibitors - cellular aspects and novel approaches for tolerance

The immune response against therapeutic clotting factors VIII and IX (FVIII and FIX) is a major adverse event that can effectively thwart their effectiveness in correcting bleeding disorders. Thus, a significant number of haemophilia patients form antibodies, called inhibitors, which neutralize the procoagulant functions of therapeutic cofactors FVIII (haemophilia A) or FIX (haemophilia B). Understanding the cellular and molecular aspects of inhibitor formation is critical to designing tolerogenic therapies for clinical use. This review will focus on the basis of the immune response to FVIII, in particular, and will discuss emerging efforts to not only reduce immunogenicity but also to prevent and/or reverse inhibitor formation.

Suppression of inhibitor formation against FVIII in a murine model of hemophilia A by oral delivery of antigens bioencapsulated in plant cells

Hemophilia A is the X-linked bleeding disorder caused by deficiency of coagulation factor VIII (FVIII). To address serious complications of inhibitory antibody formation in current replacement therapy, we created tobacco transplastomic lines expressing FVIII antigens, heavy chain (HC) and C2, fused with the transmucosal carrier, cholera toxin B subunit. Cholera toxin B-HC and cholera toxin B-C2 fusion proteins expressed up to 80 or 370 µg/g in fresh leaves, assembled into pentameric forms, and bound to GM1 receptors. Protection of FVIII antigen through bioencapsulation in plant cells and oral delivery to the gut immune system was confirmed by immunostaining. Feeding of HC/C2 mixture substantially suppressed T helper cell responses and inhibitor formation against FVIII in mice of 2 different strain backgrounds with hemophilia A. Prolonged oral delivery was required to control inhibitor formation long-term. Substantial reduction of inhibitor titers in preimmune mice demonstrated that the protocol could also reverse inhibitor formation. Gene expression and flow cytometry analyses showed upregulation of immune suppressive cytokines (transforming growth factor β and interleukin 10). Adoptive transfer experiments confirmed an active suppression mechanism and revealed induction of CD4(+)CD25(+) and CD4(+)CD25(-) T cells that potently suppressed anti-FVIII formation. In sum, these data support plant cell-based oral tolerance for suppression of inhibitor formation against FVIII.

Oral delivery of human biopharmaceuticals, autoantigens and vaccine antigens bioencapsulated in plant cells

Among 12billion injections administered annually, unsafe delivery leads to >20million infections and >100million reactions. In an emerging new concept, freeze-dried plant cells (lettuce) expressing vaccine antigens/biopharmaceuticals are protected in the stomach from acids/enzymes but are released to the immune or blood circulatory system when plant cell walls are digested by microbes that colonize the gut. Vaccine antigens bioencapsulated in plant cells upon oral delivery after priming, conferred both mucosal and systemic immunity and protection against bacterial, viral or protozoan pathogens or toxin challenge. Oral delivery of autoantigens was effective against complications of type 1 diabetes and hemophilia, by developing tolerance. Oral delivery of proinsulin or exendin-4 expressed in plant cells regulated blood glucose levels similar to injections. Therefore, this new platform offers a low cost alternative to deliver different therapeutic proteins to combat infectious or inherited diseases by eliminating inactivated pathogens, expensive purification, cold storage/transportation and sterile injections.

Mechanism of oral tolerance induction to therapeutic proteins

Oral tolerance is defined as the specific suppression of humoral and/or cellular immune responses to an antigen by administration of the same antigen through the oral route. Due to its absence of toxicity, easy administration, and antigen specificity, oral tolerance is a very attractive approach to prevent unwanted immune responses that cause a variety of diseases or that complicate treatment of a disease. Many researchers have induced oral tolerance to efficiently treat autoimmune and inflammatory diseases in different animal models. However, clinical trials yielded limited success. Thus, understanding the mechanisms of oral tolerance induction to therapeutic proteins is critical for paving the way for clinical development of oral tolerance protocols. This review will summarize progress on understanding the major underlying tolerance mechanisms and contributors, including antigen presenting cells, regulatory T cells, cytokines, and signaling pathways. Potential applications, examples for therapeutic proteins and disease targets, and recent developments in delivery methods are discussed.

Factor VIII inhibitors in hemophilia A: rationale and latest evidence

Factor VIII (FVIII) replacement therapy is the foundation of treatment in hemophilia A and is effective unless a patient develops an alloantibody (inhibitor) against exogenous FVIII. Inhibitor development is currently the most significant treatment complication seen in patients with hemophilia and is associated with considerable morbidity and a decreased quality of life. The development of an inhibitor is the result of a complex interaction between a patient's immune system and genetic and environmental risk factors. The mainstay of treatment is the eradication of the inhibitor through immune tolerance. This review summarizes the current evidence regarding inhibitor risk factors, eradication, and hemostatic bypassing agents.

Progress toward inducing immunologic tolerance to factor VIII

A major problem in treating hemophilia A patients with therapeutic factor VIII (FVIII) is that 20% to 30% of these patients produce neutralizing anti-FVIII antibodies. These antibodies block (inhibit) the procoagulant function of FVIII and thus are termed "inhibitors." The currently accepted clinical method to attempt to eliminate inhibitors is immune tolerance induction (ITI) via a protocol requiring intensive FVIII treatment until inhibitor titers drop. Although often successful, ITI is extremely costly and is less likely to succeed in patients with high-titer inhibitors. During the past decade, significant progress has been made in clarifying mechanisms of allo- and autoimmune responses to FVIII and in suppression of these responses. Animal model studies are suggesting novel, less costly methods to induce tolerance to FVIII. Complementary studies of anti-FVIII T-cell responses using blood samples from human donors are identifying immunodominant T-cell epitopes in FVIII and possible targets for tolerogenic efforts. Mechanistic experiments using human T-cell clones and lines are providing a clinically relevant counterpoint to the animal model studies. This review highlights recent progress toward the related goals of lowering the incidence of anti-FVIII immune responses and promoting durable, functional immune tolerance to FVIII in patients with an existing inhibitor.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

[Immune mechanisms involved in the development and eradication of anti-factor VIII alloantibodies in hemophilia]

Hemophilia A is an X-linked hereditary bleeding disorder caused by a congenital deficiency in blood coagulation factor VIII (FVIII). Therapy to prevent or treat bleeding is replacement of FVIII. The most significant complication of treatment in patients with hemophilia A is the development of alloantibodies that inhibit FVIII activity, termed inhibitors. In the presence of inhibitors, replacement of the missing clotting factor with FVIII preparations becomes less effective. Once replacement therapy is ineffective, morbidity increases. It remains unsolved to prevent inhibitor formation. The only strategy is long-term administration of a large quantity of FVIII in an attempt to eradicate the inhibitors through immune tolerance. However, little is known about the mechanisms involved in the induction of tolerance. This review will focus on the current understanding of why inhibitors develop and can be eradicated. The development of inhibitors by intravenous infusions of FVIII without adjuvant poses an intriguing challenge to immunologists.

Functional aspects of factor VIII expression after transplantation of genetically-modified hematopoietic stem cells for hemophilia A

BACKGROUND

Major complications with respect to the development of gene therapy treatments for hemophilia A include low factor VIII (fVIII) expression and humoral immune responses resulting in inhibitory anti-fVIII antibodies. We previously achieved sustained curative fVIII activity levels in hemophilia A mice after nonmyeloablative transplantation of genetically-modified hematopoietic stem cells (HSCs) encoding a B-domain deleted porcine fVIII (BDDpfVIII) transgene with no evidence of an immune response.

METHODS

Mouse HSCs were transduced using MSCV-based recombinant virus encoding BDDpfVIII and transplanted into hemophilia A mice. Transplanted mice were followed for donor cell engraftment, fVIII expression and activity, and generation of anti-fVIII immune response.

RESULTS

We now show that: (i) the protein expressed by hematopoietic cells has a specific activity similar to that of purified protein; (ii) BDDpfVIII expressed from hematopoietic cells effectively induces thrombus formation, which is shown using a new method of in vivo analysis of fVIII function; (iii) naïve and pre-immunized mice receiving HSC gene therapy are nonresponsive to challenges with recombinant human fVIII; (iv) nonresponsiveness is not broken after stringent challenges with BDDpfVIII; and (v) T cells from these mice are unresponsive to BDDpfVIII presentation. Furthermore, stem cells isolated from donors with high titer anti-human fVIII antibodies show no defects in donor cell engraftment or the ability to express BDDpfVIII.

CONCLUSIONS

These results demonstrate that HSC gene therapy can be an effective alternative treatment for individuals with hemophilia A and may benefit patients by inducing immunological nonresponsiveness to fVIII replacement products.

Immunomodulation for inhibitors in hemophilia A: the important role of Treg cells

Approximately 25-30% of the hemophilia A patients develop inhibitory antibodies against Factor VIII (FVIII) following protein-replacement therapy. This problem is also thought to occur following gene-replacement therapy. Recently, many approaches have been investigated to modulate FVIII-specific immune responses in either protein-replacement or gene therapy hemophilia A mouse models. Several promising protocols have been demonstrated to successfully prevent or modulate the formation of anti-FVIII antibodies, including methods to manipulate antigen presentation, development of less immunogenic FVIII proteins, or formulations or gene therapy protocols to evade immune responses, as well as immunomodulation strategies to target either T- and/or B-cell responses. Most of these successful protocols involve the induction of activated Treg cells to create a regulatory immune environment during tolerance induction. Innovative strategies to overcome pre-existing anti-FVIII immune responses and induce long-term tolerance in primed subjects still need to be developed.

Oral delivery of bioencapsulated coagulation factor IX prevents inhibitor formation and fatal anaphylaxis in hemophilia B mice

To address complications of pathogenic antibody or life-threatening anaphylactic reactions in protein replacement therapy for patients with hemophilia or other inherited protein deficiencies, we have developed a prophylactic protocol using a murine hemophilia B model. Oral delivery of coagulation factor IX fused with cholera toxin beta-subunit (with or without a furin cleavage site; CTB-FFIX or CTB-FIX), expressed in chloroplasts (up to 3.8% soluble protein or 0.4 mg/g leaf tissue), bioencapsulated in plant cells, effectively blocked formation of inhibitory antibodies (undetectable or up to 100-fold less than controls). Moreover, this treatment eliminated fatal anaphylactic reactions that occurred after four to six exposures to intravenous F.IX. Whereas only 20-25% of control animals survived after six to eight F.IX doses, 90-93% of F.IX-fed mice survived 12 injections without signs of allergy or anaphylaxis. Immunostaining confirmed delivery of F.IX to Peyer's patches in the ileum. Within 2-5 h, feeding of CTB-FFIX additionally resulted in systemic delivery of F.IX antigen. This high-responder strain of hemophilia B mice represents a new animal model to study anaphylactic reactions. The protocol was effective over a range of oral antigen doses (equivalent to 5-80 microg recombinant F.IX/kg), and controlled inhibitor formation and anaphylaxis long-term, up to 7 months (approximately 40% life span of this mouse strain). Oral antigen administration caused a deviant immune response that suppressed formation of IgE and inhibitory antibodies. This cost-effective and efficient approach of antigen delivery to the gut should be applicable to several genetic diseases that are prone to pathogenic antibody responses during treatment.

Suppression of FVIII inhibitor formation in hemophilic mice by delivery of transgene modified apoptotic fibroblasts

The development of inhibitory antibodies to factor VIII (FVIII) is currently the most significant complication of FVIII replacement therapy in the management of patients with severe hemophilia A. Immune tolerance protocols for the eradication of inhibitors require daily delivery of intravenous FVIII for at least 6 months and are unsuccessful in 20-40% of treated patients. We hypothesize that tolerance can be induced more efficiently and reliably by delivery of FVIII antigen within autologous apoptotic cells (ACs). In this study, we demonstrated suppression of the T cell and inhibitor responses to FVIII by infusion of FVIII expression vector modified apoptotic syngeneic fibroblasts in both naive and preimmunized hemophilia A mice. ACs without FVIII antigen exerted modest generalized immune suppression mediated by anti-inflammatory signals. However, FVIII expressing apoptotic syngeneic fibroblasts produced much stronger antigen-specific immune suppression. Mice treated with these fibroblasts generated CD4+ T cells that suppressed the immune response to FVIII after adoptive transfer into naive recipients and antigen-specific CD4+CD25+ regulatory T cells (Tregs) that inhibited the proliferation of FVIII responsive effector T cells in vitro. These preclinical results demonstrate the potential for using FVIII vector modified autologous ACs to treat high-titer inhibitors in patients with hemophilia A.

The molecular mechanisms of immunomodulation and tolerance induction to factor VIII

Successful factor (F) VIII replacement therapy in hemophilia A patients is confounded by the generation of inhibitory anti-FVIII antibodies (Ab) in 25-30% of treated patients. These antibodies, termed 'inhibitors', significantly increase morbidity within the hemophilia population and lower the quality of life for these patients. For the past 30 years, immune tolerance induction (ITI) has been the standard therapy to elicit immunological tolerance to FVIII in the clinic. ITI works well in approximately 75% of patients, but it is expensive, can take years to show effect and is in many cases practically challenging. Therefore, new immunological tolerance induction strategies are now being designed and tested in hemophilia A animal models. This review attempts to provide a comprehensive description, at both the cellular and molecular levels, of these novel advances in tolerance induction and immunomodulation of FVIII. We begin by briefly reviewing why and how the immune system generates a protective response against exogenous FVIII. This leads to a discussion of the latest advances in FVIII tolerance/immunomodulation technology. These advances include interesting methodologies to induce B cell specific tolerance in FVIII primed humans and animals, as well as newer T cell-specific therapies that modify and/or block co-stimulation. We also discuss methods to manipulate FVIII loading of antigen-presenting cells.

Role of regulatory T cells in tolerance to coagulation factors

The immune response to coagulation factors VIII or IX, in particular formation of inhibitory antibodies, complicates treatment of hemophilia. Therefore, a number of recent studies in animal models have explored novel approaches toward induction of immune tolerance in protein or gene replacement therapy. Strong evidence has emerged that regulatory T cells (Treg) are an important component of the mechanism by which tolerance is maintained and inhibitor formation, a T help dependent response, is prevented. Limited data in patients also support this concept. In particular, CD4+ CD25+ FoxP3+ Treg, whether naturally occurring or induced, have been invoked in suppression of antibody and of cytotoxic T lymphocyte responses to the therapeutic clotting factor. This review summarizes the data on this emerging concept of Treg-mediated regulation of the immune response in treatment of hemophilia, strategies and mechanisms of Treg induction and function, and the implications for development of immune tolerance protocols.

Inhibitor development

The immune response to factor VIII and the development of inhibitory antibodies is a complex multi-factorial process involving a variety of immune regulatory genes and cells, several of which have the potential to determine risk. A better understanding of the mechanisms involved will increase the likelihood of development of new therapeutic options for patients with hemophilia. This review summarizes genetic and non-genetic risk factors currently under evaluation, and the potential modulative effect of the von Willebrand factor on factor VIII immuno- and antigenicity. In addition, the role of T-regulatory cells in the pathogenicity of inhibitors will be discussed.

Transient blockade of the inducible costimulator pathway generates long-term tolerance to factor VIII after nonviral gene transfer into hemophilia A mice

Formation of inhibitory antibodies is a common problem encountered in clinical treatment for hemophilia. Human factor VIII (hFVIII) plasmid gene therapy in hemophilia A mice also leads to strong humoral responses. We demonstrate that short-term therapy with an anti-ICOS monoclonal antibody to transiently block the inducible costimulator/inducible costimulator ligand (ICOS/ICOSL) signaling pathway led to sustained tolerance to hFVIII in hFVIII plasmid-treated hemophilia A mice and allowed persistent, high-level FVIII functional activity (100%-300% of normal). Anti-ICOS treatment resulted in depletion of ICOS(+)CD4(+) T cells and activation of CD25(+)Foxp3(+) Tregs in the peripheral blood, spleen, and lymph nodes. CD4(+) T cells from anti-ICOS-treated mice did not proliferate in response to hFVIII stimulation and produced high levels of regulatory cytokines, including interleukin-10 and transforming growth factor-beta. Moreover, CD4(+)CD25(+) Tregs from tolerized mice adoptively transferred dominant tolerance in syngeneic hFVIII plasmid-treated hemophilia A mice and reduced the production of antibodies against FVIII. Anti-ICOS-treated mice tolerized to hFVIII generated normal primary and secondary antibody responses after immunization with the T-dependent antigen, bacteriophage Phix 174, indicating maintenance of immune competency. Our data indicate that transient anti-ICOS monoclonal antibody treatment represents a novel single-agent immunomodulatory strategy to overcome the immune responses against transgene product after gene therapy.

Dynamics of factor VIII interactions determine its immunologic fate in hemophilia A

Procoagulant factor VIII (FVIII) is either produced endogenously under physiologic conditions, or administered exogenously as a therapeutic hemostatic drug in patients with hemophilia A. In the circulation, FVIII interacts with a multitude of glycoproteins, and may be used for coagulation at the sites of bleeding, eliminated by scavenger cells, or processed by the immune system, either as a self-constituent or as a foreign antigen. The fate of FVIII is dictated by the immune status of the individual, the location of FVIII in the body at a given time point, and the inflammatory microenvironment. It also depends on the local concentration of FVIII and of each interacting partner, and on the affinity of the respective interactions. FVIII, by virtue of its promiscuity, thus constitutes the core of a dynamic network that links the coagulation cascade, cells of the immune system, and, presumably, the inflammatory compartment. We describe the different interactions that FVIII is prone to establish during its life cycle, with a special focus on players of the innate and adaptive immune response. Lessons can be learned from understanding the dynamics of FVIII interactions--lessons that should pave the way to the conception of long-lasting hemostatic drugs devoid of iatrogenic immunogenicity.

T-cell responses over time in a mild hemophilia A inhibitor subject: epitope identification and transient immunogenicity of the corresponding self-peptide

BACKGROUND

Antibodies that neutralize factor (F) VIII activity, clinically referred to as 'inhibitors', complicate the treatment of hemophilia A patients; current tolerance and bypass strategies are extremely costly and sometimes ineffective. The development of inhibitors requires T-cell help.

OBJECTIVES

We characterized T-cell responses of a subject with mild hemophilia A with missense genotype A2201P for one year following his initial inhibitor response, with the goals of defining the primary epitope(s) and its (their) MHC Class II restriction. We investigated the possible involvement of regulatory T cells in modulating immune responses.

PATIENTS/METHODS

The subject developed high-titer FVIII-neutralizing antibodies (250 BU mL(-1)) that declined over time to 8 BU ml(-1). His clotting activity was initially impaired (3%) but returned to baseline (8-10%) within four weeks. MHC Class II tetramers were used to analyze his CD4 T cells, which were stimulated with peptides spanning the C2 domain. Responses of total and CD25-depleted CD4 cells to sequences containing A2201 (native), P2201 (hemophilic), and other predicted T-cell epitopes were evaluated.

RESULTS AND CONCLUSIONS

An HLA-DRA-DRB1*0101 restricted T-cell epitope containing the wild-type A2201 sequence was identified. Interestingly, peptides containing A2201 were recognized by CD4 T cells at all time points, whereas a P2201 peptide was recognized only near the initial peak response. The responsiveness of CD25-depleted CD4 cells to an A2201 peptide was enhanced 11 and 19 weeks following inhibitor detection, suggesting the possible involvement of CD4+CD25+ regulatory T cells in modulating immune responses. Patient-derived T-cell clones proliferated in response to C2 protein and to peptides containing A2201 but not P2201.

Assessment of in vitro cytokine response in hemophilia A patients with or without factor VIII inhibitory antibody

Factor VIII (FVIII) inhibitor antibodies are produced in a proportion of hemophilia A patients. Development of anti-FVIII inhibitor antibodies is a T cell-dependent response, mediated by FVIII specific CD4(+) T cells. This study was performed to investigate the contribution of T helper (Th) cell-mediated cytokine response in inhibitor production. Peripheral blood mononuclear cells (PBMCs) were obtained from hemophilia A patients with (n = 14) or without inhibitor (n = 14) and from normal individuals (n = 14). Following stimulation of PBMCs with rFVIII and phytohemagglutinin (PHA) mitogen, the secreted cytokines, interferon-gamma (IFN-gamma), interleukin-10 (IL-10), and transforming growth factor-beta1 (TGF-beta1), in culture supernatant and the proliferative response were assessed using sandwich ELISA and (3)H-thymidine incorporation, respectively. No significant proliferative response to FVIII was observed, whereas PHA induced a strong response in all groups. No cytokine secretion was observed in response to FVIII stimulation. Although PHA induced IL-10, TGF-beta1 and IFN-gamma secretion in all groups, the level of IFN-gamma was significantly lower in hemophilia A patients than in normal individuals (p < 0.0001). The levels of TGF-beta1 and IL-10 were similarly higher in patients compared with normal subjects, but the difference was not statistically significant. Lack of FVIII-induced proliferative response and cytokine production together with reduced secretion of PHA-induced IFN-gamma in both groups of patients suggest involvement of nonspecific immunosuppression possibly due to hepatitis C virus (HCV) infection observed in the majority of patients.

Mechanisms of action of immune tolerance induction against factor VIII in patients with congenital haemophilia A and factor VIII inhibitors

In its most severe form, haemophilia A is a life-threatening haemorrhagic bleeding disorder that is caused by mutations in the factor VIII (FVIII) gene. About 25% of patients who receive replacement therapy with intravenous FVIII products develop neutralising antibodies (FVIII inhibitors) that inhibit the function of substituted FVIII. Long-term application of high or low doses of FVIII has evolved as an effective strategy for eradicating antibodies and inducing long-lasting immune tolerance. Despite clinical experience with the therapy, little is known about the immunological mechanisms that cause the down modulation of FVIII-specific immune responses or the induction of long-lasting immune tolerance against FVIII. This review summarises current knowledge of the immunological mechanisms that might be involved in the induction of immune tolerance against FVIII in patients with haemophilia A who have FVIII inhibitors. In addition to data from patients with haemophilia A, data from patients who have had organ transplants or have immune-related disorders, such as autoimmune diseases, are considered as well as data from animal models.