Restricted BV gene usage by factor VIII-reactive CD4+ T cells in inhibitor-positive patients with severe hemophilia A

Study of the T-cell receptor repertoire by CDR3 spectratyping

The T-cell receptor (TCR) is the key player within the so called immunological synapse and the analysis of its repertoire offers a picture of both versatility and wideness of the whole immune T-cell compartment. Among the different approaches applied to its study the so-called spectratyping identifies the pattern of the third complementarity determining region (CDR3) length distribution in each one of the beta variable (TRBV) subfamilies encoded by the corresponding genes. This technique consists in a CDR3 fragment analysis through capillary electrophoresis, performed after cell separation, RNA extraction and reverse transcriptase PCR. This review will run through the most relevant studies which have tried to dissect the TCR repertoire usage in patients with different immune-mediated and infective diseases as well as solid or haematologic malignancies.

T cells from hemophilia A subjects recognize the same HLA-restricted FVIII epitope with a narrow TCR repertoire

Factor VIII (FVIII)-neutralizing antibodies ("inhibitors") are a serious problem in hemophilia A (HA). The aim of this study was to characterize HLA-restricted T-cell responses from a severe HA subject with a persistent inhibitor and from 2 previously studied mild HA inhibitor subjects. Major histocompatibility complex II tetramers corresponding to both of the severe HA subject's HLA-DRA-DRB1 alleles were loaded with peptides spanning FVIII-A2, C1, and C2 domains. Interestingly, only 1 epitope was identified, in peptide FVIII_2194-2213, and it was identical to the HLA-DRA*01-DRB1*01:01-restricted epitope recognized by the mild HA subjects. Multiple T-cell clones and polyclonal lines having different avidities for the peptide-loaded tetramer were isolated from all subjects. Only high- and medium-avidity T cells proliferated and secreted cytokines when stimulated with FVIII_2194-2213 T-cell receptor β (TCRB) gene sequencing of 15 T-cell clones from the severe HA subject revealed that all high-avidity clones expressed the same TCRB gene. High-throughput immunosequencing of high-, medium-, and low-avidity cells sorted from a severe HA polyclonal line revealed that 94% of the high-avidity cells expressed the same TCRB gene as the high-avidity clones. TCRB sequencing of clones and lines from the mild HA subjects also identified a limited TCRB gene repertoire. These results suggest a limited number of epitopes in FVIII drive inhibitor responses and that the T-cell repertoires of FVIII-responsive T cells can be quite narrow. The limited diversity of both epitopes and TCRB gene usage suggests that targeting of specific epitopes and/or T-cell clones may be a promising approach to achieve tolerance to FVIII.

A cellular viewpoint of anti-FVIII immune response in hemophilia A

A large proportion of hemophilia A patients who receive replacement therapy, develop an immune response toward the infused factor VIII (FVIII). In this review, we discuss recent progress in several aspects of the anti-FVIII immune response, focusing on the sites of FVIII endocytosis (marginal zone of the spleen and bleeding site), the type of antigen-presenting cells (dendritic cells, macrophages and B cells) and endocytic receptors, implicated in FVIII presentation to T cells during primary and secondary immune response. Although it is becoming increasingly clear that regulatory T cells are involved in FVIII tolerance in healthy subjects and potentially in patients without inhibitors, we would like to demonstrate that little is known about the different T cells subsets and the cytokines network, which are also crucial for the development of allo- and autoimmune diseases. As more information on these issues becomes available, a better understanding of the role of each immune cells compartment in promoting FVIII tolerance or inhibitors development might lead to new strategies to promote FVIII tolerance in hemophilia A patients.

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.

Factor VIII inhibitors: role of von Willebrand factor on the uptake of factor VIII by dendritic cells

In patients with haemophilia A, factor VIII (FVIII) therapy leads to the development of anti-FVIII alloantibodies that inhibit FVIII pro-coagulant activity, in up to 25% of the cases. At a time when efficient viral screening procedures are at place, development of inhibitors poses the greatest threat to haemophilia A patients. Various risk factors, both patient and product-related, are responsible for the development of inhibitory antibodies. The role of FVIII-specific CD4+ T lymphocytes in the initiation of the humoral immune response to exogenous FVIII has been well. In view of their capacity to stimulate naïve T cells, dendritic cells (DCs) play a central role in the initiation of the primary immune response. Thus, in the context of a primary alloimmunization against FVIII, i.e. when FVIII-specific B lymphocytes are not there to take up FVIII from the circulation and to serve as antigen presenting cells (APCs), DCs are the only cell type that internalize FVIII, leading to activation of FVIII-specific CD4+ T lymphocytes. von Willebrand factor (VWF) present in plasma-derived FVIII therapeutic concentrates, is known to act as a chaperone molecule for procoagulant FVIII. In addition to its role in reducing the 'antigenicity' of FVIII, the role of VWF in the reduction of the 'immunogenicity' of therapeutic FVIII in patients with haemophilia A has also been suggested. We have recently demonstrated that VWF protects FVIII from being endocytosed by human DCs and subsequently being presented to FVIII-specific T cells. We propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen presenting cells.

A role for exposed mannosylations in presentation of human therapeutic self-proteins to CD4+ T lymphocytes

Several therapeutic self-proteins elicit immune responses when administered to patients. Such adverse immune responses reduce drug efficacy. To induce an immune response, a protein must interact with different immune cells, including antigen-presenting cells, T cells, and B cells. Each cell type recognizes distinct immunogenic patterns on antigens. Mannose-terminating glycans have been identified as pathogen-associated molecular patterns that are essential for internalization of microbes by antigen-presenting cells, leading to presentation. Here, we have investigated the importance of exposed mannosylation on an immunogenic therapeutic self-protein, procoagulant human factor VIII (FVIII). Administration of therapeutic FVIII to hemophilia A patients induces inhibitory anti-FVIII antibodies in up to 30% of the cases. We demonstrate that entry of FVIII into human dendritic cells (DC) leading to T cell activation, is mediated by mannose-terminating glycans on FVIII. Further, we identified macrophage mannose receptor (CD206) as a candidate endocytic receptor for FVIII on DC. Saturation of mannose receptors on DC with mannan, and enzymatic removal of mannosylated glycans from FVIII lead to reduced T cell activation. The interaction between FVIII and CD206 was blocked by VWF, suggesting that, under physiological conditions, the intrinsic mannose-dependent immunogenicity of FVIII is quenched by endogenous immunochaperones. These data provide a link between the mannosylation of therapeutic self-proteins and their iatrogenic immunogenicity. Such a link would be of special relevance in the context of replacement therapy where mechanisms of central and peripheral tolerance have not been established during ontogeny because of the absence of the antigen.

Immune response against therapeutic factor VIII in hemophilia A patients—A survey of probable risk factors

A number of diseases are treated by passive administration of human proteins. Human coagulation factor VIII (FVIII) is one such protein which is administered to hemophilia A patients in order to manage and treat hemorrhagic incidences. This mode of therapy suffers from the side effect of generating anti-FVIII antibodies (inhibitors) which neutralizes the function of the infused FVIII. At a time when efficient viral screening procedures are at place, development of inhibitors poses the greatest threat to such a therapy. Various predisposing factors, both patient and product-related, are responsible for the development of inhibitory antibodies. A proper understanding of these "risk-factors" would eventually help to better design therapeutic regimen to tackle hemophilia A.

Identification and removal of a promiscuous CD4+ T cell epitope from the C1 domain of factor VIII

BACKGROUND

The development of inhibitors in hemophiliacs is a severe complication of factor VIII (FVIII) replacement therapy and is a process driven by FVIII specific T helper cells.

OBJECTIVES

To finely map T cell epitopes within the whole FVIII protein in order to investigate the possibility of engineering FVIII variants with reduced propensity for inhibitor development.

PATIENTS AND METHODS

T cell lines were generated from five patients with severe hemophilia who had developed inhibitors, and were screened for T cell proliferation against pools of overlapping peptides spanning the entire B domain deleted (BDD) FVIII sequence. Positive peptide pools were decoded by screening individual peptides against the T cell lines. Positive peptides, and mutants thereof, were tested for their ability to bind major histocompatibility complex (MHC) Class II and stimulate T cell proliferation in a panel of healthy donors. The activities of the corresponding mutant proteins were assessed via chromogenic assay.

RESULTS

One peptide, spanning FVIII amino acids 2098-2112, elicited a vigorous response from one hemophiliac donor, induced strong T cell responses in the panel of healthy donors and bound to a number of HLA-DR alleles. Mutations were made in this peptide that removed its ability to stimulate T cells of healthy donors and to bind to MHC Class II while retaining full activity when incorporated into a mutant BDD-FVIII protein.

CONCLUSIONS

Fine T cell epitope mapping of the entire FVIII protein is feasible, although challenging, and this knowledge may be used to create FVIII variants which potentially have reduced immunogenicity.

Factor VIII alloantibodies in hemophilia

PURPOSE OF REVIEW

The development of an inhibitory response to factor VIII (FVIII) remains a puzzling challenge both for clinicians and scientists, not to mention the difficulties of maintaining hemostasis in patients producing inhibitors.

RECENT FINDINGS

Three main research lines have been explored in recent months. The mechanisms by which an anti-FVIII antibody response is elicited in patients has been examined at both the B- and T-cell levels, with particular emphasis on the generation of specific B- and T-cell clones. The hemophilia A mouse model has served to confirm the main characteristics of the anti-FVIII immune response in terms of T-cell dependency and memorization of the response. Novel strategies for the prevention and downregulation of inhibitors have emerged, with special interest in antigen-specific approaches.

SUMMARY

Although the ultimate goal, preventing or suppressing inhibitor formation in patients, is not yet achieved, the research activity developed over the past months brings us forward in that direction.

Inhibitors in hemophilia A

Factor VIII (FVIII) replacement therapy remains the mainstay in hemophilia A care. The major complication of replacement therapy is formation of antibodies, which inhibit FVIII activity, thus dramatically reducing treatment efficiency. The present review summarizes the accumulated knowledge on epitopes of FVIII inhibitors and mechanisms of their inhibitory effects. FVIII inhibitors most frequently target the A2, C2 and A3 domains of FVIII and interfere with important interactions of FVIII at various stages of its functional pathway; a class of FVIII inhibitors inactivates FVIII by proteolysis. We discuss therapeutic approaches currently used for treatment of hemophilia A patients with inhibitors and analyze the factors that influence the outcome. The choice between options should depend on the level of inhibitors and consideration of efficacy, safety, and availability of particular regimens. Advances of basic science open avenues for alternative targeted, specific and long-lasting treatments, such as the use of peptide decoys for blocking FVIII inhibitors, bypassing them with human/porcine FVIII hybrids, neutralizing FVIII-reactive CD4 T cells with anti-clonotypic antibodies, or inducing immune tolerance to FVIII with the use of universal CD4 epitopes or by genetic approaches.