Haemophilia (2014), 20 (Suppl. 5), 1–20
DOI: 10.1111/hae.12468
ORIGINAL ARTICLE
Managing Haemophilia for Life: 4th Haemophilia Global Summit EZ-YUSTE,§ M. LAFFAN,¶ J . A S T E R M A R K , * G . D O L A N , † T . H I L B E R G , ‡ V . J I M EN R . L A S S I L A , * * S . L O B E T , † † C . M A R T I N O L I ‡ ‡ and C . - F . P E R N O § § *Department of Hematology and Vascular Disorders, Sk ane University Hospital, Malm€ o/Lund, Sweden; †Department of Haematology, University Hospital, Queen’s Medical Centre, Nottingham, UK; ‡Department of Sports Medicine, University Wuppertal, Wuppertal, Germany; §Hospital Universitario La Paz, Madrid, Spain; ¶Imperial College, London, UK; **Helsinki University Central Hospital, Helsinki, Finland; ††Division of Physical Medicine and Rehabilitation & Haemostasis and Thrombosis Unit, Division of Haematology, Cliniques Universitaires Saint-Luc, Brussels, Belgium; ‡‡University of Genoa, Genoa, Italy; and §§University of Rome ‘Tor Vergata’, Rome, Italy
Summary. The 4th Haemophilia Global Summit was held in Potsdam, Germany, in September 2013 and brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. The programme was designed by an independent Scientific Steering Committee of haemophilia experts and explored global perspectives in haemophilia care, discussing practical approaches to the optimal management of haemophilia now and in the future. The topics outlined in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care globally. In this supplement from the meeting, Jan Astermark reviews current understanding of risk factors for the development of inhibitory antibodies and discusses whether this risk can be modulated and minimized. Factors key to the improvement of joint health in people with haemophilia are explored, with
Carlo Martinoli and Vıctor Jimenez-Yuste discussing the utility of ultrasound for the early detection of haemophilic arthropathy. Other aspects of care necessary for the prevention and management of joint disease in people with haemophilia are outlined by Thomas Hilberg and Sebastian Lobet, who highlight the therapeutic benefits of physiotherapy and sports therapy. Riitta Lassila and Carlo-Federico Perno describe current knowledge surrounding the risk of transmission of infectious agents via clotting factor concentrates. Finally, different types of extended halflife technology are evaluated by Mike Laffan, with a focus on the practicalities and challenges associated with these products. Keywords: global, haemophilia, half-life, inhibitor, joint damage, pathogen
Introduction G. DOLAN ON BEHALF OF THE 4TH HAEMOPHILIA GLOBAL SUMMIT SCIENTIFIC STEERING COMMITTEE*
On behalf of the Scientific Steering Committee*, I welcome you to the supplement from the 4th HaemoCorrespondence: Gerry Dolan, Department of Haematology, University Hospital, Queen’s Medical Centre, City Hospital Campus, Nottingham, UK. Tel.: +44 7725 947 998 e-mail: [email protected] © 2014 John Wiley & Sons Ltd
philia Global Summit, a meeting that brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. My distinguished colleagues on the Steering Committee, Jan Astermark (Sweden), Cedric Hermans (Belgium), Andreas Tiede (Germany), Jerzy Windyga (Poland) and I, faced the challenge of designing a
1
2
G. DOLAN et al.
programme that covered the topical areas of interest for haemophilia A, haemophilia B and other bleeding disorders and ensuring the presentations remained relevant to the multidisciplinary, global audience working in very different environments. On the basis of our own experience and feedback from delegates at previous Global Summit meetings, we presented a programme which aimed to share best practice in haemophilia care. The programme explored global perspectives in haemophilia care and current opportunities for the management of haemophilia, and also provided the opportunity to share clinical experience and best practice on the optimal management of haemophilia at all life stages. Emerging techniques in laboratory medicine were reviewed and areas of specialized interest, including rare disorders and specialist services in haemophilia care, were
considered. Understanding progress in outcome assessment and its potential to affect haemophilia management were also discussed and future areas of research for improving haemophilia care identified. The topics explored in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care now and in the future. *Jan Astermark (Sweden), Gerry Dolan (UK), Cedric Hermans (Belgium), Andreas Tiede (Germany), Jerzy Windyga (Poland). The Managing Haemophilia for Life Global Summit was sponsored by funding from Pfizer for the 4th consecutive year. As with previous meetings, an independent faculty determined the structure and scientific content of the meeting.
Inhibitor development in haemophilia A: can the risks be minimized? J. ASTERMARK E-mail: [email protected]
Summary The number of preclinical and clinical studies performed in the area of inhibitor development in haemophilia has markedly increased over the years and the understanding of the underlying complexity and immune mechanisms increases year by year. However, several issues must still be addressed and it remains largely unclear why some patients experience this immune response to replacement therapy whereas others do not, despite having the same type of causative mutation. Available data indicate that a genetic profile consistent with inhibitor risk is required for inhibitor development to occur but once established, both nongenetic factors and immune-regulatory molecules interact and together influence the final outcome in many patients. The following review focuses on the current view of risk factors for the development of inhibitory antibodies and whether this risk can be modulated and minimized.
Introduction Treatment of haemophilia using replacement of the deficient factor has substantially improved over recent decades and life expectancy for a young boy suffering from severe haemophilia A is today, in most developed countries, similar to that of his healthy peers [1,2]. However, severe adverse effects of replacement Haemophilia (2014), 20 (Suppl. 5), 1--20
therapy, such as the development of neutralizing inhibitory antibodies, remain a threat and should be considered in the management of patients. Most inhibitory antibodies will be eliminated by the use of immune tolerance induction (ITI) with or without immunosuppression, but ITI is costly and the outcome unpredictable [3]. Therefore, it is important to fully elucidate the factors that influence inhibitor development in one-third of patients with severe haemophilia A. This overview will summarize the current view of these risk factors in light of the immune response taking place, and will address the issue of whether it will be possible to minimize risk in the future.
Mechanisms of inhibitor/antibody development The mechanisms by which inhibitory antibodies develop have been carefully studied for several years and major advances in our understanding have been made [4,5]. However, much still remains to be resolved, and it is clear that there are several processes that potentially influence the outcome. These include the methods by which antigen-presenting cells (APC) process and present the endocytosed factor VIII molecule to the T-helper cells, the nature of the T and B cells and the profile of the immune-regulatory molecules, including both cell-bound molecules and those secreted into the circulation (Fig. 1). In addition, regu© 2014 John Wiley & Sons Ltd
MANAGING HAEMOPHILIA FOR LIFE
Fig. 1. Mechanisms of FVIII antibody development. Infused and endocytosed FVIII molecules will be proteolytically degraded in the antigen presenting cell (APC) and peptides defined by the human leucocyte antigen (HLA) class II molecules presented for the inactive CD4+ T-helper (TH) cell via the T-cell receptor. Co-stimulatory signals are needed to elicit the immune response and release immune-regulatory molecules (cytokines). The activated T cell then interacts with the B cells, promoting differentiation and antibody formation. In addition, memory T and B cells of importance for subsequent exposures are formed.
latory T cells with suppressor activities are of major importance and a number of initiatives are now underway to fully appreciate the impact of these cells and to define how this knowledge may be used to modify the immune response [6]. Different subsets of these T cells have been described, such as CD4+ CD25+ FoxP3+ Treg cells, IL-10-producing Tr1 cells, transforming growth factor-b-producing Th3 cells and CD8+ Treg cells. Interestingly, an immune response not dependent on T-helper cells has recently been suggested, but so far the clinical significance of this type of immune response is not completely clear [7]. It appears that predominantly low-affinity antibodies are produced by this route and may therefore be of relevance for non-inhibitory or non-neutralizing antibodies. The reason why these antibodies can be identified in some patients but not others, and are also found in patients without haemophilia, is not known.
3
mutations were higher than in patients with intron 22 inversions (pooled OR = 3.6, 95% confidence interval [95% CI], 2.3–5.7 and OR = 1.4, 95% CI, 1.1–1.8, respectively) confirming the relatively high risk for inhibitors associated with these mutation types [12]. However, a high frequency of inhibitors has also been reported for other mutations. The current view is that, besides these null mutations, small deletions/insertions outside A-runs, splice-site mutation at conserved nucleotides at position 1/2, and certain missense mutations, e.g. Arg593>Cys, Tyr2105>Cys, Arg2150>His, Arg2163>His, Trp2229>Cys and Pro2300>Leu, will also confer a relatively high inhibitor risk [13]. The question can also be raised as to whether a cross-reactive material-negative mutation with no circulating antigen, such as the intron 22 inversion, which only causes inhibitors in 20% of patients, really should be classified as a high-risk mutation or rather a protective mutation instead. Further light on this was recently provided by Pandey et al. who described that endogenous FVIII synthesis from the inverted F8 locus may modulate the immune response [14]. They actually found that the levels of F8 mRNA and intracellular FVIII in subjects with intron 22 inversions were similar to those of healthy subjects. They therefore suggested that most patients with intron 22 inversions were, in fact, tolerized against FVIII, which could explain the inhibitor rate of only 20%. The importance of the HLA class II molecules is easily appreciated when considering that these molecules will determine the peptides to be presented to the T-helper cells [15]. If only peptides with sequences previously recognized by the immune system and not able to elicit an immune response are presented, then an immune reaction against the infused factor will not occur (Fig. 2). However, if this is not the case and immunogenic peptides with foreign sequences are
Decisive factors for inhibitor risk The most thoroughly evaluated risk factor for inhibitor development is the type of F8 mutation. In the earliest reports in the 1990s, large deletions, nonsense mutations and inversions were defined as high-risk mutations, as the highest percentages of inhibitor patients were observed in these subgroups [8,9]. The risk is not, however, consistent among patients with these mutations as has been observed in family studies in which high rates of discordance have been found between siblings with the same mutation [10,11]. In a recent meta-analysis by Gouw et al., the inhibitor risks in patients with large deletions and nonsense © 2014 John Wiley & Sons Ltd
Fig. 2. Schematic presentation of the combined outcome for the type of mutation and HLA class II alleles. No inhibitory antibodies will be formed in patient 1 who has a point mutation, whereas in patient 2 who has a deletion, there will be a significant risk of this occurring. The two patients have the same HLA class II alleles, but only in patient 2 will these alleles potentially be able to pick up foreign immunogenic non-self peptides from the degraded FVIII molecule to be presented to the T-helper cell.
Haemophilia (2014), 20 (Suppl. 5), 1--20
4
G. DOLAN et al.
presented, then the reaction can take place. Whether the final outcome of this will be inhibitory antibodies produced by the plasma cells or not will then depend on the levels of a variety of immune-regulatory elements. Perhaps due to the heterogeneity of the HLA system and the repertoire of peptides that can be bound, consistent associations with inhibitor development have not been observed across studies. The strongest and most frequently observed relationship has been with the HLA DR B*1501 allele, but several other alleles are candidates. Pandey et al. have made an effort to define a pharmacogenetic algorithm by which the immune response can be predicted based on the number of putative T-cell epitopes in the infused protein and the HLA class II molecules [14]. The findings are interesting, but how useful this algorithm will be in the clinical setting is not possible to predict at this early stage. The concept of other immune-regulatory molecules – such as cytokines, chemokines and cell-bound molecules – affecting the immune response was first suggested by findings from the Malm€ o International Brother Study (MIBS) [16–18]. This is, however, not a phenomenon exclusive to haemophilia. For example, the susceptibility to variant Creutzfeldt–Jakob disease (vCJD) is modified by the prion protein gene (PRNP) codon 129 and polymorphisms in regulatory genes [19]. Moreover, the responsiveness and vulnerability to the HIV virus seem to be regulated by multiple host genetic immune-regulatory factors [20]. Several of the initial MIBS findings have indeed been confirmed in later studies, including the association between IL-10 and TNFA polymorphisms and inhibitors [21–23]. In addition, other candidate genes have been reported [24–26]. The primary outcome findings of the Hemophilia Inhibitor Genetics Study (HIGS) were recently published and these data further add to the complexity of potential significant immune pathways [27]. HIGS was an association study using a candidate gene panel of single nucleotide polymorphisms (SNPs) in immune response genes from 833 subjects to detect odds ratios of 1.5–3.0 with a power of 80–99% in three different multicentre cohorts, i.e. the HIGS, MIBS and HGDS (Hemophilia Growth and Development Study). Brother pairs, concordant or discordant for inhibitors, as well as singletons with or without inhibitors, were enrolled. Fifty-five per cent of the patients had a history of inhibitory antibodies with a Bethesda titre above 1 BU mL 1. In 80% of these cases, the inhibitor response was of the high-responding type with a peak titre above 5 BU mL 1. Eightyeight per cent of the enrolled subjects had severe haemophilia A with a basal factor level
DOI: 10.1111/hae.12468
ORIGINAL ARTICLE
Managing Haemophilia for Life: 4th Haemophilia Global Summit EZ-YUSTE,§ M. LAFFAN,¶ J . A S T E R M A R K , * G . D O L A N , † T . H I L B E R G , ‡ V . J I M EN R . L A S S I L A , * * S . L O B E T , † † C . M A R T I N O L I ‡ ‡ and C . - F . P E R N O § § *Department of Hematology and Vascular Disorders, Sk ane University Hospital, Malm€ o/Lund, Sweden; †Department of Haematology, University Hospital, Queen’s Medical Centre, Nottingham, UK; ‡Department of Sports Medicine, University Wuppertal, Wuppertal, Germany; §Hospital Universitario La Paz, Madrid, Spain; ¶Imperial College, London, UK; **Helsinki University Central Hospital, Helsinki, Finland; ††Division of Physical Medicine and Rehabilitation & Haemostasis and Thrombosis Unit, Division of Haematology, Cliniques Universitaires Saint-Luc, Brussels, Belgium; ‡‡University of Genoa, Genoa, Italy; and §§University of Rome ‘Tor Vergata’, Rome, Italy
Summary. The 4th Haemophilia Global Summit was held in Potsdam, Germany, in September 2013 and brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. The programme was designed by an independent Scientific Steering Committee of haemophilia experts and explored global perspectives in haemophilia care, discussing practical approaches to the optimal management of haemophilia now and in the future. The topics outlined in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care globally. In this supplement from the meeting, Jan Astermark reviews current understanding of risk factors for the development of inhibitory antibodies and discusses whether this risk can be modulated and minimized. Factors key to the improvement of joint health in people with haemophilia are explored, with
Carlo Martinoli and Vıctor Jimenez-Yuste discussing the utility of ultrasound for the early detection of haemophilic arthropathy. Other aspects of care necessary for the prevention and management of joint disease in people with haemophilia are outlined by Thomas Hilberg and Sebastian Lobet, who highlight the therapeutic benefits of physiotherapy and sports therapy. Riitta Lassila and Carlo-Federico Perno describe current knowledge surrounding the risk of transmission of infectious agents via clotting factor concentrates. Finally, different types of extended halflife technology are evaluated by Mike Laffan, with a focus on the practicalities and challenges associated with these products. Keywords: global, haemophilia, half-life, inhibitor, joint damage, pathogen
Introduction G. DOLAN ON BEHALF OF THE 4TH HAEMOPHILIA GLOBAL SUMMIT SCIENTIFIC STEERING COMMITTEE*
On behalf of the Scientific Steering Committee*, I welcome you to the supplement from the 4th HaemoCorrespondence: Gerry Dolan, Department of Haematology, University Hospital, Queen’s Medical Centre, City Hospital Campus, Nottingham, UK. Tel.: +44 7725 947 998 e-mail: [email protected] © 2014 John Wiley & Sons Ltd
philia Global Summit, a meeting that brought together an international faculty of haemophilia experts and delegates from multidisciplinary backgrounds. My distinguished colleagues on the Steering Committee, Jan Astermark (Sweden), Cedric Hermans (Belgium), Andreas Tiede (Germany), Jerzy Windyga (Poland) and I, faced the challenge of designing a
1
2
G. DOLAN et al.
programme that covered the topical areas of interest for haemophilia A, haemophilia B and other bleeding disorders and ensuring the presentations remained relevant to the multidisciplinary, global audience working in very different environments. On the basis of our own experience and feedback from delegates at previous Global Summit meetings, we presented a programme which aimed to share best practice in haemophilia care. The programme explored global perspectives in haemophilia care and current opportunities for the management of haemophilia, and also provided the opportunity to share clinical experience and best practice on the optimal management of haemophilia at all life stages. Emerging techniques in laboratory medicine were reviewed and areas of specialized interest, including rare disorders and specialist services in haemophilia care, were
considered. Understanding progress in outcome assessment and its potential to affect haemophilia management were also discussed and future areas of research for improving haemophilia care identified. The topics explored in this supplement were selected by the Scientific Steering Committee for their relevance and potential to influence haemophilia care now and in the future. *Jan Astermark (Sweden), Gerry Dolan (UK), Cedric Hermans (Belgium), Andreas Tiede (Germany), Jerzy Windyga (Poland). The Managing Haemophilia for Life Global Summit was sponsored by funding from Pfizer for the 4th consecutive year. As with previous meetings, an independent faculty determined the structure and scientific content of the meeting.
Inhibitor development in haemophilia A: can the risks be minimized? J. ASTERMARK E-mail: [email protected]
Summary The number of preclinical and clinical studies performed in the area of inhibitor development in haemophilia has markedly increased over the years and the understanding of the underlying complexity and immune mechanisms increases year by year. However, several issues must still be addressed and it remains largely unclear why some patients experience this immune response to replacement therapy whereas others do not, despite having the same type of causative mutation. Available data indicate that a genetic profile consistent with inhibitor risk is required for inhibitor development to occur but once established, both nongenetic factors and immune-regulatory molecules interact and together influence the final outcome in many patients. The following review focuses on the current view of risk factors for the development of inhibitory antibodies and whether this risk can be modulated and minimized.
Introduction Treatment of haemophilia using replacement of the deficient factor has substantially improved over recent decades and life expectancy for a young boy suffering from severe haemophilia A is today, in most developed countries, similar to that of his healthy peers [1,2]. However, severe adverse effects of replacement Haemophilia (2014), 20 (Suppl. 5), 1--20
therapy, such as the development of neutralizing inhibitory antibodies, remain a threat and should be considered in the management of patients. Most inhibitory antibodies will be eliminated by the use of immune tolerance induction (ITI) with or without immunosuppression, but ITI is costly and the outcome unpredictable [3]. Therefore, it is important to fully elucidate the factors that influence inhibitor development in one-third of patients with severe haemophilia A. This overview will summarize the current view of these risk factors in light of the immune response taking place, and will address the issue of whether it will be possible to minimize risk in the future.
Mechanisms of inhibitor/antibody development The mechanisms by which inhibitory antibodies develop have been carefully studied for several years and major advances in our understanding have been made [4,5]. However, much still remains to be resolved, and it is clear that there are several processes that potentially influence the outcome. These include the methods by which antigen-presenting cells (APC) process and present the endocytosed factor VIII molecule to the T-helper cells, the nature of the T and B cells and the profile of the immune-regulatory molecules, including both cell-bound molecules and those secreted into the circulation (Fig. 1). In addition, regu© 2014 John Wiley & Sons Ltd
MANAGING HAEMOPHILIA FOR LIFE
Fig. 1. Mechanisms of FVIII antibody development. Infused and endocytosed FVIII molecules will be proteolytically degraded in the antigen presenting cell (APC) and peptides defined by the human leucocyte antigen (HLA) class II molecules presented for the inactive CD4+ T-helper (TH) cell via the T-cell receptor. Co-stimulatory signals are needed to elicit the immune response and release immune-regulatory molecules (cytokines). The activated T cell then interacts with the B cells, promoting differentiation and antibody formation. In addition, memory T and B cells of importance for subsequent exposures are formed.
latory T cells with suppressor activities are of major importance and a number of initiatives are now underway to fully appreciate the impact of these cells and to define how this knowledge may be used to modify the immune response [6]. Different subsets of these T cells have been described, such as CD4+ CD25+ FoxP3+ Treg cells, IL-10-producing Tr1 cells, transforming growth factor-b-producing Th3 cells and CD8+ Treg cells. Interestingly, an immune response not dependent on T-helper cells has recently been suggested, but so far the clinical significance of this type of immune response is not completely clear [7]. It appears that predominantly low-affinity antibodies are produced by this route and may therefore be of relevance for non-inhibitory or non-neutralizing antibodies. The reason why these antibodies can be identified in some patients but not others, and are also found in patients without haemophilia, is not known.
3
mutations were higher than in patients with intron 22 inversions (pooled OR = 3.6, 95% confidence interval [95% CI], 2.3–5.7 and OR = 1.4, 95% CI, 1.1–1.8, respectively) confirming the relatively high risk for inhibitors associated with these mutation types [12]. However, a high frequency of inhibitors has also been reported for other mutations. The current view is that, besides these null mutations, small deletions/insertions outside A-runs, splice-site mutation at conserved nucleotides at position 1/2, and certain missense mutations, e.g. Arg593>Cys, Tyr2105>Cys, Arg2150>His, Arg2163>His, Trp2229>Cys and Pro2300>Leu, will also confer a relatively high inhibitor risk [13]. The question can also be raised as to whether a cross-reactive material-negative mutation with no circulating antigen, such as the intron 22 inversion, which only causes inhibitors in 20% of patients, really should be classified as a high-risk mutation or rather a protective mutation instead. Further light on this was recently provided by Pandey et al. who described that endogenous FVIII synthesis from the inverted F8 locus may modulate the immune response [14]. They actually found that the levels of F8 mRNA and intracellular FVIII in subjects with intron 22 inversions were similar to those of healthy subjects. They therefore suggested that most patients with intron 22 inversions were, in fact, tolerized against FVIII, which could explain the inhibitor rate of only 20%. The importance of the HLA class II molecules is easily appreciated when considering that these molecules will determine the peptides to be presented to the T-helper cells [15]. If only peptides with sequences previously recognized by the immune system and not able to elicit an immune response are presented, then an immune reaction against the infused factor will not occur (Fig. 2). However, if this is not the case and immunogenic peptides with foreign sequences are
Decisive factors for inhibitor risk The most thoroughly evaluated risk factor for inhibitor development is the type of F8 mutation. In the earliest reports in the 1990s, large deletions, nonsense mutations and inversions were defined as high-risk mutations, as the highest percentages of inhibitor patients were observed in these subgroups [8,9]. The risk is not, however, consistent among patients with these mutations as has been observed in family studies in which high rates of discordance have been found between siblings with the same mutation [10,11]. In a recent meta-analysis by Gouw et al., the inhibitor risks in patients with large deletions and nonsense © 2014 John Wiley & Sons Ltd
Fig. 2. Schematic presentation of the combined outcome for the type of mutation and HLA class II alleles. No inhibitory antibodies will be formed in patient 1 who has a point mutation, whereas in patient 2 who has a deletion, there will be a significant risk of this occurring. The two patients have the same HLA class II alleles, but only in patient 2 will these alleles potentially be able to pick up foreign immunogenic non-self peptides from the degraded FVIII molecule to be presented to the T-helper cell.
Haemophilia (2014), 20 (Suppl. 5), 1--20
4
G. DOLAN et al.
presented, then the reaction can take place. Whether the final outcome of this will be inhibitory antibodies produced by the plasma cells or not will then depend on the levels of a variety of immune-regulatory elements. Perhaps due to the heterogeneity of the HLA system and the repertoire of peptides that can be bound, consistent associations with inhibitor development have not been observed across studies. The strongest and most frequently observed relationship has been with the HLA DR B*1501 allele, but several other alleles are candidates. Pandey et al. have made an effort to define a pharmacogenetic algorithm by which the immune response can be predicted based on the number of putative T-cell epitopes in the infused protein and the HLA class II molecules [14]. The findings are interesting, but how useful this algorithm will be in the clinical setting is not possible to predict at this early stage. The concept of other immune-regulatory molecules – such as cytokines, chemokines and cell-bound molecules – affecting the immune response was first suggested by findings from the Malm€ o International Brother Study (MIBS) [16–18]. This is, however, not a phenomenon exclusive to haemophilia. For example, the susceptibility to variant Creutzfeldt–Jakob disease (vCJD) is modified by the prion protein gene (PRNP) codon 129 and polymorphisms in regulatory genes [19]. Moreover, the responsiveness and vulnerability to the HIV virus seem to be regulated by multiple host genetic immune-regulatory factors [20]. Several of the initial MIBS findings have indeed been confirmed in later studies, including the association between IL-10 and TNFA polymorphisms and inhibitors [21–23]. In addition, other candidate genes have been reported [24–26]. The primary outcome findings of the Hemophilia Inhibitor Genetics Study (HIGS) were recently published and these data further add to the complexity of potential significant immune pathways [27]. HIGS was an association study using a candidate gene panel of single nucleotide polymorphisms (SNPs) in immune response genes from 833 subjects to detect odds ratios of 1.5–3.0 with a power of 80–99% in three different multicentre cohorts, i.e. the HIGS, MIBS and HGDS (Hemophilia Growth and Development Study). Brother pairs, concordant or discordant for inhibitors, as well as singletons with or without inhibitors, were enrolled. Fifty-five per cent of the patients had a history of inhibitory antibodies with a Bethesda titre above 1 BU mL 1. In 80% of these cases, the inhibitor response was of the high-responding type with a peak titre above 5 BU mL 1. Eightyeight per cent of the enrolled subjects had severe haemophilia A with a basal factor level
