Seraph 100 Microbind Affinity Blood Filter Does Not Clear Antibiotics: An Analysis of Antibiotic Concentration Data from PURIFY-OBS

INTRODUCTION

Novel hemoperfusion systems are emerging for the treatment of sepsis. These devices can directly remove pathogens, pathogen-associated molecular patterns, cytokines, and other inflammatory markers from circulation. However, significant safety concerns such as potential antibiotic clearance need to be addressed prior to these devices being used in large clinical studies.

METHODS

Prospective, observational study of 34 participants undergoing treatment with the Seraph 100® Microbind Affinity Blood Filter (Seraph 100) device at 6 participating sites in the USA. Patients were included for analysis if they had a record of receiving an antibiotic concurrent with Seraph 100 treatment. Patients were excluded if there was missing information for blood flow rate. Blood samples were drawn pre- and post-filter at 1 h and 4 h after treatment initiation. These average pre- and post-filter time-concentration observations were then used to estimate antibiotic clearance in L/h (CLSeraph) due to the Seraph 100 device.

RESULTS

Of the 34 participants in the study, 17 met inclusion and exclusion criteria for the antibiotic analysis. Data were obtained for 7 antibiotics (azithromycin, cefazolin, cefepime, ceftriaxone, linezolid, piperacillin, and vancomycin) and one beta-lactamase inhibitor. Mean CLSeraph for the antibiotics investigated ranged from -0.57 to 0.47 L/h. No antibiotic had a CLSeraph statistically significant from 0.

DISCUSSION/CONCLUSION

The Seraph 100 did not significantly clear any measured antibiotic in clinical samples. These data give further evidence to suggest that these therapies may be safely administered to critically ill patients and will not impact concentrations of administered antibiotics.

Anti-FVIII antibodies in Black and White hemophilia A subjects: do F8 haplotypes play a role?

The most common complication in hemophilia A (HA) treatment, affecting 25% to 30% of patients with severe HA, is the development of alloimmune inhibitors that foreclose the ability of infused factor VIII (FVIII) to participate in coagulation. Inhibitors confer significant pathology on affected individuals and present major complexities in their management. Inhibitors are more common in African American patients, and it has been hypothesized that this is a consequence of haplotype (H)-treatment product mismatch. F8 haplotypes H1 to H5 are defined by nonsynonymous single-nucleotide polymorphisms encoding sequence variations at FVIII residues 1241, 2238, and 484. Haplotypes H2 to H5 are more prevalent in individuals with Black African ancestry, whereas 80% to 90% of the White population has the H1 haplotype. This study used an established multiplex fluorescence immunoassay to determine anti-FVIII antibody titers in plasma from 394 individuals with HA (188 Black, 206 White), measuring their binding to recombinant full-length H1 and H2 and B-domain-deleted (BDD) H1/H2, H3/H5, and H4 FVIII proteins. Inhibitor titers were determined using a chromogenic assay and linear B-cell epitopes characterized using peptide microarrays. FVIII-reactive antibodies were readily detected in most individuals with HA, with higher titers in those with a current inhibitor, as expected. Neither total nor inhibitory antibody titers correlated with F8 haplotype mismatches, and peptides with D1241E and M2238V polymorphisms did not comprise linear B-cell epitopes. Interestingly, compared with the full-length FVIII products, the BDD-FVIII proteins were markedly more reactive with plasma antibodies. The stronger immunoreactivity of BDD-FVIII suggests that B-domain removal might expose novel B-cell epitopes, perhaps through conformational rearrangements of FVIII domains.

Hemophilia A subjects with an intron-22 gene inversion mutation show CD4+ T-effector responses to multiple epitopes in FVIII

Background

Almost half of severe hemophilia A (HA) is caused by an intron 22 inversion mutation (Int22Inv), which disrupts the 26-exon F8 gene. Inverted F8 mRNA exons 1-22 are transcribed, while F8B mRNA, containing F8 exons 23-26, is transcribed from a promoter within intron 22. Neither FVIII activity nor FVIII antigen (cross-reacting material, CRM) are detectable in plasma of patients with an intron-22 inversion.

Objectives

To test the hypothesis that (putative) intracellular synthesis of FVIII proteins encoded by inverted F8 and F8B mRNAs confers T-cell tolerance to almost the entire FVIII sequence, and to evaluate the immunogenicity of the region encoded by the F8 exon 22-23 junction sequence.

Patients/Methods

Peripheral blood mononuclear cells (PBMCs) from 30 severe or moderate HA subjects (17 with an Int22Inv mutation) were tested by ELISPOT assays to detect cytokine secretion in response to FVIII proteins and peptides and to map immunodominant T-cell epitopes. Potential immunogenicity of FVIII sequences encoded by the F8 exon 22-23 junction region was also tested using peptide-MHCII binding assays.

Results

Eight of the Int22Inv subjects showed robust cytokine secretion from PBMCs stimulated with FVIII proteins and/or peptides, consistent with earlier publications from the Conti-Fine group. Peptide ELISPOT assays identified immunogenic regions of FVIII. Specificity for sequences encoded within F8 mRNA exons 1-22 and F8B mRNA was confirmed by staining Int22Inv CD4⁺ T cells with peptide-loaded HLA-Class II tetramers. FVIII peptides spanning the F8 exon 22-23 junction (encoding M2124-V2125) showed limited binding to MHCII proteins and low immunogenicity, with cytokine secretion from only one Int22Inv subject.

Conclusions

PBMCs from multiple subjects with an Int22Inv mutation, with and without a current FVIII inhibitor, responded to FVIII epitopes. Furthermore, the FVIII region encoded by the exon 22-23 junction sequence was not remarkably immunoreactive and is therefore unlikely to contain an immunodominant, promiscuous CD4⁺ T-cell epitope. Our results indicate that putative intracellular expression of partial FVIII proteins does not confer T-cell tolerance to FVIII regions encoded by inverted F8 mRNA or F8B mRNA.

Human-Immune-System (HIS) humanized mouse model (DRAGA: HLA-A2.HLA-DR4.Rag1KO.IL-2RγcKO.NOD) for COVID-19

We report a Human Immune System (HIS)-humanized mouse model ("DRAGA": HLA-A2.HLA-DR4.Rag1KO.IL-2 RγcKO.NOD) for COVID-19 research. DRAGA mice express transgenically HLA-class I and class-II molecules in the mouse thymus to promote human T cell development and human B cell Ig-class switching. When infused with human hematopoietic stem cells from cord blood reconstitute a functional human immune system, as well as human epi/endothelial cells in lung and upper respiratory airways expressing the human ACE2 receptor for SARS-CoV-2. The DRAGA mice were able to sustain SARS-CoV-2 infection for at least 25 days. Infected mice showed replicating virus in the lungs, deteriorating clinical condition, and human-like lung immunopathology including human lymphocyte infiltrates, microthrombi and pulmonary sequelae. Among the intra-alveolar and peri-bronchiolar lymphocyte infiltrates, human lung-resident (CD103⁺) CD8⁺ and CD4⁺ T cells were sequestered in epithelial (CD326⁺) lung niches and secreted granzyme B and perforin, suggesting anti-viral cytotoxic activity. Infected mice also mounted human IgG antibody responses to SARS-CoV-2 viral proteins. Hence, HIS-DRAGA mice showed unique advantages as a surrogate in vivo human model for studying SARS-CoV-2 immunopathological mechanisms and testing the safety and efficacy of candidate vaccines and therapeutics.

Prospective Assessment of SARS-CoV-2 Seroconversion (PASS) study: an observational cohort study of SARS-CoV-2 infection and vaccination in healthcare workers

BACKGROUND

SARS-CoV-2 is a recently emerged pandemic coronavirus (CoV) capable of causing severe respiratory illness. However, a significant number of infected people present as asymptomatic or pauci-symptomatic. In this prospective assessment of at-risk healthcare workers (HCWs) we seek to determine whether pre-existing antibody or T cell responses to previous seasonal human coronavirus (HCoV) infections affect immunological or clinical responses to SARS-CoV-2 infection or vaccination.

METHODS

A cohort of 300 healthcare workers, confirmed negative for SARS-CoV-2 exposure upon study entry, will be followed for up to 1 year with monthly serology analysis of IgM and IgG antibodies against the spike proteins of SARS-CoV-2 and the four major seasonal human coronavirus - HCoV-OC43, HCoV-HKU1, HCoV-229E, and HCoV-NL63. Participants will complete monthly questionnaires that ask about Coronavirus Disease 2019 (COVID-19) exposure risks, and a standardized, validated symptom questionnaire (scoring viral respiratory disease symptoms, intensity and severity) at least twice monthly and any day when any symptoms manifest. SARS-CoV-2 PCR testing will be performed any time participants develop symptoms consistent with COVID-19. For those individuals that seroconvert and/or test positive by SARS-CoV-2 PCR, or receive the SARS-CoV-2 vaccine, additional studies of T cell activation and cytokine production in response to SARS-CoV-2 peptide pools and analysis of Natural Killer cell numbers and function will be conducted on that participant's cryopreserved baseline peripheral blood mononuclear cells (PBMCs). Following the first year of this study we will further analyze those participants having tested positive for COVID-19, and/or having received an authorized/licensed SARS-CoV-2 vaccine, quarterly (year 2) and semi-annually (years 3 and 4) to investigate immune response longevity.

DISCUSSION

This study will determine the frequency of asymptomatic and pauci-symptomatic SARS-CoV-2 infection in a cohort of at-risk healthcare workers. Baseline and longitudinal assays will determine the frequency and magnitude of anti-spike glycoprotein antibodies to the seasonal HCoV-OC43, HCoV-HKU1, HCoV-229E, and HCoV-NL63, and may inform whether pre-existing antibodies to these human coronaviruses are associated with altered COVID-19 disease course. Finally, this study will evaluate whether pre-existing immune responses to seasonal HCoVs affect the magnitude and duration of antibody and T cell responses to SARS-CoV-2 vaccination, adjusting for demographic covariates.

A Human-Immune-System (HIS) humanized mouse model (DRAGA: HLA-A2. HLA-DR4. Rag1 KO.IL-2Rγc KO. NOD) for COVID-19

We report the first Human Immune System (HIS)-humanized mouse model (“DRAGA”: HLA-A2.HLA-DR4.Rag1KO.IL-2RγcKO.NOD) for COVID-19 research. This mouse is reconstituted with human cord blood-derived, HLA-matched hematopoietic stem cells. It engrafts human epi/endothelial cells expressing the human ACE2 receptor for SARS-CoV-2 and TMPRSS2 serine protease co-localized on lung epithelia. HIS-DRAGA mice sustained SARS-CoV-2 infection, showing deteriorated clinical condition, replicating virus in the lungs, and human-like lung immunopathology including T-cell infiltrates, microthrombi and pulmonary sequelae. Among T-cell infiltrates, lung-resident (CD103⁺) CD8⁺ T cells were sequestered in epithelial (CD326⁺) lung niches and secreted granzyme B and perforin, indicating cytotoxic potential. Infected mice also developed antibodies against the SARS-CoV-2 viral proteins. Hence, HIS-DRAGA mice showed unique advantages as a surrogate in vivo human model for studying SARS-CoV-2 immunopathology and for testing the safety and efficacy of candidate vaccines and therapeutics.

Inhibitors-Recent insights

The development of inhibitory antibodies to therapeutic factor VIII (FVIII) in haemophilia A (HA) patients is the major complication in treatment/prevention of haemorrhages. The reasons some HA patients develop inhibitors while others do not remain unclear. This review briefly summarizes our understanding of anti-FVIII immune responses, the roles of T cells, both effector and regulatory, and generally discusses the interplay between FVIII and the immune system, both in factor replacement therapy and gene therapy, with some comparisons to factor IX and haemophilia B therapies. Notably, we propose that the prevailing observed active tolerance to FVIII in both HA and non-HA individuals rests to greater or lesser extents on peripherally induced immune tolerance. We also propose that the immune systems of inhibitor-negative HA patients do not merely ignore therapeutic FVIII, but rather have immunologically assessed and actively tolerized the patients to exogenous FVIII. Induction of such peripheral immune tolerance may further be triggered in HA patients who failed to tolerize upon initial FVIII exposure by 'appropriate' stimulation of their immune system, eg by immune tolerance induction therapy via intensive FVIII therapy, by oral administration of FVIII, by cellular therapies or by gene therapy directed to immuno-tolerogenic sites such as the liver.

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.

The humanized DRAGA mouse (HLA-A2. HLA-DR4. RAG1 KO. IL-2R g c KO. NOD) establishes inducible and transmissible models for influenza type A infections

We have engineered a Human Immune System (HIS)-reconstituted mouse strain (DRAGA mouse: HLA-A2. HLA-DR4. Rag1 KO. IL-2Rγc KO. NOD) in which the murine immune system has been replaced by a long-term, functional HIS via infusion of CD34⁺ hematopoietic stem cells (HSC) from cord blood. Herein, we report that the DRAGA mice can sustain inducible and transmissible H1N1 and H3N2 influenza A viral (IAV) infections. DRAGA female mice were significantly more resilient than the males to the H3N2/Aichi infection, but not to H3N2/Hong Kong, H3N2/Victoria, or H1N1/PR8 sub-lethal infections. Consistently associated with large pulmonary hemorrhagic areas, both human and murine Factor 8 mRNA transcripts were undetectable in the damaged lung tissues but not in livers of DRAGA mice advancing to severe H1N1/PR8 infection. Infected DRAGA mice mounted a neutralizing anti-viral antibody response and developed lung-resident CD103 T cells. These results indicate that the DRAGA mouse model for IAV infections can more closely approximate the human lung pathology and anti-viral immune responses compared to non-HIS mice. This mouse model may also allow further investigations into gender-based resilience to IAV infections, and may potentially be used to evaluate the efficacy of IAV vaccine regimens for humans.

Factor VIII: Perspectives on Immunogenicity and Tolerogenic Strategies

Therapeutic treatment of bleeds with FVIII can lead to an antibody response that effectively inhibits its function. Herein, we review the factors that contribute to this immunogenicity and possible ways to overcome it.

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.

Predicting CD4 T-cell epitopes based on antigen cleavage, MHCII presentation, and TCR recognition

Accurate predictions of T-cell epitopes would be useful for designing vaccines, immunotherapies for cancer and autoimmune diseases, and improved protein therapies. The humoral immune response involves uptake of antigens by antigen presenting cells (APCs), APC processing and presentation of peptides on MHC class II (pMHCII), and T-cell receptor (TCR) recognition of pMHCII complexes. Most in silico methods predict only peptide-MHCII binding, resulting in significant over-prediction of CD4 T-cell epitopes. We present a method, ITCell, for prediction of T-cell epitopes within an input protein antigen sequence for given MHCII and TCR sequences. The method integrates information about three stages of the immune response pathway: antigen cleavage, MHCII presentation, and TCR recognition. First, antigen cleavage sites are predicted based on the cleavage profiles of cathepsins S, B, and H. Second, for each 12-mer peptide in the antigen sequence we predict whether it will bind to a given MHCII, based on the scores of modeled peptide-MHCII complexes. Third, we predict whether or not any of the top scoring peptide-MHCII complexes can bind to a given TCR, based on the scores of modeled ternary peptide-MHCII-TCR complexes and the distribution of predicted cleavage sites. Our benchmarks consist of epitope predictions generated by this algorithm, checked against 20 peptide-MHCII-TCR crystal structures, as well as epitope predictions for four peptide-MHCII-TCR complexes with known epitopes and TCR sequences but without crystal structures. ITCell successfully identified the correct epitopes as one of the 20 top scoring peptides for 22 of 24 benchmark cases. To validate the method using a clinically relevant application, we utilized five factor VIII-specific TCR sequences from hemophilia A subjects who developed an immune response to factor VIII replacement therapy. The known HLA-DR1-restricted factor VIII epitope was among the six top-scoring factor VIII peptides predicted by ITCall to bind HLA-DR1 and all five TCRs. Our integrative approach is more accurate than current single-stage epitope prediction algorithms applied to the same benchmarks. It is freely available as a web server (http://salilab.org/itcell).

FVIII proteins with a modified immunodominant T-cell epitope exhibit reduced immunogenicity and normal FVIII activity

Factor VIII (FVIII)-neutralizing antibodies (inhibitors) are a serious complication in hemophilia A (HA). The peptide FVIII2194-2213 contains an immunodominant HLA-DRA*01-DRB1*01:01 (DRB1*01:01)-restricted epitope recognized by CD4+ T-effector cells from HA subjects. The aim of this study was to identify amino acid substitutions to deimmunize this epitope while retaining procoagulant function and expression levels comparable to those of wild-type (WT) FVIII proteins. The shortest DRB1*01:01-binding peptide was FVIII2194-2205, and residues important for affinity were identified as F2196, M2199, A2201, and S2204. T-cell proliferation experiments with Ala-substituted FVIII2194-2205 peptides identified F2196A as a substitution that abrogated proliferation of clones specific for the WT sequence. T-cell clones that were stimulated by recombinant WT-FVIII-C2 (rWT-FVIII-C2) protein did not proliferate when cultured with rFVIII-C2-F2196A, indicating the immunogenic peptide includes a naturally processed T-cell epitope. Additional amino acid substitutions at F2196 and M2199 were evaluated by peptide-MHC class II (MHCII)-binding assays, T-cell proliferation assays, epitope prediction algorithms, and sequence homologies. Six B-domain-deleted (BDD)-FVIII proteins with substitutions F2196A, F2196L, F2196K, M2199A, M2199W, or M2199R were produced. Proliferation of T-cell clones and polyclonal lines in response to rBDD-FVIII-F2196K and rBDD-FVIII-M2199A was reduced compared with responses to WT-BDD-FVIII. The BDD-FVIII-F2196K sequence modification appears to be the most promising sequence variant tested here, due to its effectiveness at eliminating DRB1*01:01-restricted immunogenicity, low potential immunogenicity in the context of other MHCII alleles, expression level comparable to WT-BDD-FVIII, and retained procoagulant activity. These results provide proof of principle for the design of less immunogenic FVIII proteins targeted to specific subsets of HA patients.

A unique major histocompatibility complex Class II-binding register correlates with HLA-DR11-associated immunogenicity of the major K blood group antigen

BACKGROUND

Kell is a glycoprotein expressed on red blood cells (RBCs). Its K and k variants contain either Met (K antigen) or Thr (k antigen) at Position 193, respectively. Development of anti-K after K-mismatched antigen exposure via blood transfusions or pregnancy can destroy RBCs, leading to hemolytic transfusion reactions and hemolytic disease of the fetus and newborn. The immunogenicity of overlapping 15-mer Kell peptides with M193 or T193 at every possible position was investigated previously. Interestingly, Peptide W179 to M193, with the polymorphic M193T residue at the peptide's C-terminus, was the most effective at stimulating CD4 T cells from a series of K-immunized women.

STUDY DESIGN AND METHODS

This study investigates the basis for HLA restriction of anti-K immune responses. Major histocompatibility complex Class II (MHCII)-binding prediction algorithms and quantitative peptide-MHCII-binding assays were employed to determine the binding registers; anchor residues; and affinities of wild-type, truncated, and sequence-modified K and k peptides. Predictions were generated using Immune Epitope Database and ProPred algorithms. Competitive peptide-MHCII-binding assays utilized 12 recombinant HLA-DR proteins, K and k peptides, and high-affinity MHCII-restricted reference peptides.

RESULTS

The peptide-MHCII-binding assays identified a unique K peptide-binding register (W179-S187) restricted to HLA-DRB1*11:01, in addition to partially overlapping binding registers that included the K/k M193T polymorphic site and that bound promiscuously to multiple HLA-DR proteins.

CONCLUSION

Three partially overlapping MHCII-binding motifs for HLA-DRB1*11:01 result in high-avidity K-peptide binding, which may contribute to HLA-DR11-restricted immunogenicity associated with the K allele.

Hemophilia A inhibitor treatment: the promise of engineered T-cell therapy

Hemophilia A is a bleeding disorder caused by mutations in the gene encoding factor VIII (FVIII), a cofactor protein that is essential for normal blood clotting. Approximately, 1 in 3 patients with severe hemophilia A produce neutralizing antibodies (inhibitors) that block its biologic function in the clotting cascade. Current efforts to eliminate inhibitors consist of repeated FVIII injections under what is termed an "ITI" protocol (Immune Tolerance Induction). However, this method is extremely costly and approximately 30% of patients undergoing ITI do not achieve peripheral tolerance. Human T regulatory cells (Tregs) have been proposed as a new strategy to treat this antidrug antibody response, as well as other diseases. Polyclonal Tregs are nonspecific and could potentially cause general immunosuppression. Novel approaches to induce tolerance to FVIII include the use of engineered human and mouse antigen-specific Tregs, or alternatively antigen-specific cytotoxic cells, to delete, anergize, or kill FVIII-specific lymphocytes. In this review, we discuss the current state of engineered T-cell therapies, and we describe the recent progress in applying these therapies to induce FVIII-specific tolerance.

A unique MHCII-binding register correlates with HLA-associated immunogenicity of the major K blood group antigen

Kell is a glycoprotein expressed on red blood cells (RBCs). Its K and k isoforms contain either Met (K antigen) or Thr (k antigen) at position 193. Development of anti-K antibodies following K-mismatched blood transfusions can destroy RBCs, while cross-placental transfer of anti-K antibodies can destroy RBC precursors, resulting in Hemolytic Disease of the Newborn. HLA-DR11, DR13 and DR15 are over-represented among K-immunized patient populations. The immunogenicity of overlapping 15-mer Kell peptides with M193 or T193 at every possible position was investigated by Stephen et al (Blood. 2012;119(23):5563-5574). Surprisingly, peptide W179-M193, with the polymorphic M193T site at the peptide’s C-terminus, was the most effective at stimulating CD4 T cells from K-immunized women. The present study utilized MHCII binding prediction algorithms and quantitative peptide-MHCII binding assays to determine the binding registers, anchor residues and affinities of wild-type, truncated, and sequence-modified Kell peptides. Predictions were generated using IEDB and Propred algorithms. Competitive peptide-MHCII binding assays utilized 13 recombinant HLA-DRB1 monomers, Kell peptides and high-affinity reference peptides. Interestingly, the assays identified a unique binding register (W179-S187) restricted to HLA-DR11 and DR15, in addition to the polymorphic site, suggesting stronger MHCII avidity may explain the increased disease susceptibility associated with these alleles.

Engineering less immunogenic and antigenic FVIII proteins

The development of neutralizing antibodies against blood coagulation factor VIII (FVIII), referred to clinically as "inhibitors", is the most challenging and deleterious adverse event to occur following intravenous infusions of FVIII to treat hemophilia A. Inhibitors occlude FVIII surfaces that must bind to activated phospholipid membranes, the serine proteinase factor IXa, and other components of the 'intrinsic tenase complex' in order to carry out its important role in accelerating blood coagulation. Inhibitors develop in up to one of every three patients, yet remarkably, a substantial majority of severe hemophilia A patients, who circulate no detectable FVIII antigen or activity, acquire immune tolerance to FVIII during initial infusions or else after intensive FVIII therapy to overcome their inhibitor. The design of less immunogenic FVIII proteins through identification and modification ("de-immunization") of immunodominant T-cell epitopes is an important goal. For patients who develop persistent inhibitors, modification of B-cell epitopes through substitution of surface-exposed amino acid side chains and/or attachment of bulky moieties to interfere with FVIII attachment to antibodies and memory B cells is a promising approach. Both experimental and computational methods are being employed to achieve these goals. Future therapies for hemophilia A, as well as other monogenic deficiency diseases, are likely to involve administration of less immunogenic proteins in conjunction with other novel immunotherapies to promote a regulatory cellular environment promoting durable immune tolerance.

Avidity of human T cell receptor engineered CD4(+) T cells drives T-helper differentiation fate

The role of the T cell receptor (TCR) in antigen recognition and activation of T lymphocytes is well established. However, how the TCR affects T-helper differentiation/skewing is less well understood, particularly for human CD4(+) (CD4) T cell subsets. Here we investigate the role of TCR specific antigen avidity in differentiation and maintenance of human Th1, Th2 and Th17 subsets. Two human TCRs, both specific for the same peptide antigen but with different avidities, were cloned and expressed in human CD4 T cells. These TCR engineered cells were then stimulated with specific antigen in unskewed and T-helper skewed conditions. We show that TCR avidity can control the percentage of IL-4 and IFN-γ co-expression in unskewed TCR engineered cells, that effector function can be maintained in a TCR avidity-dependent manner in skewed TCR engineered cells, and that increased TCR avidity can accelerate Th1 skewing of TCR engineered cells.

Phenotypes of allo- and autoimmune antibody responses to FVIII characterized by surface plasmon resonance

Evidence of antibody isotype/subtype switching may provide prognostic value regarding the state of immune responses to therapeutic proteins, e.g. anti-factor VIII (FVIII) antibodies that develop in many hemophilia A patients, clinically termed “inhibitors”. A sensitive, high- information-content surface plasmon resonance (SPR) assay has been developed to quantify IgG subtype distributions and the domain specificity of anti-drug antibodies. Plasma samples from 22 subjects with an allo- or auto-immune reaction to FVIII were analyzed. Pre-analytical treatment protocols were developed to minimize non-specific binding and specific matrix interference due to von Willebrand factor-FVIII interactions. The dynamic range for IgG quantification was 0.2–5 µg/ml (∼1–33 nM), allowing characterization of inhibitor-positive samples. Subtype-specific monoclonal antibodies were used to quantify the IgG subtype distribution of FVIII-specific antibodies. Most samples obtained from multiply-infused inhibitor subjects contained IgG4 antibodies. Several distinct phenotypes were assigned based on the IgG subtype distribution: IgG₁, IgG₄, IgG₁ & IgG₄, and IgG₁, IgG₂ & IgG₄. An IgG₁-only response was found in mild/moderate HA subjects during early FVIII infusions, and analysis of serial samples followed antibody class switching as several subjects’ immune responses developed. Competition studies utilizing a recombinant FVIII-C2 domain indicated 40–80% of FVIII-specific antibodies in most samples were directed against this domain.

Dangerous liaisons: how the immune system deals with factor VIII

Only a fraction of patients with hemophilia A develop a neutralizing antibody (inhibitor) response to therapeutic infusions of factor VIII. Our present understanding of the underlying causes of the immunogenicity of this protein is limited. In the past few years, insights into the uptake and processing of FVIII by antigen-presenting cells (APCs) have expanded significantly. Although the mechanism of endocytosis remains unclear, current data indicate that FVIII enters APCs via its C1 domain. Its subsequent processing within endolysosomes allows for presentation of a heterogeneous collection of FVIII-derived peptides on major histocompatibility complex (MHC) class II, and this peptide-MHC class II complex may then be recognized by cognate effector CD4(+) T cells, leading to anti-FVIII antibody production. Here we aim to summarize recent knowledge gained about FVIII processing and presentation by APCs, as well as the diversity of the FVIII-specific T-cell repertoire in mice and humans. Moreover, we discuss possible factors that can drive FVIII immunogenicity. We believe that increasing understanding of the immune recognition of FVIII and the cellular mechanisms of anti-FVIII antibody production will lead to novel therapeutic approaches to prevent inhibitor formation in patients with hemophilia A.

Haemophilia management: time to get personal?

The possibility of alloimmunization in patients receiving protein replacement therapy depends on (at least) three risk factors, which are necessary concomitantly but insufficient alone. The first is the degree of structural difference between the therapeutic protein and the patient's own endogenous protein, if expressed. Such differences depend on the nature of the disease mutation and the pre-mutation endogenous protein structure as well as on post-translational changes and sequence-engineered alterations in the therapeutic protein. Genetic variations in the recipients' immune systems comprise the second set of risk determinants for deleterious immune responses. For example, the limited repertoire of MHC class II isomers encoded by a given person's collection of HLA genes may or may not be able to present a 'foreign' peptide(s) produced from the therapeutic protein - following its internalization and proteolytic processing - on the surface of their antigen-presenting cells (APCs). The third (and least characterized) variable is the presence or absence of immunologic 'danger signals' during the display of foreign-peptide/MHC-complexes on APCs. A choice between existing therapeutic products or the manufacture of new proteins, which may be less immunogenic in some patients or patient populations, may require prior definition of the first two of these variables. This leads then to the possibility of developing personalized therapies for disorders due to genetic deficiencies in endogenous proteins, such as haemophilia A and B. [Correction made after online publication 11 July 2011: several critical corrections have been made to the abstract].

T-cell responses in two unrelated hemophilia A inhibitor subjects include an epitope at the factor VIII R593C missense site

BACKGROUND

Development of neutralizing anti-factor (F)VIII antibodies ('inhibitors') is a serious clinical problem in hemophilia A. Increased inhibitor risk has been associated with certain FVIII missense substitutions, including R593C in the A2 domain.

OBJECTIVES

The aim of the present study was to identify T-cell epitopes in FVIII and characterize T-cell responses in two unrelated hemophilia A subjects sharing F8-R593C and HLA-DRB1*1101 genotypes. We hypothesized that the hemophilic substitution site coincides with an important T-cell epitope.

PATIENTS/METHODS

The binding affinities of peptides for recombinant HLA-DR proteins were measured and compared with epitope prediction results. CD4+ T cells were stimulated using peptides and stained with fluorescent, peptide-loaded tetramers.

RESULTS

The inhibitor subjects, but not HLA-matched controls, had high-avidity HLA-DRB1*1101-restricted T-cell responses against FVIII(589-608), which contains the hemophilic missense site. Antigen-specific T cells secreted Th1 and Th2 cytokines and proliferated in response to FVIII and FVIII(592-603). FVIII(589-608) bound with physiologically relevant (micromolar) IC(50) values to recombinant DR0101, DR1101 and DR1501 proteins.

CONCLUSIONS

Hemophilia A patients with R593C missense substitutions and these HLA haplotypes had an increased incidence of inhibitors in our cohorts, supporting a paradigm in which presentation of FVIII epitopes containing the wild-type R593 influences inhibitor risk in this hemophilia A sub-population.

HLA-DR-restricted T-cell responses to factor VIII epitopes in a mild haemophilia A family with missense substitution A2201P

An HLA-DRA-DRB1*0101-restricted T-cell epitope in the factor VIII (FVIII) C2 domain occurred in a mild haemophilia A patient with missense substitution FVIII-A2201P. His T cells responded to synthetic peptides FVIII(2186-2205) and FVIII(2194-2213) (J Thromb Haemost 2007; 5: 2399). T cells from family members with genotype FVIII-A2201P were analysed to determine if FVIII-specific T cells occur in individuals with a haemophilic mutation but no clinically significant inhibitor response. Fluorescent MHC class II tetramers corresponding to subjects'HLA-DRB1 types were loaded with 20-mer peptides and utilized to label antigen-specific CD4+ T cells. T-cell responses to peptides spanning the FVIII-C2 sequence were evaluated. T cells recognizing specific peptides were cloned, and antigen specificity was verified by proliferation assays. Plasma and/or purified IgG samples were tested for FVIII inhibitory activity. CD4+ T cells and T-cell clones from two brothers who shared the DRB1*0101 allele responded to FVIII(2194-2213). A haemophilic cousin's HLA-DRA-DRB1*1104-restricted response to FVIII(2202-2221) was detected only when CD4+CD25+ cells were depleted. A great uncle and two obligate carriers had no detectable FVIII-C2-specific T cells. Concentrated IgG from the brother without a clinical inhibitor response showed a low-titre FVIII inhibitor. FVIII-specific T cells and inhibitory IgG were found in a previously infused, haemophilic subject who had a sub-clinical FVIII inhibitor. CD4+CD25+ depleted T cells from a non-infused haemophilic cousin recognized an overlapping FVIII epitope, indicating a latent HLA-DRA-DRB1*1104-restricted T-cell response to FVIII. Specific T-cell responses to FVIII can occur without clinically significant inhibitors.

Distinct lineages of human T-cell clones, including Th17/Th1 cells, isolated at different stages of anti-factor VIII immune responses (48.26)

Approximately 25% of hemophilia A patients, who lack functional factor VIII (FVIII), develop neutralizing antibodies after receiving therapeutic infusions of FVIII. HLA-DRA-DRB1*0101-restricted T-cell clones that respond to FVIII2194-2213 were isolated from a hemophilia A subject (the proband) ~5 and 21 months following his development of a high-titer FVIII neutralizing antibody response, and from his brother who has not developed a clinically significant antibody response. Three clones obtained from the proband at 5 months were TH17/TH1-polarized and two were TH1/TH2-polarized. The 8 proband clones isolated at 21 months were TH2-polarized, indicating T-cell lineage changes over time. The 6 clones from the brother were TH1-polarized, indicating that B-cell tolerance to FVIII can be maintained even with circulating antigen-specific TH1-polarized cells. Several TCRBV-D-J junction sequences were identified suggesting differences in TCR interaction with the MHC class II-peptide complex. Current investigations are examining the response of clones to altered peptide ligands. This is the first evidence that TH17/TH1-polarized cells play a role in hemophilic immune responses to FVIII; it is also the first detailed characterization of antigen-specific TH17/TH1 clones isolated using standard culture conditions.

Research Support: Bayer Hemophilia Award, CSL Behring Hemophilia Research Award, NIH.

Covalent inactivation of factor VIII antibodies from hemophilia A patients by an electrophilic FVIII Analog

The antigen-binding sites of antibodies (Abs) can express enzyme-like nucleophiles that react covalently with electrophilic compounds. We examined the irreversible and specific inactivation of antibodies (Abs) to Factor VIII (FVIII) responsible for failure of FVIII replacement therapy in hemophilia A (HA) patients. Electrophilic analogs of FVIII (E-FVIII) and its C2 domain (E-C2) were prepared by placing the strongly electrophilic phosphonate groups at surface-exposed Lys side chains of diverse antigenic epitopes. IgG Abs to FVIII from HA patients formed stable immune complexes with E-FVIII and E-C2 that were refractory to dissociation by SDS treatment and boiling, procedures that dissociate noncovalent Ab-antigen complexes. The rate-limiting step in the reaction was formation of the initial noncovalent complexes. Conversion of the initial complexes to the irreversible state occurred rapidly. The antigenic epitopes of E-FVIII were largely intact, and most of the Abs were consumed covalently. E-FVIII expressed poor FVIII cofactor activity in clotting factor assays. Nonspecific interference by E-FVIII in clotting factor function was not evident. Treatment with E-FVIII, and to a lesser extent E-C2, irreversibly relieved the FVIII inhibitory effect of HA IgG in clotting factor assays. Small FVIII peptides did not display useful reactivity, highlighting the diverse epitope specificities of the Abs and the conformational character of FVIII epitopes. E-FVIII is a prototype reagent able to attain irreversible and specific inactivation of pathogenic Abs.

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.

ELISA system for detection of immune responses to FVIII: a study of 246 samples and correlation with the Bethesda assay

Inhibitors of FVIII are usually IgG polyclonal antibodies that develop as alloimmune responses in patients with congenital haemophilia A or as autoimmune responses resulting in acquired haemophilia. Their recognition can be difficult, especially when the titre is low. Furthermore, results from a Bethesda assay often require several days as samples are referred to a specialty laboratory. The aim of this study is to assess the utility of an ELISA system for detecting immune responses to FVIII. A total of 246 plasma samples submitted from 176 individuals with immune responses to FVIII, as verified with the Bethesda assay, and samples from 50 control subjects were tested for the presence of FVIII-specific IgG using an ELISA-based assay. Paired sera from 18 of the patients were also tested by the ELISA. Of the 246 samples that were positive for a FVIII inhibitor by the Bethesda assay, 235 (95.5%) were also positive by ELISA. The regression coefficient, using Log BU was r = 0.82. The correlation data were strengthened when 27 inhibitor samples were diluted further. There was a strong correlation between ELISA results for the 18-paired serum and plasma samples (r = 0.99). There is a strong correlation between the ELISA and Bethesda methods in detecting immune responses to FVIII. The ELISA provides rapid screening that could be available well in advance of confirmation by the Bethesda assay.

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

BACKGROUND

The development of anti-factor VIII (FVIII) neutralizing antibodies (inhibitors) is a significant obstacle to FVIII replacement therapy.

OBJECTIVE

As mucosal administration of an antigen may induce immune tolerance we have evaluated the efficacy of mucosal antigen exposure to achieve tolerance to FVIII.

METHODS

We investigated the effects of oral and nasal administration of the purified FVIII C2 domain (FVIII-C2) to FVIII-deficient BALB/c mice prior to FVIII protein challenge. Mice received oral or nasal doses of FVIII-C2, followed by a subcutaneous challenge of either FVIII-C2 or FVIII. The development of anti-FVIII inhibitors, cytokine production by splenocytes in vitro, and adoptive transfer assays were analyzed.

RESULTS AND CONCLUSIONS

Mucosal administration of FVIII-C2 decreases the titer of anti-FVIII-C2 inhibitors after FVIII-C2 challenge, and decreases the percentage of FVIII-C2 specific antibodies after challenge with full-length FVIII. Tolerance induction to FVIII-C2 is associated with increased IL-10 production by splenocytes in vitro, and can be adoptively transferred to naïve mice. This study is the first to demonstrate that tolerance to the FVIII-C2 domain can be induced via the mucosal route. Based on these results, the potential use of FVIII-specific mucosal tolerance induction as an immunotherapy treatment for anti-FVIII inhibitor development warrants further investigation.

Immunodominant T-cell epitopes in the factor VIII C2 domain are located within an inhibitory antibody binding site

Formation of inhibitor antibodies to factor VIII (FVIII) is a major complication of FVIII replacement therapy for hemophilia A patients, and it occurs through a T-cell dependent process. The C2 domain of FVIII contains epitopes that are recognized by antibody inhibitors. We have examined regions of the C2 domain that form epitopes for T cells in mice congenitally deficient in FVIII. We obtained CD4(+)T cells from mice immunized by intravenous infusion of therapeutic doses of recombinant human FVIII (rFVIII), or by subcutaneous injections of rFVIII or recombinant human C2 domain in adjuvant. In all cases, the T cells recognized most strongly and consistently two overlapping peptides that spanned residues 2191 to 2220 of the C2 domain. Analysis of the crystal structure of human factor VIII C2 bound to a human monoclonal antibody, BO2C11, showed these residues also constitute part of a human alloimmune B-cell epitope (Spiegel et al., Blood 2001; 98: 13-19). This region includes one of the "hydrophobic spike" protrusions, consisting of M2199 and F2200, as well as the basic residues R2215 and R2220. These residues contribute to membrane binding and to association with von Willebrand factor (vWF). These findings suggest that a major T-cell epitope in the C2 domain recognized by hemophilic mice is located within the same region that binds to inhibitors, vWF, and activated membranes.

New insights into binding interfaces of coagulation factors V and VIII and their homologues lessons from high resolution crystal structures

The large, multifunctional proteins Factors V and VIII are cofactors in the coagulation cascade and possess a similar domain structure, A1-A2-B-A3-C1-C2. The C domains are related to the discoidin protein family, while the A domains are homologous to the copper-binding protein ceruloplasmin. After proteolytic activation, Factors V and VIII behave as peripheral membrane proteins, binding to negatively charged membranes containing phosphatidylserine, primarily via specific sites on their C2 domains. This type of membrane surface is exposed at sites of tissue damage, where platelets have become activated. The cofactors then accelerate sequential proteolytic activations that occur at critical control points in the blood coagulation cascade via complex formation with specific serine proteinases. Here we compare recent structural and functional studies of the C2 domains of Factors V and VIII, and discuss their respective roles. The membrane-binding motifs consist of several exposed hydrophobic side chains surrounded by a ring of basic residues, and the C2 domains appear poised to insert their hydrophobic "feet" into the membrane interior as basic residues interact favorably with phosphatidylserine head groups. In line with their physiological roles, the membrane-binding surfaces of the C2 domains display a good deal of mobility. We then extend our analysis to other members of the discoidin protein family, which perform diverse physiological functions involving signaling pathways at cell surfaces. Finally, structural similarities between discoidin proteins and the topologically distinct but functionally related membrane-binding "classic C2 domains", including signal-transduction proteins such as Protein Kinase C and phospholipases, are noted.

Structure of a factor VIII C2 domain-immunoglobulin G4kappa Fab complex: identification of an inhibitory antibody epitope on the surface of factor VIII

The development of an immune response to infused factor VIII is a complication affecting many patients with hemophilia A. Inhibitor antibodies bind to antigenic determinants on the factor VIII molecule and block its procoagulant activity. A patient-derived inhibitory immunoglobulin G4kappa antibody (BO2C11) produced by an immortalized memory B-lymphocyte cell line interferes with the binding of factor VIII to phospholipid surfaces and to von Willebrand factor. The structure of a Fab fragment derived from this antibody complexed with the factor VIII C2 domain was determined at 2.0 A resolution. The Fab interacts with solvent-exposed basic and hydrophobic side chains that form a membrane-association surface of factor VIII. This atomic resolution structure suggests a variety of amino acid substitutions in the C2 domain of factor VIII that might prevent the binding of anti-C2 inhibitor antibodies without significantly compromising the procoagulant functions of factor VIII.

Relating structure to function: the role of the C2 domain in Factor VIII

Factor VIII is a plasma glycoprotein that becomes activated during blood coagulation. Through its association with other procoagulant components at the wound site, it leads to a tremendous acceleration in the production of thrombin, and thus to the rate at which bleeding is staunched. Deficiencies in Factor VIII can result in hemophilia A, the most common hereditary bleeding disorder. Advances in molecular biology and protein chemistry led to characterization of Factor VIII and its gene in the early 1980s. Over the past 15 years, PCR methods combined with careful clinical studies have identified many of the molecular defects associated with hemophilia A. Concurrently, basic research utilizing both plasma-derived Factor VIII and recombinant constructs has led to a greatly improved understanding of the functions of specific regions within this large glycoprotein. This review focuses on recent advances in elucidating the roles of the Factor VIII carboxy terminal C2 domain. Particular emphasis is placed upon interpreting biophysical measurements and clinical data in light of the recently obtained high-resolution crystal structure of the recombinant C2 domain. Some interesting directions for future experiments are also suggested.

Hemophilic factor VIII C1- and C2-domain missense mutations and their modeling to the 1.5-angstrom human C2-domain crystal structure

Factor VIII C domains contain key binding sites for von Willebrand factor (vWF) and phospholipid membranes. Hemophilic patients were screened for factor VIII C-domain mutations to provide a well-characterized series. Mutated residues were localized to the high-resolution C2 structure and to a homology model of C1. Of 30 families found with mutations in the C domains, there were 14 missense changes, and 9 of these were novel. Of the missense mutations, 10 were associated with reduced vWF binding and 8 were at residues with surface-exposed side chains. Six of the 10 mutants had nearly equivalent factor VIII clotting activity and antigen level, suggesting that reduced vWF binding could cause hemophilia by reducing factor VIII stability in circulation. When the present series was combined with previously described mutations from an online international database, 11 C1 and C2 mutations in patients with mild or moderately severe hemophilia A were associated with antibody-inhibitor development in at least one affected individual. Of these substitutions, 6 occurred at surface-exposed residues. As further details of the C1 structure and its interface with C2 become available, and as binding studies are performed on the plasma of more patients with hemophilic C-domain mutations, prediction of surface binding sites should improve, allowing confirmation by site-specific mutagenesis of surface-exposed residues.

Structure of the C2 domain of human factor VIII at 1.5 A resolution

Human factor VIII is a plasma glycoprotein that has a critical role in blood coagulation. Factor VIII circulates as a complex with von Willebrand factor. After cleavage by thrombin, factor VIIIa associates with factor IXa at the surface of activated platelets or endothelial cells. This complex activates factor X (refs 6, 7), which in turn converts prothrombin to thrombin in the presence of factor Va (refs 8, 9). The carboxyl-terminal C2 domain of factor VIII contains sites that are essential for its binding to von Willebrand factor and to negatively charged phospholipid surfaces. Here we report the structure of human factor VIII C2 domain at 1.5 A resolution. The structure reveals a beta-sandwich core, from which two beta-turns and a loop display a group of solvent-exposed hydrophobic residues. Behind the hydrophobic surface lies a ring of positively charged residues. This motif suggests a mechanism for membrane binding involving both hydrophobic and electrostatic interactions. The structure explains, in part, mutations in the C2 region of factor VIII that lead to bleeding disorders in haemophilia A.

The polymerization pocket "a" within the carboxyl-terminal region of the gamma chain of human fibrinogen is adjacent to but independent from the calcium-binding site

The carboxyl-terminal region of the gamma chain of fibrinogen is involved in calcium binding, fibrin polymerization, factor XIIIa-mediated cross-linking, and binding to the platelet fibrin(ogen) receptor. Protein fragments encoding amino acids Val143 to Val411 (rFbggammaC30) or Val143 to Leu427 (gamma'C30) from the carboxyl end of the gamma or gamma' chains, respectively, of human fibrinogen were expressed in yeast (Pichia pastoris) and characterized as to their cross-linking by factor XIIIa, polymerization pocket, and calcium-binding site. rFbggammaC30 and gamma'C30 were both readily cross-linked by factor XIIIa, but only rFbggammaC30 was capable of inhibiting thrombin-induced platelet aggregation. Two mutants, gammaC30-Q329R and gammaC30-D364A, which were based on the three-dimensional structure of the polymerization pocket within rFbggammaC30 and on information derived from naturally occurring mutant fibrinogens, were also expressed and characterized. rFbggammaC30 inhibited (desAA)fibrin polymerization in a dose-dependent manner, while the two mutant forms did not. Similarly, rFbggammaC30 and gamma'C30 were protected from plasmin degradation by the presence of Ca2+ or the peptide Gly-Pro-Arg-Pro, indicating that a functional Ca2+-binding site and polymerization pocket are contained within each of these fragments. The mutant fragments, however, were protected from plasmin only by metal ions, while no protective effect was conferred by GPRP or by any other peptide tested. These results indicate that the polymerization pocket "a", which binds the peptide GPRP, functions independently from the nearby calcium-binding site and that amino acids Gln329 and Asp364 play a crucial role in fibrin polymerization.

The primary fibrin polymerization pocket: three-dimensional structure of a 30-kDa C-terminal gamma chain fragment complexed with the peptide Gly-Pro-Arg-Pro

After vascular injury, a cascade of serine protease activations leads to the conversion of the soluble fibrinogen molecule into fibrin. The fibrin monomers then polymerize spontaneously and noncovalently to form a fibrin gel. The primary interaction of this polymerization reaction is between the newly exposed N-terminal Gly-Pro-Arg sequence of the alpha chain of one fibrin molecule and the C-terminal region of a gamma chain of an adjacent fibrin(ogen) molecule. In this report, the polymerization pocket has been identified by determining the crystal structure of a 30-kDa C-terminal fragment of the fibrin(ogen) gamma chain complexed with the peptide Gly-Pro-Arg-Pro. This peptide mimics the N terminus of the alpha chain of fibrin. The conformational change in the protein upon binding the peptide is subtle, with electrostatic interactions primarily mediating the association. This is consistent with biophysical experiments carried out over the last 50 years on this fundamental polymerization reaction.

Crystal structure of a 30 kDa C-terminal fragment from the gamma chain of human fibrinogen

BACKGROUND

Blood coagulation occurs by a cascade of zymogen activation resulting from minor proteolysis. The final stage of coagulation involves thrombin generation and limited proteolysis of fibrinogen to give spontaneously polymerizing fibrin. The resulting fibrin network is covalently crosslinked by factor XIIIa to yield a stable blood clot. Fibrinogen is a 340 kDa glycoprotein composed of six polypeptide chains, (alphabetagamma)2, held together by 29 disulfide bonds. The globular C terminus of the gamma chain contains a fibrin-polymerization surface, the principal factor XIIIa crosslinking site, the platelet receptor recognition site, and a calcium-binding site. Structural information on this domain should thus prove helpful in understanding clot formation.

RESULTS

The X-ray crystallographic structure of the 30 kDa globular C terminus of the gamma chain of human fibrinogen has been determined in one crystal form using multiple isomorphous replacement methods. The refined coordinates were used to solve the structure in two more crystal forms by molecular replacement; the crystal structures have been refined against diffraction data to either 2.5 A or 2.1 A resolution. Three domains were identified in the structure, including a C-terminal fibrin-polymerization domain (P), which contains a single calcium-binding site and a deep binding pocket that provides the polymerization surface. The overall structure has a pronounced dipole moment, and the C-terminal residues appear highly flexible.

CONCLUSIONS

The polymerization domain in the gamma chain is the most variable among a family of fibrinogen-related proteins and contains many acidic residues. These residues contribute to the molecular dipole moment in the structure, which may allow electrostatic steering to guide the alignment of fibrin monomers during the polymerization process. The flexibility of the C-terminal residues, which contain one of the factor XIIIa crosslinking sites and the platelet receptor recognition site, may be important in the function of this domain.