Autoimmune Antibody in a Hemorrhagic Patient Interacts With Thrombin-Activated Factor XIII in a Unique Manner

Antibodies against Noncatalytic B Subunit of Factor XIII Inhibit Activation of Factor XIII and Fibrin Crosslinking

BACKGROUND

 Coagulation factor XIII (FXIII) is a proenzyme of plasma transglutaminase. It comprises two catalytic A subunits (FXIII-A) and two carrier B subunits (FXIII-B). We previously reported that alloantibodies against FXIII-B could promote FXIII clearance in a patient with congenital FXIII-B deficiency who had received infusions of plasma-derived human FXIII (A2B2 heterotetramer).

OBJECTIVES

 We aimed to investigate whether anti-FXIII-B antibodies affect the catalytic function of FXIII.

METHODS

 FXIII activation and fibrin crosslinking were examined in the presence of patient plasma, isolated patient IgG, or rat anti-FXIII-B monoclonal antibodies.

RESULTS

 Alloantibody levels were increased by repeated infusions of plasma-derived A2B2 heterotetramer, which enhanced binding to the functionally important FXIII-B sushi domains. The patient plasma strongly inhibited cleavage of the FXIII-A activation peptide, amine incorporation, and fibrin crosslinking in normal plasma. Furthermore, anti-FXIII-B alloantibodies blocked the formation of the complex of FXIII-B with FXIII-A, and fibrinogen. Rat monoclonal antibodies against the 10th sushi domain of FXIII-B inhibited the incorporation of FXIII-B to fibrin, FXIII activation (i.e., cleavage of FXIII-A activation peptide), and ultimately fibrin crosslinking in normal plasma, independent of their effect on heterotetramer assembly with FXIII-A. Alloantibody binding to the A2B2 heterotetramer blocked the access of thrombin to the FXIII-A cleavage site, as indicated by the reaction of the alloantibodies to the A2B2 heterotetramer and FXIII-B, but not to FXIII-A.

CONCLUSION

 Anti-FXIII-B antibodies binding to the A2B2 heterotetramer and FXIII-B inhibited FXIII activation and its crosslinking function despite being directed against its noncatalytic subunit (FXIII-B).

Cloning of human anti-factor XIII monoclonal antibody dissects mechanisms of polyclonal antibodies in a single patient

BACKGROUND

Coagulation factor XIII (FXIII) consists of 2 A (FXIII-A) and 2 B (FXIII-B) subunits that cross-link and strengthen the hemostatic fibrin thrombus; thus, abnormal bleeding occurs when FXIII is significantly reduced. Autoimmune-acquired FXIII deficiency (AiF13D) is characterized by lethal bleeding secondary to the development of autoantibodies against FXIII. However, since anti-FXIII autoantibodies are polyclonal, the mechanism underlying FXIII dysfunction is unclear.

OBJECTIVES

The objective of this study was to dissect the inhibitory mechanisms of polyclonal anti-FXIII autoantibodies.

METHODS

In this study, we prepared the human monoclonal antibodies (hmAbs) from the peripheral blood of an 86-year-old man with AiF13D by using a new complementary DNA cloning method and analyzed the properties of each autoantibody.

RESULTS

Seventeen clones obtained from hmAbs were divided into the following 3 groups: dissociation inhibitors of FXIII-A₂B₂ (6 clones), assembly inhibitors of FXIII-A₂B₂ (3 clones), and nonneutralizing/inhibitory hmAbs (8 clones). Dissociation inhibitors strongly inhibited fibrin cross-linking and amine incorporation. Assembly inhibitors extracted FXIII-A from FXIII-A₂B₂, strongly inhibited binding of FXIII-A to FXIII-B, and activation peptide cleavage. However, the patient's plasma presented a strong inhibition of A₂B₂ heterodimer assembly but only a slight inhibition of thrombin-Ca²⁺-dependent dissociation, suggesting that the assembly inhibitors concealed the effect of dissociation inhibitors in plasma. By contrast, nonneutralizing antibodies had little effect on the function of FXIII, suggesting that nonneutralizing hmAbs (and/or dissociation inhibitors and/or assembly inhibitors) promoted the clearance of FXIII-A from the blood.

CONCLUSION

Cloning of anti-FXIII autoantibodies enabled us to not only elucidate the mechanism and pathophysiology of AiF13D but also develop a completely new type of anticoagulant.

Structure-Based Design of FXIIIa-Blockers: Addressing a Transient Hydrophobic Pocket in the Active Site of FXIIIa

Blood coagulation factor XIII (FXIII, F13) is considered to be a promising target for anticoagulants with reduced bleeding risk because of its unique position in the coagulation cascade downstream of thrombin. However, until now, no potent drug addressing FXIII has been available, indeed no compound has even entered clinical trials yet. In 2013, we published the co-crystal structure of FXIII in the active state (FXIIIa°), thereby providing a detailed map of the active site for the rational design of potent FXIIIa blockers. Here we report, for the first time, a structure-based approach to improving the affinity of FXIIIa inhibitors. FXIII was crystallized in complex with a methyl thiazole moiety to address a novel transient hydrophobic pocket close to the catalytic center. By subsequent structure-based design to rationalize the introduction of an ethyl ester, the potency of the inhibitor was improved significantly compared to that of the parent lead compound. The occupancy of the hydrophobic pocket described here might turn out to be a key step in the development of a potent reversible and orally available FXIIIa blocker.

Immunoproteomic Identification of Noncarbohydrate Antigens Eliciting Graft-Specific Adaptive Immune Responses in Patients with Bovine Pericardial Bioprosthetic Heart Valves

PURPOSE

This case-control retrospective discovery study is to identify antigenic bovine pericardium (BP) proteins that stimulate graft-specific humoral immune response in patients implanted with glutaraldehyde fixed bovine pericardial (GFBP) heart valves.

EXPERIMENTAL DESIGN

Banked serum is collected from age- and sex-matched patients who received either a GFBP or mechanical heart valve replacement. Serum IgG is isolated and used to generate poly-polyclonal antibody affinity chromatography columns from each patient. Native and deglycosylated BP protein extracts are separately added to individual patient affinity chromatography columns, with unbound proteins washed through the column. Proteins captured in the affinity chromatography columns are submitted for proteomic identification. Differences between GFBP and mechanical heart valve replacement recipients are analyzed with Gaussian linearized modeling.

RESULTS

Carbohydrate antigens overwhelm protein capture in the column, requiring BP protein deglycosylation prior to affinity chromatography. Nineteen BP protein antigens, which stimulated graft-specific IgG production, are identified in patients who received GFBP valve replacements. Identified antigens are significantly over-represented for calcium-binding proteins.

CONCLUSIONS AND CLINICAL RELEVANCE

Patients implanted with GFBP valves develop a graft-specific humoral immune response toward BP protein antigens, with 19 specific antigens identified in this work. The molecular functions of over-represented antigens, specifically calcium-binding proteins, may aid in understanding the underlying factors that contribute to structural valve deterioration.

Acquired factor XIII deficiency: A review

Acquired factor XIII (FXIII) deficiency is a rare bleeding disorder that can manifest with spontaneous or delayed life-threatening hemorrhage. Causes of acquired deficiency include immune-mediated inhibition, as well as non-immune FXIII hyperconsumption or hyposynthesis. The occurrence of acquired FXIII deficiency can be idiopathic or may be associated with comorbidities, such as malignancies or autoimmune disorders. Recognition of acquired FXIII deficiency and its underlying cause is imperative, as treatment options vary depending on the etiology. Diagnosis requires quantitative FXIII testing in addition to supplemental inhibitor studies if the clinical situation suggests an immune-mediated pathophysiology. Treatment may involve FXIII replacement, antifibrinolytic administration, and/or inhibitor eradication. However, treatment targets and thresholds are undefined in acquired FXIII deficiency. This review will focus on the clinical characteristics, diagnostic issues and therapeutic options for both immune and non-immune acquired FXIII deficiency. Cases are described to illustrate the clinical features of acquired FXIII deficiency.

Auto- and alloantibodies against factor XIII: laboratory diagnosis and clinical consequences

Acquired FXIII deficiencies caused by autoantibodies against FXIII subunits represent rare but very severe bleeding diatheses. Alloantibodies in FXIII-deficient patients also cause life-threatening bleeding complications, but they develop extremely rarely. In this review we provide an overview of the diagnosis and classification of anti-FXIII antibodies and analyze 48 patients with autoimmune FXIII deficiency and four additional FXIII-deficient patients who developed anti-FXIII alloantibody. The patients were collected from peer-reviewed publications from which relevant data could be extracted. With the exception of two cases the antibodies were directed against FXIII-A. The difficulties in the diagnosis of FXIII deficiency in the presence of anti-FXIII antibodies are discussed and a scheme for the functional classification of the anti-FXIII antibodies is recommended. The three main categories are neutralizing and non-neutralizing antibodies and antibodies with combined effect. The methods being used for detecting and quantifying the inhibitory effect on FXIII activation and on the transglutaminase activity of activated FXIII are summarized and techniques for the classification of neutralizing anti-FXIII antibodies are outlined. The importance of clearance studies in these cases is emphasized. Binding assays, useful for the identification of non-neutralizing and combined type antibodies, were collected from the literature and their informative power is demonstrated by examples. The most frequently occurring bleeding symptoms in patients with anti-FXIII antibodies were soft tissue bleeding; intracranial bleedings also occurred, but less frequently than in inherited FXIII deficiency. Treatment of such patients is extremely challenging; the main aim should be eradication of the antibody.

Rapid immunochromatographic test for detection of anti-factor XIII A subunit antibodies can diagnose 90 % of cases with autoimmune haemorrhaphilia XIII/13

Autoimmune haemorrhaphilia XIII/13 (AH13) is an acquired lifethreatening bleeding disorder due to anti-factor XIII (FXIII) autoantibodies (auto-Abs). AH13 patients may die of haemorrhage without correct diagnosis and proper treatment because of lack of awareness and the absence of rapid easy-to-use tests specific for this disease. Currently, the definitive diagnosis is established by cumbersome and time-consuming laboratory tests such as dot-blot assays and enzymelinked immunosorbent assays (ELISA), and therefore these tests are generally not carried out. To save AH13 patients’ lives, there is an urgent necessity for developing a rapid test for FXIII auto-Abs. We first generated and characterised mouse monoclonal antibodies (mAb) against human FXIII A subunit (FXIII-A), and then developed a rapid immunochromatographic test (ICT) for detection of anti-FXIII-A auto- Abs using one mAb with a dissociation constant of 9.3 × 10-11 M. The auto-Ab-FXIII-A complex was captured by the mAb on a nitrocellulose membrane and visualised by Au-conjugated anti-human IgG Ab. Mixing with healthy control plasma improved the detection of auto-Abs in patients having extremely low levels of FXIII-A. The specificity and sensitivity of the ICT were 87% and 94%, respectively. We also detected auto-Abs against activated FXIII (FXIIIa) in three patients by pre-converting FXIII to FXIIIa by thrombin treatment. ICT values were significantly inversely correlated with FXIII activity levels, indicating an association between the quantity of anti-FXIII autoantibodies and AH13. This reliable rapid ICT assay can be applied to a point-of-care test to detect anti-FXIII-A auto-Abs, and will contribute to early diagnosis and treatment of AH13.

Autoimmune acquired factor XIII deficiency due to anti-factor XIII/13 antibodies: A summary of 93 patients

Autoimmune acquired factor XIII (F13) deficiency or autoimmune hemophilia-like disease (hemorrhaphilia) resulted from the generation of anti-F13 antibodies (AH13) is a severe bleeding disorder that occurs mainly in the elderly. Although rare, the number of patients diagnosed with AH13 has recently increased. To improve understanding of this disease, the author summarized 93 ever reported/diagnosed AH13 cases. About 50% of cases were idiopathic. In the remaining half of the patients, autoimmune diseases and malignancies were the most common underlying diseases. Intramuscular and subcutaneous bleeding were the most frequently reported symptoms. Hemorrhage was the cause of death in 13 patients. In 4 patients, the diagnosis was established after hemorrhagic death. Therefore, physicians/hematologists must raise the awareness of AH13 as a life-threatening disease. Most patients were treated with F13 concentrates to arrest bleeding and with prednisolone and cyclophosphamide to eradicate anti-F13 autoantibodies. AH13 cases tend to become chronic and intractable and require close follow-up over an extended period.

Anti-factor XIII A subunit (FXIII-A) autoantibodies block FXIII-A2 B2 assembly and steal FXIII-A from native FXIII-A2 B2

BACKGROUND

Autoimmune hemophilia-like disease (hemorrha-philia or hemorrhagic disorder) caused by anti-factor XIII antibodies (termed AH13) or 'autoimmune FXIII deficiency' is a life-threatening bleeding disorder. AH13 was thought to be rare worldwide.

OBJECTIVES

Because the number of diagnosed AH13 cases has recently been increasing, at least in Japan, we conducted a nationwide survey supported by the Japanese Ministry of Health, Labor, and Welfare, and explored the pathologic mechanism(s) of AH13.

METHODS

We diagnosed AH13 cases during the last 11 years according to the presence of anti-FXIII autoantibodies confirmed by a dot blot assay and ELISA, and characterized 33 of these both immunologically and biochemically.

RESULTS

The AH13 cases were immunologically classified into three types, Aa, Ab, and B. Type Aa autoantibodies, observed in 27 cases, were directed against the native FXIII A subunit (FXIII-A), and blocked FXIII activation. The autoantibodies not only prevented assembly of new FXIII-A2 B2 heterotetramers, but also removed FXIII-A from native FXIII-A2 B2 heterotetramers by forming an FXIII-A-IgG complex. Type Ab autoantibodies, detected in three cases, preferentially bound to activated FXIII-A and inhibited its activity. Type Aa and Ab autoantibodies were 'neutralizing' FXIII antibodies (or FXIII inhibitors), and thus could be screened with functional assays. Type B antibodies, detected in two cases, were non-neutralizing anti-FXIII B subunit (FXIII-B) autoantibodies that possibly accelerated the clearance of FXIII, and thus could be diagnosed exclusively with immunologic methods.

CONCLUSION

There are three major types of anti-FXIII autoantibody, with distinct targets and mechanisms that cause AH13.

A child with acquired factor XIII deficiency: case report and literature review

Factor XIII (FXIII) deficiency is a rare bleeding disorder, which can result in life threatening hemorrhage. Rarer still is acquired FXIII deficiency, in which the disorder is due to autoantibodies that inhibit the factor. To describe one of the youngest reported patients with this condition. To discuss the challenges we encountered in monitoring response with the available assays. To review the literature and provide a review of all acquired FXIII cases. We present the case of our patient, a 9-year-old girl with acquired FXIII deficiency. We present a comprehensive review of all acquired FXIII deficiency cases reported globally in English, with focus on clinical presentation, diagnostic assays, treatment and prognosis. There is no current standard for therapy and measuring response to therapy can be complicated by limitations of assays in the presence of inhibitors. Clinicians should be aware of acquired FXIII deficiency as a potentially life threatening bleeding disorder even in young children. The case presented illustrates a young patient with acquired FXIII deficiency with a good clinical response to cryoprecipitate and difficulty in hemostasis monitoring utilizing clinically available assays.

Acquired factor XIII deficiency: a therapeutic challenge

Less than 60 cases of acquired factor (F)XIII deficiencies have been reported, most having distinct clinical features. To illustrate the therapeutic challenges of acquired FXIII inhibitors, we report a case of a 65-year-old patient with no previous bleeding history who suddenly developed massive haemorrhages associated to a strong and isolated FXIII inhibitor. No underlying disorder has been detected till now after three years of follow-up. Despite aggressive treatment with prednisone, rituximab, cyclophosphamide, immunoglobulin, immunoadsorption and immune tolerance his inhibitor is still present, although at low titre and with a clinical benefit since the patient has no more bleed since more than one year. Moreover the patient had a venous thromboembolic complication. After a review of the management of acquired FXIII deficiency patients and based on the management of acquired haemophilia we discuss a possible strategy for such difficult cases.

Severe bleeding complications caused by an autoantibody against the B subunit of plasma factor XIII: a novel form of acquired factor XIII deficiency

Acquired factor XIII (FXIII) deficiency due to autoantibody against FXIII is a very rare severe hemorrhagic diathesis. Antibodies directed against the A subunit of FXIII, which interfere with different functions of FXIII, have been described. Here, for the first time, we report an autoantibody against the B subunit of FXIII (FXIII-B) that caused life-threatening bleeding in a patient with systemic lupus erythematosus. FXIII activity, FXIII-A2B2 complex, and individual FXIII subunits were undetectable in the plasma, whereas platelet FXIII activity and antigen were normal. Neither FXIII activation nor its activity was inhibited by the antibody, which bound to structural epitope(s) on both free and complexed FXIII-B. The autoantibody highly accelerated the elimination of FXIII from the circulation. FXIII supplementation combined with immunosuppressive therapy, plasmapheresis, immunoglobulin, and anti-CD20 treatment resulted in the patient's recovery. FXIII levels returned to around 20% at discharge and after gradual increase the levels stabilized above 50%.

Comparison of coagulation factor XIII content and concentration in cryoprecipitate and fresh-frozen plasma

BACKGROUND

For patients with plasma coagulation factor XIII (pFXIII) deficiency, recommended means of replacement include infusions of fresh-frozen plasma (FFP), cryoprecipitate, or (where available) factor (F)XIII concentrates. Quantitative differences in pFXIII concentration in FFP and cryoprecipitate are not well defined and were, therefore, the subject of this study.

STUDY DESIGN AND METHODS

FFP and cryoprecipitate (10 bags each from blood group O donors) were analyzed to quantify pFXIII activity and antigen. Coagulation FVIII, fibrinogen, and von Willebrand factor (VWF) were also quantitated.

RESULTS

Mean (+/-SD) pFXIII activity in cryoprecipitate and FFP bags was 60 +/- 30 and 288 +/- 77 U per bag, respectively, and pFXIII antigen and activity levels were concordant. Other comparisons (mean +/- SD) between cryoprecipitate and FFP, respectively, were as follows: coagulation FVIII activity, 133 +/- 37 and 265 +/- 83 U per bag; fibrinogen content (Clauss kinetic assay), 183 +/- 44 and 725 +/- 199 mg per bag; VWF antigen content, 181 +/- 53 and 218 +/- 70 U per bag; VWF ristocetin cofactor activity, 168 +/- 34 and 221 +/- 65 U per bag; VWF collagen-binding activity, 164 +/- 40 and 208 +/- 71 U per bag; and fluid (plasma) volumes per bag, 21.3 +/- 2.7 and 245 +/- 29 mL.

CONCLUSION

In contrast to other cryoprecipitable coagulation proteins, pFXIII is only mildly enriched in cryoprecipitate when compared with FFP (approx. two- to threefold). Although both products can provide effective pFXIII replacement, FFP may be preferred when infusion volume is not a major consideration and pFXIII concentrates are not available. VWF is substantially enriched in cryoprecipitate (approx. ninefold compared with its concentration in FFP), with VWF activity content exceeding that of FVIII by approximately 26 percent on average.

Acquired coagulation inhibitor-associated bleeding disorders: an update

Acquired blood coagulation inhibitors are circulating immunoglobulins that neutralize the activity of a specific coagulation protein or accelerate its clearance from the plasma, thus causing a bleeding tendency. In this review, we focus on the nonhemophilic inhibitors of coagulation, i.e. the autoantibodies occurring in individuals without a pre-existent coagulation defect, reporting the most recent advances in the pathophysiology, diagnosis and treatment of these rare acquired bleeding disorders.

Severe hypodysfibrinogenemia in compound heterozygotes of the fibrinogen AαIVS4 + 1G>T mutation and an AαGln328 truncation (fibrinogen Keokuk)

Two siblings with hypofibrinogenemia have lifelong trauma-related bleeding. Recently, the brother experienced recurrent thrombosis after cryoprecipitate infusions following surgery. The sister had 6 miscarriages. Plasma clots in each were resistant to compression and fibrinolysis and were soluble in 5 M urea. Examination by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) revealed only the presence of crosslinked γ–γ fibrin chain dimers without high polymers of αn. Fibrin clots contained an abnormal 35-kDa constituent recognized by an antibody to the mature fibrinogen Aα–chain residues 241-476 but not by antibodies to Aα219-348 or Aα349-406. DNA analysis revealed a heterozygous CAA → TAA mutation at the codon for amino acid 328 of the Aα gene in these siblings and 2 asymptomatic family members. The Gln328stop mutation (fibrinogen Keokuk) predicted a 46% truncation and the production of a 35-kDa Aα chain. Analysis of purified fibrinogen revealed expression of the abnormal Aα chain in 4 family members but found no normal fibrinogen in the 2 hypofibrinogenemic patients. This paradox was resolved when they and their asymptomatic mother were found to be heterozygous for a second Aα mutation, a GT → TT splice site mutation in intron 4 (IVS4 + 1 G> T). However, compound heterozygosity for both mutations was required for the expression of severe hypodysfibrinogenemia and for clinical symptoms.

Transglutaminases: crosslinking enzymes with pleiotropic functions

Blood coagulation, skin-barrier formation, hardening of the fertilization envelope, extracellular-matrix assembly and other important biological processes are dependent on the rapid generation of covalent crosslinks between proteins. These reactions--which are catalysed by transglutaminases--endow the resulting supramolecular structure with extra rigidity and resistance against proteolytic degradation. Some transglutaminases function as molecular switches in cytoskeletal scaffolding and modulate protein-protein interactions. Having knowledge of these enzymes is essential for understanding the aetiologies of diverse hereditary diseases of the blood and skin, and various autoimmune, inflammatory and degenerative conditions.

Acquired hemophilia

Acquired hemophilia is a serious coagulopathy usually affecting the elderly, persons with autoimmune disorders and, infrequently, women in the immediate postpartum period. It is due to autoantibodies directed against specific domains of the factor VIII molecule, leading to inhibition of factor VIII binding to von Willebrand factor, to activated factor IX or to negatively charged phospholipids. This results in bleeding into the skin, muscles, gastrointestinal and genitourinary tracts, and other sites. Mixing patient plasma with normal plasma prolongs the activated partial thromboplastin time of the normal plasma and the Bethesda assay provides a quantitative estimate of the strength of the inhibitor. The selection of therapeutic concentrates for the management of acute bleeding is related to the titer of the inhibitor; if less than 5 Bethesda Units, human factor VIII may be effective, but higher titer inhibitors usually respond only to porcine factor VIII, recombinant factor VIIa or activated prothrombin complex concentrates. Corticosteroid treatment leads to disappearance of the autoantibody in 50% of patients; cyclophosphamide and cyclosporine are effective in many who do not respond to steroids. Occasionally, high dose intravenous immunoglobulin or immunosorbent columns transiently decrease inhibitor titers and enable control of bleeding. Other autoantibodies have been described against factors V, VII, XI and, rarely, factor XIII and prothrombin. New approaches in the management of autoimmune disease and, especially, methods to establish tolerance are in development.

Spontaneous inhibitors to coagulation factors

Spontaneous inhibitors to coagulation factors are autoantibodies that usually appear in the elderly, but may also occur in patients with immunological disorders such as lupus, lymphoma, asthma or drug reactions. Most antibodies are directed against factor VIII, but any coagulation protein may be affected. They should be suspected in individuals who previously had normal haemostasis, but who now begin to experience bleeding into the skin and muscles, or suffer haemorrhages after routine procedures such as insertion of vascular catheters, intramuscular injections, or minor surgery. The haemostasis laboratory is critical in identifying the particular inhibitor and quantitating its potency. Factor VIII inhibitors prolong the partial thromboplastin time (PTT) but not the prothrombin time (PT), and incubating mixtures of patient plasma and normal plasma enhances the prolongation of the clotting time. The Bethesda assay provides a rough assessment of inhibitor potency. Inhibitors of von Willebrand factor prolong the bleeding time and impair ristocetin-induced platelet aggregation. Factor V inhibitors are associated with a prolonged PTT and PT, not correctable with normal plasma. Patients will often have a history of exposure to bovine thrombin in fibrin glue. The antibodies most difficult to recognize are those that alter fibrin polymerization or stabilization. Abnormal clot retraction or clot solubility in urea solutions are an important clue. The management of these disorders depends on characterization of the inhibitor, and using appropriate clotting factor concentrates to control acute bleeding. For example, recombinant human factor VIII or desmopressin may be effective for patients with low titre factor VIII inhibitors, whereas porcine factor VIII, recombinant factor Vlla, or prothrombin complex concentrates stem bleeding in those with high titres. Inhibitors of von Willebrand factor may be amenable to desmopressin, cryoprecipitate, or von Willebrand factor concentrates. Some patients with factor V inhibitors have responded to platelet transfusions, as the platelet factor V may be shielded from the autoantibody. Bleeding due to factor XIII inhibitors may be managed with fibrogammin, a factor XIII concentrate. All patients should be treated for underlying disorders and given drugs such as corticosteroids and cytotoxic agents to suppress inhibitor formation. Major advances in new immunosuppressive technologies, such as monoclonal B-cell antibodies, offer hope of more effective therapies for spontaneous inhibitors to coagulation factors.

Influence of a natural and a synthetic inhibitor of factor XIIIa on fibrin clot rheology

We investigated the origins of greater clot rigidity associated with FXIIIa-dependent cross-linking. Fibrin clots were examined in which cross-linking was controlled through the use of two inhibitors: a highly specific active-center-directed synthetic inhibitor of FXIIIa, 1,3-dimethyl-4,5-diphenyl-2[2(oxopropyl)thio]imidazolium trifluoromethylsulfonate, and a patient-derived immunoglobulin directed mainly against the thrombin-activated catalytic A subunits of thrombin-activated FXIII. Cross-linked fibrin chains were identified and quantified by one- and two-dimensional gel electrophoresis and immunostaining with antibodies specific for the alpha- and gamma-chains of fibrin. Gamma-dimers, gamma-multimers, alpha(n)-polymers, and alpha(p)gamma(q)-hybrids were detected. The synthetic inhibitor was highly effective in preventing the production of all cross-linked species. In contrast, the autoimmune antibody of the patient caused primarily an inhibition of alpha-chain cross-linking. Clot rigidities (storage moduli, G') were measured with a cone and plate rheometer and correlated with the distributions of the various cross-linked species found in the clots. Our findings indicate that the FXIIIa-induced dimeric cross-linking of gamma-chains by itself is not sufficient to stiffen the fibrin networks. Instead, the augmentation of clot rigidity was more strongly correlated with the formation of gamma-multimers, alpha(n)-polymers, and alpha(p)gamma(q)-hybrid cross-links. A mechanism is proposed to explain how these cross-linked species may enhance clot rigidity.