Acquired hemophilia A (AHA) is a rare hemorrhagic disease in which autoantibodies against coagulation factor VIII- (FVIII-) neutralizing antibodies (inhibitors) impair the intrinsic coagulation system. As the inhibitors developed in AHA are autoantibodies, the disease may have an autoimmune cause and is often associated with autoimmune disease. Although acute hemorrhage associated with AHA may be fatal and is costly to treat, AHA is often unrecognized or misdiagnosed. AHA should thus be considered in the differential diagnosis particularly in postpartum women and the elderly with bleeding tendency or prolonged activated partial thromboplastin time. Cross-mixing tests and measurement of FVIII-binding antibodies are useful to confirm AHA diagnosis. For treatment of acute hemorrhage, hemostatic therapy with bypassing agents should be provided. Unlike in congenital hemophilia A with inhibitors, in which immune tolerance induction therapy using repetitive infusions of high-dose FVIII concentrates is effective for inhibitor eradication, immune tolerance induction therapy has shown poor efficacy in treating AHA. Immunosuppressive treatment should thus be initiated to eradicate inhibitors as soon as the diagnosis of AHA is confirmed.
During the course of treatment for autoimmune disease, patients with no history of bleeding sometimes suddenly present with severe ecchymoses or muscle hematoma. In such cases, acquired coagulation factor deficiencies, including acquired hemophilia A (AHA), should be considered in the differential diagnosis of the cause of bleeding [
While AHA has a high mortality rate, estimated at up to 33%, it has decreased in tandem with the advancement of therapeutic interventions since the 1980s [
In contrast to the FVIII-neutralizing inhibitors that develop in congenital hemophilia A after FVIII-replacement therapy, which are alloantibodies, the FVIII-neutralizing inhibitors that develop in AHA are autoantibodies. It is well known that approximately 50% of patients with AHA have or have had immune system disorders, such as autoimmune diseases and lymphoproliferative disorders. This fact, as well as knowledge that autoantibodies play a central role in AHA pathogenesis, indicates that modulation of the immune system or the autoimmune mechanism that generates autoantibodies is involved in AHA.
AHA patients often present with severe and massive bleeding, which is responsible for their relatively high mortality rate. The most commonly affected organ is the skin, especially at the site of injection or contusion, which often manifests severe ecchymoses. Subsequently, intramuscular and gastrointestinal/intra-abdominal bleedings are often involved. It is notable that hemarthroses most commonly appear in congenital hemophilia A but seldom occur or cause joint damage in AHA [
FVIII is a cofactor for activated factor IX (FIXa) that forms the Xase (tenase) complex in the presence of Ca2+ and phospholipids and is essential for the intrinsic coagulation system responsible for blood clotting; therefore, FVIII deficiency causes dysfunction of the intrinsic system and reduces thrombin generation, resulting in a bleeding disorder. FVIII is mainly synthesized in the liver as a 2,351 amino acid and 330-kDa single-chain precursor glycoprotein with a functional domain structure (A1-A2-B-A3-C1-C2) (Figure
Structure of the coagulation factor VIII (FVIII) molecule. The numbers indicate amino acid positions. Plasma FVIII is a heterodimer composed of a heavy chain (domains A1, A2, and B) and a light chain (domains A3, C1, and C2). Noncovalent binding of FVIII with von Willebrand factor (VWF) protects circulating FVIII from being inactivated by activated protein C. The binding sites of VWF, phospholipids (PL), and other coagulation factors (activated factor IX [FIXa], factor X [FX], and activated FX [FXa]) are also indicated. FVIII is cleaved and activated by thrombin and FXa at residues 372 and 740 within the heavy chain and at residue 1689 within the light chain. Inhibitors impair FVIII activation by interfering with thrombin-catalyzed cleavage or FVIII interactions with VWF, FIXa, FX, and PL. AR: acidic region.
The majority of FVIII inhibitors observed in AHA, which are polyclonal autoantibodies, and in congenital hemophilia A, which are polyclonal alloantibodies, bind to the A2 (454–509), A3 (1804–1819), or C2 domains (2181–2243) [
Previous studies of CD4 T-cell subsets (Th1, Th2, and Th3) specific for FVIII revealed that alloantibodies in congenital hemophilia A consist of Th1-dependent immunoglobulin (Ig) G1 and IgG2 and Th2-dependent IgG4. However, AHA autoantibodies are often IgG4 autoantibodies and less frequently IgG1 and IgG2 autoantibodies. Further, FVIII-neutralizing activity is correlated with the presence of IgG4 autoantibodies [
Most alloantibodies developed in congenital hemophilia A patients undergoing FVIII replacement therapy, which are classified as type I inhibitors of first-order kinetics, inactivate FVIII at a rate linearly correlated with their concentration and are able to completely inhibit FVIII activity at high concentrations. In contrast to the kinetics of the interaction between FVIII and the inhibitors in congenital hemophilia A, the kinetics of the interaction in AHA display a nonlinear inhibitory profile. Specifically, these type II inhibitors show a rapid initial inactivation phase followed by a slower equilibrium phase during which some residual FVIII activity (FVIII:C) is detectable even after incubation at maximum concentrations of inhibitors for a sufficient period (Figure
Kinetics of type I and type II inhibitors.
In a study of the physiological activities of AHA inhibitors, Lacroix-Desmazes et al. identified a subset of inhibitors in congenital hemophilia A that hydrolyze FVIII, resulting in FVIII inactivation [
In approximately 50% of AHA patients, especially elderly patients, autoantibody development against factor VIII is idiopathic [
Conditions associated with acquired hemophilia A.
Idiopathic | Malignancy |
Autoimmune diseases | Squamous cell cancer |
Rheumatoid arthritis | Lymphoproliferative diseases |
Systemic lupus erythematosus | Chronic lymphocytic leukemia |
Myasthenia gravis | non-Hodgkin lymphoma |
Multiple sclerosis | Multiple myeloma |
Thyroid dysfunction | Medical agents |
Autoimmune hemolytic anemia | Antibiotics |
Inflammatory bowel diseases | Penicillins |
Pemphigus | Sulfonamides |
Graft versus host disease | Chloramphenicol |
Pregnancy | Anticonvulsants (phenytoin) |
Antihypertensive (methyldopa) | |
Bacillus Calmette-Guérin vaccination |
AHA is often associated with autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, myasthenia gravis, multiple sclerosis, thyroid dysfunction, and autoimmune hemolytic anemia. Observation of an association between AHA and inflammatory bowel disease, pemphigus, and graft versus host disease (GVHD) has been also reported, indicating that AHA has an autoimmune origin. In fact, up to 20% of all AHA patients present with autoimmune disorders [
AHA is associated with pregnancy in approximately 10% of cases [
Underlying malignancy in either solid or nonsolid form presents in approximately 10% of AHA patients and commonly develops in elderly patients. An important consideration is that, as the incidence of both solid tumor as well as AHA increases with aging, inhibitors might be detected coincidentally in patients with solid tumor. Patients with lymphoproliferative diseases complicating AHA, which include chronic lymphocytic leukemia, non-Hodgkin lymphoma, and multiple myeloma [
Reactions associated with drug hypersensitivity have been implicated in the onset of AHA. Suspected medications include antibiotics (penicillin, sulfonamides, and chloramphenicol), anticonvulsants (phenytoin), antihypertensive agents (methyldopa), and bacillus Calmette-Guérin vaccination [
Anti-FVIII autoantibodies are developed in the context of dysfunction of immune system, as discussed above. Knowledge of the detailed molecular biological mechanism of inhibitor generation has accumulated gradually over the past decades.
Variants of the polymorphic cytotoxic T lymphocyte antigen-4 (CTLA-4) gene, which is found on the surface of activated and regulatory T-cells, have been associated with autoimmune diseases [
Recently, B-cell activating factor belonging to the tumor necrosis factor family (BAFF), also referred to as BlyS, has been found to regulate the immune system. Known to be involved in the survival and maturation of B-cells [
In a previous study, we found BAFF levels to be significantly higher in congenital hemophilia A patients with inhibitors compared to healthy controls or hemophilia A patients without inhibitors [
The first step in diagnosis of AHA is tracking signs of bleeding tendency, particularly in the elderly, in the clinical setting and testing for prolongation of activated partial thromboplastin time (APTT) in the laboratory. The next step is review of patient medical history by consideration of the impact of any underlying conditions associated with AHA. APTT prolongation reflects decreased levels of coagulation intrinsic factors VIII and IX, as well as decreased levels of factors XI and XII, prekallikrein, and high molecular weight kininogen, which are involved in the contact system of coagulation. However, since reduction of proteins involved in the contact system is not associated with bleeding tendencies [
To diagnose AHA, measurement of FVIII:C is essential, and consecutive determination of inhibitor titer is a requisite in cases of decreased level of FVIII:C. While APTT is prolonged in patients with low levels of FVIII:C by anti-FVIII neutralizing autoantibodies, PT, fibrinogen and VWF levels, and platelet count are within normal limits and platelet function is normal. Since thrombocytopenia, PT and APTT prolongation, and decreased levels of fibrinogen are often observed in DIC patients who are erroneously diagnosed with AHA, consideration of these findings is helpful in differentiation of DIC from AHA.
Several cross-mixing studies have been performed to examine whether APTT prolongation results from a deficiency of intrinsic factor(s) or inhibitor. In one such study, addition of an equal volume of normal control plasma to patient’s plasma was found to correct the APTT value to the normal range in coagulation-factor-deficient patients but not AHA patients [
Cross-mixing test for detection of lupus anticoagulants or inhibitor of coagulation factor. A convex upward curve indicates the presence of inhibitors, including lupus anticoagulants and coagulation factor-neutralizing antibodies, while a convex downward curve indicates the presence of a factor deficiency.
APTT is prolonged in the presence of lupus anticoagulants that interfere with the assembly and activity of the FXa-FVa-Ca2+ phospholipid complex. Lupus anticoagulants are polyclonal immunoglobulins that bind to phospholipids and proteins associated with the cell membrane and show nonspecific inhibitory effects that result in prolongation of both APTT and PT. From the perspective of laboratory testing, since intrinsic coagulation factor activity, including that of FVIII, appears to decrease in the presence of lupus anticoagulants, it is often difficult to distinguish AHA from lupus anticoagulants even by a mixing test. If the results of a mixing test indicate the presence of an inhibitor, the lupus anticoagulant is therefore evaluated by phospholipid-sensitive functional-coagulation assay, such as the dilute Russell’s viper-venom time assay [
When the presence of an inhibitor is suspected, the targeted factor should be identified and the extent of inhibitory activity quantified. For the quantification of FVIII inhibitors, the Bethesda assay is the most commonly used laboratory test worldwide [
Although these assays are useful for determination of titers of alloantibodies against FVIII in congenital hemophilia A patients with type I kinetics, exact determination of autoantibody titer in AHA is difficult in patients with type II kinetics, in whom the acquired inhibitor-FVIII complex may show some residual FVIII:C, even in the presence of high concentrations of inhibitors. Therefore, measurement of levels of FVIII-binding antibodies is necessary for performing meaningful clinical assessment of the inhibitors present in AHA [
Flow cytometric analysis of factor VIII- (FVIII-) binding antibodies. Plasma samples from 20 normal healthy volunteers (normal pooled plasma), 3 acquired hemophilia A patients, 4 congenital hemophilia A patients without inhibitors, and 10 congenital hemophilia A patients with inhibitors were assessed using the following procedure. Human recombinant FVIII (rFVIII) was bound to red fluorescent carboxylated polystyrene microbeads (Cyto-Plex polystyrene microbeads) and a certain number of human rFVIII-bound microbeads were added to serially diluted suspected plasma. After incubation and washing, PE-labeled anti-human IgG antibody was added to the microbeads. After additional incubation and washing, fluorescent intensity was measured using a FACScan flow cytometer. The fluorescence intensity of the anti-human IgG antibody bound to human rFVIII on the microbead surface was expressed as the geometric mean (shown in arbitrary units). The dotted line shows a tentative cutoff value for the inhibitor with the highest geometric mean value in plasma without inhibitor. NP: normal plasma.
Favorable outcome in AHA depends on selection of an appropriate therapeutic approach based on early, correct diagnosis. The therapeutic strategy should aim for the achievement of 2 targets: control of bleeding and eradication of inhibitors.
Bleeding episodes in AHA are often severe and life threatening and presents with severe anemia. As massive subcutaneous or intramuscular hemorrhage may continuously worsen if left untreated, provision of immediate hemostatic therapy and monitoring of its efficacy by observation of improvement in anemia and clinical manifestations is required. The first-line treatment for severe bleeding episodes, especially in patients with high titers of inhibitors, is administration of bypassing agents [
Another hemostatic treatment, the provision of inhibitor-neutralizing therapy with administration of FVIII concentrates at a level sufficient for neutralizing inhibitors, may also be beneficial for these patients. However, it is difficult to determine the quantity of FVIII required and calculate the half-life of the infused FVIII owing to the presence of type II inhibitors in AHA. In contrast, the requisite quantity of FVIII for neutralizing type I inhibitor in congenital hemophilia A can be determined theoretically. Therefore, frequent monitoring of hemostatic functioning accompanied by measurement of FVIII:C and/or APTT should be performed while providing neutralizing therapy to AHA patients. Administration of desmopressin, which stimulates the release of FVIII and VWF from endothelial cells and can provide a transient rise in FVIII:C levels to therapeutic levels [
As with congenital hemophilia A with inhibitors, suppression and eradication of inhibitors are essential for normalization of hemostatic function and elimination of the risk of hemorrhage in AHA. For this, provision of immunosuppressive therapy is critical. In some cases of postpartum and drug-induced acquired hemophilia that resolves spontaneously, immunosuppressive therapy may be unnecessary [
Agents used in immunosuppressive therapy for suppression of inhibitors include immunosuppressive agents such as prednisone, azathioprine, and cyclosporine and antineoplastic agents such as cyclophosphamide (CPA), mercaptopurine, and vincristine. Among these, administration of prednisone alone or in combination with CPA has been a common strategy. Combined prednisone-CPA administration has been reported to yield favorable outcomes [
There is no evidence that one immunosuppressive therapy is clinically superior to all others in treating AHA or that a certain therapy should be chosen depending on inhibitor titer or the hemorrhagic status. Therefore, first-line treatment is determined by evaluation of disease condition and consideration of possible adverse effects [
AHA is characterized by the presence of an autoimmune mechanism that alone or accompanied by autoimmune disease, aging, pregnancy, or drug exposure causes breakdown of immune tolerance to FVIII associated with CD4 T-cells and results in development of autoantibodies against FVIII. In addition to treatment for acute bleeding, which is often required for AHA patients, immune suppression is essential for eradication of the inhibitors that play a central role in AHA pathogenesis. While provision of immunosuppression therapies, such as combined prednisone-CPA therapy, is currently the first-line treatment, administration of anti-CD20 monoclonal antibody (rituximab) appears to be a promising alternative treatment for AHA. Consideration of the findings regarding the association between the autoimmune mechanism responsible for AHA development and the innate immune system presented here and further elucidation of this association in future research will provide for better understanding of AHA pathophysiology and the development of novel therapies for eradication of inhibitors.
The authors declare that there is no conflict of interests regarding the publication of this paper.