Chapter 127: Antibody-Mediated Coagulation Factor Deficiencies

Antibodies directed against coagulation factors can develop as an acquired, autoimmune phenomenon. These “circulating anticoagulants” or “inhibitors” were recognized as early as 1906 as a cause of an acquired bleeding disorder.¹ The most common coagulation factor targeted in autoimmunity is factor VIII. The key feature that distinguishes antibody-mediated from other acquired coagulation factor deficiencies, such as impaired synthesis (e.g., a result of vitamin K deficiency) or increased consumption (e.g., in disseminated intravascular coagulation), is the ability of the patient’s plasma to inhibit the coagulation of normal plasma. Inhibitors also can develop in response to replacement therapy in patients with congenital coagulation factor deficiencies as discussed in Chap. 123.

DEFINITIONS AND EPIDEMIOLOGY

The incidence of autoantibodies to factor VIII, which is the most commonly targeted coagulation factor in autoimmunity, is 1.4 per million people per year.^2,3,4 The associated clinical condition is called acquired hemophilia A. Approximately 40 to 50 percent of acquired hemophilia A patients have underlying conditions, including other autoimmune disorders (e.g., rheumatoid arthritis and systemic lupus erythematosus), malignancy, pregnancy, or a history consistent with a drug reaction.⁵ The remaining idiopathic cases most commonly occur in elderly patients of either sex with the median age at diagnosis being in the mid-70s.

MECHANISMS OF ANTIBODY DEVELOPMENT

Even though adaptive immunity provides a unique ability to recognize a nearly infinite range of antigenic determinants, mechanisms of immunologic tolerance exist that reduce the probability of autoimmunity.⁶ Self non-self discrimination provides the key foundation upon which immune activity can be specifically directed toward potential pathogens.⁶ However, self non-self discrimination alone does not possess the capacity to distinguish innocuous antigens from antigens associated with a real threat of infection.⁷ As a result, an elaborate network of innate immune factors also exist that recognize potential danger in the form of cellular injury or conserved determinants on pathogens themselves, often referred to as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs), respectively.^7,8 Activation of immune cell function following exposure to PAMPs or DAMPs provide the necessary signals required for an efficient immunologic response to foreign antigen.^7,8,9

The development of anti–factor VIII antibodies following factor VIII infusion in individuals with hemophilia A provides a classic example of the deleterious outcome of alloantibody formation following exposure to alloantigen. In this scenario, the factor VIII protein is foreign to the patients; consequently, central tolerance to the factor VIII protein does not occur. In contrast, acquired hemophilia results from loss of previous tolerance to a self antigen.^10,11,12

For alloantibody development in patients with hemophilia A, individual variability in factor VIII levels accounts for some of the divergent level of tolerance to factor VIII observed. However, some individuals with undetectable levels of factor VIII antigen fail to generate factor VIII inhibitors, regardless of factor VIII exposure. Although these individuals would not be predicted to be tolerized to factor VIII, 70 to 80 percent of patients with baseline factor VIII levels of less than 1 percent do not develop an immune response to repeated dosing and are considered tolerized.^13,14,15,16 For the 20 to 30 percent of patients who develop inhibitors there are both genetic and nongenetic risk factors for inhibitor development. Patients with a positive family history of inhibitors, those who have large factor VIII gene deletions, and nonwhites have a higher risk of inhibitor development.^17,18,19,20 The non–factor VIII genes—interleukin-10, tumor necrosis factor-α, and cytotoxic T-lymphocyte antigen 4–318 allele—are associated with inhibitor development.^17,18,19,20 Nongenetic risk factors, such as infusing factor at the time of a “danger” signal (e.g., a surgical procedure), intense factor exposure, and prophylaxis versus no prophylaxis, also are associated with inhibitor development.¹⁶ Patients who receive factor at the time of a “danger” signal may experience sufficient tissue injury to provide the necessary immune activation through DAMPs. Furthermore, it remains possible that low grade and potentially clinically undetectable infection may provide low levels of PAMPs that could likewise stimulate anti–factor VIII antibodies following factor VIII exposure. However, while PAMPs and/or DAMPs may provide the important immune activation signals,^21,22 several studies using animal models suggest that significant factor VIII antibody development can occur in the absence of known tissue injury or DAMP exposure.²³ Consistent with this, immune activation can occur in the apparent absence of DAMPs or PAMPs toward several model antigens.²⁴ Unique B-cell populations, especially those in the spleen, can rapidly respond to bloodborne antigens in the absence of any identifiable PAMPs or tissue injury, suggesting that these cells may be uniquely poised to respond to factor VIII.²⁵ Consistent with this, in experimental models, splenectomy can significantly inhibit factor VIII inhibitor development following factor VIII exposure,^26,27 suggesting that several of these unique B-cell populations may be involved in the development of factor VIII antibodies irrespective of DAMP or PAMP exposure.^25,26

Although examples of antigens inducing B-cell activation in the absence of known DAMPs or PAMPs exist, most of these antigens require crosslinking of cell-surface B-cell receptors for efficient activation and therefore reflect highly repetitive antigenic structures.²⁸ In contrast, factor VIII represents a soluble antigen with little inherent predicted crosslinking ability. Most soluble antigen of this type actually induce tolerance following injection, likely because of the inability of soluble monovalent antigens to adequately crosslink and thereby stimulate B-cell receptors. Although factor VIII can exist in a soluble, monovalent form, it remains possible that factor VIII may form complexes with higher-molecular-weight species and thus form a network of factor VIII antigens that may serve as a suitable substrate for efficient B-cell receptor crosslinking and subsequent activation. Consistent with this, induction of tolerance to factor VIII by exposure to high levels of factor VIII may partially reflect a saturation of sites for factor VIII complex formation,²⁹ which may, in turn, result in B-cell exposure to high levels of soluble, monovalent factor VIII. However, if this occurs, studies suggest that it likely takes place independent of interactions with von Willebrand factor, the primary binding partner of factor VIII, or its own coagulant activity.³⁰ Clearly, there is much more to learn regarding the immunologic factors responsible for factor VIII inhibitor development.

In contrast to generating alloantibodies following factor VIII infusion, some patients generate autoantibodies against factor VIII, which can result in acquired factor VIII deficiency. As coagulation typically occurs at sites of inflammation and injury where DAMPs presumably are generated, tolerance to factor VIII may unfortunately be lost in these settings. Additionally, nonproteolytic and proteolytic degradation of coagulation proteins potentially could present neoepitopes. However, the fact that the development of acquired factor VIII deficiency is rare (1.4 per million population) provides a testimony to the ability of the immune system to discriminate efficiently between infectious non-self from noninfectious self.¹² Essentially, nothing is known about the breakdown of tolerance in patients that develop autoantibodies to coagulation factors.

MOLECULAR PATHOLOGY

Factor VIII inhibitors in congenital and acquired hemophilia nearly always consist of a polyclonal immunoglobulin (Ig) G population. Although IgG₄ accounts for only 5 percent of the total IgG in normal plasma, it usually is a major, but not the sole, component of the anti–factor VIII antibody population.³¹ IgG₄ antibodies do not fix complement, which has been cited as a reason that immune complex disease is not observed in factor VIII inhibitor patients. However, it is more likely that factor VIII simply is not present in sufficient quantity to form enough immune complex deposition to mediate tissue damage.

Factor VIII contains a sequence of domains designated A1-A2-B-ap-A3-C1-C2 (Chap. 123). During the activation of factor VIII by thrombin, the B and ap domains are released, producing an A1/A2/A3-C1-C2 activated factor VIII heterotrimer.³² Anti–factor VIII antibodies in both congenital and acquired hemophilia A inhibitor are primarily directed to the A2 and C2 domains, although antibodies to all domains have been described.^33,34,35 The similarity in the properties of antibodies in congenital and acquired hemophilia, which represent very different immunologic settings, suggests that intrinsic structural features in the factor VIII molecule are an important determinant driving the immune response. Epitope spreading from a single “problem” epitope, which has been implicated in some autoantibody phenomena,³⁶ does not appear to be a property of factor VIII inhibitors because anti–C2 antibodies can occur in the absence of anti–A2 antibodies and vice versa.

The only known biologic function of factor VIII is to become proteolytically activated and participate as a cofactor for factor IXa during intrinsic pathway factor X activation on phospholipid membranes. Theoretically, antibodies could inhibit factor VIII procoagulant function in several ways, including blocking the binding of factor VIIIa to factor IXa, factor X, or phospholipid, or by interfering with the proteolytic activation of factor VIII. Some anti-A2 antibodies map to a region bounded by Arg484-Ile508³⁷ and inhibit activated factor VIII by blocking its ability to bind factor X.³⁸ Anti-C2 antibodies bind to the NH₂-terminal half of the C2 domain.³⁹ Anti-C2 antibodies have been identified that inhibit the binding of activated factor VIII to phospholipid membranes,⁴⁰ which is critical for its interaction with the platelet surfaces. However, the C2 domain also apparently contributes to the binding of factor VIII to its activators, thrombin and factor Xa.^41,42,43 Consistent with this, anti–C2 inhibitors have been identified that block factor VIII activation.^41,44

Factor VIII inhibitors also have been identified in approximately 20 percent of normal healthy donors.⁴⁵ These inhibitors inhibit factor VIII activity in pooled normal plasma, but not autologous plasma, indicating that they are not autoantibodies, but rather alloantibodies directed against an unidentified polymorphism. Anti–factor VIII IgG also has been identified in all normal plasmas tested by affinity chromatography on immobilized factor VIII.⁴⁶ The increased sensitivity of the method is a consequence of its ability to resolve anti–factor VIII antibodies from anti–anti–factor VIII idiotypic antibodies that also are present. Idiotypic regulation has been proposed as a mechanism for controlling autoantibody activity in vivo.⁴⁷

CLINICAL FEATURES

Acquired hemophilia A patients usually present with spontaneous bleeding, which often is severe and life- or limb-threatening, although large cohort studies have shown that approximately 30 percent of patients do not require hemostatic management.^2,48 Patients with acquired hemophilia are more likely to have a severe bleeding diathesis than congenital hemophilia A inhibitor patients.⁴⁹ Additionally, in contrast to patients with congenital hemophilia A, hemarthrosis in these patients is rare. The reasons for these differences is puzzling, especially in light of the fact that the properties of factor VIII inhibitors in the two patient populations is similar. As noted above, inhibitors can block factor VIII function in several ways. Conceivably, unidentified mechanistic differences in inhibitor action account for the difference in clinical severity. Factor VIII inhibitors sometimes resolve spontaneously. However, it is not possible to predict in which subset of patients this will occur.

LABORATORY FEATURES AND DIFFERENTIAL DIAGNOSIS

The new onset of an acquired bleeding disorder should immediately lead to screening tests that include an activated partial thromboplastin time (aPTT), a prothrombin time, and a platelet count. Patients with acquired hemophilia A have a prolonged aPTT resulting from decreased or absent factor VIII activity in the intrinsic pathway of blood coagulation. The autoantibody inhibits the factor VIII in the plasma from normal individuals, which forms the basis of mixing study that is used to screen for inhibitors. The presence of a prolonged aPTT in a mixing study establishes the diagnosis of a circulating anticoagulant. Specific factor assays then are performed to determine whether a specific coagulation factor inhibitor or a lupus anticoagulant is present. The activity of other intrinsic pathway coagulation factors may be decreased in the presence of high titer factor VIII inhibitors. However, the levels of these factors normalize at increasing dilutions of patient plasma, whereas factor VIII activity remains decreased.

Once the identity of an inhibitor has been established, its titer is determined using the Bethesda assay.⁵⁰ Inhibitors frequently take minutes to hours to maximally inhibit factor VIII. Therefore, dilutions of patient plasma are preincubated with normal plasma for 2 hours at 37°C. The inhibitor titer is defined as the dilution of patient plasma that produces 50 percent inhibition of the factor VIII activity and is expressed in Bethesda units per milliliter (BU/mL). Inhibitors are classified informally as low titer or high titer when the titers are less than 5 BU/mL or greater than 5 to 10 BU/mL, respectively. The Bethesda assay has been modified by the addition of 0.1 M imidazole, pH 7.4, and by diluting test plasma into factor VIII–deficient plasma during the preincubation phase to prevent assay variation resulting from pH changes and adsorptive losses of factor VIII.⁵¹ This “Nijmegen” modification of the Bethesda assay decreases false-positive low-titer inhibitors.⁵² Patients with acquired hemophilia often have measurable residual factor VIII activity. This activity may cause an underestimate of the inhibitory titer. Preanalytical heat treatment has been proposed as a simple way to denature factor VIII to allow for more accurate determination of titer in both patients with acquired hemophilia A and patients with congenital hemophilia A who may have infused factor VIII.^53,54

Factor VIII inhibitors are classified based on the kinetics and extent of inactivation of factor VIII in plasma.⁵⁵ Type I inhibitors follow second-order kinetics and inactivate factor VIII completely, which would be expected for a simple bimolecular antigen-antibody reaction. Type II inhibitors inactivate factor VIII incompletely and display more complex kinetics of inhibition. Hemophilia A inhibitor patients and acquired hemophilia A patients tend to have type I and type II inhibitors, respectively.⁵⁶ However, the borderline between type I and type II inhibitors is not always clear and the distinction is not useful clinically. Additionally in a recent observational study of patients with acquired hemophilia in the United Kingdom, factor VIII levels and inhibitor titers at presentation were not predictive of the severity of bleeding events. The median factor VIII level and inhibitory titers were nearly identical for patients with fatal bleeding events compared to those who did not require treatment for their bleeding symptoms.²

TREATMENT

The severe bleeding that often is the presenting feature of this disorder requires urgent action to establish a diagnosis and initiate therapeutic measures. Ideally, this is carried out in a setting where factor VIII inhibitors can be identified and quantitated and where there is subspecialty expertise in the management of bleeding disorders. Invasive procedures should be performed only if absolutely necessary and venipuncture should be kept to a minimum given the risk of significant bleeding.⁵⁷

Treatment of patients with acquired hemophilia A depends on the inhibitor titer. Although no prospective trials are available, clinical experience indicates that patients with a factor VIII inhibitor titer of less than 5 BU/mL often are treated successfully with sufficient doses of recombinant or plasma-derived factor VIII to neutralize the inhibitor. Patients with titers between 5 and 10 BU/mL also may respond to factor VIII, whereas those with titers greater than 10 BU/mL generally do not respond. Formulas exist to calculate the amount of factor VIII needed to treat a patient, but these are rough estimates at best. The efficacy of factor VIII concentrates was lower than that of bypassing agents in a large registry study, which was likely secondary to challenges in appropriately dosing the factor VIII concentrate.⁴⁸

Desmopressin can be administered by intravenous, subcutaneous, or intranasal routes and results in an increase in plasma von Willebrand factor levels and factor VIII activity.⁵⁸ Its potential use is in patients with baseline factor VIII levels greater than 5 IU/dL and minor bleeding. However, like factor VIII concentrates, response is not predictable and close monitoring of hemostatic efficacy and factor VIII levels is needed.

Factor VIII bypassing agents, which drive the coagulation mechanism through the extrinsic pathway, are the mainstays of management of patients with a high titer of an inhibitor. Two agents, recombinant activated factor VII (rFVIIa; NovoSeven RT) and plasma-derived antiinhibitor coagulant complex (AICC; FEIBA VH Immuno, also called activated prothrombin complex concentrate [aPCC]) are commercially available and approved by the U.S. Food and Drug Administration for treatment of acquired hemophilia A. Although no comparative trials have been done, analysis of the European Acquired Haemophilia (EACH2) Registry showed similar hemostatic efficacy between rFVIIa and aPCC at approximately 90 percent.⁴⁸ Similar hemostatic efficacy between rFVIIa and aPCC has been seen in the treatment of congenital hemophilia A with inhibitors.⁵⁹ The recommended dose range of rFVIIa for the treatment of patients with acquired hemophilia is 70 to 90 mcg/kg repeated every 2 to 3 hours until hemostasis is achieved. aPCC is given at doses of 50 to 100 U/kg every 8 to 12 hours, but should not exceed 200 U/kg per day. Lower doses (50 to 75 U/kg) are used for mild bleeding, whereas higher doses (100 U/kg) are given for severe limb or life-threatening bleeding. Treatment should be continued until there are clear signs of clinical improvement.

Although there are similar rates of efficacy between the two available bypassing agents, not all patients respond. Additionally, there are no widely accepted methods available for predicting response to therapy or monitoring patients on therapy. The use of thromboelastography and the thrombin generation assays as a predictor of response to therapy in congenital hemophilia A and inhibitors has been reported but large clinical studies linking clinical data to outcome are lacking, leaving clinical response as the only available monitoring option.⁶⁰

The major serious adverse event associated with bypassing agents is thrombosis. The EACH2 Registry reported similar rates in patients treated with rFVIIa or aPCC.⁴⁸ However, the risk of thrombosis is considered low when used for approved indications at the recommended doses. The incidence of thrombosis in patients with acquired hemophilia A treated with bypassing agents appears higher than that for patients with congenital hemophilia. This is probably because of cardiovascular risk factors in the acquired hemophilia population given their age and associated medical conditions. Escalating doses of either bypassing agent or combination of the two agents should be done with caution, especially in older patients.

Factor VIII inhibitors usually cross-react poorly with porcine factor VIII.⁶¹ A commercial plasma-derived porcine factor VIII concentrate was useful in the treatment of factor VIII inhibitor patients for approximately 20 years,⁶² but was discontinued in 2004 because of viral contamination of the product. Porcine factor VIII has the advantage of potentially being guided by laboratory monitoring of recovery of factor VIII activity in plasma. However, the development of antiporcine factor VIII antibodies may preclude its long-term use. A phase II/III clinical trial of a recombinant porcine factor VIII product has been completed in patients with acquired hemophilia A⁶³ and a phase II trial has been completed in congenital hemophilia inhibitor patients.⁶⁴

Although acquired inhibitors may remit spontaneously, fatal bleeding may occur up to several months after the initial diagnosis, even in patients who present with mild bleeding. Therefore, immunosuppressive therapy at the time of diagnosis to eradicate the inhibitor is recommended.⁵⁷ A variety of immunosuppressive agents have been used, including cyclophosphamide, azathioprine, cyclosporine, intravenous immunoglobulin, and rituximab. Immune tolerance induction using human factor VIII similar to what is done for patients with congenital hemophilia A and inhibitors has been used successfully. Additionally, plasmapheresis and immunoadsorption of the inhibitory antibody have been used.

First-line immunosuppressive regimens at many centers consist of glucocorticoids alone or glucocorticoids combined with cyclophosphamide.⁶⁵ No appropriately powered randomized studies have been performed, so the information available is from a single small randomized study, case reports, national surveys, and large registry data. The single randomized trial of 31 patients comparing prednisone and cyclophosphamide showed no difference in the treatment arms. A national registry study also showed no difference with 76 percent of patients achieving complete remission in the steroid arm and 78 percent in the steroids plus cytotoxic agent arm.⁶⁶ The EACH2 Registry has the largest reported experience with 331 patients and reported a higher rate of stable complete remission at 70 percent for patients treated with steroids and cyclophosphamide compared with 48 percent for steroids alone and 59 percent for rituximab containing regimens. Extensive analysis to control for potential confounding factors in this non-randomized study confirmed that stable complete remission was more likely with a steroid and cyclophosphamide than steroids alone (odds ratio of 3.25). The median time to remission was 5 weeks in patients treated with steroids alone or steroids and cyclophosphamide and 10 weeks in patients treated with rituximab. There have been no studies that have shown a difference in long-term outcomes including survival and sustained remission.^57,67

The rarity of this disease, the severity of bleeding at onset, and the delay in diagnosis of these patients has all contributed to the lack of controlled trials. Given the lack of controlled trial clinical management decisions are guided from the limited data available and clinical judgment.

ANTI–FACTOR V AND ANTITHROMBIN ANTIBODIES

Thrombin and factor V inhibitors are discussed together because of their frequent coexistence in immune responses to commercial products that contain thrombin. Thrombin products have been used widely in surgical and endoscopic procedures. It has been estimated that more than 500,000 patients are treated annually with products containing thrombin.⁶⁸ Thrombin is used either alone or as a component of fibrin sealants, which consist of fibrinogen and thrombin preparations that are mixed together at the wound site to form a topical fibrin clot.⁶⁹ Additionally, factor XIII sometimes is added to crosslink and stabilize the clot.

Fibrin sealants contain thrombin and fibrinogen derived from human plasma, whereas stand-alone thrombin products are prepared from bovine plasma. Both types of products are heavily contaminated with other plasma proteins, including factor V and prothrombin.^70,71 Almost all patients exposed to bovine proteins develop a detectable immune response. In half of these patients antibovine antibodies cross-react with human thrombin, factor V, or prothrombin.⁶⁸ Usually, these antibodies are subclinical.⁷² However, mild to life-threatening hemorrhage can occur, especially if the titer of anti–human factor V antibodies is high. The risk of bleeding is higher in patients who receive bovine thrombin products more than once because of the development of a secondary immune response.

There have been no clinical trials comparing the safety and efficacy of fibrin sealants to stand-alone thrombin products. Because fibrin sealants are composed mainly of human proteins, they may be less immunogenic. However, anti–factor V antibodies have been reported in a patient receiving fibrin sealant.⁷³ There currently is no stand-alone human thrombin product. It seems likely that the development of highly purified plasma-derived or recombinant products containing human thrombin in the presence or absence of human fibrinogen would decrease the incidence of antithrombin and anti–factor V antibodies.⁷⁰

Autoantibodies to thrombin are rare. However, the mechanisms of action of antithrombin antibodies have been studied extensively because of the wealth of information about thrombin structure and function.^74,75,76,77 In contrast, approximately half of the 105 cases of inhibitory anti–factor V antibodies reported and reviewed between 1955 and 1997 appeared to be autoantibodies not associated with the exposure to bovine thrombin products.⁷² β-Lactam antibiotics also are associated with anti–factor V autoantibodies and may partly explain the increased incidence with surgery. In approximately 20 percent of cases of autoantibody formation, no underlying disease was identified. Anti–factor V autoantibodies have been identified rarely in patients with autoimmune diseases, solid tumors, and monoclonal gammopathies. In addition to autoantibody formation, alloantibodies to factor V have developed in patients with severe factor V deficiency in response to replacement therapy with fresh-frozen plasma.

Patients with inhibitory antibodies to factor V have prolonged prothrombin and aPTT, low factor V levels, and a normal thrombin time. The diagnosis of a factor V inhibitor is based on the specific loss of factor V coagulant activity when patient and normal plasma are mixed in a coagulation assay. The antibody titer can be defined as in the factor VIII Bethesda assay as the dilution of test plasma that produces 50 percent inhibition of factor V activity.

Not all patients with factor V inhibitors have hemorrhagic manifestations. Factor V inhibitors anecdotally produce a less serious bleeding disorder than factor VIII inhibitors. The relationship between inhibitor titer and bleeding has not been studied. The reported incidence of bleeding has been higher in patients with autoantibodies to factor V compared to anti–factor V antibodies in patients receiving bovine thrombin. However, this may reflect a bias resulting from the reason the patient sought medical attention.

Factor V contains an A1-A2-B-A3-C1-C2 domain structure that is homologous to factor VIII. Also, like factor VIII, the N-terminal half of the factor V C2 domain contains a phospholipid-binding site⁷⁸ that is necessary for normal procoagulant function⁷⁹ and is targeted by factor V inhibitors.^80,81

ANTIPROTHROMBIN ANTIBODIES

Antiprothrombin antibodies are most commonly associated with the antiphospholipid syndrome. The antiphospholipid syndrome is caused by lupus anticoagulants, which are defined as antibodies that produce phospholipid-dependent prolongation of in vitro coagulation assays. Anionic phospholipids participate as cofactors for the lupus anticoagulant binding to protein antigens, primarily β₂-glycoprotein I⁸² and prothrombin.⁸³ The antibody–antigen complexes compete for the binding of coagulation factors to the phospholipid present in coagulation assays and produce the lupus anticoagulant phenomenon.

The role of prothrombin in the generation of lupus anticoagulant activity initially was suggested from studies of a bleeding patient with severe hypoprothrombinemia. However, in the absence of hypoprothrombinemia, lupus anticoagulants do not produce a bleeding diathesis and bleeding in patients with lupus anticoagulants is uncommon.⁸⁴ Antiprothrombin antibodies are associated with an increased incidence of thrombosis in these patients.⁸⁵ In patients with antiprothrombin antibodies and hypoprothrombinemia, precipitating, noninhibitory antibodies are present and prothrombin antigen levels are low, indicating that the hypoprothrombinemia is the result of rapid clearance of antigen–antibody complexes.⁸⁶ However, most patients with lupus anticoagulants have demonstrable antiprothrombin antibodies but do not have hypoprothrombinemia.⁸⁷ Thus, antibody-mediated hypoprothrombinemia appears to represent a relatively uncommon evolution of the autoimmune response to prothrombin in patients with lupus anticoagulants.

ANTIBODIES TO COMPONENTS OF THE PROTEIN C SYSTEM

An acquired inhibitor to protein C associated with a fatal thrombotic disorder has been reported,⁸⁸ but evidently is rare. In contrast, there is a relatively high prevalence of pathogenic anti-protein S antibodies. Inhibitory antibodies to protein S were detected in five of 15 patients with acquired protein S deficiency.⁸⁹ Anti–protein S antibodies, but not antibodies to cardiolipin, β₂-glycoprotein I, prothrombin, or protein C, appear to be a risk factor for acquired activated protein C (APC) resistance, defined as APC resistance in the absence of the factor V Leiden mutation, and for deep venous thrombosis.⁹⁰ Additionally, antibodies to the endothelial cell protein receptor have been identified that are associated with fetal death in patients with the antiphospholipid syndrome.⁹¹

ACQUIRED ANTIBODIES TO OTHER COAGULATION FACTORS

Clinically significant antibodies to coagulation factors other than factor VIII, factor V, and prothrombin that produce acquired bleeding disorders are sufficiently rare that they merit case reports, which are only incompletely listed here. In contrast to acquired hemophilia A, acquired hemophilia B is extremely rare.^92,93 Patients with antifibrinogen antibodies have been identified either who are asymptomatic with abnormal laboratory values⁹⁴ or who have abnormal bleeding.⁹⁵ Patients with abnormal bleeding associated with acquired inhibitors to factor VII,⁹⁶ factor X,⁹⁷ factor XI,⁹⁸ or factor XIII^{98,99,100,101,102,103,104,105,106,107} also have been described. The development of alloantibodies against von Willebrand factor occurs in patients with type 3 von Willebrand disease in response to treatment with plasma concentrates that contain von Willebrand factor.¹⁰⁸ Acquired von Willebrand disease can be caused by adsorption of von Willebrand factor to tumor cells, loss of high-molecular-weight von Willebrand factor multimers at as well as by autoantibodies to von Willebrand factor.¹⁰⁹