Fc Gamma Receptors and Complement Component 3 Facilitate Anti-fVIII Antibody Formation.

Anti-factor VIII (fVIII) alloantibodies, which can develop in patients with hemophilia A, limit the therapeutic options and increase morbidity and mortality of these patients. However, the factors that influence anti-fVIII antibody development remain incompletely understood. Recent studies suggest that Fc gamma receptors (FcγRs) may facilitate recognition and uptake of fVIII by recently developed or pre-existing naturally occurring anti-fVIII antibodies, providing a mechanism whereby the immune system may recognize fVIII following infusion. However, the role of FcγRs in anti-fVIII antibody formation remains unknown. In order to define the influence of FcγRs on the development of anti-fVIII antibodies, fVIII was injected into WT or FcγR knockout recipients, followed by evaluation of anti-fVIII antibodies. Anti-fVIII antibodies were readily observed following fVIII injection into FcγR knockouts, with similar anti-fVIII antibody levels occurring in FcγR knockouts as detected in WT mice injected in parallel. As antibodies can also fix complement, providing a potential mechanism whereby anti-fVIII antibodies may influence anti-fVIII antibody formation independent of FcγRs, fVIII was also injected into complement component 3 (C3) knockout recipients in parallel. Similar to FcγR knockouts, C3 knockout recipients developed a robust response to fVIII, which was likewise similar to that observed in WT recipients. As FcγRs or C3 may compensate for each other in recipients only deficient in FcγRs or C3 alone, we generated mice deficient in both FcγRs and C3 to test for potential antibody effector redundancy in anti-fVIII antibody formation. Infusion of fVIII into FcγRs and C3 (FcγR × C3) double knockouts likewise induced anti-fVIII antibodies. However, unlike individual knockouts, anti-fVIII antibodies in FcγRs × C3 knockouts were initially lower than WT recipients, although anti-fVIII antibodies increased to WT levels following additional fVIII exposure. In contrast, infusion of RBCs expressing distinct alloantigens into FcγRs, C3 or FcγR × C3 knockout recipients either failed to change anti-RBC levels when compared to WT recipients or actually increased antibody responses, depending on the target antigen. Taken together, these results suggest FcγRs and C3 can differentially impact antibody formation following exposure to distinct alloantigens and that FcγRs and C3 work in concert to facilitate early anti-fVIII antibody formation.

Reduced bone formation in males and increased bone resorption in females drive bone loss in hemophilia A mice.

Hemophilia A (HA), a rare X-linked recessive genetic disorder caused by insufficient blood clotting factor VIII, leaves affected individuals susceptible to spontaneous and traumatic hemorrhage. Although males generally exhibit severe symptoms, due to variable X inactivation, females can also be severely impacted. Osteoporosis is a disease of the skeleton predisposing patients to fragility fracture, a cause of significant morbidity and mortality and a common comorbidity in HA. Because the causes of osteoporosis in HA are unclear and in humans confounded by other traditional risk factors for bone loss, in this study, we phenotyped the skeletons of F8 total knockout (F8 TKO) mice, an animal model of severe HA. We found that trabecular bone accretion in the axial and appendicular skeletons of male F8 TKO mice lagged significantly between 2 and 6 months of age, with more modest cortical bone decline. By contrast, in female mice, diminished bone accretion was mostly limited to the cortical compartment. Interestingly, bone loss was associated with a decline in bone formation in male mice but increased bone resorption in female mice, a possible result of sex steroid insufficiency. In conclusion, our studies reveal a sexual dimorphism in the mechanism driving bone loss in male and female F8 TKO mice, preventing attainment of peak bone mass and strength. If validated in humans, therapies aimed at promoting bone formation in males but suppressing bone resorption in females may be indicated to facilitate attainment of peak mass in children with HA to reduce the risk for fracture later in life.

A microengineered vascularized bleeding model that integrates the principal components of hemostasis.

Hemostasis encompasses an ensemble of interactions among platelets, coagulation factors, blood cells, endothelium, and hemodynamic forces, but current assays assess only isolated aspects of this complex process. Accordingly, here we develop a comprehensive in vitro mechanical injury bleeding model comprising an "endothelialized" microfluidic system coupled with a microengineered pneumatic valve that induces a vascular "injury". With perfusion of whole blood, hemostatic plug formation is visualized and "in vitro bleeding time" is measured. We investigate the interaction of different components of hemostasis, gaining insight into several unresolved hematologic issues. Specifically, we visualize and quantitatively demonstrate: the effect of anti-platelet agent on clot contraction and hemostatic plug formation, that von Willebrand factor is essential for hemostasis at high shear, that hemophilia A blood confers unstable hemostatic plug formation and altered fibrin architecture, and the importance of endothelial phosphatidylserine in hemostasis. These results establish the versatility and clinical utility of our microfluidic bleeding model.

Marginal zone B cells are critical to factor VIII inhibitor formation in mice with hemophilia A.

Although factor VIII (FVIII) replacement therapy can be lifesaving for patients with hemophilia A, neutralizing alloantibodies to FVIII, known as inhibitors, develop in a significant number of patients and actively block FVIII activity, making bleeding difficult to control and prevent. Although a variety of downstream immune factors likely regulate inhibitor formation, the identification and subsequent targeting of key initiators in inhibitor development may provide an attractive approach to prevent inhibitor formation before amplification of the FVIII immune response occurs. As the initial steps in FVIII inhibitor development remain incompletely understood, we sought to define early regulators of FVIII inhibitor formation. Our results demonstrate that FVIII localizes in the marginal sinus of the spleen of FVIII-deficient mice shortly after injection, with significant colocalization with marginal zone (MZ) B cells. FVIII not only colocalizes with MZ B cells, but specific removal of MZ B cells also completely prevented inhibitor development following FVIII infusion. Subsequent rechallenge with FVIII following MZ B-cell reconstitution resulted in a primary antibody response, demonstrating that MZ B-cell depletion did not result in FVIII tolerance. Although recipient exposure to the viral-like adjuvant polyinosinic:polycytidylic acid enhanced anti-FVIII antibody formation, MZ B-cell depletion continued to display similar effectiveness in preventing inhibitor formation following FVIII infusion in this inflammatory setting. These data strongly suggest that MZ B cells play a critical role in initiating FVIII inhibitor formation and suggest a potential strategy to prevent anti-FVIII alloantibody formation in patients with hemophilia A.

Platelet-Microcapsule Hybrids Leverage Contractile Force for Targeted Delivery of Hemostatic Agents.

We report a cell-mediated, targeted drug delivery system utilizing polyelectrolyte multilayer capsules that hybridize with the patient's own platelets upon intravenous administration. The hybridized platelets function as the sensor and actuator for targeted drug delivery and controlled release in our system. These capsules are biochemically and mechanically tuned to enable platelet adhesion and capsule rupture upon platelet activation and contraction, enabling the targeted and controlled "burst" release of an encapsulated biotherapeutic. As platelets are the "first responders" in the blood clot formation process, this platelet-hybridized system is ideal for the targeted delivery of clot-augmenting biotherapeutics wherein immediate therapeutic efficacy is required. As proof-of-concept, we tailored this system to deliver the pro-clotting biotherapeutic factor VIII for hemophilia A patients that have developed inhibitory antifactor VIII antibodies. The polyelectrolyte multilayer capsules physically shield the encapsulated factor VIII from the patient's inhibitors during circulation, preserving its bioactivity until it is delivered at the target site via platelet contractile force. Using an in vitro microfluidic vascular injury model with factor VIII-inhibited blood, we demonstrate a 3.8× increase in induced fibrin formation using capsules loaded with factor VIII at a concentration an order of magnitude lower than that used in systemic delivery. We further demonstrate that clot formation occurs 18 min faster when factor VIII loaded capsules are used compared to systemic delivery at the same concentration. Because platelets are integral in the pathophysiology of thrombotic disorders, cancer, and innate immunity, this paradigm-shifting smart drug delivery system can be similarly applied to these diseases.

High-affinity, noninhibitory pathogenic C1 domain antibodies are present in patients with hemophilia A and inhibitors.

Inhibitor formation in hemophilia A is the most feared treatment-related complication of factor VIII (fVIII) therapy. Most inhibitor patients with hemophilia A develop antibodies against the fVIII A2 and C2 domains. Recent evidence demonstrates that the C1 domain contributes to the inhibitor response. Inhibitory anti-C1 monoclonal antibodies (mAbs) have been identified that bind to putative phospholipid and von Willebrand factor (VWF) binding epitopes and block endocytosis of fVIII by antigen presenting cells. We now demonstrate by competitive enzyme-linked immunosorbent assay and hydrogen-deuterium exchange mass spectrometry that 7 of 9 anti-human C1 mAbs tested recognize an epitope distinct from the C1 phospholipid binding site. These mAbs, designated group A, display high binding affinities for fVIII, weakly inhibit fVIII procoagulant activity, poorly inhibit fVIII binding to phospholipid, and exhibit heterogeneity with respect to blocking fVIII binding to VWF. Another mAb, designated group B, inhibits fVIII procoagulant activity, fVIII binding to VWF and phospholipid, fVIIIa incorporation into the intrinsic Xase complex, thrombin generation in plasma, and fVIII uptake by dendritic cells. Group A and B epitopes are distinct from the epitope recognized by the canonical, human-derived inhibitory anti-C1 mAb, KM33, whose epitope overlaps both groups A and B. Antibodies recognizing group A and B epitopes are present in inhibitor plasmas from patients with hemophilia A. Additionally, group A and B mAbs increase fVIII clearance and are pathogenic in a hemophilia A mouse tail snip bleeding model. Group A anti-C1 mAbs represent the first identification of pathogenic, weakly inhibitory antibodies that increase fVIII clearance.

A subset of high-titer anti-factor VIII A2 domain antibodies is responsive to treatment with factor VIII.

The primary B-cell epitopes of factor VIII (fVIII) are in the A2 and C2 domains. Within the C2 domain, antibody epitope and kinetics are more important than inhibitor titer in predicting pathogenicity in a murine bleeding model. To investigate this within the A2 domain, the pathogenicity of a diverse panel of antihuman fVIII A2 domain monoclonal antibodies (MAbs) was tested in the murine model. MAbs were injected into hemophilia A mice, followed by injection of human B domain-deleted fVIII. Blood loss after a 4-mm tail snip was measured. The following anti-A2 MAbs were tested: high-titer type 1 inhibitors 4A4, 2-76, and 1D4; 2-54, a high-titer type 2 inhibitor; B94, a type 2 inhibitor; and noninhibitory MAbs GMA-012, 4C7, and B25. All high-titer type 1 MAbs produced blood loss that was significantly greater than control mice, whereas all non-inhibitory MAbs produced blood loss that was similar to control. The type 2 MAbs were not pathogenic despite 2-54 having an inhibitor titer of 34 000 BU/mg immunoglobulin G. In addition, a patient with a high-titer type 2 anti-A2 inhibitor who is responsive to fVIII is reported. The discrepancy between inhibitor titer and bleeding phenotype combined with similar findings in the C2 domain stress the importance of inhibitor properties not detected in the standard Bethesda assay in predicting response to fVIII therapy.