B cell–activating factor modulates the factor VIII immune response in hemophilia A

Inhibitors of factor VIII (FVIII) remain the most challenging complication of FVIII protein replacement therapy in hemophilia A (HA). Understanding the mechanisms that guide FVIII-specific B cell development could help identify therapeutic targets. The B cell–activating factor (BAFF) cytokine family is a key regulator of B cell differentiation in normal homeostasis and immune disorders. Thus, we used patient samples and mouse models to investigate the potential role of BAFF in modulating FVIII inhibitors. BAFF levels were elevated in pediatric and adult HA inhibitor patients and decreased to levels similar to those of noninhibitor controls after successful immune tolerance induction (ITI). Moreover, elevations in BAFF levels were seen in patients who failed to achieve FVIII tolerance with anti-CD20 antibody–mediated B cell depletion. In naive HA mice, prophylactic anti-BAFF antibody therapy prior to FVIII immunization prevented inhibitor formation and this tolerance was maintained despite FVIII exposure after immune reconstitution. In preimmunized HA mice, combination therapy with anti-CD20 and anti-BAFF antibodies dramatically reduced FVIII inhibitors via inhibition of FVIII-specific plasma cells. Our data suggest that BAFF may regulate the generation and maintenance of FVIII inhibitors and/or anti-FVIII B cells. Finally, anti-CD20/anti-BAFF combination therapy may be clinically useful for ITI

Emerging therapies for hemophilia: controversies and unanswered questions [version 1; referees: 4 approved]

Several new therapies for hemophilia have emerged in recent years. These strategies range from extended half-life factor replacement products and non-factor options with improved pharmacokinetic profiles to gene therapy aiming for phenotypic cure. While these products have the potential to change hemophilia care dramatically, several challenges and questions remain regarding broader applicability, long-term safety, and which option to pursue for each patient. Here, we review these emerging therapies with a focus on controversies and unanswered questions in each category

Potentiation of thrombin generation in hemophilia A plasma by coagulation factor VIII and characterization of antibody-specific inhibition.

Development of inhibitory antibodies to coagulation factor VIII (fVIII) is the primary obstacle to the treatment of hemophilia A in the developed world. This adverse reaction occurs in 20-30% of persons with severe hemophilia A treated with fVIII-replacement products and is characterized by the development of a humoral and neutralizing immune response to fVIII. Patients with inhibitory anti-fVIII antibodies are treated with bypassing agents including recombinant factor VIIa (rfVIIa). However, some patients display poor hemostatic response to bypass therapy and improved treatment options are needed. Recently, we demonstrated that fVIII inhibitors display widely variable kinetics of inhibition that correlate with their respective target epitopes. Thus, it was hypothesized that for antibodies that display slow rates of inhibition, supplementation of rfVIIa with fVIII would result in improved thrombin generation and be predictive of clinical responses to this novel treatment regimen. In order to test this hypothesis, 10 murine monoclonal antibodies (MAbs) with non-overlapping epitopes spanning fVIII, differential inhibition titers, and inhibition kinetics were studied using a thrombin generation assay. Of the 3 MAbs with high inhibitory titers, only the one with fast and complete (classically defined as "type I") kinetics displayed significant inhibition of thrombin generation with no improvement upon supplementation of rfVIIa with fVIII. The other two MAbs that displayed incomplete (classically defined as "type II") inhibition did not suppress the potentiation of thrombin generation by fVIII. All antibodies that did not completely inhibit fVIII activity demonstrated potentiation of thrombin generation by the addition of fVIII as compared to rfVIIa alone. In conclusion, fVIII alone or in combination with rfVIIa corrects the thrombin generation defect produced by the majority of anti-fVIII MAbs better than single agent rfVIIa. Therefore, combined fVIII/rfVIIa therapy may provide better hemostatic control than current therapy in some patients with anti-fVIII inhibitors

Influence of inhibitory titer on residual fVIII activity and peak thrombin generation.

<p>The inhibitor titer for each MAb was compared with the peak thrombin generation of fVIII alone or fVIII + rfVIIa at the immediate (solid circles, r² = 0.36) or 1 hr (open circles, r² = 0.09) time points (A.) and with residual fVIII activity at the immediate (solid circles, r² = 0.004) or 1 hr (r² = 0.07) time points (B.).</p

Characteristics of anti-fVIII MAbs.

<p>Characteristics of anti-fVIII MAbs.</p

TGA parameters for anti-fVIII MAbs.

<p>ETP, peak thrombin and index velocity are presented as ratios compared to fVIII deficient plasma supplemented with 1 U/ml fVIII in the absence of any anti-fVIII MAb.</p><p>ETP – endogenous thrombin potential.</p

Amount of fVIII needed to restore thrombin generation of rfVIIa.

<p>Varying concentrations of fVIII were mixed 1∶1 with rfVIIa at a final concentration of 2.25 µg/ml to determine the level of fVIII activity necessary for restoration of thrombin generation of rfVIIa to fVIII at 1 U/ml. The resulting data was transformed using manufacturer’s software to yield thrombin generation curves (A), peak thrombin concentration (B) and index velocity (C). Error bars represent sample standard deviation.</p

Epitope map of non-overlapping MAbs.

<p>The relative epitopes of anti-fVIII MAbs are shown. MAbs 4A4, 2–54, 1D4 and 2–93 target non-overlapping epitopes in the A2 domain. MAbs G38 and 2–113 target non-overlapping epitopes in the A3 domain at residues 1690–1817 and 1818–1916, respectively. MAbs I109 and 2–77 target non-overlapping epitopes in the C2 domain. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0048172#pone.0048172-Meeks1" target="_blank">[13]</a>.</p

Inhibition of thrombin generation by group 1 anti-fVIII MAbs.

<p>Thrombin generation curves are shown for fVIII deficient plasma (negative control), fVIII deficient plasma with 1 U/ml of fVIII (positive control) and 1 U/ml of fVIII in the presence of 5 µg/ml of specified group 1 MAb. Data represent mean ± sample standard deviation for 3 experiments where duplicates of each measurement were taken and averaged.</p