The diversity of the immune response to the A2 domain of human factor VIII

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186

A normal hemostatic response to vascular injury requires both factor VIII (FVIII) and von Willebrand factor (VWF). In plasma, VWF and FVIII normally circulate as a noncovalent complex, and each has a critical function in the maintenance of hemostasis. Furthermore, the interaction between VWF and FVIII plays a crucial role in FVIII function, immunogenicity, and clearance, with VWF essentially serving as a chaperone for FVIII. Several novel recombinant FVIII (rFVIII) therapies for hemophilia A have been in clinical development, which aim to increase the half-life of FVIII (∼12 hours) and reduce dosing frequency by utilizing bioengineering techniques including PEGylation, Fc fusion, and single-chain design. However, these approaches have achieved only moderate increases in halflife of 1.5-to 2-fold compared with marketed FVIII products. Clearance of PEGylated rFVIII, rFVIIIFc, and rVIII-SingleChain is still regulated to a large extent by interaction with VWF. Therefore, the half-life of VWF (∼15 hours) appears to be the limiting factor that has confounded attempts to extend the half-life of rFVIII. A greater understanding of the interaction between FVIII and VWF is required to drive novel bioengineering strategies for products that either prolong the survival of VWF or limit VWF-mediated clearance of FVIII. (Blood.

Section: Fviii Immunogenicity: Role For Vwfmentioning

confidence: 99%

Life in the shadow of a dominant partner: the FVIII-VWF association and its clinical implications for hemophilia A

Pipe

Montgomery

Pratt

et al. 2016

173

186

“…Characterization of the structural and functional properties of anti-A2 and anti-C2 domain monoclonal antibodies (mAbs) has advanced our understanding of the epitope spectrum of anti-fVIII antibodies [12][13][14][15] and their mechanisms of inhibition. [16][17][18][19] However, there is increasing evidence of the C1 domain's contribution to fVIII function and immunogenicity.…”

Section: Introductionmentioning

confidence: 99%

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

Batsuli

Deng

Healey

et al. 2016

Self Cite

Key Points• C1 domain antibodies with low inhibitor titers by the Bethesda assay are pathogenic in mice due to increased fVIII clearance.• Monoclonal and patientderived polyclonal anti-fVIII C1 domain antibodies recognize similar B-cell epitopes.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 hydrogendeuterium 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. (Blood. 2016;128(16):2055-2067

“…Group E MAbs inhibit fVIII light chain cleavage at Arg1689. 8 In this study, we compared the pathogenicity of the various groups of anti-A2 MAbs in a murine in vivo bleeding model.…”

Section: Introductionmentioning

confidence: 99%

“…5 We have characterized the diversity of a large panel of anti-A2 and anti-C2 monoclonal antibodies (MAbs) that were produced in a murine hemophilia A immunogenicity model. [6][7][8] For the C2 antibodies, 5 groups of structural B-cell epitopes were defined on the basis of patterns of overlapping epitopes. Group A, AB, and B antibodies correspond to classical anti-C2 antibodies that inhibit the binding of fVIII and fVIIIa to phospholipid and VWF.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

Eubanks

Baldwin

Markovitz

et al. 2016

Self Cite

• Discrepancies exist between inhibitor titer and bleeding phenotype in anti-fVIII antibodies.• A subset of high-titer type 2 anti-A2 inhibitors can be treated with fVIII.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.