B-Cell and T-Cell Epitopes in Anti-factor VIII Immune Responses

Abstract

Adequate hemostasis is achieved for many hemophilia A patients by infusion of plasma-derived or recombinant factor VIII (FVIII), but unfortunately, a significant subset of patients develop an immune response in which anti-FVIII antibodies, referred to clinically as "inhibitors," interfere with its procoagulant activity. Inhibitors are the subset of anti-FVIII antibodies that bind to surfaces on FVIII (B-cell epitopes) that are important for its proper functioning in coagulation. Less antigenic FVIII molecules may be designed by identifying and then modifying the amino acid sequences of inhibitor B-cell epitopes. Conversely, characterization of these epitopes can yield important information regarding functionally important surfaces on FVIII. The production of inhibitor antibodies is driven by T cells. T cells recognize FVIII as foreign when FVIII-derived peptides bind to major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells. The class II-peptide complexes must then be recognized by T-cell receptors (TCRs). T-cell stimulation requires sustained association of antigen-presenting cells and T cells through formation of a class II-peptide-TCR complex, and peptide sequences that mediate this association are termed "T-cell epitopes." MHC class II tetramers that bind FVIII-derived peptides and recognize antigen-specific TCRs are proving useful in the characterization of human leukocyte antigen-restricted T-cell responses to FVIII.

CD4+ T-cell epitopes associated with antibody responses after intravenously and subcutaneously applied human FVIII in humanized hemophilic E17 HLA-DRB1*1501 mice

“…6,7 The identification of CD4 ϩ T-cell epitopes of FVIII has therefore been the subject of previous studies in mice and humans. [11][12][13][14][15][16][17][18] We used humanized E17 HLA-DRB1*1501 mice to identify CD4 ϩ T-cell epitopes of FVIII in the context of HLA-DRB1*1501. We identified 8 major FVIII peptide regions that contain CD4 ϩ T-cell epitopes.…”

“…The available information on FVIII peptides presented in the context of specific human MHC class II molecules is limited. Several studies used peripheral blood cells of patients and healthy controls 11 to identify CD4 ϩ T-cell epitopes in the A2 domain, 12 A3 domain, 13 and C2 domain of FVIII. 14 However, these studies lack information on the specific MHC class II molecules associated with the FVIII peptides identified.…”

Factor VIII gene variants and inhibitor risk in African American hemophilia A patients

“…The peptides corresponded to both versions of sequences encoded by the 4 ns-SNPs identified previously, 13 and they were designed to potentially allow binding to recombinant MHC(HLA-DRB1) proteins in multiple registers within the peptide-binding groove. 19 The ability of 11 HLA-DRB1 recombinant proteins, whose corresponding alleles are found in ;74% and ;40% of European and African American populations, respectively, 20 to present peptides containing these sequence variants was characterized by quantitative peptide-MHC binding assays measuring displacement of biotinylated reference peptides specific for each HLA-DRB1. 8 Binding affinity predictions were calculated using ProPred 21 (www.imtech.res.in/raghava/propred) and NetMHCIIPan2 22 (www.cbs.dtu.…”

High-resolution mapping of epitopes on the C2 domain of factor VIII by analysis of point mutants using surface plasmon resonance

“…This experimental approach may be used to target functional B-cell epitopes, including critical residues in antigenic loops in the FVIII A2 domain and in other regions of FVIII, [46][47][48][49][50][51][52] in designing novel FVIII muteins that could provide useful bypass therapy options for inhibitor patients. Because their sequences would be closer to that of the FVIII used to treat the original bleeding disorder, the risk of provoking new T-cell responses and subsequent new inhibitors [53][54][55][56][57][58] to such rationally designed therapeutic FVIII muteins might also be lowered, in comparison with porcine FVIII used as bypass therapy. We expect that sequence modifications to neutralize immunodominant B-cell and T-cell epitopes will eventually be a feature of therapeutic FVIII proteins targeted to patients with refractory inhibitor responses, as well as to patients with poor prognostic factors such as high-risk F8 gene mutations or a family history of inhibitors.…”