Human CD4+ T‐cell epitope repertoire on the C2 domain of coagulation factor VIII

Approximately 25% of severe hemophilia A patients develop antibodies (Ab) that neutralize the procoagulant function of factor (F)VIII (inhibitors). Autoimmune FVIII inhibitors may develop in individuals without congenital FVIII deficiency and cause acquired hemophilia. Low titers of anti‐FVIII Ab may be present in hemophilia A patients without inhibitors and in healthy blood donors. FVIII‐specific CD4+ T‐cells drive the synthesis of anti‐FVIII Ab. We examined the epitope repertoire of CD4+ T‐cells from 15 healthy subjects, 10 hemophilia A patients without inhibitors, 11 hemophilia A patients with inhibitors, and six acquired hemophilia patients. Blood CD4+ T‐cells were challenged in proliferation assays with a panel 16 overlapping synthetic peptides, spanning the sequence of the FVIII C2 domain. The sequence region 2291–2330 contained the most frequently and strongly recognized peptides in each of the four subject groups. Crystallographic B factor data and the location of these peptides within the three‐dimensional structure of the C2 domain confirm that this region has a high degree of solvent exposure and flexibility within the peptide backbone, which are structural features typical of immunodominant universal CD4+ epitopes. Furthermore, this sequence region overlaps inhibitor‐binding sites, suggesting that CD4+ T‐cells recognizing peptide sequences within this region might be involved in inhibitor synthesis. The sequence regions 2191–2210 (recognized strongly by each study group except hemophilia A patients with inhibitors) and 2241–2290 (recognized primarily by acquired hemophilia patients and healthy subjects) share the same structural features, and also overlap inhibitor‐binding sites. Although similar, there appear to be important differences in the CD4+ epitope repertoires of congenital and acquired hemophilia patients.