T cell recognition of the A2 domain of coagulation factor VIII in hemophilia patients and healthy subjects

Hemophilia A patients treated with coagulation factor VIII (FVIII), and also some healthy subjects, may develop anti‐FVIII antibodies (Ab), whose synthesis is driven by FVIII‐specific CD4+ T cells. Some Ab block the procoagulant function of FVIII (inhibitors). Many inhibitors recognize epitopes on the FVIII A2 domain. Here, we have sought to identify A2 epitopes recognized by CD4+ T cells. We tested the proliferative response of CD4+ blood lymphocytes (BL) from hemophilia patients and healthy subjects, to overlapping synthetic peptides spanning the A2 domain sequence. Many A2 peptides induced proliferative responses of CD4+ BL from one or more subjects. The peptide‐induced responses were strongest in hemophilia patients with inhibitors, weakest in healthy subjects. A2 peptides comprising residues 371–400, 621–650 and 671–690 elicited frequent and strong responses in hemophilia A patients, and especially in those with inhibitors. Healthy subjects recognized frequently only the sequence 371–400. A three‐dimensional model of the A2 domain suggests that these CD4+ epitope sequences have structural features typical of ‘universal’ CD4+ T epitopes.