HLA-DR-restricted T-cell responses to factor VIII epitopes in a mild haemophilia A family with missense substitution A2201P

An HLA-DRA-DRB1 *0101-restricted T-cell epitope in the factor VIII (FVIII) C2 domain occurred in a mild haemophilia A patient with missense substitution FVIII-A2201P. His T cells responded to synthetic peptides FVIII2186-2705 and FVIII2194-2213 (J Thromb Haemost 2007; 5: 2399). T cells from family members with genotype A2201P were analysed to determine if FVIII-specific T cells occur in individuals with a haemophilic mutation but no clinically significant inhibitor response. Fluorescent MHC class II tetramers corresponding to subjects' HLA-DRB1 types were loaded with 20-mer peptides and utilized to label antigen-specific CD4+ T cells. T-cell responses to peptides spanning the FVIII-C2 sequence were evaluated. T cells recognizing specific peptides were cloned, and antigen specificity was verified by proliferation assays. Plasma and/or purified IgG samples were tested for FVIII inhibitory activity. CD4+ T cells and T-cell clones from two brothers who shared the DRB1 *0101 allele responded to FVIII2194-2213. A haemophilic cousin's HLA-DRA-DRB1 *1104-restricted response to FVIII2202-2221 was detected only when CD4+CD25+ cells were depleted. A great uncle and two obligate carriers had no detectable FVIII-C2-specific T cells. Concentrated IgG from the brother without a clinical inhibitor response showed a low-titre FVIII inhibitor. FVIII-specific T cells and inhibitory IgG were found in a previously infused, haemophilic subject who had a sub-clinical FVIII inhibitor. CD4+CD25+ depleted T cells from a non-infused haemophilic cousin recognized an overlapping FVIII epitope, indicating a latent HLA-DRA-DRB1 *1104-restricted T-cell response to FVIII. Specific T-cell responses to FVIII can occur without clinically significant inhibitors.