T-cell responses in two unrelated hemophilia A inhibitor subjects include an epitope at the factor VIII R593C missense site.

BACKGROUND: Development of neutralizing anti-factor (F)VIII antibodies ('inhibitors') is a serious clinical problem in hemophilia A. Increased inhibitor risk has been associated with certain FVIII missense substitutions, including R593C in the A2 domain. OBJECTIVES: The aim of the present study was to identify T-cell epitopes in FVIII and characterize T-cell responses in two unrelated hemophilia A subjects sharing F8-R593C and HLA-DRB1*1101 genotypes. We hypothesized that the hemophilic substitution site coincides with an important T-cell epitope. PATIENTS/METHODS: The binding affinities of peptides for recombinant HLA-DR proteins were measured and compared with epitope prediction results. CD4+ T cells were stimulated using peptides and stained with fluorescent, peptide-loaded tetramers. RESULTS: The inhibitor subjects, but not HLA-matched controls, had high-avidity HLA-DRB1*1101-restricted T-cell responses against FVIII(589-608), which contains the hemophilic missense site. Antigen-specific T cells secreted Th1 and Th2 cytokines and proliferated in response to FVIII and FVIII(592-603). FVIII(589-608) bound with physiologically relevant (micromolar) IC(50) values to recombinant DR0101, DR1101 and DR1501 proteins. CONCLUSIONS: Hemophilia A patients with R593C missense substitutions and these HLA haplotypes had an increased incidence of inhibitors in our cohorts, supporting a paradigm in which presentation of FVIII epitopes containing the wild-type R593 influences inhibitor risk in this hemophilia A sub-population.

Factor VIII-specific B cell responses in haemophilia A patients with inhibitors.

Development of inhibitory antibodies to factor VIII (FVIII) provides a major complication of replacement therapy in patients with haemophilia A. The risk of inhibitor formation is influenced by the underlying FVIII gene defect. Moreover, genetic determinants in the promoter region of IL-10 and TNFalpha have been linked to an increased risk of inhibitor development. Recent cohort-studies have provided evidence that the risk of inhibitor formation is linked to intensity of treatment. Eradication of FVIII inhibitors can be achieved by frequent infusion of high dosages of FVIII, so-called immune tolerance induction (ITI). Until now, the mechanisms involved in downmodulation of the immune response to FVIII during ITI have not been unraveled. Studies performed in an animal model for haemophilia A have suggested that elimination of FVIII-specific memory B cells by high dosages of FVIII contributes to the decline in FVIII inhibitor levels during ITI. Limited knowledge is available with respect to the development and persistence of FVIII-specific memory B cells in patients with haemophilia A. Two recent studies suggest that the frequency of peripheral FVIII-specific memory B cells in haemophilia A patients with inhibitors range from <0.01 to 0.40% of that of total IgG(+) B cells. No or very low frequencies of FVIII-specific memory B cells are observed in haemophilia A patients without inhibitors and in patients treated successfully by ITI. Possible implications of these findings are discussed in the context of currently available information on the role of antigen-specific memory B cells and long-living antibody producing plasma cells in humoral immunity.