Rupture of T Cell Tolerance to Self Factor VIII in Mild/Moderate Hemophilia A.

Some patients with mild/moderate hemophilia A develop anti-Factor VIII (FVIII) antibodies following treatment with FVIII. In rare cases, the patient antibodies neutralize normal FVIII but do not recognize the patient FVIII. In most cases, the antibodies cross-react with the patient FVIII thereby changing the patient’s bleeding phenotype into that of a severe hemophilia A patient. We recently investigated the CD4+ T cells of a patient with mild hemophilia A, who produced antibodies recognizing only exogenous normal FVIII. Likewise, the patient CD4+ T cells recognized normal FVIII but not the patient FVIII. These observations raised the question of the specificity of FVIII-specific T cells occurring in mild/moderate hemophilia A patients and who developed an immune response towards both exogenous and self FVIII. We therefore investigated the specificity of CD4+ T lymphocytes in a mild hemophilia A patient (Ba), carrying a substitution Pro2292His in the FVIII gene, who produced high titer inhibitor antibodies recognizing both self and normal FVIII. The patient’s antibodies exclusively recognized the FVIII C2 domain, as determined in immunoprecipitation experiments using recombinant FVIII fragments produced in reticulocyte lysate. Eradication of the inhibitor was not successful despite using several therapeutic options (plasma exchanges, cyclophosphamide, low dose (25 U/kg) immune tolerance induction with plasma-derived Factor VIII and eventually anti-CD20 monoclonal antibody). Patient Ba FVIII specific T cells were expanded using dendritic cells. Out of 20 microcultures initiated with a total of 2 x 10⁶ T cells, 3 cell lines specifically recognised FVIII. The frequency of FVIII-specific T cells in blood of this patient is therefore at least 1/700.000 CD4+ T cells. Patient Ba T cells were cloned using as antigen presenting cells an autologous lymphoblastoid cell line producing a non inhibitory anti-FVIII IgG4 antibody. Two FVIII-specific T cell clones were successfully derived. In control experiments, no FVIII-specific T cell clones could be derived from normal individuals. Both Ba T cell clones were activated by recombinant FVIII fragments encompassing the C1 and C2 domains. T cell activation was compared in presence of normal and His2292 recombinant FVIII. In the presence of 1 IU/ml normal or His2292 FVIII, clone 4E1 produced 0,793 ± 0,024 ng/ml and 0,277 ± 0,087 ng/ml IFN-γ, respectively, whereas clone 3F9 secreted 0,208 ± 0,07 and 0,238 ± 0,021 ng/ml, respectively. The concentration of normal FVIII inducing the same IFN-γ secretion as 1 IU/ml His2292 FVIII was only 0,45 IU/ml for clone 4E1 whereas it was 1,3 IU/ml for clone 3F9. Accordingly, one T cell clone recognizes patient His2292 FVIII at least as well as normal FVIII. These observations demonstrate that in a patient with mild/moderate hemophilia A who develops an immune response to his own FVIII, the T cell tolerance to self FVIII can also be broken. Such a cellular response to self FVIII may render restoration of tolerance to self and exogenous FVIII more difficult.