Induction of factor VIII‐specific unresponsiveness by intrathymic factor VIII injection in murine hemophilia A

Summary.  Background:  Hemophilia A is a congenital bleeding disorder caused by a deficiency of coagulation factor VIII. Approximately 30% of hemophilia A patients develop inhibitors against FVIII following replacement therapy. We have reported that neonatal exposure of FVIII antigen can induce antigen‐specific immune tolerance by interferon‐γ (IFN‐γ)‐dependent T‐cell anergy in hemophilia A mice. Objective:  The thymus plays crucial roles in self‐tolerance, with negative selection of self‐reactive effector T cells and positive selection of self‐reactive regulatory T cells. We investigated the possibility of the induction of antigen‐specific immune tolerance by intrathymic injection of FVIII in hemophilia A mice. Methods:  Hemophilia A mice were injected with recombinant FVIII into the thymus under real‐time high‐resolution image guidance. Results:  Anti‐FVIII inhibitory antibody titers in mice challenged with intravenous administration of FVIII were significantly lower in mice ( n  = 22) that had received thymic FVIII injection than in mice ( n  = 18) without thymic injection (9.4 ± 2.3 vs. 122.5 ± 27.6 BU mL −1 , respectively, P  =   0.00078). The CD4 + T cells from thymic‐injected mice could not proliferate or produce interleukin (IL)‐2, IL‐12 and IFN‐γ in response to FVIII. The CD4 + CD25 + T cells generated from thymic‐treated mice but not from naïve mice efficiently suppressed the in vitro proliferative response of CD4 + T cells and blocked the in vivo development of anti‐FVIII antibodies in the adoptive transfer. Conclusion:  These data suggest that intrathymic administration of FVIII could result in immune tolerance by induction of FVIII‐specific regulatory T cells.