Immunosuppressive FVIII-Specific Human Cartregs in Hemophilia a

Hemophilia A is an X-linked disorder, in which mutations in the coagulation Factor VIII (FVIII) gene lead to a loss of FVIII function and serious bleeding episodes. These episodes can be treated with recombinant FVIII protein replacement. Unfortunately, ~25% of hemophilia A patients produce inhibitory anti-FVIII antibodies because of lack of tolerance. Thus, it is necessary to develop effective tolerogenic therapies to prevent, as well as reverse, inhibitor formation. Previously, we generated engineered antigen-specific regulatory T cells (Tregs), created by transduction of a recombinant T-cell receptor (TCR) isolated from a hemophilia A subject's T cell clone. The resulting engineered T cells bind MHC tetramers, proliferate in response to a specific FVIII epitope, and suppress effector responses to FVIII.

In this study, we engineered a FVIII-specific chimeric antigen receptor (ANS8CAR) using a FVIII-specific scFv derived from a synthetic phage display library. Following initial experiments in naïve CD4 T cells, this CAR was introduced into human Tregs. Western blot and specific staining with FVIII verified CAR expression. Transduced ANS8CAR Tregs proliferated in response to FVIII and were able to suppress the proliferation of FVIII-specific T effector cells in vitro. Additionally, the proliferation of T effector cells with different FVIII domain specificity was suppressed as well when ANS8CAR-transduced Tregs were activated with FVIII. Thus, engineered cells are able to promote bystander suppression. Cytokine expression of ANS8CAR-transduced Tregs was comparable to expression of untransduced and TCR-transduced Tregs indicating that the regulatory phenotype of Tregs was not negatively influenced by ANS8CAR expression.

In conclusion, CAR-transduced Tregs seem to be a promising alternative to TCR-transduced Tregs for a future tolerogenic treatment of hemophilia A patients with inhibitory FVIII-specific antibodies.

Supported by NIH grants HL061883 and HL126727 (DWS), the Society of Thrombosis and Hemostasis Research, and the Günter Landbeck Excellence Award (AS).