Towards the Care of Hemophilia a Patients Using Induced Pluripotent Stem Cell (iPSC)-Derived Megakaryocytes (iMks) Expressing Coagulation Factor (F) VIII

We and others have shown that FVIII expressed ectopically in platelets (pFVIII) is stored in α-granules, released at sites of vascular injury and restores hemostasis in FVIII^null mice, even in the presence of neutralizing antibodies to FVIII. These studies support the concept that unlike therapeutic interventions that correct plasma FVIII, pFVIII may be a useful therapy in hemophilia A with intractable inhibitors and significant bleeds. We have also demonstrated this approach has several limitations that may make pFVIII gene therapy bone marrow transplantation (BMT) strategies problematic: 1) pFVIII is not equivalent to plasma FVIII and its efficacy in joint and intracranial bleeds has yet to be shown, especially in the presence of inhibitors, and 2) pFVIII expressed during megakaryopoiesis can cause injury to the Mks, potentially exacerbating post-BMT thrombocytopenia. We propose an alternative strategy: interval prophylactic infusions of FVIII-containing platelets generated from patient-specific iMks expressing either human B-domain-deleted (BDD) FVIII or variants of this FVIII that have greater stability and longer half-lives; making them especially efficacious as pFVIII as we previously demonstrated. iPSCs are a renewable source of cells that can be pre-screened prior to clinical usage for lines that express optimal levels of pFVIII and also release optimal numbers of platelets after differentiation into iMks. Such iPSCs were transfected with a self-inactivating lentivirus containing cDNA for one of three FVIII variants: wildtype BDD FVIII (WT FVIII), R1645H PACE/furin cleavage site FVIII (FVIII^R1645H), and amino acid 1645 to 1648 deletion FVIII (FVIIIΔ). FVIII^R1645H and FVIIIΔ show greater stability and consequently greater specific activity with no increase in injuring Mks. All FVIII variants were expressed using the MK-specific Cxcl4 promoter and were shown to be effective in several bleeding models in FVIII^null mice. Differentiated and transduced iMKs were analyzed for RNA and protein expression. All of the FVIII variant iMKs expressed at least forty-fold higher levels of mRNA compared to the non-transduced control (n=6) and protein was expressed at >550 pg/10⁶ CD42b⁺ iMKs (n=6). Transduced MKs released FVIII into the supernatant when activated by thrombin showing the pFVIII was likely stored in α-granules. Annexin staining was the same between FVIII-expressing iMKs and control iMks suggesting that the level of pFVIII did not cause the iMks to become apoptotic. To test the ability of FVIII-expressing iMKs to correct the coagulopathy in hemophilia A, 5x10⁵ iMKs were added to FVIII^null murine whole blood and evaluated for clot formation using rotational thromboelastometry (ROTEM). Each FVIII variant showed a decrease in clotting time, clot formation time, and an increase in maximum clot firmness when compared to the non-transduced control (n=4). These data show that iMKs expressing FVIII variants can improve hemostasis in a whole blood clotting assay. Our next goal is to generate sufficient platelets from these iMKs to test for correction of the bleeding diathesis in immunodeficient FVIII^null mice and to determine their efficacy in improving hemostasis in a number of clinically relevant hemostatic models.