Platelet gene therapy corrects the hemophilic phenotype in immunocompromised hemophilia A mice transplanted with genetically manipulated human cord blood stem cells.

Our previous studies have demonstrated that platelet FVIII (2bF8) gene therapy can improve hemostasis in hemophilia A mice, even in the presence of inhibitory antibodies, but none of our studies has targeted human cells. Here, we evaluated the feasibility for lentivirus (LV)-mediated human platelet gene therapy of hemophilia A. Human platelet FVIII expression was introduced by 2bF8LV-mediated transduction of human cord blood (hCB) CD34(+) cells followed by xenotransplantation into immunocompromised NSG mice or NSG mice in an FVIII(null) background (NSGF8KO). Platelet FVIII was detected in all recipients that received 2bF8LV-transduced hCB cells as long as human platelet chimerism persisted. All NSGF8KO recipients (n = 7) that received 2bF8LV-transduced hCB cells survived tail clipping if animals had greater than 2% of platelets derived from 2bF8LV-transduced hCB cells, whereas 5 of 7 survived when human platelets were 0.3% to 2%. Whole blood clotting time analysis confirmed that hemostasis was improved in NSGF8KO mice that received 2bF8LV-transduced hCB cells. We demonstrate, for the first time, the feasibility of 2bF8LV gene delivery to human hematopoietic stem cells to introduce FVIII expression in human platelets and that human platelet-derived FVIII can improve hemostasis in hemophilia A.

Platelet gene therapy by lentiviral gene delivery to hematopoietic stem cells restores hemostasis and induces humoral immune tolerance in FIX(null) mice.

Here, we developed a clinically translatable platelet gene therapy approach for hemophilia B. Platelet-targeted FIX (2bF9) expression was introduced by transplantation of hematopoietic stem cells (HSCs) transduced with 2bF9 lentivirus (LV). Sustained therapeutic levels of platelet-FIX expression were obtained in FIX(null) mice that received 2bF9 LV-transduced HSCs. Approximately 6-39% of the platelets expressed FIX in the transduced recipients, which was sufficient to rescue the bleeding diathesis in FIX(null) mice in tail clipping models. Sequential bone marrow transplantation demonstrated that platelet-FIX expression in the secondary recipients was sustained, leading to phenotypic correction. Notably, none of the transduced recipients developed anti-FIX antibodies after platelet gene therapy. Only one of the nine recipients developed a low titer of inhibitory antibodies (1.6 BU/ml) after challenge with rhFIX. These data suggest that platelet gene therapy can not only restore hemostasis but also induce immune tolerance in hemophilia B mice, indicating that this approach may be a promising strategy for gene therapy of hemophilia B in humans.

Correction of murine Bernard-Soulier syndrome by lentivirus-mediated gene therapy.

Bernard-Soulier syndrome (BSS) is an inherited bleeding disorder caused by a defect in the platelet glycoprotein (GP) Ib-IX-V complex. The main treatment for BSS is platelet transfusion but it is often limited to severe bleeding episodes or surgical interventions due to the risk of alloimmunization. We have previously reported successful expression of human GPIbα (hGPIbα) in human megakaryocytes using a lentiviral vector (LV) encoding human GP1BA under control of the platelet-specific integrin αIIb promoter (2bIbα). In this study, we examined the efficacy of this strategy for the gene therapy of BSS using GPIbα(null) as a murine model of BSS. GPIbα(null) hematopoietic stem cells (HSC) transduced with 2bIbα LV were transplanted into lethally irradiated GPIbα(null) littermates. Therapeutic levels of hGPIbα expression were achieved that corrected the tail bleeding time and improved the macrothrombocytopenia. Sequential bone marrow (BM) transplants showed sustained expression of hGPIbα with similar phenotypic correction. Antibody response to hGPIbα was documented in 1 of 17 total recipient mice but was tolerated without any further treatment. These results demonstrate that lentivirus-mediated gene transfer can provide sustained phenotypic correction of murine BSS, indicating that this approach may be a promising strategy for gene therapy of BSS patients.

Factor IX ectopically expressed in platelets can be stored in alpha-granules and corrects the phenotype of hemophilia B mice.

We developed 2bF9 transgenic mice in a hemophilia B mouse model with the expression of human factor IX (FIX) under control of the platelet-specific integrin alphaIIb promoter, to determine whether ectopically expressing FIX in megakaryocytes can enable the storage of FIX in platelet alpha-granules and corrects the murine hemophilia B phenotype. FIX was detected in the platelets and plasma of 2bF9 transgenic mice by both antigen and activity assays. Approximately 90% of total FIX in blood was stored in platelets, most of which is releasable on activation of platelets. Immunostaining demonstrated that FIX was expressed in platelets and megakaryocytes and stored in alpha-granules. All 2bF9 transgenic mice survived tail clipping, suggesting that platelet-derived FIX normalizes hemostasis in the hemophilia B mouse model. This protection can be transferred by bone marrow transplantation or platelet transfusion. However, unlike our experience with platelet FVIII, the efficacy of platelet-derived FIX was limited in the presence of anti-FIX inhibitory antibodies. These results demonstrate that releasable FIX can be expressed and stored in platelet alpha-granules and that platelet-derived FIX can correct the bleeding phenotype in hemophilia B mice. Our studies suggest that targeting FIX expression to platelets could be a new gene therapy strategy for hemophilia B.

Targeting FVIII expression to endothelial cells regenerates a releasable pool of FVIII and restores hemostasis in a mouse model of hemophilia A.

The natural cell type(s) that synthesize and release factor VIII (FVIII) into the circulation are still not known with certainty. In vitro studies indicate that artificial expression of FVIII in endothelial cells produces an intracellular pool of FVIII that can be mobilized together with its carrier protein, von Willebrand factor (VWF), by agonists. Here, we show that expression of human B-domain deleted FVIII (hFVIII) in the vascular endothelium of otherwise FVIII-deficient mice results in costorage of FVIII and VWF in endothelial Weibel-Palade bodies and restores normal levels and activity of FVIII in plasma. Stored FVIII was mobilized into the circulation by subcutaneous administration of epinephrine. Human FVIII activity in plasma was strictly dependent on the presence of VWF. Endothelial-specific expression of hFVIII rescued the bleeding diathesis of hemophilic mice lacking endogenous FVIII. This hemostatic function of endothelial cell-derived hFVIII was suppressed in the presence of anti-FVIII inhibitory antibodies. These results suggest that targeting FVIII expression to endothelial cells may establish a releasable pool of FVIII and normalize plasma FVIII level and activity in hemophilia A, but does not prevent the inhibitory effect of anti-FVIII antibodies on the hemostatic function of transgene-derived hFVIII as is seen with platelet-derived FVIII expression.

Syngeneic transplantation of hematopoietic stem cells that are genetically modified to express factor VIII in platelets restores hemostasis to hemophilia A mice with preexisting FVIII immunity.

Although genetic induction of factor VIII (FVIII) expression in platelets can restore hemostasis in hemophilia A mice, this approach has not been studied in the clinical setting of preexisting FVIII inhibitory antibodies to determine whether such antibodies would affect therapeutic engraftment. We generated a line of transgenic mice (2bF8) that express FVIII only in platelets using the platelet-specific alphaIIb promoter and bred this 2bF8 transgene into a FVIII(null) background. Bone marrow (BM) from heterozygous 2bF8 transgenic (2bF8(tg+/-)) mice was transplanted into immunized FVIII(null) mice after lethal or sublethal irradiation. After BM reconstitution, 85% of recipients survived tail clipping when the 1100-cGy (myeloablative) regimen was used, 85.7% of recipients survived when 660-cGy (nonmyeloablative) regimens were used, and 60% of recipients survived when the recipients were conditioned with 440 cGy. Our further studies showed that transplantation with 1% to 5% 2bF8(tg+/-) BM cells still improved hemostasis in hemophilia A mice with inhibitors. These results demonstrate that the presence of FVIII-specific immunity in recipients does not negate engraftment of 2bF8 genetically modified hematopoietic stem cells, and transplantation of these hematopoietic stem cells can efficiently restore hemostasis to hemophilic mice with preexisting inhibitory antibodies under either myeloablative or nonmyeloablative regimens.