In the past two decades, several groups have been instrumental in developing novel strategies for hemophilia A gene therapy using platelets as a target (11C20)

In the past two decades, several groups have been instrumental in developing novel strategies for hemophilia A gene therapy using platelets as a target (11C20). FVIII gene therapy restores hemostasis in the presence of anti-FVIII inhibitory antibodies and induces immune tolerance in hemophilia A. Keywords: platelets, gene therapy, immune tolerance, hemophilia A, factor VIII Introduction Platelets are the second most common type of cells found in blood, with approximately 1011 newly produced daily to replenish the old platelets in the body (1, 2). Aged platelets undergo apoptosis and are phagocytosed by scavenger cells in the spleen and liver (3C5). It is increasingly recognized that platelets play fundamental roles not only in hemostasis and thrombosis but also in innate and adaptive immunity. The roles of platelets in the immune response have been extensively reviewed in many papers (6C8), but few studies indicate the role of platelets in immune tolerance. Recent studies that target platelets for Nadolol gene therapy reveal the potential role of platelets in immune tolerance induction (9, 10). Platelets are loaded with abundant bioactive proteins and circulate in the blood, serving as both a storage depot and trafficking vehicle in circulation. Due to these characteristics, platelets may be a unique target for gene therapy of diseases. In the past two decades, several groups have been instrumental in developing novel strategies for hemophilia A gene therapy using platelets as a target (11C20). It has been shown that ectopic expression of factor VIII (FVIII) in platelets directed by either the glycoprotein (GP) Ib or the GPIIb (IIb) promoter can lead to the storage of FVIII in platelet -granules and that platelet-derived FVIII can improve hemostasis in hemophilia A mice even in the presence of anti-FVIII inhibitory antibodies (referred to as inhibitors) (13, 15, 17, 21). In addition to achieving hemostatic efficacy, studies have demonstrated that lentivirus-mediated platelet-specific FVIII gene delivery under control of the IIb promoter (2bF8) to hematopoietic stem cells (HSCs) can induce antigen-specific immune tolerance in hemophilia A mice even with preexisting anti-FVIII immunity (22C24). In this review, we discuss the potential mechanisms of platelet-targeted FVIII expression in restoring hemostasis for hemophilia A in the presence of anti-FVIII inhibitors and inducing immune tolerization after platelet-specific gene therapy. Platelets Shield Neoprotein From Being Recognized by the Immune System Platelets could be an ideal target for gene therapy of hemophilia A as they can store neoprotein FVIII together with its carrier protein von Willebrand factor (VWF) in -granules and act as delivery vehicles in blood circulation. It has been shown that when FVIII expression is introduced by HSC transduction with 2bF8 lentivirus followed by transplantation, FVIII expression is detected only in platelets, but not in plasma of hemophilia A mice (14, 17, 22, 23). Plasma FVIII is undetectable in 2bF8-transduced recipients even with a platelet-FVIII level as high as 30C35 mU/108 platelets (corresponding to ~60C70% of FVIII in whole blood in normal wild-type C57BL/6 mice) (22, 23). Thus, neoprotein FVIII stored in platelets may avoid direct exposure to the immune system during the normal physiological condition, which may reduce the potential to elicit immune responses against the neoprotein. Indeed, neither inhibitory nor non-inhibitory anti-FVIII Nadolol antibodies were detected after platelet-specific FVIII gene therapy via 2bF8 lentivirus-mediated bone marrow or HSC transduction followed by transplantation. The efficacy in phenotypic correction and immune tolerance induction was further confirmed through sequential bone marrow transplantations in secondary and tertiary Nadolol recipients (14, 17, 22, 25). The effectiveness of platelet-targeted gene therapy has been further confirmed in hemophilia A rats (26) and hemophilia A dogs (27). Shi et al. recently developed a hemophilia A rat model, in which nearly the entire rat FVIII gene is inverted, with a severe spontaneous bleeding phenotype and a high incidence of inhibitor development upon rhFVIII infusion (26). Of note, the severe hemophilic phenotype in hemophilia Nadolol A rats is fully rescued after platelet-targeted FVIII expression. When platelet-FVIII expression was introduced into hemophilia A rats after transplantation of 2bF8 genetically manipulated Nadolol bone marrow cells from 2bF8 transgenic rats, the spontaneous bleeding phenotype was rescued with no inhibitor development even though animals were continuously exposed to platelet-FVIII after bone marrow transplantation (26). Using a large animal model, hemophilia A dogs, Du and coworkers demonstrated that 2bF8 lentivirus-mediated HSC transduction followed by transplantation improved hemostasis in hemophilia A dogs and animals were well-tolerized to 2bF8 lentivirus-introduced neoprotein with no detectable inhibitor development in treated animals (27). In contrast to platelet-specific FVIII expression, it has been shown that targeting FVIII expression to hematopoietic cells under a constitutively active promoter may trigger anti-FVIII immune responses. Wang et al. (20) utilized the intraosseous delivery of a lentiviral vector targeting FVIII Mouse monoclonal to Fibulin 5 to platelets (under the GPIb promoter, G-F8-LV) and a lentiviral vector with constitutive FVIII expression (under the elongation factor 1 promoter, E-F8-LV). After a FVIII gene transfer by injecting E-F8-LVs into tibias in.