Heterogeneity of the immune response to adenovirus-mediated factor VIII gene therapy in different inbred hemophilic mouse strains

Unexpected enhancement of FVIII immunogenicity by endothelial expression in lentivirus-transduced and transgenic mice

Factor VIII (FVIII) replacement therapy for hemophilia A is complicated by development of inhibitory antibodies (inhibitors) in ∼30% of patients. Because endothelial cells (ECs) are the primary physiologic expression site, we probed the therapeutic potential of genetically restoring FVIII expression selectively in ECs in hemophilia A mice (FVIIInull). Expression of FVIII was driven by the Tie2 promoter in the context of lentivirus (LV)-mediated in situ transduction (T2F8LV) or embryonic stem cell-mediated transgenesis (T2F8Tg). Both endothelial expression approaches were associated with a strikingly robust immune response. Following in situ T2F8LV-mediated EC transduction, all FVIIInull mice developed inhibitors but had no detectable plasma FVIII. In the transgenic approach, the T2F8Tg mice had normalized plasma FVIII levels, but showed strong sensitivity to developing an FVIII immune response upon FVIII immunization. A single injection of FVIII with incomplete Freund adjuvant led to high titers of inhibitors and reduction of plasma FVIII to undetectable levels. Because ECs are putative major histocompatibility complex class II (MHCII)-expressing nonhematopoietic, "semiprofessional" antigen-presenting cells (APCs), we asked whether they might directly influence the FVIII immune responses. Imaging and flow cytometric studies confirmed that both murine and human ECs express MHCII and efficiently bind and take up FVIII protein in vitro. Moreover, microvascular ECs preconditioned ex vivo with inflammatory cytokines could functionally present exogenously taken-up FVIII to previously primed CD4+/CXCR5+ T follicular helper (Tfh) cells to drive FVIII-specific proliferation. Our results show an unanticipated immunogenicity of EC-expressed FVIII and suggest a context-dependent role for ECs in the regulation of inhibitors as auxiliary APCs for Tfh cells.

Gene Therapy for Coagulation Disorders

Molecular genetic details of the human coagulation system were among the first successes of the genetic revolution in the 1980s. This information led to new molecular diagnostic strategies for inherited disorders of hemostasis and the development of recombinant clotting factors for the treatment of the common inherited bleeding disorders. A longer term goal of this knowledge has been the establishment of gene transfer to provide continuing access to missing or defective hemostatic proteins. Because of the relative infrequency of inherited coagulation factor disorders and the availability of safe and effective alternative means of management, the application of gene therapy for these conditions has been slow to realize clinical application. Nevertheless, the tools for effective and safe gene transfer are now much improved, and we have started to see examples of clinical gene therapy successes. Leading the way has been the use of adeno-associated virus-based strategies for factor IX gene transfer in hemophilia B. Several small phase 1/2 clinical studies using this approach have shown prolonged expression of therapeutically beneficial levels of factor IX. Nevertheless, before the application of gene therapy for coagulation disorders becomes widespread, several obstacles need to be overcome. Immunologic responses to the vector and transgenic protein need to be mitigated, and production strategies for clinical grade vectors require enhancements. There is little doubt that with the development of more efficient and facile strategies for genome editing and the application of other nucleic acid-based approaches to influence the coagulation system, the future of genetic therapies for hemostasis is bright.

Characterization of a genetically engineered mouse model of hemophilia A with complete deletion of the F8 gene

ESSENTIALS: Anti-factor VIII (FVIII) inhibitory antibody formation is a severe complication in hemophilia A therapy. We genetically engineered and characterized a mouse model with complete deletion of the F8 coding region. F8(TKO) mice exhibit severe hemophilia, express no detectable F8 mRNA, and produce FVIII inhibitors. The defined background and lack of FVIII in F8(TKO) mice will aid in studying FVIII inhibitor formation.

BACKGROUND

The most important complication in hemophilia A treatment is the development of inhibitory anti-Factor VIII (FVIII) antibodies in patients after FVIII therapy. Patients with severe hemophilia who express no endogenous FVIII (i.e. cross-reacting material, CRM) have the greatest incidence of inhibitor formation. However, current mouse models of severe hemophilia A produce low levels of truncated FVIII. The lack of a corresponding mouse model hampers the study of inhibitor formation in the complete absence of FVIII protein.

OBJECTIVES

We aimed to generate and characterize a novel mouse model of severe hemophilia A (designated the F8(TKO) strain) lacking the complete coding sequence of F8 and any FVIII CRM.

METHODS

Mice were created on a C57BL/6 background using Cre-Lox recombination and characterized using in vivo bleeding assays, measurement of FVIII activity by coagulation and chromogenic assays, and anti-FVIII antibody production using ELISA.

RESULTS

All F8 exonic coding regions were deleted from the genome and no F8 mRNA was detected in F8(TKO) mice. The bleeding phenotype of F8(TKO) mice was comparable to E16 mice by measurements of factor activity and tail snip assay. Similar levels of anti-FVIII antibody titers after recombinant FVIII injections were observed between F8(TKO) and E16 mice.

CONCLUSIONS

We describe a new C57BL/6 mouse model for severe hemophilia A patients lacking CRM. These mice can be directly bred to the many C57BL/6 strains of genetically engineered mice, which is valuable for studying the impact of a wide variety of genes on FVIII inhibitor formation on a defined genetic background.

Pancreatic transduction by helper-dependent adenoviral vectors via intraductal delivery

Pancreatic gene transfer could be useful to treat several diseases, such as diabetes mellitus, cystic fibrosis, chronic pancreatitis, or pancreatic cancer. Helper-dependent adenoviral vectors (HDAds) are promising tools for gene therapy because of their large cloning capacity, high levels of transgene expression, and long-term persistence in immunocompetent animals. Nevertheless, the ability of HDAds to transduce the pancreas in vivo has not been investigated yet. Here, we have generated HDAds carrying pancreas-specific expression cassettes, that is, driven either by the elastase or insulin promoter, using a novel and convenient plasmid family and homologous recombination in bacteria. These HDAds were delivered to the pancreas of immunocompetent mice via intrapancreatic duct injection. HDAds, encoding a CMV-GFP reporter cassette, were able to transduce acinar and islet cells, but transgene expression was lost 15 days postinjection in correlation with severe lymphocytic infiltration. When HDAds encoding GFP under the control of the specific elastase promoter were used, expression was detected in acinar cells, but similarly, the expression almost disappeared 30 days postinjection and lymphocytic infiltration was also observed. In contrast, long-term transgene expression (>8 months) was achieved with HDAds carrying the insulin promoter and the secretable alkaline phosphatase as the reporter gene. Notably, transduction of the liver, the preferred target for adenovirus, was minimal by this route of delivery. These data indicate that HDAds could be used for pancreatic gene therapy but that selection of the expression cassette is of critical importance to achieve long-term expression of the transgene in this tissue.

Antigenic liposomes displaying CD22 ligands induce antigen-specific B cell apoptosis

Antibodies confer humoral immunity but can also be harmful when they target an autoantigen, alloantigen, allergen, or biotherapeutic. New strategies are needed for antigen-specific suppression of undesired antibody responses, particularly to T cell-dependent protein antigens, because they elicit T cell help. Here we show that liposomal nanoparticles, displaying both antigen and glycan ligands of the inhibitory coreceptor CD22, induce a tolerogenic program that selectively causes apoptosis in mouse and human B cells. These SIGLEC-engaging tolerance-inducing antigenic liposomes (STALs, where SIGLEC is defined as sialic acid-binding Ig-like lectin) induced robust antigen-specific tolerance to protein antigens in mice, preventing subsequent immune response to challenge with the same antigen. Since development of inhibitory antibodies to FVIII is a serious problem in treatment of hemophilia A patients, we investigated the potential of this approach for inducing tolerance to FVIII in a hemophilia mouse model. STALs prevented formation of inhibitory FVIII antibodies, allowing for effective administration of FVIII to hemophilia mice to prevent bleeding. These findings suggest that STALs could be used to eliminate or prevent harmful B cell-mediated immune responses.

Animal models of hemophilia

The X-linked bleeding disorder hemophilia is caused by mutations in coagulation factor VIII (hemophilia A) or factor IX (hemophilia B). Unless prophylactic treatment is provided, patients with severe disease (less than 1% clotting activity) typically experience frequent spontaneous bleeds. Current treatment is largely based on intravenous infusion of recombinant or plasma-derived coagulation factor concentrate. More effective factor products are being developed. Moreover, gene therapies for sustained correction of hemophilia are showing much promise in preclinical studies and in clinical trials. These advances in molecular medicine heavily depend on availability of well-characterized small and large animal models of hemophilia, primarily hemophilia mice and dogs. Experiments in these animals represent important early and intermediate steps of translational research aimed at development of better and safer treatments for hemophilia, such a protein and gene therapies or immune tolerance protocols. While murine models are excellent for studies of large groups of animals using genetically defined strains, canine models are important for testing scale-up and for long-term follow-up as well as for studies that require larger blood volumes.

Factor VIII delivered by haematopoietic stem cell-derived B cells corrects the phenotype of haemophilia A mice

The main impediments to clinical application of haematopoietic stem cell (HSC) gene therapy for treatment of haemophilia A are the bone marrow transplant-related risks and the potential for insertional mutagenesis caused by retroviral vectors. To circumvent these limitations, we have adapted a non-myeloablative conditioning regimen and directed factor VIII (FVIII) protein synthesis to B lineage cells using an insulated lentiviral vector containing an immunoglobulin heavy chain enhancer-promoter. Transplantation of lentiviral vector-modified HSCs resulted in therapeutic levels of FVIII in the circulation of all transplanted mice for the duration of the study (six months). Immunostaining of spleen cells showed that the majority of FVIII was synthesised by B220+ B cells and CD138+ plasma cells. Subsequent challenge with recombinant FVIII elicited at most a minor anti-FVIII antibody response, demonstrating induction of immune hyporesponsiveness. All transplant recipients exhibited clot formation and survived tail clipping, indicating correction of their haemophilic phenotype. Therapeutic levels of FVIII could be transferred to secondary recipients by bone marrow transplantation, confirming gene transfer into long-term repopulating HSCs. Moreover, short-term therapeutic FVIII levels could also be achieved in secondary recipients by adoptive transfer of HSC-derived splenic B cells. Our findings support pursuit of B cell-directed protein delivery as a potential clinical approach to treat haemophilia A and other disorders correctable by systemically distributed proteins.

A role for thrombin in the initiation of the immune response to therapeutic factor VIII

Administration of human factor VIII (FVIII) to FVIII knockout hemophilia mice is a useful small animal model to study the physiologic response in patients iatrogenically immunized to this therapeutic protein. These mice manifest a robust, T cell-dependent, antibody response to exogenous FVIII treatment, even when encountered through traditionally tolerogenic routes. Thus, FVIII given via these routes elicits both T- and B-cell responses, whereas a control, foreign protein, such as ovalbumin (OVA), is poorly immunogenic. When FVIII is heat inactivated, it loses function and much of its immunogenicity. This suggests that FVIII's immunogenicity is principally tied to its function and not its structure. If mice are treated with the anticoagulant warfarin, which depletes other coagulation factors including thrombin, there is a reduced immune response to FVIII. Furthermore, when mice are treated with the direct thrombin inhibitor, hirudin, the T-cell responses and the serum anti-FVIII antibody concentrations are again significantly reduced. Notably, when FVIII is mixed with OVA, it acts to increase the immune response to OVA. Finally, administration of thrombin with OVA is sufficient to induce immune responses to OVA. Overall, these data support the hypothesis that formation of thrombin through the procoagulant activity of FVIII is necessary to induce costimulation for the immune response to FVIII treatment.

The impact of antigen expression in antigen-presenting cells on humoral immune responses against the transgene product

Treatment of genetic diseases by gene therapy is hampered by immune responses against the transgene product. Promoter choice has been shown to be an important parameter of the presence or absence of antibodies against the transgene product after gene transfer. Here, the generality of some of these observations was tested by comparing different murine strains and different transgene products. We show immunological unresponsiveness for human apolipoprotein (apo) A-I in six murine strains after transfer with E1E3E4-deleted adenoviral vectors containing hepatocyte-specific expression cassettes. However, differences in the induction of a humoral immune response against human apo A-I after gene transfer with vectors driven by the major histocompatibility complex class II Ebeta promoter and the ubiquitously active cytomegalovirus promoter were not consistent in these six murine strains. Furthermore, use of a potent hepatocyte-specific expression cassette did not prevent a humoral immune response against human plasminogen in C57BL/6 mice. In contrast, human microplasminogen transfer resulted in stable expression in the absence of an immune response against the transgene product. Taken together, the molecular design of strategies to abrogate or induce an immune response against the transgene product may be hampered by the multitude of parameters affecting the outcome, thus limiting the external validity of results.

Suppression of FVIII inhibitor formation in hemophilic mice by delivery of transgene modified apoptotic fibroblasts

The development of inhibitory antibodies to factor VIII (FVIII) is currently the most significant complication of FVIII replacement therapy in the management of patients with severe hemophilia A. Immune tolerance protocols for the eradication of inhibitors require daily delivery of intravenous FVIII for at least 6 months and are unsuccessful in 20-40% of treated patients. We hypothesize that tolerance can be induced more efficiently and reliably by delivery of FVIII antigen within autologous apoptotic cells (ACs). In this study, we demonstrated suppression of the T cell and inhibitor responses to FVIII by infusion of FVIII expression vector modified apoptotic syngeneic fibroblasts in both naive and preimmunized hemophilia A mice. ACs without FVIII antigen exerted modest generalized immune suppression mediated by anti-inflammatory signals. However, FVIII expressing apoptotic syngeneic fibroblasts produced much stronger antigen-specific immune suppression. Mice treated with these fibroblasts generated CD4+ T cells that suppressed the immune response to FVIII after adoptive transfer into naive recipients and antigen-specific CD4+CD25+ regulatory T cells (Tregs) that inhibited the proliferation of FVIII responsive effector T cells in vitro. These preclinical results demonstrate the potential for using FVIII vector modified autologous ACs to treat high-titer inhibitors in patients with hemophilia A.

A murine model for induction of long-term immunologic tolerance to factor VIII does not require persistent detectable levels of plasma factor VIII and involves contributions from Foxp3+ T regulatory cells

Under certain instances, factor VIII (FVIII) stimulates an immune response, and the resulting neutralizing antibodies present a significant clinical challenge. Immunotherapies to re-establish or induce long-term tolerance would be beneficial, and an in-depth knowledge of mechanisms involved in tolerance induction is essential to develop immune-modulating strategies. We have developed a murine model system for studying mechanisms involved in induction of immunologic tolerance to FVIII in hemophilia A mice. We used lentiviral vectors to deliver the canine FVIII transgene to neonatal hemophilic mice and demonstrated that induction of long-term FVIII tolerance could be achieved. Hemophilia A mice are capable of mounting a robust immune response to FVIII after neonatal gene transfer, and tolerance induction is dependent on the route of delivery and type of promoter used. High-level expression of FVIII was not required for tolerance induction and, indeed, tolerance developed in some animals without evidence of detectable plasma FVIII. Tolerance to FVIII could be adoptively transferred to naive hemophilia recipient mice, and FVIII-stimulated splenocytes isolated from tolerized mice expressed increased levels of interleukin-10 and decreased levels of interleukin-6 and interferon-gamma. Finally, induction of FVIII tolerance mediated by this protocol is associated with a FVIII-expandable population of CD4(+)CD25(+)Foxp3(+) regulatory T cells.

Nanocapsule-delivered Sleeping Beauty mediates therapeutic Factor VIII expression in liver sinusoidal endothelial cells of hemophilia A mice

Liver sinusoidal endothelial cells are a major endogenous source of Factor VIII (FVIII), lack of which causes the human congenital bleeding disorder hemophilia A. Despite extensive efforts, gene therapy using viral vectors has shown little success in clinical hemophilia trials. Here we achieved cell type-specific gene targeting using hyaluronan- and asialoorosomucoid-coated nanocapsules, generated using dispersion atomization, to direct genes to liver sinusoidal endothelial cells and hepatocytes, respectively. To highlight the therapeutic potential of this approach, we encapsulated Sleeping Beauty transposon expressing the B domain-deleted canine FVIII in cis with Sleeping Beauty transposase in hyaluronan nanocapsules and injected them intravenously into hemophilia A mice. The treated mice exhibited activated partial thromboplastin times that were comparable to those of wild-type mice at 5 and 50 weeks and substantially shorter than those of untreated controls at the same time points. Further, plasma FVIII activity in the treated hemophilia A mice was nearly identical to that in wild-type mice through 50 weeks, while untreated hemophilia A mice exhibited no detectable FVIII activity. Thus, Sleeping Beauty transposon targeted to liver sinusoidal endothelial cells provided long-term expression of FVIII, without apparent antibody formation, and improved the phenotype of hemophilia A mice.

Correction of murine hemophilia A following nonmyeloablative transplantation of hematopoietic stem cells engineered to encode an enhanced human factor VIII variant using a safety-augmented retroviral vector

Insertional mutagenesis by retroviral vectors is a major impediment to the clinical application of hematopoietic stem cell gene transfer for the treatment of hematologic disorders. We recently developed an insulated self-inactivating gammaretroviral vector, RMSinOFB, which uses a novel enhancer-blocking element that significantly decreases genotoxicity of retroviral integration. In this study, we used the RMSinOFB vector to evaluate the efficacy of a newly bioengineered factor VIII (fVIII) variant (efVIII)--containing a combination of A1 domain point mutations (L303E/F309S) and an extended partial B domain for improved secretion plus A2 domain mutations (R484A/R489A/P492A) for reduced immunogenicity--toward successful treatment of murine hemophilia A. In cell lines, efVIII was secreted at up to 6-fold higher levels than an L303E/F309S A1 domain-only fVIII variant (sfVIIIDeltaB). Most important, when compared with a conventional gammaretroviral vector expressing sfVIIIDeltaB, lower doses of RMSin-efVIII-OFB-transduced hematopoietic stem cells were needed to generate comparable curative fVIII levels in hemophilia A BALB/c mice after reduced-intensity total body irradiation or nonmyeloablative chemotherapy conditioning regimens. These data suggest that the safety-augmented RMSin-efVIII-OFB platform represents an encouraging step in the development of a clinically appropriate gene addition therapy for hemophilia A.

Transient blockade of the inducible costimulator pathway generates long-term tolerance to factor VIII after nonviral gene transfer into hemophilia A mice

Formation of inhibitory antibodies is a common problem encountered in clinical treatment for hemophilia. Human factor VIII (hFVIII) plasmid gene therapy in hemophilia A mice also leads to strong humoral responses. We demonstrate that short-term therapy with an anti-ICOS monoclonal antibody to transiently block the inducible costimulator/inducible costimulator ligand (ICOS/ICOSL) signaling pathway led to sustained tolerance to hFVIII in hFVIII plasmid-treated hemophilia A mice and allowed persistent, high-level FVIII functional activity (100%-300% of normal). Anti-ICOS treatment resulted in depletion of ICOS(+)CD4(+) T cells and activation of CD25(+)Foxp3(+) Tregs in the peripheral blood, spleen, and lymph nodes. CD4(+) T cells from anti-ICOS-treated mice did not proliferate in response to hFVIII stimulation and produced high levels of regulatory cytokines, including interleukin-10 and transforming growth factor-beta. Moreover, CD4(+)CD25(+) Tregs from tolerized mice adoptively transferred dominant tolerance in syngeneic hFVIII plasmid-treated hemophilia A mice and reduced the production of antibodies against FVIII. Anti-ICOS-treated mice tolerized to hFVIII generated normal primary and secondary antibody responses after immunization with the T-dependent antigen, bacteriophage Phix 174, indicating maintenance of immune competency. Our data indicate that transient anti-ICOS monoclonal antibody treatment represents a novel single-agent immunomodulatory strategy to overcome the immune responses against transgene product after gene therapy.

Antigen-specific tolerance of human alpha1-antitrypsin induced by helper-dependent adenovirus

As efficient and less toxic virus-derived gene therapy vectors are developed, a pressing problem is to avoid immune response to the therapeutic gene product. Secreted therapeutic proteins potentially represent a special problem, as they are readily available to professional antigen-presenting cells throughout the body. Some studies suggest that immunity to serum proteins can be avoided in some mouse strains by using tissue-specific promoters. Here we show that expression of human alpha1-antitrypsin (AAT) was nonimmunogenic in the immune-responsive strain C3H/HeJ, when expressed from helper-dependent (HD) vectors using ubiquitous as well as tissue-specific promoters. Coadministration of less immunogenic HD vectors with an immunogenic first-generation vector failed to immunize, suggesting immune suppression rather than immune stealth. Indeed, mice primed with HD vectors were tolerant to immune challenge with hAAT emulsified in complete Freund's adjuvant. Such animals developed high-titer antibodies to coemulsified human serum albumin, showing that tolerance was antigen specific. AAT-specific T cell responses were depressed in tolerized animals, suggesting that tolerance affects both T and B cells. These results are consistent with models of high-dose tolerance of B cells and certain other suppressive mechanisms, and suggest that a high level of expression from HD vectors can be sufficient to induce specific immune tolerance to serum proteins.

In vivo models of haemophilia - status on current knowledge of clinical phenotypes and therapeutic interventions

Animal models have contributed immensely to the understanding of and the improvement in treatment of haemophilia A and B. First, establishment of haemophilic dog colonies provided an invaluable opportunity to investigate the diseases and later, the advances in gene technologies resulting in small haemophilic animal models were a milestone in the preclinical research making it possible to address some of the many unanswered questions. This review provides an overview of animal models used in the study of haemophilia as well as a short overview of the contributions resulting from studies in these models.

Transgene expression levels and kinetics determine risk of humoral immune response modeled in factor IX knockout and missense mutant mice

Immune responses leading to antibody-mediated elimination of the transgenic protein are a concern in gene replacement for congenital protein deficiencies, for which hemophilia is an important model. Although most hemophilia B patients have circulating non-functional but immunologically crossreactive factor IX (FIX) protein (CRM+ phenotype), inciting factors for FIX neutralizing antibody (inhibitor) development have been studied in crossreactive material-negative (CRM-) animal models. For this study, determinants of FIX inhibitor development were compared in hemophilia B mice, in which circulating FIX protein is absent (CRM- factor IX knockout (FIXKO) model) or present (CRM+ missense R333Q-hFIX model) modeling multiple potential therapies. The investigations compare for the first time different serotypes of adeno-associated virus (AAV) vectors (AAV2 and AAV1), each at multiple doses, in the setting of two different FIX mutations. The comparisons demonstrate in the FIXKO background (CRM- phenotype) that neither vector serotype nor vector particle number independently determine the inhibitor trigger, which is influenced primarily by the level and kinetics of transgene expression. In the CRM+ missense background, inhibitor development was never stimulated by AAV gene therapy or protein therapy, despite the persistence of lymphocytes capable of responding to FIX with non-inhibitory antibodies. This genotype/phenotype is strongly protective against antibody formation in response to FIX therapy.

Induction of partial immune tolerance to factor VIII through prior mucosal exposure to the factor VIII C2 domain

BACKGROUND

The development of anti-factor VIII (FVIII) neutralizing antibodies (inhibitors) is a significant obstacle to FVIII replacement therapy.

OBJECTIVE

As mucosal administration of an antigen may induce immune tolerance we have evaluated the efficacy of mucosal antigen exposure to achieve tolerance to FVIII.

METHODS

We investigated the effects of oral and nasal administration of the purified FVIII C2 domain (FVIII-C2) to FVIII-deficient BALB/c mice prior to FVIII protein challenge. Mice received oral or nasal doses of FVIII-C2, followed by a subcutaneous challenge of either FVIII-C2 or FVIII. The development of anti-FVIII inhibitors, cytokine production by splenocytes in vitro, and adoptive transfer assays were analyzed.

RESULTS AND CONCLUSIONS

Mucosal administration of FVIII-C2 decreases the titer of anti-FVIII-C2 inhibitors after FVIII-C2 challenge, and decreases the percentage of FVIII-C2 specific antibodies after challenge with full-length FVIII. Tolerance induction to FVIII-C2 is associated with increased IL-10 production by splenocytes in vitro, and can be adoptively transferred to naïve mice. This study is the first to demonstrate that tolerance to the FVIII-C2 domain can be induced via the mucosal route. Based on these results, the potential use of FVIII-specific mucosal tolerance induction as an immunotherapy treatment for anti-FVIII inhibitor development warrants further investigation.

Persistent expression of factor VIII in vivo following nonprimate lentiviral gene transfer

Hemophilia A is a clinically important coagulation disorder caused by the lack or abnormality of plasma coagulation factor VIII (FVIII). Gene transfer of the FVIII cDNA to hepatocytes using lentiviral vectors is a potential therapeutic approach. We investigated the efficacy of feline immunodeficiency virus (FIV)-based vectors in targeting hepatocytes and correcting FVIII deficiency in a hemophilia A mouse model. Several viral envelope glycoproteins were screened for efficient FIV vector pseudotyping and hepatocyte transduction. The GP64 glycoprotein from baculovirus Autographa californica multinuclear polyhedrosis virus pseudo-typed FIV efficiently and showed excellent hepatocyte tropism. The GP64-pseudotyped vector was stable in the presence of human or mouse complement. Inclusion of a hybrid liver-specific promoter (murine albumin enhancer/human alpha1-antitrypsin promoter) further enhanced transgene expression in hepatocytes. We generated a GP64-pseudotyped FIV vector encoding the B domain-deleted human FVIII coding region driven by the liver-specific promoter, with 2 beneficial point mutations in the A1 domain. Intravenous vector administration conferred sustained FVIII expression in hemophilia A mice for several months without the generation of anti-human FVIII antibodies and resulted in partial phenotypic correction. These findings demonstrate the utility of GP64-pseudotyped FIV lentiviral vectors for targeting hepatocytes to correct disorders associated with deficiencies of secreted proteins.