Intravenous administration of an E1/E3-deleted adenoviral vector induces tolerance to factor IX in C57BL/6 mice

Toward Personalized Gene Therapy: Characterizing the Host Genetic Control of Lentiviral-Vector-Mediated Hepatic Gene Delivery

The success of lentiviral vectors in curing fatal genetic and acquired diseases has opened a new era in human gene therapy. However, variability in the efficacy and safety of this therapeutic approach has been reported in human patients. Consequently, lentiviral-vector-based gene therapy is limited to incurable human diseases, with little understanding of the underlying causes of adverse effects and poor efficacy. To assess the role that host genetic variation has on efficacy of gene therapy, we characterized lentiviral-vector gene therapy within a set of 12 collaborative cross mouse strains. Lentiviral vectors carrying the firefly luciferase cDNA under the control of a liver-specific promoter were administered to female mice, with total-body and hepatic luciferase expression periodically monitored through 41 weeks post-vector administration. Vector copy number per diploid genome in mouse liver and spleen was determined at the end of this study. We identified major strain-specific contributions to overall success of transduction, vector biodistribution, maximum luciferase expression, and the kinetics of luciferase expression throughout the study. Our results highlight the importance of genetic variation on gene-therapeutic efficacy; provide new models with which to more rigorously assess gene therapy approaches; and suggest that redesigning preclinical studies of gene-therapy methodologies might be appropriate.

A Recombinant Chimeric Ad5/3 Vector Expressing a Multistage Plasmodium Antigen Induces Protective Immunity in Mice Using Heterologous Prime-Boost Immunization Regimens

An ideal malaria vaccine should target several stages of the parasite life cycle and induce antiparasite and antidisease immunity. We have reported a Plasmodium yoelii chimeric multistage recombinant protein (P. yoelii linear peptide chimera/recombinant modular chimera), engineered to express several autologous T cell epitopes and sequences derived from the circumsporozoite protein and the merozoite surface protein 1. This chimeric protein elicits protective immunity, mediated by CD4(+) T cells and neutralizing Abs. However, experimental evidence, from pre-erythrocytic vaccine candidates and irradiated sporozoites, has shown that CD8(+) T cells play a significant role in protection. Recombinant viral vectors have been used as a vaccine platform to elicit effective CD8(+) T cell responses. The human adenovirus (Ad) serotype 5 has been tested in malaria vaccine clinical trials with excellent safety profile. Nevertheless, a major concern for the use of Ad5 is the high prevalence of anti-vector neutralizing Abs in humans, hampering its immunogenicity. To minimize the impact of anti-vector pre-existing immunity, we developed a chimeric Ad5/3 vector in which the knob region of Ad5 was replaced with that of Ad3, conferring partial resistance to anti-Ad5 neutralizing Abs. Furthermore, we implemented heterologous Ad/protein immunization regimens that include a single immunization with recombinant Ad vectors. Our data show that immunization with the recombinant Ad5/3 vector induces protective efficacy indistinguishable from that elicited by Ad5. Our study also demonstrates that the dose of the Ad vectors has an impact on the memory profile and protective efficacy. The results support further studies with Ad5/3 for malaria vaccine development.

Role of antigen-specific regulatory CD4+CD25+ T cells in tolerance induction after neonatal IP administration of AAV-hF.IX

Neonatal AAV8-mediated Factor IX (F.IX) gene delivery was applied as a model for exploring mechanisms of tolerance induction during immune ontogeny. Intraperitoneal delivery of AAV8/ Factor IX (hF.IX) during weeks 1–4 of life, over a 20-fold dose range, directed stable hF.IX expression, correction of coagulopathy in F.IX-null hemophilia B mice, and induction of tolerance to hF.IX; however, only primary injection at 1–2 days of life enabled increasing AAV8-mediated hF.IX expression after re-administration, due to the absence of anti-viral capsid antibodies. Adoptive splenocyte transfer from tolerized mice demonstrated induction of CD4⁺CD25⁺ T regulatory (T_reg) populations that specifically suppressed anti-hF.IX antibody responses, but not responses to third party antigen. Induction of hF.IX antibodies was only observed in tolerized mice after in vivo CD4⁺CD25⁺ cell depletion and hF.IX challenge. Thus, primary injection of AAV during a critical period in the first week of life does not elicit antiviral responses, enabling re-administration of AAV and augmentation of hF.IX levels. Expansion of hF.IX-specific CD4⁺CD25⁺ T_regs has a major role in tolerance induction early in immune ontogeny. Neonatal gene transfer provides a useful approach for defining the ontogeny of immune responses and may suggest approaches for inducing tolerance in the context of genetic therapies.

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.

miRNA-mediated silencing in hepatocytes can increase adaptive immune responses to adenovirus vector-delivered transgenic antigens

Adenovirus vectors based on human serotype 5 can induce potent CD8 T cell responses to vector-encoded transgenic antigens. However, the individual contribution of different cell types expressing antigen upon adenovirus vector injection to the generation of antigen-directed adaptive immune responses is poorly understood so far. We investigated the role of hepatocytes, skeletal muscle, and hematopoietic cells for the induction of cellular and humoral immune responses by miRNA-mediated tissue-specific silencing of antigen expression. Using hepatitis B small surface antigen (HBsAg) as the vector-encoded transgene we show that adenovirus vector dissemination from an intramuscular (i.m.) injection site into the liver followed by HBsAg expression in hepatocytes can limit early priming of CD8 T cells and the generation of anti-HBsAg antibody responses. However, hepatocyte-specific miRNA122a-mediated silencing of HBsAg expression overcame these limitations. Early clonal expansion of K(b)/S(190-197)-specific CD8 T cells was significantly enhanced and improved polyfunctionality of CD8 T cells was found. Furthermore, miRNA122a-mediated antigen silencing induced significantly higher anti-HBsAg antibody titers allowing an up to 100-fold vector dose reduction. These results indicate that miRNA-mediated regulation of antigen expression in the context of adenovirus vectors can significantly improve transgene product-directed immune responses. This finding could be of interest for future adenovirus vaccine vector development.

Nonredundant roles of IL-10 and TGF-β in suppression of immune responses to hepatic AAV-factor IX gene transfer

Hepatic gene transfer using adeno-associated viral (AAV) vectors has been shown to efficiently induce immunological tolerance to a variety of proteins. Regulatory T-cells (Treg) induced by this route suppress humoral and cellular immune responses against the transgene product. In this study, we examined the roles of immune suppressive cytokines interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) in the development of tolerance to human coagulation factor IX (hF.IX). Interestingly, IL-10 deficient C57BL/6 mice receiving gene transfer remained tolerant to hF.IX and generated Treg that suppressed anti-hF.IX formation. Effects of TGF-β blockade were also minor in this strain. In contrast, in C3H/HeJ mice, a strain known to have stronger T-cell responses against hF.IX, IL-10 was specifically required for the suppression of CD8(+) T-cell infiltration of the liver. Furthermore, TGF-β was critical for tipping the balance toward an regulatory immune response. TGF-β was required for CD4(+)CD25(+)FoxP3(+) Treg induction, which was necessary for suppression of effector CD4(+) and CD8(+) T-cell responses as well as antibody formation. These results demonstrate the crucial, nonredundant roles of IL-10 and TGF-β in prevention of immune responses against AAV-F.IX-transduced hepatocytes.

BALB/c mice show impaired hepatic tolerogenic response following AAV gene transfer to the liver

Following adeno-associated virus (AAV) gene transfer to the liver, both C57BL/6 and BALB/c mice show long-term expression of nonself transgene antigens along with the absence of a transgene-specific immune response. However, in this study, we report that despite the equal ability to induce T-cell tolerance to vector-encoded antigens, the underlying mechanisms are entirely different in these two strains. We have previously shown that in C57BL/6 mice, cytotoxic T lymphocyte (CTL) responses to systemic AAV-delivered antigens are suppressed by combined actions of hepatic regulatory T cells (Tregs), Kupffer cells, and hepatic suppressive cytokines. In stark contrast, our present findings reveal that such tolerogenic response is not induced in the liver of BALB/c mice systemically administered with AAV. As a result, these mice fail to suppress a transgene-specific CTL response induced by a strong immunogenic challenge and express dramatically reduced levels of AAV-encoded antigen. Interestingly, there was active B-cell tolerance to the transgene antigen, which was mediated by splenic Tregs. We conclude that lack of tolerance induction in the liver renders BALB/c mice susceptible to CTL-mediated clearance of transduced hepatocytes.

Induction of immune tolerance to FIX by intramuscular AAV gene transfer is independent of the activation status of dendritic cells

The nature of viral vectors is suggested to be a significant contributor to undesirable immune responses subsequent to gene transfer. Such viral vectors, recognized as danger signals by the host immune system, activate dendritic cells (DCs), causing unwanted antivector and/or transgene product immunity. We recently reported efficient induction of immune tolerance to coagulation factor IX (FIX) by direct intramuscular injection of adeno-associated virus (AAV)-FIX. AAV vectors are nonpathogenic and elicit minimal inflammatory response. We hypothesized that the nonpathogenic nature of AAV plays a critical role in induction of tolerance after AAV gene transfer. We observed inefficient recruitment and activation of DCs subsequent to intramuscular injection of AAV. To further validate our hypothesis, we examined immune responses to FIX after intramuscular injection of AAV with simultaneous activation of DCs. We were able to achieve phenotypic and functional activation of DCs after administration of lipopolysaccharide and anti-CD40 antibody. However, we observed efficient induction of FIX tolerance irrespective of DC activation in mice with different genetic and major histocompatibility complex backgrounds. Furthermore, activation of DCs did not exaggerate the immune response induced after intramuscular injection of AAV serotype 2 vector. Our results demonstrate that induction of FIX tolerance after AAV gene transfer is independent of DC activation status.

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.

Muscle as a target for supplementary factor IX gene transfer

Immune responses to the factor IX (F.IX) transgene product are a concern in gene therapy for the X-linked bleeding disorder hemophilia B. The risk for such responses is determined by several factors, including the vector, target tissue, and others. Previously, we have demonstrated that hepatic gene transfer with adeno-associated viral (AAV) vectors can induce F.IX-specific immune tolerance. Muscle-derived F.IX expression, however, is limited by a local immune response. Here, skeletal muscle was investigated as a target for supplemental gene transfer. Given the low invasiveness of intramuscular injections, this route would be ideal for secondary gene transfer, thereby boosting levels of transgene expression. However, this is feasible only if immune tolerance established by compartmentalization of expression to the liver extends to other sites. Immune tolerance to human F.IX established by prior hepatic AAV-2 gene transfer was maintained after subsequent injection of AAV-1 or adenoviral vector into skeletal muscle, and tolerized mice failed to form antibodies or an interferon (IFN)-gamma(+) T cell response to human F.IX. A sustained increase in systemic transgene expression was obtained for AAV-1, whereas an increase after adenoviral gene transfer was transient. A CD8(+) T cell response specifically against adenovirus-transduced fibers was observed, suggesting that cytotoxic T cell responses against viral antigens were sufficient to eliminate expression in muscle. In summary, the data demonstrate that supplemental F.IX gene transfer to skeletal muscle does not break tolerance achieved by liver-derived expression. The approach is efficacious, if the vector for muscle gene transfer does not express immunogenic viral proteins.

Expression of pIX Gene Induced by Transgene Promoter: Possible Cause of Host Immune Response in First-Generation Adenoviral Vectors

First-generation (FG) adenoviral vectors (AdVs) have been widely used not only for gene therapy but also for basic studies. Because vectors of this type lack the E1A gene that is essential for the expression of other viral genes, their expression levels in target cells have been considered low. However, we found that the viral pIX gene, located immediately downstream of the inserted expression unit of the transgene, was significantly coexpressed with the transgene in cells infected with FG AdV. Whereas CAG and SRalpha promoters activated the pIX promoter considerably through their enhancer effects, the EF1alpha promoter hardly did. Moreover, when the expression unit was inserted in the rightward orientation, not only the pIX protein but also a fusion protein consisting of the N-terminal part of transgene product and pIX were sometimes coexpressed with the transgene product through an aberrant splicing mechanism. In in vivo experiments, a LacZ-expressing AdV bearing the CAG promoter caused an elevation of alanine aminotransferase, but an AdV bearing the EF1alpha promoter produced no detectable levels. Whereas the FG AdV expressing human growth hormone under the control of the CAG promoter maintained a high hormone level for less than 1 month, the FG AdV under the control of the EF1alpha promoter maintained a high level for at least 6 months. These results suggest that pIX coexpression may be one of the main causes of AdV-induced immune responses, and that the EF1alpha promoter is probably valuable for the long-term expression of FG AdV. Thus, the in vivo utility of FG AdV should be reevaluated.

Sustained phenotypic correction in a mouse model of hypoalphalipoproteinemia with a helper-dependent adenovirus vector

We examined the efficacy and host response to the adenovirus (Ad)-mediated delivery of human apolipoprotein A-I (APOA1) gene to the liver of APOA1(-/-) mice. Administration of a first-generation vector (FGAd-AI) resulted in a transient appearance of APOA1 in plasma and induced an anti-APOA1 antibody titer, whereas treatment with a helper-dependent vector (HDAd-AI) resulted in sustained APOA1 expression without inducing an antibody titer. With these results, we studied the effects of FGAd vectors on APOAI expression by HDAd-AI vector. Co-treatment with an FGAd vector inhibited HDAd-AI- mediated APOA1 expression independent of transgene cassettes, but only FGAd-AI induced a humoral response. Furthermore, APOA1 mRNA levels in mice co-treated with FGAd vectors were much lower than those expected from the vector copy number, suggesting that DNA of FGAd vectors interferes with the HDAd-AI vector's APOA1 promoter. A single treatment with an HDAd-AI vector produced a supraphysiological plasma APOA1 level that gradually declined to about half the normal human level over the course of 2 years, associated with a plasma cholesterol level that is persistently higher than that in controls. This investigation provides the proof of principle that liver-directed HDAd gene delivery is effective for the long-term phenotypic correction of monogenic hypoalphalipoproteinemia.

Effect of tolerance induction to immunodominant T-cell epitopes of Sendai virus on gene expression following repeat administration to lung

Sendai virus (SeV) is able to transfect airway epithelial cells efficiently in vivo. However, as with other viral vectors, repeated administration leads to reduced gene expression. We have investigated the impact of inducing immunological tolerance to immunodominant T-cell epitopes on gene expression following repeated administration. Immunodominant CD4 and CD8 T-cell peptide epitopes of SeV were administered to C57BL/6 mice intranasally 10 days before the first virus administration with transmission-incompetent F-protein-deleted DeltaF/SeV-GFP. At 21 days after the first virus administration, mice were again transfected with DeltaF/SeV. To avoid interference of anti-GFP antibodies, the second transfection was carried out with DeltaF/SeV-lacZ. At 2 days after the final transfection lung beta-galactosidase expression, T-cell proliferation and antibody responses were measured. A state of 'split tolerance' was achieved with reduced T-cell proliferation, but no impact on antiviral antibody production. There was no enhancement of expression on repeat administration; instead, T-cell tolerance was, paradoxically, associated with a more profound extinction of viral expression. Multiple immune mechanisms operate to eradicate viruses from the lung, and these findings indicate that impeding the adaptive T-cell response to the immunodominant viral epitope is not sufficient to prevent the process.

Fully deleted adenovirus persistently expressing GAA accomplishes long-term skeletal muscle glycogen correction in tolerant and nontolerant GSD-II mice

Glycogen storage disease type II (GSD-II) patients manifest symptoms of muscular dystrophy secondary to abnormal glycogen storage in cardiac and skeletal muscles. For GSD-II, we hypothesized that a fully deleted adenovirus (FDAd) vector expressing hGAA via nonviral regulatory elements (PEPCK promoter/ApoE enhancer) would facilitate long-term efficacy and decrease propensity to generate anti-hGAA antibody responses against hepatically secreted hGAA. Intravenous delivery of FDAdhGAA into GAA-tolerant or nontolerant GAA-KO mice resulted in long-term hepatic secretion of hGAA. Specifically, nontolerant mice achieved complete reversal of cardiac glycogen storage and near-complete skeletal glycogen correction for at least 180 days and tolerant mice for minimally 300 days coupled with the preservation of muscle strength. Anti-hGAA antibody levels in both mouse strains were significantly less relative to those previously generated by CMV-driven hGAA expression in nontolerant GAA-KO mice. However, plasma GAA levels decreased in nontolerant GAA-KO mice despite long-term intrahepatic GAA expression from the persistent vector. This intriguing result is discussed in light of other examples of "tolerance" induction by gene-transfer-based approaches.

Permanent partial phenotypic correction and tolerance in a mouse model of hemophilia B by stem cell gene delivery of human factor IX

Immune responses against an introduced transgenic protein are a potential risk in many gene replacement strategies to treat genetic disease. We have developed a gene delivery approach for hemophilia B based on lentiviral expression of human factor IX in purified hematopoietic stem cells. In both normal C57Bl/6J and hemophilic 129/Sv recipient mice, we observed the production of therapeutic levels of human factor IX, persisting for at least a year with tolerance to human factor IX antigen. Secondary and tertiary recipients also demonstrate long-term production of therapeutic levels of human factor IX and tolerance, even at very low levels of donor chimerism. Furthermore, in hemophilic mice, partial functional correction of treated mice and phenotypic rescue is achieved. These data show the potential of a stem cell approach to gene delivery to tolerize recipients to a secreted foreign transgenic protein and, with appropriate modification, may be of use in developing treatments for other genetic disorders.

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.

Rapid Functional Exhaustion and Deletion of CTL following Immunization with Recombinant Adenovirus

Replication-deficient adenoviruses (recombinant adenovirus (rec-AdV)) expressing different transgenes are widely used vectors for gene therapy and vaccination. In this study, we describe the tolerization of transgene-specific CTL following administration of beta-galactosidase (beta gal)-recombinant adenovirus (Ad-LacZ). Using MHC class I tetramers to track beta gal-specific CTL, we found that a significant expansion of beta gal-specific CTL was restricted to a very narrow dose range. Functional analysis revealed that adenovirus-induced beta gal-specific CTL produced only very low amounts of effector cytokines and were unable to exhibit cytolytic activity in a 51Cr release assay. Furthermore, Ad-LacZ vaccination failed to efficiently clear established beta gal-positive tumors. The impaired function of Ad-LacZ-induced CTL correlated with the presence of persisting beta gal Ag in the liver. A further increase in the peripheral Ag load by injection of Ad-LacZ into SM-LacZ transgenic mice which express beta gal as self-Ag exclusively in peripheral nonlymphoid organs, resulted in the physical deletion of beta gal-specific CTL. Our results indicate first that CTL deletion in the course of adenoviral vaccination is preceded by their functional impairment and second, that the outcome of rec-AdV vaccination depends critically on the Ag load in peripheral tissues.

Prospects and problems of gene therapy: an update

The gene therapy approach can vary from delivering extra copies of a gene, through modifications of a genome using the properties of ribozymes or chimeraplasts, to injection of modified cells. For the treatment of genetic deficits the ultimate goal would be the repair of the mutated gene in the target tissue(s). The techniques required for such an approach are emerging, albeit slowly. Therefore, delivery of an extra copy of a normal gene in a specific vector remains the predominant approach. Moreover, this method finds wider applications in gene therapy relating to disorders other than heritable defects, e.g., malignancies, cardiovascular diseases and infections. The major and most intensive areas of research are: i) vectors and delivery methods, ii) regulation of transgene expression and iii) stability of expression. Targeting of the therapeutic gene is being accomplished by using viral vectors or non-viral delivery systems, either ex vivo or in vivo. The choice of vectors and delivery routes depends on the nature of the target cells and the required levels and stability of expression. Although there have been the first positive clinical results and significant technical achievements over the past 2 years, there are still obstacles to the development of effective clinical products and these remain largely unchanged. The most important barriers are the low levels and stability of expression and immune responses to vectors and/or gene products. The safety aspects of gene therapy have become painfully evident with the first death conclusively linked to gene therapy. The progress in AAV and lentiviral vectors, improved regulation of transgene expression and advances in stem cell technology are among the recent most exciting developments.

Creation of a mouse expressing defective human factor IX

The majority of cases of human hemophilia B are the result of missense mutations in the coagulation factor IX gene and defective circulating factor IX is detectable in most patients. The available mouse factor IX knockout models of hemophilia B (FIXKO mouse) reproduce the bleeding phenotype of human hemophilia B, but because the models produce no factor IX they fail to reproduce the dominant human phenotype. We have created a human factor IX mouse model of hemophilia B (R333Q-hFIX mouse) by homologous recombination in embryonic stem cells. The mouse expresses no mouse factor IX, but instead expresses a missense mutant human factor IX from the mouse FIX promoter. Mutant human factor IX mRNA transcript and circulating human factor IX are detectable throughout development, but factor IX activity is less than 1% and the mouse exhibits the hemophilic phenotype. When R333Q-hFIX mice were challenged by intramuscular injection of adeno-associated virus expressing human factor IX, factor IX expression without the development of antibodies was observed. In contrast, given the same treatment, FIXKO mice consistently develop antibodies. Our R333Q-hFIX mice strain will complement the FIXKO mice for studying factor IX circulating kinetics and gene therapy. (Blood. 2004;104:1733-1739)

Factors influencing therapeutic efficacy and the host immune response to helper-dependent adenoviral gene therapy in hemophilia A mice

BACKGROUND

Adenoviral-based methods of gene therapy have been ineffective at providing sustained factor (F)VIII expression in outbred populations of large animal hemophilic models primarily due to the immunogenicity of these vectors. Improvements have been made in vector design leading to the development of the helper-dependent adenoviral (HD) system. Unfortunately, it remains unclear whether these modifications are sufficient to circumvent the induction of inhibitor formation associated with adenoviral gene transfer.

OBJECTIVE

To develop an HD vector capable of mediating sustained FVIII expression and to determine the variables that influence inhibitor development.

METHODS

HD vectors were constructed encoding the canine FVIII B-domain deleted transgene under the control of either the cytomegalovirus (CMV) promoter or a tissue-restricted hybrid element consisting of five HNF-1 binding sites, located upstream of the human FVIII proximal promoter. Inbred and outbred populations of hemophilic mice were treated, and monitored for vector-induced toxicity, therapeutic efficacy, and inhibitor formation.

RESULTS

When HD vectors utilizing the CMV promoter were administered, all hemophilic mice developed high levels of FVIII inhibitors. In contrast, vectors under the control of the HNF/FVIII element were capable of achieving sustained elevations of FVIII for over 6 months. Strain-specific differences were also observed, with outbred animals showing a greater propensity towards inhibitor development in response to treatment.

CONCLUSIONS

HD vectors can be used to provide long-term FVIII expression in hemophilic animals, but treatment outcome and the induction of inhibitors is dependent on a number of variables including the transgene promoter, the vector dose, and the genetic background of the host.

Neonatal gene transfer with a retroviral vector results in tolerance to human factor IX in mice and dogs

The effect of neonatal gene transfer on antibody formation was determined using a retroviral vector (RV) expressing human factor IX (hFIX). Normal mice from different strains injected intravenously with RV as newborns achieved therapeutic levels of hFIX without antibody production and were tolerant as adults to challenge with hFIX. Neonatal hemophilia B mice that received different amounts of RV achieved stable and dose-related expression of hFIX without anti-hFIX antibody formation. After protein challenge, antibody formation was markedly reduced for animals that expressed hFIX at levels higher than 14 ng/mL (0.3% of normal). However, antibodies developed for animals that received the lowest dose of RV and expressed hFIX at approximately 2 ng/mL before protein challenge. In dogs, neonatal injection of a high dose of RV resulted in 500 ng/mL hFIX in plasma without antibody formation. We conclude that neonatal gene transfer with RV does not induce antibody responses to hFIX in mice or dogs and that mice achieving levels greater than 3 x 10-10 M hFIX are usually tolerant to protein injection as adults. Low-dose gene therapy or frequent protein injections in the neonatal period might induce tolerance to subsequent injections of protein with a low risk for adverse effects.

Targeting lentiviral vector expression to hepatocytes limits transgene-specific immune response and establishes long-term expression of human antihemophilic factor IX in mice

Stable gene replacement by in vivo administration of lentiviral vectors (LVs) has therapeutic potential for metabolic disorders and other systemic diseases. We studied the expression of intracellular and secreted proteins by LVs in immunocompetent mice. Liver, spleen, and bone marrow cells were efficiently transduced. However, transgene expression, driven by a ubiquitous promoter, was limited by transgene-specific cellular and humoral immune responses, leading to the clearance of transduced cells. After green fluorescent protein (GFP) gene transfer, the liver showed infiltration of CD8(+) cytotoxic T cells, and GFP-specific CD8(+) T cells were isolated from the spleen. After human factor IX (hF.IX) gene transfer, anti-hF.IX antibodies were induced. These immune responses were not detected in mice injected with heat-inactivated or genome-lacking LVs or in GFP-transgenic mice, indicating that they were specifically triggered by transgene expression in vivo. Intriguingly, selective targeting of LV expression to hepatocytes limited the immune responses to the transgenes. By this approach, high levels of hF.IX, potentially in the therapeutic range, were reached and maintained long term in immunocompetent mice, without inducing antibody formation. These results prompt further studies in relevant animal models to explore the potential of in vivo LV administration for the gene therapy of hemophilias and other liver-based diseases.

Immune responses to adeno-associated virus and its recombinant vectors

Recombinant adeno-associated virus (rAAV) vectors have emerged as highly promising for use in gene transfer for a variety of reasons, including lack of pathogenicity and wide host range. In addition, all virus-encoded genes have been removed from standard rAAV vectors, resulting in their comparatively low intrinsic immunogenicity. For gene replacement strategies, transgenes encoded by rAAV vectors may induce less robust host immune responses than other vectors in vivo. However, under appropriate conditions, host immune responses can be generated against rAAV-encoded transgenes, raising the potential for their use in vaccine development. In this review, we summarize current understanding of the generation of both undesirable and beneficial host immune responses directed against rAAV and encoded transgenes, and how they might be exploited for optimal use of this promising vector system.

Induction of immune tolerance to coagulation factor IX antigen by in vivo hepatic gene transfer

Gene replacement therapy is an attractive approach for treatment of genetic disease, but may be complicated by the risk of a neutralizing immune response to the therapeutic gene product. There are examples of humoral and cellular immune responses against the transgene product as well as absence of such responses, depending on vector design and the underlying mutation in the dysfunctional gene. It has been unclear, however, whether transgene expression can induce tolerance to the therapeutic antigen. Here, we demonstrate induction of immune tolerance to a secreted human coagulation factor IX (hF.IX) antigen by adeno-associated viral gene transfer to the liver. Tolerized mice showed absence of anti-hF.IX and substantially reduced in vitro T cell responses after immunization with hF.IX in adjuvant. Tolerance induction was antigen specific, affected a broad range of Th cell subsets, and was favored by higher levels of transgene expression as determined by promoter strength, vector dose, and mouse strain. Hepatocyte-derived hF.IX expression induced regulatory CD4(+) T cells that can suppress anti-hF.IX formation after adoptive transfer. With a strain-dependent rate of success, tolerance to murine F.IX was induced in mice with a large F.IX gene deletion, supporting the relevance of these data for treatment of hemophilia B and other genetic diseases.

In utero gene transfer of human factor IX to fetal mice can induce postnatal tolerance of the exogenous clotting factor

The fundamental hypotheses behind fetal gene therapy are that it may be possible (1) to achieve immune tolerance of transgene product and, perhaps, vector; (2) to target cells and tissues that are inaccessible in adult life; (3) to transduce a high percentage of rapidly proliferating cells, and in particular stem cells, with relatively low absolute virus doses leading to clonal transgene amplification by integrating vectors; and (4) to prevent early disease manifestation of genetic diseases. This study provides evidence vindicating the first hypothesis; namely, that intravascular prenatal administration of an adenoviral vector carrying the human factor IX (hFIX) transgene can induce immune tolerance of the transgenic protein. Following repeated hFIX protein injection into adult mice, after prenatal vector injection, we found persistence of blood hFIX and absence of hFIX antibodies in 5 of 9 mice. Furthermore, there was substantial hFIX expression after each of 2 reinjections of vector without detection of hFIX antibodies. In contrast, all adult mice that had not been treated prenatally showed a rapid loss of the injected hFIX and the development of high hFIX antibody levels, both clear manifestations of a strong immune reaction.

Hemophilia gene therapy: update

Gene transfer is an exciting and potentially important treatment approach for hemophilia A and B. Four phase I clinical trials of the safety of gene transfer in hemophilia A or B have been completed and two more trials are currently underway. The results of these trials indicate that gene transfer in hemophilia with the vectors and doses used is safe and well tolerated. Efforts continue to understand the basic biology and improve the efficiency of gene transfer.

Humoral immune response to recombinant adenovirus and adeno-associated virus after in utero administration of viral vectors in mice

Adenovirus and adeno-associated virus vector-mediated gene delivery is limited by the induction of a humoral immune response that prevents readministration. To determine whether viral delivery in the "preimmune" fetus would produce dose- or time-dependent tolerance, we evaluated the humoral immune response after sequential pre- and postnatal virus administration. We evaluated six injection route and viral dose combinations of adenovirus (intra-amniotic, intrahepatic, and intramuscular injection at 4 x 10(8) and 2 x 10(9) particles/fetus) at d 15 postconception (p.c.); three route and dose combinations at d 13 p.c. (intramuscular injection at 1 x 10(8), 3 x 10(8), and 5 x 10(8) particles/fetus); and one route and dose combination of adeno-associated virus (intramuscular at 2.5 x 10(10) genome copies/fetus) at d 15 p.c. In utero injection of either viral vector at any route and dose combination resulted in the production of low titers of neutralizing antivirus and antitransgene (beta-galactosidase) antibodies. This primary immune response only partially blocked transgene expression after the readministration of viral vectors postnatally. However, delivery of the virus postnatally triggered an immune response that completely blocked transgene expression after a third viral injection. Together, these results suggest that, for B6/129 F1 mice, in utero injection of recombinant adenovirus or adeno-associated virus between d 13 and 15 p.c. does not induce tolerance to the viral vector or transgene product.

Correction of the nonlinear dose response improves the viability of adenoviral vectors for gene therapy of Fabry disease

Systemic administration of recombinant adenoviral vectors for gene therapy of chronic diseases such as Fabry disease can be limited by dose-dependent toxicity. Because administration of a high dose of Ad2/CMVHI-alpha gal encoding human alpha-galactosidase A results in expression of supraphysiological levels of the enzyme, we sought to determine whether lower doses would suffice to correct the enzyme deficiency and lysosomal storage abnormality observed in Fabry mice. Reducing the dose of Ad2/CMVHI-alpha gal by 10-fold (from 10(11) to 10(10) particles/mouse) resulted in a greater than 200-fold loss in transgene expression. In Fabry mice, the reduced expression of alpha-galactosidase A, using the lower dose of Ad2/CMVHI-alpha gal, was associated with less than optimal clearance of the accumulated glycosphingolipid (GL-3) from the affected lysosomes. It was determined that this lack of linearity in dose response was not due to an inability to deliver the recombinant viral vectors to the liver but rather to sequestration, at least in part, of the viral vectors by the Kupffer cells. This lack of correlation between dose and expression levels could be obviated by supplementing the low dose of Ad2/CMVHI-alpha gal with an unrelated adenoviral vector or by depleting the Kupffer cells before administration of Ad2/CMVHI-alpha gal. Prior removal of the Kupffer cells, using clodronate liposomes, facilitated the use of a 100-fold lower dose of Ad2/CMVHI-alpha gal (10(9) particles/mouse) to effect the nearly complete clearance of GL-3 from the affected organs of Fabry mice. These results suggest that practical strategies that minimize the interaction between the recombinant adenoviral vectors and the reticuloendothelial system (RES) may improve the therapeutic window of this vector system. In this regard, we showed that pretreatment of mice with gamma globulins also resulted in significantly enhanced adenovirus-mediated transduction and expression of alpha-galactosidase A in the liver.

Transient blocking of both B7.1 (CD80) and B7.2 (CD86) in addition to CD40-CD40L interaction fully abrogates the immune response following systemic injection of adenovirus vector

Blockade of the CD40-CD40L and CD80/CD86-CD28 costimulatory pathways represents a strategy to inhibit the immune response against Ad vectors designed for gene therapy applications. Since most previous studies have used a CTLA4-Ig fusion molecule binding to both CD80 and CD86, the respective roles of these B7 molecules remained undefined. We have studied the effect of blocking monoclonal Abs (mAbs) directed against the costimulatory molecules CD40L, CD80 and CD86, alone or in different combinations, on the humoral and cellular immune responses against Ad. Groups of mice were transiently treated with each combination of blocking mAbs upon systemic injection of a first Ad vector. Combinations of anti-CD80 + anti-CD86 or anti-CD40L + anti-CD86 mAbs resulted in strong inhibition of the immune response against Ad. Using either of these mAb pairs, a second vector could be administered 1 month after the first injection but with lower efficiency than in naive animals. Thus, CD86 stands as the pivotal B7 molecule involved in the development of the immune response against Ad. However, only the blockade of both CD80 and CD86 in addition to CD40L fully inhibited the humoral and cellular responses against the Ad vector, such that readministration after 1 month was as efficient as in naive animals. At the time of readministration, treated animals had regained their ability to mount a normal immune response to the second Ad vector, showing that tolerance was not induced.

Sustained phenotypic correction of hemophilia B dogs with a factor IX null mutation by liver-directed gene therapy

Hemophilia B is an X-linked coagulopathy caused by absence of functional coagulation factor IX (FIX). Using adeno-associated virus (AAV)-mediated, liver-directed gene therapy, we achieved long-term (> 17 months) substantial correction of canine hemophilia B in 3 of 4 animals, including 2 dogs with an FIX null mutation. This was accomplished with a comparatively low dose of 1 x 10(12) vector genomes/kg. Canine FIX (cFIX) levels rose to 5% to 12% of normal, high enough to result in nearly complete phenotypic correction of the disease. Activated clotting times and whole blood clotting times were normalized, activated partial thromboplastin times were substantially reduced, and anti-cFIX was not detected. The fourth animal, also a null mutation dog, showed transient expression (4 weeks), but subsequently developed neutralizing anti-cFIX (inhibitor). Previous work in the canine null mutation model has invariably resulted in inhibitor formation following treatment by either gene or protein replacement therapies. This study demonstrates that hepatic AAV gene transfer can result in sustained therapeutic expression in a large animal model characterized by increased risk of a neutralizing anti-FIX response.

Sustained delivery of therapeutic concentrations of human clotting factor IX--a comparison of adenoviral and AAV vectors administered in utero

BACKGROUND

Prenatal somatic gene therapy has been considered for genetic disorders presenting with morbidity at birth. Haemophilia is associated with an increased risk of catastrophic perinatal bleeding complications such as intracranial haemorrhage, which could be prevented by gene transfer in utero. Prenatal gene therapy may be more promising than postnatal treatment, as the fetus may be more amenable to uptake and integration of therapeutic DNA and the immaturity of its immune system may permit life-long immune tolerance of the transgenic protein, thus avoiding the dominant problem in haemophilia treatment, the formation of inhibitory antibodies.

METHODS

Adenovirus serotype 5-derived or AAV serotype 2-derived vectors carrying human clotting factor IX (hfIX) cDNA or a reporter gene were administered intramuscularly, intraperitoneally or intravascularly to late-gestation mouse fetuses. Both vector types were evaluated with respect to the kinetics of hfIX delivery to the systemic circulation and possible immune responses against the vector or the transgene product.

RESULTS

Mice treated in utero by intramuscular injection of an adenoviral vector carrying hfIX cDNA exhibited high-level gene expression at birth and therapeutic--albeit continuously decreasing--plasma concentrations of hfIX over the entire 6 months of the study. Adenoviral vector spread to multiple organs was detected by polymerase chain reaction (PCR). Intramuscular, intraperitoneal or intravascular application of AAV vectors carrying hfIX cDNA led to much lower plasma concentrations of hfIX shortly after birth, which appeared to decline during the first month of life but stabilized in some of the mice at detectable levels. No signs of immune responses were found, either against the different viral vectors or against hfIX.

CONCLUSION

This study demonstrates for the first time that sustained systemic delivery of a therapeutic protein can be achieved by prenatal gene transfer. It thus shows the feasibility of gene therapy in utero and provides a basis for considering this concept as a preventive therapeutic strategy for haemophilia and perhaps also for other plasma protein deficiencies.

Muscle-directed gene transfer and transient immune suppression result in sustained partial correction of canine hemophilia B caused by a null mutation

The X-linked bleeding disorder hemophilia B is caused by absence of functional blood coagulation factor IX (F9) and can be treated by adeno-associated viral (AAV) mediated gene transfer to skeletal muscle. The safety of this approach is currently being evaluated in a phase I clinical trial. Efficacy of this and several other gene therapy strategies has been addressed in hemophilia B dogs, an important preclinical model of the disease. While previously published data demonstrated sustained expression of canine F9 in dogs with a missense mutation in the gene F9, we show here that AAV-mediated canine F9 gene transfer to skeletal muscle of hemophilia B dogs carrying a null mutation of F9 (causing an early stop codon and an unstable mRNA) results in induction of inhibitory anti-canine F9 at comparable vector doses (1 x 10(12) vector genomes/kg). Thus, the risk of inhibitor formation following AAV-mediated F9 gene therapy may be influenced by the nature of the underlying mutation in F9. Transient immune suppression with cyclophosphamide at the time of vector administration blocked formation of anti-canine F9 antibodies in the one animal treated with this approach. Treatment with this combination of gene transfer and transient immune modulation has resulted in sustained expression (>8 months) of canine F9 at levels sufficient for partial correction of coagulation parameters.

Adenovirus-mediated factor VIII gene expression results in attenuated anti-factor VIII-specific immunity in hemophilia A mice compared with factor VIII protein infusion

Hemophilia A patients are typically treated by factor VIII (FVIII) protein replacement, an expensive therapy that induces FVIII-specific inhibitors in approximately 30% of patients with severe hemophilia. FVIII gene therapy has the potential to improve the current treatment protocols. In this report, we used a hemophilia A mouse model to compare the humoral and cellular immune responses between an E1/E2a/E3-deficient adenovirus expressing human FVIII directed by a liver-specific albumin promoter and purified recombinant FVIII protein infusion. Adenovirus-mediated FVIII expression did not elicit detectable CD4+ or CD8+ T cell responses and induced a weak antibody immune response to FVIII. In contrast, FVIII protein administration resulted in a potent anti-FVIII antibody response and moderate CD4+ T cell response. Furthermore, hemophiliac mice preimmunized with FVIII protein infusion to induce anti-FVIII immunity, and subsequently treated by adenovirus-mediated FVIII gene therapy, expressed therapeutic levels of FVIII despite the presence of low levels of anti-FVIII antibodies. No FVIII was detected in the plasma of mice with intermediate or high antibody levels, although anti-FVIII antibody levels in some vector-treated animals declined. The data support the hypothesis that liver-specific gene therapy-mediated expression of FVIII may be less immunogenic than traditional protein replacement therapy.

Gene therapy for haemophilia: the end of a 'royal pathology' in the third millennium?

Haemophilia is an ideal condition for gene therapy because of its monogenetic character and the fact that it requires only a small amount of the expressed protein to achieve palliation. To date, research in the field of gene therapy for haemophilia has largely relied on retroviruses, adenoviruses and adeno-associated viruses as transfer vectors and the major aims will be to achieve stable longlasting in vivo expression of factors VIII or IX (FVIII or FIX) at therapeutic levels. Two clinical trials have been approved by the US Food and Drug Administration (FDA), using miniadenovirus FVIII and the intrahepatic and intramuscular delivery of adeno-associated virus FIX. In the third millennium, haemophilia treatment should encompass more ambitious goals through gene replacement, to result in permanent and safe haemophilia 'eradication', making haemophilia a part of the history of medicine.