Enhanced transduction of mouse bone marrow-derived dendritic cells by repetitive infection with self-complementary adeno-associated virus 6 combined with immunostimulatory ligands

Optimization of AAV vectors to target persistent viral reservoirs

Gene delivery of antiviral therapeutics to anatomical sites where viruses accumulate and persist is a promising approach for the next generation of antiviral therapies. Recombinant adeno-associated viruses (AAV) are one of the leading vectors for gene therapy applications that deliver gene-editing enzymes, antibodies, and RNA interference molecules to eliminate viral reservoirs that fuel persistent infections. As long-lived viral DNA within specific cellular reservoirs is responsible for persistent hepatitis B virus, Herpes simplex virus, and human immunodeficiency virus infections, the discovery of AAV vectors with strong tropism for hepatocytes, sensory neurons and T cells, respectively, is of particular interest. Identification of natural isolates from various tissues in humans and non-human primates has generated an extensive catalog of AAV vectors with diverse tropisms and transduction efficiencies, which has been further expanded through molecular genetic approaches. The AAV capsid protein, which forms the virions' outer shell, is the primary determinant of tissue tropism, transduction efficiency, and immunogenicity. Thus, over the past few decades, extensive efforts to optimize AAV vectors for gene therapy applications have focused on capsid engineering with approaches such as directed evolution and rational design. These approaches are being used to identify variants with improved transduction efficiencies, alternate tropisms, reduced sequestration in non-target organs, and reduced immunogenicity, and have produced AAV capsids that are currently under evaluation in pre-clinical and clinical trials. This review will summarize the most recent strategies to identify AAV vectors with enhanced tropism and transduction in cell types that harbor viral reservoirs.

AAV2/IL-12 gene delivery into dendritic cells (DC) enhances CTL stimulation above other IL-12 applications: Evidence for IL-12 intracrine activity in DC

Adoptive transfer of antigen-specific cytotoxic T lymphocytes (CTL) holds significant promise in treating cancer and Th1 response cytokines are critical for their stimulation. Recently we reported that interleukin 7-(IL-7) and interferongamma-(IFNγ) autocrine/T cell gene delivery resulted in superior ex vivo CTL stimulation over paracrine/DC delivery. IL-12 is yet another important Th1 cytokine which affects both DC and T cells. Here, using adeno-associated virus Type 2 (AAV2) gene delivery, IL-12-paracrine/DC gene delivery gave significantly superior stimulation of carcinoembryonic antigen (CEA)-specific CTL killing over that induced by autocrine gene delivery (or exogenous IL-12 addition). This is surprising as both AAV2/IL-12-treated T cells and DC secreted approximately the same level of IL-12. Paracrine IL-12 gene delivery also resulted in highest IL-12/IL-10 secretion ratio by DC and highest CD40, CD80, CD83 and CD86 expression. Moreover, AAV2/IL-12-DC stimulated the highest T-cell IFNγ production, highest T cell proliferation, highest CD69+/CD8+ levels, and lowest level of CD25+/CD4+ Treg. These data strongly suggest that the primary activity of IL-12 during CTL generation is upon the DC. These data are also consistent with there being novel activity for IL-12 within the DC itself, not involving its surface receptor; an “intracrine” activity. Given the plethora of IL-12 studies, these data also suggest that this gene delivery comparison approach could be useful for uncovering new cytokine activities and mechanism(s) of action gone unrecognized by conventional immunologic assays. Finally, these data further suggest AAV2/IL-12 intracrine gene delivery into DC may have utility in immunotherapy protocols involving antigen-specific CTL.

Use of Adeno-associated viral vectors to improve delivery of a DNA vaccine against dengue virus

Dengue virus (DENV) remains a significant healthcare and socioeconomic burden for endemic countries. Attempts to produce a safe and effective vaccine have been unsuccessful so far, making this task one of the top priorities in the field. We have previously shown that an EDIII-based DNA vaccine is able to induce neutralizing, long-lasting and highly specific antibody responses for all four DENV serotypes in mice using gene-gun delivery technology. Here, we describe the use of recombinant Adeno-associated viral vectors as an alternative DNA delivery platform, in combination with different immunization schedules, to simplify the vaccination protocol without compromising the induction of neutralizing antibody responses. Our results demonstrate that using viral vectored-platforms to deliver genetic vaccines could potentially reduce the number of doses required to induce a sustained DENV-neutralizing response, thus facilitating the implementation and deployment of the vaccine in developing countries.

Towards Clinical Implementation of Adeno-Associated Virus (AAV) Vectors for Cancer Gene Therapy: Current Status and Future Perspectives

Adeno-associated virus (AAV) vectors have gained tremendous attention as in vivo delivery systems in gene therapy for inherited monogenetic diseases. First market approvals, excellent safety data, availability of large-scale production protocols, and the possibility to tailor the vector towards optimized and cell-type specific gene transfer offers to move from (ultra) rare to common diseases. Cancer, a major health burden for which novel therapeutic options are urgently needed, represents such a target. We here provide an up-to-date overview of the strategies which are currently developed for the use of AAV vectors in cancer gene therapy and discuss the perspectives for the future translation of these pre-clinical approaches into the clinic.

Muscle-Directed Delivery of an AAV1 Vector Leads to Capsid-Specific T Cell Exhaustion in Nonhuman Primates and Humans

With the US Food and Drug Administration (FDA) and European Medicines Agency (EMA) approvals for Zolgensma, Luxturna, and Glybera, recombinant adeno-associated viruses (rAAVs) are considered efficient tools for gene transfer. However, studies in animals and humans demonstrate that intramuscular (IM) AAV delivery can trigger immune responses to AAV capsids and/or transgenes. IM delivery of rAAV1 in humans has also been described to induce tolerance to rAAV characterized by the presence of capsid-specific regulatory T cells (Tregs) in periphery. To understand mechanisms responsible for tolerance and parameters involved, we tested 3 muscle-directed administration routes in rhesus monkeys: IM delivery, venous limb perfusion, and the intra-arterial push and dwell method. These 3 methods were well tolerated and led to transgene expression. Interestingly, gene transfer in muscle led to Tregs and exhausted T cell infiltrates in situ at both day 21 and day 60 post-injection. In human samples, an in-depth analysis of the functionality of these cells demonstrates that capsid-specific exhausted T cells are detected after at least 5 years post-vector delivery and that the exhaustion can be reversed by blocking the checkpoint pathway. Overall, our study shows that persisting transgene expression after gene transfer in muscle is mediated by Tregs and exhausted T cells.

An Engineered AAV6-Based Vaccine Induces High Cytolytic Anti-Tumor Activity by Directly Targeting DCs and Improves Ag Presentation

We have previously shown that an AAV6-based vaccine generates high levels of antigen-specific CD8+ T cells. Further modifications described here led to significantly increased levels of antigen-specific CD8+ and CD4+ T cells, enhanced formation of memory cells, and superior antigen-specific killing capacity in a murine model. By tracking reporter-gene-positive dendritic cells, we showed that they were directly targeted with modified AAV6 in vivo. Our vaccine's anti-cancer potential was evaluated with the antigen ovalbumin against a B16F10 melanoma cell line stably expressing ovalbumin. The vaccination showed superior protection in a murine model of metastatic melanoma. The vaccination significantly delayed solid tumor growth but did not completely prevent tumor development. We show that tumors in immunized mice escaped vaccine-induced killing by losing ovalbumin expression. The vaccine induced massive tumor infiltration with NK and CD8+ T cells with upregulated PD-1 expression. Thus, a vaccination of a combination of anti-PD-1 antibodies demonstrated significant improvement in the treatment efficacy. To summarize, we showed that a bioengineered AAV6-based vaccine elicits strong and long-lasting cellular and humoral responses against an encoded antigen. To increase AAV vaccine efficiency and mitigate tumor escape through antigen loss, we intended to target several antigens in combination with treatments targeting the tumor microenvironment.

Transcriptional Profiles of Murine Bone Marrow-Derived Dendritic Cells in Response to Peste des Petits Ruminants Virus

Background: Peste des petits ruminants virus (PPRV) is the causative agent of PPR, which can cause an acute, highly contagious and fatal disease of sheep and goats, resulting in significant economic losses for commercial animal husbandry due to its high mortality and morbidity. As professional antigen-presenting cells, dendritic cells (DCs) play a unique role in innate immunity. This study aimed to gain a deeper understanding of the transcriptional response of bone marrow-derived dendritic cells (BMDCs) stimulated with PPRV. Results: Transcriptional profiling was performed using RNA sequencing. Herein, we reported that compared to untreatedBMDCs, 4492 differentially expressed genes (DEGs) were identified following PPRV stimulation, out of these DEGs 2311 were upregulated and 2181 were downregulated, respectively. A total of three gene ontology (GO) term clusters of biological process, cell component and molecular function were significantly enriched in 963 GO terms in the PPRV-stimulated BMDCs. These GO clusters were related to inflammatory response, cell division and vacuole, anchoring junction, positive regulation of cellular component and nucleoside binding. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of DEGs were enriched in a chemokine signaling pathway, protein processing in endoplasmic reticulum, cell cycle and mTOR signaling pathway. Additionally, identified DEGs of BMDCs were further validated by qRT-PCR and the results were in accordance with the change of the genes. This study suggested the effects of PPRV stimulation on the maturation and function of BMDCs. Conclusion: We found that the dramatic BMDCs transcriptome changes triggered were predominantly related to an inflammatory response and chemokine signaling pathway.

Proteomic analysis of murine bone marrow derived dendritic cells in response to peste des petits ruminants virus

Peste des petits ruminants virus (PPRV) poses a great threat to livestock husbandry, especially goat farming due to its high mortality and morbidity. Dendritic cells (DCs), as the principal stimulators of naive Th cells were widely used in antigen processing and presenting. In the previous study, we tested the effects of PPRV on murine bone marrow derived dendritic cells (BMDCs) including surface markers and cytokines. While the aim of this study is to detect the proteomic profile of BMDCs stimulated with PPRV towards key proteins involved in. Following PPRV stimulation, 110 differentially expressed proteins (DEPs) were identified through iTRAQ labelling with LC-MS/MS approach, of which 94 DEPs were up-regulated and 16 DEPs were down-regulated, respectively. Among them 15 out of 110 DGPs were related to innate immune system, three were involved in cell apoptosis, RPS15a and Smox were related to translation of viral mRNA. Additionally, western blot analysis showed identical results to iTRAQ analysis. There will be profound significance for understanding antigen-presenting of BMDCs after stimulation with PPRV.

Recombinant AAV-CEA Tumor Vaccine in Combination with an Immune Adjuvant Breaks Tolerance and Provides Protective Immunity

Carcinoembryonic antigen (CEA) is a human glycoprotein involved in cellular adhesion and expressed during human fetal development. Although expression of CEA largely ceases prior to birth, several human epithelial cancers, including colorectal, gastric, squamous esophageal, and breast carcinomas have been known to overexpress CEA, suggesting its potential as an immunotherapeutic target. Using a transgenic mouse model constitutively expressing human CEA in a spatiotemporal manner as a self-protein and a syngeneic mouse colon cancer cell line, MC38-CEA, overexpressing CEA, we tested the potential of a novel genetic immunotherapy approach against CEA-expressing tumors, using recombinant adeno-associated virus vector encoding CEA (rAAV-CEA) and appropriately timed immune adjuvant application. Results of the study demonstrated breaking of immune tolerance for CEA with this vaccine regimen and an anti-tumor response, resulting in tumor-free survival. Furthermore, tumor challenge of CEA-vaccinated mice with parental MC38 cells not expressing CEA did not result in protection from tumor development, confirming that the protection against tumor development is CEA specific. The study illustrates the feasibility of utilizing rAAV vectors in combination with an immunostimulatory adjuvant to break tolerance to weakly immunogenic self-antigens and for an anti-tumor response.

Regulatory and Exhausted T Cell Responses to AAV Capsid

Recombinant adeno-associated viruses (AAVs) are quickly becoming the preferred viral vector for viral gene delivery for the treatment of a wide variety of genetic disorders. However, since their use in a clinical trial targeting hemophilia B patients 10 years ago, immune responses to the AAV capsid appear to have hampered some of the early clinical gene transfer efficacy. Indeed, AAV-based gene transfer has been shown to reactivate capsid-specific memory T cells, which have correlated with a decline in AAV-transduced tissue in some patients. Importantly, clinical trials have also shown that this reactivation can be quelled by administering time-course taper of glucocorticoid steroids before or after dosing. More recently, two clinical studies have shown that AAV gene transfer is not only able to induce a deleterious immune response, but also can result in the initiation of a tolerance to the AAV capsid mediated by regulatory T cells and exhausted T cells. This article reviews clinical trials describing immune responses to AAV, as well as the mechanisms responsible for immune tolerance in chronic infections and how it could apply to AAV-based gene transfer. A better understanding of both cytotoxic and tolerogenic immune responses to recombinant AAV will lead to safer gene transfer protocols in patients.

Anti-GITR therapy promotes immunity against malignant glioma in a murine model

Regulatory T cells (Tregs) are potently immunosuppressive cells that accumulate within the glioma microenvironment. The reduction in their function and/or trafficking has been previously shown to enhance survival in preclinical models of glioma. Glucocorticoid-induced TNFR-related protein (GITR) is a tumor necrosis factor superfamily receptor enriched on Tregs that has shown promise as a target for immunotherapy. An agonistic antibody against GITR has been demonstrated to inhibit Tregs in a number of models and has only been recently addressed in glioma. In this study, we examined the modality of the antibody function at the tumor site as opposed to the periphery as the blood-brain barrier prevents efficient antibody delivery to brain tumors. Mice harboring established GL261 tumors were treated with anti-GITR monotherapy and were shown to have a significant increase in overall survival (p < 0.01) when antibodies were injected directly into the glioma core, whereas peripheral antibody treatment only had a modest effect. Peripheral treatment resulted in a significant decrease in granzyme B (GrB) expression by Tregs, whereas intratumoral treatment resulted in both a decrease in GrB expression by Tregs and their selective depletion, which was largely mediated by FcγR-mediated destruction. We also discovered that anti-GITR treatment results in the enhanced survival and functionality of dendritic cells (DCs)-a previously unreported effect of this immunotherapy. In effect, this study demonstrates that the targeting of GITR is a feasible and noteworthy treatment option for glioma, but is largely dependent on the anatomical location in which the antibodies are delivered.

Early interaction of adeno-associated virus serotype 8 vector with the host immune system following intramuscular delivery results in weak but detectable lymphocyte and dendritic cell transduction

Following in vivo recombinant adeno-associated virus (rAAV)-based gene transfer, adaptive immune responses specific to the vector or the transgene product have emerged as a potential roadblock to successful clinical translation. The occurrence of such responses depends on several parameters, including the route of vector administration as well as the viral serotype and the genome configuration, either self-complementary (sc) or single-stranded (ss). These parameters influence rAAV vector-associated immunity by modulating the crosstalk between the vector and the host immune system, including vector ability to interact or even transduce lymphoid tissues in general and antigen-presenting cells (APCs) in particular. Little is known about immune cell populations that are targeted in vivo by rAAV vectors. Moreover, the transduction of dendritic cells is still controversial and not directly demonstrated. Here, we show that intramuscular administration of an sc rAAV8 vector in the mouse leads to a rapid distribution of viral genomes in the lymphoid tissues that is associated with transgene expression. Transduced cells were detected in follicular areas of the spleen and the draining lymph nodes. In addition to B and T lymphocytes, transduced professional APCs were detected although at very low frequency. In addition, viral genomes and transgene transcripts were also detected in these cell populations after ss rAAV8 vector administration. Although the functional significance of those observations needs further explorations, our results highlight an early and intricate interaction between the rAAV vector upon its in vivo delivery and the host immune system.

Efficient lysis of epithelial ovarian cancer cells by MAGE-A3-induced cytotoxic T lymphocytes using rAAV-6 capsid mutant vector

MAGE-A3 is highly expressed in epithelial ovarian cancer (EOC), making it a promising candidate for immunotherapy. We investigated whether dendritic cells (DCs) transduced with a rAAV-6 capsid mutant vector Y445F could elicit effective MAGE-A3-specific anti-tumor cytotoxic T lymphocyte (CTL) responses in vitro. MAGE-A3 was cloned and rAAV-6-MAGE-A3 purified, followed by proviral genome detection using real-time PCR. Immunofluorescence detection of rAAV-6-Y445F-MAGE-A3-transduced DCs demonstrated 60% transduction efficiency. Fluorescent in situ hybridization analysis confirmed chromosomal integration of rAAV vectors. Flow cytometric analysis of transduced DCs showed unaltered expression of critical monocyte-derived surface molecules with retention of allo-stimulatory activity. Co-culture of autologous T lymphocytes with MAGE-A3-expressing DCs produced CTLs that secreted IFN-γ, and efficiently killed MAGE-A3+ EOC cells. This form of rAAV-based DC immunotherapy, either alone or more likely in combination with other immune-enhancing protocols, may prove useful in the clinical setting for management of EOC.

Rationally designed capsid and transgene cassette of AAV6 vectors for dendritic cell-based cancer immunotherapy

Dendritic cell (DC)-based immunotherapy has recently demonstrated a great potential for clinical applications; however, additional progress in the methods of tumor-specific antigen delivery to DCs is necessary for the further development of anti-tumor vaccines. To this end, a capsid-optimized adeno-associated virus serotype 6 (AAV6-T492V+S663V) vector was developed by site-directed mutagenesis of surface-exposed serine (S) and threonine (T) residues, which have a critical role in intracellular trafficking of AAV vectors. This double-mutant AAV6 vector had ∼ 5-fold greater transduction efficiency in monocyte-derived DCs (moDCs) compared with wild-type (WT)-AAV6 vectors. The increase in the transduction efficiency correlated with the improved nuclear translocation of AAV6-T492V+S663V over that of the WT-AAV6 vector. Additional studies of the CD11c promoter identified critical regulatory elements that fit into the AAV expression cassette and drive EGFP expression in moDCs. Development of a chimeric promoter (chmCD11c) that contains functional modules of CD11c and a Simian virus (SV40) enhancer element dramatically increased the EGFP expression in moDCs. MoDCs transduced by the capsid-optimized AAV6 vector carrying human prostate-specific antigen (hPSA) driven by CBA (AAV6-T492V+S663V-CBA-hPSA) or chmCd11c (AAV6-T492V+S663V-chmCD11c-hPSA) generated specific T-cell clone proliferation and superior cytotoxic T lymphocytes (CTLs) with higher killing capability against human prostate adenocarcinoma cells, LNCaP, compared with WT-AAV6 induced CTLs. Taken together, these studies suggest that optimization of capsid and promoter components of AAV vectors can be a useful approach for efficient targeting of moDCs and may prove to be a promising tool for cancer immunotherapy.

Current Challenges and Future Directions in Recombinant AAV-Mediated Gene Therapy of Duchenne Muscular Dystrophy

Various characteristics of adeno-associated virus (AAV)-based vectors with long-term safe expression have made it an exciting transduction tool for clinical gene therapy of Duchenne muscular dystrophy (DMD). Although host immune reactions against the vector as well as transgene products were detected in some instances of the clinical studies, there have been promising observations. Methods of producing AAV vectors for considerable in vivo experimentation and clinical investigations have been developed and a number of studies with AAV vector-mediated muscle transduction were attempted. Notably, an intravenous limb perfusion transduction technique enables extensive transgene expression in the skeletal muscles without noticeable adverse events. Furthermore, cardiac transduction by the rAAV9-microdystrophin would be promising to prevent development of cardiac dysfunction. Recent achievements in transduction technology suggest that long-term transgene expression with therapeutic benefits in DMD treatment would be achieved by the rAAV-mediated transduction strategy with an adequate regimen to regulate host immune response.

Oral vaccination with adeno-associated virus vectors expressing the Neu oncogene inhibits the growth of murine breast cancer

Recombinant adeno-associated viruses (AAV) have been used for therapeutic gene transfer. These vectors offer a number of advantages including resistance to the effects of pH, a broad cellular tropism, efficient gene transfer, persistence of gene expression, and little toxicity. AAV vectors; however, at high doses can induce humoral and cellular immune responses. While potentially problematic for replacement gene therapy, this effect may be advantageous for antitumor vaccination. We examined the activity of an oral and intramuscular antitumor vaccination using AAV serotypes 5 and 6 expressing a truncated neu oncogene in a neu-positive murine TUBO breast cancer model. Mice receiving a single oral administration of AAV5-neu or AAV6-neu demonstrated improved survival. Oral vaccination significantly improved survivals compared with intramuscular vaccination. Mice vaccinated with AAV6-neu survived longer than those treated with AAV5-neu. Vaccination with AAV5-neu or AAV6-neu induced both humoral and cellular immune responses against the NEU antigen. These responses were more robust in the mice undergoing oral vaccination compared with mice receiving the intramuscular vaccination. Protection from tumor was long lasting with 80% of the animals treated with oral AAV6-neu surviving a re-challenge with TUBO cells at 120 and 320 days post-vaccination. Further evaluation of AAV-based vectors as tumor vaccines is warranted.

High-efficiency transduction of human monocyte-derived dendritic cells by capsid-modified recombinant AAV2 vectors

Phosphorylation of surface-exposed tyrosine residues negatively impacts the transduction efficiency of recombinant AAV2 vectors. Pre-treatment of cells with specific cellular serine/threonine kinase inhibitors also significantly increased the transduction efficiency of AAV2 vectors. We reasoned that site-directed mutagenesis of surface-exposed serine residues might allow the vectors to evade phosphorylation and thus lead to higher transduction efficiency. Each of the 15 surface-exposed serine (S) residues was substituted with valine (V) residues, and the transduction efficiency of three of these mutants, S458V, S492V and S662V, was increased by up to ≈ 20-fold in different cell types. The S662V mutant was efficient in transducing human monocyte-derived dendritic cells (moDCs), a cell type not readily amenable to transduction by the conventional AAV vectors, and did not induce any phenotypic changes in these cells. Recombinant S662V-AAV2 vectors encoding a truncated human telomerase (hTERT) gene were generated and used to stimulate cytotoxic T cells (CTLs) against target cells. S662V-AAV2-hTERT vector-transduced DCs resulted in rapid, specific T-cell clone proliferation and generation of robust CTLs, which led to specific cell lysis of K562 cells. These studies suggest that high-efficiency transduction of moDCs by serine-modified AAV2 vectors is feasible, which supports the potential utility of these vectors for future human DCs vaccine studies.

Efficient induction of anti-tumor immunity by a TAT-CEA fusion protein vaccine with poly(I:C) in a murine colorectal tumor model

Protein vaccines may be a useful strategy for cancer immunotherapy because recombinant tumor antigen proteins can be produced on a large scale at relatively low cost and have been shown to be safe for clinical application. However, protein vaccines have historically exhibited poor immunogenicity; thus, an improved strategy is needed for successful induction of immune responses. TAT peptide is a protein transduction domain composed of an 11-amino acid peptide (TAT(47-57): YGRKKRRQRRR). The positive charge of this peptide allows protein antigen fused with it to improve cell penetration. Poly(I:C) is a synthetic double-stranded RNA that is negatively charged and favors interaction with the cationic TAT peptide. Poly(I:C) has been reported on adjuvant role in tumor vaccine through promotion of immune responses. Therefore, we demonstrated that vaccine with a mixture of TAT-CEA fusion protein and poly(I:C) can induce anti-tumor immunity in a murine colorectal tumor model. Splenocytes from mice vaccinated with a mixture of TAT-CEA fusion protein and poly(I:C) effectively induced CEA-specific IFN-γ-producing T cells and showed cytotoxic activity specific for MC-38-cea2 tumor cells expressing CEA. Vaccine with a mixture of TAT-CEA fusion protein and poly(I:C) delayed tumor growth in MC-38-cea-2 tumor-bearing mice. Depletion of CD8(+) T cells and NK cells reversed the inhibition of tumor growth in an MC-38-cea2-bearing mice, indicating that CD8(+) T cells and NK cells are responsible for anti-tumor immunity by vaccine with a mixture of TAT-CEA fusion protein and poly(I:C). Taken together, these results suggest that poly(I:C) could be used as a potent adjuvant to induce the anti-tumor immunity of a TAT-CEA fusion protein vaccine in a murine colorectal tumor model.

Optimized transduction of human monocyte-derived dendritic cells by recombinant adeno-associated virus serotype 6

Dendritic cells are the key antigen-presenting cells involved in the initiation of the adaptive immune response. Recombinant adeno-associated viruses (rAAVs) can transduce dendritic cells and have gained attention as potential vaccines capable of stimulating T cell immunity. Here we show that rAAV2 pseudotyped with type 6 capsid (rAAV2/6) exhibits significantly higher tropism for human monocyte-derived dendritic cells (MoDCs) than other serotypes and variants. Transduction was abolished by a single lysine-to-alanine mutation within the AAV6 capsid previously shown to inhibit binding to heparin. However, unlike rAAV2, soluble heparin did not inhibit rAAV2/6 transduction of MoDCs. Further enhancement of MoDC transduction was observed after mutation of Tyr-731 in the capsid of AAV6 consistent with a report that tyrosine residues are phosphorylated, leading to ubiquitination of capsids during uptake. Pseudotyped rAAV2/6 vectors containing a Y731F mutation minimally altered the immunophenotype of MoDCs, which retained their immunostimulatory ability and were able to stimulate an antigen-specific CD8(+) T cell clone. These findings should assist in the development of rAAV2/6 as a vaccine vector.

Regulation of the activity of an adeno-associated virus vector cancer vaccine administered with synthetic Toll-like receptor agonists

Recombinant adeno-associated virus (rAAV) is being tested as a vaccine vector, but the cellular immune responses elicited in animal tumor models have not been completely protective. The adjuvant effects of the TLR7 agonist, imiquimod, and the TLR9 agonist, ODN1826, were tested with rAAV expressing the melanoma antigen, Trp2. Mice immunized with rAAV-TRP2 and either TLR agonist alone generated T-helper-1 antitumor immune responses. Antitumor activity in all experiments was still incomplete. Furthermore, antitumor activity was not achieved when the combination of ODN1826 and imiquimod was used as adjuvant. In vitro, the combination increased IL-10 production by dendritic cells. In vivo, the combination reduced T-helper-1 response and dendritic cell activation and increased myeloid suppressor cells; regulatory T cells were not significantly modulated. Depletion of myeloid derived suppressor cells enhanced the antitumor activity of immunization with rAAV-TRP2 and the imiquimod-ODN1826 combination; depletion of regulatory T cells did not. TLR7 and TLR9 agonists can be used to enhance the immune response to rAAV immunogens, but antagonism can be observed when combined. Suppressor mechanisms, including those mediated by myeloid cells, may negatively regulate the antitumor immune response.

AAV's anatomy: roadmap for optimizing vectors for translational success

Adeno-Associated Virus based vectors (rAAV) are advantageous for human gene therapy due to low inflammatory responses, lack of toxicity, natural persistence, and ability to transencapsidate the genome allowing large variations in vector biology and tropism. Over sixty clinical trials have been conducted using rAAV serotype 2 for gene delivery with a number demonstrating success in immunoprivileged sites, including the retina and the CNS. Furthermore, an increasing number of trials have been initiated utilizing other serotypes of AAV to exploit vector tropism, trafficking, and expression efficiency. While these trials have demonstrated success in safety with emerging success in clinical outcomes, one benefit has been identification of issues associated with vector administration in humans (e.g. the role of pre-existing antibody responses, loss of transgene expression in non-immunoprivileged sites, and low transgene expression levels). For these reasons, several strategies are being used to optimize rAAV vectors, ranging from addition of exogenous agents for immune evasion to optimization of the transgene cassette for enhanced therapeutic output. By far, the vast majority of approaches have focused on genetic manipulation of the viral capsid. These methods include rational mutagenesis, engineering of targeting peptides, generation of chimeric particles, library and directed evolution approaches, as well as immune evasion modifications. Overall, these modifications have created a new repertoire of AAV vectors with improved targeting, transgene expression, and immune evasion. Continued work in these areas should synergize strategies to improve capsids and transgene cassettes that will eventually lead to optimized vectors ideally suited for translational success.

Effective transduction by self-complementary adeno-associated viruses of human dendritic cells with no alteration of their natural characteristics

BACKGROUND

Effective gene delivery techniques are required to genetically manipulate dendritic cells (DCs). We therefore investigated the feasibility of using various self-complementary recombinant adeno-associated virus (scAAV) serotypes to deliver genes to human DCs.

METHODS

Monocytes isolated from healthy volunteers were differentiated to immature DCs (iDC) by incubation with interleukin (IL)-4 and granulocyte macrophage colony-stimulating factor. The iDCs were transduced with scAAV1, 2, 3, 4, 5, 6 or 8 at various multiplicities of infection (MOIs). Transduction efficiency (TE), cell viability and functional characteristics of the transduced DCs were evaluated.

RESULTS

TE of scAAV was three-fold greater than TE of conventional recombinant adeno-associated virus with a single-stranded genome. The TEs of scAAV2, 5, and 6 were much higher than those of the other scAAVs; at 1000 MOI, the TEs were 22.2% +/- 9.5%, 27.0% +/- 8.8% and 28.4% +/- 6.0%, respectively. Exposure of iDCs to 5000 MOI of these viruses increased their TEs, leading to the transduction of nearly the entire DC population. In addition, gene transfer by scAAV did not cause any cytotoxicity. Flow cytometric analysis of scAAV-transduced DCs showed no changes in surface marker profiles. Moreover, transduced cells maintained their functional properties, as represented by active antigen uptake. These cells could efficiently differentiate into mature DCs, as shown by their release of IL-12, the substantial loss of antigen-uptake activity, and the ability of T-cell stimulation.

CONCLUSIONS

These findings strongly indicate that scAAVs, especially subtypes 2, 5 and 6, hold a promising potential as gene delivery tools in human DCs.

Activation of cytotoxic T lymphocytes against CML28-bearing tumors by dendritic cells transduced with a recombinant adeno-associated virus encoding the CML28 gene

Induction of anti-tumor immune responses by dendritic cells (DCs) transduced with a recombinant adeno-associated virus type 2 (rAAV2) encoding tumor antigens is considered a promising approach for cancer vaccine development. CML28, a novel antigen with the properties of cancer/ testis (CT) antigens, is an attractive target for antigen-specific immunotherapy. Here we investigated the feasibility of inducing CML28-specific cytotoxic T lymphocyte (CTL) responses using DCs transduced with the rAAV2 vectors containing the CML28 gene (rAAV/CML28). Using an adenovirus-free packaging system, rAAV/CML28 was generated. The transduction efficiency of rAAV/CML28 in DCs increased in a multiplicity of infection (MOI)-dependent manner. The rAAV/CML28 transduction did not impair DC maturation, but even enhanced the CD80 expression. The rAAV/CML28-transduced DCs induced CML28-specific CTLs which exhibited a MHC class I-mediated antigen-specific lytic activity against CML28-bearing tumor cell lines (HepG2 and MCF-7) as well as the primary leukemia blasts. These findings suggest that rAAV/CML28-transduced DCs vaccine may serve as a feasible approach for the treatment of CML28-associated cancers.

Co-administration of carcinoembryonic antigen and HIV TAT fusion protein with CpG-oligodeoxynucleotide induces potent antitumor immunity

Although dendritic cells (DC) have been well demonstrated as a strong cellular adjuvant for a tumor vaccine, there are several limitations for clinical application. A protein-based vaccine using a potent adjuvant is an appealing approach for tumor antigen-specific immunotherapy because of their simplicity, safety, efficacy and capacity for repeated administration. CpG-oligodeoxynucleotides (ODN) have been used as adjuvants to stimulate innate and adaptive immune responses for cancer treatment. The authors evaluated the adjuvant effects of CpG-ODN in a vaccine incorporating recombinant fusion protein of the HIV TAT PTD domain and carcinoembryonic antigen (TAT-CEA). Mice vaccinated with TAT-CEA and CpG-ODN (TAT-CEA + CpG) showed enhanced CEA-specific immunity, including cytotoxic T-lymphocytes (CTL) activity and interferon (IFN)-gamma secreting T cells compared with CEA and CpG-ODN (CEA + CpG) or TAT-CEA vaccination alone. Vaccination with TAT-CEA + CpG elicited Th1-based responses, as indicated by the higher ratio of immunoglobulin (Ig)G2a antibody/IgG1 antibodies specific for CEA. The survival rate was significantly increased after vaccination with TAT-CEA + CpG in a tumor model using MC38/CEA2. Furthermore, the TAT-CEA +/- CpG vaccine groups showed similar antitumor immunity to the CEA peptide-pulsed DC (CEA peptide/DC) vaccine groups. These data suggest that coadministration of TAT fusion protein with CpG-ODN may serve as a potential formulation for enhancing antitumor activity.

The role of the adeno-associated virus capsid in gene transfer

Adeno-associated virus (AAV) is one of the most promising viral gene transfer vectors that has been shown to effect long-term gene expression and disease correction with low toxicity in animal models, and is well tolerated in human clinical trials. The surface of the AAV capsid is an essential component that is involved in cell binding, internalization, and trafficking within the targeted cell. Prior to developing a gene therapy strategy that utilizes AAV, the serotype should be carefully considered since each capsid exhibits a unique tissue tropism and transduction efficiency. Several approaches have been undertaken in an effort to target AAV vectors to specific cell types, including utilizing natural serotypes that target a desired cellular receptor, producing pseudotyped vectors, and engineering chimeric and mosaic AAV capsids. These capsid modifications are being incorporated into vector production and purification methods that provide for the ability to scale-up the manufacturing process to support human clinical trials. Protocols for small-scale and large-scale production of AAV, as well as assays to characterize the final vector product, are presented here. The structures of AAV2, AAV4, and AAV5 have been solved by X-ray crystallography or cryo-electron microscopy (cryo-EM), and provide a basis for rational vector design in developing customized capsids for specific targeting of AAV vectors. The capsid of AAV has been shown to be remarkably stable, which is a desirable characteristic for a gene therapy vector; however, recently it has been shown that the AAV serotypes exhibit differential susceptibility to proteases. The capsid fragmentation pattern when exposed to various proteases, as well as the susceptibility of the serotypes to a series of proteases, provides a unique fingerprint for each serotype that can be used for capsid identity validation. In addition to serotype identification, protease susceptibility can also be utilized to study dynamic structural changes that must occur for the AAV capsid to perform its various functions during the virus life cycle. The use of proteases for structural studies in solution complements the crystal structural studies of the virus. A generic protocol based on proteolysis for AAV serotype identification is provided here.

Effects of sustained antiangiogenic therapy in multistage prostate cancer in TRAMP model

Antiangiogenic therapy is a promising alternative for prostate cancer growth and metastasis and holds great promise as an adjuvant therapy. The present study evaluated the potential of stable expression of angiostatin and endostatin before the onset of neoplasia and during the early and late stages of prostate cancer progression in transgenic adenocarcinoma of mouse prostate (TRAMP) mice. Groups of 5-, 10-, and 18-week-old male TRAMP mice received recombinant adeno-associated virus-6 encoding mouse endostatin plus angiostatin (E+A) by i.m. injection. The effects of therapy were determined by sacrificing groups of treated mice at defined stages of tumor progression and following cohorts of similarly treated mice for long-term survival. Results indicated remarkable survival after recombinant adeno-associated virus-(E+A) therapy only when the treatment was given at an earlier time, before the onset of high-grade neoplasia, compared with treatment given for invasive cancer. Interestingly, early-stage antiangiogenic therapy arrested the progression of moderately differentiated carcinoma to poorly differentiated state and distant metastasis. Immunohistochemical analysis of the prostate from treated mice indicated significantly lower endothelial cell proliferation and increased tumor cell apoptosis. Vascular endothelial growth factor receptor (VEGFR)-2 expression was significantly down-regulated in tumor endothelium after treatment but not VEGFR-1. Analysis of the neuroendocrine marker synaptophysin expression indicated that antiangiogenic therapy given at an early-stage disease reduced neuroendocrine transition of the epithelial tumors. These studies indicate that stable endostatin and angiostatin gene therapy may be more effective for minimally invasive tumors rather than advanced-stage disease.

Immunity to adeno-associated virus-mediated gene transfer in a random-bred canine model of Duchenne muscular dystrophy

Recombinant adeno-associated virus (rAAV)-mediated gene transfer has shown promise for treating diseases in various animal models including the mdx mouse model of Duchenne muscular dystrophy (DMD). In many cases, however, preclinical studies in inbred mice have not successfully predicted human clinical responses. To assess the potential clinical utility of treating human DMD patients by AAV-mediated gene delivery, we performed a series of direct intramuscular injections in random-bred wild-type dogs. AAV serotypes 2 and 6 carrying different promoter-transgene cassettes were produced as previously described for murine studies and administered intramuscularly. The injection sites were biopsied at various time points and analyzed for transgene expression and immunohistochemical analysis. In contrast to the generally nonimmunogenic nature of these vectors in murine studies, both AAV2 and AAV6 vectors elicited robust cellular immune responses regardless of the transgene expressed, the cellular specificity of the promoter, and the muscle type injected. Viral purification by various methods did not diminish T cell-mediated infiltration. Our data indicate that AAV2 and AAV6 capsid proteins can elicit primary cellular immune responses when injected into the skeletal muscle of random-bred dogs, and suggest the possibility of cellular immunity to AAV vectors in humans.

Major subsets of human dendritic cells are efficiently transduced by self-complementary adeno-associated virus vectors 1 and 2

Dendritic cells (DC) are antigen-presenting cells pivotal for inducing immunity or tolerance. Gene transfer into DC is an important strategy for developing immunotherapeutic approaches against infectious pathogens and cancers. One of the vectors previously described for the transduction of human monocytes or DC is the recombinant adeno-associated virus (rAAV), with a genome conventionally packaged as a single-stranded (ss) molecule. Nevertheless, its use is limited by the poor and variable transduction efficiency of DC. In this study, AAV type 1 (AAV1) and AAV2 vectors, which expressed the enhanced green fluorescent protein and were packaged as ss or self-complementary (sc) duplex strands, were used to transduce different DC subsets generated ex vivo and the immunophenotypes, states of differentiation, and functions of the subsets were carefully examined. We show here for the first time that a single exposure of monocytes (M(o)) or CD34(+) progenitors (CD34) to sc rAAV1 or sc rAAV2 leads to high transduction levels (5 to 59%) of differentiated M(o)-DC, M(o)-Langerhans cells (LC), CD34-LC, or CD34-plasmacytoid DC (pDC), with no impact on their phenotypes and functional maturation of these cells, compared to those of exposure to ss rAAV. Moreover, we show that all these DC subpopulations can also be efficiently transduced after commitment to their differentiation pathways. Furthermore, these DC subsets transduced with sc rAAV1 expressing a tumor antigen were potent activators of a CD8(+)-T-cell clone. Altogether, these results show the high potential of sc AAV1 and sc AAV2 vectors to transduce ex vivo conventional DC, LC, or pDC or to directly target them in vivo for the design of new DC-based immunotherapies.

Induction of robust immune responses against human immunodeficiency virus is supported by the inherent tropism of adeno-associated virus type 5 for dendritic cells

The ability of adeno-associated virus serotype 1 to 8 (AAV1 to AAV8) vectors expressing the human immunodeficiency virus type 1 (HIV-1) Env gp160 (AAV-HIV) to induce an immune response was evaluated in BALB/c mice. The AAV5 vector showed a higher tropism for both mouse and human dendritic cells (DCs) than did the AAV2 vector, whereas other AAV serotype vectors transduced DCs only poorly. AAV1, AAV5, AAV7, and AAV8 were more highly expressed in muscle cells than AAV2. An immunogenicity study of AAV serotypes indicates that AAV1, AAV5, AAV7, and AAV8 vectors expressing the Env gp160 gene induced higher HIV-specific humoral and cell-mediated immune responses than the AAV2 vector did, with the AAV5 vector producing the best responses. Furthermore, mice injected with DCs that had been transduced ex vivo with an AAV5 vector expressing the gp160 gene elicited higher HIV-specific cell-mediated immune responses than did DCs transduced with AAV1 and AAV2 vectors. We also found that AAV vectors produced by HEK293 cells and insect cells elicit similar levels of antigen-specific immune responses. These results demonstrate that the immunogenicity of AAV vectors depends on their tropism for both antigen-presenting cells (such as DCs) and non-antigen-presenting cells (such as muscular cells) and that AAV5 is a better vector than other AAV serotypes. These results may aid in the development of AAV-based vaccine and gene therapy.