Recombinant adeno-associated virus for the generation of autologous, gene-modified tumor vaccines: evidence for a high transduction efficiency into primary epithelial cancer cells

Intratumoral immunotherapy for melanoma

Selection of suitable tumor-associated antigens is a major challenge in the development of effective cancer vaccines. Intratumoral (i.t.) immunotherapy empowers the immune system to mount T cell responses against tumor-associated antigens which are most immunogenic. To mediate systemic tumor regression, i.t. immunotherapy must generate systemic T cell responses that can target distant metastases beyond the initially treated tumor mass. Now that promising preclinical results and some initial success in clinical trials have been obtained, we here review i.t. immunotherapy-related preclinical and clinical studies, their mechanisms of action and future prospects.

The Potential of Gene Transfer into Primary B-CLL Cells Using Recombinant Virus Vectors

Despite recent advances, chronic lymphocytic leukemia (CLL) as the most common leukemia remains a largely incurable disease. Modern treatment options include novel drugs like purine analogues, monoclonal antibodies and transplantation strategies. Moreover, gene transfer of immunostimulatory molecules is another, but still experimental approach that can be used to potentiate immune responses against leukemic cells. CD40 ligand (CD40L) was shown to be a promising molecule for immunotherapy of B-CLL playing a critical role in immune activation. However, CLL B cells are resistant to transduction with most currently available vector systems. Improving the efficiency and specificity of gene vectors is critical for the success of gene therapy in this area. Using replication defective adenovirus encoding CD40L (Ad-CD40L), immunologic and clinical responses were seen in CLL patients after infusion of autologous Ad-CD40L-CLL cells in a recent phase I trial. Due to the immunogenic nature of adenovirus vectors, alternative vector systems are currently explored. Recombinant adeno-associated virus (rAAV) was shown to enable efficient transduction of primary B-CLL cells. By use of a library of AAV clones with randomly modified capsids, receptor-targeting mutants with a tropism for CLL cells can be selected. Furthermore, helper-virus free Epstein-Barr virus (EBV)-based gene transfer vectors hold promise for development of CLL-targeted vaccines after remaining safety issues will be resolved. Herpes simplex virus (HSV)-based vectors, especially HSV amplicons, have favorable features for B-CLL gene transfer including high transduction efficiency, ability to infect postmitotic cells and a large packaging capacity. The challenge for the future will be to transfer these alternative vector systems into clinic and allow the detection of a CLL-specific immune response by use of defined tumor antigens. This will make it possible to establish the potential clinical role of gene therapy for CLL patients.

Recombinant adeno-associated virus type 2-mediated gene transfer into human keratinocytes is influenced by both the ubiquitin/proteasome pathway and epidermal growth factor receptor tyrosine kinase

Efficient gene delivery into keratinocytes is a prerequisite for successful skin gene therapy. Vectors based on recombinant adeno-associated virus type 2 (rAAV-2) offer several promising features that make them attractive for cutaneous applications. However, highly efficient gene delivery may be hampered by different cellular factors, including lack of viral receptors, impairment of cytoplasmic trafficking or limitations in viral second-strand synthesis. This study was undertaken to find factors that influence rAAV-2-mediated in vitro gene transfer into human keratinocytes and, consequently, ways to optimize gene delivery. Transduction experiments using rAAV-2 vectors expressing green fluorescent protein (GFP) demonstrated that impaired cellular trafficking of vector particles and high levels of autophosphorylation at epidermal growth factor receptor tyrosine kinase (EGF-R TK) have a negative influence on gene transfer into keratinocytes. Treatment of keratinocytes with proteasome inhibitor MG132 resulted in a transient augmentation of GFP expression in up to 37% of cells. Treatment with EGF-R TK inhibitors (quinazoline type) enhanced transgene expression in 10-14.5% of the cells. Gene expression was stable for more than 10 weeks and persisted until proliferative senescence occurred. This stable gene expression allows speculation that keratinocyte stem cells have initially been transduced. These findings might have relevance for the use of rAAV-2 vectors in skin gene therapy: transient enhancement of rAAV-2 transduction with proteasome inhibitors might be useful for genetic promotion of wound healing or skin-directed vaccination. Treatment with quinazolines may increase rAAV-2 transduction of keratinocyte stem cells, which is important for gene therapy approaches to inherited diseases.

Suicide gene therapy of sarcoma cell lines using recombinant adeno-associated virus 2 vectors

Soft-tissue sarcomas are mesenchymal tumors that respond poorly to systemic chemotherapy. Suicide gene therapy may be an alternative treatment strategy. Here we show a high susceptibility of human sarcoma cell lines for recombinant adeno-associated virus 2 (rAAV-2) suicide vectors: connective tissue sarcoma (HS-1), fibrosarcoma (HT-1080), Ewing sarcoma (RD-ES), Askin tumor (SK-N-MC), rhabdomyosarcoma (A-204) and soft-tissue sarcoma (WSKL-1). Several vectors containing the thymidine kinase (TK) gene under the control of either the cytomegalovirus promoter or the elongation-factor 1 alpha (EF1alpha) promoter were cloned and tested. Higher expression levels of the transgene were observed in the sarcoma lines when using the EF1alpha-suicide gene-containing vectors. A complete eradication of rAAV-2-EF1alpha-TK/eGFP (TK/enhanced green fluorescent protein fusion gene)-transduced tumor cells was shown following exposure to ganciclovir (2.5 microg/ml) in vitro, while at this dose level > 90% of mock-transduced tumor cells survived. Xenotransplantation tumor models (intraperitoneal, subcutaneous) for the human sarcoma cell line HS-1 were established in nonobese diabetic/severe-combined immunodeficient mice. Mice transplanted with rAAV-2-EF1alpha-TK/eGFP-transduced and ganciclovir-exposed tumor cells survived > 5 months while in the nontransduced group all mice had died approximately 1 month after inoculation. These data hold promise for further development of rAAV-2-based suicide gene therapy of sarcomas.

Adeno-associated virus serotypes 1 to 5 mediated tumor cell directed gene transfer and improvement of transduction efficiency

BACKGROUND

Gene therapy is an attractive new approach for the treatment of cancer. Therefore, the development of efficient vector systems is of crucial importance in this field. Different adeno-associated virus (AAV) serotypes have been characterized so far, which show considerable differences in tissue tropism. Consequently, we aimed to characterize the most efficient serotype for this application.

METHODS

To exclude all influences other than those provided by the capsid, all serotypes contained the same transgene cassette flanked by the AAV2 inverted terminal repeats. We systematically compared these vectors for efficiency in human cancer cell directed gene transfer. In order to identify limiting steps, the influence of second-strand synthesis and proteasomal degradation of AAV in a poorly transducible cell line were examined.

RESULTS

AAV2 was the most efficient serotype in all solid tumor cells and primary melanoma cells with transduction rates up to 98 +/- 0.3%. Transduction above 70% could be reached with serotypes 1 (in cervical and prostate carcinoma) and 3 (in cervical, breast, prostate and colon carcinoma) using 1000 genomic particles per cell. In the colon carcinoma cell line HT-29 proteasomal degradation limited AAV1-AAV4-mediated gene transfer. Moreover, inefficient second-strand synthesis prevents AAV2-mediated transgene expression in this cell line.

CONCLUSIONS

Recent advances in AAV-vector technology suggest that AAV-based vectors can be used for cancer gene therapy. Our comparative analysis revealed that, although AAV2 is the most promising candidate for such an application, serotypes 1 and 3 are valid alternatives. Furthermore, the use of self-complementary AAV vectors and proteasome inhibitors significantly improves cancer cell transduction.

Augmentation of antitumor activity of a recombinant adeno-associated virus carcinoembryonic antigen vaccine with plasmid adjuvant

Recombinant adeno-associated virus 2 (rAAV) vectors have been successfully used for sustained expression of therapeutic genes. The potential of using rAAV as a cancer vaccine vector and the impact of a bacterial plasmid adjuvant on this activity were investigated. C57BL/6 mice received a single intramuscular injection of rAAV expressing the human tumor-associated antigen, carcinoembryonic antigen (CEA). Three weeks later, when CEA expression was optimal, a bacterial plasmid containing methylated DNA motifs was injected into the same muscle. Mice were challenged 1 week later with syngeneic MC38 tumor cells stably expressing CEA. Immunization with rAAV-CEA alone resulted in sustained transgene expression and the elicitation of a humoral immune response to CEA. Cellular immune response, however, was weak, and tumor protection was not significant. In contrast, immunization with rAAV-CEA and the plasmid adjuvant resulted in stronger cellular immune response to CEA and tumor protection. The addition of plasmid adjuvant increased both myeloid dendritic cell recruitment in situ and CEA-specific T-helper-1-associated immune response. These data indicate that robust rAAV transgene expression of a tumor antigen followed by transient plasmid delivery to recruit and activate dendritic cells is an effective method of eliciting antitumor cellular immune responses.

Efficient gene transfer of HIV-1-specific short hairpin RNA into human lymphocytic cells using recombinant adeno-associated virus vectors

The cellular introduction of short, interfering RNA leads to sequence-specific degradation of homologous mRNA, a process termed RNA interference (RNAi). Here, we report that recombinant adeno-associated virus 2 (rAAV-2) can be used to transfer short hairpin (sh) RNA expression cassettes genetically into human cells. HIV-1 replication was suppressed by >95% in H9 cells and primary human lymphocytes that expressed shRNA targeting the first exon of the viral transactivator protein tat compared to control cells. rAAV-2 integrated stably into the host genome, leading to long-term expression of tat shRNA. Our findings demonstrate the utility of rAAV-2 for the genetic transfer of shRNA expression cassettes into human cells, providing an alternative to using retroviral vectors as RNAi delivery systems.

Engagement of the B-cell antigen receptor (BCR) allows efficient transduction of ZAP-70-positive primary B-CLL cells by recombinant adeno-associated virus (rAAV) vectors

Engagement of the B-cell antigen receptor (BCR) by crosslinking of the surface immunoglobulin (sIg) homodimer was studied for recombinant adeno-associated virus (rAAV)-mediated gene transfer into B-cell chronic lymphocytic leukaemia (B-CLL) cells. Leukemic cells obtained from 20 patients were stimulated with anti-sIg-directed antibodies and transduced with rAAV vectors coding for enhanced green fluorescent protein (EGFP) (AAV/EGFP) or CD40L (AAV/CD40L). Transduction of B-CLL cells was enhanced after BCR engagement compared to unstimulated controls (P=0.0356). BCR crosslinking induced a significant, dose- and time-dependent upregulation of heparan sulfate proteoglycan (HSPG), the primary receptor for AAV, on B-CLL cells (mean: 38.2 versus 1.7%; P=0.0006). A correlation of HSPG expression after BCR crosslinking with transduction efficiency by AAV/EGFP (P=0.0153) and AAV/CD40L (P=0.0347) was observed. High expression of zeta-associated protein 70 (ZAP-70) in B-CLL cells correlated with a better transduction efficiency by AAV/EGFP (P<0.0001) and AAV/CD40L (P=0.002), respectively: 48 h after transduction of ZAP-70-positive samples, transgene expression was seen in a mean of 33.8% (s.e.m. 3.7%) and 28.9% (s.e.m. 6.7%) of cells, respectively, and could be specifically blocked by heparin, a soluble competitor of HSPG (P<0.0001). In summary, engagement of the BCR on ZAP-70 positive B-CLL cells allows efficient rAAV-mediated gene delivery.

Parvovirus vectors for cancer gene therapy

Parvoviruses comprise a group of single-stranded DNA viruses with greater potential for gene therapy applications. Unique characteristics of paroviruses, such as non-pathogenicity, antioncogenicity and methods of efficient recombinant vector production, have drawn more attention towards utilising parvovirus-based vectors in cancer gene therapy. Although > 30 different parvoviruses have been identified so far, recombinant vectors derived from adeno-associated virus (AAV), minute virus of mice (MVM), LuIII and parvovirus H1 have been successfully tested in many preclinical models of human diseases, including cancer. The present article will focus on the potential of non-replicating and autonomously replicating parvoviral vectors in cancer gene therapy, including strategies that target tumour cells directly or indirectly.

Efficient gene transfer of CD40 ligand into primary B-CLL cells using recombinant adeno-associated virus (rAAV) vectors

B cells of chronic lymphocytic leukemia (B-CLL) are resistant to transduction with most currently available vector systems. Using an optimized adenovirus-free packaging system, recombinant adeno-associated virus (rAAV) vectors coding for the enhanced green fluorescent protein (AAV/EGFP) and CD40 ligand (AAV/CD40L) were packaged and highly purified resulting in genomic titers up to 3 × 1011/mL. Cells obtained from 24 patients with B-CLL were infected with AAV/EGFP or AAV/CD40L at a multiplicity of infection (MOI) of 100 resulting in transgene expression in up to 97% of cells as detected by flow cytometry 48 hours after infection. Viral transduction could be specifically blocked by heparin. Transduction with AAV/CD40L resulted in up-regulation of the costimulatory molecule CD80 not only on infected CLL cells but also on noninfected bystander leukemia B cells, whereas this effect induced specific proliferation of HLA-matched allogeneic T cells. Vaccination strategies for patients with B-CLL using leukemia cells infected ex vivo by rAAV vectors now seems possible in the near future.

Transduction of ovarian cancer cells: a recombinant adeno-associated viral vector compared to an adenoviral vector

Recombinant adeno-associated virus (rAAV) vectors have emerged as vehicles for gene therapy. In addition, anti-neoplastic properties have been attributed to wild-type AAV. To take advantage of both features and to overcome technical problems associated with rAAV preparation, we developed a production method in which rAAV particles are amplified in an infectious cycle in the presence of wtAAV. This results in a 10(3)-10(4)-fold amplification of rAAV input particles. rAAV-GFP particles generated by this method were used to transduce ovarian cancer cell lines to evaluate their potential in ovarian cancer gene therapy, in comparison to a rAd-GFP vector. The transduction efficiency of NIH-OVCAR3, MDAH 2774 and SKOV3 cells with rAAV-GFP particles was low (< 1%) and did not improve by increasing the number of particles/cell. Repeated administration and continued exposure of NIH-OVCAR3 and MDAH 2774 improved transduction to over 3%. In contrast, these cell lines were more efficiently transduced by rAAV-GFP in the presence of adenovirus (approximately 15%) and by rAd-GFP (> 50%). These results indicate that in contrast to rAd vectors, rAAV particles are not suitable for therapeutic gene transfer in ovarian cancer cells unless efficient help can be provided to mediate ss to ds DNA conversion.

Standard heparin, low molecular weight heparin, low molecular weight heparinoid, and recombinant hirudin differ in their ability to inhibit transduction by recombinant adeno-associated virus type 2 vectors

Recombinant adeno-associated virus type 2 (rAAV) is a promising vector for in vivo gene therapy. Transduction by rAAV requires binding to heparan sulfate proteoglycan on the cell surface, and heparin can block this binding. Because heparin is administered to most patients undergoing cardiovascular gene transfer in order to prevent thrombotic events, it is important to identify anticoagulants which do not interfere with rAAV transduction. Therefore, we examined the influence of different anticoagulants on rAAV transduction in vitro. rAAV transduction was inhibited by 40.5 +/- 7.9% at heparin concentrations of 0.1 U/ml, and by 81.7 +/- 3.6% at 1.0 U/ml. The low molecular weight (LMW) heparin tinzaparin inhibited rAAV transduction by 20.2 +/- 3.8% at 0.1 U/ml and 37.1 +/- 1.8% at 1.0 U/ml. The inhibitory effect was significantly weaker compared with heparin at 1.0 U/ml, (P < 0.01). The LMW heparinoid danaparoid inhibited rAAV transduction by 8.8 +/- 3.5% at 0.1 U/ml (P < 0.01 compared with heparin). In contrast, recombinant hirudin did not interfere at all with rAAV transduction. In summary, the results demonstrate that inhibition of rAAV transduction by heparin occurs rapidly and at therapeutically used concentrations. LMW heparinoids and above all recombinant hirudin might be alternatives for heparin when vascular gene transfer with rAAV requires transient anticoagulation.

The adenovirus E4 ORF6 and E1b 55 kDa proteins cooperate in a p53-independent manner to enhance transduction by recombinant adeno-associated virus vectors

The observation that exposure of target cells to genotoxic stress or adenovirus infection enhances recombinant adeno-associated virus (rAAV) transduction is an important lead towards defining the rAAV transduction mechanism, and has significant implications for the exploitation of rAAV in gene therapy applications. The adenovirus-mediated enhancement of rAAV transduction has been mapped to the E4 ORF6 gene, and expression of E4 ORF6 alone has been considered necessary and sufficient to mediate this effect. Since p53 subserves an important function in the cellular response to genotoxic stress, and interacts with the E4 ORF6 gene product during adenovirus infection, we hypothesized that p53 function might be essential to the rAAV enhancement resulting from these cellular insults. In the current study, using the p53-null cell lines H1299 and Saos-2, we find that p53 is not essential to either genotoxic stress or adenovirus-mediated enhancement of rAAV transduction. We further demonstrate using HeLa, H1299 and Saos-2 cells that E4 ORF6 expression alone is not sufficient to enhance rAAV transduction and that coexpression of the adenovirus E1b 55 kDa protein is necessary. Together, these observations indicate that the mechanism by which adenovirus infection enhances rAAV transduction involves cooperative and interdependent functions of the E4 ORF6 and E1b 55 kDa proteins that are p53-independent.

Transduction of hepatocellular carcinoma (HCC) using recombinant adeno-associated virus (rAAV): in vitro and in vivo effects of genotoxic agents

BACKGROUND/AIMS

Adeno-associated virus (AAV) is an attractive tool for gene therapy. Here we investigated the in vitro and in vivo transduction of hepatocellular carcinoma (HCC) cells by an AAV vector and the efficacy of different strategies to enhance the transduction of the tumor.

METHODS

Transduction efficiency was determined by analyzing AAV-mediated beta-galactosidase gene (rAAV/lacZ) expression.

RESULTS

Adenovirus help or pretreatment of HCC cells with y-irradiation or with the topoisomerase inhibitor etoposide resulted in marked enhancement of cell transduction in vitro. In vivo studies in nude mice with subcutaneous HCC tumors showed that HCC cells were not transduced by AAV vector alone. However, co-infection of the tumor with adenovirus allowed an efficient expression of the reporter gene but only at the sites of vector injection. Previous gamma-irradiation of subcutaneous tumors with 1800 rad was able to improve transduction of HCC cells (up to 30%) using recombinant AAV. Continuous i.p. infusion of etoposide in buffalo rats harboring HCC tumors in the liver resulted in transduction of normal liver tissue and also of very small neoplastic lesions (<2 mm) but no transduction was observed in tumors bigger than 2 mm. To analyze this phenomenon we determined etoposide concentration in hepatic tissue. Our results revealed high concentrations of the drug in non-tumoral tissue but almost undetectable levels in big tumor nodules.

CONCLUSIONS

Our results indicate that while both radiotherapy and etoposide enhance transduction of tumor cells by rAAV in vitro, only radiotherapy increases tumor transduction in vivo. Our data suggest the existence of a barrier which limits in vivo the diffusion of chemotherapeutic agents to well-established HCC nodules.

Superior gene transfer into solid tumour cells than into human mobilised peripheral blood progenitor cells using helpervirus-free adeno-associated viral vector stocks

Autologous peripheral blood progenitor cell (PBPC) grafts can be contaminated with tumour cells that potentially give rise to relapse following myeloablative therapy and PBPC transplantation. Adeno-associated virus (AAV)-based vectors produced by a new adenovirus-free technique are a gene delivery system which may be applicable for tumour cell purging. To test for the host range of these vectors, solid tumours of clinical relevance and normal CD34+ PBPC were selected as target cells for an AAV-vector, encoding the green-fluorescent protein (GFP) as the indicator gene. At a multiplicity of infection (MOI) of 100: 79.94% +/- 14.36% (mean +/- SEM) of the connective tissue sarcoma cell line (HS-1) and 64.84% +/- 6.91% of the cervical carcinoma cell line cells (HeLa-RC) expressed GFP while the other cell lines tested (1 ovarian tumour, 1 germ cell tumour, 1 osteosarcoma, 2 small cell lung cancer) ranged between 2.82% and 11.94%. Optimising the transduction protocol by use of higher MOIs of up to 500 and by pretreatment with the tyrosine kinase inhibitor, genistein, resulted in up to 95.97% and 94.10% green-fluorescent HS-1 and HeLa-RC cells, respectively. In contrast, only 1.39% +/- 0.51% of the normal haematopoietic CD34+ progenitor cells expressed GFP at a MOI of 100. The differential infectivity between HS-1 and CD34+ cells was maintained after tumour cell spiking in leucapheresis products. Our observations suggest that AAV-based vectors may prove useful for purging of autologous PBPC grafts from solid tumour cells.

Recombinant AAV-2 harboring gfp-antisense/ribozyme fusion sequences monitor transduction, gene expression, and show anti-HIV-1 efficacy

Vector-mediated delivery of potentially antivirally active genes is a key step in somatic gene therapy including therapeutic approaches against AIDS. A crucial technical prerequisite is to monitor DNA transfer into target cells. Here, we describe recombinant infectious particles derived from the adeno-associated virus type 2 (AAV-2) that are suitable to deliver effective HIV-1-directed antisense and ribozyme genes into target cells. To monitor transduction, we designed and tested a number of fusions between indicator-coding sequences of luciferase or gfp with effective HIV-1-directed antisense or ribozyme sequences. The combination of an indicator function and an antiviral func- tion in cis allows successful identification of transduced cells and measurement of effects on the replication of HIV-1 in antisense/ribozyme-expressing cells only. The fusion genes were shown to express the indicator genes. Inhibition of HIV-1 replication mediated by the antisense/ribozyme portion of the fusion transcripts was similar to parental constructs and neither acute nor long-term toxicity of fusion genes and their gene products was observed. These results suggest the use of rAAV constructs described here as tools to study the transducibility of target cells, gene expression and efficacy of HIV-1-directed antisense and ribozyme genes.

Adeno-associated virus-mediated gene delivery

Several gene delivery vehicles are being developed for somatic gene therapy and each of these vectors has unique properties which makes them appropriate for different human disease applications. Recombinant adeno-associated viral (rAAV) vectors are proving themselves to be safe and efficacious for the long-term expression of proteins and correction of genetic diseases following a single administration. The increasing number of tissues and diseases being targeted with rAAV vectors demonstrates their versatility and has resulted in different approaches for enhancing vector performance. Improving the methods for large-scale manufacturing, and accumulating safety and efficacy data in animals and humans are areas of intense research.

Efficient gene transfer into human keratinocytes with recombinant adeno-associated virus vectors

Gene transfer into the skin is a promising approach to treat inherited or acquired dermatological diseases and systemic monogenic deficiencies. For this purpose, the efficient and sustained gene delivery into keratinocytes is of critical importance. Recombinant adeno-associated virus (rAAV) vectors hold the potential to achieve a long-term gene transfer into various human organs. In order to evaluate this potential for skin gene therapy, human keratinocytes were transduced in vitro with rAAV vectors encoding the reporter genes beta-galactosidase (rAAV/LacZ) or green fluorescent protein (rAAV/GFP). Using rAAV/LacZ at a multiplicity of infection (MOI) of five transducing particles per cell, up to 70% of human keratinocytes were transduced within 48 h. This effect was independent of individual skin donors and different body areas serving as the source for keratinocyte isolation. rAAV had no significant influence on cell viability, but induced a growth arrest in transduced keratinocytes. This growth arrest was overcome by replating cells in fresh media. rAAV/GFP-transduced keratinocytes could be passaged several times, expressed GFP for up to 50 days, and passed the transgene to their daughter cells, suggesting that keratinocyto precursor cells were also transduced. Taken together, the results suggest that rAAV is a promising gene transfer vehicle for skin gene therapy.