Optimizing lentiviral vector formulation conditions for efficient ex vivo transduction of primary human T cells in chimeric antigen receptor T-cell manufacturing

BACKGROUND AIMS

Chimeric antigen receptor (CAR) T-cell products are commonly generated using lentiviral vector (LV) transduction. Optimal final formulation buffer (FFB) supporting LV stability during cryostorage is crucial for cost-effective manufacturing.

METHODS

To identify the ideal LV FFB composition for ex vivo CAR-T production, primary human T cells were transduced with vesicular stomatitis virus G-protein (VSV-G) -pseudotyped LVs (encoding a reporter gene or an anti-CD19-CAR). The formulations included phosphate-buffered saline (PBS), HEPES, or X-VIVOTM 15, and stabilizing excipients. The functional and viral particle titers and vector copy number were measured after LV cryopreservation and up to 24 h post-thaw incubation. CAR-Ts were produced with LVs in selected FFBs, and the resulting cells were characterized.

RESULTS

Post-cryopreservation, HEPES-based FFBs provided higher LV functional titers than PBS and X-VIVOTM 15, and 10% trehalose-20 mM MgCl2 improved LV transduction efficiency in PBS and 50 mM HEPES. Thawed vectors remained stable at +4°C, while a ≤ 25% median decrease in the functional titer occurred during 24 h at room temperature. Tested excipients did not enhance LV post-thaw stability. CAR-Ts produced using LVs cryopreserved in 10% trehalose- or sucrose-20 mM MgCl2 in 50 mM HEPES showed comparable transduction rates, cell yield, viability, phenotype, and in vitro functionality.

CONCLUSION

A buffer consisting of 10% trehalose-20 mM MgCl2 in 50 mM HEPES provided a feasible FFB to cryopreserve a VSV-G -pseudotyped LV for CAR-T-cell production. The LVs remained relatively stable for at least 24 h post-thaw, even at ambient temperatures. This study provides insights into process development, showing LV formulation data generated using the relevant target cell type for CAR-T-cell manufacturing.

Corrigendum: Optimized riboswitch-regulated AAV vector for VEGF-B gene therapy

[This corrects the article DOI: 10.3389/fmed.2022.1052318.].

Optimized riboswitch-regulated AAV vector for VEGF-B gene therapy

Gene therapy would greatly benefit from a method to regulate therapeutic gene expression temporally. Riboswitches are small RNA elements that have been studied for their potential use in turning transgene expression on or off by ligand binding. We compared several tetracycline and toyocamycin-inducible ON-riboswitches for a drug responsive transgene expression. The tetracycline-dependent K19 riboswitch showed the best control and we successfully applied it to different transgenes. The induction of gene expression was 6- to 10-fold, dose-dependent, reversible, and occurred within hours after the addition of a clinically relevant tetracycline dose, using either plasmid or adeno-associated virus (AAV) vectors. To enhance the switching capacity, we further optimized the gene cassette to control the expression of a potential therapeutic gene for cardiovascular diseases, VEGF-B. Using two or three riboswitches simultaneously reduced leakiness and improved the dynamic range, and a linker sequence between the riboswitches improved their functionality. The riboswitch function was promoter-independent, but a post-transcriptional WPRE element in the expression cassette reduced its functionality. The optimized construct was a dual riboswitch at the 3' end of the transgene with a 100 bp linker sequence. Our study reveals significant differences in the function of riboswitches and provides important aspects on optimizing expression cassette designs. The findings will benefit further research and development of riboswitches.

Functional roles of the membrane-associated AAV protein MAAP

With a limited coding capacity of 4.7 kb, adeno-associated virus (AAV) genome has evolved over-lapping genes to maximise the usage of its genome. An example is the recently found ORF in the cap gene, encoding membrane-associated accessory protein (MAAP), located in the same genomic region as the VP1/2 unique domain, but in a different reading frame. This 13 KDa protein, unique to the dependovirus genus, is not homologous to any known protein. Our studies confirm that MAAP translation initiates from the first CTG codon found in the VP1 ORF2. We have further observed MAAP localised in the plasma membrane, in the membranous structures in close proximity to the nucleus and to the nuclear envelope by co-transfecting with plasmids encoding the wild-type AAV (wt-AAV) genome and adenovirus (Ad) helper genes. While keeping VP1/2 protein sequence identical, both inactivation and truncation of MAAP translation affected the emergence and intracellular distribution of the AAV capsid proteins. We have demonstrated that MAAP facilitates AAV replication and has a role in controlling Ad infection. Additionally, we were able to improve virus production and capsid integrity through a C-terminal truncation of MAAP while other modifications led to increased packaging of contaminating, non-viral DNA. Our results show that MAAP plays a significant role in AAV infection, with profound implications for the production of therapeutic AAV vectors.

Benchmarking of Scale-X Bioreactor System in Lentiviral and Adenoviral Vector Production

We have previously produced viral vectors (lentiviral vector, adenoviral vector, and adeno-associated viral vector) in small and in commercial scale in adherent cells using Pall fixed-bed iCELLis^® bioreactor. Recently, a company called Univercells has launched a new fixed-bed bioreactor with the same cell growth surface matrix material, but with different fixed-bed structure than is used in iCELLis bioreactor. We sought to compare the new scale-X™ hydro bioreactor (2.4 m²) and iCELLis Nano system (2.67 m²) to see if the difference has any effect on cell growth or lentiviral vector and adenoviral vector productivity. Runs were performed using parameters optimized for viral vector production in iCELLis Nano bioreactor. Cell growth was monitored by counting nuclei, as well as by following glucose consumption and lactate production. In both bioreactor systems, cells grew well, and the cell distribution was found quite homogeneous in scale-X bioreactor. Univercells scale-X bioreactor was proven to be at least equally efficient or even improved in both lentiviral vector and adenoviral vector production. Based on the results, the same protocol and parameters used in viral vector production in iCELLis bioreactor can also be successfully used for the production in scale-X bioreactor system.

Evaluation of the Single-Use Fixed-Bed Bioreactors in Scalable Virus Production

The accelerating development of gene therapy from research towards clinical trials and beyond has elevated the demand for practical viral vector-manufacturing solutions. The use of disposable upstream technology is gaining traction in clinical manufacturing. Packed-bed or fixed-bed reactors, where column is packed with immobilized biocatalyst particles providing surface to constrain the cells in a particular region of the reactor, have been widely used in bioprocessing applications since mid-1900s. However, the world's first single-use, fully integrated, high cell density, fixed-bed bioreactor was launched only approximately a decade ago. By now, several single-use, fixed-bed technology solutions have been developed in a small scale. Scaling-up the manufacturing can be challenging and for commercial-scale manufacturing, there is practically only one single-use, good manufacturing practice-compliant option available. This study reviews the latest, fully disposable, fixed-bed bioreactors; compares the virus production in the different systems; and discusses important manufacturing cost-related topics. It is predicted that single-use, fixed-bed bioreactors will receive even more attention in the field of viral vector manufacturing and commercialization, especially with the need for higher virus titers and virus yields.

Detection of lentiviral suicide gene therapy in C6 rat glioma using hyperpolarised [1-13 C]pyruvate

Hyperpolarised [1-¹³ C]pyruvate MRI has shown promise in monitoring therapeutic efficacy in a number of cancers including glioma. In this study, we assessed the pyruvate response to the lentiviral suicide gene therapy of herpes simplex virus-1 thymidine kinase with the prodrug ganciclovir (HSV-TK/GCV) in C6 rat glioma and compared it with traditional MR therapy markers. Female Wistar rats were inoculated with 10⁶ C6 glioma cells. Treated animals received intratumoural lentiviral HSV-TK gene transfers on days 7 and 8 followed by 2-week GCV therapy starting on day 10. Animals were repeatedly imaged during therapy using volumetric MRI, diffusion and relaxation mapping, as well as metabolic [1-¹³ C]pyruvate MRS imaging. Survival (measured as time before animals reached a humane endpoint and were euthanised) was assessed up to day 30 posttherapy. HSV-TK/GCV gene therapy lengthened the median survival time from 12 to 25 days. This was accompanied by an apparent tumour growth arrest, but no changes in diffusion or relaxation parameters in treated animals. The metabolic response was more evident in the case-by-case analysis than in the group-level analysis. Treated animals also showed a 37 ± 15% decrease (P < 0.05, n = 5) in lactate-to-pyruvate ratio between therapy weeks, whereas a 44 ± 18% increase (P < 0.05, n = 6) was observed in control animals. Hyperpolarised [1-¹³ C]pyruvate MRI can offer complementary metabolic information to traditional MR methods to give a more comprehensive picture of the slowly developing gene therapy response. This may benefit the detection of the successful therapy response in patients.

Development of Large-Scale Downstream Processing for Lentiviral Vectors

The interest in lentiviral vectors (LVs) has increased prominently for gene therapy applications, but few have reached the later stages of clinical trials. The main challenge has remained in scaling up the manufacturing process for the fragile vector to obtain high titers for in vivo usage. We have previously scaled up the LV production to iCELLis 500, being able to produce up to 180 L of harvest material in one run with perfusion. The following challenge considers the purification and concentration of the product to meet titer and purity requirements for clinical use. We have developed a downstream process, beginning with clarification, buffer exchange, and concentration, by tangential flow filtration. This is followed by a purification step using single membrane-based anion exchange chromatography and final formulation with tangential flow filtration. Different materials and conditions were compared to optimize the process, especially for the chromatography step that has been the bottleneck in lentiviral vector purification scale-up. The final infectious titer of the lentiviral vector product manufactured using the optimized scale-up process was determined to be 1.97 × 10⁹ transducing units (TU)/mL, which can be considered as a high titer for lentiviral vectors.

Preclinical Proof-of-Concept, Analytical Development, and Commercial Scale Production of Lentiviral Vector in Adherent Cells

The therapeutic efficacy of a lentiviral vector (LV) expressing the herpes simplex virus thymidine kinase (HSV-TK) was studied in an immunocompetent rat glioblastoma model. Intraperitoneal ganciclovir injections (50 mg/kg/day) were administered for 14 consecutive days, resulting in reduced tumor volumes as monitored by MRI. Survival analyses revealed a significant improvement among the LV-expressing HSV-TK (LV-TK)/ganciclovir-treated animals when compared to non-treated control rats. However, a limiting factor in the use of LV has been the suboptimal small-scale production in flasks. Our aim during the translation phase, prior to entering the final pre-clinical and early clinical phases, was to develop a scalable, robust, and disposable manufacturing process for LV-TKs. We also aimed to minimize future process changes and enable production upscaling to make the process suitable for larger patient populations. The upstream process relies on fixed-bed iCELLis technology and transient plasmid transfection. This is the first time iCELLis 500 commercial-scale bioreactor was used for LV production. A testing strategy to determine the pharmacological activity of LV-TK drug product by measuring cell viability was developed, and the specificity of the potency assay was also proven. In this paper we focus on upstream process development while showing analytical development and the proof-of-concept of LV-TK functionality.

Tissue damage drives co-localization of NF-κB, Smad3, and Nrf2 to direct Rev-erb sensitive wound repair in mouse macrophages

Although macrophages can be polarized to distinct phenotypes in vitro with individual ligands, in vivo they encounter multiple signals that control their varied functions in homeostasis, immunity, and disease. Here, we identify roles of Rev-erb nuclear receptors in regulating responses of mouse macrophages to complex tissue damage signals and wound repair. Rather than reinforcing a specific program of macrophage polarization, Rev-erbs repress subsets of genes that are activated by TLR ligands, IL4, TGFβ, and damage-associated molecular patterns (DAMPS). Unexpectedly, a complex damage signal promotes co-localization of NF-κB, Smad3, and Nrf2 at Rev-erb-sensitive enhancers and drives expression of genes characteristic of multiple polarization states in the same cells. Rev-erb-sensitive enhancers thereby integrate multiple damage-activated signaling pathways to promote a wound repair phenotype.

DOI:http://dx.doi.org/10.7554/eLife.13024.001

Process Development of Adenoviral Vector Production in Fixed Bed Bioreactor: From Bench to Commercial Scale

Large-scale vector manufacturing for phase III and beyond has proven to be challenging. Upscaling the process with suspension cells is increasingly feasible, but many viral production applications are still applicable only in adherent settings. Scaling up the adherent system has proven to be troublesome. The iCELLis(®) disposable fixed-bed bioreactors offer a possible option for viral vector manufacturing in large quantities in an adherent environment. In this study, we have optimized adenovirus serotype 5 manufacturing using iCELLis Nano with a cultivation area up to 4 m(2). HEK293 cell cultivation, infection, and harvest of the virus (by lysing the cells inside the bioreactor) proved possible, reaching total yield of up to 1.6×10(14) viral particles (vp)/batch. The iCELLis 500 is designed to satisfy demand for large-scale requirements. Inoculating a large quantity of cell mass into the iCELLis 500 was achieved by first expanding the cell mass in suspension. Upscaling the process into an iCELLis 500/100 m(2) cultivation area cassette was practical and produced up to 6.1×10(15) vp. Flask productivity per cm(2) in iCELLis Nano and iCELLis 500 was in the same range. As a conclusion, we showed for the first time that iCELLis 500 equipment has provided an effective way to manufacture large batches of adenoviral vectors.

Differential Promoter Methylation of Macrophage Genes Is Associated With Impaired Vascular Growth in Ischemic Muscles of Hyperlipidemic and Type 2 Diabetic Mice: Genome-Wide Promoter Methylation Study

RATIONALE

Hyperlipidemia and type 2 diabetes mellitus (T2DM) severely impair adaptive vascular growth responses in ischemic muscles. This is largely attributed to dysregulated gene expression, although details of the changes are unknown.

OBJECTIVE

To define the role of promoter methylation in adaptive vascular growth in hyperlipidemia (LDLR(-/-)ApoB(100/100)) and T2DM (IGF-II/LDLR(-/-)ApoB(100/100)) mouse models of hindlimb ischemia.

METHODS AND RESULTS

Unilateral hindlimb ischemia was induced by ligating femoral artery. Perfusion was assessed using ultrasound, and capillary and arteriole parameters were assessed using immunohistochemistry. Genome-wide methylated DNA sequencing was performed with DNA isolated from ischemic muscle, tissue macrophages (Mϕs), and endothelial cells. Compared with the controls, hyperlipidemia and T2DM mice showed impaired perfusion recovery, which was associated with impaired angiogenesis and arteriogenesis. Genome-wide proximal promoter DNA methylation analysis suggested differential patterns of methylation in Mϕ genes in ischemic muscles. Classically activated M1-Mϕ gene promoters, including Cfb, Serping1, and Tnfsf15, were significantly hypomethylated, whereas alternatively activated M2-Mϕ gene promoters, including Nrp1, Cxcr4, Plxnd1, Arg1, Cdk18, and Fes, were significantly hypermethylated in Mϕs isolated from hyperlipidemia and T2DM ischemic muscles compared with controls. These results combined with mRNA expression and immunohistochemistry showed the predominance of proinflammatory M1-Mϕs, compared with anti-inflammatory and proangiogenic M2-Mϕs in hyperlipidemia and T2DM ischemic muscles.

CONCLUSIONS

We found significant promoter hypomethylation of genes typical for proinflammatory M1-Mϕs and hypermethylation of anti-inflammatory, proangiogenic M2-Mϕ genes in hyperlipidemia and T2DM ischemic muscles. Epigenetic alterations modify Mϕ phenotype toward proinflammatory M1 as opposed to anti-inflammatory, proangiogenic, and tissue repair M2 phenotype, which may contribute to the impaired adaptive vascular growth under these pathological conditions.

Soluble vascular endothelial growth factor receptor-1 improves therapeutic efficacy of suicide gene therapy in an angiogenesis-independent manner

Abstract Malignant gliomas (MGs) are highly vascularized, aggressive brain cancers carrying a dismal prognosis. Because of their high vascularity, anti-angiogenic therapy is a potential treatment option. Indeed, the anti-vascular endothelial growth factor (VEGF) antibody bevacizumab has demonstrated promising results in clinical trials. Similarly, adenovirus-medicated Herpes simplex virus thymidine kinase and ganciclovir (AdHSV-tk/GCV) suicide gene therapy has established itself in clinical trials as a potential novel therapeutic strategy for MGs. In this study, we demonstrate the feasibility of combining adenovirus-mediated soluble VEGF receptor-1 anti-angiogenic gene therapy with AdHSV-tk/GCV suicide gene therapy to treat experimental MGs. Our results reveal that, apart from inhibiting angiogenesis, other anti-tumor mechanisms, such as reduction of infiltration by tumor-associated macrophages/microglia, may contribute to the improved therapeutic benefit of combination therapy.

Biodistribution and antitumor effect of Cetuximab-targeted lentivirus

Viral vectors are central tools for gene therapy. Targeting of the vector to desired tissues followed by expression of the therapeutic gene forms one of the most critical points in effective therapy. In this study we used streptavidin-displaying lentivirus conjugated to biotinylated anti-epidermal growth factor receptor (EGFR) antibody (Cetuximab) to target vector specifically to ovarian tumors. Biodistribution of the targeted virus was studied in nude mice with orthotropic SKOV-3m human ovarian carcinoma xenografts. Radiolabeled antibodies were conjugated to streptavidin-displaying lentiviruses and biodistribution of the virus after the intravenous delivery to tumor-bearing mice was monitored up to 6 days using combined SPECT/CT imaging modality. Organ samples were collected post mortem and specific organ activities were measured. The integration of lentivirus vectors in collected tissue samples was analyzed using qPCR and the expression of green fluorescent protein (GFP)-transgene was tested by enzyme-linked immunosorbent assay. Our results showed that lentiviruses conjugated to Cetuximab (Cet-LV) or control human IgG (IgG-LV) accumulated mainly to the liver and spleen of the mice and to lower extent to lung, kidneys and tumors. Strikingly, in 50% of the mice injected with cetuximab-targeted lentivirus no tumor tissue was found, whereas the remaining half showed a significant decrease in tumor size. We hypothesize/present data that lentivirus-mediated INF-αβ production together with tumor targeting could function as an effective antitumor treatment.

Oxidative stress-regulated lentiviral TK/GCV gene therapy for lung cancer treatment

Nuclear factor erythroid-2 related factor 2 (Nrf2) is a transcription factor that regulates protection against a wide variety of toxic insults to cells, including cytotoxic cancer chemotherapeutic drugs. Many lung cancer cells harbor a mutation in either Nrf2 or its inhibitor Keap1 resulting in permanent activation of Nrf2 and chemoresistance. In this study, we sought to examine whether this attribute could be exploited in cancer suicide gene therapy by using a lentiviral (LV) vector expressing herpes simplex virus thymidine kinase (HSV-TK/GCV) under the regulation of antioxidant response element (ARE), a cis-acting enhancer sequence that binds Nrf2. In human lung adenocarcinoma cells in which Nrf2 is constitutively overexpressed, ARE activity was found to be high under basal conditions. In this setting, ARE-HSV-TK was more effective than a vector in which HSV-TK expression was driven by a constitutively active promoter. In a mouse xenograft model of lung cancer, suicide gene therapy with LV-ARE-TK/GCV was effective compared with LV-PGK-TK/GCV in reducing tumor size. We conclude that ARE-regulated HSV-TK/GCV therapy offers a promising approach for suicide cancer gene therapy in cells with high constitutive ARE activity, permitting a greater degree of therapeutic targeting to those cells.

Baculovirus-mediated vascular endothelial growth factor-D(ΔNΔC) gene transfer induces angiogenesis in rabbit skeletal muscle

BACKGROUND

Occluded arteries and ischemic tissues cannot always be treated by angioplasty, stenting or by-pass-surgery. Under such circumstances, viral gene therapy may be useful in inducing increased blood supply to ischemic area. There is evidence of improved blood flow in ischemic skeletal muscle and myocardium in both animal and human studies using adenoviral vascular endothelial growth factor (VEGF) gene therapy. However, the expression is transient and repeated gene transfers with the same vector are inefficient due to immune responses.

METHODS

Different baculoviral vectors pseudotyped with or without vesicular stomatitis virus glycoprotein (VSV-G) and/or carrying woodchuck hepatitis virus post-transcriptional regulatory element (Wpre) were tested both in vitro and in vivo. VEGF-D(ΔNΔC) was used as therapeutic transgene and lacZ as a control. In vivo efficacy was evaluated as capillary enlargement and transgene expression in New Zealand White (NZW) rabbit skeletal muscle.

RESULTS

A statistically significant capillary enlargement was detected 6 days after gene transfer in transduced areas compared to the control gene transfers with baculovirus and adenovirus encoding β-galactosidase (lacZ). Substantially improved gene transfer efficiency was achieved with a modified baculovirus pseudotyped with VSV-G and carrying Wpre. Dose escalation experiments revealed that either too large volume or too many virus particles caused inflammation and necrosis in the target tissue, whereas 10(9) plaque forming units injected in multiple aliquots resulted in transgene expression with only mild immune reactions.

CONCLUSIONS

We show the first evidence of biologically significant baculoviral gene transfer in skeletal muscle of NZW rabbits using VEGF-D(ΔNΔC) as a therapeutic transgene.

Endothelium-specific overexpression of human vascular endothelial growth factor-D in mice leads to increased tumor frequency and a reduced lifespan

BACKGROUND

Vascular endothelial growth factors (VEGFs) are central mediators in vascular development and lymphangiogenesis. VEGF-D contributes to the growth and formation of blood and lymphatic vessels, although its biological role is still somewhat unclear.

METHODS

Transgenic mice, which express the mature form of human VEGF-D under endothelium-specific Tie1 promoter, were produced by the lentiviral perivitelline-injection method. The mice were followed up to generation F(5) and the effect of the transgene was analyzed.

RESULTS

Transgenic mice had a high expression of human (h)VEGF-D in the endothelium in several tissues, such as kidney, liver, lung and spleen. However, transgenic mice developed tumors in lungs, kidneys, liver, mammary glands and lymph nodes upon aging and their mortality was also increased as a result of other pathological conditions. Hind limb ischemia was surgically induced in these mice and they were analyzed 1, 2 and 3  weeks after the ischemia operation. No significant differences were found in hVEGF-D mRNA expression, the number of capillaries or tissue repair between ischemic transgenic mice and transgene negative littermates.

CONCLUSIONS

It is concluded that targeted unregulated long-term expression of hVEGF-D in endothelium may not be useful and reduces the life span of transgenic mice.

Avidin-biotin technology in targeted therapy

IMPORTANCE OF THE FIELD

The goal of drug targeting is to increase the concentration of the drug in the vicinity of the cells responsible for disease without affecting healthy cells. Many approaches in cancer treatment are limited because of their broad range of unwanted side effects on healthy cells. Targeting can reduce side effects and increase efficacy of drugs in the patient.

AREAS COVERED IN THIS REVIEW

Avidin, originally isolated from chicken eggs, and its bacterial analogue, streptavidin, from Streptomyces avidinii, have extremely high affinity for biotin. This unique feature is the basis of avidin-biotin technology. This article reviews the current status of avidin-biotin systems and their use for pretargeted drug delivery and vector targeting.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the following approaches using the avidin-biotin system: i) targeting antibodies and therapeutic molecules are administered separately leading to a reduction of drug dose in normal tissues compared with conventional (radio)immunotherapies; ii) introducing avidin gene into specific tissues by local gene transfer, which subsequently can sequester and concentrate considerable amounts of therapeutic ligands; and iii) enabling transductional targeting of gene therapy vectors.

TAKE HOME MESSAGE

Avidin and biotin technology has proved to be an extremely versatile tool with broad applications, such as pretargeting, delivering avidin gene into cells enabling targeting of biotinylated compounds and targeting of viral vectors.

Avidin fusion protein-expressing lentiviral vector for targeted drug delivery

One of the main objectives of cancer therapy is to enhance the effectiveness of the drug by concentrating it at the target site and to minimize the undesired side effects to nontarget cells. We have previously constructed a fusion protein, Lodavin, consisting of avidin and the endocytotic part of the low-density lipoprotein receptor, and demonstrated its applicability to transient drug targeting in vivo. In this study we produced a lentiviral vector expressing this fusion protein and evaluated its safety and efficacy. The results showed that lentivirus-mediated gene transfer led to long-term avidin fusion protein expression on glioma cells and that the receptor was able to bind biotinylated compounds. Repeated administration was proven feasible and the optimal time frame(s) for administration of biotinylated therapeutic and/or imaging compounds was elucidated. Intravenous or intracranial injection of the virus into BDIX rats led to the production of antibodies against transgene (avidin), but repeated administration of the vector was unable to boost this effect. Neutralizing antibodies against the lentivirus were also detected. Furthermore, we showed that the anti-avidin antibodies did not significantly affect the ligand-binding capacity of the avidin fusion protein. The therapeutic efficacy of avidin fusion protein in tumor treatment was tested in vitro with biotinylated and nonbiotinylated nanoparticles loaded with paclitaxel. In vivo applicability of lentivirus was studied in the BDIX rat glioma model, in which high receptor expression was detected in the tumor area. The lentivirus-mediated delivery of the avidin fusion protein thus represents a potential approach for the repeated targeting of cytotoxic compounds to cancer cells.

A 96-well format for a high-throughput baculovirus generation, fast titering and recombinant protein production in insect and mammalian cells

Background

Baculovirus expression vector system (BEVS) has become a standard in recombinant protein production and virus-like particle preparation for numerous applications.

Findings

We describe here protocols which adapt baculovirus generation into 96-well format.

Conclusion

The established methodology allows simple baculovirus generation, fast virus titering within 18 h and efficient recombinant protein production in a high-throughput format. Furthermore, the produced baculovirus vectors are compatible with gene expression in vertebrate cells in vitro and in vivo.

Optimized self-excising Cre-expression cassette for mammalian cells

We observed that overexpression of Cre recombinase in 293 T cells has toxic effects and that the chicken beta-actin promoter is active in Escherichia coli, causing expression of Cre in bacteria. This led to significant problems in the cloning of Cre/loxP constructs. Leaky Cre-expression in E. coli, and toxicity of the Cre overexpression in mammalian cells, were solved by constructing a novel silent self-inactivating Cre (SSi-Cre) expression cassette. The SSi-Cre is based on modified loxP sites flanking the Cre/Int/DsRed fusion gene containing a Cre coding sequence interrupted by an intron, which prevents leaky expression of Cre in E. coli. Additionally, this system contains a reporter gene to visualize Cre activity by fluorescent microscopy. The SSi-Cre cassette provides a universal strategy for the generation of Cre/loxP constructs, as well as a solution to the toxicity caused by the overexpression of Cre in target cells. SSi-Cre should thus provide a useful tool for various applications based on the Cre/loxP system.