Transduction rate constant as more reliable index quantifying efficiency of retroviral gene delivery

Rapid antiviral assay using QD-tagged fish virus as imaging nanoprobe

Development of rapid antiviral assays can expedite the process of screening potential agents against viral pathogens. In the present study, fluorescent quantum dots (QDs) incorporated with infectious pancreatic necrosis virus (IPNV) were used as imaging nanoprobes to detect the threshold amount of poly I:C (an interferon inducer) required to induce zebrafish cells into an antiviral state against IPNV. QD-IPNV hybrids were formed by colloidal clustering of negatively charged QDs and IPNV, using the cationic polymer polybrene (50 μg/mL). To test the screening potential of the QD-IPNV hybrids for anti-IPNV drug candidates, zebrafish ZF4 cells primed with the immunostimulant poly I:C at concentrations of 1, 5, and 10 μg/mL for 6h were used as a model system. After poly I:C treatment, cells were exposed to the QD-IPNV hybrids for 6h at a multiplicity of infection (MOI) of 5. The anti-IPNV effectiveness of poly I:C was assessed via fluorescence intensity of the QDs. Our results showed that ZF4 cells primed with poly I:C at 10 μg/mL were highly protected from IPNV challenge (i.e., no detection of QD fluorescence). In summary, a rapid and efficient cell-based imaging platform has been developed for assessing the anti-IPNV activity of poly I:C on ZF4 cells using QD-IPNV hybrids. This approach may be applied to a wider range of fish species and fish pathogenic viruses.

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Genetic modification of human hematopoietic cells: preclinical optimization of oncoretroviral-mediated gene transfer for clinical trials

This chapter provides information about the oncoretroviral transduction of human hematopoietic stem/ progenitor cells under clinically applicable conditions. We describe in detail a short -60 h transduction protocol which consistently yields transduction efficiencies in the range of 30-50% with five different oncoretroviral vectors. We discuss a number of parameters that affect transduction efficiency, including the oncoretroviral vector characteristics, the vector stock collection, the source of CD34+ cells and transduction conditions.

Improvement of the anodic bioelectrocatalytic activity of mixed culture biofilms by a simple consecutive electrochemical selection procedure

The increasing threats of viral diseases have gained worldwide attention in recent years. Quite a few infectious diseases are still lacking effective prevention or treatment. The pace of developing antiviral agents could be expedited by the availability of quick and efficient drug screening platforms. In this study, quantum dot (QD), an emerging probe for biological imaging and medical diagnostics, was employed to form complexes with virus and used as fluorescent imaging probes for exploring potential antiviral therapeutics. Inorganic CdSe/ZnS QDs synthesized in organic phase were encapsulated by amphiphilic alginate to attain biocompatible water-soluble QDs via phase transfer. Virus employed for this study was dengue virus which is a notorious one in tropical and subtropical regions of the world. To construct a QD–virus imaging modality capable of providing meaningful information, preservation of viral infectivity after tagging virus with QDs is of utmost importance. In order to form colloidal complexes of QD–virus, electrostatic repulsion force generated from both negatively charged virus and QDs was neutralized by various concentrations of cationic polybrene. Results showed that BHK-21 cells infected with dengue viruses incorporated with QDs exhibited bright fluorescence intracellularly within 30 min. To demonstrate the potency of QD–virus complexes as bioprobes for screening antiviral agents, BHK-21 cells were incubated for one hour with allophycocyanin purified from blue-green algae and then infected with QD–virus complexes. Based on the developed cell-based imaging assay, allophycocyanin with concentration of 125 μg/mL led to extremely weak intracellular fluorescence post-infection of QD–virus complexes for 30 min. That is, the efficacy of anti-dengue viral activity of the algae extract was clearly illustrated by the inorganic–organic hybrid platform constructed in current study.

Differential interaction of retroviral vector with target cell: quantitative effect of cellular receptor, soluble proteoglycan, and cell type on gene delivery efficiency

Retroviral vectors are powerful tools for gene therapy and stem cell engineering. To improve efficiency of retroviral gene delivery, quantitative understanding of interactions of a retroviral vector and a cell is crucial. Effects of nonspecific adsorption of retrovirus on a cell via proteoglycans and receptor-mediated binding of retrovirus to a cell on overall transduction efficiency were quantified by combining a mathematical model and experimental data. Results represented by transduction rate constant, a lumped parameter of overall transduction efficiency, delineated that chondroitin sulfate C (CSC) plays dual roles as either enhancer or inhibitor of retroviral transduction, depending on its concentrations in the retroviral supernatant. At the concentration of 20 microg/mL, CSC enhanced the transduction efficiency up to threefold but inhibited more than sevenfold at the concentration of 100 microg/mL. Transduction rate constants for amphotropic retroviral infection of NIH 3T3 cells under phosphate-depleted culture condition showed a proportional relationship between cellular receptor density on a cell and transduction efficiency. It was finally shown that amphotropic retrovirus transduced human fibroblast HT1080 cells more efficiently than NIH 3T3 cells. On the contrary, the transduction efficiency of NIH 3T3 cells by vesicular stomatitis virus G protein pseudotyped retroviruses was eightfold higher than that of HT1080 cells. This study implies usefulness of using quantitative analysis of retroviral transduction in understanding and optimizing retroviral gene delivery systems for therapeutic approaches to tissue engineering.

Incorporation of quantum dots on virus in polycationic solution

Developing methods to label viruses with fluorescent moieties has its merits in elucidating viral infection mechanisms and exploring novel antiviral therapeutics. Fluorescent quantum dots (QDs), an emerging probe for biological imaging and medical diagnostics, were employed in this study to tag retrovirus encoding enhanced green fluorescent protein (EGFP) genes. Electrostatic repulsion forces generated from both negatively charged retrovirus and QDs were neutralized by cationic Polybrene, forming colloidal complexes of QDs-virus. By examining the level of EGFP expression in 3T3 fibroblast cells treated with QDs-tagged retroviruses for 24 hours, the infectivity of retrovirus incorporated with QDs was shown to be only slightly decreased. Moreover, the imaging of QDs can be detected in the cellular milieu. In summary, the mild method developed here makes QDs-tagged virus a potential imaging probe for direct tracking the infection process and monitoring distribution of viral particles in infected cells.

Functional assessment of the engraftment potential of gammaretrovirus-modified CD34+ cells, using a short serum-free transduction protocol

The successful transduction and engraftment of human mobilized peripheral blood (MBP) CD34(+) cells are determined to a large extent by the ex vivo cell-processing conditions. In preparation for upcoming clinical trials, we investigated essential culture parameters and devised a short and efficient gammaretroviral transduction protocol entailing minimal manipulation of MBP CD34(+) cells. The engraftment potential and in vivo transgene expression in the progeny of repopulating CD34(+) cells were measured to assess the functionality of CD34(+) cells transduced under these conditions. Using a competitive in vivo repopulation assay in nonobese diabetic/severe combined immunodeficient mice, we demonstrate equivalent engraftment of CD34(+) cells transduced under serum-free conditions as compared with CD34(+) cells cultured with serum. We also took advantage of this in vivo model to demonstrate that ex vivo manipulation of CD34(+) cells can be shortened to 60 hr, using 36 hr of prestimulation and two cycles of transduction 12 hr apart. These minimally manipulated CD34(+) cells engraft in a manner similar to cells transduced under longer protocols and the vector-encoded transgene is expressed at the same frequency in cells derived from repopulating CD34(+) cells in vivo. We have thus developed a short and efficient human MBP CD34(+) transduction protocol under serum-free conditions that is suitable and broadly applicable for phase I clinical trials.

Downstream processing of oncoretroviral and lentiviral gene therapy vectors

Retroviral vectors from both oncoretroviral and lentiviral origins have a great potential as gene delivery vehicles. A number of research groups have devoted considerable effort to the development of large-scale production strategies for retroviral vectors. However, the manufacturing of clinical-grade vectors for gene therapy, especially for in vivo applications, additionally requires scaleable purification strategies to remove the contaminants present in the harvested supernatants while preserving the functionality of the vectors. In this article, we review recent advances made in the field of downstream processing of retroviral vectors. The methods currently described in the literature for clarification, concentration and purification of retroviral vectors will be presented, with special emphasis on novel chromatography methods that open up the possibility to selectively and efficiently purify retroviruses on a large-scale. Problems associated with stability and quantification of retroviral particles will be outlined and future challenges will be discussed.

Silencing of retroviral vector transduced LacZ reporter gene by frameshift mutation

Moloney murine leukemia virus-based vector expressing Escherichia coli beta-galactosidase (lacZ) as reporter gene and the transposon Tn5 neomycin resistance (neo) gene was transduced at low-multiplicity of infections into NIH 3T3 cells. Geneticin (G418)-resistant cells were recloned and cell lines containing beta-galactosidase positive or beta-galactosidase negative cells were obtained. Both positive and negative cell lines contained a single proviral copy at distinct integration sites. RNA complementary to lacZ was detected in beta-galactosidase positive as well as in one of three investigated beta-galactosidase negative cell lines. DNA sequence analysis of proviral LacZ gene in beta-galactosidase negative cell line C6 showed a single nucleotide insertion at position 1567 resulting in reading frame shift and translational stop codon at position 1629. This mutation explains the enzyme inactivation. The absence of beta-galactosidase after retroviral transduction of LacZ reproter gene may be a consequence of definite mutation but not a consequence of ineffective transduction or transcriptional inactivation of transgene.

Determination of infectious retrovirus concentration from colony-forming assay with quantitative analysis

The colony formation assay is the most commonly used titration method for defining the concentration of replication-incompetent murine leukemia virus-derived retroviral vectors. However, titer varies with target cell type and number, transduction time, and concentration of polycation (e.g., Polybrene). Moreover, because most of the viruses cannot encounter target cells due to Brownian motion, their short half-lives, and the requirement for target cell division for activity, the actual infectious retrovirus concentration in the collected supernatant is higher than the viral titer. Here we correlate the physical viral particle concentration with the infectious virus concentration and colony formation titer with the help of a mathematical model. Ecotropic murine leukemia retrovirus supernatant, collected from the GP+E86/LNCX retroviral vector producer cell line, was concentrated by centrifugation and further purified by a sucrose density gradient. The physical concentration of purified viral vectors was determined by direct particle counting with an electron microscope. The concentrations of fresh and concentrated supernatant were determined by a quantitative reverse transcriptase activity assay. Titration of all supernatants by neomycin-resistant colony formation assay was also performed. There were 767 +/- 517 physical viral particles per infectious CFU in the crude viral supernatant. However, the infectious viral concentration determined by mathematical simulation was 143 viral particles per infectious unit, which is more consistent with the concentration determined by particle counting in purified viral solution. Our results suggest that the mathematical model can be used to extract a more accurate and reliable concentration of infectious retrovirus.