Host receptor-targeted therapeutic approach to counter pathogenic New World mammarenavirus infections.

Five New World mammarenaviruses (NWMs) cause life-threatening hemorrhagic fever (HF). Cellular entry by these viruses is mediated by human transferrin receptor 1 (hTfR1). Here, we demonstrate that an antibody (ch128.1/IgG1) which binds the apical domain of hTfR1, potently inhibits infection of attenuated and pathogenic NWMs in vitro. Computational docking of the antibody Fab crystal structure onto the known structure of hTfR1 shows an overlapping receptor-binding region shared by the Fab and the viral envelope glycoprotein GP1 subunit that binds hTfR1, and we demonstrate competitive inhibition of NWM GP1 binding by ch128.1/IgG1 as the principal mechanism of action. Importantly, ch128.1/IgG1 protects hTfR1-expressing transgenic mice against lethal NWM challenge. Additionally, the antibody is well-tolerated and only partially reduces ferritin uptake. Our findings provide the basis for the development of a novel, host receptor-targeted antibody therapeutic broadly applicable to the treatment of HF of NWM etiology.

Amino acid residues involved in the heparin-binding activity of murine IL-12 in the context of an antibody-cytokine fusion protein.

An antibody-cytokine fusion protein, composed of the murine single-chain cytokine interleukin-12 (IL-12) genetically fused to a human IgG3 specific for the human tumor-associated antigen HER2/neu maintains antigen binding, cytokine bioactivity, and IL-12 heparin-binding activity. This latter property is responsible for the binding of the cytokine to glycosaminoglycans (GAGs) on the cell surface and the extracellular matrix and has been implicated in modulating IL-12 bioactivity. Previous studies indicate that the p40 subunit of human and murine IL-12 is responsible for the heparin-binding activity of this heterodimeric cytokine. In the present study we used bioinformatic analysis and site-directed mutagenesis to develop a version of the antibody-(IL-12) fusion protein without heparin-binding activity. This was accomplished by replacing the basic arginine (R) and lysine (K) residues in the cluster of amino acids 254-260 (RKKEKMK) of the murine IL-12 p40 subunit by the neutral non-polar amino acid alanine (A), generating an AAAEAMA mutant fusion protein. ELISA and flow cytometry demonstrated that the antibody fusion protein lacks heparin-binding activity but retains antigen binding. A T-cell proliferation assay showed IL-12 bioactivity in this construct. However, the IL-12 bioactivity is decreased compared to its non-mutated counterpart, which is consistent with an ancillary role of the heparin-binding site of IL-12 in modulating its activity. Thus, we have defined a cluster of amino acid residues with a crucial role in the heparin-binding activity of murine IL-12 in the context of an antibody-cytokine fusion protein.

Sub-ångström cryo-EM structure of a prion protofibril reveals a polar clasp.

The atomic structure of the infectious, protease-resistant, ß-sheet-rich and fibrillar mammalian prion remains unknown. Through the cryo-EM method MicroED, we reveal the sub-ångström-resolution structure of a protofibril formed by a wild-type segment from the ß2-α2 loop of the bank vole prion protein. The structure of this protofibril reveals a stabilizing network of hydrogen bonds that link polar zippers within a sheet, producing motifs we have named 'polar clasps'.

Antibody-mediated targeting of the transferrin receptor in cancer cells / Anticuerpos que reconocen el receptor de transferrina en células tumorales

Abstract: Iron is essential for cell growth and is imported into cells in part through the action of transferrin (Tf), a protein that binds its receptor (TfR1 or CD71) on the surface of a cell, and then releases iron into endosomes. TfR1 is a single pass type-II transmembrane protein expressed at basal levels in most tissues. High expression of TfR1 is typically associated with rapidly proliferating cells, including various types of cancer. TfR1 is targeted by experimental therapeutics for several reasons: its cell surface accessibility, constitutive endocytosis into cells, essential role in cell growth and proliferation, and its overexpression by cancer cells. Among the therapeutic agents used to target TfR1, antibodies stand out due to their remarkable specificity and affinity. Clinical trials are being conducted to evaluate the safety and efficacy of agents targeting TfR1 in cancer patients with promising results. These observations suggest that therapies targeting TfR1 as direct therapeutics or delivery conduits remain an attractive alternative for the treatment of cancers that overexpress the receptor.

Resumen: El hierro es esencial para el crecimiento celular. Es transportado dentro de las células con la ayuda de la transferrina (Tf), proteína que se une a su receptor (TfR1 o CD71) en la superficie celular y libera el hierro dentro de los endosomas. El TfR1 es una proteína de membrana tipo II que se sobreexpresa en muchos tejidos debido al requerimiento de las células para importar hierro unido a Tf. La sobreexpresión de TfR1 se ha asociado con células que proliferan rápidamente, incluyendo los diferentes tipos de cáncer. El TfR1 se ha empleado como blanco terapéutico por diversos motivos: su accesibilidad a la superficie celular, su capacidad de internalizarse constitutivamente en las células, su papel esencial en el crecimiento y la proliferación celular, así como por su sobreexpresión en las células tumorales proliferantes. Entre los agentes terapéuticos dirigidos contra el TfR1 destacan los anticuerpos, por su alta especificidad, estabilidad y propiedades estructurales. Se han realizado diversos ensayos clínicos para evaluar la seguridad y la eficacia de los anticuerpos que reconocen el TfR1 en pacientes con cáncer y se han obtenido resultados prometedores. Estas observaciones sugieren que las terapias con fundamento en el reconocimiento de TfR1, ya sea como terapia directa o empleados como acarreadores, representan una alternativa muy atractiva de tratamiento contra los diferentes tipos de cáncer que sobreexpresan este receptor.

Antibody-mediated targeting of the transferrin receptor in cancer cells.

Iron is essential for cell growth and is imported into cells in part through the action of transferrin (Tf), a protein that binds its receptor (TfR1 or CD71) on the surface of a cell, and then releases iron into endosomes. TfR1 is a single pass type-II transmembrane protein expressed at basal levels in most tissues. High expression of TfR1 is typically associated with rapidly proliferating cells, including various types of cancer. TfR1 is targeted by experimental therapeutics for several reasons: its cell surface accessibility, constitutive endocytosis into cells, essential role in cell growth and proliferation, and its overexpression by cancer cells. Among the therapeutic agents used to target TfR1, antibodies stand out due to their remarkable specificity and affinity. Clinical trials are being conducted to evaluate the safety and efficacy of agents targeting TfR1 in cancer patients with promising results. These observations suggest that therapies targeting TfR1 as direct therapeutics or delivery conduits remain an attractive alternative for the treatment of cancers that overexpress the receptor.

Antibody engineering and therapeutics, The Annual Meeting of the Antibody Society: December 8-12, 2013, Huntington Beach, CA.

The 24th Antibody Engineering & Therapeutics meeting brought together a broad range of participants who were updated on the latest advances in antibody research and development. Organized by IBC Life Sciences, the gathering is the annual meeting of The Antibody Society, which serves as the scientific sponsor. Preconference workshops on 3D modeling and delineation of clonal lineages were featured, and the conference included sessions on a wide variety of topics relevant to researchers, including systems biology; antibody deep sequencing and repertoires; the effects of antibody gene variation and usage on antibody response; directed evolution; knowledge-based design; antibodies in a complex environment; polyreactive antibodies and polyspecificity; the interface between antibody therapy and cellular immunity in cancer; antibodies in cardiometabolic medicine; antibody pharmacokinetics, distribution and off-target toxicity; optimizing antibody formats for immunotherapy; polyclonals, oligoclonals and bispecifics; antibody discovery platforms; and antibody-drug conjugates.

Rationale and preclinical efficacy of a novel anti-EMP2 antibody for the treatment of invasive breast cancer.

Despite significant advances in biology and medicine, the incidence and mortality due to breast cancer worldwide is still unacceptably high. Thus, there is an urgent need to discover new molecular targets. In this article, we show evidence for a novel target in human breast cancer, the tetraspan protein epithelial membrane protein-2 (EMP2). Using tissue tumor arrays, protein expression of EMP2 was measured and found to be minimal in normal mammary tissue, but it was upregulated in 63% of invasive breast cancer tumors and in 73% of triple-negative tumors tested. To test the hypothesis that EMP2 may be a suitable target for therapy, we constructed a fully human immunoglobulin G1 (IgG1) antibody specific for a conserved domain of human and murine EMP2. Treatment of breast cancer cells with the anti-EMP2 IgG1 significantly inhibited EMP2-mediated signaling, blocked FAK/Src signaling, inhibited invasion, and promoted apoptosis in vitro. In both human xenograft and syngeneic metastatic tumor monotherapy models, anti-EMP2 IgG1 retarded tumor growth without detectable systemic toxicity. This antitumor effect was, in part, attributable to a potent antibody-dependent cell-mediated cytotoxicity response as well as direct cytotoxicity induced by the monoclonal antibody. Together, these results identify EMP2 as a novel therapeutic target for invasive breast cancer.

Paclitaxel-loaded PCL-TPGS nanoparticles: in vitro and in vivo performance compared with Abraxane®.

The purpose of this work was to develop Cremophor(®) EL-free nanoparticles (NPs) loaded with Paclitaxel (PTX) in order to improve the drug i.v. pharmacokinetic profile and to evaluate its activity against commercially available formulations such as Taxol(®) and Abraxane(®). PTX-loaded poly(ε-caprolactone)-alpha tocopheryl polyethylene glycol 1000 succinate (PCL-TPGS) NPs were prepared using three different techniques: (i) by nanoprecipitation (NPr-method), (ii) by emulsion-solvent evaporation homogenized with an Ultra-Turrax(®) (UT-method) and (iii) by emulsion-solvent evaporation homogenized with an ultrasonicator (US-method). The NPs prepared by US-method showed the smallest size and the highest drug content. The NPs exhibited a slow and continuous release of PTX. The in vitro anti-tumoral activity was assessed using two human breast cancer cell lines (MCF-7 and MDA-MB-231) with the WTS assay. Cytotoxicity studies with both cell lines showed that PTX-loaded PCL-TPGS NPs exhibited better anti-cancer activity compared to PTX solution and the commercial formulation Abraxane(®) at different concentrations. Importantly, in the case of triple negative MDA-MB-231 breast cancer cells, the IC50 value for PTX-loaded PCL-TPGS NPs was 7.8 times lower than Abraxane(®). Finally, in vivo studies demonstrated that PTX-loaded PCL-TPGS NPs exhibited longer systemic circulation time and slower plasma elimination rate than Taxol(®) and Abraxane(®). Therefore, the novel NPs investigated might be an alternative nanotechnological platform for PTX delivery system in cancer chemotherapy.

Polymalic acid nanobioconjugate for simultaneous immunostimulation and inhibition of tumor growth in HER2/neu-positive breast cancer.

Breast cancer remains the second leading cause of cancer death among women in the United States. Breast cancer prognosis is particularly poor in case of tumors overexpressing the oncoprotein HER2/neu. A new nanobioconjugate of the Polycefin(TM) family of anti-cancer drugs based on biodegradable and non-toxic polymalic acid (PMLA) was engineered for a multi-pronged attack on HER2/neu-positive breast cancer cells. An antibody-cytokine fusion protein consisting of the immunostimulatory cytokine interleukin-2 (IL-2) genetically fused to an antibody specific for human HER2/neu [anti-HER2/neu IgG3-(IL-2)] was covalently attached to the PMLA backbone to target HER2/neu expressing tumors and ensure the delivery of IL-2 to the tumor microenvironment. Antisense oligonucleotides (AON) were conjugated to the nanodrug to inhibit the expression of vascular tumor protein laminin-411 in order to block tumor angiogenesis. It is shown that the nanobioconjugate was capable of specifically binding human HER2/neu and retained the biological activity of IL-2. We also showed the uptake of the nanobioconjugate into HER2/neu-positive breast cancer cells and enhanced tumor targeting in vivo. The nanobioconjugate exhibited marked anti-tumor activity manifested by significantly longer animal survival and significantly increased anti-HER2/neu immune response in immunocompetent mice bearing D2F2/E2 murine mammary tumors that express human HER2/neu. The combination of laminin-411 AON and antibody-cytokine fusion protein on a single polymeric platform results in a new nanobioconjugate that can act against cancer cells through inhibition of tumor growth and angiogenesis and the orchestration of an immune response against the tumor. The present Polycefin(TM) variant may be a promising agent for treating HER2/neu expressing tumors and demonstrates the versatility of the Polycefin(TM) nanobioconjugate platform.

A novel IgE antibody targeting the prostate-specific antigen as a potential prostate cancer therapy.

BACKGROUND: Prostate cancer (PCa) is the second leading cause of cancer deaths in men in the United States. The prostate-specific antigen (PSA), often found at high levels in the serum of PCa patients, has been used as a marker for PCa detection and as a target of immunotherapy. The murine IgG1 monoclonal antibody AR47.47, specific for human PSA, has been shown to enhance antigen presentation by human dendritic cells and induce both CD4 and CD8 T-cell activation when complexed with PSA. In this study, we explored the properties of a novel mouse/human chimeric anti-PSA IgE containing the variable regions of AR47.47 as a potential therapy for PCa. Our goal was to take advantage of the unique properties of IgE in order to trigger immune activation against PCa. METHODS: Binding characteristics of the antibody were determined by ELISA and flow cytometry. In vitro degranulation was determined by the release of ß-hexosaminidase from effector cells. In vivo degranulation was monitored in human FcεRIα transgenic mice using the passive cutaneous anaphylaxis assay. These mice were also used for a vaccination study to determine the in vivo anti-cancer effects of this antibody. Significant differences in survival were determined using the Log Rank test. In vitro T-cell activation was studied using human dendritic cells and autologous T cells. RESULTS: The anti-PSA IgE, expressed in murine myeloma cells, is properly assembled and secreted, and binds the antigen and FcεRI. In addition, this antibody is capable of triggering effector cell degranulation in vitro and in vivo when artificially cross-linked, but not in the presence of the natural soluble antigen, suggesting that such an interaction will not trigger systemic anaphylaxis. Importantly, the anti-PSA IgE combined with PSA also triggers immune activation in vitro and in vivo and significantly prolongs the survival of human FcεRIα transgenic mice challenged with PSA-expressing tumors in a prophylactic vaccination setting. CONCLUSIONS: The anti-PSA IgE exhibits the expected biological properties and is capable of triggering immune activation and anti-tumor protection. Further studies on this antibody as a potential PCa therapy are warranted.

Insights into the mechanism of cell death induced by saporin delivered into cancer cells by an antibody fusion protein targeting the transferrin receptor 1.

We previously developed an antibody-avidin fusion protein (ch128.1Av) that targets the human transferrin receptor 1 (TfR1) and exhibits direct cytotoxicity against malignant B cells in an iron-dependent manner. ch128.1Av is also a delivery system and its conjugation with biotinylated saporin (b-SO6), a plant ribosome-inactivating toxin, results in a dramatic iron-independent cytotoxicity, both in malignant cells that are sensitive or resistant to ch128.1Av alone, in which the toxin effectively inhibits protein synthesis and triggers caspase activation. We have now found that the ch128.1Av/b-SO6 complex induces a transcriptional response consistent with oxidative stress and DNA damage, a response that is not observed with ch128.1Av alone. Furthermore, we show that the antioxidant N-acetylcysteine partially blocks saporin-induced apoptosis suggesting that oxidative stress contributes to DNA damage and ultimately saporin-induced cell death. Interestingly, the toxin was detected in nuclear extracts by immunoblotting, suggesting the possibility that saporin might induce direct DNA damage. However, confocal microscopy did not show a clear and consistent pattern of intranuclear localization. Finally, using the long-term culture-initiating cell assay we found that ch128.1Av/b-SO6 is not toxic to normal human hematopoietic stem cells suggesting that this critical cell population would be preserved in therapeutic interventions using this immunotoxin.

Targeting HER2/neu with a fully human IgE to harness the allergic reaction against cancer cells.

Breast and ovarian cancer are two of the leading causes of cancer deaths among women in the United States. Overexpression of the HER2/neu oncoprotein has been reported in patients affected with breast and ovarian cancers, and is associated with poor prognosis. To develop a novel targeted therapy for HER2/neu expressing tumors, we have constructed a fully human IgE with the variable regions of the scFv C6MH3-B1 specific for HER2/neu. This antibody was expressed in murine myeloma cells and was properly assembled and secreted. The Fc region of this antibody triggers in vitro degranulation of rat basophilic cells expressing human FcεRI (RBL SX-38) in the presence of murine mammary carcinoma cells that express human HER2/neu (D2F2/E2), but not the shed (soluble) antigen (ECD(HER2)) alone. This IgE is also capable of inducing passive cutaneous anaphylaxis in a human FcεRIα transgenic mouse model, in the presence of a cross-linking antibody, but not in the presence of soluble ECD(HER2). Additionally, IgE enhances antigen presentation in human dendritic cells and facilitates cross-priming, suggesting that the antibody is able to stimulate a secondary T-cell anti-tumor response. Furthermore, we show that this IgE significantly prolongs survival of human FcεRIα transgenic mice bearing D2F2/E2 tumors. We also report that the anti-HER2/neu IgE is well tolerated in a preliminary study conducted in Macaca fascicularis (cynomolgus) monkeys. In summary, our results suggest that this IgE should be further explored as a potential therapeutic against HER2/neu overexpressing tumors, such as breast and ovarian cancers.

The transferrin receptor and the targeted delivery of therapeutic agents against cancer.

BACKGROUND: Traditional cancer therapy can be successful in destroying tumors, but can also cause dangerous side effects. Therefore, many targeted therapies are in development. The transferrin receptor (TfR) functions in cellular iron uptake through its interaction with transferrin. This receptor is an attractive molecule for the targeted therapy of cancer since it is upregulated on the surface of many cancer types and is efficiently internalized. This receptor can be targeted in two ways: 1) for the delivery of therapeutic molecules into malignant cells or 2) to block the natural function of the receptor leading directly to cancer cell death. SCOPE OF REVIEW: In the present article we discuss the strategies used to target the TfR for the delivery of therapeutic agents into cancer cells. We provide a summary of the vast types of anti-cancer drugs that have been delivered into cancer cells employing a variety of receptor binding molecules including Tf, anti-TfR antibodies, or TfR-binding peptides alone or in combination with carrier molecules including nanoparticles and viruses. MAJOR CONCLUSIONS: Targeting the TfR has been shown to be effective in delivering many different therapeutic agents and causing cytotoxic effects in cancer cells in vitro and in vivo. GENERAL SIGNIFICANCE: The extensive use of TfR for targeted therapy attests to the versatility of targeting this receptor for therapeutic purposes against malignant cells. More advances in this area are expected to further improve the therapeutic potential of targeting the TfR for cancer therapy leading to an increase in the number of clinical trials of molecules targeting this receptor. This article is part of a Special Issue entitled Transferrins: molecular mechanisms of iron transport and disorders.

Lethal iron deprivation induced by non-neutralizing antibodies targeting transferrin receptor 1 in malignant B cells.

A number of antibodies have been developed that induce lethal iron deprivation (LID) by targeting the transferrin receptor 1 (TfR1/CD71) and either neutralizing transferrin (Tf) binding, blocking internalization of the receptor and/or inducing its degradation. We have developed recombinant antibodies targeting human TfR1 (ch128.1 and ch128.1Av), which induce receptor degradation and are cytotoxic to certain malignant B-cells. We now show that internalization of TfR1 bound to these antibodies can lead to its sequestration and degradation, as well as reduced Tf uptake, and the induction of a transcriptional response consistent with iron deprivation, which is mediated in part by downstream targets of p53. Cells resistant to these antibodies do not sequester and degrade TfR1 after internalization of the antibody/receptor complex, and accordingly maintain their ability to internalize Tf. These findings are expected to facilitate the rational design and clinical use of therapeutic agents targeting iron import via TfR1 in hematopoietic malignancies.

An antibody-based multifaceted approach targeting the human transferrin receptor for the treatment of B-cell malignancies.

We previously developed an antibody-avidin fusion protein (ch128.1Av) targeting the human transferrin receptor 1 (TfR1, also known as CD71), which demonstrates direct in vitro cytotoxicity against malignant hematopoietic cells. This cytotoxicity is attributed to its ability to decrease the level of TfR1 leading to lethal iron deprivation. We now report that ch128.1Av shows the ability to bind the Fcγ receptors and the complement component C1q, suggesting that it is capable of eliciting Fc-mediated effector functions such as antibody-dependent cell-mediated cytotoxicity and complement-mediated cytotoxicity. In addition, in 2 disseminated multiple myeloma xenograft mouse models, we show that a single dose of ch128.1Av results in significant antitumor activity, including long-term survival. It is interesting to note that the parental antibody without avidin (ch128.1) also shows remarkable in vivo anticancer activity despite its limited in vitro cytotoxicity. Finally, we demonstrate that ch128.1Av is not toxic to pluripotent hematopoietic progenitor cells using the long-term cell-initiating culture assay suggesting that these important progenitors would be preserved in different therapeutic approaches, including the in vitro purging of cancer cells for autologous transplantation and in vivo passive immunotherapy. Our results suggest that ch128.1Av and ch128.1 may be effective in the therapy of human multiple myeloma and potentially other hematopoietic malignancies.

Visualization and quantification of cytotoxicity mediated by antibodies using imaging flow cytometry.

Conventional approaches for the detection of antibody dependent cell-mediated cytotoxicity (ADCC) activity rely on quantification of the release of traceable compounds from target cells or flow cytometry analysis of population-wide phenomena. We report a new method for the direct imaging and quantification of ADCC of cancer cells. The proposed method using imaging flow cytometry combines the statistical power of flow cytometry with the analytical advantages of cell imaging, providing a novel and more comprehensive perspective of effector/target cell interactions during ADCC events. With this method we can quantify and show in detail the morphological changes in target and effector cells, their apoptotic index, the physical interaction between effector and target cells, and a directional transfer of cytosolic contents from effector to target cells. As a model system we used the therapeutic anti-CD20 antibody rituximab to target CFSE labeled Ramos human Burkitt's lymphoma cells, to CMTPX-labeled human monocytic U-937 effector cells. We expect that similar studies using different effector and target cell populations may contribute to the pre-clinical evaluation of therapeutic antibodies and help to identify mechanisms that could be beneficial in the immunotherapy of cancer.

Inhibition of NF-kappaB and Akt pathways by an antibody-avidin fusion protein sensitizes malignant B-cells to cisplatin-induced apoptosis.

Multiple myeloma (MM) is an incurable disease of malignant plasma cells. Recent therapeutic advancements have resulted in improved response rates, however, there is no improvement in overall survival, therefore, new therapeutics are needed. Since the transferrin receptor is upregulated on the surface of MM cells, we previously developed an antibody fusion protein consisting of an IgG3 specific for the human transferrin receptor 1 (TfR1, CD71) genetically fused to avidin at its carboxy-terminus (ch128.1Av). We have previously shown that ch128.1Av exhibits intrinsic cytotoxicity against certain malignant B-cells by disrupting the cycling of the TfR and decreasing TfR cell surface expression resulting in lethal iron starvation. In addition, ch128.1Av can sensitize malignant cells to apoptosis induced by gambogic acid, a herbal drug used in Chinese medicine. In this study, we hypothesized that ch128.1Av may also sensitize drug-resistant malignant B-cells to chemotherapeutic agents by inhibiting key survival pathways. In this study we show that ch128.1Av sensitizes malignant B-cells to apoptosis induced by cisplatin (CDDP). The sensitization by ch128.1Av resulted in the inhibition of the constitutively activated Akt and NF-kappaB survival/antiapoptotic pathways and downstream decreased expression of antiapoptotic gene products such as BclxL and survivin. The direct role of the inhibition of the Akt and NF-kappaB pathways by ch128.1Av in CDDP-mediated cytotoxicity was demonstrated by the use of specific chemical inhibitors and siRNA which mimicked the effects of ch128.1Av. Overall, this study provides evidence of the therapeutic potential of ch128.1Av as a chemo-sensitizing agent in drug-resistant tumor cells.

A versatile targeting system with lentiviral vectors bearing the biotin-adaptor peptide.

BACKGROUND: Targeted gene transduction in vivo is the ultimate preferred method for gene delivery. We previously developed targeting lentiviral vectors that specifically recognize cell surface molecules with conjugated antibodies and mediate targeted gene transduction both in vitro and in vivo. Although effective in some experimental settings, the conjugation of virus with antibodies is mediated by the interaction between protein A and the Fc region of antibodies, which is not as stable as covalent conjugation. We have now developed a more stable conjugation strategy utilizing the interaction between avidin and biotin. METHODS: We inserted the biotin-adaptor-peptide, which was biotinylated by secretory biotin ligase at specific sites, into our targeting envelope proteins, enabling conjugation of the pseudotyped virus with avidin, streptavidin or neutravidin. RESULTS: When conjugated with avidin-antibody fusion proteins or the complex of avidin and biotinylated targeting molecules, the vectors could mediate specific transduction to targeted cells recognized by the targeting molecules. When conjugated with streptavidin-coated magnetic beads, transduction by the vectors was targeted to the locations of magnets. CONCLUSIONS: This targeting vector system can be used for broad applications of targeted gene transduction using biotinylated targeting molecules or targeting molecules fused with avidin.