Background: Venetoclax-based combination therapies have provided treatment options for patients with acute myeloid leukemia (AML) who are unfit for intensive chemotherapy. However, venetoclax resistance is common, and for such patients, the prognosis is dismal, and treatment approaches with different mechanisms of action are urgently needed. γδ T cells are a promising candidate owing to their good safety profile and cytotoxic effects in various types of cancers but are mostly unstudied in AML. Methods: Here we used flow cytometry to profile the subtype and memory phenotype of peripheral blood γδ T cells in AML patients and investigate the feasibility of using donor-derived Vγ9Vδ2 T cells to treat AML as both a single agent and in combination with venetoclax. Additionally, we used bioluminescence imaging to examine the effect of donor-derived Vγ9Vδ2 T cells on AML xenograft models alone and in combination with venetoclax. Results: We observed that Vδ2 T cells were less abundant and the TEMRA (terminally differentiated effector memory) phenotype was more prevalent as compared with that of healthy donors, suggesting that replenishing patients with Vδ2 T cells may be an effective treatment option. We found that donor-derived Vγ9Vδ2 T cells that Vγ9Vδ2 T cells efficiently induced apoptosis in AML cells from eight cell lines and three primary cultures in an effector-to-target cell ratio-dependent manner. Moreover, Vγ9Vδ2 T cells showed potent cytotoxicity against the venetoclax-resistant OCI-AML3 cell line and remained potent in the presence of venetoclax. Treatment with Vγ9Vδ2 T cells significantly extended survival in two AML xenograft models established with the aggressive Molm-13 and the venetoclax-resistant OCI-AML3 cell lines. An additive effect of venetoclax and Vγ9Vδ2 T cells was observed in the latter model. Conclusions: Overall, these findings suggest Vγ9Vδ2 T cells as a promising “off-the-shelf” immunotherapy approach for AML patients, especially for patients with venetoclax-resistant disease.

Background: Gamma delta(γδ) T cells have been associated with improved graft-versus-leukemia effect, leukemia-free-survival, and reduced risk of relapse in acute myeloid leukemia (AML). Variable delta 2 (Vδ2) cells are a subset of γδ T cells that are known to have a strong anti-cancer effect in a variety of malignancies, and increased Vδ2 cells have also been associated with better outcomes in chronic lymphocytic leukemia (CLL) and B cell acute lymphocytic leukemia. Earlier studies have indicated that AML blasts can disrupt T cell development. However, the effect of AML on γδ T cell memory phenotypes is not known. Naïve γδ cells are double-positive for CD27 and CD45RA and can differentiate into CD45RA-negative central memory (CM) γδ T cells in response to pAg stimulation. CM γδ T cells have the greatest potential for proliferation, and additional pAg stimulation causes them to further differentiate into effector memory (EM) γδ T cells. EM γδ T cells do not express CD27 or CD45RA, and though they are less proliferative than CM γδ T cells, they express more cytokines. EM γδ T cells can further differentiate into terminally differentiated effector memory (TEMRA) γδ T cells, which exhibit the highest cytotoxicity but the lowest proliferation. TEMRA γδ T cells also regain CD45RA expression. Imbalances of these γδ T cell memory phenotypes have been associated with poorer treatment responses and outcomes in CLL and myeloma. In this study, we have investigated γδ T cell memory phenotypes in AML patients. Additionally, we have tested ex-vivo expanded donor-derived Vδ2 γδ T cells for their effect on AML cells. We hypothesized that AML alters the memory phenotypes of γδ T cells and that ex-vivo expanded donor-derived Vδ2 γδ T cells induce a potent apoptotic effect on AML cell lines and primary AML cells. Methods: To examine γδ T cell memory phenotypes in AML patients, we have developed a multi-parameter flow cytometry assay (8-color panel) using a Miltenyi Biotec MACSQuant16. Peripheral blood mononuclear cells from healthy donors (n=10) and AML patients (n=14) were used in this study. To investigate the potential of donor-derived γδ T cell therapy for AML, we co-cultured freshly thawed Vδ2 γδ T cells (GMP-grade frozen γδ T cells were obtained from CytoMed Therapeutics Limited, Singapore) overnight at effector-to-target cell ratios (E:T) of 2:1, 5:1, and 10:1 with 8 AML cell lines (OCI-AML2, OCI-AML3, U937, THP1, Kasumi-1, MV4-11, Molm-13, and Molm-14), as well as primary AML cells derived from patient samples. We analyzed the effect of the γδ T cells on apoptosis induction in the AML cells using flow cytometry. Results: We did not observe any significant changes in the percentages of naïve or EM γδ T cell memory phenotype populations between samples from AML patients and healthy donors. However, we found significantly lower CM and significantly higher TEMRA γδ T cell populations in AML patients than in healthy individuals, with the mean CM percentage dropping from 52.47% ± 13.62% to 22.09% ± 10.07% (p<0.0001) and the mean TEMRA percentage increasing from 15.61% ± 10.08% to 35.55% ± 24.07% (p<0.05). As CM cells are the most proliferative memory phenotype and TEMRA cells are the least, this decrease indicates that γδ T cell populations have less capacity to proliferate in AML patients than in healthy individuals. This also suggests that adoptive transfer of donor-derived γδ T cells may provide a therapeutic opportunity for AML. Next, we determined the cytotoxic effect of donor-derived Vδ2 γδ T cells. When co-cultured with AML cells, the donor-derived γδ cells exerted a potent E:T-dependent apoptotic effect on all leukemic cell lines tested, including the TP53-mutant Kasumi-1 and FLT3-mutant Molm-13, Molm-14, and MV4-11 cells. The mean percentage of apoptotic AML cells was 41.30% ± 22.74% for the 2:1 E:T ratio, 67.52% ± 22.94% for the 5:1 ratio, and 77.54% ± 14.07% for the 10:1 ratio. One-way ANOVA confirmed an E:T ratio-dependent increase in apoptotic AML cells (p<0.0001). Experiments investigating the effect of Vδ2 γδ T cells on primary AML cells and in vivo studies are ongoing. Conclusion: γδ T cell memory phenotype profiles from AML patients showed a marked decrease in CM γδ T cells and increase in TEMRA γδ T cells, suggesting a more limited proliferative capacity in AML. Furthermore, expanded donor-derived Vδ2 γδ T cells induced E:T-dependent apoptosis in AML cell lines and may be a promising off-the-shelf treatment option for AML patients.

Abstract Background: Adoptive cellular therapy (ACT) has been transformative in the treatment of hematological malignancies. Limitations of ACTs include difficulty identifying suitable tumor antigens for solid tumors, the ability to target only one antigen per construct which commonly results in secondary resistance due to antigen escape, and largely autologous approaches which are challenging in heavily pre-treated patients with poor marrow function and those with rapidly progressive disease who may not receive their personalised cellular products in time. This is a phase I study evaluating a novel off-the-shelf allogenic mRNA-electroporated NKG2D ligand-targeting CAR-grafted γδ T cell therapy. NKG2DLs are a group of 8 types of stress-induced cancer antigens which are preferentially and widely expressed on tumor cells from diverse tissue origins but are not typically present on normal tissue. NKG2DL-targeting γδ T cells will be manufactured from peripheral blood mononuclear cells isolated from healthy donors enrolled on the donor protocol of the ANGELICA trial. ANGELICA is a phase I study evaluating the safety, tolerability and recommended phase 2 dose (RP2D) of NKG2DL-targeting CAR-grafted γδ T cells in patients with treatment refractory tumors. Methods: Patients will be enrolled in a 3+3 design. Dose escalation will be performed at 3 dose levels: 1x108, 3x108 and 1x109 per infusion (adjusted for body weight). Lymphodepletion with fludarabine 25mg/m2/day and cyclophosphamide 250mg/m2/day will given for 3 days and completed at least 2 days prior to the first cycle of treatment. Patients will receive 4 doses of weekly infusions for the first cycle, with up to 5 subsequent infusions every 2 months as maintenance. Patients will receive intravenous zoledronic acid 1mg prior to each cellular infusion and subcutaneous IL-2 1x106 IU/m2 within 2 hours of each cellular infusion as pre-clinical data demonstrated cancer cell sensitisation and prolonged γδ T cell survival with these adjuncts. Dose-limiting toxicities will be assessed over the first 8 weeks. Adverse events, response rates (RECIST v1.1), survival outcomes and immunomonitoring (immune cell phenotyping and serum cytokine analysis) will be assessed. The trial is currently enrolling healthy donors; enrolment of patients at the first dose level will begin in February 2024. NCT05302037 TABLE 1: NAND Dose Level and Schedule Dose Level Dose per infusion Infusion schedule Number of patients 1 Weight 65kg and above: 1x108.Weight less than 65kg: 1.5x106. Cycle 1: weekly x 4 infusions. Maintenance phase: 2-monthly x 5 infusions. 3-6 2 Weight 65kg and above: 3x108.Weight less than 65kg: 4.6x106. 3-6 3 Weight 65kg and above: 1x109.Weight less than 65kg: 1.5x107. 3-6 Citation Format: Joan Choo, Wee Kiat Tan, Lucas Luk, Jieming Zeng, Teck Guan Soh, Sou Yen Soon, Jedidah Lieow, Calista Wong, Mei Yan Pang, Sudipto Bari, Michelle Poon, Liang Piu Koh, Wee Joo Chng, Anand Jeyasekharan, Lip Kun Tan, Esther Chan, Raghav Sundar. A phase I trial to evaluate allogeneic NKG2DL-targeting chimeric antigen receptor-grafted γδ T cells in subjects with advanced solid tumors or hematological malignancies (the ANGELICA Trial) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr CT160.

BACKGROUND:Redirection of natural killer (NK) cells with chimeric antigen receptors (CAR) is attractive in developing off-the-shelf CAR therapeutics for cancer treatment. However, the site-specific integration of a CAR gene into NK cells remains challenging.METHODS:In the present study, we genetically modified human induced pluripotent stem cells (iPSCs) with a zinc finger nuclease (ZFN) technology to introduce a cDNA encoding an anti-EpCAM CAR into the adeno-associated virus integration site 1, a "safe harbour" for transgene insertion into human genome, and next differentiated the modified iPSCs into CAR-expressing iNK cells.RESULTS:We detected the targeted integration in 4 out of 5 selected iPSC clones, 3 of which were biallelically modified. Southern blotting analysis revealed no random integration events. iNK cells were successfully derived from the modified iPSCs with a 47-day protocol, which were morphologically similar to peripheral blood NK cells, displayed NK phenotype (CD56+CD3-), and expressed NK receptors. The CAR expression of the iPSC-derived NK cells was confirmed with RT-PCR and flow cytometry analysis. In vitro cytotoxicity assay further confirmed their lytic activity against NK cell-resistant, EpCAM-positive cancer cells, but not to EpCAM-positive normal cells, demonstrating the retained tolerability of the CAR-iNK cells towards normal cells.CONCLUSION:Looking ahead, the modified iPSCs generated in the current study hold a great potential as a practically unlimited source to generate anti-EpCAM CAR iNK cells.

V gamma 9V delta 2 T cell-based anticancer immunotherapy has shown some promise in early-phase clinical trials but there is still large room for improvement. Using the extracellular domain of the human NKG2D, a stimulatory receptor expressed by V gamma 9V delta 2 T cells, we constructed NKG2D ligand-specific chimeric antigen receptors (CARs). We adopted a non-viral CAR approach via mRNA electroporation to modify V gamma 9V delta 2 T cells and demonstrated that, upon interaction with the NKG2D ligand-positive cancer cells, the CARs substantially enhanced the cytotoxic activity of the modified cells toward multiple cultured solid tumor cell lines, including those resistant to Zometa treatment. Repeated doses of the CAR-expressing cells resulted in tumor regression in mice with established tumors, extending median survival time by up to 132% as compared to the PBS control group. The findings suggest clinical potential for RNA CAR-modified V gamma 9V delta 2 T cells to treat a wide variety of NKG2D ligand-expressing cancers.

Using induced pluripotent stem cells (iPSCs) to derive chimeric antigen receptor-modified T (CAR-T) cells has great industrial potential. A previous study used αβ T cell-derived CAR-modified iPSCs to produce CAR-T cells. However, these αβ T cells are restricted to autologous use and only recognize single cancer antigen. To make CAR-T alternative for allogeneic use, we reprogrammed γδ T cell into iPSCs (γδ T-iPSCs) to circumvent the risk of graft-versus-host disease. To target multiple cancer-associated antigens, we used an “NK cell-promoting” protocol to differentiate γδ T-iPSCs and to induce expression of natural killer receptors (NKRs). Through such two-step strategy, mimetic γδ T cells endowed with an array of NKRs and thus designated as “γδ natural killer T (γδ NKT) cells” were derived. With no/low-level expression of inhibitory killer cell immunoglobulin-like receptors (KIRs) and immune checkpoint receptors, γδ NKT cells may provide a potent “off-the-shelf” cytotoxic cell source to recognize multiple ubiquitous antigens in a broad spectrum of cancers.

Vγ9Vδ2 T cells are a minor subset of lymphocytes in the peripheral blood that has been extensively investigated for their tolerability, safety and anticancer efficacy. A hindrance to the broad application of these cells for adoptive cellular immunotherapy has been attaining clinically appropriate numbers of Vγ9Vδ2 T cells. Furthermore, Vγ9Vδ2 T cells exist at low frequencies among cancer patients. We, therefore, sought to conceive an economical method that allows for a quick and robust large-scale expansion of Vγ9Vδ2 T cells. A two-step protocol was developed, in which peripheral blood mononuclear cells (PBMCs) from healthy donors or cancer patients were activated with Zometa and interleukin (IL)-2, followed by co-culturing with gamma-irradiated, CD64-, CD86- and CD137L-expressing K562 artificial antigen-presenting cells (aAPCs) in the presence of the anti-CD3 antibody OKT3. We optimized the co-culture ratio of K562 aAPCs to immune cells, and migrated this method to a G-Rex cell growth platform to derive clinically relevant cell numbers in a Good Manufacturing Practice (GMP)-compliant manner. We further include a depletion step to selectively remove αβ T lymphocytes. The method exhibited high expansion folds and a specific enrichment of Vγ9Vδ2 T cells. Expanded Vγ9Vδ2 T cells displayed an effector memory phenotype with a concomitant down-regulated expression of inhibitory immune checkpoint receptors. Finally, we ascertained the cytotoxic activity of these expanded cells by using nonmodified and chimeric antigen receptor (CAR)–engrafted Vγ9Vδ2 T cells against a panel of solid tumor cells. Overall, we report an efficient approach to generate highly functional Vγ9Vδ2 T cells in massive numbers suitable for clinical application in an allogeneic setting.

Current donor cell-dependent strategies can only produce limited "made-to-order" therapeutic natural killer (NK) cells for limited patients. To provide unlimited "off-the-shelf" NK cells that serve many recipients, we designed and demonstrated a holistic manufacturing scheme to mass-produce NK cells from induced pluripotent stem cells (iPSCs). Starting with a highly accessible human cell source, peripheral blood cells (PBCs), we derived a good manufacturing practice-compatible iPSC source, PBC-derived iPSCs (PBC-iPSCs) for this purpose. Through our original protocol that excludes CD34+ cell enrichment and spin embryoid body formation, high-purity functional and expandable NK cells were generated from PBC-iPSCs. Above all, most of these NK cells expressed no killer cell immunoglobulin-like receptors (KIRs), which renders them unrestricted by recipients' human leukocyte antigen genotypes. Hence, we have established a practical "from blood cell to stem cells and back with less (less KIRs)" strategy to generate abundant "universal" NK cells from PBC-iPSCs for a wide range of patients.

Human pluripotent stem cells (hPSCs) provide a promising platform to produce dendritic cell (DC) vaccine. To streamline the production process, we investigated a unique antigen-loading strategy that suits this novel platform. Specifically, we stably modified hPSCs using tumour antigen genes in the form of a full-length tumour antigen gene or an artificial tumour antigen epitope-coding minigene. Such antigenically modified hPSCs were able to differentiate into tumour antigen-presenting DCs. Without conventional antigen-loading, DCs derived from the minigene-modified hPSCs were ready to prime a tumour antigen-specific T cell response and further expand these specific T cells in restimulation processes. These expanded tumour antigen-specific T cells were potent effectors with central memory or effector memory phenotype. Thus, we demonstrated that immunocompetent tumour antigen-loaded DCs can be directly generated from antigenically modified hPSCs. Using such strategy, we can completely eliminate the conventional antigen-loading step and significantly simplify the production of DC vaccine from hPSCs.

Given their intrinsic ability to home to tumor sites, endothelial progenitor cells (EPCs) are attractive as cellular vehicles for targeted cancer gene therapy. However, collecting sufficient EPCs is one of the challenging issues critical for effective clinical translation of this new approach. In this study, we sought to explore whether human induced pluripotent stem (iPS) cells could be used as a reliable and accessible cell source to generate human EPCs suitable for cancer treatment. We used an embryoid body formation method to derive CD133(+)CD34(+) EPCs from human iPS cells. The generated EPCs expressed endothelial markers such as CD31, Flk1, and vascular endothelial-cadherin without expression of the CD45 hematopoietic marker. After intravenous injection, the iPS cell-derived EPCs migrated toward orthotopic and lung metastatic tumors in the mouse 4T1 breast cancer model but did not promote tumor growth and metastasis. To investigate their therapeutic potential, the EPCs were transduced with baculovirus encoding the potent T cell costimulatory molecule CD40 ligand. The systemic injection of the CD40 ligand-expressing EPCs stimulated the secretion of both tumor necrosis factor-α and interferon-γ and increased the caspase 3/7 activity in the lungs with metastatic tumors, leading to prolonged survival of the tumor bearing mice. Therefore, our findings suggest that human iPS cell-derived EPCs have the potential to serve as tumor-targeted cellular vehicles for anticancer gene therapy.

Invariant natural killer T (iNKT) cells are a unique lymphocyte subpopulation that mediates antitumor activities upon activation. A current strategy to harness iNKT cells for cancer treatment is endogenous iNKT cell activation using patient-derived dendritic cells (DCs). However, the limited number and functional defects of patient DCs are still the major challenges for this therapeutic approach. In this study, we investigated whether human embryonic stem cells (hESCs) with an ectopically expressed CD1d gene could be exploited to address this issue. Using a lentivector carrying an optimized expression cassette, we generated stably modified hESC lines that consistently overexpressed CD1d. These modified hESC lines were able to differentiate into DCs as efficiently as the parental line. Most importantly, more than 50% of such derived DCs were CD1d+. These CD1d-overexpressing DCs were more efficient in inducing iNKT cell response than those without modification, and their ability was comparable to that of DCs generated from monocytes of healthy donors. The iNKT cells expanded by the CD1d-overexpressing DCs were functional, as demonstrated by their ability to lyse iNKT cell-sensitive glioma cells. Therefore, hESCs stably modified with the CD1d gene may serve as a convenient, unlimited, and competent DC source for iNKT cell-based cancer immunotherapy.

The interaction between CD40 ligand (CD40L) and CD40 can directly inhibit growth of CD40-positive carcinoma cells and may indirectly inhibit tumor growth through coordination of immune responses. Many efforts in CD40L cancer gene therapy have been focused on direct CD40L gene transfer into malignant target cells. This in vivo gene therapy approach relies on high-efficiency gene transfer and could be technically challenging for the treatment of certain cancers, especially multisite metastases. We report herein an alternative means of using the tumor-homing property of neural stem cells (NSCs) to deliver CD40L molecules into tumor tissues. NSCs were derived from human induced pluripotent stem cells, transduced in vitro with a baculoviral vector encoding CD40L, and intravenously injected into immunocompetent mice with orthotopic and metastatic breast cancers. Through a bystander mechanism of intercellular transfer of CD40L from the donor NSCs to tumor target cells, the treatment impeded tumor growth, leading to prolonged survival of the tumor-bearing mice. We further showed that compared with the stem cell-based gene therapy that employed a suicide gene, the CD40L immunogene therapy did not cause liver and kidney injury in the treated mice. This new approach may be particularly valuable for metastatic cancer treatments after systemic stem cell administration.

Integrative gene transfer using retroviruses to express reprogramming factors displays high efficiency in generating induced pluripotent stem cells (iPSCs), but the value of the method is limited because of the concern over mutagenesis associated with random insertion of transgenes. Site-specific integration into a preselected locus by engineered zinc-finger nuclease (ZFN) technology provides a potential way to overcome the problem. Here, we report the successful reprogramming of human fibroblasts into a state of pluripotency by baculoviral transduction-mediated, site-specific integration of OKSM (Oct3/4, Klf4, Sox2, and c-myc) transcription factor genes into the AAVS1 locus in human chromosome 19. Two nonintegrative baculoviral vectors were used for cotransduction, one expressing ZFNs and another as a donor vector encoding the four transcription factors. iPSC colonies were obtained at a high efficiency of 12% (the mean value of eight individual experiments). All characterized iPSC clones carried the transgenic cassette only at the ZFN-specified AAVS1 locus. We further demonstrated that when the donor cassette was flanked by heterospecific loxP sequences, the reprogramming genes in iPSCs could be replaced by another transgene using a baculoviral vector-based Cre recombinase-mediated cassette exchange system, thereby producing iPSCs free of exogenous reprogramming factors. Although the use of nonintegrating methods to generate iPSCs is rapidly becoming a standard approach, methods based on site-specific integration of reprogramming factor genes as reported here hold the potential for efficient generation of genetically amenable iPSCs suitable for future gene therapy applications.

Intravenously injected neural stem cells (NSCs) can infiltrate both primary and metastatic tumor sites; thus, they are attractive tumor-targeting vehicles for delivering anticancer agents. However, because the systemic distribution of the injected NSCs involves normal organs and might induce off-target actions leading to unintended side effects, clinical applications of this approach is impeded. Given that the vesicular stomatitis virus glycoprotein (VSV-G) can promote the formation of multinucleated syncytia to kill cells in a pH-dependent manner, we engineered a pH sensor of VSV-G and generated a novel VSV-G mutant that efficiently promotes syncytium formation at the tumor extracellular pH (pH(e)) but not at pH 7.4. Using transduced NSCs derived from induced pluripotent stem cells (iPSCs), the VSV-G mutant was delivered into mice with metastatic breast cancers in the lung through tail vein injection. Compared with the conventional stem cell-based gene therapy that uses the herpes simplex virus thymidine kinase (HSVtk) suicide gene, this treatment did not display toxicity to normal non-targeted organs while retaining therapeutic effects in tumor-bearing organs. Our findings demonstrate the effectiveness of a new approach for achieving tumor-selective killing effects following systemic stem cell administration. Its potential in stem cell-based gene therapy for metastatic cancer is worthy of further exploration.

Human embryonic stem cell-derived dendritic cells (hESC-DCs) may potentially provide a platform to generate "off-the-shelf" therapeutic cancer vaccines. To apply hESC-DCs for cancer immunotherapy in a semiallogeneic setting, it is crucial for these cells to "jump-start" adaptive antitumor immunity before their elimination by host alloreaction. In this study, we investigated whether CD1d upregulation in hESC-DCs may exploit invariant NKT (iNKT) cell adjuvant activity and boost antitumor immunity. Using a baculoviral vector carrying the CD1d gene, we produced CD1d-overexpressing hESC-DCs and demonstrated that the upregulated CD1d was functional in presenting α-galactosylceramide for iNKT cell expansion. Pulsed with melanoma Ag recognized by T cell 1 peptide, the CD1d-overexpressing hESC-DCs displayed enhanced capability to prime CD8(+) T cells without relying on α-galactosylceramide loading. Blocking the CD1d with Ab reduced the immunogenicity, suggesting the importance of hESC-DC and iNKT cell interaction in this context. The CD1d-overexpressing hESC-DCs also induced a proinflammatory cytokine profile that may favor the T cell priming. Moreover, a similar immunostimulatory effect was observed when the CD1d upregulation strategy was applied in human monocyte-derived dendritic cells. Therefore, our study suggests that the upregulation of CD1d in hESC-DCs provides a novel strategy to enhance their immunogenicity. This approach holds potential for advancing the application of hESC-DCs into human cancer immunotherapy.

Transient gene expression is one possible approach to manipulate the signaling pathways that control the proliferation and differentiation of human embryonic stem (hES) cells. We tested in hES cells a range of baculoviral vectors with a human elongation factor-1alpha promoter and various viral regulatory elements and observed the most dramatic augmenting effect on the transient expression when the promoter was used together with the human cytomegalovirus immediate-early gene enhancer and the woodchuck hepatitis virus post-transcriptional regulatory elements. This vector provided a 1.6-fold increase in the percentage of transduced cells (up to 72%) over a vector containing the elongation factor-1alpha promoter alone. The effective baculoviral transduction of hES cells did not affect cell proliferation, expression of embryonic stem cell markers and teratoma formation. This new viral vector for temporary transgene expression might become a useful tool for developmental biology studies and biomedical applications of hES cells.

Transient genetic manipulation of human neurons without chromosomal integration of the transgene would be valuable but has been challenging due to the quiescent nature of these postmitotic cells. In this study, we developed a set of baculoviral vectors for transient transduction in nondividing neurons derived from human embryonic stem cells (hESCs). Using a baculoviral vector equipped with the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE), we observed a quick onset of transgene expression as early as day 1 after baculoviral transduction and a high efficiency of up to 80%. Strong transgene expression in the cultured human neurons was observed for more than 1 month and the signal was easily detectable even after 3 months. Using two baculoviral vectors carrying different transgenes, we found that co-transduction at a single neuron level was possible. After transplantation into the brain of nude mice, the baculovirus-transduced human neurons were integrated into the mouse brain and maintained transgene expression for at least 4 weeks, portending the usefulness of this technique in assisting neural transplantation. Therefore, by mediating efficient transient gene expression, baculoviral vectors can provide useful tools for both basic gene function studies in human neurons and therapeutic applications of these cells.