Croix B. Uncovering receptor-ligand interactions using a high-avidity CRISPR activation screening platform

The majority of clinically approved drugs target proteins that are secreted or cell surface bound. However, further advances in this area have been hindered by the challenging nature of receptor deorphanization, as there are still many secreted and cell-bound proteins with unknown binding partners. Here, we developed an advanced screening platform that combines CRISPR-CAS9 guide-mediated gene activation (CRISPRa) and high-avidity bead-based selection. The CRISPRa platform incorporates serial enrichment and flow cytometry-based monitoring, resulting in substantially improved screening sensitivity for well-known yet weak interactions of the checkpoint inhibitor family. Our approach has successfully revealed that siglec-4 exerts regulatory control over T cell activation through a low affinity trans-interaction with the costimulatory receptor 4-1BB. Our highly efficient screening platform holds great promise for identifying extracellular interactions of uncharacterized receptor-ligand partners, which is essential to develop next-generation therapeutics, including additional immune checkpoint inhibitors.

Efficient clinical-grade γ-retroviral vector purification by high-speed centrifugation for CAR T cell manufacturing

γ-Retroviral vectors (γ-RV) are powerful tools for gene therapy applications. Current clinical vectors are produced from stable producer cell lines which require minimal further downstream processing, while purification schemes for γ-RV produced by transient transfection have not been thoroughly investigated. We aimed to develop a method to purify transiently produced γ-RV for early clinical studies. Here, we report a simple one-step purification method by high-speed centrifugation for γ-RV produced by transient transfection for clinical application. High-speed centrifugation enabled the concentration of viral titers in the range of 10⁷-10⁸ TU/mL with >80% overall recovery. Analysis of research-grade concentrated vector revealed sufficient reduction in product- and process-related impurities. Furthermore, product characterization of clinical-grade γ-RV by BioReliance demonstrated two-logs lower impurities per transducing unit compared with regulatory authority-approved stable producer cell line vector for clinical application. In terms of CAR T cell manufacturing, clinical-grade γ-RV produced by transient transfection and purified by high-speed centrifugation was similar to γ-RV produced from a clinical-grade stable producer cell line. This method will be of value for studies using γ-RV to bridge vector supply between early- and late-stage clinical trials.

Detection of response to tumor microenvironment-targeted cellular immunotherapy using nano-radiomics

Immunotherapies, including cell-based therapies, targeting the tumor microenvironment (TME) result in variable and delayed responses. Thus, it has been difficult to gauge the efficacy of TME-directed therapies early after administration. We investigated a nano-radiomics approach (quantitative analysis of nanoparticle contrast-enhanced three-dimensional images) for detection of tumor response to cellular immunotherapy directed against myeloid-derived suppressor cells (MDSCs), a key component of TME. Animals bearing human MDSC-containing solid tumor xenografts received treatment with MDSC-targeting human natural killer (NK) cells and underwent nanoparticle contrast-enhanced computed tomography (CT) imaging. Whereas conventional CT-derived tumor metrics were unable to differentiate NK cell immunotherapy tumors from untreated tumors, nano-radiomics revealed texture-based features capable of differentiating treatment groups. Our study shows that TME-directed cellular immunotherapy causes subtle changes not effectively gauged by conventional imaging metrics but revealed by nano-radiomics. Our work provides a method for noninvasive assessment of TME-directed immunotherapy potentially applicable to numerous solid tumors.

Toward a Scalable Purification Protocol of GaLV-TR-Pseudotyped Lentiviral Vectors

Lentiviral vectors (LV) that are used in research and development as well as in clinical trials are in majority vesicular stomatitis virus G glycoprotein (VSVg) pseudotyped. The predominance of this pseudotype choice for clinical gene therapy studies is largely due to a lack of purification schemes for pseudotypes other than VSVg. In this study, we report for the first time the development of a new downstream process protocol allowing high-yield production of stable and infectious gibbon ape leukemia virus (GaLV)-TR-LV particles. We identified critical conditions in tangential flow filtration (TFF) and chromatographic steps for preserving the infectivity/functionality of LV during purification. This was carried out by identifying for each step, the critical parameters affecting LV infectivity, including pH, salinity, presence of stabilizers, temperature, and by defining the optimal order of these steps. A three-step process was developed for GaLV-TR-LV purification consisting of one TFF and two chromatographic steps (ion-exchange chromatography and size exclusion chromatography) permitting recoveries of >27% of infectious particles. With this process, purified GaLV-pseudotyped LV enabled the transduction of 70% human CD34⁺ cells in the presence of the Vectofusin-1 peptide, whereas in the same conditions nonpurified vector transduced only 9% of the cells (multiplicity of infection 20). Our protocol will allow for the first time the purification of GaLV-TR-LV that are biologically active, stable, and with sufficient recovery in the perspective of preclinical studies and clinical applications. Obviously, further optimizations are required to improve final vector yields.

Generation of Chimeric Antigen Receptor T Cells Using Gammaretroviral Vectors

Manufacturing chimeric antigen receptor (CAR)-modified T cells requires incorporation of the CAR transgene, for which viral vectors are most often used. Here, we describe the generation of CAR T cells using primary human T cells and a non-self-inactivating gammaretroviral vector encoding a CAR transgene. The gammaretroviral vector is produced by 293T cells transiently transfected with DNA plasmids encoding necessary components of the viral vector. The resulting viral particles efficiently infect activated T cells and integrate the CAR transgene into the genome of dividing cells for stable expression.

NK Cells Expressing a Chimeric Activating Receptor Eliminate MDSCs and Rescue Impaired CAR-T Cell Activity against Solid Tumors

Solid tumors are refractory to cellular immunotherapies in part because they contain suppressive immune effectors such as myeloid-derived suppressor cells (MDSCs) that inhibit cytotoxic lymphocytes. Strategies to reverse the suppressive tumor microenvironment (TME) should also attract and activate immune effectors with antitumor activity. To address this need, we developed gene-modified natural killer (NK) cells bearing a chimeric receptor in which the activating receptor NKG2D is fused to the cytotoxic ζ-chain of the T-cell receptor (NKG2D.ζ). NKG2D.ζ-NK cells target MDSCs, which overexpress NKG2D ligands within the TME. We examined the ability of NKG2D.ζ-NK cells to eliminate MDSCs in a xenograft TME model and improve the antitumor function of tumor-directed chimeric antigen receptor (CAR)-modified T cells. We show that NKG2D.ζ-NK cells are cytotoxic against MDSCs, but spare NKG2D ligand-expressing normal tissues. NKG2D.ζ-NK cells, but not unmodified NK cells, secrete proinflammatory cytokines and chemokines in response to MDSCs at the tumor site and improve infiltration and antitumor activity of subsequently infused CAR-T cells, even in tumors for which an immunosuppressive TME is an impediment to treatment. Unlike endogenous NKG2D, NKG2D.ζ is not susceptible to TME-mediated downmodulation and thus maintains its function even within suppressive microenvironments. As clinical confirmation, NKG2D.ζ-NK cells generated from patients with neuroblastoma killed autologous intratumoral MDSCs capable of suppressing CAR-T function. A combination therapy for solid tumors that includes both NKG2D.ζ-NK cells and CAR-T cells may improve responses over therapies based on CAR-T cells alone.

Characterization of an alternative packaging system derived from the cat RD114 retrovirus for gene delivery

BACKGROUND

Retroviral vectors derived from the Moloney murine leukemia virus (MLV) are widely used in gene therapy. Pseudotyping of these vectors with the cat RD114 retrovirus envelope increases their potential for delivering genes into human hematopoietic cells. In the present study, we have further investigated the potential of the RD114 retrovirus in gene therapy. We describe and characterize an alternative retroviral packaging system derived from the RD114 retrovirus.

METHODS

RD114-derived recombinant retroviruses were produced transiently by transfection of 293T cells, and viral titers were assessed on TE671 cells by measuring the percentage of infected green fluorescent protein (GFP) positive cells by fluorescence-activated cell sorter (FACS) analysis. Purified human hematopoietic cells (lymphocytes and CD34(+) cells) were activated and transduced on retronectin-coated plates. Two days later, the percentage of GFP positive cells was evaluated by FACS analysis.

RESULTS

We demonstrate that RD114 viral particles could package MLV transfer vectors, and that, in addition to its natural envelope, RD114 cores could be efficiently pseudotyped by the Gibbon ape leukemia, the MLV-amphotropic and the vesicular stomatitis virus G protein envelopes. Furthermore, we found that RD114 viral particles were highly efficient to transduce human lymphocytes and CD34(+) cells.

CONCLUSIONS

This is the first demonstration that replication-defective RD114 viral particles can be generated and used for efficient gene delivery into human hematopoietic cells. We conclude that RD114-derived vectors could be useful in the field of gene therapy.

Generation of EBV-specific cytotoxic T cells that are resistant to calcineurin inhibitors for the treatment of posttransplantation lymphoproliferative disease

Epstein-Barr virus (EBV)-driven posttransplantation lymphoproliferative disease (PTLD) is a serious complication of immunosuppression after either stem cell transplantation (SCT) or solid organ transplantation (SOT). Adoptive transfer of EBV-specific cytotoxic T lymphocytes (EBV-CTLs) is an effective prophylaxis and treatment for PTLD after SCT, but not for PTLD after SOT when pharmacologic immunosuppression cannot be discontinued. We report the generation of calcineurin (CN) mutants that render EBV-CTL resistant to the immunosuppressants tacrolimus (FK506) and cyclosporin A (CsA): mutant CNa12 confers resistance to CsA but not FK506, and mutant CNa22 confers resistance to FK506 but not CsA, whereas mutant CNb30 renders CTLs resistant to both calcineurin inhibitors. Untransduced EBV-CTLs do not proliferate in the presence of FK506/CsA. However, EBV-CTLs transduced with a retroviral vector coding for these mutants retain the ability to both proliferate and secrete normal levels of interferon-gamma in the presence therapeutic levels of FK506 (CNa12), CsA (CNa22), or both (CNb30). The cytotoxicity and phenotype of EBV-CTL lines were unaffected by expression of these mutant CNs. This approach should allow effective immunotherapy with EBV-CTLs in the SOT setting without risking the graft by reduction in immunosuppression, and represents a generic approach to improving immunotherapy in the face of immunosuppression.

Generation of Epstein-Barr virus-specific cytotoxic T lymphocytes resistant to the immunosuppressive drug tacrolimus (FK506)

Adoptive transfer of autologous Epstein-Barr virus-specific cytotoxic T lymphocytes (EBV-CTLs) to solid organ transplant (SOT) recipients has been shown safe and effective for the treatment of EBV-associated posttransplantation lymphoproliferative disorders (PTLDs). SOT recipients, however, require the continuous administration of immunosuppressive drugs to prevent graft rejection, and these agents may significantly limit the long-term persistence of transferred EBV-CTLs, precluding their use as prophylaxis. Tacrolimus (FK506) is one of the most widely used immunosuppressive agents in SOT recipients, and its immunosuppressive effects are largely dependent on its interaction with the 12-kDa FK506-binding protein (FKBP12). We have knocked down the expression of FKBP12 in EBV-CTLs using a specific small interfering RNA (siRNA) stably expressed from a retroviral vector and found that FKBP12-silenced EBV-CTLs are FK506 resistant. These cells continue to expand in the presence of the drug without measurable impairment of their antigen specificity or cytotoxic activity. We confirmed their FK506 resistance and anti-PTLD activity in vivo using a xenogenic mouse model, suggesting that the proposed strategy may be of value to enhance EBV-specific immune surveillance in patients at high risk of PTLD after transplantation.

Model systems for examining effects of leukemia-associated oncogenes in primary human CD34+ cells via retroviral transduction

The use of primary human cells to model cancer initiation and progression is now within the grasp of investigators. It has been nearly a decade since the first defined genetic elements were introduced into primary human epithelial and fibroblast cells to model oncogenesis. This approach has now been extended to the hematopoietic system, with the first described experimental transformation of primary human hematopoietic cells. Human cell model systems will lead to a better understanding of the species and cell type specific signals necessary for oncogenic initiation and progression, and will allow investigators to interrogate the cancer stem cell hypothesis using a well-defined hierarchical system that has been studied for decades. The molecular and biochemical link between self-renewal and differentiation can now be experimentally approached using primary human cells. In addition, the models that result from these experiments are likely to generate highly relevant systems for use in identification and validation of potential therapeutic targets as well as testing of small molecule therapeutics. We describe here the methodologies and reagents that are used to examine the effects of leukemia fusion protein expression on primary human hematopoietic cells, both in vitro and in vivo.

Genetic manipulation of tumor-specific cytotoxic T lymphocytes to restore responsiveness to IL-7

Adoptive transfer of antigen-specific cytotoxic T lymphocytes (CTLs) can induce objective clinical responses in patients with malignant diseases. The option of providing a proliferative and survival advantage to adoptively transferred CTLs remains a challenge to improve their efficacy. Host lymphodepletion and administration of recombinant interleukin-2 (IL-2) are currently used to improve CTL survival and expansion after adoptive transfer, but these approaches are frequently associated with significant side effects and may increase proliferation of T regulatory cells. IL-7 is a crucial homeostatic cytokine that has been safely administered as a recombinant protein. However, while IL-7 induces robust expansion of naive and memory T lymphocytes, the lack of expression of the IL-7 receptor alpha chain (IL-7Ralpha) by CTLs precludes their response to this cytokine. We found that CTLs can be genetically modified to re-express IL-7Ralpha, and that this manipulation restores the response of these cells to IL-7 without apparent modification of their antigen specificity or dependency, and without changing their response to other common gamma (gammac) chain cytokines. This approach may allow selective expansion of CTLs without the unwanted effects associated with IL-2.

Co-expression of cytokine and suicide genes to enhance the activity and safety of tumor-specific cytotoxic T lymphocytes

The antitumor effect of adoptively transferred tumor-specific cytotoxic T lymphocytes (CTLs) is impaired by the limited capacity of these cells to expand within the tumor microenvironment. Administration of interleukin 2 (IL-2) has been used to overcome this limitation, but the systemic toxicity and the expansion of unwanted cells, including regulatory T cells, limit the clinical value of this strategy. To discover whether transgenic expression of lymphokines by the CTLs themselves might overcome these limitations, we evaluated the effects of transgenic expression of IL-2 and IL-15 in our model of Epstein Barr Virus-specific CTLs (EBV-CTLs). We found that transgenic expression of IL-2 or IL-15 increased the expansion of EBV-CTLs both in vitro and in vivo in a severe combined immunodeficiency disease (SCID) mouse model and enhanced antitumor activity. Although the proliferation of these cytokine genes transduced CTLs remained strictly antigen dependent, clinical application of this approach likely requires the inclusion of a suicide gene to deal with the potential development of T-cell mutants with autonomous growth. We found that the incorporation of an inducible caspase-9 suicide gene allowed efficient elimination of transgenic CTLs after exposure to a chemical inducer of dimerization, thereby increasing the safety and feasibility of the approach.

Epstein Barr virus specific cytotoxic T lymphocytes expressing the anti-CD30zeta artificial chimeric T-cell receptor for immunotherapy of Hodgkin disease

Adoptive transfer of Epstein Barr virus (EBV)-specific cytotoxic T-lymphocytes (EBV-CTLs) has shown that these cells persist in patients with EBV(+) Hodgkin lymphoma (HD) to produce complete tumor responses. Treatment failure, however, occurs if a subpopulation of malignant cells in the tumor lacks or loses expression of EBV antigens. We have therefore determined whether we could prepare EBV-CTLs that retained the antitumor activity conferred by their native receptor while expressing a chimeric antigen receptor (CAR) specific for CD30, a molecule highly and consistently expressed on malignant Hodgkin Reed-Sternberg cells. We made a CD30CAR and were able to express it on 26% (+/- 11%) and 22% (+/- 5%) of EBV-CTLs generated from healthy donors and HD patients, respectively. These CD30CAR(+) CTLs killed both autologous EBV(+) cells through their native receptor and EBV(-)/CD30(+) targets through their major histocompatibility complex (MHC)-unrestricted CAR. A subpopulation of activated T cells also express CD30, but the CD30CAR(+) CTLs did not impair cellular immune responses, probably because normal T cells express lower levels of the target antigen. In a xenograft model, CD30CAR(+) EBV-CTLs could be costimulated by EBV-infected cells and produce antitumor effects even against EBV(-)/CD30(+) tumors. EBV-CTLs expressing both a native and a chimeric antigen receptor may therefore have added value for treatment of HD.

MYCN-Directed Centrosome Amplification Requires MDM2-Mediated Suppression of p53 Activity in Neuroblastoma Cells

The MYC family oncogenes cause transformation and tumor progression by corrupting multiple cellular pathways, altering cell cycle progression, apoptosis, and genomic instability. Several recent studies show that MYCC (c-Myc) expression alters DNA repair mechanisms, cell cycle checkpoints, and karyotypic stability, and this is likely partially due to alterations in centrosome replication control. In neuroblastoma cell lines, MYCN (N-Myc) expression induces centrosome amplification in response to ionizing radiation. Centrosomes are cytoplasmic domains that critically regulate cytokinesis, and aberrations in their number or structure are linked to mitotic defects and karyotypic instability. Whereas centrosome replication is linked to p53 and Rb/E2F-mediated cell cycle progression, the mechanisms downstream of MYCN that generate centrosome amplification are incompletely characterized. We hypothesized that MDM2, a direct transcriptional target of MYCN with central inhibitory effects on p53, plays a role in MYC-mediated genomic instability by altering p53 responses to DNA damage, facilitating centrosome amplification. Herein we show that MYCN mediates centrosome amplification in a p53-dependent manner. Accordingly, inhibition of the p53-MDM2 interaction with Nutlin 3A (which activates p53) completely ablates the MYCN-dependent contribution to centrosome amplification after ionizing radiation. We further show that modulating MDM2 expression levels by overexpression or RNA interference-mediated posttranscriptional inhibition dramatically affects centrosome amplification in MYCN-induced cells, indicating that MDM2 is a necessary and sufficient mediator of MYCN-mediated centrosome amplification. Finally, we show a significant correlation between centrosome amplification and MYCN amplification in primary neuroblastoma tumors. These data support the hypothesis that elevated MDM2 levels contribute to MYCN-induced genomic instability through altered regulation of centrosome replication in neuroblastoma.

Evaluation of different protocols for gene transfer into non-obese diabetes/severe combined immunodeficiency disease mouse repopulating cells

PURPOSE

Although gene transfer with retroviral vectors has shown distinct clinical success in defined settings, efficient genetic manipulation of hematopoietic progenitor cells remains a challenge. To address this issue we have evaluated different transduction protocols and retroviral constructs in the non-obese diabetes (NOD)/severe combined immunodeficiency disease (SCID) xenograft model.

METHODS

An extended transduction protocol requiring 144 h of in vitro manipulation was compared to a more conventional protocol requiring 96 h only.

RESULT

While pretransplantation analysis of cells transduced with a retroviral vector, expressing the enhanced green fluorescent protein (EGFP) marker gene, demonstrated significantly higher overall transduction rates for the extended protocol (33.6 +/- 2.3 vs. 22.1 +/- 3.8%), EGFP expression in CD34+ cells before transplantation (4.0 +/- 0.9 vs. 11.6 +/- 2.5%), engraftment of human cells in NOD/SCID bone marrow 4 weeks after transplantation (4.5 +/- 1.7 vs. 36.5 +/- 9.4%) and EGFP expression in these cells (0 +/- 0 vs. 11.3 +/- 2.8%) were significantly impaired. When the 96 h protocol was used in combination with the spleen focus forming virus (SFFV)/murine embryonic stem cell (MESV) hybrid vector SFbeta11-EGFP, high transduction rates for CD45+ (41.0 +/- 5.3%) and CD34+ (38.5 +/- 3.7%) cells prior to transplantation, as well as efficient human cell engraftment in NOD/SCID mice 4 weeks after transplantation (32.4 +/- 3.5%), was detected. Transgene expression was observed in B-lymphoid (15.9 +/- 2.0%), myeloid (36.5 +/- 3.5%) and CD34+ cells (10.1 +/- 1.5%).

CONCLUSION

Our data show that CD34+ cells maintained in cytokines for multiple days may differentiate and loose their capacity to contribute to the haematological reconstitution of NOD/SCID mice. In addition, the SFFV/MESV hybrid vector SFbeta11-EGFP allows efficient transduction of and gene expression in haematopoietic progenitor cells.

Hematopoietic stem cell transduction and amplification in large animal models

Progress in retroviral gene transfer to large animal hematopoietic stem cells (HSCs) has led to efficient, reproducible long-term marking in both canine and nonhuman primate models. Successes for HSC gene therapy have occurred in the severe combined immunodeficiency setting, in which transduced cells have a selective advantage. However, for most diseases, the therapeutic transgene does not confer a sufficient survival advantage, and increasing the percentage of gene-marked cells in vivo will be necessary to observe a therapeutic effect. In vivo amplification should expand the potential of HSC gene therapy, and progress in this area has benefited greatly from the use of large animal models where efficacy and toxicity have often not correlated with results in murine models. To date, the best results have been observed with O(6)-methylguanine-DNA methyltransferase (MGMT) selection, with which increases in gene-marked repopulating cells have been maintained long-term, likely because of the toxicity of 1,3-bis-(2-chloroethyl)-1-nitrosourea and temozolomide to quiescent HSCs. Using MGMT selection, long-term marking levels exceeding 50% can now be routinely attained with minimal toxicity. There is cause to be optimistic that HSC gene therapy with in vivo amplification will soon allow the treatment of several genetic and infectious diseases.

Correction of canine X-linked severe combined immunodeficiency by in vivo retroviral gene therapy

X-linked severe combined immunodeficiency (XSCID) is characterized by profound immunodeficiency and early mortality, the only potential cure being hematopoietic stem cell (HSC) transplantation or gene therapy. Current clinical gene therapy protocols targeting HSCs are based upon ex vivo gene transfer, potentially limited by the adequacy of HSC harvest, transduction efficiencies of repopulating HSCs, and the potential loss of their engraftment potential during ex vivo culture. We demonstrate an important proof of principle by showing achievement of durable immune reconstitution in XSCID dogs following intravenous injection of concentrated RD114-pseudotyped retrovirus vector encoding the corrective gene, the interleukin-2 receptor gamma chain (gamma c). In 3 of 4 dogs treated, normalization of numbers and function of T cells were observed. Two long-term-surviving animals (16 and 18 months) showed significant marking of B lymphocytes and myeloid cells, normalization of IgG levels, and protective humoral immune response to immunization. There were no adverse effects from in vivo gene therapy, and in one dog that reached sexual maturity, sparing of gonadal tissue from gene transfer was demonstrated. This is the first demonstration that in vivo gene therapy targeting HSCs can restore both cellular and humoral immunity in a large-animal model of a fatal immunodeficiency.

Hematopoietic stem cell gene therapy with drug resistance genes: an update

Transfer of drug resistance genes into hematopoietic stem cells (HSCs) has promise for the treatment of a variety of inherited, that is, X-linked severe combined immune deficiency, adenosine deaminase deficiency, thalassemia, and acquired disorders, that is, breast cancer, lymphomas, brain tumors, and testicular cancer. Drug resistance genes are transferred into HSCs either for providing myeloprotection against chemotherapy-induced myelosuppression or for selecting HSCs that are concomitantly transduced with another gene for correction of an inherited disorder. In this review, we describe ongoing experimental approaches, observations from clinical trials, and safety concerns related to the drug resistance gene transfer.

Efficient characterisation of human cell–bioceramic interactions in vitro and in vivo by using enhanced GFP-labelled mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) were transfected using four retroviral pseudotypes, amphotropic murine leukemia viruses 4070 (MuLV-10A1), a modification of amphotropic pseudotype 4073 (A71G, Q74K, V139M), gibbon ape leukemia virus (GaLV), or feline endogenous virus (RD114) encoding the neomycin resistance (Neo(r)) gene and enhanced green fluorescent protein (eGFP) as genetic markers. It was observed that the MuLV4073 was the most efficient pseudotype for hMSC transfection. The proliferation and differentiation characteristics of eGFP-labelled hMSCs were not significantly different from control hMSCs. G418 selected eGFP-labelled cells were cultured for 3 weeks on two porous, commercially available calcium phosphate bioceramics, a "synthetic hydroxyapatite" and a "deproteinised bone", before implantation into NOD/SCID mice for up to 4 weeks. The eGFP-labelled hMSCs could be readily visualised by their intense green fluorescence both in vitro and in vivo. In "synthetic hydroxyapatite" implants the cells remained in a monolayer, whereas in "deproteinised bone" implants mineralised tissues were detected by histology, scanning electron microscopy and energy dispersive X-ray spectrometry. From the results, it is concluded that the use of eGFP-labelled hMSCs is an effective tool to trace the fate of hMSCs and evaluate the interactions between cells and ceramics both in vitro and in vivo. This is of great value in prospective assessments of these cell populations for use in tissue engineering applications.

Gene transfer to repopulating human CD34+ cells using amphotropic-, GALV-, or RD114-pseudotyped HIV-1-based vectors from stable producer cells

A novel, stable human immunodeficiency virus type 1 vector packaging system, STAR, was tested for its ability to transduce human cord blood CD34+ progenitor cells assayed both in vitro and after transplantation into NOD/SCID mice. Vectors pseudotyped with three different gammaretrovirus envelopes were used: the amphotropic MLV envelope (MLV-A), a modified gibbon ape leukemia virus envelope (GALV+), and a modified feline endogenous virus RD114 envelope (RDpro). Gene transfer to freshly thawed CD34+ cells in the absence of cytokines was very low. Addition of cytokines increased gene transfer efficiency significantly and this was further augmented if the cells were prestimulated for 24 h. Concentration of the vectors (15-fold) by low-speed centrifugation increased gene transfer to CD34+ cells in vitro even further. More than 90% of cells were transduced with a single exposure to the RDpro vector as determined by GFP expression using flow cytometry. The two other pseudotypes transduced approximately 65-70% of the cells under the same conditions. Transplantation of CD34+ cells prestimulated for 24 h and then transduced with a single exposure to concentrated vector revealed that the RDpro vector transduced 55.1% of NOD/SCID repopulating human cells, which was significantly higher than the MLV-A (12.6%)- or GALV+ (25.1%)-pseudotyped vectors.

Factors influencing the titer and infectivity of lentiviral vectors

Lentiviral vectors have undergone several generations of design improvement to enhance their biosafety and expression characteristics, and have been approved for use in human clinical studies. Most preclinical studies with these vectors have employed easily assayed marker genes for the purpose of determining vector titers and transduction efficiencies. Naturally, the adaptation of these vector systems to clinical use will increasingly involve the transfer of genes whose products may not be easily measured, meaning that the determination of vector titer will be more complicated. One method for determining vector titer that can be universally employed on all human immunodeficiency virus type 1-based lentiviral vector supernatants involves the measurement of Gag (p24) protein concentration in vector supernatants by immunoassay. We have studied the effects that manipulation of several variables involved in vector design and production by transient transfection have on vector titer and infectivity. We have determined that manipulation of the amount of transfer vector, packaging, and envelope plasmids used to transfect the packaging cells does not alter vector infectivity, but does influence vector titer. We also found that modifications to the transfer vector construct, such as replacing the internal promoter or transgene, do not generally alter vector infectivity, whereas inclusion of the central polypurine tract in the transfer vector increases vector infectivity on HEK293 cells and human umbilical cord blood CD34+ hematopoietic progenitor cells (HPCs). The infectivities of vector supernatants can also be increased by harvesting at early time points after the initiation of vector production, collection in serum-free medium, and concentration by ultracentrifugation. For the transduction of CD34+ HPCs, we found that the simplest method of increasing vector infectivity is to pseudotype vector particles with the RD114 envelope instead of vesicular stomatitis virus G glycoprotein (VSV-G).

Human cytotoxic T lymphocytes with reduced sensitivity to Fas-induced apoptosis

Effector-memory T cells expressing Fas (Apo-1/CD95) are switched to an apoptotic program by cross-linking with Fas-ligand (FasL). Consequently, tumors that express FasL can induce apoptosis of infiltrating Fas-positive T lymphocytes and subdue any antitumor host immune response. Since Epstein-Barr virus (EBV)-associated tumors such as Hodgkin lymphoma (HL) and nasopharyngeal carcinoma (NPC) express FasL, we determined whether EBV-specific cytotoxic T lymphocytes (EBV-CTLs) could be modified to resist this evasion strategy. We show that long-term down-modulation of Fas can be achieved in EBV-CTLs by transduction with small interfering RNA (siRNA) encoded in a retrovirus. Modified T cells resisted Fas/FasL-mediated apoptosis compared with control cells and showed minimal cleavage of the caspase3 substrate poly(ADP-ribose) polymerase (PARP) protein after Fas engagement. Prolonged Fas stimulation selected a uniformly Fas(low) and FasL resistant population. Removal of responsiveness to this single death signal had no other discernible effects on EBV-CTLs. In particular, it did not lead to their autonomous growth since the modified EBV-CTLs remained polyclonal, and their survival and proliferation retained dependence on antigen-specific stimulation and on the presence of other physiologic growth signals. EBV-CTLs with knocked down Fas should have a selective functional and survival advantage over unmodified EBV-CTLs in the presence of tumors expressing FasL and may be of value for adoptive cellular therapy.

RD114-pseudotyped retroviral vectors kill cancer cells by syncytium formation and enhance the cytotoxic effect of the TK/GCV gene therapy strategy

BACKGROUND

Wild-type RD114 virus is capable of generating syncytia during its replication, and it is believed that cell-free viruses direct the fusion of neighboring cells. The RD114 envelope (Env) that mediates this fusion event is now widely used to pseudotype retroviral and lentiviral vectors in gene therapy. Indeed, vectors pseudotyped with RD114 Env are very efficient to transfer genes into human hematopoietic cells, and they are resistant to human complement inactivation. In this study, we have tested the potential of RD114-pseudotyped vectors produced from the FLYRD18 packaging cell line to induce syncytia.

METHODS

RD114-pseudotyped vectors produced from the FLYRD18 packaging cells were added on tumor cell lines, and the formation of syncytia was assessed by microscopy after cell fixation and methylene blue staining. The kinetics of syncytium formation was analyzed by time-lapse microscopy. Finally, the cytotoxic effect of RD114-pseudotyped vectors was measured by the MTT assay on tumor cells, and in combination with the TK/GCV strategy.

RESULTS

We have found that these vectors were able to mediate cell-to-cell fusion of human tumor cell lines. A few hours after addition of the vector, cells started to aggregate to form syncytia that eventually evolved toward cell death 48 h postinfection. RD114-pseudotyped vectors were very efficient at killing human cancer cells, and they were also able to enhance dramatically the cytotoxic effect of the TK/GCV strategy.

CONCLUSIONS

These findings indicate that RD114-pseudotyped vectors used alone, or in combination with a suicide gene therapy approach, have great potential for the treatment of cancer.

Lentiviral vectors pseudotyped with minimal filovirus envelopes increased gene transfer in murine lung

A human immunodeficiency virus (HIV)-based vector pseudotyped with the Ebola Zaire (EboZ) viral envelope glycoprotein (GP) was recently shown to transduce murine airway epithelia cells in vivo. In this study, the vector was further redesigned to improve gene transfer and also to increase safety. We used mutant EboZ envelopes for pseudotyping, which resulted in higher titers and increased transduction of airway cells in vivo compared to vectors pseudotyped with wild-type EboZ GP. As these envelopes lack regions associated with toxicity of the wild-type EboZ GP, they should also be safer to use for pseudotyping of lentiviral vectors. In addition, lentiviral vectors were created based on feline immunodeficiency virus and shown to have similar efficiency of transduction compared to HIV-based vectors. The creation of lentiviral vectors with highly engineered EboZ envelopes improved the performance of the system and should also increase its safety since only minimal regions of the EboZ envelope, which lack the toxic domain, are used.

Direct comparison of RD114-pseudotyped versus amphotropic-pseudotyped retroviral vectors for transduction of rhesus macaque long-term repopulating cells

Recently, RD114 (feline endogenous retrovirus envelope protein)-pseudotyped retroviral particles have been shown to transduce human NOD/SCID repopulating cells efficiently. In this study, we compared directly transduction of repopulating cells with RD114-pseudotyped vector to that with standard amphotropic vector in the rhesus macaque model. G-CSF/SCF-mobilized CD34(+) rhesus peripheral blood cells were cultured in the presence of SCF, Flt-3 ligand, and MGDF on Retronectin-coated flasks. To assess directly the ability of the two pseudotypes to transduce primitive cells, both vectors were added simultaneously to the target cells every 24 h, for a total of four exposures in 96 h. The cells were reinfused after the animals received 1000 cGy total body irradiation. At the end of transduction, gene marking efficiency of CFU was higher with amphotropic LNL6 vector (mean 88.4%) vs RD114-G1Na vector (mean 18.5%). After long-term engraftment in three animals, total neo gene marking levels were 4-5% in PBMNCs and 1.5-4% in granulocytes. The RD114-G1Na marking levels were consistently higher in granulocytes than in mononuclear cells, while amphotropic LNL6 marking levels were higher in PBMNCs than in granulocytes. The differential gene marking patterns suggest that RD114 and amphotropic vectors may target distinct progenitor or stem cell populations. There was no clear advantage for RD114-pseudotyped vectors in this predictive preclinical model in terms of overall long-term marking levels; however, optimization of transduction conditions by increasing m.o.i. or inducing the receptor could potentially improve results with this novel vector system.

Prolonged multilineage clonal hematopoiesis in a rhesus recipient of CD34 positive cells marked with a RD114 pseudotyped oncoretroviral vector

The ability to efficiently transfer a gene into repopulating hematopoietic stem cells would create many therapeutic opportunities. We have evaluated the ability of particles bearing an alternative envelope protein, that of the feline endogenous virus (RD114), to transduce stem cells in a nonhuman primate autologous transplantation model using rhesus macaques. We have previously shown this pseudotyped vector to be superior to the amphotropic vector at transducing cells in umbilical cord blood capable of establishing hematopoiesis in immunodeficient mice. Gene transfer efficiency as reflected by the number of genetically modified cells in hematopoietic tissues varied among the five monkeys studied from low levels (<1%) in three animals to much higher levels in two (20-60%). An animal that exhibited extremely high levels for several weeks was found by vector genome insertion site analysis to have reconstitution predominantly with a single clone of cells. This variability among animals is in keeping with computer simulations of reconstitution with limiting numbers of stem cells genetically modified at about 10% efficiency. Our studies provide insights into the biology of hematopoietic reconstitution and suggest approaches for increasing stem cell targeted gene transfer efficiency.

Genetic marking as an approach to studying in vivo hematopoiesis: progress in the non-human primate model

Retroviral insertion site analysis following transplantation of marked hematopoietic stem cells (HSCs) is a powerful method for studying hematopoiesis in vivo. High-level gene transfer efficiency was achieved in murine models in the late 1980s, but early human gene transfer protocols into hematopoietic stem and progenitor cells using the murine methodology showed consistently poor results. The utility of non-human primates as pre-clinical models has since become apparent. Modifications in retroviral transduction conditions have resulted in stable long-term gene transfer efficiency as high as 15-20% to primitive repopulating cells in non-human primate models. This has permitted, for the first time in a large animal model, tracking of individual stem and progenitor cell clones via insertion site analysis, an advantage over competitive transplantation studies, which cannot firmly evaluate the number or life span of individual clones contributing to hematopoiesis. Retroviral tracking studies in mice suggest that stable hematopoiesis may be dominated by a small number of clones, but these studies have been limited by insensitive detection methods, low numbers of transplanted stem cells, and limited life span of immunodeficient mice. Autologous transplantation studies in non-human primates have just begun and have the potential to shed light on controversial issues such as the number of clones contributing to stable hematopoiesis, clonal succession, and lineage commitment, as well as the effect of clinically relevant manipulations such as cytokines, chemotherapy, and radiation on hematopoiesis. These approaches will have significant impact in studying various aspects of stem cell biology including the phenomenon of stem cell plasticity.