Early induction of cytokine release syndrome by rapidly generated CAR T cells in preclinical models

Cytokine release syndrome (CRS) is a significant side-effect of conventional chimeric antigen receptor (CAR) T-cell therapy. To facilitate patient accessibility, short-term (st) CAR T cells, which are administered to patients only 24 h after vector exposure, are in focus of current investigations. Their impact on the incidence and severity of CRS has been poorly explored. Here, we evaluated CD19-specific stCAR T cells in preclinical models. In co-culture with tumor cells and monocytes, stCAR T cells exhibited anti-tumoral activity and potent release of CRS-related cytokines (IL-6, IFN-γ, TNF-α, GM-CSF, IL-2, IL-10). When administered to NSG-SGM3 mice, stCAR T cells, but not conventional CAR T cells, induced severe acute adverse events within 24 h, including hypothermia and weight loss, as well as high body scores, independent of the presence of tumor target cells. Human (IFN-γ, TNF-α, IL-2, IL-10) and murine (MCP-1, IL-6, G-CSF) cytokines, typical for severe CRS, were systemically elevated. Our data highlight potential safety risks of rapidly manufactured CAR T cells and suggest NSG-SGM3 mice as sensitive model for their preclinical safety evaluation.

Heterogeneity of layer 4 in visual areas of rhesus macaque cortex

Recently, single-cell RNA-sequencing (scRNA-seq) has enabled unprecedented insights to the cellular landscape of the brains of many different species, among them the rhesus macaque as a key animal model. Building on previous, broader surveys of the macaque brain, we closely examined five immediately neighboring areas within the visual cortex of the rhesus macaque: V1, V2, V4, MT and TEO. To facilitate this, we first devised a novel pipeline for brain spatial archive - the BrainSPACE - which enabled robust archiving and sampling from the whole unfixed brain. SnRNA-sequencing of ~100,000 nuclei from visual areas V1 and V4 revealed conservation within the GABAergic neuron subtypes, while seven and one distinct principle neuron subtypes were detected in V1 and V4, respectively, all most likely located in layer 4. Moreover, using small molecule fluorescence in situ hybridization, we identified cell type density gradients across V1, V2, V4, MT, and TEO appearing to reflect the visual hierarchy. These findings demonstrate an association between the clear areal specializations among neighboring areas with the hierarchical levels within the visual cortex of the rhesus macaque.

A new age of precision gene therapy

Gene therapy has become a clinical reality as market-approved advanced therapy medicinal products for the treatment of distinct monogenetic diseases and B-cell malignancies. This Therapeutic Review aims to explain how progress in genome editing technologies offers the possibility to expand both therapeutic options and the types of diseases that will become treatable. To frame these impressive advances in the context of modern medicine, we incorporate examples from human clinical trials into our discussion on how genome editing will complement currently available strategies in gene therapy, which still mainly rely on gene addition strategies. Furthermore, safety considerations and ethical implications, including the issue of accessibility, are addressed as these crucial parameters will define the impact that gene therapy in general and genome editing in particular will have on how we treat patients in the near future.

AAV vectors displaying bispecific DARPins enable dual-control targeted gene delivery

Precise delivery of genes to therapy-relevant cells is crucial for in vivo gene therapy. Receptor-targeting as prime strategy for this purpose is limited to cell types defined by a single cell-surface marker. Many target cells are characterized by combinations of more than one marker, such as the HIV reservoir cells. Here, we explored the tropism of adeno-associated viral vectors (AAV2) displaying designed ankyrin repeat proteins (DARPins) mono- and bispecific for CD4 and CD32a. Cryo-electron tomography revealed an unaltered capsid structure in the presence of DARPins. Surprisingly, bispecific AAVs transduced CD4/CD32a double-positive cells at much higher efficiencies than single-positive cells, even if present in low amounts in cell mixtures or human blood. This preference was confirmed when vector particles were systemically administered into mice. Cell trafficking studies revealed an increased cell entry rate for bispecific over monospecific AAVs. When equipped with an HIV genome-targeting CRISPR/Cas cassette, the vectors prevented HIV replication in T cell cultures. The data provide proof-of-concept for high-precision gene delivery through tandem-binding regions on AAV. Reminiscent of biological products following Boolean logic AND gating, the data suggest a new option for receptor-targeted vectors to improve the specificity and safety of in vivo gene therapy.

CAR Gene Delivery by T-cell Targeted Lentiviral Vectors is Enhanced by Rapamycin Induced Reduction of Antiviral Mechanisms

Lentiviral vectors (LV) have become the dominant tool for stable gene transfer into lymphocytes including chimeric antigen receptor (CAR) gene delivery to T cells, a major breakthrough in cancer therapy. Yet, room for improvement remains, especially for the latest LV generations delivering genes selectively into T cell subtypes, a key requirement for in vivo CAR T cell generation. Toward improving gene transfer rates with these vectors, whole transcriptome analyses on human T lymphocytes are conducted after exposure to CAR-encoding conventional vectors (VSV-LV) and vectors targeted to CD8+ (CD8-LV) or CD4+ T cells (CD4-LV). Genes related to quiescence and antiviral restriction are found to be upregulated in CAR-negative cells exposed to all types of LVs. Down-modulation of various antiviral restriction factors, including the interferon-induced transmembrane proteins (IFITMs) is achieved with rapamycin as verified by mass spectrometry (LC-MS). Strikingly, rapamycin enhances transduction by up to 7-fold for CD8-LV and CD4-LV without compromising CAR T cell activities but does not improve VSV-LV. When administered to humanized mice, CD8-LV results in higher rates of green fluorescent protein (GFP) gene delivery. Also in vivo CAR T cell generation is improved in kinetics and tumor control, however to a moderate extent, leaving room for improvement by optimizing the rapamycin administration schedule. The data favor multi-omics approaches for improvements in gene delivery.

Substantially improved gene transfer to interneurons with second-generation glutamate receptor-targeted DART-AAV vectors

BACKGROUND

Adeno-associated viral vectors (AAVs) are a widely used gene transfer platform in neuroscience. Although naturally AAV serotypes can have preferences for certain tissues, selectivity for particular cell types in the CNS does not exist. Towards interneuron targeting, capsid engineering of AAV2 including display of the designed ankyrin repeat protein (DARPin) 2K19 specific for the glutamate receptor subunit 4 (GluA4) at the N-terminus of the VP2 capsid protein has been established. The resulting AAV-VP2N is highly specific for interneurons, but exhibits rather moderate transduction efficiencies.

METHODS

Two alternative insertion sites for 2K19 in the GH2/GH3 loop of capsid proteins VP1 (AAV-VP1L) or VP2 (AAV-VP2L) were exploited to yield second generation GluA4-AAVs. Having packaged reporter genes under ubiquitous promoters, the vectors were characterized for biochemical properties as well as gene delivery into cell lines and rat hippocampal slice cultures. Electrophysiological recordings monitored the functional properties of transduced cells.

RESULTS

Compared to AAV-VP2N, the second-generation vectors, especially AAV-VP1L, achieved about 2-fold higher genomic titers as well as a substantially improved GluA4 binding. Improvements in gene transfer activities were 18-fold on GluA4-overexpressing A549 cells and five-fold on rat hippocampal organotypic slice cultures reaching approximately 60 % of all parvalbumin positive interneurons upon a single administration. The spiking behaviour of transduced cells was unaltered and characteristic for a heterogeneous group of interneurons.

CONCLUSION

The substantially improved gene transfer activity of the second generation GluA4-targeted AAV combined with low toxicity makes this vector an attractive tool for interneuron-directed gene transfer with unrestricted promotor and transgene choice.

CD62L as target receptor for specific gene delivery into less differentiated human T lymphocytes

Chimeric antigen receptor (CAR)-expressing T cells are a complex and heterogeneous gene therapy product with variable phenotype compositions. A higher proportion of less differentiated CAR T cells is usually associated with improved antitumoral function and persistence. We describe in this study a novel receptor-targeted lentiviral vector (LV) named 62L-LV that preferentially transduces less differentiated T cells marked by the L-selectin receptor CD62L, with transduction rates of up to 70% of CD4+ and 50% of CD8+ primary T cells. Remarkably, higher amounts of less differentiated T cells are transduced and preserved upon long-term cultivation using 62L-LV compared to VSV-LV. Interestingly, shed CD62L neither altered the binding of 62L-LV particles to T cells nor impacted their transduction. The incubation of 2 days of activated T lymphocytes with 62L-LV or VSV-LV for only 24 hours was sufficient to generate CAR T cells that controlled tumor growth in a leukemia tumor mouse model. The data proved that potent CAR T cells can be generated by short-term ex vivo exposure of primary cells to LVs. As a first vector type that preferentially transduces less differentiated T lymphocytes, 62L-LV has the potential to circumvent cumbersome selections of T cell subtypes and offers substantial shortening of the CAR T cell manufacturing process.

FcγRIIA-specific DARPins as novel tools in blood cell analysis and platelet aggregation

Fc receptors are involved in a variety of physiologically and disease relevant responses. Among them, FcγRIIA (CD32a) is known for its activating functions in pathogen recognition and platelet biology, and, as potential marker of T lymphocytes latently infected with HIV-1. The latter has not been without controversy due to technical challenges complicated by T-B cell conjugates and trogocytosis as well as a lack of antibodies distinguishing between the closely related isoforms of FcγRII. To generate high-affinity binders specific for FcγRIIA, libraries of designed ankyrin repeat proteins (DARPins) were screened for binding to its extracellular domains by ribosomal display. Counter-selection against FcγRIIB eliminated binders cross-reacting with both isoforms. The identified DARPins bound FcγRIIA with no detectable binding for FcγRIIB. Their affinities for FcγRIIA were in the low nanomolar range and could be enhanced by cleavage of the His-tag and dimerization. Interestingly, complex formation between DARPin and FcγRIIA followed a two-state reaction model and discrimination from FcγRIIB was based on a single amino acid residue. In flow cytometry, DARPin F11 detected FcγRIIA⁺ cells even when they made up less than 1% of the cell population. Image stream analysis of primary human blood cells confirmed that F11 caused dim but reliable cell surface staining of a small subpopulation of T lymphocytes. When incubated with platelets, F11 inhibited their aggregation equally efficient as antibodies unable to discriminate between both FcγRII isoforms. The selected DARPins are unique novel tools for platelet aggregation studies as well as the role of FcγRIIA for the latent HIV-1 reservoir.

APPsα rescues CDK5 and GSK3β dysregulation and restores normal spine density in Tau transgenic mice

The Tau protein can be phosphorylated by numerous kinases. In Alzheimer's disease (AD) hyperphosphorylated Tau species accumulate as neurofibrillary tangles that constitute a major hallmark of AD. AD is further characterized by extracellular Aβ plaques, derived from the β-amyloid precursor protein APP. Whereas Aβ is produced by amyloidogenic APP processing, APP processing along the competing non-amyloidogenic pathway results in the secretion of neurotrophic and synaptotrophic APPsα. Recently, we demonstrated that APPsα has therapeutic effects in transgenic AD model mice and rescues Aβ-dependent impairments. Here, we examined the potential of APPsα to regulate two major Tau kinases, GSK3β and CDK5 in THY-Tau22 mice, a widely used mouse model of tauopathy. Immunohistochemistry revealed a dramatic increase in pathologically phosphorylated (AT8 and AT180) or misfolded Tau species (MC1) in the hippocampus of THY-Tau22 mice between 3 and 12 months of age. Using a highly sensitive radioactive kinase assay with recombinant human Tau as a substrate and immunoblotting, we demonstrate an increase in GSK3β and CDK5 activity in the hippocampus of THY-Tau22 mice. Interestingly, AAV-mediated intracranial expression of APPsα in THY-Tau22 mice efficiently restored normal GSK3β and CDK5 activity. Western blot analysis revealed upregulation of the CDK5 regulatory proteins p35 and p25, indicating CDK5 hyperactivation in THY-Tau22 mice. Strikingly, AAV-APPsα rescued p25 upregulation to wild-type levels even at stages of advanced Tau pathology. Sarkosyl fractionation used to study the abundance of soluble and insoluble phospho-Tau species revealed increased soluble AT8-Tau and decreased insoluble AT100-Tau species upon AAV-APPsα injection. Moreover, AAV-APPsα reduced misfolded (MC1) Tau species, particularly in somatodendritic compartments of CA1 pyramidal neurons. Finally, we show that AAV-APPsα upregulated PSD95 expression and rescued deficits in spine density of THY-Tau22 mice. Together our findings suggest that APPsα holds therapeutic potential to mitigate Tau-induced pathology.

Dasatinib is a potent enhancer for CAR T cell generation by CD3-targeted lentiviral vectors

CD3-targeted lentiviral vectors (CD3-LVs) mediate selective transduction of human T lymphocytes in vitro and in vivo while simultaneously activating the targeted cells. Previously, we have demonstrated that CD3-LV leads to downmodulation of the CD3:T cell receptor (TCR) complex. We therefore hypothesized that inhibition of CD3 phosphorylation by Src/Abl tyrosine kinase inhibitors such as dasatinib results in enhancement of gene delivery by T cell-targeted LVs. Indeed, dasatinib treatment of T cells prior to incubation with CD3-LV increased reporter gene delivery by 3- to 10-fold. Moreover, the presence of dasatinib enhanced selective transduction into non-activated target cells present in whole blood. When combined with delivery of the CD19-chimeric antigen receptor (CAR) gene, dasatinib increased CAR T cell numbers by close to 10-fold. Importantly, the short-term exposure of T cells to dasatinib during vector incubation did not interfere with tumor cell killing by the resulting CAR T cells and rather came along with less upregulated exhaustion markers and a more naive phenotype. Our data suggest that dasatinib prevents CD3-LV-induced phosphorylation and CD3:TCR intake, thereby increasing the amount of CD3-LV bound to the cell surface. This is the first description of dasatinib as transduction enhancer, an activity particularly relevant for CAR T cell generation with CD3-LV.

Expansion of T memory stem cells with superior anti-tumor immunity by Urolithin A-induced mitophagy

T memory stem cells (T_SCM) display increased self-renewal and prolonged survival capabilities, thus preventing T cell exhaustion and promoting effective anti-tumor T cell responses. T_SCM cells can be expanded by Urolithin A (UA), which is produced by the commensal gut microbiome from foods rich in ellagitannins and is known to improve mitochondrial health. Oral UA administration to tumor-bearing mice conferred strong anti-tumor CD8⁺ T cell immunity, whereas ex vivo UA pre-treated T cells displayed improved anti-tumor function upon adoptive cell transfer. UA-induced T_SCM formation depended on Pink1-mediated mitophagy triggering cytosolic release of the mitochondrial phosphatase Pgam5. Cytosolic Pgam5 dephosphorylated β-catenin, which drove Wnt signaling and compensatory mitochondrial biogenesis. Collectively, we unravel a critical signaling pathway linking mitophagy to T_SCM formation and suggest that the well-tolerated metabolic compound UA represents an attractive option to improve immune therapy.

In vivo generation of CAR T cells in the presence of human myeloid cells

Pre-clinical humanized mouse models are a powerful tool to evaluate immunotherapies. NSG-SGM3 mice reconstituted with human stem cells (huSGM3) develop pronounced human myeloid cells due to transgenic expression of stem cell factor, granulocyte-macrophage colony-stimulating factor, and interleukin-3 (IL-3) compared with the widely used humanized NSG (huNSG) model. We assessed in vivo generation of CD19-CAR T cells in huSGM3 mice upon single intravenous injection of the T cell-specific lentiviral vectors (LVs) CD4-LV and CD8-LV. While in vivo CAR T cell generation was clearly detectable in individual mice, generation appeared less efficient than previously observed for huNSG mice. Especially for the CD4-LV group, this correlated with increased IL-15 and decreased GM-CSF levels, indicating activation of monocytes and macrophages. Co-culture assays identified macrophages as a potential barrier for gene transfer. Refining CD4-LV and CD8-LV with a less immunogenic surface by using modified packaging cells substantially improved the transduction of lymphocytes in vitro in the presence of macrophages, as well in vivo in huSGM3 mice. Notably, two mice that developed less CAR T cells showed high interferon-α or -β levels before vector injection. Our data emphasize the relevance of innate immune responses for in vivo generation of CAR T cells, which can be overcome by vector surface engineering.

APPsα Rescues Tau-Induced Synaptic Pathology

Alzheimer's disease (AD) is histopathologically characterized by Aβ plaques and the accumulation of hyperphosphorylated Tau species, the latter also constituting key hallmarks of primary tauopathies. Whereas Aβ is produced by amyloidogenic APP processing, APP processing along the competing nonamyloidogenic pathway results in the secretion of neurotrophic and synaptotrophic APPsα. Recently, we demonstrated that APPsα has therapeutic effects in transgenic AD model mice and rescues Aβ-dependent impairments. Here, we examined the potential of APPsα to mitigate Tau-induced synaptic deficits in P301S mice (both sexes), a widely used mouse model of tauopathy. Analysis of synaptic plasticity revealed an aberrantly increased LTP in P301S mice that could be normalized by acute application of nanomolar amounts of APPsα to hippocampal slices, indicating a homeostatic function of APPsα on a rapid time scale. Further, AAV-mediated in vivo expression of APPsα restored normal spine density of CA1 neurons even at stages of advanced Tau pathology not only in P301S mice, but also in independent THY-Tau22 mice. Strikingly, when searching for the mechanism underlying aberrantly increased LTP in P301S mice, we identified an early and progressive loss of major GABAergic interneuron subtypes in the hippocampus of P301S mice, which may lead to reduced GABAergic inhibition of principal cells. Interneuron loss was paralleled by deficits in nest building, an innate behavior highly sensitive to hippocampal impairments. Together, our findings indicate that APPsα has therapeutic potential for Tau-mediated synaptic dysfunction and suggest that loss of interneurons leads to disturbed neuronal circuits that compromise synaptic plasticity as well as behavior.SIGNIFICANCE STATEMENT Our findings indicate, for the first time, that APPsα has the potential to rescue Tau-induced spine loss and abnormal synaptic plasticity. Thus, APPsα might have therapeutic potential not only because of its synaptotrophic functions, but also its homeostatic capacity for neuronal network activity. Hence, APPsα is one of the few molecules which has proven therapeutic effects in mice, both for Aβ- and Tau-dependent synaptic impairments and might therefore have therapeutic potential for patients suffering from AD or primary tauopathies. Furthermore, we found in P301S mice a pronounced reduction of inhibitory interneurons as the earliest pathologic event preceding the accumulation of hyperphosphorylated Tau species. This loss of interneurons most likely disturbs neuronal circuits that are important for synaptic plasticity and behavior.

Precision Medicine: In Vivo CAR Therapy as a Showcase for Receptor-Targeted Vector Platforms

Chimeric antigen receptor (CAR) T cells are a cancer immunotherapy of extremes: Unprecedentedly effective, but complex and costly to manufacture, they are not yet a therapeutic option for all who would benefit. This disparity has motivated worldwide efforts to simplify treatment. Among the proposed solutions, the generation of CAR T cells directly in the patient, i.e. in vivo, is arguably simultaneously the most technically challenging and clinically useful approach to convert CAR therapy from a cell-based autologous medicinal product into a universally applicable off-the-shelf treatment. Here we review the current state-of-the-art of in vivo CAR therapy, focusing especially on the vector technologies used. These cover lentiviral vectors, adenovirus-associated vectors as well as synthetic polymer nanocarriers and lipid nanoparticles. Proof-of-concept, i.e. the generation of CAR cells directly in mouse models, has been demonstrated for all vector platforms. Receptor-targeting of vector particles is crucial, as it can prevent CAR gene delivery into off-target cells, thus reducing toxicities. We discuss the properties of the vector platforms, such as their immunogenicity, potency, and modes of CAR delivery (permanent versus transient). Finally, we outline the work required to advance in vivo CAR therapy from proof-of-concept to a robust, scalable technology for clinical testing.

Time 2EVOLVE: predicting efficacy of engineered T-cells - how far is the bench from the bedside?

Immunotherapy with gene engineered CAR and TCR transgenic T-cells is a transformative treatment in cancer medicine. There is a rich pipeline with target antigens and sophisticated technologies that will enable establishing this novel treatment not only in rare hematological malignancies, but also in common solid tumors. The T2EVOLVE consortium is a public private partnership directed at accelerating the preclinical development of and increasing access to engineered T-cell immunotherapies for cancer patients. A key ambition in T2EVOLVE is to assess the currently available preclinical models for evaluating safety and efficacy of engineered T cell therapy and developing new models and test parameters with higher predictive value for clinical safety and efficacy in order to improve and accelerate the selection of lead T-cell products for clinical translation. Here, we review existing and emerging preclinical models that permit assessing CAR and TCR signaling and antigen binding, the access and function of engineered T-cells to primary and metastatic tumor ligands, as well as the impact of endogenous factors such as the host immune system and microbiome. Collectively, this review article presents a perspective on an accelerated translational development path that is based on innovative standardized preclinical test systems for CAR and TCR transgenic T-cell products.

Monitoring CAR T cell generation with a CD8-targeted lentiviral vector by single-cell transcriptomics

Quantifying gene expression in individual cells can substantially improve our understanding about complex genetically engineered cell products such as chimeric antigen receptor (CAR) T cells. Here we designed a single-cell RNA sequencing (scRNA-seq) approach to monitor the delivery of a CD19-CAR gene via lentiviral vectors (LVs), i.e., the conventional vesicular stomatitis virus (VSV)-LV and the CD8-targeted CD8-LV. LV-exposed human donor peripheral blood mononuclear cells (PBMCs) were evaluated for a panel of 400 immune response-related genes including LV-specific probes. The resulting data revealed a trimodal expression for the CAR and CD8A, demanding a careful distribution-based identification of CAR T cells and CD8+ lymphocytes in scRNA-seq analysis. The fraction of T cells expressing high CAR levels was in concordance with flow cytometry results. More than 97% of the cells hit by CD8-LV expressed the CD8A gene. Remarkably, the majority of the potential off-target cells were in fact on-target cells, resulting in a target cell selectivity of more than 99%. Beyond that, differential gene expression analysis revealed the upregulation of restriction factors in CAR-negative cells, thus explaining their protection from CAR gene transfer. In summary, we provide a workflow and subsetting approach for scRNA-seq enabling reliable distinction between transduced and untransduced cells during CAR T cell generation.

Lentiviral and adeno-associated vectors efficiently transduce mouse T lymphocytes when targeted to murine CD8

Preclinical studies on gene delivery into mouse lymphocytes are often hampered by insufficient activity of lentiviral (LV) and adeno-associated vectors (AAVs) as well as missing tools for cell type selectivity when considering in vivo gene therapy. Here, we selected designed ankyrin repeat proteins (DARPins) binding to murine CD8. The top-performing DARPin was displayed as targeting ligand on both vector systems. When used on engineered measles virus (MV) glycoproteins, the resulting mCD8-LV transduced CD8+ mouse lymphocytes with near-absolute (>99%) selectivity. Despite its lower functional titer, mCD8-LV achieved 4-fold higher gene delivery to CD8+ cells than conventional VSV-LV when added to whole mouse blood. Addition of mCD8-LV encoding a chimeric antigen receptor (CAR) specific for mouse CD19 to splenocytes resulted in elimination of B lymphocytes and lymphoma cells. For display on AAV, the DARPin was inserted into the GH2-GH3 loop of the AAV2 capsid protein VP1, resulting in a DARPin-targeted AAV we termed DART-AAV. Stocks of mCD8-AAV contained similar genome copies as AAV2 but were >20-fold more active in gene delivery in mouse splenocytes, while exhibiting >99% specificity for CD8+ cells. These results suggest that receptor targeting can overcome blocks in transduction of mouse splenocytes.

OS06.2A Local immunotherapy of glioblastoma via AAV-mediated gene transfer of checkpoint inhibitors in combination with CAR-NK cells

BACKGROUND

Glioblastoma (GB) is the most common primary brain tumor which is characterized by low immunogenicity of tumor cells and prevalent immunosuppression in the tumor microenvironment (TME). Since expression of PD-L1 on GB cells has been described, immunotherapy with checkpoint inhibitors (CIs) may be a promising approach for GB treatment. However, systemic administration of CIs bears the risk of autoimmune-like side effects, while the intratumoral drug concentration reached may not be sufficient.

METHODS

We studied delivery of CIs through targeted Adeno-associated viral vectors (AAVs) encoding an anti PD-1 immunoadhesin (aPD-1) as a novel approach towards local immunotherapy in the syngeneic GL261-HER2 glioma model. Tumor cell-specific delivery was achieved by targeting HER2 via a specific designed ankyrin repeat protein (DARPin). We investigated the effects of this strategy alone and in combination with local injection of HER2-specific CAR-NK cells (NK-92/5.28.z), which have already shown efficacy in preclinical GB models and are currently under investigation in the CAR2BRAIN phase I clinical trial. Furthermore, aPD-1 functionality and cellular response to viral transduction as well as compatibility of both therapy approaches has been evaluated in various in vitro models.

RESULTS

HER2-AAV transduction efficacy of GB cells correlated with HER2 expression level, while target cells did not show anti-viral responses upon transduction. After transduction with aPD-1 HER2-AAVs, aPD-1 immunoadhesin was secreted in a time-dependent manner, bound its target on PD-1-expressing cells and was able to re-activate T-cells due to PD-1 blockade. AAV-transduction did not interfere with CAR-NK cell mediated tumor cell lysis. Biodistribution studies in mice revealed the presence of aPD-1 up to 10 days after a single HER2-AAV injection. In subcutaneous GL261-HER2 tumors, local treatment with HER2-AAVaPD-1 or HER2-AAVIgG-Fc+ NK-92/5.28.z therapy had no significant effect, whereas combination therapy profoundly delayed tumor growth.

CONCLUSIONS

Local therapy with aPD-1 encoding HER2-AAVs in combination with NK-92/5.28.z cells is a promising novel strategy for GB immunotherapy with the potential to enhance efficacy and reduce side effects.

Proceedings From the First International Workshop at Sidra Medicine: "Engineered Immune Cells in Cancer Immunotherapy (EICCI): From Discovery to Off-the-Shelf Development", 15th-16th February 2019, Doha, Qatar

The progress in the isolation and characterization of tumor antigen (TA)-specific T lymphocytes and in the genetic modification of immune cells allowed the clinical development of adoptive cell therapy (ACT). Several clinical studies highlighted the striking clinical activity of T cells engineered to express either Chimeric Antigen (CAR) or T Cell (TCR) Receptors to target molecularly defined antigens expressed on tumor cells. The breakthrough of immunotherapy is represented by the approval of CAR-T cells specific for advanced or refractory CD19⁺ B cell malignancies by both the Food and Drug Administration (FDA) and the European Medicinal Agency (EMA). Moreover, advances in the manufacturing and gene editing of engineered immune cells contributed to the selection of drug products with desired phenotype, refined specificity and decreased toxicity. An important step toward the optimization of CAR-T cell therapy is the development of "off-the shelf" T cell products that allow to reduce the complexity and the costs of the manufacturing and to render these drugs available for a broad number of cancer patients. The Engineered Immune Cells in Cancer Immunotherapy (EICCI) workshop hosted in Doha, Qatar, renowned experts, from both academia and industry, to present and discuss the progress on both pre-clinical and clinical development of genetically modified immune cells, including advances in the "off-the-shelf" manufacturing. These experts have addressed also organizational needs and hurdles for the clinical grade production and application of these biological drugs.

Designed Ankyrin Repeat Protein (DARPin) to target chimeric antigen receptor (CAR)-redirected T cells towards CD4+ T cells to reduce the latent HIV+ cell reservoir

Chimeric Antigen Receptor (CAR)-redirected T cells show great efficacy in the patient-specific therapy of hematologic malignancies. Here, we demonstrate that a DARPin with specificity for CD4 specifically redirects and triggers the activation of CAR engineered T cells resulting in the depletion of CD4+ target cells aiming for elimination of the human immunodeficiency virus (HIV) reservoir.

CD8-Specific Designed Ankyrin Repeat Proteins Improve Selective Gene Delivery into Human and Primate T Lymphocytes

Adoptive T cell immunotherapy in combination with gene therapy is a promising treatment concept for chronic infections and cancer. Recently, receptor-targeted lentiviral vectors (LVs) were shown to enable selective gene transfer into particular types of lymphocytes both in vitro and in vivo. This approach might facilitate the genetic engineering of a patient's own T lymphocytes, possibly even shifting this concept from personalized medicine to an off-the shelf therapy in future. Here, we describe novel high-affinity binders for CD8 consisting of designed ankyrin repeat proteins (DARPins), which were selected to bind to the CD8 receptor of human and nonhuman primate (NHP) cells. These binders were identified by ribosome display screening of DARPin libraries using recombinant human CD8 followed by receptor binding analysis on primary lymphocytes. CD8-targeted LVs (CD8-LVs) were then generated that delivered genes exclusively and specifically to human and NHP T lymphocytes by using the same targeting domain. These CD8-LVs were as specific for human T lymphocytes as their single-chain variable fragment-based counterpart, but they could be produced to higher titers. Moreover, they were superior in transducing cytotoxic T cells both in vitro and in vivo when equal particle numbers were applied. Since the here described CD8-LVs transduced primary T lymphocytes from NHP and human donors equally well, they offer the opportunity for preclinical studies in different animal models including large animals such as NHPs without the need for modifications in vector design.

In vivo generated human CAR T cells eradicate tumor cells

Chimeric antigen receptor (CAR) T cells are in prime focus of current research in cancer immunotherapy. Facilitating CAR T cell generation is among the top goals. We have recently demonstrated direct in vivo generation of human CD19-CAR T cells by targeting CD8⁺ cells using lentiviral vectors (LVs). The anti-tumor potency of in vivo generated CAR T cells was assessed in human PBMC-transplanted NSG mice carrying i.v. injected CD19⁺ Nalm-6 tumor cells. A single injection of CD8-targeted LV delivering CD19-CAR was sufficient to completely eliminate the tumor cells from bone marrow and spleen, whereas control animals contained high levels of CD19⁺ cells. Tumor elimination was due to in vivo generated CAR⁺ cells. Notably, these were not only composed of T lymphocytes but also included CAR⁺ natural killer cells (NK and NKT). This is the first demonstration of tumor elimination by in vivo generated human CAR T cells.

Highly Efficient and Selective CAR-Gene Transfer Using CD4- and CD8-Targeted Lentiviral Vectors

Chimeric antigen receptor (CAR)-modified T cells have revealed promising results in the treatment of cancer, but they still need to overcome various hurdles, including a complicated manufacturing process. Receptor-targeted lentiviral vectors (LVs) delivering genes selectively to T cell subtypes may facilitate and improve CAR T cell generation, but so far they have resulted in lower gene delivery rates than conventional LVs (vesicular stomatitis virus [VSV]-LV). To overcome this limitation, we studied the effect of the transduction enhancer Vectofusin-1 on gene delivery to human T cells with CD4- and CD8-targeted LVs, respectively, encoding a second-generation CD19-CAR in conjunction with a truncated version of the low-affinity nerve growth factor receptor (ΔLNGFR) as reporter. Vectofusin-1 significantly enhanced the gene delivery of CD4- and CD8-LVs without a loss in target cell selectivity and killing capability of the generated CAR T cells. Notably, delivery rates mediated by VSV-LV were substantially reduced by Vectofusin-1. Interestingly, a transient off-target signal in samples treated with Vectofusin-1 was observed early after transduction. However, this effect was not caused by uptake and expression of the transgene in off-target cells, but rather it resulted from cell-bound LV particles having ΔLNGFR incorporated into their surface. The data demonstrate that gene transfer rates in the range of those mediated by VSV-LVs can be achieved with receptor-targeted LVs.

Tumor-Specific Delivery of Immune Checkpoint Inhibitors by Engineered AAV Vectors

Immune checkpoint inhibitors (ICIs) can block distinct receptors on T cells or tumor cells thus preventing T cell inactivation and tumor immune escape. While the clinical response to treatment with ICIs in cancer patients is impressive, this therapy is often associated with a number of immune-related adverse events. There is therefore a need to explore innovative strategies of tumor-specific delivery of ICIs. Delivery of therapeutic proteins on a genetic level can be accomplished with viral vectors including those derived from adeno-associated virus (AAV). Here, we assessed the tumor-targeted Her2-AAV, a receptor-targeted AAV vector binding to the tumor antigen Her2/neu for cell entry, as vehicle for ICI gene delivery. Initially, we packaged the coding sequence of a scFv-Fc fusion protein directed against mouse programmed cell death protein-1 (PD-1) into Her2-AAV. Upon transduction of Her2/neu⁺ RENCA cells, AAV-encoded αPD-1 was readily detectable in the cell culture supernatant and revealed specific binding to its target antigen. In vivo, in BALB/c mice bearing subcutaneous RENCA-Her2/neu tumors, Her2-AAV mediated specific gene delivery into tumor tissue upon intravenous administration as verified by luciferase gene transfer and in vivo imaging thus demonstrating unimpaired tumor-targeting by Her2-AAV vectors in immunocompetent animals. When delivering the αPD-1 gene, levels of ICI were similar in tumor tissue for Her2-AAV and AAV2 but substantially reduced in liver for Her2-AAV. When combined with chemotherapy a tendency for reduced progression of tumor growth was documented for Her2-AAV treated mice. To get closer to the clinical situation, AAV constructs that deliver the complete coding sequence of the therapeutic antibody nivolumab which is directed against human PD-1 were generated next. The AAV-Nivolumab constructs were expressed and released from transduced MDA-MB-453 cells in vitro and from RENCA-Her2/neu cells upon intratumoral as well as intravenous administration in vivo. Antibody processing and expression levels were further improved through optimization of construct design. In conclusion, we provide proof-of-principle for redirecting the biodistribution of ICIs from liver and serum to tumor tissue by the use of engineered AAV vectors. This strategy can be easily combined with other types of immunotherapeutic concepts.

Ligand Coupling to the AAV Capsid for Cell-Specific Gene Transfer

Cell entry of AAV vectors is initiated by contacting the cell surface attachment receptor. This process can be rationally engineered through mutating the contact residues on the AAV capsid and covalently coupling targeting ligands to the capsid surface that exhibit high affinity for a cell surface protein of choice. This way, selective gene delivery to target-receptor positive cell types has been achieved. Two methods for coupling targeting ligands to the AAV capsid can be distinguished. Genetic coupling is achieved through expressing fusion proteins composed of the capsid protein VP2 and the targeting ligand in packaging cells. Biochemical coupling involves split-intein-mediated protein trans-splicing between the mutated AAV capsid and the targeting ligand. While genetic coupling is restricted to designed ankyrin repeat proteins as targeting ligand, biochemical coupling tolerates single-chain antibody fragments as well.

In vivo generation of human CD19-CAR T cells results in B-cell depletion and signs of cytokine release syndrome

Chimeric antigen receptor (CAR) T cells brought substantial benefit to patients with B-cell malignancies. Notwithstanding, CAR T-cell manufacturing requires complex procedures impeding the broad supply chain. Here, we provide evidence that human CD19-CAR T cells can be generated directly in vivo using the lentiviral vector CD8-LV specifically targeting human CD8+ cells. Administration into mice xenografted with Raji lymphoma cells and human peripheral blood mononuclear cells led to CAR expression solely in CD8+ T cells and efficacious elimination of CD19+ B cells. Further, upon injection of CD8-LV into mice transplanted with human CD34+ cells, induction of CAR T cells and CD19+ B-cell depletion was observed in 7 out of 10 treated animals. Notably, three mice showed elevated levels of human cytokines in plasma. Tissue-invading CAR T cells and complete elimination of the B-lymphocyte-rich zones in spleen were indicative of a cytokine release syndrome. Our data demonstrate the feasibility of in vivo reprogramming of human CD8+ CAR T cells active against CD19+ cells, yet with similar adverse effects currently notorious in the clinical practice.

Surface-Engineered Lentiviral Vectors for Selective Gene Transfer into Subtypes of Lymphocytes

Lymphocytes have always been among the prime targets in gene therapy, even more so since chimeric antigen receptor (CAR) T cells have reached the clinic. However, other gene therapeutic approaches hold great promise as well. The first part of this review provides an overview of current strategies in lymphocyte gene therapy. The second part highlights the importance of precise gene delivery into B and T cells as well as distinct subtypes of lymphocytes. This can be achieved with lentiviral vectors (LVs) pseudotyped with engineered glycoproteins recognizing lymphocyte surface markers as entry receptors. Different strategies for envelope glycoprotein engineering and selection of the targeting ligand are discussed. With a CD8-targeted LV that was recently used to achieve proof of principle for the in vivo reprogramming of CAR T cells, these vectors are becoming a key tool to genetically engineer lymphocytes directly in vivo.

A Library-Based Screening Strategy for the Identification of DARPins as Ligands for Receptor-Targeted AAV and Lentiviral Vectors

Delivering genes selectively to the therapeutically relevant cell type is among the prime goals of vector development. Here, we present a high-throughput selection and screening process that identifies designed ankyrin repeat proteins (DARPins) optimally suited for receptor-targeted gene delivery using adeno-associated viral (AAV) and lentiviral (LV) vectors. In particular, the process includes expression, purification, and in situ biotinylation of the extracellular domains of target receptors as Fc fusion proteins in mammalian cells and the selection of high-affinity binders by ribosome display from DARPin libraries each covering more than 1012 variants. This way, DARPins specific for the glutamate receptor subunit GluA4, the endothelial surface marker CD105, and the natural killer cell marker NKp46 were generated. The identification of DARPins best suited for gene delivery was achieved by screening small-scale vector productions. Both LV and AAV particles displaying the selected DARPins transduced only cells expressing the corresponding target receptor. The data confirm that a straightforward process for the generation of receptor-targeted viral vectors has been established. Moreover, biochemical analysis of a panel of DARPins revealed that their functional cell-surface expression as fusion proteins is more relevant for efficient gene delivery by LV particles than functional binding affinity.

Virotherapy Research in Germany: From Engineering to Translation

Virotherapy is a unique modality for the treatment of cancer with oncolytic viruses (OVs) that selectively infect and lyse tumor cells, spread within tumors, and activate anti-tumor immunity. Various viruses are being developed as OVs preclinically and clinically, several of them engineered to encode therapeutic proteins for tumor-targeted gene therapy. Scientists and clinicians in German academia have made significant contributions to OV research and development, which are highlighted in this review paper. Innovative strategies for "shielding," entry or postentry targeting, and "arming" of OVs have been established, focusing on adenovirus, measles virus, parvovirus, and vaccinia virus platforms. Thereby, new-generation virotherapeutics have been derived. Moreover, immunotherapeutic properties of OVs and combination therapies with pharmacotherapy, radiotherapy, and especially immunotherapy have been investigated and optimized. German investigators are increasingly assessing their OV innovations in investigator-initiated and sponsored clinical trials. As a prototype, parvovirus has been tested as an OV from preclinical proof-of-concept up to first-in-human clinical studies. The approval of the first OV in the Western world, T-VEC (Imlygic), has further spurred the involvement of investigators in Germany in international multicenter studies. With the encouraging developments in funding, commercialization, and regulatory procedures, more German engineering will be translated into OV clinical trials in the near future.

Clinical development of CAR T cells-challenges and opportunities in translating innovative treatment concepts

Chimeric antigen receptor (CAR) T cell therapy, together with checkpoint inhibition, has been celebrated as a breakthrough technology due to the substantial benefit observed in clinical trials with patients suffering from relapsed or refractory B‐cell malignancies. In this review, we provide a comprehensive overview of the clinical trials performed so far worldwide and analyze parameters such as targeted antigen and indication, CAR molecular design, CAR T cell manufacturing, anti‐tumor activities, and related toxicities. More than 200 CAR T cell clinical trials have been initiated so far, most of which aim to treat lymphoma or leukemia patients using CD19‐specific CARs. An increasing number of studies address solid tumors as well. Notably, not all clinical trials conducted so far have shown promising results. Indeed, in a few patients CAR T cell therapy resulted in severe adverse events with fatal outcome. Of note, less than 10% of the ongoing CAR T cell clinical trials are performed in Europe. Taking lead from our analysis, we discuss the problems and general hurdles preventing efficient clinical development of CAR T cells as well as opportunities, with a special focus on the European stage.

Distinct in vivo roles of secreted APP ectodomain variants APPsα and APPsβ in regulation of spine density, synaptic plasticity, and cognition

Increasing evidence suggests that synaptic functions of the amyloid precursor protein (APP), which is key to Alzheimer pathogenesis, may be carried out by its secreted ectodomain (APPs). The specific roles of APPsα and APPsβ fragments, generated by non-amyloidogenic or amyloidogenic APP processing, respectively, remain however unclear. Here, we expressed APPsα or APPsβ in the adult brain of conditional double knockout mice (cDKO) lacking APP and the related APLP2. APPsα efficiently rescued deficits in spine density, synaptic plasticity (LTP and PPF), and spatial reference memory of cDKO mice. In contrast, APPsβ failed to show any detectable effects on synaptic plasticity and spine density. The C-terminal 16 amino acids of APPsα (lacking in APPsβ) proved sufficient to facilitate LTP in a mechanism that depends on functional nicotinic α7-nAChRs. Further, APPsα showed high-affinity, allosteric potentiation of heterologously expressed α7-nAChRs in oocytes. Collectively, we identified α7-nAChRs as a crucial physiological receptor specific for APPsα and show distinct in vivo roles for APPsα versus APPsβ. This implies that reduced levels of APPsα that might occur during Alzheimer pathogenesis cannot be compensated by APPsβ.

CD30-targeted oncolytic viruses as novel therapeutic approach against classical Hodgkin lymphoma

Classical Hodgkin lymphoma (cHL) is a hematopoietic malignancy with a characteristic cellular composition. The tumor mass is made up of infiltrated lymphocytes and other cells of hematologic origin but only very few neoplastic cells that are mainly identified by the diagnostic marker CD30. While most patients with early stage cHL can be cured by standard therapy, treatment options for relapsed or refractory cHL are still not sufficient, although immunotherapy-based approaches for the treatment of cHL patients have gained ground in the last decade. Here, we suggest a novel therapeutic concept based on oncolytic viruses selectively destroying the CD30⁺-positive cHL tumor cells. Relying on a recently described CD30-specific scFv we have generated CD30-targeted measles virus (MV-CD30) and vesicular stomatitis virus (VSV-CD30). For VSV-CD30 the VSV glycoprotein G reading frame was replaced by those of the CD30-targeted MV glycoproteins. Both viruses were found to be highly selective for CD30-positive cells as demonstrated by infection of co-cultures of target and non-target cells as well as through blocking infection by soluble CD30. Notably, VSV-CD30 yielded much higher titers than MV-CD30 and resulted in a more rapid and efficient killing of cultivated cHL-derived cell lines. Mouse tumor models revealed that intratumorally, as well as systemically injected VSV-CD30, infected cHL xenografts and significantly slowed down tumor growth resulting in a substantially prolonged survival of tumor-bearing mice. Taken together, the data support further preclinical testing of VSV-CD30 as novel therapeutic agent for the treatment of cHL and other CD30⁺-positive malignancies.

Covalent coupling of high-affinity ligands to the surface of viral vector particles by protein trans-splicing mediates cell type-specific gene transfer

We have established a novel approach for the covalent coupling of large polypeptides to the surface of fully assembled adeno-associated viral gene transfer vector (AAV) particles via split-intein mediated protein-trans-splicing (PTS). This way, we achieved selective gene transfer to distinct cell types. Single-chain variable fragments (scFvs) or designed ankyrin repeat proteins (DARPins), exhibiting high-affinity binding to cell surface receptors selectively expressed on the surface of target cells, were coupled to AAV particles harboring mutations in the capsid proteins which ablate natural receptor usage. Both, the AAV capsid protein VP2 and multiple separately produced targeting ligands recognizing Her2/neu, EpCAM, CD133 or CD30 were genetically fused with complementary split-intein domains. Optimized coupling conditions led to an effective conjugation of each targeting ligand to the universal AAV capsid and translated into specific gene transfer into target receptor-positive cell types in vitro and in vivo. Interestingly, PTS-based AAVs exhibited significantly less gene transfer into target receptor-negative cells than AAVs displaying the same targeting ligand but coupled genetically. Another important consequence of the PTS technology is the possibility to now display scFvs or other antibody-derived domain formats harboring disulfide-bonds in a functionally active form on the surface of AAV particles. Hence, the custom combination of a universal AAV vector particle and targeting ligands of various formats allows for an unprecedented flexibility in the generation of gene transfer vectors targeted to distinct cell types.

Enhancing the Oncolytic Activity of CD133-Targeted Measles Virus: Receptor Extension or Chimerism with Vesicular Stomatitis Virus Are Most Effective

Therapy resistance and tumor recurrence are often linked to a small refractory and highly tumorigenic subpopulation of neoplastic cells, known as cancer stem cells (CSCs). A putative marker of CSCs is CD133 (prominin-1). We have previously described a CD133-targeted oncolytic measles virus (MV-CD133) as a promising approach to specifically eliminate CD133-positive tumor cells. Selectivity was introduced at the level of cell entry by an engineered MV hemagglutinin (H). The H protein was blinded for its native receptors and displayed a CD133-specific single-chain antibody fragment (scFv) as targeting domain. Interestingly, MV-CD133 was more active in killing CD133-positive tumors than the unmodified MV-NSe despite being highly selective for its target cells. To further enhance the antitumoral activity of MV-CD133, we here pursued arming technologies, receptor extension, and chimeras between MV-CD133 and vesicular stomatitis virus (VSV). All newly generated viruses including VSV-CD133 were highly selective in eliminating CD133-positive cells. MV-CD46/CD133 killed in addition CD133-negative cells being positive for the MV receptors. In an orthotopic glioma model, MV-CD46/CD133 and MV^SCD-CD133, which encodes the super cytosine deaminase, were most effective. Notably, VSV-CD133 caused fatal neurotoxicity in this tumor model. Use of CD133 as receptor could be excluded as being causative. In a subcutaneous tumor model of hepatocellular cancer, VSV-CD133 revealed the most potent oncolytic activity and also significantly prolonged survival of the mice when injected intravenously. Compared to MV-CD133, VSV-CD133 infected a more than 10⁴-fold larger area of the tumor within the same time period. Our data not only suggest new concepts and approaches toward enhancing the oncolytic activity of CD133-targeted oncolytic viruses but also raise awareness about careful toxicity testing of novel virus types.

A Two-Phase Expansion Protocol Combining Interleukin (IL)-15 and IL-21 Improves Natural Killer Cell Proliferation and Cytotoxicity against Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue malignancy in children. Despite intensive research in recent decades the prognosis for patients with metastatic or relapsed diseases has hardly improved. New therapeutic concepts in anti-tumor therapy aim to modulate the patient’s immune system to increase its aggressiveness or targeted effects toward tumor cells. Besides surgery, radiotherapy and chemotherapy, immune activation by direct application of cytokines, antibodies or adoptive cell therapy are promising approaches. In the last years, adoptive transfer of natural killer (NK) cells came into the focus of translational medicine, because of their high cytotoxic potential against transformed malignant cells. A main challenge of NK cell therapy is that it requires a high amount of functional NK cells. Therefore, ex vivo NK cell expansion protocols are currently being developed. Many culturing strategies are based on the addition of feeder or accessory cells, which need to be removed prior to the clinical application of the final NK cell product. In this study, we addressed feeder cell-free expansion methods using common γ-chain cytokines, especially IL-15 and IL-21. Our results demonstrated high potential of IL-15 for NK cell expansion, while IL-21 triggered NK cell maturation and functionality. Hence, we established a two-phase expansion protocol with IL-15 to induce an early NK cell expansion, followed by short exposure to IL-21 that boosted the cytotoxic activity of NK cells against RMS cells. Further functional analyses revealed enhanced degranulation and secretion of pro-inflammatory cytokines such as interferon-γ and tumor necrosis factor-α. In a proof of concept in vivo study, we also observed a therapeutic effect of adoptively transferred IL-15 expanded and IL-21 boosted NK cells in combination with image guided high precision radiation therapy using a luciferase-transduced RMS xenograft model. In summary, this two-phased feeder cell-free ex vivo culturing protocol combined efficient expansion and high cytolytic functionality of NK cells for treatment of radiation-resistant RMS.

Immune Modulatory Cell Therapy for Hemophilia B Based on CD20-Targeted Lentiviral Gene Transfer to Primary B Cells

Gene-modified B cells expressing immunoglobulin G (IgG) fusion proteins have been shown to induce tolerance in several autoimmune and other disease models. However, lack of a vector suitable for gene transfer to human B cells has been an obstacle for translation of this approach. To overcome this hurdle, we developed an IgG-human factor IX (hFIX) lentiviral fusion construct that was targeted to specifically transduce cells expressing human CD20 (hCD20). Receptor-specific retargeting by mutating envelope glycoproteins of measles virus (MV)-lentiviral vector (LV) and addition of a single-chain variable fragment specific for hCD20 resulted in gene delivery into primary human and transgenic hCD20 mouse B cells with high specificity. Notably, this protocol neither required nor induced activation of the B cells, as confirmed by minimal activation of inflammatory cytokines. Using this strategy, we were able to demonstrate induction of humoral tolerance, resulting in suppression of antibody formation against hFIX in a mouse model of hemophilia B (HB). In conclusion, transduction of receptor-specific retargeted LV into resting B cells is a promising method to develop B cell therapies for antigen-specific tolerance induction in human disease.

Receptor-Targeted Nipah Virus Glycoproteins Improve Cell-Type Selective Gene Delivery and Reveal a Preference for Membrane-Proximal Cell Attachment

Receptor-targeted lentiviral vectors (LVs) can be an effective tool for selective transfer of genes into distinct cell types of choice. Moreover, they can be used to determine the molecular properties that cell surface proteins must fulfill to act as receptors for viral glycoproteins. Here we show that LVs pseudotyped with receptor-targeted Nipah virus (NiV) glycoproteins effectively enter into cells when they use cell surface proteins as receptors that bring them closely enough to the cell membrane (less than 100 Å distance). Then, they were flexible in receptor usage as demonstrated by successful targeting of EpCAM, CD20, and CD8, and as selective as LVs pseudotyped with receptor-targeted measles virus (MV) glycoproteins, the current standard for cell-type specific gene delivery. Remarkably, NiV-LVs could be produced at up to two orders of magnitude higher titers compared to their MV-based counterparts and were at least 10,000-fold less effectively neutralized than MV glycoprotein pseudotyped LVs by pooled human intravenous immunoglobulin. An important finding for NiV-LVs targeted to Her2/neu was an about 100-fold higher gene transfer activity when particles were targeted to membrane-proximal regions as compared to particles binding to a more membrane-distal epitope. Likewise, the low gene transfer activity mediated by NiV-LV particles bound to the membrane distal domains of CD117 or the glutamate receptor subunit 4 (GluA4) was substantially enhanced by reducing receptor size to below 100 Å. Overall, the data suggest that the NiV glycoproteins are optimally suited for cell-type specific gene delivery with LVs and, in addition, for the first time define which parts of a cell surface protein should be targeted to achieve optimal gene transfer rates with receptor-targeted LVs.

Pseudotyping of lentiviral vectors (LVs) with glycoproteins from other enveloped viruses has not only often been revealing in mechanistic studies of particle assembly and entry, but is also of practical importance for gene delivery. LVs pseudotyped with engineered glycoproteins allowing free choice of receptor usage are expected to overcome current limitations in cell-type selectivity of gene transfer. Here we describe for the first time receptor-targeted Nipah virus glycoproteins as important step towards this goal. LV particles carrying the engineered Nipah virus glycoproteins were substantially more efficient in gene delivery than their state-of-the-art measles virus-based counterparts, now making the production of receptor-targeted LVs for clinical applications possible. Moreover, the data define for the first time the molecular requirements for membrane fusion with respect to the position of the receptor binding site relative to the cell membrane, a finding with implications for the molecular evolution of paramyxoviruses using proteinaceous receptors for cell entry.

CD30 Receptor-Targeted Lentiviral Vectors for Human Induced Pluripotent Stem Cell-Specific Gene Modification

Cultures of induced pluripotent stem cells (iPSCs) often contain cells of varying grades of pluripotency. We present novel lentiviral vectors targeted to the surface receptor CD30 (CD30-LV) to transfer genes into iPSCs that are truly pluripotent as demonstrated by marker gene expression. We demonstrate that CD30 expression is restricted to SSEA4(high) cells of human iPSC cultures and a human embryonic stem cell line. When CD30-LV was added to iPSCs during routine cultivation, efficient and exclusive transduction of cells positive for the pluripotency marker Oct-4 was achieved, while retaining their pluripotency. When added during the reprogramming process, CD30-LV solely transduced cells that became fully reprogrammed iPSCs as confirmed by co-expression of endogenous Nanog and the reporter gene. Thus, CD30-LV may serve as novel tool for the selective gene transfer into PSCs with broad applications in basic and therapeutic research.

Viral gene transfer of APPsα rescues synaptic failure in an Alzheimer's disease mouse model

Alzheimer's disease (AD) is characterized by synaptic failure, dendritic and axonal atrophy, neuronal death and progressive loss of cognitive functions. It is commonly assumed that these deficits arise due to β-amyloid accumulation and plaque deposition. However, increasing evidence indicates that loss of physiological APP functions mediated predominantly by neurotrophic APPsα produced in the non-amyloidogenic α-secretase pathway may contribute to AD pathogenesis. Upregulation of APPsα production via induction of α-secretase might, however, be problematic as this may also affect substrates implicated in tumorigenesis. Here, we used a gene therapy approach to directly overexpress APPsα in the brain using AAV-mediated gene transfer and explored its potential to rescue structural, electrophysiological and behavioral deficits in APP/PS1∆E9 AD model mice. Sustained APPsα overexpression in aged mice with already preexisting pathology and amyloidosis restored synaptic plasticity and partially rescued spine density deficits. Importantly, AAV-APPsα treatment also resulted in a functional rescue of spatial reference memory in the Morris water maze. Moreover, we demonstrate a significant reduction of soluble Aβ species and plaque load. In addition, APPsα induced the recruitment of microglia with a ramified morphology into the vicinity of plaques and upregulated IDE and TREM2 expression suggesting enhanced plaque clearance. Collectively, these data indicate that APPsα can mitigate synaptic and cognitive deficits, despite established pathology. Increasing APPsα may therefore be of therapeutic relevance for AD.

Surface-Engineered Viral Vectors for Selective and Cell Type-Specific Gene Delivery

Recent progress in gene transfer technology enables the delivery of genes precisely to the application-relevant cell type ex vivo on cultivated primary cells or in vivo on local or systemic administration. Gene vectors based on lentiviruses or adeno-associated viruses can be engineered such that they use a cell surface marker of choice for cell entry instead of their natural receptors. Binding to the surface marker is mediated by a targeting ligand displayed on the vector particle surface, which can be a peptide, single-chain antibody, or designed ankyrin repeat protein. Examples include vectors that deliver genes to specialized endothelial cells or lymphocytes, tumor cells, or particular cells of the nervous system with potential applications in gene function studies and molecular medicine.

In vitro incorporation of a cell-binding protein to a lentiviral vector using an engineered split intein enables targeted delivery of genetic cargo

Gene therapy represents a promising therapeutic paradigm for addressing many disorders, but the absence of a vector that can be robustly and reproducibly functionalized with cell-homing functionality to mediate the delivery of genetic cargo specifically to target cells following systemic administration has stood as a major impediment. In this study, a high-affinity protein-protein pair comprising a splicing-deficient naturally split intein was used as molecular Velcro to append a HER2/neu-binding protein (DARPin) onto the surface of a binding-deficient, fusion-competent lentivirus. HER2/neu-specific lentiviruses created using this in vitro pseudotyping approach were able to deliver their genetic reporter cargo specifically to cells that express the target receptor at high levels in a co-culture. We envision that the described technology could provide a powerful, broadly applicable platform for the incorporation of cell-targeting functionality onto viral vectors.

Exclusive Transduction of Human CD4+ T Cells upon Systemic Delivery of CD4-Targeted Lentiviral Vectors

Playing a central role in both innate and adaptive immunity, CD4(+) T cells are a key target for genetic modifications in basic research and immunotherapy. In this article, we describe novel lentiviral vectors (CD4-LV) that have been rendered selective for human or simian CD4(+) cells by surface engineering. When applied to PBMCs, CD4-LV transduced CD4(+) but not CD4(-) cells. Notably, also unstimulated T cells were stably genetically modified. Upon systemic or intrasplenic administration into mice reconstituted with human PBMCs or hematopoietic stem cells, reporter gene expression was predominantly detected in lymphoid organs. Evaluation of GFP expression in organ-derived cells and blood by flow cytometry demonstrated exclusive gene transfer into CD4(+) human lymphocytes. In bone marrow and spleen, memory T cells were preferentially hit. Toward therapeutic applications, we also show that CD4-LV can be used for HIV gene therapy, as well as for tumor therapy, by delivering chimeric Ag receptors. The potential for in vivo delivery of the FOXP3 gene was also demonstrated, making CD4-LV a powerful tool for inducible regulatory T cell generation. In summary, our work demonstrates the exclusive gene transfer into a T cell subset upon systemic vector administration opening an avenue toward novel strategies in immunotherapy.

Receptor-targeted lentiviral vectors are exceptionally sensitive toward the biophysical properties of the displayed single-chain Fv

An increasing number of applications require the expression of single-chain variable fragments (scFv) fusion proteins in mammalian cells at the cell surface membrane. Here we assessed the CD30-specific scFv HRS3, which is used in immunotherapy, for its ability to retarget lentiviral vectors (LVs) to CD30 and to mediate selective gene transfer into CD30-positive cells. Fused to the C-terminus of the type-II transmembrane protein hemagglutinin (H) of measles virus and expressed in LV packaging cells, gene transfer mediated by the released LV particles was inefficient. A series of point mutations in the scFv framework regions addressing its biophysical properties, which substantially improved production and increased the melting temperature without impairing its kinetic binding behavior to CD30, also improved the performance of LV particles. Gene transfer into CD30-positive cells increased ∼100-fold due to improved transport of the H-scFv protein to the plasma membrane. Concomitantly, LV particle aggregation and syncytia formation in packaging cells were substantially reduced. The data suggest that syncytia formation can be triggered by trans-cellular dimerization of H-scFv proteins displayed on adjacent cells. Taken together, we show that the biophysical properties of the targeting ligand have a decisive role for the gene transfer efficiency of receptor-targeted LVs.

CD105 is a surface marker for receptor-targeted gene transfer into human long-term repopulating hematopoietic stem cells

Hematopoietic stem cells (HSCs) are an important target cell population for gene therapy since they can reconstitute the entire hematopoietic system. HSC-enriched cell populations can be recognized based on cell surface marker expression, such as CD34, which is broadly expressed on immature and partially differentiated cells. In mice, co-expression of CD34 and CD105 was previously shown to be relatively more specific for the most immature, long-term repopulating HSCs. Here, we evaluated whether CD105, which is expressed on 30%-80% of CD34(+) cells, is a marker also for human long-term repopulating HSCs. Therefore, we tracked the mature progeny of CD34(+) cells transduced with the CD105-targeted lentiviral vector CD105-LV in xenotolerant mice. Transduction was blocked with soluble CD105 protein confirming specificity. Importantly, CD105-LV transduced human CD34(+) cells engrafted in NOD-scid IL2Rγ(-/-) mice with up to 20% reporter gene-positive cells detected long term in all human hematopoietic lineages in bone marrow (BM), spleen, and blood. In addition, competitive repopulation experiments in mice showed a superior engraftment of CD105-LV transduced CD34(+) cells in BM and spleen compared with cells transduced with a conventional nontargeted lentiviral vector. Thus, human CD34(+)/CD105(+) cells are enriched for early HSCs with high repopulating capacity. Targeting this cell population with CD105-LV offers a novel gene transfer strategy to reach high engraftment rates of transduced cells and highlights the applicability of receptor-targeted vectors to trace cell subsets offering an alternative to prospective isolation by surface markers.

Selective lentiviral gene delivery to CD133-expressing human glioblastoma stem cells

Glioblastoma multiforme (GBM) is a deadly primary brain malignancy. Glioblastoma stem cells (GSC), which have the ability to self-renew and differentiate into tumor lineages, are believed to cause tumor recurrence due to their resistance to current therapies. A subset of GSCs is marked by cell surface expression of CD133, a glycosylated pentaspan transmembrane protein. The study of CD133-expressing GSCs has been limited by the relative paucity of genetic tools that specifically target them. Here, we present CD133-LV, a lentiviral vector presenting a single chain antibody against CD133 on its envelope, as a vehicle for the selective transduction of CD133-expressing GSCs. We show that CD133-LV selectively transduces CD133+ human GSCs in dose-dependent manner and that transduced cells maintain their stem-like properties. The transduction efficiency of CD133-LV is reduced by an antibody that recognizes the same epitope on CD133 as the viral envelope and by shRNA-mediated knockdown of CD133. Conversely, the rate of transduction by CD133-LV is augmented by overexpression of CD133 in primary human GBM cultures. CD133-LV selectively transduces CD133-expressing cells in intracranial human GBM xenografts in NOD.SCID mice, but spares normal mouse brain tissue, neurons derived from human embryonic stem cells and primary human astrocytes. Our findings indicate that CD133-LV represents a novel tool for the selective genetic manipulation of CD133-expressing GSCs, and can be used to answer important questions about how these cells contribute to tumor biology and therapy resistance.