Irene C. Schneider

Specific gene transfer to neurons, endothelial cells and hematopoietic progenitors with lentiviral vectors

We present a flexible and highly specific targeting method for lentiviral vectors based on single-chain antibodies recognizing cell-surface antigens. We generated lentiviral vectors specific for human CD105+ endothelial cells, human CD133+ hematopoietic progenitors and mouse GluA-expressing neurons. Lentiviral vectors specific for CD105 or for CD20 transduced their target cells as efficiently as VSV-G pseudotyped vectors but discriminated between endothelial cells and lymphocytes in mixed cultures. CD133-targeted vectors transduced CD133+ cultured hematopoietic progenitor cells more efficiently than VSV-G pseudotyped vectors, resulting in stable long-term transduction. Lentiviral vectors targeted to the glutamate receptor subunits GluA2 and GluA4 exhibited more than 94% specificity for neurons in cerebellar cultures and when injected into the adult mouse brain. We observed neuron-specific gene modification upon transfer of the Cre recombinase gene into the hippocampus of reporter mice. This approach allowed targeted gene transfer to many cell types of interest with an unprecedented degree of specificity.

Pseudotyping lentiviral vectors with the wild-type measles virus glycoproteins improves titer and selectivity

We pseudotyped HIV-1 vectors with cytoplasmic tail-truncated envelope glycoproteins of a wild-type (WT) measles virus (MV). The particles entered the lymphatic cells exclusively through the signaling lymphocyte activation molecule (SLAM, CD150), whereas particles pseudotyped with the MV vaccine strain glycoproteins also recognized the ubiquitous membrane cofactor protein (CD46) as receptor and had less specific cell entry. MVWT-HIV vectors reached titers of 108 t.u. ml−1, which were up to 10-fold higher than those of MVVac-HIV vectors, and discriminated between SLAM-positive and SLAM-negative cells, also in mixed cell cultures. As these vectors transduce primary human cells more efficiently than vesicular stomatitis virus-G pseudotyped vectors do, they are promising candidates for gene transfer to human lymphocytes and certain epithelial cells.

Specific gene delivery to liver sinusoidal and artery endothelial cells

Different types of endothelial cells (EC) fulfill distinct tasks depending on their microenvironment. ECs are therefore difficult to genetically manipulate ex vivo for functional studies or gene therapy. We assessed lentiviral vectors (LVs) targeted to the EC surface marker CD105 for in vivo gene delivery. The mouse CD105-specific vector, mCD105-LV, transduced only CD105-positive cells in primary liver cell cultures. Upon systemic injection, strong reporter gene expression was detected in liver where mCD105-LV specifically transduced liver sinusoidal ECs (LSECs) but not Kupffer cells, which were mainly transduced by nontargeted LVs. Tumor ECs were specifically targeted upon intratumoral vector injection. Delivery of the erythropoietin gene with mCD105-LV resulted in substantially increased erythropoietin and hematocrit levels. The human CD105-specific vector (huCD105-LV) transduced exclusively human LSECs in mice transplanted with human liver ECs. Interestingly, when applied at higher dose and in absence of target cells in the liver, huCD105-LV transduced ECs of a human artery transplanted into the descending mouse aorta. The data demonstrate for the first time targeted gene delivery to specialized ECs upon systemic vector administration. This strategy offers novel options to better understand the physiological functions of ECs and to treat genetic diseases such as those affecting blood factors.

T cell receptor gene transfer exclusively to human CD8 + cells enhances tumor cell killing

Transfer of tumor-specific T-cell receptor (TCR) genes into patient T cells is a promising strategy in cancer immunotherapy. We describe here a novel vector (CD8-LV) derived from lentivirus, which delivers genes exclusively and specifically to CD8(+) cells. CD8-LV mediated stable in vitro and in vivo reporter gene transfer as well as efficient transfer of genes encoding TCRs recognizing the melanoma antigen tyrosinase. Strikingly, T cells genetically modified with CD8-LV killed melanoma cells reproducibly more efficiently than CD8(+) cells transduced with a conventional lentiviral vector. Neither TCR expression levels, nor the rate of activation-induced death of transduced cells differed between both vector types. Instead, CD8-LV transduced cells showed increased granzyme B and perforin levels as well as an up-regulation of CD8 surface expression in a small subpopulation of cells. Thus, a possible mechanism for CD8-LV enhanced tumor cell killing may be based on activation of the effector functions of CD8(+) T cells by the vector particle displaying OKT8-derived CD8-scFv and an increase of the surface density of CD8, which functions as coreceptor for tumor-cell recognition. CD8-LV represents a powerful novel vector for TCR gene therapy and other applications in immunotherapy and basic research requiring CD8(+) cell-specific gene delivery.

Cell entry targeting restricts biodistribution of replication-competent retroviruses to tumour tissue.

Virotherapy is currently being developed for many different types of viruses including replication-competent murine leukaemia virus (MLV) as a novel tool in cancer therapy. However, there is the risk of insertional mutagenesis associated with this virus, making careful preclinical studies necessary before its first application in man. We have previously generated conditionally replication-competent MLV variants that require activation by tumour-associated proteases to become infectious. Here we analysed in a comparative study the spreading of non-targeted and of such tumour-targeted MLV variants to tumour and extratumoural organs in immunodeficient mice. Both virus types were able to efficiently infect tumour cells after systemic administration. The non-targeted virus, however, also infected extratumoural organs like bone marrow, spleen and liver efficiently. In contrast, the targeted viruses revealed in a quantitative analysis of virus spreading an up to 500-fold more selective infection of tumour tissue than the non-targeted virus. The data raise serious doubts about a safe clinical use of non-targeted MLV. Engineering the virus to become activatable by tumour-associated proteases can significantly improve the safety of MLV.

Resting lymphocyte transduction with measles virus glycoprotein pseudotyped lentiviral vectors relies on CD46 and SLAM

The measles virus (MV) glycoproteins hemagglutinin (H) and fusion (F) were recently shown to mediate transduction of resting lymphocytes by lentiviral vectors. MV vaccine strains use CD46 or signaling lymphocyte activation molecule (SLAM) as receptor for cell entry. A panel of H protein mutants derived from vaccine strain or wild-type MVs that lost or gained CD46 or SLAM receptor usage were investigated for their ability to mediate gene transfer into unstimulated T lymphocytes. The results demonstrate that CD46 is sufficient for efficient vector particle association with unstimulated lymphocytes. For stable gene transfer into these cells, however, both MV receptors were found to be essential.

Retroviral Display and High Throughput Screening

Retroviruses distinguish themselves from all other mammalian viruses by their abilities to infect and propagate in mammalian cells without causing a cytopathic effect and to stably integrate their genetic information into the genome of the host cell. These unique properties make them an ideal platform for the display and directed evolution of proteins in a mammalian cell environment. This review will describe the essentials about retrovirus biology and then discuss in detail display and screening strategies that have been developed during the past 15 years of retroviral display technology.

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.

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.

Displaying Tetra‐Membrane Spanning Claudins on Enveloped Virus‐Like Particles for Cancer Immunotherapy

Virus-like particles (VLPs) displaying foreign antigens have become an important tool in vaccination including the induction of immune responses against self-antigens. Claudin 6 (CLDN6) has been identified as tumor-associated antigen and is therefore a potential target for tumor vaccination strategies. However, as tetra-membrane spanning protein its incorporation into VLPs while preserving a native fold is challenging. Here, we attempted the incorporation of a panel of engineered CLDN6 variants into the membrane of retrovirus-derived VLPs. Interestingly, wild-type CLDN6 revealed the most efficient display. VLPs presenting CLDN6 or CLDN9 derived from different donor species were produced and preservation of their native confirmation was demonstrated by antibody binding assays. VLPs displaying murine CLDN6 were used to immunize mice. Antibodies recognizing native CLDN6 as displayed on cell surfaces and mediating complement-dependent cytotoxicity were elicited in vaccinated animals. The data suggest applications of CLDN6 displaying VLPs in cancer immunotherapy.