Expression and release of stable and active forms of murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) targeted to different subcellular compartments

A murine model of acute myeloid leukemia with Evi1 overexpression and autocrine stimulation by an intracellular form of GM-CSF in DA-3 cells

The poor treatment response of acute myeloid leukemia (AML) overexpressing high-risk oncogenes such as EVI1, demands specific animal models for new treatment evaluations. Evi1 is a common site of activating integrations in murine leukemia virus (MLV)-induced AML and in retroviral and lentiviral gene-modified HCS. Still, a model of overt AML induced by Evi1 has not been generated. Cell lines from MLV-induced AML are growth factor-dependent and non-transplantable. Hence, for the leukemia maintenance in the infected animals, a growth factor source such as chronic immune response has been suggested. We have investigated whether these leukemias are transplantable if provided with growth factors. We show that the Evi1(+)DA-3 cells modified to express an intracellular form of GM-CSF, acquired growth factor independence and transplantability and caused an overt leukemia in syngeneic hosts, without increasing serum GM-CSF levels. We propose this as a general approach for modeling different forms of high-risk human AML using similar cell lines.

Genetically modified autoactivated cells expressing intracellular forms of GM-CSF as a model for regulated administration of cytokines

The application of cytokines for immunotherapy is frequently hampered by undesirable side effects. To avoid systemic effects, cytokines can be directly expressed in the target cells by using gene transfer. However, the uncontrolled cellular secretion of cytokines could still exert some undesirable bystander effects. Therefore, it is important to develop additional methods for a more restricted administration of cytokines. Recently, using the murine granulocyte-macrophage colony-stimulating factor (mGM-CSF), we have demonstrated that cytokines can be targeted to different subcellular compartments as stable and biologically active proteins. This model could be used as a method of highly restricted administration of cytokines. Here, as model for the proof of principle, we have used a cell line (DA-3) strictly dependent on mGM-CSF for growth and demonstrated that these cells acquired autonomous growth after gene modification with plasmids encoding either extracellular or intracellular forms of mGM-CSF. Cell lines expressing secreted forms of mGM-CSF displayed the highest rates of autonomous growth and released substantial amounts of mGM-CSF. However, cell lines expressing intracellular forms of mGM-CSF also acquired autonomous growth induced by a mechanism of restricted autocrine stimulation and did not release detectable mGM-CSF to the extracellular medium. Cocultivation experiments of DA-3 cell lines expressing intracellular mGM-CSF with unmodified cells showed that there was no activation of the bystander cells. Taken together, these results support the concept that genes encoding intracellular cytokines may be used to provide the desired effect of cytokines on the target cells while avoiding the side effects of their uncontrolled secretion.

Recombinant replication-restricted VSV as an expression vector for murine cytokines

Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus that rapidly and efficiently shuts down the production of host cell-encoded proteins and utilizes the cell's protein production machinery to express high levels of virally encoded proteins. In an effort to take advantage of this characteristic of VSV, we have employed a reverse genetics system to create recombinant forms of VSV encoding a variety of murine cytokines. Previous studies have revealed that cells infected with recombinant VSV that lack expression of the surface glycoprotein (G protein), designated deltaG-VSV, more efficiently express and secrete recombinant proteins than do recombinant "wild-type" VSV. Therefore, murine cytokine-expressing recombinants were produced as deltaG viruses. Propagation of these deltaG viruses in cells that transiently express G protein in vitro results in G-complemented virions that can infect cells, shut down host protein synthesis, and express at high levels each virally encoded protein (including the designated cytokine). We assessed the ability of each deltaG-VSV construct to express recombinant cytokine by infecting BHK cells and then monitoring/measuring the production of the desired cytokine. When possible, the bioactivity of the cytokine products was also measured. The results presented here reveal that large quantities of bioactive cytokines can be produced rapidly and inexpensively using deltaG-VSV as a protein expression system.