Factors affecting the performance of different long terminal repeats in the retroviral vector

A Promising Vector for TCR Gene Therapy: Differential Effect of siRNA, 2A Peptide, and Disulfide Bond on the Introduced TCR Expression

Adoptive immunotherapy using TCR gene-modified T-lymphocytes is an attractive strategy for targeting malignancies. However, TCR mispairings between endogenous and introduced TCR chains are a major concern, as they may induce mixed TCRs with unknown specificities and may reduce the expression of therapeutic TCRs. To overcome these problems, we have recently established a novel retroviral siTCR vector encoding small-interfering RNAs (siRNAs) to knockdown endogenous TCR genes for the efficient expression of therapeutic TCRs. In this study, to improve the efficacy of siTCR vectors, we developed 2A peptide-based siTCR vectors that could increase the expression levels of transduced TCRs compared with internal promoter-based siTCR vectors. We also evaluated the efficacy of an siTCR strategy and the addition of a new interchain disulfide bond created by cysteine modification. We found that the effect of the cysteine modification depended on TCR variations, while the siTCR strategy improved the expression of all TCRs tested. Furthermore, the combined effect of the siTCR and cysteine modification strategies was highly significant for certain TCRs. Therefore, our novel siTCR technology, in isolation or in combination with another strategy, may open the door to effective immunotherapy for cancer patients.Molecular Therapy - Nucleic Acids (2012) 1, e63. doi:10.1038/mtna.2012.52; published online 18 December 2012.

Development of murine leukemia virus-based retroviral vectors with a minimum possibility of cis-activation

The possibility of insertional mutagenesis in retroviral gene therapy can be reduced by using a vector lacking the enhancer sequence present in the U3 of the long-terminal repeats. However, such vectors suffer from many pitfalls. We attempted to improve a murine leukemia virus-based retroviral vector containing the enhancer-free U3, first by making it easier to construct a producer line and then by introducing the cellular RPL10 promoter as an internal promoter. The reverse orientation of the expression cassette of the transgene was found to give higher transducing titer and higher-level gene expression. The deletion analysis revealed that the 54-bp-long sequence of U3 (34 and 20 bp present at 5' and 3' extreme ends, respectively) was sufficient for the functions of retroviral vectors. The data from the in vitro cell culture assay indicated that the final construct, ROK, containing all these features, had little cis-activation activity, even if it was placed right upstream from the RNA start site of the neighboring gene. Our data suggested that the newly developed vector might provide increased safety, while still producing high viral titer from a stable producer line and high-level gene expression in various target cells including human CD34(+) stem cells.

Nonintegrating foamy virus vectors

Foamy viruses (FVs), or spumaviruses, are integrating retroviruses that have been developed as vectors. Here we generated nonintegrating foamy virus (NIFV) vectors by introducing point mutations into the highly conserved DD35E catalytic core motif of the foamy virus integrase sequence. NIFV vectors produced high-titer stocks, transduced dividing cells, and did not integrate. Cells infected with NIFV vectors contained episomal vector genomes that consisted of linear, 1-long-terminal-repeat (1-LTR), and 2-LTR circular DNAs. These episomes expressed transgenes, were stable, and became progressively diluted in the dividing cell population. 1-LTR circles but not 2-LTR circles were found in all vector stocks prior to infection. Residual integration of NIFV vectors occurred at a frequency 4 logs lower than that of integrase-proficient FV vectors. Cre recombinase expressed from a NIFV vector mediated excision of both an integrated, floxed FV vector and a gene-targeted neo expression cassette, demonstrating the utility of these episomal vectors. The broad host range and large packaging capacity of NIFV vectors should make them useful for a variety of applications requiring transient gene expression.

Rapid alphabeta TCR-mediated responses in gammadelta T cells transduced with cancer-specific TCR genes

Adoptive T-cell transfer of in vitro cultured T cells derived from cancer patients with naturally developed immune responses has met with some success as an immunotherapeutic approach, although only a limited number of patients showed spontaneous immune responses. To find alternative ways, such as cancer-specific T-cell receptor (TCR) gene transfer, in preparation for sufficient numbers of antigen-specific T cells is an important issue in the field of adoptive T-cell therapy. Given the inherent disadvantage of alphabeta TCR transfer to other alphabeta T cells, namely the possible formation of mixed TCR heterodimers with endogenous alpha or beta TCR, we employed gammadelta T cells as a target for retroviral transfer of cancer-specific TCR and examined whether gammadelta T cells were useful as an alternative population for TCR transfer. Although retroviral transduction to gammadelta T cells with TCR alphabeta genes alone, isolated from a MAGE-A4(143-151)-specific alphabeta CD8(+) cytotoxic T lymphocyte (CTL) clone, did not provide sufficient affinity to recognize major histocompatibility (MHC)-peptide complexes due to the lack of CD8 co-receptor, gammadelta T cells co-transduced with TCR alphabeta and CD8 alphabeta genes acquired cytotoxicity against tumor cells and produced cytokines in both alphabeta- and gammadelta-TCR-dependent manners. Furthermore, alphabeta TCR and CD8-transduced gammadelta T cells, stimulated either through alphabeta TCR or gammadelta TCR, rapidly responded to target cells compared with conventional alphabeta T cells, reminiscent of gammadelta T cells. We propose alphabeta TCR-transduced gammadelta T cells as an alternative strategy for adoptive T-cell transfer.

Cloning and transcriptional activity of a novel ovarian-specific promoter from rat retrovirus-like elements

The long terminal repeats (LTRs) are the control centers for retrovirus gene expression, which possess all of the requisite signals. It has been proved that the LTRs of Moloney murine leukemia virus (MoMLV) could constitutively activate genes in diverse cell types. Recently, a retrovirus-like element, OSP-1 (ovarian-specific promoter 1), was extracted from rat ovary according to the LTRs of MoMLV, whose name was derived from the fact of ovarian-specific transcription. It was reasonable to speculate that the tissue-specificity was acquired through mutations and that there should be abound other mutants, active or inactive. In the present study, we isolated several homologous sequences to OSP-1 and detected their function. Consequently, one of them could also drive target gene expression specifically to ovarian cell lines and was named OSP-2 which shared 98% similarity to OSP-1. On the other hand, we picked up other two closest sequences and proved them inactive, which was 97% and 95% similar to OSP-1, respectively. Sequence analysis revealed the different mutations around/within the binding sites of transcriptional factors that might play important roles in tissue-specificity. In summary, we extracted a novel ovarian-specific promoter as well as other nonfunctional mutants, which in part shed light on the study of ovarian-specific transcription. In addition, it also provided a new tool in cancer gene therapy and to create transgenic animals.

Bioluminescent evaluation of the therapeutic effects of total body irradiation in a murine hematological malignancy model

OBJECTIVE

We investigated the utility of in vivo bioluminescence imaging (BLI) in assessing the therapeutic effects of total body irradiation (TBI) in a murine hematological malignancy model.

MATERIALS AND METHODS

The suspension of Ba/F3 cells transduced with firefly luciferase and p190 BCR-ABL genes was exposed to ionizing radiation, and viable cell numbers and bioluminescent signals were measured serially. Mice intravenously inoculated with the cells underwent TBI at various doses. In vivo BLI was performed repeatedly until spontaneous death, and whole-body bioluminescence signals were determined as an indicator of whole-body tumor burden.

RESULTS

In the cell culture study, bioluminescence signals generally reflected viable cell numbers, despite some overestimation immediately after irradiation. Sublethal TBI in mice transiently depressed the increase in whole-body signals and prolonged survival. Spontaneous death occurred at similar signal levels regardless of radiation dose. A significant negative correlation was found between survival and whole-body signal early after TBI. Significant dose dependence was demonstrated for both survival and signal increase early after TBI and was more evident for signal increase. Lethally irradiated mice without bone marrow transplantation died while showing weak signals. In mice receiving lethal TBI and syngeneic bone marrow transplantation, signal reduction and prolongation of survival were prominent, and whole-body signals at death were similar to those in nonirradiated or sublethally irradiated mice.

CONCLUSION

In vivo BLI allows longitudinal, quantitative evaluation of the response to TBI in mice of a hematological malignancy model. Antitumor effects can be assessed early and reliably using in vivo BLI.

Development of an in vitro cell culture assay system for measuring the activation of a neighbouring gene by the retroviral vector

BACKGROUND

The use of retroviral vectors has shown an actual clinical benefit in a few inherited diseases. However, the occurrence of cases of leukemia after the X-SCID gene therapy trial raised concerns about the safety of insertional mutagenesis inherent to the biology of the retrovirus. Although the retrovirus has long been known to integrate into the host chromosome, and thus have the potential to activate the nearby gene, there has been no convenient method of studying or assaying such a cis-activation phenomenon.

METHODS

In the present study, we report an in vitro assay system in which the effect of retroviral integration on the expression of the neighbouring gene can be studied. In this system, a retroviral vector and the neighbouring reporter gene were constructed in a single plasmid as if it had integrated into the chromosome.

RESULTS

Using this assay, we found that the full-length long terminal repeat (LTR) could indeed activate the neighbouring gene expression from a distance and the magnitude of its activation was highly increased when this LTR was placed in the vicinity of the transcription start site of the gene, whereas the truncated LTR exerted little influence.

CONCLUSIONS

This assay system might provide a useful tool for selecting the appropriate vector structure, as well as for studying the molecular mechanism underlying the cis-activation by the viral LTR.