Screening retroviral packaging cells for highly efficient virus production by using a combined selection procedure

Generation of serum-stabilized retroviruses: Reduction of α1,3gal-epitope synthesis in a murine NIH3T3-derived packaging cell line by expression of chimeric glycosyltransferases

Retroviral vectors released from mouse-derived packaging cell lines are inactivated in human sera by naturally occurring antibodies due to the recognition of Galalpha1,3Galbeta1,4GlcNAc (alphagal-epitope) decorated surface proteins. In this study, an extensive analysis of the glycosylation potential of NIH3T3-derived PA317 packaging cells using combined MALDI/TOF-MS and HPAE-PAD reveals that 34% of the N-glycan moiety represents alphagal-epitope containing structures. Stable expression of glycosyltransferases and transport signal chimeras has been demonstrated to represent an efficient tool to alter cell- and species-specific glycosylation (Grabenhorst and Conradt, 1999. J. Biol. Chem. 274, 36107-36116). In order to reduce alphagal-epitope synthesis selected chimeric glycosyltransferases were constructed by fusing Golgi-signal sequences for compartment-specific localization with the catalytic domain of alpha2,3-sialyltransferase (ST3). Stable expression of these constructs in these cells resulted in a significant reduced alphagal-epitope synthesis, and moreover, a release of retroviral vectors showing an up to 3.5-fold increase in serum stability. Thus, our results suggest that the stably transfected cells stably transfected with chimeric glycosyltransferases compete efficiently with endogenous alpha1,3-galactosyltransferase. This approach allows favored glycodesign and we anticipate the applicability of such improved retroviral vectors produced by glycosylation engineered host cells for in vivo gene therapy and, furthermore, suggest the therapeutic benefit of this technology for xenotransplantation.

Gene expression analysis of murine cells producing amphotropic mouse leukaemia virus at a cultivation temperature of 32 and 37 degrees C

Cultivation of retrovirus packaging cells at 32 degrees C represents a common procedure to achieve high titres in mouse retrovirus production. Gene expression profiling of mouse NIH 3T3 cells producing amphotropic mouse leukaemia virus 4070A revealed that 10 % of the 1176 cellular genes investigated were regulated by temperature shift (37/32 degrees C), while 5 % were affected by retrovirus infection. Strikingly, retrovirus production at 32 degrees C activated the cholesterol biosynthesis/transport pathway and caused an increase in plasma membrane cholesterol levels. Furthermore, these conditions resulted in transcriptional activation of smoothened (smo), patched (ptc) and gli-1; Smo, Ptc and Gli-1, as well as cholesterol, are components of the Sonic hedgehog (Shh) signalling pathway, which directs pattern formation, diversification and tumourigenesis in mammalian cells. These findings suggest a link between cultivation at 32 degrees C, production of MLV-A and the Shh signalling pathway.

The temperature stability of mouse retroviruses depends on the cholesterol levels of viral lipid shell and cellular plasma membrane

To delineate parameters contributing to the extracellular lifetime of retroviral vectors, we carried out stability tests of retroviruses derived from cell lines of different origin and kept under different cultivation conditions. Results show that amphotropic mouse retroviruses (MLV-A) derived from human and hamster cells exhibit 2- to 3-fold higher half-lives compared to retroviruses from mouse cells. Cultivation at 32 degrees C has been reported to yield high virus titers. However, the benefit of virus production in mouse cells at 32 degrees C is controversial. In our hands the cultivation temperature affected, hitherto not noticed, the half-life time of MLV-A. The 37/32 degrees C shift resulted in a 3-fold decrease of viral half-lifes compared to MLV-A released from mouse cells at 37 degrees C. Thus, MLV-A released at 37 degrees C is phenotypically different from MLV-A synthesized at 32 degrees C. Increased virus stability was inversely correlated with the level of cholesterol in the viral membrane. Finally, depletion of viral cholesterol in vitro resulted in intact virus with increased thermal stability. Thus, retrovirus lability depends on the host cell and parallels the cholesterol amount in the viral lipid shell.

Complement-protected amphotropic retroviruses from murine packaging cells

The application of retroviruses generated from murine cells for in vivo gene therapy is restricted primarily because of the rapid inactivation of these viruses by the human complement system. To circumvent this disadvantageous property of murine retroviruses we have generated infectious amphotropic retroviruses that exhibit strong protection against human complement attack. The membrane of these viruses contains a fusion protein, DAFF2A, that is composed of the catalytic domain of the human complement regulatory protein (CRP) decay-accelerating factor (DAF) and the envelope protein of the amphotropic murine leukemia virus (MuLV) 4070A (EnvA). The fusion of two other CRPs, MCP and CD59, to the same amphotropic Env moiety did not lead to equivalent results. The fusion protein DAFF2A was stably expressed in mouse NIH 3T3-based helper cells and independently identified with either alpha-DAF MAb or alpha-Env PAb on the cell membrane. Western blot analysis confirmed the expected molecular weight of the fusion protein. Viral titers obtained from NIH 3T3 helper cell pools were 5 x 10(5) CFU for wild-type amphotropic EnvA virus and 1 x 10(5) CFU for DAFF2A virus, respectively. By blocking the catalytic domain of DAF by pretreatment with alpha-DAF MAb DAFF2A, recombinant virions could be converted to wild-type with respect to sensitivity against human serum. Since the method for producing virions that are protected against human serum should be applicable to any cell type it offers a novel tool for human in vivo gene therapy.

Poor expression of MDR1 transgene in HeLa cells by bicistronic Moloney murine leukemia virus-based vector

Cotransfer of a therapeutic gene together with the human MDR1 gene provides an opportunity to increase the number of transduced marrow cells, expressing the therapeutic gene, by in vivo selection for MDR1. We have used an Lg-MDR1-IRES-neo (LgMIN) retroviral vector, containing MDR1 and neo genes, separated by the EMCV IRES. Human HeLa or canine CTAC cells, transduced with GALV env pseudotyped LgMIN at an MOI of less than 0.01 to ensure 1 proviral copy/genome, were selected with either G418 for neo expression or colchicine for MDR1 expression. The titer determined on HeLa cells with G418 selection was eight-fold higher than that with colchicine selection. In contrast, the same viral supernatant exhibited only a 1.4-fold difference between neo- and MDR1-based viral titer values for CTAC cells. The transduced HeLa cells, with one intact proviral copy per genome, exhibited a 55-fold higher resistance to G418 but only a 4-fold higher resistance to colchicine and a 2-fold higher resistance to Taxol compared with nontransduced cells. About 23% of the transduced cell population did not express vector-derived P-glycoprotein (P-gp) as detected by anti-human P-gp MAb MRK-16. This could explain the difference in viral titers obtained on CTAC cells but not that obtained on HeLa cells. The vector-mediated increase in expression of P-gp was about 20-fold higher in CTAC cells as compared with HeLa cells. These results indicated suppression of expression of vector-derived MDR1 in HeLa cells, in contrast with CTAC cells. To investigate further the possible reasons for this difference, genomic DNA was isolated from the G418-resistant individual colonies of infected cells and analyzed by PCR for full-length proviral MDR1. For transduced CTAC and HeLa cells, selected at a G418 concentration of 1 mg/ml, PCR detected aberrant forms of MDR1 in 17 to 25% of colonies tested. The aberrant forms consisted of MDR1 genes with 2- and 0.7-kb deletions. DNA sequencing across the 2-kb and the 0.7-kb deletion junction suggests cryptic splicing in the producer cell line as the origin of these deletions. The 2-kb deletion corresponds to MDR1 mRNA cryptic splicing via donor (codon 113) and acceptor (codon 773). The 0.7-kb deletion corresponds to splicing via the same donor and a different acceptor (codon 344). When transduced HeLa cells were selected at a higher concentration of G418 (3 mg/ml), the aberrant forms were detected at an increased frequency of about 50% of colonies tested. These results indicate that vector-derived MDR1 is a poor selective marker in HeLa cells but not in CTAC cells and that deletions, which inactivated the MDR1 gene in a bicistronic Mo-MuLV vector, may provide an advantage for expression of the second transgene in HeLa cells.