Inhibition of murine leukemia virus Pr65gag cleavage in vitro and in vivo by hypertonic medium

Effect of osmotic pressure on the production of retroviral vectors: Enhancement in vector stability

The use of Moloney murine leukaemia virus (MoMLV) derived retroviral vectors in gene therapy requires the production of high titer preparations. However, obtaining high titers of infective MoMLV retroviral vectors is difficult due to the vector inherent instability. In this work the effect of the cell culture medium osmotic pressure upon the virus stability was studied. The osmolality of standard medium was raised from 335 up to 500 mOsm/kg using either ionic (sodium chloride) or non-ionic osmotic agents (sorbitol and fructose). It was observed that, independently of the osmotic agent used, the infectious vector inactivation rate was inversely correlated with the osmolality used in the production media; therefore, the use of high medium osmolalities enhanced vector stability. For production purposes a balance must be struck between cell yield, cell productivity and retroviral stability. From the conditions tested herein sorbitol addition, ensuring osmolalities between 410 and 450 mOsm/kg, yields the best production conditions; NaCl hampered the viral infectious production while fructose originates lower cell yields. Lipid extractions were performed for cholesterol and phospholipid analyses showing that more stable viral vectors had a 10% reduction in the cholesterol content. A similar reduction in cholesterol was observed in the producer cells. A detailed analysis of the major phospholipids composition, type and fatty acid content, by mass spectrometry did not show significant changes, confirming the decrease in the cholesterol to phospholipids ratio in the viral membrane as the major reason for the increased vector stability.

Characterization of a virus-specific proteolytic activity processing the gag precursor of the simian sarcoma-associated virus

The proteolytic processing of the gag precursor polypeptide pr65gag of simian sarcoma-associated virus (SSAV) has been studied in vivo and in vitro. In SSAV-infected cells (i.e., in vivo) proteins of 52 and 38 kDa and the viral protein p30 could be immunoprecipitated with anti-p30 serum. This cleavage pattern is only in part imitated by in vitro cleavage of the isolated pr65gag with avian myeloblastosis virus (AMV) protease p15. However, in vitro incubation of isolated pr65gag with detergent-disrupted SSAV particles generated products identical in size to those found in vivo, i.e., proteins of 52 and 38 kDa and p30. The extent of cleavage is dependent on the concentration of the disrupted virions added to the incubation mixture. Studies with protease inhibitors suggest that the SSAV enzyme is a serine-type protease like that of other mammalian retroviruses and unlike the protease of avian viruses. The SSAV protease activity eluted from a molecular sieve column in a range of about 10-15 kDa reflecting the molecular weight of the murine leukemia virus (MuLV) protease (Mr = 13.5K). Thus, it appears that there is a close similarity between the proteolytic enzymes present in different mammalian retroviruses such as MuLV and SSAV.