Polykaryocytosis induced by vesicular stomatitis virus infection in BHK-21 cells

Optimization of production of vesicular stomatitis virus (VSV) in suspension serum-free culture medium at high cell density

During the last decade, oncolytic viruses such as vesicular stomatitis virus (VSV) have gained tremendous popularity as efficient vaccines for infectious diseases as well as for the treatment of cancer. Our laboratory has developed two stable cell lines, 293SF-3F6 (derived from HEK293A cells) and SF-BMAdR cells (a variant of A549 that expresses the E1 region of human adenovirus). These two cell lines were adapted to grow efficiently in suspension culture and in serum-free medium. In this report we evaluated the production of a recombinant VSV expressing the green fluorescent protein (VSV-GFP) in these two stable cell lines. At a relatively low cell density of 500,000 cells per ml, 293SF-3F6 produced 4.6 times more infectious particles than SF-BMAdR cells. There was a positive correlation between volumetric virus titer and cell density up to 2.E + 07 cells/ml. A fed-batch process using an in-house medium and feed was developed to support the growth of 293SF-3F6 cells up to a concentration of 1.E + 07 cells/ml for infection at higher cell density and VSV production at high titer. Shifting the temperature from 37 °C to 34 °C at infection time improved VSV titer up to 3.3 fold. After scaling up the optimal condition from small scale (3 ml) to larger volumes (50 & 200 ml), the maximal volumetric titer obtained using the 293SF-3F6 cells was in average 2.9E + 10 extracellular infectious particles/ml. In conclusion, our data demonstrated that 293SF-3F6 cells, for which a cGMP master cell bank is available, is a performant cell line to scale up VSV production in suspension culture using serum-free medium.

Neuroattenuation of vesicular stomatitis virus through picornaviral internal ribosome entry sites

Vesicular stomatitis virus (VSV) is potent and a highly promising agent for the treatment of cancer. However, translation of VSV oncolytic virotherapy into the clinic is being hindered by its inherent neurotoxicity. It has been demonstrated that selected picornaviral internal ribosome entry site (IRES) elements possess restricted activity in neuronal tissues. We therefore sought to determine whether the picornavirus IRES could be engineered into VSV to attenuate its neuropathogenicity. We have used IRES elements from human rhinovirus type 2 (HRV2) and foot-and-mouth disease virus (FMDV) to control the translation of the matrix gene (M), which plays a major role in VSV virulence. In vitro studies revealed slowed growth kinetics of IRES-controlled VSVs in most of the cell lines tested. However, in vivo studies explicitly demonstrated that IRES elements of HRV2 and FMDV severely attenuated the neurovirulence of VSV without perturbing its oncolytic potency.

Role of membrane phospholipids and glycolipids in cell-to-cell fusion by VSV

To identify membrane components of CER cells interacting with vesicular stomatitis virus (VSV) during fusion at acidic pH (fusion from without, FFWO) two different approaches have been used, i.e. (i) treating the whole cells with enzymes and (ii) testing the ability of isolated membrane molecules to interfere with FFWO. Phospholipase A2 and C digestion of cells greatly reduced syncytia formation, pointing towards the involvement of lipid structures as target sites for VSV. Cell susceptibility to FFWO was also reduced after neuraminidase, beta-galactosidase or periodate treatment, suggesting that carbohydrate residues may participate in a complex receptor structure required for virus fusion. When membrane molecules were examined separately for their ability to inhibit viral FFWO, phosphatidylserine, phosphatidylinositol, sphingomyelin, cholesterol and GM3 ganglioside were found to be active, confirming the role of membrane lipid moiety in the cell surface structures involved in the early phases of VSV infection.

Vesicular stomatitis virus-infected cells fuse when the intracellular pool of functional M protein is reduced in the presence of G protein

Five highly cytolytic strains of both Indiana and New Jersey serotypes of vesicular stomatitis virus were shown to induce cell fusion in BHK-21 and R(B77) cells. Inhibition of protein synthesis after the eclipse period of viral replication is a prerequisite for vesicular stomatitis virus-induced cell fusion. Pulse-chase experiments showed that inhibition of protein synthesis would lead to a drastic reduction in the intracellular pool of M protein as compared with other proteins. A temperature-sensitive mutant defective in M protein function (G31) was the only mutant of the five complementation groups to spontaneously induce polykaryocytes at the nonpermissive temperature. Previously, G protein has been shown to play a role in vesicular stomatitis virus-induced cell fusion. These results suggest that the combination of the presence of G protein on the virus-infected cell surface and the absence of functional M protein or a reduced level of intracellular M protein promotes cell fusion. On the basis of this study, we propose that vesicular stomatitis virus infection can induce cell fusion when the functional M protein pool declines to a critical level while G protein remains on the cell surface.

Requirement of carbohydrates for cell fusion induced by vesicular stomatitis virus

The fusion of BHK-21-KB cells by vesicular stomatitis virus was not induced in Eagle's minimal essential medium without glucose. In medium containing glucose, the rate of polykaryocyte formation decreased as the concentration of glucose was reduced below 5 mM. However, no reduction in virus production 24 hr after infection was seen under this condition. Addition of pyruvate or mannose to the culture medium caused a reversal of cell fusing activity. Cell fusion and virus growth were significantly suppressed by sodium azide and 2,4-dinitrophenol.

Effect of certain inhibitors of glycoprotein synthesis on cell fusion induced by vesicular stomatitis virus

The effect of certain metabolic inhibitors on the fusion of BHK-21 cells induced by vesicular stomatitis virus (VSV) was studied. The polykaryocyte formation in infected cells and virus growth were inhibited by 2-deoxy-D-glucose and D-glucosamine. Host-cell proteins synthesis was suppressed profoundly in both BHK-21-KB and B cells infected with VSV. On the other hand, glycoprotein synthesis was significantly enhanced during the polykaryocyte formation in BHK-21-KB cells, while it was suppressed in BHK-21-B cells which were not sensitive to cell fusion by VSV.

Comparative studies on cytopathic effects induced by vesicular stomatitis virus in two cell types

Infection of mouse L cells with vesicular stomatitis virus (VSV) leads to an extensive cell fusion, while porcine kidney stable (PS) cells infected with VSV show only cell rounding. Therefore, comparative morphological studies on the infection of the two cell lines were carried out using a transmission or scanning electron microscope and an immunofluorescence microscope. PS cells infected with VSV contrasted to L cells infected with the same virus in the following two points; (1) the principal site of VSV maturation was the intracytoplasmic vacuolar membrane in PS cells and the plasma membrane in L cells. However, it was found that viral glycoprotein was present on the cell surface of infected PS cells; (2) the morphological changes at the cell surface of infected PS cells occurred much earlier and were severer than those at the cell surface of infected L cells. From these observations, we discuss the possibility that the surfaceembrane of PS cells is too sensitive to the VSV-induced cell damage to cause cell fusion.