Search

The culture environment exerts a major effect on the glycosylation pattern of recombinant human interferon-gamma (IFN-gamma) produced by Chinese-hamster ovary (CHO) cells. The recombinant IFN-gamma is heterogeneous and consists of a mixture of fully (2N), partially (1N) and non-glycosylated (0N) glycoforms, and throughout batch cultures there is a decline in the proportion of fully glycosylated IFN-gamma. Glucose and glutamine, nutrients that are depleted early in such cultures, were prima facie candidates for causing such a shift in glycoform profile. Batch feeding of these nutrients did not prevent the decline in 2N glycoform, but the glycosylation pattern of IFN-gamma was affected by the initial glutamine concentration in the culture. Under different serum-free environments the extent of IFN-gamma glycosylation was affected by (1) the concentration of BSA, (2) the quality of BSA, (3) the lipid composition of the culture medium and (4) the presence of surfactants. Moreover, the inclusion of serum in cultures caused changes in the molecular masses of the major glycoforms, that was indicative of cleavage of the core polypeptide. The results reported emphasize the necessity of considering the effects of culture media on product quality as well as on product quantity during process optimization.

The role of bovine serum albumin in mammalian cell cultures and the possibility of its substitution by other components in a serum-free medium has been investigated. In this study, BSA was shown to be important for growth and product formation in CHO cells expressing recombinant human interferon-γ. There were indications that its stimulating growth effect was dependent on the source of BSA used and probably was related to the purification procedure used for the production of the desired albumin fraction. Cell growth did not occur in the absence of BSA but at low concentration (1 mg ml(-1)) it was stimulated by the addition of a combination of a commercial lipid mixture plus Pluronic F68. However, under the latter conditions IFN-γ production was adversely effected. The importance of individual lipid components was investigated using a statistical approach based on a Plackett-Burman design. Linoleic acid was identified as a positive variable for cell growth while cholesterol was identified as a negative variable for both cell growth and IFN-γ production. When a combination of linoleic acid plus Pluronic F68 was included in the formulation of low BSA medium, cell growth was similar to that at high BSA concentration (5 mg ml(-1)) but the IFN-γ concentration was significantly reduced (ca. 45%).

The physiology of a recombinant Chinese hamster ovary cell line in glucose-limited chemostat culture was studied over a range of dilution rates (D = 0.008 to 0.20 h(-1)). The specific growth rate (micro) deviated from D at low dilution rates due to an increased specific death rate. Extrapolation of these data suggested a minimum specific growth rate of 0.011 h(-1) (micro(max) = 0.025 h(-1)) The metabolism at each steady state was characterized by determining the metabolic quotients for glucose, lactate, ammonia, amino acids, and interferon-gamma (IFN-gamma). The specific rate of glucose uptake increased linearly with mu, and the saturation constant for glucose (K(s)) was calculated to be 59.6 microM. There was a linear increase in the rate of lactate production with a higher yield of lactate from glucose at high growth rates. The decline in the rate of production of lactate, alanine, and serine at low growth rate was consistent with the limitation of the glycolytic pathway by glucose. The specific rate of IFN-gamma production increased with mu in a manner indicative of a growth-related product. Despite changes in the IFN-gamma production rate and cell physiology, the pattern of IFN-gamma glycosylation was similar at all except the lowest growth rates where there was increased production of nonglycosylated IFN-gamma.

The importance of serum-free medium components on the growth of Chinese hamster ovary (CHO) cells and production of recombinant human interferon(IFN)-gamma was investigated. The complexity of the medium led to the adoption of a statistical optimization approach based on a Plackett-Burman design. From this analysis a set of nutritional components was identified as important for cell growth and recombinant protein production. Glycine was identified as an important determinant of specific growth rate, whereas for cell production bovine serum albumin (BSA), phenylalanine and tyrosine were also identified as important. BSA, sodium pyruvate, glutamate, methionine, proline, histidine, hydroxyproline, tyrosine and phenylalanine were shown to be important for IFN-gamma production. Other medium components, such as insulin, arginine, aspartate and serine produced an inhibitory effect on both cell growth and IFN-gamma production. The effect of the stimulatory nutrients as a whole group was tested by increasing their concentration in the medium. A significant improvement in specific cell growth rate, cell production and IFN-gamma production (up to 45%) was achieved on both shake-flask and fermentor cultures. An increase in the medium concentration of the negative variables had only a small inhibitory effect (approximately 10%) on the same parameters. Analysis of the effects of the group of stimulatory amino acids and BSA on CHO cell growth showed that the effect of the former was independent of BSA.

Factors affecting cell growth and antibody production in a mouse hybridoma were investigated. Antibody was produced during the growth and decline phases of a batch culture with an increase in the specific rate of antibody production during the decline phase. The specific rate of antibody production was also increased in cells arrested by 2 mM thymidine, suggesting that cell proliferation and antibody production can be uncoupled. Reduced serum concentrations resulted in lower cell growth rates but increased antibody production rates. However, this trend was reversed in hybridomas which had been arrested by thymidine, since the highest antibody production rate was associated with high serum concentrations. Likewise, in proliferating cells, the optimum pH for antibody production (pH 6.8) was lower than the optimum pH for cell growth (pH 7.2), whereas in thymidine-blocked cells, the highest antibody production rate was at pH 7.2. High antibody production rates and product yields were also associated with low growth rates in continuous cultures. The possibility that antibody was under cell cycle control was investigated in synchronized hybridoma cultures. Antibody production occurred during G1 and G2 with a decline in the M phase and evidence of a further decline in the S phase. Thus antibody production was not restricted to the G1 and S phase in this hybridoma.

A Chinese hamster ovary (CHO) cell line expressing recombinant human interferon-gamma (IFN-gamma) was grown under glucose limitation in a chemostate at a constant dilution rate of 0.015 h(-1) with glucose feed concentrations of 2.75 mM and 4.25 mM. The changes in cell concentration that accompanied changes in the glucose feed concentration indicated that the cells were glucose-limited. The cell yield on glucose remained constant, but there was a decline in residual glucose concentration and a reduced lactate yield from glucose in the latter stages of the culture. The consumption rates for many of the essential amino acids were increased later in the culture. The volumetric rate of interferon-gamma production was maintained throughout the course of this culture, indicating that IFN-gamma expression was stable under these conditions. However, the specific rate of IFN-gamma production was significantly lower at the higher glucose feed concentration. Under glucose limitation, the proportion of fully glycosylated IFN-gamma produced by these cells was less than that produced in the early stages of batch cultures. The proportion of fully glycosylated IFN-gamma increased during transient periods of glucose excess, suggesting that the culture environment influences the glycosylation of IFN-gamma.

Recombinant human interferon-gamma production by Chinese hamster ovary cells was restricted to the growth phase of batch cultures in serum-free medium. The specific interferon production rate was highest during the initial period of exponential growth but declined subsequently in parallel with specific growth rate. This decline in specific growth rate and interferon productivity was associated with a decline in specific metabolic activity as determined by the rate of glucose uptake and the rates of lactate and ammonia production. The ammonia and lactate concentrations that had accumulated by the end of the batch culture were not inhibitory to growth. Glucose was exhausted by the end of the growth phase but increased glucose concentrations did not improve the cell yield or interferon production kinetics. Analysis of amino acid metabolism showed that glutamine and asparagine were exhausted by the end of the growth phase, but supplementation of these amino acids did not improve either cell or product yields. When glutamine was omitted from the growth medium there was no cell proliferation but interferon production occurred, suggesting that recombinant protein production can be uncoupled from cell proliferation.

Recombinant human interferon-gamma (Hu-IFN-gamma) produced by Chinese-hamster ovary (CHO) cells was analysed by immunoprecipitation and SDS/PAGE. Up to twelve molecular-mass variants were secreted by this cell line. Three variants were recovered after enzymic removal of all N-linked oligosaccharides or when glycosylation was inhibited by tunicamycin. The presence of three polypeptide forms rather than a single form suggested that proteolytic cleavage had occurred at two sites in both the glycosylated and non-glycosylated forms. Proteolytically cleaved IFN-gamma was more prevalent in cell lysates than in the secreted glycoprotein. In common with naturally produced IFN-gamma, both fully glycosylated IFN-gamma (asparagine residues 28 and 100 occupied) and partially glycosylated product (thought to be substituted at position Asn28) were secreted. This was deduced from the Mr of the glycosylated products and the relative amounts of sialic acid expressed by each variant. In contrast with naturally produced IFN-gamma, non-glycosylated IFN-gamma was also secreted by the transfected CHO cells. When the cells were grown in batch culture in serum-free medium under pH and dissolved-oxygen control, the proportion of non-glycosylated IFN-gamma increased from 3 to 5% after 3 h, to 30% of the total IFN-gamma present after 195 h. This change in the proportion of glycosylated protein produced was not seen when metabolically labelled IFN-gamma was incubated for 96 h with cell-free supernatant from actively growing CHO cells. This implied that an alteration in intracellular glycosylation was occurring rather than a degradation of oligosaccharide side chains after secretion. The decrease in IFN-gamma glycosylation was independent of the glucose concentration in the culture medium, but could be related to specific growth and IFN-gamma production rates, as these declined steadily after 50 h of culture, in line with the increased production of non-glycosylated IFN-gamma.