Recombinant human interferon-gamma. Differences in glycosylation and proteolytic processing lead to heterogeneity in batch culture

Dissecting cell death pathways in fed-batch bioreactors

Chinese hamster ovary (CHO) cells are widely used for production of biologics including therapeutic monoclonal antibodies. Cell death in CHO cells is a significant factor in biopharmaceutical production, impacting both product yield and quality. Apoptosis has previously been described as the major form of cell death occurring in CHO cells in bioreactors. However, these studies were undertaken when less was known about non-apoptotic cell death pathways. Here, we report the occurrence of non-apoptotic cell death in an industrial antibody-producing CHO cell line during fed-batch culture. Under standard conditions, crucial markers of apoptosis were not observed despite a decrease in viability towards the end of the culture; only by increasing stress within the system did we observe caspase activation indicative of apoptosis. In contrast, markers of parthanatos and ferroptosis were observed during standard fed-batch culture, indicating that these non-apoptotic cell death pathways contribute to viability loss under these conditions. These findings pave the way for targeting non-conventional cell death pathways to improve viability and biologic production in CHO cells.

Insights into the quality of recombinant proteins produced by two different Bombyx mori expression systems

The silkworm, Bombyx mori, is an attractive host for recombinant protein production due to its high expression efficiency, quality, and quantity. Two expression systems have been widely used for recombinant protein production in B. mori: baculovirus/silkworm expression system and transgenic silkworm expression system. Both expression systems enable high protein production, but the qualities of the resulting recombinant proteins have not been well evaluated. In this study, we expressed bovine interferon γ (IFN-γ) using the two systems and examined the quality of the resulting proteins in terms of N-glycosylation and protein cleavage. Both expression systems successfully produced IFN-γ as an N-glycoprotein. Although the production in the baculovirus/silkworm expression system was much more efficient than that in the transgenic silkworm expression system, unexpected variants of IFN-γ were also produced in the former system due to the different N-glycosylation and C-terminal truncations. These results indicate that while high protein production could be achieved in the baculovirus/silkworm expression system, unintentional protein modification might occur, and therefore protein expression in the transgenic silkworm expression system is preferable from the point-of-view of N-glycosylation of the recombinant protein and evasion of unexpected attack by a protease in B. mori.

Fusion of a Novel Native Signal Peptide Enhanced the Secretion and Solubility of Bioactive Human Interferon Gamma Glycoproteins in Nicotiana benthamiana Using the Bamboo Mosaic Virus-Based Expression System

Plant viruses may serve as expression vectors for the efficient production of pharmaceutical proteins in plants. However, the downstream processing and post-translational modifications of the target proteins remain the major challenges. We have previously developed an expression system derived from Bamboo mosaic virus (BaMV), designated pKB19, and demonstrated its applicability for the production of human mature interferon gamma (mIFNγ) in Nicotiana benthamiana. In this study, we aimed to enhance the yields of soluble and secreted mIFNγ through the incorporation of various plant-derived signal peptides. Furthermore, we analyzed the glycosylation patterns and the biological activity of the mIFNγ expressed by the improved pKB19 expression system in N. benthamiana. The results revealed that the fusion of a native N. benthamiana extensin secretory signal (SS^Ext) to the N-terminal of mIFNγ (designated SS^Ext mIFNγ) led to the highest accumulation level of protein in intracellular (IC) or apoplast washing fluid (AWF) fractions of N. benthamiana leaf tissues. The addition of 10 units of 'Ser-Pro' motifs of hydroxyproline-O-glycosylated peptides (HypGPs) at the C-terminal end of SS^Ext mIFNγ (designated SS^Ext mIFNγ(SP)₁₀) increased the solubility to nearly 2.7- and 1.5-fold higher than those of mIFNγ and SS^Ext mIFNγ, respectively. The purified soluble SS^Ext mIFNγ(SP)₁₀ protein was glycosylated with abundant complex-type N-glycan attached to residues N⁵⁶ and N¹²⁸, and exhibited biological activity against Sindbis virus and Influenza virus replication in human cell culture systems. In addition, suspension cell cultures were established from transgenic N. benthamiana, which produced secreted SS^Ext mIFNγ(SP)₁₀ protein feasible for downstream processing. These results demonstrate the applicability of the BaMV-based vector systems as a useful alternative for the production of therapeutic proteins, through the incorporation of appropriate fusion tags.

Low glucose depletes glycan precursors, reduces site occupancy and galactosylation of a monoclonal antibody in CHO cell culture

Controlled feeding of glucose has been employed previously to enhance the productivity of recombinant glycoproteins but there is a concern that low concentrations of glucose could limit the synthesis of precursors of glycosylation. Here we investigate the effect of glucose depletion on the metabolism, productivity and glycosylation of a chimeric human-llama monoclonal antibody secreted by CHO cells. The cells were inoculated into media containing varying concentrations of glucose. Glucose depletion occurred in cultures with an initial glucose ≤5.5 mM and seeded at low density (2.5 × 10(5) cells/mL) or at high cell inoculum (≥2.5 × 10(6) cells/mL) at higher glucose concentration (up to 25 mM). Glucose-depleted cultures produced non-glycosylated Mabs (up to 51%), lower galactosylation index (GI <0.43) and decreased sialylation (by 85%) as measured by mass spectrometry and HPLC. At low glucose a reduced intracellular pool of nucleotides (0.03-0.23 fmoles/cell) was measured as well as a low adenylate energy charge (<0.57). Low glucose also reduced GDP-sugars (by 77%) and UDP-hexosamines (by 90%). The data indicate that under glucose deprivation, low levels of intracellular nucleotides and nucleotide sugars reduced the availability of the immediate precursors of glycosylation. These results are important when applied to the design of fed-batch cultures.

The availability of glucose to CHO cells affects the intracellular lipid-linked oligosaccharide distribution, site occupancy and the N-glycosylation profile of a monoclonal antibody

The glycosylation pattern of a chimeric heavy chain antibody (EG2) produced from CHO cells was affected by the glucose concentration (0-25mM) of cultures established at high density (>10(6)ml(-1)) over 24h. The resulting proportion of non-glycosylated Mab was directly correlated to the exposure time of cells to media depleted of glucose. Deprivation of glucose for the full 24h resulted in a 45% non-glycosylated Mab fraction. Analysis of steady state levels of intracellular lipid-linked oligosaccharides (LLOs) showed that under glucose limitation there was a reduction in the amount of full length LLO (Glc3Man9GlcNac2), with a concomitant increase in the smaller mannosyl-glycans (Man2-5GlcNAc2). Glycan microheterogeneity was quantified by galactosylation and sialylation indices (GI and SI) which showed a direct correlation to the cell specific glucose uptake. The GI increased to 0.83 following media supplementation with a cocktail of uridine, manganese and galactose. This is significantly higher than for a fully humanized antibody (DP12) produced under the similar conditions or for similar antibodies reported in the literature. The high GI of the chimeric antibody (EG2) may be due to its low molecular weight and unusual structure. These findings are important in relation to the low substrate that may occur in fed-batch cultures for Mab production.

Process characterization of hCTLA4Ig production in transgenic rice cell cultures using a 3-L bioreactor

Most of the technical know-how and experience of bioreactor engineering is applicable to plant cell cultures. In this study, transgenic rice cell cultures using RAmy3D promoter were used for the production of human cytotoxic T-lymphocyte antigen 4-immunoglobulin (hCTLA4Ig). In process aspect, the rice cells during production phase are strongly influenced by hydrodynamic stresses, such as shear stress and bubble burst. Therefore, the effects of agitation and aeration rates on cell growth and hCTLA4Ig production were investigated in a 3-L multi-bioreactor. By increasing over 240 rpm, the detrimental effects on cell growth and hCTLA4Ig production were observed. At an aeration rate of 0.3 vvm, relative cell viability sharply decreased 2 days earlier than those of lower aeration rates. In addition, it was confirmed that the specific yields and the specific productivity at 0.3 vvm were superior to those values at 0.05 vvm. Overall, higher aeration rate showed the improved hCTLA4Ig production in combination experiment. High aeration rates in general, however, have an undesired effect as excessive aeration was found to negatively affect the quality of hCTLA4Ig. Consequently, the hydrodynamic conditions must be tightly regulated during bioreactor operation in order to enhance hCTLA4Ig productivity and quality in transgenic rice cell cultures.

Fluorophore-labeled carbohydrate analysis of immunoglobulin fusion proteins: Correlation of oligosaccharide content with in vivo clearance profile

CTLA4 is a membrane receptor on cytotoxic T cells whose interaction with the B7 counterreceptor on B cells is important in alloantigen responses. Soluble recombinant human and murine CTLA4 were produced using either Chinese hamster ovary or NS-0 cell lines. Expression vectors were constructed containing the gene coding for the extracellular domain of CTLA4 fused to either human lgG1 hinge, CH2, and CH3 domains or murine lgG2a hinge, CH2, and CH3 domain genes. These glycoproteins were produced in hollow-fiber or packed-bed-type bioreactors and purified from conditioned media by protein A affinity chromatography. Batches of purified CTLA4lg were analyzed for size, composition, and isoelectric point (pl) patterns by standard protein methods; oligosaccharide and monosaccharide profiles using several carbohydrate specific techniques; and in vivo clearance profiles using a murine model. Significant differences were observed between lots in their pl, clearance, and crbohydrate profiles. Higher overall pl values correlated with accelerated alpha-phase clearance and changes in oligosaccharide composition as determined by lectin binding analysis and electrophoresis of fluorophore-conjugated carbohydrates. Preparations exhibiting slower clearance profiles had oligosaccharides with higher quantities of N-acetylneuraminic acid and were predominantly of an N-linked biantennary complex-type. Conversely, batches with accelerated clearance profiles had less detectable N-acetylneuraminic acid. Oligosaccharides from murine CTLA4lg produced in NS-0 cells had terminal N-glycolylneuraminic acid but no detectable N-acetylneuraminic acid and had concomitant accelerated clearance. These data suggest that the presence and quantity of N-acetylneuraminic acid is an important component in predicting CTLA4lg plasma clearance rates and that production lots can be analyzed for oligosaccharide heterogeneity and sialic acid content by electrophoresis of fluorophore-conjugated carbohydrates. (c) 1995 John Wiley & Sons, Inc.

Building high affinity human antibodies by altering the glycosylation on the light chain variable region in N-acetylglucosamine-supplemented hybridoma cultures

We attempted to improve antibody affinity by varying glycosylation on the light chain variable region. The human hybridoma line HB4C5 produces an antibody reactive to lung adenocarcinoma, which possess a N-glycosylated carbohydrate chain on the light chain hypervariable region. It has been shown that altering this carbohydrate structure can be accomplished by varying the level of N-acetylglucosamine in glucose free medium, a change in the carbohydrate chain could be induced which resulted in modifying antigen binding. By culturing the cells in media containing more than 20 mM N-acetylglucosamine, cells produced antibody with 10 fold improved affinity as compared with antibody produced in 20 mM glucose-containing medium. A newly induced light chain glycoform produced in the N-acetylglucosamine-containing medium was shown to be responsible for this antigen binding enhancement. Addition of glucose in the N-acetylglucosamine-containing media led to decreased antibody affinity and slightly inhibited production of a new light chain in a dose-dependent manner. Combination of 20 mM N-acetylglucosamine and 0.5 mM glucose gave a higher antibody production without the decrease of the antigen binding. These results indicate that optimization of N-glycosylation on the light chain, which leads to higher antigen binding, can be accomplished by adjusting a ratio of glucose and N-acetylglucosamine in the culture medium.

N-linked glycosylation of GP5 of porcine reproductive and respiratory syndrome virus is critically important for virus replication in vivo

It has been proposed that the N-linked glycan addition at certain sites in GP5 of porcine reproductive and respiratory syndrome virus (PRRSV) is important for production of infectious viruses and viral infectivity. However, such specific N-linked glycosylation sites do not exist in some field PRRSV isolates. This implies that the existence of GP5-associated glycan per se is not vital to the virus life cycle. In this study, we found that mutation of individual glycosylation sites at N30, N35, N44, and N51 in GP5 did not affect virus infectivity in cultured cells. However, the mutants carrying multiple mutations at N-linked glycosylation sites in GP5 had significantly reduced virus yields compared with the wild-type (wt) virus. As a result, no viremia and antibody response were detected in piglets that were injected with a mutant without all N-linked glycans in GP5. These results suggest that the N-linked glycosylation of GP5 is critically important for virus replication in vivo. The study also showed that removal of N44-linked glycan from GP5 increased the sensitivity of mutant virus to convalescent-phase serum samples but did not elicit a high-level neutralizing antibody response to wt PRRSV. The results obtained from the present study have made significant contributions to better understanding the importance of glycosylation of GP5 in the biology of PRRSV.

Enhanced erythropoietin heterogeneity in a CHO culture is caused by proteolytic degradation and can be eliminated by a high glutamine level

The molecular heterogeneity of recombinant humanerythropoietin (EPO) increased during the course of abatch culture of transfected Chinese hamster ovary(CHO) cells grown in serum-free medium. This wasshown by both an increased molecular weight and pIrange of the isolated EPO at the end of the culture. However, analysis of the N-glycan structures of themolecule by fluorophore-assisted carbohydrateelectrophoresis (FACE) and HPLC anion exchangechromatography indicated a consistent pattern ofglycosylation. Seven glycoforms were identified, thepredominant structure being a fully sialylatedtetra-antennary glycan. The degree of sialylationwas maintained throughout the culture. Analysis ofthe secreted EPO indicated a time-dependent increasein the molecular weight band width of the peptideconsistent with proteolytic degradation. A highglutamine concentration (16-20 mM) in the culturedecreased the apparent degradation of the EPO.

Expression of recombinant human interferon-γ with antiviral activity in the bi-cistronic baculovirus-insect/larval system

A bi-cistronic baculovirus-insect/larval system containing a polyhedron promoter, an internal ribosome entry site (IRES), and an egfp gene was developed as a cost-effective platform for the production of recombinant human interferon gamma (rhIFN-γ). There was no significant difference between the amounts of rhIFN-γ produced in the baculovirus-infected Spodoptera frugiferda 21 cells grown in serum-free medium and the serum-supplemented medium, while the Trichoplusia ni (T. ni) and Spodoptera exigua (S. exigua) larvae afforded rhIFN-γ amounting to 1.08±0.04 and 9.74±0.35 µg/mg protein respectively. The presence of non-glycosylated and glycosylated rhIFN-γ was confirmed by immunoblot and lectin blot. The immunological activity of purified rhIFN-γ, with 96% purity by Nickel (II)-nitrilotriacetic acid (Ni-NTA) affinity chromatography, was similar to that commercially available. Moreover, the rhIFN-γ protein from T. ni had more potent antiviral activity. These findings suggest that this IRES-based expression system is a simple and inexpensive alternative for large-scale protein production in anti-viral research.

Protein glycosylation control in mammalian cell culture: past precedents and contemporary prospects

Protein glycosylation is a post-translational modification of paramount importance for the function, immunogenicity, and efficacy of recombinant glycoprotein therapeutics. Within the repertoire of post-translational modifications, glycosylation stands out as having the most significant proven role towards affecting pharmacokinetics and protein physiochemical characteristics. In mammalian cell culture, the understanding and controllability of the glycosylation metabolic pathway has achieved numerous successes. However, there is still much that we do not know about the regulation of the pathway. One of the frequent conclusions regarding protein glycosylation control is that it needs to be studied on a case-by-case basis since there are often conflicting results with respect to a control variable and the resulting glycosylation. In attempts to obtain a more multivariate interpretation of these potentially controlling variables, gene expression analysis and systems biology have been used to study protein glycosylation in mammalian cell culture. Gene expression analysis has provided information on how glycosylation pathway genes both respond to culture environmental cues, and potentially facilitate changes in the final glycoform profile. Systems biology has allowed researchers to model the pathway as well-defined, inter-connected systems, allowing for the in silico testing of pathway parameters that would be difficult to test experimentally. Both approaches have facilitated a macroscopic and microscopic perspective on protein glycosylation control. These tools have and will continue to enhance our understanding and capability of producing optimal glycoform profiles on a consistent basis.

Liquid chromatographic profiling of monosaccharide concentrations in complex cell-culture media and fermentation broths

A solid phase extraction, liquid chromatography and fluorescence (SPE-RPLC-FL) based protocol for the determination of free monosaccharides in highly complex raw material powders and formulated fermentation feedstocks and broths has been developed. Monosaccharides within sample extracts were derivatised pre-column with anthranilic acid and the derivatives separated using reversed-phase LC with fluorescence detection. Using a 2.1 mm × 50 mm 1.8 µm Zorbax Eclipse XDB-C18 column, a flow rate of 0.4 mL min-1 and an acetonitrile gradient in a sodium acetate buffer (pH 4.3; 50 mmol L-1) the baseline resolution of glucosamine, mannosamine, galactosamine, galactose, mannose, glucose, ribose, xylose, fucose and sialic acid within 20 minutes was achieved. Pre-column derivatisation involved combining a 30 mg mL-1 solution of anthranilic acid in a 1 : 1 ratio with an aqueous standard prior to injection. Standard analytical performance criteria were used for evaluation purposes, with the method found to exhibit LOD's as low as 10 fmol, and be linear and precise (%RSD < 2.2% (n = 7). The method was applied to the analysis of a range of highly complex biopharmaceutical production samples, including yeastolate powders, chemically defined media and in-process fermentation broth samples. Sample preparation involved passing an aqueous sample through a C18 solid phase extraction cartridge to trap hydrophobic peptides and vitamins, with recovery of all test sugars exceeding 90%. Finally, standard statistical analysis was performed on samples taken from different lots in order to estimate lot-to-lot variability.

Glucose-limited chemostat culture of Chinese hamster ovary cells producing recombinant human interferon-gamma

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.

Glycosylation: An intrinsic sign of "danger"

The "danger" model of immunity posits that the immune system is triggered by endogenous danger signals, rather than exogenous non-self signals per se. It has been proposed that danger signals may consist of both intracellular "pre-packed" molecules released from damaged cells and stress-induced proteins. Here we focus on glycosylation aberrancies as a unifying concept for danger signaling. According to this proposition glycosylation patterns reliably reflect cellular phenotypic state and appearance of altered carbohydrate structures may constitute a pivotal phenotypic alteration that alarms the immune system to danger and initiates immunity. Viewed from this vantage point, healthy cells avert immune recognition by virtue of their normal terminal glycosylation patterns. By contrast, abnormal cells display and release glycoproteins and glycolipids with aberrant terminal glycosylation trees, which in turn immunologically flag these cells. Diverse carbohydrate-binding receptors are expressed on immune cells and are used to detect these phenotypic changes. Thus, in addition to the "pre-packed" and stress-induced signals this glycosylation-based signal represents an endogenous signal reliably reflecting the cell phenotypic status, enabling the immune system to monitor the tissue/cell's physical condition and to respond accordingly.

Temperature Reduction in Cultures of hGM-CSF-expressing CHO Cells: Effect on Productivity and Product Quality

We have demonstrated that temperature reduction from 37 to 33 degrees C in the culture of a CHO cell line producing recombinant human granulocyte macrophage colony stimulating factor (CHO-K1-hGM-CSF) leads to a reduced growth rate, increased cell viability, improved cellular productivity, and decreased cell metabolism. In the present study, CHO-K1-hGM-CSF cells were cultured in a biphasic mode: first, a 37 degrees C growth phase for achieving a high cell number, followed by a production phase where the culture temperature was shifted to 33 degrees C. The maximum cell density was not affected after temperature reduction while cell viability remained above 80% for a further 3.7 days in the culture kept at the lower temperature, when compared to the control culture maintained at 37 degrees C. Furthermore, the total rhGM-CSF production increased 6 times in the culture shifted to 33 degrees C. Because the quality and hence the in vivo efficacy of a recombinant protein might be affected by numerous factors, we have analyzed the N- and O-glycosylation of the protein produced under both cell culture conditions using high-pH anion-exchange chromatography and complementary mass spectrometry techniques. The product quality data obtained from the purified protein preparations indicated that decreasing temperature had no significant effect on the rhGM-CSF glycosylation profiles, including the degree of terminal sialylation. Moreover, both preparations exhibited the same specific in vitro biological activity. These results revealed that the employed strategy had a positive effect on the cell specific productivity of CHO-K1-hGM-CSF cells without affecting product quality, representing a novel procedure for the rhGM-CSF production process.

Influence of intracellular nucleotide and nucleotide sugar contents on recombinant interferon-gamma glycosylation during batch and fed-batch cultures of CHO cells

Both the macroheterogeneity of recombinant human IFN-gamma produced by CHO cells and intracellular levels of nucleotides and sugar nucleotides, have been characterized during batch and fed-batch cultures carried out in different media. Whereas PF-BDM medium was capable to maintain a high percentage of the doubly- glycosylated glycoforms all over the process, mono-glycosylated and non-glycosylated forms increased during the batch culture using SF-RPMI medium. Intracellular level of UTP was higher in PF-BDM all over the batch culture compared to the SF-RPMI process. UDP-Gal accumulated only during the culture performed in PF-BDM medium, probably as a consequence of the reduced UDP-Glc synthesis flux in SF-RPMI medium. When the recombinant CHO cells were cultivated in fed-batch mode, the UTP level remained at a relatively high value in serum-containing RPMI and its titer increased during the fed-phase indicating an excess of biosynthesis. Besides, an accumulation of UDP-Gal occurred as well. Those results all together indicate that UTP and UDP-Glc syntheses in CHO cells cultivated in SF-RPMI medium in batch process, could be limiting during the glycosylation processes of the recombinant IFN-gamma. At last, the determination of the energetic status of the cells over the three studied processes suggested that a relationship between the adenylate energy charge and the glycosylation macroheterogeneity of the recombinant IFN-gamma may exist.

An on-line method for the reduction of fouling of spin-filters for animal cell perfusion cultures

The main limitation in the use of spin-filters during perfusion cultures of animal cells was revealed to be filter fouling. This phenomenon involves cell-sieve interactions as well as cell attachment to, and growth on, the filter surface. The cell attachment effect has been analysed in the present study during long-term perfusion simulations with CHO animal cells. It was demonstrated that at low filter acceleration, below 6.2 m/s2, a high perfusion rate of 25 cm/h induced rapid filter pore clogging within 3 days, whereas increasing the filter acceleration to 25 m/s2 increased filter longevity from 3 to 25 days, for filters with a pore size of 8.5 microm. Increasing the filter pore size to 14.5 microm improved filter longevity by 84% with less viable and dead cell deposits on the filter surface. However, it was demonstrated that filter longevity was not necessarily dependent on the amount of cell deposit on the filter surface. In the second part of this study, ultrasonic technology was used to reduce filter fouling. Filter vibration, induced by a piezo actuator, improved filter longevity by 113% during CHO cells perfusion cultures.

Optimisation of the cellular metabolism of glycosylation for recombinant proteins produced by Mammalian cell systems

Many biopharmaceuticals are now produced as secreted glycoproteins from mammalian cell culture. The glycosylation profile of these proteins is essential to ensure structural stability and biological and clinical activity. However, the ability to control the glycosylation is limited by our understanding of the parameters that affect the heterogeneity of added glycan structures. It is clear that the glycosylation process is affected by a number of factors including the 3-dimensional structure of the protein, the enzyme repertoire of the host cell, the transit time in the Golgi and the availability of intracellular sugar-nucleotide donors. From a process development perspective there are many culture parameters that can be controlled to enable a consistent glycosylation profile to emerge from each batch culture. A further, but more difficult goal is to control the culture conditions to enable the enrichment of specific glycoforms identified with desirable biological activities. The purpose of this paper is to discuss the cellular metabolism associated with protein glycosylation and review the attempts to manipulate, control or engineer this metabolism to allow the expression of human glycosylation profiles in producer lines such as genetically engineered Chinese hamster ovary (CHO) cells.

Animal cell cultures: recent achievements and perspectives in the production of biopharmaceuticals

There has been a rapid increase in the number and demand for approved biopharmaceuticals produced from animal cell culture processes over the last few years. In part, this has been due to the efficacy of several humanized monoclonal antibodies that are required at large doses for therapeutic use. There have also been several identifiable advances in animal cell technology that has enabled efficient biomanufacture of these products. Gene vector systems allow high specific protein expression and some minimize the undesirable process of gene silencing that may occur in prolonged culture. Characterization of cellular metabolism and physiology has enabled the design of fed-batch and perfusion bioreactor processes that has allowed a significant improvement in product yield, some of which are now approaching 5 g/L. Many of these processes are now being designed in serum-free and animal-component-free media to ensure that products are not contaminated with the adventitious agents found in bovine serum. There are several areas that can be identified that could lead to further improvement in cell culture systems. This includes the down-regulation of apoptosis to enable prolonged cell survival under potentially adverse conditions. The characterization of the critical parameters of glycosylation should enable process control to reduce the heterogeneity of glycoforms so that production processes are consistent. Further improvement may also be made by the identification of glycoforms with enhanced biological activity to enhance clinical efficacy. The ability to produce the ever-increasing number of biopharmaceuticals by animal cell culture is dependent on sufficient bioreactor capacity in the industry. A recent shortfall in available worldwide culture capacity has encouraged commercial activity in contract manufacturing operations. However, some analysts indicate that this still may not be enough and that future manufacturing demand may exceed production capacity as the number of approved biotherapeutics increases.

Effect of culture pH on erythropoietin production by Chinese hamster ovary cells grown in suspension at 32.5 and 37.0°C

To investigate the effect of culture pH in the range of 6.85-7.80 on cell growth and erythropoietin (EPO) production at 32.5 and 37.0 degrees C, serum-free suspension cultures of recombinant CHO cells (rCHO) were performed in a bioreactor with pH control. Lowering culture temperature from 37.0 to 32.5 degrees C suppressed cell growth, but cell viability remained high for a longer culture period. Regardless of culture temperature, the highest specific growth rate (mu) and maximum viable cell concentration were obtained at pH values of 7.00 and 7.20, respectively. Like mu, the specific consumption rates of glucose and glutamine decreased at 32.5 degrees C compared to 37.0 degrees C. In addition, they increased with increasing culture pH. Culture pH at 32.5 degrees C affected specific EPO productivity (q(EPO)) in a different fashion from that at 37 degrees C. At 37 degrees C, the q(EPO) was fairly constant in the pH range of 6.85-7.80, while at 32.5 degrees C, the q(EPO) was significantly influenced by culture pH. The highest q(EPO) was obtained at pH 7.00 and 32.5 degrees C, and its value was approximately 1.5-fold higher than that at pH 7.00 and 37.0 degrees C. The proportion of acidic EPO isoforms, which is a critical factor for high in vivo biological activity of EPO, was highest in the stationary phase of growth, regardless of culture temperature and pH. Although cell viability rapidly decreased in death phase at both 32.5 and 37.0 degrees C, the significant degradation of produced EPO, probably by the action of proteases released from lysed cells, was observed only at 37.0 degrees C. Taken together, through the optimization of culture temperature and pH, a 3-fold increase in maximum EPO concentration and a 1.4-fold increase in volumetric productivity were obtained at pH 7.00 and 32.5 degrees C when compared with those at 37.0 degrees C. These results demonstrate the importance of optimization of culture temperature and pH for enhancing EPO production in serum-free, suspension culture of rCHO cells.

NMR characterization of the interaction between the C-terminal domain of interferon-gamma and heparin-derived oligosaccharides

Interferons are cytokines that play a complex role in the resistance of mammalian hosts to pathogens. IFNgamma (interferon-gamma) is secreted by activated T-cells and natural killer cells. IFNgamma is involved in a wide range of physiological processes, including antiviral activity, immune response, cell proliferation and apoptosis, as well as the stimulation and repression of a variety of genes. IFNgamma activity is modulated by the binding of its C-terminal domain to HS (heparan sulphate), a glycosaminoglycan found in the extracellular matrix and at the cell surface. In the present study, we analysed the interaction of isolated heparin-derived oligosaccharides with the C-terminal peptide of IFNgamma by NMR, in aqueous solution. We observed marked changes in the chemical shifts of both peptide and oligosaccharide compared with the free state. Our results provide evidence of a binding through electrostatic interactions between the charged side chains of the protein and the sulphate groups of heparin that does not induce specific conformation of the C-terminal part of IFNgamma. Our data also indicate that an oligosaccharide size of at least eight residues displays the most efficient binding.

Glycation and post-translational processing of human interferon-gamma expressed in Escherichia coli

Until recently, nonenzymatic glycosylation (glycation) was thought to affect the proteins of long living eukaryotes only. However, in a recent study (Mironova, R., Niwa, T., Hayashi, H., Dimitrova, R., and Ivanov, I. (2001) Mol. Microbiol. 39, 1061-1068), we have shown that glycation takes place in Escherichia coli as well. In the present study, we demonstrate that the post-translational processing (proteolysis and covalent dimerization) observed with cysteineless recombinant human interferon-gamma (rhIFN-gamma) is tightly associated with its in vivo glycation. Our results show that, at the time of isolation, rhIFN-gamma contained early (but not advanced) glycation products. Using reverse phase high performance liquid chromatography in conjunction with fluorescence measurements, enzyme-linked immunosorbent assay, and mass spectrometry, we found that advanced glycation end products arose in rhIFN-gamma during storage. The latter were identified mainly in the Arg/Lys-rich C terminus of the protein, which was also the main target of proteolysis. Mass spectral analysis and N-terminal sequencing revealed four major (Arg140/Arg141, Phe137/Arg138, Met135/Leu136, and Lys131/Arg132) and two minor (Lys109/Ala110 and Arg90/Asp91) cleavage sites in this region. Tryptic peptide mapping indicated that the covalent dimers of rhIFN-gamma originating during storage were formed mainly by lateral cross-linking of the monomer subunits. Antiviral assay showed that proteolysis lowered the antiviral activity of rhIFN-gamma, whereas covalent dimerization completely abolished it.

Effects of buffering conditions and culture pH on production rates and glycosylation of clinical phase I anti-melanoma mouse IgG3 monoclonal antibody R24

R24, a mouse IgG3 monoclonal antibody (MAb) against ganglioside GD3 (Neu5Acalpha8Neu5Acalpha3Gal beta4Glcbeta1Cer), can block tumor growth as reported in a series of clinical trials in patients with metastatic melanoma. The IgG molecule basically contains an asparagine-linked biantennary complex type oligosaccharide on the C(H)2 domain of each heavy chain, which is necessary for its in vivo effector function. The purpose of this study was to investigate the biotechnological production and particularly the glycosylation of this clinically important MAb in CO(2)/HCO(3) (-) (pH 7.4, 7.2, and 6.9) and HEPES buffered serum-free medium. Growth, metabolism, and IgG production of hybridoma cells (ATCC HB-8445) were analyzed on a 2-L bioreactor scale using fed-batch mode. Specific growth rates (mu) and MAb production rates (q(IgG)) varied significantly with maximum product yields at pH 6.9 (q(IgG) = 42.9 microg 10(-6) cells d(-1), mu = 0.30 d(-1)) and lowest yields in pH 7.4 adjusted batches (q(IgG) = 10.8 microg 10(-6) cells d(-1), mu = 0.40 d(-1)). N-glycans were structurally characterized by high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray-ionization quadrupole time-of-flight (ESI-QTOF) mass spectrometry (MS). The highest relative amounts of agalacto and monogalacto biantennary complex type oligosaccharides were detected in the pH 7.2 (46% and 38%, respectively) and pH 6.9 (44% and 40%, respectively) cultivations and the uppermost quantities of digalacto (fully galactosylated) structures in the pH 7.4 (32%) and the HEPES (26%) buffered fermentation. In the experiments with HEPES buffering, antibodies with a molar Neu5Ac/Neu5Gc ratio of 3.067 were obtained. The fermentations at pH 7.2 and 6.9 resulted in almost equal molar Neu5Ac/Neu5Gc ratios of 1.008 and 0.985, respectively, while the alkaline shift caused a moderate overexpression of Neu5Ac deduced from the Neu5Ac/Neu5Gc quotient of 1.411. Different culture buffering gave rise to altered glycosylation pattern of the MAb R24. Consequently, a detailed molecular characterization of MAb glycosylation is generally recommended as a part of the development of MAbs for targeted in vivo immunotherapy to assure biochemical consistency of product lots and oligosaccharide-dependent biological activity.

FORTIFICATION OF A PROTEIN-FREE CELL CULTURE MEDIUM WITH PLANT PEPTONES IMPROVES CULTIVATION AND PRODUCTIVITY OF AN INTERFERON-γ–PRODUCING CHO CELL LINE

A strong tendency is currently emerging to remove not only serum but also any product of animal origin from animal cell culture media during production of recombinant proteins. This should facilitate downstream processing and improve biosafety. One way consists in the fortification of protein-free nutritive media with plant protein hydrolysates. To investigate the effects of plant peptones on mammalian cell cultivation and productivity, CHO 320 cells, a clone of CHO K1 cells genetically modified to secrete human interferon-gamma (IFN-gamma), were first adapted to cultivation in suspension in a protein-free medium. Both cell growth and IFN-gamma secretion were found to be equivalent to those reached in serum-containing medium. Eight plant peptones, selected on the basis of their content in free amino acids and oligopeptides, as well as molecular weight distribution of oligopeptides, were tested for their ability to improve culture parameters. These were improved in the presence of three peptones, all having an important fraction of oligopeptides ranging from 1 to 10 kDa and a small proportion of peptides higher than 10 kDa. These peptones do not seem to add significantly to the nutritive potential to basal protein-free nutritive medium. Nevertheless, supplementation of an oligopeptide-enriched wheat peptone improved cell growth by up to 30% and IFN-gamma production by up to 60% in shake-flask experiments. These results suggest that the use of plant peptones with potential growth factor-like or antiapoptotic bioactivities could improve mammalian cell cultivation in protein-free media while increasing the product biosafety.

Expression of recombinant cytoplasmic yeast pyruvate carboxylase for the improvement of the production of human erythropoietin by recombinant BHK-21 cells

Recently, a recombinant yeast pyruvate carboxylase expressed in the cytoplasm of BHK-21 cells was shown to reconstitute the missing link between glycolysis and TCA, thus increasing the flux of glucose into the TCA and resulting in a higher intracellular ATP content. Now, these metabolically engineered cells have been additionally transfected with a plasmid bearing the gene for human erythropoietin. EPO yield and substrate-specific productivity of the recombinant BHK-21 cells have been compared to control cells without the PYC2-gene but transfected with the plasmid coding for the expression of the selection genes and EPO. PYC2-expressing clones showed a 2-fold higher glucose-specific productivity and a 2-fold higher product concentration in a continuously perfused bioreactor. Moreover, the PYC2 expression enabled the cells to become more resistant to low glucose concentrations in the culture medium. They could produce at nearly maximum productivity under glucose-limiting conditions of 0.05-1 gl(-1) that guaranteed a reduced accumulation of lactate in fed-batch production systems. Due to the fact that PYC2-expressing cells are characterized by reduced glucose consumption, a prolonged production phase in bioreactors can be maintained. Based on the demand not to fall short of 80% cell viability for the production, EPO could be produced for 2 days (30%) longer compared to the control due to a more economic exploitation of glucose, and the prolonged viability period of the cells using a batch cultivation driven by glutamine limitation.

Involvement of conserved hydrophobic residues in the CTLD of human lectin-like oxidized LDL receptor in ligand binding

We previously identified the hydrophilic residues that are essential for ligand binding in the C-type lectin-like domain (CTLD) of human lectin-like oxidized LDL receptor (hLOX-1). To provide a more detailed understanding of ligand binding, we selected in the present study 13 conserved hydrophobic residues in the CTLD of hLOX-1 for mutagenesis analysis. The selected residues were replaced either by Ser (drastic mutation) or by size- and structure-based alternative hydrophobic residues (conserved mutation). Mutation targeted at F228, Y238, and G232 deprived hLOX-1 of ligand binding without alteration of protein expression and localization. In contrast, drastic mutation introduced into positions W203, W215, and W217 resulted in mislocalization, whereas conserved mutation at the same sites resulted in clones with similar cell surface localization and ligand binding to native hLOX-1. Our results indicate that F228, Y238, and G232 are essential for ligand binding, while W203, W215, W217, and L206 play a structural role.

Characterization of residues and sequences of the carbohydrate recognition domain required for cell surface localization and ligand binding of human lectin-like oxidized LDL receptor

Lectin-like oxidized low-density lipoprotein receptor (LOX-1) has been cloned from human aortic endothelial cells, and has a sequence identical to that from human lung. Previous studies showed that human LOX-1 can recognize modified LDL, apoptotic cells and bacteria. To further explore the relationship between the structure and function of LOX-1, a mutagenesis study was carried out. Our results showed that the carbohydrate recognition domain (CRD) was the ligand-binding domain of human LOX-1. We also investigated the sequences and residues in CRD that were essential for protein cell surface localization and ligand binding. LOX-1s carrying a mutation on each of six Cys in CRD resulted in a variety of N-glycosylation and failed to be transported to the cell surface. This was strong evidence for the involvement of all six Cys in the intrachain disulfide bonds required for proper folding, processing and transport of LOX-1. The C-terminal sequence (KANLRAQ) was also essential for protein folding and transport, while the four final residues (LRAQ) were involved in maintaining receptor function. Both positive charged (R208, R209, H226, R229 and R231) and non-charged hydrophilic (Q193, S198, S199 and N210) residues were involved in ligand binding, suggesting that ligand recognition of LOX-1 is not merely dependent on the interaction of positively charged residues with negatively charged ligands.

Multiple cell culture factors can affect the glycosylation of Asn-184 in CHO-produced tissue-type plasminogen activator

Human tissue-type plasminogen activator (t-PA) contains a variably occupied glycosylation site at Asn-184 in naturally produced t-PA and in t-PA produced in recombinant Chinese hamster ovary (CHO) cells. The presence of an oligosaccharide at this site has previously been shown to reduce specific activity and fibrin binding. In this report, the site occupancy of t-PA is shown to increase gradually over the course of batch and fed-batch CHO cultures. Additional cell culture factors, including butyrate and temperature, are also shown to influence the degree of glycosylation. In each of these cases, conditions with decreased growth rate correlate with increased site occupancy. Investigations using quinidine and thymidine to manipulate the cell cycle distribution of cultures further support this correlation between site occupancy and growth state. Comparison of the cell cycle distribution across the range of cell culture factors investigated shows a consistent relationship between site occupancy and the fraction of cells in the G(0)/G(1) phase of the cell cycle. These results support a correlation between growth state and site occupancy, which fundamentally differs from site occupancy trends previously observed and illustrates the importance of the growth profile of CHO cultures in producing consistently glycosylated recombinant glycoproteins.

Adaptation of mammalian cells to growth in serum-free media

A three-step protocol is described for adapting an anchorage-dependent, serum-dependent recombinant mammalian cell lineage to high density serum-free suspension culture. The objective is a cell lineage that is well-suited for the manufacture of a recombinant protein. The first step of the protocol generates an anchorage-independent cell lineage by culturing trypsin-treated cells in spinner flasks using serum-containing medium. The second step adapts the lineage to serum-free medium through a series of serum reduction steps in the presence of defined growth-promoting additives. The third step adapts the lineage to high-cell-density conditions by culturing the cells in a bioreactor in a manner that allows development of tolerance to growth-inhibiting substances released by the cells. Examples are presented for the use of this protocol for recombinant CHO cells.

Tetracycline-controlled expression of glycosylated porcine interferon-gamma in mammalian cells

Tetracycline-controlled expression plasmids that allow inducible expression of proteins in mammalian cells (Gossen & Bujard, 1992), have been used to express porcine interferon-gamma in the RK-13 rabbit kidney cell line. Following neomycin selection, stable clones produced recombinant, glycosylated porcine interferon-gamma (rGPoIFN-gamma) only after removal of tetracycline (Tc). Southern blot analysis of one clone showed that approximately 50 copies of IFN-gamma cDNA were present in the cell genome. In the absence of Tc, stable clones secreted large amounts of rGPoIFN-gamma (up to 16 microg/ml) into the medium supplemented with 10% FCS and high glucose concentration. Molecular weight comparison of 35S-Methionine, labelled rGPoIFN-gamma with natural leukocytic IFN-gamma after immunoprecipitation, revealed 4 major glycoforms with apparent Mr of 27,000; 25,000; 20,000 and 18,500, that are almost identical in both IFN-gamma species. In both cases, all 4 glycoforms resolved into 2 polypeptide monomers with apparent Mr of 16,500 and 14,500 upon deglycosylation with N-glycosydase F. The biological activity of rGPoIFN-gamma was in the same range as that of natural leukocytic PoIFN-gamma (2 x 10(6) U/mg). Eventually, this recombinant mammalian IFN-gamma should constitute a very useful substitute for leukocyte PoIFN-gamma in in vitro or in vivo experiments.

Disparate intracellular processing of human IL-12 preprotein subunits: atypical processing of the P35 signal peptide

IL-12 is a heterodimeric cytokine produced by APC that critically regulates cell-mediated immunity. Because of its crucial function during immune responses, IL-12 production is stringently regulated, in part through transcriptional control of its p35 subunit, which requires the differentiative effects of IFN-gamma for expression. To determine whether post-transcriptional aspects of IL-12 production might be regulated, we examined intracellular protein processing of each subunit. We report here that p40 and p35 subunits are processed by disparate pathways. Whereas processing of p40 conforms to the cotranslational model of signal peptide removal concomitant with translocation into the endoplasmic reticulum (ER), processing of p35 does not. Translocation of the p35 preprotein into the ER was not accompanied by cleavage of the signal peptide; rather, removal of the p35 signal peptide occurred via two sequential cleavages. The first cleavage took place within the ER, and the cleavage site localized to the middle of the hydrophobic region of the signal peptide. Although the preprotein was glycosylated upon entry into the ER, its glycosylation status did not affect primary cleavage. Subsequently, the remaining portion of the p35 signal peptide was removed by a second cleavage, possibly involving a metalloprotease, concomitant with additional glycosylation and secretion. Secretion could be inhibited by mutation of the second cleavage site or by inhibition of glycosylation with tunicamycin. In contrast, p40 secretion was not affected by inhibition of glycosylation. Our findings demonstrate that IL-12 subunits are processed by disparate pathways and suggest new modalities for regulation of IL-12 production.

Engineering Chinese hamster ovary cells to maximize sialic acid content of recombinant glycoproteins

We have engineered two Chinese hamster ovary cell lines secreting different recombinant glycoproteins to express high levels of human beta1,4-galactosyltransferase (GT, E.C. 2.4.1.38) and/or alpha2, 3-sialyltransferase (ST, E.C. 2.4.99.6). N-linked oligosaccharide structures synthesized by cells overexpressing the glycosyltransferases showed greater homogeneity compared with control cell lines. When GT was overexpressed, oligosaccharides terminating with GlcNAc were significantly reduced compared with controls, whereas overexpression of ST resulted in sialylation of >/=90% of available branches. As expected, GT overexpression resulted in reduction of oligosaccharides terminating with GlcNAc, whereas overexpression of ST resulted in sialylation of >/=90% of available branches. The more highly sialylated glycoproteins had a significantly longer mean residence time in a rabbit model of pharmacokinetics. These experiments demonstrate the feasibility of genetically engineering cell lines to produce therapeutics with desired glycosylation patterns.

Sequence pattern for the occurrence of N-glycosylation in proteins

To further understand the occurrence of N-glycosylation, 21 nonhomologous proteins with Asn-x-Ser/Thr sequence were investigated. The results showed that some oligopeptides with Gly residues (G-x-y or y-x-G) are adjacent to the N-glycosylated sequences. These oligopeptides are not only essential for the structure and function of the proteins, but they are also found to be often proteolytic processing sites. These properties suggest that these oligopeptides may be a "sequence pattern" for the occurrence of N-glycosylation. The implications of the findings for protein structure and function are discussed.

Relationship between recombinant activated protein C secretion rates and mRNA levels in baby hamster kidney cells

Analysis of 12 baby hamster kidney (BHK) clones in exponential growth revealed a linear relationship between cell-specific recombinant activated protein C (APC) production rates and APC mRNA levels. This correlation indicated that mRNA levels limited APC productivity. Two strategies were employed to increase APC mRNA levels and APC productivity. First, sodium butyrate was added to increase mRNA levels by two- to sixfold in five APC-producing clones to obtain up to 2.7-fold increase in APC production rate. The second strategy was to retransfect an APC-producing BHK cell line with a vector containing additional APC cDNA and a mutant DHFR. This mutant DHFR gene allowed the selection of retransfected clones in higher MTX concentrations. Over two-fold higher mRNA levels were obtained in these retransfected clones and the cell-specific APC production rate increased twofold. At the highest level of APC secretion, increases in mRNA levels did not result in higher rates of APC production. Analysis of the intracellular APC content revealed a possible saturation in the secretory pathway at high mRNA levels. The relation between mRNA level and APC secretion rate was also investigated in batch culture. The levels of total cellular RNA, APC mRNA, and beta-actin mRNA were relatively stable while cells were in the exponential growth phase, but rapidly decreased when cells reached the stationary phase. The decline of cell-specific APC mRNA levels correlated with a decline in APC secretion rates, which indicated that the mRNA levels continued to limit the rates beyond the exponential phase and into the declining growth and stationary phases of batch APC production.

Metabolic effects on recombinant interferon-gamma glycosylation in continuous culture of Chinese hamster ovary cells

Asparagine linked (N-linked) glycosylation is an important modification of recombinant proteins, because the attached oligosaccharide chains can significantly alter protein properties. Potential glycosylation sites are not always occupied with oligosaccharide, and site occupancy can change with the culture environment. To investigate the relationship between metabolism and glycosylation site occupancy, we studied the glycosylation of recombinant human interferon-gamma (IFN-gamma) produced in continuous culture of Chinese hamster ovary cells. Intracellular nucleotide sugar levels and IFN-gamma glycosylation were measured at different steady states which were characterized by central carbon metabolic fluxes estimated by material balances and extracellular metabolite rate measurements. Although site occupancy varied over a rather narrow range, we found that differences correlated with the intracellular pool of UDP-N-acetylglucosamine + UDP-N-acetylgalactosamine (UDP-GNAc). Measured nucleotide levels and estimates of central carbon metabolic fluxes point to UTP depletion as the cause of decreased UDP-GNAc during glucose limitation. Glucose limited cells preferentially utilized available carbon for energy production, causing reduced nucleotide biosynthesis. Lower nucleoside triphosphate pools in turn led to lower nucleotide sugar pools and reduced glycosylation site occupancy. Subsequent experiments in batch and fed-batch culture have confirmed that UDP-sugar concentrations are correlated with UTP levels in the absence of glutamine limitation. Glutamine limitation appears to influence glycosylation by reducing amino sugar formation and hence UDP-GNAc concentration. The influence of nucleotide sugars on site occupancy may only be important during periods of extreme starvation, since relatively large changes in nucleotide sugar pools led to only minor changes in glycosylation.

Glycan engineering of proteins with whole living yeast cells expressing rat liver alpha2,3-sialytransferase in the porous cell wall

The N-glycans of recombinant proteins produced via the secretory pathway of cultured mammalian cells are often undersialylated, and insect cells lack sialytransferases. Undersialylated glycoproteins are rapidly cleared from the circulation, compromising the effect of pharmaceuticals. We show that incubation with living Saccharomyces cerevisiae cells expressing the catalytic ectodomain of rat liver alpha2,3-sialyltransferase (ST3Ne) in the porous cell wall resulted in sialylation of glycoproteins. The Km values of the yeast enzyme for several substrates were similar to those of recombinant ST3Ne from insect cells and of authentic ST3N. The yeast strain provides an inexpensive self-perpetuating source of ST3N activity for glycan engineering of recombinant proteins.

Real-time isoform analysis by two-dimensional chromatography of a monoclonal antibody during bioreactor fermentations

A humanized monoclonal antibody specific for L-selectin exhibits two distinct isoforms that are distinguishable by a charged group modification on one of the antibody light chains. The added charge allows baseline separation of the isoforms by anion-exchange chromatography. Since this modification most likely results from specific enzymatic activity within the Golgi complex, it is possible that fermentation conditions may affect the relative amounts of the isoforms produced. Herein is described a two-dimensional chromatographic method for quantifying the relative amounts of the isoforms from fermentation broths, in real time.

The heparan sulfate binding sequence of interferon-gamma increased the on rate of the interferon-gamma-interferon-gamma receptor complex formation

Interferon-gamma (IFNgamma), in common with a number of growth factors, binds both to heparan sulfate or heparin-related molecules and to a specific high affinity receptor (IFNgammaR). Using surface plasmon resonance technology, kinetic analysis of the IFNgamma. IFNgammaR complex formation was performed with the extracellular part of IFNgammaR immobilized on a sensor chip. At the sensor chip surface, IFNgamma was bound by two IFNgammaR molecules with an affinity in the nanomolar range (0.68 nM). This binding was characterized by an important on rate, kon = 7.3 x 10(6) M-1.s-1, and an off rate, koff = 5 x 10(-3).s-1. This binding assay was used to investigate a possible role of heparin in the IFNgamma.IFNgammaR complex formation. In contrast to growth factors for which binding to heparin is usually required for high affinity receptor interaction, we found in this study that IFNgamma bound to heparin displayed a strongly reduced affinity for its receptor. This is consistent with the fact that a cluster of basic amino acids (KTGKRKR, called the C1 domain) in the carboxyl-terminal sequence of the cytokine was involved both in heparin and receptor recognition. To understand how a single domain of IFNgamma could be implicated in two discrete functions (i.e. binding to heparin and to IFNgammaR), we also analyzed in a detailed manner the role of the IFNgamma carboxyl-terminal sequence in receptor binding. Using forms of IFNgamma, with carboxyl terminus truncations of defined regions of the heparin binding sequence, we found that the C1 domain functioned by increasing the on rate of the IFNgamma.IFNgammaR binding reaction but was not otherwise required for the stability of the complex. Interactions between the IFNgamma carboxyl-terminal domain and IFNgammaR could increased the association rate of the reaction either by increasing the number of encounters between the two molecules or by favoring productive collisions. The mechanisms by which heparan sulfate regulates IFNgamma activity may thus include both control of selective protease cleavage events, which directly affect the cytokine activity, and also an ability to modulate the interaction of IFNgamma with the IFNgammaR via competitive binding to the C1 domain.

The glycosylation heterogeneity of recombinant human IFN-gamma

The cloning of the cDNA for human interferon-gamma (IFN-gamma) has resulted in its expression in Escherichia coli, baculovirus-infected insect cells, Chinese hamster ovary (CHO) cells, and the mammary gland of transgenic mice. Large quantities of highly purified recombinant IFN-gamma have been generated, aided by the use of highly specific neutralizing monoclonal antibodies, with a view to its production as a human therapeutic protein. The primary source of structural heterogeneity for IFN-gamma during its production in mammalian expression systems is glycosylation, which can profoundly affect the three-dimensional structure of a glycoprotein and its biological function. A number of analytical approaches have been developed recently to allow a detailed analysis of the carbohydrate structures associated with IFN-gamma, the principal advances being in the areas of capillary electrophoresis and mass spectrometry. The implementation of these high-resolution analytical tools to determine the glycosylation profile of IFN-gamma makes it one of the best characterized recombinant glycoproteins. Recombinant human IFN-gamma acts as a model secretory glycoprotein, typifying the intrinsic glycosylation processing events associated with production of a potential therapeutic glycoprotein.

The relationship between intracellular UDP-N-acetyl hexosamine nucleotide pool and monoclonal antibody production in a mouse hybridoma

The effect of intracellular UDP-N-acetyl hexosamine accumulation in a murine hybridoma was investigated using tunicamycin and ammonium chloride. The treatment of cells with tunicamycin resulted in the inhibition of glycosylation of the secreted monoclonal antibody (IgGl) and cell growth arrest. Tunicamycin concentrations of 0.01-0.1 microgram ml-1 resulted in the formation of both glycosylated and non-glycosylated heavy chains of the immunoglobulin, whereas complete inhibition of protein glycosylation was observed at higher concentrations of tunicamycin. Tunicamycin treatment also resulted in a dose dependent accumulation of UDP-N-acetyl hexosamine. It was concluded that the specific monoclonal antibody production rate (qMab) was not dependent on the extent of glycosylation. Treatment of cells with NH4Cl also resulted in dose dependent accumulation of UDP-N-acetyl hexosamine. Supplementation of cultures with 10 mM NH4Cl resulted in a 40% reduction in cell growth rate and a 36% increase in the qMab. The data suggest that the reduction in growth rate and not UDP-N-acetyl hexosamine accumulation was the cause of increased qMab in the ammonium supplemented cultures. Ammonium chloride did not affect the extent of protein glycosylation. We conclude that UDP-N-acetyl hexosamine does not act as a mediator of enhanced rates of monoclonal antibody synthesis in the hybridoma cell culture system.