Expression of Human Interleukin-2 in Recombinant Baby Hamster Kidney, Ltk−, and Chinese Hamster Ovary Cells

Discovery and characterization of hydroxylysine O-glycosylation in an engineered IL-2 fusion protein

In the present study, an engineered interleukin-2 (IL-2) fusion protein consisting of an anti-human serum albumin nanobody linked by ASTKG and a (G₄S)₂ linker to IL-2 was constructed. Liquid chromatography-mass spectrometry (LC-MS) characterization was performed on the intact molecule and at the peptide level. The LC-MS molecular mass analysis for the engineered fusion protein showed the appearance of unreported +340 Da peaks, apart from the expected O-glycosylation-related peaks in the IL-2 domain. Through a combination analysis of a K120R mutated molecule (The lysine at the position of 120 was mutated to arginine while the rest amino acid sequence remain unchanged), the possibility of a non-cleaved valine-histidine-serine signal peptide was ruled out and the presence of hydroxylysine (HyK) O-glycosylation in the ASTKG linker was confirmed. HyK O-glycosylation have been reported in other proteins such as collagen, which occurs in the conserved Gly-Xaa-HyK motif and is catalyzed by lysyl hydroxylase-3 complex. The present study showed high similar conserved motif of HyK-O-glycosylation in collagen, implying the HyK O-glycosylation in the engineered IL-2 possibly was catalyzed by the Chinese hamster ovary homolog of enzymes promoting HyK O-glycosylation in collagen. Bioactivity testing results revealed that HyK-O-glycosylation had no obvious effect on the in vitro activity of engineered IL-2. Our study is the first to report HyK-O-glycosylation modifications in therapeutic proteins through LC-MS characterization and in vitro activity analysis, which expands the scope of post-translational modification knowledge of therapeutic proteins.

An efficient system for bioconjugation based on a widely applicable engineered O-glycosylation tag

Bioconjugates are an important class of therapeutic molecules. To date, O-glycan-based metabolic glycoengineering has had limited use in this field, due to the complexities of the endogenous O-glycosylation pathway and the lack of an O-glycosylation consensus sequence. Here, we describe the development of a versatile on-demand O-glycosylation system that uses a novel, widely applicable 5 amino acid O-glycosylation tag, and a metabolically engineered UDP-galactose-4-eperimase (GALE) knock-out cell line. Optimization of the primary sequence of the tag enables the production of Fc-based proteins with either single or multiple O-glycans with complexity fully controlled by media supplementation. We demonstrate how the uniformly labeled proteins containing exclusively N-azido-acetylgalactosamine are used for CLICK chemistry-based bioconjugation to generate site-specifically fluorochrome-labeled antibodies, dual-payload molecules, and bioactive Fc-peptides for applications in basic research and drug discovery. To our knowledge, this is the first description of generating a site-specific O-glycosylation system by combining an O-glycosylation tag and a metabolically engineered cell line.

Glycoengineering of Chinese hamster ovary cells for enhanced erythropoietin N-glycan branching and sialylation

Sialic acid, a terminal residue on complex N-glycans, and branching or antennarity can play key roles in both the biological activity and circulatory lifetime of recombinant glycoproteins of therapeutic interest. In order to examine the impact of glycosyltransferase expression on the N-glycosylation of recombinant erythropoietin (rEPO), a human α2,6-sialyltransferase (ST6Gal1) was expressed in Chinese hamster ovary (CHO-K1) cells. Sialylation increased on both EPO and CHO cellular proteins as observed by SNA lectin analysis, and HPLC profiling revealed that the sialic acid content of total glycans on EPO increased by 26%. The increase in sialic acid content was further verified by detailed profiling of the N-glycan structures using mass spectra (MS) analysis. In order to enhance antennarity/branching, UDP-N-acetylglucosamine: α-1,3-D-mannoside β1,4-N-acetylglucosaminyltransferase (GnTIV/Mgat4) and UDP-N-acetylglucosamine:α-1,6-D-mannoside β1,6-N-acetylglucosaminyltransferase (GnTV/Mgat5), was incorporated into CHO-K1 together with ST6Gal1. Tri- and tetraantennary N-glycans represented approximately 92% of the total N-glycans on the resulting EPO as measured using MS analysis. Furthermore, sialic acid content of rEPO from these engineered cells was increased ∼45% higher with tetra-sialylation accounting for ∼10% of total sugar chains compared to ∼3% for the wild-type parental CHO-K1. In this way, coordinated overexpression of these three glycosyltransferases for the first time in model CHO-K1 cell lines provides a mean for enhancing both N-glycan branching complexity and sialylation with opportunities to generate tailored complex N-glycan structures on therapeutic glycoproteins in the future.

The effect of ammonia on the O-linked glycosylation of granulocyte colony-stimulating factor produced by chinese hamster ovary cells

Ammonium ion concentrations ranging from 0 to 10 mM are shown to significantly reduce the sialylation of granuiocyte colony-stimulating factor (G-CSF) produced by recombinant Chinese hamster ovary cells. Specifically, the degree of completion of the final reaction in the O-linked glycosylation pathway, the addition of sialic acid in an alpha(2,6) linkage to N-acetylgalactosamine, is reduced by NH(4) (+) concentrations of as low as 2 mM. The effect of ammonia on sialylation is rapid, sustained, and does not affect the secretion rate of G-CSF. Additionally, the effect can be mimicked using the weak base chloroquine, suggesting that the effect is related to the weak base characteristics of ammonia. In support of this hypothesis, experiments using brefeldin A suggest that the addition of sialic acid in an alpha(2,6) linkage to N-acetylgalactosamine occurs in the trans-Golgi compartment prior to the trans-Golgi network, which would be expected under normal conditions to have a slightly acidic pH in the range from 6.5 to 6.75. Ammonium ion concentrations of 10 mM would be expected to reduce significantly the differences in pH between acidic intracellular compartments and the cytoplasm. The pH-activity profile for the CHO O-linked alpha(2,6) sialytransferase using monosialylated G-CSF as a substrate reveals a twofold decrease in enzymatic activity across the pH range from 6.75 to 7.0.Mathematical modeling of this sialylation reaction supports the hypothesis that this twofold decrease in sialyltransferase activity resulting from an ammoniainduced increase in trans-Golgi pH could produce the observed decrease in G-CSF sialylation. (c) 1995 John Wiley & Sons, Inc.

Differentiating factors between erythropoiesis-stimulating agents: a guide to selection for anaemia of chronic kidney disease

Endogenous erythropoietin (EPO) consists of a central polypeptide core covered by post-translationally linked carbohydrates. Three of the four currently available erythropoiesis stimulating agents (ESA)--epoetin-alpha, epoetin-beta and epoetin-omega- are composed of an identical amino acid sequence, but glycosylation varies as a result of type- and host cell-specific differences in the production process. Epoetin-alpha and epoetin-beta resemble each other with respect to molecular characteristics and pharmacokinetic data, although epoetin-beta has a higher molecular weight, a lower number of sialylated glycan residues and possibly slight pharmacokinetic advantages such as a longer terminal elimination half-life. A serious adverse effect of long-term administration of ESA is pure red cell aplasia. This effect has been observed predominantly with subcutaneous use of epoetin-alpha produced outside the US after albumin was removed from the formulation. In comparison with the intravenous route, subcutaneous administration of epoetin has been reported to have a dose-sparing effect in some studies. Epoetin-beta has been the subject of studies aimed at proving efficacy with a reduced administration frequency but results are not unequivocal. Epoetin-omega is produced in a different host cell than all other erythropoietic agents, hence glycosylation and pharmacokinetics are different. Small-scale clinical studies found epoetin-omega to be slightly more potent than epoetin-alpha. Epoetin-delta is a recently approved agent produced by human cells that are genetically engineered to transcribe and translate the EPO gene under the control of a newly introduced regulatory DNA sequence. However, epoetin-delta is not yet on the market and few data are available. The erythropoietin analogue darbepoetin-alpha carries two additional glycosylation sites that permit a higher degree of glycosylation. Consequently, in comparison with the other epoetins, darbepoetin-alpha has a longer serum half-life and a higher relative potency, which further increases with extension of the administration interval. Dosage requirements of darbepoetin-alpha do not appear to differ between the intravenous and subcutaneous routes of administration. The less frequent administration of darbepoetin-alpha in comparison to the other epoetins may reduce drug costs in the long term, but the variability in dosage or dosage frequency required within a single patient is high. Further studies should be aimed at defining predictors of the individual demand for erythropoietic agents, thereby allowing nephrologists to prescribe a cost-effective, individualised regimen.

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.

Process development of a recombinant antibody/interleukin-2 fusion protein expressed in protein-free medium by BHK cells

The production, purification and stability of quality (in terms of integrity and glycosylation) of an antibody/interleukin-2 fusion protein with potential application in tumour-targeted therapy expressed in BHK21 cells are described. Consistency of the product throughout time was determined by analysis of glycosylation of the fusion protein using MALDI-TOF mass spectroscopy and HPAEC-PAD combined with product integrity studies by SDS-PAGE and Western blotting. These investigations showed consistent expression in terms of integrity and of three major oligosaccharide structures of the fusion protein after 62 generations. The data obtained at this stage indicated the suitability of the cell line for production purposes. Different approaches for the production of this protein were subsequently carried out. The relative productivity of the recombinant fusion protein and general performance of the cells in two different protein-free medium (PFM) culture systems, continuous chemostat and continuous perfusion using a Centritech centrifuge as a cell retention device, were studied. The results indicate that the chemostat culture resulted in more stable and controllable nutrient environment, which could indicate better product consistency, in accordance with what has been observed under serum-containing conditions, in relation to the perfusion culture. Finally, product obtained from the chemostat culture was analysed and purified. The purification process was optimised with an increase in the overall yield from 38 to 70% being obtained, a significant improvement with important consequences for the implementation of an industrial-scale culture system. In conclusion, it was possible to produce and purify the recombinant antibody/interleukin-2 fusion protein assuring the quality and stability of the product in terms of integrity and glycosylation. Therefore, a candidate production process was established.

Expression and characterisation of equine interleukin 2 and interleukin 4

In the present study, we describe the expression of equine IL2 and IL4. The cDNA of equine IL2 or IL4 was cloned in a mammalian expression vector, containing c-terminal myc- and six histidines His(6)-epitopes for recognition and purification of equine cytokines. The vector constructs were used for transfection of chinese hamster ovary (CHO) cells. Purified equine cytokines were characterised by western blotting. Equine IL2 was secreted with a molecular weight of approximately 17.1kDa, whereas IL4 was expressed in three different sizes of 17.1, 19.6 and 22.1kDa, probably due to different glycosylation modifications. The biological activities of both cytokines were tested by proliferation assays using leukoagglutinin (LAG) prestimulated equine PBMC. Both, equine IL2 and IL4 induced dose-dependent lymphocyte proliferation. In contrast to IL4, IL2 supported the proliferation of B cells.

Posttranslationally processed forms of the human chemokine HCC-1

HCC-1 is the only CC-chemokine known so far which circulates in nanomolar concentrations in human plasma. Its physiological function is not well defined. Posttranslational processing of HCC-1 was shown to modulate its biological properties. In this study several different processed forms of HCC-1 were isolated. Western blot analysis of human plasma extracts revealed a HCC-1 immunoreactive double band at 8-10 kDa indicating the presence of two distinct HCC-1 peptides. These peptides were isolated from a peptide library of human blood filtrate and represent predominantly HCC-1 (1-74) and glycosylated HCC-1 (1-74). Glycosylated HCC-1 exhibits a molecular mass of 9621 Da due to O-glycosylation at position 7 (Ser-7) with two N-acetylneuraminic acids and the disaccharide N-acetylgalactosamine galactose. Furthermore N-terminally truncated HCC-1 (3-74) and HCC-1 (4-74) were identified in the peptide library. In hemofiltrate approximately 3% of total HCC-1 represents HCC-1 (3-74) and approximately 1% represents HCC-1 (4-74) whereas the major products are nonglycosylated HCC-1 (1-74) and glycosylated HCC-1 (1-74). Our data imply that HCC-1 (1-74), HCC-1 (3-74), HCC-1 (4-74) and glycosylated HCC-1 (1-74) circulate in human blood. The N-terminal processing and modification of HCC-1 might be of importance in displaying its full biological activity.

Metabolic shifts do not influence the glycosylation patterns of a recombinant fusion protein expressed in BHK cells

BHK-21 cells expressing a human IgG-IL2 fusion protein, with potential application in tumor-targeted therapy, were grown under different nutrient conditions in a continuous system for a time period of 80 days. At very low-glucose (< 0.5 mM) or glutamine (< 0. 2 mM) concentrations, a shift toward an energetically more efficient metabolism was observed. Cell-specific productivity was maintained under metabolically shifted growth conditions and at the same time an almost identical intracellular ATP content, obtained by in vivo (31)P NMR experiments, was observed. No significant differences in the oligosaccharide structures were detected from the IgG-IL2 fusion protein preparations obtained by growing cells under the different metabolic states. By using oligosaccharide mapping and MALDI/TOF-MS, only neutral diantennary oligosaccharides with or without core alpha1-6-linked fucose were detected that carried no, one or two beta1-4-linked galactose. Although the O-linked oligosaccharide structures that are present in the IL2 moiety of the protein were studied with less detail, the data obtained from the hydrazinolysis procedure point to the presence of the classical NeuAcalpha2-3Galbeta1-3GalNAc structure. Here, it is shown that under different defined cellular metabolic states, the quality of a recombinant product in terms of O- and N-linked oligosaccharides is stable, even after a prolonged cultivation period. Moreover, unaffected intracellular ATP levels under the different metabolic states were observed.

Effects of ammonia on CHO cell growth, erythropoietin production, and glycosylation

The effect of ammonium chloride was determined on a culture of CHO cells transfected with the human erythropoietin (EPO) gene. Cell growth was inhibited above a culture concentration of 5 mM NH(4)Cl with an IC-50 determined to be 33 mM. The specific production of EPO increased with the addition of NH(4)Cl above 5 mM. At 10 mM NH(4)Cl, the final cell density after 4 days in culture was significantly lower but the final yield of EPO was significantly higher. This appeared to be due to continued protein production after cell growth had ceased. The metabolic effects of added NH(4)Cl included higher specific consumption rates of glucose and glutamine and an increased rate of production of alanine, glycine, and glutamate. The EPO analyzed from control cultures had a molecular weight range of 33-39 kDa and an isoelectric point range of 4.06-4.67. Seven distinct isoforms of the molecule were identified by two-dimensional electrophoresis. This molecular heterogeneity was ascribed to variable glycosylation. Complete enzymatic de-glycosylation resulted in a single molecular form with a molecular mass of 18 kDa. Addition of NH(4)Cl to the cultures caused a significant increase in the heterogeneity of the glycoforms as shown by an increased molecular weight and pI range. Enzymatic de-sialylation of the EPO from the ammonia-treated and control cultures resulted in identical electrophoretic patterns. This indicated that the effect of ammonia was in the reduction of terminal sialylation of the glycan structures which accounted for the increased pI. Selective removal of the N-glycan structures by PNGase F resulted in two bands identified as the O-glycan linked structure (19 kDa) and the completely de-glycosylated structure (18 kDa). The proportion of the O-linked glycan structure was reduced, and its pI increased in cultures to which ammonia was added. Thus, the glycosylation pattern altered by the presence of ammonia included a reduction in terminal sialylation of all the glycans and a reduction in the content of the O-linked glycan. The addition of a sialidase inhibitor to the cultures had no effect on the ammonia-induced increase in EPO heterogeneity. Also, the effect of ammonia on glycosylation could not be mimicked using the weak base chloroquine in our system.

Improved IL-2 detection for determination of helper T lymphocyte precursor frequency in limiting dilution assay

In the context of allogeneic bone marrow transplantation, an accurate estimate of the risk of developing graft-versus-host disease (GVHD) is of major interest. The pre-transplant frequency of donor's helper T-lymphocyte precursors (HTLp) directed against host's antigens may be helpful in predicting this risk. This technique relies on an indirect measurement of interleukin-2 (IL-2) secreted by the HTLp, as assessed by the proliferation of an IL-2 dependent cell line. Many authors use the murine CTLL-2 cell line in this assay, but these cells do not respond to the presence of minute amounts of IL-2 in the culture medium, and thus do not discriminate between the absence or the presence of very low levels of IL-2. We therefore decided to compare CTLL-2 with another IL-2 dependent cell line, the murine A9.12 cell line. A comparison was made using serial dilutions of recombinant human IL-2, limiting dilutions of baby hamster kidney (BHK) cells transfected with human IL-2 gene and in the context of clinical tests performed for the detection of pre-transplant HTLp. Both the sensitivity and reliability of the tests were better using A9.12. We conclude that the A9.12 cell line might be a more suitable tool for pre-transplant HTLp determinations before allogeneic bone marrow transplantation or whenever low IL-2 levels are to be measured.

Peptide, disulfide, and glycosylation mapping of recombinant human thrombopoietin from ser1 to Arg246

Thrombopoietin (TPO) is a hematopoietic factor involved in the regulation of megakaryocytopoiesis. Full length recombinant human TPO (332 residues) has been expressed in BHK cells and purified to homogeneity using conventional means. Peptide, disulfide, and glycosylation mapping of human TPO from residues 1 to 246 has been carried out using liquid chromatography-electrospray mass spectrometry (LC-ESMS). A modification of the ramped orifice method of Carr and co-workers [Carr et al. (1993) Protein Sci. 2, 183-196] is employed, providing additional information for assignment of the LC-ESMS chromatograms. With the modification, b- and y-series peptide ions are produced via front-end CID which confirms the mass-based assignments. The results of our analysis of TPO indicate that the amino acid sequence of TPO 1-246 is as expected from the transfected cDNA with complete cleavage of the signal peptide. Two unique disulfides are formed between the four cysteines in the cytokine domain of TPO: Cys7-Cys151 and Cys29-Cys85. The glycosylation map indicates the position, occupancy, and structures of the N- and O-glycans in TPO 1-246. In addition, site specific structural characterization of the PNGase F-liberated N-glycans has been performed following purification by high-pH anionic exchange chromatography with pulsed amperometric detection (HPAEC-PAD); the results corroborate the LC-ESMS data. The N-glycans are of the complex type with the core-fucosylated disialylated biantennary and trisialylated triantennary structures predominating. The O-glycans are of the mucin type with the monosialylated and disialylated GalGalNAc-S/T structures predominating. Furthermore, we propose that the C-terminal domain of TPO be further divided into two domains on the basis of sequence homology among the cloned sequences and glycosylation/structural features: an N-glycan domain (154-246) and an O-glycan domain (247-332).

Human interleukin-2 (IL-2) expressed by transfected mammalian cells

cDNA for human interleukin-2 (IL-2) was cloned into the pRc/RSV vector for expression in animal cells. Baby hamster kidney (BHK-21) cells and Chinese hamster ovary (CHO) cells were transfected several times using calcium phosphate and electroporation methods with the construct pRc/RSV SIGIL2. Different transfection efficiencies were obtained. The biological test on CTLL-2 (mouse cytotoxic lymphocytes) showed that the kinetics of cell proliferation were different from those of rIL-2 (recombinant IL-2) expressed in bacteria and in BHK cells. When high concentrations of rIL-2 were applied, an inhibitory effect on CTLL-2 was observed when bacterial product was used, whilst rBHK interleukin caused no inhibition. Recombinant BHK IL2 induced a slower response of CTLL-2 cells at the beginning of the cultivation, however, prolonged activity was detected at the later stage of the experiment.

Glutamine and tetrapeptide repeat variations affect the biological activity of different mouse interleukin-2 alleles

Mouse interleukin-2 (IL-2) was thought to be encoded by a single allele. We have recently described N-terminal differences in five IL-2 molecules from nine mouse strains analyzed (Matesanz, F., Alcina, A. and Pellicer, A., Immunogenetics 1993. 38: 300). In this study, we isolated and sequenced the cDNA of three polymorphic IL-2 molecules and constructed two recombinant IL-2 molecules to cover representative structural changes and to address the functional significance of these changes using human and mouse cellular assays in vitro. Apart from punctual codon changes, major differences include an expanding CAG codon (translated into glutamine) and the presence of the tetrapeptide Pro-Thr-Ser-Ser repeated 1, 2, or 3.5 times which is also present once in human IL-2. This tetrapeptide repeat includes an O-glycosylation site. These recombinant IL-2 proteins were expressed at high levels in bacteria and purified by preparative SDS-PAGE with a complete activity recovery. Differences in growth-inducing activity on mouse primary splenocytes were observed in some of them, although no differences were observed in proliferative stimulation of CTLL cells. In human peripheral blood lymphocytes and the T cell line Kit-225, the growth stimulation capacity was inversely dependent on the size of the glutamine stretch and the number of tetrapeptide repeats. These results suggest an evolutionary adaptation of the mouse IL-2/IL-2 receptor system that maintains polyglutamine extensions in the IL-2 molecule. In summary, mouse IL-2 polymorphism results in different bioactivities which may determine susceptibility or resistance to disease.

Characterization of changes in the glycosylation pattern of recombinant proteins from BHK-21 cells due to different culture conditions

The N-glycosylation patterns of a genetically engineered human interleukin-2 variant glycoprotein (IL-Mu6), produced by BHK-21 cells from long-term suspension and microcarrier cultures in the presence and absence of fetal calf serum were compared. IL-Mu6 was used as a model protein in studying the effect of different controlled cell culture conditions on the expression of N-glycans in recombinant glycoproteins. IL-Mu6 contains a single amino acid substitution (Glu100<==>Asn) generating a potential N-glycosylation recognition site (Asn100-Xxx-Thr/Ser) in addition to the natural O-glycosylation at position Thr3. Parallel cell cultivations were carried out in two continuously perfused 2.5-liter stirred bioreactors under defined culture conditions. Major differences were found in the glycoprotein products obtained during these different cultivation conditions. Serum-free cultures resulted in a higher level of terminal sialylation and proximal alpha 1-6 fucosylation. The ratio of O- to N-glycans as well as the amount of nonglycosylated product and the antennarity of N-linked carbohydrates in the model protein exhibited major differences depending on the presence or absence of serum, the condition of growth and the cultivation procedure.

Site-specific detection and structural characterization of the glycosylation of human plasma proteins lecithin:cholesterol acyltransferase and apolipoprotein D using HPLC/electrospray mass spectrometry and sequential glycosidase digestion

Site-specific structural characterization of the glycosylation of human lecithin:cholesterol acyltransferase (LCAT) was carried out using microbore reversed-phase high performance liquid chromatography coupled with electrospray ionization mass spectrometry (HPLC/ESIMS). A recently described mass spectrometric technique involving monitoring of carbohydrate-specific fragment ions during HPLC/ESIMS was employed to locate eight different groups of glycopeptides in a digest of a human LCAT protein preparation. In addition to the four expected N-linked glycopeptides of LCAT, a di-O-linked glycopeptide was detected, as well as three additional glycopeptides. Structural information on the oligosaccharides from all eight glycopeptides was obtained by sequential glycosidase digestion of the glycopeptides followed by HPLC/ESIMS. All four potential N-linked glycosylation sites (Asn20, Asn84, Asn272, and Asn384) of LCAT were determined to contain sialylated triantennary and/or biantennary complex structures. Two unanticipated O-linked glycosylation sites were identified at Thr407 and Ser409 of the LCAT O-linked glycopeptide, each of which contain sialylated galactose beta 1-->3N-acetylgalactosamine structures. The three additional glycopeptides were determined to be from a copurifying protein, apolipoprotein D, which contains potential N-linked glycosylation sites at Asn45 and Asn78. These glycopeptides were determined to bear sialylated triantennary oligosaccharides or fucosylated sialylated biantennary oligosaccharides. Previous studies of LCAT indicated that removal of the glycosylation site at Asn272 converts this protein to a phospholipase (Francone OL, Evangelista L, Fielding CJ, 1993, Biochim Biophys Acta 1166:301-304). Our results indicate that the carbohydrate structures themselves are not the source of this functional discrimination; rather, it must be mediated by the structural environment around Asn272.

Detection of intracellular interleukin 2: evidence for novel immunologically related forms of the lymphokine

At present, few data are available on intracellular interleukin 2 (IL-2) and its posttranscriptional regulation. Unlike other lymphokines, IL-2 does not accumulate within the cell, but is rapidly secreted following its production. The process of detection and biochemical characterization of intracellular IL-2 involved using a high producer subclone of the Jurkat T-lymphoma line as a source for IL-2, in combination with a two-step separation protocol and a sensitive detection method. Following phytohemagglutinin (PHA)/4 beta-phorbol 12-myristate 13 acetate (TPA) stimulation, a 14 kDa molecule could be visualized in Western blots by means of two monoclonal anti-IL-2 antibodies possessing different epitope specificities. This molecule exhibited biological activity of IL-2 as determined by a murine cytotoxic T-cell proliferation assay. In addition to this biologically active form of the lymphokine, a strongly immunoreactive protein with a molecular weight of 54 kDa (P54) was found in Jurkat cell lysates. Further biochemical characterization of this intracellular variant revealed an isoelectric point similar to that of secreted forms of IL-2. All attempts to split the 54 kDa molecule into smaller subunits failed, and no biological IL-2 activity could be measured in response to P54. However, the appearance of this high molecular weight variant followed clear-cut time kinetics. The highest concentration of P54 was found to occur after 2 h of stimulation. Thereafter its concentration decreased continuously, while the amount of the biologically active 14 kDa variant increased under ongoing stimulation. One possible explanation for these results is that P54 may represent an immature form of IL-2 that is tightly linked to a carrier molecule.

Evaluation of the new serum-free medium (MDSS2) for the production of different biologicals: use of various cell lines

The presence of serum in cell culture raises safety problems for the production of biologicals, thus a new serum-free medium (MDSS2) was developed. The evaluation of this medium for the growth of different cell lines (BHK-21 C13, BSR and Vero) has shown that cells grew in this medium similarly to standard serum-containing medium, independently of the culture system used: in static (as monolayer) as well as in agitated systems (in suspension in spinner and perfusion reactors). BHK-21 and BSR cells grew as aggregate cultures and could proliferate in both static and agitated culture systems. Vero cells stayed attached to a substrate and proliferated equally in static and in agitated microcarrier-culture systems. The cell densities obtained with BHK-21 cells depended only on the culture system used. They ranged from 2-3 x 10(6) to 6-12 x 10(6) cells per ml for static batch and perfusion reactor cultures respectively. The cell concentration was 3 to 6 times higher than in classical cultures performed in serum-containing medium. The cell densities obtained with Vero cells were indistinguishable from those obtained in serum-containing medium, whatever the cell culture system used. These cell lines have been used for the production of rabies virus. With respect to BHK-21 and BSR, similar production rates of rabies glycoprotein have been found as in the standard roller bottle process. The production of rabies virus and of viral glycoprotein by Vero cells cultivated in serum-free medium was augmented 1.5-fold and 2.5-fold, respectively, when compared to serum-containing medium. A recombinant BHK-21 cell line, producing human IL-2, can also proliferate in MDSS2, after addition of insulin. The specific IL-2 production rate was augmented 3-4-fold in comparison to serum-containing medium. For the cells tested, the MDSS2 serum-free medium is a good growth and production medium. Its use for cultivating other cell lines and/or for the production of other biologicals is discussed.

Cultural and physiological factors affecting expression of recombinant proteins

The variability in expression of recombinant proteins has been analyzed with regard to (a) comparison of clones from the same transfection experiment; (b) comparison of the same genetic construct in different cell lines; (c) the effect of the culture system used (free suspension, aggregate suspension, and microcarrier); and (d) physiochemical parameters in long-term (100d) culture in a macroporous fixed bed bioreactor (FBR). Differences in product expression between clones were accompanied by differences in growth rates, metabolic kinetics, and ability to grow in suspension as opposed to attached culture. The single most important factor affecting product expression when comparing constructs (for SEAP and IgG), cell lines (BHK 21 and myeloma), and culture systems was whether cells were grown in an attached or suspension mode. Thus key factors could be related to cell morphology (suspension versus monolayer), the presence of microenvironments and physiological stress to control growth rate. The relationship of key process parameters to volumetric and specific rAb productivity of the FBR was investigated in a partial factorial experiment with a rBHK cell line. The highest productivity levels are associated with a combination of the highest values tested for re-cycle (195 ml min-1) and dilution rates (1 d-1) and glutamine concentration (2.5 mmol 1-1), plus the lowest values for bead size (2 mm) and inoculum density (10(7) m1-1). Together with data from fluidised bed cultures, these results suggest that higher productivity is not primarily the result of greater cell numbers within the system but more the physicochemical definition of the system.

Purification and chemical characterization of beta-trace protein from human cerebrospinal fluid: its identification as prostaglandin D synthase

beta-Trace protein from pooled human CSF was purified to homogeneity. An apparent molecular mass of 23-29 kDa was determined for the polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Amino-terminal sequencing of the polypeptide yielded the unique amino acid sequence APEAQVSVQPNFQQDKFLGRWFSA. Alignment of amino acid sequences obtained from tryptic peptides with the sequence previously deduced from a cDNA clone isolated by other investigators allowed the identification of beta-trace protein as prostaglandin D synthase [prostaglandin-H2 D-isomerase; (5Z, 13E)-(15S)-9 alpha, 11 alpha-epidioxy-15-hydroxyprosta-5,13-dienoate D-isomerase; EC 5.3.99.2]. A conservative amino acid exchange (Thr instead of Ser) was detected at amino acid position 154 of the beta-trace polypeptide chain in the corresponding tryptic peptide. The two N-glycosylation sites of the polypeptide were shown to be almost quantitatively occupied by carbohydrate. Carbohydrate compositional as well as methylation analysis indicated that Asn29 and Asn56 bear exclusively complex-type oligosaccharide structures (partially sialylated with alpha 2-3- and/or alpha 2-6-linked N-acetylneuraminic acid) that are almost quantitatively alpha 1-6 fucosylated at the proximal N-acetylglucosamine; approximately 70% of these molecules contain a bisecting N-acetylglucosamine. Agalacto structures as well as those with a peripheral fucose are also present.

Culture pH affects expression rates and glycosylation of recombinant mouse placental lactogen proteins by Chinese hamster ovary (CHO) cells

Glycosylation patterns and specific expression rates of the recombinant protein mouse placental lactogen-I (mPL-I) by Chinese hamster ovary (CHO) cells varied significantly over the extracellular pH (pHe) range of 6.1 to 8.7. The maximum specific mPL-I expression rates occurred between pHe 7.6 and 8.0. The pHe effect on protein expression was confirmed using a different CHO cell expressing the unglycosylated recombinant protein mouse placental lactogen-II (mPL-II). Decreases in the extent of glycosylation of mPL-I were observed at low (below 6.9) and high (above 8.2) pHe values. The pHe dependent variations in mPL-I accumulation in the supernatant as well as in glycosylation patterns were not the result of enzymatic degradation in the culture medium.

Structures of sialylated oligosaccharides of human erythropoietin expressed in recombinant BHK-21 cells

The native structures of the Asn-linked oligosaccharides and the O-glycans at Ser126 of human erythropoietin expressed from recombinant BHK cells have been elucidated. Enzymatically released N-glycans were studied by methylation analyses, fast-atom-bombardment mass spectrometry as well as one- and two-dimensional 1H-NMR spectrometry at 600 MHz. Many (82.7%) were found to be tetraantennary N-acetyllactosamine-type (22.8% with one, 3.6% with two and 0.4% with three N-acetyllactosamine repeats) being tetrasialylated (41%), trisialylated (29.6%) and disialylated (12.2%). A few (9.7%; 4.1% 2,4-branched, 5.6%, 2,6-branched) of the chains were triantennary (5.4% trisialyl, 4.3% disialyl) and 4.6% were of the disialyl diantennary type. Almost all of the innermost GlcNAc residues were alpha 1-6 fucosylated and NeuAc was exclusively alpha 2-3 linked to Gal beta 1-4GlcNAc-R; 60% of the protein was found to be O-glycosylated at Ser126; structures were monosialylated (70%) or disialylated (30%) forms of the Gal beta 1-3GalNAc core type. Glycosylation patterns at individual Asn-Xaa-Thr/Ser sites were determined by analytical high-pH anion-exchange chromatography with pulsed amperometric detection. Only tetraantennary chains with 0-3 N-acetyllactosamine repeats were detected at Asn38 and Asn83, while almost all of the di- and triantennary oligosaccharides were attached to Asn24. Batch analysis of different preparations of recombinant erythropoietin revealed the high reproducibility of the production procedure. Structures containing terminal GalNAc-GlcNAc were detected in small amounts in a few batches.

Analysis of glycoform of O-glycan from human myeloma immunoglobulin A1 by gas-phase hydrazinolysis following pyridylamination of oligosaccharides

A comparative study was made on the glycoform of O-glycan from human myeloma immunoglobulin A1. By gas-phase hydrazinolysis, O-glycan was released from its hinge portion. The released oligosaccharide was pyridylaminated and separated by a two-dimensional analytical method of gel filtration and reverse-phase HPLC. Four major pyridylamino derivatives (P1-P4) were obtained. The neutral component (P4) among them was identified as Gal beta 1,3GalNAc-PA by cochromatography with an authentic standard pyridylamino sugar. The desialylation of the other components indicated the largest P1 and middle size P2 components possibly corresponded to a disialylated structure, NeuAc alpha 2,3Gal beta 1,3(NeuAc alpha 2,6)GalNAc-PA, and a monosialylated component, NeuAc alpha 2,3Gal beta 1,3GalNAc-PA, respectively. The structural assignment of P3 is still incomplete. Four similar components were also detected in bovine fetuin whose relative content (P1:P2: P3:P:4) was 16:43:19:22. The relative content (%) of P1-P4 (glycoform) in IgA1 from the healthy control was 10.1 +/- 3.3, 48.2 +/- 4.6, 7.0 +/- 2.6, and 34.7 +/- 4.5. The glycoform of O-glycan on IgA1 thus appears the same for any individual. Analysis of IgA1 myeloma protein indicated glycoforms distinct from those of the healthy controls. The relative content of these component could be classed as 2:8:0:90 (Type I, only one case designated as Kita), 5:24:3:68 (Type II, seven cases), and 9:41:5:45 (Type III, four cases). Thus, the results for IgA1 myeloma protein indicate that at least three glycoforms of O-glycan are possible for the IgA1 hinge structure. However, only one glycoform was found in the healthy controls.

Site-specific O-glycosylation of human granulocyte/macrophage colony-stimulating factor secreted by yeast and animal cells

To compare the site specificity of O-glycosylation in lower and higher eukaryotes, we expressed human granulocyte/macrophage colony-stimulating factor (hGM-CSF) in the yeast Saccharomyces cerevisiae and in COS-1 cells. Analyses of specific hGM-CSF mutants secreted by yeast led to the conclusion that efficient O-glycosylation in yeast requires residues S9 and T10. However, only S9 is used as an attachment point for an extended O-glycosyl chain in a 15.5-kDa hGM-CSF form. A 14.5-kDa hGM-CSF form, secreted by yeast, appears substituted by single mannosyl residues at both positions S9 and T10, indicating that O-glycosylation at T10 inhibits extension of the O-glycosyl chain attached to S9. As in yeast cells, the addition of O-glycosyl chains to hGM-CSF secreted by COS-1 cells requires the presence of S9 and T10 residues. These results demonstrate that, inspite of different biosynthetic routes, the selection of O-glycosylation sites is similar between lower and higher eukaryotes.

The Oligosaccharides of Glycoproteins: Bioprocess Factors Affecting Oligosaccharide Structure and their Effect on Glycoprotein Properties

In this review, we organize the recent data concerning the effects of bioprocess factors on the oligosaccharide structure of human therapeutic glycoproteins, with particular emphasis on the influence of the host cell. We also discuss the effect of oligosaccharide structure on glycoprotein properties, including antigenicity, immunogenicity and plasma clearance rate.

Enzyme sensor-FIA-system for on-line monitoring of glucose, lactate and glutamine in animal cell cultures

Enzyme sensors for glucose, lactate and glutamine were connected via flow-injection analysis (FIA) devices to two different bioprocesses. They were used for on-line process control of perfused bioreactor systems containing mammalian cell lines producing a monoclonal antibody and recombinant interleukin-2. The biosensor system gives direct access to important process data which can be used as control parameters for long term cell cultivation systems.

Sialylated carbohydrate chains of recombinant human glycoproteins expressed in Chinese hamster ovary cells contain traces of N-glycolylneuraminic acid

HPLC analysis of sialic acids released from recombinant variants of human tissue plasminogen activator, human chimeric plasminogen activator, human erythropoietin, and human follitropin, expressed in Chinese hamster ovary cells, demonstrates for each glycoprotein the presence of N-acetylneuraminic acid and N-glycolylneuraminic acid in a ratio of 97:3. Structural analysis by 500 MHz1H-NMR spectroscopy, of the enzymatically released N-linked carbohydrate chains of chimeric plasminogen activator and of erythropoietin, showed that alpha 2-3 linked N-glycolylneuraminic acid can occur in different N-acetyllactosamine type antennary structures.

Glycosylation of the envelope glycoprotein from a polytropic murine retrovirus in two different host cells

A polytropic recombinant retrovirus containing the envelope gene of Friend mink cell focus-inducing virus plus the remainder of the genome of an amphoropic murine leukemia virus was propagated on mouse embryo fibroblasts and mink lung cells. Virus particles, metabolically labeled with [2-3H]mannose, were harvested from the culture supernatants and lysed with detergents. The viral envelope glycoprotein was isolated from the lysates by immunoaffinity chromatography and purified by preparative SDS/PAGE. Oligosaccharides were liberated by sequential treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H and peptide-N4-(N-acetyl-beta-glucosaminyl) asparagine amidase F and fractionated by high-performance liquid chromatography. Individual glycans were characterized chromatographically, by methylation analyses and in part, by enzymic microsequencing. The results demonstrated that viral glycoproteins, synthesized in mouse embryo fibroblasts, carried as major constituents partially fucosylated diantennary, 2,4- and 2,6-branched triantennary and tetraantennary complex type N-glycans with 0-4 sialic acid residues and only small amounts of high-mannose type species with 5-9 mannose residues. As a characteristic feature, part of the complex type glycans contained additional Gal(alpha 1-3) substituents. Glycoprotein obtained from virions propagated on mink lung cells, contained partially fucosylated diantennary and 2,4-branched triantennary oligosaccharides with 1-3 sialic acid residues, in addition to trace amounts of high-mannose type species with 8 or 9 mannose residues. Thus, the results reveal that predominantly, the complex type N-glycans of the retroviral envelope glycoprotein display cell-specific variations including differences in oligosaccharide branching, sialylation and substitution by additional Gal(alpha 1-3) residues.

Carbohydrate structures of a human tissue plasminogen activator variant expressed in recombinant Chinese hamster ovary cells

The carbohydrate structures of a genetically engineered human tissue plasminogen activator variant bearing a single N-glycosylation site at Asn 448 are reported. After isolation of the tryptic glycopeptide and liberation of the N-linked carbohydrates by polypeptide:N-glycosidase F, 6 major oligosaccharide fractions were separated by HPLC on NH2-bonded phase. Their structures were determined by compositional and methylation analyses combined with fast atom bombardment mass spectrometry. Seventy percent of the carbohydrates were of the biantennary complex type with fucose at the proximal GlcNAc and zero, one or two alpha 2-3 linked NeuAc. The remainder were triantennary structures with one, two or three NeuAc.

Production of recombinant human interleukin-2 with BHK cells in a hollow fibre and a stirred tank reactor with protein-free medium

For the production of recombinant human interleukin-2 (IL-2) two different culture processes, a 1-2 liter homogeneous stirred bubble-free aerated system and a dense cell hollow fibre bioreactor were compared. Cultivations were carried out with serum- or protein-free medium formulations. In the stirred culture 0.75 mg IL-2 were produced with 1 l of perfused medium at a maximum cell number of 3 X 10(10). The product yield in the hollow fibre module was only 0.23 mg l-1 at a maximum cell number of 6 X 10(10). In contrast to results with hybridoma or EBV-transformed cell lines, in which hollow fibre bioreactors showed comparable efficiency to perfused stirred tank reactors, the tissue-like cell density is disadvantageous as adherent cells tend to stick together leaving insufficient intercellular space for removal of product.