Silkworm Pupae Function as Efficient Producers of Recombinant Glycoproteins with Stable-Isotope Labeling

Baculovirus-infected silkworms are promising bioreactors for producing recombinant glycoproteins, including antibodies. Previously, we developed a method for isotope labeling of glycoproteins for nuclear magnetic resonance (NMR) studies using silkworm larvae reared on an artificial diet containing ¹⁵N-labeled yeast crude protein extract. Here, we further develop this method by introducing a technique for the expression of isotope-labeled glycoproteins by silkworm pupae, which has several potential advantages relative to larvae-based techniques in terms of production yield, ease of handling, and storage. Here, we fed fifth instar larvae an artificial diet with an optimized composition containing [methyl-¹³C]methionine, leading to pupation. Nine-day-old pupae were then injected with recombinant Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid for expression of recombinant human immunoglobulin G (IgG). From the whole-body homogenates of pupae, 0.35 mg/pupa of IgG was harvested, which is a yield that is five times higher than can be obtained from larvae. Recombinant IgG, thus prepared, exhibited mainly three kinds of pauci-mannose-type oligosaccharides and had a ¹³C-enrichment ratio of approximately 80%. This enabled selective observation of NMR signals originating from the methionyl methyl group of IgG, confirming its conformational integrity. These data demonstrate the utility of silkworm pupae as factories for producing recombinant glycoproteins with amino-acid-selective isotope labeling.

Efficient N-Glycosylation of the Heavy Chain Tailpiece Promotes the Formation of Plant-Produced Dimeric IgA

Production of monomeric IgA in mammalian cells and plant expression systems such as Nicotiana benthamiana is well-established and can be achieved by co-expression of the corresponding light and heavy chains. In contrast, the assembly of dimeric IgA requires the additional expression of the joining chain and remains challenging especially in plant-based systems. Here, we examined factors affecting the assembly and expression of HER2 binding dimeric IgA1 and IgA2m(2) variants transiently produced in N. benthamiana. While co-expression of the joining chain resulted in efficient formation of dimeric IgAs in HEK293F cells, a mixture of monomeric, dimeric and tetrameric variants was detected in plants. Mass-spectrometric analysis of site-specific glycosylation revealed that the N-glycan profile differed between monomeric and dimeric IgAs in the plant expression system. Co-expression of a single-subunit oligosaccharyltransferase from the protozoan Leishmania major in N. benthamiana increased the N-glycosylation occupancy at the C-terminal heavy chain tailpiece and changed the ratio of monomeric to dimeric IgAs. Our data demonstrate that N-glycosylation engineering is a suitable strategy to promote the formation of dimeric IgA variants in plants.

Method establishment for discerned immunogenicity assessment of a recombinant glycoprotein containing nonhuman sialic acid Neu5Gc residues

AIM

Recombinant glycoprotein produced in nonhuman mammalian cell lines can be modified with the immunogenic nonhuman sialic N-glycolylneuraminic acid (Neu5Gc). We describe here a validated method for detection of antidrug antibodies against both protein and Neu5Gc-containing glycan epitopes.

RESULTS

An electrochemiluminescent method was established with drug conjugates as capture and detection reagents. Rabbit antidrug polyclonal antibodies were used as the positive control for protein moiety-specific antibodies, while chicken anti-Neu5Gc polyclonal antibodies were used as the positive control for antibodies against Neu5Gc glycan epitope. Specificity to Neu5Gc was verified by signal inhibition with bovine γ-globulin that contains Neu5Gc.

CONCLUSION

The assay illustrated here discerns the immunogenicity of the protein backbone and the sialic acid Neu5Gc glycan moiety of a recombinant protein containing Neu5Gc.

Exploring Site-Specific N-Glycosylation of HEK293 and Plant-Produced Human IgA Isotypes

The full potential of recombinant Immunoglobulin A as therapeutic antibody is not fully explored, owing to the fact that structure-function relationships of these extensively glycosylated proteins are not well understood. Here monomeric IgA1, IgA2m(1), and IgA2m(2) variants of the anti-HER2 antibody (IgG1) trastuzumab were expressed in glyco-engineered Nicotiana benthamiana plants and in human HEK293-6E cells. All three IgA isotypes were purified and subjected to biophysical and biochemical characterization. While no differences in assembly, antigen binding, and glycosylation occupancy were observed, both systems vary tremendously in terms of glycan structures and heterogeneity of glycosylation. Mass-spectrometric analysis of site-specific glycosylation revealed that plant-produced IgAs carry mainly complex-type biantennary N-glycans. HEK293-6E-produced IgAs, on the contrary, showed very heterogeneous N-glycans with high levels of sialylation, core-fucose, and the presence of branched structures. The site-specific analysis revealed major differences between the individual N-glycosylation sites of each IgA subtype. Moreover, the proline-rich hinge region from HEK293-6E cell-derived IgA1 was occupied with mucin-type O-glycans, whereas IgA1 from N. benthamiana displayed numerous plant-specific modifications. Interestingly, a shift in unfolding of the CH2 domain of plant-produced IgA toward lower temperatures can be observed with differential scanning calorimetry, suggesting that distinct glycoforms affect the thermal stability of IgAs.

Transient Glyco-Engineering to Produce Recombinant IgA1 with Defined N- and O-Glycans in Plants

The production of therapeutic antibodies to combat pathogens and treat diseases, such as cancer is of great interest for the biotechnology industry. The recent development of plant-based expression systems has demonstrated that plants are well-suited for the production of recombinant monoclonal antibodies with defined glycosylation. Compared to immunoglobulin G (IgG), less effort has been undertaken to express immunoglobulin A (IgA), which is the most prevalent antibody class at mucosal sites and a promising candidate for novel recombinant biopharmaceuticals with enhanced anti-tumor activity. Here, we transiently expressed recombinant human IgA1 against the VP8* rotavirus antigen in glyco-engineered ΔXT/FT Nicotiana benthamiana plants. Mass spectrometric analysis of IgA1 glycopeptides revealed the presence of complex biantennary N-glycans with terminal N-acetylglucosamine present on the N-glycosylation site of the CH2 domain in the IgA1 alpha chain. Analysis of the peptide carrying nine potential O-glycosylation sites in the IgA1 alpha chain hinge region showed the presence of plant-specific modifications including hydroxyproline formation and the attachment of pentoses. By co-expression of enzymes required for initiation and elongation of human O-glycosylation it was possible to generate disialylated mucin-type core 1 O-glycans on plant-produced IgA1. Our data demonstrate that ΔXT/FT N. benthamiana plants can be engineered toward the production of recombinant IgA1 with defined human-type N- and O-linked glycans.

Using glyco-engineering to produce therapeutic proteins

INTRODUCTION

Glycans are increasingly important in the development of new biopharmaceuticals with optimized efficacy, half-life, and antigenicity. Current expression platforms for recombinant glycoprotein therapeutics typically do not produce homogeneous glycans and frequently display non-human glycans which may cause unwanted side effects. To circumvent these issues, glyco-engineering has been applied to different expression systems including mammalian cells, insect cells, yeast, and plants.

AREAS COVERED

This review summarizes recent developments in glyco-engineering focusing mainly on in vivo expression systems for recombinant proteins. The highlighted strategies aim at producing glycoproteins with homogeneous N- and O-linked glycans of defined composition.

EXPERT OPINION

Glyco-engineering of expression platforms is increasingly recognized as an important strategy to improve biopharmaceuticals. A better understanding and control of the factors leading to glycan heterogeneity will allow simplified production of recombinant glycoprotein therapeutics with less variation in terms of glycosylation. Further technological advances will have a major impact on manufacturing processes and may provide a completely new class of glycoprotein therapeutics with customized functions.

Effect of the developmental stage and tissue position on the expression and glycosylation of recombinant glycoprotein GA733-FcK in transgenic plants

The influence of developmental stage and position (top, middle, and base) of leaves and stem tissues on the expression and glycosylation pattern of a recombinant therapeutic protein -GA733-FcK- was observed in transgenic seedlings during a 16-week growth period. RNA expression gradually increased with age in the middle and basal leaves and decreased in top leaves after 14 weeks. The protein expression level at all leaf positions increased until 14 weeks and slightly decreased at 16 weeks; it was lower in yellow leaves than in green leaves. In stem, protein expression gradually decreased from the top to the base. The glycosylation patterns of GA733-FcK were analyzed from 10 to 16 weeks. The plant-specific glycans increased in the top leaves at 14 weeks, but only slightly changed in the middle and basal leaves. The structure of glycans varied with tissue position. The glycosylation level in the top and middle leaves increased until 12 and 14 weeks, respectively, and decreased thereafter, whereas it decreased in basal leaves until 14 weeks and increased at 16 weeks. In stem, all three sections showed high-mannose type glycan structures. The area size of the glycans was significantly higher in the top stem than in both the middle and basal stems, and it was smaller in yellow leaves than in green leaves. The glycan profiles were similar between green and yellow leaves until 16 weeks. Thus, biomass-harvesting time should be optimized to obtain recombinant therapeutic proteins with ideal glycan structure profiles.

Comparative evaluation of the Bio-Rad Geenius HIV-1/2 Confirmatory Assay and the Bio-Rad Multispot HIV-1/2 Rapid Test as an alternative differentiation assay for CLSI M53 algorithm-I

INTRODUCTION

The CLSI-M53-A, Criteria for Laboratory Testing and Diagnosis of Human Immunodeficiency Virus (HIV) Infection; Approved Guideline includes an algorithm in which samples that are reactive on a 4th generation EIA screen proceed to a supplemental assay that is able to confirm and differentiate between antibodies to HIV-1 and HIV-2. The recently CE-marked Bio-Rad Geenius HIV-1/2 Confirmatory Assay was evaluated as an alternative to the FDA-approved Bio-Rad Multispot HIV-1/HIV-2 Rapid Test which has been previously validated for use in this new algorithm.

METHODS

This study used reference samples submitted to the Canadian - NLHRS and samples from commercial sources. Data was tabulated in 2×2 tables for statistical analysis; sensitivity, specificity, predictive values, kappa and likelihood ratios.

RESULTS

The overall performance of the Geenius and Multispot was very high; sensitivity (100%, 100%), specificity (96.3%, 99.1%), positive (45.3, 181) and negative (0, 0) likelihood ratios respectively, high kappa (0.96) and low bias index (0.0068). The ability to differentiate HIV-1 (99.2%, 100%) and HIV-2 (98.1%, 98.1%) Ab was also very high.

CONCLUSION

The Bio-Rad Geenius HIV-1/2 Confirmatory Assay is a suitable alternative to the validated Multispot for use in the second stage of CLSI M53 algorithm-I. The Geenius has additional features including traceability and sample and cassette barcoding that improve the quality management/assurance of HIV testing. It is anticipated that the CLSI M53 guideline and assays such as the Geenius will reduce the number of indeterminate test results previously associated with the HIV-1 WB and improve the ability to differentiate HIV-2 infections.