Rapid transient production in plants by replicating and non-replicating vectors yields high quality functional anti-HIV antibody

Abstract

Background: The capacity of plants and plant cells to produce large amounts of recombinant protein has been well established. Due to advantages in terms of speed and yield, attention has recently turned towards the use of transient expression systems, including viral vectors, to produce proteins of pharmaceutical interest in plants. However, the effects of such high level expression from viral vectors and concomitant effects on host cells may affect the quality of the recombinant product.

Methodology/principal findings: To assess the quality of antibodies transiently expressed to high levels in plants, we have expressed and characterised the human anti-HIV monoclonal antibody, 2G12, using both replicating and non-replicating systems based on deleted versions of Cowpea mosaic virus (CPMV) RNA-2. The highest yield (approximately 100 mg/kg wet weight leaf tissue) of affinity purified 2G12 was obtained when the non-replicating CPMV-HT system was used and the antibody was retained in the endoplasmic reticulum (ER). Glycan analysis by mass-spectrometry showed that the glycosylation pattern was determined exclusively by whether the antibody was retained in the ER and did not depend on whether a replicating or non-replicating system was used. Characterisation of the binding and neutralisation properties of all the purified 2G12 variants from plants showed that these were generally similar to those of the Chinese hamster ovary (CHO) cell-produced 2G12.

Conclusions: Overall, the results demonstrate that replicating and non-replicating CPMV-based vectors are able to direct the production of a recombinant IgG similar in activity to the CHO-produced control. Thus, a complex recombinant protein was produced with no apparent effect on its biochemical properties using either high-level expression or viral replication. The speed with which a recombinant pharmaceutical with excellent biochemical characteristics can be produced transiently in plants makes CPMV-based expression vectors an attractive option for biopharmaceutical development and production.

Figure 1. Schematic representation of the plant expression cassettes used to express 2G12.

White boxes represent CPMV coding sequences, light and dark grey boxes represent 2G12 light and heavy chains respectively.

Figure 2. Electrophoretic analysis of protein-A purified 2G12 variants produced by delRNA-2 + RNA-1 or with CPMV-HT.

12% SDS-PAGE under reducing conditions was used to separate 4 µg of 2G12 preparations for Coomassie staining and 50 ng for immunological detection of samples from ^CHO2G12 (1), ^CPMV2G12HL (2), ^HT2G12HL (3), ^CPMV2G12HEL (4), and ^HT2G12HEL (5). A sample of size markers (visible on the Coomassie-stained gel but not the blots) was loaded between lanes 1 and 2.

Figure 3. Relative abundances of glycoforms for each of the purified 2G12 variants with diagrammatic structures of each glycotype.

Data was generated by mass-spectrometric analysis of the two predicted tryptic peptides representing the same glycosylation site (EEQYN²⁹⁷STYR and TKPREEQYN²⁹⁷STYR). N-glycan structure abbreviations are given according to http://www.proglycan.com.

Figure 4. Ratios of the binding rate signals obtained by SPR on the purified 2G12 variants.

Values were derived using protein-A, protein-L, and gp120 surfaces represented by linear regressions. (A) protein-L/protein-A ratio. (B) gp120/protein-A ratio. (C) gp120/protein-L ratio.