Although many recombinant proteins have been produced in seeds at high yields without adverse effects on the plant, endoplasmic reticulum (ER) stress and aberrant localization of endogenous or recombinant proteins have also been reported. The production of murine interleukin-10 (mIL-10) in Arabidopsis thaliana seeds resulted in the de novo formation of ER-derived structures containing a large fraction of the recombinant protein in an insoluble form. These bodies containing mIL-10 were morphologically similar to Russell bodies found in mammalian cells. We confirmed that the compartment containing mIL-10 was enclosed by ER membranes, and 3D electron microscopy revealed that these structures have a spheroidal shape. Another feature shared with Russell bodies is the continued viability of the cells that generate these organelles. To investigate similarities in the formation of Russell-like bodies and the plant-specific protein bodies formed by prolamins in cereal seeds, we crossed plants containing ectopic ER-derived prolamin protein bodies with a line accumulating mIL-10 in Russell-like bodies. This resulted in seeds containing only one population of protein bodies in which mIL-10 inclusions formed a central core surrounded by the prolamin-containing matrix, suggesting that both types of protein aggregates are together removed from the secretory pathway by a common mechanism. We propose that, like mammalian cells, plant cells are able to form Russell-like bodies as a self-protection mechanism, when they are overloaded with a partially transport-incompetent protein, and we discuss the resulting challenges for recombinant protein production.

VHHs or nanobodies are widely acknowledged as interesting diagnostic and therapeutic tools. However, for some applications, multivalent antibody formats, such as the dimeric VHH-Fc format, are desired to increase the functional affinity. The scope of this study was to compare transient expression of diagnostic VHH-Fc antibodies in Nicotiana benthamiana leaves with their stable expression in Arabidopsis thaliana seeds and Pichia pastoris. To this end, VHH-Fc antibodies targeting green fluorescent protein or the A. thaliana seed storage proteins (albumin and globulin) were produced in the three platforms. Differences were mainly observed in the accumulation levels and glycosylation patterns. Interestingly, although in plants oligomannosidic N-glycans were expected for KDEL-tagged VHH-Fcs, several VHH-Fcs with an intact KDEL-tag carried complex-type N-glycans, suggesting a dysfunctional retention in the endoplasmic reticulum. All VHH-Fcs were equally functional across expression platforms and several outperformed their corresponding VHH in terms of sensitivity in ELISA.

Since the serendipitous discovery 20 years ago of bona fide camelid heavy-chain antibodies, their single-domain antigen-binding fragments, known as VHHs or nanobodies, have received a progressively growing interest. As a result of the beneficial properties of these stable recombinant entities, they are currently highly valued proteins for multiple applications, including fundamental research, diagnostics, and therapeutics. Today, with the original patents expiring, even more academic and industrial groups are expected to explore innovative VHH applications. Here, we provide a thorough overview of novel implementations of VHHs as research and diagnostic tools, and of the recently evaluated production platforms for several VHHs and VHH-derived antibody formats.

Antibodies and antibody derived fragments are excellent tools for the detection and purification of proteins. However, only few antibodies targeting Arabidopsis seed proteins are currently available. Here, we evaluate the process to make antibody libraries against crude protein extracts and more particularly to generate a VHH phage library against native Arabidopsis thaliana seed proteins. After immunising a dromedary with a crude Arabidopsis seed extract, we cloned the single-domain antigen-binding fragments from their heavy-chain only antibodies in a phage display vector and selected nanobodies (VHHs) against native Arabidopsis seed proteins. For 16 VHHs, the corresponding antigens were identified by affinity purification and MS/MS analysis. They were shown to bind the major Arabidopsis seed storage proteins albumin and globulin (14 to albumin and 2 to globulin). All 16 VHHs were suitable primary reagents for the detection of the Arabidopsis seed storage proteins by ELISA. Furthermore, several of the anti-albumin VHHs were used successfully for storage protein localisation via electron microscopy. The easy cloning, selection and production, together with the demonstrated functionality and applicability, strongly suggest that the VHH antibody format will play a more prominent role in future protein research, in particular for the study of native proteins.

A wide variety of recombinant proteins has been produced in the dicot model plant, Arabidopsis thaliana. Many of these proteins are targeted for secretion by means of an N terminal endoplasmic reticulum (ER) signal peptide. In addition, they can also be designed for ER retention by adding a C terminal H/KDEL-tag. Despite extensive knowledge of the protein trafficking pathways, the final protein destination, especially of such H/KDEL-tagged recombinant proteins, is unpredictable. In this respect, glycoproteins are ideal study objects. Microscopy experiments reveal their deposition pattern and characterization of their N-glycans aids in elucidating the trafficking. Here, we combine microscopy and N glycosylation data generated in Arabidopsis leaves and seeds, and highlight the lack of a decent understanding of heterologous protein trafficking.

Summary Nanobodies ® ( VHH s) provide powerful tools in therapeutic and biotechnological applications. Nevertheless, for some applications, bivalent antibodies perform much better, and for this, an F c chain can be fused to the VHH domain, resulting in a bivalent homodimeric VHH ‐ F c complex. However, the production of bivalent antibodies in Escherichia coli is rather inefficient. Therefore, we compared the production of VHH 7 and VHH 7‐ F c as antibodies of interest in A rabidopsis seeds for detecting prostate‐specific antigen ( PSA ), a well‐known biomarker for prostate cancer in the early stages of tumour development. The influence of the signal sequence (camel versus plant) and that of the F c chain origin (human, mouse or pig) were evaluated. The accumulation levels of VHH s were very low, with a maximum of 0.13% VHH of total soluble protein ( TSP ) in homozygous T 3 seeds, while VHH ‐ F c accumulation levels were at least 10‐ to 100‐fold higher, with a maximum of 16.25% VHH ‐ F c of TSP . Both the camel and plant signal peptides were efficiently cleaved off and did not affect the accumulation levels. However, the F c chain origin strongly affected the degree of proteolysis, but only had a slight influence on the accumulation level. Analysis of the m RNA levels suggested that the low amount of VHH s produced in A rabidopsis seeds was not due to a failure in transcription, but rather to translation inefficiency, protein instability and/or degradation. Most importantly, the plant‐produced VHH 7 and VHH 7‐ F c antibodies were functional in detecting PSA and could thus be used for diagnostic applications.

Transgenic plants for the production of high-value recombinant complex and/or glycosylated proteins are a promising alternative for conventional systems, such as mammalian cells and bacteria. Many groups use plants as production platform for antibodies and antibody fragments. Here, we describe how bivalent camel-like antibodies can be produced in leaves and seeds. Camel-like antibodies are fusions of the antigen-binding domain of heavy chain camel antibodies (VHH) with an Fc fragment of choice. Transient expression in Nicotiana benthamiana leaves allows the production of VHH-Fc antibodies within a few days after the expression plasmid has been obtained. Generation of stable Arabidopsis thaliana transformants allows production of scalable amounts of VHH-Fc antibodies in seeds within a year. Further, we describe how the in planta-produced VHH-Fc antibodies can be quantified by Western blot analysis with Fc-specific antibodies.