The endoplasmic reticulum (ER) is the main site of protein synthesis, folding, and secretion to other organelles. The capacity of the ER to process proteins is limited, and excessive accumulation of unfolded and misfolded proteins can induce ER stress, which is associated with plant diseases. Here, a transgenic Arabidopsis system was established to express anti-cancer monoclonal antibodies (mAbs) that recognize the tumor-associated antigen GA733-2. Monoclonal antibody (mAb) CO17-1A recognize a tumor-associated epitope expressed on the colorectal cancer cell surface. The ER retention Lys-Asp-Glu-Leu (KDEL) motif sequence was added to the C-terminus of the heavy chain to retain anti-colorectal cancer mAbs in the ER, consequently boosting mAb production. Agrobacterium-mediated floral dip transformation was used to generate T1 transformants, and homozygous T4 seeds obtained from transgenic Arabidopsis plants expressing anti-colorectal cancer mAbs were used to confirm the physiological effects of KDEL tagging. Germination rates were not significantly different between both plants expressing mAb CO without KDEL mAb CO (CO plant) and mAb CO with KDEL mAb COK (COK plant). However, COK plants primary root lengths were shorter than those of CO plants and non-transgenic Arabidopsis plants in in vitro media. Most ER stress-related genes, with the exception of bZIP28 and IRE1a, were upregulated in COK plants compared to CO plants. Western blot and SDS-PAGE analyses showed that COK plants exhibited up to five times higher expression and mAb amounts than plants. Enhanced expression in mAb COK plants was confirmed by immunohistochemical analyses. mAb COK was distributed across most of the area of leaf tissues, whereas mAb CO was mainly distributed in extracellular areas. Surface plasmon resonance analyses revealed that mAb CO and mAb COK possessed equivalent or slightly better binding activities to antigen EpCAM compared to a commercially available parental antibody. N-glycosylation analysis showed that mAb CO had plant specific residues whereas mAb COK mainly showed an oligo-mannose N-glycan structure without the plant specific glycan residues. In this study, the reduction of plant growth and biomass induced by ER retention signal peptide might be only in in vitro conditions, and thus should be carefully considered for the initial screening for transgenic lines on culture media. Taken together, nevertheless the fusion of ER retention signal peptide is an effective approach for enhancing the yields of recombinant proteins in vivo.

Viral hemorrhagic septicemia (VHS) is an important infectious disease in fish worldwide caused by viral hemorrhagic septicemia virus (VHSV). VHSV is the causative agent of serious systemic diseases in fish, affecting a number of teleost fish species. In this study, VHSV glycoprotein (G), including its epitope, as a subunit vaccine candidate, was expressed in tobacco plant ( Nicotiana tabacum ). The recombinant gene, VHSVG, was fused to the immunoglobulin Fc fragment and extended with the endoplasmic reticulum (ER) retention signal (KDEL) to generate VHSVG-FcK. The recombinant expression vector for VHSVG-FcK was transferred into Agrobacterium tumefaciens (LBA4404), and plant transformation was conducted N. tabacum . Polymerase chain reaction (PCR) was performed to confirm gene insertion and VHSVG-FcK protein expression was confirmed by immunoblot analysis. VHSVG-FcK protein was successfully purified from tobacco plant leaves. Furthermore, ELISA analysis showed that mice serum immunized with the plant-derived VHSVG-FcK (VHSVGP-FcK) had a high absorbance against VHSVG-FcK, indicating that the plant-derived recombinant subunit vaccine protein VHSVG-FcK can induce immune response. Taken together, this recombinant vaccine protein can be expressed in plant expression systems and can be appropriately assembled to be functional in immunogenicity.

Epidermal cell adhesion molecule (EpCAM) is a tumor-associated antigen (TAA), which has been considered as a cancer vaccine candidate. The EpCAM protein fused to the fragment crystallizable region of immunoglobulin G (IgG) tagged with KDEL endoplasmic reticulum (ER) retention signal (EpCAM-FcK) has been successfully expressed in transgenic tobacco (Nicotiana tabacum cv. Xanthi) and purified from the plant leaf. In this study, we investigated the ability of the plant-derived EpCAM-FcK (EpCAM-FcK P ) to elicit an immune response in vivo. The animal group injected with the EpCAM-FcK P showed a higher differentiated germinal center (GC) B cell population (~9%) compared with the animal group injected with the recombinant rhEpCAM-Fc chimera (EpCAM-Fc M ). The animal group injected with EpCAM-FcK P (~42%) had more differentiated T follicular helper cells (Tfh) than the animal group injected with EpCAM-Fc M (~7%). This study demonstrated that the plant-derived EpCAM-FcK fusion antigenic protein induced a humoral immune response in mice.

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Transgenic plants expressing immunoglobulin (Ig) M Fc-fused Prostate acid phosphatase (PAP) antigenic proteins (PAP-IgM FcK) and J-chain proteins were generated by Agrobacterium-mediated transformation. The Fc region was tagged with the ER retention motif (KDEL) to make PAP-IgM FcK. Two transgenic plants were crossed together to generate F1 expressing both PAP-IgM FcK and J-chain proteins (PAP-IgM FcK × J-chain). PCR and RT-PCR analyses confirmed the transgene insertion and mRNA transcription of PAP-IgM FcK and J-chain in leaf tissue of PAP-IgM FcK × J-chain F1 plant. Western blot confirmed the expression of PAP-IgM FcK × J-chain protein. Size exclusion (SEC)-high performance liquid chromatography (HPLC) and Bio-transmission electron microscope (TEM) analyses were performed to show the size and shape of the PAP- IgM FcK × J-chain fusion proteins. These results suggest that PAP-IgM FcK with J-chain can be produced in plant expression system with plant crossing.