Patent Application
20240173398
The present invention relates to the recombinant production of a protein of interest in a prokaryotic or eukaryotic host cell wherein the protein of interest is obtained in a correctly folded and stable form. The protein of interest may be a difficult-to-make polypeptide for use as a vaccine or a pharmaceutical. The protein of interest is co-expressed with or fused to a ‘fold promoter’, which may be a VHH antibody recognizing the said protein.
Newly synthesized proteins typically need to fold to become functional. This folding process involves forming (hydrogen-bonded) secondary structures (such as α-helices and β-sheets) and the burial of hydrophobic residues into the protein's hydrophobic core to acquire their tertiary structure (for review see: (Balchin et al., 2020; Trombetta and Parodi, 2003). Many secretory proteins, such as immunoglobulins, serum albumin, or RNase A, also form disulfide bonds from initially reduced cysteines. A failure to fold correctly leads to non-functional proteins, aggregation, and/or rapid degradation by dedicated proteases. Many proteins assemble into multi-subunit complexes (quaternary structure), examples being immunoglobulins or hemoglobin. Subunits, which cannot find an appropriate partner subunit, are typically eliminated by cellular quality control systems.
Protein folding is typically catalyzed by dedicated folding enzymes called chaperones, whereby the available chaperone repertoire depends on where folding occurs. The eukaryotic cytosol harbors numerous chaperones, including members of the Hsp70 and Hsp90 ATPases and the TRIC complex, which confines folding intermediates into a large folding chamber and releases them when folding is complete. Folding can occur already during synthesis (co-translational folding) or in a posttranslational manner.
Secretory proteins are transported in an unfolded state through a so-called translocon into the lumen of the endoplasmic reticulum (ER). These include numerous proteins used in pharmaceutical applications, such as immunoglobulins, immune-regulatory factors, or hormones. The folding environment of the ER is rather different from the cytosolic one: while the cytosol is reducing, the ER lumen allows stabilizing disulfide bonds to form—catalyzed by a set of specific enzymes that includes protein disulfide isomerases (PDIs). Likewise, folding inside the ER is often coupled to another ER-specific posttranslational modification, namely N-glycosylation, which enables glycosylation-assisted folding mechanisms involving lectin-like molecules. The ER is also capable of ATP-dependent protein folding, using ER-specific Hsp70 and Hsp90-type family members. However, the ER lacks GroEL/Hsp60/TRIC complex-type chaperones typical of the cytosolic folding environment. After having passed quality control, secretory proteins exit the ER, traverse the Golgi, where additional modifications such as proteolytic processing or changes to the sugar chains might occur.
Protein folding in prokaryotes, such as Escherichia coli, shows similarities but also differences to eukaryotic folding. Instead of the TRIC complex, the E. coli cytosol contains a smaller and simpler GroE-GroL complex. Secretion in E. coli typically ends in the periplasmic space, which harbors disulfide-bonding enzymes, but lacks glycosylation, glycosylation-coupled folding, and ATP-dependent folding enzymes.
Recombinant expression of proteins is not only important for basic research, but also it is a key process in the area of biotechnology, in particular for the production of pharmaceutically relevant biologicals. Several expression systems are available for recombinant expression. Cytoplasmic expression in E.coli is the easiest to implement—because of the fast growth and straightforward genetic manipulation of this bacterium, because of economic growth media, and its safe handling. However, many target proteins do not fold correctly in the E.coli cytosol, in particular targets that usually fold in a secretory compartment and rely on correct disulfide-bonding and/or glycosylation-assisted folding.
There are several solutions to this problem, such as switching to another expression system, e.g., to secretory expression from mammalian CHO cells. However, the latter is far more time consuming and expensive, particularly when it comes to large-scale expressions. Another solution is to manipulate the cytosolic folding system of bacteria such as E.coli, for example, by making this folding environment less reducing and by ectopic cytoplasmic expression of a periplasmic disulfide isomerase (DsbC) (Bessette et al., 1999; Levy et al., 2001). Meanwhile, such strains are commercially available, e.g., the E.coli NEB Express Shuffle strain from New England Biolabs.
However, even when using bacterial strains that have a modified cytosolic folding system, certain polypeptides are still not correctly folded. Thus there is a need for further improving the folding of recombinant proteins produced in a bacterial host cell.
A first aspect of the invention relates to a method for the recombinant production of a polypeptide of interest in a host cell, e.g. a prokaryotic host cell, or a eukaryotic host cell comprising the steps:
In a particular embodiment, the polypeptide of interest is selected from a receptor binding domain (RBD) of a coronavirus spike protein, e.g. the RBD of the OC43, HKU1, 229E, NL63, SARS-CoV-1, MERS-COV or SARS-CoV-2 spike protein.
A further aspect of the invention relates to a nucleic molecule encoding a fusion polypeptide comprising a polypeptide of interest and a fold promoter for the polypeptide of interest, wherein the fold promoter is a VVH antibody directed against the polypeptide of interest.
A further aspect of the invention relates to a set of nucleic molecules comprising a first nucleic acid molecule encoding a polypeptide of interest and a second nucleic acid molecule encoding a fold promoter for the polypeptide of interest, wherein the fold promoter is a VHH antibody directed against the polypeptide of interest.
A further aspect of the invention relates to a host cell comprising a nucleic molecule or a set of nucleic acid molecules as described above.
In a particular embodiment, the host cell is a prokaryotic host cell, e.g. an E.coli cell.
A further aspect of the invention relates to a polypeptide selected from a receptor binding domain (RBD) of a coronavirus spike protein, e.g. the RBD of the OC43, HKU1, 229E, NL63, SARS-CoV-1, MERS-CoV or SARS-CoV-2 spike protein, in correctly folded form, particularly produced in a prokaryotic host cell.
In a particular embodiment, the polypeptide is the RBD of the SARS-CoV-2 spike protein in correctly folded form, particularly produced in a prokaryotic host cell.
A further aspect of the invention relates to a complex of an RBD of a coronavirus spike protein and a fold-enhancing VHH antibody.
A further aspect of the invention relates to an RBD as described above for use in medicine.
A further aspect of the invention relates to a polypeptide of interest for use as a vaccine, wherein the polypeptide of interest is attached to a heterologous amino acid sequence having a length of at least about 5 amino acids, which has a negative net charge of at least about −5.
The present inventors explored the use of E.coli as a host cell for an economic recombinant production of the RBD (receptor-binding domain) of the SARS-CoV-2 spike protein. The RBD binds ACE2 during viral entry into target cells (Lan et al., 2020; Wang et al., 2020; Yan et al., 2020; Zhou et al., 2020a). Therefore, the RBD is a prime target for therapeutic intervention—be it for the production of neutralizing antibodies or as a constituent of vaccines. The RBD gets natively folded inside the ER lumen; it contains four structural disulfide bonds and gets N-glycosylated at two sites.
The inventors observed that the RBD failed to fold when expressed in the E. coli cytoplasm, even when using a strain that allows disulfide-bonding in the cytoplasm (NEB Shuffle). The small soluble fraction was then largely trapped in a non-productive complex with the chaperone GroE. This is in line with previous reports of failed folding of coronavirus RBDs in E. coli (Jegouic et al., 2020; Chen et al., 2005). Failure of correct folding was further confirmed by the finding that the soluble RBD fraction was not recognized by the super-neutralizing anti-RBD VHH antibody Re5D06, which is disclosed among a large set of other anti-RBD VHH antibodies in EP 20 188 541, EP 21 151 145, and EP 21 151 159, the contents of which are herein incorporated by reference.
Additional attempts of testing numerous variants of periplasmic RBD expression also gave unacceptably low yields. The still best variant was a fusion to the periplasmic disulfide interexchange protein G (DsbG), which yielded a soluble DsbG-RBD fusion that behaved in gel filtration like a folded protein. Nevertheless, the periplasmic expression workflow requires an osmotic shock protocol to release the periplasmic fraction, which cannot be easily implemented for an industrial-scale production; and yields were moderate at best.
The inventors discovered that a correct folding of the RBD domain could be enforced in the E.coli cytosol by its co-expression or fusion with “fold-promoting” anti-RBD VHH antibodies.
To this end, the inventors explored numerous ways to obtain a correctly folded RBD through expression in the E.coli cytosol. Eventually, the inventors tried the unconventional approach of testing systematically if co-expressing specific VHH antibodies could force the RBD to fold properly in the NEB Shuffle Express strain. Indeed, amongst 32 tested anti-RBD VHH antibodies, seven had the desired RBD-fold-promoting effect, namely: Re6A11, Re6B07, Re7E02, Re9C07, Re9D02, Re9F06, Re9G12 and Re11H04.
All of these seven VHH antibodies compete for the same RBD epitope (defined by Re7E02; see
Accordingly, the present invention provides a covalent fusion polypeptide of a protein of interest, e.g. a coronavirus RBD and a fold promoting VHH antibody, a non-covalent complex of a protein of interest, e.g. a coronavirus RBD, and a fold promoting VHH antibody. The covalent fusion polypeptide optionally comprises further domains, e.g. a purification domain, e.g. an N-terminal cleavable purification tag, and/or a spacer located between the polypeptide of interest and the fold promoting VHH antibody.
The term “protein of interest” generally relates to any globular protein, protein fragment or protein domain that folds inefficiently when produced in a heterologous system.
The term “fold-enhancing VHH antibody” refers to a VHH antibody, which when co-expressed or fused to the protein of interest leads to an increase of the amount of correctly folded protein when expressed in a host cell, particularly in a prokaryotic host cell such as E. coli.
A “non-competing antibody” may be VHH antibody, or any other type of antibody or antigen-binding fragment thereof, and is directed against an epitope on the protein of interest, which is different from the epitope, against which the fold-enhancing VHH antibody is directed. Preferably, the non-competing antibody is a monovalent antibody, i.e. an antibody having a single antigen-binding site. More preferably, the non-competing antibody is a VHH antibody. The non-competing antibody and the fold-enhancing VHH antibody can bind to the protein of interest simultaneously. In certain embodiments, the non-competing antibody selectively binds to the protein of interest in correctly folded form, e.g. by binding to a conformational epitope on the protein of interest
A list of fold-enhancing VHH antibodies Re6A11, Re6B07, Re7E02, Re9C07, Re9D02, Re9F06, Re9G12, and Re11H04, and of the non-competing VHH antibody R6D05 and their CDR1, CDR2, and CDR3 sequences is shown in the following Table 1, as well as main epitope-binding Re6H06 and fold-enhancing VHH antibodies Re21D01 and Re21H01 as expressed in yeast:
For cytoplasmic expression in E.coli, VHH are preferred that contain only a single disulfide bond, i.e. Re6A11, Re6B07, Re7E02, Re9C07, Re9D02, Re9F06, Re9G12, and Re11H04.
Re6A11, Re6B07, Re7E02, Re9C07, Re9D02, Re9F06, Re9G12, and Re11H04 as well as Re21D01 and Re21H01 are suitable for secretory expression in eukaryotes such as yeast.
The VHH antibody as described herein is particularly a monoclonal VHH antibody characterized by a specific amino acid sequence. The VHH antibody may be produced in a prokaryotic host cell, a yeast cell or a mammalian cell. In certain embodiments, the VHH antibody is non-glycosylated. In certain embodiments, the VHH antibody is glycosylated, wherein a carbohydrate structure may be derived from a glycosylation site introduced into the VHH sequence and/or from a fusion partner.
A VHH antibody is characterized by (i) a CDR3 sequence, (ii) a combination of a CDR1 sequence, a CDR2 sequence, and a CDR3 sequence, (iii) by a complete VHH sequence, or (iv) by competition with a specific reference antibody. Specific CDR and VHH sequences are provided in the Tables, Figures and the Sequence Listing.
According to the present invention, sequences related to the above sequences are encompassed. These related sequences are defined by having a minimum identity to a specifically indicated amino acid sequence, e.g. a CDR or VHH sequence. This identity is indicated over the whole length of the respective reference sequence and may be determined by using well known algorithms such as BLAST.
In particular embodiments, a related CDR3 sequence has an identity of at least 80% or at least 90% or at least 95% to a specifically indicated CDR3 sequence, e.g. a substitution of 1, 2, or 3 amino acids.
In particular embodiments, a related combination of a CDR1 sequence, a CDR2 sequence, and a CDR3 sequence has an identity of at least 80% or at least 90% or at least 95% to a specifically indicated combination of a CDR1 sequence, a CDR2 sequence, and a CDR3 sequence, e.g. a substitution of 1, 2, 3, 4, 5 or 6 amino acids by different amino acids.
In particular embodiments, a related VHH sequence has an identity of least 70%, at least 80%, at least 90%, at least 95% or at least 99% to a VHH sequence, e.g. a substitution of 1, 2, 3, 4, 5 or up to 20 amino acids.
Further, the invention refers to a VHH antibody, which competes with a specific VHH antibody disclosed herein for the binding to the SARS-CoV-2 spike protein S1 domain. In certain embodiments, a competing VHH antibody binds the same or an overlapping epitope on the SARS-CoV-2 spike protein S1 domain, particularly the receptor-binding domain (RBD) of the SARS-CoV-2 S1 domain as a specific VHH antibody disclosed herein. For example, the invention refers to a fold-enhancing VHH antibody, which competes with VHH antibodies Re6B07 or Re7E02.
Competition is measured either as an at least 90% loss of fluorescence staining of the polypeptide of interest, e.g. the SARS-CoV-2 spike protein by a fluorophore-labelled candidate VHH antibody in the presence of a 100-fold molar excess of an unlabelled competitor VHH antibody, e.g. Re7E02, or conversely as an at least 90% loss of polypeptide staining, e.g. spike protein-staining by a fluorophore-labelled VHH antibody, e.g. Re7E02 in the presence of a 100-fold molar excess of the unlabelled candidate VHH antibody as a competitor.
In their first experiments, the inventors co-expressed the VHH antibody and the RBD from two different plasmids. This was not quite ideal as the expression levels of the two polypeptides turned out to be different. Therefore, the presently preferred solution is to express the fold promoter and the RBD as a single fusion polypeptide comprising (i) optionally a cleavable N-terminal purification tag, e.g. the His14-SUMO module, (ii) the fold-promoting VHH, (iii) a spacer, e.g. a 39 residues long flexible, hydrophilic and negatively charged spacer, and (iv) the RBD. Such an entity (a His14-SUMO-VHH Re6A11-spacer-SARS-CoV-2-RBD fusion) was found to be well expressed in the cytoplasm of E. coli NEB Shuffle, soluble and purifiable in a single Ni (II) chelate step (
The interpretation of proper RBD-folding in the presence of those VHH antibodies rests on several lines of evidence. First, their co-expression or fusion increased soluble RBD expression. Second, it counteracted trapping by GroE. Third, binary fold-promoting VHH-RBD complexes or VHH-RBD fusions were easily obtained by co-expression and behaved, e.g., during gel filtration, like properly folded entities. Fourth, those binary complexes bound a non-competing antibody, e.g. complementary VHH Re5D06 with high affinity and specificity (
To obtain ultimate proof for correct folding of the RBD obtained under such expression conditions, the inventors crystallized a trimeric complex comprising the fold-promoting VHH antibody Re6A11, the RBD, and the neutralizing VHH antibody Re5D06 and solved the structure of this complex to a resolution of 1.75 Å (
Another important detail is that the fold promoter produces only a comparably small clash with ACE2, which is sufficient to preclude ACE2-binding but leaves most of the ACE2-binding interface of the RBD exposed (
The fold promoter Re6A11 not only leaves the best epitope for neutralization fully exposed but should even improve its presentation to the immune system because it prevents masking of this epitope by ACE2 (
In the aforementioned Re6A11-RBD fusion, the inventors linked the two entities with a long, negatively charged spacer of 39 residues in length and a net charge of -13. The length is needed to safely bridge the 56 Å distance between the C-terminus of Re6A11 and the N-terminus of the RBD. The negatively charged spacer serves, however, potentially yet another function, namely, to confer binding to cationic lipids or Al3+-based colloids that are typical components/carriers of vaccine formulations. This binding should concentrate the antigen on the surface of the particulate carriers and thus increase immunogenicity by presenting the antigen in a multivalent form.
The present invention provides a fusion polypeptide comprising a polypeptide of interest and a VHH antibody further comprising a heterologous amino acid sequence as a spacer having a length of at least about 5 amino acids and preferably up 100 amino acids, particularly about 5 to 50 amino acids. In certain embodiments, this polypeptide is selected from viral antigens that might trigger a neutralizing antibody response in humans or animals. The spacer has a negative net charge, particularly a negative net charge of at least about −5, more particularly a negative net charge of about −8 to about −30 or even more. The negative net charge of the spacer results from an excess of negatively charged amino acids therein. Preferably, the spacer has a content of at least about 15%, at least about 20% or least about 25% of negatively charged amino acids such as Asp and Glu. Further, the spacer may have a content of at least about 80%, or at least about 90% and up to about 100% of hydrophilic amino acids, particularly selected from Gly, Ser, Glu, Asp, Thr, Pro, and Gln.
The RBD co-expression with a fold-stabilizing VHH antibody as fold promoter or the RBD expression as a fusion with a fold-stabilizing VHH antibody a host cell like E. coli is thus a viable option to produce an anti-SARS-CoV-2 vaccine.
One can expand this concept to other vaccine candidates, such as RBDs from other coronaviruses that cause infections in humans (OC43, HKU1, 229E, NL63, SARS1, and MERS) or farm animals (e.g., swine and bovine coronaviruses).
The concept can be further expanded to other biotechnologically relevant biologicals, where a VHH can assist folding and production without interfering with the intended downstream application.
Further, the present invention provides a polypeptide of interest, particularly for use as a vaccine, wherein the polypeptide is attached to a heterologous amino acid sequence having a length of at least about 5 amino acids and preferably up 100 amino acids, particularly about 5 to 50 amino acids. In certain embodiments, this polypeptide is selected from viral antigens that might trigger a neutralizing antibody response in humans or animals. The heterologous amino acid sequence has a negative net charge, particularly a negative net charge of at least about −5, more particularly a negative net charge of about −8 to about −30 or even more. The negative net charge of the heterologous amino acid sequence results from an excess of negatively charged amino acids therein. Preferably, the heterologous amino acid sequence has a content of at least about 15%, at least about 20% or least about 25% of negatively charged amino acids such as Asp and Glu. Further, the heterologous amino acid sequence may have a content of at least about 80%, or at least about 90% and up to about 100% of hydrophilic amino acids, particularly selected from Gly, Ser, Glu, Asp, Thr, Pro, and Gln.
As indicated above, the heterologous spacer and the heterologous amino acid sequence have a negative net charge. This will increase the interaction with a positively charged compound, such as a polyamine, e.g. spermidine, a cationic polypeptide, e.g. protamine, a cationic lipid or an Al3+-based colloid or a particle comprising such a positively charged compound. Such a positively charged compound or particle is a typical component and/or carrier of a vaccine formulation. By means of this interaction, a non-covalent complex of the polypeptide and a positively charged compound or a particle comprising a positively charged compound will be generated.
Thereby, the polypeptide will be concentrated on the surface of a positively charged particulate carrier and thus increase immunogenicity by presenting the antigen in a multivalent form.
Thus, the invention provides a non-covalent complex of a polypeptide comprising a heterologous spacer or a heterologous amino acid sequence as described and positively charged compound including a particle comprising a positively charged compound. This complex is particularly useful as a component of immunogenic composition, e.g. an immunogenic composition for use as vaccine, e.g. in human medicine or in veterinary medicine, or for use in the production of antibodies, e.g. in an experimental animal.
To test if the fold-promoting effect of VHHs is restricted to production in E. coli or also applies to production in eukaryotes, the inventors expressed a SARS-CoV-2 RBD in the yeast Pichia pastoris as a secreted protein. This involves folding and disulfide-bonding in the endoplasmic reticulum as well as transport through the secretory pathway. This also involves quality control steps that need to be passed, for example, before the protein can exit the ER.
When expressed as a single protein, the RBD was not produced to detectable levels in the culture supernatant (
In the following, particular embodiments of the specification are listed.
According to the invention VHH, CDR1, CDR2 and CDR3 sequences characterized by SEQ IDs NO. 42 to SEQ ID NO. 55 relate in particular to the expression in eukaryotic host cells, in particular yeast such as Pichia pastoris.
HeLa cells were transiently transfected to express the SARS-CoV-2 spike protein. Following fixation, cells were stained for 1 hour with fluorophore-labeled Re10B10 (5 nM, green) and Re7E02 (15 nM, red) in the presence of the indicated unlabeled VHH competitors. Competitor (150 nM) was added 20 min prior to the labeled nanobodies. Cells were imaged by confocal laser scanning microscopy. For each competitor, the binding site on the RBD (epitope 1 or 2) are indicated. Re10B10 and Re5D06 define the competition class for epitope 1, while the fold promoters Re6A11, Re6B07, Re7E02, Re9C07, Re9D02, Re9G12, and Re11H04 compete for epitope 2 and thus bind to a different site.
A) An expression plasmid was constructed that encodes a fusion with an N-terminal His-SUMO tag, VHH-Re6A11, a 39 residues long acidic spacer (net charge: −13), and the SARS-CoV-2 RBD. E. coli NEB Express Shuffle was transformed with this plasmid, and plasmid-containing transformants were selected with 50 μg/ml kanamycin. Bacteria were grown in TB medium, and T5/lac-controlled expression was induced at 25° C. with 100 μM IPTG for 3 hours. Cells were recovered by centrifugation, resuspended in 50 mM Tris/HCl PH 7.5, 300 mM NaCl, 20 mM imidazole/HCl PH 7.5, and lysed by ultrasonication. The soluble fraction was obtained by ultracentrifugation. 45 ml soluble fraction (corresponding to 750 ml culture) was bound to a 1.2 ml Ni(II) EDTA-amide chelate column. Non-bound material was washed off with sonication buffer, and the Re6A11-RBD fusion was eluted by cleaving the His-SUMO-tag with the SUMO Ulp1 protease. Panel shows analysis of the experiment by SDS-PAGE (Coomassie-staining). Note the strong Re6A11-RBD fusion band in the eluted fraction. The yield was approximately 8 mg Re6A11-RBD fusion per liter of culture.
B) An analytical version of the experiment from panel a. Where indicated, 1 μM untagged Re5D06 was added to the binding reaction; elution was with lysis buffer containing 500 mM imidazole, which leaves the His14-SUMO fusion intact. Co-elution of Re5D06 with His14-SUMO-Re6A11-RBD indicates a specific RBD. Re5D06 interaction. The endogenous E. coli proteins present in the soluble starting lysate can be considered as negative controls for the binding reaction.
The complex was produced by co-expressing all three components in the cytoplasm of E. coli NEB Shuffle Express, then purified by double-tag purification (Frey and Görlich, 2014a; Frey and Görlich, 2014b) followed by gel filtration. The homogeneous complex was crystallized, an x-ray diffraction dataset was recorded at the Swiss Light Source synchrotron, and the structure solved by molecular replacement to a resolution of 1.75 Å and an Rfree of 0.229.
A) Crystal structure of the Re9F06·RBD·Re5D06 complex in surface (left) or ribbon representation (right). The intramolecular disulfide bonds (shown as yellow sticks) are labeled.
B) Ribbon representation of the Re9F06·RBD·Re5D06 complex as in A, but with RBD-bound ACE2 shown as transparent brown surface. Docking of ACE2 is based on alignment of the RBD with the SARS-CoV-2 RBD·ACE2 complex (PDB ID 7KMS; (Zhou et al., 2020b); RMSD=0.986 Å). Note the frontal clash of ACE2 with Re5D06. The clash between Re9F06 and ACE2 is minor and caused by CDR2 and the VHH scaffold protruding into ACE2; despite the clash, Re9F06 and ACE2 contact different residues on the RBD.
C) Comparison of the RBD structure in the ternary Re9F06·RBD·Re5D06 complex with the structure of the RBD as reported by PDB 6YZ5. Note that the folds are identical, with only minor deviations in some flexible loop structures.
The SARS-CoV-2 RBD from
To test if the fold-promoting effect is restricted to production in E. coli or also applies to eukaryotes, the inventors expressed a SARS-CoV-2 RBD in the yeast Pichia pastoris. This domain comprised residues 334-526 of the S1 spike and carried an N343D mutation to eliminate an N-glycosylation site. The RBD was cloned under the control of a methanol-inducible AOX1 promoter. It was fused behind an Ost1 secretion signal (for transport into the ER) and an a-factor propeptide with a C-terminal Kex2 cleavage site. This expression cassette was genomically integrated using homologous recombination and selection for a G418 resistance marker.
Three additional RBD strains with co-integrated VHH expression cassettes (selected through Zeocin resistance marker) were constructed for co-expressing either the epitope 1-binder Re6H06 or the epitope 2-binders Re21D01 or Re21H01. The resulting strains were cultivated, induced for 48 hours with 0.5% methanol (added twice) before analyzing the pattern of secreted proteins in the culture supernatant by SDS-PAGE and Coomassie-staining.
The VHH antibodies expressed upon methanol induction as expected (marked by °. No secretion of RBD was evident without an additional VHH or when the main epitope-binder Re6H06 was co-expressed. However, a major RBD band (marked by *) was evident when Re21D01 or Re21H01 were co-expressed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21154077.8 | Jan 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/052087 | 1/28/2022 | WO |
