Construction of an Integration Vector with a Chimeric Signal Peptide for the Expression of Monoclonal Antibodies in Mammalian Cells

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. This article constructs a novel integrated vector that can express monoclonal antibodies in mammalian cells. This study has certain innovation, especially in the expression of monoclonal antibodies on vectors, providing more choices. However, there are still many issues in the article that need to be addressed before publication.

2. Some major question

2.1 It is noted that your manuscript needs careful editing by someone with expertise in technical English editing paying particular attention to English grammar, and sentence structure， I think many description was  not clear to the reader.

2.2 In general, there is a lack of explanation of replicates and statistical methods used in the study.  Theres was less Statistically analyze the differences between different groups.

2.3 The conclusion needs to be revised, it should be more concise and highlight key points.

 

3. Other small mistake like:

3.1 Page 85 The website address should be correct showing.

 

3.2 The Figure could be more draw reasonable.

3.3 The Figure 2 should be highlight key points.

3.4 Figure 5 1-3 should be describe as Lane 1-3.

 

4. Statistics: I am not sure if Figure 6 requires significant statistical differences.

5. Reference: The reference format should be consistent, as there are many small errors inside. For example:

5.1) some reference  The NO. following the number, however other following the month date

Author Response

The team of authors would like to thank the reviewer for his appreciation of our work and important comments.

1.This article constructs a novel integrated vector that can express monoclonal antibodies in mammalian cells. This study has certain innovation, especially in the expression of monoclonal antibodies on vectors, providing more choices. However, there are still many issues in the article that need to be addressed before publication.

2. Some major question

2.1 It is noted that your manuscript needs careful editing by someone with expertise in technical English editing paying particular attention to English grammar, and sentence structure， I think many description was not clear to the reader.

Thank you for taking the time to provide us with your feedback. We have given your comment due consideration and engaged the services of an editor with a deep knowledge of grammar to revise the text. The text has been revised to enhance its quality and clarity.

 

2.2 In general, there is a lack of explanation of replicates and statistical methods used in the study. Theres was less Statistically analyze the differences between different groups.

Thank you for your comment. We have taken your remark into account and made appropriate changes to the Materials and Methods section, adding a description of the statistical methods used.

2.8. Statistical analysis

Statistical analyses were conducted using GraphPad Prism software (version 9.3.1; GraphPad Software, San Diego, CA, USA). Differences were considered statisti-cally significant at p < 0.05.

 

2.3 The conclusion needs to be revised, it should be more concise and highlight key points.

Thanks for the comment. The conclusion has been revised with a focus on concision and emphasis on the key findings of the study. The text now highlights the main achievements:

As part of the research program, an integration vector for the expression of recombinant proteins in mammalian cells was developed. This vector incorporates an artificial signaling sequence (176), which is a key feature of the construct. The vector was successfully used to produce a monoclonal antibody, designated 10H10, which exhibits specificity for the E surface protein of flaviviruses. The design of the vector includes several regulatory elements: the Gaussia luciferase signal sequence, IRES, P2A peptide, and furin cleavage site, which ensure the proper processing of the synthesized polypeptide and the assembly of the antibody's heavy and light chains. Analysis of the culture fluid from the clones showed antibody production in over 95% of cases. The level of antibody production by adhesion culturing, without optimization of conditions, was found to be 15 mg/L. These results confirm the promising potential of the proposed vector for the production of full-length antibodies and other recombinant proteins in cell systems. Please note that any amendments to the text are highlighted in purple.

We hope that you will be satisfied with the revised version.

3. Other small mistake like:

3.1 Page 85 The website address should be correct showing. The codon optimization was made for expression in with the on-line Codon Optimisation Tool software

 Thanks for the comment. We have amended the link in accordance with your suggestions.

The variable sequence of the gene encoding the 10H10 heavy chain (GenBank OK483332) and the variable sequence of the gene encoding the 10H10 light chain (GenBank OL448869) were retrieved from the GenBank database and optimized for codon composition for CHO cells using the online Codon Optimisation Tool software https://www.idtdna.com/pages/tools/codon-optimization-tool).

 

3.2 The Figure could be more draw reasonable.

Thanks for the comment. We conducted a review of the figures and implemented changes to enhance their clarity and informational value. The figure captions were enhanced with detailed explanations to provide greater clarity on key aspects of the study. Furthermore, the graphical component of the figure has been redesigned to more accurately reflect the main results and the logic of the experiment. We trust that the revised version meets your expectations.

 

3.3 The Figure 2 should be highlight key points.

Thanks for the comment. We have revised Figure 2 and made changes to enhance its clarity and informational value.

Figure 2. Schematic representation of the expression cassette. Key elements of the construct: 176 – nucleotide sequence encoding a hybrid signal peptide from luciferase (Cypridina noctiluca) and fi-broin (Dendrolimus spectabilis), facilitating protein export from the cell; Furin – nucleotide sequence encoding the proteolytic site for the cellular protease furin; P2A – nucleotide sequence encoding the self-cleaving P2A peptide; GL – nucleotide sequence encoding the Gaussia luciferase signal se-quence, facilitating protein export from the cell; EMCV IRES – internal ribosome entry site. Additional elements: CMV promoter – CMV promoter region; Puro – nucleotide sequence encoding the antibiotic resistance factor for puromycin.

3.4 Figure 5 1-3 should be describe as Lane 1-3.

 Thanks for the comment. We have revised Figure 5 and made changes to enhance its clarity and informational value:

Figure 5. Electrophoresis in 15% SDS-PAGE. Lane 1 – recombinant 10H10ch antibody under de-naturing conditions (concentration ~ 10 µg/well); lane 2 – murine 10H10 antibody under dena-turing conditions (concentration ~ 20 µg/well); lane 3 – molecular weight markers (250–10 kDa). The recombinant 10H10ch antibody was isolated using affinity chromatography. The murine 10H10 was isolated using a caprylic acid purification protocol from mouse ascites fluid.

 

4. Statistics: I am not sure if Figure 6 requires significant statistical differences.

Thanks for the comment. We concur that in this instance, the indication of statistical differences between the groups in Figure 6 is superfluous, given that the figure's primary objective is to illustrate the retention of the recombinant antibody's functional properties. To provide a clear example, the figure also includes data for the original mouse antibody, allowing for a qualitative comparison. This approach emphasises the key message of the study, eliminating the need for statistical analysis to confirm the main message of the figure.

 

5. Reference: The reference format should be consistent, as there are many small errors inside. For example:

5.1) some reference The NO. following the number, however other following the month date

Thanks for the comment. You are correct in your observation that there were technical inconsistencies in the submitted references. We have conducted a comprehensive review and updated the formatting of the references to align with the journal's standards. Your feedback has enabled us to enhance the quality of the data presentation.

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the authors developed an integrative vector pVEAL3 for the expression of full-length recombinant monoclonal antibodies in mammalian cells. The vector includes a sequence encoding an artificial chimeric signal peptide 176, as well as various regulatory elements such as the Gaussia luciferase signal sequence, IRES, the P2A peptide, and a furin cleavage site. Both the P2A peptide (self-cleaving) and IRES (internal ribosome entry site) are usually used for the co-expressionof  two or more protein on a single vector and to coordinate the synthesis levels of the antibody chains. However, the use of an IRES-element often results in reduced protein expression of downstream genes, ranging from 6 to 100%, making this system unpredictable, which is why self-cleaving 2A peptides are a better choice (see ref. 24). So I do not understand why this IRES sequence was added, because it is located after the antibody gene sequence (HC) and before the puromycin gene sequence and contribute nothing. The explanation in lines 197-198: “The configuration used in our work theoretically reduces the synthesis level of the selection factor, allowing for the selection of clones with higher yields of the target protein” does not provide proper clarification. So what does it  such a configuration do in reality? Concluding, either a P2A peptide or an IRES sequence should be used. Please explain this in the manuscript.

Both, Peptide 176 and Gaussia luciferase signal sequence facilitate protein export from the cell. Please explain why both are used – it may be sufficient to use only peptide 176  once or twice. Is it necessary to use such signals before LC and HC sequences separately?

In the caption of Figure 5, the amount of antibody loaded on the lines (in μg) should be added. Please also add an explanation of whether antibodies after purification were loaded in both cases.

In Figure 6 - add the clear label “mouse 10H10” in the first line of the legend.

Ref 15 – please add the reference in English or describe briefly the used standard cloning techniques.

 Altogether, I suggest major revision.

Author Response

Reviewer 2

The team of authors would like to thank the reviewer for his appreciation of our work and important comments.

In this manuscript, the authors developed an integrative vector pVEAL3 for the expression of full-length recombinant monoclonal antibodies in mammalian cells. The vector includes a sequence encoding an artificial chimeric signal peptide 176, as well as various regulatory elements such as the Gaussia luciferase signal sequence, IRES, the P2A peptide, and a furin cleavage site. Both the P2A peptide (self-cleaving) and IRES (internal ribosome entry site) are usually used for the co-expressionof  two or more protein on a single vector and to coordinate the synthesis levels of the antibody chains. However, the use of an IRES-element often results in reduced protein expression of downstream genes, ranging from 6 to 100%, making this system unpredictable, which is why self-cleaving 2A peptides are a better choice (see ref. 24). So I do not understand why this IRES sequence was added, because it is located after the antibody gene sequence (HC) and before the puromycin gene sequence and contribute nothing. The explanation in lines 197-198: “The configuration used in our work theoretically reduces the synthesis level of the selection factor, allowing for the selection of clones with higher yields of the target protein” does not provide proper clarification. So what does it such a configuration do in reality? Concluding, either a P2A peptide or an IRES sequence should be used. Please explain this in the manuscript.

We completely agree with the remark! You are correct in your assertion that the use of the IRES element often results in decreased expression of downstream genes, which makes it an unreliable tool for synthesising target proteins. However, in our study, we used the IRES element not for the synchronous expression of two subunits, but as a tool to create a controlled selection of clones with high productivity. Theoretically, the placement of an IRES in this configuration could result in a reduction of the expression level with respect to a selection factor (puromycin) by 50%. This enables the selection of clones with higher productivity of the target protein, as they will be less sensitive to antibiotic selection. This approach has been shown to improve the overall yield of the target protein in a number of published studies, including: Ho S. C. L. et al. Comparison of internal ribosome entry site (IRES) and Furin-2A (F2A) for monoclonal antibody expression level and quality in CHO cells //PloS one. – 2013. – vol. 8. – no. 5. – P. 3247; Ho S. C. L. et al. IRES-mediated Tricistronic vectors for enhancing generation of high monoclonal antibody expressing CHO cell lines //Journal of biotechnology. – 2012. – vol. 157. – no. 1. – pp. 130-139. The use of IRES or its modifications (e.g., IRESsatt) has been shown to reduce the proportion of non-expressing clones and increase productivity. To ensure synchronised expression of antibody heavy and light chains, we selected a superior and more robust tool, the self-replicating peptide 2A. The manuscript has been updated to provide a detailed explanation of how IRES is employed in our work and to highlight its significance in the selection of highly productive clones, despite its inherent unpredictability with respect to expression. Additionally, the text has been expanded to provide a comprehensive rationale for the selection of the 2A peptide for synchronising the expression of heavy and light antibody chains. The relevant information is included in the Results and Discussion section:

In the process of developing expression vectors for the synthesis of complex proteins, it is essential to ensure that the subunits that comprise the complex protein are expressed at a synchronous level. This is in addition to the necessity of obtaining highly productive clones. The utilisation of the IRES element may result in an unanticipated level of expression of genes situated downstream of it in the cassette, which may fluctuate considerably, from 6% to 100% [17, 33]. Nevertheless, this feature can be employed as a tool for the selection of highly productive clones. To illustrate, reducing the expression level to 50% theoretically favours the selection of clones with higher productivity of the target protein due to the action of a selective antibiotic. As evidenced by existing literature, the utilisation of IRES or its derivatives (e.g., IRES-satt) has been demonstrated to facilitate a reduction in the proportion of non-expressing clones, thereby enhancing the overall yield of the target protein. [2,34–35].

Design of an expression vector for full-length antibody production requires the control of the ratio of light and heavy chains to ensure proper tetrameric antibody structure formation. Studies show that maintaining an equivalent or slightly excess expression of the light chain compared to the heavy chain is necessary. In order to guarantee the synchronised expression of heavy and light chains of antibodies, it is recommended that the use of self-splicing peptide 2A be employed, as evidenced by a number of studies [2, 6, 36]. This approach allows for the coordinated synthesis of both chains. Nevertheless, it is acknowledged that the efficacy of 2A peptide cleavage may be insufficient to optimise system performance [15]. To mitigate this effect, a furin proteolysis site was incorporated into the developed construct, thereby compensating for the deficiencies of the 2A peptide and enhancing the efficiency of the expression of the target proteins [17]. In the developed vector, the sequence encoding the light chain is placed before the heavy chain to enhance antibody expression and minimize the risk of undesired oligomerization [37].

From here on our edits are highlighted in green.

Both, Peptide 176 and Gaussia luciferase signal sequence facilitate protein export from the cell. Please explain why both are used – it may be sufficient to use only peptide 176 once or twice. Is it necessary to use such signals before LC and HC sequences separately?

Thank You for valuable comment. We have developed a new signal peptide, 176, but given the lack of preliminary data on its efficacy in this expression system, we believe it would be too risky to use it simultaneously for both subunits of the antibody (LC and HC). To mitigate risks and obtain reliable data, we opted to utilise a combination of two signal peptides: the novel peptide 176 for one subunit and the widely used Gaussia luciferase signal peptide sequence for the other. This approach enables us to assess the functionality of peptide 176 and its interaction with the expression system. Going forward, we intend to undertake comparative studies of various combinations of signal peptides, including a variant utilising peptide 176 for both antibody chains. A comparable approach to optimising protein export using signal peptides has been successfully deployed in studies such as: You M. et al. Efficient mAb production in CHO cells with optimized signal peptide, codon, and UTR //Applied microbiology and biotechnology. – 2018. – vol. 102. – pp. 5953-5964; Haryadi R. et al. Optimization of heavy chain and light chain signal peptides for high level expression of therapeutic antibodies in CHO cells //PloS one. – 2015. – vol. 10. – no. 2. – P. 116878. The current design employs two signal peptides to validate the new peptide 176.

In the caption of Figure 5, the amount of antibody loaded on the lines (in μg) should be added. Please also add an explanation of whether antibodies after purification were loaded in both cases.

Thanks for the comment. the caption to the figure has been changed:

Figure 5. Electrophoresis in 15% SDS-PAGE. Lane 1 – recombinant 10H10ch antibody under denaturing conditions (concentration ~ 10 µg/well); lane 2 – murine 10H10 antibody under denaturing conditions (concentration ~ 20 µg/well); lane 3 – molecular weight markers (250–10 kDa). The recombinant 10H10ch antibody was isolated using affinity chromatography. The murine 10H10 was isolated using a caprylic acid purification protocol from mouse ascites fluid.

In Figure 6 - add the clear label “mouse 10H10” in the first line of the legend.

Thanks for the comment. the caption to the figure has been changed:

Figure 6. The results of the interaction of the recombinant 10H10ch and murine 10H10 antibodies with recombinant fragments of flavivirus envelope proteins are as follows: TEF1 represents the 1+2 do-mains of the E protein from tick-borne encephalitis virus; ZEF1 denotes the 1+2 domains of the E protein from Zika virus; WEF1 refers to the 1+2 domains of the E protein from West Nile virus; DEF1 comprises the 1+2 domains of the E protein from Dengue virus; and TNS1 indicates the non-structural protein 1 from tick-borne encephalitis virus.

Ref 15 – please add the reference in English or describe briefly the used standard cloning techniques.

Construction of the Integration Vector pVEAL3-10H10ch added a brief description of the standard cloning methods:

The variable sequence of the gene encoding the 10H10 heavy chain (GenBank OK483332) and the variable sequence of the gene encoding the 10H10 light chain (GenBank OL448869) were retrieved from the GenBank database and optimized for codon composition for CHO cells using the online Codon Optimisation Tool software https://www.idtdna.com/pages/tools/codon-optimization-tool). The final nucleotide sequence was supplemented with sequences encoding the 176 signal peptide, the constant region of the light chain, the cleavage site for cellular protease furin, the self-cleaving P2A peptide, the Gaussia luciferase signal sequence, and the constant region of the heavy chain (CH1-CH2-CH3), as well as restriction sites for NheI and BstXI. The synthesized nucleotide sequence of 10H10 was cloned into the pVEAL3 vector using standard cloning techniques [26]. Polymerase chain reaction (PCR) was used to amplify the synthesized nucleotide sequence with Q5 High-Fidelity DNA polymerase. The primers 10H10-pVL3_F (5'-aaaaaaaaagctagCACCACCATGATGCGGACCCTGATCCTGGCTGTGCTGCTGCTG-3') and 10H10-BstXI_R (5'-aaaaaaGTGCAGCTGCTGCTGGAGGGCACGG-3') were used for amplification. The PCR products and plasmid vector pVEAL3 were then treated with NheI and BstXI restriction endonucleases and purified using the Gel Ex-traction Kit (Qiagen, Germany) according to the manufacturer's instructions. Ligation of the PCR product with the pVEAL3 vector was performed using T4 DNA ligase (SibEnzyme, Russia). The resulting constructs were introduced into competent Escherichia coli cells (NebStable strain) by transformation. The primary structure of the expression vector was verified by Sanger sequencing at the Genomics Centre of the Siberian Branch of the Russian Academy of Sciences (Novosibirsk).

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Summary:  A vector was designed and tested for expression of a monoclonal antibody in Chinese Hamster Ovary cells.  The design included a feature that integrated the plasmid into the gene for stable long term expression.  The light and heavy chains were expressed from one plasmid, which ensured equal levels of light and heavy chains.  The yield was 15 mg antibody per liter of culture medium.  The chimeric recombinant antibody recognized the target epitope on the surface of flavivirus E proteins.  It was concluded that the integration plasmid vector developed in this work was useful for producing functional antibodies.

   

 

1.     Figure 4.  Please define the abbreviation K.  Is it potassium? Or Lysine? Or kanamycin?

2.     In this reviewer’s experience a critical feature in vector design is the Kozak sequence.  The following quote describes the Kozak sequence.

“The Kozak sequence is a nucleic acid motif that indicates the start site for protein translation in most eukaryotic mRNA transcripts. It's the sequence that surrounds the AUG start codon of an open reading frame. The Kozak sequence directs the ribosome and pre-initiation complex to the start codon, which helps ensure the correct protein sequence is translated. 

The consensus Kozak sequence is GCCGCCACCAUGG, where the A in the AUG start codon is coordinate 1. A purine (usually A) in position -3 is important for efficient translation initiation. If a purine isn't present in position -3, a G at position +4 is essential. 

The Kozak sequence affects the likelihood that a ribosome will recognize and translate the start codon. A weak Kozak sequence could mean that the mRNAs encode proteins that need to be expressed at low levels or have a regulatory role. A strong Kozak sequence enhances the recognition of the correct AUG codon, while a weak Kozak sequence might lead to translation initiation from a downstream start codon.”

            Does the AUG start site in your vector have the Kozak sequence?

 

Author Response

Reviewer 3

The team of authors would like to thank the reviewer for his appreciation of our work and important comments.

1. Figure 4. Please define the abbreviation K.  Is it potassium? Or Lysine? Or kanamycin?

Thanks for the comment. This was a technical issue. The K+ symbol has been replaced with the phrase 'positive control' and the K- symbol with 'negative control'.

2. In this reviewer’s experience a critical feature in vector design is the Kozak sequence. The following quote describes the Kozak sequence.

“The Kozak sequence is a nucleic acid motif that indicates the start site for protein translation in most eukaryotic mRNA transcripts. It's the sequence that surrounds the AUG start codon of an open reading frame. The Kozak sequence directs the ribosome and pre-initiation complex to the start codon, which helps ensure the correct protein sequence is translated.

The consensus Kozak sequence is GCCGCCACCAUGG, where the A in the AUG start codon is coordinate 1. A purine (usually A) in position -3 is important for efficient translation initiation. If a purine isn't present in position -3, a G at position +4 is essential.

The Kozak sequence affects the likelihood that a ribosome will recognize and translate the start codon. A weak Kozak sequence could mean that the mRNAs encode proteins that need to be expressed at low levels or have a regulatory role. A strong Kozak sequence enhances the recognition of the correct AUG codon, while a weak Kozak sequence might lead to translation initiation from a downstream start codon.”

 

            Does the AUG start site in your vector have the Kozak sequence?

Thanks for the comment. In the construction of our vector, we considered the necessity of including the Kozak sequence. As you correctly identified, not all nucleotide residues in the structure are of equal importance. The most important residues are from -4 to -1 and +4. However, we were unable to change the position of +4, so we have included the CACC sequence before the ATG. To ensure successful protein synthesis, our vector also incorporates the Kozak consensus sequence. Furthermore, the CACC motif was included before the start codon. Furthermore, we have included a brief description in the article text.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors improved the manuscript according to my suggestions.