Rapid development of stable transgene CHO cell lines by CRISPR/Cas9-mediated site-specific integration into C12orf35

Revisiting the impact of genomic hot spots: C12orf35 locus as a hot spot and engineering target

Traditional Chinese hamster ovary (CHO) cell line development is based on random integration (RI) of transgene that causes clonal variation and subsequent large-scale clone screening. Therefore, site-specific integration (SSI) of transgenes into genomic hot spots has recently emerged as an alternative method for cell line development. However, the specific mechanisms underlying hot spot site formation remain unclear. In this study, we aimed to generate landing pad (LP) cell lines via the RI of transgenes encoding fluorescent reporter proteins flanked by recombination sites to facilitate recombinase-mediated cassette exchange. The RI-based LP cell line expressing high reporter levels with spontaneous C12orf35 locus deletion exhibited similar reporter fluorescent protein levels compared to targeted integrants with an identical reporter LP construct at the CHO genome hot spot, the C12orf35 locus. Additionally, Resf1, a C12orf35 locus gene, knockout (KO) in the RI-based LP cell line with conserved C12orf35 increased reporter expression levels, comparable to those in cell lines with C12orf35 locus disruption. These results indicate that the effect of SSI into the C12orf35 locus, a genomic hot spot, on high-level transgene expression was caused by C12orf35 disruption. In contrast to C12orf35 KO, KO at other well-known hot spot sites at specific loci of genes, including Fer1L4, Hprt1, Adgrl4, Clcc1, Dop1b, and Ddc, did not increase transgene expression. Overall, our findings suggest that C12orf35 is a promising engineering target and a hot spot for SSI-based cell line development.

The new frontier in CHO cell line development: From random to targeted transgene integration technologies

Cell line development represents a crucial step in the development process of a therapeutic glycoprotein. Chinese hamster ovary (CHO) cells are the most frequently employed mammalian host cell system for the industrial manufacturing of biologics. The predominant application of CHO cells for heterologous recombinant protein expression lies in the relative simplicity of stably introducing ectopic DNA into the CHO host cell genome. Since CHO cells were first used as expression host for the industrial production of biologics in the late 1980s, stable genomic transgene integration has been achieved almost exclusively by random integration. Since then, random transgene integration had become the gold standard for generating stable CHO production cell lines due to a lack of viable alternatives. However, it was eventually demonstrated that this approach poses significant challenges on the cell line development process such as an increased risk of inducing cell line instability. In recent years, significant discoveries of new and highly potent (semi)-targeted transgene integration systems have paved the way for a technological revolution in the cell line development sector. These advanced methodologies comprise the application of transposase-, recombinase- or Cas9 nuclease-mediated site-specific genomic integration techniques, which enable a scarless transfer of the transgene expression cassette into transcriptionally active loci within the host cell genome. This review summarizes recent advancements in the field of transgene integration technologies for CHO cell line development and compare them to the established random integration approach. Moreover, advantages and limitations of (semi)-targeted integration techniques are discussed, and benefits and opportunities for the biopharmaceutical industry are outlined.

A precise and sustainable doxycycline-inducible cell line development platform for reliable mammalian cell engineering with gain-of-function mutations

For mammalian synthetic biology research, multiple orthogonal and tunable gene expression systems have been developed, among which the tetracycline (Tet)-inducible system is a key tool for gain-of-function mutations. Precise and long-lasting regulation of genetic circuits is necessary for the effective use of these systems in genetically engineered stable cell lines. However, current cell line development strategies, which depend on either random or site-specific integration along with antibiotic selection, are unpredictable and unsustainable, limiting their widespread use. To overcome these issues, we aimed to establish a Robust Overexpression via Site-specific integration of Effector (ROSE) system, a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated streamlined Tet-On3G-inducible master cell line (MCL) development platform. ROSE MCLs equipped with a landing pad facilitated the transcriptional regulation of various effector genes via recombinase-mediated cassette exchange. Long-term investigation revealed that the modular design of genetic payloads and integration sites significantly affected the induction capacity and stability, with ROSE MCLs exhibiting exceptional induction performance. To demonstrate the versatility of our platform, we explored its efficiency for the precise regulation of selection stringency, manufacturing of therapeutic antibodies with tunable expression levels and timing, and transcription factor engineering. Overall, this study demonstrated the effectiveness and reliability of the ROSE platform, highlighting its potential for various biological and biotechnological applications.

Promoting the production of challenging proteins via induced expression in CHO cells and modified cell-free lysates harboring T7 RNA polymerase and mutant eIF2α

Chinese hamster ovary (CHO) cells are crucial in biopharmaceutical production due to their scalability and capacity for human-like post-translational modifications. However, toxic proteins and membrane proteins are often difficult-to-express in living cells. Alternatively, cell-free protein synthesis can be employed. This study explores innovative strategies for enhancing the production of challenging proteins through the modification of CHO cells by investigating both, cell-based and cell-free approaches. A major result in our study involves the integration of a mutant eIF2 translation initiation factor and T7 RNA polymerase into CHO cell lysates for cell-free protein synthesis. This resulted in elevated yields, while eliminating the necessity for exogenous additions during cell-free production, thereby substantially enhancing efficiency. Additionally, we explore the potential of the Rosa26 genomic site for the integration of T7 RNA polymerase and cell-based tetracycline-controlled protein expression. These findings provide promising advancements in bioproduction technologies, offering flexibility to switch between cell-free and cell-based protein production as needed.

Gene copy number, gene configuration and LC/HC mRNA ratio impact on antibody productivity and product quality in targeted integration CHO cell lines

The augmentation of transgene copy numbers is a prevalent approach presumed to enhance transcriptional activity and product yield. CHO cell lines engineered via targeted integration (TI) offer an advantageous platform for investigating the interplay between gene copy number, mRNA abundance, product yield, and product quality. Our investigation revealed that incrementally elevating the gene copy numbers of both IgG heavy chain (HC) and light chain (LC) concurrently resulted in the attainment of plateaus in mRNA levels and product titers, notably occurring beyond four to five gene copies integrated at the same TI site. Furthermore, maintaining a fixed gene copy number while varying the position of genes within the vector influenced the LC/HC mRNA ratio, which subsequently exerted a substantial impact on product titer. Moreover, manipulation of the LC/HC gene ratio through the introduction of surplus LC gene copies led to heightened LC mRNA expression and a reduction in the levels of high molecular weight species. It is noteworthy that the effects of excess LC on product titer were dependent on the specific molecule under consideration. The strategic utilization of PCR tags enabled precise quantification of transcription from each expression slot within the vector, facilitating the identification of highly expressive and less expressive slots. Collectively, these findings significantly enhance our understanding of stable antibody production in TI CHO cell lines.

Manipulating gene expression levels in mammalian cell factories: An outline of synthetic molecular toolboxes to achieve multiplexed control

Mammalian cells have developed dedicated molecular mechanisms to tightly control expression levels of their genes where the specific transcriptomic signature across all genes eventually determines the cell's phenotype. Modulating cellular phenotypes is of major interest to study their role in disease or to reprogram cells for the manufacturing of recombinant products, such as biopharmaceuticals. Cells of mammalian origin, for example Chinese hamster ovary (CHO) and Human embryonic kidney 293 (HEK293) cells, are most commonly employed to produce therapeutic proteins. Early genetic engineering approaches to alter their phenotype have often been attempted by "uncontrolled" overexpression or knock-down/-out of specific genetic factors. Many studies in the past years, however, highlight that rationally regulating and fine-tuning the strength of overexpression or knock-down to an optimum level, can adjust phenotypic traits with much more precision than such "uncontrolled" approaches. To this end, synthetic biology tools have been generated that enable (fine-)tunable and/or inducible control of gene expression. In this review, we discuss various molecular tools used in mammalian cell lines and group them by their mode of action: transcriptional, post-transcriptional, translational and post-translational regulation. We discuss the advantages and disadvantages of using these tools for each cell regulatory layer and with respect to cell line engineering approaches. This review highlights the plethora of synthetic toolboxes that could be employed, alone or in combination, to optimize cellular systems and eventually gain enhanced control over the cellular phenotype to equip mammalian cell factories with the tools required for efficient production of emerging, more difficult-to-express biologics formats.

Enhancing Cell Line Stability by CRISPR/Cas9-Mediated Site-Specific Integration Based on Histone Modifications

In traditional cell line design pipelines, cost- and time-intensive long-term stability studies must be performed due to random integration of the transgene into the genome. By this, integration into epigenetically silenced regions can lead to silencing of the recombinant promoter over time. Site-specific integration into regions with active chromatin structure can overcome this problem and lead to strong and stable gene expression. Here, we describe a detailed protocol to identify integration sites with epigenetically preferable properties by chromatin immunoprecipitation sequencing and use them for stable and strong gene expression by applying CRISPR/Cas9. Furthermore, the examination of the integration sites with focus on Cas9-targeted sequencing with nanopores is described.

Construction and application of a multifunctional CHO cell platform utilizing Cre/lox and Dre/rox site-specific recombination systems

During the development of traditional Chinese hamster ovary (CHO) cell lines, target genes randomly integrate into the genome upon entering the nucleus, resulting in unpredictable productivity of cell clones. The characterization and screening of high-yielding cell lines is a time-consuming and expensive process. Site-specific integration is recognized as an effective approach for overcoming random integration and improving production stability. We have designed a multifunctional expression cassette, called CDbox, which can be manipulated by the site-specific recombination systems Cre/lox and Dre/rox. The CDbox expression cassette was inserted at the Hipp11(H11) locus hotspot in the CHO-K1 genome using CRISPR/Cas9 technology, and a compliant CHO-CDbox cell platform was screened and obtained. The CHO-CDbox cell platform was transformed into a pool of EGFP-expressing cells using Cre/lox recombinase-mediated cassette exchange (RMCE) in only 2 weeks, and this expression remained stable for at least 75 generations without the need for drug stress. Subsequently, we used the Dre/rox system to directly eliminate the EGFP gene. In addition, two practical applications of the CHO-CDbox cell platform were presented. The first was the quick construction of the Pembrolizumab antibody stable expression strain, while the second was a protocol for the integration of surface-displayed and secreted antibodies on CHO cells. The previous research on site-specific integration of CHO cells has always focused on the single functionality of insertion of target genes. This newly developed CHO cell platform is expected to offer expanded applicability for protein production and gene function studies.

Identification of a New Integration Site and Study on Site-Specific Integration in CHO-K1 Cells

Site-specific integration is an important approach used to address the problem of unstable cell lines in industry. In this study, we observed a reduction in the gene copy number and antibody production in a CHOK1 cell line BA03 capable of high antibody expression. We identified a new integration site named locus 7 in the intron region of the parva gene through sequencing, FISH, and genome walking. We demonstrate that the integration of the exogenous gene at this locus does not affect the transcription of the parva and, therefore, has a minimal impact on cell growth. We designed sgRNA and donor vectors to integrate the etanercept-coding gene into locus 7 and obtained a cell line, SSI-4. We performed a passaged stability study on SSI-4 and proved the possibility of the stable, site-specific integration of exogenous genes at this locus in terms of integration site, copy number, expression level, and cell growth. In summary, our study has identified a new integration site suitable for site-specific integration, which lays the foundation for the subsequent development of site-specific integration cell lines.

CRISPR-Cas9 Mediated Stable Expression of Exogenous Proteins in the CHO Cell Line through Site-Specific Integration

Chinese hamster ovary (CHO) cells are a popular choice in biopharmaceuticals because of their beneficial traits, including high-density suspension culture, safety, and exogenously produced proteins that closely resemble natural proteins. Nevertheless, a decline in the expression of exogenous proteins is noted as culture time progresses. This is a consequence of foreign gene recombination into chromosomes by random integration. The current investigation employs CRISPR-Cas9 technology to integrate foreign genes into a particular chromosomal location for sustained expression. Results demonstrate the successful integration of enhanced green fluorescent protein (EGFP) and human serum albumin (HSA) near base 434814407 on chromosome NC_048595.1 of CHO-K1 cells. Over 60 successive passages, monoclonal cell lines were produced that consistently expressed all relevant external proteins without discernible variation in expression levels. In conclusion, the CHO-K1 cell locus, NC_048595.1, proves an advantageous locus for stable exogenous protein expression. This study provides a viable approach to establishing a CHO cell line capable of enduring reliable exogenous protein expression.

Engineered CHO cells as a novel AAV production platform for gene therapy delivery

The Herpes simplex virus (HSV)-based platform for production of recombinant adeno-associated viral vectors (rAAVs) yields higher titers and increased percentage of full capsids when compared to the triple transient transfection (TTT) method. However, this platform currently faces two major challenges. The first challenge is the reliance on commercial media, sometimes supplemented with serum, leading to costly manufacturing and a high risk for introduction of adventitious agents. The second challenge is that the production of HSV-1 relies on adherent complementing Vero cells (V27), making it difficult to scale up. We engineered serum-free-adapted CHO cells expressing key HSV-1 entry receptors, HVEM and/or Nectin-1 to address the first challenge. Using high-throughput cloning methods, we successfully selected a HVEM receptor-expressing clone (CHO-HV-C1) that yields 1.62 × 109, 2.51 × 109, and 4.07 × 109 viral genome copies/mL with rAAV6.2-GFP, rAAV8-GFP, and rAAV9-GFP vectors respectively, within 24 h post rHSV-1 co-infection. Moreover, CHO-HV-C1-derived rAAVs had comparable in vitro transduction, infectivity, and biodistribution titers to those produced by TTT. The second challenge was addressed via engineering CHO-HV-C1 cells to express HSV-1 CP27. These cells successfully produced rHSV-1 vectors, but with significantly lower titers than V27 cells. Taken together, the CHO/HSV system provides a novel, scalable, reduced cost, serum-free AAV manufacturing platform.

Cell factory engineering: Challenges and opportunities for synthetic biology applications

The production of high-quality recombinant proteins is critical to maintaining a continuous supply of biopharmaceuticals, such as therapeutic antibodies. Engineering mammalian cell factories presents a number of limitations typically associated with the proteotoxic stress induced upon aberrant accumulation of off-pathway protein folding intermediates, which eventually culminate in the induction of apoptosis. In this review, we will discuss advances in cell engineering and their applications at different hierarchical levels of control of the expression of recombinant proteins, from transcription and translational to posttranslational modifications and subcellular trafficking. We also highlight challenges and unique opportunities to apply modern synthetic biology tools to the design of programmable cell factories for improved biomanufacturing of therapeutic proteins.

Nanopore Cas9-targeted sequencing enables accurate and simultaneous identification of transgene integration sites, their structure and epigenetic status in recombinant Chinese hamster ovary cells

The integration of a transgene expression construct into the host genome is the initial step for the generation of recombinant cell lines used for biopharmaceutical production. The stability and level of recombinant gene expression in Chinese hamster ovary (CHO) can be correlated to the copy number, its integration site as well as the epigenetic context of the transgene vector. Also, undesired integration events, such as concatemers, truncated, and inverted vector repeats, are impacting the stability of recombinant cell lines. Thus, to characterize cell clones and to isolate the most promising candidates, it is crucial to obtain information on the site of integration, the structure of integrated sequence and the epigenetic status. Current sequencing techniques allow to gather this information separately but do not offer a comprehensive and simultaneous resolution. In this study, we present a fast and robust nanopore Cas9-targeted sequencing (nCats) pipeline to identify integration sites, the composition of the integrated sequence as well as its DNA methylation status in CHO cells that can be obtained simultaneously from the same sequencing run. A Cas9-enrichment step during library preparation enables targeted and directional nanopore sequencing with up to 724× median on-target coverage and up to 153 kb long reads. The data generated by nCats provides sensitive, detailed, and correct information on the transgene integration sites and the expression vector structure, which could only be partly produced by traditional Targeted Locus Amplification-seq data. Moreover, with nCats the DNA methylation status can be analyzed from the same raw data without prior DNA amplification.

Improving recombinant protein production in CHO cells using the CRISPR-Cas system

Chinese hamster ovary (CHO) cells are among the most widely used mammalian cell lines in the biopharmaceutical industry. Therefore, it is not surprising that significant efforts have been made around the engineering of CHO cells using genetic engineering methods such as the CRISPR-Cas system. In this review, we summarize key recent studies that have used different CRISPR-Cas systems such as Cas9, Cas13 or dCas9 fused with effector domains to improve recombinant protein (r-protein) production in CHO cells. Here, every relevant stage of production was considered, underscoring the advantages and limitations of these systems, as well as discussing their bottlenecks and probable solutions. A special emphasis was given on how these systems could disrupt and/or regulate genes related to glycan composition, which has relevant effects over r-protein properties and in vivo activity. Furthermore, the related promising future applications of CRISPR to achieve a tunable, reversible, or highly stable editing of CHO cells are discussed. Overall, the studies covered in this review show that despite the complexity of mammalian cells, the synthetic biology community has developed many mature strategies to improve r-protein production using CHO cells. In this regard, CRISPR-Cas technology clearly provides efficient and flexible genetic manipulation and allows for the generation of more productive CHO cell lines, leading to more cost-efficient production of biopharmaceuticals, however, there is still a need for many emerging techniques in CRISPR to be reported in CHO cells; therefore, more research in these cells is needed to realize the full potential of this technology.

CRISPR-interceded CHO cell line development approaches

For industrial production of recombinant protein biopharmaceuticals, Chinese hamster ovary (CHO) cells represent the most widely adopted host cell system, owing to their capacity to produce high-quality biologics with human-like posttranslational modifications. As opposed to random integration, targeted genome editing in genomic safe harbor sites has offered CHO cell line engineering a new perspective, ensuring production consistency in long-term culture and high biotherapeutic expression levels. Corresponding the remarkable advancements in knowledge of CRISPR-Cas systems, the use of CRISPR-Cas technology along with the donor design strategies has been pushed into increasing novel scenarios in cell line engineering, allowing scientists to modify mammalian genomes such as CHO cell line quickly, readily, and efficiently. Depending on the strategies and production requirements, the gene of interest can also be incorporated at single or multiple loci. This review will give a gist of all the most fundamental recent advancements in CHO cell line development, such as different cell line engineering approaches along with donor design strategies for targeted integration of the desired construct into genomic hot spots, which could ultimately lead to the fast-track product development process with consistent, improved product yield and quality.

A comprehensive evaluation of stable expression "hot spot" in the ScltI gene of Chinese hamster ovary cells

The Chinese hamster ovary (CHO) cell is the most widely used biopharmaceutical expression system, but its long-term expression is unstable. This issue can be effectively addressed by site-specific integration of exogenous genes into the genome. Therefore, exogenous protein sites with stable expression in the CHO cell genome must be identified. CRISPR/Cas9 technology was used in this study to integrate various exogenous genes into the ScltI site as a "hot spot" at the CHO-K1 cell genome NW_003614095.1, and the stability and adaptability of exogenous genes expressed at the site were investigated. Flow cytometry sorting technology was used to obtain positive monoclonal cell lines that expressed either intracellular protein green fluorescent protein (EGFP) or secretory protein human serum albumin (HSA). For 60 passages, the positive monoclonal cell lines' cell growth cycles and exogenous protein expression were both observed. The results demonstrated that integrating the gene encoding exogenous proteins into the ScltI site had no effect on cell growth. The fluorescence intensity of EGFP was similar after 60 passages, and the expression of HSA increased slightly. Additionally, the super-monomeric protein VWF hydrolase (ADAMTS13) (190 kDa), human coagulation factor VII (FVII) (55 kDa), and interferon α2b (12 kDa) were integrated into the ScltI site for expression. In conclusion, the site located in the first exon of the ScltI gene within the CHO-K1 cell genome NW_003614095.1 is an ideal "hot spot" for the stable expression of various exogenous proteins. KEY POINTS: • The site-specific integration strategy of an exogenous gene in CHO cells was established for the ScltI site. • The genes for EGFP and HSA were site-directed integrated and stably expressed at the ScltI site. • The ScltI site fulfills the expression of exogenous proteins of different molecular weight sizes (15-190 kDa).

Targeted integration in CHO cells using CRIS-PITCh/Bxb1 recombinase-mediated cassette exchange hybrid system

Recombinant Chinese hamster ovary (CHO) cell line development for complex biotherapeutic production is conventionally based on the random integration (RI) approach. Due to the lack of control over the integration site and copy number, RI-generated cell pools are always coupled with rigorous screening to find clones that satisfy requirements for production titers, quality, and stability. Targeted integration into a well-defined genomic site has been suggested as a possible strategy to mitigate the drawbacks associated with RI. In this work, we employed the CRISPR-mediated precise integration into target chromosome (CRIS-PITCh) system in combination with the Bxb1 recombinase-mediated cassette exchange (RMCE) system to generate an isogenic transgene-expressing cell line. We successfully utilized the CRIS-PITCh system to target a 2.6 kb Bxb1 landing pad with homology arms as short as 30 bp into the upstream region of the S100A gene cluster, achieving a targeting efficiency of 10.4%. The platform cell line (PCL) with a single copy of the landing pad was then employed for the Bxb1-mediated landing pad exchange with an EGFP encoding cassette to prove its functionality. Finally, to accomplish the main goal of our cell line development method, the PCL was applied for the expression of a secretory glycoprotein, human recombinant soluble angiotensin-converting enzyme 2 (hrsACE2). Taken together, on-target, single-copy, and stable expression of the transgene over long-term cultivation demonstrated our CRIS-PITCh/RMCE hybrid approach might possibly improve the cell line development process in terms of timeline, specificity, and stability. KEY POINTS: • CRIS-PITCh system is an efficient method for single copy targeted integration of the landing pad and generation of platform cell line • Upstream region of the S100A gene cluster of CHO-K1 is retargetable by recombinase-mediated cassette exchange (RMCE) approach and provides a stable expression of the transgene • CRIS-PITCh/Bxb1 RMCE hybrid system has the potential to overcome some limitations of the random integration approach and accelerate the cell line development timeline.

Enhancing stability of recombinant CHO cells by CRISPR/Cas9-mediated site-specific integration into regions with distinct histone modifications

Chinese hamster ovary (CHO) cells are the most important platform for producing biotherapeutics. Random integration of a transgene into epigenetically instable regions of the genome results in silencing of the gene of interest and loss of productivity during upstream processing. Therefore, cost- and time-intensive long-term stability studies must be performed. Site-specific integration into safe harbors is a strategy to overcome these limitations of conventional cell line design. Recent publications predict safe harbors in CHO cells based on omics data sets or by learning from random integrations, but those predictions remain theory. In this study, we established a CRISPR/Cas9-mediated site-specific integration strategy based on ChIP-seq data to improve stability of recombinant CHO cells. Therefore, a ChIP experiment from the exponential and stationary growth phase of a fed-batch cultivation of CHO-K1 cells yielded 709 potentially stable integration sites. The reporter gene eGFP was integrated into three regions harboring specific modifications by CRISPR/Cas9. Targeted Cas9 nanopore sequencing showed site-specific integration in all 3 cell pools with a specificity between 23 and 73%. Subsequently, the cells with the three different integration sites were compared with the randomly integrated donor vector in terms of transcript level, productivity, gene copy numbers and stability. All site-specific integrations showed an increase in productivity and transcript levels of up to 7.4-fold. In a long-term cultivation over 70 generations, two of the site-specific integrations showed a stable productivity (>70%) independent of selection pressure.

Hybrid cell line development system utilizing site-specific integration and methotrexate-mediated gene amplification in Chinese hamster ovary cells

Site-specific integration has emerged as a promising strategy for streamlined and predictable Chinese hamster ovary (CHO) cell line development (CLD). However, the low specific productivity of the targeted integrants limits their practical application. In this study, we developed a hybrid CLD platform combining site-specific integration of a transgene and dihydrofolate reductase/methotrexate (DHFR/MTX)-mediated gene amplification to generate high-producing recombinant CHO cell lines. We used the CRISPR/Cas9-based recombinase-mediated cassette exchange landing pad platform to integrate the DHFR expression cassette and transgene landing pad into a CHO genomic hot spot, C12orf35 locus, of DHFR-knockout CHO-K1 host cell lines. When subjected to various MTX concentrations up to 1 μM, EGFP-expressing targeted integrants showed a 3.6-fold increase in EGFP expression in the presence of 200 nM MTX, accompanied by an increase in the DHFR and EGFP copy number. A single-step 200 nM MTX amplification increased the specific monoclonal antibody (mAb) productivity (q_mAb) of recombinant mAb-producing targeted integrants by 2.8-folds, reaching a q_mAb of 9.1–11.0 pg/cell/day. Fluorescence in situ hybridization analysis showed colocalization of DHFR and mAb sequences at the intended chromosomal locations without clear amplified arrays of signals. Most MTX-amplified targeted integrants sustained recombinant mAb production during long-term culture in the absence of MTX, supporting stable gene expression in the amplified cell lines. Our study provides a new CLD platform that increases the productivity of targeted integrants by amplifying the transgene copies.

A physical model for dynamic assembly of human salivary stem/progenitor microstructures

The in vitro reconstructions of human salivary glands in service of their eventual medical use represent a challenge for tissue engineering. Here, we present a theoretical approach to the dynamical formation of acinar structures from human salivary cells, focusing on observed stick-slip radial expansion as well as possible growth instabilities. Our findings demonstrate the critical importance of basement membrane remodeling in controlling the growth process.

Novel CRISPR/Cas9-mediated knockout of LIG4 increases efficiency of site-specific integration in Chinese hamster ovary cell line

AIM

To investigate the impact of deficiency of LIG4 gene on site-specific integration in CHO cells.

RESULTS

CHO cells are considered the most valuable mammalian cells in the manufacture of biological medicines, and genetic engineering of CHO cells can improve product yield and stability. The traditional method of inserting foreign genes by random integration (RI) requires multiple rounds of screening and selection, which may lead to location effects and gene silencing, making it difficult to obtain stable, high-yielding cell lines. Although site-specific integration (SSI) techniques may overcome the challenges with RI, its feasibility is limited by the very low efficiency of the technique. Recently, SSI efficiency has been enhanced in other mammalian cell types by inhibiting DNA ligase IV (Lig4) activity, which is indispensable in DNA double-strand break repair by NHEJ. However, this approach has not been evaluated in CHO cells. In this study, the LIG4 gene was knocked out of CHO cells using CRISPR/Cas9-mediated genome editing. Efficiency of gene targeting in LIG4^-/--CHO cell lines was estimated by a green fluorescence protein promoterless reporter system. Notably, the RI efficiency, most likely mediated by NHEJ in CHO, was inhibited by LIG4 knockout, whereas SSI efficiency strongly increased 9.2-fold under the precise control of the promoter in the ROSA26 site in LIG4^-/--CHO cells. Moreover, deletion of LIG4 had no obvious side effects on CHO cell proliferation.

CONCLUSIONS

Deficiency of LIG4 represents a feasible strategy to improve SSI efficiency and suggests it can be applied to develop and engineer CHO cell lines in the future.

Cell Engineering and Cultivation of Chinese Hamster Ovary Cells for the Development of Orthogonal Eukaryotic Cell-free Translation Systems

The investigation of protein structures, functions and interactions often requires modifications to adapt protein properties to the specific application. Among many possible methods to equip proteins with new chemical groups, the utilization of orthogonal aminoacyl-tRNA synthetase/tRNA pairs enables the site-specific incorporation of non-canonical amino acids at defined positions in the protein. The open nature of cell-free protein synthesis reactions provides an optimal environment, as the orthogonal components do not need to be transported across the cell membrane and the impact on cell viability is negligible. In the present work, it was shown that the expression of orthogonal aminoacyl-tRNA synthetases in CHO cells prior to cell disruption enhanced the modification of the pharmaceutically relevant adenosine A2a receptor. For this purpose, in complement to transient transfection of CHO cells, an approach based on CRISPR/Cas9 technology was selected to generate a translationally active cell lysate harboring endogenous orthogonal aminoacyl-tRNA synthetase.

Generation of a Host Cell line containing a MAR-rich landing pad for site-specific integration and expression of transgenes

In recent years, targeted gene integration (TI) has been introduced as a strategy for the generation of recombinant mammalian cell lines for the production of biotherapeutics. Besides reducing the immense heterogeneity within a pool of recombinant transfectants, TI also aims at shortening the duration of the current cell line development process. Here we describe the generation of a host cell line carrying Matrix‐Attachment Region (MAR)‐rich landing pads (LPs), which allow for the simultaneous and site‐specific integration of multiple genes of interest (GOIs). We show that several copies of each chicken lysozyme 5'MAR‐based LP containing either BxB1 wild type or mutated recombination sites, integrated at one random chromosomal locus of the host cell genome. We further demonstrate that these LP‐containing host cell lines can be used for the site‐specific integration of several GOIs and thus, generation of transgene‐expressing stable recombinant clones. Transgene expression was shown by site‐specific integration of heavy and light chain genes coding for a monospecific antibody (msAb) as well as for a bi‐specific antibody (bsAb). The genetic stability of the herein described LP‐based recombinant clones expressing msAb or bsAb was demonstrated by cultivating the recombinant clones and measuring antibody titers over 85 generations. We conclude that the host cell containing multiple copies of MAR‐rich landing pads can be successfully used for stable expression of one or several GOIs.

Endogenous BiP reporter system for simultaneous identification of ER stress and antibody production in Chinese hamster ovary cells

As the biopharmaceutical industry expands, improving the production of therapeutic proteins using Chinese hamster ovary (CHO) cells is important. However, excessive and complicated protein production causes protein misfolding and triggers endoplasmic reticulum (ER) stress. When ER stress occurs, cells mediate the unfolded protein response (UPR) pathway to restore protein homeostasis and folding capacity of the ER. However, when the cells fail to control prolonged ER stress, UPR induces apoptosis. Therefore, monitoring the degree of UPR is required to achieve high productivity and the desired quality. In this study, we developed a fluorescence-based UPR monitoring system for CHO cells. We integrated mGFP into endogenous HSPA5 encoding BiP, a major ER chaperone, and the primary ER stress activation sensor, using CRISPR/Cas9-mediated targeted integration. The mGFP expression level changed according to the ER stress induced by chemical treatment and batch culture in the engineered cell line. Using this monitoring system, we demonstrated that host cells and recombinant CHO cell lines with different mean fluorescence intensities (MFI; basal expression levels of BiP) possess a distinct capacity for stress culture conditions induced by recombinant protein production. Antibody-producing recombinant CHO cell lines were generated using site-specific integration based on host cells equipped with the BiP reporter system. Targeted integrants showed a strong correlation between productivity and MFI, reflecting the potential of this monitoring system as a screening readout for high producers. Taken together, these data demonstrate the utility of the endogenous BiP reporter system for the detection of real-time dynamic changes in endogenous UPR and its potential for applications in recombinant protein production during CHO cell line development.

How to train your cell - Towards controlling phenotypes by harnessing the epigenome of Chinese hamster ovary production cell lines

Recent advances in omics technologies and the broad availability of big datasets have revolutionized our understanding of Chinese hamster ovary cells in their role as the most prevalent host for production of complex biopharmaceuticals. In consequence, our perception of this "workhorse of the biopharmaceutical industry" has successively shifted from that of a nicely working, but unknown recombinant protein producing black box to a biological system governed by multiple complex regulatory layers that might possibly be harnessed and manipulated at will. Despite the tremendous progress that has been made to characterize CHO cells on various omics levels, our understanding is still far from complete. The well-known inherent genetic plasticity of any immortalized and rapidly dividing cell line also characterizes CHO cells and can lead to problematic instability of recombinant protein production. While the high mutational frequency has been a focus of CHO cell research for decades, the impact of epigenetics and its role in differential gene expression has only recently been addressed. In this review we provide an overview about the current understanding of epigenetic regulation in CHO cells and discuss its significance for shaping the cell's phenotype. We also look into current state-of-the-art technology that can be applied to harness and manipulate the epigenetic network so as to nudge CHO cells towards a specific phenotype. Here, we revise current strategies on site-directed integration and random as well as targeted epigenome modifications. Finally, we address open questions that need to be investigated to exploit the full repertoire of fine-tuned control of multiplexed gene expression using epigenetic and systems biology tools.

Rational design and construction of multi-copy biomanufacturing islands in mammalian cells

Cell line development is a critical step in the establishment of a biopharmaceutical manufacturing process. Current protocols rely on random transgene integration and amplification. Due to considerable variability in transgene integration profiles, this workflow results in laborious screening campaigns before stable producers can be identified. Alternative approaches for transgene dosage increase and integration are therefore highly desirable. In this study, we present a novel strategy for the rapid design, construction, and genomic integration of engineered multiple-copy gene constructs consisting of up to 10 gene expression cassettes. Key to this strategy is the diversification, at the sequence level, of the individual gene cassettes without altering their protein products. We show a computational workflow for coding and regulatory sequence diversification and optimization followed by experimental assembly of up to nine gene copies and a sentinel reporter on a contiguous scaffold. Transient transfections in CHO cells indicates that protein expression increases with the gene copy number on the scaffold. Further, we stably integrate these cassettes into a pre-validated genomic locus. Altogether, our findings point to the feasibility of engineering a fully mapped multi-copy recombinant protein 'production island' in a mammalian cell line with greatly reduced screening effort, improved stability, and predictable product titers.

Recent advances in CHO cell line development for recombinant protein production

Recombinant proteins used in biomedical research, diagnostics and different therapies are mostly produced in Chinese hamster ovary cells in the pharmaceutical industry. These biotherapeutics, monoclonal antibodies in particular, have shown remarkable market growth in the past few decades. The increasing demand for high amounts of biologics requires continuous optimization and improvement of production technologies. Research aims at discovering better means and methods for reaching higher volumetric capacity, while maintaining stable product quality. An increasing number of complex novel protein therapeutics, such as viral antigens, vaccines, bi- and tri-specific monoclonal antibodies, are currently entering industrial production pipelines. These biomolecules are, in many cases, difficult to express and require tailored product-specific solutions to improve their transient or stable production. All these requirements boost the development of more efficient expression optimization systems and high-throughput screening platforms to facilitate the design of product-specific cell line engineering and production strategies. In this minireview, we provide an overview on recent advances in CHO cell line development, targeted genome manipulation techniques, selection systems and screening methods currently used in recombinant protein production.

Naming CHO cells for bio-manufacturing: Genome plasticity and variant phenotypes of cell populations in bioreactors question the relevance of old names

Chinese Hamster Ovary [CHO] cells are the workhorse for production of modern biopharmaceuticals. They are however immortalized cells with a high propensity for genetic change. Judging from published culture records, CHO cell populations have undergone hundreds of population doublings since their origin in the late 1950s. Different cell populations were established and named from 1 to 3 decades after their generation, such as CHO-Pro-, CHO-K1, CHO-DG44, CHO-S, CHO-DUK, CHO-DXB-11 to indicate origin and certain phenotypic features. These names are commonly used in scientific publications still today. This article discusses the relevance of such names. We argue that they provide a false sense of identity. To substantiate this, we provide the long (and poorly recorded) history of CHO cells as well as their highly complex genetics. Finally, we suggest an alternative naming system for CHO cells which provides more relevant information. While the implementation of a new naming convention will require substantial discussions among members of the relevant community, it should improve interpretation and comparability between laboratories. This, in turn will help scientific communities and industrial users to attain and further the full potential of CHO cells.

Targeted integration into pseudo attP sites of CHO cells using CRISPR/Cas9

Chinese hamster ovary (CHO) cells are regarded as a prominent host for manufacturing therapeutic proteins. Although conventional strategies for generating recombinant proteins in CHO cells depend on the random integration of a gene of interest (GOI), these established techniques occasionally result in genetically heterogeneous cell lines, which causes diminished expression of the recombinant proteins in the long run. Production instability can be reduced by SSI and creates stable cell lines with a consistent expression of the GOI. In this experiment, we demonstrate the targeted incorporation of a reporter cassette in two PhiC31 pseudo attP sites of CHO cells exploiting the homology-directed repair (HDR) generated by the CRISPR/Cas9 platform. Genes encoding GFP and puromycin resistance marker were precisely inserted into these loci via CRISPR/Cas9. Stable cell lines were suitably produced following antibiotic selection. Junction PCR and fluorescence assay determined targeted integration and expression homogeneity of the reporter cassette, respectively. Taken together, our results indicate the possibility of these two PhiC31 pseudo attP sites as the target sites for site-specific integration of a transgene mediated by CRISPR/Cas9. Furthermore, higher knock-in efficiency and expression homogeneity was observed in the pseudo attP site associated with chromosome 6 compared to the pseudo attP site from chromosome 3.

Genetic rearrangement during site specific integration event facilitates cell line development of a bispecific molecule

Site specific integration (SSI) expression systems offer robust means of generating highly productive and stable cell lines for traditional monoclonal antibodies. As complex modalities such as antibody‐like molecules comprised of greater than two peptides become more prevalent, greater emphasis needs to be placed on the ability to produce appreciable quantities of the correct product of interest (POI). The ability to screen several transcript stoichiometries could play a large role in ensuring high amounts of the correct POI. Here we illustrate implementation of an SSI expression system with a single site of integration for development and production of a multi‐chain, bi‐specific molecule. A SSI vector with a single copy of all of the genes of interest was initially selected for stable Chinese hamster ovary transfection. While the resulting transfection pools generated low levels of the desired heterodimer, utilizing an intensive clone screen strategy, we were able to identify clones having significantly higher levels of POI. In‐depth genotypic characterization of clones having the desirable phenotype revealed that a duplication of the light chain within the landing pad was responsible for producing the intended molecule. Retrospective transfection pool analysis using a vector configuration mimicking the transgene configuration found in the clones, as well as other vector configurations, yielded more favorable results with respect to % POI. Overall, the study demonstrated that despite the theoretical static nature of the SSI expression system, enough heterogeneity existed to yield clones having significantly different transgene phenotypes/genotypes and support production of a complex multi‐chain molecule.

Controlling Ratios of Plasmid-Based Double Cut Donor and CRISPR/Cas9 Components to Enhance Targeted Integration of Transgenes in Chinese Hamster Ovary Cells

Chinese hamster ovary (CHO) cells are the most valuable expression host for the commercial production of biotherapeutics. Recent trends in recombinant CHO cell-line development have focused on the site-specific integration of transgenes encoding recombinant proteins over random integration. However, the low efficiency of homology-directed repair upon transfection of Cas9, single-guide RNA (sgRNA), and the donor template has limited its feasibility. Previously, we demonstrated that a double-cut donor (DCD) system enables highly efficient CRISPR/Cas9-mediated targeted integration (TI) in CHO cells. Here, we describe several CRISPR/Cas9 vector systems based on DCD templates using a promoter trap-based TI monitoring cell line. Among them, a multi-component (MC) system consisting of an sgRNA/DCD vector and Cas9 expression vector showed an approximate 1.5-fold increase in knock-in (KI) efficiency compared to the previous DCD system, when a systematically optimized relative ratio of sgRNA/DCD and Cas9 vector was applied. Our optimization efforts revealed that concurrently increasing sgRNA and DCD components relative to Cas9 correlated positively with KI efficiency at a single KI site. Furthermore, we explored component bottlenecks, such as effects of sgRNA components and applicability of the MC system on simultaneous double KI. Taken together, we improved the DCD vector design by tailoring plasmid constructs and relative component ratios, and this system can be widely used in the TI strategy of transgenes, particularly in CHO cell line development and engineering.

Hydroxyurea selection for enhancement of homology-directed targeted integration of transgenes in CHO cells

Site-specific integration via genome editing technologies has been implemented in Chinese hamster ovary (CHO) cells for predictable and efficient cell line development and engineering. Various strategies have been employed to enhance knock-in (KI) efficiency for precise homology-directed repair (HDR)-mediated targeted integration of transgenes in CHO cells. Given the cell cycle-dependent regulation of the DNA damage repair pathway, cell cycle synchronization to the HDR-favored S/G2 phase has been successfully utilized in mammalian cells, but the effect is limited in CHO cells. Here, we describe a cell cycle enrichment method to increase HDR-mediated KI efficiency in CHO cells. Existing G1 cell cycle synchronization methods showed transient cell cycle arrest and did not improve KI efficiency. Rather than cell cycle arrest with a high concentration of chemicals followed by a release step, cells were incubated in the presence of a lower concentration of hydroxyurea (HU) to enrich cells in the S phase. HU selection allowed for robust S phase enrichment of CHO cells by up to 70 % and maintained cell viability. This short-term selection resulted in improved KI efficiency by 1.2-1.5 fold compared with cells in the control condition. Overall, this approach serves as a simple and effective strategy for enhancement of site-specific genome engineering in CHO cells.

Chromatin-modifying elements for recombinant protein production in mammalian cell systems

Mammalian cells are the preferred choice system for the production of complex molecules, such as recombinant therapeutic proteins. Although the technology for increasing the yield of proteins has improved rapidly, the process of selecting, identifying as well as maintaining high-yield cell clones is still troublesome, time-consuming and usually uncertain. Optimization of expression vectors is one of the most effective methods for enhancing protein expression levels. Several commonly used chromatin-modifying elements, including the matrix attachment region, ubiquitous chromatin opening elements, insulators, stabilizing anti-repressor elements can be used to increase the expression level and stability of recombinant proteins. In this review, these chromatin-modifying elements used for the expression vector optimization in mammalian cells are summarized, and future strategies for the utilization of expression cassettes are also discussed.

A Site-Specific Integration Reporter System That Enables Rapid Evaluation of CRISPR/Cas9-Mediated Genome Editing Strategies in CHO Cells

Targeted gene knockout and site-specific integration (SSI) are powerful genome editing techniques to improve the development of industrially relevant Chinese hamster ovary (CHO) cell lines. However, past efforts to perform SSI in CHO cells are characterized by low efficiencies. Moreover, numerous strategies proposed to boost SSI efficiency in mammalian cell types have yet to be evaluated head to head or in combination to appreciably boost efficiencies in CHO. To enable systematic and rapid optimization of genome editing methods, the SSIGNAL (site-specific integration and genome alteration) reporter system is developed. This tool can analyze CRISPR (clustered regularly interspaced palindromic repeats)/Cas9 (CRISPR-associated protein 9)-mediated disruption activity alone or in conjunction with SSI efficiency. The reporter system uses green and red dual-fluorescence signals to indicate genotype states within four days following transfection, facilitating rapid data acquisition via standard flow cytometry instrumentation. In addition to describing the design and development of the system, two of its applications are demonstrated by first comparing transfection conditions to maximize CRISPR/Cas9 activity and subsequently assessing the efficiency of several promising SSI strategies. Due to its sensitivity and versatility, the SSIGNAL reporter system may serve as a tool to advance genome editing technology.

Optimized CRISPR/Cas9 strategy for homology-directed multiple targeted integration of transgenes in CHO cells

Site-specific integration has emerged as a promising strategy for precise Chinese hamster ovary (CHO) cell line engineering and predictable cell line development (CLD). CRISPR/Cas9 with the homology-directed repair (HDR) pathway enables precise integration of transgenes into target genomic sites. However, inherent recalcitrance to HDR-mediated targeted integration (TI) of transgenes results in low targeting efficiency, thus requiring a selection process to find a targeted integrant in CHO cells. Here, we explored several parameters that influence the targeting efficiency using a promoter-trap-based single- or double-knock-in (KI) monitoring system. A simple change in the donor template design by the addition of single-guide RNA recognition sequences strongly increased KI efficiency (2.9-36.0 fold), depending on integration sites and cell culture mode, compared to conventional circular donor plasmids. Furthermore, sequential and simultaneous KI strategies enabled us to obtain populations with ~1-4% of double-KI cells without additional enrichment procedures. Thus, this simple optimized strategy not only allows efficient CRISPR/Cas9-mediated TI in CHO cells but also paves the way for the applicability of multiplexed KIs in one experimental step without the need for sequential and independent CHO-CLD procedures.

Key Challenges in Designing CHO Chassis Platforms

Following the success of and the high demand for recombinant protein-based therapeutics during the last 25 years, the pharmaceutical industry has invested significantly in the development of novel treatments based on biologics. Mammalian cells are the major production systems for these complex biopharmaceuticals, with Chinese hamster ovary (CHO) cell lines as the most important players. Over the years, various engineering strategies and modeling approaches have been used to improve microbial production platforms, such as bacteria and yeasts, as well as to create pre-optimized chassis host strains. However, the complexity of mammalian cells curtailed the optimization of these host cells by metabolic engineering. Most of the improvements of titer and productivity were achieved by media optimization and large-scale screening of producer clones. The advances made in recent years now open the door to again consider the potential application of systems biology approaches and metabolic engineering also to CHO. The availability of a reference genome sequence, genome-scale metabolic models and the growing number of various “omics” datasets can help overcome the complexity of CHO cells and support design strategies to boost their production performance. Modular design approaches applied to engineer industrially relevant cell lines have evolved to reduce the time and effort needed for the generation of new producer cells and to allow the achievement of desired product titers and quality. Nevertheless, important steps to enable the design of a chassis platform similar to those in use in the microbial world are still missing. In this review, we highlight the importance of mammalian cellular platforms for the production of biopharmaceuticals and compare them to microbial platforms, with an emphasis on describing novel approaches and discussing still open questions that need to be resolved to reach the objective of designing enhanced modular chassis CHO cell lines.

The M4 insulator, the TM2 matrix attachment region, and the double copy of the heavy chain gene contribute to the enhanced accumulation of the PHB-01 antibody in tobacco plants

The expression of recombinant proteins in plants is a valuable alternative to bioreactors using mammalian cell systems. Ease of scaling, and their inability to host human pathogens, enhance the use of plants to generate complex therapeutic products such as monoclonal antibodies. However, stably transformed plants expressing antibodies normally have a poor accumulation of these proteins that probably arise from the negative positional effects of their flanking chromatin. The induction of boundaries between the transgenes and the surrounding DNA using matrix attachment regions (MAR) and insulator elements may minimize these effects. With the PHB-01 antibody as a model, we demonstrated that the insertion of DNA elements, the TM2 (MAR) and M4 insulator, flanking the transcriptional cassettes that encode the light and heavy chains of the PHB-01 antibody, increased the protein accumulation that remained stable in the first plant progeny. The M4 insulator had a stronger effect than the TM2, with over a twofold increase compared to the standard construction. This effect was probably associated with an enhancer-promoter interference. Moreover, transgenic plants harboring two transcriptional units encoding for the PHB-01 heavy chain combined with both TM2 and M4 elements enhanced the accumulation of the antibody. In summary, the M4 combined with a double transcriptional unit of the heavy chain may be a suitable strategy for potentiating PHB-01 production in tobacco plants.

An Efficient Exogenous Gene Insertion Site in CHO Cells with High Transcription Level to Enhance AID-Induced Mutation

Antibodies have been extensively used for the purpose of scientific research, clinical diagnosis, and therapy. Combination of in vitro somatic hypermutation and mammalian cell surface display has been an efficient technology for antibody or other proteins optimization, in which the efficiency of activation-induced cytidine deaminase (AID) mutations in genes is one of the most important key factors. Gene transcriptional level has been found to be positively proportional to AID-induced mutation frequency. Thus, construction of the cell clone bearing a gene of interest (GOI) with high transcription level can increase AID-induced mutations. In this study, a retargetable gene cassette is inserted onto predetermined chromosome site (ywhae gene site) which is among the genes with the highest as well as stable transcription, and is found that one subsite is suitable to be retargeted for efficient protein display in Chinese hamster ovary (CHO) cells. The resultant cell clone (T31) has higher and more stable transcription/expression than CHO-puro clone which was previously established through the strategy of random insertion followed by a high-throughput selection. It also possesses a significantly higher mutation frequency to GOI than CHO-puro cells; thus, it is a better clone for the in vitro improvement of antibody affinity, and probably other properties.

Synthetic biology and healthcare

Through the application of the engineering paradigm of ‘design–build–test–learn’ allied to recent advances in DNA sequencing, bioinformatics and, critically, the falling cost of DNA synthesis, Synthetic Biology promises to make existing therapies more accessible and be at the centre of the development of new types of advanced therapies. As existing pharmaceutical companies integrate Synthetic Biology tools into their normal ways of working, existing products are being produced by cheaper and more sustainable methods. Vaccine design and production is becoming driven by the molecular design allied to rapidly scalable production methods to combat the threat of pandemics and the ability of pathogens to escape the immune system by mutation. Advanced therapies, such as chimeric antigen receptor T cell therapy, are able to capitalise on the tools of Synthetic Biology to design new proteins and molecular ‘kill switches’ as well as design scalable and effective vectors for cellular transduction. This review highlights how Synthetic Biology is having an impact across the various therapeutic modalities from existing products to new therapies.

Systematic Evaluation of Site-Specific Recombinant Gene Expression for Programmable Mammalian Cell Engineering

Many branches of biology depend on stable and predictable recombinant gene expression, which has been achieved in recent years through targeted integration of the recombinant gene into defined integration sites. However, transcriptional levels of recombinant genes in characterized integration sites are controlled by multiple components of the integrated expression cassette. Lack of readily available tools has inhibited meaningful experimental investigation of the interplay between the integration site and the expression cassette components. Here we show in a systematic manner how multiple components contribute to final net expression of recombinant genes in a characterized integration site. We develop a CRISPR/Cas9-based toolbox for construction of mammalian cell lines with targeted integration of a landing pad, containing a recombinant gene under defined 5' proximal regulatory elements. Generated site-specific recombinant cell lines can be used in a streamlined recombinase-mediated cassette exchange for fast screening of different expression cassettes. Using the developed toolbox, we show that different 5' proximal regulatory elements generate distinct and robust recombinant gene expression patterns in defined integration sites of CHO cells with a wide range of transcriptional outputs. This approach facilitates the generation of user-defined and product-specific gene expression patterns for programmable mammalian cell engineering.

Development of a recombinant human IL-15·sIL-15Rα/Fc superagonist with improved half-life and its antitumor activity alone or in combination with PD-1 blockade in mouse model

Recombinant human interleukin-15 (IL-15) is a potent cancer immunotherapeutic candidate due to its excellent immune stimulating effects. Previous work demonstrated that IL-15 appeared with short half-life in circulation system, while the complex with its receptor can prolong the half-life as well as benefit its activities in vivo. Therefore, IL-15 complex was more favorably considered for clinical development. Herein we developed IL-15·sIL-15Rα/Fc, a complex comprising of IL-15 and the extracellular region of its receptor alpha subunit which fused to Immunoglobulin G (IgG1) Fc to further prolong the half-life in plasma. Through transient gene expression in HEK293 cells, we expressed the superagonist by co-transfection of plasmids encoding IL-15 and sIL-15Rα/Fc respectively, yielding 36 mg/L of product after purification. Pharmacokinetic study demonstrated that the combination profoundly prolonged the half-life of IL-15 to 13.1 h in mice, about 18 folds longer than that of IL-15 monomer which is around 0.7 h. The bioactivity of the superagonist was characterized by CTLL-2 cells proliferation assay in vitro, showing its capability of stimulating the expansion of memory CD8⁺ T cells (cluster of differentiation) in mouse spleen. Using a HT-29 xenograft NOD-SCID mouse model, we observed tumor growth inhibition in all groups that received the superagonist, indicating its anti-tumor efficacy via stimulating infused human immune cells. In addition, combo cancer treatment by IL-15·sIL-15Rα/Fc and programmed death-1 (PD-1) antibody have shown stronger inhibitory effects as compared with treatment with either single molecule. Therefore, we developed IL-15·sIL-15Rα/Fc to be a long half-life potential cancer immunotherapy candidate that can be applied alone or in synergy with PD-1/PD-L1 blockade.