Assessment of UCOE on Recombinant EPO Production and Expression Stability in Amplified Chinese Hamster Ovary Cells

Expression of UCOE and HSP27 Molecular Elements to Improve the Stable Protein Production on HEK293 Cells

Recombinant proteins represent one of the greatest achievements of modern pharmaceutical biotechnology, as they are increasingly used across nearly all branches of medicine to treat a wide range of conditions. In response to this demand, various cell engineering approaches have been developed to improve their expression. Some of these approaches involve the use of genetic elements that prevent the silencing of the gene of interest, as well as the generation of resistant cell lines to inhibit or avoid programmed cell death (PCD). This research focuses on analyzing the effects of overexpression of UCOE elements and the HSP27 protein, both individually and together, on the production of human rIFNγ in HEK293 cells. Our results show that 4-Kb UCOE elements have no effect on protein production in HEK293 cells, while overexpression of HSP27 prolongs the stationary phase during growth kinetics. The Qp of rIFNγ is 96-fold higher in clones containing the HSP27/UCOE combination compared to the clone containing only UCOE elements or to the control HEK293 cells. These results correlate with the MCP analyses, which showed that overexpression of HSP27 decreased the expression of Bax, caspase 3, cytochrome C, Beclin, and LC3II mRNA. Finally, this study suggests the potential utility of a cell engineering approach based on the overexpression of the human HSP27 protein for enhancing the production of recombinant viruses and proteins in HEK293 cells.

Barriers Composed of tRNA Genes Can Complement the Benefits of a Ubiquitous Chromatin Opening Element to Enhance Transgene Expression

Random integration of transgenes into host cell genomes often occurs in epigenetically unstable regions, leading to variable and unreliable transgene expression. To address this, biomanufacturing organizations frequently employ barrier elements, such as the widely-used ubiquitous chromatin opening element (UCOE). We have compared UCOE barrier activity against a barrier provided by tRNA genes. We demonstrate that the tRNA genes provide a more effective barrier than a UCOE in preventing transgene silencing in Chinese hamster ovary (CHO) cells. Nevertheless, the UCOE offers other benefits, increasing expression strongly, albeit transiently, and reducing production variability. Both the UCOE and tRNA genes counteract the repressive heterochromatin mark H3K9me3, but only the tRNA genes sustain euchromatic H3K27ac and recruitment of RNA polymerase II (Pol II) throughout long-term culture. A hybrid combining these distinct types of elements can provide benefits of both, enhancing expression in a more enduring manner. This synthetic hybrid offers potential for biomanufacturing applications.

Impacts of Nucleosome Positioning Elements and Pre-Assembled Chromatin States on Expression and Retention of Transgenes

BACKGROUND/OBJECTIVES

Transgene applications, ranging from gene therapy to the development of stable cell lines and organisms, rely on maintaining the expression of transgenes. To date, the use of plasmid-based transgenes has been limited by the loss of their expression shortly after their delivery into the target cells. The short-lived expression of plasmid-based transgenes has been largely attributed to host-cell-mediated degradation and/or silencing of transgenes. The development of chromatin-based strategies for gene delivery has the potential to facilitate defining the requirements for establishing epigenetic states and to enhance transgene expression for numerous applications.

METHODS

To assess the impact of "priming" plasmid-based transgenes to adopt accessible chromatin states to promote gene expression, nucleosome positioning elements were introduced at promoters of transgenes, and vectors were pre-assembled into nucleosomes containing unmodified histones or mutants mimicking constitutively acetylated states at residues 9 and 14 of histone H3 or residue 16 of histone H4 prior to their introduction into cells, then the transgene expression was monitored over time.

RESULTS

DNA sequences capable of positioning nucleosomes could positively impact the expression of adjacent transgenes in a distance-dependent manner in the absence of their pre-assembly into chromatin. Intriguingly, the pre-assembly of plasmids into chromatin facilitated the prolonged expression of transgenes relative to plasmids that were not pre-packaged into chromatin. Interactions between pre-assembled chromatin states and nucleosome positioning-derived effects on expression were also assessed and, generally, nucleosome positioning played the predominant role in influencing gene expression relative to priming with hyperacetylated chromatin states.

CONCLUSIONS

Strategies incorporating nucleosome positioning elements and the pre-assembly of plasmids into chromatin prior to nuclear delivery can modulate the expression of plasmid-based transgenes.

Enhancing Antibody-Specific Productivity: Unraveling the Impact of XBP1s Overexpression and Glutamine Availability in SP2/0 Cells

Augmentation of glycoprotein synthesis requirements induces endoplasmic reticulum (ER) stress, activating the unfolded protein response (UPR) and triggering unconventional XBP1 splicing. As a result, XBP1s orchestrates the expression of essential genes to reduce stress and restore homeostasis. When this mechanism fails, chronic stress may lead to apoptosis, which is thought to be associated with exceeding a threshold in XBP1s levels. Glycoprotein assembly is also affected by glutamine (Gln) availability, limiting nucleotide sugars (NS), and preventing compliance with the increased demands. In contrast, increased Gln intake synthesizes ammonia as a by-product, potentially reaching toxic levels. IgA2m(1)-producer mouse myeloma cells (SP2/0) were used as the cellular mammalian model. We explored how IgA2m(1)-specific productivity (qIgA2m(1)) is affected by (i) overexpression of human XBP1s (h-XBP1s) levels and (ii) Gln availability, evaluating the kinetic behavior in batch cultures. The study revealed a two and a five-fold increase in qIgA2m(1) when lower and higher levels of XBP1s were expressed, respectively. High h-XBP1s overexpression mitigated not only ammonia but also lactate accumulation. Moreover, XBP1s overexpressor showed resilience to hydrodynamic stress in serum-free environments. These findings suggest a potential application of h-XBP1s overexpression as a feasible and cost-effective strategy for bioprocess scalability.

Effects of genome instability of parental CHO cell clones on chromosome number distribution and recombinant protein production in parent-derived subclones

Chinese hamster ovary (CHO) cells are the de facto standard host cells for biopharmaceuticals, and there is great interest in developing methods for constructing stable production cell lines. In this study, clones with a wide chromosome number distribution were selected from isolated antibody-producing strains, and subclones obtained from these clones were evaluated. The transgene copy number varied between the subclones. Even among subclones with similar copy numbers of antibody genes and maintained insertion sites, clones with different productivity were generated. Although the chromosome number distribution differed between these subclones, there was no correlation between the variability in chromosome number after cloning (genome instability) and productivity. Most of the subclones obtained from a parental strain with a wide chromosome number had the same wide chromosome number distribution as the parental strain. Less frequently, cells with less variation (remaining in one distribution) in chromosome number were isolated from cells with a wide chromosome number distribution, from which subclones with less variation in chromosome number were obtained when subcloning was performed again. These results imply that the characteristics of clones with chromosomal instability are inherited by subclones, and thus provide a better understanding of cell line stability/instability.

Long term culture promotes changes to growth, gene expression, and metabolism in CHO cells that are independent of production stability

Phenotypic stability of Chinese hamster ovary (CHO) cells over long term culture (LTC) presents one of the most pressing challenges in the development of therapeutic protein manufacturing processess. However, our current understanding of the consequences of LTC on recombinant (r-) CHO cell lines is still limited, particularly as clonally-derived cell lines present distinct production stability phenotypes. This study evaluated changes of culture performance, global gene expression, and cell metabolism of two clonally-derived CHO cell lines with a stable or unstable phenotype during the LTC (early [EP] vs. late [LP] culture passages). Our findings indicated that LTC altered the behavior of CHO cells in culture, in terms of growth, overall gene expression, and cell metabolism. Regardless whether cells were categorized as stable or unstable in terms of r-protein production, CHO cells at LP presented an earlier decline in cell viability and loss of any observable stationary phase. These changes were parallelled by the upregulation of genes involved in cell proliferation and survival pathways (i.e., MAPK/ERK, PI3K-Akt). Stable and unstable CHO cell lines both showed increased consumption of glucose and amino acids at LP, with a parallel accumulation of greater amounts of lactate and TCA cycle intermediates. In terms of production stability, we found that decreased r-protein production in the unstable cell line directly correlated to the loss in r-gene copy number and r-mRNA expression. Our data revealed that LTC produced ubiquitious effects on CHO cell phenotypes, changes that were rooted in alterations in cell transcriptome and metabolome. Overall, we found that CHO cells adapted their cellular function to proliferation and survival during the LTC, some of these changes may well have limited effects on overall yield or specific productivity of the desired r-product, but they may be critical toward the capacity of cells to handle r-proteins with specific molecular features.

Use of ubiquitous chromatin opening elements (UCOE) as tools to maintain transgene expression in biotechnology

Amongst the most important outputs of the biopharmaceutical industry are recombinant proteins, many of which are produced by integrating transgenes into the genomes of mammalian cells. However, expression is highly variable and can be unstable during prolonged culture. This is often due to epigenetic mechanisms silencing the transgenes. To combat this problem, vectors have been engineered to include ubiquitous chromatin opening elements (UCOEs) that protect against silencing. Here, we recount the evidence that UCOEs can modify chromatin environments and benefit biomanufacturing.

Effects of ubiquitous chromatin opening element (UCOE) on recombinant anti-TNFα antibody production and expression stability in CHO-DG44 cells

To date, the production of antibodies (mAbs) usually faces the risks of transgene expression reduction and instability, especially after long-time culture. The inclusion of ubiquitous chromatin opening element (UCOE) into expression vectors was reported to enhance protein production and maintain transgene expression stability in CHO cell lines. Thus, we investigate the effects of UCOE on recombinant monoclonal anti-TNFα antibody (mAbTNFα) production and expression stability in CHO-DG44 cells. In our study, non-UCOE and UCOE-based vectors encoding mAbTNFα were constructed and introduced into the CHO-DG44 cells. Cell pools and single-cell clones were obtained by selecting transfected cells with G418, amplifying them by treatment with methotrexate (MTX), and isolating them by limiting dilution. The effects of UCOE on mAb production and stable transgene expression in transfected cells were analyzed via the correlation between mAb yields and mRNA expression level variations, and gene copy number changes. The UCOE pool exhibited higher mAb yield compared to non-UCOE pool. The UCOE was associated with higher transgene transcriptional activity, leading to improvement of mAb production after MTX-mediated gene amplification. The incorporation of UCOE generated cells allowed isolation of greater numbers of positive clones with higher expression. Despite the slightly decreased mAb yield, UCOE clones still retain stable long-term expression in the absence of selective pressure, which was explained by the loss of transgene copies rather than due to the decline of transcriptional activity. In addition, the purified mAb had primary chemical and biological characteristics similar to those of adalimumab. The results showed that the incorporation of UCOE within vectors provides significant advantages in the generation of high-producing clones, enhancement of mAb production, and improvement of gene expression stability.

Enhancement of anti-TNFα monoclonal antibody production in CHO cells through the use of UCOE and DHFR elements in vector construction and the optimization of cell culture media

Recently, there has been a high demand for anti-tumor necrosis factor-α monoclonal antibodies (mAbTNFα) in the treatment of rheumatoid arthritis and other autoimmune diseases. Thus, efficient strategies and stable high-producing cell lines need to be established to increase antibody production. In this study, we describe an efficient approach to establish a mAbTNFα high-producing clone through the optimization of expression vectors and cell culture media. The ubiquitous chromatin opening element (UCOE) and dihydrofolate reductase (DHFR)-based vectors encoding mAbTNFα were introduced into the CHO-DG44 cells using lipofection. Clones were obtained by selecting transfected cells with G418, amplifying them by treatment with methotrexate, and isolating them by limiting dilution. Different media formulated with commercial feeds and media were also screened to develop an improved medium. The antibody produced by the selected clone was purified, characterized, and compared to standard adalimumab. Using our established protocol, a cell clone obtained from stable mAbTNFα-expressing cell pools showed a 3.8-fold higher antibody titer compared to stable cell pools. Furthermore, the highest antibody yield of selected clones cultured in fed-batch mode using improved medium was 2450 ± 30 µg/mL, which was 13.2-fold higher than that of stable cell pool cultivated in batch mode using a basal medium. The purified antibody had primary chemical and biological characteristics similar to those of adalimumab. Therefore, the use of UCOE and DHFR vectors in combination with the optimization of cell culture media may help in establishing stable and high-producing CHO cell lines for therapeutic antibody production.

Overexpression of transcription factor BLIMP1/prdm1 leads to growth inhibition and enhanced secretory capacity in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells present inherent limitations for processing and secretion of large amounts of recombinant proteins, especially for those requiring complex post-translational processing. To tackle these limitations, we engineered CHO host cells (CHOK1 and CHOS) to overexpress the transcription factor BLIMP1/prdm1 (a master regulator of the highly-secreting phenotype of antibody-producing plasma cells), generating novel CHO cell lines (referred to as CHOB). The CHOB cell lines exhibited decreased cell densities, prolonged stationary phase and arrested cell cycle in G1/G0 phase but simultaneously had significantly greater product titre for recombinant IgG1 (> 2-fold increase) coupled with a significantly greater cell-specific productivities (> 3-fold increase). We demonstrated that the improved productive phenotype of CHOB cells resulted from a series of changes to cell physiology and metabolism. CHOB cells showed a significantly greater ER size and increased protein synthesis and secretion capacity compared to control cells. In addition, CHOB cells presented a metabolic profile that favoured energy production to support increased recombinant protein production. This study indicated that a cell engineering approach based on BLIMP1 expression offers great potential for improving the secretory capacity of CHO cell hosts utilised for manufacture of recombinant biopharmaceuticals. Our findings also provides a greater understanding of the relationship between cell growth and productivity, valuable generic information for improving productive phenotypes for CHO cell lines during industrial cell line development.

Characterization of metabolic responses, genetic variations, and microsatellite instability in ammonia-stressed CHO cells grown in fed-batch cultures

BACKGROUND

As bioprocess intensification has increased over the last 30 years, yields from mammalian cell processes have increased from 10's of milligrams to over 10's of grams per liter. Most of these gains in productivity can be attributed to increasing cell densities within bioreactors. As such, strategies have been developed to minimize accumulation of metabolic wastes, such as lactate and ammonia. Unfortunately, neither cell growth nor biopharmaceutical production can occur without some waste metabolite accumulation. Inevitably, metabolic waste accumulation leads to decline and termination of the culture. While it is understood that the accumulation of these unwanted compounds imparts a suboptimal culture environment, little is known about the genotoxic properties of these compounds that may lead to global genome instability. In this study, we examined the effects of high and moderate extracellular ammonia on the physiology and genomic integrity of Chinese hamster ovary (CHO) cells.

RESULTS

Through whole genome sequencing, we discovered 2394 variant sites within functional genes comprised of both single nucleotide polymorphisms and insertion/deletion mutations as a result of ammonia stress with high or moderate impact on functional genes. Furthermore, several of these de novo mutations were found in genes whose functions are to maintain genome stability, such as Tp53, Tnfsf11, Brca1, as well as Nfkb1. Furthermore, we characterized microsatellite content of the cultures using the CriGri-PICR Chinese hamster genome assembly and discovered an abundance of microsatellite loci that are not replicated faithfully in the ammonia-stressed cultures. Unfaithful replication of these loci is a signature of microsatellite instability. With rigorous filtering, we found 124 candidate microsatellite loci that may be suitable for further investigation to determine whether these loci may be reliable biomarkers to predict genome instability in CHO cultures.

CONCLUSION

This study advances our knowledge with regards to the effects of ammonia accumulation on CHO cell culture performance by identifying ammonia-sensitive genes linked to genome stability and lays the foundation for the development of a new diagnostic tool for assessing genome stability.

Evaluation of the Effects of Human Beta-Interferon Scaffold Attachment Region (IFN-SAR) on Expression of Vascular Endothelial Growth Factor-Fc (VEGF-Fc) Fusion Protein Expression in Chinese Hamster Ovary (CHO) Cells

Background: Recombinant anti-vascular endothelial growth factor (VEGF) monoclonal antibodies and Fc-fusion proteins have been widely used for the effective treatment of retinal neovascular diseases. In this regard, VEGFR-Fc fusions, which act as strong VEGF inhibitors, have been approved for the treatment of age-related macular degeneration (AMD) and diabetic macular edema (DME). Production of monoclonal antibodies and Fc-fusion proteins relies on mammalian host systems such as Chinese hamster ovary (CHO) cells. Application of genomic regulatory elements including scaffold/matrix attachment regions (SAR/MARs) can profoundly affect recombinant protein expression in CHO cells. Methods: To construct the VEGFR-Fc expression vectors, the enhanced green fluorescent protein (EGFP) gene was replaced by the VEGFR-Fc coding sequence in pEGFP-SAR-puro and pEGFP-puro vectors. Recombinant plasmids were transfected to CHO-K1 cells using TurboFect transfection reagent. VEGFR-Fc expression was evaluated in transiently transfected cells as well as stable cell pools and clones using an enzyme-linked immunosorbent assay (ELISA). Results: IFN-SAR showed no significant effect on transient expression of VEGFR-Fc during 72 h of culture. However, a 2.2-fold enhancement in VEGFR-Fc fusion protein titer was observed in IFN-SAR containing stable cell pools. Further evaluation of the VEGFR-Fc expression level in single-cell clones also indicated that clones with the highest VEGFR-Fc expression belonged to the pools transfected with IFN-SAR construct. Conclusion: Our results indicate that the incorporation of IFN-SAR in expression vector can increase the expression of VEGFR-Fc in stable cell pools as well as single-cell clones. In contrast, transient expression of the fusion protein was not affected by IFN-SAR. More studies are needed to investigate the mechanism underlying this effect, including the analysis of mRNA expression and gene copy number in stable cell pools as well as clonal 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.

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.

Expression vector cassette engineering for recombinant therapeutic production in mammalian cell systems

Human tissue plasminogen activator was the first recombinant therapy protein that successfully produced in Chinese hamster ovary cells in 1986 and approved for clinical use. Since then, more and more therapeutic proteins are being manufactured in mammalian cells, and the technologies for recombinant protein production in this expression system have developed rapidly, with the optimization of both upstream and downstream processes. One of the most promising strategies is expression vector cassette optimization based on the expression vector cassette. In this review paper, these approaches and developments are summarized, and the future strategy on the utilizing of expression cassettes for the production of recombinant therapeutic proteins in mammalian cells is discussed.

Development of a new promoter to avoid the silencing of genes in the production of recombinant antibodies in chinese hamster ovary cells

Background

The production of recombinant proteins in mammalian cell lines is one of the most important areas in biopharmaceutical industry. Viral transcriptional promoters are widely used to express recombinant proteins in mammalian cell lines. However, these promoters are susceptible to silencing, thus limiting protein productivity. Some CpG islands can avoid the silencing of housekeeping genes; for that reason, they have been used to increase the production of recombinant genes in cells of animal origin. In this study, we evaluated the CpG island of the promoter region of the β-actin gene of Cricetulus griseous (Chinese hamster), associated to the Cytomegalovirus (CMV) promoter, to increase recombinant antibodies production in Chinese Hamster Ovary (CHO) cells.

Results

We focused on the non-coding region of CpG island, which we called RegCG. RegCG behaved as a promoter, whose transcriptional activity was mainly commanded by the CAAT and CArG boxes of the proximal promoter. However, the transcription started mainly at the intronic region before the proximal transcription start site. While the CMV promoter was initially more powerful than RegCG, the latter promoter was more resistant to silencing than the CMV promoter in stable cell lines, and its activity was improved when combined with the CMV promoter. Thereby, the chimeric promoter was able to maintain the expression of recombinant antibodies in stable clones for 40 days at an average level 4 times higher than the CMV promoter. Finally, the chimeric promoter showed compatibility with a genetic amplification system by induction with methotrexate in cells deficient in the dihydrofolate reductase gene.

Conclusions

We have generated an efficient synthetic hybrid transcription promoter through the combination of RegCG with CMV, which, in stable cell lines, shows greater activity than when both promoters are used separately. Our chimeric promoter is compatible with a genetic amplification system in CHO DG44 cells and makes possible the generation of stable cell lines with high production of recombinant antibodies. We propose that this promoter can be a good alternative for the generation of clones expressing high amount of recombinant proteins, essential for industrial applications.

Multiplexed Digital mRNA Expression Analysis Profiles System-Wide Changes in mRNA Abundance and Responsiveness of UPR-Specific Gene Expression Changes During Batch Culture of Recombinant Chinese Hamster Ovary Cells

The unfolded protein response (UPR) signaling pathway is viewed as critical for setting the effectiveness of recombinant protein expression in CHO cells. In this study, Nanostring nCounter technology is used to study expression of a group of genes associated with cellular processes linked to UPR activation under ER stress and the changing environment of a batch culture. Time course induction of ER stress, using tunicamycin (TM), shows a group of genes such as Chop, Trb3, Sqstm1, Grp78, and Herpud1 respond rapidly to TM inhibition of N-glycosylation, while others such as Atf5, Odz4, and Birc5 exhibits a delayed response. In batch culture, expression of "classical" UPR markers only increases when cells enter decline phase. In addition to providing a detailed analysis of the expression of process-relevant UPR markers during batch culture and in response to imposed chemical stress, we also highlighted six genes (Herpud1, Odz4, Sqstm1, Trb3, Syvn1, and Birc5) associated with the perception of ER stress responses in recombinant CHO cells. Herpud1 (involved in ER-associated degradation) exhibits a rapid (primary) response to stress and its relationship (and that of the other five genes) to the overall cellular UPR may identify novel targets to modulate recombinant protein production in CHO cells.

A chimeric HS4 insulator-scaffold attachment region enhances transgene expression in transfected Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are one of the most commonly used expression systems for the production of recombinant proteins but low levels of transgene expression and transgene silencing are frequently encountered. Epigenetic regulatory elements such as the chicken β-globin locus control region hypersensitive site 4 (HS4) and scaffold/matrix attachment regions (S/MARs) have positive effects on transgene expression. In this study, a chimeric HS4-SAR was cloned upstream or downstream of an enhanced green fluorescent protein (eGFP) expression cassette in a eukaryotic vector, and the resulting vectors were transfected into CHO cells. eGFP was detected by flow cytometry. Real-time quantitative PCR (qPCR) was used to determine copy numbers of the stably transfected cells. And fluorescence in situ hybridization (FISH) was used to detect the status of vector in the host cell chromosome. The results showed that HS4-SAR positioned downstream of the expression cassette could enhance eGFP expression by 4.83-fold compared with the control vector. There may not be a relationship between transgene copy number and gene expression level. HS4-SAR did not appear to alter the integration of the transgene into the host cell chromosome or its position in the chromosome. We found a synthetic chimeric HS4-SAR positively increased transgene expression in CHO cells.