Hyperosmolality in CHO cell culture: effects on the proteome.

Chinese hamster ovary (CHO) cells are the most commonly used host cell lines for therapeutic protein production. Exposure of these cells to highly concentrated feed solution during fed-batch cultivation can lead to a non-physiological increase in osmolality (> 300 mOsm/kg) that affects cell physiology, morphology, and proteome. As addressed in previous studies (and indeed, as recently addressed in our research), hyperosmolalities of up to 545 mOsm/kg force cells to abort proliferation and gradually increase their volume-almost tripling it. At the same time, CHO cells also show a significant hyperosmolality-dependent increase in mitochondrial activity. To gain deeper insight into the molecular mechanisms that are involved in these processes, as detailed in this paper, we performed a comparative quantitative label-free proteome study of hyperosmolality-exposed CHO cells compared with control cells. Our analysis revealed differentially expressed key proteins that mediate mitochondrial activation, oxidative stress amelioration, and cell cycle progression. Our studies also demonstrate a previously unknown effect: the strong regulation of proteins can alter both cell membrane stiffness and permeability. For example, we observed that three types of septins (filamentous proteins that form diffusion barriers in the cell) became strongly up-regulated in response to hyperosmolality in the experimental setup. Overall, these new observations correlate well with recent CHO-based fluxome and transcriptome studies, and reveal additional unknown proteins involved in the response to hyperosmotic pressure by over-concentrated feed in mammalian cells.Key points• First-time comparative proteome analysis of CHO cells exposed to over-concentrated feed.• Discovery of membrane barrier-forming proteins up-regulation under hyperosmolality.• Description of mitochondrial and protein chaperones activation in treated cells.

Mapping the molecular basis for growth related phenotypes in industrial producer CHO cell lines using differential proteomic analysis.

BACKGROUND: The ability to achieve high peak viable cell density earlier in CHO cell culture and maintain an extended cell viability throughout the production process is highly desirable to increase recombinant protein yields, reduce host cell impurities for downstream processing and reduce the cost of goods. In this study we implemented label-free LC-MS/MS proteomic profiling of IgG4 producing CHO cell lines throughout the duration of the cell culture to identify differentially expressed (DE) proteins and intracellular pathways associated with the high peak viable cell density (VCD) and extended culture VCD phenotypes. RESULTS: We identified key pathways in DNA replication, mitotic cell cycle and evasion of p53 mediated apoptosis in high peak VCD clonally derived cell lines (CDCLs). ER to Golgi vesicle mediated transport was found to be highly expressed in extended culture VCD CDCLs while networks involving endocytosis and oxidative stress response were significantly downregulated. CONCLUSION: This investigation highlights key pathways for targeted engineering to generate desirable CHO cell phenotypes for biotherapeutic production.

Overexpressed exocyst complex component 3-like 1 spontaneously induces apoptosis.

Exocyst complex component 3-like 1 (EXOC3L1), which regulates insulin secretion, is ubiquitously present in heart, lung, liver, spleen, kidney, muscle, cerebellum, pituitary, adrenal grand, and pancreatic islets. Its deduced amino acid sequence has 31% identity and 53% similarity with Sec6, so they are considered isoforms. Since Sec6 suppresses apoptosis via HSP27, we investigated the involvement of EXOC3L1 expression in apoptosis. We found that overexpressed EXOC3L1 in Chinese hamster ovary cells significantly reduced cultured cell numbers. It also significantly increased apoptotic DNA ladder, caspase 3 activity, and cleavage of caspase 3 compared with the control. Thus, although Sec6 reduces apoptosis by increasing HSP27 phosphorylation, overexpressed EXOC3L1 alone can spontaneously induce apoptosis without apoptotic stimulators or inducers.

Investigating the influence of physiologically relevant hydrostatic pressure on CHO cell batch culture.

Chinese hamster ovary (CHO) cells have been the most commonly used mammalian host for large-scale commercial production of therapeutic proteins, such as monoclonal antibodies. Enhancement of productivity of these CHO cells is one of the top priorities in the biopharmaceutical industry to reduce manufacturing cost. Although there are many different methods (e.g. temperature, pH, feed) to improve protein production in CHO cells, the role of physiologically relevant hydrostatic pressure in CHO cell culture has not been reported yet. In this study, four different hydrostatic pressures (0, 30, 60, and 90 mmHg) were applied to batch CHO cells, and their cell growth/metabolism and IgG1 production were examined. Our results indicate that hydrostatic pressure can increase the maximum cell concentration by up to 50%. Moreover, overall IgG1 concentration on Day 5 showed that 30 mmHg pressure can increase IgG1 production by 26%. The percentage of non-disulphide-linked antibody aggregates had no significant change under pressure. Besides, no significant difference was observed between 30 mmHg and no pressure conditions in terms of cell clumping formation. All these findings are important for the optimization of fed-batch or perfusion culture for directing cell growth and improving antibody production.

Characterization of glycoprotein IIb/IIIa-specific alloantibodies induced by cross-strain platelet immunization in mice.

BACKGROUND: We previously described a mouse model in which platelet immunization between selected strains leads to production of alloantibodies and severe autoimmune thrombocytopenia and mimics the human condition posttransfusion purpura (PTP). This report describes studies defining epitopes recognized by these alloantibodies. STUDY DESIGN: Hybridomas were produced from spleen cells of immunized mice. Glycoprotein (GP) targets of resulting monoclonal antibodies were characterized by immunoprecipitation using platelets from the immunizing strains. Antigens defined by single amino acid (AA) polymorphisms recognized by monoclonal antibodies were identified by mutagenizing target glycoproteins expressed in Chinese hamster ovary cells and observing the effects on antibody binding. RESULTS: Three monoclonal antibodies (417.1, 417.3, 425.1) were produced that recognized GPIIb on immunizing platelets. Monoclonal antibodies 417.1 and 417.3 both required G111 and 425.1 required V37, located on the beta propeller domain of GPIIb, for binding to platelets from the immunizing strains C57 and PWK, respectively. Injection of 417.3 and 425.1 into mice caused platelet destruction only in mice with GPIIb containing the targeted AAs. CONCLUSIONS: Findings made provide evidence that alloantibodies produced by mice experiencing thrombocytopenia in a mouse model of PTP are specific for single AA polymorphisms that differ in GPIIb/IIIa integrin of the immunizing and immunized strains and therefore closely resemble the potent alloantibodies found in patients with PTP. The observations show that naturally occurring single AA differences in GPIIb/IIIa integrin of various mouse strains are highly immunogenic in the mouse strains studied and readily induce antibodies comparable to human platelet antigen-specific antibodies found in transfused and pregnant humans.

Systematically gap-filling the genome-scale metabolic model of CHO cells.

OBJECTIVE: Chinese hamster ovary (CHO) cells are the leading cell factories for producing recombinant proteins in the biopharmaceutical industry. In this regard, constraint-based metabolic models are useful platforms to perform computational analysis of cell metabolism. These models need to be regularly updated in order to include the latest biochemical data of the cells, and to increase their predictive power. Here, we provide an update to iCHO1766, the metabolic model of CHO cells. RESULTS: We expanded the existing model of Chinese hamster metabolism with the help of four gap-filling approaches, leading to the addition of 773 new reactions and 335 new genes. We incorporated these into an updated genome-scale metabolic network model of CHO cells, named iCHO2101. In this updated model, the number of reactions and pathways capable of carrying flux is substantially increased. CONCLUSIONS: The present CHO model is an important step towards more complete metabolic models of CHO cells.

Expanded Chinese hamster organ and cell line proteomics profiling reveals tissue-specific functionalities.

Chinese hamster ovary (CHO) cells are the predominant production vehicle for biotherapeutics. Quantitative proteomics data were obtained from two CHO cell lines (CHO-S and CHO DG44) and compared with seven Chinese hamster (Cricetulus griseus) tissues (brain, heart, kidney, liver, lung, ovary and spleen) by tandem mass tag (TMT) labeling followed by mass spectrometry, providing a comprehensive hamster tissue and cell line proteomics atlas. Of the 8470 unique proteins identified, high similarity was observed between CHO-S and CHO DG44 and included increases in proteins involved in DNA replication, cell cycle, RNA processing, and chromosome processing. Alternatively, gene ontology and pathway analysis in tissues indicated increased protein intensities related to important tissue functionalities. Proteins enriched in the brain included those involved in acidic amino acid metabolism, Golgi apparatus, and ion and phospholipid transport. The lung showed enrichment in proteins involved in BCAA catabolism, ROS metabolism, vesicle trafficking, and lipid synthesis while the ovary exhibited enrichments in extracellular matrix and adhesion proteins. The heart proteome included vasoconstriction, complement activation, and lipoprotein metabolism enrichments. These detailed comparisons of CHO cell lines and hamster tissues will enhance understanding of the relationship between proteins and tissue function and pinpoint potential pathways of biotechnological relevance for future cell engineering.

Characterization of glutathione proteome in CHO cells and its relationship with productivity and cholesterol synthesis.

Glutathione (GSH) plays a central role in the redox balance maintenance in mammalian cells. Previous studies of industrial Chinese hamster ovary cell lines have demonstrated a relationship between GSH metabolism and clone productivity. However, a thorough investigation is required to understand this relationship and potentially highlight new targets for cell engineering. In this study, we have modulated the GSH intracellular content of an industrial cell line under bioprocess conditions to further elucidate the role of the GSH synthesis pathway. Two strategies were used: the variation of cystine supply and the direct inhibition of the GSH synthesis using buthionine sulfoximine (BSO). Over time of the bioprocess, a correlation between intracellular GSH and product titer has been observed. Analysis of metabolites uptake/secretion rates and proteome comparison between BSO-treated cells and nontreated cells has highlighted a slowdown of the tricarboxylic acid cycle leading to a secretion of lactate and alanine in the extracellular environment. Moreover, an adaptation of the GSH-related proteome has been observed with an upregulation of the regulatory subunit of glutamate-cysteine ligase and a downregulation of a specific GSH transferase subgroup, the Mu family. Surprisingly, the main impact of BSO treatment was observed on a global downregulation of the cholesterol synthesis pathways. As cholesterol is required for protein secretion, it could be the missing piece of the puzzle to finally elucidate the link between GSH synthesis and productivity.

Random epigenetic modulation of CHO cells by repeated knockdown of DNA methyltransferases increases population diversity and enables sorting of cells with higher production capacities.

Chinese hamster ovary (CHO) cells produce a large share of today's biopharmaceuticals. Still, the generation of satisfactory producer cell lines is a tedious undertaking. Recently, it was found that CHO cells, when exposed to new environmental conditions, modify their epigenome, suggesting that cells adapt their gene expression pattern to handle new challenges. The major aim of the present study was to employ artificially induced, random changes in the DNA-methylation pattern of CHO cells to diversify cell populations and consequently increase the finding of cell lines with improved cellular characteristics. To achieve this, DNA methyltransferases and/or the ten-eleven translocation enzymes were downregulated by RNA interference over a time span of ∼16 days. Methylation analysis of the resulting cell pools revealed that the knockdown of DNA methyltransferases was highly effective in randomly demethylating the genome. The same approach, when applied to stable CHO producer cells resulted in (a) an increased productivity diversity in the cell population, and (b) a higher number of outliers within the population, which resulted in higher specific productivity and titer in the sorted cells. These findings suggest that epigenetics play a previously underestimated, but actually important role in defining the overall cellular behavior of production clones.

Process Optimization using High Throughput Automated Micro-Bioreactors in Chinese Hamster Ovary Cell Cultivation.

Optimization of bioprocesses to increase the yield of desired products is of importance in the biopharmaceutical industry. This can be achieved by strain selection and by developing bioprocess parameters. Shake flasks have been used for this purpose. They, however, lack the capability to control the process parameters such as pH and dissolved oxygen (DO). This limitation can be overcome with the help of an automated micro-bioreactor. These bioreactors mimic cultivation at a larger scale. One of the major advantages of this system is the integration of the Design of Experiment (DOE) in the software. This integration enables establishing a design where multiple process parameters can be varied simultaneously. The critical process parameters and optimum bioprocess conditions can be analyzed within the software. The focus of the work presented here is to introduce the user to the steps involved in process design in the software and incorporation of the DOE within the cultivation run.

Dynamic resource allocation drives growth under nitrogen starvation in eukaryotes.

Cells can sense changes in their extracellular environment and subsequently adapt their biomass composition. Nutrient abundance defines the capability of the cell to produce biomass components. Under nutrient-limited conditions, resource allocation dramatically shifts to carbon-rich molecules. Here, we used dynamic biomass composition data to predict changes in growth and reaction flux distributions using the available genome-scale metabolic models of five eukaryotic organisms (three heterotrophs and two phototrophs). We identified temporal profiles of metabolic fluxes that indicate long-term trends in pathway and organelle function in response to nitrogen depletion. Surprisingly, our calculations of model sensitivity and biosynthetic cost showed that free energy of biomass metabolites is the main driver of biosynthetic cost and not molecular weight, thus explaining the high costs of arginine and histidine. We demonstrated how metabolic models can accurately predict the complexity of interwoven mechanisms in response to stress over the course of growth.

Enzyme capacity-based genome scale modelling of CHO cells.

Chinese hamster ovary (CHO) cells are most prevalently used for producing recombinant therapeutics in biomanufacturing. Recently, more rational and systems approaches have been increasingly exploited to identify key metabolic bottlenecks and engineering targets for cell line engineering and process development based on the CHO genome-scale metabolic model which mechanistically characterizes cell culture behaviours. However, it is still challenging to quantify plausible intracellular fluxes and discern metabolic pathway usages considering various clonal traits and bioprocessing conditions. Thus, we newly incorporated enzyme kinetic information into the updated CHO genome-scale model (iCHO2291) and added enzyme capacity constraints within the flux balance analysis framework (ecFBA) to significantly reduce the flux variability in biologically meaningful manner, as such improving the accuracy of intracellular flux prediction. Interestingly, ecFBA could capture the overflow metabolism under the glucose excess condition where the usage of oxidative phosphorylation is limited by the enzyme capacity. In addition, its applicability was successfully demonstrated via a case study where the clone- and media-specific lactate metabolism was deciphered, suggesting that the lactate-pyruvate cycling could be beneficial for CHO cells to efficiently utilize the mitochondrial redox capacity. In summary, iCHO2296 with ecFBA can be used to confidently elucidate cell cultures and effectively identify key engineering targets, thus guiding bioprocess optimization and cell engineering efforts as a part of digital twin model for advanced biomanufacturing in future.

A platform for context-specific genetic engineering of recombinant protein production by CHO cells.

An increasing number of engineered therapeutic recombinant proteins with unpredictable manufacturability are currently filling industrial cell line development pipelines. These proteins can be "difficult-to-express" (DTE) in that production of a sufficient quantity of correctly processed recombinant product by engineered mammalian cells is difficult to achieve. In these circumstances, identification of appropriate cell engineering strategies to increase yield is difficult as constraints are cell line and product-specific. Here we describe and validate the development of a high-throughput microscale platform for multiparallel testing of multiple functional genetic components at varying stoichiometry followed by assessment of their effect on cell functional performance. The platform was used to compare and identify optimal cell engineering solutions for both transient and stable production of a model DTE IgG1 monoclonal antibody. We simultaneously tested the functional effect of 32 genes encoding discrete ER or secretory pathway components, each at varying levels of expression and utilized in different combinations. We show that optimization of functional gene load and relative stoichiometry is critical and optimal cell engineering solutions for stable and transient production contexts are significantly different. Our analysis indicates that cell engineering workflows should be cell line, protein product and production-process specific; and that next-generation cell engineering technology that enables precise control of the relative expression of multiple functional genetic components is necessary to achieve this.

Quantification of the dynamics of population heterogeneities in CHO cultures with stably integrated fluorescent markers.

Cell population heterogeneities and their changes in mammalian cell culture processes are still not well characterized. In this study, the formation and dynamics of cell population heterogeneities were investigated with flow cytometry and stably integrated fluorescent markers based on the lentiviral gene ontology (LeGO) vector system. To achieve this, antibody-producing CHO cells were transduced with different LeGO vectors to stably express single or multiple fluorescent proteins. This enables the tracking of the transduced populations and is discussed in two case studies from the field of bioprocess engineering: In case study I, cells were co-transduced to express red, green, and blue fluorescent proteins and the development of sub-populations and expression heterogeneities were investigated in high passage cultivations (total 130 days). The formation of a fast-growing and more productive population was observed with a simultaneous increase in cell density and product titer. In case study II, different preculture growth phases and their influence on the population dynamics were investigated in mixed batch cultures with flow cytometry (offline and automated). Four cell line derivatives, each expressing a different fluorescent protein, were generated and cultivated for different time intervals, corresponding to different growth phases. Mixed cultures were inoculated from them, and changes in the composition of the cell populations were observed during the first 48 h of cultivation with reduced process productivity. In summary, we showed how the dynamics of population heterogeneities can be characterized. This represents a novel approach to investigate the dynamics of cell population heterogeneities under near-physiological conditions with changing productivity in mammalian cell culture processes.

Improvement of Intracellular Traffic System by Overexpression of KDEL Receptor 1 in Antibody-Producing CHO Cells.

The localization of soluble endoplasmic reticulum (ER) chaperones in the cell organelle is mediated by the C-terminal KDEL (lysine, aspartic acid, glutamic acid and leucine) motif. This motif is recognized by the KDEL receptor, a seven-transmembrane protein that cycles between the ER and cis-Golgi to capture missorted KDEL chaperones from post-ER compartments in a pH-dependent manner. The KDEL receptor's target chaperones have a substantial role in protein folding and assembly. In this study, the gene expression level of KDEL receptor 1 shows a moderate upregulation during either ER stress or growth of Chinese hamster ovary (CHO) cells in batch culture, while the ER chaperones show higher upregulation. This might indicate the possibility of saturation of the ER retention machinery or at least hindered retention during late stage batch culture in recombinant CHO cells. KDELR1 is overexpressed in a monoclonal antibody-producing CHO cell line to improve the intracellular chaperone retention rate in the ER. An increase in the specific productivity of IgG1 by 13.2% during the exponential phase, and 23.8% in the deceleration phase of batch culture is observed. This is the first study to focus on the ER retention system as a cell engineering target for enhancing recombinant protein production.

Analysis of chemometric models applied to Raman spectroscopy for monitoring key metabolites of cell culture.

The Food and Drug Administration (FDA) initiative of Process Analytical Technology (PAT) encourages the monitoring of biopharmaceutical manufacturing processes by innovative solutions. Raman spectroscopy and the chemometric modeling tool partial least squares (PLS) have been applied to this aim for monitoring cell culture process variables. This study compares the chemometric modeling methods of Support Vector Machine radial (SVMr), Random Forests (RF), and Cubist to the commonly used linear PLS model for predicting cell culture components-glucose, lactate, and ammonia. This research is performed to assess whether the use of PLS as standard practice is justified for chemometric modeling of Raman spectroscopy and cell culture data. Model development data from five small-scale bioreactors (2 × 1 L and 3 × 5 L) using two Chinese hamster ovary (CHO) cell lines were used to predict against a manufacturing scale bioreactor (2,000 L). Analysis demonstrated that Cubist predictive models were better for average performance over PLS, SVMr, and RF for glucose, lactate, and ammonia. The root mean square error of prediction (RMSEP) of Cubist modeling was acceptable for the process concentration ranges of glucose (1.437 mM), lactate (2.0 mM), and ammonia (0.819 mM). Interpretation of variable importance (VI) results theorizes the potential advantages of Cubist modeling in avoiding interference of Raman spectral peaks. Predictors/Raman wavenumbers (cm-1 ) of interest for individual variables are X1139-X1141 for glucose, X846-X849 for lactate, and X2941-X2943 for ammonia. These results demonstrate that other beneficial chemometric models are available for use in monitoring cell culture with Raman spectroscopy.

A cell-based assay for the detection of neutralizing antibodies against alemtuzumab.

Aim: The humanized anti-CD52 monoclonal antibody alemtuzumab depletes lymphocytes and is currently used to treat relapsing multiple sclerosis. During treatment, anti-alemtuzumab antibodies may develop and reduce effective lymphocyte depletion in future treatment cycles. Results: Alemtuzumab-Alexa Fluor 488 conjugate binding to the CHO-CD52 cell surface was inhibited by anti-alemtuzumab antibodies. Conclusion: In this proof-of-concept study, a CHO-CD52 cell line has been developed and used to detect the presence of anti-alemtuzumab neutralizing antibodies. This platform provides the basis of an assay for routine screening of serum for neutralizing antibodies from patients treated with alemtuzumab.

Small-scale bioreactor supports high density HEK293 cell perfusion culture for the production of recombinant Erythropoietin.

Process intensification in mammalian cell culture-based recombinant protein production has been achieved by high cell density perfusion exceeding 108 cells/mL in the recent years. As the majority of therapeutic proteins are produced in Chinese Hamster Ovary (CHO) cells, intensified perfusion processes have been mainly developed for this type of host cell line. However, the use of CHO cells can result in non-human posttranslational modifications of the protein of interest, which may be disadvantageous compared with human cell lines. In this study, we developed a high cell density perfusion process of Human Embryonic Kidney (HEK293) cells producing recombinant human Erythropoietin (rhEPO). Firstly, a small-scale perfusion system from commercial bench-top screening bioreactors was developed for <250 mL working volume. Then, after the first trial runs with CHO cells, the system was modified for HEK293 cells (more sensitive than CHO cells) to achieve a higher oxygen transfer under mild aeration and agitation conditions. Steady states for medium (20 × 106 cells/mL) and high cell densities (80 × 106 cells/mL), normal process temperature (37 °C) and mild hypothermia (33 °C) as well as different cell specific perfusion rates (CSPR) from 10 to 60 pL/cell/day were applied to study the performance of the culture. The volumetric productivity was maximized for the high cell density steady state but decreased when an extremely low CSPR of 10 pL/cell/day was applied. The shift from high to low CSPR strongly reduced the nutrient uptake rates. The results from our study show that human cell lines, such as HEK293 can be used for intensified perfusion processes.

Effects of the Selective EP2 Receptor Agonist Omidenepag on Adipocyte Differentiation in 3T3-L1 Cells.

Purpose: We aimed at comparing the effects of omidenepag (OMD) with those of prostaglandin F (FP) receptor agonists (FP agonists) on adipogenesis in mouse 3T3-L1 cells. Methods: To evaluate the agonistic activities of OMD against the mouse EP2 (mEP2) receptor, we determined cAMP contents in mEP2 receptor-expressing CHO cells by using radioimmunoassays. Overall, 3T3-L1 cells were cultured in differentiation medium for 10 days and adipocyte differentiation was assessed according to Oil Red O-stained cell areas. Changes in expression levels of the adipogenic transcription factors Pparg, Cebpa, and Cebpb were determined by using real-time polymerase chain reaction (PCR). OMD at 0.1, 1, 10, and 40 µmol/L, latanoprost free acid (LAT-A) at 0.1 µmol/L, or prostaglandin F2α (PGF2α), at 0.1 µmol/L were added to cell culture media during adipogenesis. Oil Red O-stained areas and expression patterns of transcription factor targets of OMD or FP agonists were compared with those of untreated controls. Results: The 50% effective concentration (EC50) of OMD against the mEP2 receptor was 3.9 nmol/L. Accumulations of Oil Red O-stained lipid droplets were observed inside control cells on day 10. LAT-A and PGF2α significantly inhibited the accumulation of lipid droplets; however, OMD had no effect on this process even at concentrations up to 40 µmol/L. LAT-A and PGF2α significantly suppressed Pparg, Cebpa, and Cebpb gene expression levels during adipocyte differentiation. Conversely, OMD had no obvious effects on the expression levels of these genes. Conclusions: A selective EP2 receptor agonist, OMD, did not affect the adipocyte differentiation in 3T3-L1 cells, whereas FP agonists significantly inhibited this process.

Characterization of Chinese hamster ovary cells with disparate chromosome numbers: Reduction of the amount of mRNA relative to total protein.

Chromosomes in Chinese hamster ovary (CHO) cells are labile. We have shown that high-chromosome-number CHO cells have greater potential to become robust producers of recombinant proteins. One explanation being the increase in transgene integration sites. However, high-chromosome-number cell clones produce more IgG3 following culture of single-cell clones, even under conditions that yield the same number of integrations as cells with normal chromosome numbers. Here, we characterized high-chromosome-number cells by transcriptome analysis. RNA standards were used to normalize transcriptomes of cells that had different chromosome numbers. Our results demonstrate that the mRNA ratio of ß-actin and many other genes in high-chromosome-number cells to that in normal-chromosome-number cells per cell (normalized to RNA standards) was smaller than the equivalent genomic size and cell volume ratios. Many genes encoding membrane proteins are more highly expressed in high-chromosome-number cells, probably due to differences in cell size caused by the increase in chromosomes. In addition, genes related to histone modification and lipid metabolism are differentially expressed. The reduced transcript level required per protein produced in total and the different intracellular signal transductions might be key factors for antibody production.