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Introduction

Normal somatic cells have a limited number of divisions when cultured in vitro. Once they reach a certain critical threshold, the cells enter a state of senescence arrest – this is known as the Hayflick limit. To overcome this limitation, researchers employ techniques to obtain immortalized cells capable of infinite proliferation. Such cells can be passaged long‑term and expanded stably, providing homogeneous experimental materials for basic research and drug development.

Establishing high‑quality immortalized cell lines typically involves multiple technical routes, each with its own core principles and key operational points. Based on practical experience, we summarize here the main methods for immortalization construction and provide some specific operational recommendations for several critical experimental steps.

Main Technical Routes for Immortalization Construction

The most common strategies currently used for cell immortalization include viral gene introduction, telomerase activation, and combined intervention of both pathways. Different methods suit different cell types and research purposes.

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Viral Oncogene‑Mediated Method

SV40 large T antigen is the classic tool. This protein can simultaneously bind and inhibit p53 and pRb, thereby removing two major negative regulatory nodes of the cell cycle. Immortalized cells obtained by this method often rapidly bypass the senescence phase, but during the subsequent M2 crisis, only a few cells can spontaneously activate telomerase to achieve stable passaging. This method has broad applicability, but the resulting immortalized cell lines carry a high risk of genetic variation.

Similarly, HPV E6/E7 proteins degrade p53 and inhibit pRb, respectively, with a well‑defined mechanism, and are also commonly used immortalization inducers. For B lymphocytes, the Epstein‑Barr virus (EBV) activates telomerase and interferes with the p53/pRb pathway through its encoded proteins, such as EBNA‑1 and LMP‑1, to establish lymphoblastoid immortalized cell lines.

Introduction of the Telomerase Catalytic Subunit hTERT

Introducing the hTERT gene into primary cells reconstitutes telomerase activity and maintains telomere length from the source. This strategy is more physiologically relevant; the resulting immortalized cells generally have a more stable karyotype and a lower tumorigenic risk than those derived from viral oncogene methods. The hTERT method is particularly suitable for studies requiring high genetic fidelity, such as immortalization of fibroblasts, endothelial cells, and some epithelial cells.

Combined Gene Transfection Strategy

Some cell types are insensitive to a single immortalizing gene, thus requiring combined introduction of SV40T and hTERT, or simultaneous inactivation of the p53/pRb pathway and activation of telomerase. This strategy achieves higher efficiency, but the procedure is more complex and requires continuous optimization of transfection ratios and selection conditions.

Key Techniques and Quality Control in Experimental Operations

From primary cells to stably passaged immortalized cells, multiple operational steps are involved. The refined control points are described stepwise below.

Quality Control of Primary Cell Preparation

The starting point for immortalization is high‑quality primary cells. Therefore, the trypsin digestion time should not be too long; a viability of no less than 90% by trypan blue staining is the benchmark. Seeding density directly affects subsequent gene transfer efficiency; generally, transfection or infection is performed when confluence reaches 60% to 70%.

For samples with scarce tissue sources, priority should be given to expanding the primary cells and freezing low‑passage stock cells as early as possible to prevent cell death during subsequent experiments.

Optimization of Transfection or Transduction Efficiency

When using lentiviral or retroviral vectors, the viral titer of the packaging system is the core determinant of infection success. It is recommended to determine the optimal multiplicity of infection through preliminary experiments. Polybrene can be added during infection to improve transduction efficiency, but its working concentration must be titrated according to different cell types.

Selection pressure (e.g., puromycin or neomycin) is applied 48‑72 hours after infection; the selection period is typically 7‑14 days. During this period, a control group should be set up to confirm complete death of uninfected cells before proceeding to the expansion phase.

Clonal Selection and Single‑cell Isolation

Among the cell populations that survive antibiotic selection, cells that are not fully immortalized but tolerate the selection pressure are often mixed in. Therefore, limiting dilution should be used to isolate monoclonal cells: serially dilute the cell suspension to 1‑2 cells per 100 μL, seed into 96‑well plates, and mark single‑clone wells under the microscope. The picked clones need to be expanded separately and frozen as backups.

For B‑cell immortalized cell lines established by the EBV method, note that cell clusters appearing after the crisis period are positive clones. Clusters with a plump morphology and active proliferation should be prioritized for picking and expansion.

Identification and Quality Assessment of Immortalized Cells

After obtaining candidate clones, a series of experiments must be performed to verify whether they have truly become qualified immortalized cell lines.

Proliferation Capacity Assessment

Drawing a growth curve is a basic identification method. Immortalized cells should exhibit sustained logarithmic proliferation that breaks through the senescence inflection point; they can usually be passaged continuously for more than 40 generations without signs of proliferation slowdown. A plate colony formation assay should also be performed; an ideal immortalized cell line should have a colony‑forming efficiency above 5%.

Molecular Marker Validation

PCR or Western blot detection should be performed for the introduced exogenous genes. For example, in hTERT‑mediated immortalized cells, positive telomerase activity should be detected; in SV40T‑antigen immortalized cells, T antigen expression must be confirmed. Simultaneously, changes in the expression levels of p53, p21, p16 and pRb should be examined to confirm that the two key pathways have been inactivated as expected.

Karyotype Analysis and Identity Authentication

For immortalized cell lines that need long‑term maintenance or are used for drug screening, G‑banded karyotype analysis is recommended to assess the degree of chromosomal aberrations. STR profiling should be performed approximately every 10 passages to confirm that the cell identity has not been compromised by cross‑contamination or genetic drift.

Standardized Procedures and Precautions for Cell Immortalization

Establishment Timeline and Aseptic Requirements

Establishing an immortalized cell line typically takes 2–4 months, and strict aseptic technique must be followed throughout the experiment.

Differences in Culture Medium and Serum Concentration

Different cell types have different culture requirements. For example, B‑cell immortalization commonly uses RPMI‑1640 medium supplemented with 20% fetal bovine serum; adherent cells (such as fibroblasts) commonly use DMEM medium supplemented with 10% fetal bovine serum.

Reagent Management

All reagents should be dedicated to single use to avoid cross‑contamination.

Risk of Phenotypic Changes

During long‑term passaging, immortalized cells may gradually exhibit phenotypic alterations.

Early Freezing and Stock Preservation Strategy

It is recommended to freeze at least 10 vials of low‑passage stock cells within the first few passages after selection, with each vial containing more than 1×10^6 cells, and to store them in different liquid nitrogen tanks.

Documentation and Traceability Requirements

Detailed records should be kept for each passage, including date, split ratio, cell morphology, and culture medium lot number, to form a traceable archive.

Alpha Lifetech has accumulated extensive experience in the field of cell immortalization services. For primary cells derived from different species and tissue sources, we can design optimized viral vector combinations and screening protocols. Through standardized Primary Cell Extraction and Cell Immortalization services, we ultimately deliver rigorously validated immortalized cell lines, providing reliable tool cells for downstream functional studies, drug screening, or antibody production.

FAQs

• 1. How to select the appropriate cell immortalization method based on research needs?
• 2. What cascading obstacles can substandard primary cell quality cause in immortalization experiments?
• 3. What key factors must be controlled to improve the success rate of gene transduction using viral vectors?
• 4. From which dimensions can comprehensive validation of cell immortalization outcomes be performed?
• 5. What potential problems should not be overlooked during long‑term serial cultivation of immortalized cells?

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