1.         What are cell lines?

A cell line is an immortalized tissue derived from explants of primary tissue. Cell cultures are essential for biomedical and molecular biology research, as cell lines provide a powerful model system in vitro.

Cancer cell lines, finite cell lines, healthy but immortalized cell lines, and induced pluripotent stem cells are often used instead of primary cells and cultures because they are inexpensive and easy to maintain. Biological material is available indefinitely for research. Due to the limited number of cell divisions, primary cell cultures can only be maintained for a few days before they die. These conditions limit the reproducibility of experiments because identical cells cannot be used. Cell lines, on the other hand, allow reproducible results because similar cells can be used for research. The cell line must be authenticated, free of mycoplasma and other contaminants, and from a reliable source such as the CLS or other biorepositories.

2.       What are the different types of cell lines?

2.1.               Human cell lines

Human cell lines are immortal cells grown in vitro from primary human tissue or body fluids explants. Since the turn of the 20th century, scientists have used cell lines to gain knowledge about cell biology and metabolism. Cell lines, or immortalized cell lines, have become a popular model in the cell culture literature, serving as a well-characterized and optimized entity for pharmacological research, biochemical analysis, synthesis of bioactive compounds, etc. Scientists prefer cell lines over primary cells because they are less expensive, more user-friendly, and can undergo more passages. Since cell lines are easy to manipulate and propagate, they are preferred for numerous screenings as they provide an endless material supply.

2.2.              Animal cell lines

Animal cell lines are indispensable for research in cell biology and biomedicine: the cultivation of animal cells provides an excellent model system for studying fundamental aspects of cell biology and metabolism. Animal cell cultures have been used as 2D and 3D models for studying infectious agents and drugs in various scientific investigations. A significant advantage of using an animal cell line for research is the reduced need for animal models. Because animal models are more similar to human systems, researchers can use animal cells to study various disease processes and evaluate innovative treatments in animal models before applying these findings to human patients.

2.3.              What are immortal cell lines and how are they produced?

The first cell line was created by placing a sample of human cancer cells in a liquid medium containing nutrients and growth factors (now known as cell culture medium) and allowing the cells to divide. After an extended period, it was found that the cell culture could survive and divide indefinitely (through clonal expansion). However, the cell culture continued to mutate simultaneously, resulting in different cell lines with other functional properties, even though they were derived from the same cancer stem cell.

Today, there are numerous options for the production of cell lines, including:

• Cell isolation from a cancer sample (as described above).
• Introduction of viral genes that deregulate cell cycle regulation.
• Hybridoma technology

The following are known examples of potentially immortal cell lines:

Advantages of immortalized cell lines

• Standardization: Immortalized cell lines are used by many different laboratories, resulting in a well-characterized and standardized system.
• Consistent and reproducible results: Due to their homogeneous and genetically identical nature, results obtained with immortalized cell lines are more consistent and reproducible.
• Easier culture: immortalized cell lines grow more robustly and do not need to be extracted from a living animal, making them easier to culture than cells in primary cultures.
• High protein yield: Immortalized cells grow rapidly and continuously, enabling the extraction of large amounts of proteins for biochemical studies.
• Gene expression: It is possible to generate cell lines that continuously express a specific gene, e.g. a fluorescent label or a mutated protein version.

Disadvantages of immortalized cell lines

• Lack of normality: immortalized cells divide indefinitely and can express unique gene patterns not found in normal cells, making them unsuitable for studying relatively normal cells.
• Characteristic changes: Continuous growth can lead to changes in cell characteristics over time, making the cells even more different from normal cells.
• Validation required: It is essential to regularly check the properties of the cultured cells to ensure that they have not changed over time, especially if the cells have been passaged too often.

What is an example of a cell line?

HeLa: The oldest and most widely used immortal cell line to date is the HeLa cell line. The cells were isolated from a cervical cancer sample caused by human papillomavirus 18 (HPV-18) infection. HeLa cell culture was used to obtain the polio virus, test it, and produce it to develop the first mass-produced polio vaccine.

HEK293: In the 1970s, the HEK293 cell line was derived from human embryonic kidney cells by Alex van der Eb at Utrecht University. Multiple copies of the adenovirus genes E1A and E1B confer high transfection efficiency to HEK293 cells. This makes them useful in biotechnology, especially for protein expression and gene therapy, among other applications. HEK293 cells are widely used in industrial biotechnology, where they are used to produce recombinant proteins and enzymes.

CHO: In 1957, T. T. Puck established the CHO cell line by isolating cells from the ovary of an adult Chinese hamster. This cell line has since become an indispensable model for biotechnology. The cells are used extensively to produce recombinant proteins such as therapeutic antibodies and vaccines. In toxicology, CHO cells are used as model organisms to study the effects of chemicals, drugs, and other toxins on cell growth and metabolism. The cells are used to produce monoclonal antibodies, recombinant proteins, and vaccines. More than sixty therapeutic proteins produced with CHO cells have been approved for production. This cell line has been used for decades in biotechnology for industrial biotechnology, toxicology and bioproduction. They are ideal for the production of recombinant proteins and the development of new drugs.

3.       Use of cell lines in research

The significant areas of investigation for established cell lines are cell proliferation, cell division, the study of drug metabolism, gene expression, gene function, the study of biological compounds, and the general understanding of biological processes.

The many applications of cell line research include vaccine production, drug development and discovery, single-cell studies, virus propagation, and drug expression through the production of recombinant proteins and secretion of naturally secreted proteins in a cell line. The individual applications are described in detail below:

3.1.               Recombinant protein production in mammalian and insect cells.

Recombinant proteins are produced by first cloning the gene encoding the desired protein into an expression vector. The protein is then made in a laboratory environment by microorganisms such as bacteria, yeast or animal cells. The human genes used in this process may contain non-coding DNA, known as introns, so a version without introns is produced by converting mRNA to cDNA. The expression vectors provide the required promoter, ribosome binding site, and terminator sequences. Recombinant protein production is widely used in research because it is inexpensive, efficient, and offers reasonable yields. However, some proteins require post-translational modifications that can only occur in eukaryotic cells such as yeast, insect, or mammalian cells. These modifications can be achieved by transient transfection or production in mammalian cells, producing high-quality proteins similar to natural ones. E.coli is also used to produce approved recombinant therapeutic proteins due to its well-understood genetics, rapid growth, and high production yield.

Due to their ability to synthesize proteins, eukaryotic cell lines have become indispensable for producing recombinant proteins. Other systems cannot facilitate protein folding and assembly of molecules. Recombinant protein production begins with the development of expression vectors and transfection into the host system, followed by cell selection, cloning, screening, and evaluation. Recombinant protein manufacturers need efficient and cost-effective expression hosts to meet quality and scalability standards.

3.2.              Virus production

The introduction of cell culture techniques has dramatically changed the isolation and propagation of viruses in the laboratory. Cell-based production methods offer a practical and cost-effective approach to virus isolation, detection, and identification, and more advanced process control results in a more reliable and better-characterized product with faster and shorter production cycles than animal or egg-based systems.

Cell-based manufacturing processes are critical for virus culture and vaccine production for:

• Virus detection/identification
• Exploration of host-pathogen interactions
• Structure and replication of viruses Production of vaccines

3.3.              Toxicity evaluation

In evaluating the toxicity of new drugs, chemicals, and cosmetics, animal cell cultures are becoming increasingly popular as an alternative to animal testing. The kidney and liver are the main organs used for producing and using animal cell cultures in this field.

3.4.              Cell-based production

Animal cell cultures can potentially be used to mass-produce viruses, which can then be used in vaccine production. Numerous vaccines, including those against polio, rabies, measles, varicella, and hepatitis B, have benefited from this strategy.

Animal cell cultures can be used not only to produce viruses but also to produce genetically modified products with commercial and medical applications. There are many forms of products, such as monoclonal antibodies, insulin, hormones, etc.

3.5.              Drug screening and development

Assays based on animal cell cultures are becoming an increasingly important part of the pharmaceutical industry. They are not only used for toxicity testing but also for high-throughput screening of potential drugs.

3.6.              Cancer studies

Animal cell cultures are used for biomarker and molecular research in cancer. In addition, cultured cancer cells have the potential to serve as test models for a variety of anticancer drugs. Recent cancer research aims to find methods to selectively eliminate cancer cells from populations that also contain normal primary cells.

3.7.              Virology

Occasionally, animal cell cultures are used to replicate viruses to circumvent animal testing. These replicated viruses can be used to produce vaccines and isolate and study basic viruses.

3.8.              Genetic modification

The essence of genetic engineering is to rewrite an individual's genes to produce different proteins. Transfection is the ability to introduce additional genetic material into cells. For clinical research or medical treatment, animal cell cultures can be subjected to transfection to produce a significant amount of new proteins.

3.9.              Gene treatment

Now that we know that animal cell cultures can be used for genetic engineering, we also know that genetically modified cells can be used for therapeutic purposes. A patient's cells can be removed and replaced with synthetic cells containing the required functional gene. This method is called ex vivo gene therapy. Alternatively, a viral vector can introduce the missing gene into the patient's cells.

3.10.           Treatment with stem cells

Both stem cell research and therapy use stem cell cultures derived from animal cells. In both fields, mainly mesenchymal and hematopoietic stem cells have been used. Animal cell cultures consisting of somatic cells from different animals have also been used in induced pluripotent stem cell research. Animal cell cultures have the potential to serve as tissue or organ substitutes. For example, this method can be used to produce artificial skin, which can then be used to treat people with burns or ulcers. On the other hand, research can be used to grow artificial organs such as the liver, kidney, and pancreas.