What have we learned about T cell biology from Jurkat cells?

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Jurkat T cell

‘What have we learned about T cell biology from Jurkat cells?‘. A more accurate reformulation would be ‘What have we learned from Jurkat cells that’s applicable to T cell biology and what’s not?”. Unfortunately, the answer isn’t that easy to untangle from archival data.

Much like the tumor cell line HeLa – Wikipedia before it, from the 1980s through the 1990s, Jurkat cells – Wikipedia became a heavyweight tool to understand human T-cell receptor – Wikipedia (TCR) biochemistry (1). Later, Jurkat also became a popular tool to model human T cell infection by HIV. However, very much a Curate’s egg – Wikipedia, Jurkat‘s perceived benefits remained disproportionately the focus during this time while its harms have been considered only much more recently. Meantime, 21st century technological advances have pretty much eliminated reliance on such inherently problematic transformed (tumor) cell lines.

This answer outlines

• A brief history of the Jurkat T cell line, highlighting how the imperatives of prevailing technical limitations drove its past popularity.
• how Jurkat‘s inherent limitations in decoding TCR signaling are predicated on the fact that it is a tumor cell, has mutations and is potentially contaminated, features that cast doubt on the validity of some historical results using it.

Brief History of the Jurkat T cell line

In 1977, a cell line was derived from T cells isolated from a 14 year old boy with Acute lymphoblastic leukemia – Wikipedia (ALL) (2). This cell line was eventually called Jurkat.

Back then, T cells remained very much a mystery, there was little or no consensus on how to maintain them in culture for long periods of time, a basic requirement necessary to dissect their essential properties, especially their biochemistry.

The experimental mouse model, today the backbone of immunology research, was still in its infancy as was molecular biology. Today’s technological mainstays such as gene targeting to create T cell transgenic mice and T cell transgenics with attached reporter genes to facilitate their monitoring were advances decades in the future.

Thus in this vacuum, an immortalized cell line such as Jurkat capable of being maintained in culture in perpetuity became an extremely valuable tool that in hindsight arose at the moment when most needed.

In its early years of use, Jurkat was thus used to delineate a great deal of the signal transduction pathway and molecules triggered by T-cell receptor – Wikipedia (TCR) signaling (1).

Hindsight also suggests a readily available human T cell line made such research far simpler and much cheaper. No need to draft complicated study protocols, get them reviewed and approved by Institutional review board – Wikipedia (IRB) in order to gain permission to bleed people to isolate their T cells in order to study them. Jurkat was thus a convenient tool to study aspects of human TCR biochemistry.

Limitations of Jurkat T cell line

Immortalized cell line versus primary, normal cell. During its early years of use, methods to culture human primary T cells didn’t exist. Even today, primary T cells simply can’t be maintained indefinitely in culture. Their long-term study requires stimulation, expansion, cloning and then immortalization (hybridization [mechanical fusing] with a partner tumor cell).

Yet, being a tumor cell, do results from Jurkat apply to normal human T cells in general? That’s simply unknowable since it’s impossible to know exactly what T cell stage Jurkat represents considering it got established and began to be used back when little was known about T cell development, activation, effector differentiation and memory formation. While papers routinely refer to Jurkat as a Lymphoblast – Wikipedia, at best that’s just a tenuous guess.

Mutations in many key molecules involved in the TCR signaling pathway. Tumor cells replicate uncontrollably, having broken free of biological control. Mutations in cell cycle checkpoints make such liberation possible. During its first two decades of use, how Jurkat‘s mutations might affect its TCR functioning wasn’t a focus. After all, there is an inherent tautology to unraveling signaling defects in cell lines being used to identify signaling pathways in the first place. Signaling pathways need to be comprehensively deciphered first to determine if a particular cell line has a signaling defect or two or however many the case may be.

As T cell biochemistry advanced apace by the late 1990s, some TCR signaling pathways identified using mouse T cells, T cell lines or other human T cell lines didn’t concord with those found in Jurkat. Turns out mutations in Jurkat accounted for such discrepancies.

• Today, signaling of the Phosphoinositide 3-kinase – Wikipedia (PI3K) family is known to be a central feature downstream of TCR signaling. Yet, in the late 1990s-early 2000s, two key molecules that mediate PI3K signaling were found to be missing in Jurkat cells (1). Such a fundamental signaling defect raised questions about the validity of using Jurkat as a tool to understand (human) TCR signaling (1, 3).
• A 2017 analysis uploaded to the preprint server, bioRxiv, comprehensively collates the various defective pathways and key signaling molecules missing in Jurkat (3).
  • In addition to PI3K, it points out Jurkat doesn’t express other molecules such as SHIP1 (INPP5D – Wikipedia), CTLA-4 – Wikipedia and Syk – Wikipedia, all now known to be important components of the TCR signaling pathway.
  • The authors also suggest a potentially ingenious use of such Jurkat defects, namely, to use them in reconstitution experiments to validate functionality of a given molecule in a particular pathway.
• Notwithstanding such grave TCR signaling defects, even today hundreds of papers using Jurkat continue to be published annually.
• Use of Jurkat as a model system to study HIV infection in human T cells yields a similarly confusing story, with many discordances in observations between Jurkat and primary CD4 T cells (4).

Microbial Contamination. A 2008 study reported a batch of Jurkat cells to be contaminated with a retrovirus belonging to the family of Gammaretrovirus – Wikipedia (5). Today this virus is designated as Xenotropic murine leukemia virus-related virus – Wikipedia or XMLV. Note that this study sourced its Jurkat from ATCC (company) – Wikipedia (ATCC), a major global supplier of cell lines. Open questions remain,

• Given ATCC’s Jurkat was found to be contaminated with XMLV in 2008, how many previous published studies on Jurkat used such contaminated cells?
• Are Jurkats stored in other cell bank repositories and maintained by labs around the world similarly infected?
• When did Jurkat become infected? In the 1990s when the XMLV is suspected to have arisen during xenograft studies or later?
• How does this infection influence historical results from Jurkat? Clearly, comparisons of ‘clean’ and ‘contaminated’ Jurkats are needed to figure out if and what effect this has on their TCR signaling.

Bibliography

1. Abraham, Robert T., and Arthur Weiss. “Jurkat T cells and development of the T-cell receptor signalling paradigm.” Nature Reviews Immunology 4.4 (2004): 301-308. https://www.researchgate.net/pro…

2. Schneider, Ulrich, Hans‐Ulrich Schwenk, and Georg Bornkamm. “Characterization of EBV‐genome negative “null” and “T” cell lines derived from children with acute lymphoblastic leukemia and leukemic transformed non‐Hodgkin lymphoma.” International journal of cancer 19.5 (1977): 621-626.

3. Gioia, Louis, et al. “A Genome-wide Survey of Mutations in the Jurkat Cell Line.” bioRxiv (2017): 118117. https://www.biorxiv.org/content/…

4. Markle, Tristan J., Philip Mwimanzi, and Mark A. Brockman. “HIV-1 Nef and T-cell activation: a history of contradictions.” Future virology 8.4 (2013): 391-404. https://www.researchgate.net/pro…

5. Takeuchi, Yasuhiro, Myra O. McClure, and Massimo Pizzato. “Identification of gammaretroviruses constitutively released from cell lines used for human immunodeficiency virus research.” Journal of virology 82.24 (2008): 12585-12588. Identification of Gammaretroviruses Constitutively Released from Cell Lines Used for Human Immunodeficiency Virus Research

https://www.quora.com/What-have-we-learned-about-T-cell-biology-from-Jurkat-cells/answer/Tirumalai-Kamala

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What are the effects of mycoplasmic contamination on cell cultures?

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Cell culture, Mycoplasma

Mycoplasma: some basic facts

Contamination with Mycoplasma – Wikipedia, previously known as pleuropneumonia-like organisms or PPLO, is a common bane in cell cultures, with studies routinely finding many cell lines contaminated with it (1, 2).

Though 0.2 μm or even 0.1 μm pore size of sterilizing filters are normally recommended to filter culture media constituents such as FBS/FCS (Fetal Bovine/Calf Serum) to exclude mycoplasma as well as other micro-organisms (3), mycoplasma are extremely small, 300 to 800 nm in diameter, so filtering alone may be insufficient to exclude them (4, 5). Many mycoplasma strains are also unresponsive to commonly used cell culture antibiotics. ATCC, Bionique, BioReliance, ECACC, Mycoplasma Experience are companies that provide mycoplasma screening services (5). Unfortunately in vitro culture of eukaryotic cells appears to favor mycoplasma growth (6), especially given careless hands and lack of attention to detail.

Routine testing of cell lines for mycoplasma contamination is the only way to mitigate and minimize its harmful effect on biomedical research, something well-recognized as far back as 1994 (see below from 1).

‘100% of the cultures from labs without mycoplasma testing programs were contaminated, but only 2% of the cultures from labs that tested regularly.’

Mycoplasma: cell culture effects

Stealthy by nature and insidious in effect, mycoplasma contamination is much more difficult to discern unlike contamination by bacteria or fungi, whose effects are easily detectable through microscopy or pH (Phenol Red-containing culture media turn yellow through a rapid pH drop) or turbidity changes.

Old is often gold when it comes to cell culture resources and one of the best descriptions of effects of mycoplasma contamination on cell cultures is from a 1971 review (see below from 4)

‘Gross macroscopic changes in tissue cells infected with Mycoplasma sp. can range from inapparent or minimal unsuspected alterations to cytopathology and cell destruction reminiscent of viral infections. Macroscopic changes in morphology can be so minimal that they are not suspected even though a culture can reveal high titers of Mycoplasma. Cytopathie changes may be related to depletion of arginine in the medium by the Mycoplasma…Effects of cytopathic Mycoplasma sp. have been confused with virus infections…studies of cytopathic Mycoplasma strains have emphasized the risk involved in attributing all cytopathic effects to viruses…Many properties are shared by Mycoplasma and viruses: (a) filtrability: a number of strains of Mycoplasma (78) produced cells capable of passing through a 0.22 uL MIillipore filter, and it has been found (79) that the size of the smallest filtrable unit varies under different conditions of culture; (b) electron microscopic morphology: certain pleomorphic forms of rickettsiae, ornithosis virus, and Mycoplasma can be confused with one another (80); (c) sensitivity to ether (75); (d) ability to hemagglutinate (81, 82); (e) ability to cause hemadsorption (83); (f) resistance to some antibiotics; (g) inhibition of growth by antiserum (84, 85); and (h) induction of chromosomal aberrations (86, 87).’

A 1994 review (1) summarizes mycoplasma contamination effects on cell cultures as,

‘the ability to alter their host culture’s cell function, growth, metabolism, morphology, attachment, membranes, virus propagation and yield, interferon induction and yield, cause chromosomal aberrations and damage, and cytopathic effects including plaque formation ‘

Thus, mycoplasma contamination typically

• Affect the rate of cell proliferation (1, 4, 7).
• Induce morphological changes (4, 7, see below from 1).
• Cause chromosome aberrations (1, 7, see next one below from 8).
• Influence amino acid and nucleic acid metabolism (1, 7).
• Induce cell transformation (7).
• Yield poorly reproducible results from cell lines (9, 10).
• Activate primary immune cells (11).
• Change gene expression patterns (see next one below from 8), with one study (12) suggesting as many as 10% of gene expression studies including those published in leading science journals showed evidence of mycoplasma contamination.

Bibliography

1. Ryan, John A. Understanding and managing cell culture contamination. Corning Incorporated, 1994. https://pdfs.semanticscholar.org…

2. Uphoff, Cord C., and Hans G. Drexler. “Comparative PCR analysis for detection of mycoplasma infections in continuous cell lines.” In Vitro Cellular & Developmental Biology-Animal 38.2 (2002): 79-85. https://www.dkfz.de/gpcf/fileadm…

3. Clarke, Sue, and Janette Dillon. “The Cell Culture Laboratory.” Animal Cell Culture: Essential Methods (2011): 1-31.

4. Fogh, Jørgen, Nelda B. Holmgren, and Peter P. Ludovici. “A review of cell culture contaminations.” In vitro 7.1 (1971): 26-41. A review of cell culture contaminations

5. Davis, John M. “Basic techniques and media, the maintenance of cell lines, and safety.” Animal Cell Culture: Essential Methods (2011): 91-151.

6. Razin, Shmuel, and Leonard Hayflick. “Highlights of mycoplasma research—an historical perspective.” Biologicals 38.2 (2010): 183-190. https://www.researchgate.net/pro…

7. Thraves, Peter, and Cathy Rowe. “The Quality Control of Animal Cell Lines and the Prevention, Detection and Cure of Contamination.” Animal Cell Culture: Essential Methods (2011): 255-296.

8. Chernov, V. M., O. A. Chernova, and J. T. Sanchez-Vega. “Mycoplasma contamination of cell cultures: vesicular traffic in bacteria and control over infectious agents.” Acta Naturae (англоязычная версия) 6.3 (22) (2014). https://pdfs.semanticscholar.org…

9. Callaway, Ewen. “Contamination hits cell work: Mycoplasma infestations are widespread and costing laboratories millions of dollars in lost research.” Nature 511.7511 (2014): 518-519. https://www.nature.com/polopoly_…

10. Gedye, Craig, et al. “Mycoplasma infection alters cancer stem cell properties in vitro.” Stem Cell Reviews and Reports 12.1 (2016): 156-161.

11. Heidegger, Simon, et al. “Mycoplasma hyorhinis-contaminated cell lines activate primary innate immune cells via a protease-sensitive factor.” PloS one 10.11 (2015): e0142523. http://journals.plos.org/plosone…

12. Assessing the prevalence of mycoplasma contamination in cell culture via a survey of NCBI?s RNA-seq archive. https://pdfs.semanticscholar.org…

https://www.quora.com/What-are-the-effects-of-mycoplasmic-contamination-on-cell-cultures/answer/Tirumalai-Kamala

What factors go into choosing a cell culture medium type?

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Cell culture

Appropriateness for species, research purpose, cell history/provenance and type, these are among the most important factors to consider when choosing a cell culture medium.

Appropriateness for species. The fifty year old Roswell Park Memorial Institute medium – Wikipedia or RPMI 1640 (1) for short was specially formulated for human cell culture. OTOH, Norman Iscove formulated Iscove’s Modified Dulbecco’s medium or IMDM specifically for mouse cell culture (2). These definitions aren’t set in stone but some knowledge of the history of the most commonly used cell culture media helps make better choices especially when trouble-shooting.

Research purpose spans basic animal or preclinical and basic clinical on the one hand, and therapeutic on the other hand.

Serum is a common cell culture medium component because most culture media, simple mixtures of amino acids, salts, sugars and vitamins, are just too meager to support cell culture on their own. This is why the history of cell culture is dotted with additives such as extracts from embryos, amniotic fluid, milk, colostrum, lymph, plasma and serum. Its ready availability in quantity and ease of storage meant that serum, specifically from fetal calves, eventually came to dominate cell culture.

While serum, especially fetal calf/bovine serum (FCS/FBS) is a common ingredient in basic research cell culture, it’s usually a strict no-no in therapeutic uses. Being especially rich in proteins, hormones and numerous other biochemical molecules makes serum a rich source of nutrition in cell cultures used in basic research. That same advantage, however, renders it a handicap for therapeutic purposes by offering many allogeneic/xenogeneic targets for immune responses when injected into patients, something that would nullify therapeutic benefits. Serum also presents considerable safety issues in the form of potential viral or prion contaminants. Thus, therapeutic use usually entails customized or proprietary serum-free culture media such as the X-VIVO series AIM-V, etc.

Cell history/provenance in terms of primary or culture-adapted is a key difference with the former requiring richer culture media. Culture-adapted, which could be anything from cell lines to clones to transformed cells, are by contrast hardier and capable of growing in more minimal media.

Cell type is another important consideration. For example, 2-Mercaptoethanol – Wikipedia is essential for lymphocyte culture. Whether cells are adherent or free-floating. If the former, culture also entails coating materials, usually proteins such as fibronectin or pre-coated culture-ware to make the cells stick (adhere) to the culture surface.

Better to look up the published literature as well as reference repositories such as ATCC (company) – Wikipedia rather than word of mouth or sales pitches, which likely lack relevant history, let alone critical nuances pertaining to specific cell types and cultures.

Bibliography

1. Moore, George E., Robert E. Gerner, and H. Addison Franklin. “Culture of normal human leukocytes.” Jama 199.8 (1967): 519-524.

2. Iscove, N_N, and F. Melchers. “Complete replacement of serum by albumin, transferrin, and soybean lipid in cultures of lipopolysaccharide-reactive B lymphocytes.” Journal of Experimental Medicine 147.3 (1978): 923-933. http://jem.rupress.org/content/j….

 

https://www.quora.com/What-factors-go-into-choosing-a-cell-culture-medium-type/answer/Tirumalai-Kamala

Are all HeLa cells from a commercial source equivalent to numbers or replications? I wager they are not.

Tags

Cancer cell lines, Cell line authentication, Cell line contamination, Cross-contaminated cell lines, HeLa, Henrietta Lacks, John R. Masters, Roland Nardone, Short Tandem Repeat (STR) profiling, Stanley Gartler, Walter Nelson-Rees

Cell lines like HeLa are the bread and butter of biomedical research, especially human research, yet this could be one of the safest wagers.

Laypersons are largely unaware that for decades misidentified and cross-contaminated cell lines (1) were the vast, seedy underbelly of human biomedical research, a problem well-known in the scientific community with no one, not the scientists, not the funding agencies, not the professional societies nor the scientific journals taking the necessary, largely simple steps to ensure clean up. Though the problem maybe slightly better acknowledged today, it persists and will continue to do so unless and until the key stakeholders with power, namely, funding agencies and scientific journals enforce mandatory policies, rather than continue their head-in-the-sand policy stemming from the wrongheaded notion that science is apparently self-correcting, i.e. that ‘blatantly poor science’ will correct itself.

Scientific editorials casually opine that up to a third of all human cell lines in existence may be misidentified. No way to know for sure. Cell lines circulate freely between colleagues and collaborators. For long, the attitude was casual, along the lines of ‘who has the time or money to get authenticated cell lines from a cell bank?‘. After all, the key stakeholders with the power to effect change, namely, the funding agencies and scientific journals, didn’t mandate such effort. We can surmise that the problem reached epidemic proportion if a mainstream non-science news source such as the Wall Street Journal published a detailed 2012 article on the topic. Titled ‘Lab Mistakes Hobble Cancer Studies But Scientists Slow to Take Remedies’ (2), the article identifies the key sources of the problem:

• Inexperienced or distracted technicians use the same tool when working with different samples
• A pipette or other tool isn’t sterilized
• Cultures are stored closely together
• Containers are mislabeled or labels are missed
• Cell samples from colleagues and other labs aren’t thoroughly tested

The story of cell lines in general and of HeLa in particular is thus operatic for its scale of shoddiness, both unwitting and knowing, and the steadfast courage and integrity of a handful who chose to highlight the increasingly vexing problem of cell line cross-contamination and misidentification, including some like Walter Nelson-Rees, who doggedly pursued a clean up, even in the face of vilification of their scientific reputation, with finally, vindication, though only in posterity (4).

My brief, largely graphic, history starts with this 2002 list (1) by John R. Masters, one of the heroes in this story, that says

• There was never a common stock of HeLa.
• Many different HeLa sublines exist.
• HeLa sublines are quite different from each other.

HeLa cell line heterogeneity has been observed for decades. For example, different alkaline phosphatase expression between HeLa sublines (4).

Another example, differences in expression of bone morphogenic proteins (BMPs) (5).

A recent comparison (6; the only non-peer-reviewed study I refer to in this answer) shows that dramatic karyotype differences have existed between HeLa sublines for decades.

Recently the HeLa genome was sequenced, creating more headaches (7, 8, 9) because the authors publicly posted the genome without first seeking the approval of the Lacks family. Discussions between the family and the US NIH resulted in the condition that US federal grant recipients who use and publish on HeLa need to acknowledge Henrietta Lacks and her family (10).

The HeLa story doesn’t end with the multiple sublines and their many documented and as yet undiscovered differences. HeLa is probably one of the most robust cell lines in existence. Over the decades, this characteristic feature of Hela combined with shoddy cell culture techniques and lax oversight ensured that many cell lines commonly used in human biomedical research became contaminated with HeLa (11, 12, 13).

Yet scientists continue using these contaminated cell lines with an unknown number of peer-reviewed scientific papers containing utterly erroneous and even useless data. In fact, such data is not just useless but costly because it results in wrong research avenues being pursued, sometimes for years, at tremendous cost to taxpayers (13, 14).

Finally, a consensus of sorts on cell line authentication led to the ATCC (American Tissue Culture Collection) paper Standard ‘ASN-0002 – Authentication of Human Cell Lines: Standardization of STR Profiling’ (15). A brief history and overview of current cell line authentication.

What method does ASN-002 advocate for cell line authentication? That everyone using cell lines ‘should use STR (Short Tandem Repeat) profiling for every cell line they use for every publication and for every grant’ (16, 17).

Advantages of STR.

• Accessible
• Easy to perform
• Inexpensive
• Robust

STR is the same PCR (Polymerase Chain Reaction)-based method the FBI uses on its forensic DNA samples (16).

• In STR, the PCR  amplifies across tetra- or pentanucleotide repeats
• Varying numbers of repeats produce different sized DNA fragments
• Fragments generated by each cell line are unique, and identified and assigned a numerical value by comparing to size standards.
• Result? A database of STR for cell lines that can be compared between labs, using appropriate controls and protocols.

Drawbacks of STR

• Cannot account for genetic drift, which is a common issue in cancer cells.
• Not useful for discriminating human cell lines derived from different tissues of the same person.
• So not useful for comparing HeLa sublines.
• Need guidelines on minimum number of STR loci needing to be tested for each cell line being authenticated.
• Human-specific, not much useful for other species.
• Complicated data interpretation stemming from two sources. One, human cell lines have numerous genomic changes such as chromosome duplications, mutations and rearrangements. Such chromosomal abnormalities yield complex STR profiles that are difficult to interpret. Two, PCR can create artifacts, requiring experience to diagnose correctly.

Finally a list of online resources for list of Misidentified cell lines, and databases of cross-contaminated or misidentified cell lines:
misidentified cell line[Filter – BioSample – NCBI]
Database of Cross-contaminated or Misidentified Cell Lines – ICLAC
Page on atcc.org
Page on atcc.org
Page on atcc.org
Home – BioSample – NCBI

Bibliography

1. Masters, John R. “HeLa cells 50 years on: the good, the bad and the ugly.” Nature Reviews Cancer 2.4 (2002): 315-319.
2. Lab Mistakes Hobble Cancer Studies But Scientists Slow to Take Remedies
3. American Type Culture Collection Standards Development Organization Workgroup ASN-0002. “Cell line misidentification: the beginning of the end.” Nature Reviews Cancer 10.6 (2010). Page on calis.edu.cn
4. Benham, Frances J., M. Susan Povey, and Harry Harris. “Heterogeneity of alkaline phosphatases in different HeLa lines.” Somatic cell genetics 4.1 (1978): 13-25.
5. Kochanowska, I. E., et al. “Osteogenic properties of various HeLa cell lines and the BMP family genes expression.” Annals of Transplantation 7.4 (2002): 58-62.
6. Rutledge, Samuel. “What HeLa Cells Are You Using?.” The Winnower (2014). The Winnower | DIY Scientific Publishing
7. Landry, Jonathan JM, et al. “The genomic and transcriptomic landscape of a HeLa cell line.” G3: Genes| Genomes| Genetics 3.8 (2013): 1213-1224. The Genomic and Transcriptomic Landscape of a HeLa Cell Line
8. Mittelman, David, and John H. Wilson. “The fractured genome of HeLa cells.” Genome biology 14.4 (2013): 111. Page on biomedcentral.com
9. Andrews, Brenda J., and Tracey DePellegrin. “HeLa Sequencing and Genomic Privacy: The Next Chapter.” G3: Genes| Genomes| Genetics 3.8 (2013): vii-vii. The Next Chapter
10. Greely, Henry T., and Mildred K. Cho. “The Henrietta Lacks legacy grows.” EMBO reports 14.10 (2013): 849-849. Page on embopress.org
11. Gartler, Stanley M. “Apparent HeLa cell contamination of human heteroploid cell lines.” Nature, 1968; 217 (5130): 750-751.
12. Perkel, J.M. Curing cell lines. Biotechniques 2011, 51, 85–90; Page on biotechniques.com
13. American Type Culture Collection Standards Development Organization Workgroup ASN-0002. “Cell line misidentification: the beginning of the end.” Nature Reviews Cancer 10.6 (2010). Page on calis.edu.cn
14. MacLeod, Roderick AF, Wilhelm G. Dirks, and Hans G. Drexler. “Where have all the cell lines gone?.” International Journal of Cancer 132.5 (2013): 1232-1234. Page on wiley.com
15. Barallon, Rita, et al. “Recommendation of short tandem repeat profiling for authenticating human cell lines, stem cells, and tissues.” In Vitro Cellular & Developmental Biology-Animal 46.9 (2010): 727-732.
16. Perkel, J.M. Curing cell lines. Biotechniques 2011, 51, 85–90. Page on biotechniques.com
17. Reid, Yvonne, and Joe Mintzer. “The current state of cell contamination and authentication—And what it means for biobanks.” Biopreservation and biobanking 10.3 (2012): 236-238.

https://www.quora.com/Are-all-HeLa-cells-from-a-commercial-source-equivalent-to-numbers-or-replications/answer/Tirumalai-Kamala