Ectopic human telomerase catalytic subunit expression maintains telomere length but is not sufficient for CD8+ T lymphocyte immortalization

Regulation of telomerase towards tumor therapy

Cancer is an aging-related disease, while aging plays an important role in the development process of tumor, thus the two are inextricably associated. Telomere attrition is one of the recognized hallmark events of senescence. Hence, targeting telomerase which could extends telomere sequences to treat tumors is widely favored. Cancer cells rely on high activity of telomerase to maintain a strong proliferative potential. By inhibiting the expression or protein function of telomerase, the growth of cancer cells can be significantly suppressed. In addition, the human immune system itself has a defense system against malignant tumors. However, excessive cell division results in dramatic shortening on telomeres and decline in the function of immune organs that facilitates cancer cell evasion. It has been shown that increasing telomerase activity or telomere length of these immune cells can attenuate senescence, improve cellular viability, and enhance the immunosuppressive microenvironment of tumor. In this paper, we review the telomerase-targeting progress using different anti-tumor strategies from the perspectives of cancer cells and immune cells, respectively, as well as tracking the preclinical and clinical studies of some representative drugs for the prevention or treatment of tumors.

Beyond youth: Understanding CAR T cell fitness in the context of immunological aging

Population aging, a pervasive global demographic trend, is anticipated to challenge health and social systems worldwide. This phenomenon is due to medical advancements enabling longer lifespans, with 20% of the US population soon to be over 65 years old. Consequently, there will be a surge in age-related diseases. Senescence, characterized by the loss of biological maintenance and homeostasis at molecular and cellular levels, either correlates with or directly causes age-related phenotypic changes. Decline of the immune system is a critical factor in the senescence process, with cancer being a primary cause of death in elderly populations. Chimeric antigen receptor (CAR) T cell therapy, an innovative approach, has demonstrated success mainly in pediatric and young adult hematological malignancies but remains largely ineffective for diseases affecting older populations, such as late-in-life B cell malignancies and most solid tumor indications. This limitation arises because CAR T cell efficacy heavily relies on the fitness of the patient-derived starting T cell material. Numerous studies suggest that T cell senescence may be a key driver of CAR T cell deficiency. This review examines correlates and underlying factors associated with favorable CAR T cell outcomes and explores potential experimental and clinically actionable strategies for T cell rejuvenation.

Epigenetic strategies to boost CAR T cell therapy

Chimeric antigen receptor (CAR) T cell therapy has led to a paradigm shift in cancer immunotherapy, but still several obstacles limit CAR T cell efficacy in cancers. Advances in high-throughput technologies revealed new insights into the role that epigenetic reprogramming plays in T cells. Mechanistic studies as well as comprehensive epigenome maps revealed an important role for epigenetic remodeling in T cell differentiation. These modifications shape the overall immune response through alterations in T cell phenotype and function. Here, we outline how epigenetic modifications in CAR T cells can overcome barriers limiting CAR T cell effectiveness, particularly in immunosuppressive tumor microenvironments. We also offer our perspective on how selected epigenetic modifications can boost CAR T cells to ultimately improve the efficacy of CAR T cell therapy.

Telomerase activation in the treatment of aging or degenerative diseases: a systematic review

Telomeres are protective structures that are shortened during the lifetime, resulting in aging and degenerative diseases. Subjects experiencing aging and degenerative disorders present smaller telomeres than young and healthy ones. The size of these structures can be stabilized by telomerase, an enzyme which is inactive in adult tissues but functional in fetal and newborn tissues and adult testes and ovaries. The aim of this study was to perform a systematic review to evaluate the effect of telomerase activation in the treatment of degenerative and aging disorders. We accomplished the search using the Pubmed interface for papers published from September 1985 to April 16th, 2020. We found twenty one studies that matched our eligibility criteria. I concluded that telomerase is probably a potential and safe treatment for aging and degenerative diseases, demonstrating neither side effects nor risk of cancer in the selected studies. Further studies in humans are needed to confirm safety and efficiency of this treatment.

Prolonged Persistence of Chimeric Antigen Receptor (CAR) T Cell in Adoptive Cancer Immunotherapy: Challenges and Ways Forward

CAR T cell qualities, such as persistence and functionality play important roles in determining the outcome of cancer immunotherapy. In spite of full functionality, it has been shown that poor persistence of CAR T cells can limit an effective antitumor immune response. Here, we outline specific strategies that can be employed to overcome intrinsic and extrinsic barriers to CAR T cell persistence. We also offer our viewpoint on how growing use of CAR T cells in various cancers may require modifications in the intrinsic and extrinsic survival signals of CAR T cells. We anticipate these amendments will additionally provide the rationales for generation of more persistent, and thereby, more effective CAR T cell treatments. CAR T cell qualities, such as persistence and functionality play important roles in determining the outcome of cancer immunotherapy. In spite of full functionality, it has been shown that poor persistence of CAR T cells can limit an effective antitumor immune response. Here, we outline specific strategies that can be employed to overcome intrinsic and extrinsic barriers to CAR T cell persistence. We also offer our viewpoint on how growing use of CAR T cells in various cancers may require modifications in the intrinsic and extrinsic survival signals of CAR T cells. We anticipate these amendments will additionally provide the rationales for generation of more persistent, and thereby, more effective CAR T cell treatments.

Expression and regulation of telomerase in human T cell differentiation, activation, aging and diseases

Telomeres are essential for chromosomal integrity. Telomere shortening during cell division restricts cellular proliferative capacity and leads to cellular senescence when critically shortened telomere lengths are reached. Similar to hematopoietic stem cells, T cells can upregulate telomerase activity to compensate for telomere loss incurred during proliferation in response to engagement of the T cell antigen receptor (TCR) or exposure to homeostatic cytokines. However, this compensation for telomere loss by telomerase in T cells is imperfect or limited, as shortening of T cell telomeres is observed in human aging and during in vitro longterm culture. In this review, we summarize the current state of knowledge regarding the expression and regulation of telomerase in human T cells and changes of telomerase expression during development, activation, differentiation, aging and disease conditions. In conclusion, we discuss how controlled enhancement of telomerase activity could be a potential strategy to improve T cell function in the elderly and in immunotherapy.

Ageing and the telomere connection: An intimate relationship with inflammation

Telomeres are the heterochromatic repeat regions at the ends of eukaryotic chromosomes, whose length is considered to be a determinant of biological ageing. Normal ageing itself is associated with telomere shortening. Here, critically short telomeres trigger senescence and eventually cell death. This shortening rate may be further increased by inflammation and oxidative stress and thus affect the ageing process. Apart from shortened or dysfunctional telomeres, cells undergoing senescence are also associated with hyperactivity of the transcription factor NF-κB and overexpression of inflammatory cytokines such as TNF-α, IL-6, and IFN-γ in circulating macrophages. Interestingly, telomerase, a reverse transcriptase that elongates telomeres, is involved in modulating NF-κB activity. Furthermore, inflammation and oxidative stress are implicated as pre-disease mechanisms for chronic diseases of ageing such as neurodegenerative diseases, cardiovascular disease, and cancer. To date, inflammation and telomere shortening have mostly been studied individually in terms of ageing and the associated disease phenotype. However, the interdependent nature of the two demands a more synergistic approach in understanding the ageing process itself and for developing new therapeutic approaches. In this review, we aim to summarize the intricate association between the various inflammatory molecules and telomeres that together contribute to the ageing process and related diseases.

Enhancement of the in vivo persistence and antitumor efficacy of CD19 chimeric antigen receptor T cells through the delivery of modified TERT mRNA

Chimeric antigen receptor T cell immunotherapy is a promising therapeutic strategy for treating tumors, demonstrating its efficiency in eliminating several hematological malignancies in recent years. However, a major obstacle associated with current chimeric antigen receptor T cell immunotherapy is that the limited replicative lifespan of chimeric antigen receptor T cells prohibits the long-term persistence and expansion of these cells in vivo, potentially hindering the long-term therapeutic effects of chimeric antigen receptor T cell immunotherapy. Here we showed that the transient delivery of modified mRNA encoding telomerase reverse transcriptase to human chimeric antigen receptor T cells targeting the CD19 antigen (CD19 chimeric antigen receptor T cells) would transiently elevate the telomerase activity in these cells, leading to increased proliferation and delayed replicative senescence without risk of insertion mutagenesis or immortalization. Importantly, compared to conventional CD19 chimeric antigen receptor T cells, after the transient delivery of telomerase reverse transcriptase mRNA, these CD19 chimeric antigen receptor T cells showed improved persistence and proliferation in mouse xenograft tumor models of human B-cell malignancies. Furthermore, the transfer of CD19 chimeric antigen receptor T cells after the transient delivery of telomerase reverse transcriptase mRNA enhanced long-term antitumor effects in mouse xenograft tumor models compared with conventional CD19 chimeric antigen receptor T cell transfer. The results of the present study provide an effective and safe method to improve the therapeutic potential of chimeric antigen receptor T cells, which might be beneficial for treating other types of cancer, particularly solid tumors.

A new hope in immunotherapy for malignant gliomas: adoptive T cell transfer therapy

Immunotherapy emerged as a promising therapeutic approach to highly incurable malignant gliomas due to tumor-specific cytotoxicity, minimal side effect, and a durable antitumor effect by memory T cells. But, antitumor activities of endogenously activated T cells induced by immunotherapy such as vaccination are not sufficient to control tumors because tumor-specific antigens may be self-antigens and tumors have immune evasion mechanisms to avoid immune surveillance system of host. Although recent clinical results from vaccine strategy for malignant gliomas are encouraging, these trials have some limitations, particularly their failure to expand tumor antigen-specific T cells reproducibly and effectively. An alternative strategy to overcome these limitations is adoptive T cell transfer therapy, in which tumor-specific T cells are expanded ex vivo rapidly and then transferred to patients. Moreover, enhanced biologic functions of T cells generated by genetic engineering and modified immunosuppressive microenvironment of host by homeostatic T cell expansion and/or elimination of immunosuppressive cells and molecules can induce more potent antitumor T cell responses and make this strategy hold promise in promoting a patient response for malignant glioma treatment. Here we will review the past and current progresses and discuss a new hope in adoptive T cell therapy for malignant gliomas.

Engineered T cells for cancer treatment

Adoptively transferred T cells have the capacity to traffic to distant tumor sites, infiltrate fibrotic tissue and kill antigen-expressing tumor cells. Various groups have investigated different genetic engineering strategies designed to enhance tumor specificity, increase T cell potency, improve proliferation, persistence or migratory capacity and increase safety. This review focuses on recent developments in T cell engineering, discusses the clinical application of these engineered cell products and outlines future prospects for this therapeutic modality.

Developing an in vitro model of T cell type of large granular lymphocyte leukemia

We developed a strategy that can prolong in vitro growth of T cell type of large granular lymphocyte (T-LGL) leukemia cells. Primary CD8+ lymphocytes from T-LGL leukemia patients were stably transduced with the retroviral tax gene derived from human T cell leukemia virus type 2. Expression of Tax overrode replicative senescence and promoted clonal expansion of the leukemic CD8+ T cells. These cells exhibit features characteristic of leukemic LGL, including resistance to FasL-mediated apoptosis, sensitivity to the inhibitors of sphingosine-1-phosphate receptor and IκB kinases as well as expression of cytotoxic gene products such as granzyme B, perforin and IFNγ. Collectively, these results indicate that this leukemia cell model can duplicate the main phenotype and pathophysiological characteristics of the clinical isolates of T-LGL leukemia. This model should be useful for investigating molecular pathogenesis of the disease and for developing new therapeutics targeting T-LGL leukemia.

Phenotype-independent effects of retroviral transduction in human dental pulp stem cells

An immortalized human dental pulp stem cell (DPSC) line of an odontoblastic phenotype is established to circumvent the normal programmed senescence and to maintain the cell line's usefulness as a tool for further study of cellular activity. DPSCs are isolated from human dental pulp tissues and transfected using hTERT. The influence of this process on the DPSC phenotype and the mRNA expression of oncogenes involved in cellular senescence is investigated. The results reveal an absence of altered DPSC morphology and phenotype following the exogenous introduction of the hTERT gene, which is coupled with a significant reduction in p16 mRNA expression. This provides insight into how to circumvent in vitro dental pulp stem cell death following the exogenous introduction of hTERT.

Functional assessment of pharmacological telomerase activators in human T cells

Telomeres are structures at the ends of chromosomes that shorten during cell division and eventually signal an irreversible state of growth arrest known as cellular senescence. To delay this cellular aging, human T cells, which are critical in the immune control over infections and cancer, activate the enzyme telomerase, which binds and extends the telomeres. Several different extracts from the Astragalus membranaceus root have been documented to activate telomerase activity in human T cells. The objective of this research was to compare two extracts from Astragalus membranaceus, TA-65 and HTA, for their effects on both telomerase and proliferative activity of human CD4 and CD8 T cells. Our results demonstrate that, TA-65 increased telomerase activity significantly (1.3 to 3.3-fold relative to controls) in T cell cultures from six donors tested, whereas HTA only increased telomerase levels in two out of six donors. We also demonstrate that TA-65 activates telomerase by a MAPK- specific pathway. Finally, we determine that during a three-day culture period, only the T cells treated with the TA-65 extract showed a statistically significant increase in proliferative activity. Our results underscore the importance of comparing multiple telomerase activators within the same experiment, and of including functional assays in addition to measuring telomerase activity.

Telomere lengthening in CD8(+)cells in polyclonal in vitro stimulation is associated with an increase in protein content of catalytic subunit of telomerase (hTERT)

Culturing of polyclonally activated T lymphocytes for 7 days in vitro leads to telomere lengthening in CD8(+), but not CD4(+)lymphocytes. Under these conditions, CD8(+)lymphocytes more intensively express telomerase catalytic subunit protein (hTERT) and divide more often than CD4(+)lymphocytes. It changes the ratio of CD4(+)and CD8(+)subpopulations in favor of the latter by the end of culturing.

P16/p53 expression and telomerase activity in immortalized human dental pulp cells

INTRODUCTION

Residing within human dental pulp are cells of an ectomesenchymal origin which have the potential to differentiate into odontoblast-like cells. These cells have a limited growth potential owing to the effects of cell senescence. This study examines the effects of immortalizing odontoblast-like cells on cell proliferation and mineralization by comparing transformed dental pulp stem cells (tDPSCs) and non-transformed dental pulp stem cells (nDPSCs).

RESULTS

With the exogenous expression of hTERT, tDPSCs maintained a continued expression of odontogenic markers for cell proliferation and mineralization (ALP, COL-1, DMP-1, DSPP, OCN amd OPN)as did nDPScs. Oncoprotein expression was seen in both groups except for a noted absence of p16 in the tDPSCs. nDPSCs also showed lower levels of total ALP and DNA activity in comparison to tDPSCs when assayed as well as low telomerase activity readings.

METHODS

Using a retroviral vector, exogenous human telomerase reverse transcriptase (hTERT) was expressed in tDPSCs. Both cell groups were cultured and their telomerase activities is determined using a telomerase quantification assay. Also examined were the expression of genes involved in proliferation and mineralization such as human alkaline phosphatase (ALP), β-actin, collagen 1 (col-1), core binding factor (cbfa-1), dentin matrix protein (DMP-1), dentin sialophosphoprotein (DSPP), GAPDH, hTERT, osteocalcin (OCN), osteopontin (OPN) as well as oncoproteins involved in senescence (p16, p21 and p53) using RT-PCR. DNA and alkaline phosphatase activity was assayed in both cell groups.

CONCLUSIONS

These results indicate maintainance of odontoblast-like differentiation characteristics after retroviral transformation with hTERT and suggest a possible link with a reduced p16 expression.

Adoptive therapy using antigen-specific T-cell clones

T-cell therapy involves the ex vivo isolation and expansion of antigen-specific T cells for adoptive transfer. The use of T-cell clones represents one embodiment of this approach and provides a uniform population of effector cells, so that parameters contributing to an effective response can be rigorously evaluated. T cells of defined specificity, phenotype, and function are isolated and expanded; when infused into patients, these intrinsic factors can be considered in light of extrinsic factors such as the type of conditioning regimen, cytokine support, and immunomodulatory reagents. In this chapter, 2 topics related to the use of antigen-specific T-cell clones are discussed: first, advances enabling the isolation and expansion of antigen-specific T-cell clones for human trials of adoptive therapy, and second, a contextual framework of advantages and limitations in which the use of adoptively transferred T-cell clones can be judiciously applied as a means to dissect the requirements for effective therapy.

Oxidation in the nucleotide pool, the DNA damage response and cellular senescence: Defective bricks build a defective house

Activation of persistent DNA damage response (DDR) signaling is associated with the induction of a permanent proliferative arrest known as cellular senescence, a phenomenon intrinsically linked to both tissue aging as well as tumor suppression. The DNA damage observed in senescent cells has been attributed to elevated levels of reactive oxygen species (ROS), failing DNA damage repair processes, and/or oncogenic activation. It is not clear how labile molecules such as ROS are able to damage chromatin-bound DNA to a sufficient extent to invoke persistent DNA damage and DDR signaling. Recent evidence suggests that the nucleotide pool is a significant target for oxidants and that oxidized nucleotides, once incorporated into genomic DNA, can lead to the induction of a DNA strand break-associated DDR that triggers senescence in normal cells and in cells sustaining oncogene activation. Evasion of this DDR and resulting senescence is a key step in tumor progression. This review will explore the role of oxidation in the nucleotide pool as a major effector of oxidative stress-induced genotoxic damage and DDR in the context of cellular senescence and tumorigenic transformation.

Rhadinovirus vector-derived human telomerase reverse transcriptase expression in primary T cells

The rhadinovirus herpesvirus saimiri (HVS) as a gene delivery vector allows large DNA insertions and long-termed gene expression. In the case of T-cell transduction, such vectors use the viral transformation-associated genes of HVS C488 for T-cell amplification. In this report, we investigated whether the gene for the catalytic telomerase subunit human telomerase reverse transcriptase (hTERT) can substitute for the transformation-associated genes in rhadinoviral T-cell transduction and amplification. By using virus mutants generated by en passant mutagenesis from bacterial artificial chromosomes, we observed a very early and functional transgene expression even by virus mutants without transformation-associated genes. The markers of T-cell transformation by HVS, namely CD2 hyperreactivity, overexpression of interleukin-26, and of the tyrosine kinase Lyn could neither be induced nor enhanced by ectopic hTERT expression. When the viral transformation-associated genes were replaced by the hTERT gene, it was not sufficient for growth transformation, although hTERT was efficiently transduced and functionally expressed by the rhadinovirus vector. Thus, the transformation-associated proteins StpC and Tip are responsible for the T-cell phenotype after transduction by HVS and, additionally, modulate telomerase activity independently of hTERT expression.

Immunotherapy of human cancers using gene modified T lymphocytes

Adoptive T cell therapies can produce objective clinical responses in patients with hematologic and solid malignancies. Genetic manipulation of T lymphocytes has been proposed as a means of increasing the potency and range of this anti-tumor activity. We now review how coupling expression of transgenic receptors with countermeasures against potent tumor immune evasion strategies is proving highly effective in pre-clinical models and describe how these approaches are being evaluated in human subjects.

Immortalization and Characterization of Porcine Ventricular Endocardial Endothelial Cells

Endocardial endothelial cells (EECs), which form the inner lining of the cavities of the heart, are a distinct cell population whose dysfunction can be critical in pathological conditions of heart. Insights into the role and organization of these cells in pathological states of the heart are limited mainly due to a dearth of experimental models. To date no endocardial endothelial cell line is available. The authors attempted to immortalize porcine ventricular EECs by transfecting the cells with human telomerase reverse transcriptase (hTERT). EECs immortalized by ectopic expression of hTERT exhibit phenotypic and functional characteristics similar to primary EECs. The EE cell line could be useful for the study of mechanisms involved in the interaction of EECs with the underlying myocardium and cardiac interstitium and as useful tools in understanding their role in diseased states of heart.

Telomeres and aging

Telomeres play a central role in cell fate and aging by adjusting the cellular response to stress and growth stimulation on the basis of previous cell divisions and DNA damage. At least a few hundred nucleotides of telomere repeats must "cap" each chromosome end to avoid activation of DNA repair pathways. Repair of critically short or "uncapped" telomeres by telomerase or recombination is limited in most somatic cells and apoptosis or cellular senescence is triggered when too many "uncapped" telomeres accumulate. The chance of the latter increases as the average telomere length decreases. The average telomere length is set and maintained in cells of the germline which typically express high levels of telomerase. In somatic cells, telomere length is very heterogeneous but typically declines with age, posing a barrier to tumor growth but also contributing to loss of cells with age. Loss of (stem) cells via telomere attrition provides strong selection for abnormal and malignant cells, a process facilitated by the genome instability and aneuploidy triggered by dysfunctional telomeres. The crucial role of telomeres in cell turnover and aging is highlighted by patients with 50% of normal telomerase levels resulting from a mutation in one of the telomerase genes. Short telomeres in such patients are implicated in a variety of disorders including dyskeratosis congenita, aplastic anemia, pulmonary fibrosis, and cancer. Here the role of telomeres and telomerase in human aging and aging-associated diseases is reviewed.

Telomere and adaptive immunity

The adaptive immune response relies on the ability of lymphocytes to undergo periodic massive expansion. It is an enigma how lymphocytes are able to undergo this seemingly unlimited number of cell divisions. Telomeres and telomerase play a critical role in regulation of the replicative lifespan of cells, providing a potential mechanism which lymphocytes may employ. Here I will review the recent progress of the role of telomeres and telomerase in lymphocyte differentiation, function, and aging.

Assessing ageing of individual T lymphocytes: mission impossible?

Effector T lymphocytes are the progeny of a limited number of antigen-specific precursor cells and it has been estimated that clonotypic human T cells may expand million fold on their way reaching high cell numbers that are sufficient for immune protection. Moreover, memory T cell responses are characterized by repetitive expansion of antigen-specific T cell clonotypes, and limitations in the proliferative capacity could lead to immune senescence. Because telomeres progressively shorten as a function of cell division, telomere length is a powerful indicator of the replicative in vivo history of human T lymphocytes. In this review, we summarize observations made over the last decade on telomere length dynamics of well-defined T cell populations derived from healthy donors and patients with infectious disease or cancer. We focus on T cell differentiation, T cell ageing, and natural and vaccine induced immune responses. We also discuss the scientific evidence for in vivo replicative senescence of antigen-specific T cells, and evaluate the available methods for measuring telomere lengths and telomerase activity, and their potential and limitations to increase our understanding of T cell physiology.

Loss of T cell receptor-induced Bmi-1 in the KLRG1(+) senescent CD8(+) T lymphocyte

Clones of CD8(+) T cells specific for viral antigens must avoid replicative senescence to maintain continuous production of new effector cells during chronic viral infections. In the present study, we have determined whether this capability may be related to Bmi-1, a transcriptional repressor that is required for the maintenance of hematopoietic stem cells and certain neural stem cells and that mediates its antisenescence function by inhibiting transcription of the Ink4a/Arf tumor suppressor locus. Ligation of the T cell receptor increased the levels of Bmi-1 mRNA and protein in primary CD8(+) T cells. The increased expression was reversible upon removal of antigen but could be maintained by using stimulation with the IL-2 receptor. Specific suppression of Bmi-1 by using a lentivirally encoded short hairpin RNA inhibited the proliferation of IL-2-stimulated CTLL-2 cytotoxic T cells and primary CD8(+) T cells. Ectopically expressed Bmi-1 enhanced the expansion of primary CD8(+) T cells stimulated by IL-2 and IL-7 in vitro and by homeostatic signals in vivo. Taken together, these findings indicate that Bmi-1 is required for CD8(+) T cell clonal expansion and is positively regulated by receptors that mediate this response. Therefore, the observation that the ability of the T cell receptor to induce Bmi-1 is maintained in the subset of replication-competent, antigen-experienced CD8(+) T cells that do not express the killer cell lectin-like receptor G1 (KLRG1) but is developmentally switched off in the senescent, KLRG1(+) subset suggests that Bmi-1 is a molecular determinant of the capacity of a CD8(+) T cell clone to persist during chronic viral infections.

Telomerase in T lymphocytes: use it and lose it?

The enzyme telomerase counteracts telomere loss in proliferating cells and extends their capacity for replication. The importance of telomerase is highlighted by the award of the 2006 Albert Lasker Prize for Basic Medical Research for its discovery. Malignant cells subvert telomerase induction to their advantage, and up-regulation of this enzyme confers these populations with unlimited proliferative potential with obvious detrimental consequences. However this enzyme is also essential for the lifelong maintenance of normal cell populations that have a high rate of turnover. Thymic involution in early adulthood dictates that memory T cell populations have to be maintained by continuous proliferation. This highlights the inherent paradox that telomerase down-regulation in T cells may protect against malignancy yet also lead to replicative exhaustion of repeatedly activated memory T cells. In this article, we review the data on telomerase regulation in T lymphocytes and the implications this has for the maintenance of T cell memory.

Improving T cell therapy for cancer

Adoptive transfer of antigen-specific T lymphocytes is a powerful therapy for the treatment of opportunistic disease and some virus-associated malignancies such as Epstein-Barr virus-positive post-transplant lymphoproliferative disease. However, this strategy has been less successful in patients with nonviral cancers owing to their many and varied immune evasion mechanisms. These mechanisms include downregulation of target antigens and antigen-presenting machinery, secretion of inhibitory cytokines, and recruitment of regulatory immune cells to the tumor site. With increased understanding of the tumor microenvironment and the behavior and persistence of ex vivo-manipulated, adoptively transferred T cells, two novel approaches for increasing the efficacy of T cell therapy have been proposed. The first involves genetic modification of tumor-specific T cells to improve their biological function, for example by augmenting their ability to recognize tumor cells or their resistance to tumor-mediated immunosuppression. The second requires modifications to the host environment to improve the homeostatic expansion of infused T cells or to eliminate inhibitory T cell subsets. In this review, we discuss current, promising strategies to improve adoptive T cell therapy for the treatment of cancer.

Cytokine-independent growth and clonal expansion of a primary human CD8+ T-cell clone following retroviral transduction with the IL-15 gene

Malignancies arising from retrovirally transduced hematopoietic stem cells have been reported in animal models and human gene therapy trials. Whether mature lymphocytes are susceptible to insertional mutagenesis is unknown. We have characterized a primary human CD8(+) T-cell clone, which exhibited logarithmic ex vivo growth in the absence of exogenous cytokine support for more than 1 year after transduction with a murine leukemia virus-based vector encoding the T-cell growth factor IL-15. Phenotypically, the clone was CD28(-), CD45RA(-), CD45RO(+), and CD62L(-), a profile consistent with effector memory T lymphocytes. After gene transfer with tumor-antigen-specific T-cell receptors, the clone secreted IFN-gamma upon encountering tumor targets, providing further evidence that they derived from mature lymphocytes. Gene-expression analyses revealed no evidence of insertional activation of genes flanking the retroviral insertion sites. The clone exhibited constitutive telomerase activity, and the presence of autocrine loop was suggested by impaired cell proliferation following knockdown of IL-15R alpha expression. The generation of this cell line suggests that nonphysiologic expression of IL-15 can result in the long-term in vitro growth of mature human T lymphocytes. The cytokine-independent growth of this line was a rare event that has not been observed in other IL-15 vector transduction experiments or with any other integrating vector system. It does not appear that the retroviral vector integration sites played a role in the continuous growth of this cell clone, but this remains under investigation.

Persistence of tumor infiltrating lymphocytes in adoptive immunotherapy correlates with telomere length

Transfer of autologous tumor-specific tumor infiltrating lymphocytes (TILs) in adoptive immunotherapy can mediate the regression of tumor in patients with metastatic melanoma. In this procedure, TILs from resected tumors are expanded in vitro, then administered to patients and further stimulated to proliferate in vivo by the administration of high dose IL-2. After in vitro expansion, TILs are often dominated by a few specific clonotypes, and recently it was reported that the persistence in vivo of one or more of these clonotypes correlated with positive therapeutic response. We and others have previously shown that repeated in vitro stimulation and clonal expansion of normal human T lymphocytes results in progressive decrease in telomerase activity and shortening of telomeres, ultimately resulting in replicative senescence. In the studies reported here, we therefore compared telomerase activity and telomere length in persistent and nonpersistent TIL clonotypes before transfer in vivo, and found a correlation between telomere length and clonal persistence. We also observed that TILs proliferate extensively in vivo in the days after transfer, but fail to induce substantial telomerase activity, and undergo rapid decreases in telomere length within days after transfer. Thus, in vivo loss of telomeres by clonotypes that have the shortest telomeres at the time of administration may drive these clones to replicative senescence, whereas cells with longer telomeres are able to persist and mediate antitumor effects. These findings are relevant both to predicting effectiveness of adoptive immunotherapy and in deriving strategies for improving effectiveness by sustaining telomere length.

Extension of cell life span using exogenous telomerase

Normal human somatic cells undergo limited cell division cycles and enter irreversible replication arrest called senescence. Cellular senescence of many human cell types is regulated by the length and status of telomeric sequences, which is shortened after each round of DNA replication. Telomeres can be rejuvenated by telomerase, an enzyme which carries out de novo synthesis of telomeric DNA. Telomerase is a ribonucleoprotein complex composed minimally of telomere reverse transcriptase gene (hTERT) and RNA template (hTR), and its enzyme activity in cells is primarily limited by the level of hTERT expression. Therefore, telomerase activity in cells can be reconstituted by overexpression of hTERT, frequently resulting in extension of replicative life span or immortalization. It is well established that the effect of telomerase reconstitution on cellular life span is clearly cell type-dependent because telomere shortening is not the only limiting factor of cellular life span. However, telomerase activity appears to be a requirement for cellular immortalization, irrespective of the cell types. In this article, we discuss the detailed methods to extend the in vitro replicative life span of primary human cells by ectopic expression of hTERT.

Mechanisms Regulating the Proliferative Potential of Human CD8+ T Lymphocytes Overexpressing Telomerase

In human somatic cells, including T lymphocytes, telomeres progressively shorten with each cell division, eventually leading to a state of cellular senescence. Ectopic expression of telomerase results in the extension of their replicative life spans without inducing changes associated with transformation. However, it is yet unknown whether somatic cells that overexpress telomerase are physiologically indistinguishable from normal cells. Using CD8+ T lymphocyte clones overexpressing telomerase, we investigated the molecular mechanisms that regulate T cell proliferation. In this study, we show that early passage T cell clones transduced or not with human telomerase reverse transcriptase displayed identical growth rates upon mitogenic stimulation and no marked global changes in gene expression. Surprisingly, reduced proliferative responses were observed in human telomerase reverse transcriptase-transduced cells with extended life spans. These cells, despite maintaining high expression levels of genes involved in the cell cycle progression, also showed increased expression in several genes found in common with normal aging T lymphocytes. Strikingly, late passage T cells overexpressing telomerase accumulated the cyclin-dependent inhibitors p16Ink4a and p21Cip1 that have largely been associated with in vitro growth arrest. We conclude that alternative growth arrest mechanisms such as those mediated by p16Ink4a and p21Cip1 still remained intact and regulated the growth potential of cells independently of their telomere status.

Immunosenescence, suppression and tumour progression

There are good arguments for suggesting that two seminal papers published 50 years ago can be taken as the beginning of modern tumour immunology. These papers by R. Baldwin, "Immunity to transplanted tumour: the effect of tumour extracts on the growth of homologous tumours in rats" and "Immunity to methylcholanthrene-induced tumours in inbred rats following atrophy and regression of the implanted tumours" (Br J Cancer 9:646-51 and 652-657, 1955) showed that once tumours are established, they and their products can be recognised by the adaptive immune system and rejected. However, the tumour normally co-evolves with immunity, like a parasite, rather than being suddenly introduced as in these, and many other, experimental models. Dynamics of this co-evolution are illustrated by findings that inflammation enhances tumorigenicity, yet is important to enable T cells to respond properly to tumour antigen and exert anti-tumour effects. The important thing is to maintain the balance between effective anti-tumour immunity and tumour escape and/or stimulatory mechanisms. Tumours almost always co-exist with immune defence systems over extended periods and interact chronically with T cells. The effect of this is potentially similar to other situations of chronic antigenic stress, particularly lifelong persistent virus infection, most strikingly, CMV infection. The questions briefly explored in this symposium paper are what happens when T lymphocyte clones are chronically stimulated by antigen which is not or cannot be eliminated? What are the similarities and differences between chronic antigenic stimulation by tumour antigen versus CMV antigen? What can we learn in one system which may illuminate the other?

Transformation by herpesviruses: focus on T cells

Acute T-lymphoproliferative syndromes are caused by herpesvirus saimiri (HVS) and ateles in neotropical primates; by alcelaphine herpesvirus-1 and ovine herpesvirus-2 strains in domestic cattle and other ungulates; and by the α-herpesvirus of Marek's disease in chickens. T-cell lymphoproliferation caused by these herpesviruses has short incubation periods and a rapid course when compared with retroviral disease. The B-lymphotropic Epstein–Barr virus (EBV) is also associated with some human T-cell malignancies. Analogous to EBV in B cells, HVS isolates of the subgroup C are uniquely capable of transforming human and Old World primate T lymphocytes to continuous growth in cell culture and can provide useful tools for T-cell immunology or gene transfer. Signal transduction pathways stimulated by the viral oncoproteins seem to converge at related cellular effector proteins, in total providing a proproliferative signal. However, the viral oncoproteins most likely evolved to evade immune recognition and to support persistent infection in the natural host, where these viruses are frequently apathogenic.

Mechanisms that limit the in vitro proliferative potential of human CD8+ T lymphocytes

Human T lymphocytes can be numerically expanded in vitro only to a limited extent. The cyclin-dependent kinase inhibitor p16(INK4a) is essential in the control of cellular proliferation, and its expression, in epithelial cells, is associated with irreversible growth arrest. Using long-term cultured CD8+ T lymphocytes, we have investigated the role of the p16/pRb pathway in the regulation of T cell proliferation and senescence. In this study, we describe at least two mechanisms that cause replicative growth arrest in cultured lymphocytes. The first one depends on the expression of p16(INK4a) and is directly responsible for the exit of a significant proportion of CD8+ T cells from the proliferative population. This induced p16 expression pattern is observed during each round of mitogen stimulation and is not related to activation-induced cell death. Importantly, knocking down p16(INK4a) expression allows increased proliferation of T cells. The second one is a phenomenon that resembles human fibroblast senescence, but is independent of p16(INK4a) and of telomere attrition. Interestingly, virtually all pRb proteins in the senescent population are found in the active form. Our data indicate that newly synthesized p16(INK4a) limits the proliferation of T lymphocytes that respond to mitogen, but is not required for the loss of mitogen responsiveness called senescence.

Immunotherapy and immunoselection -- tumour escape as the final hurdle

Tumours are immunogenic and are commonly infiltrated by anti-cancer effector cells. Why, then, are they not completely rejected by the host? Unfortunately, tumours are Darwinian paragons, winning the battle against the forces of natural immune selection. Some of the latter can even act as double-edged swords, actually being subverted to become pro-tumorigenic. Prevention or reversal of tumour escape from the immune response therefore offers the possibility of reconstituting effective anti-tumour immunity and remains the major challenge for 21st century tumour immunology.

Engineering Anticancer T Cells for Extended Functional Longevity

Like other somatic cells, human T lymphocytes have a finite replicative capacity in vitro, and, by implication and consistent with the limited data available, in vivo as well. An accumulation of dysfunctional T cells may be detrimental under conditions of chronic antigenic stress (chronic infection, cancer, autoimmunity). Using T cells from young donors to model the process of T cell clonal expansion in vitro under these conditions reveals age-associated increasing levels of oxidative DNA damage and microsatellite instability (MSI), coupled with decreasing DNA repair capacity, telomerase induction and telomere length, decreased levels of expression of the T cell costimulator CD28 and consequently reduced secretion of the T cell growth factor interleukin-2 (IL-2). However, data from similar experiments using T cell clones (TCCs) derived from extremely healthy very elderly donors ("successfully aged") indicate that DNA repair is better maintained, MSI less prevalent, and (already short) telomere lengths are maintained. Nonetheless, oxidative DNA damage is seen to the same extent, and clonal longevity is also similar in these clones. DNA damage levels are reduced by culture in 5% oxygen, but longevity is not improved. This may be because of the requirement for intermittent reactivation via receptor pathways dependent on free radical production in T cells. These recent findings from our international immunosenescence research consortium suggest that strategies other than telomere maintenance, better protection against free radicals, or improved DNA repair will be required for functional longevity extension of human TCCs. To obtain sufficient cells for adoptive immunotherapy of cancer, alternative avenues need exploration; currently, these include enforced expression of certain heat shock proteins and proteasome components, and interference with the expression of negative regulatory receptors expressed by T cells.

Spontaneously immortalized human T lymphocytes develop gain of chromosomal region 2p13-24 as an early and common genetic event

To gain further insight into the molecular events responsible for the extended life span and immortalization of human lymphoid cells, we analyzed a series of spontaneously immortalized, IL2-dependent human T-cell lines using molecular cytogenetic techniques. Two of the cell lines were derived from normal spleen and three from patients with Nijmegen breakage syndrome (NBS), a recessive disorder characterized by a high incidence of lymphoid malignancies. Here we show that spontaneous immortalization of the five T-cell lines was associated with the acquisition of copy number gains involving chromosomal region 2p13-24 as common early alterations. In addition, we found an amplification of 8q21-24 after prolonged propagations in all three NBS-derived cell lines as well as early development of near-tetraploidy in two of these lines. Gains involving the short arm of chromosome 2 recently were found in several lymphoid malignancies. Therefore, the cell lines described here can be used for identification and characterization of genes involved in the pathogenesis of lymphoid neoplasms and would also provide a useful tool for better understanding the mechanisms responsible for cell immortalization.

In vitro generation and life span extension of human papillomavirus type 16-specific, healthy donor-derived CTL clones

Human papillomavirus (HPV) type 16 infection is strongly associated with the development of cervical carcinoma (CxCa) in women. The HPV16-derived oncoproteins E6 and E7, responsible for both onset and maintenance of malignant transformation, are expressed constitutively in CxCa cells and represent tumor-associated Ags. As a result, E6 and E7 constitute potential targets for adoptive CTL-mediated immunotherapy of CxCa. However, the availability to date of well-characterized HPV16-specific, CxCa-reactive human CTLs is extremely limited. The current study describes the in vitro generation and isolation of HPV16 E7-specific, CxCa-reactive human CTL clones from low-frequency healthy donor-derived CD8beta-positive precursors. For this purpose, an in vitro CTL induction protocol was used involving mature monocyte-derived dendritic cells as stimulator cells loaded with an HLA-A2.1-restricted, E7(11-20)-derived high-affinity altered peptide ligand. A double tetramer-guided isolation procedure and subsequent limiting-dilution cloning resulted in Ag-specific CTL clones. Stringent CTL characterization clearly indicated Ag-specific, HLA-A2.1-restricted reactivity against different HPV16-transformed CxCa cell lines. To allow expansion of E7(11-20)-specific CTL clones to numbers required for prolonged in vitro as well as in vivo application, their life span was significantly extended by ectopic expression of human telomerase reverse transcriptase. Collectively, our results show that optimized CTL induction and stringent CTL selection procedures, followed by human telomerase reverse transcriptase-mediated life span extension will allow continued availability of low-frequency HPV16-specific, CxCa-reactive human CTL clones. This may enhance the prospects of HPV16-specific adoptive CTL immunotherapy in CxCa patients.

Telomere maintenance and cell cycle regulation in spontaneously immortalized T-cell lines from Nijmegen breakage syndrome patients

Nijmegen breakage syndrome (NBS) is a rare genetic instability syndrome associated with a high incidence of lymphoid malignancies. The NBS1 protein has been implicated in telomere biology suggesting that cells from NBS patients might have deficient telomere maintenance capacity. In this study we characterized spontaneously immortalized T-cell lines derived from three NBS patients regarding growth characteristics, telomere biology, expression of cell-cycle regulators, and response to DNA damage to understand the role of NBS1 in the immortalization process. In all the NBS T-cell lines the acquisition of an immortal phenotype was associated with telomere length stabilization, high telomerase activity, and increased mRNA expression of the catalytic subunit of telomerase (hTERT), together with c-myc up-regulation. Our findings provide evidence that telomere length maintenance was intact in the T lymphocytes in the absence of a full-length NBS protein, presumably due to the presence of an alternatively transcribed NBS protein of 70 kDa. Normal protein expression patterns for pRb and p53 in all the immortal lines coincided with altered expression of some cell-cycle proteins as well as with an impaired G1/S arrest after gamma irradiation, despite a seemingly normal p53/p21 pathway. The here described, spontaneously immortalized NBS derived T-cell lines can be useful in future analysis of the biologic effects in the NBS.

Ectopic hTERT expression extends the life span of human CD4+ helper and regulatory T-cell clones and confers resistance to oxidative stress-induced apoptosis

Human somatic cells have a limited life span in vitro. Upon aging and with each cell division, shortening of telomeres occurs, which eventually will lead to cell cycle arrest. Ectopic hTERT expression has been shown to extend the life span of human T cells by preventing this telomere erosion. In the present study, we have shown that ectopic hTERT expression extends the life span of CD4+ T helper type 1 or 2 and regulatory T-cell clones and affected neither the in vitro cytokine production profile nor their specificity for antigen. In mixed cell cultures, ectopic hTERT-expressing clones were found to expand in greater numbers than untransduced cells of the same replicative age. This ectopic hTERT-induced growth advantage was not due to an enhanced cell division rate or number of divisions following T-cell receptor-mediated activation, as determined in carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeling experiments. Moreover, the susceptibility to activation-induced cell death of both cell types was similar. However, cultures of resting hTERT-transduced T cells contained higher frequencies of Bcl-2-expressing cells and lower active caspase-3-expressing cells, compared with wild-type cells. Furthermore, hTERT-transduced cells were more resistant to oxidative stress, which causes preferential DNA damage in telomeres. Taken together, these results show that ectopic hTERT expression not only protects proliferating T cells from replicative senescence but also confers resistance to apoptosis induced by oxidative stress.

Overexpressed mortalin (mot-2)/mthsp70/GRP75 and hTERT cooperate to extend the in vitro lifespan of human fibroblasts

The lifespan of human foreskin fibroblasts (HFF5), cultured under standard in vitro conditions (including ambient atmospheric oxygen tension), was extended slightly by expression of exogenous mortalin (mot-2)/mthsp70/Grp75, but not by the catalytic subunit of telomerase, hTERT. Together, mot-2 and hTERT permitted bypass of senescence, a substantial extension of lifespan, and possibly immortalization. This is the first demonstration that mot-2 and telomerase can cooperate in the immortalization process.

[Telomerase, elixir of life for human cells?]

Telomeres are specialized structures at the end of eukaryotic chromosomes that in vertebrates constain hundreds to thousands of tandem repeats of the sequence TTAGGG. In most human somatic cells, telomeres shorten with each cell division, eventually triggering an irreversible arrest of proliferation called cellular senescence. These observations have led to a model in which telomere length reflects the mitotic history of somatic cells. Further support for this hypothesis has come from the discovery of telomerase, a unique reverse transcriptase ribonucleoprotein that has the ability to extend 3' end of telomeres. In fibroblasts, senescence is induced by telomere attrition and depends on p53 and pRb pathways triggered by one or a few critically short telomeres. Previous studies have shown that the replicative life span of various primary human cells can be prolonged by transduction of the telomerase reverse transcriptase (hTERT) gene. The hTERT expressing cells proliferate indefinitely, without undergoing any changes associated with transformation to malignancy. Rapid progress has been made towards the goal of using tumor-specific cytolytic CD8+ T lymphocytes for the immunotherapy of cancer. These cells can be expanded in vitro and, in principle, could be used for adoptive immunotherapy. One of the major problems that remains to be solved is the finite life span of normal human T lymphocytes. In an attempt to overcome this barrier three groups have introduced hTERT cDNA into human T lymphocytes and monitored its effect on their life span. In two of these studies, hTERT significantly extended the replicative life span of CD8+ T clones, whereas this was not the case in the third study using bulk T lymphocytes. Possible explanations for these discordant results are that better growth conditions avoided culture-induced stress in the study with clones, or that clones had undergone alterations leading, for example, to the inactivation of the pRb pathway during their derivation.

Differential impairment of lytic and cytokine functions in senescent human immunodeficiency virus type 1-specific cytotoxic T lymphocytes

Telomere length is abnormally short in the CD8(+) T-cell compartment of human immunodeficiency virus type 1 (HIV-1)-infected persons, likely because of chronic cell turnover. Although clonal exhaustion of CD8(+) cytotoxic T lymphocytes (CTL) has been proposed as a mechanism for loss of antigen-specific responses, the functional consequences of exhaustion are poorly understood. Here we used telomerase transduction to evaluate the impact of senescence on CTL effector functions. Constitutive expression of telomerase in an HIV-1-specific CTL clone results in enhanced proliferative capacity, in agreement with prior studies of other human cell types. Whereas the CTL remain phenotypically normal in terms of antigenic specificity and requirements for proliferation, their cytolytic and antiviral capabilities are superior to those of control CTL. In contrast, their ability to produce gamma interferon and RANTES is essentially unchanged. The selective enhancement of cytolytic function in memory CTL by ectopic telomerase expression implies that loss of this function (but not cytokine production) is a specific consequence of replicative senescence. These data suggest a unifying mechanism for the in vivo observations that telomere lengths are shortened in the CD8(+) cells of HIV-1-infected persons and that HIV-1-specific CTL are deficient in perforin. Telomerase transduction could therefore be a tool with which to explore a potential therapeutic approach to an important pathophysiologic process of immune dysfunction in chronic viral infection.

Evidence that high telomerase activity may induce a senescent-like growth arrest in human fibroblasts

Expression of the catalytic subunit of human telomerase (hTERT), in normal human fibroblasts allows them to escape replicative senescence. However, we have observed that populations of hTERT-immortalized human fibroblasts contain 3-20% cells with a senescent morphology. To determine what causes the appearance of these senescent-like cells, we used flow cytometry to select them from the population and analyzed them for various senescence markers, telomere length, and telomerase activity. This subpopulation of cells had elevated levels of p21 and hypophosphorylated Rb, but telomere length was similar to that of the immortal cells in the culture that was sorted. Surprisingly, telomerase activity in the senescent-like cells was significantly elevated compared with immortal cells from the same population, suggesting that high telomerase activity may induce the senescent phenotype. Furthermore, transfection of normal fibroblasts with a hTERT-expressing plasmid that confers high telomerase activity led to the induction of p21, a higher percentage of SA-beta-galactosidase-positive cells, and a greater number of cells entering growth arrest compared with controls. These results suggest that excessive telomerase activity may act as a hyperproliferative signal in cells and induce a senescent phenotype in a manner similar to that seen following overexpression of oncogenic Ras, Raf, and E2F1. Thus, there must be a critical threshold of telomerase activity that permits cell proliferation.

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.

Odontoblast cells immortalized by telomerase produce mineralized dentin-like tissue both in vitro and in vivo

The formation of dentin provides one well accepted paradigm for studying mineralized tissue formation. For the assembly of dentin, several cellular signaling pathways cooperate to provide neural crest-derived mesenchymal cells with positional information. Further, "cross-talk" between signaling pathways from the mesenchymal derived odontoblast cells and the epithelially derived ameloblasts during development is responsible for the formation of functional odontoblasts. These intercellular signals are tightly regulated, both temporally and spatially. When isolated from the developing tooth germ, odontoblasts quickly lose their potential to maintain the odontoblast-specific phenotype. Therefore, generation of an odontoblast cell line would be a valuable reproducible tool for studying the modulatory effects involved in odontoblast differentiation as well as the molecular events involved in mineralized dentin formation. In this study an immortalized odontoblast cell line, which has the required biochemical machinery to produce mineralized tissue in vitro, has been generated. These cells were implanted into animal models to determine their in vivo effects on dentin formation. After implantation, we observed a multistep, programmed cascade of gene expression in the exogenous odontoblasts as the dentin formed de novo. Some of the genes expressed include the dentin matrix proteins 1, 2, and 3, which are extracellular matrix molecules responsible for the ultimate formation of mineralized dentin. The biological response was also examined by histology and radiography and confirmed for mineral deposition by von Kossa staining. Thus, a transformed odontoblast cell line was created with high proliferative capacity that might ultimately be used for the regeneration and repair of dentin in vivo.

Telomerase is not an oncogene

In the decade since the telomere hypothesis of cellular aging was proposed, the two essential genes for human telomerase were cloned and characterized, allowing experimental proof of the causal relationships between telomere loss and replicative senescence, and telomerase activation and immortalization. These relationships were established using a variety of cultured human cell types from both normal and tumor tissues, and were largely confirmed in the telomerase knockout mouse. Taken together, the data provide strong support for the potential utility of telomerase detection and inhibition for cancer, and telomerase activation for degenerative diseases. The specificity of the promoter for the telomerase catalytic gene and the antigenicity of the protein product, hTERT, provide additional strategies for killing telomerase-positive tumor cells. Unfortunately, the strong link between telomerase and cancer has led some to confuse telomerase activation with cancer, and others to overstate the cancer risk of telomerase activation therapies for degenerative diseases. This review clarifies the difference between telomerase, which does not cause growth deregulation, and oncogenes, which do. It also addresses the concept of telomerase repression as a tumor suppressor mechanism early in life, with detrimental tissue degeneration and tumor-promoting consequences late in life. This extended view of the telomere hypothesis helps explain how telomerase inhibition can be therapeutic in cancer patients, while controlled telomerase activation for degenerative diseases may actually reduce, rather than increase, the frequency of age-related tumorigenesis.

Comparison of human mammary epithelial cells immortalized by simian virus 40 T-Antigen or by the telomerase catalytic subunit

We directly compared two methods of immortalizing human mammary epithelial cells (HMECs). Cells were transfected with an expression plasmid either for hTERT, the catalytic subunit of telomerase, or for the simian virus 40 (SV40) early region genes. Under standard culture conditions, HMECs were not immortalized by hTERT unless they had spontaneously ceased expression of the p16(INK4a) tumor suppressor gene. Untransfected HMECs had low levels of telomerase expression, and immortalization by both methods was associated with an increase in telomerase activity and prevention of telomere shortening. SV40-induced immortalization was accompanied by aberrant differentiation, loss of DNA damage response, karyotypic instability and, in some cases, tumorigenicity. hTERT-immortalized cells had fewer karyotypic changes, but had intact DNA damage responses, and features of normal differentiation. Although SV40-immortalized cells are useful for studies of carcinogenesis, hTERT-immortalized cells retain more properties of normal cells.

Generation of human pulmonary microvascular endothelial cell lines

The limited lifespan of human microvascular endothelial cells in cell culture represents a major obstacle for the study of microvascular pathobiology. To date, no endothelial cell line is available that demonstrates all of the fundamental characteristics of microvascular endothelial cells. We have generated endothelial cell lines from human pulmonary microvascular endothelial cells (HPMEC) isolated from adult donors. HPMEC were cotransfected with a plasmid encoding the catalytic component of telomerase (hTERT) and a plasmid encoding the simian virus 40 (SV40) large T antigen. Cells transfected with either plasmid alone had an extended lifespan, but the cultures eventually entered crisis after several months of proliferation. Only those cells that were transfected with both plasmids acquired the capacity to grow in vitro without demonstrating major crisis, and these cells have been in culture for 24 months. HPMEC isolated from two different donors were used, generating two populations of immortalized cells, HPMEC-ST1 and HPMEC-ST2. Single cell-derived clones of the immortalized cells HPMEC-ST1 exhibited growth characteristics that were similar to those of the parental HPMEC. One selected clone, HPMEC-ST1.6R, displayed all major constitutively expressed and inducible endothelial phenotypic markers, including platelet endothelial cell adhesion molecule (PECAM-1, CD31), von Willebrand factor (vWF), and the adhesion molecules, intercellular adhesion molecule (ICAM-1), vascular adhesion molecule (VCAM-1), and E-selectin. In addition, an angiogenic response was demonstrated by sprout formation on a biological extracellular matrix (Matrigel). The HPMEC-ST1.6R cells did not form tumors in nude mice. The microvascular endothelial cell line, HPMEC-ST1.6R, will be a valuable tool for the study of microvascular endothelial physiology and pathology including gene expression, angiogenesis, and tumorigenesis.

Role of p14(ARF) in replicative and induced senescence of human fibroblasts

Following a proliferative phase of variable duration, most normal somatic cells enter a growth arrest state known as replicative senescence. In addition to telomere shortening, a variety of environmental insults and signaling imbalances can elicit phenotypes closely resembling senescence. We used p53(-/-) and p21(-/-) human fibroblast cell strains constructed by gene targeting to investigate the involvement of the Arf-Mdm2-p53-p21 pathway in natural as well as premature senescence states. We propose that in cell types that upregulate p21 during replicative exhaustion, such as normal human fibroblasts, p53, p21, and Rb act sequentially and constitute the major pathway for establishing growth arrest and that the telomere-initiated signal enters this pathway at the level of p53. Our results also revealed a number of significant differences between human and rodent fibroblasts in the regulation of senescence pathways.

Transfer of the human telomerase reverse transcriptase (TERT) gene into T lymphocytes results in extension of replicative potential

In most human somatic cells telomeres progressively shorten with each cell division eventually leading to chromosomal instability and cell senescence. The loss of telomere repeats with cell divisions may also limit the replicative life span of antigen-specific T lymphocytes. Recent studies have shown that the replicative life span of various primary human cells can be prolonged by induced expression of the telomerase reverse transcriptase (hTERT) gene. To test whether introduction of hTERT can extend the life span of primary human T lymphocytes, naive CD8(+) T lymphocytes were transfected with retroviral vectors containing the hTERT gene. Transduced T-cell clones expressed high levels of telomerase and either maintained or elongated their telomere lengths upon culture for extended periods of time. Two of the transduced subclones retained a normal cloning efficiency for more than 170 population doublings (PDs). In contrast, T-cell clones transfected with control vectors exhibited progressive telomere length shortening and stopped proliferation at around 108 PDs. Telomerase-positive T clones had a normal 46,XY karyotype, maintained their cytotoxic properties, and showed very little staining for the apoptotic marker annexin-V. These results indicate that ectopic hTERT gene expression is capable of extending the replicative life span of primary human CD8(+) cytotoxic T lymphocytes. (Blood. 2001;98:597-603)