Telomere loss, senescence, and genetic instability in CD4+ T lymphocytes overexpressing hTERT

Unexpected links between cancer and telomere state

Eukaryotes possess chromosome ends known as telomeres. As telomeres shorten, organisms age, a process defined as senescence. Although uncontrolled telomere lengthening has been naturally connected with cancer developments and immortalized state, many cancers are instead characterized by extremely short, genomically unstable telomeres that may hide cancer cells from immune attack. By contrast, other malignancies feature extremely long telomeres due to absence of 'shelterin' end cap protecting factors. The reason for rampant telomere extension in these cancers had remained elusive. Hence, while telomerase supports tumor progression and escape in cancers with very short telomeres, it is possible that different - transfer based or alternative - lengthening pathways be involved in the early stage of tumorigenesis, when telomere length is intact. In this Review, I hereby discuss recent discoveries in the field of telomeres and highlight unexpected links connecting cancer and telomere state. We hope these parallelisms may inform new therapies to eradicate cancers.

Telomerase reverse transcriptase and neurodegenerative diseases

Neurodegenerative diseases (NDs) are chronic conditions that result in progressive damage to the nervous system, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and Amyotrophic lateral sclerosis (ALS). Age is a major risk factor for NDs. Telomere shortening is a biological marker of cellular aging, and telomerase reverse transcriptase (TERT) has been shown to slow down this process by maintaining telomere length. The blood-brain barrier (BBB) makes the brain a unique immune organ, and while the number of T cells present in the central nervous system is limited, they play an important role in NDs. Research suggests that NDs can be influenced by modulating peripheral T cell immune responses, and that TERT may play a significant role in T cell senescence and NDs. This review focuses on the current state of research on TERT in NDs and explores the potential connections between TERT, T cells, and NDs. Further studies on aging and telomeres may provide valuable insights for developing therapeutic strategies for age-related diseases.

Assessing the association of leukocyte telomere length with ankylosing spondylitis and rheumatoid arthritis: A bidirectional Mendelian randomization study

Background

Telomere length shortening can cause senescence and apoptosis in various immune cells, resulting in immune destabilization and ageing of the organism. In this study, we aimed to systematically assess the causal relationship of leukocyte telomere length (LTL) with ankylosing spondylitis (AS) and rheumatoid arthritis (RA) using a Mendelian randomization study.

Methods

LTL (n=472174) was obtained from the UK Biobank genome-wide association study pooled data. AS (n=229640), RA (n=212472) were obtained from FinnGen database. MR-Egger, inverse variance weighting, and weighted median methods were used to estimate the effects of causes. Cochran's Q test, MR Egger intercept test, MR-PRESSO, leave-one-out analysis, and funnel plots were used to look at sensitivity, heterogeneity, and multiple effects. Forward MR analysis considered LTL as the exposure and AS, RA as the outcome. Reverse MR analysis considered AS, RA as the exposure and LTL as the outcome.

Results

In the forward MR analysis, inverse variance-weighted and weighted median analysis results indicated that longer LTL might be associated with increased risk of AS (IVW: OR = 1.55, 95% CI: 1.14-2.11, p = 0.006). MR Egger regression analysis showed no pleiotropy between instrumental variables (IVs) (Egger intercept= 0.008, p = 0.294). The leave-one-out analysis showed that each single nucleotide polymorphism (SNP) of AS was robust to each outcome. No significant causal effects were found between AS, RA and LTL in the reverse MR analysis.

Conclusion

Longer LTL may be related with an increased risk of developing AS, and these findings provide a foundation for future clinical research on the causal association between LTL and AS.

Aplastic anemia: Etiology, molecular pathogenesis, and emerging concepts

Aplastic anemia (AA) is rare disorder of bone marrow failure which if severe and not appropriately treated is highly fatal. AA is characterized by morphologic marrow features, namely hypocellularity, and resultant peripheral cytopenias. The molecular pathogenesis of AA is not fully understood, and a uniform process may not be the culprit across all cases. An antigen-driven and likely autoimmune dysregulated T-cell homeostasis is implicated in the hematopoietic stem cell injury which ultimately founds the pathologic features of the disease. Defective telomerase function and repair may also play a role in some cases as evidenced by recurring mutations in related telomerase complex genes such as TERT and TERC. In addition, recurring mutations in BCOR/BCORL, PIGA, DNMT3A, and ASXL1 as well as cytogenetic abnormalities, namely monosomy 7, trisomy 8, and uniparental disomy of the 6p arm seem to be intimately related to AA pathogenesis. The increased incidence of late clonal disease has also provided clues to accurately describe plausible predispositions to the development of AA. The emergence of newer genomic sequencing and other techniques is incrementally improving the understanding of the pathogenic mechanisms of AA, the detection of the disease, and ultimately offers the potential to improve patient outcomes. In this comprehensive review, we discuss the current understanding of the immunobiology, molecular pathogenesis, and future directions of such for AA.

Aberrant leukocyte telomere length in Birdshot Uveitis

Purpose

Birdshot Uveitis (BU) is an archetypical chronic inflammatory eye disease, with poor visual prognosis, that provides an excellent model for studying chronic inflammation. BU typically affects patients in the fifth decade of life. This suggests that it may represent an age-related chronic inflammatory disease, which has been linked to increased erosion of telomere length of leukocytes.

Methods

To study this in detail, we exploited a sensitive standardized quantitative real-time polymerase chain reaction to determine the peripheral blood leukocyte telomere length (LTL) in 91 genotyped Dutch BU patients and 150 unaffected Dutch controls.

Results

Although LTL erosion rates were very similar between BU patients and healthy controls, we observed that BU patients displayed longer LTL, with a median of log (LTL) = 4.87 (= 74131 base pair) compared to 4.31 (= 20417 base pair) in unaffected controls (P<0.0001). The cause underpinning the difference in LTL could not be explained by clinical parameters, immune cell-subtype distribution, nor genetic predisposition based upon the computed weighted genetic risk score of genotyped validated variants in TERC, TERT, NAF1, OBFC1 and RTEL1.

Conclusions

These findings suggest that BU is accompanied by significantly longer LTL.

An antiapoptotic role for telomerase RNA in human immune cells independent of telomere integrity or telomerase enzymatic activity

Telomerase is a ribonucleoprotein complex that adds telomeric DNA to the ends of linear chromosomes. It contains two core canonical components: the essential RNA component, hTR, which provides the template for DNA synthesis, and the reverse transcriptase protein component, hTERT. Low telomerase activity in circulating peripheral blood mononuclear cells has been associated with a variety of diseases. It is unknown, however, whether telomerase, in addition to its long-term requirement for telomere maintenance, is also necessary for short-term immune cell proliferation and survival. We report that overexpression of enzymatically inactive hTR mutants protected against dexamethasone-induced apoptosis in stimulated CD4 T cells. Furthermore, hTR knockdown reproducibly induced apoptosis in the absence of any detectable telomere shortening or DNA damage response. In contrast, hTERT knockdown did not induce apoptosis. Strikingly, overexpression of hTERT protein caused apoptosis that was rescued by overexpression of enzymatically inactive hTR mutants. Hence, we propose that hTR can function as a noncoding RNA that protects from apoptosis independent of its function in telomerase enzymatic activity and long-term telomere maintenance in normal human immune cells. These results imply that genetic or environmental factors that alter hTR levels can directly affect immune cell function to influence health and disease.

p38 signaling inhibits mTORC1-independent autophagy in senescent human CD8⁺ T cells

T cell senescence is thought to contribute to immune function decline, but the pathways that mediate senescence in these cells are not clear. Here, we evaluated T cell populations from healthy volunteers and determined that human CD8+ effector memory T cells that reexpress the naive T cell marker CD45RA have many characteristics of cellular senescence, including decreased proliferation, defective mitochondrial function, and elevated levels of both ROS and p38 MAPK. Despite their apparent senescent state, we determined that these cells secreted high levels of both TNF-α and IFN-γ and showed potent cytotoxic activity. We found that the senescent CD45RA-expressing population engaged anaerobic glycolysis to generate energy for effector functions. Furthermore, inhibition of p38 MAPK signaling in senescent CD8+ T cells increased their proliferation, telomerase activity, mitochondrial biogenesis, and fitness; however, the extra energy required for these processes did not arise from increased glucose uptake or oxidative phosphorylation. Instead, p38 MAPK blockade in these senescent cells induced an increase in autophagy through enhanced interactions between p38 interacting protein (p38IP) and autophagy protein 9 (ATG9) in an mTOR-independent manner. Together, our findings describe fundamental metabolic requirements of senescent primary human CD8+ T cells and demonstrate that p38 MAPK blockade reverses senescence via an mTOR-independent pathway.

Elevated expression of pleiotrophin in lymphocytic leukemia CD19+ B cells

Pleiotrophin (PTN) has been demonstrated to be strongly expressed in many fetal tissues, but seldom in healthy adult tissues. While PTN has been reported to be expressed in many types of tumors as well as at high serum concentrations in patients with many types of cancer, to date, there has been no report that PTN is expressed in leukemia, especially in lymphocytic leukemia. We isolated the CD19(+) subset of B cells from peripheral blood from healthy adults, B-cell acute lymphocytic leukemia (B-ALL) patients, and B-cell chronic lymphocytic leukemia (B-CLL) patients and examined these cells for PTN mRNA and protein expression. We used immunocytochemistry, western blotting, and enzyme-linked immunosorbent assay to show that PTN protein is highly expressed in CD19(+) B cells from B-ALL and B-CLL patients, but barely expressed in B cells from healthy adults. We also examined PTN expression at the nucleic acid level using reverse transcription polymerase chain reaction (RT-PCR) and northern blotting and detected a high levels of PTN transcripts in the CD19(+) B cells from both groups of leukemia patients, but very few in the CD19(+) B cells from the healthy controls. Interestingly, the quantity of the PTN transcripts correlated with the severity of disease. Moreover, suppression of PTN activity with an anti-PTN antibody promoted apoptosis of cells from leukemia patients and cell lines SMS-SB and JVM-2. This effect of the anti-PTN antibody suggests that PTN may be a new target for the treatment of lymphocytic leukemia.

Telomeres and telomerase in T cells of tumor immunity

Telomeres are specific nucleoprotein structures at the end of a eukaryotic chromosomes characterized by repeats of the sequence TTAGGG and regulated by the enzyme telomerase which prevents their degradation, loss, rearrangement and end-to-end fusion. During activation, T lymphocytes actively divide, albeit through only a finite number of cell divisions due to shortening of telomeres. However, studies have demonstrated that human telomerase reverse transcriptase (hTERT), thought to be the major component regulating telomerase activity, can enhance the proliferation of T cells when overexpressed. There are many treatments for cancers, most of which are targeting the telomere and telomerase of tumor cells. However, the hTERT-transduced T cells improve their potential for proliferation, making them an appropriate cell resource for tumor adoptive immunotherapy, a procedure whereby T cells are isolated from patients, expanded ex vivo and eventually delivered back into the patients, provides a new approach for tumor therapy through improved overall survival rates in cancer patients. In this review, we will focus on the telomerase activity in T cells, the regulation of telomerase activity, and hTERT-transduced T cells used in adoptive immunotherapy for cancer.

Telomerase and primary T cells: biology and immortalization for adoptive immunotherapy

Telomeres are specialized repeats, present at the end of chromosomes, whose loss during cell division is followed by growth arrest, a central mechanism of replicative senescence in human cells. Telomere length in stem cells is maintained by telomerase, a specialized reverse transcriptase, whose function is to restore shortening telomeres. Unlike most somatic cell types, human T lymphocytes are capable of briefly reactivating telomerase expression at the time of stimulation. Telomerase expression in T lymphocytes is modulated by a variety of external stimuli and by viral infections. However, telomerase reactivation in stimulated, proliferating human T lymphocytes is limited and cannot prevent the ultimate onset of senescence. Ectopic telomerase expression can rescue human and macaque antigen-specific T cells from senescence. Primary T cells have been engineered with telomerase to have substantially extended replicative lifespans without the loss of primary cell functions or malignant transformation. 'Immortal' antigen-specific T-cell lines and clones overexpressing telomerase are an invaluable source of well-characterized quasi-primary T cells for research of T-cell biology and are potentially useful for immunotherapy of cancer and AIDS.

Viperin mRNA is a novel target for the human RNase MRP/RNase P endoribonuclease

RNase MRP is a conserved endoribonuclease, in humans consisting of a 267-nucleotide RNA associated with 7–10 proteins. Mutations in its RNA component lead to several autosomal recessive skeletal dysplasias, including cartilage-hair hypoplasia (CHH). Because the known substrates of mammalian RNase MRP, pre-ribosomal RNA, and RNA involved in mitochondrial DNA replication are not likely involved in CHH, we analyzed the effects of RNase MRP (and the structurally related RNase P) depletion on mRNAs using DNA microarrays. We confirmed the upregulation of the interferon-inducible viperin mRNA by RNAi experiments and this appeared to be independent of the interferon response. We detected two cleavage sites for RNase MRP/RNase P in the coding sequence of viperin mRNA. This is the first study providing direct evidence for the cleavage of a mRNA by RNase MRP/RNase P in human cells. Implications for the involvement in the pathophysiology of CHH are discussed.

Computational models of HIV-1 resistance to gene therapy elucidate therapy design principles

Gene therapy is an emerging alternative to conventional anti-HIV-1 drugs, and can potentially control the virus while alleviating major limitations of current approaches. Yet, HIV-1's ability to rapidly acquire mutations and escape therapy presents a critical challenge to any novel treatment paradigm. Viral escape is thus a key consideration in the design of any gene-based technique. We develop a computational model of HIV's evolutionary dynamics in vivo in the presence of a genetic therapy to explore the impact of therapy parameters and strategies on the development of resistance. Our model is generic and captures the properties of a broad class of gene-based agents that inhibit early stages of the viral life cycle. We highlight the differences in viral resistance dynamics between gene and standard antiretroviral therapies, and identify key factors that impact long-term viral suppression. In particular, we underscore the importance of mutationally-induced viral fitness losses in cells that are not genetically modified, as these can severely constrain the replication of resistant virus. We also propose and investigate a novel treatment strategy that leverages upon gene therapy's unique capacity to deliver different genes to distinct cell populations, and we find that such a strategy can dramatically improve efficacy when used judiciously within a certain parametric regime. Finally, we revisit a previously-suggested idea of improving clinical outcomes by boosting the proliferation of the genetically-modified cells, but we find that such an approach has mixed effects on resistance dynamics. Our results provide insights into the short- and long-term effects of gene therapy and the role of its key properties in the evolution of resistance, which can serve as guidelines for the choice and optimization of effective therapeutic agents.

A primary obstacle to the success of any anti-HIV treatment is HIV's ability to rapidly resist it by generating new viral strains whose vulnerability to the treatment is reduced. Gene therapies represent a novel class of treatments for HIV infection that may supplement or replace present therapies, as they alleviate some of their major shortcomings. The design of gene therapeutic agents that effectively reduce viral resistance can be aided by a quantitative elucidation of the processes by which resistance is acquired following therapy initiation. We developed a computational model that describes a patient's response to therapy and used it to quantify the influence of therapy parameters and strategies on the development of viral resistance. We find that gene therapy induces different clinical conditions and a much slower viral response than present therapies. These dictate different design principles such as a greater significance to the virus' competence in the absence of therapy. We also show that one can effectively delay emergence of resistance by delivering distinct therapeutic genes into separate cell populations. Our results highlight the differences between traditional and gene therapies and provide a basic understanding of how key controllable parameters and strategies affect resistance development.

Imetelstat (GRN163L)--telomerase-based cancer therapy

Telomeres and telomerase play essential roles in the regulation of the lifespan of human cells. While normal human somatic cells do not or only transiently express telomerase and therefore shorten their telomeres with each cell division, most human cancer cells typically express high levels of telomerase and show unlimited cell proliferation. High telomerase expression allows cells to proliferate and expand long-term and therefore supports tumor growth. Owing to the high expression and its role, telomerase has become an attractive diagnostic and therapeutic cancer target. Imetelstat (GRN163L) is a potent and specific telomerase inhibitor and so far the only drug of its class in clinical trials. Here, we report on the structure and the mechanism of action of imetelstat as well as about the preclinical and clinical data and future prospects using imetelstat in cancer therapy.

MART-1-specific melanoma tumor-infiltrating lymphocytes maintaining CD28 expression have improved survival and expansion capability following antigenic restimulation in vitro

We determined how CD8(+) melanoma tumor-infiltrating lymphocytes (TILs) isolated from two distinct phases of expansion in preparation for adoptive T cell therapy respond to melanoma Ag restimulation. We found that TILs isolated after the rapid expansion protocol (REP) phase, used to generate the final patient TIL infusion product, were hyporesponsive to restimulation with MART-1 peptide-pulsed dendritic cells, with many CD8(+) T cells undergoing apoptosis. Telomere length was shorter post-REP, but of sufficient length to support further cell division. Phenotypic analysis revealed that cell-surface CD28 expression was significantly reduced in post-REP TILs, whereas CD27 levels remained unchanged. Tracking post-REP TIL proliferation by CFSE dilution, as well as sorting for CD8(+)CD28(+) and CD8(+)CD28(-) post-REP subsets, revealed that the few CD28(+) TILs remaining post-REP had superior survival capacity and proliferated after restimulation with MART-1 peptide. An analysis of different supportive cytokine mixtures during the REP found that a combination of IL-15 and IL-21 facilitated comparable expansion of CD8(+) TILs as IL-2, but prevented the loss of CD28 expression with improved responsiveness to antigenic restimulation post-REP. These results suggest that current expansion protocols using IL-2 for melanoma adoptive T cell therapy yields largely CD8(+) T cells unable to persist and divide in vivo following Ag contact. The few CD8(+)CD28(+) T cells that remain may be the only CD8(+) TILs that ultimately survive to repopulate the host and mediate long-term tumor control. A REP protocol using IL-15 and IL-21 may greatly increase the number of CD28(+) TILs capable of long-term persistence.

Replicative potential of human natural killer cells

The replicative potential of human CD56(+) CD3(-) natural killer (NK) cells is unknown. We found that by exposing NK cells to the leukaemic cell line K562 genetically modified to express 4-1BB ligand and interleukin 15 (K562-mb15-41BBL), they expanded for up to 30 population doublings, achieving numbers that ranged from 1.6 x 10(5) to 1.2 x 10(11)% (median, 5.9 x 10(6)%; n = 7) of those originally seeded. However, NK cells eventually became unresponsive to stimulation and died. Their demise could be suppressed by enforcing the expression of the human telomerase reverse transcriptase gene (TERT). TERT-overexpressing NK cells continued to proliferate in response to K562-mb15-41BBL stimulation for more than 1 year of culture, while maintaining a normal karyotype and genotype. Long-lived NK cells had high cytotoxicity against myeloid and T-lineage leukaemic cells. They remained susceptible to genetic manipulation, becoming highly cytotoxic to B-lineage leukaemic cells after expression of anti-CD19 signaling receptors. Thus, human NK cells have a replicative potential similar to that of T lymphocytes and their lifespan can be significantly prolonged by an increase in TERT activity. We suggest that the methods described here should have many applications in studies of NK cell biology and NK cell-based therapies.

Telomerase insufficiency in rheumatoid arthritis

In rheumatoid arthritis (RA), chronically stimulated T lymphocytes sustain tissue-destructive joint inflammation. Both naïve and memory T cells in RA are prematurely aged with accelerated loss of telomeres suggesting excessive proliferative pressure or inadequate telomeric maintenance. Upon stimulation, RA naïve CD4 T cells are defective in up-regulating telomerase activity (P < 0.0001) due to insufficient induction of the telomerase component human telomerase reverse transcriptase (hTERT); T cell activation and cell cycle progression are intact. Telomerase insufficiency does not affect memory T cells or CD34 hematopoietic stem cells and is present in untreated patients and independent from disease activity. Knockdown of hTERT in primary human T cells increases apoptotic propensity (P = 0.00005) and limits clonal burst (P = 0.0001) revealing a direct involvement of telomerase in T cell fate decisions. Naïve RA CD4 T cells stimulated through the T cell receptor are highly susceptible to apoptosis, expanding to smaller clonal size. Overexpression of ectopic hTERT in naïve RA T cells conveys apoptotic resistance (P = 0.008) and restores proliferative expansion (P < 0.0001). Telomerase insufficiency in RA results in excessive T cell loss, undermining homeostatic control of the naive T cell compartment and setting the stage for lymphopenia-induced T cell repertoire remodeling. Restoring defective telomerase activity emerges as a therapeutic target in resetting immune abnormalities in RA.

Telomere uncapping during in vitro T-lymphocyte senescence

Normal lymphocytes represent examples of somatic cells that are able to induce telomerase activity when stimulated. As previously reported, we showed that, during lymphocyte long-term culture and repeated stimulations, the appearance of senescent cells is associated with telomere shortening and a progressive drop in telomerase activity. We further showed that this shortening preferentially occured at long telomeres and was interrupted at each stimulation by a transitory increase in telomere length. In agreement with the fact that telomere uncapping triggers lymphocyte senescence, we observed an increase in gamma-H2AX and 53BP1 foci as well as in the percentage of cells exhibiting DNA damage foci in telomeres. Such a DNA damage response may be related to the continuous increase of p16(ink4a) upon cell stimulation and cell aging. Remarkably, at each stimulation, the expression of shelterin genes, such as hTRF1, hTANK1, hTIN2, hPOT1 and hRAP1, was decreased. We propose that telomere dysfunction during lymphocyte senescence caused by iterative stimulations does not only result from an excessive telomere shortening, but also from a decrease in shelterin content. These observations may be relevant for T-cell biology and aging.

The effect of age and telomere length on immune function in the horse

Telomeres, specialized structures present at the ends of linear eukaryotic chromosomes, function to maintain chromosome stability and integrity. Telomeres shorten with each cell division eventually leading to replicative senescence, a process thought to be associated with age-related decline in immune function. We hypothesized that shortened PBMC telomere length is a factor contributing to immunosenescence of the aged horse. Telomere length was assessed in 19 horses ranging in age from 1 to 25 years. Mitogen-induced 3H-thymidine incorporation, total serum IgG, and pro-inflammatory cytokine expression was also determined for each horse. Relative telomere length (RTL) was highly correlated with overall age. RTL was positively correlated with 3H-thymidine incorporation and total IgG. Expression of pro-inflammatory cytokines was negatively correlated with RTL. These measures were also correlated with age, as expected. However, RTL was not correlated with immunosenescence and inflammaging in the oldest horse.

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.

Correlation between recent thymic emigrants and CD31+ (PECAM-1) CD4+ T cells in normal individuals during aging and in lymphopenic children

CD31(+)CD45RA(+)RO(-) lymphocytes contain high numbers of T cell receptor circle (TREC)-bearing T cells; however, the correlation between CD31(+)CD4(+) lymphocytes and TREC during aging and under lymphopenic conditions has not yet been sufficiently investigated. We analyzed TREC, telomere length and telomerase activity within sorted CD31(+) and CD31(-) CD4(+) lymphocytes in healthy individuals from birth to old age. Sorted CD31(+)CD45RA(+)RO(-) naive CD4(+) lymphocytes contained high TREC numbers, whereas CD31(+)CD45RA(-)RO(+) cells (comprising < or =5% of CD4(+) cells during aging) did not contain TREC. CD31(+) overall CD4(+) cells remained TREC rich despite an age-related tenfold reduction from neonatal (100 : 1000) to old age (10 : 1000). Besides a high TREC content, CD31(+)CD45RA(+)RO(-)CD4(+) cells exhibited significantly longer telomeres and higher telomerase activity than CD31(-)CD45RA(+)RO(-)CD4(+) cells, suggesting that CD31(+)CD45RA(+)RO(-)CD4(+) cells represent a distinct population of naive T cells with particularly low replicative history. To analyze the value of CD31 in lymphopenic conditions, we investigated six children after allogeneic hematopoietic stem cell transplantation (HSCT). Reemerging overall CD4(+) as well as naive CD45RA(+)RO(-)CD4(+) cells predominantly expressed CD31 and correlated well with the recurrence of TREC 5-12 months after HSCT. Irrespective of limitations in the elderly, CD31 is an appropriate marker to monitor TREC-rich lymphocytes essentially in lymphopenic children after HSCT.

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.

Telomeres, senescence, and hematopoietic stem cells

The replicative lifespan of normal somatic cells is restricted by the erosion of telomeres, which are protective caps at the ends of linear chromosomes. The loss of telomeres induces antiproliferative signals that eventually lead to cellular senescence. The enzyme complex telomerase can maintain telomeres, but its expression is confined to highly proliferative cells such as stem cells and tumor cells. The immense regenerative capacity of the hematopoietic system is provided by a distinct type of adult stem cell: hematopoietic stem cells (HSCs). Although blood cells have to be produced continuously throughout life, the HSC pool seems not to be spared by aging processes. Indeed, limited expression of telomerase is not sufficient to prevent telomere shortening in these cells, which is thought ultimately to limit their proliferative capacity. In this review, we discuss the relevance of telomere maintenance for the hematopoietic stem cell compartment and consider potential functions of telomerase in this context. We also present possible clinical applications of telomere manipulation in HSCs and new insights affecting the aging of the hematopoietic stem cell pool and replicative exhaustion.

Short telomeres and high telomerase activity in T-cell prolymphocytic leukemia

To test the role of telomere biology in T-cell prolymphocytic leukemia (T-PLL), a rare aggressive disease characterized by the expansion of a T-cell clone derived from immuno-competent post-thymic T-lymphocytes, we analyzed telomere length and telomerase activity in subsets of peripheral blood leukocytes from 11 newly diagnosed or relapsed patients with sporadic T-PLL. Telomere length values of the leukemic T cells (mean+/-s.d.: 1.53+/-0.65 kb) were all below the 1st percentile of telomere length values observed in T cells from healthy age-matched controls whereas telomere length of normal T- and B cells fell between the 1st and 99th percentile of the normal distribution. Leukemic T cells exhibited high levels of telomerase and were sensitive to the telomerase inhibitor BIBR1532 at doses that showed no effect on normal, unstimulated T cells. Targeting the short telomeres and telomerase activity in T-PLL seems an attractive strategy for the future treatment of this devastating disease.

Genetic modification of T cells for immunotherapy

Adoptive transfer of antigen-specific T cells is a promising approach for preventing progressive viral infections in immunosuppressed hosts. By contrast, effective T-cell therapy of malignant disease has proven to be much more difficult to achieve. This, in part, reflects the difficulty of isolating high avidity T cells specific for tumor-associated antigens, many of which are self-antigens that have induced some level of tolerance in the host. Even when tumor-reactive T cells can be isolated, the ability of these cells to survive in vivo and traffic to tumor sites is often impaired. Additionally, most tumors employ multiple mechanisms to escape T-cell recognition, including interference in antigen presentation, secretion of inhibitory factors and recruitment of regulatory or immunosuppressive cells. The genetic modification of T cells prior to transfer provides a potential means to overcome many of these obstacles and enhance the efficacy of T-cell therapy. This review article discusses the rationale for genetic modification of T cells, the critical steps involved in gene transfer, and potential advantages and disadvantages of strategies that are now being examined to engineer improved effector T cells for the treatment of human infectious and malignant disease.

Mechanisms of aPL-mediated thrombosis: effects of aPL on endothelium and platelets

Antiphospholipid antibodies (aPL) are associated with thrombosis and pregnancy loss in patients with systemic lupus erythematosus and antiphospholipid syndrome. Strong evidence demonstrates that aPL are pathogenic in vivo from studies that utilized animal models of thrombosis, endothelial cell activation, and pregnancy loss. However, the mechanisms by which aPL mediate disease are only partially understood, and our knowledge is limited by the polyspecificity of the antibodies, the multiple potential end-organ targets, and the variability of the clinical context in which the disease may present. This review discusses and summarizes the most current data available on molecular interactions and pathogenic mechanisms in antiphospholipid syndrome.

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.

Principles of adoptive T cell cancer therapy

The transfusion of T cells, also called adoptive T cell therapy, is an effective treatment for viral infections and has induced regression of cancer in early-stage clinical trials. However, recent advances in cellular immunology and tumor biology are guiding new approaches to adoptive T cell therapy. For example, use of engineered T cells is being tested as a strategy to improve the functions of effector and memory T cells, and manipulation of the host to overcome immunotoxic effects in the tumor microenvironment has led to promising results in early-stage clinical trials. Challenges that face the field and must be addressed before adoptive T cell therapy can be translated into routine clinical practice are discussed.

Cellular lifespan and regenerative medicine

Tissue engineering is a promising approach to aid in the treatment of a wide range of clinical disorders by developing replacement tissues for damaged or diseased organs. Such approaches, however, will require large and functional cell populations in order to produce a tissue that can replicate in vivo function. Most adult cells have limited replicative potential that limits their use in tissue engineering applications. Thus, cell populations with expanded lifespan or increased replicative potential are of interest. Stem cell-derived populations may allow the creation of large cell populations that have increased replicative potential over adult differentiated cells. In addition, ectopic human telomerase reverse transcriptase expression and induced B-cell lymphoma 2 (bcl-2) expression can allow adult cells to proliferate more extensively than unaltered cells. However, concerns for malignant transformation exist with telomerase and bcl-2 approaches. The current states of research in these areas are reviewed as they relate to tissue engineering and the cellular lifespan.

Antiphospholipid antibodies and the antiphospholipid syndrome: an update on treatment and pathogenic mechanisms

PURPOSE OF REVIEW

The antiphospholipid syndrome is a disorder of recurrent thrombosis, pregnancy loss and thrombocytopenia associated with the presence of antiphospholipid antibodies and persistently positive anticardiolipin or lupus anticoagulant positive tests. Since its recognition in the 1980s, growing interest in the field, not only with respect to diagnosis and treatment, but also regarding the pathogenesis of antiphospholipid antibodies, has emerged.

RECENT FINDINGS

First, this review addresses the recently updated classification criteria for diagnosis and treatment of the antiphospholipid syndrome. A discussion on the newly described potential beneficial roles of hydroxychloroquine and the statins for the treatment of antiphospholipid syndrome-associated clinical manifestations is included. Importantly, this article analyzes recent data that examine the molecular and intracellular events that antiphospholipid antibodies trigger in target cells, as well as new findings in the identification of the receptors for these antibodies on the membrane of those cells. A separate section discusses novel pathogenic mechanisms of antiphospholipid antibodies, including the activation of complement and their interaction with homologous catalytic domains of several serine proteases of the coagulation system.

SUMMARY

Understanding the molecular interactions and the intracellular signaling that antiphospholipid antibodies trigger, new therapeutic and targeted strategies to ameliorate clinical manifestations in patients with antiphospholipid syndrome may be established.

Impact of culture conditions on the proliferative lifespan of human T cells in vitro

BACKGROUND

In human T cells, telomerase is transiently expressed upon activation and stimulation and, as shown previously, telomerase levels are able to control the lifespan of T cells. To improve T-cell expansion it is of critical importance to understand the effects of culture parameters on telomerase activity and lifespan.

METHODS

We investigated the influence of culture condition (FCS, human AB serum and autologous serum) and stimulation (PHA/feeder cells, anti-CD3/CD28 beads) on the lifespan, clonogenicity (number of positive wells), cell cycle, telomerase activity and telomere length of T cells in vitro.

RESULTS

The proliferative lifespan of T cells expanded with PHA/feeder cells and autologous serum from different donors was doubled compared with stimulation with PHA/feeder cells and AB serum. No or only a small difference was found for T cells expanded with anti-CD3/CD28 beads and autologous or AB serum. The use of autologous serum also increased the clonogenicity to about three-fold compared with the use of AB serum or FCS, without any signs of differences in the fractions of cycling cells. Interestingly, T cells cultured with autologous serum exhibited a significantly higher telomerase activity at day 6 after stimulation and a reduced decline of telomerase activity compared with cultures with AB serum.

DISCUSSION

The use of autologous serum combined with PHA stimulation and feeder cells remarkably extends the proliferative lifespan and clonogenicity and increases the telomerase activity of human T cells in vitro. This might be useful for applications where large numbers of specific T cells are required.

In situ detection of hTERT variants in anaplastic large cell lymphoma

The expression of hTERT and its isoforms is difficult to assess in lymphoma tissues with the commonly used reverse transcription-polymerase chain reaction (RT-PCR) methods, because non-neoplastic lymphocytes expressing hTERT are always present in the lymphomatous infiltrates. The present study aimed to investigate hTERT mRNA variants in anaplastic large cell lymphoma (ALCL) (n = 38) with in situ hybridization (ISH), along with the immunodetection of hTERT protein. Probes for the identification of mRNAs containing (Bplus) and lacking (Bdel) exons 7 and 8 of the hTERT mRNA were used. Normal lymphocyte populations equally expressed both Bplus and Bdel mRNAs. Although all ALCL examined were found positive for hTERT expression with RT-PCR, hTERT mRNAs were identified in 68% of these tumors with ISH, with a higher incidence in the group bearing ALK translocations (10 out of 11; 90.9%) compared to the ALK negative group (17 out of 27; 59.3%) (PPearson's = 0.002). The same results were obtained with immunohistochemistry for hTERT. In approximately 50% of cases, only Bplus positive cells were identified, again with a higher incidence in the ALK positive compared to the ALK negative group (PPearson's = 0.016). In conclusion, ISH for hTERT mRNAs appears to be a valuable tool for the investigation of hTERT expression in lymphomas. Aberrations in hTERT variant profiles and a decline in the expression of the B deleted isoform may be associated with the pathogenesis of ALCL, especially with respect to ALK positive tumors.

Telomere length of in vivo expanded CD4(+)CD25 (+) regulatory T-cells is preserved in cancer patients

PURPOSE

CD4(+)CD25(+) regulatory T-cells (Treg) are increased in the peripheral blood of cancer patients. It remains unclear whether this is due to redistribution or active proliferation. The latter would require the upregulation of telomerase activity, whose regulation also remains unknown for Treg.

EXPERIMENTAL DESIGN

Treg and CD4(+)CD25(-) T-cells were isolated from peripheral blood of cancer patients (n=23) and healthy age-matched controls (n=17) and analyzed for their content of T-cell receptor excision circles (TREC) and for telomere length using flow-FISH, real-time PCR and Southern blotting. The in vitro regulation of telomerase of Treg was studied using PCR-ELISA in bulk cultures as well as in isolated proliferating and non-proliferating Treg.

RESULTS

Treg isolated from peripheral blood of cancer patients exhibit significantly decreased levels of TREC when compared to Treg from healthy controls. Despite their in vivo proliferation, telomere length is not further shortened in Treg from cancer patients. Accordingly, telomerase activity of Treg was readily inducible in vitro. Notably, sorting of in vitro proliferating Treg revealed a significant telomere shortening in Treg with high-proliferative capacity. The latter are characterized by shortened telomeres despite high telomerase activity.

CONCLUSIONS

Increased frequencies of Treg in peripheral blood of cancer patients are due to active proliferation rather than due to redistribution from other compartments (i.e., secondary lymphoid organs or bone marrow). In vivo expansion does not further shorten telomere length, probably due to induction of telomerase activity. In contrast, under conditions of strong in vitro stimulation telomerase induction seems to be insufficient to avoid progressive telomere shortening.

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.

Improving T cell therapy for cancer

Adoptive transfer of antigen-specific T cells has been most effective in treating cytomegalovirus (CMV) disease and Epstein-Barr virus (EBV)-associated lymphoproliferative disease (LPD). Both of these diseases develop only during periods of acute immune suppression, and both involve highly immunogenic infected cells, and thus respond well to T cell therapies. In contrast, tumours that develop in the presence of a competent immune system evolve complex immune evasion strategies to avoid and subvert T cell-mediated killing. Therefore, even T cells that display potent cytotoxic activity against tumour cells in vitro may not be effective in vivo without altering the tumour:T cell balance in favour of the T cell. This review discusses several new areas of research aimed at improving adoptive T cell therapy for the treatment of cancer, including the genetic modification of antigen-specific T cells to allow them to perform better in vivo, and conditioning the host to improve in vivo expansion and function of transferred cells.

HTLV-1 propels untransformed CD4 lymphocytes into the cell cycle while protecting CD8 cells from death

Human T cell leukemia virus type 1 (HTLV-1) infects both CD4+ and CD8+ lymphocytes, yet it induces adult T cell leukemia/lymphoma (ATLL) that is regularly of the CD4+ phenotype. Here we show that in vivo infected CD4+ and CD8+ T cells displayed similar patterns of clonal expansion in carriers without malignancy. Cloned infected cells from individuals without malignancy had a dramatic increase in spontaneous proliferation, which predominated in CD8+ lymphocytes and depended on the amount of tax mRNA. In fact, the clonal expansion of HTLV-1-positive CD8+ and CD4+ lymphocytes relied on 2 distinct mechanisms--infection prevented cell death in the former while recruiting the latter into the cell cycle. Cell cycling, but not apoptosis, depended on the level of viral-encoded tax expression. Infected tax-expressing CD4+ lymphocytes accumulated cellular defects characteristic of genetic instability. Therefore, HTLV-1 infection establishes a preleukemic phenotype that is restricted to CD4+ infected clones.

Dissociation of telomerase activity and telomere length maintenance in primitive human hematopoietic cells

Primitive human hematopoietic cells have low endogenous telomerase activity, yet telomeres are not maintained. In contrast, ectopic telomerase expression in fibroblasts and other cells leads to telomere length maintenance or elongation. It is unclear whether this disparity can be attributed to telomerase level or stems from fundamentally different telomere biology. Here, we show that telomerase overexpression does not prevent proliferation-associated telomere shortening in human hematopoietic cells, pointing to the existence of cell type-specific differences in telomere dynamics. Furthermore, we observed eventual stabilization of telomere length without detectable changes in telomerase activity during establishment of two leukemic cell lines from normal cord blood cells, indicating that additional cooperating events are required for telomere maintenance in immortalized human hematopoietic cells.

Genomic stability and functional activity may be lost in telomerase-transduced human CD8+ T lymphocytes

To obtain the large amount of T cells required for adoptive immunotherapy in a clinical setting, T-cell lifespan extension by human telomerase reverse transcriptase (hTERT) transduction is of particular interest. However, constitutive expression of hTERT is associated with malignant transformation and thus warrants a detailed evaluation of the safety of hTERT-transduced T cells before clinical application. In view of this, we performed an extensive cytogenetic analysis of hTERT-transduced MART-1 (melanoma antigen recognized by T cell 1)-and human papillomavirus type 16 (HPV16) E7-specific human CD8+ cytotoxic T lymphocytes (CTLs), reactive against melanoma and cervical carcinoma, respectively. Our results, obtained by (spectral) karyotyping and array comparative genomic hybridization, showed the development of minor chromosomal aberrations in an hTERT-transduced MART-1-specific CTL clone, whereas severe clonal aberrations were detected in an hTERT-transduced HPV16 E7-specific CTL clone. Furthermore, hTERT transduction did not protect CTLs from immunosenescence, because the HPV16 E7-specific, hTERT-transduced CTL clone showed a decreased functional activity on prolonged culture. Although the general frequency of major chromosomal aberrations in hTERT-transduced CTLs and the in vivo significance of our observations remain still unclear at this point, the currently available data suggest that clinical application of hTERT-transduced CTLs should proceed with caution.

The impact of telomere erosion on memory CD8+ T cells in patients with X-linked lymphoproliferative syndrome

Patients with X-linked lymphoproliferative syndrome (XLP) experience excessive T cell proliferation after primary Epstein-Barr virus (EBV) infection, due to mutations in the signalling lymphocyte activation molecule (SLAM) associated protein (SAP) molecule. We examined the impact of dysfunctional proliferative control on the extent of CD8+ T cell differentiation in XLP patients who recovered from primary EBV infection. Although these young patients have normal numbers of lytic and latent EBV-epitope-specific CD8+ T cells, they were extremely differentiated as defined by loss of CCR7 and CD27, low telomerase activity and very short telomeres. This was not a direct effect arising from the loss of SAP, but was due to excessive T cell stimulation due to this defect. Thus, transduction of XLP CD8+ T cells with the catalytic component of telomerase (hTERT), but not SAP, prevented telomere loss and considerably extended proliferative lifespan in vitro. These results indicate that excessive proliferation in CD8+ T cells in XLP patients may lead to end-stage differentiation and loss of functional EBV-specific CD8+ T cells through replicative senescence. This may contribute to the defective immunity found in XLP patients who survive acute EBV infection who develop EBV-related B cell lymphomas before the fourth decade of life.