The flow cytometric analysis of telomere length in antigen-specific CD8+ T cells during acute Epstein-Barr virus infection

Mantoux Test as a model for a secondary immune response in humans

The Mantoux Test (MT) is a classical delayed-type hypersensitivity (DTH) response to the intradermal injection of tuberculin purified protein derivative (PPD). It represents a cutaneous T cell mediated memory recall immune response. The test is typically used to determine immunity to tuberculosis in humans and positive reactions develop in individuals previously exposed to Mycobacterium tuberculosis, and those immunised with the Bacillus of Calmette and Guérin (BCG) vaccine. In view of its relative accessibility human skin represents a convenient tissue for the investigation of human immune responses. Using the MT, we have been able to determine that significant cellular proliferation and clonal expansion occur at the site of antigen deposition in the skin. Furthermore, cells undergoing proliferation in the skin also undergo accelerated differentiation. Taken together with other studies, in humans and in mice, these observations shed new light on the importance of the microenvironment at the site of the immune response for the proliferation and differentiation of memory T cells.

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.

A robust method for production of MHC tetramers with small molecule fluorophores

Tetramers of major histocompatibility complex molecules (MHC) are now well-established reagents for the detection of antigen-specific T cells by flow cytometry. MHC tetramers are prepared by mixing enzymatically biotinylated MHC molecules with commercial preparations of streptavidin, usually conjugated to a fluorescent phycobiliprotein such as phycoerythrin (PE) or allophycocyanin (APC). While data obtained with MHC tetramers prepared with small molecule fluorophores has been reported, considerable lot-to-lot variation among conventional streptavidin conjugates to small molecules prevents routine preparation of such reagents. We now report robust preparation of MHC tetramers with small molecule fluorophores, using a recombinant mutant of streptavidin incorporating a carboxy-terminal cysteine in each of the four identical subunits that is conjugated to maleimide derivatives of any of several small molecule fluorophores. These reagents significantly expand the versatility of the MHC tetramer methodology.

Significance of senescence for virus-specific memory T cell responses: rapid ageing during chronic stimulation of the immune system

There is a generalized age-related decline in immune responses which leads to increased susceptibility of elderly to infection and, possibly, to autoimmune disease and cancer. This is associated with phenotypic changes of CD8+ T lymphocytes that include the loss of costimulatory molecules CD28 and CD27, which are important for proliferation and cell survival of CD8+ T cells. Loss of these molecules is associated with less ability to respond to recurrent infection. Functional changes within T cells during ageing include a reduction in the number of naive T cells and a progressively limited T cell repertoire. Furthermore, persistent life-long antigenic stress upon the memory pool leads to telomere erosion and concomittant loss of proliferative capacity, a phenomenon known as replicative senesence. In this review, we discuss that replicative senescence, or clonal exhaustion, may also occur in relatively young individuals, as evidenced from HIV-infected individuals and healthy Ethiopians. We discuss data suggesting that T cell defects may arise in individuals because of chronic antigen activation leading to rapid ageing of the memory CD8+ T cell pool.

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.

Virus Specific T-Cell Responses

CD8+ and CD4+ T-cells play a key role in the maintenance of our immunity against viruses. Recent technological developments, such as the use of MHC-peptide tetrameric complexes, have permitted significant improvements in the study of these cells. It is now possible to assess precisely frequencies as well as phenotypic and functional features of virus specific T-cells from the onset of many viral infections onwards. Different virus specific T-cell populations exhibit distinct functional characteristics and can be positioned at different stages of a process of post-thymic development, which we are drawing near to understanding the significance. Still, further work is needed before consensus is reached as regards what defines and how to induce the optimal virus specific T-cell response which will confer long lasting immunological protection in humans.

Lymphocyte subpopulations from patients with primary antibody deficiency do not show increased telomere erosion

Telomere erosion and residual replicative capacity can be used as markers of the replicative history of somatic cells. We have investigated telomere length, in vitro replicative capacity and rate of telomere erosion in T and B lymphocyte populations from patients with primary antibody deficiency requiring immunoglobulin replacement therapy. We found no significant differences in telomere lengths of B cells, or of CD4+, CD8+, CD45RA+ (naive) and CD45RO+ (memory) T cell populations between patients and age matched controls. Overall, telomere length correlated inversely with age, and was reduced in memory (CD45RO+) as compared with naive (CD45RA+) T cells. In vitro long-term (6 months) cell cultures showed no differences between patients and controls in the mitogen-stimulated replicative potential of T cell subpopulations (CD4+, CD8+, CD45RA+, CD45RO+), or in the rates of telomere erosion with cellular replication in these cell populations. The rate of telomere erosion per population doubling in CD45RA+ cells, however, was greater than in CD45RO+ cells in both patients and controls. These data suggest that premature immune exhaustion is unlikely to represent a long-term complication of primary antibody deficiency.

T-cell senescence: a culprit of immune abnormalities in chronic inflammation and persistent infection

Long-lived clonal T cells deficient in CD28 expression are commonly found in patients with inflammatory syndromes and persistent infections. Considering that CD28 loss is the most consistent immunological marker of aging, we propose that, in pathological states, CD28(null) T cells represent prematurely senescent cells resulting from persistent immune activation. These unusual lymphocytes have aberrant functions that contribute to disease-related immune abnormalities, and the degree of accumulation of CD28(null) T cells predicts the severity of clinical manifestations. We suggest that understanding of the biological properties of T cells that have reached replicative senescence will influence the future management of certain diseases. Indeed, studies on the molecular basis for the loss of CD28 are already providing information on methods to functionally rescue senescent T cells.

Telomere erosion in memory T cells induced by telomerase inhibition at the site of antigenic challenge in vivo

The extent of human memory T cell proliferation, differentiation, and telomere erosion that occurs after a single episode of immune challenge in vivo is unclear. To investigate this, we injected tuberculin purified protein derivative (PPD) into the skin of immune individuals and isolated responsive T cells from the site of antigenic challenge at different times. PPD-specific CD4⁺ T cells proliferated and differentiated extensively in the skin during this secondary response. Furthermore, significant telomere erosion occurred in specific T cells that respond in the skin, but not in those that are found in the blood from the same individuals. Tissue fluid obtained from the site of PPD challenge in the skin inhibited the induction of the enzyme telomerase in T cells in vitro. Antibody inhibition studies indicated that type I interferon (IFN), which was identified at high levels in the tissue fluid and by immunohistology, was responsible in part for the telomerase inhibition. Furthermore, the addition of IFN-α to PPD-stimulated CD4⁺ T cells directly inhibited telomerase activity in vitro. Therefore, these results suggest that the rate of telomere erosion in proliferating, antigen-specific CD4⁺ T cells may be accelerated by type I IFN during a secondary response in vivo.

Will telomere erosion lead to a loss of T-cell memory?

Evidence is accumulating that elderly individuals are more susceptible to infection with organisms to which they were previously immune. This indicates that there might be a limit to the persistence of immune memory. This fact is particularly disturbing because the average life expectancy of humans has almost doubled in the past 200 years and is still increasing. We discuss mechanisms that might constrain the persistence of memory T cells and consider whether humans will suffer from memory T-cell exhaustion as life expectancy increases.

Replicative senescence of CD8 T cells: effect on human ageing

Elderly persons have been exposed to a myriad of pathogens over their lifespan. This life-long immunological history leads, in some cases, to the generation of expanded populations of memory CD8 T cells that have reached the end stage of replicative senescence. In cell culture, CD8 T cells that are subjected to repeated rounds of antigen-driven proliferation eventually show irreversible cell cycle arrest, permanent and complete loss of CD28 gene expression, apoptosis resistance, reduced gene transcription of the major stress protein in response to heat shock, and shortened telomeres compared to their CD28-expressing progenitors. Clinical studies have documented that high proportions of CD8 T cells that lack CD28 are correlated with reduced antibody response to influenza vaccination and are also an immune marker of increased risk of mortality in persons greater than 80 years of age. In addition, CD8 T cells lacking CD28 expression have been documented to have suppressive influences on immune function. Thus, senescent CD8 T cells may affect immune function both directly and indirectly by modulating other immune cell types. The potential role of senescent T cells in bone homeostasis is suggested as a potentially fruitful area for future investigation. The patterns of cytokine changes observed during the progression to senescence in cell culture are consistent with this possibility, and T cells producing these same cytokines have, in fact, been identified within the bone marrow in murine models of osteoporosis. Interestingly, CD8 T cells with markers of replicative senescence are correlated with increased osteoporotic fractures in the elderly. Thus, senescent CD8 T cells are associated with a variety of deleterious health-related outcomes, suggesting that these cells may exert pleiotropic negative effects on both immune and non-immune organ systems during ageing.

Telomere length measurements in leukocyte subsets by automated multicolor flow-FISH

BACKGROUND

Telomeres are essential protein-DNA structures at the end of chromosomes which are implicated in genome stability and cell replication. The average length of telomere repeats can be measured by in situ hybridization and flow cytometry [flow-FISH]. Such telomere length values reflect telomere shortening (resulting from cell divisions, oxidative damage and other causes) and telomere elongation (mainly resulting from telomerase activity) of the chromosome-specific telomere length inherited in the gametes. Here we report improvements in flow-FISH methodology that enable measurements of telomere length in subsets of human nucleated blood cells.

METHODS AND RESULTS

In order to measure the telomere length in granulocytes, naive T cells, memory T cells, B cells and natural killer (NK)/NKT cells within a blood sample, we combined flow-FISH with antibody-staining (Multicolor flow-FISH). Most steps in the staining protocol were automated using a 96-well microdispenser device. The minimum detectable difference in telomere length and the reproducibility of the method are in the range of 0.2-0.5 kb and measurements can be made with as few as a thousand cells.

CONCLUSIONS

Automated multicolor flow-FISH will greatly facilitate studies of telomere length regulation in subsets of nucleated blood cells, especially when only few cells are available and when differences in telomere length are small.

Ex vivo characterization of human CD8+ T subsets with distinct replicative history and partial effector functions

After antigenic challenge, naive T lymphocytes enter a program of proliferation and differentiation during the course of which they acquire effector functions and may ultimately become memory cells. In humans, the pathways of effector and memory T-cell differentiation remain poorly defined. Here we describe the properties of 2 CD8+ T-lymphocyte subsets, RA+CCR7-27+28+ and RA+CCR7-27+28-, in human peripheral blood. These cells display phenotypic and functional features that are intermediate between naive and effector T cells. Like naive T lymphocytes, both subsets show relatively long telomeres. However, unlike the naive population, these T cells exhibit reduced levels of T-cell receptor excision circles (TRECs), indicating they have undergone additional rounds of in vivo cell division. Furthermore, we show that they also share effector-type properties. At equivalent in vivo replicative history, the 2 subsets express high levels of Fas/CD95 and CD11a, as well as increasing levels of effector mediators such as granzyme B, perforin, interferon gamma, and tumor necrosis factor alpha. Both display partial ex vivo cytolytic activity and can be found among cytomegalovirus-specific cytolytic T cells. Taken together, our data point to the presence of T cells with intermediate effector-like functions and suggest that these subsets consist of T lymphocytes that are evolving toward a more differentiated effector or effector-memory stage.

The dynamics of T-cell fratricide: application of a robust approach to mathematical modelling in immunology

Fratricide between CD8(+) T lymphocytes is known to occur in HTLV-I and possibly HSV-1 and HIV-1 infection. However it is not known what effect, if any, T-cell fratricide has on the course of infection. Here we present simple mathematical techniques to investigate T-cell fratricide with particular reference to HTLV-I infection. Using a general model we predict the qualitative and quantitative effect of fratricide on HTLV-I equilibrium proviral load. We also investigate the effect of fratricide on the probability of viral clearance. We show that, surprisingly, fratricide can lead either to an increase or a decrease in equilibrium proviral load. We derive the conditions necessary for fratricide to cause a decrease in load and deduce that, for the five HTLV-I-positive patients considered here, fratricide has probably caused an increase in equilibrium load. We also estimate the percentage increase in load that is attributable to fratricide and determine the parameters that should be measured in order to improve this estimate. Finally, we show that fratricide reduces the probability of viral clearance. Mathematical modelling of HTLV-I infection, as is often the case in biology, is severely hampered by a lack of experimental data. Consequently it is difficult to know what functional form a model should take. The behaviour of complex nonlinear systems is highly model-dependent. Predictions based on theoretical models are therefore sensitive to the choice of model; this is a very severe problem that undermines and limits the success of the application of mathematics to immunology. In this paper we reduce the model dependency of the results in two ways-by considering (analytically) a general model with a minimal number of assumptions and, where this is not possible, by checking (numerically) that a wide range of models yield the same results. We therefore begin to develop two practical methods for dealing with the problem of robustness in mathematical models of the immune system.

Genetic alterations in the ageing immune system: impact on infection and cancer

The immune system, which is able to distinguish between self and non-self, is programmed to protect the organism from a huge spectrum of potential foreign invaders. Each T and B lymphocyte bears an antigen receptor of a single specificity, which is determined during development by a unique genetic mechanism that generates millions of different variants of the genes encoding the receptor molecules. When a particular antigen, such as a virus, is encountered, only those lymphocytes bearing the relevant receptors become activated and undergo massive clonal expansion. The expanded antigen-specific B cells produce antibodies, which neutralize free virus in the bloodstream, whereas the T cells, particularly the so-called CD8 T cells, actually kill cells that are infected with the virus. Once the infection is cleared, most of the expanded T cells undergo apoptosis, leaving a small number of memory cells to await future possible encounters with the same virus. During ageing, both latent and acute viral infections lead to increased morbidity and mortality, based, in large part, on the diminished ability of T cells to control the infection. To investigate the underlying mechanism of the T cell defects, we have analyzed the process of replicative senescence in human T cells. Our research has shown that following repeated stimulation with antigen in cell culture, the responding T cells eventually reach an irreversible state of cell cycle arrest, at which time they show loss of gene expression of a key T cell-specific signaling molecule required for proliferation, as well as reduced stress protein production, apoptosis resistance, shortened telomeres and inability to upregulate telomerase. Increased proportions of T cells with identical phenotypes are present in elderly individuals, suggesting that chronic/repeated stimulation of some T cells may lead to replicative senescence in vivo. Genetic modulation of this process may yield novel strategies to augment immune function in the elderly.

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.

Telomere maintenance in human B lymphocytes

Telomere shortening has been causally linked to replicative senescence in human cells. To characterize telomere-length heterogeneity in peripheral blood cells of normal individuals, we analysed the mean length of telomeric repeat sequences in subpopulations of peripheral blood leucocytes, using fluorescence in situ hybridization and flow cytometry (flow-FISH). Although the telomere length of most haematopoietic subsets was within the same range, the mean telomere length was found to be 15% higher in B compared with T lymphocytes in adult peripheral blood. Whereas telomere loss with ageing corresponded to 33 base pairs (bp) per year in T cells, telomere shortening was slower in B cells, corresponding to 15 bp per year. Separation of adult B-lymphocyte subpopulations based on CD27 expression revealed that telomere length was almost 2 kb longer in CD19+CD27+ (memory) compared with CD19+CD27- (naive) cells. Furthermore, peripheral blood B cells were activated in vitro. Whereas B-cell activation with Staphylococcus aureus Cowan strain (SAC) did not increase telomere length, a striking telomere elongation was observed when cells were stimulated with SAC and interleukin 2 to induce plasma cell differentiation. Our observations support the concept that telomere dynamics in B cells are distinct from other haematopoietic cell lineages and that telomere elongation may play an essential role in the generation of long-term B memory cells.

Simultaneous flow cytometric analysis of two cell surface markers, telomere length, and DNA content

BACKGROUND

Various protocols for estimation of telomere length in individual cells by flow cytometry using fluorescence in situ hybridization of fluorescently labeled peptide nucleic acid (PNA) probes (Flow-FISH) have been described. Combined analysis of telomere length and cell phenotype, however, remains difficult because few fluorochromes with suitable emission spectra tolerate the harsh conditions needed for DNA denaturation during hybridization of the telomere-specific PNA probe. We overcame these problems and developed a method for measuring telomere length in cell subsets characterized by the expression of two surface antigens.

METHODS

Alexa Fluor 488 and Alexa Fluor 546 were used for cell surface staining. Antigen-antibody complexes were covalently cross-linked onto the cell membrane before Flow-FISH. Cells were hybridized with a PNA probe conjugated to cyanine 5 (Cy5). Hoechst 33342 (HO342) was added for determination of cellular DNA content. For assay standardization, we added an aliquot of a single batch of 1,301 cells to each sample as an internal control before hybridization with the PNA probe. Samples were prepared in duplicate and analyzed on a standard three-laser BD LSR flow cytometer. For assay validation, the same samples were analyzed in parallel to correlate the percentage of telomere length of the sample versus 1,301 control cells to the mean size of terminal restriction fragments (TRFs) of DNA as determined by Southern gel analysis.

RESULTS

The method permitted clear identification of lymphocyte subsets in samples hybridized for Flow-FISH, with subset frequencies comparable to those of untreated samples. At a concentration of 10 nM, the Cy5-labeled telomere-specific PNA probe produced a bright fluorescence signal well separated from background. Addition of HO342 in low concentration did not interfere with Cy5 telomere fluorescence, produced adequate DNA histograms, and permitted clear identification of cell phenotype. The probe concentration of 10 nM also proved optimal for inclusion of 1,301 control cells for assay standardization. Telomere length estimations by the current method correlated highly with TRF calculations by Southern gel hybridization (r(2)= 0.9, P = 0.0003). Application of our protocol to the analysis of human CD8CD28 lymphocyte subsets showed that CD8(+bright)CD28(-) lymphocytes generally exhibit shorter telomeres than CD8(+bright)CD28(+) cells. These data concurred with previous results of telomere shortening in CD8(+)CD28(-) T cells that were obtained by using different techniques.

CONCLUSIONS

The multiparameter Flow-FISH protocol permitted rapid determination of differences in telomere length in subpopulations characterized by two surface markers without prior cell separation.

The assessment of antigen-specific CD8+ T cells through the combination of MHC class I tetramer and intracellular staining

Peptide-bound histocompatibility leukocyte antigen (HLA) class I tetramers enable a precise identification of antigen-specific CD8+ T cells using flow cytometry. The combination of this technology with intracellular staining techniques opens up significantly better ways of studying these cells than previously possible, allowing immunologists to look at their life cycle (activation and proliferation), manner of death (aging and apoptosis) and effector function (cytotoxic potential and cytokine production). In this review, we hope to provide an overview of these possibilities, as well as making specific suggestions about the use of intracellular staining techniques in the study of antigen-specific T cells. Understanding how antigen-specific cells develop and function in different circumstances and pathologies will be the key to unravelling the secrets of our cellular immune system.

Telomeres in T and B cells

Telomeres are the structures at the ends of linear chromosomes. In mammalian cells, they consist of hexanucleotide (TTAGGG) repeats, together with many associated proteins. In the absence of a compensatory mechanism, dividing cells undergo gradual telomere erosion until a critical degree of shortening results in chromosomal abnormalities and cell death or senescence. For T and B cells, the ability to undergo extensive cell division and clonal expansion is crucial for effective immune function. This article describes our current understanding of telomere-length regulation in lymphocytes and its implications for immune function.

Epstein-Barr virus-specific CD8(+) T cells that re-express CD45RA are apoptosis-resistant memory cells that retain replicative potential

During acute infection, latent and lytic Epstein-Barr virus (EBV) epitope-specific CD8(+) T cells have a CD45RO(+) CD45RA(-) phenotype. However, after resolution of the infection, a large proportion of these cells, particularly those specific for lytic viral epitopes, re-express the CD45RA molecule. The role of CD8(+) CD45RA(+) T cells in ongoing immunity to EBV and other viruses is unknown. We now demonstrate that, relative to their CD45RO(+) counterparts, the EBV-specific CD8(+) T cells that revert to CD45RA expression after acute infectious mononucleosis are not in cell cycle, have longer telomeres, and are more resistant to apoptosis partly because of increased Bcl-2 expression. However, the EBV-specific CD8(+) CD45RA(+) T cells have shorter telomeres than the total CD8(+) CD45RA(+) T-cell pool and predominantly express low levels of the CCR7 chemokine receptor, indicating that they are not naive cells. In addition, EBV-specific CD8(+) CD45RA(+) T cells can be induced to proliferate and exhibit potent cytotoxic activity against target cells loaded with specific peptide. Our results strongly suggest, therefore, that EBV-specific CD8(+) CD45RA(+) T cells represent a stabilized virus-specific memory pool and not terminally differentiated effector cells. The identification of mechanisms that enable stable virus-specific CD8(+) T cells to persist after acute infection may lead to the enhancement of antiviral immunity in immunocompromised and elderly persons.

Effector and memory T-cell differentiation: implications for vaccine development

Recent work shows that after stimulation with antigen, CD4+ and CD8+ T cells embark on a programme of proliferation that is closely linked with the acquisition of effector functions and leads ultimately to memory-cell formation. Here, we discuss the signals required for commitment to this programme of development and the factors that might influence its progression. Models of the pathways of effector and memory T-cell differentiation are discussed, and we highlight the implications of this new understanding for the optimization of vaccine strategies.

CD8+CD28- T cells: certainties and uncertainties of a prevalent human T-cell subset

Human peripheral blood CD8+ T cells comprise cells that are in different states of differentiation and under the control of complex homeostatic processes. In a number of situations ranging from chronic inflammatory conditions and infectious diseases to ageing, immunodeficiency, iron overload and heavy alcohol intake, major phenotypic changes, usually associated with an increase in CD8+ T cells lacking CD28 expression, take place. CD8+CD28- T cells are characterized by a low proliferative capacity to conventional stimulation in vitro and by morphological and functional features of activated/memory T cells. Although the nature of the signals that give origin to this T-cell subset is uncertain, growing evidence argues for the existence of an interplay between epithelial cells, molecules with the MHC-class I fold and CD8+ T cells. The possibility that the generation of CD8+CD28- T cells is the combination of TCR/CD3zeta- and regulatory factor-mediated signals as a result of the sensing of modifications of the internal environment is discussed.