The MS risk allele of CD40 is associated with reduced cell-membrane bound expression in antigen presenting cells: Implications for gene function

Human genetic and animal studies have implicated the costimulatory molecule CD40 in the development of multiple sclerosis (MS). We investigated the cell specific gene and protein expression variation controlled by the CD40 genetic variant(s) associated with MS, i.e. the T-allele at rs1883832. Previously we had shown that the risk allele is expressed at a lower level in whole blood, especially in people with MS. Here, we have defined the immune cell subsets responsible for genotype and disease effects on CD40 expression at the mRNA and protein level. In cell subsets in which CD40 is most highly expressed, B lymphocytes and dendritic cells, the MS-associated risk variant is associated with reduced CD40 cell-surface protein expression. In monocytes and dendritic cells, the risk allele additionally reduces the ratio of expression of full-length versus truncated CD40 mRNA, the latter encoding secreted CD40. We additionally show that MS patients, regardless of genotype, express significantly lower levels of CD40 cell-surface protein compared to unaffected controls in B lymphocytes. Thus, both genotype-dependent and independent down-regulation of cell-surface CD40 is a feature of MS. Lower expression of a co-stimulator of T cell activation, CD40, is therefore associated with increased MS risk despite the same CD40 variant being associated with reduced risk of other inflammatory autoimmune diseases. Our results highlight the complexity and likely individuality of autoimmune pathogenesis, and could be consistent with antiviral and/or immunoregulatory functions of CD40 playing an important role in protection from MS. © 2015 Field et al.

Single-cell-precision microplasma-induced cancer cell apoptosis

The issue of single-cell control has recently attracted enormous interest. However, in spite of the presently achievable intracellular-level physiological probing through bio-photonics, nano-probe-based, and some other techniques, the issue of inducing selective, single-cell-precision apoptosis, without affecting neighbouring cells remains essentially open. Here we resolve this issue and report on the effective single-cell-precision cancer cell treatment using the reactive chemistry of the localized corona-type plasma discharge around a needle-like electrode with the spot size ∼1 µm. When the electrode is positioned with the micrometer precision against a selected cell, a focused and highly-localized micro-plasma discharge induces apoptosis in the selected individual HepG2 and HeLa cancer cells only, without affecting any surrounding cells, even in small cell clusters. This is confirmed by the real-time monitoring of the morphological and structural changes at the cellular and cell nucleus levels after the plasma exposure.

Stochastic dynamics of interacting haematopoietic stem cell niche lineages

Since we still know very little about stem cells in their natural environment, it is useful to explore their dynamics through modelling and simulation, as well as experimentally. Most models of stem cell systems are based on deterministic differential equations that ignore the natural heterogeneity of stem cell populations. This is not appropriate at the level of individual cells and niches, when randomness is more likely to affect dynamics. In this paper, we introduce a fast stochastic method for simulating a metapopulation of stem cell niche lineages, that is, many sub-populations that together form a heterogeneous metapopulation, over time. By selecting the common limiting timestep, our method ensures that the entire metapopulation is simulated synchronously. This is important, as it allows us to introduce interactions between separate niche lineages, which would otherwise be impossible. We expand our method to enable the coupling of many lineages into niche groups, where differentiated cells are pooled within each niche group. Using this method, we explore the dynamics of the haematopoietic system from a demand control system perspective. We find that coupling together niche lineages allows the organism to regulate blood cell numbers as closely as possible to the homeostatic optimum. Furthermore, coupled lineages respond better than uncoupled ones to random perturbations, here the loss of some myeloid cells. This could imply that it is advantageous for an organism to connect together its niche lineages into groups. Our results suggest that a potential fruitful empirical direction will be to understand how stem cell descendants communicate with the niche and how cancer may arise as a result of a failure of such communication.

Atmospheric pressure gas plasma-induced colorectal cancer cell death is mediated by Nox2-ASK1 apoptosis pathways and oxidative stress is mitigated by Srx-Nrf2 anti-oxidant system

Atmospheric pressure gas plasma (AGP) generates reactive oxygen species (ROS) that induce apoptosis in cultured cancer cells. The majority of cancer cells develop a ROS-scavenging anti-oxidant system regulated by Nrf2, which confers resistance to ROS-mediated cancer cell death. Generation of ROS is involved in the AGP-induced cancer cell death of several colorectal cancer cells (Caco2, HCT116 and SW480) by activation of ASK1-mediated apoptosis signaling pathway without affecting control cells (human colonic sub-epithelial myofibroblasts; CO18, human fetal lung fibroblast; MRC5 and fetal human colon; FHC). However, the identity of an oxidase participating in AGP-induced cancer cell death is unknown. Here, we report that AGP up-regulates the expression of Nox2 (NADPH oxidase) to produce ROS. RNA interference designed to target Nox2 effectively inhibits the AGP-induced ROS production and cancer cell death. In some cases both colorectal cancer HT29 and control cells showed resistance to AGP treatment. Compared to AGP-sensitive Caco2 cells, HT29 cells show a higher basal level of the anti-oxidant system transcriptional regulator Nrf2 and its target protein sulfiredoxin (Srx) which are involved in cellular redox homeostasis. Silencing of both Nrf2 and Srx sensitized HT29 cells, leads to ROS overproduction and decreased cell viability. This indicates that in HT29 cells, Nrf2/Srx axis is a protective factor against AGP-induced oxidative stress. The inhibition of Nrf2/Srx signaling should be considered as a central target in drug-resistant colorectal cancer treatments.

Nutritional status of children with cystic fibrosis measured by total body potassium as a marker of body cell mass: Lack of sensitivity of anthropometric measures

Objective: To investigate measures aimed at defining the nutritional status of cystic fibrosis (CF) populations, this study compared standard anthropometric measurements and total body potassium (TBK) as indicators of malnutrition. Methods: Height, weight, and TBK measurements of 226 children with CF from Royal Children's Hospital, Brisbane, Australia, were analyzed. Z scores for height for age, weight for age, and weight for height were analyzed by means of the National Centre for Health Statistics reference. TBK was measured by means of whole body counting and compared with predicted TBK for age. Two criteria were evaluated with respect to malnutrition: (1) a z score < -2.0 and (2) a TBK for age <80% of predicted. Results: Males and females with CF had lower mean height-for-age and weight-for-age z scores than the National Centre for Health Statistics reference (P < .01), but mean weight-for-height z score was not significantly different. There were no significant gender differences. According to anthropometry, only 7.5% of this population were underweight and 7.6% were stunted. However, with TBK as an indicator of nutritional status, 29.9% of males and 22.0% of females were malnourished. Conclusion: There are large differences in the percentage of patients with CF identified as malnourished depending on whether anthropometry or body composition data are used as the nutritional indicator. At an individual level, weight-based indicators are not sensitive indicators of suboptimal nutritional status in CF, significantly underestimating the extent of malnutrition. Current recommendations in which anthropometry is used as the indicator of malnutrition in CF should be revised.

Estimating cell diffusivity and cell proliferation rate by interpreting IncuCyte ZOOM™ assay data using the Fisher-Kolmogorov model

Background: Standard methods for quantifying IncuCyte ZOOM™ assays involve measurements that quantify how rapidly the initially-vacant area becomes re-colonised with cells as a function of time. Unfortunately, these measurements give no insight into the details of the cellular-level mechanisms acting to close the initially-vacant area. We provide an alternative method enabling us to quantify the role of cell motility and cell proliferation separately. To achieve this we calibrate standard data available from IncuCyte ZOOM™ images to the solution of the Fisher-Kolmogorov model. Results: The Fisher-Kolmogorov model is a reaction-diffusion equation that has been used to describe collective cell spreading driven by cell migration, characterised by a cell diffusivity, D, and carrying capacity limited proliferation with proliferation rate, λ, and carrying capacity density, K. By analysing temporal changes in cell density in several subregions located well-behind the initial position of the leading edge we estimate λ and K. Given these estimates, we then apply automatic leading edge detection algorithms to the images produced by the IncuCyte ZOOM™ assay and match this data with a numerical solution of the Fisher-Kolmogorov equation to provide an estimate of D. We demonstrate this method by applying it to interpret a suite of IncuCyte ZOOM™ assays using PC-3 prostate cancer cells and obtain estimates of D, λ and K. Comparing estimates of D, λ and K for a control assay with estimates of D, λ and K for assays where epidermal growth factor (EGF) is applied in varying concentrations confirms that EGF enhances the rate of scratch closure and that this stimulation is driven by an increase in D and λ, whereas K is relatively unaffected by EGF. Conclusions: Our approach for estimating D, λ and K from an IncuCyte ZOOM™ assay provides more detail about cellular-level behaviour than standard methods for analysing these assays. In particular, our approach can be used to quantify the balance of cell migration and cell proliferation and, as we demonstrate, allow us to quantify how the addition of growth factors affects these processes individually.

Molecular Genetic Background of Tumours in Hereditary Leiomyomatosis and Renal Cell Cancer Syndrome

Hereditary Leiomyomatosis and Renal Cell Cancer (HLRCC) is a hereditary tumour predisposition syndrome. Its phenotype includes benign cutaneous and uterine leiomyomas (CLM, ULM) with high penetrance and rarer renal cell cancer (RCC), most commonly of papillary type 2 subtype. Over 130 HLRCC families have been identified world-wide but the RCC phenotype seems to concentrate in families from Finland and North America for unknown reasons. HLRCC is caused by heterozygous germline mutations in the fumarate hydratase (FH) gene. FH encodes the enzyme fumarase from mitochondrial citric acid cycle. Fumarase enzyme activity or type or site of the FH mutation are unassociated with disease phenotype. The strongest evidence for tumourigenesis mechanism in HLRCC supports a hypoxia inducible factor driven process called pseudohypoxia resulting from accumulation of the fumarase substrate fumarate. In this study, to assess the importance of gene- or exon-level deletions or amplifications of FH in patients with HLRCC-associated phenotypes, multiplex ligation-dependent probe amplification (MLPA) method was used. One novel FH mutation, deletion of exon 1, was found in a Swedish male patient with an evident HLRCC phenotype with CLM, RCC, and a family history of ULM and RCC. Six other patients with CLM and 12 patients with only RCC or uterine leiomyosarcoma (ULMS) remained FH mutation-negative. These results suggest that copy number aberrations of FH or its exons are an infrequent cause of HLRCC and that only co-occurrence of benign tumour types justifies FH-mutation screening in RCC or ULMS patients. Determination of the genomic profile of 11 HLRCC-associated RCCs from Finnish patients was performed by array comparative genomic hybridization. The most common copy number aberrations were gains of 2, 7, and 17 and losses of 13q12.3-q21.1, 14, 18, and X. When compared to aberrations of sporadic papillary RCCs, HLRCC-associated RCCs harboured a distinct DNA copy number profile and lacked many of the changes characterizing the sporadic RCCs. The findings suggest a divergent molecular pathway for tumourigenesis of papillary RCCs in HLRCC. In order to find a genetic modifier of RCC risk in HLRCC, genome-wide linkage and identical by descent (IBD) analysis studies were performed in Finnish HLRCC families with microsatellite marker mapping and SNP-array platforms. The linkage analysis identified only one locus of interest, the FH gene locus in 1q43, but no mutations were found in the genes of the region. IBD analysis yielded no convincing haplotypes shared by RCC patients. Although these results do not exclude the existence of a genetic modifier for RCC risk in HLRCC, they emphasize the role of FH mutations in the malignant tumourigenesis of HLRCC. To study the benign tumours in HLRCC, genome-wide DNA copy number and gene expression profiles of sporadic and HLRCC ULMs were defined with modern SNP- and gene-expression array platforms. The gene expression array suggests novel genes involved in FH-deficient ULM tumourigenesis and novel genes with putative roles in propagation of sporadic ULM. Both the gene expression and copy number profiles of HLRCC ULMs differed from those of sporadic ULMs indicating distinct molecular basis of the FH-deficient HLRCC tumours.

Target Cell Topography and Cytoskeletal Reorganization in Natural Killer Cell Activity

The immune system has to recognize and destroy abnormal or infected cells to maintain homeostasis. Natural killer (NK) cells directly recognize and kill transformed or virus-infected cells without prior sensitization. We have studied both virus-infected and tumor cells in order to identify the target structures involved in triggering NK activity. Mouse/human cell hybrids containing various human chromosomes were used as targets. The human chromosome responsible for activating NK cell killing was identified to chromosome number 6. The results suggest that activated NK cells recognize ligands that are encoded on human chromosome 6. We showed that the ligand on the target cell side was intercellular adhesion molecule 2 (ICAM-2). There was no difference in the level of expression of ICAM-2, however, but a drastic difference was seen in the distribution of the molecule: ICAM-2 was evenly distributed on the surface of the NK-resistant cells, but almost totally redistributed to the tip of uropods, bud-like extensions, which were absent from the parental cells. Interestingly, the gene coding for cytoskeletal linker protein ezrin has been localized to human chromosome 6, and there was a colocalization of ezrin and ICAM-2 in the uropods. Furthermore, the transfected human ezrin into NK cell-resistant cells induced uropod formation, ICAM-2 and ezrin redistribution to newly formed uropods, and sensitized target cells to NK cell killing. These data reveal a novel form of NK cell recognition: target structures are already present on normal cells; they become detectable only after abnormal redistribution into hot spots on the target cell membrane. NK cells are central players in the defence against virus infections. They inhibit the spread of infection, allowing time for specific immune responses to develop. The virus-proteins that directly activate human NK cell killing are largely unknown. We studied the sensitivity of virus-specific early proteins of Semliki Forest virus (SFV) to NK killing. The viral non-structural proteins (nsP1-4) translated early in the virus cycle were transfected in NK-resistant cells. Viral early gene nsP1 alone efficiently sensitized target cells to NK activity, and the tight membrane association of nsP1 seems to be critical in the triggering of NK killing. NsP1 protein colocalized with (redistributed) ezrin in filopodia-like structures to which the NK cells were bound. The results suggest that also in viral infections NK cells react to rapid changes in membrane topography. Based on the results of this thesis, a new model of target cell recognition of NK cells can be suggested: reorganization of the cytoskeleton induces alterations in cell surface topography, and this new pattern of surface molecules is recognized as "altered-self".

Studies on the mechanism of follitropin action at the cellular level

With a view toward understanding better the mechanism of action of follitropin, an attempt was made using granulosa cells obtained from intact immature estrogenized rats to study in short-term incubations the effect of highly purified ovine follitropin on the binding of the hormone to the cells and the associated aromatase response. A modified radioimmunoassay procedure has been used to monitor unlabeled physiologically fully active follitropin bound to the cell. A linear relationship between the actual amount of hormone bound to the cells and the estradiol produced in vitro has been established. The amount of ovine follitropin bound that can elicit a half-maximal response in estrogen production was calculated to be 400 pg. The number of follitropin binding sites per cell was 375 and the Kd of binding was 3.03 × 10−10 Image . By the addition of ovine follitropin antiserum at different time points of a 4-h incubation period, a continual need for follitropin support for estradiol production has been established.

Implications of high level pseudogene transcription in Mycobacterium leprae

Background: The Mycobacterium leprae genome has less than 50% coding capacity and 1,133 pseudogenes. Preliminary evidence suggests that some pseudogenes are expressed. Therefore, defining pseudogene transcriptional and translational potentials of this genome should increase our understanding of their impact on M. leprae physiology. Results: Gene expression analysis identified transcripts from 49% of all M. leprae genes including 57% of all ORFs and 43% of all pseudogenes in the genome. Transcribed pseudogenes were randomly distributed throughout the chromosome. Factors resulting in pseudogene transcription included: 1) co-orientation of transcribed pseudogenes with transcribed ORFs within or exclusive of operon-like structures; 2) the paucity of intrinsic stem-loop transcriptional terminators between transcribed ORFs and downstream pseudogenes; and 3) predicted pseudogene promoters. Mechanisms for translational ``silencing'' of pseudogene transcripts included the lack of both translational start codons and strong Shine-Dalgarno (SD) sequences. Transcribed pseudogenes also contained multiple ``in-frame'' stop codons and high Ka/Ks ratios, compared to that of homologs in M. tuberculosis and ORFs in M. leprae. A pseudogene transcript containing an active promoter, strong SD site, a start codon, but containing two in frame stop codons yielded a protein product when expressed in E. coli. Conclusion: Approximately half of M. leprae's transcriptome consists of inactive gene products consuming energy and resources without potential benefit to M. leprae. Presently it is unclear what additional detrimental affect(s) this large number of inactive mRNAs has on the functional capability of this organism. Translation of these pseudogenes may play an important role in overall energy consumption and resultant pathophysiological characteristics of M. leprae. However, this study also demonstrated that multiple translational ``silencing'' mechanisms are present, reducing additional energy and resource expenditure required for protein production from the vast majority of these transcripts.

Role of cell cycle regulators in development of the inner ear

The inner ear originates from an ectodermal thickening called the otic placode. The otic placode invaginates and closes to an otic vesicle, the otocyst. The otocyst epithelium undergoes morphogenetic changes and cell differentiation, leading to the formation of the labyrinth-like mature inner ear. Epithelial-mesenchymal interactions control inner ear morphogenesis, but the modes and molecules are largely unresolved. The expressions of negative cell cycle regulators in the epithelium of the early-developing inner ear have also not been elucidated. The mature inner ear comprises the hearing (cochlea) and balance (vestibular) organs that contain the nonsensory and sensory cells. In mammals, the inner ear sensory cells, called hair cells, exit the cell cycle during embryogenesis and are mitotically quiescent during late-embryonic differentiation stages and postnatally. The mechanisms that maintain this hair cell quiescense are largely unresolved. In this work I examined 1) the epithelial-mesenchymal interactions involved in inner ear morphogenesis, 2) expression of negative cell cycle regulators in the epithelium of the early developing inner ear and 3) the molecular mechanisms that maintain the postmitotic state of inner ear sensory cells. We observed that during otocyst stages, epithelial fibroblast growth factor 9 (Fgf9) communicates with the surrounding mesenchyme, where its receptors are expressed. Fgf9 inactivation leads to reduced proliferation of the surrounding vestibular mesenchyme and to the absence of semicircular canals. Semicircular canal development is blocked, since fusion plates do not form. These results show that the mesenchyme directs fusion plate formation and give direct evidence for the existence of reciprocal epithelial-mesenchymal interactions in the developing inner ear. Cyclin-dependent kinase inhibitors (CKIs) are negative regulators of proliferation. We show that the members of the Cip/Kip family of CKIs (p21Cip1, p27Kip1 and p57Kip2) are expressed in the early-developing inner ear. Our expression data suggest that CKIs divide the otic epithelium into proliferative and nonproliferative compartments that may underlie shaping of the otocyst. At later stages, CKIs regulate proliferation of the vestibular appendages, and this may regulate their continual growth. In addition to restricting proliferation, CKIs may play a role in regional differentiation of various epithelial cells. Differentiating and adult inner ear hair cells are postmitotic and do not proliferate in response to serum or mitogenic growth factors. In our study, we show that this is the result of the activity of negative cell cycle regulators. Based on expression profiles, we first focused on the retinoblastoma (Rb) gene, which functions downstream of the CKIs. Analysis of the inner ear phenotype of Rb mutant mice show, that the retinoblastoma protein regulates the postmitotic state of hair cells. Rb inactivation leads to hyperplasia of vestibular and cochlear sensory epithelia that is a result of abnormal cell cycle entry of differentiated hair cells and of delayed cell cycle exit of the hair cell precursor cells. In addition, we show that p21Cip1 and p19Ink4d cooperate in maintaining the postmitotic state of postnatal auditory hair cells. Whereas inactivation of p19Ink4d alone leads to low-level S-phase entry (Chen et al., 2003) and p21Cip1 null mutant mice have a normal inner ear phenotype, codeletion of p19Ink4d and p21Cip1 triggers high-level S-phase entry of auditory hair cells during early postnatal life, which leads to supernumerary hair cells. The ectopic hair cells undergo apoptosis in all of the mutant mice studied, DNA damage being the immediate cause of this death. These findings demonstrate that the maintenance of the postmitotic state of hair cells is regulated by Rb and several CKIs, and that these cell cycle regulators are critical for the lifelong survival of hair cells. These data have implications for the future design of therapies to induce hair cell regrowth.

Development of salinity tolerance in rice by constitutive-overexpression of genes involved in the regulation of programmed cell death

Environmental factors contribute to over 70% of crop yield losses worldwide. Of these drought and salinity are the most significant causes of crop yield reduction. Rice is an important staple crop that feeds more than half of the world’s population. However among the agronomically important cereals rice is the most sensitive to salinity. In the present study we show that exogenous expression of anti-apoptotic genes from diverse origins, AtBAG4 (Arabidopsis), Hsp70 (Citrus tristeza virus) and p35 (Baculovirus), significantly improves salinity tolerance in rice at the whole plant level. Physiological, biochemical and agronomical analyses of transgenic rice expressing each of the anti-apoptotic genes subjected to salinity treatment demonstrated traits associated with tolerant varieties including, improved photosynthesis, membrane integrity, ion and ROS maintenance systems, growth rate, and yield components. Moreover, FTIR analysis showed that the chemical composition of salinity-treated transgenic plants is reminiscent of non-treated, unstressed controls. In contrast, wild type and vector control plants displayed hallmark features of stress, including pectin degradation upon subjection to salinity treatment. Interestingly, despite their diverse origins, transgenic plants expressing the anti-apoptotic genes assessed in this study displayed similar physiological and biochemical characteristics during salinity treatment thus providing further evidence that cell death pathways are conserved across broad evolutionary kingdoms. Our results reveal that anti-apoptotic genes facilitate maintenance of metabolic activity at the whole plant level to create favorable conditions for cellular survival. It is these conditions that are crucial and conducive to the plants ability to tolerate/adapt to extreme environments.

A promoter-level mammalian expression atlas

Regulated transcription controls the diversity, developmental pathways and spatial organization of the hundreds of cell types that make up a mammal. Using single-molecule cDNA sequencing, we mapped transcription start sites (TSSs) and their usage in human and mouse primary cells, cell lines and tissues to produce a comprehensive overview of mammalian gene expression across the human body. We find that few genes are truly 'housekeeping', whereas many mammalian promoters are composite entities composed of several closely separated TSSs, with independent cell-type-specific expression profiles. TSSs specific to different cell types evolve at different rates, whereas promoters of broadly expressed genes are the most conserved. Promoter-based expression analysis reveals key transcription factors defining cell states and links them to binding-site motifs. The functions of identified novel transcripts can be predicted by coexpression and sample ontology enrichment analyses. The functional annotation of the mammalian genome 5 (FANTOM5) project provides comprehensive expression profiles and functional annotation of mammalian cell-type-specific transcriptomes with wide applications in biomedical research.

CD1 expression and CD1-restricted T cell activity in normal and tumour-bearing human liver

CD1d-restricted natural killer T (NKT) cells expressing invariant Valpha14Jalpha18 T cell receptor alpha-chains are abundant in murine liver and are implicated in the control of malignancy, infection and autoimmunity. Invariant NKT cells have potent anti-metastatic effects in mice and phase I clinical trials involving their homologues in humans are ongoing. However, invariant NKT cells are less abundant in human liver ( approximately 0.5% of hepatic T cells) than in murine liver (up to 50%) and it is not known if other hepatic T cells are CD1-restricted. We have examined expression of CD1a, CD1b, CD1c and CD1d mRNA and protein in human liver and evaluated the reactivity of mononuclear cells (MNC) from histologically normal and tumour-bearing human liver specimens against these CD1 isoforms. Messenger RNA for all CD1 isotypes was detectable in all liver samples. CD1c and CD1d were expressed at the protein level by hepatic MNC. CD1d, only, was detectable at the cell surface, but CD1c and CD1d were found at an intracellular location in significant numbers of liver MNC. CD1b was not expressed by MNC from healthy livers but was detectable within MNC in all tumour samples tested. Hepatic T cells exhibited reactivity against C1R cells expressing transfected CD1c and CD1d, but neither CD1a nor CD1b. These cells secreted interferon-gamma (IFN-gamma) but not interleukin-4 (IL-4) upon stimulation. In contrast, similar numbers of peripheral T cells released 13- and 16-fold less IFN-gamma in response to CD1c and CD1d, respectively. CD1c and CD1d expression and T cell reactivity were not altered in tumour-bearing liver specimens compared to histologically normal livers. These data suggest that, in addition to invariant CD1d-restricted NKT cells, autoreactive T cells that recognise CD1c and CD1d and release inflammatory cytokines are abundant in human liver.

Steady-state dendritic cells expressing cognate antigen terminate memory CD8+ T-cell responses

Antigen stimulation of naive T cells in conjunction with strong costimulatory signals elicits the generation of effector and memory populations. Such terminal differentiation transforms naive T cells capable of differentiating along several terminal pathways in response to pertinent environmental cues into cells that have lost developmental plasticity and exhibit heightened responsiveness. Because these cells exhibit little or no need for the strong costimulatory signals required for full activation of naive T cells, it is generally considered memory and effector T cells are released from the capacity to be inactivated. Here, we show that steadystate dendritic cells constitutively presenting an endogenously expressed antigen inactivate fully differentiated memory and effector CD8+ T cells in vivo through deletion and inactivation. These findings indicate that fully differentiated effector and memory T cells exhibit a previously unappreciated level of plasticity and provide insight into how memory and effector T-cell populations may be regulated. © 2008 by The American Society of Hematology.