CD7 is a differentiation marker that identifies multiple CD8 T cell effector subsets

The adaptive immune response of human CD8 T cells to invading pathogens involves the differentiation of naive cells into memory and effector cells. However, the lineage relationship between memory and effector cells and the differentiation of CD8 T cells into distinct subsets of effector cell subpopulations are subjects of considerable debate. CD7 identifies three populations of CD8 T cells: CD7 high (CD7(high)), low (CD7(low)), and negative (CD7(neg)) that translate into subsets with distinct functional properties. The CD7(high) subset contains naive and memory cells and the CD7(low) and CD7(neg) subsets contain effector cells. The effector cells can functionally be divided into cytokine-secreting effector CD8 T cells and lytic effector CD8 T cells. These data provide a model of human CD8 T cell differentiation in which specialized distinct subpopulations can be identified by expression of CD7.     

Development and homeostasis of T cell memory in rhesus macaque.

The rhesus macaque (RM) is a critical animal model for studies of viral pathogenesis and immunity, yet fundamental aspects of their cellular immune response remain poorly defined. One such deficiency is the lack of validated phenotypic signatures for their naive and memory T cell subsets, and the resultant unavailability of accurate information on their memory T cell development, homeostasis, and function. In this study, we report a phenotypic paradigm allowing definitive characterization of these subsets and their comprehensive functional analysis. Naive T cells are optimally delineated by their homogeneous CD95(low)CD28(high)beta(7) integrin(int) (CD4+) or CD95(low)CD28(int)CD11a(low) (CD8+) phenotypes. This subset 1) was present in blood and secondary lymph tissues, but not effector sites; 2) vastly predominated in the fetal/neonatal immune system, but rapidly diminished with postnatal age; 3) lacked IFN-gamma production capability, and specific responses to RM CMV; and 4) demonstrated low in vivo proliferative activity. CD4+ and CD8+ memory subsets were CD95(high), but otherwise phenotypically heterogeneous and included all IFN-gamma production, RM CMV-specific responses, effector site T cells, and demonstrated high in vivo proliferative activity ( approximately 10 times the naive subset). These analyses also revealed the RM "effector memory" subset within the overall memory population. This population, best defined by lack of CD28 expression, contained the majority of RM CMV-specific cells, was highly enriched in extralymphoid effector sites, and comprised an increasing proportion of total memory cells with age. The effector memory subset demonstrated similar in vivo proliferative activity and survival as CD28+ "central memory" T cells, consistent with independent homeostatic regulation.     

Differential in vivo persistence of two subsets of memory phenotype CD8 T cells defined by CD44 and CD122 expression levels

The existence of distinct subsets of memory CD8 T cells with different characteristics is now well established. In this work, we describe two subsets of mouse CD8 T cells with memory characteristics that coexist in primed thymectomized TCR-transgenic F5 mice and that share some properties with the human central and effector memory cells. The first subset corresponds to CD8 T cells generated following nucleoprotein 68 peptide priming which are CD44(int)CD122(-)nucleoprotein 68/H-2D(b) tetramer(+) and express high levels of CCR7 mRNA. In contrast, CD8 T cells in the second subset are CD44(high)CD122(+), are heterogeneous in terms of Ag specificity, and express low levels of CCR7 mRNA. We have studied the functional characteristics and the persistence of these two subsets in thymectomized mice. CD44(int) CD8 T cells persist like naive cells; i.e., they are slowly lost with time. However, surviving cells maintain their phenotype and memory characteristics for the entire life span of the animal. In contrast, CD44(high) CD8 T cells are persistent and accumulate in thymectomized but not euthymic mice. This is correlated with an increased in vivo proliferative and survival potential of these cells. These results show that acquisition of enhanced functional characteristics and long-term persistence by memory T cells are independent. This may have important consequences for the design of specific vaccine.     

Immediate early effector functions of virus-specific CD8+CCR7+ memory cells in humans defined by HLA and CC chemokine ligand 19 tetramers

Memory T cells exhibit a high degree of heterogeneity in terms of their phenotype and functional characteristics. It has been proposed that the CCR7 chemokine receptor divides memory T cell populations into central memory T cells and effector memory T cells with distinct functions in secondary immune responses. We were interested whether this hypothesis holds true in experiments performed on Ag-specific CD8(+) T cells. To identify CCR7(+) cells, we engineered a fluorescent ligand for CCR7; results with the new CC chemokine ligand 19 chemotetramer were verified by staining with a CCR7 mAb. Staining with the CC chemokine ligand 19 chemotetramer reveals two subsets within CCR7(+) cells: a CCR7(int) population containing memory cells and a CCR7(high) population containing naive T cells. Phenotypic analysis of MHC class I/peptide tetramer-positive cells revealed that HLA-A2-restricted CMV-specific CD8 T cells exhibit the lowest percentage of CCR7(+) cells (0.5-5%), while HLA-A2-restricted flu- and HLA-B8-restricted EBV-specific CD8 T cells showed the highest (45-70%). Intracellular staining of unstimulated cells revealed that both CCR7(int)- and CCR7(-)-specific CD8 T cells exhibit a detectable level of perforin. Both CCR7(int) and CCR7(-) Ag-specific CD8(+) T cells produced IFN-gamma and TNF-alpha following short-term peptide stimulation. Therefore, our finding that CCR7(+)CD8(+) T cells are able to exert immediate effector functions requires a substantial revision to the central and effector memory hypothesis.     

Functional properties and lineage relationship of CD8+ T cell subsets identified by expression of IL-7 receptor alpha and CD62L

Three major subsets of Ag-experienced CD8+ T cells have been identified according to their expression of CD62L and CD127. These markers are associated with central memory T cells (CD62L+ CD127+), effector memory T cells (CD162L- CD127+), and effector T cells (CD62L- CD127-). In this study we characterized the development of these three populations during acute and chronic viral infections and after immunization with virus-like particles and determined their lineage relation and functional and protective properties. We found that the balance between the three subsets was critically regulated by the availability of Ag and time. After initial down-regulation of CD127, the responding CD8+ T cell population down-regulated CD62L and re-expressed CD127. Dependent on Ag availability, the cells then further differentiated into CD62L- CD127- effector cells or, in the absence of Ag, re-expressed CD62L to become central memory T cells. Although all three populations efficiently produced effector cytokines such as IFN-gamma, CD62L- CD127- effector cells exhibited the highest ex vivo lytic potential. In contrast, CD62L+ CD127+ central memory T cells most efficiently produced IL-2 and proliferated extensively in vitro and in vivo upon antigenic restimulation. Strikingly, only effector and effector memory, but not central memory, T cells were able to protect against peripheral infection with vaccinia virus, whereas central memory T cells were most potent at protecting against systemic infection with lymphocytic choriomeningitis virus, indicating that the antiviral protective capacities of specific CD8+ T cell subsets are closely related to the nature of the challenging pathogen.     

Human autoreactive CD4+ T cells from naive CD45RA+ and memory CD45RO+ subsets differ with respect to epitope specificity and functional antigen avidity

T cells with specificity for self-Ags are normally present in the peripheral blood, and, upon activation, may target tissue Ags and become involved in the pathogenesis of autoimmune processes. In multiple sclerosis, a demyelinating disease of the CNS, it is postulated that inflammatory damage is initiated by CD4+ T cells reactive to myelin Ags. To investigate the potential naive vs memory origin of circulating myelin-reactive cells, we have generated myelin basic protein (MBP)- and tetanus toxoid-specific T cell clones from CD45RA+/RO- and CD45RO+/RA- CD4+ T cell subsets from the peripheral blood of multiple sclerosis patients and controls. Our results show that 1) the response to MBP, different from that to TT, predominantly emerges from the CD45RA+ subset; 2) the reactivity to immunodominant MBP epitopes mostly resides in the CD45RA+ subset; 3) in each individual, the recognition of single MBP epitopes is skewed to either subset, with no overlap in the Ag fine specificity; and 4) in spite of a lower expression of costimulatory and adhesion molecules, CD45RA+ subset-derived clones recognize epitopes with higher functional Ag avidity. These findings point to a central role of the naive CD45RA+ T cell subset as the source for immunodominant, potentially pathogenic effector CD4+ T cell responses in humans.     

Enhanced levels of costimulation lead to reduced effector/memory CD8+ T cell functionality

The role of different levels of costimulation in conjunction with signal 1 in the activation of memory CD8+ T cells remains elusive. In this study, we demonstrate, in a mouse model with the influenza nucleoprotein epitope NP68, that mouse early memory (effector/memory) CD8+ T cells that were generated with high levels of costimulation have reduced CTL functionality compared with those that were generated with low levels of costimulation. This reduction is associated with increased phosphorylation of the negative regulatory site 292 on Zap70 and a decrease in granzyme B levels. Furthermore, we show that enhanced costimulation reduces proliferation and cytokine production of effector/memory CD8+ T cells in response to intermediate and weak TCR stimulation, in contrast to previously described positive effects of costimulation on naive CD8+ T cells. This effect is associated with the expression of ICAM-1 on APCs. Together, our results indicate that enhanced costimulation can lead to reduced functionality in effector/memory CD8+ T cells. This compromised effector function of effector/memory CD8+ T cells in response to high levels of costimulation can have important implications for designing immunotherapeutic strategies to enhance immune responses.     

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.     

Human effector CD8+ T lymphocytes express TLR3 as a functional coreceptor

TLR are evolutionarily conserved molecules that play a key role in the initiation of innate antimicrobial immune responses. Through their influence on dendritic cell maturation, these receptors are also thought to indirectly shape the adaptive immune response. However, no data are currently available regarding both TLR expression and function in human CD8+ T cell subsets. We report that a subpopulation of CD8+ T cells, i.e., effector, but neither naive nor central memory cells, constitutively expresses TLR3. Moreover, the ligation of the receptor by a specific agonist in TLR3-expressing CD8+ T cells increased IFN-gamma secretion induced by TCR-dependent and -independent stimulation, without affecting proliferation or specific cytolytic activity. These results thereby suggest that TLR3 ligands can not only indirectly influence the adaptive immune response through modulation of dendritic cell activation, but also directly increase IFN-gamma production by Ag-specific CD8+ T cells. Altogether, the present work might open new perspectives for the use of TLR ligands as adjuvants for immunotherapy.     

Origin and fate of lymphocytic choriomeningitis virus-specific CD8+ T cells coexpressing the inhibitory NK cell receptor Ly49G2

CD8+ T cells that coexpress the inhibitory NK cell receptor, Ly49G2 (G2), are present in immunologically naive C57BL/6 mice but display Ags found on memory T cells. To assess how G2+CD8+ cells relate to bona fide memory cells, we examined the origin and fate of lymphocytic choriomeningitis virus (LCMV)-induced G2+CD8+ cells. During early (day 4) acute LCMV infection, both G2+ and G2-CD8+ T cell subsets underwent an attrition in number and displayed an activation (CD69(high)1B11(high)CD62L(low)) phenotype. By day 8, both subsets synthesized IFN-gamma in response to immunodominant LCMV peptides, though the expansion of G2+ cells was less than that of G2- cells. Adoptive transfer experiments with purified G2- or G2+CD8+ cells from naive mice indicated that the LCMV-specific G2+ subset was derived from a pre-existing G2+ population and not generated from G2- cells responding to LCMV infection. Their participation in the LCMV-specific T cell response increased with age, reflecting an increase in the size of the pre-existing G2+ pool. Following establishment of stable LCMV memory, the proportion of CD8+ cells coexpressing G2 was reduced in comparison to naive controls, presumably due to displacement by G2- LCMV-specific memory cells. LCMV-specific G2+ cells were present in the memory pool, but at low frequencies, and they did not exhibit the typical phenotypic changes of reactivation during secondary challenge. We suggest that G2+CD8+ cells represent a cell lineage distinct from bona fide memory T cells, but that they can participate in an acute virus-specific T cell response.     

EBV-specific CD8+ T cell memory: relationships between epitope specificity, cell phenotype, and immediate effector function

EBV infection in humans induces CD8+ T cell memory to viral epitopes derived from both lytic and latent cycle Ags. We have analyzed the relationship between the phenotype and function of the memory pool of T cells specific for these Ags. Lytic epitope-specific populations were heterogeneous in terms of CD45RO/RA and CD28 expression, whereas latent epitope-specific populations were uniformly CD45RO+ and CD28+, consistent with the higher antigenic challenge from lytic epitopes driving some memory cells toward a CD45RA+, CD28- phenotype. However, both types of memory population showed immediate epitope-specific cytotoxicity and type 1 cytokine production in ex vivo assays. Cytotoxic function was not associated with preactivated T cells, as EBV-specific populations were negative for activation markers such as CD69 or CD38, nor could cytotoxic function be ascribed to CD27- or CD56+ subsets, as such cells were not detected in EBV-specific memory. Furthermore, cytotoxicity was not limited to CD45RA+ and/or CD28- fractions, but also was observed in CD45RO+, CD28+ populations in lytic and latent epitope-specific memory. Cytokine (IFN-gamma, TNF-alpha) responses, measured by intracytoplasmic staining after peptide stimulation, also were detectable in CD45RO+ and RA+ subsets as well as CD28+ and CD28- subsets. Of other markers that were heterogeneous in both lytic and latent epitope populations, CCR7 gave the best discrimination of functionality; thus, CCR7+ cells consistently failed to give an IFN-gamma or TNF-alpha response, whereas many CCR7- cells were responsive. Our data are consistent with effector functions having a broad distribution among phenotypically distinct subsets of "effector memory" cells that have lost the CCR7 marker.     

CD8+ T Lymphocyte Expansion,Proliferation and Activation in Dengue Fever

Dengue fever induces a robust immune response, including massive T cell activation. The level of T cell activation may, however, be associated with more severe disease. In this study, we explored the level of CD8+ T lymphocyte activation in the first six days after onset of symptoms during a DENV2 outbreak in early 2010 on the coast of São Paulo State, Brazil. Using flow cytometry we detected a progressive increase in the percentage of CD8+ T cells in 74 dengue fever cases. Peripheral blood mononuclear cells from 30 cases were thawed and evaluated using expanded phenotyping. The expansion of the CD8+ T cells was coupled with increased Ki67 expression. Cell activation was observed later in the course of disease, as determined by the expression of the activation markers CD38 and HLA-DR. This increased CD8+ T lymphocyte activation was observed in all memory subsets, but was more pronounced in the effector memory subset, as defined by higher CD38 expression. Our results show that most CD8+ T cell subsets are expanded during DENV2 infection and that the effector memory subset is the predominantly affected sub population.     

Kinetics of in vivo proliferation and death of memory and naive CD8 T cells: parameter estimation based on 5-bromo-2'-deoxyuridine incorporation in spleen, lymph nodes, and bone marrow

To study naive and memory CD8 T cell turnover, we performed BrdU incorporation experiments in adult thymectomized C57BL/6 mice and analyzed data in a mathematical framework. The following aspects were novel: 1) we examined the bone marrow, in addition to spleen and lymph nodes, and took into account the sum of cells contained in the three organs; 2) to describe both BrdU-labeling and -delabeling phase, we designed a general mathematical model, in which cell populations were distinguished based on the number of divisions; 3) to find parameters, we used the experimentally determined numbers of total and BrdU(+) cells and the BrdU-labeling coefficient. We treated mice with BrdU continuously via drinking water for up to 42 days, measured by flow cytometry BrdU incorporation at different times, and calculated the numbers of BrdU(+) naive (CD44(int/low)) and memory (CD44(high)) CD8 T cells. By fitting the model to data, we determined proliferation and death rates of both subsets. Rates were confirmed using independent sets of data, including the numbers of BrdU(+) cells at different times after BrdU withdrawal. We found that both doubling time and half-life of the memory population were approximately 9 wk, whereas for the naive subset the doubling time was almost 1 year and the half-life was roughly 7 wk. Our findings suggest that the higher turnover of memory CD8 T cells as compared with naive CD8 T cells is mostly attributable to a higher proliferation rate. Our results have implications for interpreting physiological and abnormal T cell kinetics in humans.     

Effector CD4 T cells are biochemically distinct from the memory subset: evidence for long-term persistence of effectors in vivo.

Memory T cell responses are believed to be mediated by long-lived memory T cells that arise directly from a subset of short-lived, activated effector T cells that have reverted to the resting state. Although widely accepted, definitive proof that memory T cells arise from effectors is lacking because of the inability to reliably distinguish these subsets based on known phenotypic or functional parameters. We have used a biochemical approach to distinguish effector and memory CD4 T cell subsets and follow the differentiative fate of effector cells in vivo. When examined biochemically, effector and memory CD4 T cells are strikingly distinct and exhibit qualitative and quantitative differences in tyrosine phosphorylation. These effector-specific patterns were identical in effectors derived either from naive CD4 T cells (primary effectors) or memory CD4 T cells (memory effectors). To monitor the fate of effector cells in vivo, Ag-activated CD4+ TCR-transgenic T cells were transferred into irradiated BALB/c mice. These TCR-transgenic CD4 T cells persisted in adoptive hosts for several months, gave a recall response to Ag, yet exhibited effector-specific biochemical profiles. These results suggest that a subset of effector CD4 T cells can persist in vivo and contribute to long-term immunity by mediating secondary immune responses.     

Profound enhancement of the IL-12/IL-18 pathway of IFN-gamma secretion in human CD8+ memory T cell subsets via IL-15

Human memory CD8(+) T cell subsets, termed central memory and effector memory T cells, can be identified by expression of CD45RA, CD62 ligand (CD62L), and CCR7. Accordingly, functional differences have been described for each subset, reflecting unique roles in immunological memory. The common gamma-chain cytokines IL-15 and IL-7 have been shown to induce proliferation and differentiation of human CD8(+) T cell subsets, as well as increased effector functions (i.e., cytokines, cytotoxicity). In this study, we observed that addition of IL-15 or IL-7 to cultures of human CD8(+) T cells profoundly enhanced the IL-12-IL-18 pathway of IFN-gamma production. Importantly, IL-15 and IL-7 lowered the threshold concentrations of IL-12 and IL-18 required for induction of IFN-gamma by 100-fold. Comparison of IL-15 and IL-7 demonstrated that IL-15 was superior in its ability to enhance IL-12-IL-18-induced IFN-gamma, without evidence of a synergistic effect between IL-15 and IL-7. We also observed that IL-15- and IL-7-mediated enhancement of IL-12-IL-18-induced IFN-gamma production was a functional property of effector memory CD8(+) T cells. Despite a lack of association between cell division and acquisition of IL-12-IL-18-induced IFN-gamma, down-regulation of CD62L expression correlated well with increased IL-12-IL-18-induced IFN-gamma. Purified central memory T cells stimulated with IL-15 and IL-7 down-regulated CD62L and acquired potent IL-12-IL-18-induced IFN-gamma similar to effector memory T cells. Thus, in addition to its known role in development of T cell memory, IL-15 may amplify memory CD8(+) T cell effector functions by increasing sensitivity to proinflammatory cytokine stimulation.     

Recombinant interleukin-7 induces proliferation of naive macaque CD4+ and CD8+ T cells in vivo

Interleukin-7 (IL-7) regulates T-cell homeostasis, and its availability is augmented in lymphopenic hosts. Naive CD8+ T cells transferred to lymphopenic mice acquire a memory-like phenotype, raising the possibility that IL-7 is the biological mediator of this effect. Here, we provide direct evidence that IL-7 induces the acquisition of memory-cell markers not only in CD8+ T cells but also in CD4+ T-cell subsets in immune-competent Indian rhesus macaques. The increase of these memory-like populations was dependent on the dose of the cytokine, and these cells were found in the blood as well as secondary lymphoid organs. Memory-like CD4+ and CD8+ T cells acquired the ability to secrete tumor necrosis factor alpha and, to a lesser extent, gamma interferon following stimulation with a cognate antigen. The phenotypic change observed in naive T cells was promptly reversed after discontinuation of IL-7. Importantly, IL-7 induced cycling of both CD4+ and CD8+ central memory and effector memory T cells, demonstrating its contribution to the maintenance of the entire T-cell pool. Thus, IL-7 may be of benefit in the treatment of iatrogenic or virus-induced T-cell depletion.     

Three memory subsets of human CD8+ T cells differently expressing three cytolytic effector molecules

Multicolor flow cytometric analysis for the expression of three effector molecules, i.e., perforin (Per), granzyme A (GraA), and granzyme B (GraB), in human CD8(+) T cells demonstrated that they included five subpopulations, implying the following pathway for the differentiation of CD8(+) T cells: Per(-)GraA(-)GraB(-)-->Per(-)GraA(+)GraB(-)-->Per(low)GraA(+)GraB(-)--> Per(low)GraA(+)GraB(+)-->Per(high)GraA(+)GraB(+). The analysis of the expression of these molecules in the subsets classified by the combination of the expression of CCR7 and CD45RA or by that of CD27, CD28, and CD45RA showed that functional CD8(+) T cell subsets could be partially identified by these phenotypic classifications. However, the functional subsets could be precisely identified by the classification using five cell surface markers or three cell surface markers and three cytolytic molecules. Per(-)GraA(-)GraB(-) and Per(-/low)GraA(+)GraB(-) cells were predominantly found in CCR5(-)CCR7(+) and CCR5(high/low)CCR7(-) subsets, respectively, of CD8(+) T cells expressing the CD27(+)CD28(+)CD45RA(-) phenotype, whereas Per(low)GraA(+)GraB(+) cells were found in the CCR5(low)CCR7(-) subset of those expressing this phenotype and in a part of the CCR5(-/low)CCR7(-) subset of those expressing the CD27(-/low)CD28(-)CD45RA(-/+) phenotype. Ex vivo EBV-specific CD8(+) T cells, which were Per(low/-)GraA(+)GraB(-/+) cells, hardly or very weakly killed the target cells, indicating that these were not effector T cells. These findings suggest that the Per(-)GraA(-)GraB(-), Per(-/low)GraA(+)GraB(-), and Per(low)GraA(+)GraB(+) cells were central memory, early effector memory, and late effector memory T cells, respectively. Per(-/low)GraA(+)GraB(-) cells gained GraB expression after TCR stimulation, indicating that early effector memory T cells could differentiate into late effector and effector T cells. The present study showed the existence of three memory subsets and the pathway for their differentiation.     

A CD8+/CD103high T cell subset regulates TNF-mediated chronic murine ileitis

Recruitment of lymphocytes to sites of inflammation requires the sequential engagement of adhesion molecules and chemokine receptors. Of these, the lectin-like molecule CD44 has been particularly implicated in inflammatory trafficking. Using a TNF-driven model of chronic ileitis (i.e., B6.129P-Tnf(Delta)(ARE) mice) that recapitulates many features of Crohn's disease, we demonstrate dynamic changes in the expression and functional state of CD44 on CD8+ T cells. These cells coexpress CD44 and L-selectin, giving them a surface phenotype similar to that of central memory T cells. Yet functionally they exhibit the phenotype of effector T cells, because they produce IFN-gamma. Unexpectedly, depletion of the CD8+ population had no effect on the severity of ileitis. Further analyses showed a second CD8+ population that lacked CD44, but expressed CD103, produced TGF-beta, inhibited the proliferation of CD4+ in vitro, and attenuated adoptively transferred ileitis in vivo, most likely counteracting the proinflammatory role of the CD44(high) subset. Collectively, these data suggest that the presence or absence of CD44 and CD103 on the CD8+ lymphocyte surface defines functionally distinct subsets of CD8+ T cells in vivo. These inflammation-driven populations exert distinct roles during the development of chronic ileitis, and influence the balance of effector and regulatory functions in the chronically inflamed small intestine.     

Suppression of memory CD8 T cell generation and function by tryptophan catabolism

Dendritic cell-derived indoleamine 2,3-dioxygenase (IDO) suppresses naive T cell proliferation and induces their apoptosis by catalyzing tryptophan, and hence is essential for the maintenance of peripheral tolerance. However, it is not known whether memory T cells are subject to the regulation by IDO-mediated tryptophan catabolism, as memory T cells respond more rapidly and vigorously than their naive counterparts and are resistant to conventional costimulatory blockade. In this study, we present the evidence that memory CD8+ T cells are susceptible to tryptophan catabolism mediated by IDO. We found that overexpression of IDO in vivo attenuated the generation of both central memory CD8+ T cells (T(CM)) and effector memory CD8+ T cells (T(EM)) while suppressing IDO activity promoted their generation. Moreover, IDO overexpression suppressed the effector function of T(CM) cells or T(CM) cell-mediated allograft rejection as well as their proliferation in vivo. Interestingly, T(CM) cells were resistant to apoptosis induced by tryptophan catabolism. However, IDO overexpression did not suppress the effector function of T(EM) cells or T(EM) cell-mediated allograft rejection, suggesting that T(EM) cells, unlike T(CM) cells, do not require tryptophan for their effector function once they are generated. This study provides insight into the mechanisms underlying the differential regulation of memory T cell responsiveness and has clinical implications for vaccination or tolerance induction.     

Molecular signatures distinguish human central memory from effector memory CD8 T cell subsets

Memory T cells are heterogeneous in terms of their phenotype and functional properties. We investigated the molecular profiles of human CD8 naive central memory (T(CM)), effector memory (T(EM)), and effector memory RA (T(EMRA)) T cells using gene expression microarrays and phospho-protein-specific intracellular flow cytometry. We demonstrate that T(CM) have a gene expression and cytokine signaling signature that lies between that of naive and T(EM) or T(EMRA) cells, whereas T(EM) and T(EMRA) are closely related. Our data define the molecular basis for the different functional properties of central and effector memory subsets. We show that T(EM) and T(EMRA) cells strongly express genes with known importance in CD8 T cell effector function. In contrast, T(CM) are characterized by high basal and cytokine-induced STAT5 phosphorylation, reflecting their capacity for self-renewal. Altogether, our results distinguish T(CM) and T(EM)/T(EMRA) at the molecular level and are consistent with the concept that T(CM) represent memory stem cells.