Cutting edge: memory CD8 T cell maturation occurs independently of CD8alphaalpha

Differentiation of Memory CD8 T Cells Unravel Gene Expression Pattern Common to Effector and Memory Precursors

Long-term immunological protection relies on the differentiation and maintenance of memory lymphocytes. Since the knowledge of memory generation has been centered on in vivo models of infection, there are obstacles to deep molecular analysis of differentiating subsets. Here we defined a novel in vitro CD8 T cell activation and culture regimen using low TCR engagement and cytokines to generate differentiated cells consistent with central memory-like cells, as shown by surface phenotype, gene expression profile and lack of cytotoxic function after challenge. Our results showed an effector signature expressed by in vitro memory precursors and their plasticity under specific conditions. Moreover, memory CD8 T cells conferred long-term protection against bacterial infection and slowed in vivo tumor growth more efficiently than effector cells. This model may allow further understanding of CD8 T cell memory molecular differentiation subsets and be suited for generating cells to be used for immunotherapy.

Diverse developmental pathways of intestinal intraepithelial lymphocytes

The intestinal epithelial barrier is patrolled by resident intraepithelial lymphocytes (IELs) that are involved in host defence against pathogens, wound repair and homeostatic interactions with the epithelium, microbiota and nutrients. Intestinal IELs are one of the largest populations of lymphocytes in the body and comprise several distinct subsets, the identity and lineage relationships of which have long remained elusive. Here, we review advances in unravelling the complexity of intestinal IEL populations, which comprise conventional αβ T cell receptor (TCRαβ)⁺ subsets, unconventional TCRαβ⁺ and TCRγδ⁺ subsets, group 1 innate lymphoid cells (ILC1s) and ILC1-like cells. Although these intestinal IEL lineages have partially overlapping effector programmes and recognition properties, they have strikingly different developmental pathways. We suggest that evolutionary pressure has driven the recurrent generation of cytolytic effector lymphocytes to protect the intestinal epithelial layer, but they may also precipitate intestinal inflammatory disorders, such as coeliac disease.

Differential Requirement of Cd8 Enhancers E8I and E8VI in Cytotoxic Lineage T Cells and in Intestinal Intraepithelial Lymphocytes

CD8 expression in T lymphocytes is tightly regulated by the activity of at least six Cd8 enhancers (E8_I-E8_VI), however their complex developmental stage-, subset-, and lineage-specific interplays are incompletely understood. Here we analyzed ATAC-seq data on the Immunological Genome Project database and identified a similar developmental regulation of chromatin accessibility of a subregion of E8_I, designated E8_I-core, and of E8_VI. Loss of E8_I-core led to a similar reduction in CD8 expression in naïve CD8⁺ T cells and in IELs as observed in E8 _I ^-/- mice, demonstrating that we identified the core enhancer region of E8_I. While E8 _VI ^-/- mice displayed a mild reduction in CD8 expression levels on CD8SP thymocytes and peripheral CD8⁺ T cells, CD8 levels were further reduced upon combined deletion of E8_I-core and E8_VI. Moreover, activated E8 _I -core^-/- E8 _VI ^-/- CD8⁺ T cells lost CD8 expression to a greater degree than E8 _I -core^-/- and E8 _VI ^-/- CD8⁺ T cells, suggesting that the combined activity of both enhancers is required for establishment and maintenance of CD8 expression before and after TCR activation. Finally, we observed a severe reduction of CD4 CTLs among the TCRβ⁺CD4⁺ IEL population in E8 _I -core^-/- but not E8 _VI ^-/- mice. Such a reduction was not observed in Cd8a ^-/- mice, indicating that E8_I-core controls the generation of CD4 CTLs independently of its role in Cd8a gene regulation. Further, the combined deletion of E8_I-core and E8_VI restored CD4 CTL subsets, suggesting an antagonistic function of E8_VI in the generation of CD4 CTLs. Together, our study demonstrates a complex utilization and interplay of E8_I-core and E8_VI in regulating CD8 expression in cytotoxic lineage T cells and in IELs. Moreover, we revealed a novel E8_I-mediated regulatory mechanism controlling the generation of intestinal CD4 CTLs.

Acetylation of the Cd8 Locus by KAT6A Determines Memory T Cell Diversity

How functionally diverse populations of pathogen-specific killer T cells are generated during an immune response remains unclear. Here, we propose that fine-tuning of CD8αβ co-receptor levels via histone acetylation plays a role in lineage fate. We show that lysine acetyltransferase 6A (KAT6A) is responsible for maintaining permissive Cd8 gene transcription and enabling robust effector responses during infection. KAT6A-deficient CD8(+) T cells downregulated surface CD8 co-receptor expression during clonal expansion, a finding linked to reduced Cd8α transcripts and histone-H3 lysine 9 acetylation of the Cd8 locus. Loss of CD8 expression in KAT6A-deficient T cells correlated with reduced TCR signaling intensity and accelerated contraction of the effector-like memory compartment, whereas the long-lived memory compartment appeared unaffected, a result phenocopied by the removal of the Cd8 E8I enhancer element. These findings suggest a direct role of CD8αβ co-receptor expression and histone acetylation in shaping functional diversity within the cytotoxic T cell pool.

Transcriptional profiling of peripheral CD8+T cell responses to SIVΔnef and SIVmac251 challenge reveals a link between protective immunity and induction of systemic immunoregulatory mechanisms

Immunization of macaques with attenuated simian immunodeficiency virus (SIV) with deletions in nef (SIVΔnef) is shown to elicit protective immunity to infection by pathogenic SIV, yet the mechanisms that orchestrate protection and prevent pathogenesis remains unknown. We utilized whole-genome transcriptional profiling to reveal molecular signatures of protective immunity in circulating CD8+ T cells of rhesus macaques vaccinated with SIVmac239Δnef and challenged with pathogenic SIVmac251. Our findings suggest that protective immunity to pathogenic SIV infection induced by SIVmac239∆nef is associated with balanced induction of T cell activation and immunoregulatory mechanisms and dampened activation of interferon-induced signaling pathways and cytolytic enzyme production as compared with pathogenic SIVmac251 infection of unvaccinated controls. We provide evidence that protective immunity to SIVmac251 correlates with induction of biomarkers of T cell activation, differentiation, signaling, and adhesion that were down regulated in unvaccinated controls. The study highlights potential immunomodulatory networks associated with protective immunity against the virus.

Transcriptional control of CD4 and CD8 coreceptor expression during T cell development

The differentiation and function of peripheral helper and cytotoxic T cell lineages is coupled with the expression of CD4 and CD8 coreceptor molecules, respectively. This indicates that the control of coreceptor gene expression is closely linked with the regulation of CD4/CD8 lineage decision of DP thymocytes. Research performed during the last two decades revealed comprehensive mechanistic insight into the developmental stage- and subset/lineage-specific regulation of Cd4, Cd8a and Cd8b1 (Cd8) gene expression. These studies provided important insight into transcriptional control mechanisms during T cell development and into the regulation of cis-regulatory networks in general. Moreover, the identification of transcription factors involved in the regulation of CD4 and CD8 significantly advanced the knowledge of the transcription factor network regulating CD4/CD8 cell-fate choice of DP thymocytes. In this review, we provide an overview of the identification and characterization of CD4/CD8 cis-regulatory elements and present recent progress in our understanding of how these cis-regulatory elements control CD4/CD8 expression during T cell development and in peripheral T cells. In addition, we describe the transcription factors implicated in the regulation of coreceptor gene expression and discuss how these factors are integrated into the transcription factor network that regulates CD4/CD8 cell-fate choice of DP thymocytes.

Cd8 enhancer E8I and Runx factors regulate CD8α expression in activated CD8+ T cells

Cd8a and Cd8b1 coreceptor gene (Cd8) expression is tightly controlled during T-cell development by the activity of five Cd8 enhancers (E8(I)-E8(V)). Here we demonstrate a unique transcriptional program regulating CD8 expression during CD8(+) effector T-cell differentiation. The Cd8 enhancer E8(I) and Runx/core-binding factor-β (CBFβ) complexes were required for the establishment of this regulatory circuit, because E8(I)-, Runx3-, or CBFβ-deficient CD8(+) T cells down-regulated CD8α expression during activation. This finding correlated with enhanced repressive histone marks at the Cd8a promoter in the absence of E8(I), and the down-regulation of CD8α expression could be blocked by treating E8(I)-, Runx3-, or CBFβ-deficient CD8(+) T cells with the histone deacetylase inhibitor trichostatin A. Moreover, Runx/CBFβ complexes bound the Cd8ab gene cluster in activated CD8(+) T cells, suggesting direct control of the Cd8a locus. However, CD8(+) effector T cells maintained high levels of CD8α when CBFβ was conditionally deleted after activation. Thus, our data suggest an E8(I)- and Runx3/CBFβ-dependent epigenetic programming of the Cd8a locus during T-cell activation, leading to Runx/CBFβ complex-independent maintenance of CD8α expression in effector T cells.

Mucosal memory CD8⁺ T cells are selected in the periphery by an MHC class I molecule

The presence of immune memory at pathogen-entry sites is a prerequisite for protection. Nevertheless, the mechanisms that warrant immunity at peripheral interfaces are not understood. Here we show that the nonclassical major histocompatibility complex (MHC) class I molecule thymus leukemia antigen (TL), induced on dendritic cells interacting with CD8αα on activated CD8αβ(+) T cells, mediated affinity-based selection of memory precursor cells. Furthermore, constitutive expression of TL on epithelial cells led to continued selection of mature CD8αβ(+) memory T cells. The memory process driven by TL and CD8αα was essential for the generation of CD8αβ(+) memory T cells in the intestine and the accumulation of highly antigen-sensitive CD8αβ(+) memory T cells that form the first line of defense at the largest entry port for pathogens.

High content cellular immune profiling reveals differences between rhesus monkeys and men

A better understanding of similarities and differences in the composition of the cellular immune system in non-human primates (NHPs) compared with human subjects will improve the interpretation of preclinical studies. It will also aid in addressing the usefulness of NHPs as subjects for studying chronic diseases, vaccine development and immune reconstitution. We employed high content colour flow cytometry and analysed simultaneously the expression of CD3, CD4, CD8alpha, CD8beta, CD16/CD56, CD45RA, CCR7, CD27, CD28, CD107a and the interleukin-7 receptor alpha-chain (IL-7Ralpha) in peripheral blood mononuclear cells (PBMCs) of 27 rhesus macaques and 16 healthy human subjects. Regulatory T cells (Tregs) were identified using anti-CD3, -CD4, -CD25, -FoxP3, and -IL-7Ralpha monoclonal antibodies. Responsiveness to IL-7 was gauged in a signal transducer and activation of transcription 5 (STAT-5) phosphorylation assay. Human and NHP PBMCs showed a similar T-cell composition pattern with some remarkable differences. Similarities: human and NHP CD4(+) and CD8(+) cells showed a similar STAT-5 phosphorylation pattern in response to IL-7. Multicolour flow cytometric analysis identified a CD4(+) CD8alphaalpha(+) CD8alphabeta(+) T-cell population in NHPs as well as in human subjects that expressed the degranulation marker CD107a and may represent a unique CD4(+) T-cell subset endowed with cytotoxic capacity. Differences: we identified in PBMCs from NHPs a higher proportion (5.16% in CD3(+) T cells) of CD8alphaalpha(+) T cells when compared with human donors (1.22% in CD3(+) T cells). NHP CD8alphaalpha(+) T cells produced tumour necrosis factor-alpha / interferon-gamma (TNF-alpha/IFN-gamma) or TNF-alpha, whereas human CD8alphaalpha(+) T cells produced simultaneously TNF-alpha/IFN-gamma and IL-2. A minor percentage of human CD8(+) T cells expressed CD25(bright) and FoxP3 (0.01%). In contrast, 0.07% of NHP CD8(+) T cells exhibited the CD25(bright) FoxP3(+) phenotype. PBMCs from NHPs showed less IL-7Ralpha-positive events in all T-cell subsets including CD4(+) Tregs (median 5%) as compared with human (median 12%). The data visualize commonalities and differences in immune cell subsets in humans and NHPs, most of them in long-lived memory cells and cells with suppressive functions. This provides a matrix to assess future efforts to study diseases and vaccines in NHPs.

Plasticity in programming of effector and memory CD8 T-cell formation

CD8(+) T cells (also called cytotoxic T lymphocytes) play a major role in protective immunity against many infectious pathogens and can eradicate malignant cells. The path from naive precursor to effector and memory CD8(+) T-cell development begins with interactions between matured antigen-bearing dendritic cells (DCs) and antigen-specific naive T-cell clonal precursors. By integrating differences in antigenic, costimulatory, and inflammatory signals, a developmental program is established that governs many key parameters associated with the ensuing response, including the extent and magnitude of clonal expansion, the functional capacities of the effector cells, and the size of the memory pool that survives after the contraction phase. In this review, we discuss the multitude of signals that drive effector and memory CD8(+) T-cell differentiation and how the differences in the nature of these signals contribute to the diversity of CD8(+) T-cell responses.

A critical role for direct TLR2-MyD88 signaling in CD8 T-cell clonal expansion and memory formation following vaccinia viral infection

Recent advances have suggested a crucial role of the innate immunity in shaping adaptive immune responses. How activation of innate immunity promotes adaptive T-cell responses to pathogens in vivo is not fully understood. It has been thought that Toll-like receptor (TLR)-mediated control of adaptive T-cell responses is mainly achieved by the engagement of TLRs on antigen-presenting cells to promote their maturation and function. In this study, we showed that direct TLR2-myeloid differentiating factor 88 (MyD88) signaling in CD8 T cells was also required for their efficient clonal expansion by promoting the survival of activated T cells on vaccinia viral infection in vivo. Effector CD8 T cells that lacked direct TLR2-MyD88 signaling did not survive the contraction phase to differentiate into long-lived memory cells. Furthermore, we observed that direct TLR2 ligation on CD8 T cells promoted CD8 T-cell proliferation and survival in vitro in a manner dependent on the phosphatidylinositol 3-kinase (PI3K)-Akt pathway activation and that activation of Akt controlled memory cell formation in vivo. These results identify a critical role for intrinsic TLR2-MyD88 signaling and PI3K-Akt pathway activation in CD8 T-cell clonal expansion and memory formation in vivo and could lead to the development of new vaccine approaches.

Doubting the TCR coreceptor function of CD8alphaalpha

"The beginning of wisdom is found in doubting; by doubting we come to question, and by seeking we may come upon the truth." -Pierre Abélard. CD8 is a glycoprotein expressed on hematopoietic cells. Two isoforms of CD8, CD8alphabeta and CD8alphaalpha, have been identified that are distinct in their expression and function. Whereas CD8alphabeta serves as a T cell receptor (TCR) coreceptor to enhance the functional avidity and is constitutively expressed on MHC class I-restricted T cells, CD8alphaalpha marks T cells that are distinct from the conventional thymus-selected and MHC-restricted CD4(+) or CD8alphabeta(+) T cells. Inconsistent with a coreceptor function, CD8alphaalpha decreases antigen sensitivity of the TCR, and it can be transiently or permanently expressed on T cells, regardless of the MHC restriction of the TCR or the presence of conventional coreceptors. Together, these observations indicate that CD8alphaalpha on T cells marks a differentiation stage and that it likely functions as a TCR corepressor to negatively regulate T cell activation.

Extent of stimulation controls the formation of memory CD8 T cells

Only a small fraction of effector CD8 T cells survives to become long-lived memory cells, whereas the majority of them die after an acute infection. What controls the formation of memory CD8 T cells remains mostly unknown. In this study, we showed CD8 T cells primed earlier during vaccinia viral infection received stronger stimulation, divided more extensively, and survived better than those primed later, leading to generation of a larger memory pool. Despite differentiation into effectors, the late-primed CD8 T cells lacked full cell division, displayed increased apoptosis, and failed to develop into memory cells, suggesting that the extent of stimulation influences the survival of effector CD8 T cells. We further demonstrated that the extent of stimulation, which included both the duration and the levels of antigenic stimulation/costimulation, during priming determined the formation of memory CD8 T cells via controlling the extent of Akt activation, and functional suppression of Akt led to defective CD8 memory formation in vivo. Collectively, our data suggest that the extent of stimulation controls CD8 memory formation via activation of Akt and may provide important insights into the design of effective vaccines.

CD8 alpha is expressed by human monocytes and enhances Fc gamma R-dependent responses

Background

CD8α enhances the responses of antigen-specific CTL activated through TCR through binding MHC class I, favoring lipid raft partitioning of TCR, and inducing intracellular signaling. CD8α is also found on dendritic cells and rat macrophages, but whether CD8α enhances responses of a partner receptor, like TCR, to activate these cells is not known. TCR and FcR, use analogous or occasionally interchangeable signaling mechanisms suggesting the possibility that CD8α co-activates FcR responses. Interestingly, CD8α+ monocytes are often associated with rat models of disease involving immune-complex deposition and FcR-mediated pathology, such as arthritis, glomerulonephritis, ischaemia, and tumors. While rat macrophages have been shown to express CD8α evidence for CD8α expression by mouse or human monocytes or macrophages was incomplete.

Results

We detected CD8α, but not CD8β on human monocytes and the monocytic cell line THP-1 by flow cytometry. Reactivity of anti-CD8α mAb with monocytes is at least partly independent of FcR as anti-CD8α mAb detect CD8α by western blot and inhibit binding of MHC class I tetramers. CD8α mRNA is also found in monocytes and THP-1 suggesting CD8α is synthesized by monocytes and not acquired from other CD8α+ cell types. Interestingly, CD8α from monocytes and blood T cells presented distinguishable patterns by 2-D electrophoresis. Anti-CD8α mAb alone did not activate monocyte TNF release. In comparison, TNF release by human monocytes stimulated in a FcR-dependent manner with immune-complexes was enhanced by inclusion of anti-CD8α mAb in immune-complexes.

Conclusion

Human monocytes express CD8α. Co-engagement of CD8α and FcR enhances monocyte TNF release, suggesting FcR may be a novel partner receptor for CD8α on innate immune cells.

Anti-TCR antibody treatment activates a novel population of nonintestinal CD8 alpha alpha+ TCR alpha beta+ regulatory T cells and prevents experimental autoimmune encephalomyelitis

CD8alphaalpha+CD4-TCRalphabeta+ T cells are a special lineage of T cells found predominantly within the intestine as intraepithelial lymphocytes and have been shown to be involved in the maintenance of immune homeostasis. Although these cells are independent of classical MHC class I (class Ia) molecules, their origin and function in peripheral lymphoid tissues are unknown. We have recently identified a novel subset of nonintestinal CD8alphaalpha+CD4-TCRalphabeta+ regulatory T cells (CD8alphaalpha Tregs) that recognize a TCR peptide from the conserved CDR2 region of the TCR Vbeta8.2-chain in the context of a class Ib molecule, Qa-1a, and control- activated Vbeta8.2+ T cells mediating experimental autoimmune encephalomyelitis. Using flow cytometry, spectratyping, and real-time PCR analysis of T cell clones and short-term lines, we have determined the TCR repertoire of the CD8alphaalpha regulatory T cells (Tregs) and found that they predominantly use the TCR Vbeta6 gene segment. In vivo injection of anti-TCR Vbeta6 mAb results in activation of the CD8alphaalpha Tregs, inhibition of the Th1-like pathogenic response to the immunizing Ag, and protection from experimental autoimmune encephalomyelitis. These data suggest that activation of the CD8alphaalpha Tregs present in peripheral lymphoid organs other than the gut can be exploited for the control of T cell-mediated autoimmune diseases.

Effector and memory CTL differentiation

Technological advances in recent years have allowed for an ever-expanding ability to analyze and quantify in vivo immune responses. MHC tetramers, intracellular cytokine staining, an increasing repertoire of transgenic and "knockout" mice, and the detailed characterization of a variety of infectious models have all facilitated more precise and definitive analyses of the generation and function of cytotoxic T lymphocytes (CTL). Understanding the mechanisms behind the differentiation of effector and memory CTL is of increasing importance to develop vaccination strategies against a variety of established and emerging infectious diseases. This review focuses on recent advances in our understanding of how effector and memory CTL differentiate and survive in vivo in response to viral or bacterial infection.

Immunophenotypic patterns of CD8+ T cell subsets expressing CD8alphaalpha and IL-7Ralpha in viremic, aviremic and slow progressor HIV-1-infected subjects

Evidence from animal models suggests that the expression of CD8alphaalpha homodimer on CD8(+) T cells plays a key role in the generation of long-lived memory cells. Here, we studied the quantitative alterations of CD8(+) T cell subsets expressing CD8alphaalpha, interleukin-7 receptor (IL-7Ralpha) and activation markers in HIV-1-infected individuals including aviremic, viremic and slow progressor subjects using eight-color flow cytometry. Compared to slow progressor subjects, expression of CD8alphaalpha was significantly reduced in aviremic and viremic patients and this reduction occurred mainly within central memory cell subsets and not in naive and effector memory compartments. Persistence of antigenemia leads to IL-7Ralpha loss mainly on central and pre-terminal memory CD8(+) T cell subsets in viremic patients but not in slow progressor subjects. Compared to aviremic and viremic patients, slow progressor subjects had lower levels of IL-7 and reduced activated cells. The expression of CD8alphaalpha was not significantly related to IL-7Ralpha although negative associations were evidenced within all CD8(+) T cell subsets. Collectively, these results further advance the characterization of immunophenotypic patterns of CD8(+) T cell subsets expressing CD8alphaalpha/IL-7Ralpha and provide new insights into the ability of HIV-1 infection to alter memory cell population.

Interleukin-7 receptor expression: intelligent design

Interleukin-7 (IL-7) is produced by stromal cells in lymphoid tissues and is required for the development of T cells and for their persistence in the periphery. Unlike many other cytokines that act on lymphocytes, IL-7 production by stromal cells is not substantially affected by extrinsic stimuli. So, the amount of available IL-7 protein is thought to be regulated by the rate that it is scavenged by T cells. As we review here, there is mounting evidence indicating that the amount of IL-7 receptor expressed on a cell not only determines how vigorously the cell responds to IL-7, but it can also determine how efficiently the cell consumes IL-7 and, therefore, affect the supply of this limiting resource in the niche.

The Tower of Babel of CD8+ T-cell memory: known facts, deserted roads, muddy waters, and possible dead ends

Adequate antigen stimulation can lead to permanent modifications of primed cells and to the generation of memory T cells that have astonishingly improved capacities to deal with antigen. The overall properties of memory T cells (increased survival, precocious and increased division capacities, and improved effector functions) can be used to identify this unique cell type. However, each immune response may lead to the generation of multiple primed types that do not necessarily possess all these characteristics. It is not known whether these different cell types are just side products of the immune reaction or whether they are involved in disease control. Control of different infections may involve different challenges and lead to the generation of different types of immune reactions. Our major challenge is to unravel this complexity, but we must overcome our handicapped experimental tests and our imperfect a priori definitions.

Protective immunity towards intracellular pathogens

Immunity towards intracellular pathogens is often dependent upon the generation of CD8(+) memory T cells, which provide long-lasting and effective protection. Over the past few years, we have gained novel insights into the heterogeneity of CD8(+) T cells, time points of lineage commitment, and lineage relationships between subpopulations. These studies suggest that memory CD8(+) T cells progressively develop from naïve cells early during the immune response and further differentiate unidirectionally into short-living effector cells. We have also learnt that different memory subsets play distinct roles in conferring protection: whereas effector memory T cells are able to provide immediate protection but are not necessarily maintained long-term, central memory T cells have the potential for constant self-renewal. Thus, neither subset really fulfills all criteria of memory. As protective effector memory cells can develop from central memory cells, vaccination strategies should focus on induction of a balanced ratio of the two memory T cell subsets.

Divergent Generation of Heterogeneous Memory CD4 T Cells

Mechanisms for the generation of memory CD4 T cells and their delineation into diverse subsets remain largely unknown. In this study, we demonstrate in two Ag systems, divergent generation of heterogeneous memory CD4 T cells from activated precursors in distinct differentiation stages. Specifically, we show that influenza hemagglutinin- and OVA-specific CD4 T cells activated for 1, 2, and 3 days, respectively, exhibit gradations of differentiation by cell surface phenotype, IFN-gamma production, and proliferation, yet all serve as direct precursors for functional memory CD4 T cells when transferred in vivo into Ag-free mouse hosts. Using a conversion assay to track the immediate fate of activated precursors in vivo, we show that day 1- to 3-activated cells all rapidly convert from an activated phenotype (CD25(high)IL-7R(low)CD44(high)) to a resting memory phenotype (IL-7R(high)CD25(low)CD44(high)) 1 day after antigenic withdrawal. Paradoxically, stable memory subset delineation from undifferentiated (day 1- to 2-activated) precursors was predominantly an effector memory (CD62L(low)) profile, with an increased proportion of central memory (CD62L(high)) T cells arising from more differentiated (day 3-activated) precursors. Our findings support a divergent model for generation of memory CD4 T cells directly from activated precursors in multiple differentiation states, with subset heterogeneity maximized by increased activation and differentiation during priming.

Programming, demarcating, and manipulating CD8 + T‐cell memory

In response to infection, antigen-specific CD8+ T cells undergo massive expansion in numbers, acquire effector mechanisms, and disseminate throughout the body. The expansion phase is followed by a contraction (death) phase, where 90-95% of antigen-specific CD8+ T cells are eliminated. The remaining antigen-specific CD8+ T cells form the initial memory pool, which can be stably maintained for life. In this review, we discuss evidence that early events after infection 'program' CD8+ T cells to expand, contract, and generate memory in a fashion that is largely insensitive to the duration of infection or antigen display. Recent data demonstrate, despite numerical stability, that memory CD8+ T-cell populations undergo phenotypic and functional changes with time after immunization. However, the early suggestion that specific markers can be used to identify memory CD8+ T cells has not been supported by recent studies. Thus, we argue that specific functional characteristics, such as the ability to persist and undergo vigorous secondary expansion leading to elevated memory cell numbers, remain the best markers of 'good' memory cells. Finally, we discuss experimental approaches to manipulate and accelerate generation of CD8+ T cells with memory characteristics, and how these systems can inform both basic and applied immunology.

Do CD8 effector cells need IL-7R expression to become resting memory cells?

The role for IL-7R expression in the differentiation of effector T cells into resting memory remains controversial. Here, using a conditional IL-7R transgenic model, we were able to test directly whether CD8 effector T cells require IL-7R expression for their differentiation into resting memory cells. In the absence of IL-7R expression, effector cells transferred into "full" hosts underwent a protracted and unremitting contraction compared with IL-7R-expressing control cells and were unable to develop into long-term resting memory cells. Surprisingly, when the same effector cells were transferred into empty T-cell-deficient hosts, they could generate long-lived fully functional resting memory cells independently of IL-7R expression. Formation of these latter cells was found to be dependent on IL-15, because the same IL-7R-deficient effector cells were rapidly lost from IL-15-deficient hosts, having a half-life of less than 40 hours. Therefore, our data suggest that, under physiological conditions, both IL-7 and IL-15 synergize to promote the formation of memory cells directly by limiting the contraction of effectors that occurs following an immune response and that reexpression of IL-7R is a key checkpoint in the regulation of this process.

Committed to memory: lineage choices for activated T cells

The mechanisms for the generation of memory T cells and their delineation into heterogeneous subsets remain unknown. The linear model for memory T-cell generation from differentiated effector cells has been favored, although there is evidence that memory T cells can emerge directly from naive T cells undergoing homeostatic expansion and from activated T cells lacking effector functions. Here, we discuss the evidence from diverse studies of memory generation that support a new 'intersecting pathway' model for memory T-cell generation in which antigen-driven effector differentiation and homeostasis-driven memory differentiation follow distinct but analogous pathways. Antigen withdrawal during effector differentiation enables intersection with the memory pathway through a pre-memory intermediate, and memory heterogeneity is influenced by homeostasis, migration and persistence in vivo.