IL-7 and IL-15 differentially regulate CD8+ T-cell subsets during contraction of the immune response

A role for IL-15 in the migration of effector CD8 T cells to the lung airways following influenza infection

The cytokines generated locally in response to infection play an important role in CD8 T cell trafficking, survival, and effector function, rendering these signals prime candidates for immune intervention. In this paper, we show that localized increases in the homeostatic cytokine IL-15 induced by influenza infection is responsible for the migration of CD8 effector T cells to the site of infection. Moreover, intranasal delivery of IL-15-IL-15Rα soluble complexes (IL-15c) specifically restores the frequency of effector T cells lost in the lung airways of IL-15-deficient animals after influenza infection. Exogenous IL-15c quantitatively augments the respiratory CD8 T cell response, and continued administration of IL-15c throughout the contraction phase of the anti-influenza CD8 T cell response magnifies the resultant CD8 T cell memory generated in situ. This treatment extends the ability of these cells to protect against heterologous infection, immunity that typically depreciates over time. Overall, our studies describe what to our knowledge is a new function for IL-15 in attracting effector CD8 T cells to the lung airways and suggest that adjuvanting IL-15 could be used to prolong anti-influenza CD8 T cell responses at mucosal surfaces to facilitate pathogen elimination.

IL-2 complex treatment can protect naive mice from bacterial and viral infection

IL-2 complexes have substantial effects on the cellular immune system, and this approach is being explored for therapeutic application in infection and cancer. However, the impact of such treatments on subsequent encounter with pathogens has not been investigated. In this study, we report that naive mice treated with a short course of IL-2 complexes show enhanced protection from newly encountered bacterial and viral infections. IL-2 complex treatment expands both the NK and CD8 memory cell pool, including a recently described population of preexisting memory-phenotype T cells responsive to previously unencountered foreign Ags. Surprisingly, prolonged IL-2 complex treatment decreased CD8 T cell function and protective immunity. These data reveal the impact of cytokine complex treatment on the primary response to infection.

Differential localization of effector and memory CD8 T cell subsets in lymphoid organs during acute viral infection

It is unclear where within tissues subsets of effector and memory CD8 T cells persist during viral infection and whether their localization affects function and long-term survival. Following lymphocytic choriomeningitis virus infection, we found most killer cell lectin-like receptor G1 (KLRG1)(lo)IL-7R(hi) effector and memory cells, which are long-lived and high proliferative capacity, in the T cell zone of the spleen. In contrast, KLRG1(hi)IL-7R(lo) cells, which appear terminally differentiated and have shorter life spans, were exclusively localized to the red pulp. KLRG1(lo)IL-7R(hi) T cells homed to the T cell zone using pertussis toxin-sensitive chemokine receptors and appeared to contact gp38(+) stromal cells, which produce the chemokines CCL19 and CCL21 and the T cell survival cytokine IL-7. The transcription factors T-bet and B lymphocyte-induced maturation protein-1 controlled effector CD8 T cell splenic migration. Effector CD8 T cells overexpressing T-bet homed to the red pulp, whereas those lacking B lymphocyte-induced maturation protein-1 homed to the T cell zone. Upon memory formation, CD62L(+) memory T cells were predominantly found in the T cell zone, whereas CD62L(-) cells were found in the red pulp. Thus, effector and memory CD8 T cell subset localization within tissues is linked to their differentiation states, and this may identify anatomical niches that regulate their longevity and homeostasis.

Interleukin-2 receptor signaling: at the interface between tolerance and immunity

Interleukin-2 receptor (IL-2R) signaling regulates tolerance and immunity. Here, we review recent work concerning the structure, signaling, and function of the IL-2R, emphasizing the contribution of IL-2 for T cell-dependent activity in vivo. IL-2R signaling influences two discrete aspects of immune responses by CD8(+) T cells, terminal differentiation of effector cells in primary responses, and aspects of memory recall responses. IL-2 also delivers essential signals for thymic development of regulatory T (Treg) cells and later to promote their homeostasis and function. Each of these outcomes on T effector and Treg cells requires distinct amounts of IL-2R signaling, with low IL-2R signaling sufficient for many key aspects of Treg cells. Thus, tolerance is readily maintained and favored with limited IL-2.

Differential effects of STAT5 and PI3K/AKT signaling on effector and memory CD8 T-cell survival

During viral infection, effector CD8 T cells contract to form a population of protective memory cells that is maintained by IL-7 and IL-15. The mechanisms that control effector cell death during infection are poorly understood. We investigated how short- and long-lived antiviral CD8 T cells differentially used the survival and cell growth pathways PI3K/AKT and JAK/STAT5. In response to IL-15, long-lived memory precursor cells activated AKT significantly better than short-lived effector cells. However, constitutive AKT activation did not enhance memory CD8 T-cell survival but rather repressed IL-7 and IL-15 receptor expression, STAT5 phosphorylation, and BCL2 expression. Conversely, constitutive STAT5 activation profoundly enhanced effector and memory CD8 T-cell survival and augmented homeostatic proliferation, AKT activation, and BCL2 expression. Taken together, these data illustrate that effector and memory cell viability depends on properly balanced PI3K/AKT signaling and the maintenance of STAT5 signaling.

The persistence of T cell memory

T cell memory is a crucial feature of the adaptive immune system in the defense against pathogens. During the last years, numerous studies have focused their efforts on uncovering the signals, inflammatory cues, and extracellular factors that support memory differentiation. This research is beginning to decipher the complex gene network that controls memory programming. However, how the different signals, that a T cell receives during the process of differentiation, interplay to trigger memory programming is still poorly defined. In this review, we focus on the most recent advances in the field and discuss how T cell receptor signaling and inflammation control CD8 memory differentiation.

Early events governing memory CD8+ T-cell differentiation

Understanding the regulation of the CD8(+) T-cell response and how protective memory cells are generated has been intensely studied. It is now appreciated that a naive CD8(+) T cell requires at least three signals to mount an effective immune response: (i) TCR triggering, (ii) co-stimulation and (iii) inflammatory cytokines. Only recently have we begun to understand the molecular integration of those signals and how early events regulate the fate decisions of the responding CD8(+) T cells. This review will discuss the recent findings about both the extracellular and intracellular factors that regulate the destiny of responding CD8(+) T cells.

STAT5 is critical to maintain effector CD8+ T cell responses

During an immune response, most effector T cells die, whereas some are maintained and become memory T cells. Factors controlling the survival of effector CD4(+) and CD8(+) T cells remain unclear. In this study, we assessed the role of IL-7, IL-15, and their common signal transducer, STAT5, in maintaining effector CD4(+) and CD8(+) T cell responses. Following viral infection, IL-15 was required to maintain a subpopulation of effector CD8(+) T cells expressing high levels of killer cell lectin-like receptor subfamily G, member 1 (KLRG1), and lower levels of CD127, whereas IL-7 and IL-15 acted together to maintain KLRG1(low)CD127(high) CD8(+) effector T cells. In contrast, effector CD4(+) T cell numbers were not affected by the individual or combined loss of IL-15 and IL-7. Both IL-7 and IL-15 drove phosphorylation of STAT5 within effector CD4(+) and CD8(+) T cells. When STAT5 was deleted during the course of infection, both KLRG1(high)CD127(low) and KLRG1(low)CD127(high) CD8(+) T cells were lost, although effector CD4(+) T cell populations were maintained. Furthermore, STAT5 was required to maintain expression of Bcl-2 in effector CD8(+), but not CD4(+), T cells. Finally, IL-7 and IL-15 required STAT5 to induce Bcl-2 expression and to maintain effector CD8(+) T cells. Together, these data demonstrate that IL-7 and IL-15 signaling converge on STAT5 to maintain effector CD8(+) T cell responses.

Contracting the 'mus cells'--does down-sizing suit us for diving into the memory pool?

Maintenance of T-cell homeostasis is critical for normal functioning of the immune system. After thymocyte selection, T cells enter the peripheral lymphoid organs, where they are maintained as naive cells. Transient disruption of homeostasis occurs when naive T cells undergo antigen-driven expansion and acquire effector functions. Effector T cells then either undergo apoptosis (i.e. contraction at the population level) or survive to become memory cells. This apoptotic process is crucial: it resets T-cell homeostasis, promotes protective immunity, and limits autoimmunity. Although initial studies using in vitro models supported a role for death receptor signaling, more recent in vivo studies have implicated Bcl-2 family members as being critical for the culling of T-cell responses. While several Bcl-2 family members likely contribute to T-cell contraction, the pro-apoptotic molecule Bim and its anti-apoptotic antagonist Bcl-2 are essential regulators of the process. This review discusses the progress made in our understanding of the mechanisms underlying contraction of T-cell responses and how some cells avoid this cell death and become memory T cells.

Generation of effector CD8+ T cells and their conversion to memory T cells

Immunological memory is a cardinal feature of adaptive immunity. We are now beginning to elucidate the mechanisms that govern the formation of memory T cells and their ability to acquire longevity, survive the effector-to-memory transition, and mature into multipotent, functional memory T cells that self-renew. Here, we discuss the recent findings in this area and highlight extrinsic and intrinsic factors that regulate the cellular fate of activated CD8+ T cells.

Distinct roles for IL-2 and IL-15 in the differentiation and survival of CD8+ effector and memory T cells

IL-2 provides a memory differentiation signal to CD8+ T cells during the primary response that impacts the ability of the subsequent memory pool to mount a successful recall response. In this study, we find that although primary effector CTL development is modestly decreased in the absence of IL-2, the persistence of short-term and long-term effector memory CD8+ T cells on pathogen clearance is greatly diminished. Furthermore, secondary challenge of CD8+ memory T cells lacking the high-avidity IL-2R results in a failure to repopulate the effector pool. The role of IL-2 in promoting effector differentiation is not shared with the highly related cytokine, IL-15. Although IL-15 supports the survival of effector CD8+ T cells after pathogen clearance, its absence does not impair either primary or secondary effector CTL differentiation, nor does it impact the differentiation of long-term effector memory CD8+ T cells. These findings indicate a unique role for IL-2, but not IL-15, in promoting the differentiation not only of primary effector CD8+ T cells, but also of CD8+ memory T cells capable of secondary effector differentiation.

Early signals during CD8 T cell priming regulate the generation of central memory cells

The CD8(+) T cell response to infection is characterized by the appearance of short-lived (CD127(low) killer cell lectin-like receptor G 1-high) and memory-precursor (CD127(high) killer cell lectin-like receptor G 1-low) effector cells. How and when central-memory T (T(CM); CD62L(high) CCR7(+)) cell and effector-memory T(T(EM); CD62L(low) CCR7(-)) cell subsets are established remains unclear. We now show that the T(CM) cell lineage represents an early developmental branchpoint during the CD8(+) T cell response to infection. Central-memory CD8(+) T cells could be identified prior to the peak of the CD8(+) T cell response and were enriched in lymphoid organs. Moreover, the kinetics and magnitude of T(CM) cell development were dependent on the infectious agent. Furthermore, the extent of early Ag availability, which regulated programmed death-1 and CD25 expression levels, controlled the T(CM)/T(EM) cell lineage decision ultimately through IL-2 and IL-15 signaling levels. These observations identify key early signals that help establish the T(CM)/T(EM) cell dichotomy and provide the means to manipulate memory lineage choices.

Cytotoxic potential of lung CD8(+) T cells increases with chronic obstructive pulmonary disease severity and with in vitro stimulation by IL-18 or IL-15

Lung CD8(+) T cells might contribute to progression of chronic obstructive pulmonary disease (COPD) indirectly via IFN-gamma production or directly via cytolysis, but evidence for either mechanism is largely circumstantial. To gain insights into these potential mechanisms, we analyzed clinically indicated lung resections from three human cohorts, correlating findings with spirometrically defined disease severity. Expression by lung CD8(+) T cells of IL-18R and CD69 correlated with severity, as did mRNA transcripts for perforin and granzyme B, but not Fas ligand. These correlations persisted after correction for age, smoking history, presence of lung cancer, recent respiratory infection, or inhaled corticosteroid use. Analysis of transcripts for killer cell lectin-like receptor G1, IL-7R, and CD57 implied that lung CD8(+) T cells in COPD do not belong to the terminally differentiated effector populations associated with chronic infections or extreme age. In vitro stimulation of lung CD8(+) T cells with IL-18 plus IL-12 markedly increased production of IFN-gamma and TNF-alpha, whereas IL-15 stimulation induced increased intracellular perforin expression. Both IL-15 and IL-18 protein expression could be measured in whole lung tissue homogenates, but neither correlated in concentration with spirometric severity. Although lung CD8(+) T cell expression of mRNA for both T-box transcription factor expressed in T cells and GATA-binding protein 3 (but not retinoic acid receptor-related orphan receptor gamma or alpha) increased with spirometric severity, stimulation of lung CD8(+) T cells via CD3epsilon-induced secretion of IFN-gamma, TNF-alpha, and GM-CSF, but not IL-5, IL-13, and IL-17A. These findings suggest that the production of proinflammatory cytokines and cytotoxic molecules by lung-resident CD8(+) T cells contributes to COPD pathogenesis.

Gamma(c) deficiency precludes CD8+ T cell memory despite formation of potent T cell effectors

Several cytokines (including IL-2, IL-7, IL-15, and IL-21) that signal through receptors sharing the common gamma chain (gamma(c)) are critical for the generation and peripheral homeostasis of naive and memory T cells. Recently, we demonstrated that effector functions fail to develop in CD4(+) T cells that differentiate in the absence of gamma(c). To assess the role of gamma(c) cytokines in cell-fate decisions that condition effector versus memory CD8(+) T cell generation, we compared the response of CD8(+) T cells from gamma(c)(+) or gamma(c)(-) P14 TCR transgenic mice after challenge with lymphocytic choriomeningitis virus. The intrinsic IL-7-dependent survival defect of gamma(c)(-) naive CD8(+) T cells was corrected by transgenic expression of human Bcl-2. We demonstrated that although gamma(c)-dependent signals are dispensable for the initial expansion and the acquisition of cytotoxic functions following antigenic stimulation, they condition the terminal proliferation and differentiation of CD8(+) effector T cells (i.e., KLRG1(high) CD127(low) short-lived effector T cells) via the transcription factor, T-bet. Moreover, the gamma(c)-dependent signals that are critical for memory T cell formation are not rescued by Bcl2 overexpression. Together, these data reveal an unexpected divergence in the requirement for gamma(c) cytokines in the differentiation of CD4(+) versus CD8(+) cytotoxic T lymphocytes.

Once a killer, always a killer: from cytotoxic T cell to memory cell

The control of the differentiation pathways followed by responding CD8(+) T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.

Memory CD8+ T cell differentiation

In response to infection or effective vaccination, naive antigen-specific CD8+ T cells undergo a dramatic highly orchestrated activation process. Initial encounter with an appropriately activated antigen-presenting cell leads to blastogenesis and an exponential increase in antigen-specific CD8+ T cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in formation of both primary effector and long-lived memory cells. Current findings have emphasized the heterogeneity of effector and memory cell populations with the description of multiple cellular subsets based on phenotype, function, and anatomic location. Yet, only recently have we begun to dissect the underlying factors mediating the temporal control of the development of distinct effector and memory CD8+ T cell sublineages. In this review we will focus on the requirements for mounting an effective CD8+ T cell response and highlight the elements regulating the differentiation of effector and memory subsets.

In vitro detection of cytotoxic T and NK cells in peripheral blood of patients with various drug-induced skin diseases

BACKGROUND

Cytotoxic cells are involved in most forms of drug-induced skin diseases. Till now, no in vitro test addressed this aspect of drug-allergic responses. Our report evaluates whether drug-induced cytotoxic cells can be detected in peripheral blood of nonacute patients with different forms of drug hypersensitivity, and also whether in vitro detection of these cells could be helpful in drug-allergy diagnosis.

METHODS

GranzymeB enzyme-linked immunosorbent spot-forming (ELISPOT) and cell surface expression of the degranulation marker CD107a were evaluated on peripheral blood mononuclear cells from 12 drug-allergic patients in remission state and 16 drug-exposed healthy controls.

RESULTS

In 10/12 allergic patients culprit but not irrelevant drug elicited granzymeB release after 48-72 h stimulation. It was clearly positive in patients with high proliferative response to the drug, measured in lymphocyte transformation tests. In patients, who showed moderate or low proliferation and low drug-response in granzymeB ELISPOT, overnight preincubation with interleukin (IL)-7/IL-15 enhanced drug-specific granzymeB release and allowed to clearly identify the offending agent. CD107a staining was positive on CD4+/CD3+, CD8+/CD3+ T cells as well as CD56+/CD3- natural killer cells. None of the drug-exposed healthy donors reacted to the tested drugs and allergic patients reacted only to the offending, but not to tolerated drugs.

CONCLUSION

GranzymeB ELISPOT is a highly specific in vitro method to detect drug-reacting cytotoxic cells in peripheral blood of drug-allergic patients even several years after disease manifestation. Together with IL-7/IL-15 preincubation, it may be helpful in indentifying the offending drug even in some patients with weak proliferative drug-response.

CD4+ T cell regulation of CD25 expression controls development of short-lived effector CD8+ T cells in primary and secondary responses

Both CD4(+) T cell help and IL-2 have been postulated to "program" activated CD8(+) T cells for memory cell development. However, the linkage between these two signals has not been well elucidated. Here we have studied effector and memory CD8(+) T cell differentiation following infection with three pathogens (Listeria monocytogenes, vesicular stomatitis virus, and vaccinia virus) in the absence of both CD4(+) T cells and IL-2 signaling. We found that expression of CD25 on antigen-specific CD8(+) T cells peaked 3-4 days after initial priming and was dependent on CD4(+) T cell help, likely through a CD28:CD80/86 mediated pathway. CD4(+) T cell or CD25-deficiency led to normal early effector CD8(+) T cell differentiation, but a subsequent lack of accumulation of CD8(+) T cells resulting in overall decreased memory cell generation. Interestingly, in both primary and recall responses KLRG1(high) CD127(low) short-lived effector cells were drastically diminished in the absence of IL-2 signaling, although memory precursors remained intact. In contrast to previous reports, upon secondary antigen encounter CD25-deficient CD8(+) T cells were capable of undergoing robust expansion, but short-lived effector development was again impaired. Thus, these results demonstrated that CD4(+) T cell help and IL-2 signaling were linked via CD25 up-regulation, which controls the expansion and differentiation of antigen-specific effector CD8(+) T cells, rather than "programming" memory cell traits.

IL-15 regulates both quantitative and qualitative features of the memory CD8 T cell pool

Memory T cells are critical for immunity to various intracellular pathogens. Recent studies have indicated that CD8 secondary memory cells, induced by prime-boost approaches, show enhanced protective function compared with primary memory cells and exhibit phenotypic and functional characteristics that distinguish them from primary memory cells. However, little is known about the cytokine requirements for generation and maintenance of boosted memory CD8 T cells. We studied the role of IL-15 in determining the size and composition of the secondary (2 degrees) memory CD8 T cell pool induced by Listeria monocytogenes infection in mice. Following boosting, IL-15-deficient animals failed to generate a subset of CD8 effector memory cells, including a population of IL-7Ralpha(low) cells, which were prominent among secondary memory cells in normal mice. IL-15 deficiency also resulted in changes within the IL-7Ralpha(high)CD62L(low) subset of 2 degrees memory CD8 T cells, which expressed high levels of CD27 but minimal granzyme B. In addition to these qualitative changes, IL-15 deficiency resulted in reduced cell cycle and impaired Bcl-2 expression by 2 degrees memory CD8 T cells, suggesting a role for IL-15 in supporting both basal proliferation and survival of the pool. Analogous qualitative differences in memory CD8 T cell populations were observed following a primary response to Sendai virus in IL-15(-/-) animals. Collectively, these findings demonstrate that IL-15 plays an important role in dictating the composition rather than simply the maintenance of the CD8 memory pool.

Memories that last forever: strategies for optimizing vaccine T-cell memory

For acute self-limiting infections a vaccine is successful if it elicits memory at least as good as the natural experience; however, for persistent and chronic infections such as HIV, hepatitis C virus (HCV), human papillomavirus (HPV), and human herpes viruses, this paradigm is not applicable. At best, during persistent virus infection the person must be able to maintain the integrity of the immune system in equilibrium with controlling replicating virus. New vaccine strategies are required that elicit both potent high-avidity CD8(+) T-cell effector/memory and central memory responses that can clear the nidus of initial virus-infected cells at mucosal surfaces to prevent mucosal transmission or significantly curtail development of disease. The objective of an HIV-1 T-cell vaccine is to generate functional CD8(+) effector memory cells at mucosal portals of virus entry to prevent viral transmission. In addition, long-lived CD8(+) and CD4(+) central memory cells circulating through secondary lymphoid organs and resident in bone marrow, respectively, are needed to provide a concerted second wave of defense that can contain virus at mucosal surfaces and prevent systemic dissemination. Further understanding of factors which can influence long-lived effector and central memory cell differentiation will significantly contribute to development of effective T-cell vaccines. In this review we will focus on discussing mechanisms involved in T-cell memory and provide promising new approaches toward expanding current vaccine strategies to enhance antiviral memory.

Memory-like CD8+ T cells generated during homeostatic proliferation defer to antigen-experienced memory cells

Naive T cells proliferate in response to lymphopenia and acquire the phenotypic and functional qualities of memory T cells, providing enhanced protection against infection. How well memory-like T cells generated during lymphopenia-induced homeostatic proliferation (HP)-memory differentiate into secondary memory cells and compete with Ag-experienced true-memory cells is unknown. We found that CD8(+) HP-memory T cells generated robust responses upon infection and produced a secondary memory population comparable to true-memory cells in the absence of competition. However, when true-memory and HP-memory T cells competed during infection, HP-memory cells contributed less to the effector population, contracted earlier, and formed fewer secondary memory cells. Furthermore, HP- and true-memory cells demonstrated distinct chemokine receptor expression and localization within the spleen during infection, indicating differential access to signals necessary for secondary memory formation. Thus, HP-memory T cells provide protection without compromising the true-memory population. Differences in HP- and true-memory T cells may reveal the basis of competition for limited resources within the memory-T cell compartment.

Opposing effects of TGF-beta and IL-15 cytokines control the number of short-lived effector CD8+ T cells

An effective immune response against infectious agents involves massive expansion of CD8(+) T cells. Once the infection is cleared, the majority of these effector cells die through unknown mechanisms. How is expansion controlled to maximize pathogen clearance and minimize immunopathology? We found, after Listeria infection, plasma transforming growth factor beta (TGF-beta) titers increased concomitant with the expansion of effector CD8(+) T cells. Blocking TGF-beta signaling did not affect effector function of CD8(+) T cells. However, TGF-beta controlled effector cell number by lowering Bcl-2 amounts and selectively promoting the apoptosis of short-lived effector cells. TGF-beta-mediated apoptosis of this effector subpopulation occurred during clonal expansion and contraction, whereas interleukin-15 (IL-15) promoted their survival only during contraction. We demonstrate that the number of effector CD8(+) T cells is tightly controlled by multiple extrinsic signals throughout effector differentiation; this plasticity should be exploited during vaccine design and immunotherapy against tumors and autoimmune diseases.

New insights into the regulation of T cells by gamma(c) family cytokines

Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

Diversity in CD8(+) T cell differentiation

CD8(+) T cells are key effector cells of the adaptive immune system, however their activity must be tightly regulated to allow pathogen clearance whilst preventing immunopathology and autoimmunity. In this review, we summarise the diversity of responses that CD8(+) T cells make to antigenic stimulation with a focus on how CD8(+) T cell responses are regulated to achieve different immune outcomes. In particular, we discuss phenotypic diversity during tolerance induction as well as signals that drive effector and memory cell differentiation in response to infection.

Surviving the crash: transitioning from effector to memory CD8+ T cell

One outcome of infection is the formation of long-lived immunological memory, which provides durable protection from symptomatic re-infection. In response to infection or vaccination, T cells undergo dramatic proliferation and differentiate into effector T cells that mediate removal of the pathogen. Following pathogen clearance, the majority of effector cells die, restoring lymphocyte homeostasis. However, a small number of antigen-specific cells survive and seed the memory T cell population. Here, we focus on recent advances in identifying the key proteins and transcription factors that allow a portion of effector CD8(+) T cells to persist after contraction of the immune response, forming a memory cell population programmed for long-term self-renewal and survival. We also examine new findings addressing the role of environmental cues such as cytokines and co-stimulatory molecules in CD8(+) memory T cell formation and how the cell-extrinsic cues influence the molecular players of intracellular pathways important for memory formation.