Control of homeostasis of CD8+ memory T cells by opposing cytokines

Murine lymph node-derived stromal cells effectively support survival but induce no activation/proliferation of peripheral resting T cells in vitro

Little is known about the homeostatic mechanisms by which the levels of peripheral lymphocytes are maintained. The survival of naïve T cells in vivo must be maintained by some factors that have not been characterized in an in vitro culture system. In this study, we established a culture system of stromal cells derived from murine lymph nodes and investigated the action of the stromal cells in supporting the survival of resting T cells in vitro. Most of the T cells cocultured with the stromal cells did not die, and the supernatant of cultured stromal cells increase the viability of T cells. This T-cell survival-supporting activity was maintained for more than 7 days. Although interleukin (IL)-4, IL-6, IL-7, and interferon-beta also rescued peripheral T cells from spontaneous cell death, medium-soluble and heat-sensitive factor(s) derived from the stromal cells supported the survival of T cells more effectively and for a longer time than did these cytokines. T cells maintained in the culture system with the stromal cells appeared to remain in a resting G0/G1 state and did not show remarkable DNA synthesis. From these results, it is presumed that some soluble factor(s) other than the tested cytokines that have been identified as supporting T-cell survival are produced from lymph node stromal cells. These factor(s) play an important role in maintenance of resting T cells.

Anti-human immunodeficiency virus-gag CD8+ memory T cells generated in vitro from Listeria-immunized mice

The goal of vaccination is the generation of immune memory, an immune state that permits rapid and intense recall responses to a pathogen. Considerable effort is being made to understand the nature of memory T cells. We report here that by extending the length of in vitro culture following a single restimulation with specific peptide, preparations of highly enriched, highly active antigen-specific CD8+ memory T cells could be obtained. These cultures were begun with splenocytes from mice primed by infection either with an attenuated strain of Listeria monocytogenes or vaccinia virus, both expressing the human immunodeficiency virus-1-gag gene. In the cultures, antigen-specific cytotoxic T lymphocyte (CTL) activity reached a maximum at about 9 days and thereafter fell to negligible values. Concomitant with the fall of CTL activity, however, we observed enrichment for a subset of CD11ahigh antigen-specific gag-tetramerpos CD8+ T cells. The cells showed little or no 4-hr CTL activity, but had high delayed (18-hr) CTL activity, and very high cytolytic activity after restimulation. They rapidly expressed interferon-gamma production. Their growth and survival after sorting was completely dependent on interleukin-2 or -15. As few as 5000 of the fluorescence-activated cell sorting-purified cells protected recipients against challenge 3 months after transfer. In response to the challenge, the cells repopulated lymphoid and non-lymphoid organs and showed a sizeable increase in number. The cells therefore demonstrate high protective activity for long periods of time. These cultured cells are thus a potential source of enriched natural memory T cells for reperfusion studies and in which the mechanisms that underlie the generation, differentiation and persistence of memory can be examined.

Homeostasis of naive and memory CD4+ T cells: IL-2 and IL-7 differentially regulate the balance between proliferation and Fas-mediated apoptosis

Cytokines play a crucial role in the maintenance of polyclonal naive and memory T cell populations. It has previously been shown that ex vivo, the IL-7 cytokine induces the proliferation of naive recent thymic emigrants (RTE) isolated from umbilical cord blood but not mature adult-derived naive and memory human CD4(+) T cells. We find that the combination of IL-2 and IL-7 strongly promotes the proliferation of RTE, whereas adult CD4(+) T cells remain relatively unresponsive. Immunological activity is controlled by a balance between proliferation and apoptotic cell death. However, the relative contributions of IL-2 and IL-7 in regulating these processes in the absence of MHC/peptide signals are not known. Following exposure to either IL-2 or IL-7 alone, RTE, as well as mature naive and memory CD4(+) T cells, are rendered only minimally sensitive to Fas-mediated cell death. However, in the presence of the two cytokines, Fas engagement results in a high level of caspase-dependent apoptosis in both RTE as well as naive adult CD4(+) T cells. In contrast, equivalently treated memory CD4(+) T cells are significantly less sensitive to Fas-induced cell death. The increased susceptibility of RTE and naive CD4(+) T cells to Fas-induced apoptosis correlates with a significantly higher IL-2/IL-7-induced Fas expression on these T cell subsets than on memory CD4(+) T cells. Thus, IL-2 and IL-7 regulate homeostasis by modulating the equilibrium between proliferation and apoptotic cell death in RTE and mature naive and memory T cell subsets.

Some interfaces of dendritic cell biology

The field of dendritic cell (DC) biology is robust, with several new approaches to analyze their role in vivo and many newly recognized functions in the control of immunity and tolerance. There also is no shortage of mysteries and challenges. To introduce this volume, I would like to summarize four interfaces of DC research with other lines of investigation and highlight some current issues. One interface is with hematopoiesis. DCs constitute a distinct lineage of white blood cell development with some unique features, such as their origin from both lymphoid and myeloid progenitors, the existence of several distinct subsets, and an important final stage of differentiation termed "maturation," which occurs in response to inflammation and infection, and is pivotal for determining the subsequent immune response. A second interface is with lymphocyte biology. DCs are now known to influence many different classes of lymphocytes (B, NK, NKT) and many types of T cell responses (Th1/Th2, regulatory T cells, peripheral T cell deletion), not just the initial priming or induction of T cell-mediated immunity, which was the first function to be uncovered. DCs are sentinels, controlling many of the afferent or inductive limbs of immune function, alerting the immune system and controlling its early decisions. A third interface is with cell biology. This is a critical discipline to understand at the subcellular and molecular levels the distinct capacities of DCs to handle antigens, to move about the body in a directed way, to bind and activate lymphocytes, and to exert many quality controls on the type of responses, for both tolerance and immunity. A fourth interface is with medicine. Here DCs are providing new approaches to disease pathogenesis and therapy. This interface is perhaps the most demanding, because it requires research with humans. Human research currently is being slowed by the need to deal with many challenges in the design of such studies, and the need to excite, attract and support the young scientists who are essential to move human investigation forward. Nonetheless, DCs are providing new opportunities to study patients and the many clinical conditions that involve the immune system.

Interleukin 7 and T cell receptor signals regulate homeostasis of CD4 memory cells

Immunological memory depends on the long-term maintenance of memory T cells. Although the factors that maintain CD8 T cell memory are well understood, those responsible for CD4 memory are not well defined. We have shown here that interleukin 7 (IL-7) was an important survival factor for CD4 memory T cells that together with T cell receptor (TCR) signals regulated homeostasis of the CD4 memory population in lymphopenic conditions and in the intact immune system. Thus, IL-7 contributes to the maintenance of all naive and memory T cell subsets, and therefore controls the overall size of the T cell pool.

Suppressor of cytokine signaling-1 regulates signaling in response to interleukin-2 and other gamma c-dependent cytokines in peripheral T cells

Suppressor of cytokine signaling-1 (SOCS-1) is an essential regulator of cytokine signaling. SOCS-1-/- mice die before weaning with a complex disease characterized by fatty degeneration and necrosis of the liver. This disease is mediated by interferon (IFN) gamma as neonatal mortality fails to occur in SOCS-1-/-IFNgamma-/- mice. However, the immune system of healthy SOCS-1-/-IFNgamma-/- mice is dysregulated with a reduced ratio of CD4:CD8 T cells and increases in some aspects of T cell activation. SOCS-1-/-IFNgamma-/- mice also die before their wild type and IFNgamma-/- counterparts with a range of inflammatory conditions including pneumonia, gut infiltration, and skin ulceration, suggesting that SOCS-1 controls not only IFNgamma signaling, but also other immunoregulatory factors. This study shows that T cells from SOCS-1-deficient mice display hypersensitivity to cytokines that act through the gammac receptor. SOCS-1 expression is induced by interleukin (IL) 2, IL-4, IL-7, and IL-15, and SOCS-1-deficient T cells show increased proliferation and prolonged survival in response to IL-2 and IL-4. Furthermore, IL-2 induced increased STAT5 phosphorylation and CD44 expression in SOCS-1-deficient T cells compared with controls. Hypersensitivity to gammac-dependent cytokines may contribute to abnormal T cell function, as well as the pathology observed in mice lacking SOCS-1.

Mechanisms of enhanced antigen-specific T cell response following vaccination with a novel peptide-based cancer vaccine and systemic interleukin-2 (IL-2)

Systemic interleukin-2 (IL-2) therapy has been shown to enhance the clinical efficacy of peptide-based cancer vaccines. However, the mechanisms involved in this complex response remain poorly defined. IL-2 is known to be a potent T cell growth factor, but recent studies suggest that IL-2 is also involved in the regulation of T cell immune responses by increasing the susceptibility of proliferating T cells to apoptosis. Using an adoptive transfer model, we demonstrate that the administration of systemic IL-2 significantly enhances the primary and memory immune responses following peptide-based vaccination. In order to define the mechanisms of IL-2 therapy on the antigen-specific T cell response, the kinetics of T cell proliferation, apoptosis, and trafficking were explored. Systemic IL-2 therapy did not appear to alter the kinetics of T cell proliferation immediately following vaccination, but did prolong the proliferative response. Furthermore, IL-2 therapy did not significantly influence apoptosis of proliferating T cells. Such therapy did, however, potentiate L-selectin (CD62L) downregulation on activated antigen-specific T cells, and altered their trafficking confirming their potential therapeutic value. Our findings support the use of systemic IL-2 following peptide-based vaccination, and suggest that IL-2 therapy enhances the primary and memory immune responses by prolonging the proliferative response and altering the trafficking of antigen-specific T cells.

Regulation of peripheral T-cell homeostasis by receptor signalling

The peripheral T-cell pool is maintained at a constant size throughout life. This is achieved through tight homeostatic regulation of the different T-cell subsets. T cells appear to be able to sense whether the T-cell compartment is full and, if it is not, they are able to respond by undergoing cell division. Recent advances have identified that both signalling through the T-cell receptor and a variety of cytokines are essential for the regulation of these responses.

IL-2 and IL-15 manifest opposing effects on activation of nuclear factor of activated T cells

IL-2 and IL-15 are cytokines involved in T cell activation and death. Their non-shared receptors, IL-2Ralpha and IL-15Ralpha, are important in the homeostasis of lymphocytes as evidenced by gene deletion studies. How these cytokine/receptor systems affect T cell antigen receptor signaling pathways is poorly understood. Here, we show that the IL-2 and IL-15 cytokine/receptor alpha systems regulate activation of nuclear factor of activated T cells (NF-AT) in opposing ways. IL-15Ralpha increased while IL-2Ralpha decreased basal NF-AT activation status in a Jurkat transient transfection model. The effect of each of the alpha chain receptors on NF-AT activation was further opposed by addition of the respective cytokine. These effects were inhibited by anti-cytokine and anti-cytokine receptor reagents as well as by inhibitors of TCR signaling. These results suggest a novel pathway of cytokine action to regulate T cell signaling, activation, death, and homeostasis.

Proliferation and differentiation potential of human CD8+ memory T-cell subsets in response to antigen or homeostatic cytokines

Four human CD8+ T-cell subsets, naive (CCR7+CD45RA+), central memory (TCM, CCR7+CD45RA-), effector memory (TEM, CCR7-CD45RA-), and CD45RA+ effector memory cells (TEMRA, CCR7-CD45RA+) were compared for their capacity to proliferate and differentiate in response to antigen or homeostatic cytokines. Cytokine responsiveness and interleukin-15 receptor expression were low in naive T cells and progressively increased from TCM to TEM and TEMRA. In contrast, the capacity to accumulate in response to T-cell receptor (TCR) or cytokine stimulation showed a reciprocal pattern and was associated with resistance to cell death and Bcl-2 expression. Whereas all TCR-stimulated cells acquired a CD45RA-CCR7- phenotype, cytokine-stimulated cells maintained their phenotype with the exception of TCM cells, which expressed CCR7, CD45RA, and perforin in various combinations. Single CD8+ TCM cells, but not TEM cells, could be expanded with cytokines, and the obtained clones displayed several distinct phenotypes, suggesting that TCM cells are heterogeneous. Consistently, CCR4 expression in the CD8+ TCM pool discriminated CCR4+ type 2 polarized cells (Tc2) and CCR4-CTL precursors. Finally, ex vivo bromodeoxyuridine (BrdU) incorporation experiments revealed that memory subsets have different in vivo proliferation rates, with CCR4-TCM having the highest turnover and TEMRA the lowest. These results show that human CD8+ memory T-cell subsets have different proliferation and differentiation potentials in vitro and in vivo. Furthermore, they suggest that TEMRA cells are generated from a TCM subset upon homeostatic proliferation in the absence of antigen.

Parasites and immune responses: memory illusion?

Immunological memory responses to intracellular protozoa and extracellular helminths govern host resistance and susceptibility to reinfection. Humans and livestock living in parasitic disease endemic regions face continuous exposure from a very early age that often leads to asymptomatic chronic infection over their entire lifespan. Fundamental immunological studies suggest that the generation of T-cell memory is driven by tightly coordinated innate and adaptive cellular immune responses rapidly triggered following initial host infection. A key distinguishing feature of immune memory maintenance between the majority of parasitic diseases and most bacterial or viral diseases is long-term antigen persistence. Consequently, functional parasite immune memory is in a continuous, dynamic flux between activation and deactivation producing functional parasite killing or functional memory cell death. In this sense, T-cell immune memory can be regarded as "memory illusion." Furthermore, due to the finite capacity of memory lymphocytes to proliferate, continuous parasite antigen stimulation may exceed a threshold level at some point in the chronically infected host. This may result in suboptimal effector immune memory leading to host susceptibility to reinfection, or immune dysregulation yielding disease reactivation or immune pathology. The goal of this review is to highlight, through numerous examples, what is currently known about T-cell immune memory to parasites and to provide compelling hypotheses on the survival and maintenance of parasite "memory illusion." These novel concepts are discussed in the context of rationale parasite vaccine design strategies.

Renewal of peripheral CD8+ memory T cells during secondary viral infection of antibody-sufficient mice

Kinetic studies and short pulses of injected 5-bromo-2-deoxyuridine have been used to analyze the development and renewal of peripheral CD8(+) memory T cells in the lungs during primary and secondary respiratory virus infections. We show that developing peripheral CD8(+) memory T cells proliferate during acute viral infection with kinetics that are indistinguishable from those of lymphoid CD8(+) memory T cells. Secondary exposure to the same virus induces a new round of T cell proliferation and extensive renewal of the peripheral and lymphoid CD8(+) memory T cell pools in both B cell-deficient mice and mice with immune Abs. In mice with virus-specific Abs, CD8(+) T cell proliferation takes place with minimal inflammation or effector cell recruitment to the lungs. The delayed arrival of CD8(+) memory T cells to the lungs of these animals suggests that developing memory cells do not require the same inflammatory signals as effector cells to reach the lung airways. These studies provide important new insight into mechanisms that control the maintenance and renewal of peripheral memory T cell populations during natural infections.

IL-15 promotes the survival of naive and memory phenotype CD8+ T cells

IL-15 stimulates the proliferation of memory phenotype CD44(high)CD8(+) T cells and is thought to play a key role in regulating the turnover of these cells in vivo. We have investigated whether IL-15 also has the capacity to affect the life span of naive phenotype (CD44(low)) CD8(+) T cells. We report that IL-15 promotes the survival of both CD44(low) and CD44(high) CD8(+) T cells, doing so at much lower concentrations than required to induce proliferation of CD44(high) cells. Rescue from apoptosis was associated with the up-regulation of Bcl-2 in both cell types, whereas elevated expression of Bcl-x(L) was observed among CD44(high) but not CD44(low) CD8(+) cells. An investigation into the role of IL-15R subunits in mediating the effects of IL-15 revealed distinct contributions of the alpha- and beta- and gamma-chains. Most strikingly, IL-15R alpha was not essential for either induction of proliferation or promotion of survival by IL-15, but did greatly enhance the sensitivity of cells to low concentrations of IL-15. By contrast, the beta- and gamma-chains of the IL-15R were absolutely required for the proliferative and pro-survival effects of IL-15, although it was not necessary for CD44(high)CD8(+) cells to express higher levels of IL-15R beta than CD44(low) cells to proliferate in response to IL-15. These results show that IL-15 has multiple effects on CD8 T cells and possesses the potential to regulate the life span of naive as well as memory CD8(+) T cells.

Therapeutic use of IL-2 to enhance antiviral T-cell responses in vivo

Interleukin (IL)-2 is currently used to enhance T-cell immunity but can have both positive and negative effects on T cells. To determine whether these opposing results are due to IL-2 acting differently on T cells depending on their stage of differentiation, we examined the effects of IL-2 therapy during the expansion, contraction and memory phases of the T-cell response in lymphocytic choriomeningitis virus (LCMV)-infected mice. IL-2 treatment during the expansion phase was detrimental to the survival of rapidly dividing effector T cells. In contrast, IL-2 therapy was highly beneficial during the death phase, resulting in increased proliferation and survival of virus-specific T cells. IL-2 treatment also increased proliferation of resting memory T cells in mice that controlled the infection. Virus-specific T cells in chronically infected mice also responded to IL-2 resulting in decreased viral burden. Thus, timing of IL-2 administration and differentiation status of the T cell are critical parameters in designing IL-2 therapies.

Contrasting urban and rural lifestyles of memory CD8+ T cells

Memory CD8(+) T cells in lymphoid tissue exhibit an unexpectedly high apoptosis rate, while memory cells located in several nonlymphoid tissues do not. This may be because the lymphoid CD8(+) memory T cell repertoire is being continuously updated, while the tissue repertoire is more stable.

Homeostatic proliferation but not the generation of virus specific memory CD8 T cells is impaired in the absence of IL-15 or IL-15Ralpha

The generation and efficient maintenance of antigen specific memory T cells is essential for long-lasting immunological protection. Antigen specific memory CD8 T cells are known to be maintained via antigen-independent homeostatic proliferation. However, signals that drive memory T cell generation and/or influence the slow turnover of memory T cells are unknown. Recently, IL-15 has received attention for its potential effect on memory CD8 T cells. In this report we examine the role of IL-15 in the generation and maintenance of virus specific memory CD8 T cells using mice deficient in either IL-15 or the IL-15 receptor a chain. Both cytokine and receptor deficient mice mount a robust CD8 T cell response to infection with lymphocytic choriomeningitis virus (LCMV) that is initially only slightly lower than in control mice. Further, virus specific memory CD8 T cells are generated in both IL-15 -/- and IL-15Ralpha -/- mice. However, longitudinal analysis reveals a slow attrition of LCMV specific memory CD8 T cells in the absence of IL-15 signals. Indeed, direct examination of homeostatic proliferation reveals a severe defect in the turnover of antigen specific memory CD8 T cells in the absence of IL-15. Together these results suggest that IL-15 is not essential for the generation of memory CD8 T cells, but is required for homeostatic proliferation to maintain populations of memory cells over long periods of time.

IL-15R alpha expression on CD8+ T cells is dispensable for T cell memory

The generation and maintenance of immunological memory requires the activation, expansion, and persistent proliferation of antigen-specific T cells. Recent work suggests that IL-15 may be important for this process. Surprisingly, we now find that expression of the high-affinity receptor for IL-15, IL-15R alpha, on T cells is dispensable for the generation or maintenance of memory CD8(+) T cells. By contrast, IL-15R alpha expression on cells other than T cells is absolutely critical for this function. These findings may be related to IL-15R alpha's ability to present IL-15 in trans to low-affinity IL-15R beta gamma(c) receptors on memory CD8(+) T cells. These unexpected results provide insights into how IL-15R alpha supports memory CD8(+) T cells.

Determinants of HIV-specific CD8 T-cell responses in HIV-infected pediatric patients and enhancement of HIV-gag-specific responses with exogenous IL-15

Cellular immune responses play a central role in controlling HIV-1 infection. HIV-specific IFN-gamma production by CD8 T cells was evaluated in 17 HLA-A2+ HIV-infected pediatric patients (age range 1 month to 16 years) in an ELISPOT assay. Most patients (15/17) exhibited responses to HIV-gag, followed by responses to envelope gp120, gp41, and V3 loop. Only 7 patients responded to all four antigenic peptides. Treatment-related immune reconstitution of CD4 T cells was associated with increase in gag-specific responses, but these declined with prolonged viral suppression. Exogenous IL-15 resulted in augmentation of HIV-gag-specific response in 71% of patients, while IL-2 and IL-7 had variable effects, augmenting responses in 25% patients. Thus, HIV-specific CD8 T-cell responses are dependent on both CD4 T-cell help and antigenic stimulation. The cytokine IL-15 may be a useful modality as adjunctive therapy to augment HIV-specific memory CD8 T cells.

T cell fitness determined by signal strength

Two potential outcomes confront proliferating antigen-stimulated naive T cells: differentiation to effector and memory cells, or deletion. How stimulation affects cell fate is unclear. Autonomous CD8+ T cell differentiation has been proposed, but this does not explain the abortive proliferation of T cells induced by immature dendritic cells. Here we show that human and mouse CD4+ and CD8+ T cells receiving short or weak stimulation of the T cell receptor proliferate in response to interleukin 2 (IL-2) but are not 'fit' because they die by neglect, fail to proliferate in response to IL-7 and IL-15 and disappear in vivo. Conversely, prolonged or strong stimulation promotes 'fitness' by enhancing survival and cytokine responsiveness. Our results are consistent with the concept that signal strength drives progressive T cell differentiation and the acquisition of fitness.

Expression of CD57 defines replicative senescence and antigen-induced apoptotic death of CD8+ T cells

Virus-specific CD8(+) T-cell responses play a pivotal role in limiting viral replication. Alterations in these responses, such as decreased cytolytic function, inappropriate maturation, and limited proliferative ability could reduce their ability to control viral replication. Here, we report on the capacity of HIV-specific CD8(+) T cells to secrete cytokines and proliferate in response to HIV antigen stimulation. We find that a large proportion of HIV-specific CD8(+) T cells that produce cytokines in response to cognate antigen are unable to divide and die during a 48-hour in vitro culture. This lack of proliferative ability of HIV-specific CD8(+) T cells is defined by surface expression of CD57 but not by absence of CD28 or CCR7. This inability to proliferate in response to antigen cannot be overcome by exogenous interleukin-2 (IL-2) or IL-15. Furthermore, CD57 expression on CD8(+) T cells, CD4(+) T cells, and NK cells is a general marker of proliferative inability, a history of more cell divisions, and short telomeres. We suggest, therefore, that the increase in CD57(+) HIV-specific CD8(+) T cells results from chronic antigen stimulation that is a hallmark of HIV infection. Thus, our studies define a phenotype associated with replicative senescence in HIV-specific CD8(+) T cells, which may have broad implications to other conditions associated with chronic antigenic stimulation.

Oligoclonal expansions of antigen-specific CD8+ T cells in aged mice

Oligoclonal T cell expansions (TCE) are common in old humans and mice. It is not known whether an Ag-specific response becomes more oligoclonal with age, and, if so, how this might alter biological responses or compromise the immune response, thus contributing to the immunodeficiency of aging. We used a tumor antigen response to study these questions. Early on, antigen reactive T cell numbers at the site of tumor injection were lower and clonally more restricted in old mice. Subsequently, long-term oligoclonal TCE emerged in the blood and spleen of old mice. IL-15 was not necessary for development of TCE in the blood. Overall, the data pointed to a dysregulated immune response in old mice, perhaps due to lack of optimal IL-2 and CD4 help at the earliest stages and a lack of an efficient local peritoneal CTL response. This was associated with a deficient humoral response and, likely, persistence of tumor cells or tumor antigens. Perhaps the spleen is the site of persistence which explains clonal TCE observed primarily in PBL and spleen. The TCE appear to be inefficient as they are often anergic. As a result an occasional peritoneal or splenic tumor may arise in old mice.

Cytokine control of memory T-cell development and survival

Evidence has accumulated that cytokines have a fundamental role in the differentiation of memory T cells. Here, we follow the CD8+ T cell from initial activation to memory-cell generation, indicating the checkpoints at which cytokines determine the fate of the T cell. Members of the common cytokine-receptor gamma-chain (gammac)-cytokine family--in particular, interleukin-7 (IL-7) and IL-15--act at each stage of the immune response to promote proliferation and survival. In this manner, a stable and protective, long-lived memory CD8+ T-cell pool can be propagated and maintained.

Coadministration of HIV vaccine vectors with vaccinia viruses expressing IL-15 but not IL-2 induces long-lasting cellular immunity

Vaccine efficacy is determined largely by cellular and humoral immunity as well as long-lasting immunological memory. IL-2 and IL-15 were evaluated in vaccinia vectors expressing HIV gp160 for the establishment of an effective vaccine strategy. Both IL-2 and IL-15 in the vaccinia vector induced strong and long-lasting antibody-mediated immunity as well as a short-term cytotoxic T cell response against HIV gp120. In addition, IL-15 also supported robust CD8+ T cell-mediated long-term immunity, whereas the CD8+ T cell-mediated immunity induced by IL-2 was short-lived. Moreover, we found that the cytokine milieu at the time of priming had surprisingly persistent effects on the character of the memory CD8 T cells long afterward with respect to their fate, functional activities, cytokine receptor expression, and antigen-independent proliferation.

Turnover of memory-phenotype CD8+ T cells

Memory-phenotype (CD44(hi)) T cells are presumed to represent the long-lived progeny of T cells responding to various environmental antigens. For CD8+ T cells, the background rate of proliferation (turnover) of memory-phenotype cells is increased following exposure to infectious agents. This increase in turnover is controlled by interferons (IFN-I and IFN-gamma) and is mediated by IL-15. Unlike IFNs, IL-15 is directly stimulatory for CD44(hi) CD8+ cells. In addition to controlling proliferation of these cells, IL-15 may also play a vital role in keeping CD44(hi) CD8+ cells alive.

Memory CD8+ T cells provide an early source of IFN-gamma

During the non-Ag-specific early phase of infection, IFN-gamma is believed to be primarily provided by NK and NKT cells in response to pathogen-derived inflammatory mediators. To test whether other cell types were involved in early IFN-gamma release, IFN-gamma-producing cells were visualized in spleens and lymph nodes of LPS-injected mice. In addition to NK and NKT cells, IFN-gamma was also detected in a significant fraction of CD8(+) T cells. CD8(+) T cells represented the second major population of IFN-gamma-producing cells in the spleen ( approximately 30%) and the majority of IFN-gamma(+) cells in the lymph nodes ( approximately 70%). LPS-induced IFN-gamma production by CD8(+) T cells was MHC class I independent and was restricted to CD44(high) (memory phenotype) cells. Experiments performed with C3H/HeJ (LPS-nonresponder) mice suggested that CD8(+) T cells responded to LPS indirectly through macrophage/dendritic cell-derived IFN-alpha/beta, IL-12, and IL-18. IFN-gamma was also detected in memory CD8(+) T cells from mice injected with type I IFN or with poly(I:C), a synthetic dsRNA that mimics early activation by RNA viruses. Taken together, these results suggest that in response to bacterial and viral products, memory T cells may contribute to innate immunity by providing an early non-Ag-specific source of IFN-gamma.

Studies on the production of IL-15 in HIV-infected/AIDS patients

IL-15 is essential for the development and differentiation of NK cells. It selectively induces proliferation of CD8+ memory T lymphocytes. Despite its importance in both innate and adaptive immune responses, little is known about its production in HIV-infected persons. We report here that IL-15 levels are significantly decreased in the sera of HIV-infected/AIDS patients compared to control sera. We also show that PBMC from the infected patients are compromised in their ability to respond with enhanced production of IL-15 upon exposure to HSV-1. The decreased production of IL-15 occurs despite a comparable increase in IL-15 mRNA in the PBMC of HIV-infected and healthy HIV-seronegative donors when exposed to HSV-1. The HSV-stimulated patients' PBMC exhibited less NK activity compared to similarly treated normal PBMC. These results suggest that a compromised ability of PBMC from HIV-infected individuals to induce IL-15 production in response to a viral stimulus may be a reason of their compromised innate and adaptive immunity.

Regulation of the generation and maintenance of T-cell memory: a direct, default pathway from effectors to memory cells

Memory T cells are derived directly from effector cells without need for additional antigen, TcR triggering or induced cytokines. A large fraction of effectors can become memory cells without division, supporting a default pathway with little further differentiation. This suggests that the same signals during infection/vaccination determine the extent and nature of both effector and memory cell development.

Lineage relationship and protective immunity of memory CD8 T cell subsets

Memory CD8 T cells can be divided into two subsets, central (T(CM)) and effector (T(EM)), but their lineage relationships and their ability to persist and confer protective immunity are not well understood. Our results show that T(CM) have a greater capacity than T(EM) to persist in vivo and are more efficient in mediating protective immunity because of their increased proliferative potential. We also demonstrate that, following antigen clearance, T(EM) convert to T(CM) and that the duration of this differentiation is programmed within the first week after immunization. We propose that T(CM) and T(EM) do not necessarily represent distinct subsets, but are part of a continuum in a linear naive --> effector --> T(EM) --> T(CM) differentiation pathway.

Eradication of systemic B-cell tumors by genetically targeted human T lymphocytes co-stimulated by CD80 and interleukin-15

The genetic transfer of antigen receptors provides a means to rapidly generate autologous tumor-reactive T lymphocytes. However, recognition of tumor antigens by cytotoxic T cells is only one step towards effective cancer immunotherapy. Other crucial biological prerequisites must be fulfilled to expand tumor-reactive T cells that retain a functional phenotype, including in vivo cytolytic activity and the ability to travel to tumor sites without prematurely succumbing to apoptosis. We show that these requirements are met by expanding peripheral blood T cells genetically targeted to the CD19 antigen in the presence of CD80 and interleukin-15 (IL-15). T cells expanded in the presence of IL-15 uniquely persist in tumor-bearing severe combined immunodeficiency (SCID)-Beige mice and eradicate disseminated intramedullary tumors. Their anti-tumor activity is further enhanced by in vivo co-stimulation. In addition, transduced T cells from patients with chronic lymphocytic leukemia (CLL) effectively lyse autologous tumor cells. These findings strongly support the clinical feasibility of this therapeutic strategy.

Tubulitis in renal allograft rejection: role of transforming growth factor-beta and interleukin-15 in development and maintenance of CD103+ intraepithelial T cells

BACKGROUND

Renal tubules normally show no lymphocyte infiltration, but tubulitis is a feature of renal allograft rejection with many intratubular T cells expressing CD8 and CD103 (the alphaEbeta7 integrin). We investigated the development and maintenance of allospecific CD103 T cells within the tubular microenvironment.

METHODS

Mixed lymphocyte cultures were supplemented with transforming growth factor (TGF)-beta1 to model the expression and function of CD103 observed in situ on intratubular lymphocytes. Immunocytochemical techniques were used to identify cells coexpressing CD8 and interleukin (IL)-15Ralpha, to enumerate proliferating intratubular T cells, and to quantify IL-15 expression within the tubules of control and rejection-graded transplant biopsy specimens. These results were compared with a parallel analysis of the phenotype and proliferation of allospecific T cells expanded in vitro in the presence of TGF-beta1 and IL-15.

RESULTS

TGF-beta1 only induced the expression of adhesive CD103 after at least one cycle of alloantigen-specific cell division in vitro. In the renal allograft, a similar proportion of intratubular T cells was observed to proliferate during and after acute rejection. Tubular epithelial cells expressed IL-15 constitutively, whereas intratubular CD8 T cells expressed IL-15 receptor alpha. IL-15 and TGF-beta1 synergized to promote expansion and survival of allospecific CD8 CD103 T cells in vitro, but IL-15 down-regulated perforin expression.

CONCLUSIONS

These results suggest that activated, allospecific CD8 T cells are recruited to tubules during acute rejection where they encounter TGF-beta, up-regulate CD103 expression, and bind E-cadherin. A proportion of these cells proliferates and is maintained in a state of low perforin expression by the combined action of TGF-beta and IL-15.

H2-M3-restricted memory T cells: persistence and activation without expansion

H2-M3-restricted T cells respond more rapidly to primary Listeria monocytogenes infection than conventional MHC class Ia-restricted T cells. Reinfection with L. monocytogenes, while inducing explosive proliferation of H2-K(d)-restricted T cells, does not stimulate significant expansion of H2-M3-restricted CTL. These disparate responses to reinfection are apparent within 5 days of primary L. monocytogenes infection. However, H2-M3-restricted memory T cells are generated, and are indistinguishable from classically restricted T cells in terms of cell surface memory markers and longevity. Early responses of H2-M3- and H2-K(d)-restricted memory T cells to reinfection are similar, with increases in size and expression of activation markers. Interestingly, priming of H2-M3-restricted T cells with an L. monocytogenes-derived N-formyl peptide plus anti-CD40 generates memory T cells that expand upon re-exposure to Ag during L. monocytogenes infection. Our data indicate that disparate H2-M3- and MHC class Ia-restricted memory T cell responses reflect intrinsic differences between these T cell populations. Although distinct proliferative programs appear to be hardwired in these populations during primary L. monocytogenes infection, under different inflammatory circumstances M3-restricted T cell populations can maintain the ability to expand upon re-exposure to Ag.

The crystal structure of a TL/CD8alphaalpha complex at 2.1 A resolution: implications for modulation of T cell activation and memory

TL is a nonclassical MHC class I molecule that modulates T cell activation through relatively high-affinity interaction with CD8alphaalpha. To investigate how the TL/CD8alphaalpha interaction influences TCR signaling, we characterized the structure of the TL/CD8alphaalpha complex using X-ray crystallography. Unlike antigen-presenting molecules, the TL antigen-binding groove is occluded by specific conformational changes. This feature eliminates antigen presentation, severely hampers direct TCR recognition, and prevents TL from participating in the TCR activation complex. At the same time, the TL/CD8alphaalpha interaction is strengthened through subtle structure changes in the TL alpha3 domain. Thus, TL functions to sequester and redirect CD8alphaalpha away from the TCR, modifying lck-dependent signaling.

Pathways to a robust immune response in the elderly

Circumstantial evidence suggests that infectious disease is the major cause of morbidity and mortality in the elderly, and immune-system dysfunction may contribute to this finding. Because innate and humoral immunity seem to be relatively unaffected by aging and because the T-cell compartment shows marked age-associated alterations, this article focuses on the association between T cells and aging. Longitudinal studies suggest that immune parameters, which predominantly are related to T cells, can be clustered to yield an IRP that is predictive of mortality in the elderly. Determining the IRP also may be helpful in younger individuals, particularly those under chronic antigenic stress (eg, patients with cancer or chronic infections) who experience premature aging of the immune system. Some changes in T cells can be modeled in clonal cultures in vitro to discover new biomarkers of immune aging. These biomarkers, which need to be validated in vivo, could be used to refine IRP. Interventions to selectively target changes that are identified as part of IRP may improve the health and quality of life of the elderly, reduce healthcare costs, and avoid potential unwanted side effects of global intervention approaches, such as triggering or exacerbating autoimmunity and inflammation.

Hide, shield and strike back: how HIV-infected cells avoid immune eradication

Viruses that induce chronic infections can evade immune responses. HIV is a prototype of this class of pathogen. Not only does it mutate rapidly and make its surface components difficult to access by neutralizing antibodies, but it also creates cellular hideouts, establishes proviral latency, removes cell-surface receptors and destroys immune effectors to escape eradication. A better understanding of these strategies might lead to new approaches in the fight against AIDS.

Origin of late-onset autoimmune disease

Autoimmune disease in the elderly is hypothesized to be caused by an imbalance in T-cell expansion and deletion after an encounter with self-antigens. A decrease in thymic output leads to a decreased pool of naive T cells in the periphery and to increased oligoclonal expansion of T cells. This expansion may be caused by stimulation with autoantigens that drive high-affinity interactions with self-antigens. Accumulation of presenescent, apoptosis-resistant, and proinflammatory T cells results in the growth of these autoreactive T cells. A decreased T-cell activation response that occurs with age leads to several defects that diminish the immune response.

IL-15 enhances survival and function of HIV-specific CD8+ T cells

HIV-specific CD8(+) T cells are prone to undergo apoptosis, and this may affect their ability to control HIV infection. Because CD8-mediated immune responses play a key role in controlling HIV infection, enhancing the survival and effector function of HIV-specific CD8(+) T cells may augment their ability to control HIV virus. We show here that interleukin 15 (IL-15) potently inhibits spontaneous and CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells. IL-15 inhibits apoptosis in both CD45RA(-)CD62L(-) and CD45RA(+)CD62L(-) effector memory subpopulations of these cells. Furthermore, IL-15 greatly enhances the survival of HIV-specific CD8(+) T cells in long-term cultures. Finally, IL-15 directly enhances activation, interferon gamma (IFNgamma) production, and direct ex vivo cytotoxicity of HIV-specific CD8(+) T cells. Thus, IL-15 potently enhances the survival and effector function of HIV-specific CD8(+) T cells and, therefore, may prove useful in augmenting the antiviral function of these cells.

Cytokines and T cell homeostasis

In recent years it has become apparent that the long-term survival of T cells requires continuous contact with external stimuli. At least two types of stimuli, namely self antigens and cytokines, are involved in maintaining T cell viability. As discussed here, the factors controlling T cell survival and turnover in vivo differ considerably from one T cell subset to another.

Memory T cells and vaccines

T lymphocytes play a central role in the generation of a protective immune response in many microbial infections. After immunization, dendritic cells take up microbial antigens and traffic to draining lymph nodes where they present processed antigens to naïve T cells. These naïve T cells are stimulated to proliferate and differentiate into effector and memory T cells. Activated, effector and memory T cells provide B cell help in the lymph nodes and traffic to sites of infection where they secrete anti-microbial cytokines and kill infected cells. At least two types of memory cells have been defined in humans based on their functional and migratory properties. T central-memory (T(CM)) cells are found predominantly in lymphoid organs and can not be immediately activated, whereas T effector-memory (T(EM)) cells are found predominantly in peripheral tissue and sites of inflammation and exhibit rapid effector function. Most currently licensed vaccines induce antibody responses capable of mediating long-term protection against lytic viruses such as influenza and small pox. In contrast, vaccines against chronic pathogens that require cell-mediated immune responses to control, such as malaria, Mycobacterium tuberculosis (TB), human immunodeficiency virus (HIV) and hepatitis C virus (HCV), are currently not available or are ineffective. Understanding the mechanisms by which long-lived cellular immune responses are generated following vaccination should facilitate the development of safe and effective vaccines against these emerging diseases. Here, we review the current literature with respect to memory T cells and their implications to vaccine development.

Tissue distribution of target antigen has a decisive influence on the outcome of adoptive cancer immunotherapy

Adoptive transfer of allogeneic T cells has unmatched efficacy to eradicate leukemic cells. We therefore sought to evaluate in kinetic terms interactions between T cells and allogeneic leukemic cells. T cells primed against the model B6(dom1) minor histocompatibility antigen were adoptively transferred in irradiated B10 (B6(dom1)-positive) and congenic B10.H7(b) (B6(dom1)-negative) recipients, some of which were also injected with EL4 leukemia/lymphoma cells (B6(dom1)-positive). A key finding was that the tissue distribution of the target epitope dramatically influenced the outcome of adoptive cancer immunotherapy. Widespread expression of B6(dom1) in B10 recipients induced apoptosis and dysfunction of antigen-specific T cells. Furthermore, in leukemic B10 and B10.H7(b) hosts, a massive accumulation of effector/memory B6(dom1)-specific T cells was detected in the bone marrow, the main site of EL4 cell growth. The accumulation of effector/memory cells in recipient bone marrow was EL4 dependent, and its kinetics was different from that observed in recipient spleen. We conclude that strategies must be devised to prevent apoptosis of adoptively transferred T cells confronted with a high antigen load and that local monitoring of the immune response at the site of tumor growth may be mandatory for a meaningful assessment of the efficacy of adoptive immunotherapy.

CD28 costimulation and immunoaffinity-based selection efficiently generate primary gene-modified T cells for adoptive immunotherapy

The introduction of an inducible suicide gene has been proposed as a strategy to exploit the antitumor reactivity of donor T cells after allogeneic hematopoietic stem cell transplantation but permit control of graft-versus-host disease. However, there are several obstacles to this approach that may impair the ability of T cells to function and survive in vivo. These include the requirement for in vitro activation or long-term culture to introduce the transgene and obtain therapeutic cell numbers, the toxicity of drug selection to enrich transduced cells, and the immunogenicity of the transgene-encoded products. Here we have developed a transduction and selection strategy for generating large numbers of polyclonal T cells transduced with a retroviral vector encoding the human low-affinity nerve growth factor receptor (LNGFR) for selection and a Fas-based suicide construct (LV'VFas). Ligation of CD28 in conjunction with a T-cell receptor signal permitted efficient transduction, substantially promoted T-cell growth, and contributed to the generation of gene-modified T cells that retained clonal diversity, functional properties, and a homing receptor profile similar to untransduced peripheral blood lymphocytes. Microbeads conjugated directly to antibody specific to LNGFR significantly improved the immunomagnetic selection of LV'VFas-modified T cells and assisted in scaling of the selection procedure to therapeutic cell numbers. Thus, these studies identified a strategy that requires only a brief ex vivo culture and does not use drug selection to obtain large numbers of functional gene-modified polyclonal T cells that can be used for adoptive immunotherapy.

Cutting edge: homeostatic proliferation of peripheral T lymphocytes is regulated by clonal competition

Homeostatic proliferation functions to maintain peripheral T cell numbers and is regulated by cytokines. In this study, we provide evidence that T cell homeostasis is also regulated by clonal competition. Naive polyclonal T cells divided when transferred to TCR transgenic hosts, as did monoclonal T cells when transferred to TCR transgenic hosts of differing clonotype. However, T cells did not divide in hosts of identical clono-type. Transgenic T cell proliferation was inhibited in irradiated hosts of the same clonotype, while cotransferred nontransgenic T cells proliferated extensively. These results show that clonal competition is a component of homeostasis that may contribute to selection of the peripheral T cell repertoire.

Post-transplant renal tubulitis: the recruitment, differentiation and persistence of intra-epithelial T cells

Tubulitis is used by the Banff protocol as a major criterion to grade acute renal allograft rejection. This review integrates results from in vitro and in vivo studies to develop a chronological model to explain the development and functions of tubular inflammation during the rejection process. Proteoglycan-immobilized chemokines are the primary motivators for the vectorial recruitment of specific immune cell populations from the blood, through the endothelium and interstitial tissues to the renal tubules. After penetration of the basement membrane, T cells encounter TGF-beta that can induce expression of the alphaEbeta7 integrin on proliferating cells. This allows adhesion to E-cadherin on the baso-lateral surfaces of tubular epithelial cells and provides an explanation for the epithelial-specific cytotoxicity observed during acute rejection. Tubular epithelium is also a rich source of IL-15 that can stimulate IL-15 receptor-expressing intratubular CD8+ T cells. This anti-apoptotic microenvironment may explain the long-term persistence of cycling T cells within intact tubules after episodes of acute rejection. These memory-like T cells may have local immunoregulatory properties, including the production of additional TGF-beta, but could also modify normal tubular homeostasis resulting in epithelial to mesenchymal transdifferentiation, tubulointerstitial fibrosis and, ultimately, graft failure.

Efficient induction of primary and secondary T cell-dependent immune responses in vivo in the absence of functional IL-2 and IL-15 receptors

IL-2 and IL-15 are thought to be important cytokines for T cell-dependent immune responses. Mice deficient in IL-2, IL-2Ralpha, and IL-2Rbeta are each characterized by a rapid lethal autoimmune lymphoproliferative disorder that complicates their use in studies aimed at investigating the role of these cytokines and receptors for immune responses in vivo. We have previously characterized a novel transgenic (Tg) mouse on the IL-2Rbeta-/- genetic background (Tg-/- mice) that lacks autoimmune disease but still contains peripheral T cells that are nonresponsive to IL-2 and IL-15. In the present study, these mice were used to investigate the extent by which IL-2 and IL-15 are essential for T cell immunity in vivo. Tg-/- mice generated near normal primary and secondary Ab responses to OVA, readily mounted first and second set allogeneic skin graft rejection responses, and developed primary and recall CD8 T cell responses to vaccinia virus. However, Tg-/- mice generated a slightly lower level of IgG2a Abs to OVA, exhibited a somewhat delayed first set skin graft rejection response with lower allo-specific CTL, and developed a significantly lower number of IFN-gamma-producing vaccinia-specific CD8+ T cells. Thus, although T effector function is somewhat impaired, T cell immunity is largely functional in the absence of IL-2- and IL-15-induced signaling through IL-2Rbeta.