Linear differentiation of cytotoxic effectors into memory T lymphocytes

Increasing the CD4+ T cell precursor frequency leads to competition for IFN-gamma thereby degrading memory cell quantity and quality

The precursor frequency of naive CD4(+) T cells shows an inverse relationship with the number of memory cells generated after exposure to cognate Ag. Using the lymphocytic choriomeningitis virus (LCMV) model, we show here that only when the initial number of naive virus-specific CD4(+) T cell precursors is low (< or =10(4) per spleen) do they give rise to abundant and homogeneous memory cells that are CD62L(low), IL-7R(high), and imbued with an enhanced capacity to produce cytokine, proliferate, and survive over time. Furthermore, memory cells derived from a high naive precursor number show functional deficits upon secondary exposure to virus. The negative effect of higher naive precursor frequency was not attributable to competition for limiting amounts of Ag, because LCMV-naive CD4(+) TCR-transgenic CD4 T cells were recruited into the LCMV-induced response even when their initial number was high. Instead, the T cells appear to compete for direct IFN-gamma signals as they differentiate into memory cells. These results are consistent with a model of T cell development in which the most fit effector T cells that receive sufficient direct IFN-gamma signals are selected to differentiate further into memory cells.

T lymphocyte engineering ex vivo for cancer and infectious disease

BACKGROUND

Basic research contributions towards the molecular and cellular understanding of immune mediated control of cancer and infectious diseases have created opportunities to develop new forms of T-cell-based vaccination for cancer and chronic infections like HIV. In the past two decades, there has been a dramatic increase in the number of cell therapy clinical trials around the world aimed at enhancing antitumor immunity, restoring immune function to infectious diseases and augmenting vaccine efficacy.

OBJECTIVE

To provide a review of new and emerging methods of T lymphocyte engineering, gene transfer to T cells and clinical trials.

METHODS

A review of recent clinical trials, along with a brief historical perspective, with a focus on challenges and recent advances in the field and requirements for successful T-cell therapies.

CONCLUSION

Advances in the technological approaches and methods for ex vivo manipulation have led to T lymphocytes endowed with enhanced potency and unique functions, with promise as the new generation of infused therapeutics.

Influencing the fates of CD4 T cells on the path to memory: lessons from influenza

In the face of emerging infectious diseases caused by rapidly evolving and highly virulent pathogens, such as influenza, we are challenged to develop innovative vaccine strategies that can induce lasting protection. Since CD4 T cells are needed to generate and maintain protective B-cell and CD8 T-cell immunity, and can also mediate additional protective mechanisms, vaccines should ideally elicit efficient CD4 T cell, in addition to CD8 T and B-cell responses. We outline here the process of CD4 T-cell differentiation from naïve to effector and from effector to memory with an emphasis on how exposure to microbial products and variables in antigen presentation can impact the functional quality and heterogeneity of activation-based CD4 T-cell subsets in vitro and in vivo. We discuss the impact of different phases of antigen recognition, the inflammatory milieu, acute versus chronic antigen presentation, and the contribution of residual antigen depots on CD4 T-cell effector differentiation and the formation and maintenance of CD4 T-cell memory. We propose that novel vaccine strategies, which incorporate both microbial products and antigen targeting, may provide a flexible and long-lived memory CD4 T-cell pool.

Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates

An important question in memory development is understanding the differences between effector CD8 T cells that die versus effector cells that survive and give rise to memory cells. In this study, we provide a comprehensive phenotypic, functional, and genomic profiling of terminal effectors and memory precursors. Using killer cell lectin-like receptor G1 as a marker to distinguish these effector subsets, we found that despite their diverse cell fates, both subsets possessed remarkably similar gene expression profiles and functioned as equally potent killer cells. However, only the memory precursors were capable of making interleukin (IL) 2, thus defining a novel effector cell that was cytotoxic, expressed granzyme B, and produced inflammatory cytokines in addition to IL-2. This effector population then differentiated into long-lived protective memory T cells capable of self-renewal and rapid recall responses. Experiments to understand the signals that regulate the generation of terminal effectors versus memory precursors showed that cells that continued to receive antigenic stimulation during the later stages of infection were more likely to become terminal effectors. Importantly, curtailing antigenic stimulation toward the tail end of the acute infection enhanced the generation of memory cells. These studies support the decreasing potential model of memory differentiation and show that the duration of antigenic stimulation is a critical regulator of memory formation.

Effector CD8 T cell development: a balancing act between memory cell potential and terminal differentiation

Immune responses to infection are optimally designed to generate large numbers of effector T cells while simultaneously minimizing the collateral damage of their potentially lethal actions and generating memory T cells to protect against subsequent encounter with pathogens. Much remains to be discovered about how these equally essential processes are balanced to enhance health and longevity and, more specifically, what factors control effector T cell expansion, differentiation, and memory cell formation. The innate immune system plays a prominent role in the delicate balance of these decisions. Insights into these questions from recent work in the area of effector CD8 T cell differentiation will be discussed.

Proliferative expansion and acquisition of effector activity by memory CD4+ T cells in the lungs following pulmonary virus infection

The memory CD4+ T cell response to the respiratory syncytial virus (RSV) attachment (G) protein in the lungs of primed BALB/c mice undergoing challenge pulmonary RSV infection is dominated by effector T cells expressing a single Vbeta-chain, Vbeta14. We have used Vbeta14 expression to examine the kinetics of the activation, accumulation, and acquisition of the effector activity of memory CD4+ T cells responding to pulmonary infection. This analysis revealed that proliferative expansion and effector CD4+ T cell differentiation preferentially occur in the respiratory tract following rapid activation within and egress from the lymph nodes draining the respiratory tract. These findings suggest that, in response to natural infection at a peripheral mucosal site such as the lungs, memory CD4+ T cell expansion and differentiation into activated effector T cells may occur predominantly in the peripheral site of infection rather than exclusively in the lymph nodes draining the site of infection.

Differential impact of the transcriptional repressor Gfi1 on mature CD4+ and CD8+ T lymphocyte function

The transcriptional repressor Gfi1 is a nuclear zinc-finger protein that is expressed in T cell precursors in the thymus, but is down-regulated in mature, resting T cells. Gfi1 expression rises transiently to levels seen in thymocytes upon antigenic activation. We show here that lack of Gfi1 causes delayed cell cycle entry and apoptosis after antigenic stimulation in both mature CD4+ and CD8+ T cells ex vivo. DNA micro-array analysis demonstrated that this correlated with an up-regulation of the death receptor CD95, the proapoptotic factors Bad and Apaf1 and the cell cycle inhibitor p21, and a down-regulation of Bcl-2 expression in Gfi1-/- T cells. Surprisingly, while Gfi1-deficient CD4+ T cells showed the same defective behavior in vivo, Gfi1-deficient CD8+ T cells showed no aberration in vivo and were fully able to mount an anti-viral immune response. This indicates that Gfi1 exerts different functions in CD4+ and CD8+ T cells very likely by maintaining different genetic programs in both cell types, and appears to be essential for the CD4 helper T cell immune response but dispensable for the function of cytotoxic CD8+ T cells.

Long-lived virus-reactive memory T cells generated from purified cytokine-secreting T helper type 1 and type 2 effectors

Many vaccination strategies and immune cell therapies aim at increasing the numbers of memory T cells reactive to protective antigens. However, the differentiation lineage and therefore the optimal generation conditions of CD4 memory cells remain controversial. Linear and divergent differentiation models have been proposed, suggesting CD4 memory T cell development from naive precursors either with or without an effector-stage intermediate, respectively. Here, we address this question by using newly available techniques for the identification and isolation of effector T cells secreting effector cytokines. In adoptive cell transfers into normal, nonlymphopenic mice, we show that long-lived virus-specific memory T cells can efficiently be generated from purified interferon γ–secreting T helper (Th) type 1 and interleukin (IL)-4– or IL-10–secreting Th2 effectors primed in vitro or in vivo. Importantly, such effector-derived memory T cells were functional in viral challenge infections. They proliferated vigorously, rapidly modulated IL-7 receptor expression, exhibited partial stability and flexibility of their cytokine patterns, and exerted differential effects on virus-induced immunopathology. Thus, cytokine-secreting effectors can evade activation-induced cell death and develop into long-lived functional memory cells. These findings demonstrate the efficiency of linear memory T cell differentiation and encourage the design of vaccines and immune cell therapies based on differentiated effector T cells.

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.

Discriminating between different pathways of memory CD8+ T cell differentiation

Despite the rapid accumulation of quantitative data on the dynamics of CD8(+) T cell responses following acute viral or bacterial infections of mice, the pathways of differentiation of naive CD8(+) T cells into memory during an immune response remain controversial. Currently, three models have been proposed. In the "stem cell-associated differentiation" model, following activation, naive T cells differentiate into stem cell-like memory cells, which then convert into terminally differentiated short-lived effector cells. In the "linear differentiation" model, following activation, naive T cells first differentiate into effectors, and after Ag clearance, effectors convert into memory cells. Finally, in the "progressive differentiation" model, naive T cells differentiate into memory or effector cells depending on the amount of specific stimulation received, with weaker stimulation resulting in formation of memory cells. This study investigates whether the mathematical models formulated from these hypotheses are consistent with the data on the dynamics of the CD8(+) T cell response to lymphocytic choriomeningitis virus during acute infection of mice. Findings indicate that two models, the stem cell-associated differentiation model and the progressive differentiation model, in which differentiation of cells is strongly linked to the number of cell divisions, fail to describe the data at biologically reasonable parameter values. This work suggests additional experimental tests that may allow for further discrimination between different models of CD8(+) T cell differentiation in acute infections.

Duration of antigen expression in vivo following DNA immunization modifies the magnitude, contraction, and secondary responses of CD8+ T lymphocytes

The duration of Ag expression in vivo has been reported to have a minimal impact on both the magnitude and kinetics of contraction of a pathogen-induced CD8(+) T cell response. In this study, we controlled the duration of Ag expression by excising the ear pinnae following intradermal ear pinnae DNA immunization. This resulted in decreased magnitude, accelerated contraction and differentiation, and surprisingly greater secondary CD8(+) T cell responses. Furthermore, we found delayed and prolonged Ag presentation in the immunized mice; however, this presentation was considerably decreased when the depot Ag was eliminated. These findings suggest that the magnitude and the contraction phase of the CD8(+) T cell response following intradermal DNA immunization is regulated by the duration rather than the initial exposure to Ag.

IL-2 regulates expansion of CD4+ T cell populations by affecting cell death: insights from modeling CFSE data

It is generally accepted that IL-2 influences the dynamics of populations of T cells in vitro and in vivo. However, which parameters for cell division and/or death are affected by IL-2 is not well understood. To get better insights into the potential ways of how IL-2 may influence the population dynamics of T cells, we analyze data on the dynamics of CFSE-labeled polyclonal CD4(+) T lymphocytes in vitro after anti-CD3 stimulation at different concentrations of exogenous IL-2. Inferring cell division and death rates from CFSE-delabeling experiments is not straightforward and requires the use of mathematical models. We find that to adequately describe the dynamics of T cells at low concentrations of exogenous IL-2, the death rate of divided cells has to increase with the number of divisions cells have undergone. IL-2 hardly affects the average interdivision time. At low IL-2 concentrations 1) fewer cells are recruited into the response and successfully complete their first division; 2) the stochasticity of cell division is increased; and 3) the rate, at which the death rate increases with the division number, increases. Summarizing, our mathematical reinterpretation suggests that the main effect of IL-2 on the in vitro dynamics of naive CD4(+) T cells occurs by affecting the rate of cell death and not by changing the rate of cell division.

Viral vector vaccines make memory T cells against malaria

Vaccines that comprise attenuated viral vectors encoding antigens from target pathogens generate potent T-cell responses. One such pathogen is malaria, and in particular the liver stage of its life cycle. Immunogenicity and efficacy studies in animals and humans have revealed the generation of memory T cells of both the central and effector phenotypes, depending on the viral vectors used in the malaria vaccination regime (viral species and serotype, combination and sequence for prime-boost) and suggest a divergence in their protective role. Being able to influence the memory T-cell make-up in a rational manner may allow us to develop more efficacious vaccines.

Differential survival of cytotoxic T cells and memory cell precursors

It is widely assumed that the development of memory CD8 T cells requires the escape of CTLs from programmed cell death. We show in this study that although serine protease inhibitor 6 (Spi6) is required to protect clonal bursts of CTLs from granzyme B-induced programmed cell death, it is not required for the development of memory cells. This conclusion is reached because memory cell precursors down-regulate both Spi6 and granzyme B, unlike CTLs, and they do not require Spi6 for survival. These findings suggest that memory CD8 T cells are derived from progenitors that are refractory to self-inflicted damage, rather than derived from fully differentiated CTLs.

CD8 single-cell gene coexpression reveals three different effector types present at distinct phases of the immune response

To study in vivo CD8 T cell differentiation, we quantified the coexpression of multiple genes in single cells throughout immune responses. After in vitro activation, CD8 T cells rapidly express effector molecules and cease their expression when the antigen is removed. Gene behavior after in vivo activation, in contrast, was quite heterogeneous. Different mRNAs were induced at very different time points of the response, were transcribed during different time periods, and could decline or persist independently of the antigen load. Consequently, distinct gene coexpression patterns/different cell types were generated at the various phases of the immune responses. During primary stimulation, inflammatory molecules were induced and down-regulated shortly after activation, generating early cells that only mediated inflammation. Cytotoxic T cells were generated at the peak of the primary response, when individual cells simultaneously expressed multiple killer molecules, whereas memory cells lost killer capacity because they no longer coexpressed killer genes. Surprisingly, during secondary responses gene transcription became permanent. Secondary cells recovered after antigen elimination were more efficient killers than cytotoxic T cells present at the peak of the primary response. Thus, primary responses produced two transient effector types. However, after boosting, CD8 T cells differentiate into long-lived killer cells that persist in vivo in the absence of antigen.

Principles of adoptive T cell cancer therapy

The transfusion of T cells, also called adoptive T cell therapy, is an effective treatment for viral infections and has induced regression of cancer in early-stage clinical trials. However, recent advances in cellular immunology and tumor biology are guiding new approaches to adoptive T cell therapy. For example, use of engineered T cells is being tested as a strategy to improve the functions of effector and memory T cells, and manipulation of the host to overcome immunotoxic effects in the tumor microenvironment has led to promising results in early-stage clinical trials. Challenges that face the field and must be addressed before adoptive T cell therapy can be translated into routine clinical practice are discussed.

Temporal cross-talk between TCR and STAT signals for CD8 T cell effector differentiation

The strength and duration of signaling through surface receptors is a primary means of controlling cell fate decisions. In adaptive immunity, Ag-initiated T cell stimulation is secondarily regulated by cytokines. We here summarize evidence for temporal control of a gene expression program in naive CD8 T cells. It is initiated in response to TCR engagement but relies on secondary signaling from cytokine receptors to be sustained and to allow development of full effector capacity. This mechanism permits cytokine receptor signaling to rescue abortive TCR signaling, such as that induced in response to weak or partial TCR agonists. Indeed, limiting TCR-initiated signaling on the Ras/ERK pathway may be complemented by STAT activation. Thus, TCR- and cytokine-driven activation of transcription factors and epigenetic modifications may act in concert in a temporally staggered process to establish the functional program of effector CD8 T cells. Based on gene expression profiling, molecular targets whose activation or inactivation may boost or dampen CD8 T cell effectors are also identified. Manipulation of these targets may, respectively, increase anti-tumor responses or prevent graft-versus-host reactions.

IL-12-programmed long-term CD8+ T cell responses require STAT4

Immunological adjuvants activate innate immune cells for Ag presentation and elicitation of cytokines like IL-12 that promote T cell expansion and effector differentiation. An important but elusive aim for most immunization strategies is to produce memory T cells that provide durable immunity. Recent evidence demonstrates that the context of Ag presentation instructionally programs T cells for short- and long-term responses. However, the role and mechanisms by which cytokines like IL-12 condition CD8 T cells for long-term responses remain relatively uncharacterized. In this study, we show that brief exposure (20 h) of naive TCR-transgenic CD8 cells to IL-12 during Ag stimulation leads to transient phosphorylation of STAT4 for robust effector differentiation. Moreover, the IL-12-induced STAT4 engenders greater clonal expansion of the Ag-activated CD8 cells by regulating the expression of the transcriptional factor Bcl3- and Bcl2-related genes that promote survival of Ag-activated CD8 cells. Remarkably, the IL-12-conditioned CD8 T cells demonstrate increased sensitivity to IL-7 and IL-15, whereby they are rendered "fit" for homeostatic self-renewal as well as augmented CD4-dependent recall responses that are effective at controlling Salmonella infection in vivo. This information provides new insights into mechanisms by which IL-12 conditions CD8 T cells for long-term immunity, which is likely to benefit development of new strategies for the use of IL-12 in infectious diseases and cancer.

T cell memory

T cell memory induced by prior infection or vaccination provides enhanced protection against subsequent microbial infections. The processes involved in generating and maintaining T cell memory are becoming better understood due to recent technological advances in identifying memory T cells and monitoring their behavior and function in vivo. Memory T cells develop in response to a progressive set of cues-starting with signals from antigen-loaded, activated antigen-presenting cells (APCs) and inflammatory mediators induced by the innate immune response, to the poorly defined subsequent signals triggered as the immune response wanes toward homeostasis. The persistence of the resting memory T cells that eventually develop is regulated by cytokines. This chapter discusses recent findings on how memory T cells develop to confer long-term protective immunity.

Not-so-great expectations: re-assessing the essence of T-cell memory

We are often taught that secondary, or memory, responses by lymphocytes are more vigorous than primary responses. An expectation commonly associated with this notion is that the initial encounter with a pathogen should result in immunity to re-infection. Although this outcome is sometimes the case, it is not universally true. In this review, we propose a unified model of T-cell memory to explain the apparent successes and failures of eliciting vaccine-like protection from prior encounters with pathogens. We speculate that memory T cells arise as an invariant consequence of clonal selection during an immune response. The quality of memory T cells, however, seems to vary in the degree to which they have acquired effector characteristics and, thus, their ability to confer immunity to re-infection. Although not all memory T cells possess the embellished attributes of fully developed effector cells, they all seem to share the rudimentary quality of preserving an antigen specificity that has proven itself useful. We suggest that the ability to maintain the integrity of the T-cell repertoire, more than establishing immunity to re-infection, may represent the fundamental form of memory for the adaptive immune system.

CD127+CCR5+CD38+++ CD4+ Th1 effector cells are an early component of the primary immune response to vaccinia virus and precede development of interleukin-2+ memory CD4+ T cells

The stages of development of human antigen-specific CD4+ T cells responding to viral infection and their differentiation into long-term memory cells are not well understood. The inoculation of healthy adults with vaccinia virus presents an opportunity to study these events intensively. Between days 11 and 14 postinoculation, there was a peak of proliferating CCR5+CD38+++ CD4+ effector cells which contained the cytotoxic granule marker T-cell intracellular antigen 1 and included gamma interferon (IFN-gamma)-producing vaccinia virus-specific CD4+ T cells. The majority of these initial vaccinia virus-specific CD4+ T cells were CD127+ and produced interleukin-2 (IL-2) but not CTLA-4 in response to restimulation in vitro. Between days 14 and 21, there was a switch from IFN-gamma and IL-2 coexpression to IL-2 production only, coinciding with a resting phenotype and an increased in vitro proliferation response. The early CCR5+CD38+++ vaccinia virus-specific CD4+ T cells were similar to our previous observations of human immunodeficiency virus (HIV)-specific CD4+ T cells in primary HIV type 1 (HIV-1) infection, but the vaccinia virus-specific cells expressed much more CD127 and IL-2 than we previously found in their HIV-specific counterparts. The current study provides important information on the differentiation of IL-2+ vaccinia virus-specific memory cells, allowing further study of antiviral effector CD4+ T cells in healthy adults and their dysfunction in HIV-1 infection.

Natural variation in HIV infection: Monte Carlo estimates that include CD8 effector cells

Viral load and CD4 T-cell counts in patients infected with the human immunodeficiency virus (HIV) are commonly used to guide clinical decisions regarding drug therapy or to assess therapeutic outcomes in clinical trials. However, random fluctuations in these markers of infection can obscure clinically significant change. We employ a Monte Carlo simulation to investigate contributing factors in the expected variability in CD4 T-cell count and viral load due solely to the stochastic nature of HIV infection. The simulation includes processes that contribute to the variability in HIV infection including CD4 and CD8 T-cell population dynamics as well as T-cell activation and proliferation. The simulation results may reconcile the wide range of variabilities in viral load observed in clinical studies, by quantifying correlations between viral load measurements taken days or weeks apart. The sensitivity of variability in T-cell count and viral load to changes in the lifetimes of CD4 and CD8 T-cells is investigated, as well as the effects of drug therapy.

Unidirectional development of CD8+ central memory T cells into protective Listeria-specific effector memory T cells

Three distinct subsets of antigen-experienced CD8(+) T cells have been identified so far: short-living effector T cells (T(EC)) and two long-living subsets, described as central (T(CM)) and effector memory (T(EM)) T cells. The lineage relationships of these subpopulations as well as their involvement in protection have not yet been conclusively determined. We recently described a novel marker combination (CD127 and CD62L) to identify all three major CD8(+) T cell subsets in mice infected with Listeria monocytogenes (L.m.). Extensive lineage relationship analyses on highly purified subpopulations after in vitro and in vivo stimulation demonstrated that T(CM) can develop into T(EM) or T(EC), whereas T(EM) can only progress to T(EC) cells. Short-living T(EC) never regained a T(EM) or T(CM) phenotype. These data strongly suggest a hierarchical and unidirectional order of developmental stages. In vivo priming protocols that preferentially induced one of the different CD8(+) T cell subsets demonstrated that predominance of T(EM) (CD40 stimulation) correlated best with effective protection against L.m., whereas generation of neither T(CM) (by immunization with heat-killed L.m.) nor T(EC) (by systemic co-administration of CpG during primary infection) conferred substantial long-term protective immunity. These findings have important implications for the design of more effective T cell-based vaccines.

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.

Increased level and longevity of protective immune responses induced by DNA vaccine expressing the HIV-1 Env glycoprotein when combined with IL-21 and IL-15 gene delivery

We investigated the ability of a plasmid-derived IL-21 delivered alone or in combination with the IL-15 gene to regulate immune responses to the HIV-1 envelope (Env) glycoprotein induced by DNA vaccination. Mice were injected with the gp140DeltaCFI(HXB2/89.6) vector expressing a modified Env glycoprotein with C-terminal mutations intended to mimic a fusion intermediate, in which the most divergent region encoding the variable V1, V2, and V3 domains of CXCR4-tropic HxB2 virus was replaced with the dual-tropic 89.6 viral strain. Using a recombinant vaccinia virus expressing 89.6 Env glycoprotein (vBD3) in a mouse challenge model, we observed that IL-21 plasmid produced sustained resistance to viral transmission when injected 5 days after DNA vaccination. Moreover, IL-21 in a synergistic manner with IL-15 expression vector augmented the vaccine-induced recall responses to the vBD3 challenge compared with those elicited by immunization in the presence of either cytokine alone. The synergistic combination of IL-21 and IL-15 plasmids promoted expansion of CD8+CD127+ memory T cell pools specific for a subdominant HLA-A2-restricted Env(121-129) epitope (KLTPLCVTL). Our results also show that coimmunization with IL-21 and IL-15 plasmid combination resulted in enhanced CD8+ T cell function that was partially independent of CD4+ T cell help in mediating protection against vBD3 challenge. Furthermore, the use of IL-21 and IL-15 genes was able to increase Ab-dependent cellular cytotoxicity and complement-dependent lysis of Env-expressing target cells through augmentation of Env-specific IgG Ab levels. These data indicate that the plasmid-delivered IL-21 and IL-15 can increase the magnitude of the response to DNA vaccines.

Control of CD4 + T‐cell memory by cytokines and costimulators

During T-cell priming, cytokines and costimulatory molecules provide important signals that determine the magnitude and quality of the response. Although the functions of defined cytokines and costimulators in the primary T-cell response are well characterized, much less is known about how these factors contribute to memory T-cell development and survival. Since memory cells are thought to be long-lived progeny of the primary response, it is conceivable that the same signals shaping initial T-cell expansion and differentiation also contribute to memory generation. Here, we review evidence and show novel data on the role of the cytokines interleukin-2 (IL-2) and IL-7 and the costimulator CD28 in CD4+ memory T-cell development. We emphasize that transient IL-2 and CD28 signals during priming imprint a long-lasting survival advantage in primed T cells, thus contributing to the persistence of a memory population. The requirement for IL-2 and CD28 signals is not linked to promoting T-cell division and expansion but most likely due to their capacity to (i) promote effector cell differentiation; (ii) induce survival proteins, and, as we discuss in more detail; (iii) program expression of receptors for 'memory survival factors' such as IL-7. Studies exploring the therapeutic potential of these insights are also discussed.

Differentiation of memory B and T cells

In the past few years progress has been made in understanding the molecular mechanisms that underlie the initial generation, and the ensuing differentiation and maintenance, of humoral and cellular immunity. Although B and T cell immunological memory contribute to protective immunity through fundamentally distinct effector functions, interesting analogies are becoming apparent between the two memory compartments. These include heterogeneity in function, anatomical location and phenotype, which probably relate to differential environmental cues during the early priming events as well as the later differentiation phases. Detailed definition of the molecular and cellular signals involved in the development of immunological memory, and the relative contributions of different memory subsets to protective immunity, remains an important goal.

Intratumor CD4 T-Cell Accumulation Requires Stronger Priming than for Expansion and Lymphokine Secretion

T cells need to migrate to and accumulate inside tumors before mediating rejection of the tumor. The number of specific T cells inside tumors may depend on the efficiency of priming in the draining lymph node (DLN), intratumor deletion, suppressive phenomena, or both. We used monoclonal anti-male antigen CD4 (Marilyn) T cells and tumor cell lines expressing or not the corresponding antigen (Dby) to analyze CD4 T-cell accumulation in tumors. Priming by MHC II(+) or MHC II(-) male splenocytes or Dby(+) tumor cells induced similar Marilyn T-cell expansion in the DLN and recirculation in other lymph nodes and capacity to produce IFN-gamma. However, intratumor accumulation was different for each priming condition. In mice with Dby(-) tumors, MHC II(+) male splenocyte priming induced greater, although not statistically significant, Marilyn T-cell accumulation in the tumors than MHC II(-) male splenocyte priming. In mice with Dby(+) tumors, priming in the tumor DLN induced less Marilyn T-cell intratumor accumulation than priming by MHC II(+) male splenocytes. We saw comparable differences for Marilyn T-cell accumulation in gut lamina propria, suggesting that priming affects effector T-cell accumulation in inflamed tissues. Mature dendritic cells were loaded with graded doses of Dby peptide to control for antigen-presenting cell characteristics during priming. We observed similar proliferation, with higher concentrations inducing higher intratumor accumulation. Thus, intratumor accumulation requires stronger stimulation than for proliferation or the capacity to secrete lymphokines. In this system, priming intensity alone can explain the number of intratumor T cells without having to call for intratumor deletion or suppression phenomena.

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.

The generation of protective memory-like CD8+ T cells during homeostatic proliferation requires CD4+ T cells

Antigen-specific memory T cells are a critical component of protective immunity because of their increased frequency and enhanced reactivity after restimulation. However, it is unclear whether 'memory-like' T cells generated during lymphopenia-induced homeostatic proliferation can also offer protection against pathogens. Here we show that homeostatic proliferation-induced memory (HP-memory) CD8(+) T cells controlled bacterial infection as effectively as 'true' memory CD8(+) T cells, but their protective capacity required the presence of CD4(+) T cells during homeostatic proliferation. The necessity for CD4 help was overcome, however, if the HP-memory CD8(+) T cells lacked expression of TRAIL (tumor necrosis factor-related apoptosis-inducing ligand; also called Apo-2L). Thus, like conventional CD8(+) memory T cells, the protective function of HP-memory CD8(+) T cells shows dependence on CD4(+) T cell help.

Determination of the functional status of alloreactive T cells by interferon-gamma kinetics

BACKGROUND

A fundamental limitation of in vitro immunologic tests in the field of transplantation is that existing functional tests poorly correlate with in vivo immune responses such as rejection, tolerance, or absence of rejection due to immunosuppression. It would be helpful to have a measure of T lymphocyte responsiveness that reliably reflects these conditions.

METHODS

C57BL/6J mice received skin transplants from BALB/c donors with: a) no treatment, b) treatment with CsA, or c) treatment with CTLA-4Ig, alpha-CD40L mAb, and alpha-CD25 mAb. Syngeneic skin transplants served as controls. Recipient splenocytes were co-cultured with irradiated donor splenocytes and culture supernatant was harvested once a day for 5 consecutive days. IFN-gamma levels were measured by ELISA.

RESULTS

Splenocytes obtained from non-transplanted mice responded to specific alloantigen stimulation (primary response) at least 2 days later than the splenocytes from mice which had rejected skin grafts (effector/memory response). Splenocytes from mice treated with CsA after skin transplants had no response to third-party alloantigen, but showed an effector/memory pattern of IFN-gamma elaboration with donor cell stimulation (immunosuppression), although the IFN-gamma levels were not as high as those mice with unmodified graft rejection. Mice treated with combined CTLA4Ig, alpha-CD40L and alpha-CD25 accepted skin grafts without further immunosuppression. Splenocytes from these tolerant mice showed a primary response to the third-party and failed to secrete detectable IFN-gamma in the presence of donor cells (tolerance).

CONCLUSION

This assay clearly differentiated the functional status of the alloreactive T cells, including primary alloimmune response, effector/memory response, immunosuppressed T cell response, and donor specific tolerance.

Secondary memory CD8+ T cells are more protective but slower to acquire a central-memory phenotype

The formation of memory CD8 T cells is an important goal of vaccination. However, although widespread use of booster immunizations in humans generates secondary and tertiary CD8 T cell memory, experimental data are limited to primary CD8 T cell memory. Here, we show that, compared with primary memory CD8 T cells, secondary memory CD8 T cells exhibit substantially delayed conversion to a central–memory phenotype, as determined by CD62L expression and interleukin (IL)-2 production. This delayed conversion to a central–memory phenotype correlates with reduced basal proliferation and responsiveness to IL-15, although in vitro coculture with a high concentration of IL-15 is capable of inducing proliferation and CD62L upregulation. Functionally, secondary memory CD8 T cells are more protective in vivo on a per cell basis, and this may be explained by sustained lytic ability. Additionally, secondary memory CD8 T cells are more permissive than primary memory CD8 T cells for new T cell priming in lymph nodes, possibly suggesting a mechanism of replacement for memory T cells. Thus, primary and secondary memory CD8 T cells are functionally distinct, and the number of encounters with antigen influences memory CD8 T cell function.

CD4 memory T cells survive and proliferate but fail to differentiate in the absence of CD40

Secondary T cell responses are enhanced because of an expansion in numbers of antigen-specific (memory) cells. Using major histocompatibility complex class II tetramers we have tracked peptide-specific endogenous (non–T cell receptor transgenic) CD4 memory T cells in normal and in costimulation-deficient mice. CD4 memory T cells were detectable after immunization for more than 200 days, although decay was apparent. Memory cells generated in CD40 knockout mice by immunization with peptide-pulsed wild-type dendritic cells survived in the absence of CD40 and proliferated when boosted with peptide (plus adjuvant) in a CD40-independent fashion. However, differentiation of the memory cells into cytokine-producing effector cells did not occur in the absence of CD40. The data indicate that memory cells can be generated without passing through the effector cell stage.

Effector T Cell Differentiation and Memory T Cell Maintenance Outside Secondary Lymphoid Organs

Naive T cell circulation is restricted to secondary lymphoid organs. Effector and memory T cells, in contrast, acquire the ability to migrate to nonlymphoid tissues. In this study we examined whether nonlymphoid tissues contribute to the differentiation of effector T cells to memory cells and the long-term maintenance of memory T cells. We found that CD4, but not CD8, effector T cell differentiation to memory cells is impaired in adoptive hosts that lack secondary lymphoid organs. In contrast, established CD4 and CD8 memory T cells underwent basal homeostatic proliferation in the liver, lungs, and bone marrow, were maintained long-term, and functioned in the absence of secondary lymphoid organs. CD8 memory T cells found in nonlymphoid tissues expressed both central and effector memory phenotypes, whereas CD4 memory T cells displayed predominantly an effector memory phenotype. These findings indicate that secondary lymphoid organs are not necessary for the maintenance and function of memory T cell populations, whereas the optimal differentiation of CD4 effectors to memory T cells is dependent on these organs. The ability of memory T cells to persist and respond to foreign Ag independently of secondary lymphoid tissues supports the existence of nonlymphoid memory T cell pools that provide essential immune surveillance in the periphery.

Homeostasis of the memory T cell pool

Memory T cells are critical for the establishment of long-term immunity. The number of memory T cells formed at the conclusion of the primary response is strongly influenced by the number of effector T cells generated in the response, but some factors can additionally enhance the efficiency and quality of memory cell recruitment. Homeostasis of the memory T cell pool depends on cytokine-mediated regulation of cell survival and proliferation. This review discusses factors that influence both the development and the maintenance of the memory T cell pool.

Identification of the immunodominant HY H2-D(k) epitope and evaluation of the role of direct and indirect antigen presentation in HY responses

Minor histocompatibility Ags derive from self-proteins and provoke allograft rejection and graft-vs-host disease in MHC-matched donor-recipient combinations. In this study, we define the HYD(k) epitope of the HY minor histocompatibility Ag as the 8mer peptide RRLRKTLL derived from the Smcy gene. Using HY tetramers, the response to this peptide was found to be immunodominant among the four characterized MHC class I-restricted HY epitopes (HYD(k)Smcy (defined here), HYK(k)Smcy, HYD(b)Uty, and HYD(b)Smcy). Indirect presentation stimulated a robust primary HYD(k)Smcy response. Indirect presentation and priming of HY-specific CD8+ T cells is also operative in the presence of a full MHC mismatch. To determine whether the indirect route of Ag presentation is required for HY priming, female parent into F1 (H2bxk) female recipient bone marrow chimeras were immunized with male cells of the other parental haplotype, limiting presentation to the direct pathway. The dominant H2b HY response (HYD(b)Uty) was dependent on indirect presentation. However, the dominant H2k HY response (HYD(k)Smcy) could be stimulated efficiently by the direct pathway. In contrast, secondary expansion of both HYD(k)Smcy and HYD(b)Uty-specific CD8+ T cells was effective only when Ag was presented by the direct route. Transgenic overproduction of Smcy mRNA within the immunizing cells resulted in a corresponding increase in the HYD(k)Smcy, HYD(b)Smcy, and HYK(k)Smcy-specific CD8+ T cell responses when presented via the direct pathway but did not enhance indirect presentation demonstrating the independent regulation of MHC class I-peptide occupancy in the two Ag-processing pathways.

Lack of ICAM-1 on APCs during T cell priming leads to poor generation of central memory cells

ICAM-1/LFA-1 interactions are known to enhance T cell/APC interactions and to promote T cell activation and cytokine secretion. We have analyzed the consequences of ICAM-1-mediated signaling on the generation of memory T cell subsets. We report that lack of ICAM-1 on APCs, but not on T cells, leads to poor T cell activation and proliferation in vitro and in vivo, and that the defect can be compensated by Ag dose, exogenous IL-2, additional costimulation, and by increasing responder T cell density on APCs. ICAM-1-null mice do not respond to immunization with OVA peptide, but immunization with OVA or with Salmonella typhimurium leads to good T cell proliferation 7-10 days later, and clearance of a challenge infection is equivalent to that of wild-type mice. However, when followed over time, recall proliferation and antibacterial immunity decay rapidly in ICAM-1-null mice, while recall cytokine responses are unaffected. The decline in immunity is not related to poor survival of T cells activated on ICAM-1-null APCs, or to poor generation of effectors in ICAM-1-null mice. Phenotypic analysis of T cells stimulated on ICAM-1-null APCs reveals preferential generation of CD44(high) CD62L(low) effector memory cells (T(EM)) over CD44(high) CD62L(high) central memory cells (T(CM)). Further, while the proportion of naive:memory T cells is similar in unmanipulated wild-type and ICAM-1-null mice, there is an accumulation of T(EM) cells, and a high T(EM):T(CM) ratio in aging ICAM-1-null mice. Together, the data indicate that signaling through LFA-1 during T cell activation may be involved in commitment to a proliferation-competent memory pool.

Host-reactive CD8+ memory stem cells in graft-versus-host disease

Graft-versus-host disease (GVHD) is caused by alloreactive donor T cells that trigger host tissue injury. GVHD develops over weeks or months, but how this immune response is maintained over time is unknown. In mouse models of human GVHD, we identify a new subset of postmitotic CD44(lo)CD62L(hi)CD8(+) T cells that generate and sustain all allogeneic T-cell subsets in GVHD reactions, including central memory, effector memory and effector CD8(+) T cells, while self-renewing. These cells express Sca-1, CD122 and Bcl-2, and induce GVHD upon transfer into secondary recipients. The postmitotic CD44(lo)CD62L(hi)CD8(+) T cells persist throughout the course of GVHD, are generated in the initial phase in response to alloantigens and dendritic cells and require interleukin-15. Thus, their long life, ability to self-renew and multipotentiality define these cells as candidate memory stem cells. Memory stem cells will be important targets for understanding and influencing diverse chronic immune reactions, including GVHD.

Interleukin-7-dependent expansion and persistence of melanoma-specific T cells in lymphodepleted mice lead to tumor regression and editing

Active-specific immunotherapy with dendritic cells loaded with peptide derived from the melanoma antigen, gp100, failed to mediate regression of established B16F10 melanoma in normal mice. Dendritic cell vaccination induced activation and subsequent deletion of adoptively transferred naive CD8+ T-cell receptor transgenic (pmel-1) T cells specific for gp100 in normal mice. In lymphodepleted mice, dendritic cell vaccination produced greater T-cell expansion, long-term persistence of memory T cells, and tumor regression. Most tumors that persisted in the presence of functional memory T cells had either lost or exhibited reduced expression of MHC class I or gp100 proteins. In contrast to other naive T cells, pmel-1 T cells adoptively transferred to lymphodepleted mice exhibited faster proliferation and a more differentiated phenotype after exposure to peptide-pulsed dendritic cells. Proliferation and persistence of pmel-1 T cells was highly dependent on interleukin-7 (IL-7) in irradiated mice, and IL-15 when IL-7 was neutralized, two critical homeostatic cytokines produced in response to the irradiation-induced lymphodepletion.

Anti-CD3 priming generates heterogeneous antigen-specific memory CD4 T cells

Anti-CD3 activation of peripheral T cells is used in adoptive immunotherapy for cancer and HIV infection, but the long-term fate of anti-CD3-primed T cells in vivo is not known. In this study, we demonstrate that anti-CD3-mediated activation of influenza hemagglutinin (HA)-specific TCR-transgenic CD4 T cells results in generation of a long-lived HA-specific memory CD4 T cell population when transferred into lymphocyte-deficient and intact mouse hosts. This anti-CD3-primed memory population is indistinguishable from HA peptide-primed memory CD4 T cells in terms of phenotype, rapid recall function, and enhanced proliferative capacity. Moreover, anti-CD3 priming generates phenotypically heterogeneous memory subsets in lymphoid and non-lymphoid sites. Our results suggest that anti-CD3 has potential efficacy in generating memory responses in adoptive immunotherapies and vaccines and that the tissue distribution and maintenance of heterogeneous lymphoid and non-lymphoid memory T cell subsets are a stochastic process that can occur independent of antigen or TCR specificity.

Transient loss of MHC class I tetramer binding after CD8+ T cell activation reflects altered T cell effector function

Engagement of the Ag receptor on naive CD8+ T cells by specific peptide-MHC complex triggers their activation/expansion/differentiation into effector CTL. The frequency of Ag-specific CD8+ T cells can normally be determined by the binding of specific peptide-MHC tetramer complexes to TCR. In this study we demonstrate that, shortly after Ag activation, CD8+ T cells transiently lose the capacity to efficiently bind peptide-MHC tetramer complexes. This transient loss of tetramer binding, which occurs in response to naturally processed viral peptide during infection in vitro and in vivo, is associated with reduced signaling through the TCR and altered/diminished effector activity. This change in tetramer binding/effector response is likewise associated with a change in cell surface TCR organization. These and related results suggest that early during CD8+ T cell activation, there is a temporary alteration in both cell surface Ag receptor display and functional activity that is associated with a transient loss of cognate tetramer binding.

A Novel Role of CD30/CD30 Ligand Signaling in the Generation of Long-Lived Memory CD8+ T Cells

Memory CD8+ T cells can be divided into two subsets, central memory (T(CM)) and effector memory (T(EM)) CD8+ T cells. We found that CD30, a member of the TNFR-associated factor (TRAF)-linked TNFR superfamily, signaling is involved in differentiation of long-lived CD8+ T(CM) cells following Listeria monocytogenes infection. Although CD8+ T(EM) cells transiently accumulated in the nonlymphoid tissues of CD30 ligand (CD153-/-) mice after infection, long-lived memory CD8+ T(CM) cells were poorly generated in these mice. CCR7 mRNA expression was down-regulated in CD8+ T cells of the spleen of CD153-/- mice in vivo and the expression was up-regulated in CD8+ T(EM) cells by anti-CD30 mAb cross-linking in vitro. These results suggest that CD30/CD30 ligand signaling plays an important role in the generation of long-lived memory CD8+ T cells at least partly by triggering homing receptors for T(CM) cells.

IL-7 Receptor Expression Levels Do Not Identify CD8+ Memory T Lymphocyte Precursors following Peptide Immunization

Identification of the mechanisms underlying the survival of effector T cells and their differentiation into memory T lymphocytes are critically important to understanding memory development. Because cytokines regulate proliferation, differentiation, and survival of T lymphocytes, we hypothesized that cytokine signaling dictates the fate of effector T cells. To follow cytokine receptor expression during T cell responses, we transferred murine TCR transgenic T cells into naive recipients followed by immunization with peptide emulsified in adjuvant or pulsed on dendritic cells. Our findings did not correlate IL-7R alpha-chain and IL-2R beta-chain expression on effector CD8+ cells with the generation of memory T lymphocytes. However, we could correlate the extent of IL-7R alpha expression down-regulation on effector T cells with the level of inflammation generated by the immunization. Furthermore, our findings showed that the maintenance of a high level of IL-7R expression by effector T cells at the peak of the response does not preclude their death. This suggests that maintenance of IL-7R expression is not sufficient to prevent T cell contraction. Thus, our results indicate that expression of the IL-7R is not always a good marker for identifying precursors of memory T cells among effectors and that selective expression of the IL-7R by effector T cells should not be used to predict the success of vaccination.

Effect of the CTL proliferation program on virus dynamics

Experiments have established that CTLs do not require continuous antigenic stimulation for expansion. Instead, responses develop by a process of programmed proliferation which involves approximately 7-10 antigen-independent cell divisions, the generation of effector cells and the differentiation into memory cells. The effect of this program on the infection dynamics and the advantages gained by the program have, however, not been explored yet. We investigate this with mathematical models. We find that more programmed divisions can make virus clearance more efficient because CTL division continues to occur independent from antigenic stimulation when virus load drops to low levels. This results in stronger effector activity at low virus loads, and in a higher chance of virus extinction. On the other hand, the more programmed divisions occur, the less efficient the response is at preventing high acute virus loads and thus acute symptoms. The reason is that the programmed divisions are independent from antigenic stimulation, and an increase in virus load does not speed up the rate of CTL expansion. We hypothesize that the 7-10 programmed divisions observed in vivo represent an optimal solution to this trade-off which maximizes the chances to clear, while preventing excessive acute pathology. If the CTLs fail to clear the virus, however, we find that the properties of the programmed proliferation model are very similar to those derived from models which assume continuous antigenic stimulation. We discuss these results in the context of data from murine virus infections and explore implications for virus dynamics in CD4 helper-deficient hosts.

Maintenance of long-term tumour-specific T-cell memory by residual dormant tumour cells

LacZ (Gal)-reactive immune cells were transferred into athymic nu/nu mice inoculated with Gal-expressing syngeneic tumour cells (ESbL-Gal) in order to study tumour-protective T-cell memory. This transfer prevented tumour outgrowth in recipients and resulted in the persistence of a high frequency of Gal-specific CD8(+) T cells in the bone marrow and spleen. In contrast, such Ag-specific memory CD8(+) T cells were not detectable by peptide-major histocompatibility complex (MHC) multimer staining in animals that had not previously received an antigenic challenge. Even though CD44(hi) memory T cells from the bone marrow showed a significantly higher turnover rate, as judged by bromodeoxyuridine (BrdU) incorporation, than respective cells from spleen or lymph nodes, as well as in comparison to CD44(lo) naïve T cells, these findings suggest that tumour-associated antigen (TAA) from residual dormant tumour cells are implicated in maintaining high frequencies of long-term surviving Gal-specific memory CD8(+) T cells. Memory T cells could be recruited to the peritoneal cavity by tumour vaccination of immunoprotected nu/nu mice and exhibited ex vivo antitumour reactivity. Long-term immune memory and tumour protection could be maintained over four successive transfers between tumour-inoculated recipients, which involved periodic antigenic restimulation in vivo prior to reisolating the cells for adoptive transfer. Using a cell line (ESbL-Gal-BM) that was established from dormant tumour cells isolated from the bone marrow of immunoprotected animals, it could be demonstrated that the tumour cells had up-regulated the expression of MHC class I molecules and down-regulated the expression of several adhesion molecules during the in vivo passage. Our results suggest that the bone marrow microenvironment has special features that are of importance for the maintenance of tumour dormancy and immunological T-cell memory, and that a low level of persisting antigen favours the maintenance of Ag-specific memory T cells over irrelevant memory T cells.

Does programmed CTL proliferation optimize virus control?

CD8 T-cell or cytotoxic T-lymphocyte responses develop through an antigen-independent proliferation and differentiation program. This is in contrast to the previous thinking, which was that continuous antigenic stimulation was required. This Opinion discusses why nature has chosen the proliferation program and how it compares to continuous stimulation. Although the two mechanisms should not lead to significantly different dynamics during chronic infection, they do make a difference in acute infection. We argue that programmed proliferation is better at clearance, whereas continuous stimulation is better at limiting acute symptoms. The 7-10 programmed cell divisions observed in vivo might be an optimization of this trade-off. We also discuss the conditions under which the program does or does not require CD4 T-cell help for clearance.

Genetically programmed B lymphocytes are highly efficient in inducing anti-virus protective immunity mediated by central memory CD8 T cells

The hallmarks of specific T cell immunity include proliferative expansion, acquisition of effector function and memory T cell formation. Here, we used priming with B lymphocytes transgenic for the dominant epitope (NP366-374) of the influenza virus nucleoprotein, to study the characteristics of the CD8 T cell memory response in C57Bl/6 mice and elucidate which subset of CD8 T cells memory mediates protection from disease. We found that (i) the size of the memory CTL response is independent of the priming dose and is similar to that induced by the live virus, (ii) priming with a low dose (3 x 10(2)cells/inoculum) of transgenic B lymphocytes confers a protective memory CTL response, and (iii) protection from disease is mediated by central memory (T(CM)) CD8 T cells.

Chemokines: control of primary and memory T-cell traffic

Motility is a hallmark of leukocytes, and breakdown in the control of migration contributes to many inflammatory diseases. Chemotactic migration of leukocytes largely depends on adhesive interaction with the substratum and recognition of a chemoattractant gradient. Chemokines are secreted proteins and have emerged as key controllers of integrin function and cell locomotion. Numerous distinct chemokines exist that target all types of leukocytes, including hematopoietic precursors, leukocytes of the innate immune system, as well as naive memory, and effector lymphocytes. The combinatorial diversity in responsiveness to chemokines ensures the proper tissue distribution of distinct leukocyte subsets under normal and pathological conditions. Inflammatory chemokines are readily detected in lesional tissue and local cellular infiltrates carry corresponding chemokine receptors. Blocking of inflammatory chemokines represents a promising strategy for the development of novel anti-inflammatory therapeutics. This review focuses on a separate class of chemokines, termed homeostatic chemokines, with steady-state production at diverse sites, including primary and secondary lymphoid tissues as well as peripheral (extralymphoid) tissues. More precisely, we discuss the chemokines involved in T-cell traffic during the initiation of adaptive immunity and compare the distinct migration properties of short-lived effector T cells and long-lived memory T cells. Memory T cells are currently classified according to the presence of the lymph node-homing receptor CCR7 into CCR7+ central memory T (T(CM)) cells and CCR7- effector memory T (T(EM)) cells. For better understanding memory T-cell function, we propose the distinction of a third category, termed peripheral immune surveillance T (T(PS)) cells, which typically reside in healthy peripheral tissues, such as skin, lung, and gastrointestinal tract. Localization and relocation of memory T cells is strictly related to their function in recall responses. Therefore, detailed knowledge of their generation and tissue distribution may help to design better vaccination strategies.

Fetal expression of Fas ligand is necessary and sufficient for induction of CD8 T cell tolerance to the fetal antigen H-Y during pregnancy

Interaction of Fas with Fas ligand (FasL) is known to play a role in peripheral tolerance mediated by clonal deletion of Ag-specific T cells. We have assessed the requirement for Fas/FasL interactions during induction of tolerance to the fetus. Using H-Y-specific TCR transgenic mice, we have previously demonstrated that exposure of maternal T cells to H-Y expressed by male fetuses results in deletion of 50% of H-Y-specific maternal T cells. The remaining H-Y-specific T cells were hyporesponsive to H-Y as assayed by decreased proliferative ability and CTL activity. To determine whether Fas/FasL interactions contribute to induction of maternal T cell tolerance, responsiveness to fetal H-Y was assessed in H-Y-specific TCR transgenic pregnant females that were deficient in functional Fas or FasL. Surprisingly, both deletion and nondeletion components of tolerance were abrogated in TCR transgenic H-Y-specific lpr (Fas-deficient) or gld (FasL-deficient) pregnant females. Experiments further revealed that expression of FasL by the fetus, but not by the mother, is necessary and sufficient for both components of maternal T cell tolerance to fetal Ags. Fas interaction with fetal FasL is thus critical for both deletion and hyporesponsiveness of H-Y-reactive CD8+ T cells during pregnancy.

Generation and maintenance of immunological memory

The key feature of the adaptive immune response is its specificity and the ability to generate and maintain memory. Preexisting antibodies in the circulation and at the mucosa provide the first line of defense against re-infection by extracellular as well as intracellular pathogens. Memory T cells are an important second line of defense against intracellular pathogens, and in particular against microbes that can cause chronic or latent infection. In this article we will review our current understanding of the generation and maintenance of B cell and T cell memory.