Resting memory CD8+ T cells are hyperreactive to antigenic challenge in vitro

Persistence of memory CD8 T cells in MHC class I-deficient mice

An understanding of how T cell memory is maintained is crucial for the rational design of vaccines. Memory T cells were shown to persist indefinitely in major histocompatibility complex (MHC) class I-deficient mice and retained the ability to make rapid cytokine responses upon reencounter with antigen. In addition, memory CD8 T cells, unlike naïve cells, divided without MHC-T cell receptor interactions. This "homeostatic" proliferation is likely to be important in maintaining memory T cell numbers in the periphery. Thus, after naïve CD8 T cells differentiate into memory cells, they evolve an MHC class I-independent "life-style" and do not require further stimulation with specific or cross-reactive antigen for their maintenance.

Selective expansion of cross-reactive CD8(+) memory T cells by viral variants

The role of memory T cells during the immune response against random antigenic variants has not been resolved. Here, we show by simultaneous staining with two tetrameric major histocompatibility complex (MHC)-peptide molecules, that the polyclonal CD8(+) T cell response against a series of natural variants of the influenza A nucleoprotein epitope is completely dominated by infrequent cross-reactive T cells that expand from an original memory population. Based on both biochemical and functional criteria, these cross-reactive cytotoxic T cells productively recognize both the parental and the mutant epitope in vitro and in vivo. These results provide direct evidence that the repertoire of antigen-specific T cells used during an infection critically depends on prior antigen encounters, and indicate that polyclonal memory T cell populations can provide protection against a range of antigenic variants.

Expression of the Transcription Factor Lung Krüppel-Like Factor Is Regulated by Cytokines and Correlates with Survival of Memory T Cells In Vitro and In Vivo

The transcription factor lung Krüppel-like factor (LKLF) is involved in naive T cell survival. Expression of LKLF is rapidly down-regulated upon T cell stimulation, raising the question of whether LKLF is reexpressed after activation, and what factors are required for such reexpression. Furthermore, the expression of LKLF in resting memory cells has not been determined. Here, we use the OT-I TCR transgenic mouse system to address these issues. LKLF was found to be reexpressed following culture of activated CD8 T cells in certain cytokines (IL-2, IL-7) but not others (IL-12) known to influence CTL development. Interestingly, induction of LKLF reexpression corresponded with long-term T cell survival and development of memory T cell phenotype. Furthermore, using OT-I cells stimulated in vivo, we demonstrated that Ag induced rapid LKLF down-regulation and that the factor is expressed by in vivo-derived memory T cells.

Pretransplant Frequency of Donor-Specific, IFN-γ-Producing Lymphocytes Is a Manifestation of Immunologic Memory and Correlates with the Risk of Posttransplant Rejection Episodes

While matching for MHC Ags improves renal allograft survival, closely matched grafts sometimes fail due to rejection, and poorly matched allografts are often well tolerated by the recipient. The severity of the rejection process may partially depend on the presence of environmentally primed T cells in the recipient that cross-react with donor Ags. To test for the presence of primed, donor-specific T cells in humans before transplantation, we used an enzyme-linked immunospot assay for detection of allospecific cytokines produced by individual human PBLs. We demonstrate that this approach detects cytokine production at single cell resolution and detects production of IFN-γ only when there is defined immunologic priming, thus representing a measure of primed donor-specific immunity. Because the environmental Ag exposure of the recipient is not a function of the HLA mismatch between donor and potential recipient, the number of HLA mismatches may not correlate with the frequency of pretransplant, donor-specific IFN-γ-producing PBLs. Studies of donor-specific IFN-γ-producing lymphocytes in a cohort of patients being evaluated for renal transplantation corroborated this hypothesis. Moreover, for recipients of both living and cadaver renal allografts, the pretransplant frequency of donor-specific memory cells correlated with the posttransplant risk of developing acute rejection episodes. This improved ability to define the strength of the allospecific immune response by enzyme-linked immunospot assay may allow improved pairing of recipients with donors and identification of kidney allograft donor-recipient pairs at high risk for acute rejection, thus permitting targeted interventions aimed at prolonging graft survival.

Strong TCR Ligation Without Costimulation Causes Rapid Onset of Fas-Dependent Apoptosis of Naive Murine CD4+ T Cells

Activation-induced cell death of T cells typically occurs late in the primary response after a prior proliferative response. Here, we describe a novel form of cell death in which purified naive murine CD4+ cells undergo apoptosis within 18 h in vitro after strong TCR ligation. Such rapid-onset TCR-mediated death of T cells does not involve cell division and is Fas-dependent, inhibited by CD28 (and IL-6) costimulation and enhanced by IL-4 and IL-7; by contrast, spontaneous death of CD4+ cells cultured alone is Fas-independent and inhibited by IL-4 and IL-7. TCR-mediated Fas-dependent death of CD4+ cells is prevented by combined TCR/Fas ligation and by drugs that inhibit calcineurin-dependent signaling and mitogen-activated protein kinase MEK1 activation.

Primed T Cells Are More Resistant to Fas-Mediated Activation-Induced Cell Death than Naive T Cells

Memory T cells respond in several functionally different ways from naive T cells and thus function as efficient effector cells. In this study we showed that primed T cells were more resistant to Fas-mediated activation-induced cell death (AICD) than naive T cells using OVA-specific TCR transgenic DO10 mice and Fas-deficient DO10 lpr/lpr mice. We found that apoptosis was efficiently induced in activated naive T cells at 48 and 72 h after Ag restimulation (OVA peptide; 0.3 and 3 μM), whereas apoptosis was not significantly increased in activated primed T cells at 24–72 h after Ag restimulation. We further showed that the resistance to AICD in primed T cells was due to the decreased sensitivity to apoptosis induced by Fas-mediated signals, but TCR-mediated signaling equally activated both naive and primed T cells to induce Fas and Fas ligand expressions. Furthermore, we demonstrated that primed T cells expressed higher levels of Fas-associated death domain-like IL-1β-converting enzyme inhibitory protein (FLIP), an inhibitor of Fas-mediated apoptosis, at 24–48 h after Ag restimulation than naive T cells. In addition, Bcl-2 expression was equally observed between activated naive and primed T cells after Ag restimulation. Thus, these results indicate that naive T cells are sensitive to Fas-mediated AICD and are easily deleted by Ag restimulation, while primed/memory T cells express higher levels of FLIP after Ag restimulation, are resistant to Fas-mediated AICD, and thus function as efficient effector cells for a longer period.

Following the development of a CD4 T cell response in vivo: from activation to memory formation

The in vivo differentiation of CD4 T cells from naive to memory cells was followed after their adoptive transfer together with syngeneic dendritic cells into MHC mismatched adoptive hosts lacking lymphocytes and NK cells. Functional and molecular changes were measured as the antigenic stimulus, provided by the cotransferred dendritic cells, disappeared. Memory cells as opposed to effector cells show an inversion in the relative expression of Bcl-2 family members in favor of antiapoptotic molecules, and compared with naive cells they have an increased ratio of bcl-xL to bcl-2. They differ qualitatively from naive T cells, suggesting that accelerated CD4 memory responses can occur without the need for increased frequencies of specific T cells.

Memory CD44(int) CD8 T cells show increased proliferative responses and IFN-gamma production following antigenic challenge in vitro

F5 TCR transgenic mice challenged in vivo with peptide generate long-lived primed CD8 T cells that hyper-proliferate in response to peptide in vitro. These primed CD8 T cells can be subdivided into three distinct populations on the basis of CD44 cell surface expression. In this report, we show that among primed CD8 T cells, those expressing intermediate levels of CD44 appear to be true memory T cells by the measurement of a variety of characteristics. Indeed, these cells hyper-proliferate in response to peptide re-stimulation in vitro, and produce IFN-gamma with faster kinetics and at higher levels than naive populations in vitro. We also show that CD8 T cells expressing high levels of CD44 express several activation markers and cycle in vivo in the absence of antigen. However, this population is unable to respond to peptide stimulation in vitro as measured by both proliferation and IFN-gamma secretion. The origin and specificity of these cells is unknown. These results provide evidence that memory CD8 T cells are functionally different from naive CD8 T cells both in terms of proliferation and cytokine secretion. They identify the CD8/CD44(int) T cells as the population responsible for hyper-reactivity in vitro.

Enhancement of the Listeria monocytogenes p60-Specific CD4 and CD8 T Cell Memory by Nonpathogenic Listeria innocua

The contact of T cells to cross-reactive antigenic determinants expressed by nonpathogenic environmental micro-organisms may contribute to the induction or maintenance of T cell memory. This hypothesis was evaluated in the model of murine Listeria monocytogenes infection. The influence of nonpathogenic L. innocua on the L. monocytogenes p60-specific T cell response was analyzed. We show that some CD4 T cell clones raised against purified p60 from L. monocytogenes cross-react with p60 purified from L. innocua. The L. monocytogenes p60-specific CD4 T cell clone 1A recognized the corresponding L. innocua p60 peptide QAAKPAPAPSTN, which differs only in the first amino acid residue. In vitro experiments revealed that after L. monocytogenes infection of APCs, MHC class I-restricted presentation of p60 occurs, while MHC class II-restricted p60 presentation is inhibited. L. innocua-infected cells presented p60 more weakly but equally well in the context of both MHC class I and MHC class II. In contrast to these in vitro experiments the infection of mice with L. monocytogenes induced a strong p60-specific CD4 and CD8 T cell response, while L. innocua infection failed to induce p60-specific T cells. L. innocua booster infection, however, expanded p60-specific memory T cells induced by previous L. monocytogenes infection. In conclusion, these findings suggest that infection with a frequently occurring environmental bacterium such as L. innocua, which is nonpathogenic and not adapted to intracellular replication, can contribute to the maintenance of memory T cells specific for a related intracellular pathogen.

Functional differences between memory and naive CD8 T cells

To determine how murine memory and naive T cells differ, we generated large numbers of long-lived memory CD8(+) T cells and compared them to naive cells expressing the same antigen-specific receptor (T cell receptor; TCR). Although both populations expressed similar levels of TCR and CD8, on antigen stimulation in vitro memory T cells down-regulated their TCR faster and more extensively and secreted IFN-gamma and IL-2 faster than naive T cells. Memory cells were also larger, and when freshly isolated from mice they contained perforin and killed target cells without having to be restimulated. They further differed from naive cells in requiring IL-15 for proliferation and in having a greater tendency to undergo apoptosis in vitro. On antigen stimulation in vivo, however, they proliferated more rapidly than naive cells. These findings suggest that, unlike naive T cells, CD8 memory T cells are intrinsically programmed to rapidly express their effector functions in vivo without having to undergo clonal expansion and differentiation.

Ikaros sets thresholds for T cell activation and regulates chromosome propagation

T cell activation involves the sustained accumulation of T cell receptor (TCR) and IL-2 receptor (IL-2R) mediated signaling events that promote cell cycle entry and progression. The Ikaros family of nuclear factors regulate this process by providing thresholds overcome by receptor signaling. T cells with reduced levels of Ikaros activity require fewer TCR engagement events for activation, exhibit a greater proliferative response to IL-2, and are less sensitive to inhibitors of TCR and IL-2R signaling. Upon T cell activation, Ikaros proteins localize in a higher-order chromatin structure where they colocalize with components of the DNA replication machinery. Proliferating T cells with reduced Ikaros activity display chromosome abnormalities. We propose that participation of Ikaros in higher-order chromatin structures controls cell cycle transitions and restricts DNA replication.

Function and regulation of memory CD4 T cells

The development of peripheral naive CD4 T cells is dependent on the success of positive selection of immature T cells in the thymus. Only thymocytes that express a T cell receptor (TCR) capable of recognizing self-MHC with low affinity are selected for survival and differentiation into mature naive T cells. Although the TCR of naive T cells has to maintain self-tolerance, it also propagates naive CD4 T cell proliferation on recognition of appropriate foreign peptide associated with MHC class II on antigen-presenting cells (APCs). Naive CD4 T cells that successfully engage foreign peptide undergo further differentiation that leads to the maturation of a select few into the memory T cell pool. Although the requirements that lead to memory T cell development are currently not known, functional changes have been described that are thought to be associated with the greater efficiency with which memory T cells respond to antigen. This article will discuss differences associated with signaling through the TCR of naive and memory CD4 T cells and describe unique control mechanisms imposed on memory CD4 T cells that are likely to have ari sen to counterbalance the altered TCR signaling.

Distinct kinetics of cytokine production and cytolysis in effector and memory T cells after viral infection

In the present study, naive T cells were compared with in vivo generated effector and memory T cells expressing the same TCR specific for lymphocytic choriomeningitis virus. Upon restimulation in vitro, the same minimal concentrations of the full agonist peptide p33 and also of weak and partial agonist peptides were required for proliferation of naive, effector and memory T cells, indicating no difference in threshold of activation. However, activation kinetics were distinct. While effector cytotoxic T cells exhibited immediate ex vivo lytic effector function, naive and memory T cells required 12 h and more exposure to antigen to develop lytic activity. However, both effector and memory T cells contained IFN-gamma mRNA in vivo and required less than 3 h for secretion of cytokines upon restimulation in vitro. In contrast, naive T cells did not contain IFN-gamma mRNA and required more than 12 h for cytokine secretion. Our results show that memory T cells exhibit a unique phenotype in that they produce cytokines and commit to proliferation as rapidly as effector cells, whereas they resemble naive T cells in the time requirement for development of cytolytic function.

Kinetics of the response of naive and memory CD8 T cells to antigen: similarities and differences

We have studied the kinetics of the antigen induced response of naive and memory CD8 T cells expressing a transgenic T cell receptor (TCR) specific for the glycoprotein peptide amino acid 33-41 (GP33) of the lymphocytic choriomeningitis virus (LCMV). Memory T cells were generated in vivo by adoptive transfer of LCMV TCR transgenic T cells into normal recipient mice, followed by LCMV infection. The results demonstrated that the cell cycle progression and kinetics of TCR down-modulation, CD25 and CD69 up-regulation were identical in naive and memory T cells after antigen recognition. Moreover, the two T cell populations did not differ in respect of activation thresholds and in their proliferative capacities neither in vitro nor in vivo. However, memory CD8 T cells could be more rapidly induced to become cytolytic and to secrete high levels of interleukin-2 and interferon-gamma than naive T cells. LCMV GP33-specific CD8 memory T cells were only slightly more efficient in reducing LCMV titers in the spleen but were far more effective than naive LCMV GP33-specific T cells in controlling subcutaneous tumor growth of B16.F10 melanoma cells which expressed the LCMV GP33 epitope as tumor-associated antigen. Thus, in our experiments the main difference between CD8 memory T cells and naive cells is the ability of the former to rapidly acquire effector cell functions.

Differential Role of CTLA-4 in Regulation of Resting Memory Versus Naive CD4 T Cell Activation

Regulation of peripheral T cell responses is critical for preserving self tolerance. Memory T cells have a lower threshold for activation through the TCR and are thought to be less dependent on costimulation than naive T cells, suggesting a requirement for more stringent regulation of memory T cells. We have recently shown that CD4 engagement apart from the TCR results in the inactivation of memory, but not naive, CD4 T cells. We show here that this inhibition requires ligation of CTLA-4, in that blocking CTLA-4-B7 interactions restores memory CD4 T cell responsiveness. Early signaling through CTLA-4 is possible because resting memory, but not naive, CD4 T cells contain intracellular stores of CTLA-4 that are continuously recycled between the cytoplasm and the cell surface. This mechanism ensures that low intensity TCR engagements, which are thought to be important for peripheral T cell longevity, do not cause memory T cell activation but instead raise their threshold for costimulatory signals. This may give memory T cells an extended lifespan with a reduced risk of inappropriate activation.

Differentiation of Naive CTL to Effector and Memory CTL: Correlation of Effector Function with Phenotype and Cell Division

Phenotypically and functionally, the early steps of T cell differentiation are not well characterized. In addition, the effector T cell stage shares several phenotypic characteristics with memory T cells, which has made the analysis of T cell memory difficult. In this study, we have investigated in vitro and in vivo the differentiation of naive CTL into effector and memory CTL as a function of cell division using lymphocytic choriomeningitis virus-specific TCR-transgenic spleen cells labeled with the vital dye carboxyfluorescein diacetate, succinimidyl ester. The following major points emerged. 1) During the first nine cell divisions, the investigated cell surface markers were strongly modulated. 2) The TCR was stepwise down-regulated during viral infection. 3) Cytotoxic effector function was acquired within one cell division and was retained during the next four to five divisions. 4) In vitro, CTL reached a CD44highCD62L+ memory phenotype after 6–10 cell divisions and required restimulation to exert effector function. 5) Lymphocytic choriomeningitis virus memory mice contained two distinct memory populations: a CD44highCD62L− population, predominately located in the spleen and exerting rapid effector function, and a CD44highCD62L+ population found in the spleen and the lymph nodes, which had lost immediate effector function. This finding suggests that two types of memory CTL exist. The correlation between CD62L expression, effector function, and Ag persistence is discussed.

Tolerant CD8 T Cells Induced by Multiple Injections of Peptide Antigen Show Impaired TCR Signaling and Altered Proliferative Responses In Vitro and In Vivo

The mechanisms responsible for peripheral CD8 T cell tolerance to foreign Ags remain poorly understood. In this study we have characterized the state of CD8 T cell tolerance induced in F5 TCR transgenic mice by multiple peptide injections in vivo. The tolerant state of CD8 T cells is characterized by impaired proliferative responses, increased sensitivity to cell death, and failure to acquire cytotoxic effector function after in vitro antigenic challenge. In vivo monitoring of CD8 T cell proliferation using 5-carboxyfluorescein diacetate succinimidyl ester showed that a large subset of the tolerant T cell population failed to divide in response to peptide. TCR down-regulation could not account for this loss of responsiveness to Ag since recombination-activating gene-1 (RAG-1)−/−F5 CD8 T cell responses were similar to those of RAG-1−/−F5 × RAG-1−/− F1 T lymphocytes, which express lower levels of the transgenic TCR. Analysis of early signal transduction in tolerant CD8 T cells revealed high basal levels of cytoplasmic calcium as well as impaired calcium mobilization and tyrosine phosphorylation after cross-linking of CD3ε and CD8α. Together these data indicate that repeated exposure to soluble antigenic peptide in vivo can induce a state of functional tolerance characterized by defective TCR signaling, impaired proliferation, and increased sensitivity to cell death.

Protective heterologous antiviral immunity and enhanced immunopathogenesis mediated by memory T cell populations

A basic principle of immunology is that prior immunity results in complete protection against a homologous agent. In this study, we show that memory T cells specific to unrelated viruses may alter the host's primary immune response to a second virus. Studies with a panel of heterologous viruses, including lymphocytic choriomeningitis (LCMV), Pichinde (PV), vaccinia (VV), and murine cytomegalo (MCMV) viruses showed that prior immunity with one of these viruses in many cases enhanced clearance of a second unrelated virus early in infection. Such protective immunity was common, but it depended on the virus sequence and was not necessarily reciprocal. Cell transfer studies showed that both CD4 and CD8 T cell populations from LCMV-immune mice were required to transfer protective immunity to naive hosts challenged with PV or VV. In the case of LCMV-immune versus naive mice challenged with VV, there was an enhanced early recruitment of memory phenotype interferon (IFN) gamma-secreting CD4(+) and CD8(+) cells into the peritoneal cavity and increased IFN-gamma levels in this initial site of virus replication. Studies with IFN-gamma receptor knockout mice confirmed a role for IFN-gamma in mediating the protective effect by LCMV-immune T cell populations when mice were challenged with VV but not PV. In some virus sequences memory cell populations, although clearing the challenge virus more rapidly, elicited enhanced IFN-gamma-dependent immunopathogenesis in the form of acute fatty necrosis. These results indicate that how a host responds to an infectious agent is a function of its history of previous infections and their influence on the memory T cell pool.

In vitro induction of naive cytotoxic T lymphocytes with complexes of peptide and recombinant MHC class I molecules coated onto beads: role of TCR/ligand density

We previously reported that complexes of peptide with soluble single-chain recombinant MHC (SC-MHC) class I molecules are able to induce cytotoxic T lymphocytes (CTL) in vitro in a murine system with an efficiency comparable to that observed with peptide-pulsed dendritic cells as antigen-presenting cells. In this report, we have assessed the capacity of preformed peptide/SC-Kd complexes in monomeric or dimeric form as well as of peptide/SC-Kd-loaded beads to generate in vitro specific CTL responses from naive DBA/2 spleen cells. Peptide/SC-Kd-coated beads were consistently more efficient. We evaluated the role of costimulatory molecules, using monoclonal antibodies anti-CD80 or anti-CD86. In addition, the capacity of peptide/SC-Kd-coated beads to generate a CTL response from purified naive CD8+ T cells was ascertained. Taken together, the results indicate that, under our conditions, CTL priming does not require the participation of co-stimulatory molecules and is the consequence of a direct interaction between the cognate TCR on peptide-specific CTL precursors and the peptide/SC-Kd-loaded beads. Titration of the amount of preformed complexes of SC-Kd and peptide 170-179 of HLA-CW3 that need to be coated onto the beads to prime CTL precursors shows an activation threshold which can be calculated to be between 25000 and 50000 complexes. In effect, in cultures stimulated with specific peptide CW3/SC-Kd complexes representing less than 50% occupancy of the total (10(5)) complexes on the beads, no peptide-specific cytolytic activity was observed. These results suggest that the efficiency of the primary CTL induction depends on the density of specific peptide/SC-Kd complexes present on the beads.

Protection against lethal encephalomyocarditis virus infection in the absence of serum-neutralizing antibodies

Although the ability of serum-neutralizing antibodies to protect against picornavirus infection is well established, the contribution of cell-mediated immunity to protection is uncertain. Using major histocompatibility complex class II-deficient (RHAbeta-/-) mice, which are unable to mediate CD4(+) T-lymphocyte-dependent humoral responses, we demonstrated antibody-independent protection against lethal encephalomyocarditis virus (EMCV) infection in the natural host. The majority of RHAbeta-/- mice inoculated with 10(4) PFU of attenuated Mengo virus (vMC24) resolved infection and were resistant to lethal challenge with the highly virulent, serotypically identical cardiovirus, EMCV. Protection in these mice was in the absence of detectable serum-neutralizing antibodies. Depletion of CD8(+) T lymphocytes prior to lethal EMCV challenge ablated protection in vMC24-immunized RHAbeta-/- mice. The CD8(+) T-lymphocyte-dependent protection observed in vivo may, in part, be the result of cytotoxic T-lymphocyte (CTL) activity, as CD8(+) T splenocytes exhibited in vitro cytolysis of EMCV-infected targets. The existence of virus-specific CD8(+) T-lymphocyte memory in these mice was demonstrated by increased expression of cell surface activation markers CD25, CD69, CD71, and CTLA-4 following antigen-specific reactivation in vitro. Although recall response in vMC24-immunized RHAbeta-/- mice was intact and effectual shortly after immunization, protection abated over time, as only 3 of 10 vMC24-immunized RHAbeta-/- mice survived when rechallenged 90 days later. The present study demonstrating CD8(+) T-lymphocyte-dependent protection in the absence of serum-neutralizing antibodies, coupled with our previous results indicating that vMC24-specific CD4(+) T lymphocytes confer protection against lethal EMCV in the absence of prophylactic antibodies, suggests the existence of nonhumoral protective mechanisms against picornavirus infections.

Shifting T-cell activation thresholds in autoimmunity and determinant spreading

The best-characterized autoimmune T-cell response is that to myelin basic protein (MBP). MBP has classically been regarded as a sequestered antigen that does not cause negative selection. This view has been fostered by the observation that T-cell receptor-transgenic T cells that are specific for the "immunodominant determinant" on the molecule, MBP:Ac1-11, persist as naive cells in MBP-expressing H-2u mice. The same T cells, however, can cause autoimmune pathology once they have been primed by environmental stimulation to become memory cells. Once the autoimmune response to Ac1-11 has been engaged, determinant spreading occurs and second-wave T-cell responses that are specific for weaker, "cryptic" determinants like MBP:121-140 develop. Although the nature of these cryptic determinants has been enigmatic, recent studies using MBP-/- mice have provided new insights. These studies showed that MBP is not a sequestered antigen, but one that causes negative selection; as MBP:121-140 is actually the immunodominant determinant in MBP-/- mice, it tolerizes high avidity clones in MBP+/+ mice, making it appear cryptic. Based on this new information, we attempt here to redefine the MBP-specific repertoire within the theoretical framework of the threshold model for negative selection, and we propose a model of shifting T-cell activation thresholds to explain how ignorant/naive T cells can become effector cells of autoimmune pathology and why this effector cell repertoire spreads.

Persisting viruses and chronic inflammation: understanding their relation to autoimmunity

Viral infections may induce and sustain autoimmune processes via several and overlapping mechanisms. We outline how chronic inflammation, sustained by persisting viruses, may be "the prerequisite" for initiation and maintenance of the multistep process leading to autoimmunity. Chronic inflammation may favour priming of autoreactive T cells which have escaped thymic tolerance and are able to mount a cross-reactive response to self-mimicking antigens carried by viruses in the periphery. Moreover, chronic inflammation and persisting viruses can synergistically support autoimmunity through other relevant mechanisms: unveiling of cryptic self-epitopes, determinant spreading, activation of dendritic cells, constant priming of new autoreactive T cells, and efficient generation and restimulation of memory cells. Therefore, viruses seem to play a key role among the many environmental factors which, together with the genetic background, have been implicated in the pathogenesis of autoimmune diseases. We will also discuss some hypotheses explaining why autoimmunity is a rare event.

Qualitative Differences Between Naive and Memory T Cells Make a Major Contribution to the More Rapid and Efficient Memory CD8+ T Cell Response

CD8+ T cells are present at a higher frequency following a primary response, and these memory cells exhibit qualitative differences from naive cells. The importance of these differences vs increased precursor frequency in making a memory response more rapid and efficient has been unclear. Adoptive transfer of 2C TCR transgenic CD8+ T cells into normal recipients, followed by i.p. challenge with allogeneic P815 tumor, results in a long-lived memory population that includes both endogenous host CD8+ T cells and 2C cells. The 2C cells can be identified, using 1B2 mAb specific for the TCR, and thus used as an indicator of the properties of the memory cells. The memory cells have a heterogeneous surface phenotype, and their distribution in lymphoid organs, blood, and peripheral sites is distinct from that of naive cells. Upon rechallenge with Ag, memory cells access the peritoneal cavity much more rapidly than do naive cells (12 h vs 5 days). This appears to result from a requirement for naive cells to interact with Ag before they can efficiently migrate to inflammatory sites, while this is not required for memory cells. In addition, memory cells exhibit some cytolytic activity before rechallenge with Ag, and potent cytolytic activity is present in the peritoneal cavity within 12 h of rechallenge. Comparison of primary and memory responses in mice having similar frequencies of Ag-specific precursors demonstrated that the more rapid migration and the immediate effector function of at least some memory cells contribute very substantially to making a memory response at a peripheral site more rapid and efficient.

Evolution of a complex T cell receptor repertoire during primary and recall bacterial infection

The mechanisms underlying the genesis and maintenance of T cell memory remain unclear. In this study, we examined the evolution of a complex, antigen-specific T cell population during the transition from primary effector to memory T cells after Listeria monocytogenes infection. T cell populations specific for listeriolysin O (LLO)91-99, the immunodominant epitope recognized by H2-Kd-restricted T lymphocytes, were directly identified in immune spleens using tetrameric H2-Kd-epitope complexes. The T cell receptor (TCR) Vbeta repertoire of specific T cells was determined by direct, ex vivo staining with a panel of mAbs. We demonstrate that LLO91-99-specific, primary effector T cell populations have a diverse TCR Vbeta repertoire. Analyses of memory T cell populations demonstrated similar TCR diversity. Furthermore, experiments with individual mice demonstrated that primary effector and memory T cells have indistinguishable TCR repertoires. Remarkably, after reinfection with L. monocytogenes, LLO91-99-specific T cells have a narrower TCR repertoire than do primary effector or memory T cells. Thus, our studies show that the TCR repertoire of primary effector T lymphocytes is uniformly transmitted to memory T cells, whereas expansion of memory T cells is selective.

In Vivo Persistence of CD8 Polarized T Cell Subsets Producing Type 1 or Type 2 Cytokines

Naive CD8 T cells can be polarized into effectors producing the type 1 cytokines IFN-γ and IL-2 or the type 2 cytokines IL-4, IL-5, and IL-10, respectively. To study whether the polarized cytokine phenotype of the effectors is stable, we generated highly cytotoxic hemagglutinin (HA) peptide-specific CD8 Tc1 and Tc2 (cytotoxic CD8 T cells producing type 1 or type 2 cytokines) effectors from Clone-4 TCR-transgenic mice, which were adoptively transferred into syngeneic adult thymectomized irradiated and bone marrow-reconstituted recipients. The highly activated blast-size, CD25+ Tc1 and Tc2 effectors gave rise to homogeneous resting CD25−CD44highLy6Chigh Ag-specific populations, which persisted for at least 13 wk after adoptive transfer. These memory CD8 T cells, recovered 13 wk after transfer of Tc1 or Tc2 effectors, still produced either the type 1 or type 2 cytokines, i.e., IFN-γ, or IL-4 and IL-5, respectively, upon restimulation with APCs loaded with the HA peptide, but not in the absence of Ag. The amounts of IL-2 detected in the supernatants of Tc1 and Tc2 memory populations were comparable to that in memory CD4 cells, and both Tc1 and Tc2 memory cells became cytotoxic upon restimulation. Thus, cytokine-polarized CD8 memory T cells are a source of a variety of cytokines, which were classically considered helper cytokines, opening new perspectives on their function as regulatory cells in an immune response.

Distinct methylation of the interferon gamma (IFN-gamma) and interleukin 3 (IL-3) genes in newly activated primary CD8+ T lymphocytes: regional IFN-gamma promoter demethylation and mRNA expression are heritable in CD44(high)CD8+ T cells

Differential genomic DNA methylation has the potential to influence the development of T cell cytokine production profiles. Therefore, we have conducted a clonal analysis of interferon (IFN)-gamma and interleukin (IL)-3 gene methylation and messenger (m)RNA expression in primary CD8+ T cells during the early stages of activation, growth, and cytokine expression. Despite similar distributions and densities of CpG methylation sites, the IFN-gamma and IL-3 promoters exhibited differential demethylation in the same T cell clone, and heterogeneity between clones. Methylation patterns and mRNA levels were correlated for both genes, but demethylation of the IFN-gamma promoter was widespread across >300 basepairs in clones expressing high levels of IFN-gamma mRNA, whereas demethylation of the IL-3 promoter was confined to specific CpG sites in the same clones. Conversely, the majority of clones expressing low or undetectable levels of IFN-gamma mRNA exhibited symmetrical methylation of four to six of the IFN-gamma promoter CpG sites. Genomic DNA methylation also has the potential to influence the maintenance or stability of T cell cytokine production profiles. Therefore, we also tested the heritability of IFN-gamma gene methylation and mRNA expression in families of clones derived from resting CD44(low)CD8+ T cells or from previously activated CD44(high)CD8+ T cells. The patterns of IFN-gamma gene demethylation and mRNA expression were faithfully inherited in all clones derived from CD44(high) cells, but variable in clones derived from CD44(low) cells. Overall, these findings suggest that differential genomic DNA methylation, including differences among cytokine genes, among individual T cells, and among T cells with different activation histories, is an important feature of cytokine gene expression in primary T cells.

Endogenous myelin basic protein inactivates the high avidity T cell repertoire

To study the contribution of endogenous myelin basic protein (MBP) to the positive and/or negative selection of the MBP-specific T cell repertoire, we studied the T cell response to MBP in MBP-deficient shiverer and MBP-expressing congenic C3H mice. Immunization with MBP induced a vigorous T cell response in shiverer mice directed against a single I-Ak- restricted immunodominant determinant, the core of which is peptide MBP:79-87 (DENPVVHFF). Injection of this peptide induced a high avidity T cell repertoire in shiverer mice that primarily consisted of clones capable of recognizing the native MBP protein in addition to the peptide itself. These data show that endogenous MBP is not required for the positive selection of an MBP-specific T cell repertoire. C3H mice, in contrast, were selectively unresponsive to the MBP protein and injection of MBP:79-87 peptide induced a low avidity repertoire that could be stimulated only by the peptide, not by the protein. Therefore, endogenous MBP induced profound inactivation of high avidity clones specific for the immunodominant determinant making that determinant appear cryptic.

T cell memory

Immunological memory can be defined as the faster and stronger response of an animal that follows reexposure to the same antigen. By this definition, it is an operational property of the whole animal or the immune system. Memory cells express a different pattern of cell surface markers, and they respond in several ways that are functionally different from those of naive cells. Murine memory cells are CD44 high and low in the expression of activation markers such as CD25 (IL-2R), whereas human memory cells are CD45RA-, CD45RO+. In contrast to naive cells, memory cells secrete a full range of T cell cytokines and can be polarized to secrete particular restricted patterns of secretion for both CD4 and CD8 T cells. The requirements for the activation of memory cells for proliferation and cytokine production are not quite as strict as those of naive cells, but costimulation in the broad sense is required for optimum responses and for responses to suboptimum antigen concentrations. It would appear that memory cells can persist in the absence of antigenic stimulation and persist as nondividing cells. Reencounter with the same antigen can expand the population to a new, stable, higher level and generate a separate population of CD44 high effectors that may be required for protection, while competition from other antigens can drive it down to a lower stable level. It is unclear how or where memory cells arise, but once generated they have different pathways of recirculation and homing.

Protective immunity does not correlate with the hierarchy of virus-specific cytotoxic T cell responses to naturally processed peptides

Infection of C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV) stimulates major histocompatibility complex class I-restricted cytotoxic T cells (CTLs), which normally resolve the infection. Three peptide epitopes derived from LCMV have been shown to bind the mouse class I molecule H-2 Db and to stimulate CTL responses in LCMV-infected mice. This report describes the identity and abundance of each CTL epitope after their elution from LCMV-infected cells. Based on this information, peptide abundance was found to correlate with the magnitude of each CTL response generated after infection with LCMV. Subsequent experiments, performed to determine the antiviral capacity of each CTL specificity, indicate that the quantitative hierarchy of CTL activity does not correlate with the ability to protect against LCMV infection. This report, therefore, indicates that immunodominant epitopes should be defined, not only by the strength of the CTL response that they stimulate, but also by the ability of the CTLs to protect against infection.

Protective Immunity Does Not Correlate with the Hierarchy of Virus-specific Cytotoxic T Cell Responses to Naturally Processed Peptides

Infection of C57BL/6 mice with lymphocytic choriomeningitis virus (LCMV) stimulates major histocompatibility complex class I–restricted cytotoxic T cells (CTLs), which normally resolve the infection. Three peptide epitopes derived from LCMV have been shown to bind the mouse class I molecule H-2 Db and to stimulate CTL responses in LCMV-infected mice. This report describes the identity and abundance of each CTL epitope after their elution from LCMV-infected cells. Based on this information, peptide abundance was found to correlate with the magnitude of each CTL response generated after infection with LCMV. Subsequent experiments, performed to determine the antiviral capacity of each CTL specificity, indicate that the quantitative hierarchy of CTL activity does not correlate with the ability to protect against LCMV infection. This report, therefore, indicates that immunodominant epitopes should be defined, not only by the strength of the CTL response that they stimulate, but also by the ability of the CTLs to protect against infection.

MHC Class I Is Required for Peripheral Accumulation of CD8+ Thymic Emigrants

MHC molecules influence the fate of T lymphocytes at two important stages of their differentiation. Recognition of self peptide/MHC complexes in the thymus determines whether immature T cells should live and mature into immunocompetent T cells or whether they should die. In the periphery, recognition of Ags presented by MHC molecules induces T cell activation, proliferation, and differentiation into effector/memory T cells. We describe in this work a third role that MHC molecules play in T cell physiology. CD8+ thymic emigrants require presence of MHC class I molecules in the periphery to seed the peripheral lymphoid organs. Numbers of CD8+ T cells are reduced severely in both the thymus and the periphery of β2-microglobulin-deficient (β2m−/−) mice. When grafted with wild-type (β2m+/+) thymic epithelium, immature β2m−/− T cells that populate the graft develop into functional mature CD8+ cells. However, significant numbers of peripheral CD8+ cells in grafted β2m−/− mice can be observed only after injection of MHC class I-expressing cells in the periphery. Thus, naive T cells in the periphery do not passively await antigenic stimulation, but actively engage in interactions with self MHC molecules that may promote their survival.

High-resolution characterization of cytokine-producing alloreactivity in naive and allograft-primed mice

BACKGROUND

Whether alloreactive T cells in a naive host derive from naive or memory T cells remains unclear. It is also unclear whether graft rejection alters the phenotype of these T cells. Proliferation assays and cytokine enzyme-linked immunosorbent assays performed on culture supernatants do not differentiate primary T-cell alloreactivity from recall responses in allograft-primed mice, suggesting that these methods are inadequate measures of the alloreactive immune repertoire.

METHODS

To better characterize alloreactivity in naive and skin allograft-primed mice, we used a modified, high-resolution cytokine enzyme-linked immunosorbent spot assay capable of detecting cytokine production over short time periods.

RESULTS

Twenty-four-hour analysis of alloreactivity in mice that rejected fully MHC-disparate skin allografts revealed a high frequency of interferon (IFN)-gamma- and interleukin (IL)-4-producing, L-selectin-negative T cells, consistent with a memory phenotype. In contrast, 24-hr allostimulation of T cells from naive mice resulted in IL-2 production with minimal secretion of IFN-gamma or IL-4. The frequency of IL-2 producers was low and their phenotype was L-selectin positive, suggesting that they were naive and not memory T cells. When maintained in culture for 48 hr, however, the T cells from the primary mixed lymphocyte reaction began producing IFN-gamma, consistent with in vitro priming.

CONCLUSIONS

The primary alloresponse does not seem to involve clones that have been preprimed by environmental antigens, but instead behaves similarly to self-MHC-restricted immunity directed toward prototypic protein antigens: T cells with a naive phenotype are specifically induced to differentiate into high-frequency memory populations. These findings may have important implications for therapeutic induction of allograft tolerance.

CD8+ Memory T Cells (CD44high, Ly-6C+) Are More Sensitive than Naive Cells (CD44low, Ly-6C−) to TCR/CD8 Signaling in Response to Antigen

Memory CD8+ T cells from mice previously primed with alloantigen (alloAg) can respond in vitro to IL-2 and purified class I alloAg presented on microspheres, while no response can be detected using cells from naive mice. Similar results have been obtained using cells from OT-1 mice expressing a transgenic TCR that is specific for OVA257–264 (SIINFEKL) peptide bound to H-2Kb. A population of resting memory cells (defined on the basis of low forward scatter and CD44high, Ly-6C+, CD25−, CD69− surface phenotype) that is present in the OT-1 mice exhibits a substantially higher sensitivity to Ag-stimulation than do naive cells (CD44low, Ly-6C−) expressing the same TCR. CD44high cells respond vigorously to H-2Kb immobilized on microspheres and pulsed with peptide, while CD44low cells respond weakly and only at high class I density and peptide concentration. The Ag-presenting surface only has ligands for TCR and CD8 (class I and peptide), thus ruling out the possibility that differences are due to ligand binding by other adhesion or costimulatory receptors that are expressed at high levels on the memory cells. Experiments using anti-TCR mAb as the stimulus and coimmobilized non-Ag class I as a ligand for CD8 suggest that the difference between naive and memory cells may be at the level of stimulation through the TCR. Thus, in addition to expressing increased levels of adhesion receptors that may enhance responses to Ag on APCs, memory CD8+ T cells appear to be intrinsically more sensitive than naive cells to stimulation through the TCR/CD8 complex.

Counting antigen-specific CD8 T cells: a reevaluation of bystander activation during viral infection

Viral infections induce extensive T cell proliferation in vivo, but the specificity of the majority of the responding T cells has not been defined. To address this issue we used tetramers of MHC class I molecules containing viral peptides to directly visualize antigen-specific CD8 T cells during acute LCMV infection of mice. Based on tetramer binding and two sensitive assays measuring interferon-gamma production at the single-cell level, we found that 50%-70% of the activated CD8 T cells were LCMV specific [2 x 10(7) virus-specific cells/spleen]. Following viral clearance, antigen-specific CD8 T cell numbers dropped to 10(6) per spleen and were maintained at this level for the life of the mouse. Upon rechallenge with LCMV, there was rapid expansion of memory T cells, but after infection with the heterologous vaccinia virus there was no detectable change in the numbers of LCMV-specific memory CTL. Therefore, much of the CD8 T cell expansion seen during viral infection represents antigen-specific cells and warrants a revision of our current thinking on the size of the antiviral response.

The duration of antigenic stimulation determines the fate of naive and effector T cells

It is known that T cells engage antigen-presenting cells (APCs) in a stable interaction that results in sustained TCR signaling. We show here that the duration of this process is critical in determining whether T cells will be activated or deleted. Whereas naive T cells require approximately 20 hr of sustained signaling to be committed to proliferation, effector T cells become committed after only 1 hr but die following activation if antigenic stimulation is prolonged. Costimulation by anti-CD28 facilitates T cell activation by decreasing the time of commitment and by protecting T cells from death. These findings explain in quantitative terms the essential requirement for professional APCs in T cell priming and show that the duration of antigenic stimulation is the major factor determining the fate of naive and effector T cells.

Hepatocytes induce functional activation of naive CD8+ T lymphocytes but fail to promote survival

Intraperitoneal peptide injection of TCR-transgenic mice or expression of antigen in hepatocytes leads to an accumulation in the liver of specific apoptotic CD8+ T cells expressing activation markers. To determine whether liver cells are capable of directly activating naive CD8+ T cells, we have studied the ability of purified hepatocytes to activate TCR-transgenic CD8+ T cells in vitro. We show that hepatocytes which do not express CD80 and CD86 co-stimulatory molecules are able to induce activation and effective proliferation of specific naive CD8+ T cells in the absence of exogenously added cytokines, a property only shared by professional antigen-presenting cells (APC). Specific T cell proliferation induced by hepatocytes was comparable in magnitude to that seen in response to dendritic cells and was independent of CD4+ T cell help or bystander professional APC co-stimulation. During the first 3 days, the same number of divisions was observed in co-cultures of CD8+ T cells with either hepatocytes or splenocytes. Both APC populations induced expression of early T cell activation markers and specific cytotoxic T lymphocyte (CTL) activity. However, in contrast to T cells activated by splenocytes, T cells activated by hepatocytes lost their cytolytic function after 3 days of co-culture. This correlated with death of activated T cells, suggesting that despite efficient activation, proliferation and transient CTL function, T cells activated by hepatocytes did not survive. Death could be prevented by adding antigen-expressing splenocytes or exogenous IL-2 to the co-culture, indicating that hepatocytes are not involved in direct killing of CD8+ T cells but rather fail to promote survival. Dying cells acquired a CD8(low) TCR(low) B220+ phenotype similar to the one described for apoptotic intrahepatic T cells, suggesting an alternative model to account for the origin of these cells in the liver. The importance of these findings for the understanding of peripheral tolerance and the ability of liver grafts to be accepted is discussed.

The minimal number of antigen-major histocompatibility complex class II complexes required for activation of naive and primed T cells

Although previous studies have shown that 50-200 antigen-major histocompatibility complex complexes (Ag-MHC) are sufficient to stimulate significant secretion of interleukin (IL)-2 from MHC class II-restricted T cell hybridomas, there have been no studies of this nature on more physiologically relevant T cell populations. In this study we have analyzed the ligand requirements for stimulation of responses from naive and previously primed T cells derived from T cell receptor (TCR)-transgenic animals whose TCR is specific for the pigeon cytochrome c (PCC) 88-104 peptide presented by I-Ek. Primed T cells were as sensitive as the previously reported T cell hybridomas, requiring about 100 Ag-MHC complexes to synthesize readily detectable quantities of IL-2, whereas naive T cells required 15 times more ligand to produce equivalent quantities of IL-2. Similarly, primed T cells required about 40 Ag-MHC complexes to produce a significant proliferative response, whereas naive T cells required about 400 complexes. In contrast to these results, naive and primed T cells showed similar ligand requirements when early events in the T cell activation pathway were analyzed; i.e. TCR down-modulation, CD69 and CD25 expression, and blast transformation. A further analysis of IL-2 and IL-2R expression indicated: 1) The first synthesis of IL-2 was detected at the same ligand concentration in both primed and naive T cells, but primed T cells made much more IL-2 as the ligand concentrations increased; 2) primed T cells expressed about fivefold more IL-2 receptor (R) than naive T cells, despite the fact that the antigen dose-response curves with respect to the percentage of cells expressing IL-2R were identical. These results suggest that naive and primed T cells have the same threshold with respect to the number of Ag-MHC complexes required to initiate T cell activation, but that due to the inefficient expression of IL-2 and IL-2R, engagement of more complexes is needed to enable naive T cells to synthesize the necessary amounts of these two molecules to allow T cells to go through a complete cycle of replication.

Recruitment, activation and proliferation of CD8+ memory T cells in an immunoprivileged site

The capacity of a memory cytotoxic T lymphocyte (CTL) population to protect against viral infections is well established, but the processes underlying this protection are less well understood. We have used heterotypic intranasal immunization with influenza A/X31 (H3N2) to protect against a subsequent infection with the neurovirulent influenza A/WSN (H1N1) in either the cerebrospinal fluid or the immunoprivileged brain parenchyma. Viral clearance from both sites was associated with a local infiltration and proliferation of A/WSN-specific CD8+ T cells. Infection in the cerebrospinal fluid elicited a proliferative response in the draining lymph nodes, an anti-H1N1 serum antibody response and an increase in the extracerebral A/WSN-specific CTL precursor frequency. In contrast, infection in the brain parenchyma elicited no lymph node proliferative response or serum antibody response and caused a transient decrease in the extracerebral CTL precursor frequency. Thus the memory CTL population protected against an intracerebral viral infection independent of any immune response occurring in systemic lymphoid tissue.

A functional and kinetic comparison of antiviral effector and memory cytotoxic T lymphocyte populations in vivo and in vitro

To analyze the critical parameters for effective antiviral cytotoxic T lymphocyte (CTL) activity in vivo, control of lymphocytic choriomeningitis virus (LCMV) infection in the spleen was studied after adoptive transfer of different spleen cell populations into preinfected recipients. The quantitative, qualitative and kinetic requirements for virus control were defined and related to in vitro assays to compare the antiviral protective function of CTL from naive, acutely infected and memory mice. Treatment of mice with an established but limited LCMV infection by adoptive transfer of spleen cells from acutely LCMV-infected mice led to complete virus elimination mainly mediated by donor-derived CD8+ T cell-mediated, perforin-dependent cytotoxicity. Since virus is continuously spreading and the number of infected target cells rapidly increases, the time until target cell lysis is achieved was critical: if release of viral progeny was not prevented early, additional time to perform effector function did not improve overall virus control. When the function of various cell populations was compared in this model, we found that CTL from naive and memory mice perform considerably less well than CTL from acutely infected mice. In vitro studies indicated that this is probably due to the fact that they can not fulfill the limiting time requirements for immediate antiviral protection: while CTL from acutely infected mice can perform lytic effector function immediately, memory CTL require a considerable reactivation time before they can lyse infected target cells. This reactivation does not necessarily involve cell division. These findings illustrate how critical time limitations are for CTL to mediate early control of a dynamic virus infection in vivo.

Immunological memory

Most of the antigen-specific T and B cells participating in the primary immune response are rapidly eliminated, but some of the cells survive and become long-lived memory cells. There have been a number of recent developments on the features and functions of memory cells.

Factors controlling the turnover of T memory cells

Most of the T cells participating in the primary immune response are rapidly eliminated, but small numbers of these cells survive and differentiate into long-lived memory cells. Information on the life history of memory cells can be obtained by studying the component of memory-phenotype T cells found in normal animals; these cells are presumed to represent memory cells specific for various environmental antigens. For CD8+ cells, in vivo exposure to viruses and certain other infectious agents causes a large proportion of memory-phenotype (CD44hi) cells to enter the cell cycle. In this situation, stimulation of CD44hi CD8+ cells does not seem to require T-cell receptor ligation and appears to reflect release of various cytokines, especially type I interferon. The capacity of infectious agents to induce non-antigen-specific stimulation of T cells may play a role in boosting the survival of memory cells and perhaps also in providing an adjuvant function during the primary response.