Peripheral clonal deletion of antiviral memory CD8+ T cells

Central tolerance shapes the neutralizing B cell repertoire against a persisting virus in its natural host

Viral mimicry of host cell structures has been postulated to curtail the B cell receptor (BCR) repertoire against persisting viruses through tolerance mechanisms. This concept awaits, however, experimental testing in a setting of natural virus-host relationship. We engineered mouse models expressing a monoclonal BCR specific for the envelope glycoprotein of lymphocytic choriomeningitis virus (LCMV), a naturally persisting mouse pathogen. When the heavy chain of the LCMV-neutralizing antibody KL25 was paired with its unmutated ancestor light chain, most B cells underwent receptor editing, a behavior reminiscent of autoreactive clones. In contrast, monoclonal B cells expressing the same heavy chain in conjunction with the hypermutated KL25 light chain did not undergo receptor editing but exhibited low levels of surface IgM, suggesting that light chain hypermutation had lessened KL25 autoreactivity. Upon viral challenge, these IgMlow cells were not anergic but up-regulated IgM, participated in germinal center reactions, produced antiviral antibodies, and underwent immunoglobulin class switch as well as further affinity maturation. These studies on a persisting virus in its natural host species suggest that central tolerance mechanisms prune the protective antiviral B cell repertoire.

Soluble monomeric human programmed cell death-ligand 1 inhibits the functions of activated T cells

Introduction

The presence of soluble human programmed cell death-ligand 1 (shPD-L1) in the blood of patients with cancer has been reported to be negatively correlated with disease prognosis. However, little information exists about the mechanisms underlying high levels of shPD-L1 for promoting disease progression.

Methods

In this study, we first analyzed the correlations between shPD-L1 and apoptosis of T cells in patients with cancer, then tested the effect of shPD-L1 on T-cell functions and the production of regulatory T cells.

Results

We found that the apoptosis of human peripheral PD-1+CD4+ T cells was significantly elevated in patients with cancer compared with healthy donors and was positively correlated with circulating PD-L1 levels in patients with cancer. In vitro, monomeric shPD-L1 significantly inhibited the proliferation, cytokine secretion, and cancer cell-killing activity of peripheral blood mononuclear cells (PBMCs) activated by either agonist antibodies or HATac (high-affinity T cell activation core)-NYE (NY-ESO-1 antigen). It also promoted CD4+ T cells to express forkhead family transcription factor 3 (FoxP3) for the conversion of induced T regulatory cells, which was more significant than that mediated by soluble human PD-L1 fusion protein (shPD-L1-Fc).

Discussion

These results confirm that soluble PD-L1 could be a candidate for inhibiting the functions of activated T cells, promoting peripheral tolerance to tumor cells, and implicating in system tumor immune escape in addition to the tumor microenvironment. This is an important mechanism explaining the negative correlation between peripheral blood PD-L1 levels and cancer prognosis. Therefore, understanding the roles of hPD-L1 in peripheral blood will be helpful for the development of precision immunotherapy programs in treating various tumors.

Chronic Viral Infection Promotes Efficient Germinal Center B Cell Responses

Persistent viral infections subvert key elements of adaptive immunity. To compare germinal center (GC) B cell responses in chronic and acute lymphocytic choriomeningitis virus infection, we exploit activation-induced deaminase (AID) fate-reporter mice and perform adoptive B cell transfer experiments. Chronic infection yields GC B cell responses of higher cellularity than acute infections do, higher memory B cell and antibody secreting cell output for longer periods of time, a better representation of the late B cell repertoire in serum immunoglobulin, and higher titers of protective neutralizing antibodies. GC B cells of chronically infected mice are similarly hypermutated as those emerging from acute infection. They efficiently adapt to viral escape variants and even in hypermutation-impaired AID mutant mice, chronic infection selects for GC B cells with hypermutated B cell receptors (BCRs) and neutralizing antibody formation. These findings demonstrate that, unlike for CD8⁺ T cells, chronic viral infection drives a functional, productive, and protective GC B cell response.

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Chronic viral infection elicits potent and sustained germinal center (GC) responses

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Chronic infection triggers prolonged plasma cell and memory B cell output from GCs

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GC B cells hypermutate efficiently and are potently selected in chronic infection

Cancer and HIV-1 Infection: Patterns of Chronic Antigen Exposure

The main role of the human immune system is to eliminate cells presenting foreign antigens and abnormal patterns, while maintaining self-tolerance. However, when facing highly variable pathogens or antigens very similar to self-antigens, this system can fail in completely eliminating the anomalies, leading to the establishment of chronic pathologies. Prototypical examples of immune system defeat are cancer and Human Immunodeficiency Virus-1 (HIV-1) infection. In both conditions, the immune system is persistently exposed to antigens leading to systemic inflammation, lack of generation of long-term memory and exhaustion of effector cells. This triggers a negative feedback loop where effector cells are unable to resolve the pathology and cannot be replaced due to the lack of a pool of undifferentiated, self-renewing memory T cells. In addition, in an attempt to reduce tissue damage due to chronic inflammation, antigen presenting cells and myeloid components of the immune system activate systemic regulatory and tolerogenic programs. Beside these homologies shared between cancer and HIV-1 infection, the immune system can be shaped differently depending on the type and distribution of the eliciting antigens with ultimate consequences at the phenotypic and functional level of immune exhaustion. T cell differentiation, functionality, cytotoxic potential and proliferation reserve, immune-cell polarization, upregulation of negative regulators (immune checkpoint molecules) are indeed directly linked to the quantitative and qualitative differences in priming and recalling conditions. Better understanding of distinct mechanisms and functional consequences underlying disease-specific immune cell dysfunction will contribute to further improve and personalize immunotherapy. In the present review, we describe relevant players of immune cell exhaustion in cancer and HIV-1 infection, and enumerate the best-defined hallmarks of T cell dysfunction. Moreover, we highlight shared and divergent aspects of T cell exhaustion and T cell activation to the best of current knowledge.

TIGIT limits immune pathology during viral infections

Co-inhibitory pathways have a fundamental function in regulating T cell responses and control the balance between promoting efficient effector functions and restricting immune pathology. The TIGIT pathway has been implicated in promoting T cell dysfunction in chronic viral infection. Importantly, TIGIT signaling is functionally linked to IL-10 expression, which has an effect on both virus control and maintenance of tissue homeostasis. However, whether TIGIT has a function in viral persistence or limiting tissue pathology is unclear. Here we report that TIGIT modulation effectively alters the phenotype and cytokine profile of T cells during influenza and chronic LCMV infection, but does not affect virus control in vivo. Instead, TIGIT has an important effect in limiting immune pathology in peripheral organs by inducing IL-10. Our data therefore identify a function of TIGIT in limiting immune pathology that is independent of viral clearance.

TIGIT is a lymphocyte co-inhibitory receptor that can limit type 1 and cytotoxic T cell responses and maintain immunological tolerance. Here the authors show that TIGIT also limits immune pathology during LCMV or influenza infections in mice by driving IL-10 expression without negatively affecting the viral load.

Antigen-specific immune decoys intercept and exhaust autoimmunity to prevent disease

Relapsing-remitting patterns of many autoimmune diseases such as multiple sclerosis (MS) are perpetuated by a recurring circuit of adaptive immune cells that amplify in secondary lymphoid organs (SLOs) and traffic to compartments where antigen is abundant to elicit damage. Some of the most effective immunotherapies impede the migration of immune cells through this circuit, however, broadly suppressing immune cell migration can introduce life-threatening risks for patients. We developed antigen-specific immune decoys (ASIDs) to mimic tissues targeted in autoimmunity and selectively intercept autoimmune cells to preserve host tissue. Using Experimental Autoimmune Encephalomyelitis (EAE) as a model, we conjugated autoantigen PLP_139-151 to a microporous collagen scaffold. By subcutaneously implanting ASIDs after induction but prior to the onset of symptoms, mice were protected from paralysis. ASID implants were rich with autoimmune cells, however, reactivity to cognate antigen was substantially diminished and apoptosis was prevalent. ASID-implanted mice consistently exhibited engorged spleens when disease normally peaked. In addition, splenocyte antigen-presenting cells were highly activated in response to PLP rechallenge, but CD3⁺ and CD19 ⁺ effector subsets were significantly decreased, suggesting exhaustion. ASID-implanted mice never developed EAE relapse symptoms even though the ASID material had long since degraded, suggesting exhausted autoimmune cells did not recover functionality. Together, data suggested ASIDs were able to sequester and exhaust immune cells in an antigen-specific fashion, thus offering a compelling approach to inhibit the migration circuit underlying autoimmunity.

Coinhibitory receptors and CD8 T cell exhaustion in chronic infections

PURPOSE OF REVIEW

To describe the recent data on the role of coinhibitory receptors, such as PD-1, Tim-3, CD160, as mediators of the 'exhaustion' of virus-specific CD8 T cells in chronic infections and particularly in HIV.

RECENT FINDINGS

Exhaustion of chronic virus-specific CD8 T cells is a dynamic process characterized by altered differentiation, impaired function, and compromised proliferation/survival profile of these cells. This process is mediated by coinhibitory receptors expressed on the surface of virus-specific CD8 T cells and an orchestrated function of centrally connected pathways. Coexpression of several coinhibitory receptors characterizes severely exhausted virus-specific CD8 T cells. Several studies suggest a synergistic action, instead of a redundant role, of the different receptors. In-vivo manipulation of the coinhibitory network can rejuvenate exhausted virus-specific CD8 T cell responses and constrain replication of chronic viruses, including HIV.

SUMMARY

Revealing the molecular basis of virus-specific CD8 T cell exhaustion in chronic infections is critical for the understanding of the disease pathogenesis and the designing of novel vaccines aiming to enhance the cytolytic arm of the immune system. This is of particular interest for the development of immunotherapies in the context of a functional cure for HIV.

Peptide-based immunotherapy: a novel strategy for allergic disease

The T-cell component of the antigen-specific immune response is the target of various novel interventions to modify chronic immunologic disorders, such as allergic diseases. Recent clinical trials have evaluated the safety and efficacy of therapeutic vaccines consisting of short, synthetic, allergen-derived peptides, corresponding to T-cell epitopes from the eliciting antigen. The main advantage of such an approach is the reduction in systemic, immunoglobulin E-mediated adverse events compared with existing whole allergen immunotherapy, often referred to as 'allergy shots'. T-cell peptide epitopes, although capable of inducing immunologic tolerance, are short linear structures that have reduced ability to cross-link mast cell- and basophil-bound immunoglobulin E. The precise mechanism of tolerance induction remains incompletely defined. However, recent data indicate that peptide therapy induces/expands a population of antigen-specific regulatory T-cells. A novel form of treatment combining efficacy with a substantially decreased occurrence of adverse events is likely to have a major impact on the management and prevalence of allergic diseases. Furthermore, the principles of epitope-specific therapy hold promise for the development of therapeutic vaccines for the treatment of autoimmune diseases.

Between Scylla and Charybdis: The role of the human immune system in the pathogenesis of hepatitis C

Hepatitis C virus (HCV) frequently elicits only mild immune responses so that it can often establish chronic infection. In this case HCV antigens persist and continue to stimulate the immune system. Antigen persistence then leads to profound changes in the infected host’s immune responsiveness, and eventually contributes to the pathology of chronic hepatitis. This topic highlight summarizes changes associated with chronic hepatitis C concerning innate immunity (interferons, natural killer cells), adaptive immune responses (immunoglobulins, T cells, and mechanisms of immune regulation (regulatory T cells). Our overview clarifies that a strong anti-HCV immune response is frequently associated with acute severe tissue damage. In chronic hepatitis C, however, the effector arms of the immune system either become refractory to activation or take over regulatory functions. Taken together these changes in immunity may lead to persistent liver damage and cirrhosis. Consequently, effector arms of the immune system will not only be considered with respect to antiviral defence but also as pivotal mechanisms of inflammation, necrosis and progression to cirrhosis. Thus, avoiding Scylla - a strong, sustained antiviral immune response with inital tissue damage - takes the infected host to virus-triggered immunopathology, which ultimately leads to cirrhosis and liver cancer - the realm of Charybdis.

PC61 (anti-CD25) treatment inhibits influenza A virus-expanded regulatory T cells and severe lung pathology during a subsequent heterologous lymphocytic choriomeningitis virus infection

Prior immunity to influenza A virus (IAV) in mice changes the outcome to a subsequent lymphocytic choriomeningitis virus (LCMV) infection and can result in severe lung pathology, similar to that observed in patients that died of the 1918 H1N1 pandemic. This pathology is induced by IAV-specific memory CD8(+) T cells cross-reactive with LCMV. Here, we discovered that IAV-immune mice have enhanced CD4(+) Foxp3(+) T-regulatory (Treg) cells in their lungs, leading us to question whether a modulation in the normal balance of Treg and effector T-cell responses also contributes to enhancing lung pathology upon LCMV infection of IAV-immune mice. Treg cell and interleukin-10 (IL-10) levels remained elevated in the lungs and mediastinal lymph nodes (mLNs) throughout the acute LCMV response of IAV-immune mice. PC61 treatment, used to decrease Treg cell levels, did not change LCMV titers but resulted in a surprising decrease in lung pathology upon LCMV infection in IAV-immune but not in naive mice. Associated with this decrease in pathology was a retention of Treg in the mLN and an unexpected partial clonal exhaustion of LCMV-specific CD8(+) T-cell responses only in IAV-immune mice. PC61 treatment did not affect cross-reactive memory CD8(+) T-cell proliferation. These results suggest that in the absence of IAV-expanded Treg cells and in the presence of cross-reactive memory, the LCMV-specific response was overstimulated and became partially exhausted, resulting in a decreased effector response. These studies suggest that Treg cells generated during past infections can influence the characteristics of effector T-cell responses and immunopathology during subsequent heterologous infections. Thus, in humans with complex infection histories, PC61 treatment may lead to unexpected results.

Antigen amount dictates CD8+ T-cell exhaustion during chronic viral infection irrespective of the type of antigen presenting cell

Chronic viral infections lead to CD8(+) T-cell exhaustion, characterized by impaired cytokine secretion and loss of proliferative capacity. While viral load and T-cell dysfunction correlate, it is currently unclear whether the quality of a cell type presenting antigen determines the degree of T-cell exhaustion or if the overall amount of antigen recognized by T cells promotes exhaustion. We found that chronic lymphocytic chorio-meningitis virus infection led to decreased CD8(+) T-cell exhaustion in DC-MHC class I (MHCI) mice, in which CD8(+) T cells can only recognize antigen on DCs. However, this increase in CD8(+) T-cell function came at the expense of fatal immunopathology. Additional antigen recognition on nonhematopoietic cells in DC-MHCI mice promoted T-cell exhaustion and avoidance of immunopathology. Likewise, increased numbers of antigen-expressing hematopoietic cells, as well as a selective elevation of the number of DCs as the only cell type presenting antigen in DC-MHCI mice, resulted in compromised T-cell function. These results favor a scenario in which the overall amount of antigen exposure, rather than the type of cell engaging with virus-specific CD8(+) T cells, is responsible for their functional exhaustion. Furthermore, exhaustion of virus-specific CD8(+) T cells leads to avoidance of life-threatening immunopathology.

Vaccination alters the balance between protective immunity, exhaustion, escape, and death in chronic infections

While T cell-based vaccines have the potential to provide protection against chronic virus infections, they also have the potential to generate immunopathology following subsequent virus infection. We develop a mathematical model to investigate the conditions under which T cells lead to protection versus adverse pathology. The model illustrates how the balance between virus clearance and immune exhaustion may be disrupted when vaccination generates intermediate numbers of specific CD8 T cells. Surprisingly, our model suggests that this adverse effect of vaccination is largely unaffected by the generation of mutant viruses that evade T cell recognition and cannot be avoided by simply increasing the quality (affinity) or diversity of the T cell response. These findings should be taken into account when developing vaccines against persistent infections.

Tricks with tetramers: how to get the most from multimeric peptide-MHC

The development of fluorochrome-conjugated peptide-major histocompatibility complex (pMHC) multimers in conjunction with continuing advances in flow cytometry has transformed the study of antigen-specific T cells by enabling their visualization, enumeration, phenotypic characterization and isolation from ex vivo samples. Here, we bring together and discuss some of the 'tricks' that can be used to get the most out of pMHC multimers. These include: (1) simple procedures that can substantially enhance the staining intensity of cognate T cells with pMHC multimers; (2) the use of pMHC multimers to stain T cells with very-low-affinity T-cell receptor (TCR)/pMHC interactions, such as those that typically predominate in tumour-specific responses; and (3) the physical grading and clonotypic dissection of antigen-specific T cells based on the affinity of their cognate TCR using mutant pMHC multimers in conjunction with new approaches to the molecular analysis of TCR gene expression. We also examine how soluble pMHC can be used to examine T-cell activation, manipulate T-cell responses and study allogeneic and superantigen interactions with TCRs. Finally, we discuss the problems that arise with pMHC class II (pMHCII) multimers because of the low affinity of TCR/pMHCII interactions and lack of 'coreceptor help'.

Animal models using lymphocytic choriomeningitis virus

This unit includes protocols for inducing systemic infection and persistent infection of mice with lymphocytic choriomeningitis virus (LCMV). Methods used to measure T cell responses to LCMV are then described. A protocol to assess anti-LCMV immunity in vivo is also included. Support protocols for preparing LCMV stocks and measuring LCMV titers using a plaque assay are also included. Finally, a support protocol for detecting anti-LCMV antibodies by ELISA is presented.

Clonal focusing of epitope-specific CD8+ T lymphocytes in rhesus monkeys following vaccination and simian-human immunodeficiency virus challenge

To afford the greatest possible immune protection, candidate human immunodeficiency virus (HIV) vaccines must generate diverse and long-lasting CD8(+) T lymphocyte responses. In the present study, we evaluate T-cell receptor Vbeta (variable region beta) gene usage and a CDR3 (complementarity-determining region 3) sequence to assess the clonality of epitope-specific CD8(+) T lymphocytes generated in rhesus monkeys following vaccination and simian-human immunodeficiency virus (SHIV) challenge. We found that vaccine-elicited epitope-specific CD8(+) T lymphocytes have a clonal diversity comparable to those cells generated in response to SHIV infection. Moreover, we show that the clonal diversity of vaccine-elicited CD8(+) T-lymphocyte responses is dictated by the epitope sequence and is not affected by the mode of antigen delivery to the immune system. Clonal CD8(+) T-lymphocyte populations persisted following boosting with different vectors, and these clonal cell populations could be detected for as long as 4 years after SHIV challenge. Finally, we show that the breadth of these epitope-specific T lymphocytes transiently focuses in response to intense SHIV replication. These observations demonstrate the importance of the initial immune response to SHIV, induced by vaccination or generated during primary infection, in determining the clonal diversity of cell-mediated immune responses and highlight the focusing of this clonal diversity in the setting of high viral loads. Circumventing this restricted CD8(+) T-lymphocyte clonal diversity may present a significant challenge in the development of an effective HIV vaccine strategy.

Evasion of host immune responses by tumours and viruses

Viruses and tumours use various mechanisms to avoid immune surveillance. Oncogenic viruses have achieved a balance with the immune system through evolutionary time to ensure long-term persistence. Mutations that promote escape mechanisms favouring tumour growth to the detriment of host survival through reproductive age offer no selective advantage and will not generally be maintained in the viral genome that persists in nature. Conventional (non-oncogenic) and tumour viruses interact with various immune mediators and T cells in different ways. Oncogenic viruses cannot operate solely in the context of a lytic cycle, though this may be characteristic of the initial phase of infection that is limited by the acute immune response. Some oncogenic viruses interact with normal cellular growth control and signalling mechanisms. Synthesis of key viral proteins may be tightly controlled in replicating cells that are subject to T cell surveillance, such as basal epithelia, while productive infection occurs in non-proliferating progeny that are lost under normal physiological conditions, such as desquamating epithelia. Tumorigenesis may be an aberrant consequence of the molecular mechanisms needed to maintain this pattern of viral growth regulation in the context of the cell cycle. Vaccines designed to limit the acute phase of infection with cell-free oncogenic viruses should be as effective as those for conventional viruses.

Respiratory syncytial virus infects and abortively replicates in the lungs in spite of preexisting immunity

Respiratory syncytial virus (RSV) is a major cause of bronchiolitis and viral pneumonia in young children and a serious health risk in immunocompromised individuals and the elderly. Immunity to RSV is not completely understood. In this work, we established a method for monitoring RSV infection by real-time PCR and applied this method for analysis of RSV replication in vivo in the cotton rat model in naïve animals and in animals rendered immune to RSV by prior RSV infection. We found that even though no virus could be isolated from the lungs of RSV-challenged immune animals, RSV infection in fact took place and an accumulation of viral RNA transcripts was observed. This type of replication, therefore, can be termed "abortive," as RSV is capable of entering the cells in the lungs of immune animals, yet the production of progeny viruses is impaired. Similar patterns of RSV gene expression gradient were observed between naïve and reinfected animals, indicating that the skewing of mRNA gradient of viral gene expression, a mechanism documented during latent infection by other viruses, is not likely to be responsible for abortive replication of RSV during reinfection. We found that passive administration of antibodies to RSV prevents productive infection normally accompanied by viral release in the lung, but it does not prevent abortive replication of the virus. To the best of our knowledge, this is the first evidence of abortive replication of RSV in vivo.

Immunopathogenesis in hepatitis C virus cirrhosis

HCV (hepatitis C virus) has a high propensity to persist and to cause chronic hepatitis C, eventually leading to cirrhosis. Since HCV itself is not cytopathic, liver damage in chronic hepatitis C is commonly attributed to immune-mediated mechanisms. HCV proteins interact with several pathways in the host's immune response and disrupt pathogen-associated pattern recognition pathways, interfere with cellular immunoregulation via CD81 binding and subvert the activity of NK (natural killer) cells as well as CD4(+) and CD8(+) T-cells. Finally, HCV-specific T-cells become increasingly unresponsive and apparently disappear, owing to several possible mechanisms, such as escape mutations in critical viral epitopes, lack of sufficient help, clonal anergy or expansion of regulatory T-cells. The role of neutralizing antibodies remains uncertain, although it is still possible that humoral immunity contributes to bystander damage of virally coated cells via antibody-dependent cellular cytotoxicity. Cytotoxic lymphocytes kill HCV-infected cells via the perforin/granzyme pathway, but also release Fas ligand and inflammatory cytokines such as IFNgamma (interferon gamma). Release of soluble effector molecules helps to control HCV infection, but may also destroy uninfected liver cells and can attract further lymphocytes without HCV specificity to invade the liver. Bystander damage of these non-specific inflammatory cells will expand the tissue damage triggered by HCV infection and ultimately activate fibrogenesis. A clear understanding of these processes will eventually help to develop novel treatment strategies for HCV liver disease, independent from direct inhibition of HCV replication.

Costimulatory molecules as immunotherapeutic targets in systemic lupus erythematosus

T cells undergo full and productive activation when they traffic to lymph nodes where they encounter dendritic cells displaying foreign antigen in the context of MHC molecules on their surface. Recognition of these antigen-MHC complexes by the T cell's receptor for antigen, or T cell receptor, provides the first of two obligate signals needed to drive cell proliferation. The second antigen-independent signal is provided by the costimulatory receptor, CD28, as it engages its ligand on the antigen-presenting cells. Failure of the T cell to receive this second signal after antigen recognition leaves the T cell in a state of anergy. Understanding the role of T cell costimulatory receptors in T cell activation has led to the development of novel approaches for regulating immune responses in subjects with cancer or autoimmune disease by experimentally triggering or blocking costimulatory receptor signaling. In this review, we will discuss, first, several costimulatory pathways known to participate or regulate the progression of autoimmune disease, and, second, how manipulation of T cell costimulation and/or costimulation blockade has been used to treat systemic lupus erythematosus.

Gammaherpesvirus persistence alters key CD8 T-cell memory characteristics and enhances antiviral protection

In herpesvirus infections, the virus persists for life but is contained through T-cell-mediated immune surveillance. How this immune surveillance operates is poorly understood. Recent studies of other persistent infections have indicated that virus persistence is associated with functional deficits in the CD8(+) T-cell response. To test whether this is the case in a herpesvirus infection, we used a mutant murine gammaherpesvirus that is defective in its ability to persist in the host. By comparing the immune response to this virus with a revertant virus that can persist, we were able to dissect the changes in the antiviral CD8(+) T-cell response that are induced by virus persistence. Surprisingly, persistently infected mice controlled a secondary challenge infection more rapidly than nonpersistently infected mice, indicating enhanced rather than diminished effector functions. Consistent with this, virus-specific CD8 T cells from these mice exhibited faster upregulation of the cytotoxic mediator granzyme B. Another unexpected finding was that CD8(+) T cells from neither infection responded efficiently to homeostatic cytokines. The unresponsiveness of the memory cells from the nonpersistently infected mice appears to be linked to the prolonged replication of virus within the lungs. Other changes seen in different chronic infection models were also observed, such as changes in Bcl-2 levels, interleukin-2 production, and the immunodominance hierarchy. These data show persistence of gammaherpesvirus type 68 alters the properties of CD8(+) T cells and illustrates that immune surveillance does not require CD8 T cells with the same attributes as "classical" memory CD8(+) T cells.

Translating innate immunity into immunological memory: implications for vaccine development

Vaccination is the most effective means of preventing infectious diseases. Despite the success of many vaccines, there is presently little knowledge of the immunological mechanisms that mediate their efficacy. Such information will be critical in the design of future vaccines against old and new infectious diseases. Recent advances in immunology are beginning to provide an intellectual framework with which to address fundamental questions about how the innate immune system shapes adaptive immunity. In this review, we summarize current knowledge about how the innate immune system modulates the quantity and quality of long-term T and B cell memory and protective immune responses to pathogens. In addition, we point out unanswered questions and identify critical challenges, the solution of which, we believe, will greatly facilitate the rational design of novel vaccines against a multitude of emerging infections.

Immunotherapeutic relief from persistent infections and amyloid disorders

Persistent infections and amyloid disorders afflict a significant number of people worldwide. It would appear at first glance that the treatment of these afflictions should be entirely unrelated; however, in both cases components of the adaptive immune system have been harnessed in an attempt to provide some therapeutic relief. Given that the ability of a pathogen to establish persistence often depends in part on a shortcoming of the adaptive immune response, it seems logical to devise immunotherapies with the intention of supplementing (or replacing) the insufficient immunologic element. A case in point is an intervention referred as immunocytotherapy, which relies upon the adoptive transfer of pathogen-specific T lymphocytes into a persistently infected host. Remarkably, the adoptively transferred T lymphocytes not only have the capacity to clear the persistent infection, but can also provide the recipient with protection against subsequent rechallenge (i.e., immunologic memory). Treatment of amyloid disorders (e.g., Alzheimer disease, sporadic inclusion-body myositis) with a similar therapeutic approach is complicated by the fact that the aberrant protein accumulations are self-derived. Focusing the adaptive response on these aberrant self-proteins has the potential to result in autoimmune pathology. This review critically evaluates the importance of immunotherapeutic approaches for the treatment of persistent infections and amyloid disorders, and attempts to delineate the interventions that are most likely to succeed in an exceedingly complex disorder such as sporadic inclusion-body myositis.

The impact of CD4+CD25+ Treg on tumor specific CD8+ T cell cytotoxicity and cancer

There is sufficient evidence to suggest that tumor growth elicits specific immune responses, including CD8(+) and CD4(+) T cell responses that may delay tumor growth and could potentially be harnessed to eradicate cancer. Nevertheless the frequent outcome of cancer is lethality associated with uncontrolled growth and dissemination of tumor cells. The failure of the immune response may be naturally programmed and related to a specific subpopulation of CD4(+)CD25(+) regulatory T cells, whose function is to protect us against autoimmunity. Recent investigations have shed light on the in vivo behavior and functions of these cells. It is becoming evident that a major impact of these cells is on the cytolytic action of specific CD8(+) T cells that target the tumor. Inhibition of cytotoxicity is dependent on TGF-beta signaling by the effector cells. Thus, targeting immune regulation may provide a promising approach to the immune therapy of cancer. This approach however could also have unexpected deleterious consequences, as surprising new observations indicate that regulatory T cells can also delay tumor growth by independent mechanisms that relate to their cross talk with the innate immune response to cancer.

The role of bystander T cells in CNS pathology and pathogen clearance

It is generally accepted that both self- and pathogen-specific T lymphocytes have the potential to mediate immunopathogenesis and contribute to a variety of human ailments. Despite this unfortunate tendency to induce tissue injury, these cells are guided by interactions with peptide-loaded major histocompatibility complexes (MHC) and adhere appropriately to a vital evolutionary constraint imposed by the host: specificity. More recently, a series of studies have demonstrated that bystander T cells of an irrelevant specificity can bypass peptide/MHC restriction and become active participants in immunopathology. This review critically evaluates the role of bystander T cells in immunopathogenesis and pathogen clearance in the periphery as well as the central nervous system and attempts to establish the likelihood of their participation in human disease.

On natural and artificial vaccinations

This review summarizes the general parameters of cell- and antibody-mediated immune protection and the basic mechanisms responsible for what we call immunological memory. From this basis, the various successes and difficulties of vaccines are evaluated with respect to the role of antigen in maintaining protective immunity. Based on the fact that in humans during the first 12-48 months maternal antibodies from milk and serum protect against classical acute childhood and other infections, the concept is developed that maternal antibodies attenuate most infections of babies and infants and turn them into effective vaccines. If this "natural vaccination" under passive protective conditions does not occur, acute childhood diseases may be severe, unless infants are actively vaccinated with conventional vaccines early enough, i.e., in synchronization with the immune system's maturation. Although vaccines are available against the classical childhood diseases, they are not available for many seemingly milder childhood infections such as gastrointestinal and respiratory infections; these may eventually trigger immunopathological diseases. These changing balances between humans and infections caused by changes in nursing habits but also in hygiene levels may well be involved in changing disease patterns including increased frequencies of certain autoimmune and degenerative diseases.

The evolution of antigen-specific CD8+ T cell responses after natural primary infection of humans with Epstein-Barr virus

Epstein-Barr virus (EBV) is a persistent, gamma-herpes virus that infects 90% of the human population. Primary infection, particularly if it is delayed until adolescence or beyond, may cause acute infectious mononucleosis and persistent infection may be associated with the development of several malignancies. CD8(+) T cells play a critical role in controlling both the primary and persistent phases of infection. This review summarises work that has been done characterising the primary immune responses to EBV. It goes on to describe the down regulation of the primary immune response and to discuss some of the factors that may be involved in determining the death or survival of populations of antigen-specific CD8(+) T cells. Finally it describes features of the populations of memory cells that mediate the long-term control of EBV in healthy seropositive individuals. The studies show differences in the responses to epitopes from lytic cycle versus latent proteins and highlight the complexity of naturally occurring, in vivo, immune responses. A clear understanding of the means by which CD8(+) T cells control EBV is important if we are to successfully develop vaccines and other forms of immunotherapy for the virus and its related malignancies.

Viral escape and T cell exhaustion in hepatitis C virus infection analysed using Class I peptide tetramers

Hepatitis C virus (HCV) has infected over 170 million people world wide, and in the majority sets up a chronic infection associated with hepatic inflammation. How it evades host immunity, particularly CD8+ T cells (CTL) is unclear, but two major factors are likely to operate, viral escape mutation and T cell exhaustion. We have investigated the role of CTL in control of infection during acute disease using Class I peptide tetramers. Although the immune response is quite diverse and numerous epitopes can be targeted, we observe that, especially during acute disease, one epitope (NS3 1073-81) is commonly recognised in HLA-A2 positive individuals. However, the levels of response to this epitope (and others) are very much lower if persistence is established. We examined in detail whether the cause of this low level of reactivity is due to mutation within the epitope. We find that, in fact this epitope is highly conserved during chronic infection, at a clonal level, between individuals, and over time. Thus, although variation within the epitope does occur, lack of reactivity in peripheral blood against this epitope in chronic disease, and loss of control of virus cannot be explained entirely by viral escape. Escape through mutation probably does play an important role in persistence of HCV, but we also discuss other mechanisms which lead to attenuation of T cell responses which may be important in determining the outcome.

Antigen-driven T-cell turnover

A mathematical model is developed to characterize the distribution of cell turnover rates within a population of T lymphocytes. Previous models of T-cell dynamics have assumed a constant uniform turnover rate; here we consider turnover in a cell pool subject to clonal proliferation in response to diverse and repeated antigenic stimulation. A basic framework is defined for T-cell proliferation in response to antigen, which explicitly describes the cell cycle during antigenic stimulation and subsequent cell division. The distribution of T-cell turnover rates is then calculated based on the history of random exposures to antigens. This distribution is found to be bimodal, with peaks in cell frequencies in the slow turnover (quiescent) and rapid turnover (activated) states. This distribution can be used to calculate the overall turnover for the cell pool, as well as individual contributions to turnover from quiescent and activated cells. The impact of heterogeneous turnover on the dynamics of CD4(+) T-cell infection by HIV is explored. We show that our model can resolve the paradox of high levels of viral replication occurring while only a small fraction of cells are infected.

Uncertainties - discrepancies in immunology

The many immunological observations and results from in-vitro or in-vivo experiments vary, and their interpretations differ enormously. A major problem is that within a normal distribution of biological phenomena, which are measurable with many methods, virtually anything is possible. Within a coevolutionary context, the definition of biologically relevant thresholds is an important key to improve our understanding of weaknesses and strengths of the immune system. This review is a personal view, comparing textbook rules and experiments using model antigens with observations on immunity against infections or tumors to critically evaluate our perception and understanding of specificity, affinity maturation, antigen presentation, selection of the class of the immune response, immunological memory and protective immunity, positive selection of T cells and self/nonself discrimination.

Cellular immune responses against hepatitis C virus: the evidence base 2002

Hepatitis C virus (HCV) is an RNA virus which is estimated to persistently infect about 170 million people worldwide. After acute infection, there is an initial period during which long-term outcome is decided. There is strong evidence that the cellular immune responses, involving both CD4+ and CD8+ T lymphocytes, are involved at this stage and it is their effectiveness which determines outcome. What is not understood is what determines their effectiveness. The most important component of this is likely to be some aspect of epitope selection, itself dictated by host MHC. Thus, to understand host immunity to HCV, we need to have a detailed understanding of the peptides involved in T lymphocyte responses. In this review, we discuss the peptide epitopes that have been identified so far, and their potential significance. We relate this to a scheme of host defence which may be useful for understanding natural and vaccine-induced immunity.

Exhaustion of cytotoxic T cells during adoptive immunotherapy of virus carrier mice can be prevented by B cells or CD4+ T cells

Rapid disappearance of antiviral CTL after transfusion into persistently infected individuals is a serious limitation of adoptive immunotherapy protocols. In the mouse model of persistent infection with lymphocytic choriomeningitis virus (LCMV) naive or immune virus-specific donor CD8+ T cells are exhausted after transfusion into carrier recipients with similar kinetics. Here we show that cotransfusion of immune CD4+ T cells prevents exhaustion of immune CD8+ T cells. Interestingly, cotransfer of primed B cells also prevented CD8+ T cell exhaustion in carriers even in the absence of T helper cells. This effect required the presence of immune B cells as repetitive treatment with hyperimmune serum led to the generation of antibody escape mutants. A combination of primed CD4+ T cells and primed B cells enhanced antiviral effects and prevented exhaustion also of naive CD8+ T cells. One key factor for prevention of CD8+ T cell exhaustion was the antiviral effect of the cotransfused cells thus reducing the time that CD8+ T cells are confronted with a high systemic viral load. These findings have implications for improving adoptive immunotherapy for persistent human viral infections.

Critical role for perforin-, Fas/FasL-, and TNFR1-mediated cytotoxic pathways in down-regulation of antigen-specific T cells during persistent viral infection

Viral persistence following infection with invasive strains of lymphocytic choriomeningitis virus (LCMV) can be achieved by selective down-regulation of virus-specific T lymphocytes. High viral burden in the onset of infection drives responding cells into functional unresponsiveness (anergy) that can be followed by their physical elimination. In this report, we studied down-regulation of the virus-specific CD8(+)-T-cell response during persistent infection of adult mice with LCMV, with emphasis on the role of perforin-, Fas/FasL-, or tumor necrosis factor receptor 1 (TNFR1)-mediated cytolysis in regulating T-cell homeostasis. The results reveal that the absence of perforin, Fas-ligand, or TNFR1 has no significant effect on the kinetics of proliferation and functional inactivation of virus-specific CD8(+) T cells in the onset of chronic LCMV infection. However, these molecules play a critical role in the homeostatic regulation of T cells, influencing the longevity of the virus-specific CD8(+)-T-cell population once it has become anergic. Thus, CD8(+) T cells specific to the dominant LCMV NP(396-404) epitope persist in an anergic state for at least 70 days in perforin-, FasL-, or TNFR1-deficient mice, but they were eliminated by day 30 in C57BL/6 controls. These effects were additive as shown by a deficit of apoptotic death of NP(396-404) peptide-specific CD8(+) T cells in mice lacking both perforin and TNFR1. This suggests a role for perforin-, FasL-, and TNFR1-mediated pathways in down-regulation of the antiviral T cell response during persistent viral infection by determining the fate of antigen-specific T cells. Moreover, virus-specific anergic CD8(+) T cells in persistently infected C57BL/6 mice contain higher levels of Bcl-2 and Bcl-XL than functionally intact T cells generated during acute LCMV infection. In the case of proapoptotic factors, Bax expression did not differ between T-cell populations and Bad was below the limit of detection in all samples. As expression of the Bcl-2 family members controls susceptibility to apoptosis, this finding may provide a molecular basis for the survival of anergic cells under conditions of prolonged antigen stimulation.

Peripheral T-cell tolerance defined through transgenic mouse studies

Selection in the thymus restricted by MHC and self-peptide shapes the diverse reactivities of the T-cell population which subsequently seeds into the peripheral tissues, in anticipation of the universe of pathogen antigens to which the organism may be exposed. A necessary corollary is the potential for T-cell self-reactivity (autoimmunity) in the periphery. Transgenic mouse models in which transgene expression in the thymus is prevented or excluded, have been particularly useful for determining the immunological outcome when T-cells encounter transgene-encoded 'self' antigen in peripheral tissues. Data suggest that non-mutually exclusive mechanisms of T-cells 'ignoring' self-antigen, T-cell deletion, T-cell anergy and T-cell immunoregulation have evolved to prevent self-reactivity while maintaining T-cell diversity. The peripheral T-cell repertoire, far from being static following maturation through the thymus, is in a dynamic stated determined by these peripheral selective and immunoregulatory influences. This article reviews the evidence with particular reference to CD8+ive T-cells.

Three-cell interactions in T cell-mediated suppression? A mathematical analysis of its quantitative implications

Aiming to further our understanding of T cell-mediated suppression, we investigate the plausibility of the hypothesis that regulatory T cells suppress other T cells (target cells), while both cells are conjugated with one APC. We use a mathematical model to analyze the proliferation inhibition scored during in vitro suppression assays. This model is a radical simplification of cell culture reality, assuming that thymidine incorporation is proportional to the number of target cells that would instantaneously form conjugates with APCs that are free of regulatory cells. According to this model the inhibition index should be mainly determined by the number of regulatory cells per APC and should be insensitive to the number of target cells. We reanalyzed several published data sets, confirming this expectation. Furthermore, we demonstrate that the instantaneous inhibition index has an absolute limit as a function of the number of regulatory cells per APC. By calculating this limit we find that the model can explain the data under two non-mutually exclusive conditions. First, only approximately 15% of APCs used in the suppression assays form conjugates with T cells. Second, the growth of the regulatory cell population depends on the target cells, such that the number of regulatory cells per APC increases when they are cocultured with target cells and overcomes its limit. However, if neither of these testable conditions is fulfilled, then one could conclude that suppression in vitro does not require the formation of multicellular conjugates.

Visualizing T cell competition for peptide/MHC complexes: a specific mechanism to minimize the effect of precursor frequency

In vivo antigenic competition of naive CD4+ TCR transgenic T cells was visualized by tracking cell division. Competition reduced both recruitment into cell division and burst size per recruited precursor cell, minimizing the effect of differences in precursorfrequency while maintaining the dose-response relationship with antigen. Competition was restricted to T cells of the same specificity, indicating that cells were competing for access to Ag-MHC complexes rather than for Ag nonspecific factors. Moreover, the qualitative distinction between the responses to i.v. peptide and s.c. peptide/CFA was unaffected by precursor frequency. These data explain the paradoxical ability of the immune system to tailor responses to the type and dose of Ag even in individuals with large differences in initial precursor frequency.

Modelling T-cell-mediated suppression dependent on interactions in multicellular conjugates

Tolerance to peripheral body antigens involves multiple mechanisms, namely T-cell-mediated suppression of potentially autoimmune cells. Recent in vivo and in vitro evidence indicates that regulatory T cells suppress the response of effector T cells by a mechanism that requires the simultaneous conjugation of regulatory and effector T cells with the same antigen-presenting cell (APC). Despite this strong requirement, it is not yet clear what happens while both cells are conjugated. Several hypotheses are discussed in the literature. Suppression may result from simple competition of regulatory and effector cells for activation resources on the APC; regulatory T cells may deliver an inhibitory signal to effector T cells in the same conjugate; or effector T cells may acquire the regulatory phenotype during their interaction with regulatory T cells. The present article tries to further our understanding of T-cell-mediated suppression, and to narrow-down the number of candidate mechanisms. We propose the first general formalism describing the formation of multicellular conjugates of T cells and APCs. Using this formalism we derive three particular models, representing alternative mechanisms of T-cell-mediated suppression. For each model, we make phase plane and bifurcation analysis, and identify their pros and cons in terms of the relationship with the large body of experimental observations on T-cell-mediated suppression. We argue that accounting for the quantitative details of adoptive transfers of tolerance requires models with bistable regimes in which either regulatory cells or effectors cells dominate the steady state. From this analysis, we conclude that the most plausible mechanism of T-cell-mediated suppression requires that regulatory T cells actively inhibit the growth of effector T cells, and that the maintenance of the population of regulatory T cells is dependent on the effector T cells. The regulatory T cell population may depend on a growth factor produced by effector T cells and/or on a continuous differentiation of effector cells to the regulatory phenotype.

CD8(+) T-cell selection, function, and death in the primary immune response in vivo

The primary immune response to Epstein Barr virus (EBV) is characterized by striking proliferation of EBV-specific CD8(+) T cells. In this study we have investigated the clonal composition and functional properties of the cells mediating this primary response and have analyzed the mechanisms that control the downregulation of the primary response and the selection of memory cells. We show that massively expanded T-cell clones often dominate the primary antigen-specific T-cell response. Despite the enormous extent of expansion, the virus-specific T cells express high levels of intracellular perforin and are potently cytotoxic. They are, however, functionally heterogeneous in their ability to secrete proinflammatory cytokines, with subpopulations of the antigen-specific T cells being hyporesponsive. The primary response is closely regulated, and the majority of cells are programmed to die via a cytokine-rescuable pathway, leaving only small populations of memory T cells surviving. Comparison of the clonal composition of primary and memory responses in vivo shows that the clones that dominate the primary response are relatively heavily culled during the downregulation of the primary response and the establishment of T-cell memory.

Antigen-specific elimination of T cells induced by oligomerized hemagglutinin (HA) 306-318

In a previous study we reported that oligomerized T cell epitopes "superactivated" CD4+ T cells. These oligomers, consisting of 12-16 copies of a peptide epitope derived from the hemagglutinin protein of influenza virus (HA306-318), induced a specific T cell response in amounts as little as 5 pg/ml. We now show that the improved antigenicity of these multimerized epitopes can also be utilized to induce "high zone tolerance". Tolerization, similar to activation, occurred at about 3 logs lower concentration of oligomer than of peptide. HA306-318-specific T cell cultures became nonresponsive to stimulation with peptide after incubation with 0.5-5 microg/ml HA306-318 12-mer. The nonresponsiveness was accompanied by a drastic down-regulation of the TCR and by T cell elimination by apoptotic cell death. In contrast, stimulation with peptide even at 50 microg/ml led to temporary induction of anergy. Consequently, induction of tolerance with the oligomer was permanent and no recovery of the cultures was seen in recall experiments 12-14 days after high zone exposure to the 12-mer.

Can Langerhans cell UV injury and dendritic cell infection by immunosuppressive viruses induce immunologic tolerance? First part

Though it has not been described by the HIV1-AIDS consensual thinkers, we propose the thesis according to which some immunocyte populations or their interactions can achieve tolerance or split immunologic tolerance in patients suffering from this disease. The proposed concept is based on data registered: 1) after Langerhans cell local injury by UV irradiation in their physiologic localisations, the skin and mucosae: and 2) after systemic infections of dendritic cells by immunosuppressive viruses, including some animal retroviruses, measles, lymphocytic choriomeningitis viruses, and HIV1. We can describe both types or manifestations concerning local UV injury and systemic viral infection combined in an AIDS patient, who received a PUVA irradiation before and at the beginning of his virostatic treatment, which he has now successfully received for eight years. The conditions of this patient differ from those of non-UV victim patients of the cohort to which he belongs, for two reasons: 1) the numbers of both his CD4+ and his suppressor T cells have been reduced to near 0 after UV irradiation; and b) his viral load, which has been exponentially reduced, has not reached, after eight years, the asymptotic part of the VL curve, characterized by PCR non-assessable viral load, which the non-UV victims of the cohort have reached. We wonder if, in this patient, there do not coexist two sorts of immunologic phenomena: 1) one bound to CTL, the numbers of which are increased, whose cytotoxicity is added to the virostatic action; and 2) one bound to the virus attack restriction by an immunologic tolerance. Contrary to the cohort controls, in whom an allogenic skin graft was only accepted for 20 days, the graft of the same donor has been tolerated in the UV victim patient for 70 days. This patient will be discussed in the second part of this editorial: the first part is devoted to the description of several conditions in which Langerhans cell injury by UV, or dendritic cell infections by measles, lymphocytic choriomeningitis viruses or animal retroviruses are followed, not only by immunodepression manifestations, but by immunologic tolerance.

Immunodominance among EBV-derived epitopes restricted by HLA-B27 does not correlate with epitope abundance in EBV-transformed B-lymphoblastoid cell lines

Using synthetic peptides, the HLA-B27-restricted CTL response to EBV in asymptomatic virus carriers has been mapped to four epitope regions in EBV latent cycle Ags. One of these peptide-defined epitopes (RRIYDLIEL) tends to be immunodominant and is recognized in the context of all three B27 subtypes studied, B*2702, B*2704, and B*2705. The other peptide-defined epitopes induce responses only in the context of one subtype, the immunogenic combinations being RRARSLSAERY/B*2702, RRRWRRLTV/B*2704, and FRKAQIQGL/B*2705. We used immunoaffinity chromatography to isolate the naturally presented viral peptides associated with these MHC class I molecules on the surface of EBV-transformed B-LCL. Using CTL reconstitution assays in conjunction with mass spectrometry, we established that the naturally processed and presented peptides are identical with the previously identified synthetic sequences. Despite the subtype-specific immunogenicity of three of the four epitopes, all four epitope peptides were found in association with each of the three different HLA-B27 subtypes. Indeed, those peptides that failed to induce a response in the context of a particular HLA-B27 subtype were frequently presented at greater abundance by that subtype than were the immunogenic peptides. Furthermore, among the peptides that did induce a response, immunodominance did not correlate with epitope abundance; in fact the immunodominant RRIYDLIEL epitope was least abundant, being present at less than one copy per cell. The relationship of this unexpected finding to the persistence of EBV is discussed.

The In Vivo Fate of APCs Displaying Minor H Antigen and/or MHC Differences Is Regulated by CTLs Specific for Immunodominant Class I-Associated Epitopes

The goal of this work was to evaluate the fate of APCs following interactions with T cells in unprimed mice with a normal T cell repertoire. We elaborated a model in which male adherent peritoneal mononuclear cells were injected into the foreleg footpads of naive female recipients mismatched for either minor or major histocompatibility Ags. At various times after injection, APC numbers in the draining (axillary and brachial) lymph nodes were assessed using a Ube1y gene-specific PCR assay. Our experimental model was designed so that the number of APCs expressing the priming epitope was similar to what is observed under real life conditions. Thus, early after injection, the frequency of afferent lymph-derived APCs expressing the priming epitope was in the range of 101–102/106 lymph node cells. We found that APCs presenting some, but not all, nonself epitopes were killed rapidly after entrance into the lymph nodes. Rapid elimination of APCs occurred following interactions with MHC class I-restricted, but not class II-restricted, T cells and was observed when APCs presented an immunodominant (B6dom1/H7a), but not a nondominant (HY), epitope. Killing of APCs was mediated partly, but not exclusively, by perforin-dependent process. We propose that killing of APCs by CTLs specific for immunodominant MHC class I-restricted epitopes may be instrumental in regulating the intensity, duration, and diversity of T cell responses.

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.

Mature T lymphocyte apoptosis--immune regulation in a dynamic and unpredictable antigenic environment

Apoptosis of mature T lymphocytes preserves peripheral homeostasis and tolerance by countering the profound changes in the number and types of T cells stimulated by diverse antigens. T cell apoptosis occurs in at least two major forms: antigen-driven and lymphokine withdrawal. These forms of death are controlled in response to local levels of IL-2 and antigen in a feedback mechanism termed propriocidal regulation. Active antigen-driven death is mediated by the expression of death cytokines such as FasL and TNF. These death cytokines engage specific receptors that assemble caspase-activating protein complexes. These signaling complexes tightly regulate cell death but are vulnerable to inherited defects. Passive lymphokine withdrawal death may result from the cytoplasmic activation of caspases that is regulated by mitochondria and the Bcl-2 protein. The human disease, Autoimmune Lymphoproliferative Syndrome (ALPS) is due to dominant-interfering mutations in the Fas/APO-1/CD95 receptor and other components of the death pathway. The study of ALPS patients reveals the necessity of apoptosis for preventing autoimmunity and allows the genetic investigation of apoptosis in humans. Immunological, cellular, and molecular evidence indicates that throughout the life of a T cell, apoptosis may be evoked in excessive, harmful, or useless clonotypes to preserve a healthy and balanced immune system.

General and specific immunosuppression caused by antiviral T-cell responses

Immunosuppression caused by the non-cytopathic lymphocytic choriomeningitis virus (LCMV) (an RNA virus) is mediated by antiviral cytotoxic T cells that destroy LCMV-infected cells, also of the immune system. While this immunopathological destruction of antigen-presenting cells, macrophages and follicular dendritic cells and of some CD4+ T cells causes general immunosuppression and impairs immune response to third party antigens, it also enhances exhaustion/deletion of LCMV-specific CD8+ T-cell responses. LCMV seems in addition to infect neutralizing antibody-producing B cells via the specific receptor; immunopathological LCMV specific CD8+ T-cell-mediated elimination of these infected B cells (but not of uninfected internal virus antigen-specific B cells) causes a highly specific immunosuppression that delays neutralizing antibody responses and thereby enhances virus persistence. Both generalized and specific immunosuppression by CD8+ T-cell-mediated immunopathology may be involved in human infections with HIV, hepatitis B virus or hepatitis C virus.

The impact of variation in the number of CD8(+) T-cell precursors on the outcome of virus infection

We investigated the role of varying the initial number of naive antiviral CTL precursors on the dynamics of LCMV-DOCILE infection. C57BL/6 mice, exhibiting LCMV-specific CTLp frequencies of about 50, are protected against virus persistence over a range of infectious doses up to 10(4) pfu. With 10-fold higher doses, a 100-fold increase in CTLp is required to restore virus control. With doses above 10(6) pfu, elevation of the initial CTLp number leads only to lethal immunopathology. Similarly, a 1000-fold increase in the number of initial naïve CTLp enhances the overall kinetics of virus elimination, but cannot limit early virus spread within the first 48 h after low-dose infection (500 pfu). Increases in initial naïve virus-specific CTLp numbers are of limited benefit in antiviral control. In addition to the number of virus-specific T cells, the time period needed to reach cytolytic effector function is a limiting parameter.