A functionally compromised intermediate in extrathymic CD8+ T cell deletion

Identification of two major types of age-associated CD8 clonal expansions with highly divergent properties

CD8 memory T cells are tightly regulated in young, healthy individuals but are often perturbed in aged animals by the appearance of large CD8 T cell clones. These clones are associated with impaired immunity in the aged. The molecular basis of this phenomenon remains unclear. Here, it is shown that the issue is confused by the fact that the clones are heterogeneous. Some clones bear high, and others, low levels of integrin alpha(4) (itgalpha4). These subtypes differ by multiple criteria. They appear in mice of different ages, concentrate in different tissues, and have different stabilities in vivo and responses to stimulation in vitro. itgalpha4(high), but not itgalpha4(low), CD8 clonal expansions have several characteristics consistent with a chronically stimulated phenotype. These properties include lowered levels of CD8, decreased expression of some cytokine receptors, and elevated expression of various inhibitory receptors, including the programmed death-1 (PD1) receptor and the killer cell lectin-like receptor G1 (KLRG1). The characteristics of itgalpha4(high) clonal expansions suggest that they may arise from age-dependent alterations in antigen expression and tolerance. These data redefine CD8 clonal expansions into at least two distinct entities and indicate that there are multiple mechanisms that drive age-related alterations of CD8 T cell homeostasis.

Continued maturation of thymic emigrants in the periphery

Developing thymocytes are selected for recognition of molecules encoded by the major histocompatibility complex, purged of self-reactive cells and committed to either the CD4 or CD8 lineage. The 1% of thymocytes that complete these tasks emigrate and join the population of peripheral lymphocytes. Whether T cell maturation is complete at the time of thymic exit has been a subject of debate. Using mice transgenic for green fluorescent protein driven by the recombination activating gene 2 promoter to identify recent thymic emigrants, we now show that T cell differentiation continues post-thymically, with progressive maturation of both surface phenotype and immune function. In addition, the relative contribution of CD4 and CD8 recent thymic emigrants was modulated as they entered the peripheral T cell pool. Thus, T cell maturation and subset contribution are both finalized in the lymphoid periphery.

Antiapoptotic function of NF-kappaB in T lymphocytes is influenced by their differentiation status: roles of Fas, c-FLIP, and Bcl-xL

Inducible protection from apoptosis in vivo controls the size of cell populations. An important question in this respect is how differentiation affects mechanisms of apoptosis regulation. Among mature T lymphocytes, the NF-kappaB/Rel transcription factors are coupled to receptors that control cell population sizes by concurrently regulating survival and multiplication. In the present study, we used a transgenic inhibitor of NF-kappaB/Rel signaling to investigate the role of this pathway in proliferation and death of mature T cells in vivo. The results indicate that NF-kappaB integrates two critical yet distinct molecular pathways preventing apoptosis affected by the death receptor Fas, coordinately regulating levels of FLIP and Bcl-x(L) in primary T cells. Surprisingly, NF-kappaB blockade preferentially impacted naive as compared to memory T cells. The Fas/FasL pathway was linked to these findings by evidence that the abnormalities imposed by NF-kappaB inhibition were ameliorated by Fas deficiency, particularly for the CD4(+) lineage. Moreover, levels of an inhibitor of Fas-mediated apoptosis, c-FLIP, were diminished in cells expressing the transgenic inhibitor. NF-kappaB was also linked to T cell survival in vivo by mediating induction of Bcl-x(L): restoration of Bcl-x(L) levels reversed the preferential deficit of naive T cells, differentially impacting the CD4 and CD8 subsets. These results show that promoting survival and effective multiplication are central roles for NF-kappaB in T lymphoid homeostasis in vivo, but this effect and its underlying mechanisms are influenced by the developmental state of the lymphocyte.

CD4(+) and CD8(+) anergic T cells induced by interleukin-10-treated human dendritic cells display antigen-specific suppressor activity

Interleukin-10 (IL-10)-treated dendritic cells (DCs) induce an alloantigen- or peptide-specific anergy in various CD4(+) and CD8(+) T-cell populations. In the present study, we analyzed whether these anergic T cells are able to regulate antigen-specific immunity. Coculture experiments revealed that alloantigen-specific anergic CD4(+) and CD8(+) T cells suppressed proliferation of syngeneic T cells in a dose-dependent manner. The same effect was observed when the hemagglutinin-specific CD4(+) T-cell clone HA1.7 or tyrosinase-specific CD8(+) T cells were cocultured with anergic T cells of the same specificity. Anergic T cells did not induce an antigen-independent bystander inhibition. Suppression was dependent on cell-to-cell contact between anergic and responder T cells, required activation by antigen-loaded DCs, and was not mediated by supernatants of anergic T cells. Furthermore, anergic T cells displayed an increased extracellular and intracellular expression of cytotoxic T-lymphocyte antigen (CTLA)-4 molecules, and blocking of the CTLA-4 pathway restored the T-cell proliferation up to 70%, indicating an important role of the CTLA-4 molecule in the suppressor activity of anergic T cells. Taken together, our experiments demonstrate that anergic T cells induced by IL-10-treated DCs are able to suppress activation and function of T cells in an antigen-specific manner. Induction of anergic T cells might be exploited therapeutically for suppression of cellular immune responses in allergic or autoimmune diseases with identified (auto) antigens.

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.

In situ tolerance within the central nervous system as a mechanism for preventing autoimmunity

Multiple sclerosis (MS) is believed to be an autoimmune disease in which autoreactive T cells infiltrate the central nervous system (CNS). Animal models of MS have shown that CNS-specific T cells are present in the peripheral T cell repertoire of healthy mice and cause autoimmune disease only when they are activated by immunization. T cell entry into the CNS is thought to require some form of peripheral activation because the blood-brain barrier prohibits trafficking of this tissue by naive cells. We report here that naive T cells can traffic to the CNS without prior activation. Comparable numbers of T cells are found in the CNS of both healthy recombinase activating gene (Rag)(-/)- T cell receptor (TCR) transgenic mice and nontransgenic mice even when the transgenic TCR is specific for a CNS antigen. Transgenic T cells isolated from the CNS that are specific for non-CNS antigens are phenotypically naive and proliferate robustly to antigenic stimulation in vitro. Strikingly, transgenic T cells isolated from the CNS that are specific for myelin basic protein (MBP) are also primarily phenotypically naive but are unresponsive to antigenic stimulation in vitro. Mononuclear cells from the CNS of MBP TCR transgenic but not nontransgenic mice can suppress the response of peripheral MBP-specific T cells in vitro. These results indicate that naive MBP-specific T cells can traffic to the CNS but do not trigger autoimmunity because they undergo tolerance induction in situ.

Direct detection of apoptotic cells in peripheral blood from highly pathogenic SHIV-inoculated monkey

Apoptosis in peripheral blood leukocytes (PBL) has been estimated by the enhancement of spontaneous apoptosis after in vitro culture, because apoptotic cells have not been observed directly in freshly isolated PBL in the course of HIV/AIDS. In monkeys infected with a highly pathogenic simian/human immunodeficiency virus (SHIV), which corresponds to rapid progressors of HIV infection, a high frequency of apoptotic cells was directly detected in fresh PBL by electron-microscopic studies. Peripheral blood apoptosis transiently occurred after intense plasma viremia, and peaking at 3 weeks postinfection; occurrence was not limited specifically to lymphocytes, but also occurred in other types of leukocytes. Apoptosis in peripheral lymph nodes was also detected following intense plasma viremia. However, the in vivo apoptosis was not detected in nonpathogenic SHIV-infected monkeys that showed no cell loss. Thus, we directly showed the apoptosis of PBL, which might be associated with pathogenic SHIV produced during the time of plasma viremia.

Transgenic expression of Fas ligand on thyroid follicular cells prevents autoimmune thyroiditis

"Immune privilege" is defined as tissue resistance to aggression by specifically activated lymphocytes, and involves the interaction between Fas expressed on infiltrating cells and Fas ligand (FasL) constitutively expressed on the target tissue. To test whether ectopic expression of FasL on thyrocytes could prevent autoimmune aggression of the thyroid by activated lymphoid cells, three lines of transgenic mice expressing low, intermediate, and high levels of functional FasL on thyroid follicular cells were generated. Experimental autoimmune thyroiditis was induced by immunization with mouse thyroglobulin. In all of the experiments, the effects were dependent on the level of FasL expression. Low and intermediate expression had no or only weak preventive effects, respectively, whereas high FasL expression strongly inhibited lymphocytic infiltration of the thyroid. Anti-mouse thyroglobulin-proliferative and cytotoxic T cell responses, as well as autoantibody production, were diminished in transgenic mice expressing high levels of FasL relative to controls. Furthermore, in these latter mice Th1 responses to mouse thyroglobulin were profoundly down-regulated, uncovering a new potential role for FasL in peripheral tolerance to organ-specific Ags. In sum, the prevention of experimental autoimmune thyroiditis by FasL on thyrocytes is dependent on the level of FasL expression.

Differential requirement for CD80 and CD80/CD86-dependent costimulation in the lung immune response to an influenza virus infection

The CD28 costimulatory pathway is critical to T cell activation. Blockade of the interaction of CD28 with its ligands CD80 and CD86 using CTLA4-Ig has been proposed as a therapy for a number of immune-based disorders. We have used a murine model of influenza virus infection to study the role of CD28-dependent costimulation in the development of antiviral immune responses. In vivo treatment with CTLA4-Ig to block the interaction of CD28 with CD80 and CD86 reduced virus-specific cytotoxicity and IFN-gamma production by bronchoalveolar lavage fluid CD8+ T lymphocytes in vitro. It also resulted in decreased numbers of virus-specific CD8+ T lymphocytes in the bronchoalveolar lavage fluid, lung, and spleen and lowered virus-specific Ab titers. Mice treated with CTLA4-Ig were able to control and clear the virus infection, but this was delayed compared with controls. Treatment with Y100F-Ig, a mutant form of CTLA4-Ig which selectively binds to CD80 and blocks the CD28-CD80 interaction leaving CD28-CD86 binding intact, did not affect Ab production, spleen cytotoxic precursors, or clearance of virus. However, Y100F-Ig treatment had a clear effect on lung effector cell function. Secretion of IFN-gamma by bronchoalveolar lavage fluid CD8+ T lymphocytes in vitro was decreased, and the number of virus-specific CD8+ T lymphocytes in the bronchoalveolar lavage fluid and lungs of infected mice was reduced. These results indicate that CD28-dependent costimulation is important in the antiviral immune response to an influenza virus infection. The individual CD28 ligand, CD80, is important for some lung immune responses and cannot always be compensated for by CD86.

Characterization of CD8+ T lymphocytes that persist after peripheral tolerance to a self antigen expressed in the pancreas

As a result of expression of the influenza hemagglutinin (HA) in the pancreatic islets, the repertoire of HA-specific CD8+ T lymphocytes in InsHA transgenic mice (D2 mice expressing the HA transgene under control of the rat insulin promoter) is comprised of cells that are less responsive to cognate Ag than are HA-specific CD8+ T lymphocytes from conventional mice. Previous studies of tolerance induction involving TCR transgenic T lymphocytes suggested that a variety of different mechanisms can reduce avidity for Ag, including altered cell surface expression of molecules involved in Ag recognition and a deficiency in signaling through the TCR complex. To determine which, if any, of these mechanisms pertain to CD8+ T lymphocytes within a conventional repertoire, HA-specific CD8+ T lymphocytes from B10.D2 mice and B10.D2 InsHA transgenic mice were compared with respect to expression of cell surface molecules, TCR gene utilization, binding of tetrameric KdHA complexes, lytic mechanisms, and diabetogenic potential. No evidence was found for reduced expression of TCR or CD8 by InsHA-derived CTL, nor was there evidence for a defect in triggering lytic activity. However, avidity differences between CD8+ clones correlated with their ability to bind KdHA tetramers. These results argue that most of the KdHA-specific T lymphocytes in InsHA mice are not intrinsically different from KdHA-specific T lymphocytes isolated from conventional animals. They simply express TCRs that are less avid in their binding to KdHA.

Anergic CD8+ T Cells Can Persist and Function In Vivo

Using a mouse model system, we demonstrate that anergic CD8+ T cells can persist and retain some functional capabilities in vivo, even after the induction of tolerance. In TCR Vβ5 transgenic mice, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction. CD8low cells are characterized by an activated phenotype, are functionally compromised in vitro, and are slated for deletion in vivo. We now demonstrate that CD8low cells derive from a proliferative compartment, but do not divide in vivo. CD8low cells persist in vivo with a t1/2 of 3–5 days, in contrast to their in vitro t1/2 of 0.5–1 day. During this unexpectedly long in vivo life span, CD8low cells are capable of producing IFN-γ in vivo despite their inability to proliferate or to kill target cells in vitro. CD8low cells also accumulate at sites of inflammation, where they produce IFN-γ. Therefore, rather than withdrawing from the pool of functional CD8+ T cells, anergic CD8low cells retain a potential regulatory role despite losing their capacity to proliferate. The ability of anergic cells to persist and function in vivo adds another level of complexity to the process of tolerance induction in the lymphoid periphery.

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.

Chronic deletion, escape from deletion and activation of mouse mammary tumor virus superantigen-reactive T cells in C57BL/10 mice

Though C57BL/10 mice express the mouse mammary tumor virus superantigens (sag) encoded by Mtv-8 and Mtv-9, it has been thought that these sag do not bind to the MHC class II molecule H2-Ab and consequently do not affect the T cell repertoire. However, we show that cells bearing TCR Vbeta chains specific for Mtv-8 and -9 sag are chronically deleted in C57BL/10 mice. Thymocytes and peripheral T cells escaping deletion by Mtv sag display a small reduction in the level of cell surface CD4. T cells escaping thymic deletion respond variably to endogenous Mtv sag with some, but not all, reactive populations appearing overrepresented in the activated/memory subset. The data suggest that in normal mice fine modulation of coreceptor expression levels may be a common way by which thymocytes escape elimination, that systems utilizing potentially Mtv sag-reactive TCR on a C57BL background may be inappropriate for the measurement of the affinity of TCR/MHC/peptide interactions required in thymic selection, and that detection of the activity of human sag may be aided by analysis of CD4 levels and activation markers on T cells in conjunction with studies of the frequency of cells bearing specific TCRVbeta chains.

Chronic Modulation of the TCR Repertoire in the Lymphoid Periphery

Using TCR Vβ5 transgenic mice as a model system, we demonstrate that the induction of peripheral tolerance can mold the TCR repertoire throughout adult life. In these mice, three distinct populations of peripheral T cells are affected by chronic selective events in the lymphoid periphery. First, CD4+Vβ5+ T cells are deleted in the lymphoid periphery by superantigens encoded by mouse mammary tumor viruses-8 and -9 in an MHC class II-dependent manner. Second, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction by a process that depends upon neither mouse mammary tumor virus-encoded superantigens nor MHC class II expression. Third, a population of CD4−CD8−Vβ5+ T cells arises in the lymphoid periphery in an age-dependent manner. We analyzed the TCR Vα repertoire of each of these cellular compartments in both Vβ5 transgenic and nontransgenic C57BL/6 mice as a function of age. This analysis revealed age-related changes in the expression of Vα families among different cellular compartments, highlighting the dynamic state of the peripheral immune repertoire. Our work indicates that the chronic processes maintaining peripheral T cell tolerance can dramatically shape the available TCR repertoire.

A role for perforin in downregulating T-cell responses during chronic viral infection

Cytotoxic T cells secrete perforin to kill virus-infected cells. In this study we show that perforin also plays a role in immune regulation. Perforin-deficient (perf -/-) mice chronically infected with lymphocytic choriomeningitis virus (LCMV) contained greater numbers of antiviral T cells compared to persistently infected +/+ mice. The enhanced expansion was seen in both CD4 and CD8 T cells, but the most striking difference was in the numbers of LCMV-specific CD8 T cells present in infected perf -/- mice. Persistent LCMV infection of +/+ mice results in both deletion and anergy of antigen-specific CD8 T cells, and our results show that this peripheral "exhaustion" of activated CD8 T cells occurred less efficiently in perf -/- mice. This excessive accumulation of activated CD8 T cells resulted in immune-mediated damage in persistently infected perf -/- mice; approximately 50% of these mice died within 2 to 4 weeks, and mortality was fully reversed by in vivo depletion of CD8 T cells. This finding highlights an interesting dichotomy between the role of perforin in viral clearance and immunopathology; perforin-deficient CD8 T cells were unable to clear the LCMV infection but were capable of causing immune-mediated damage. Finally, this study shows that perforin also plays a role in regulating T-cell-mediated autoimmunity. Mice that were deficient in both perforin and Fas exhibited a striking acceleration of the spontaneous lymphoproliferative disease seen in Fas-deficient (lpr) mice. Taken together, these results show that the perforin-mediated pathway is involved in downregulating T-cell responses during chronic viral infection and autoimmunity and that perforin and Fas act independently as negative regulators of activated T cells.

Chronic intravenous injections of antigen induce and maintain tolerance in T cell receptor-transgenic mice

Antigen-specific T cell tolerance can be induced by systemic injection of high-dose antigen. In particular, a single intravenous (i.v.) injection of influenza virus hemagglutinin peptide in HNT-TCR transgenic mice induces T cell tolerance through thymocyte apoptosis as well as anergy and deletion of peripheral CD4+ T cells. We now show that this tolerance is reversed after 8 weeks probably due to the short in vivo half-life of the peptide. Since durable tolerance is required for this strategy to be of therapeutic value, we tested whether weekly i.v. injections of peptide (up to 12 weeks) could maintain the CD4+ T cell tolerance. Each injection induces a profound deletion of thymocytes, although their level recovers before the next injection. Therefore, during the treatment period, the thymus undergoes cycles of contraction/expansion. In the periphery, the number of CD4+ T cells is stably decreased and the persisting CD4+ T cells are hyporeactive both in vitro and in vivo. This tolerance is essentially peripheral since comparable results were obtained in thymectomized HNT-TCR mice injected weekly. Our data show that stable antigen-specific tolerance can be induced by repeated i.v. injections of antigen. These findings might have implications for the treatment of T cell-mediated autoimmune diseases.

Viral immune evasion due to persistence of activated T cells without effector function

We examined the regulation of virus-specific CD8 T cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection of mice. Our study shows that within the same persistently infected host, different mechanisms can operate to silence antiviral T cell responses; CD8 T cells specific to one dominant viral epitope were deleted, whereas CD8 T cells responding to another dominant epitope persisted indefinitely. These virus-specific CD8 T cells expressed activation markers (CD69(hi), CD44(hi), CD62Llo) and proliferated in vivo but were unable to elaborate any antiviral effector functions. This unresponsive phenotype was more pronounced under conditions of CD4 T cell deficiency, highlighting the importance of CD8- CD4 T cell collaboration in controlling persistent infections. Importantly, in the presence of CD4 T cell help, adequate CD8 effector activity was maintained and the chronic viral infection eventually resolved. The persistence of activated virus-specific CD8 T cells without effector function reveals a novel mechanism for silencing antiviral immune responses and also offers new possibilities for enhancing CD8 T cell immunity in chronically infected hosts.

Viral immune evasion due to persistence of activated T cells without effector function

We examined the regulation of virus-specific CD8 T cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection of mice. Our study shows that within the same persistently infected host, different mechanisms can operate to silence antiviral T cell responses; CD8 T cells specific to one dominant viral epitope were deleted, whereas CD8 T cells responding to another dominant epitope persisted indefinitely. These virus-specific CD8 T cells expressed activation markers (CD69(hi), CD44(hi), CD62Llo) and proliferated in vivo but were unable to elaborate any antiviral effector functions. This unresponsive phenotype was more pronounced under conditions of CD4 T cell deficiency, highlighting the importance of CD8- CD4 T cell collaboration in controlling persistent infections. Importantly, in the presence of CD4 T cell help, adequate CD8 effector activity was maintained and the chronic viral infection eventually resolved. The persistence of activated virus-specific CD8 T cells without effector function reveals a novel mechanism for silencing antiviral immune responses and also offers new possibilities for enhancing CD8 T cell immunity in chronically infected hosts.

RAG reexpression and DNA recombination at T cell receptor loci in peripheral CD4+ T cells

Under most circumstances, allelic exclusion at the T cell receptor (TCR)beta locus is tightly regulated. Here, we describe a system in which TCRbeta allelic exclusion is overcome as a result of V(D)J recombination in peripheral CD4+ T cells. In TCRbeta chain transgenic mice, tolerogen-mediated chronic peripheral selection against cells expressing the transgene leads to surface expression of endogenous TCRbeta chains. Peripheral CD4+ T cells reexpress the recombination activating genes, RAG1 and RAG2, and contain signal end intermediates indicative of ongoing V(D)J recombination. The rescue from deletion of mature T cells expressing newly generated TCRbeta chains suggests that receptor revision plays a role in the maintenance of peripheral T cell tolerance.

CD4 T cell tolerance to human C-reactive protein, an inducible serum protein, is mediated by medullary thymic epithelium

Inducible serum proteins whose concentrations oscillate between nontolerogenic and tolerogenic levels pose a particular challenge to the maintenance of self-tolerance. Temporal restrictions of intrathymic antigen supply should prevent continuous central tolerization of T cells, in analogy to the spatial limitation imposed by tissue-restricted antigen expression. Major acute-phase proteins such as human C-reactive protein (hCRP) are typical examples for such inducible self-antigens. The circulating concentration of hCRP, which is secreted by hepatocytes, is induced up to 1,000-fold during an acute-phase reaction. We have analyzed tolerance to hCRP expressed in transgenic mice under its autologous regulatory regions. Physiological regulation of basal levels (<10(-9) M) and inducibility (>500-fold) are preserved in female transgenics, whereas male transgenics constitutively display induced levels. Surprisingly, crossing of hCRP transgenic mice to two lines of T cell receptor transgenic mice (specific for either a dominant or a subdominant epitope) showed that tolerance is mediated by intrathymic deletion of immature thymocytes, irrespective of widely differing serum levels. In the absence of induction, hCRP expressed by thymic medullary epithelial cells rather than liver-derived hCRP is necessary and sufficient to induce tolerance. Importantly, medullary epithelial cells also express two homologous mouse acute-phase proteins. These results support a physiological role of "ectopic" thymic expression in tolerance induction to acute-phase proteins and possibly other inducible self-antigens and have implications for delineating the relative contributions of central versus peripheral tolerance.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Cutting Edge: Multiple Mechanisms of Peripheral T Cell Tolerance to the Fetal “Allograft”

The fetus represents a foreign entity to the maternal immune system, yet this “natural” allograft is not normally rejected. This unique situation provides a physiologic system to evaluate peripheral tolerance in which the maternal immune system is challenged with relatively rare Ags not previously encountered in the thymus. Using H-Y-specific TCR transgenic mice, we demonstrate that T cells specific for fetal Ags decrease in an Ag-specific manner during pregnancy and remain low postpartum, the result of an encounter with fetal cells expressing the appropriate MHC/peptide complexes. The finding that placental trophoblasts can induce Fas-mediated death of T cells is consistent with peripheral clonal deletion as one mechanism of tolerance. The remaining clonotypic T cells are unresponsive to antigenic stimulation, although neither TCR nor coreceptor is down-regulated. Our study demonstrates that specific recognition of fetal allogeneic Ags by maternal T cells results in tolerance induction of reactive T cells via multiple mechanisms.

A range of CD4 T cell tolerance: partial inactivation to organ-specific antigen allows nondestructive thyroiditis or insulitis

T cell receptor (TCR) transgenic mice specific for hen egg lysozyme (HEL) were crossed with mice expressing HEL on the thyroid epithelium, on pancreatic islet beta cells, or systemically. Depending on the pattern of HEL expression, deletion of double-positive thymocytes ranged from minimal to complete, and peripheral CD4 cells exhibited graded reduction in TCR expression, in vitro responsiveness, and in vivo helper ability. CD4 cells were least tolerant in TCR/thyroid-HEL and TCR/islet-HEL mice, which developed an extensive lymphocytic thyroiditis or insulitis that nevertheless did not eliminate HEL-expressing endocrine cells. Autoreactive CD4 clones thus escape the thymus under a range of circumstances, retain sufficient function to initiate subclinical autoimmune inflammation when self-antigens are concentrated in the thyroid or pancreas, and may regulate progression of subclinical inflammation to destructive autoimmune disease.

Perturbation of the T lymphocyte lineage in transgenic mice expressing a constitutive repressor of nuclear factor (NF)-kappaB

Members of the nuclear factor (NF)-kappaB/Rel family transcription factors are induced during thymic selection and in mature T lymphocytes after ligation of the T cell antigen receptor (TCR). Despite these findings, disruption of individual NF-kappaB/Rel genes has revealed no intrinsic defect in the development of mature T cells, perhaps reflecting functional redundancy. To circumvent this possibility, the T cell lineage was targeted to express a trans-dominant form of IkappaBalpha that constitutively represses the activity of multiple NF-kappaB/Rel proteins. Transgenic cells expressing this inhibitor exhibit a significant proliferative defect, which is not reversed by the addition of exogenous interleukin-2. Moreover, mitogenic stimulation of splenocytes leads to increased apoptosis of transgenic T cells as compared with controls. In addition to deregulated T cell growth and survival, transgene expression impairs the development of normal T cell populations as evidenced by diminished numbers of TCRhi CD8 single-positive thymocytes. This defect was significantly amplified in the periphery and was accompanied by a decrease in CD4(+) T cells. Taken together, these in vivo findings indicate that the NF-kappaB/Rel signaling pathway contains compensatory components that are essential for the establishment of normal T cell subsets.

Mechanisms underlying T-cell tolerance

To understand how the immune system manages to disarm potentially harmful T cells with reactivity against self-antigens, many immunologists have turned to model systems in which transgenic expression of TCRs, foreign antigens, or both, allows visualization of otherwise undetectable mechanisms. Recent data have offered several new insights into the variables that dictate the mechanism for tolerance employed by peripheral T cells. Some of these mechanisms can also dampen ongoing immune responses against foreign antigens and provide a strikingly powerful supplement to the primary mechanism of tolerance, thymic clonal deletion.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.