Tolerance induction in double specific T-cell receptor transgenic mice varies with antigen

Qualitative and Quantitative Requirements for CD4+ T Cell-Mediated Antiviral Protection

CD4+ Th cells deliver the cognate and cytokine signals that promote the production of protective virus-neutralizing IgG by specific B cells and are also able to mediate direct antiviral effector functions. To quantitatively and qualitatively analyze the antiviral functions of CD4+ Th cells, we generated transgenic mice (tg7) expressing an MHC class II (I-Ab)-restricted TCR specific for a peptide derived from the glycoprotein (G) of vesicular stomatitis virus (VSV). The elevated precursor frequency of naive VSV-specific Th cells in tg7 mice led to a markedly accelerated and enhanced class switching to virus-neutralizing IgG after immunization with inactivated VSV. Furthermore, in contrast to nontransgenic controls, tg7 mice rapidly cleared a recombinant vaccinia virus expressing the VSV-G (Vacc-IND-G) from peripheral organs. By adoptive transfer of naive tg7 CD4+ T cells into T cell-deficient recipients, we found that 105 transferred CD4+ T cells were sufficient to induce isotype switching after challenge with a suboptimal dose of inactivated VSV. In contrast, naive transgenic CD4+ T cells were unable to adoptively confer protection against peripheral infection with Vacc-IND-G. However, tg7 CD4+ T cells that had been primed in vitro with VSV-G peptide were able to adoptively transfer protection against Vacc-IND-G. These results demonstrate that the antiviral properties of CD4+ T cells are governed by the differentiation status of the CD4+ T cell and by the type of effector response required for virus elimination.

Protection against lymphocytic choriomeningitis virus infection induced by a reduced peptide bond analogue of the H-2Db-restricted CD8(+) T cell epitope GP33

Recent investigations have suggested that pseudopeptides containing modified peptide bonds might advantageously replace natural peptides in therapeutic strategies. We have generated eight reduced peptide bond Psi(CH2-NH) analogues corresponding to the H-2Db-restricted CD8(+) T cell epitope (called GP33) of the glycoprotein of the lymphocytic choriomeningitis virus. One of these pseudopeptides, containing a reduced peptide bond between residues 6 and 7 (Psi(6-7)), displayed very similar properties of binding to major histocompatibility complex (MHC) and recognition by T cell receptor transgenic T cells specific for GP33 when compared with the parent peptide. We assessed in vitro and in vivo the proteolytic resistance of GP33 and Psi(6-7) and analyzed its contribution to the priming properties of these peptides. The Psi(6-7) analogue exhibited a dramatically increased proteolytic resistance when compared with GP33, and we show for the first time that MHC-peptide complexes formed in vivo with a pseudopeptide display a sustained half-life compared with the complexes formed with the natural peptide. Furthermore, in contrast to immunizations with GP33, three injections of Psi(6-7) in saline induced significant antiviral protection in mice. The enhanced ability of Psi(6-7) to induce antiviral protection may result from the higher stability of the analogue and/or of the MHC-analogue complexes.

Dual MHC Class I and Class II Restriction of a Single T Cell Receptor: Distinct Modes of Tolerance Induction by Two Classes of Autoantigens

In the final stages of thymic development, immature T cells undergo three distinct processes (positive selection, negative selection, and lineage commitment) that all depend on interactions of thymocyte TCRs with MHC molecules. It is currently thought that TCRs are preferentially restricted by either MHC class I or class II molecules. In this report, we present direct evidence that the TCR previously described as H-Y/H-2Db specific cross-reacts with H-2IAb if expressed in CD4+ cells. We also demonstrate an increase in thymocyte numbers in H-Y TCR-trangenic mice deficient in MHC class II, suggesting a relatively discrete form of negative selection by MHC class II compared with that induced by H-Y/H-2Db. We propose that inability to generate CD4+ T cells expressing H-Y TCR in different experimental settings may be due to tolerance to self-MHC class II. These results, therefore, support an intriguing possibility that tolerance to self may influence and/or interfere with the outcome of the lineage commitment.

Antigen-Induced Coreceptor Down-Regulation on Thymocytes Is Not a Result of Apoptosis

The various stages of T cell development are typically characterized by the expression level of the two coreceptors, CD4 and CD8. During the CD4+CD8+ (double-positive, DP) stage of development, thymocytes that perceive a low avidity signal through the TCR go on to differentiate (positive selection), and ultimately down-regulate one coreceptor to express either CD4 or CD8. Alternatively, thymocytes that perceive a high avidity signal down-regulate both coreceptors and are induced to die via apoptosis (negative selection). However, it has recently been suggested that positively selected thymocytes may also partially down-regulate both coreceptors before up-regulating the one coreceptor that is ultimately expressed. This would imply that coreceptor down-regulation (dulling) is not a consequence of commitment to the death pathway. To explore this possibility, we have utilized an in vitro assay to demonstrate that dulling occurred in response to both positive and negative selecting ligands in vitro, was not a result of nonspecific membrane perturbation, was not dependent on the type of APC, and occurred before death in vitro. Furthermore, when thymocyte apoptosis was blocked, CD4 and CD8 were down-regulated in response to TCR stimulation. These data suggest that dulling in response to TCR ligation is distinct from death, and support a model in which DP dulling occurs during both positive and negative selection. The biological implications of this phenomenon are discussed.

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

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

Presence or Absence of TGF-β Determines IL-4-Induced Generation of Type 1 or Type 2 CD8 T Cell Subsets

CD8+ T cells often differentiate into highly cytotoxic cells, secreting a Th1-like or type 1 cytokine pattern characterized by the production of IFN-γ. However, cytotoxic, and in some reports, noncytotoxic, type 2 cells that secrete IL-4, IL-5, or IL-10 instead of IFN-γ, can be generated when CD8+ T cells are primed in the presence of IL-4. Here, we show that IL-4 can also generate typical CD8 type 1 responses. Indeed, while presence of TGF-β biases the development of CD8 T cells that, then, produce little cytolytic activity and IFN-γ, addition of IL-4 results in the recovery of cytotoxicity and IFN-γ production. The cooperative effects of TGF-β and IL-4 imply dual functions, not only for IL-4, but also for TGF-β. Indeed, depending on the presence or absence of IL-4, TGF-β either inhibits or induces the generation of type 1 CD8+ T cells. Physiologically, the ratio of local IL-4/TGF-β concentration may therefore be a critical element in determining the outcome of T cell responses to pathogen and autoantigens. It allows CD8 T cells to switch from an immunotolerant state in the presence of only TGF-β or IL-4, to an immunocompetent proinflammatory type 1 state in the absence or presence of both cytokines.

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

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

Visualization of CD4/CD8 T cell commitment

A system to innocuously visualize T cell lineage commitment is described. Using a "knock-in" approach, we have generated mice expressing a beta-galactosidase reporter in place of CD4; expression of beta-galactosidase in these animals appears to be an accurate and early indicator of CD4 gene transcription. We have exploited this knock-in line to trace CD4/CD8 lineage commitment in the thymus, avoiding important pitfalls of past experimental approaches. Our results argue in favor of a selective model of thymocyte commitment, demonstrating a fundamentally symmetrical process: engagement of either class of major histocompatibility complex (MHC) molecule by a differentiating CD4(+)CD8(+) cell can give rise to T cell antigen receptor (TCR)hi thymocytes of either lineage. Key findings include (a) direct demonstration of a substantial number of CD4-committed, receptor/coreceptor-mismatched cells in MHC class II- deficient mice, a critical prediction of the selective model; (b) highly efficient rescue of such "mismatched" intermediates by forced expression of CD8 in a TCR transgenic line, and an explanation of why previous experiments of this nature were less successful-a major past criticism of the selective model; (c) direct demonstration of an analogous, though smaller, population of CD8-committed mismatched intermediates in class I-deficient animals. Finally, we found no evidence of a CD4 default pathway.

CD5 expression is developmentally regulated by T cell receptor (TCR) signals and TCR avidity

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4(-)CD8(-) (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56(lck). At the CD4(+)CD8(+) (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR-major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4(+) or CD8(+) (single positive, SP) thymocytes is induced by engagement of the alpha/beta-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR-MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.

Cutting Edge: Virus-Activated CD8 T Cells and Lymphokine-Activated NK Cells Express the Mast Cell Function-Associated Antigen, An Inhibitory C-Type Lectin

The mast cell function-associated Ag (MAFA) is an inhibitory C-type lectin that was originally identified on the cell surface of a rat mucosal mast cell line, RBL-2H3. We have cloned the mouse homologue of the rat MAFA gene, and Northern blot analysis revealed that mouse MAFA (mMAFA) gene expression was strongly induced in effector CD8 T cells and lymphokine-activated NK cells but not in effector CD4 T cells and in mouse mast cells. Moreover, mMAFA gene expression was only found in effector CD8 T cells that had been primed in vivo with live virus because in vitro activated CD8 T cells did not express mMAFA. Primary sequence comparison revealed a high degree of conservation (89% similarity) between rat MAFA and mMAFA. Thus, the MAFA molecule in the mouse is a putative inhibitory receptor on anti-viral CD8 T cells induced in vivo and on NK cells.

Degree of TCR Internalization and Ca2+ Flux Correlates with Thymocyte Selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

Dendritic cells induce autoimmune diabetes and maintain disease via de novo formation of local lymphoid tissue

Activation of autoreactive T cells can lead to autoimmune diseases such as insulin-dependent diabetes mellitus (IDDM). The initiation and maintenance of IDDM by dendritic cells (DC), the most potent professional antigen-presenting cells, were investigated in transgenic mice expressing the lymphocytic choriomeningitis virus glycoprotein (LCMV-GP) under the control of the rat insulin promoter (RIP-GP mice). We show that after adoptive transfer of DC constitutively expressing the immunodominant cytotoxic T lymphocyte (CTL) epitope of the LCMV-GP, RIP-GP mice developed autoimmune diabetes. Kinetic and functional studies of DC-activated CTL revealed that development of IDDM was dependent on dose and timing of antigenic stimulation. Strikingly, repeated CTL activation by DC led to severe destructive mononuclear infiltration of the pancreatic islets but also to de novo formation of islet-associated organized lymphoid structures in the pancreatic parenchyma. In addition, repetitive DC immunization induced IDDM with lymphoid neogenesis also in perforin-deficient RIP-GP mice, illustrating that CD8(+) T cell-dependent inflammatory mechanisms independent of perforin could induce IDDM. Thus, DC presenting self-antigens not only are potent inducers of autoreactive T cells, but also help to maintain a peripheral immune response locally; therefore, the induction of autoimmunity against previously ignored autoantigens represents a potential hazard, particularly in DC-based antitumor therapies.

Transient alteration of T cell fine specificity by a strong primary stimulus correlates with T cell receptor down-regulation

P14 mice expressing a transgenic TCR specific for the lymphocytic choriomeningitis virus glycoprotein p33 epitope were used to study the induction of CTL effector activity by a variety of ligands. Surprisingly, p33 variants which are weaker agonists for the P14 TCR than the wild-type p33 peptide were able to induce more potent effectors with a broader range of cytolytic specificity. Similarly, low concentrations of p33 were more effective than higher concentrations. These results correlated with no or only moderate TCR down-regulation by variants of p33 and low p33 concentrations. This phenotype observed after 18 h of culture was transient as progressive restoration of reactivity was observed at 42 or 66 h in the cultures stimulated with high p33 concentrations and this correlated with recovery of TCR surface levels. TCR down-regulation was blocked by src family kinase inhibitors. These findings indicate that the specificity of a T cell can be fine-tuned by the nature of the primary stimulus correlating with surface TCR level and imply an important role for src family kinases in the differential regulation of surface TCR levels upon TCR engagement by different ligand/MHC complexes.

Comparison of activation versus induction of unresponsiveness of virus-specific CD4+ and CD8+ T cells upon acute versus persistent viral infection

The functional status of CD4+ T cells during establishment of persistent infection with the noncytopathic lymphocytic choriomeningitis virus was assessed and compared to that of cytotoxic CD8+ T cells. Functionality of virus-specific CD4+ T cells was measured by proliferative responses, cytokine secretion, cognate help, and IFNgamma-mediated protection against challenge infection with recombinant vaccinia virus. Functional CD4+ T cells were induced early after infection and remained measurable up to 6 weeks but then were rendered unresponsive. In contrast, CD8+ T cells were functionally inactivated within 10-15 days. Importantly, functional inactivation of virus-specific CD4+ T cells during persistent viral infection seemed to be critical for the survival of the host.

Antigen persistence and time of T-cell tolerization determine the efficacy of tolerization protocols for prevention of skin graft rejection

We studied antigen-specific T-cell tolerization therapy using skin transplantation across a defined minor histocompatibility antigen difference. Specific tolerization protocols using short-lived peptide or long-lived spleen cells presenting the peptide as antigen prevented graft rejection without immunosuppression when started before or as long as 10 days after transplantation. Peptide-induced T-cell tolerance was transient, and antigen presentation by the graft was not sufficient to maintain tolerance. In contrast, transfer of antigen-expressing lymphoid cells induced long-lasting tolerance correlating with donor cell chimerism. These findings show that antigen-specific tolerization can induce graft acceptance even when begun after transplantation and that long-term graft survival depends on persistence of the tolerizing antigen.

Tumor-Infiltrating Lymphocytes Exhibiting High Ex Vivo Cytolytic Activity Fail to Prevent Murine Melanoma Tumor Growth In Vivo

The identification of tumor-associated Ags recognized by CD8+ CTL and prevention of tumor outgrowth by adoptive transfer of these CTL demonstrates that CD8+ T cells play a major role in antitumor immunity. We have generated B16.F10 melanoma cells that express the glycoprotein epitope amino acid 33–41 (GP33) of the lymphocytic choriomeningitis virus (LCMV) to examine antitumor CD8+ T cell response in C57BL/6 mice immune to LCMV and in mice transgenic for the LCMV GP33-specific P14 TCR (P14 TCR mice). We find that B16.F10GP33 tumor cells grew in syngeneic C57BL/6 mice without inducing T cell tolerance. LCMV infection or adoptive transfer of LCMV-specific effector T cells delayed but did not prevent growth of preestablished tumors in these mice. However, B16.F10GP33 tumor cells were rejected in mice immune to LCMV and in mice treated with LCMV-specific effector T cells on the same day as the tumor. Surprisingly, B16.F10GP33 tumor cells grew in P14 TCR transgenic mice despite an abundance of tumor-associated Ag-specific CD8+ T cells. In these mice, freshly isolated tumor-infiltrating lymphocytes exhibited an activated phenotype and displayed high GP33-specific cytolytic activity when assessed ex vivo. Thus, B16.F10GP33 melanoma cells are able to initiate, but not to sustain, a GP33-specific CTL response sufficient to clear the tumor enduringly.

Receptor-Specific Allelic Exclusion of TCRVα-Chains During Development

Expression of a single Ag receptor on lymphocytes is maintained via allelic exclusion that generates cells with a clonal receptor repertoire. We show in normal mice and mice expressing functionally rearranged TCRαβ transgenes that allelic exclusion at the TCRα locus is not operational in immature thymocytes, whereas most mature T cells express a single TCRVα-chain. TCRVα allelic exclusion in mature thymocytes is regulated through a CD45 tyrosine phosphatase-mediated signal during positive selection. Using functional and genetic systems for selection of immature double TCRVα+ thymocytes, we show that peptide-specific ligand recognition provides the signal for allelic exclusion, i.e., mature T cells maintain expression of the ligand-specific TCRVα-chain, but lose the nonfunctional receptor. Whereas activation of TCRVβ-chains or CD3ε leads to receptor internalization, TCRVα ligation promotes retention of the TCR on the cell surface. Although both TCRVα- and TCRVβ-chains trigger phosphotyrosine signaling, only the TCRVβ-chain mediates membrane recruitment of the GTPase dynamin. These data indicate that TCRVα-directed signals for positive selection control allelic exclusion in T cells, and that developmental signals can select for single receptor usage.

Beta-galactoside-binding protein secreted by activated T cells inhibits antigen-induced proliferation of T cells

We have used mRNA differential display PCR to search for genes induced in activated T cells and have found the LGALS1 (lectin, galactoside-binding, soluble) gene to be strongly up-regulated in effector T cells. The protein coded by the LGALS1 gene is a beta-galactoside-binding protein (betaGBP), which is released by cells as a monomeric negative growth factor but which can also associate into homodimers (galectin-1) with lectin properties. Northern blot analysis revealed that ex vivo isolated CD8+ effector T cells induced by a viral infection expressed high amounts of LGALS1 mRNA, whereas LGALS1 expression was almost absent in resting CD8+ T cells. LGALS1 expression could be induced in CD4+ and CD8+ T cells upon activation with the cognate peptide antigen and high levels of LGALS1 expression were found in concanavalin A-activated T cells but not in lipopolysaccharide-activated B cells. Gel filtration and Western blot analysis revealed that only monomeric betaGBP was released by activated CD8+ T cells and in vitro experiments further showed that recombinant betaGBP was able to inhibit antigen-induced proliferation of naive and antigen-experienced CD8+ T cells. Thus, these data indicate a role of betaGBP as an autocrine negative growth factor for CD8+ T cells.

No requirement for V(D)J recombination in p53-deficient thymic lymphoma

The p53 tumor suppressor is activated in response to a variety of cellular stress signals, although specific in vivo signals that trigger tumor suppression are unknown. In mouse thymocytes, where p53 inactivation leads to tumorigenesis, several observations suggest that V(D)J recombination of T-cell receptor (TCR) loci could provide a DNA damage signal triggering p53-dependent apoptosis and tumor suppression. Inactivation of p53 would allow V(D)J driven mutation of additional cancer genes, facilitating tumorigenesis. Here, we show that mice with a p53 deficiency in thymocytes and unable to carry out V(D)J recombination are not impaired in the development of thymoma. Recombination-activating gene (RAG) deficiencies were introduced into both p53-/- mice and TgTDeltaN transgenic mice, a strain in which 100% of the mice develop thymoma due to thymocyte-specific inactivation of p53 by a simian virus 40 T-antigen variant. V(D)J recombination was dispensable for tumorigenesis since thymomas developed with or without the RAG-1 or RAG-2 gene, although some delay was observed. When V(D)J recombination was suppressed by expression of rearranged TCR transgenes, 100% of the TgTDeltaN mice developed thymoma, surprisingly with reduced latency. Further introduction of a RAG deficiency into these mice had no impact on the timing or frequency of tumorigenesis. Finally, karyotype and chromosome painting analyses showed no evidence for TCR gene translocations in p53-deficient thymomas, although abundant aneuploidy involving frequent duplication of certain chromosomes was present. Thus, contrary to the current hypothesis, these studies indicate that signals other than V(D)J recombination promote p53 tumor suppression in thymocytes and that the mechanism of tumorigenesis is distinct from TCR translocation oncogene activation.

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.

Dendritic cells efficiently induce protective antiviral immunity

Cytotoxic T lymphocytes (CTL) are essential for effective immunity to various viral infections. Because of the high speed of viral replication, control of viral infections imposes demanding functional and qualitative requirements on protective T-cell responses. Dendritic cells (DC) have been shown to efficiently acquire, transport, and present antigens to naive CTL in vitro and in vivo. In this study, we assessed the potential of DC, either pulsed with the lymphocytic choriomeningitis virus (LCMV)-specific peptide GP33-41 or constitutively expressing the respective epitope, to induce LCMV-specific antiviral immunity in vivo. Comparing different application routes, we found that only 100 to 1,000 DC had to reach the spleen to achieve protective levels of CTL activation. The DC-induced antiviral immune response developed rapidly and was long lasting. Already at day 2 after a single intravenous immunization with high doses of DC (1 x 10(5) to 5 x 10(5)), mice were fully protected against LCMV challenge infection, and direct ex vivo cytotoxicity was detectable at day 4 after DC immunization. At day 60, mice were still protected against LCMV challenge infection. Importantly, priming with DC also conferred protection against infections in which the homing of CTL into peripheral organs is essential: DC-immunized mice rapidly cleared an infection with recombinant vaccinia virus-LCMV from the ovaries and eliminated LCMV from the brain, thereby avoiding lethal choriomeningitis. A comparison of DC constitutively expressing the GP33-41 epitope with exogenously peptide-pulsed DC showed that in vivo CTL priming with peptide-loaded DC is not limited by turnover of peptide-major histocompatibility complex class I complexes. We conclude that the priming of antiviral CTL responses with DC is highly efficient, rapid, and long lasting. Therefore, the use of DC should be considered as an efficient means of immunization for antiviral vaccination strategies.

Defective TCR expression in transgenic mice constructed using cDNA-based alpha- and beta-chain genes under the control of heterologous regulatory elements

We describe the generation of ovalbumin (OVA)-specific, MHC class II-restricted alpha beta T cell receptor (TCR) transgenic mice. Initial attempts at generating these transgenic mice utilized heterologous regulatory elements to drive the expression of cDNA genes encoding the separate alpha- and beta-chains of the TCR. Unexpectedly, T cells bearing the transgenic alpha beta TCR failed to emerge from the thymus in these mice, although the transgenes did modify endogenous TCR expression. However, subsequent modification of the approach which enabled expression of the TCR beta-chain under the control of its natural regulatory elements generated mice whose peripheral T cells expressed the transgenic TCR and were capable of antigen-dependent proliferation. These results show that successful generation of MHC class II-restricted, OVA-specific alpha beta TCR transgenic mice was dependent upon combining cDNA- and genomic DNA-based constructs for expression of the respective alpha- and beta-chains of the TCR.

Constitutive expression of Bcl-xL or Bcl-2 prevents peptide antigen-induced T cell deletion but does not influence T cell homeostasis after a viral infection

We examined the CD8+ T cell response to lymphocytic choriomeningitis virus (LCMV) in mice doubly transgenic for an LCMV-specific TCR and for either bcl-xL or bcl-2. Clonal down-sizing of the anti-viral CD8+ T cell response and the generation of T cell memory was not influenced by constitutive expression of these anti-apoptotic proteins in T cells. Expression of Bcl-xL or Bcl-2 did, however, prevent LCMV peptide-induced peripheral deletion of mature CD8+ T cells in vivo and apoptosis of activated LCMV-specific effector T cells in vitro. The CD8+ T cells "rescued" by Bcl-xL or Bcl-2 from peptide antigen-induced cell death were anergic and this could not be reversed by addition of IL-2 in vitro or by adoptive transfer into antigen-free recipient mice followed by LCMV infection in vivo. Taken together, we show here that 1) Bcl-xL or Bcl-2 are functionally equivalent in their ability to modulate CD8+ T cell survival in vivo, 2) distinct apoptosis signaling pathways exist in CD8+ T cells, one that can be inhibited by Bcl-2 or Bcl-xL and one that cannot be blocked, and 3) apoptosis of CD8+ effector T cells during the declining phase of an immune response is not prevented by constitutive expression of the anti-apoptotic proteins Bcl-xL and Bcl-2.

Mechanisms of viral clearance and persistence

Using examples predominantly drawn from study of the lymphocytic choriomeningitis virus (LCMV) model system, this review describes the mechanisms involved in control of virus infections by the cell-mediated immune response, and some of the different strategies viruses have evolved to evade such immune clearance so that they can persist in their hosts. The important role played by the CD8+ cytotoxic lymphocyte (CTL) response in clearance of many systemic virus infections is discussed; and it is emphasized that although CD8+ CTL are classically thought of as lymphocytes which mediate lysis of virus-infected target cells, the principal mechanism by which CD8+ T cells effect clearance of persistent and many acute virus infections via production of antiviral cytokines such as tumour necrosis factor-alpha and interferon-gamma, not via destruction of virus-producing cells. To avoid immune-mediated clearance, viruses frequently use a combination of several different strategies. These can be grouped into mechanisms for avoiding recognition by the immune response (such as establishing latent infections, replicating in immune-privileged sites, down-regulating the expression of immune recognition signals on the surface of infected cells, or undergoing antigenic variation); and mechanisms for suppressing the immune response. The latter include generalized immune suppression mechanisms, and strategies for more precisely disabling the specific immune response such as inducing tolerance or exhaustion of virus-specific CTL. The value of understanding both immune clearance mechanisms and viral evasion strategies in the rational design of immune-based therapies to combat persistent virus infections is discussed.

Virus-specific MHC-class II-restricted TCR-transgenic mice: effects on humoral and cellular immune responses after viral infection

A transgenic mouse expressing MHC class II-restricted TCR with specificity for a lymphocytic choriomeningitis virus (LCMV) glycoprotein-derived T helper cell epitope was developed to study the role of LCMV-specific CD4+ T cells in virus infection in vivo. The majority of CD4+ T cells in TCR transgenic mice expressed the transgenic receptor, and LCMV glycoprotein-specific TCR transgenic CD4+ T cells efficiently mediated help for the production of LCMV glycoprotein-specific isotype-switched antibodies. In contrast, LCMV glycoprotein-specific TCR transgenic mice exhibited a drastically reduced ability to provide help for the generation of antibody responses specific for the virus-internal nucleoprotein, indicating that intramolecular/intrastructural help is limited to antigens that are accessible to B cells on the viral surface. Antiviral cellular immunity was studied with noncytopathic LCMV and recombinant cytopathic vaccinia virus expressing the LCMV glycoprotein. TCR transgenic mice failed to efficiently control LCMV infection, demonstrating that functional LCMV-specific CD4+ T cells--even if activated and present at extremely high frequencies--cannot directly mediate protective immunity against LCMV. Despite the fact that LCMV-primed CD4+ T cells from TCR transgenic mice as well as from control mice showed low MHC class II-restricted cytotoxic activity in vivo, this did not correlate with protection against LCMV replication in vivo. In contrast, CD4+ T cells from TCR-transgenic mice mediated efficient protection against infection with recombinant vaccinia virus. These results further support the need for different immune effector functions for protective immunity against different viral infections.

TNF Receptor-Deficient Mice Reveal Striking Differences Between Several Models of Thymocyte Negative Selection

Central tolerance depends upon Ag-mediated cell death in developing thymocytes. However, the mechanism of induced death is poorly understood. Among the known death-inducing proteins, TNF was previously found to be constitutively expressed in the thymus. The role of TNF in thymocyte negative selection was therefore investigated using TNF receptor (TNFR)-deficient mice containing a TCR transgene. TNFR-deficient mice displayed aberrant negative selection in two models: an in vitro system in which APC are cultured with thymocytes, and a popular in vivo system in which mice are treated with anti-CD3 Abs. In contrast, TNFR-deficient mice displayed normal thymocyte deletion in two Ag-induced in vivo models of negative selection. Current models of negative selection and the role of TNFR family members in this process are discussed in light of these results.

Normal Responsiveness of CTLA-4-Deficient Anti-Viral Cytotoxic T Cells

CTLA-4 has been proposed to negatively regulate immune responses, and mice deficient for CTLA-4 expression succumb to a lymphoproliferative disorder within a few weeks after birth. This study assessed the responsiveness of CTLA-4-deficient T cells expressing a class I-restricted TCR specific for lymphocytic choriomeningitis virus (LCMV). The kinetics of T cell proliferation were studied in vitro after stimulation of T cells with full and partial T cell agonists. No gross abnormalities in CTLA-4-deficient T cells could be detected. Using adoptive transfer experiments, T cell responses were also measured in vivo after infection with LCMV. Low dose infection with LCMV leads to strong expansion of specific T cells followed by a reduction in T cells that parallels the elimination of Ag. The kinetics of T cell expansion and elimination after low dose LCMV infection were not affected by the absence of CTLA-4. High dose infection of mice with LCMV leads to a transient expansion of T cells followed by T cell exhaustion, where all specific T cells are eliminated. T cell exhaustion also occurred in the absence of CTLA-4. Thus, surprisingly, the absence of CTLA-4 did not interfere with T cell activation, down-regulation of ongoing T cell responses after the elimination of Ag, or the exhaustion of T cell responses in the presence of excessive amounts of Ag.

Four types of Ca2+ signals in naive CD8+ cytotoxic T cells after stimulation with T cell agonists, partial agonists and antagonists

Stimulation of T cells via the T cell receptor (TCR) leads to an increase intracellular in free Ca2+ levels ([Ca2+]i) and the activation of the MAP kinase signaling pathway. This study analyzes for the first time Ca2+ fluxes in naive cytotoxic T cells stimulated with full agonists, partial agonists, or antagonists. Four different types of Ca2+ responses could be observed. Full agonists triggered a strong and sustained increase in [Ca2+]i. In contrast, partial T cell agonists induced either a strong but transient Ca2+ flux or very low to no increases in [Ca2+]i, while T cell antagonists failed to induce any measurable Ca2+ flux. The ability of peptides to induce elevated [Ca2+]i perfectly paralleled their ability to trigger TCR internalization and T cell activation. Thus, stimulation of naive cytotoxic T cells with a panel of defined altered peptide ligands reveals a consistent picture, where Ca2+ fluxes predict agonist, partial agonist and antagonist properties of peptides.

Spontaneous autoimmune diabetes in monoclonal T cell nonobese diabetic mice

It has been established that insulin-dependent diabetes mellitus (IDDM) in nonobese diabetic (NOD) mice results from a CD4+ and CD8+ T cell-dependent autoimmune process directed against the pancreatic beta cells. The precise roles that beta cell-reactive CD8+ and CD4+ T cells play in the disease process, however, remain ill defined. Here we have investigated whether naive beta cell-specific CD8+ and CD4+ T cells can spontaneously accumulate in pancreatic islets, differentiate into effector cells, and destroy beta cells in the absence of other T cell specificities. This was done by introducing Kd- or I-Ag7-restricted beta cell-specific T cell receptor (TCR) transgenes that are highly diabetogenic in NOD mice (8.3- and 4.1-TCR, respectively), into recombination-activating gene (RAG)-2-deficient NOD mice, which cannot rearrange endogenous TCR genes and thus bear monoclonal TCR repertoires. We show that while RAG-2(-/-) 4.1-NOD mice, which only bear beta cell-specific CD4+ T cells, develop diabetes as early and as frequently as RAG-2+ 4.1-NOD mice, RAG-2(-/-) 8.3-NOD mice, which only bear beta cell-specific CD8+ T cells, develop diabetes less frequently and significantly later than RAG-2(+) 8.3-NOD mice. The monoclonal CD8+ T cells of RAG-2(-/-) 8.3-NOD mice mature properly, proliferate vigorously in response to antigenic stimulation in vitro, and can differentiate into beta cell-cytotoxic T cells in vivo, but do not efficiently accumulate in islets in the absence of a CD4+ T cell-derived signal, which can be provided by splenic CD4+ T cells from nontransgenic NOD mice. These results demonstrate that naive beta cell- specific CD8+ and CD4+ T cells can trigger diabetes in the absence of other T or B cell specificities, but suggest that efficient recruitment of naive diabetogenic beta cell-reactive CD8+ T cells to islets requires the assistance of beta cell-reactive CD4+ T cells.

TRAF2 is essential for JNK but not NF-kappaB activation and regulates lymphocyte proliferation and survival

TRAF2 is believed to mediate the activation of NF-kappaB and JNK induced by the tumor necrosis factor receptor (TNFR) superfamily, which elicits pleiotropic responses in lymphocytes. We have investigated the physiological roles of TRAF2 in these processes by expressing a lymphocyte-specific dominant negative form of TRAF2, thereby blocking this protein's effector function. We find that the TNFR superfamily signals require TRAF2 for activation of JNK but not NF-kappaB. In addition, we show that TRAF2 induces NF-kappaB-independent antiapoptotic pathways during TNF-induced apoptosis. Inhibition of TRAF2 leads to splenomegaly, lymphadenopathy, and an increased number of B cells. These findings indicate that TRAF2 is involved in the regulation of lymphocyte function and growth in vivo.

A novel Bcl-x isoform connected to the T cell receptor regulates apoptosis in T cells

We define a novel Bcl-x isoform, Bcl-x gamma, that is generated by alternative splicing and characterized by a unique 47 amino acid C-terminus. Bcl-x gamma is expressed primarily in thymocytes, where it may depend on an interaction between the TCR and host MHC products, and in mature T cells, where its expression is associated with ligation of the T cell receptor. Overexpression of Bcl-x gamma in T cells inhibits activation-induced apoptosis; inhibition of Bcl-x gamma, after stable expression of Bcl-x gamma antisense cDNA, enhances activation-induced apoptosis. In contrast to other Bcl-x isoforms, cells that fail to express Bcl-x gamma after CD3 ligation undergo programmed cell death, while activated T cells that express Bcl-x gamma are spared. Identification of Bcl-x gamma helps provide a molecular explanation of T cell activation and death after antigen engagement.

A mechanism for the major histocompatibility complex-linked resistance to autoimmunity

Certain major histocompatibility complex (MHC) class II haplotypes encode elements providing either susceptibility or dominant resistance to the development of spontaneous autoimmune diseases via mechanisms that remain undefined. Here we show that a pancreatic beta cell-reactive, I-Ag7-restricted, transgenic TCR that is highly diabetogenic in nonobese diabetic mice (H-2(g7)) undergoes thymocyte negative selection in diabetes-resistant H-2(g7/b), H-2(g7/k), H-2(g7/q), and H-2(g7/nb1) NOD mice by engaging antidiabetogenic MHC class II molecules on thymic bone marrow-derived cells, independently of endogenous superantigens. Thymocyte deletion is complete in the presence of I-Ab, I-Ak + I-Ek or I-Anb1 + I-Enb1 molecules, partial in the presence of I-Aq or I-Ak molecules alone, and absent in the presence of I-As molecules. Mice that delete the transgenic TCR develop variable degrees of insulitis that correlate with the extent of thymocyte deletion, but are invariably resistant to diabetes development. These results provide an explanation as to how protective MHC class II genes carried on one haplotype can override the genetic susceptibility to an autoimmune disease provided by allelic MHC class II genes carried on a second haplotype.

Distinct roles for LFA-1 and CD28 during activation of naive T cells: adhesion versus costimulation

Efficient T cell activation requires the engagement of a variety of ligand/receptor molecules in addition to T cell receptor (TCR)-major histocompatibility complex (MHC)/peptide interactions. The leukocyte function antigen 1 (LFA-1) and the CD28 glycoprotein have both been implicated in T cell activation. The present study dissects the roles of LFA-1 and CD28 in the activation of naive virus-specific CD8+ T cells. We demonstrate that LFA-1 facilitates T cell activation by lowering the amounts of antigen necessary for T cell activation. In the absence of LFA-1, 100-fold more antigen was required for T cell-antigen-presenting cell (APC) conjugation and all subsequent events of T cell activation, including TCR down-regulation, Ca2+-flux, T cell proliferation, and lytic effector cell induction. Thus, LFA-1 facilitates the functional triggering of TCRs by promoting adhesion of T cells to APCs but does not affect T cell activation otherwise. In contrast, CD28 played an entirely different role during T cell activation. CD28 reduced the number of TCRs that had to be triggered for T cell activation and allowed activation of T cells by low affinity ligands. CD28 but not LFA-1 prevented induction of T cell unresponsiveness after stimulation of TCRs. These results demonstrate that LFA-1 and CD28 exhibit distinct, nonoverlapping ways to influence T cell activation and suggest that the terms costimulation and signal 2 should be revisited.

Evidence for rapid disappearance of initially expanded HIV-specific CD8+ T cell clones during primary HIV infection

Down-regulation of the initial burst of viremia during primary HIV infection is thought to be mediated predominantly by HIV-specific cytotoxic T lymphocytes, and the appearance of this response is associated with major perturbations of the T cell receptor repertoire. Changes in the T cell receptor repertoire of virus-specific cytotoxic T lymphocytes were analyzed in patients with primary infection to understand the failure of the cellular immune response to control viral spread and replication. This analysis demonstrated that a significant number of HIV-specific T cell clones involved in the primary immune response rapidly disappeared. The disappearance was not the result of mutations in the virus epitopes recognized by these clones. Evidence is provided that phenomena such as high-dose tolerance or clonal exhaustion might be involved in the disappearance of these monoclonally expanded HIV-specific cytotoxic T cell clones. These findings should provide insights into how HIV, and possibly other viruses, elude the host immune response during primary infection.

Peptide-induced T cell receptor down-regulation on naive T cells predicts agonist/partial agonist properties and strictly correlates with T cell activation

Recent experiments defining T cell agonists, partial agonists and antagonists have suggested that the T cell can discriminate between subtle differences in interactions leading to T cell activation. To further understand the complexities of T cell activation, we have analyzed the requirements for the induction of a variety of effector functions using naive T cells and a variety of altered peptide ligands. Using a strong agonist peptide, massive T cell receptor (TCR) down-regulation correlated with a wide range of effector functions that were all induced above the same threshold peptide concentration. Interestingly, the kinetics of TCR down-regulation correlated with the concentration of the peptide, whereas the maximal degree of TCR down-regulation correlated with the induction of all monitored effector functions. A selected group of altered peptide ligands was also examined that were able to render target cells susceptible for lysis by effector cytotoxic T lymphocytes. The extent of TCR down-regulation induced by these peptides corresponded to the induction of a subset of effector functions. These studies have shown that the extent of TCR down-regulation defines the strength of TCR-mediated "signal 1" which correlates with the spectrum of effector functions activated within the T cell. Thus, activation of different T cell functions requires the triggering of distinct numbers of TCR. The different parameters that influence TCR down-regulation define important distinctions between our results and previously reported findings with T cell clones and may outline decisive parameters for the consequences of T cell activation in vivo.

Self antigens expressed by solid tumors Do not efficiently stimulate naive or activated T cells: implications for immunotherapy

Induction and maintenance of cytotoxic T lymphocyte (CTL) activity specific for a primary endogenous tumor was investigated in vivo. The simian virus 40 T antigen (Tag) expressed under the control of the rat insulin promoter (RIP) induced pancreatic beta-cell tumors producing insulin, causing progressive hypoglycemia. As an endogenous tumor antigen, the lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) was introduced also under the control of the RIP. No significant spontaneous CTL activation against GP was observed. However, LCMV infection induced an antitumor CTL response which efficiently reduced the tumor mass, resulting in temporarily normalized blood glucose levels and prolonged survival of double transgenic RIP(GP x Tag2) mice (137 +/- 18 d) as opposed to control RIP-Tag2 mice (88 +/- 8 d). Surprisingly, the tumor-specific CTL response was not sustained despite the facts that the tumor cells continued to express MHC class I and LCMV-GP-specific CTLs were present and not tolerized. Subsequent adoptive transfer of virus activated spleen cells into RIP(GP x Tag2) mice further prolonged survival (168 +/- 11 d), demonstrating continued expression of the LCMV-GP tumor antigen and MHC class I. The data show that the tumor did not spontaneously induce or maintain an activated CTL response, revealing a profound lack of immunogenicity in vivo. Therefore, repetitive immunizations are necessary for prolonged antitumor immunotherapy. In addition, the data suggest that the risk for induction of chronic autoimmune diseases is limited, which may encourage immunotherapy against antigens selectively but not exclusively expressed by the tumor.

Differentiation of CD4(high)CD8(low) coreceptor-skewed thymocytes into mature CD8 single-positive cells independent of MHC class I recognition

Thymocytes with a CD4(hi)CD8(lo) coreceptor-skewed (CRS) phenotype have been shown to contain precursors for CD8 single-positive (SP) thymocytes, in addition to precursors for CD4 SP cells. The selection mechanisms that stimulate CD4(hi)CD8(lo) cells to revert to the CD8 lineage are not known. Mice transgenic (tg) for the major histocompatibility complex (MHC) class I-restricted P14 T cell receptor (TCR), on the H-2bm13 background, generate a large number of CD4(hi)CD8(lo) CRS thymocytes. We analyzed the developmental potential and the differentiation requirements of the CD4(hi)CD8(lo) population of these mice. Using reaggregate thymic organ cultures (RTOC), we observed that these cells efficiently and almost exclusively differentiate into CD8 SP cells. Differentiation occurred independent of whether or not the MHC haplotype of the thymic stroma corresponds to the MHC restriction of the tg TCR. Loss of CD4 was independent of thymic stroma, up-regulation of CD8 to full levels was dependent on thymic stroma but independent of MHC haplotype. After trypsin treatment and overnight incubation, these CRS cells re-expressed CD8 but failed to re-express CD4, indicating that they are in the process of terminating CD4 synthesis. CD8 SP cells derived from the CRS cells proliferate in response to peptide-pulsed antigen-presenting cells. Our data suggest that CD4(hi)CD8(lo) CRS thymocytes bearing the P14 tg TCR have completed positive selection and differentiate autonomously into functionally competent CD8 SP cells.

Functional management of an antiviral cytotoxic T-cell response

Lymphocytic choriomeningitis virus (LCMV) is known to induce strong, polyclonal cytotoxic T-lymphocyte (CTL) responses. Using a set of variant peptides derived from the major CTL epitope of LCMV, we analyzed the functional fine specificity of the LCMV-specific CTL response. During the primary response, almost all the tested peptides were recognized. In contrast, the secondary response was purged of all minor cross-reactivities and very few peptides were significantly recognized. This study is the first demonstration of the functional maturation of a T-cell response and has important clinical and biological implications.

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.

Normal thymic selection, normal viability and decreased lymphoproliferation in T cell receptor-transgenic CTLA-4-deficient mice

CTLA-4 is a T cell surface receptor essential for the negative regulation of T cell activation. In the CTLA-4-deficient mouse, a dramatic accumulation of activated peripheral T cells effects extensive damage to host tissues, resulting in mortality within 5 weeks of age. To determine whether the accumulation of activated T cells in CTLA-4(-/-) mice is due to a defect in thymic selection, we examined negative selection in CTLA-4(-/-) mice using two transgenic T cell receptor (TCR) models of thymic selection. Neither the H-Y-specific TCR nor the lymphocytic choriomeningitis virus (LCMV)-specific TCR transgenic models revealed a defect in positive or negative selection in CTLA-4(-/-) mice in vivo or in vitro. In fact, the negatively selecting phenotype of male H-YTCR-transgenic mice greatly mitigated the accumulation of activated peripheral T cells. Further, peripheral CTLA-4(-/-) T cells expressing a single LMCV-specific transgenic TCR did not have an activated phenotype, indicating that CTLA-4(-/-) T cells require specific antigen for proliferation. These results demonstrate that specific antigen is required for the lymphoproliferation observed in CTLA-4(-/-) mice, and that CTLA-4 deficiency does not lead to a gross defect in negative selection.

Specific recognition of thymic self-peptides induces the positive selection of cytotoxic T lymphocytes

To understand how thymic selection gives rise to T cells that are capable of major histocompatibility complex (MHC)-restricted recognition of antigen but are tolerant of self, we directly examined how peptide/MHC ligands expressed on thymic epithelial cells trigger the positive selection of immature thymocytes. We demonstrate that abundant self-peptides, purified from the H-2D(b) molecules of thymic epithelial cells, are specifically recognized during the positive selection of CD8+ T cells, implying that positive selection generates a repertoire of T cells that is weakly self-reactive. We also found that this recognition is somewhat cross-reactive, thereby providing an explanation for how the specific recognition of a limited repertoire of thymic self-peptides can select a diverse repertoire of T cells.

Thymic selection by a single MHC/peptide ligand produces a semidiverse repertoire of CD4+ T cells

The influence of individual peptides in thymic selection was examined in H2-M- mice, in which positive selection is directed to a single peptide, class II-associated invariant chain peptide (CLIP) bound to H2-A(b). Two sensitive in vivo approaches showed that 70%-80% of CD4+ T cells undergoing positive selection to CLIP+H2-A(b) have self-reactivity to the various peptides expressed on wild-type H2-M+ antigen-presenting cells. When these self-reactive T cells were depleted, the residual CD4+ cells displayed a polyclonal repertoire in terms of alloreactivity, responses to foreign protein antigens, and Vbeta usage. Nevertheless, studies with two T cell receptor transgenic lines suggested that the repertoire of CD4+ cells induced by CLIP was less diverse than the repertoire of CD4+ cells in normal mice. Generation of a fully diverse T cell repertoire thus requires positive selection against multiple peptides.

The interferon regulatory transcription factor IRF-1 controls positive and negative selection of CD8+ thymocytes

Little is known about the molecular mechanisms and transcriptional regulation that govern T cell selection processes and the differentiation of CD4+ and CD8+ T cells. Mice lacking the interferon regulatory transcription factor-1 (IRF-1) have reduced numbers of mature CD8+ cells within the thymus and peripheral lymphatic organs. Here we show that positive and negative T cell selection of two MHC class I-restricted TCR alphabeta transgenes, H-Y and P14, are impaired in IRF-1-/- mice. The absence of IRF-1 resulted in decreased expression of LMP2, TAP1, and MHC class I on thymic stromal cells. Despite decreased MHC class I expression on IRF-1-/- thymic stromal cells, the defect in CD8+ T cells development did not reside in the thymic environment, and IRF-1-/- stromal cells can fully support development of CD8+ thymocytes in in vivo bone marrow chimeras and in vitro reaggregation cultures. Moreover, IRF-1-/- thymocytes displayed impaired TCR-mediated signal transduction, and the induction of negative selection in TCR Tg thymocytes from IRF-1-/- mice required a 1000-fold increase in selecting peptide. We also provide evidence that IRF-1 is mainly expressed in mature, but not immature, thymocytes and that expression of IRF-1 in immature thymocytes is induced after peptide-specific TCR activation. These results indicate that IRF-1 regulates gene expression in developing thymocytes required for lineage commitment and selection of CD8+ thymocytes.

CD5- CD8 alpha beta intestinal intraepithelial lymphocytes (IEL) are induced to express CD5 upon antigen-specific activation: CD5- and CD5+ CD8 alpha beta IEL do not represent separate T cell lineages

We followed alpha beta T cell receptor (TCR) usage in subsets of gut intraepithelial lymphocytes (IEL) in major histocompatibility complex class I-restricted alpha beta TCR-transgenic (tg) mice. The proportion of tg alpha beta TCR+ CD8 alpha beta IEL is reduced compared with CD8+ splenocytes of the same animal, particularly under conventional conditions of maintenance. Further fractionation of CD8 alpha beta IEL according to the expression level of surface CD5 revealed that in conventionally housed animals tg TCR+ CD5- CD8 alpha beta IEL are as frequent as in specific pathogen-free (SPF) mice, whereas tg TCR+ CD5int or, even more pronounced, tg TCR+ CD5hi CD8 alpha beta IEL are greatly diminished when compared with mice kept under SPF conditions. Upon antigen-specific stimulation of CD5- CD8 alpha beta IEL in vitro, CD5 surface expression is up-regulated on a large fraction of cells within 48 h. Up-regulation of CD5 surface expression is further enhanced by the presence of the anti-alpha IEL monoclonal antibody 2E7. This clearly demonstrates that CD5-, and CD5+ CD8 alpha beta IEL cannot be considered as separate T cell lineages.

A regulatory role for TRAF1 in antigen-induced apoptosis of T cells

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) and TRAF1 were found as components of the TNFR2 signaling complex, which exerts multiple biological effects on cells such as cell proliferation, cytokine production, and cell death. In the TNFR2-mediated signaling pathways, TRAF2 works as a mediator for activation signals such as NF-kappaB, but the role of TRAF1 has not been previously determined. Here we show in transgenic mice that TRAF1 overexpression inhibits antigen-induced apoptosis of CD8(+) T lymphocytes. Our results demonstrate a biological role for TRAF1 as a regulator of apoptotic signals and also support the hypothesis that the combination of TRAF proteins in a given cell type determines distinct biological effects triggered by members of the TNF receptor superfamily.

Magnitude of protein tyrosine phosphorylation-linked signals determines growth versus death of thymic T lymphocytes

Using concanavalin A (Con A) as a multireceptor-reactive agonist, we studied the relationship between the growth or death of thymic T lymphocytes and the agonist concentration-dependent magnitude of the intracellularly delivered signal. Both immature and mature thymic T lymphocytes were subjected to a high concentration of Con A-mediated signal for apoptotic cell death. In this model, a number of cellular proteins including mitogen activated protein kinases were phosphorylated at tyrosine depending on the concentration of Con A. This effect was followed by corresponding increase in serine 73 phosphorylation of c-jun and transcription of c-fos. DNA fragmentation and cell membrane disruption developed concomitantly after stimulation with high concentrations of Con A. The addition of inhibitors of protein kinases which completely inhibited the growth of cells stimulated with low concentrations of Con A only partially prevented death, and even promoted DNA fragmentation of cells stimulated with high concentrations of Con A. The dissociated sensitivities of Con A-mediated cell growth and cell death to the inhibitors were, however, shown to be due to the different efficiency of inhibition of high and low levels of intracellularly delivered signals. The results indicate that the magnitude of signaling could be the principal element that determines the growth versus death of thymic T lymphocytes.