Essential Role of the E3 Ubiquitin Ligase Cbl-b in T Cell Anergy Induction

Antigen-specific immunotolerance limits the expansion of self-reactive T cells involved in autoimmune diseases. Here, we show that the E3 ubiquitin ligase Cbl-b is upregulated in T cells after tolerizing signals. Loss of Cbl-b in mice results in impaired induction of T cell tolerance both in vitro and in vivo. Importantly, rechallenge of Cbl-b mutant mice with the tolerizing antigen results in massive lethality. Moreover, ablation of Cbl-b resulted in exacerbated autoimmunity. Mechanistically, loss of Cbl-b rescues reduced calcium mobilization of anergic T cells, which was attributed to Cbl-b-mediated regulation of PLCgamma-1 phosphorylation. Our results show a critical role for Cbl-b in the regulation of peripheral tolerance and anergy of T cells.

Control of Foxp3+ CD25+CD4+ regulatory cell activation and function by dendritic cells

Naturally occurring CD4+CD25+ regulatory T (TR) cells play crucial roles in normal immunohomeostasis. CD4+CD25+ TR cells exhibit a number of interesting in vitro properties including a 'default state' of profound anergy refractory to conventional T cell stimuli. We investigated the in vitro activation requirements of CD4+CD25+ TR cells using bone marrow-derived DC, which as professional antigen presenting cells (APC) can support the activation of normal naive T cells. Comparison of different APC types revealed that LPS-matured DC were by far the most effective at breaking CD4+CD25+ TR cell anergy and triggering proliferation, and importantly their IL-2 production. Examination of Foxp3, a key control gene for CD4+CD25+ TR cells, showed this to be stably expressed even during active proliferation. Although CD4+CD25+ TR cell proliferation was equivalent to that of CD25- cells their IL-2 production was considerably less. Use of IL-2-/- mice demonstrated that the DC stimulatory ability was not dependent on IL-2 production; nor did IL-15 appear crucial but was, at least in part, related to costimulation. DC also blocked normal CD4+CD25+ TR cell-mediated suppression partially via IL-6 secretion. DC therefore possess novel mechanisms to control the suppressive ability, expansion and/or differentiation of CD4+CD25+ TR cells in vivo.

Aging, autoimmunity and arthritis: T-cell senescence and contraction of T-cell repertoire diversity - catalysts of autoimmunity and chronic inflammation

Rheumatoid arthritis (RA), like many other autoimmune syndromes, is a disease of adults, with the highest incidence rates reported in the elderly. The immune system undergoes profound changes with advancing age that are beginning to be understood and that need to be incorporated into the pathogenetic models of RA. The age-related decline in thymic function causes extensive remodeling of the T-cell system. Age-dependent changes in T-cell homeostasis are accelerated in patients with RA. The repertoire of naive and memory T cells is less diverse, possibly as a result of thymic insufficiency, and it is biased towards autoreactive cells. Presenescent T cells emerge that are resistant to apoptosis and that often expand to large clonal populations. These cells are under the regulatory control of nonconventional costimulatory molecules, display potent effector functions, and appear to be critical in the synovial and extra-articular manifestations of RA.

Physiological signals and oncogenesis mediated through Crk family adapter proteins

The viral Crk oncogene (v-Crk) is known to induce sarcomas in chicken and its cellular homologs c-Crk I, c-Crk II, and Crk-like (CRKL) have been implicated in many signal transduction events. These include cell differentiation, cell migration, and the induced nonresponsiveness of T-cells to stimulation of the T-cell receptor (TCR), a state known as anergy. CRKL is also the most prominent substrate of the Bcr-Abl oncoprotein which causes human chronic myelogenous leukemias (CML). The modular composition of the Crk family adapters which largely consist of Src homology (SH2 and SH3) domains has prompted an intensive search for physiological and pathological upstream and downstream signalling partners which selectively bind to these adapters. Upstream proteins include various receptors and large multisite docking proteins, while several protein kinases and guanine nucleotide release proteins (GNRPs) have been suggested to function downstream of c-Crk and CRKL. Most Crk/CRKL SH2- and SH3-binding proteins contain several docking sites with considerable sequence similarity. Thus the binding requirements of Crk/CRKL SH2 and SH3 domains are now well defined, providing a basis for the design of small inhibitory molecules to block the function of these adapter proteins. The enzymatic cascades activated through Crk family adapters are only partially known, but stress kinases (SAPKs/JNKs) and the GTPase Rap1, as well as the B-Raf isoform of the Raf protein kinases, are affected in some systems. Several yet unidentified, highly selective Crk interacting proteins detectable in specific cell types remain to be studied. More detailed analyses of the enzymatic activities triggered through Crk-type adapters will also be crucial to fully define the signalling pathways controlled by this protein family.

Loss of anergic B cells in prediabetic and new-onset type 1 diabetic patients

Although dogma predicts that under normal circumstances, potentially offensive autoreactive cells are silenced by mechanisms of immune tolerance, islet antigen-reactive B lymphocytes are known to play a crucial role in the development of autoimmunity in type 1 diabetes (T1D). Thus, participation of these cells in T1D may reflect escape from silencing mechanisms. Consistent with this concept, we found that in healthy subjects, high-affinity insulin-binding B cells occur exclusively in the anergic naive IgD(+), IgM(-) B-cell (BND) compartment. Antigen receptors expressed by these cells are polyreactive and have N-region additions, Vh usage, and charged complementarity-determining region 3 consistent with autoreactivity. Consistent with a potential early role in autoimmunity, these high-affinity insulin-binding B cells are absent from the anergic compartment of some first-degree relatives and all prediabetic and new-onset (<1 year) T1D patients tested, but return to normal levels in individuals diabetic for >1 year. Interestingly, these changes were correlated by transient loss of the entire BND compartment. These findings suggest that environmental events such as infection or injury may, by disrupting B-cell anergy, dispose individuals toward autoimmunity, the precise nature of which is specified by genetic risk factors, such as HLA alleles.

Tumor escape mechanisms from immunosurveillance: induction of unresponsiveness in a specific MHC-restricted CD4+ human T cell clone by the autologous MHC class II+ melanoma

The co-existence of tumor specific immunity with a progressing tumor is observed in a variety of experimental systems and remains one of the major paradoxes of tumor immunology. We now report that a human melanoma cell line (SMC) expressing MHC class II was able to induce clonal anergy in a specific, MHC-restricted CD4+ T cell clone (STC3). Clonal anergy is a mechanism of T lymphocyte tolerance induced by antigen receptor stimulation in the absence of co-stimulation. We observed that a CD4+ T cell clone and an autologous melanoma cell line form conjugates with each other that leads to an increase of [Ca++]i in the T cell clone; however, this interaction failed to induce IL-2 production or proliferation of the T cell clone. Furthermore, this interaction rendered this T cell clone unresponsive to subsequent stimulation. All these effects were MHC class II restricted. Therefore, the human melanoma cell line SMC was capable of delivering antigen-specific signals to the T cell clone, but did not deliver the co-stimulatory signals, e.g. a B7/CD28 interaction, necessary for full T cell activation. Transfection of the melanoma cells with an expression vector containing a B7 cDNA that resulted in subsequent expression of B7 on its cell surface rendered it into a fully competent antigen presenting cell, which is able to induce a nuclear factor binding to the IL-2 promoter in the specific T cell clone resulting in enhanced IL-2 transcription, synthesis, and T cell proliferation. These findings suggest that manipulation of co-stimulation may offer new strategies for future tumor immunotherapy.

CTLA-4 activation of phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death

The balance of T-cell proliferation, anergy and apoptosis is central to immune function. In this regard, co-receptor CTLA-4 is needed for the induction of anergy and tolerance. One central question concerns the mechanism by which CTLA-4 can induce T-cell non-responsiveness without a concurrent induction of antigen induced cell death (AICD). In this study, we show that CTLA-4 activation of the phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death. CTLA-4 ligation induced PI 3K activation as evidenced by the phosphorylation of PKB/AKT that in turn inactivated GSK-3. The level of activation was similar to that observed with CD28. CTLA-4 induced PI 3K and AKT activation also led to phosphorylation of the pro-apoptotic factor BAD as well as the up-regulation of BcL-XL. In keeping with this, CD3/CTLA-4 co-ligation prevented apoptosis under the same conditions where T-cell non-responsiveness was induced. This effect was PI 3K and PKB/AKT dependent since inhibition of these enzymes under conditions of anti-CD3/CTLA-4 co-ligation resulted in cell death. Our findings therefore define a mechanism by which CTLA-4 can induce anergy (and possibly peripheral tolerance) by preventing the induction of cell death.

Pretransplant administration of a single donor class I major histocompatibility complex molecule is sufficient for the indefinite survival of fully allogeneic cardiac allografts: evidence for linked epitope suppression

BACKGROUND

Allograft survival can be prolonged by the administration of alloantigen(s) before transplantation. Blood transfusion is the commonest form of alloantigen pretreatment currently used in clinical practice. However, for recipients of organs from cadaver donors, it is not possible to predict the identity of the organ donor in advance. Therefore, it is highly unlikely that all the alloantigens expressed by a cadaver organ donor will be represented in the alloantigen pretreatment inoculum. We have previously shown that it is not necessary to expose the recipient to the full complement of donor alloantigens to induce long-term survival of a subsequent cardiac allograft. Here, we investigated the in vivo mechanism responsible for this phenomena.

METHODS

Unresponsiveness to the mouse MHC class I molecule Kb was induced in CBA.Ca (H2k) recipients by administration of bone marrow cells from transgenic CBA mice, CBK (H2k + Kb) before transplantation of fully allogeneic and F1 vascularized cardiac allografts.

RESULTS

Pretreatment with CBK bone marrow cells resulted in the long-term survival of all cardiac allografts expressing H2-Kb. For example, C57BL/10 (H2b) and (CBKxBALB/c) F1 (H2k,d + Kb) cardiac allografts were accepted by recipients treated with CBK bone marrow cells before transplantation. In contrast, allografts that did not express Kb, such as BALB/c (H2d) or (CBAxBALB/c) F1 (H2k,d), were rejected acutely, with a median survival time (MST) of 7 and 6 days, respectively, in recipients treated with CBK bone marrow cells. Furthermore, when recipients pretreated with CBK bone marrow cells were grafted with a BALB/c heart and a CBK heart simultaneously, the BALB/c hearts were rejected (MST=10 days), whereas the CBK hearts were accepted. By contrast, in the maintenance phase (i.e., after transplantation), recipients with long-term surviving (CBKxBALB/c) F1 hearts (> 100 days) were found to accept BALB/c hearts indefinitely, whereas fourth-party B10.S (H2s) grafts were rejected (MST=7.5 days). This indicated that the allografts bearing Kb could tolerize recipients to other alloantigens expressed by the transplanted heart.

CONCLUSIONS

These data provide clear evidence for linked epitope suppression in the induction of operational tolerance in vivo.

T-cell activation through the antigen receptor. Part 2: role of signaling cascades in T-cell differentiation, anergy, immune senescence, and development of immunotherapy

Part 2 of this review on cellular activation by the T-cell antigen receptor (TCR) will highlight how TCR signaling pathways are adapted to achieve specific biologic outcomes, including different states of T-cell differentiation and the induction of T-cell tolerance. We will also explore how treatment with altered peptide ligands affects TCR signaling to change T-cell differentiation or to induce an anergy state. These changes are accomplished through alteration of protein tyrosine kinase activity, the stoichiometry of phosphorylation of immunoreceptor tyrosine-based activation motifs, intracellular free ionized calcium flux, mitogen-activated protein kinase activity, and transcriptional activation of key cytokine promoters. The CTLA-4 plays an important role in the induction and maintenance of anergy. The second theme will highlight how altered TCR signal transduction, including changes in the compartmentalization of signaling components at the TCR synapse, contributes to decreased T-cell activation during immune senescence. Finally, we will illustrate how the molecular details of TCR activation can be used to modify the function of the immune system. This includes a description of the mechanism of action of altered peptide ligands, CTLA-4Ig, and pharmacologic inhibitors of mitogen-activated protein kinases, nuclear factor kappaB, and protein kinase C cascades.

Primer: mechanisms of immunologic tolerance

Successful adaptive immunity against a broad range of pathogens depends on the diversity of randomly generated T-lymphocyte and B-lymphocyte receptors. A subset of these receptors will be self-reactive and must be regulated to prevent autoimmunity. The process of immunologic tolerance addresses this problem by either purging autoreactive receptors from the system or tuning down their reactivity sufficiently to prevent disease. Immature lymphocytes generate a novel receptor during development in the thymus or bone marrow. Engagement of self antigens by these nascent receptors leads to their purging, either by the apoptotic death of the lymphocyte or by the initiation of receptor editing, a process in which the autoreactive receptor is replaced. If the lymphocytes mature further, the activation threshold of autoreactive cells can be tuned by the co-expression of inhibitory receptors or negative signaling molecules, allowing the persistence of the receptor without an increased risk of autoimmunity. T-cell and B-cell receptors that escape these checkpoints can still be regulated in the peripheral immune system by both purging and tuning mechanisms. A separate set of mechanisms, mediated by various regulatory cells, also operates to tune peripheral receptors in a cell-extrinsic fashion. The combined action of these processes ensures that the organism does not suffer autoimmune pathology, even if autoreactive receptors are generated and maintained in the immune system.

Endothelial cell anergy is mediated by bFGF through the sustained activation of p38-MAPK and NF-kappaB inhibition

Tumors escape from immune surveillance by, among other mechanisms, the down- regulation of endothelial adhesion molecules, such as ICAM-1, and by unresponsiveness to inflammatory signals, a process mediated by angiogenic factors that is called endothelial cell anergy. Here we present the cell biological regulation of these processes. The angiogenic basic fibroblast growth factor (bFGF/FGF-2) was found to inhibit tumor necrosis factor-alpha (TNF-alpha)- induced elevation of ICAM-1, at transcriptional level. Furthermore, we found that bFGF inhibits the TNF-mediated activation of NF-kappaB by blocking phosphorylation and degradation of IkappaBalpha. We also found that bFGF induces hyperphosphorylation of p38 MAPK on endothelial cells, whereas inhibition of such kinase abrogates the effect of bFGF on the TNF-mediated activation of NF-kappaB. Thus, we suggest that bFGF acts as an inhibitor of leukocyte adhesion in tumor vessels by decreasing the ICAM-1 expression through the sustained activation of p38-MAPK and via inhibition of NF-kappaB.

Regulation of peripheral T cell tolerance by the E3 ubiquitin ligase Cbl-b

The family of the Casitas B-lineage Lymphoma (Cbl) proteins, c-Cbl, Cbl-b, and Cbl-3, function as E3 ubiquitin ligases and molecular adaptors. In particular, Cbl-b acts as a gatekeeper in T cell activation that controls activation thresholds and the requirement for co-stimulation. Loss of Cbl-b expression renders animals susceptible to antigen-triggered autoimmunity suggesting that Cbl-b is a key autoimmunity gene. In addition, Cbl-b plays a critical role in T cell anergy and escape from regulatory T cells (Treg) suppression. Modulation of Cbl-b might provide us with a unique opportunity for future immune treatment of human disorders such as autoimmunity, immunodeficiency, or cancer.

E3 ligases in T cell anergy--turning immune responses into tolerance

Peripheral tolerance is an important strategy used by the immune system to prevent self-reactive lymphocytes from attacking host tissues. A variety of mechanisms contribute to peripheral tolerance, among them activation-induced cell death, suppression by regulatory T cells, and T cell anergy or unresponsiveness. Recent work has led to a better understanding of the cell-intrinsic program that establishes T cell anergy. A major insight is that during the induction phase of anergy, incomplete stimulation (T cell receptor stimulation without costimulation) leads via calcium influx to an altered gene expression program that includes up-regulation of several E3 ubiquitin ligases. When the anergic T cells contact antigen-presenting cells, intracellular signaling proteins are monoubiquitinated and targeted for lysosomal degradation, thus decreasing intracellular signaling and also resulting in decreased stability of the T cell-antigen-presenting cell contact. We propose a molecular program leading to T cell anergy and discuss other proteins that may play a role.

The impact of cell-bound antigen transport on mucosal tolerance induction

Mucosal surfaces are exposed continuously to a flood of foreign antigens demanding a tightly controlled balance between immunity and tolerance induction. Tolerance toward food and inhaled antigens, known as oral and respiratory tolerance, respectively, evokes a body-wide nonresponsiveness against the plethora of environmental antigens. Key issues in understanding the induction of mucosal tolerance relate to the site of antigen entrance, the mechanisms of antigen transport, and the exact anatomical location where lymphocytes meet their cognate antigens. In this regard, opposing ideas have been put forward: In one scenario, antigens taken up at mucosal surfaces are considered to spread throughout the body, thus potentially evoking tolerogenic immune responses in all secondary lymphoid organs. Alternatively, tolerance induction might be confined to the draining regional lymph nodes (LN). Recent observations strongly supported the latter scenario, emphasizing the importance of regional LN and their network of afferent lymphatics in this process. In this model, air-borne and intestinal antigens are captured at mucosal sites by dendritic cells, which then migrate exclusively in a CCR7-dependent way to draining regional LN. Tolerance is then induced actively by the activation of antigen-specific T cells, which are subsequently deleted, become anergic, or alternatively, differentiate into regulatory T cells. Thus, the concept of local induction of immune responses seems to hold true for the majority of immune reactions, regardless of whether they are tolerogenic or defensive in their outcome.

Reversibility with interleukin-2 suggests that T cell anergy contributes to donor-specific hyporesponsiveness in renal transplant patients

Data from various rodent models have implicated a role for anergic T cells in the maintenance of self and transplantation tolerance. The relevance of donor-specific T cell anergy to clinical transplantation, however, has not been demonstrated. Previous studies have reported that recipients of solid organ transplant often have reduced frequencies of CD4(+) T cells with anti-donor direct pathway allospecificity after transplantation. The underlying mechanism(s) of this donor-specific hyporesponsiveness is unclear but likely to contribute to the diminished immunosuppressive requirement of transplant patients with time after transplantation. This study shows that ex vivo treatment of CD4(+) T cells from renal transplant recipients with IL-2 could specifically increase the anti-donor frequency in all the patients with evidence of donor-specific hyporesponsiveness. It also shows that the IL-2-induced recovery of anti-donor frequency is unlikely to result from nonspecific stimulation or selective clonal expansion of activated, allospecific CD4(+) T cells. Taken together, the data suggest that T cell anergy plays an important role in the direct pathway hyporesponsiveness that evolves in many human renal transplant recipients.

Evidence that induction of tolerance in vivo involves active signaling via a B7 ligand-dependent mechanism: CTLA4-Ig protects V beta 8+ T cells from tolerance induction by the superantigen staphylococcal enterotoxin B

Co-stimulation through CD28 is thought to be necessary for the activation of unprimed CD4+ T cells, which are otherwise rendered tolerant. However, we previously found that CD4+ T cell priming was normal or augmented in mice which overexpressed a soluble form of CTLA4 where co-stimulation through CD28 was abrogated. To investigate this CD4+ T cell response, we exploited the capacity of the superantigen staphylococcal enterotoxin B to stimulate T lymphocytes bearing V beta 8+, which represent approximately 30% of all CD4+ T cells. In litter-mate controls of CTLA4-Ig transgenic mice, immunization with staphylococcal enterotoxin B leads to expansion, followed by deletion of V beta 8+ T cells, and the remaining cells are tolerant when stimulated in vitro. Comparable expansion and deletion of V beta 8 T cells occurs in CTLA4-Ig transgenic mice. However, in contrast to normal mice, the remaining V beta 8+ T cells from CTLA4-Ig transgenic mice are not anergic and remain responsive to superantigen in vitro.

Differences in the Kinetics, Amplitude, and Localization of ERK Activation in Anergy and Priming Revealed at the Level of Individual Primary T Cells by Laser Scanning Cytometry

One of the potential mechanisms of peripheral tolerance is the unresponsiveness of T cells to secondary antigenic stimulation as a result of the induction of anergy. It has been widely reported that antigenic unresponsiveness may be due to uncoupling of MAPK signal transduction pathways. However, such signaling defects in anergic T cell populations have been mainly identified using immortalized T cell lines or T cell clones, which do not truly represent primary Ag-specific T cells. We have therefore attempted to quantify signaling events in murine primary Ag-specific T cells on an individual cell basis, using laser-scanning cytometry. We show that there are marked differences in the amplitude and cellular localization of phosphorylated ERK p42/p44 (ERK1/2) signals when naive, primed and anergic T cells are challenged with peptide-pulsed dendritic cells. Primed T cells display more rapid kinetics of phosphorylation and activation of ERK than naive T cells, whereas anergic T cells display a reduced ability to activate ERK1/2 upon challenge. In addition, the low levels of pERK found in anergic T cells are distributed diffusely throughout the cell, whereas in primed T cells, pERK appears to be targeted to the same regions of the cell as the TCR. These data suggest that the different consequences of Ag recognition by T cells are associated with distinctive kinetics, amplitude, and localization of MAPK signaling.

Trypanosoma cruzi surface mucin TcMuc-e2 expressed on higher eukaryotic cells induces human T cell anergy, which is reversible

Chagas' disease is a chronic, debilitating, multisystemic disorder that affects millions of people in Latin America. The protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas' disease, has a large number of O-glycosylated Thr/Ser/Pro-rich mucin molecules on its surface (TcMuc). These mucins are the main acceptors of sialic acid and have been suggested to play a role on various host-parasite interactions, such as adhesion to macrophages, protection from complement lysis, and immunomodulation of the immune response mounted by the host. To observe the immunologic effect obtained by the heterologous expression of a TcMuc gene in higher eukaryotic cells exposed to xenogeneic lymphocytes, we developed a strategy based on the transfection of a known T. cruzi mucin gene (TcMuc-e2) into Vero cells. In contrast to the brisk proliferation and activation of human lymphocytes observed at 3, 4, and 5 days induced by normal Vero cells, neither proliferation nor significant activation of human lymphocytes was observed with TcMuc-e2-transfected Vero cells. This TcMuc-e2 mucin-induced suppression of T cell response can be reversed by the addition of exogenous IL-2. In addition it was demonstrated that the immunosuppressive reaction was not related to the induction of an important degree of apoptosis in human lymphocytes. Posttranslational modification are required for the inhibitory effect that TcMuc-e2 exerts when transfected to Vero cells. O-glycosylation and sialylation are required to obtain the immunomodulatory effect as assessed by O-sialoglycoprotease and neuraminidase treatments. These results are consistent with other studies showing that surface glycoconjugates from T. cruzi and mammalian cells can induce an inhibition of the immune response.

ICAM-1 is required for T cell proliferation but not for anergy or apoptosis induced by Staphylococcus aureus enterotoxin B in vivo

The response of T lymphocytes to superantigens requires expression of the appropriate TCR V beta gene products as well as the establishment of cellular interactions mediated by adhesion molecules. To study the role of intercellular adhesion molecule (ICAM)-1 in the response in vivo to superantigens, we have analyzed the effects induced by the bacterial superantigen Staphylococcus aureus enterotoxin B (SEB) in mice which have been made genetically deficient in ICAM-1. SEB treatment of wild-type mice causes proliferation, deletion and anergy of the SEB-reactive V beta 8+ T cell population. Here we show that cellular interactions mediated by ICAM-1 are not essential for the induction of anergy or for the deletion of CD4+ V beta 8+ or CD8+ V beta 8+ T cells, but are required for the proliferation of these peripheral T lymphocytes. This is the first demonstration in vivo that the absence of the co-stimulatory signals provided by the interaction of ICAM-1 with its specific ligands impairs the proliferation of SEB-reactive T cells. Interestingly, our study showed that SEB-induced proliferation of CD8+ V beta 8+ T cells from lymph nodes (not from spleen) is independent of the interactions mediated by ICAM-1.

Vaccination with T cell receptor peptides primes anti-receptor cytotoxic T lymphocytes (CTL) and anergizes T cells specifically recognized by these CTL

We selected three peptides from the germ-line sequence of the V beta 8.2 and J beta 2.3 gene segments of the murine T cell receptor for antigen (TCR) which contained putative Kd- and Ld-restricted epitopes. Immunization of BALB/c (H-2d) mice with the V beta 8.2(67-90) 23-mer peptide 1 as well as the 15-mer V beta 8.2(95-108)-peptide 2 efficiently primed specific CD8+ cytotoxic T lymphocyte (CTL) responses in vivo against natural TCR-V beta 8.2 epitopes. V beta 8.2+ T cells were not deleted in TCR peptide-immunized mice because the fractions of V beta 8.2+ CD4+ and V beta 8.2+ CD8+ T cells in spleen and lymph nodes were not altered. The proliferative response of V beta 8.2+ T cells to stimulation by monoclonal antibody F23.2 was selectively suppressed (by 60-80%) in peptide-immunized BALB/c mice, indicating partial anergy of this T subset. Immunization of BALB/c mice with the J beta 2.3-derived peptide 3 stimulated a CD8+ CTL response against a class I-restricted epitope within this J beta segment that was also generated during natural "endogenous" processing of this self antigen. These data confirm the predictive value of major histocompatibility complex class I allele-specific motifs. The described experiments indicate that TCR peptide-primed CD8+ CTL recognize class I-restricted, natural V beta/J beta-TCR epitopes. Such anti-TCR CTL may, thus, operate in V beta-specific immunoregulation of the T cell system suppressing their functional reactivity without deleting them.

Anergic B cells constitutively present self antigen: enhanced immunoglobulin receptor-mediated presentation of antigenic determinants by B cells is hierarchical

Presentation of hen egg lysozyme (HEL) by HEL-specific B cells was studied in transgenic mice expressing anti-HEL immunoglobulin (Ig-transgenic). In T hybridoma assays, presentation of the HEL46-61 determinant by B cells from Ig-transgenic mice required 10(3)-10(4)-fold lower concentrations of HEL than were required for presentation by B cells from non-transgenic mice. In contrast, presentation of the HEL determinants 112-129 and 25-43 by HEL-specific B cells was either not significantly enhanced, or enhanced only 10-fold compared with B cells from non-transgenic mice. Enhanced presentation of HEL determinants by B cells from Ig-transgenic donors was specific for HEL, since keyhole limpet hemocyanin or synthetic HEL46-61 peptide were presented comparably by B cells from Ig-transgenic mice and non-transgenic littermates. A minimum of 1-4% Ig-transgenic B cells was required to detect enhanced presentation of HEL46-61 in vitro. Constitutive presentation of the HEL46-61 determinant, but not the HEL25-43 or HEL112-129 determinants, was detectable on anergic HEL-specific B cells from double (HEL/Ig)-transgenic mice. In the presence of exogenously added HEL, anergic B cells presented all three HEL determinants. Constitutively presented HEL46-61 was not due to endogenous synthesis of HEL antigen by anergic B cells from double-transgenic mice, as comparable levels of the HEL46-61 determinant were constitutively presented by B cells from Ig-Tg-->HEL-Tg irradiation bone marrow chimeric mice. Firstly, these results indicate that the enhanced antigen presentation mediated by Ig receptors on B cells is not equivalent for all antigenic determinants. Secondly, the data demonstrate that anergic, autoreactive B cells efficiently process and present nominal antigens in addition to constitutively presenting specific self antigen in vivo.

TCR antagonism and T cell tolerance can be independently induced in a DR-restricted, hemagglutinin-specific T cell clone

The outcome of TCR engagement with peptide-MHC is of central importance for the immune response of the host. TCR antagonism is one phenomenon known which is characterized by selective inhibition of T cell responses by non-stimulatory antigen analogs. T cell anergy is another state resulting in T cell unresponsiveness, generally characterized by lack of proliferation and lymphokine production. In the present study, the relationship between TCR antagonism and T cell anergy was examined by using protocols known to induce either phenomenon. Re-isolation experiments demonstrated that antagonized T cells were not tolerized, in that they were fully capable of responding to a subsequent antigen challenge. Conversely, while high doses of soluble antigen could efficiently induce T cell tolerance, TCR antagonists, either alone or in conjunction with suboptimal antigen doses, could not. Taken together, these data demonstrate that TCR antagonism and T cell tolerance are phenomena independent of each other.

T cells of staphylococcal enterotoxin B-tolerized autoimmune MRL-lpr/lpr mice require co-stimulation through the B7-CD28/CTLA-4 pathway for activation and can be reanergized in vivo by stimulation of the T cell receptor in the absence of this co-stimulatory signal

The CD28/CTLA-4 receptors on T cells interact with the B7 molecule on antigen-presenting cells (APC) to produce a co-stimulatory signal that determines the outcome of activation. The role of this co-stimulatory signal in T cell activation and loss of tolerance in autoimmune MRL-lpr/lpr mice has not been investigated previously. The present study examines the contribution of the CD28/CTLA-4 co-stimulatory pathway to the loss of T cell tolerance in V beta 8 transgenic MRL-lpr/lpr and (-)+/+ mice in which neonatal tolerance has been induced by the superantigen staphylococcal enterotoxin B (SEB). An artificial APC transfected with the murine B7 gene, and a CTLA-4-Ig fusion protein were used to analyze the significance of the CD28/CTLA-4 pathway in vitro. The CTLA-4-Ig fusion protein was also used to inhibit the pathway in vivo. Our results demonstrate that CD28 and CTLA-4 mRNA was overexpressed in the lymph nodes of lpr/lpr mice (MRL, C57BL/6, C3H and AKR), but not in +/+ mice of the same background strain. Lymph node T cells and thymocytes from SEB neonatally tolerized MRL-lpr/lpr mice that had undergone tolerance loss, proliferated when cultured with SEB and B7+ fibroblasts in vitro, but did not proliferate when the SEB was presented in the context of B7- fibroblasts. This in vitro tolerance loss could be prevented by blocking of B7 signaling by CTLA-4-Ig. This loss of tolerance did not occur in lymph node T cells from thymectomized MRL-lpr/lpr mice. SEB challenge of tolerized MRL-lpr/lpr mice in vivo led to weight loss, increased serum cytokine levels and depletion of V beta 8+ T cells. These effects were blocked by blocking of the co-stimulatory pathway by treatment with the CTLA-4-Ig fusion protein prior to and during challenge with SEB. T cells from thymus and lymph nodes of these mice did not proliferate later in response to stimulation in vitro with SEB even in the presence of B7+ APC. Nonresponsiveness was not due to deletion of V beta 8+ CD28+ T cells, as the number of these cells was increased after treatment with SEB and the CTLA-4-Ig fusion protein.(ABSTRACT TRUNCATED AT 250 WORDS)