Essential requirement for major histocompatibility complex recognition in T-cell tolerance induction

PD-L1 signaling on human memory CD4+ T cells induces a regulatory phenotype

Programmed cell death protein 1 (PD-1) is expressed on T cells upon T cell receptor (TCR) stimulation. PD-1 ligand 1 (PD-L1) is expressed in most tumor environments, and its binding to PD-1 on T cells drives them to apoptosis or into a regulatory phenotype. The fact that PD-L1 itself is also expressed on T cells upon activation has been largely neglected. Here, we demonstrate that PD-L1 ligation on human CD25-depleted CD4⁺ T cells, combined with CD3/TCR stimulation, induces their conversion into highly suppressive T cells. Furthermore, this effect was most prominent in memory (CD45RA⁻CD45RO⁺) T cells. PD-L1 engagement on T cells resulted in reduced ERK phosphorylation and decreased AKT/mTOR/S6 signaling. Importantly, T cells from rheumatoid arthritis patients exhibited high basal levels of phosphorylated ERK and following PD-L1 cross-linking both ERK signaling and the AKT/mTOR/S6 pathway failed to be down modulated, making them refractory to the acquisition of a regulatory phenotype. Altogether, our results suggest that PD-L1 signaling on memory T cells could play an important role in resolving inflammatory responses; maintaining a tolerogenic environment and its failure could contribute to ongoing autoimmunity.

This study shows that programmed death cell receptor ligand 1 (PD-L1) signaling in memory CD4+ T cells from healthy individuals induces a regulatory phenotype; this mechanism seems to be defective in equivalent T cells from rheumatoid arthritis patients and could be in part responsible for the pathology.

Mechanisms Underlying CD4+ Treg Immune Regulation in the Adult: From Experiments to Models

To maintain immunological balance the organism has to be tolerant to self while remaining competent to mount an effective immune response against third-party antigens. An important mechanism of this immune regulation involves the action of regulatory T-cell (Tregs). In this mini-review, we discuss some of the known and proposed mechanisms by which Tregs exert their influence in the context of immune regulation, and the contribution of mathematical modeling for these mechanistic studies. These models explore the mechanisms of action of regulatory T cells, and include hypotheses of multiple signals, delivered through simultaneous antigen-presenting cell (APC) conjugation; interaction of feedback loops between APC, Tregs, and effector cells; or production of specific cytokines that act on effector cells. As the field matures, and competing models are winnowed out, it is likely that we will be able to quantify how tolerance-inducing strategies, such as CD4-blockade, affect T-cell dynamics and what mechanisms explain the observed behavior of T-cell based tolerance.

Mechanisms that regulate peripheral immune responses to control organ-specific autoimmunity

The immune system must balance the need to maintain a diverse repertoire of lymphocytes to be able to fight infection with the need to maintain tolerance to self-proteins. The immune system places strict regulation over the ability of T cells to produce the major T cell growth factor interleukin 2 as this cytokine can influence a variety of immune outcomes. T cells require the delivery of two signals, one through the antigen receptor and a second through the costimulatory receptor CD28. The immune system uses a variety of E3 ubiquitin ligases to target signaling proteins that function downstream of the TCR and CD28 receptors. Mutations in these E3 ligases can lead to a breakdown in immune tolerance and development of autoimmunity. This paper will examine the role of a range of E3 ubiquitin ligases and signaling pathways that influence the development of T-cell effector responses and the development of organ-specific autoimmune diseases such as type 1 diabetes.

Expression of fusion IL2-B7.1(IgV+C) and effects on T lymphocytes

The search for an effective immunotherapeutic treatment for tumors is an important area of cancer research. To prepare a more effective form of the bifunctional fusion protein IL2-B7.1(IgV+C) and analyze its effect on the stimulation of T lymphocyte proliferation, we used DNAStar 5.03 software to predict the structural diversity and biochemical character of IL2-B7.1(IgV+C). We then prepared fusion protein IL2-B7.1(IgV+C) by establishing its prokaryotic expression system, and tested its effect on the stimulation of T lymphocytes in vitro. The results indicated that IL2-B7.1(IgV+C) correctly formed a secondary structure in which both IL2 and B7.1(IgV+C) maintained their original hydrophilicity and epitopes. Western blot analysis revealed that IL2-B7.1(IgV+C) was efficiently expressed. Our analysis of CTLL-2 and T-cell proliferation showed that recombinant human (rh) IL2-B7.1(IgV+C) exerted the combined stimulating effects of both rhIL2 and rh B7.1(IgV+C) on cell proliferation, and that these effects could be blocked by adding either anti-IL2 or anti-B7.1 monoclonal antibodies. A >2-fold increase in [3H]TdR incorporation compared with that of cells treated with recombinant protein IL2, or B7.1(IgV+C) alone, revealed that rhIL2-B7.1(IgV+C) had dose-dependent synergetic effects on T-cell activation in the presence of anti-CD3 monoclonal antibody. We concluded that the augmented potency of rhIL2-B7.1(IgV+C) resulted in a stronger stimulation of T-cell proliferation than either rhB7.1(IgV+C) or rhIL2 alone.

Secretion of IFN-gamma and not IL-2 by anergic human T cells correlates with assembly of an immature immune synapse

We report differences in the supramolecular organization of the immunologic synapse (IS) formed by resting and anergic human T cells with agonist peptide-loaded antigen-presenting cells (APCs). T cells reactive to influenza A hemagglutinin peptide or Fel d 1 peptide 4 were rendered both anergic and regulatory by incubation with high doses of agonist peptide in the absence of APCs. At the IS between resting T cells and peptide-loaded APCs, both CD3epsilon and CD3zeta initially accumulate within a ring or arc before redistributing within 30 minutes to single or multiple foci more central to the contact. In contrast, at synapses formed by anergized T cells, CD3epsilon and CD3zeta remained organized within an arc or ring and failed to redistribute centrally. However, intercellular communication between anergic human T cells and agonist peptide-loaded APCs was not a null event, since it triggered secretion of T-cell interferon gamma (IFN-gamma) but not, for example, interleukin 2 (IL-2). Thus, distinct organizations of CD3 at the T-cell IS correlate with different cytokine profiles; the mature IS formed by resting T cells correlates with their production of both IFN-gamma and IL-2, whereas the immature IS formed by anergic T cells seems able to facilitate IFN-gamma but not IL-2 production.

MHC class II biosynthesis by activated rat CD4+ T cells: development of repression in vitro and modulation by APC-derived signals

This study focused on synthesis of MHC class II glycoproteins (MHCII) by rat CD4(+) T-helper cells. During activation in Con A and IL-2, purified rat splenic CD4(+) T cells expressed abundant surface MHCII together with transcripts for I-A alpha/beta, invariant chain, and the type III and type IV MHC class II transactivator (CIITA). Activated thymic CD8(+)CD4(-) and CD8(+)CD4(+) T cells exhibited essentially the same phenotype. MHCII synthesis by CD4(+) T cells enabled presentation of myelin basic protein (MBP) to antigen-specific responders. T cell expression of MHCII was due to direct biosynthesis rather than adsorption from professional APC; indeed, T cell-mediated expression of MHCII was optimal in the absence of professional APC. Despite periodic reactivation with Con A during 3-4 weeks of culture, CD4(+) T cells repressed MHCII synthesis and reverted to a MHCII(-) phenotype. These short-term lines resembled established lines of MBP-specific T cells in that mitogenic activation elicited extensive blastogenesis without MHCII synthesis. Activation-dependent synthesis of MHCII however was partially restored in lines of mitogen-stimulated T cells when the cultures were reconstituted with irradiated splenic APC. These data indicate that most naive rat CD4(+) T cells exhibit activation-dependent synthesis of MHCII whereas continuously propagated T cells require an APC-derived signal to support MHCII synthesis.

Acquisition of functional MHC class II/peptide complexes by T cells during thymic development and CNS-directed pathogenesis

This study provides evidence that both rat and mouse thymic and splenic T cells express significant levels of MHC class II glycoproteins (MHCII) in vivo. Derivation of rat and mouse chimeras revealed that a major source of MHCII on thymic T cells was acquired from radioresistant host APC. Expression of MHC on thymic T cells appeared physiologically relevant because presentation of rat myelin basic protein (RMBP) by nonadherent, radiosensitive thymic T cells was associated with the adoptive transfer of tolerance. Mature MBP-specific effector T cells isolated from the CNS in both rat and mouse models of EAE also expressed significant levels of MHCII. Adoptive transfer of activated B10.PL MBP/I-A(u)-restricted TCR transgenic T cells into F1(C57BL/6 x B10.PL) mice revealed acquisition of allogeneic I-A(b) on encephalitogenic CNS-derived T cells. Overall, this study indicates that immature and mature T cells in rats and mice acquire functional MHCII in vivo during thymic development and pathogenic inflammation.

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

T-cell anergy and peripheral T-cell tolerance

The discovery that T-cell recognition of antigen can have distinct outcomes has advanced understanding of peripheral T-cell tolerance, and opened up new possibilities in immunotherapy. Anergy is one such outcome, and results from partial T-cell activation. This can arise either due to subtle alteration of the antigen, leading to a lower-affinity cognate interaction, or due to a lack of adequate co-stimulation. The signalling defects in anergic T cells are partially defined, and suggest that T-cell receptor (TCR) proximal, as well as downstream defects negatively regulate the anergic T cell's ability to be activated. Most importantly, the use of TCR-transgenic mice has provided compelling evidence that anergy is an in vivo phenomenon, and not merely an in vitro artefact. These findings raise the question as to whether anergic T cells have any biological function. Studies in rodents and in man suggest that anergic T cells acquire regulatory properties; the regulatory effects of anergic T cells require cell to cell contact, and appear to be mediated by inhibition of antigen-presenting cell immunogenicity. Close similarities exist between anergic T cells, and the recently defined CD4+ CD25+ population of spontaneously arising regulatory cells that serve to inhibit autoimmunity in mice. Taken together, these findings suggest that a spectrum of regulatory T cells exists. At one end of the spectrum are cells, such as anergic and CD4+ CD25+ T cells, which regulate via cell-to-cell contact. At the other end of the spectrum are cells which secrete antiinflammatory cytokines such as interleukin 10 and transforming growth factor-beta. The challenge is to devise strategies that reliably induce T-cell anergy in vivo, as a means of inhibiting immunity to allo- and autoantigens.

MHC class-II-restricted antigen presentation by myelin basic protein-specific CD4+ T cells causes prolonged desensitization and outgrowth of CD4- responders

T cells express MHC class II glycoproteins under various conditions of activation or inflammation. To assess whether T cell APC (T-APC) activity had long-term tolerogenic consequences, myelin basic protein (MBP)-specific rat T cells were induced to acquire MBP-derived I-A complexes to promote reciprocal antigen presentation. T-T antigen presentation caused extensive cell death among T-APC and MBP-specific T responders and caused long-term desensitization of surviving responders. Addition of the anti-I-A mAb OX6 to activated I-A+ responders inhibited T-APC activity, accelerated recovery from postactivation refractoriness, and prevented long-term loss of reactivity in responder T cells. Antigenic activation of responder T cells with irradiated T-APC induced profound losses in reactivity that lasted for over 1 month of propagation in IL-2 and was associated with preferential outgrowth of CD4- T cells. Antigen-activated CD4- T cells exhibited more rapid IL-2-dependent growth that eventually normalized compared to CD4+ T cells 1-2 months after antigen exposure. In conclusion, expression of T-APC activity by activated T cells represents an important negative feedback pathway that depletes antigen-reactive T cells and causes long-term desensitization of surviving T cells. Hence, T cell APC may be an important mechanism of self-tolerance.

Role of NFkappaB in antigen presentation and development of regulatory T cells elucidated by treatment of dendritic cells with the proteasome inhibitor PSI

Dendritic cells (DC) are the most potent antigen-presenting cells for naive T cells, due to their high expression of MHC and costimulatory molecules, but relatively little is known about the biochemical pathways that regulate this function. We used the proteasome inhibitor N-benzyloxycarbonyl-Ile-Glu(O-tert-butyl)-Ala-leucinal (PSI) to demonstrate that DC antigen presentation is NFkappaB dependent. As PSI is not a specific inhibitor of NFkappaB, we reproduced this finding using a very specific approach, namely adenoviral gene transfer of IkappaBalpha, the naturally occurring inhibitor of NFkappaB. The mechanism for this inhibition of DC antigen presentation involves at least three aspects of antigen presenting function: down-regulation of HLA class II, down-regulation of CD86, and inhibition of the immunostimulatory cytokines IL-12 and TNF-alpha. In the light of the marked down-regulation of antigen-presentation cell function, it was of interest to investigate what effects exposure to PSI-treated DC might have on T cell function. It was found that immunological tolerance was induced, as challenge of T cells previously exposed to PSI-treated DC, with normal DC from the same donor did not restore their response, despite the presence of viable T cells. There were also changes in T cell surface markers, with down-regulation of CD3 and CD25 expression, and inhibition of the production of Th1 cytokines like IL-2 and IFN-gamma. These results demonstrates that NFkappaB is an effective target for blocking DC antigen presentation and inhibiting T cell-dependent immune responses, and this has implications for the development of therapeutic agents for use in multiple conditions, including transplantation, allergy and autoimmune diseases.

Fully competent dendritic cells as inducers of T cell anergy in autoimmunity

Mature immunologically competent dendritic cells are the most efficient antigen-presenting cells that powerfully activate T cells and initiate and sustain immune responses. Indeed, dendritic cells are able to efficiently capture antigens, express high levels of costimulatory molecules, and produce the combination of cytokines required to create a powerful immune response. They are also considered to be important in initiating autoimmune disease by efficiently presenting autoantigens to self-reactive T cells that, in this case, will mount a pathogenic autoimmune reaction. Triggering T cells is not a simple on-off procedure, as T cell receptor responds to minor changes in ligand with gradations of T cell activation and effector functions. These "misfit" peptides have been called Altered Peptide Ligands, and have been shown to have important biological significance. Here, we show that fully capable dendritic cells may present, upon natural antigen processing, a self-epitope with Altered Peptide Ligands features that can unexpectedly induce anergy in a human autoreactive T cell clone. These results indicate that presentation of a self-epitope by immunologically competent dendritic cells does not always mean "danger" and show a mechanism involved in the fine balance between activation and tolerance induction in humans.

Class II MHC/Peptide Complexes Are Released from APC and Are Acquired by T Cell Responders During Specific Antigen Recognition

T cell expression of class II MHC/peptide complexes may be important for maintenance of peripheral self-tolerance, but mechanisms underlying the genesis of class II MHC glycoproteins on T cells are not well resolved. T cell APC (T-APC) used herein were transformed IL-2-dependent clones that constitutively synthesized class II MHC glycoproteins. When pulsed with myelin basic protein (MBP) and injected into Lewis rats, these T-APC reduced the severity of experimental autoimmune encephalomyelitis, whereas unpulsed T-APC were without activity. Normal MBP-reactive clones cultured without APC did not express class II MHC even when activated with mitogens and exposed to IFN-γ. However, during a 4-h culture with T-APC or macrophage APC, recognition of MBP or mitogenic activation of responder T cells elicited high levels of I-A and I-E expression on responders. Acquisition of class II MHC glycoproteins by responders was resistant to the protein synthesis inhibitor cycloheximide, coincided with transfer of a PKH26 lipophilic dye from APC to responders, and resulted in the expression of syngeneic and allogeneic MHC glycoproteins on responders. Unlike rested I-A− T cell clones, rat thymic and splenic T cells expressed readily detectable levels of class II MHC glycoproteins. When preactivated with mitogens, naive T cells acquired APC-derived MHC class II molecules and other membrane-associated proteins when cultured with xenogeneic APC in the absence of Ag. In conclusion, this study provides evidence that APC donate membrane-bound peptide/MHC complexes to Ag-specific T cell responders by a mechanism associated with the induction of tolerance.

Immunological self/nonself discrimination: integration of self vs nonself during cognate T cell interactions with antigen-presenting cells

The hypothesis is presented that immunological integration of nonefficacious vs efficacious T cell antigen receptor (TCR) signals are foundational for self/nonself discrimination and that multiple integrative mechanisms are intrinsic to the molecular to molar organization of an adaptive immune response. These integrative mechanisms are proposed to adaptively regulate expression of costimulatory signals, such that foreign proteins are associated with the expression of costimulatory signals, whereas self-proteins are associated with the lack of costimulatory signaling. Overall, this model offers several unique contributions to the study of immunology. First, this model postulates that cognate TCR/major histocompatibility complex (MHC) interactions are sufficient to adaptively mediate immunological self/nonself discrimination. This model thereby offers a unique alternative to models that largely rely on innate immunity to prime immune discrimination. Second, the integrative model argues that the immune system can simultaneously reinforce self-tolerance and promote immunity to foreign organisms at the same time and in the same location. Many alternative models presume that pathogenic self-reactive T cells do not exist at the outset of an immune response against foreign agents. Third, the integrative model uniquely predicts relationships between immunodeficiency and autoimmune pathogenesis. Fourth, this model illustrates the regulatory advantages of cognate antigen presenting cell (APC) systems (i.e., T cell or B cell APC) compared to nonspecific APC. Cognate APC systems together with the respective clonotypic responders may comprise a fundamental "network" of lymphoid cells. Such networks would have clone-specific regulatory capabilities and may be central for immunological self/nonself discrimination. Fifth, this model provides an explanation for "infectious" tolerance without creating specialized subsets of "suppressor" or "regulatory" T cells. Each mature T cell retains the potential to reinforce tolerance or mediate immunity, depending on the specific antigenic cues present in the immediate environment.

Molecular regulation of interleukin-2 expression by CD28 co-stimulation and anergy

The consequences of T-cell receptor engagement (signal 1) are profoundly affected by the presence or absence of co-stimulation (signal 2). T-cell receptor (TCR) stimulation in the absence of CD28-mediated co-stimulation not only results in little interleukin (IL)-2 production, but induces a long lasting hyporesponsive state known as T-cell clonal anergy. The addition of CD28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL-2. Our laboratory has utilized CD4+ Th 1 clones in an effort to understand the molecular events resulting in enhanced IL-2 production by co-stimulation and the inhibition of IL-2 production in anergy. Our current studies have focused on defining the post-transcriptional effects of CD28-enhanced IL-2 production. The data suggest that a major component of CD28's ability to regulate IL-2 production occurs at the level of message stability and involves the 3'-untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co-stimulation, but rather signal 1 in the absence of IL-2 receptor signaling and proliferation. Furthermore, T-cell anergy appears to be an active negative state in which IL-2 production is inhibited both at the level of signal transduction and by cis-dominant repression at the level of the IL-2 promoter.

Partial agonism elicits an enduring phase of T-cell-medicated antigen presentation

Previous studies have shown that the anti-CD4 mAb W3/25 strongly enhances T cell APC (T-APC) activity. In this study, single positive CD4+ and double negative (DN) (CD4-CD8-) T-helper cells specific for the 55-69 or 72-86 sequence of guinea pig (GP) myelin basic protein (GPMBP) were used to study CD4 regulation of T-APC activity. Clones were cultured with irradiated SPL and GPMBP or rat (R) MBP for 2-3 days, were propagated in IL-2 for another 1-3 days, were irradiated, and were used as T-APC. DN T cells specific for GP55-69 effectively presented GPMBP and were superior APC compared to other CD4+ T cells for presentation of this antigen. In contrast, DN T cells specific for the dominant encephalitogenic 72-86 determinant did not effectively present the agonist GPMBP but potently presented the partial agonist RMBP. The heightened APC activity of DN T cells reflected the lack of CD4 because the anti-CD4 mAb W3/25 promoted T-APC activity of CD4+ T cells to those levels expressed by DN T cells. Overall, T cells with potent reactivity to GPMBP or RMBP were subsequently unable to present that antigen, whereas T cells exhibiting partial or low antigen reactivities were highly effective APC for presentation of that antigen. The unrelated antigen conalbumin was presented by MBP-specific clones only when added to culture with a specific partial agonist. Together, these data indicate that partially agonistic MHC ligands promote prolonged expression of T-APC activity and that DN T cells may be specialized to mediate postactivational antigen presentation.

Class II MHC/peptide complexes on T cell antigen-presenting cells: agonistic antigen recognition inhibits subsequent antigen presentation

Previous studies have shown that tolerogenic anti-CD4 (W3/25) and anti-LFA-1 mAb (LRTC1) which block T cell activation paradoxically enhance T cell-mediated antigen presentation. Lasting T cell APC (T-APC) activity requires and initial exposure of T cells to these mAb in the presence of professional APC and antigen. This study revealed a central mechanism regulating the duration of T-APC activity. T cell recognition of class II MHC complexes of T-APC catalyzed a rapid decay in the presentation of agonistic antigens, whereas partial agonistic signals decayed at a shower rate. Likewise, blockade of agonistic T-T cell autorecognition by these mAb led to the persistence of agonistic MHC/antigen on T-APC. The best predictor of T-APC activity was related to the ability of clonal T cells to respond to antigen presented by neighboring T cells. Strong responders were inefficient T-APC, whereas inefficient responders were strong T-APC. Addition of irradiated myelin basic protein (MBP0-specific responders to T-APC cultures specifically inhibited the subsequent presentation of MBP but not conalbumin, and vice versa. T-APC presentation of antigen to responder T cells also resulted in reduced surface expression of class II MHC I-A glycoproteins on T-APC. These findings indicate that agonistic recognition of antigen of T-APC specifically inhibits subsequent presentation of that antigen, whereas antagonistic MHC/antigen complexes are preserved for an enduring T-APC activity.

T:T Antigen Presentation by Activated Murine CD8+ T Cells Induces Anergy and Apoptosis

Using an IL-2-secreting, noncytolytic, H-Y-specific, CD8+ T cell clone, the functional consequences of Ag presentation by T cells to T cells were investigated. Incubation of the T cells with H-Y-soluble peptide led to nonresponsiveness to Ag rechallenge. This was due to the simultaneous induction of apoptosis, involving approximately 40% of the T cells, and of anergy in the surviving cells. These effects were strictly dependent upon bidirectional T:T presentation, in that exposure of C6 cells to peptide-pulsed T cells from the same clone induced proliferation but not apoptosis or anergy. The inhibitory effects of T:T presentation were not due to a lack of costimulation, since the T cells expressed levels of CD80 and CD86 higher than those detected on cultured dendritic cells and equipped them to function as efficient APCs for primary CD8+ T cell responses. Following incubation with soluble peptide, CD80 expression increased, and high levels of CTLA-4 (CD152) expression were induced. Although addition of anti-CTLA-4 Ab augmented proliferation in response to soluble peptide, no protection from apoptosis or anergy was observed. Neither Fas nor TNF-α was expressed/produced by the C6 cells, and coligation of MHC class I molecules and TCR failed to reproduce the effects of T:T presentation. Taken together, these data suggest that T:T Ag presentation induces anergy and apoptosis in murine CD8+ T cells and may reflect the regulatory consequences of T:T interactions in the course of clonal expansion in vivo.

Antigen presentation by T cells: T cell receptor ligation promotes antigen acquisition from professional antigen-presenting cells

The purpose of this study was to determine whether the clonotypic specificity of the T cell receptor influences the specificity of T cell-mediated antigen presentation. We have previously shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen, irradiated syngeneic splenocytes (IrrSPL) and tolerogenic monoclonal antibody become highly effective antigen-presenting cells (APC). In the current studies, we investigated the transfer of specific (MBP) and unrelated (conalbumin) antigens from antigen-pulsed SPL to R1 T cells. R1 T cells cultured with IrrSPL that were pulsed simultaneously with both MBP and conalbumin acquired and presented both antigens to the appropriate T cell responders in a secondary assay. These results suggested a physical transfer of major histocompatibility complex (MHC)/peptide complexes from professional APC to R1 T cells. Transfer of conalbumin from professional APC to R1 T cells required specific recognition of MBP and was optimal when both conalbumin and MBP were presented on the same group of professional APC. Antigens transfer did not occur when allogeneic SPL were used as APC. The anti-I-A mAb OX6 inhibited antigen transfer but only when added during the initiation of culture. OX6 also inhibited antigen acquisition by R1-trans, a variant of the R1 T cell line which constitutively synthesizes high levels of I-A, from MBP-pulsed IrrSPL but blockade of I-A did not inhibit antigen acquisition when soluble MBP was added directly to the culture. Despite constitutive synthesis of I-A, R1-trans T cells did not acquire guinea pig MBP from pulsed allogeneic APC. These studies demonstrate that although T cells of a particular specificity can present unrelated antigens, the cognate interaction of the T cell antigen receptor with the appropriate antigen/self-MHC complex strongly promotes acquisition of these complexes from professional APC.

Effect of major histocompatibility complex matching on the development of tolerance to primarily vascularized renal allografts: a study in miniature swine

Prevention of rejection and the induction of transplantation tolerance are two related but separable phenomena that must both be considered in the analysis of the response to a transplanted organ. It is frequently hard to separate these phenomena in assessing the outcome of clinical transplants, because patients are rarely studied in the absence of immunosuppressive agents. Use of our partially inbred miniature swine has permitted us to examine the effects of selective MHC matching on transplant survival, and the data indicate that matching has an effect on both phenomena. Prevention of early rejection with CyA was possible for all mismatches examined, although it was clearly more difficult with increasing degrees of mismatching. On the other hand, tolerance induction after cessation of the immunosuppressive agent was dependent on presence of at least one matched MHC locus between the donor and recipient, with complete class II matching appearing to be the most successful way of assuring long-term graft survival. It is also apparent from our data that although durable tolerance to primarily vascularized renal allografts could be induced across a variety of selective MHC disparities, all cases involving a class II mismatch (ie, selective class I matched or one-haplotype full MHC mismatched kidney allografts) underwent spontaneously reversible rejection crises during the early follow-up period. Such a clinical course might be unacceptable for human clinical trials, even though the transient renal dysfunction may reflect events involved in tolerance induction rather than true rejection (Gianello et al: Immunol Rev 133:19, 1993.). Indeed, we do not yet know whether or not further immunosuppressive treatment at the times of such crises may prevent rather than facilitate the induction of tolerance. On the other hand, in the case of selective two-haplotype class I mismatch the regimen utilized was capable of inducing tolerance to renal allografts in 100% of the recipients with minimal or no renal dysfunction throughout the follow-up period. Although the excellent results achieved with current antirejection agents has led to debate about the wisdom of HLA matching for cadaver transplants in terms of preventing rejection, our data would suggest that such matching might be of even greater importance for success of protocols in which attempts are made to induce transplantation tolerance. Because class II antigens are less polymorphic than are class I antigens, mismatching for class I antigens may be achievable for cadaver donor transplantation, and may provide the first situation in which these principles can be applied to clinical trials.

Effects of antigen presentation on superantigen-induced apoptosis mediated by Fas/Fas ligand interactions in human T cells

Stimulation of T cells with bacterial superantigens has several distinct functional outcomes including proliferation, anergy and apoptosis. At present however, the mechanisms that dictate whether activation, anergy, or apoptosis predominate are unclear. In this study we have investigated the effects of superantigen presentation to mature superantigen-reactive human T-cell lines. Despite expressing major histocompatibility complex (MHC) class II molecules, these lines failed to proliferate in response to superantigen in the absence of antigen-presenting cells (APC) but proliferated when minimal APC were added. In the absence of APC a significant proportion of the T cells underwent apoptosis. This response was rapid, antigen dependent and largely abolished by the addition of cyclosporin A. Interestingly the response was not blocked by the addition of a number of antibodies to cell surface molecules including MHC class II and intracellular adhesion molecule-1. Using both a bioassay and messenger RNA analysis we were able to demonstrate that stimulation of these T cells with superantigen resulted in the induction of Fas-ligand expression on the T cells and furthermore, the ability of these cells to induce apoptosis was inhibited by the addition of blocking Fas antibodies as well as a Fas-Fc fusion protein. These data demonstrate that stimulation of T cells with staphylococcal enterotoxin B induces expression of Fas-ligand resulting in T-cell apoptosis; however, the final outcome of proliferation or apoptosis is determined by the presence of APC.

Inactivation of human immunodeficiency virus (HIV)-1 envelope-specific CD8+ cytotoxic T lymphocytes by free antigenic peptide: a self-veto mechanism?

Free peptide has been found to inhibit cytotoxic T lymphocyte (CTL) activity, and veto cells bearing peptide-major histocompatibility complex (MHC) complexes have been found to inactivate CTL, but the two phenomena have not been connected. Here we show that a common mechanism may apply to both. CD8+ CTL lines or clones specific for a determinant of the human immunodeficiency virus (HIV) 1 IIIB envelope protein gp160, P18IIIB, are inhibited by as little as 10 min exposure to the minimal 10-mer peptide, I-10, within P18IIIB, free in solution, in contrast to peptide already bound to antigen-presenting cells (APC), which does not inhibit. Several lines of evidence suggest that the peptide must be processed and presented by H-2Dd on the CTL itself to the specific T cell receptor (TCR) to be inhibitory. The inhibition was not killing, in that CTL did not kill 51Cr-labeled sister CTL in the presence of free peptide, and in mixing experiments with CTL lines of different specificities restricted by the same MHC molecule, Dd, the presence of free peptide recognized by one CTL line did not inhibit the activity of the other CTL line that could present the peptide. Also, partial recovery of activity could be elicited by restimulation with cell-bound peptide, supporting the conclusion that neither fratricide nor suicide (apoptosis) was involved. The classic veto phenomenon was ruled out by failure of peptide-bearing CTL to inactivate others. Using pairs of CTL lines of differing specificity but similar MHC restriction, each pulsed with the peptide for which the other is specific, we showed that the minimal requirement is simultaneous engagement of the TCR and class I MHC molecules of the same cell. This could occur in single cells or pairs of cells presenting peptide to each other. Thus, mechanistically, the inhibition is analogous to veto, and might be called self-veto. As a clue to a possible mechanism, we found that free I-10 peptide induced apparent downregulation of expression of specific TCR as well as interleukin 2 receptor, CD69, lymphocyte function-associated antigen 1, and CD8. This self-veto effect also has implications for in vivo immunization and mechanisms of viral escape from CTL immunity.

Differential effects of interleukin-10 on the expression of HLA class II and CD1 molecules induced by granulocyte/macrophage colony-stimulating factor/interleukin-4

Interleukin (IL)-10 down-regulates HLA class II molecules, whether constitutively expressed or up-regulated by interferon-gamma or IL-4 on monocytes but not on B lymphocytes. In this study we show that IL-10 does not inhibit HLA class II expression induced by the combination granulocyte/macrophage colony-stimulating factor and IL-4 on monocytes, although it simultaneously abrogates the expression of CD1 molecules induced by the same combination of cytokines. CD1 molecules can act as element of genetic restriction for CD4- CD8- T lymphocytes, and the suppression of CD1 expression by IL-10 abolished antigen presentation to CD1-restricted CD4- CD8- T cell receptor-positive T cells. Although HLA class II expression was not down-regulated by IL-10, the antigen specific proliferative response of CD4+ T cells was nevertheless decreased. This was not caused by down-regulation of known co-stimulatory molecules such as B7.1, B7.2 and ICAM-1. IL-10 decreased the antigen specific proliferative response further by directly influencing the T lymphocytes. Our results indicate that IL-10 exerts some of its immunoregulatory functions by differential modulation of antigen presenting molecules, induced by the same combination of cytokines.

Induction of non-responsiveness in human allergen-specific type 2 T helper cells

Activation of allergen-reactive human T helper (Th)2 cells in the absence of professional antigen-presenting cells, induces non-responsiveness or anergy in these cells in vitro. This induction of anergy is accompanied by phenotypic modulation and altered cytokine production. Furthermore, peptide-treated Th2 cells fail to provide B-cell help for IgE synthesis. Recent studies indicate that impaired signal transduction via the T-cell receptor may account for the lack of responsiveness to antigenic stimulation. Here, we review present knowledge on the cell biology of non-responsive or anergic Th2 cells.

Soluble transferrin mediates targeting of hepatitis B envelope antigen to transferrin receptor and its presentation by activated T cells

In a previous study, we identified that transferrin receptor (TfR) is the receptor utilized by hepatitis B virus (HBV) to enter T cells. We demonstrated that hepatitis B envelope antigen (HBenvAg) is taken up by activated T cells via TfR, processed in endosomal compartments, and presented on class II molecules to specific CD4+ T cell clones. Herein, we report that binding to soluble ferric Tf by HBenvAg is needed in TfR-mediated endocytosis. Accordingly, presentation of HBenvAg by activated T cells is not observed in serum-free medium and is restored by addition of soluble Tf. Moreover, we provide evidence that pre-S2 and S regions of HBenvAg contain the critical residues for the interaction with soluble Tf. Our data not only explain HBV entry into a variety of host activated cells, but may also help in developing strategies to alter the course of chronic HBV infection.

Professional presentation of antigen by activated human T cells

Activated human T cells express class II molecules, but their capacity to present soluble antigens and stimulate T cells has been repeatedly questioned. Two lines of evidence indicate that T cells may indeed function as professional antigen-presenting cells. First, T cells that have been recently activated can efficiently capture, process and present tetanus toxoid to class II-restricted T cell clones. This capacity correlates with the rate of class II synthesis. Second, activated T cell clones express high levels of B7, are powerful stimulators in mixed lymphocyte reactions, and their stimulatory capacity is inhibited by soluble CTLA4 or anti-B7 antibody. Furthermore, expression of B7 can be detected in vivo on T cells from biopsies of patients with liver disease. Presentation of soluble antigen by activated T cells may play a role in the amplification of the specific response, and possibly in immunopathological states.

HLA class II molecules transduce accessory signals affecting the CD3 but not the interleukin-2 activation pathway in T blasts

MHC class II molecules play a central role in the control of the immune response, but their biologic function and mechanism of action on the surface of activated human T lymphocytes are not entirely understood. In our study, the functional role of HLA class II molecules in T-blast proliferation was investigated by analyzing in parallel the IL-2- and CD3-driven activation pathways. The results indicate that the cross-linking of class II and CD3 molecules significantly increased the CD3-mediated T-blast proliferation, while no effect was observed on the IL-2-driven cell activation. This phenomenon was not confined to either CD4+ or CD8+ subsets nor was specifically affected by CD45 triggering. Biochemical studies showed that signaling via MHC class II molecules in T blasts led to PKC membrane translocation and IP accumulation. The simultaneous triggering of CD3 and HLA class II molecules led to a synergistic effect on IP accumulation but did not increase the CD3-mediated PKC membrane translocation. Our data suggest that HLA class II molecules are involved in T-cell-T-cell interactions and can mediate accessory signals, affecting the T-lymphocyte activation state.

Humoral immune response to the insect allergen Chi t I in aquarists and fish-food factory workers

Our examination of 225 subjects who had been exposed to the insect allergen Chi t I involved the degree of allergen exposure, the exposure-associated symptoms, and their relationship to the presence of specific IgE and IgG antibodies as well as sensitization to ubiquitous allergens. It could be shown that specific IgE antibodies found in 34% of these subjects were closely associated with symptoms (P < 0.01), whereas no relationship between IgG antibodies and complaints could be observed. Conjunctivitis (63%) and rhinitis (62%) were predominant, followed by asthma (45%) and urticaria (37%). Antibody levels of patients suffering from asthma were highest. In addition, symptoms were associated with the degree of exposure. While nearly all IgE-sensitized subjects of the medium-, high-, and very high-exposure group were symptomatic, only 57% of the sensitized individuals of the low-exposure group reported complaints. Furthermore, specific IgE antibodies were most frequently present in the groups with medium (46%) and high (54.5%) exposure, whereas IgG antibodies predominated in individuals with very high exposure (69.1%). In the low-exposure group, most subjects (73.6%) had neither IgE nor IgG antibodies. In addition, within Chi t I sensitized subjects, sensitization to common allergens and elevated total IgE levels were more frequently present than within non-Chi t I sensitized individuals, indicating a predisposition to allergy.

Nonspecific regulatory mechanism of contact sensitivity: nonspecific suppressor factor (NSF)-treated intermediate cells produce a second nonspecific suppressor factor (NSFint)

Nonspecific suppressor factor (NSF), which inhibits the passive transfer of contact sensitivity (CS), is produced spontaneously from macrophage-like suppressor cells induced by intravenous administration of oxazolone (Ox)-conjugated spleen cells. NSF binds selectively to Ia-positive, cyclophosphamide (CY)-sensitive, and plastic-adherent cells (named intermediate cells) present in the normal spleen. NSF-treated intermediate cells acquire the ability to suppress the passive transfer of CS nonspecifically. In this study, NSF-treated intermediate cells were found to release a second nonspecific suppressor factor (NSFint) during a 2-hr culture, while retaining their suppressor activity. Investigation of the relationship between these two factors showed that both NSF and NSFint were trypsin-sensitive, nondialyzable proteins. However, gel chromatography revealed that NSF was about 43 kDa, while NSFint was about 20 kDa. NSF was released from macrophage-like suppressor cells after RNA-dependent protein synthesis. In contrast, production of NSFint was energy dependent but did not require protein synthesis. Intermediate cells pretreated with lysosomotropic agents, such as ammonium chloride or chloroquine, did not acquire suppressor activity nor release suppressor factors due to NSF treatment. These observations suggest that NSFint is an altered form of NSF released by the intermediate after having undergone some modification; the biochemical mechanism is not known. This study showed that the intermediate cells play an active role in the suppressor cascade of NSF.

Stimulation or tolerization of an anti-myelin basic protein T lymphocyte line with membrane fragments from antigen presenting cells

We have investigated the abilities of a cell-free supernatant of splenocytes or thymocytes, which have been incubated with myelin basic protein (MBP), and of membranes prepared by lysing these cells, to stimulate proliferation of a Lewis rat anti-MBP T lymphocyte line in vitro. The supernatant fraction, obtained by low-speed centrifugation, is thought to contain shed membrane fragments bearing class II MHC protein (Ia) and processed antigen. Almost all of 67 preparations of supernatant fraction and about a third (26/70) of the membrane preparations stimulate proliferation of the line cells in the absence of other antigen-presenting cells and antigen. Some membrane preparations bearing the synthetic peptide S69 (residues 69-89 of MBP), containing the immunodominant encephalitogenic determinant for the Lewis rat, instead of processed MBP could also stimulate proliferation. Those membrane preparations bearing either processed MBP or synthetic S69, which do not stimulate proliferation, induce a state of unresponsiveness in which the cells do not proliferate but produce inositol phosphate. Stimulation of proliferation and induction of unresponsiveness were both inhibited by anti-Ia antibody. Addition of cyclosporin A prevents induction of unresponsiveness. Addition of allogeneic splenocytes or the cell-free supernatant fraction of syngeneic or allogeneic splenocytes or thymocytes, prevents induction of unresponsiveness by providing a necessary costimulatory signal. Further fractionation of the cell-free supernatant by high-speed ultracentrifugation showed that the costimulatory signal resided in a particulate fraction which sedimented and not in the supernatant. These results indicate that the encephalitogenic peptide can induce anergy in T cells when presented on class II MHC in the absence of the costimulatory signal. Tolerizing forms of the membrane preparations which lack the costimulatory signal may be useful for in vivo treatment of autoimmune response.

Nonsegmental vitiligo: decrease of the CD45RA+ T-cell subset and evidence for peripheral T-cell activation

Peripheral T-lymphocytes from 16 randomly selected patients with nonsegmental vitiligo were labeled with monoclonal antibodies recognizing T-cell receptor (TCR) alpha/beta, TCR gamma/delta, CD3, CD4, CD8, CD45RA, CD45RO, CD11b, CD11c, CD16, CD56, CD25, CD54, and HLA-DR antigens. In comparison with matched controls, a significant decrease of the CD45RA+ subset (P less than 0.03) together with significant increase of the circulating HLA-DR+ cells (P less than 0.02) was found. No other alterations were detected. These findings may point to some autoimmune phenomena involved in the pathogenesis of the disease. The increased HLA-DR expression indicates the presence of activated peripheral T-cells. Thus, our data provide new and further evidence for T-cell dysregulation in nonsegmental vitiligo.

Treatment of rats with monoclonal anti-CD4 induces long-term resistance to streptococcal cell wall-induced arthritis

To investigate the role of CD4+ cells in the induction and maintenance of streptococcal cell wall (SCW)-induced arthritis, Lewis rats were treated with a monoclonal antibody against rat CD4 (W3/25). Injection before onset of the arthritis resulted in resistance to SCW arthritis. Treatment with anti-CD4 during ongoing arthritis induced an amelioration of the arthritis, demonstrating that CD4+ cells are involved in both the induction and effector phases of the chronic arthritis. After return of CD4+ cells to normal levels in the circulation, no arthritis occurred in protected rats, despite the continued presence of SCW in the body. Even reinjection of SCW could not induce arthritis in these rats, suggesting that tolerance to SCW had occurred. In addition, these tolerized rats were refractory to actively induced adjuvant arthritis (AA), but were susceptible to passively transferred AA. Our data imply, that (a) treatment with anti-CD4 plus SCW induces a long-term resistance to SCW-induced arthritis and adjuvant arthritis, (b) SCW and M. tuberculosis may use similar mechanisms of regulation of arthritis and (c) active peripheral suppression is not the mechanism of this nonresponsiveness.

An attenuated variant of Coxsackievirus B3 preferentially induces immunoregulatory T cells in vivo

BALB/c mice infected with the Woodruff variant of coxsackievirus group B type 3 (CVB3W) develop myocarditis mediated by autoimmune cytolytic T lymphocytes. A variant of CVB3W (designated H3-10A1) which infects the myocardium but induces minimal mortality of myocarditis compared to the parental virus was selected. Although H3-10A1 infections stimulate normal CTL responses to CVB3-infected myocytes, the autoimmune response to myocardial antigens is absent. Treatment of H3-10A1-infected mice with 50 mg of cyclophosphamide per kg of body weight, a treatment which preferentially eliminates suppressor cells, allows both the development of the autoimmune cytotoxic T-lymphocyte response and the expression of myocarditis. Similar treatment of CVB3W-infected mice had no effect on the disease. The presence of the immunoregulatory cells was confirmed by adoptive transfer of T lymphocytes from either H3-10A1 or CVB3W-infected donor mice into syngeneic CVB3W-infected recipients. Animals given H3-10A1-immune cells had minimal myocardial inflammation, while animals given CVB3W-immune lymphocytes developed enhanced cardiac disease. Elimination of the T-lymphocyte population from the donor cells prior to transfer abrogated suppression with the H3-10A1-immune population, showing that immunoregulation depended upon T lymphocytes. Both H3-10A1 and CVB3W have cross-reactive epitopes between the adenine translocator protein and the virion which are indicative of antigenic mimicry and may be the basis for the autoimmunity to cardiac antigens. These results suggest that immunoregulatory T cells may be primarily responsible for the nonpathogenicity of the H3-10A1 variant.

Genetic control of immune response and disease susceptibility by the HLA-DQ gene

The particular alleles of the HLA-DQ locus may control the low immune response to natural antigens by a dominant genetic trait through the immune suppression mediated by CD8+ suppressor T cells. The suppressor T cells may be activated by DQ-restricted and antigen-specific CD4+ suppressor/inducer T cells, because (1) a statistically significant association and linkage between low immune responsiveness to the natural antigens and the HLA-DQ gene were observed; (2) antigen-specific CD4+ T cells restricted by the DQ molecules encoded for by the HLA-DQ allele associated with low responsiveness were evidenced in many low responders; and (3) anti-HLA-DQ mAb restored the immune response to natural antigens, in some low responders. This HLA-DQ-controlled polymorphism of immune response to the natural antigens may account for the association between HLA-DQ alleles and organ-specific autoimmune diseases.

Signal transduction by HLA class II antigens expressed on activated T cells

Human T cells express HLA class II antigens upon activation. Although activated, class II+ T cells can present alloantigens under certain circumstances, the functional role of class II antigens on activated T cells remains largely unknown. Here, we report that cross-linking of HLA-DR molecules expressed on allospecific, CD4+ T clones and cell lines can function as transduction elements that trigger rapid cellular responses including tyrosine phosphorylation of cellular proteins and mobilization of Ca2+ from internal stores. The proteins phosphorylated on tyrosine were distinct from those observed after cross-linking CD4. Ligation of CD4 and class II molecules generated a synergistic effect of the intracellular free Ca2+ concentration response that required an interaction between the molecules on the cell surface. Since class II is the natural ligand for CD4, the present data suggest that class II is induced on activated T cells to regulate CD4 function, possibly by specific interaction with the CD4-associated p56lck protein tyrosine kinase.

Induction of specific clonal anergy in human T lymphocytes by Staphylococcus aureus enterotoxins

The exotoxins produced by certain strains of Staphylococcus aureus are able to stimulate powerful polyclonal proliferative responses and to induce nonresponsiveness by clonal deletion of T lymphocytes expressing the appropriate T-cell antigen receptor V beta gene products. This paper examines the ability of S. aureus enterotoxins to modulate the responsiveness of human CD4+ T lymphocytes with defined antigen specificity. It was observed that certain S. aureus toxins were able to activate and induce anergy in hemagglutinin-reactive T cells expressing V beta 3+ elements. After exposure to S. aureus enterotoxins A, B, and D in the absence of antigen-presenting cells, the T cells failed to respond to their natural ligand presented in an immunogenic form, despite enhanced proliferation to exogenous interleukin 2. The S. aureus toxin-induced anergy was associated with modulation of T-cell membrane receptors; down-regulation of the T-cell antigen receptor was concomitant with enhanced expression of CD2 and CD25. Interestingly, CD28 was increased only on stimulation, suggesting this protein may be differentially expressed by activated and anergic T cells. These results indicate that bacterial toxins are able to induce antigen-specific nonresponsiveness in human T cells, the application of which may be relevant in the regulation of T cells expressing a particular family of V beta gene products.

The endothelial cell as a regulator of T-cell function

These studies have analyzed the antigen-presenting capacities of EC. EC can transcribe, translate and express MHC class II molecules in response to IFN-gamma as well as express class I molecules. The class II dimeric structure is functional, in that allospecific CTL can efficiently kill IFN-gamma-treated EC or fibroblasts, an outcome that can be blocked by antibody to non-polymorphic regions of the class II molecule. Moreover, EC can present antigen in an MHC-restricted manner to resting T cells as well as to antigen-specific cloned T-cell lines. This ability to stimulate primary as well as secondary responses has been further confirmed by experiments using purified populations of naive and memory T cells. In this regard, EC differ from fibroblasts and other non-immune cell types in that they possess costimulator activities necessary for activation of resting T cells. As the local concentration of IL-2 has been shown to be critical in determining the fate of T cells--whether they become activated or anergic--we have investigated the ability of EC to modulate T-cell IL-2 production, believing that this may underlie their ability to act as costimulatory cells. Using PHA-stimulated peripheral blood mononuclear cells or purified CD4+ T cells we have found that EC can augment IL-2 production, typically by 3- to 8-fold. This increased IL-2 production is functional as OKT3-stimulated or sub-optimally PHA-stimulated T cells proliferate more in the presence of EC than in their absence. The major pathway by which EC augment T-cell IL-2 production is cell contact-dependent and involves the CD2:LFA-3 ligand pair. However, use of blocking mAb to CD2 and LFA-3, of PI-LFA-3, and of the immunosuppressive drug CsA has allowed us to reveal the presence of a second signalling pathway. This pathway confers a certain degree of CsA resistance on T cells, but the ligands involved have not yet been identified. We do not find a role for CD28, LFA-1:ICAM-1, VLA-4:VCAM-1 or CD44 in this system. Augmentation is independent of EC metabolism or soluble factors, as fixed cells are almost as efficient as living cells. Similar mechanisms seem also to be involved in more physiological settings, such as alloresponses. Here, proliferation can be blocked by antibodies to CD2 or LFA-3, presumably by blocking of augmented IL-2 production.(ABSTRACT TRUNCATED AT 400 WORDS)

Clonal anergy in self-reactive alpha/beta T cells is abrogated by heat-shock protein-reactive gamma/delta T cells in aged athymic nude mice

Although T cells proliferate and differentiate primarily in the thymus, athymic nude mice contain an appreciable level of T cell receptor alpha/beta and gamma/delta T cells, suggesting the existence of the extrathymic pathway in the development of both T cells. Recent studies with nude mice indicate that clonal deletion of self-reactive T cells does not occur extrathymically. In the present study, we have investigated the responsiveness of self-reactive T cells differentiating along an extrathymic pathway in aged BALB/c (H-2d, Mls-1b2a, I-E+, 7-8 month old) nude mice. Consistent with recent reports, T cells bearing V beta 3 or V beta 11, which are important for recognizing proteins encoded by the Mls-2a or the I-E allele, respectively, are readily detected in age nude mice. The V beta 3- or V beta 11-bearing T cells, however, do not proliferate in response to staphylococcal enterotoxin A which specifically stimulates V beta 3- or V beta 11-bearing T cells. When exogenous recombinant interleukin 2 was added to the culture, the V beta 3-bearing T cells in aged nude mice significantly proliferated in response to staphylococcal enterotoxin A. Aged nude mice also contained a substantial level of gamma/delta T cells which account for 15.6% of all Thy-1.2+ cells. The gamma/delta T cells proliferated and produced a significant level of interleukin 2 in response to the 65-kDa mycobacterial heat-shock protein, which is highly homologous to its eukaryotic counterpart. These results suggest that unresponsiveness of self-reactive T cells may be reversed by T cells responding to stress proteins expressed by the invading microbes and/or the stressed autologous cells.

In vivo induction of antigen-specific transplantation tolerance to Qa1a by exposure to alloantigen in the absence of T-cell help

A goal of transplantation immunology is to be able to induce antigen-specific tolerance in transplant recipients. In the present study we describe an in vivo model of antigen-specific transplantation tolerance to skin allografts using mice congenic at Qa1, a ubiquitously expressed class I-like molecule encoded to the right of H-2D. B6 mice are deficient in Qa1a-specific T-helper cells and only reject Qa1a disparate tail skin grafts when a second graft expressing additional helper determinants is also present. We report that animals initially engrafted with Qa1a disparate skin, in the absence of any source of additional help, are rendered tolerant to Qa1a disparate skin allografts despite the subsequent presence of inducer skin grafts expressing additional helper allodeterminants. The nonresponsive state is Qa1a-specific, because HY-bearing inducer grafts are rejected normally. In vitro, Qa1a-tolerant animals are specifically unable to generate anti-Qa1a T-killer cells, which provides the cellular basis for their failure in vivo to reject Qa1a skin allografts. Thus, initial exposure to Qa1a allodeterminants, in the absence of T-cell help, leads to a state of Qa1a-specific transplantation tolerance. This study suggests that antigen-specific transplantation tolerance may be induced by exposing naive T-killer cells to tissue alloantigens under conditions in which T-cell help is not generated.

The costimulatory function of antigen-presenting cells

In addition to the expression of MHC-antigen complexes and molecules which non-specifically promote cell adhesion, antigen-presenting cells (APCs) provide costimulatory activity for T-lymphocyte stimulation. Costimulatory molecules are essential for activation and multiplication: the presentation of antigen by cells lacking such molecules may lead to T-cell tolerance. In this review, Casey Weaver and Emil Unanue update information on interleukin 1 (IL-1), the original and best-studied costimulator, and investigate the nature of novel, as yet uncharacterized, costimulatory molecules.

The Florey lecture, 1989. Self-tolerance: the key to autoimmunity

'Horor autotoxicus', as it was termed by Erhlich, is a rare clinical event despite the genetic potential of every individual to mount immune responses to self-antigens. This can be explained by the fact that the developing immune system learns to recognize self-antigens and to tolerate them. The key to autoimmunity therefore lies in unravelling the mechanisms of self-tolerance. Studies of conventional models of unresponsiveness have failed to provide a definitive answer owing to the difficulty in controlling for the large number of antigen-related variables associated with self-tolerance and in following the fate of individual clones of self-reactive lymphocytes which emerge in very low numbers from the pre-immune repertoire. These problems have now been overcome by creation of transgenic mice tolerant to endogenous antigens and containing high frequencies of autoreactive T or B lymphocytes. According to the results obtained to date, different mechanisms of tolerance induction operate for self-reactive T lymphocytes compared with B lymphocytes. Thus self-tolerance in T lymphocytes appears to depend largely on clonal deletion within the thymus. By contrast, self-reactive B lymphocytes are functionally silenced without undergoing deletion provided that the transgenic B lymphocytes express both IgM and IgD on their surfaces. This dichotomy makes good sense given that the T-lymphocyte repertoire once shaped within the thymus is not subject to further mutation whereas antigen receptors on mature B lymphocytes undergo hypermutation in the periphery.

Tolerance induction in the organ-cultured thymus lobes upon intimate contact with allogeneic thymus lobes

In an organ-cultured murine fetus thymus, precursor cytotoxic T lymphocytes (pCTL) specific for alloantigens developed successfully but those reactive with self antigens were eliminated. In attempting to dissect the mechanism of self tolerance, intrathymic chimera was made by culturing two genetically disparate thymuses in close contact with each other (parabiosis of thymuses). This maneuver resulted in the induction of specific and mutual CTL tolerance. It seems that CTL tolerance was induced by clonal deletion but not by active suppression. Since 2'-deoxyguanosine treatment abolished the tolerogenic capability of the thymus, hemopoietic cells capable of migrating to and fro in the parabiotic thymuses are thought to be responsible for tolerance induction. Induction of CTL tolerance was dependent on the maturation stages of T cells in the thymus: T cells in 5-day-, but not in 7-day-cultured thymuses were susceptible to tolerance induction, indicating that T cells expressing T cell receptors at low density are susceptible to tolerance induction.

Antigen-specific inhibition of the IL-2-driven proliferation of myelin basic protein-reactive, human T cells

This report examines the antigen-specific inhibition of the IL-2-driven proliferation of autoantigen-reactive, human T cells. Human, myelin basic protein (MBP)-reactive CD4+ cell lines and clones were isolated and maintained in culture by use of IL-2 and periodic antigen stimulation. When freshly isolated antigen-presenting cells (APC) were present, MBP induced proliferation of MBP-reactive T cell populations. However, under different culture conditions, MBP reduced the IL-2-driven proliferation of some MBP-reactive T cell populations. The inhibition of IL-2-driven proliferation did not appear to require CD8+ or OKM 1+ cells since these were not detected when inhibition studies were performed at least 9 days after the last restimulation by irradiated APC and MBP. Supraoptimal concentrations of MBP were not required for inhibition of proliferation. Some heterogeneity of response was apparent since MBP inhibited the IL-2-driven proliferation of some T cell clones while for others MBP had either no effect or produced slight enhancement of proliferation. These results demonstrate an antigen-specific, in vitro immune mechanism that reduces the IL-2-dependent proliferation of autoantigen-reactive, human T cells.