Interleukin 2 production, not the pattern of early T-cell antigen receptor-dependent tyrosine phosphorylation, controls anergy induction by both agonists and partial agonists

Taming the TCR: antigen-specific immunotherapeutic agents for autoimmune diseases

Current treatments for autoimmune diseases are typically non-specific anti-inflammatory agents that affect not only the autoreactive cells but also the parts of the immune system that are required to maintain health. There is a need for the development of antigen-specific therapeutic agents that can effectively prevent the autoimmune attack while leaving the rest of the immune system functioning as normal. The simplest way to achieve this is using the autoantigen itself as a tolerizing agent; however, there is some risk involved with administering a potentially pathogenic antigen. In this review, we focus instead on the development and use of modified T cell receptor (TCR) ligands, in which the peptide ligand is modified to change the response by the T cell from a disease inducing to a protective response, and still retain the antigen-specificity necessary to target the autoreactive T cells. We review the use of modified TCR ligands as therapeutic agents in animal models of autoimmunity and in human autoimmune disease, and finally consider how they need to be improved in order to use them effectively in patients with autoimmune disease.

Prevention of graft-versus-host-disease with preserved graft-versus-leukemia-effect by ex vivo and in vivo modulation of CD4(+) T-cells

This is the first report showing that an epitope-specific ex vivo modulation of an allogeneic hematopoietic stem cell graft by the anti-human CD4 antibody MAX.16H5 IgG1 simultaneously facilitates the anti-tumor capacity of the graft (Graft-versus-leukemia effect, GvL) and the long-term suppression of the deleterious side effect Graft-versus-host-disease (GvHD). To distinguish and consolidate GvL from GvHD, the anti-human CD4 antibody MAX16.H5 IgG1 was tested in murine GvHD and tumor models. The survival rate was significantly increased in recipients receiving a MAX.16H5 IgG1 short-term (2 h) pre-incubated graft even when tumor cells were co-transplanted or when recipient mice were treated by MAX.16H5 IgG1 before transplantation. After engraftment, regulatory T-cells are generated only supporting the GvL effect. It was also possible to transfer the immune tolerance from GvHD-free recipient chimeras into third party recipient mice without the need of reapplication of MAX.16H5 IgG1 anti-human CD4 antibodies. These findings are also benefical for patients with leukemia when no matched related or unrelated donor is available and provides a safer allogeneic HSCT, which is more effective against leukemia. It also facilitates allogeneic (stem) cell transplantations for other indications (e.g., autoimmune-disorders).

Modulation of innate antigen-presenting cell function by pre-patent schistosome infection

Schistosomes are intravascular helminths that infect over 200 million people worldwide. Deposition of eggs by adult schistosomes stimulates Th2 responses to egg antigens and induces granulomatous pathology that is a hallmark of schistosome infection. Paradoxically, schistosomes require host immune function for their development and reproduction and for egress of parasite eggs from the host. To identify potential mechanisms by which immune cells might influence parasite development prior to the onset of egg production, we assessed immune function in mice infected with developing schistosomes. We found that pre-patent schistosome infection is associated with a loss of T cell responsiveness to other antigens and is due to a diminution in the ability of innate antigen-presenting cells to stimulate T cells. Diminution of stimulatory capacity by schistosome worms specifically affected CD11b⁺ cells and did not require concomitant adaptive responses. We could not find evidence for production of a diffusible inhibitor of T cells by innate cells from infected mice. Rather, inhibition of T cell responsiveness by accessory cells required cell contact and only occurred when cells from infected mice outnumbered competent APCs by more than 3∶1. Finally, we show that loss of T cell stimulatory capacity may in part be due to suppression of IL-12 expression during pre-patent schistosome infection. Modulation of CD4⁺ T cell and APC function may be an aspect of host immune exploitation by schistosomes, as both cell types influence parasite development during pre-patent schistosome infection.

The disease schistosomiasis is caused by a parasitic blood fluke found mainly in the tropics and subtropics and affects over 200 million people worldwide. Using mice to model human schistosome infection, our previous studies showed that schistosome development in the infected host is linked to host immune function, such that parasite development is impaired in hosts with immunological deficiencies. CD4⁺ T cells and cells of the monocyte/macrophage lineage are two types of immune cells that are involved in modulating schistosome development. In this study, we examined immune function in mice infected with developing schistosomes, to gain insights into how immune cells might influence parasite development. We found evidence of broad-spectrum suppression of CD4⁺ T cell responses during early schistosome infection. We also show that the loss of T cell responsiveness is due to impairment of T cell stimulation by CD11b⁺ cells. These findings suggest that exploitation of CD4⁺ T cells and monocytes/macrophages by schistosomes may involve parasite modification of the function of these cells.

Alkylating HIV-1 Nef - a potential way of HIV intervention

Background

Nef is a 27 KDa HIV-1 accessory protein. It downregulates CD4 from infected cell surface, a mechanism critical for efficient viral replication and pathogenicity. Agents that antagonize the Nef-mediated CD4 downregulation may offer a new class of drug to combat HIV infection and disease. TPCK (N-α-p-tosyl-L-phenylalanine chloromethyl ketone) and TLCK (N-α-p-tosyl-L-lysine chloromethyl ketone) are alkylation reagents that chemically modify the side chain of His or Cys residues in a protein. In search of chemicals that inhibit Nef function, we discovered that TPCK and TLCK alkylated HIV Nef.

Methods

Nef modification by TPCK was demonstrated on reducing SDS-PAGE. The specific cysteine residues modified were determined by site-directed mutagenesis and mass spectrometry (MS). The effect of TPCK modification on Nef-CD4 interaction was studied using fluorescence titration of a synthetic CD4 tail peptide with recombinant Nef-His protein. The conformational change of Nef-His protein upon TPCK-modification was monitored using CD spectrometry

Results

Incubation of Nef-transfected T cells, or recombinant Nef-His protein, with TPCK resulted in mobility shift of Nef on SDS-PAGE. Mutagenesis analysis indicated that the modification occurred at Cys55 and Cys206 in Nef. Mass spectrometry demonstrated that the modification was a covalent attachment (alkylation) of TPCK at Cys55 and Cys206. Cys55 is next to the CD4 binding motif (A₅₆W₅₇L₅₈) in Nef required for Nef-mediated CD4 downregulation and for AIDS development. This implies that the addition of a bulky TPCK molecule to Nef at Cys55 would impair Nef function and reduce HIV pathogenicity. As expected, Cys55 modification reduced the strength of the interaction between Nef-His and CD4 tail peptide by 50%.

Conclusions

Our data suggest that this Cys55-specific alkylation mechanism may be exploited to develop a new class of anti HIV drugs.

Uncoupling of T cell receptor zeta chain function during the induction of anergy by the superantigen, staphylococcal enterotoxin A

Staphylococcus aureus enterotoxins have immunomodulatory properties. In this study, we show that Staphylococcal enterotoxin A (SEA) induces a strong proliferative response in a murine T cell clone independent of MHC class II bearing cells. SEA stimulation also induces a state of hypo-responsiveness (anergy). We characterized the components of the T cell receptor (TCR) during induction of anergy by SEA. Most interestingly, TCR zeta chain phosphorylation was absent under SEA anergizing conditions, which suggests an uncoupling of zeta chain function. We characterize here a model system for studying anergy in the absence of confounding costimulatory signals.

Mesenchymal stem cells and their use as cell replacement therapy and disease modelling tool

Mesenchymal stem cells (MSCs) from adult somatic tissues may differentiate in vitro and in vivo into multiple mesodermal tissues including bone, cartilage, adipose tissue, tendon, ligament or even muscle. MSCs preferentially home to damaged tissues where they exert their therapeutic potential. A striking feature of the MSCs is their low inherent immunogenicity as they induce little, if any, proliferation of allogeneic lymphocytes and antigen-presenting cells. Instead, MSCs appear to be immunosuppressive in vitro. Their multi-lineage differentiation potential coupled to their immuno-privileged properties is being exploited worldwide for both autologous and allo-geneic cell replacement strategies. Here, we introduce the readers to the biology of MSCs and the mechanisms underlying immune tolerance. We then outline potential cell replacement strategies and clinical applications based on the MSCs immunological properties. Ongoing clinical trials for graft-versus-host-disease, haematopoietic recovery after co-transplantation of MSCs along with haematopoietic stem cells and tissue repair are discussed. Finally, we review the emerging area based on the use of MSCs as a target cell subset for either spontaneous or induced neoplastic transformation and, for modelling non-haematological mesenchymal cancers such as sarcomas.

IFN-gamma induces the erosion of preexisting CD8 T cell memory during infection with a heterologous intracellular bacterium

Memory T cells are critical for the control of intracellular pathogens and require few signals for maintenance; however, erosion of established preexisting memory CD8(+) T cells has been shown to occur during infection with heterologous viral infections. We evaluated whether this also occurs during infection with various intracellular bacteria and what mechanisms may be involved. We demonstrate that erosion of established memory is also induced during infection of mice with various intracellular bacteria, such as Listeria monocytogenes, Salmonella typhimurium, and Mycobacterium bovis (bacillus Calmette-Guérin). The extent of erosion of established CD8(+) T cell memory was dependent on the virulence of the heterologous pathogen, not persistence. Furthermore, when antibiotics were used to comprehensively eliminate the heterologous pathogen, the numbers of memory CD8(+) T cells were not restored, indicating that erosion of preexisting memory CD8(+) T cells was irreversible. Irrespective of the initial numbers of memory CD8(+) T cells, challenge with the heterologous pathogen resulted in a similar extent of erosion of memory CD8(+) T cells, suggesting that cellular competition was not responsible for erosion. After challenge with the heterologous pathogen, effector memory CD8(+) T cells were rapidly eliminated. More importantly, erosion of preexisting memory CD8(+) T cells was abrogated in the absence of IFN-gamma. These studies help reveal the paradoxical role of IFN-gamma. Although IFN-gamma promotes the control of intracellular bacterial replication during primary infection, this comes at the expense of erosion of preexisting memory CD8(+) T cells in the wake of infection with heterologous pathogens.

The proline-rich sequence of CD3epsilon as an amplifier of low-avidity TCR signaling

Engagement of peptide-MHC by the TCR induces a conformational change in CD3epsilon that exposes a proline-rich sequence (PRS) and recruits the cytoskeletal adaptor Nck. This event, which precedes phosphorylation of the CD3epsilon ITAM, has been implicated in synapse formation and T cell function. However, there is compelling evidence that responsiveness to TCR ligation is CD3epsilon PRS independent. In this study, we show that the CD3epsilon PRS is necessary for peptide-MHC-induced phosphorylation of CD3epsilon and for recruitment of protein kinase Ctheta to the immune synapse in differentiated CD8+ T lymphocytes. However, whereas these two events are dispensable for functional T cell responsiveness to high-avidity ligands, they are required for responsiveness to low-avidity ones. Thus, in at least certain T cell clonotypes, the CD3epsilon PRS amplifies weak TCR signals by promoting synapse formation and CD3epsilon phosphorylation.

Derivation and fluidity of acutely induced dysfunctional CD8+ T cells

Dysfunctional CD8(+) T (T(CD8(+))) cells lacking cytokine production have been identified in many viral infections, but their genesis is not well understood. Established results indicate that such cells could be either high avidity that enter a refractory state due to overstimulation or low avidity that are only partially stimulated. Using an acute, resolving infection model that results in rapid production of dysfunctional cells, we show that this IL2 unresponsive phenotype emerges from the low end of the avidity spectrum and is characterized by broad TCR usage and a reduced proliferation rate. Furthermore, the dysfunctional population is extremely fluid, being sustained by high Ag dose but virtually eliminated following low dose boosting. Together, these results suggest that persistence of dysfunctional cells generated in this manner depends upon continual exposure to high Ag levels and that such cells may ultimately predominate if functional cells become exhausted.

Peripheral tolerance and the qualitative characteristics of autoreactive T cell clones in primary biliary cirrhosis

Primary biliary cirrhosis is characterized by autoreactive T cells specific for the mitochondrial Ag PDC-E2(163-176). We studied the ability of eight T cell clones (TCC) specific for PDC-E2(163-176) to proliferate or become anergic in the presence of costimulation signals. TCC were stimulated with either human PDC-E2(163-176), an Escherichia coli 2-oxoglutarate dehydrogenase mimic (OGDC-E2(34-47)), or analogs with amino acid substitutions using HLA-matched allogeneic PBMC or mouse L-DR53 fibroblasts as APC. Based on their differential responses to these peptides (human PDC-E2(163-176), E. coli OGDC-E2(34-47)) in the different APC systems, TCC were classified as costimulation dependent or independent. Only costimulation-dependent TCC could become anergic. TCC with costimulation-dependent responses to OGDC-E2 become anergic to PDC-E2 when preincubated with mimic, even if costimulation is independent for PDC-E2(163-176). Anergic TCC produced IL-10. One selected TCC could not become anergic after preincubation with PDC-E2(163-176)-pulsed L-DR53 but became anergic using L-DR53 pulsed with PDC-E2 peptide analogs with a substitution at a critical TCR binding site. TCC that only respond to peptide-pulsed PBMC, but not L-DR53, proliferate with peptide-pulsed CD80/CD86-transfected L-DR53; however, anergy was not induced with peptide-pulsed L-DR53 transfected with only CD80 or CD86. These data highlight that costimulation plays a dominant role in maintaining peripheral tolerance to PBC-specific Ags. They further suggest that, under specific circumstances, molecular mimicry of an autoantigen may restore rather than break peripheral tolerance.

CD4+ T cells from simian immunodeficiency virus disease-resistant sooty mangabeys produce more IL-2 than cells from disease-susceptible species: involvement of p300 and CREB at the proximal IL-2 promoter in IL-2 up-regulation

IL-2 is an important cytokine required for the physiological function of CD4(+) T cells. Immunological unresponsiveness-anergy- of CD4(+) T cells is characterized by the inability of these cells to synthesize IL-2. Both progressive HIV infection leading to AIDS in humans and SIV infection in rhesus macaques (RM) are associated with dysregulation of IL-2 synthesis. In certain nonhuman primate species, such as sooty mangabeys (SM), SIV infection does not lead to AIDS. We have shown that this is associated with the resistance of the CD4(+) T cells from SM to undergo anergy in vitro. In this study, we show that CD4(+) T cells from SM spontaneously synthesize 2- to 3-fold higher levels of IL-2 than corresponding cells from RM. Proximal IL-2 promoter constructs derived from SM show significantly higher activity than the RM-derived constructs in primary CD4(+) T cells, which is associated with an element at approximately nt -200. Activity of both constructs was up-regulated by p300 and down-regulated by CREB to a similar degree. Chromatin immunoprecipitation analysis showed significantly higher binding of p300 and lower binding of CREB to the SM promoter in vivo. Two single nucleotide substitutions present in the SM sequence around position -200 and -180 seem to increase the affinity of these sites for the binding of transcription factors, one of which was identified as Oct-1. These unique characteristics of the proximal IL-2 promoter in SM therefore can represent one of the mechanisms contributing to the resistance of these cells to undergo anergy.

Gene transfer of programmed death ligand-1.Ig prolongs cardiac allograft survival

BACKGROUND

The CD28 homologue programmed death-1 (PD-1) and its ligands, PD-L1 and PD-L2 (which are homologous to B7), constitute an inhibitory pathway of T cell costimulation. The PD-1 pathway is of interest for immune-mediated diseases given that PD-1-deficient mice develop autoimmune diseases. We have evaluated the effect of local overexpression of a PD-L1.Ig fusion protein on cardiac allograft survival.

METHODS

Adenovirus-mediated PD-L1.Ig gene transfer was performed in F344 rat donor hearts placed in the abdominal position in Lewis recipients. Inflammatory cell infiltrates in the grafts were assessed by immunohistochemistry.

RESULTS

Allografts transduced with the PD-L1.Ig gene survived for longer periods of time compared with those receiving noncoding adenovirus or virus dilution buffer alone: median survival time (MST), 17 (range: 16-20) days vs. 11 (8-14) and 9 (8-13) days, respectively (P < 0.001). PD-L1.Ig gene transfer combined with a subtherapeutic regimen of cyclosporin A (CsA) was superior to CsA alone: MST, 25 (15-42) vs. 15 (13-19) days (P < 0.05). PD-L1.Ig gene transfer was associated with decreased numbers of CD4 cells and monocytes/macrophages infiltrating the graft (P < 0.05).

CONCLUSIONS

Localized PD-L1.Ig expression in donor hearts attenuates acute allograft rejection in a rat model. The effect is additive to that of a subtherapeutic regimen of CsA. These results suggest that targeting of PD-1 by gene therapy may inhibit acute cardiac allograft rejection in vivo.

Anti-CD45RB monoclonal antibody prolongs renal allograft survival in cynomolgus monkeys

Previously, an anti-CD45RB monoclonal antibody (mAb) has been shown to induce murine allograft tolerance. The present study was performed to assess the ability of an anti-human CD45RB mAb to prevent rejection in a monkey MHC-mismatched kidney transplant model. The recipients were allocated into the following treatment groups: (1) isotype control IgG; (2) mouse anti-human CD45RB IgG1 (6G3); (3) human-mouse chimeric anti-CD45RB-IgG1 (C6G3-IgG1); (4) human-mouse chimeric anti-CD45RB-IgG2 (C6G3-IgG2); (5) tacrolimus at a subtherapeutic dose and (6) tacrolimus and C6G3-IgG1 in combination. Monotherapy with anti-CD45RB mAb significantly prolonged renal allograft survival to a median survival of 21 days. Adding a subtherapeutic dose of tacrolimus improved the efficacy of the anti-CD45RB mAb, achieving a median survival of 85 days, whereas a subtherapeutic dose of tacrolimus alone only moderately prolonged survival to 27 days. Treatment with anti-CD45RB mAb resulted in an alteration of the CD45RB(hi) : CD45RB(lo) cell ratio in the peripheral blood. We have, for the first time, demonstrated that an anti-human CD45RB mAb (6G3) can prolong graft survival. Induction with an anti-CD45RB mAb improves the efficacy of tacrolimus in the prevention of rejection. These encouraging results indicate that an anti-CD45RB mAb may be valuable in future clinical transplantation.

Cell- rather than antibody-mediated immunity leads to the development of profound thrombocytopenia during experimental Plasmodium berghei malaria

Experimental malarial thrombocytopenia can reach life-threatening levels and is believed to be due to Abs targeting platelets for destruction by the reticuloendothelial system. However, we report that Abs account for at most 15% of platelet destruction as Plasmodium berghei-infected B cell-deficient mice exhibited profound thrombocytopenia (83%) as did C57BL/6 controls (98%). Further, no significant increase in Abs bound to intact platelets was observed during infection. P. berghei infection can enhance the activity of anti-platelet Abs as indicated by a significantly (p < 0.005) increased thrombocytopenia on day 4 of infection in mice that were administered a low dose anti-CD41 mAb compared with rat IgG1-injected controls. RAG1-/- and CD4- plus CD8-deficient mice were markedly protected from thrombocytopenia (p < 0.005) and malarial pathogenesis. CD8- or TCRgammadelta-deficient mice were not protected from thrombocytopenia and CD4-deficient mice were modestly protected. RAG1-/- mice exhibited significantly (p < 0.05) lower levels of plasma TNF, IFN-gamma, and IL-12 during infection. IFNgamma-/- and IL-12-/- mice exhibited increased survival but similar thrombocytopenia to C57BL/6 controls. Collectively, these data indicate that thrombocytopenia is necessary but not sufficient for malarial pathogenesis and Abs are not the major contributors to malarial thrombocytopenia. Rather, we propose that both CD4+ and CD8+ T cell populations play key roles in malarial thrombocytopenia; a complex bidirectional interaction between cell-mediated immunity and platelets exists during experimental severe malaria that regulates both responses.

Decreased CD4 expression by polarized T helper 2 cells contributes to suboptimal TCR-induced phosphorylation and reduced Ca2+ signaling

Polarized Th1 and Th2 cells expressing the same TCR produce distinct biochemical responses to ligand engagement. Compared to Th1 cells, Th2 cells show altered substrate tyrosine phosphorylation and a diminished or transient Ca2+ response. Here we demonstrate that agonist stimulation of Th1 cells leads to the predominant appearance of fully phosphorylated (p23) TCR zeta, substantial phosphorylation of zeta-associated protein 70 (ZAP-70), and strong elevation of intracellular Ca2+, whereas agonist stimulation of Th2 cells expressing an identical TCR results in an elevated p21:p23 TCR zeta ratio, little or no detectable ZAP-70 phosphorylation, and a more limited elevation in intracellular Ca2+. Th2 cells consistently had twofold lower surface CD4 expression as compared to Th1 cells with the same TCR. When CD4 levels in Th2 cells were raised to Th1 levels using retroviral gene transfer, the transduced cells showed greater generation of p23 phospho-zeta, measurable phosphorylation of ZAP-70, and increased Ca2+ responses. These findings suggest that the apparent qualitative differences in TCR signaling characterizing Th1 versus Th2 cells are largely the result of modest quantitative variation in CD4 expression, with decreased CD4 expression playing a significant role in attenuating the proximal signaling responsiveness of Th2 cells to TCR ligands.

Plasticity in the positive selection of T cells: affinity of the selecting antigen and IL-7 affect T cell responsiveness

The current study examines how responsiveness of T cells is affected by the avidity of the peptide/MHC engaged during positive selection of their thymocyte precursors. We used a thymus reaggregate culture system in which CD4(+)CD8(+) thymocytes from AND TCR transgenic mice were induced to undergo positive selection by pigeon cytochrome c (PCC) peptide or its analogs presented by I-E(k) class II MHC on a thymic epithelial cell line. When low-affinity peptide analogs drove positive selection, up to 100 microM was needed to produce >50% CD4(+) T cells, and these cells were highly responsive to PCC. In contrast, <0.2 microM high-affinity peptides was required to achieve similar selection efficiency, but the resultant cells failed to respond to PCC. However, these cells were not dead based on dye exclusion and capacity to respond to phorbal ester and to agonist if IL-2 was also present, supporting the view that non-responsiveness of cells selected on high-affinity peptides is a form of central T cell tolerance distinct from deletion. Cells selected on intermediate-affinity peptides showed variable responsiveness which was suppressed 5- to 10-fold by addition during reaggregate culture of antibody to the IL-7R. Similarly, supplementary IL-7 in the reaggregate culture produced CD4(+) T cells that were promiscuously responsive. Overall, this study demonstrates that the responsiveness of T cells is not rigidly controlled and that the presence of IL-7 during T cell development has the potential to negate central T cell tolerance and produce autoreactive T cells.

Transduction of naive CD4 T?cells with kinase-deficient Lck-HIV-Tat fusion protein dampens T?cell activation and provokes a switch to regulatory function

We show here that T cell differentiation can be altered by exposing naive mouse CD4 T cells to altered T cell receptor signaling, achieved by transducing them with a fusion protein consisting of a modified Lck protein lacking the kinase domain and the HIV-Tat protein transduction domain. The Lck-HIV-Tat fusion protein is internalized into naive mouse T cells within 30 min after application to the medium. Activation of transduced cells in vitro resulted in strongly reduced intracellular calcium mobilization, alterations in cytokine profile, and sustained up-regulation of CD25. The cells had suppressive activity in vitro, but no Foxp3 expression. Our data indicate that signals encountered by a naive T cell during its initial activation can profoundly influence its subsequent functional behavior and elicit T cells, which can have regulatory activity.

Mechanism of modulation of T cell responses by N-palmitoylated peptides

Small structural changes in the antigenic peptides recognized by TCR can alter the biological properties of those peptides and convert them into weak agonists, partial agonists, or antagonists of these receptors. These altered peptide ligands (APL) are usually generated by conservative amino acid substitutions at TCR contact residues. Here, we show that APL with therapeutic properties can also be generated by attachment of palmitic acid at the N terminus of the peptide without the need to modify the peptide's primary sequence. Using N-palmitoylated pigeon cytochrome-c peptide 81-104 (PALPCC(81-104)), we were able to induce T cell hyporesponsiveness to the wild-type peptide in vitro. More importantly, administration of the PALPCC(81-104 )to mice reduced the responsiveness to the native peptide when tested ex vivo. Biochemical and functional experiments indicated that the action of N-palmitoylated peptides was due to the conversion of the native peptide into a weak agonist that could then induce T cell anergy. Our results demonstrate that N-palmitoylation of antigenic peptides is a feasible strategy to generate APL, as it avoids the need to screen multiple amino acid variants of each specific antigen to identify those with therapeutic properties.

Autoreactive T Cells Persist in Rats Protected against Experimental Autoimmune Encephalomyelitis and Can Be Activated through Stimulation of Innate Immunity

Lewis rats can be rendered unresponsive to experimental autoimmune encephalomyelitis by immunization with myelin basic protein (MBP), or MBP68-86, the dominant encephalitogenic MBP epitope for this strain, administered in IFA. However, protected rats harbor potentially encephalitogenic T cells, which are maintained in an inactive state. We investigated whether these quiescent effector cells could be activated in vitro. Although these T cells respond poorly to MBP68-86, they proliferate vigorously whether cocultured with MBP68-86 and either IL-2 or IL-12, suggesting that the T cells are in a state of anergy. Moreover, we could activate these anergic T cells with peptide and cytosine-guanine dinucleotide (CpG) oligonucleotide, but not control oligonucleotide, suggesting that products of the innate immune response are capable of activating anergic autoreactive T cells. The activated T cells produced the proinflammatory cytokine, IFN-gamma in response to IL-12, and IL-6 was secreted in response to CpG oligonucleotide. IL-6 has been reported to play a role in T cell activation by blocking T regulatory/suppressor (Treg) cell-mediated suppression through a Toll-like receptor-dependent pathway. However, anti-IL-6 mAb did not block CpG activation of the anergized cells. In contrast, anti-TGF-beta(1) Ab released the unresponsive T cells from the anergic state in the presence of MBP68-86, whereas TGF-beta(1) inhibited proliferation of MBP68-86- plus CpG-activated T cells. Because TGF-beta(1) has previously been implicated in Treg activity, this finding is consistent with a role for Treg cells in maintaining autoreactive T cells in the anergic state.

Cell-cycle regulation of T-cell responses - novel approaches to the control of alloimmunity

Alloreactive T cells undergo clonal expansion before they participate in allograft rejection. Current estimates suggest that roughly 1 in 20 peripheral T cells are alloreactive, and these cells may expand at least 20-50-fold during an alloimmune response in vivo. The majority of immunosuppressive drugs currently used to facilitate graft survival in experimental models and in the clinic act to inhibit T-cell proliferation. This review focuses on 1) recent advances in monitoring alloreactive T-cell proliferation during alloimmune responses, 2) the link between cell division, anergy avoidance, and effector T-cell differentiation, and 3) an overview of growth factor receptor-coupled signal transduction pathways, with emphasis on key cell-cycle regulators that may serve as potential targets for novel immunosuppressive or tolerance-inducing strategies.

Kinetic perspectives of T cell antigen receptor signaling. A two-tier model for T cell full activation

T-cell activation consists of multiple layers of signaling events. Interleukin-2 production is of interest for many, since its expression determines a critical difference between partial and full T-cell activation. To achieve full activation of T cells, it is necessary for the T-cell antigen receptor (TCR) to be engaged for an extended period of time. However, why extended stimulation is required for full T-cell activation is not understood at the molecular level. In this review, orchestrated events of TCR signal transduction will be analyzed in a kinetic manner and connected toward the understanding of the mechanism of T-cell activation. Based on recent results, a model of the mechanism that dictates the threshold between partial and full T-cell activation is proposed.

The biological behaviour of human adenoid cystic carcinoma cells transduced with interleukin-2-gene

Adenoid cystic carcinoma (ACC) of the salivary glands is a highly infiltrative malignant tumour with a tendency for lung metastasis. Gene therapy could be a potentially effective therapy for ACC and its metastasis. The aims of the study were: To transduce interleukin-2 (IL-2) gene into an ACC cell line with predisposition for lung metastasis (ACC-M); to compare the bioactivity of the gene-transduced cells and the parent cell line in vitro and in vivo. The IL-2 gene was transduced via a bicistronic retroviral vector into the ACC-M cells. The growth rate and DNA cell cycles of the parent ACC-M, the control viral vector AmGCEN, and the gene transduced AmIL-2 cell cultures were compared quantitatively and by flow cytometry, respectively. The tumourigenic ability of the three cell lines was verified by inoculation in athymic nude mice. The tumours developed were extracted and compared quantitatively and histologically. There was no difference in the growth rate and the DNA count between the ACC-M, AmGCEN, and AmIL-2 cell cultures. In the animal experiment, both the ACC-M and AmGCEN cells stimulated lung metastasis in all the mice, whereas there was no tumour found in the 1 x 10(6) AmIL-2 cells inoculation. On 3 x 10(6) AmIL-2 cells stimulation, three out of six mice developed tumours but the mass and volume of the tumours were smaller than the other two groups. Under light microscopy, the ACC-M tumours were mainly poorly differentiated with minimal cellular matrix, whereas the AmIL-2 tumours were well differentiated with ample matrix. The transduction of IL-2 gene can reduce the tumourigenicity of ACC-M cells and induces tumour cell differentiation in mice. The IL-2 gene can be a potential effective gene for the treatment of adenoid cystic carcinoma of salivary glands and its lung metastasis.

Programmed death-1 targeting can promote allograft survival

The recently identified CD28 homolog and costimulatory molecule programmed death-1 (PD-1) and its ligands, PD-L1 and PD-L2, which are homologs of B7, constitute an inhibitory regulatory pathway of potential therapeutic use in immune-mediated diseases. We examined the expression and functions of PD-1 and its ligands in experimental cardiac allograft rejection. In initial studies, we found that most normal tissues and cardiac isografts had minimal expression of PD-1, PD-L1, or PD-L2, but intragraft induction of all three molecules occurred during development of cardiac allograft rejection. Intragraft expression of all three genes was maintained despite therapy with cyclosporin A or rapamycin, but was prevented in the early posttransplant period by costimulation blockade using CD154 or anti-inducible costimulator mAb. We prepared PD-L1.Ig and PD-L2.Ig fusion proteins and showed that each bound to activated PD-1(+) T cells and inhibited T cell functions in vitro, thereby allowing us to test the effects of PD-1 targeting on allograft survival in vivo. Neither agent alone modulated allograft rejection in wild-type recipients. However, use of PD-L1.Ig administration in CD28(-/-) recipients, or in conjunction with immunosuppression in fully MHC-disparate combinations, markedly prolonged cardiac allograft survival, in some cases causing permanent engraftment, and was accompanied by reduced intragraft expression of IFN-gamma and IFN-gamma-induced chemokines. PD-L1.Ig use also prevented development of transplant arteriosclerosis post-CD154 mAb therapy. These data show that when combined with limited immunosuppression, or in the context of submaximal TCR or costimulatory signals, targeting of PD-1 can block allograft rejection and modulate T and B cell-dependent pathologic immune responses in vivo.

Reduced self-reactivity of an autoreactive T cell after activation with cross-reactive non-self-ligand

Autoreactive CD4(+) T lymphocytes are critical to the induction of autoimmune disease, but because of the degenerate nature of T cell receptor (TCR) activation such receptors also respond to other ligands. Interaction of autoreactive T cells with other non-self-ligands has been shown to activate and expand self-reactive cells and induce autoimmunity. To understand the effect on the autoreactivity of naive cross-reactive T cells of activation with a potent nonself ligand, we have generated a TCR transgenic mouse which expresses a TCR with a broad cross-reactivity to a number of ligands including self-antigen. The activation of naive transgenic recombination activating gene (Rag)2(-)(/)(-) T cells with a potent non-self-ligand did not result in a enhancement of reactivity to self, but made these T cells nonresponsive to the self-ligand and anti-CD3, although they retained a degree of responsiveness to the non-self-ligand. These desensitized cells had many characteristics of anergic T cells. Interleukin (IL)-2 production was selectively reduced compared with interferon (IFN)-gamma. p21(ras) activity was reduced and p38 mitogen-activated protein kinase (MAPK) was relatively spared, consistent with known biochemical characteristics of anergy. Surprisingly, calcium fluxes were also affected and the anergic phenotype could not be reversed by exogenous IL-2. Therefore, activation with a hyperstimulating non-self-ligand changes functional specificity of an autoreactive T cell without altering the TCR. This mechanism may preserve the useful reactivity of peripheral T cells to foreign antigen while eliminating responses to self.

The ability of antigen, but not interleukin-2, to promote n-butyrate-induced T helper 1 cell anergy is associated with increased expression and altered association patterns of cyclin-dependent kinase inhibitors

The ability of the cell cycle inhibitor n-butyrate to induce T helper 1 (Th1) cell anergy is dependent upon its ability to block the cell cycle progression of activated Th1 cells in G1. Results reported here show that although both interleukin (IL)-2 and antigen (Ag) push Th1 cells into G1 where they are blocked by n-butyrate, only the Ag-activated Th1 cells demonstrate functional anergy once the n-butyrate has been removed from the culture. Because n-butyrate-induced Th1 cell anergy has been linked to increased expression of the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1, mechanistic experiments focused on the role of these inhibitors. It was found that when Th1 cells were reincubated in Ag-stimulated secondary cultures, the Th1 cells previously exposed to Ag and n-butyrate (anergic Th1 cells) demonstrated a cumulative increase in p21Cip1 and p27Kip1 when compared with Th1 cells previously exposed to recombinant (r)IL-2 and n-butyrate (non-anergic Th1 cells). p27Kip1 in the anergic Th1 cells from the secondary cultures was associated with cyclin-dependent kinases (cdks). In contrast, p21Cip1 in the anergic Th1 cells, although present at high levels, did not associate significantly with cdks, suggesting that p21Cip1 may target some other protein in the anergic Th1 cells. Taken together, these findings suggest that Th1 cell exposure to Ag and n-butyrate, rather than IL-2 and n-butyrate, is needed to induce the cumulative increase in p21Cip1 and p27Kip1 that is associated with the proliferative unresponsiveness in anergic Th1 cells. In addition, p21Cip1 may inhibit proliferation in the anergic Th1 cells by some mechanism other than suppression of cdks that is unique to the induction of Th1 cell anergy.

Cross-reactive antigen is required to prevent erosion of established T cell memory and tumor immunity: a heterologous bacterial model of attrition

Induction and maintenance of T cell memory is critical for the control of intracellular pathogens and tumors. Memory T cells seem to require few "maintenance signals," though often such studies are done in the absence of competing immune challenges. Conversely, although attrition of CD8(+) T cell memory has been characterized in heterologous viral models, this is not the case for bacterial infections. In this study, we demonstrate attrition of T cell responses to the intracellular pathogen Listeria monocytogenes (LM) following an immune challenge with a second intracellular bacterium, Mycobacterium bovis (bacillus Calmette-Guérin, BCG). Mice immunized with either LM or recombinant LM (expressing OVA; LM-OVA), develop a potent T cell memory response. This is reflected by peptide-specific CTL, IFN-gamma production, and frequency of IFN-gamma-secreting T cells to native or recombinant LM Ags. However, when the LM-infected mice are subsequently challenged with BCG, there is a marked reduction in the LM-specific T cell responses. These reductions are directly attributable to the effects on CD4(+) and CD8(+) T cells and the data are consistent with a loss of LM-specific T cells, not anergy. Attrition of the Ag (OVA)-specific T cell response is prevented when LM-OVA-immunized mice are challenged with a subsequent heterologous pathogen (BCG) expressing OVA, demonstrating memory T cell dependence on Ag. Although the reduction of the LM-specific T cell response did not impair protection against a subsequent LM rechallenge, for the first time, we show that T cell attrition can result in the reduction of Ag-specific antitumor (B16-OVA) immunity previously established with LM-OVA immunization.

T-cell tolerance and autoimmune diabetes

Herein we describe the major signaling events that occur in T-cells upon T-cell receptor (TCR) engagement, and the mechanisms responsible for the induction of T-cell anergy that may ultimately lead to the development of immunospecific therapies in T-cell mediated autoimmune diseases. A new type of antigen presenting molecule (dimeric MHC class-II/peptide, DEF) endowed with antigen-specific immunomodulatory effects such as induction of Th2 polarization and T-cell anergy is also described as a potential antidiabetogenic agent. According to our preliminary results, the MHC II/peptide-based approach may provide rational grounds for further development of antigen-specific immunotherapeutic agents such as human-like MHC lI/peptide chimeras endowed with efficient down-regulatory effects in CD4 T-cell-mediated autoimmune diseases such as Type 1 diabetes, multiple sclerosis, primary biliary cirrhosis, and rheumatoid arthritis.

Respiratory syncytial virus infection suppresses lung CD8+ T-cell effector activity and peripheral CD8+ T-cell memory in the respiratory tract

Respiratory syncytial virus (RSV) is a major cause of morbidity from respiratory infection in infants, young children and the elderly. No effective vaccine against RSV is currently available and studies of the natural history of RSV infection suggest repeated infections with antigenically related virus strains are common throughout an individual's lifetime. We have studied the CD8+ T-cell response during experimental murine RSV infection and found that RSV inhibits the expression of effector activity by activated RSV-specific CD8+ T cells infiltrating the lung parenchyma and the development of pulmonary CD8+ T-cell memory by interfering with TCR-mediated signaling. These data suggest a possible mechanism to explain the limited duration of protective immunity in RSV infection.

Autoreactivity, dynamic tuning and selectivity

The immune system adjusts its response to the context in which antigens, including self-antigens, are recognized. Recent observations support a conceptual framework for understanding how this may be achieved at the cellular and cell-population levels. At both levels, 'perturbations' elicit competition between excitation and de-excitation, resulting either in adaptation or in various responses. The responsiveness of individual cells is dynamically tuned, reflecting their recent experience. The tuning of T-cell activation thresholds by self-ligands facilitates positive selection and continuously regulates the level of autoreactivity in the periphery. Autoreactivity appears to be involved in regulation of the immune response, homeostasis, maintaining of the functional integrity of naïve and memory cells, and in other physiological functions.

HuM291(NUVION), A HUMANIZED Fc RECEPTOR-NONBINDING ANTIBODY AGAINST CD3, ANERGIZES PERIPHERAL BLOOD T CELLS AS PARTIAL AGONIST OF THE T CELL RECEPTOR1

BACKGROUND

Humanized Fc receptor (FcR)-nonbinding antibodies against CD3 are promising immunosuppressive agents that may overcome both the neutralizing response to and the cytokine release syndrome seen with conventional monoclonal antibodies against CD3. In addition, evidence from several murine models suggests that these recombinant antibodies may actively induce T cell unresponsiveness by a mechanism other than modulation of the T cell receptor (TCR) or T cell depletion. We hypothesized that FcR-nonbinding antibodies against CD3 could induce T cell unresponsiveness by acting as partial agonist ligands of the TCR and thus, inducing T cell anergy.

METHODS

To test this hypothesis, we examined the signaling and functional effects of HuM291 (Nuvion), a FcR-nonbinding humanized antibody against CD3, on primary human T cells.

RESULTS

Short exposure of human peripheral blood T lymphocytes to HuM291 caused a partial agonist type of signaling through the TCR characterized by incomplete phosphorylation of TCR zeta, failure to activate ZAP-70 and to phosphorylate LAT but activation of ERK-1/-2 and subsequent up-regulation of CD69 expression. These changes correlated with a dose-dependent induction of anergy in human, primary resting T cells, which was reversed by exogenous interleukin-2.

CONCLUSIONS

The tolerogenic properties of FcR-nonbinding monoclonal antibodies against CD3 correlate with its ability to reproduce the biochemical and functional effects of TCR partial agonist ligands. Thus, generation of engineered antibodies against CD3 with low TCR oligomerization potential may provide a clinically applicable partial agonist-based strategy for the prevention of polyclonal T cell responses.

Simian immunodeficiency virus (SIV) infection of a rhesus macaque induces SIV-specific CD8(+) T cells with a defect in effector function that is reversible on extended interleukin-2 incubation

A vigorous expansion of antigen-specific CD8(+) T cells lacking apparent effector function was observed in a rhesus macaque acutely infected with the simian immunodeficiency virus (SIV) strain SIVmac239. Antigen-specific CD8(+) T cells were identified using antigenic-peptide class I major histocompatibility complex tetramers. As many as 8.3% of CD8(+) cells recognized the Mamu-A*01-associated SIV epitope Gag(181-189) (CTPYDINQM); however, these cells demonstrated no effector function when presented with peptide-incubated targets, as measured by intracellular cytokine staining for gamma interferon (IFN-gamma), interleukin-2 (IL-2) production, or direct cellular lysis. Similar results were observed with three other SIV peptide antigens. Nonresponsiveness did not correlate with apoptosis of the CD8(+) cells, nor were cells from this macaque impaired in their ability to present peptide antigens. Associated with the nonresponsive state was a lack of IL-2 production and decreased IL-2 receptor expression. Exogenous IL-2 treatment for 1 week in the absence of antigenic stimulation restored antigen-specific responses and the quantitative correlation between tetramer recognition and antigen-responsive IFN-gamma secretion. This case report suggests a regulatory mechanism that may impede the effector function of antigen-specific T cells during acute infection with SIV or human immunodeficiency virus in some cases. This mechanism may participate in the failure of the immune system to limit infection.

Altered superantigenic ligands demonstrate the quantitative nature of T-cell activation

In a recent study, a superantigen mutated in the TCR binding site (staphylococcal enterotoxin B (SEB)delta61Y) was described, which behaved as a partial agonist for a Vbeta17-expressing T-cell clone. Evidence is now presented to demonstrate that there is distinct heterogeneity in the response of primary T cells to this protein. Some Vbeta17 T cells responded to SEBdelta61Y by modulating surface receptor expression consistent with activation, and by proliferating. Other Vbeta17 T cells did not proliferate, nor did they display a receptor expression phenotype consistent with activation. However, when repeatedly exposed to the altered superantigen, some of these non-responders entered cell cycle. This pattern of responses was not recapitulated by providing additional costimulation via CD28, although such treatment did induce some of the 'unresponsive' Vbeta17 T cells to upregulate the IL-2 receptor, indicative of partial activation. It was also found that the heterogeneous pattern could be replicated using very low doses of native SEB. The data are discussed in the context of models of T-cell activation in which differences in TCR ligand affinity and dose determine qualitatively different response phenotypes.

T cell effector function and anergy avoidance are quantitatively linked to cell division

We have shown previously that T cells activated by optimal TCR and CD28 ligation exhibit marked proliferative heterogeneity, and approximately 40% of these activated cells fail entirely to participate in clonal expansion. To address how prior cell division influences the subsequent function of primary T cells at the single cell level, primary CD4+ T cells were subjected to polyclonal stimulation, sorted based on the number of cell divisions they had undergone, and restimulated by ligation of TCR/CD28. We find that individual CD4+ T cells exhibit distinct secondary response patterns that depend upon their prior division history, such that cells that undergo more rounds of division show incrementally greater IL-2 production and proliferation in response to restimulation. CD4+ T cells that fail to divide after activation exist in a profoundly hyporesponsive state that is refractory to both TCR/CD28-mediated and IL-2R-mediated proliferative signals. We find that this anergic state is associated with defects in both TCR-coupled activation of the p42/44 mitogen-activated protein kinase (extracellular signal-related kinase 1/2) and IL-2-mediated down-regulation of the cell cycle inhibitor p27kip1. However, these defects are selective, as TCR-mediated intracellular calcium flux and IL-2R-coupled STAT5 activation remain intact in these cells. Therefore, the process of cell division or cell cycle progression plays an integral role in anergy avoidance in primary T cells, and may represent a driving force in the formation of the effector/memory T cell pool.

CD40 ligand trimer and IL-12 enhance peripheral blood mononuclear cells and CD4+ T cell proliferation and production of IFN-gamma in response to p24 antigen in HIV-infected individuals: potential contribution of anergy to HIV-specific unresponsiveness

It has been suggested that CD4+ T cell proliferative responses to HIV p24 Ag may be important in the control of HIV infection. However, these responses are minimal or absent in many HIV-infected individuals. Furthermore, while in vitro and in vivo responses to non-HIV recall Ags improve upon administration of highly active antiretroviral therapy, there does not appear to be a commensurate enhancement of HIV-specific immune responses. It is possible that CD4+ p24-specific T cells are deleted early in the course of infection. However, it is also possible that a discrete unresponsiveness, or anergy, contributes to the lack of proliferation to p24. To evaluate the possible contribution of unresponsiveness to the lack of CD4+ T cell proliferation to p24 in HIV-infected individuals, we attempted to overcome unresponsiveness. CD40 ligand trimer (CD40LT) and IL-12 significantly increased PBMC and CD4+ T cell proliferative responses to p24 Ag in HIV-infected, but not uninfected, individuals. No increase in proliferative response to CMV Ag was observed. CD40LT exerted its effect through B7-CD28-dependent and IL-12- and IL-15-independent mechanisms. Finally, the increase in proliferation with CD40LT and IL-12 was associated with an augmented production of IFN-gamma in most, but not all, individuals. These data suggest the possible contribution of HIV-specific unresponsiveness to the lack of CD4+ T cell proliferation to p24 Ag in HIV-infected individuals and that clonal deletion alone does not explain this phenomenon. They also indicate the potential for CD40LT and IL-12 as immune-based therapies for HIV infection.

Induction of global anergy rather than inhibitory Th2 lymphokines mediates posttrauma T cell immunodepression

Depressed mitogen-induced IL-2 and IFN-gamma responses after severe mechanical or thermal injury are postulated to result from an expansion of Th2 lymphocytes with concomitant excessive production of IL-4 and/or IL-10. Here, we simultaneously assessed proliferation and Th1 (IFN-gamma) versus Th2 (IL-10, IL-4) lymphokine production in trauma patients' isolated T cells stimulated in a costimulation sufficient, antigen presenting cell independent system (anti CD3 + anti-CD4). T cells with depressed proliferation and IL-2 production simultaneously lost IL-4, IL-10, and IFN-gamma protein and mRNA responses. Exogenous IL-12 addition did not restore IFNgamma responses, but exogenous IL-2 partially restored IL-4, IFN-gamma, and IL-10 production. Although initially partially restored by exogenous IL-2 or stimulation with PMA + ionomycin, patient T cells with persisting anergy progressively lost even these lymphokine and proliferative responses. Development of global T cell anergy was not a result of lost T cell viability or protein synthesis, since it corresponded to predominance of anergic T cells with upregulated expression of CD11b, but downregulated CD28 and CD3 expression. Thus, the subset of posttrauma patients whose isolated T cells become unresponsive experienced progressively worsening global anergy, mediated not by an increased production of Th2 lymphokines, but possibly by T cell incapacity to be activated through TCR triggering or Ca(2+) mobilization.

Induction of T cell anergy in the absence of CTLA-4/B7 interaction

Immunologic tolerance in T lymphocytes is maintained through both thymic and peripheral contributions. One peripheral tolerance mechanism is the induction of T cell anergy, a form of nonresponsiveness resulting from incomplete T cell activation, such as stimulation through the TCR in the absence of costimulation. Recent reports have suggested that engagement of the inhibitory receptor CTLA-4 by its B7 ligand is critical for the initiation of anergy. We tested the importance of CTLA-4 in anergy induction in primary T cells with an in vitro anergy system. Using both CTLA-4/B7-blocking agents and CTLA-4-deficient T cells, we found that T cell anergy can be established in the absence of CTLA-4 expression and/or function. Even in the absence of CTLA-4 signal transduction, T cells activated solely through TCR ligation lose the ability to proliferate as a result of autocrine IL-2 production upon subsequent receptor engagement. Thus, CTLA-4 signaling is not required for the development of T cell anergy.

T-cell receptor-mediated anergy of a human immunodeficiency virus (HIV) gp120-specific CD4(+) cytotoxic T-cell clone, induced by a natural HIV type 1 variant peptide

Human immunodeficiency virus type 1 (HIV-1) infection triggers a cytotoxic T-lymphocyte (CTL) response mediated by CD8(+) and perhaps CD4(+) CTLs. The mechanisms by which HIV-1 escapes from this CTL response are only beginning to be understood. However, it is already clear that the extreme genetic variability of the virus is a major contributing factor. Because of the well-known ability of altered peptide ligands (APL) to induce a T-cell receptor (TCR)-mediated anergic state in CD4(+) helper T cells, we investigated the effects of HIV-1 sequence variations on the proliferation and cytotoxic activation of a human CD4(+) CTL clone (Een217) specific for an epitope composed of amino acids 410 to 429 of HIV-1 gp120. We report that a natural variant of this epitope induced a functional anergic state rendering the T cells unable to respond to their antigenic ligand and preventing the proliferation and cytotoxic activation normally induced by the original antigenic peptide. Furthermore, the stimulation of Een217 cells with this APL generated altered TCR-proximal signaling events that have been associated with the induction of T-cell anergy in CD4(+) T cells. Importantly, the APL-induced anergic state of the Een217 T cells could be prevented by the addition of interleukin 2, which restored their ability to respond to their nominal antigen. Our data therefore suggest that HIV-1 variants can induce a state of anergy in HIV-specific CD4(+) CTLs. Such a mechanism may allow a viral variant to not only escape the CTL response but also facilitate the persistence of other viral strains that may otherwise be recognized and eliminated by HIV-specific CTLs.

T-cell recognition of melanoma-associated antigens

In this review, we summarize the significant progress that has been made in the identification of melanoma-associated antigens (MAA) recognized by cytotoxic T-lymphocytes (CTL). These antigens belong to three main groups: tumor-associated testis-specific antigens (e.g. , MAGE, BAGE, and GAGE); melanocyte differentiation antigens (e.g., tyrosinase, Melan-A/MART-1); and mutated or aberrantly expressed molecules (e.g, CDK4, MUM-1, beta-catenin). Although strong CTL activity may be induced ex vivo against most of these antigens, often in the presence of excess cytokines and antigen, a clear understanding of the functional status of CTL in vivo and their impact on tumor growth, is still lacking. Several mechanisms are described that potentially contribute to tumor cell evasion of the immune response, suggesting that any antitumor efficacy achieved by immune effectors may be offset by factors that result ultimately in tumor progression. Nevertheless, most of these MAA are currently being investigated as immunizing agents in clinical studies, the conflicting results of which are reviewed. Indeed, the therapeutic potential of MAA has still to be fully exploited and new strategies have to be found in order to achieve an effective and long-lasting in vivo immune control of melanoma growth and progression.

Partial T cell activation with an altered superantigenic ligand

T cells have the capacity to respond to ligands as full, weak, partial or null agonists, or indeed as antagonists. In the present paper, it is reported that staphylococcal enterotoxin B (SEB) mutated in a T cell receptor (TCR) contact site (SEBDelta61Y) behaves as an altered ligand for a T cell clone (AC20) that expresses the Vbeta17 TCR. The T cells were partially activated by SEBDelta61Y, as shown by TCR down-modulation and up-regulation of the IL-2 receptor. However, these cells did not secrete IL-2, IL-3, IL-4 or IFN-gamma, nor did they proliferate. Analysis of intracellular protein tyrosine phosphorylation after cellular activation provided further evidence that SEBDelta61Y could transduce a signal via the Vbeta17 TCR. The events following receptor ligation were clearly different when the T cells were stimulated with SEB or SEBDelta61Y, manifested as both quantitatively and qualitatively different patterns of phosphorylation of intracellular substrates. In contrast, only quantitative differences were apparent when a transfectant expressing the same alpha/beta TCR was stimulated with the different superantigens. Together, these results provide the first demonstration that altered TCR ligands are not restricted to peptides substituted at secondary TCR contact residues. Rather, an altered superantigenic ligand mutated in the TCR binding site can behave as a partial agonist.

Anergic T lymphocytes selectively express an integrin regulatory protein of the cytohesin family

It has been proposed that the maintenance of T cell anergy depends on the induction of negative regulatory factors. Differential display of reverse transcribed RNA was used to identify novel genes that might mediate this function in anergic Th1 clones. We report that anergic Th1 clones do indeed express a genetic program different from that of responsive T cells. Moreover, one gene, the general receptor of phosphoinositides 1 (GRP1), was selectively induced in anergic T cells. The GRP1, located in the plasma membrane, regulated integrin-mediated adhesion and was invariably associated with unresponsiveness in multiple models of anergy. T cells expressing retrovirally transduced GRP1 exhibited normal proliferation and cytokine production. However, GRP1-transduced T cells were not stable and rapidly lost GRP1 expression. Thus, although GRP1 may not directly mediate T cell anergy, it regulates cell expansion and survival, perhaps through its integrin-associated activities.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

The −180 Site of the IL-2 Promoter Is the Target of CREB/CREM Binding in T Cell Anergy

Anergic T cells display a marked decrease in their ability to produce IL-2 even in the presence of optimal TCR and costimulatory signals. Using IL-2 enhancer/promoter-driven reporter constructs, we have previously identified a region that appears to be a target for cis transcriptional repression in anergy. This region of the promoter, which shares partial homology with a consensus AP-1-binding sequence, is located about −180 bp from the transcriptional start site. In the present study, we demonstrate that cAMP response element-binding protein/cAMP response element modulator (CREB/CREM), activating transcription factor-2/c-Jun, and Jun-Jun/Oct complexes bind to this site. However, the induction of anergy by prolonged stimulation through the TCR led to an increase in binding of only the CREB/CREM complex. Furthermore, the level of binding of this complex appeared to be up-regulated in both resting and restimulated anergic T cells. Finally, an IL-2 promoter-driven reporter construct that contained a mutation that specifically reduced the binding of the CREB/CREM complex displayed a decreased ability to be affected by anergy, while a construct that contained a mutation that decreased the binding of the Jun-Jun/Oct complex was still susceptible to anergy. These findings suggest that the −180 region of the IL-2 promoter is the target of a CREB/CREM transcriptional inhibitor that contributes to the repression of IL-2 production in T cell anergy.

Effect of the bm12 class II mutation on proliferative and cytokine responses of encephalitogenic T cells in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis

The bm12 mutation in the class II I-A(b)molecule can profoundly influence experimental autoimmune disease, enhancing the development of systemic lupus erythematosus-like syndromes in NZB.H-2(bm12)mice or, conversely, abolishing the susceptibility of C57BL/6J (H-2(b)) mice to the induction of experimental autoimmune myasthenia gravis. We have studied the effect of this mutation on experimental autoimmune encephalomyelitis (EAE), induced in H-2(b)mice by myelin oligodendrocyte glycoprotein (MOG), and recently showed that MOG 35-55 peptide (pMOG 35-55), which represents the immunodominant encephalitogenic region for H-2(b)mice, is also a strong encephalitogen for H-2(bm12)mice. Nevertheless, although the differences in fine epitope specificity and TCR-Vbeta gene usage between encephalitogenic pMOG 35-55-specific T cells from H-2(b)and H-2(bm12)mice were subtle, H-2(bm12)and H-2(b)antigen presenting cells failed to effectively cross-present pMOG 35-55 non-syngeneically to I-A(b)/pMOG 33-55- and I-A(bm12)/pMOG 35-55-specific T cells, respectively. In the present study, we show that the abrogation of the response to pMOG 35-55 by the Th1 encephalitogenic pMOG 35-55-specific T cells upon non-syngeneic cross-presentation is neither due to a cytokine shift to a Th2 pattern, nor a result of anergy induction. Therefore, we suggest that presentation of pMOG 35-55 to I-A(b)/pMOG 35-55-specific T cells via the bm12 class II MHC molecule resulted in ineffective stimulation, similar to a weak agonistic effect.

Differential CD3ζ Phosphorylation Is Not Required for the Induction of T Cell Antagonism by Altered Peptide Ligands

T cells recognize foreign Ags in the form of short peptides bound to MHC molecules. Ligation of the TCR:CD3 complex gives rise to the generation of two tyrosine-phosphorylated forms of the CD3 ζ-chain, pp21 and pp23. Replacement of residues in MHC-bound peptides that alter its recognition by the TCR can generate altered peptide ligands (APL) that antagonize T cell responses to the original agonist peptide, leading to altered T cell function and anergy. This biological process has been linked to differential CD3ζ phosphorylation and generation of only the pp21 phospho-species. Here, we show that T cells expressing CD3ζ mutants, which cannot be phosphorylated, exhibit a 5-fold reduction in IL-2 production and a 30-fold reduction in sensitivity following stimulation with an agonist peptide. However, these T cells are still strongly antagonized by APL. These data demonstrate that: 1) the threshold required for an APL to block a response is much lower than for an agonist peptide to induce a response, 2) CD3ζ is required for full agonist but not antagonist responses, and 3) differential CD3ζ phosphorylation is not a prerequisite for T cell antagonism.

Induction of T Cell Anergy by Low Numbers of Agonist Ligands

Engagement of TCR by its ligand, the MHC/peptide complex, causes T cell activation. T cells respond positively to stimulation with agonists, and are inhibited by antagonist MHC/peptide ligands. Failure to induce proper conformational changes in the TCR or fast TCR/MHC dissociation are the leading models proposed to explain anergy induction by antagonist ligands. In this study, we demonstrate that presentation of between 1 and 10 complexes of agonist/MHC II by unfixed APC induces T cell anergy that persists up to 7 days and has characteristics similar to anergy induced by antagonist ligand or TCR occupancy without costimulation. Furthermore, anergy-inducing doses of hemagglutinin 306–318 peptide led to the engagement of less than 1000 TCR/CD3 complexes. Thus, engagement of a subthreshold number of TCR by either a low density of agonist/MHC or a 2–3 orders of magnitude higher density of antagonist/MHC causes anergy. Moreover, we show that anergy induced by low agonist concentrations is inhibited in the presence of IL-2 or cyclosporin A, suggesting involvement of the calcineurin signaling pathway.

T cell costimulatory blockade: new therapies for transplant rejection

Optimal T cell responses occur when T cells receive both antigen-specific signals through the T cell receptor and non-antigen-specific costimulatory signals through accessory cell surface molecules. The best understood costimulatory receptor is CD28. Signals through the T cell receptor and CD28 cooperatively induce cytokine gene expression and promote T cell proliferation and survival. Negative signals delivered through a related cell surface receptor, cytotoxic T lymphocyte antigen (CTLA-4), act to terminate immune responses and are required for normal immune homeostasis. This article reviews T cell costimulation, including the CD28/CTLA-4 system and other potential costimulatory pathways (such as CD40/CD154), the role of these pathways in normal immune responses, and the potential for the inhibition of these pathways to induce transplantation tolerance.

T-cell anergy induced by clonotype-specific antibodies: modulation of an autoreactive human T-cell clone in vitro

Monoclonal antibodies (mAb) specific for the clonotype of an autoreactive T cell may be useful reagents in the modulation of autoimmune disease. We have previously reported the generation of a set of mAb specific for the clonotypic structure of a human T-cell clone recognizing an epitope of human cartilage gp-39. This glycoprotein was recently identified as a candidate autoantigen in rheumatoid arthritis. Here, we demonstrate for the first time that small amounts of immobilized anticlonotype mAb can induce anergy in the autoreactive clone. Following the anergic stimulus, T cells failed to proliferate upon restimulation as a result of a lack of interleukin-2 (IL-2) gene transcription. In addition, a diminished interferon-gamma (IFN-gamma) production was found. Our data indicate that anergy was not a result of T-cell receptor (TCR) downmodulation or the absence of free TCR. The anergic state was induced independent of costimulation or the presence of IL-2 and no protein synthesis was required for the induction of anergy. Anticlonotype mAb-induced anergy was prevented by cyclosporin A, suggesting that active signalling via the calcium/calcineurin pathway was required for the induction of anergy. In coculture experiments, anergic T cells were found to suppress the response of reactive cells from the same clone. This bystander suppression led to 90% inhibition of peptide-induced proliferation. Together, these findings suggest that mAb to the clonotypic structure of autoreactive T cells may be suitable reagents for the functional inactivation of these T cells in autoimmune diseases.

Inhibition of Cell Cycle Progression by Rapamycin Induces T Cell Clonal Anergy Even in the Presence of Costimulation

Costimulation (signal 2) has been proposed to inhibit the induction of T cell clonal anergy by either directly antagonizing negative signals arising from TCR engagement (signal 1) or by synergizing with signal 1 to produce IL-2, which in turn leads to proliferation and dilution of negative regulatory factors. To better define the cellular events that lead to the induction of anergy, we used the immunosuppressive agent rapamycin, which blocks T cell proliferation in late G1 phase but does not affect costimulation-dependent IL-2 production. Our data demonstrate that full T cell activation (signal 1 plus 2) in the presence of rapamycin results in profound T cell anergy, despite the fact that these cells produce copious amounts of IL-2. Similar to conventional anergy (induction by signal 1 alone), the rapamycin-induced anergic cells show a decrease in mitogen-activated protein kinase activation, and these cells can be rescued by culture in IL-2. Interestingly, the rapamycin-induced anergic cells display a more profound block in IL-3 and IFN-γ production upon rechallenge. Finally, in contrast to rapamycin, full T cell activation in the presence of hydroxyurea (which inhibits the cell cycle in early S phase) did not result in anergy. These data suggest that it is neither the direct effect of costimulation nor the subsequent T cell proliferation that prevents anergy induction, but rather the biochemical events that occur upon progression through the cell cycle from G1 into S phase.

Role of interleukin-2 in superantigen-induced T-cell anergy

T-cell anergy is a state of immunological tolerance characterized by unresponsiveness to antigenic stimulation. Previous studies have shown that anergy is induced in T cells following stimulation in the absence of adequate costimulatory signals. These cells fail to respond to stimulation via the T-cell receptor (TCR), and fail to produce normal levels of interleukin-2 (IL-2). We present results here which show that low concentrations of the superantigen staphylococcal enterotoxin A (SEA) in the absence of antigen-presenting cells induced both proliferation and anergy in the A.E7 T-cell clone. Furthermore, under these conditions, the A.E7 clone remained responsive to exogenous IL-2. Fluorescence-activated cellular cytometry analysis revealed unaltered expression of the TCR/CD3 complex in the anergized clone; however, both CD4 and CD25 expression increased after 24 hr of stimulation by SEA under these conditions. Interestingly, a low level of IL-2 production was measured during the induction of anergy. Most strikingly, stimulation of the A.E7 clone by SEA in combination with exogenous IL-2 resulted in a more pronounced state of anergy. These results suggest that the induction of anergy is a process that is essentially independent of the production of IL-2.