Antigen-specific signaling by a soluble, dimeric peptide/major histocompatibility complex class II/Fc chimera leading to T helper cell type 2 differentiation

Review: Bioengineering strategies to probe T cell mechanobiology

T cells play a major role in adaptive immune response, and T cell dysfunction can lead to the progression of several diseases that are often associated with changes in the mechanical properties of tissues. However, the concept that mechanical forces play a vital role in T cell activation and signaling is relatively new. The endogenous T cell microenvironment is highly complex and dynamic, involving multiple, simultaneous cell-cell and cell-matrix interactions. This native complexity has made it a challenge to isolate the effects of mechanical stimuli on T cell activation. In response, researchers have begun developing engineered platforms that recapitulate key aspects of the native microenvironment to dissect these complex interactions in order to gain a better understanding of T cell mechanotransduction. In this review, we first describe some of the unique characteristics of T cells and the mounting research that has shown they are mechanosensitive. We then detail the specific bioengineering strategies that have been used to date to measure and perturb the mechanical forces at play during T cell activation. In addition, we look at engineering strategies that have been used successfully in mechanotransduction studies for other cell types and describe adaptations that may make them suitable for use with T cells. These engineering strategies can be classified as 2D, so-called 2.5D, or 3D culture systems. In the future, findings from this emerging field will lead to an optimization of culture environments for T cell expansion and the development of new T cell immunotherapies for cancer and other immune diseases.

Bioconjugate Strategies for the Induction of Antigen-Specific Tolerance in Autoimmune Diseases

Antigen-specific immunotherapy (ASI) holds great promise for the treatment of autoimmune diseases. In mice, administration of major histocompatibility complex (MHC) binding synthetic peptides which modulate T cell receptor (TCR) signaling under subimmunogenic conditions induces selective tolerance without suppressing the global immune responses. However, clinical translation has yielded limited success. It has become apparent that the TCR signaling pathway via synthetic peptide antigen alone is inadequate to induce an effective tolerogenic immunity in autoimmune diseases. Bioconjugate strategies combining additional immunomodulatory functions with TCR signaling can amplify the antigen-specific immune tolerance and possibly lead to the development of new treatments in autoimmune diseases. In this review, we provide a summary of recent advances in the development of bioconjugates to achieve antigen-specific immune tolerance in vivo, with the discussion focused on the underlying design principles and challenges that must be overcome to target these therapies to patients suffering from autoimmune diseases.

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

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.

The dendritic cell cytoskeleton promotes T cell adhesion and activation by constraining ICAM-1 mobility

Integrity of the dendritic cell (DC) actin cytoskeleton is essential for T cell priming, but the underlying mechanisms are poorly understood. We show that the DC F-actin network regulates the lateral mobility of intracellular cell adhesion molecule 1 (ICAM-1), but not MHCII. ICAM-1 mobility and clustering are regulated by maturation-induced changes in the expression and activation of moesin and α-actinin-1, which associate with actin filaments and the ICAM-1 cytoplasmic domain. Constrained ICAM-1 mobility is important for DC function, as DCs expressing a high-mobility ICAM-1 mutant lacking the cytoplasmic domain exhibit diminished antigen-dependent conjugate formation and T cell priming. These defects are associated with inefficient induction of leukocyte functional antigen 1 (LFA-1) affinity maturation, which is consistent with a model in which constrained ICAM-1 mobility opposes forces on LFA-1 exerted by the T cell cytoskeleton, whereas ICAM-1 clustering enhances valency and further promotes ligand-dependent LFA-1 activation. Our results reveal an important new mechanism through which the DC cytoskeleton regulates receptor activation at the immunological synapse.

Effects of cellular viscoelasticity in multiple-bond force spectroscopy

Receptor-ligand bonds are often subjected to forces that regulate their detachment via modulating off-rates. Though the dynamics of detachment is primarily controlled by the physical chemistry of adhesion molecules cellular features such as cell deformability and microvillus viscoelasticity have been shown to have an effect on it as well. In this work, Monte Carlo simulation of the rupture of multiple receptor-ligand bonds between substrate and a polymorphonuclear leukocyte (PMN) cell suspended in a Newtonian fluid is performed. It is demonstrated via various micromechanical models of the PMN cell adhered to the substrate by multiple receptor-ligand bonds that viscous drag caused by relative motion of cell suspended in a Newtonian fluid and cellular viscoelasticity modulate transmission of an applied external load to receptor-ligand bonds. It is demonstrated that due to cellular viscoelasticity the instantaneous intermolecular bond force is lower than the instantaneous applied force. It is also demonstrated that due to cellular viscoelasticity, the mean intermolecular bond rupture forces are lowered while the mean bond lifetime increases.

CD8(low)CD28(-) T Cells: A Human CD8 T-Suppressor Subpopulation With Alloantigen Specificity Induced by Soluble HLA-A2 Dimer In Vitro

CD8(+) suppressor T cells have been demonstrated to provide protection of allografts from rejection. We previously reported that soluble peptide/HLA-A2 dimer shows peptide-specific inhibitory effects on alloresponse in a coculture of peptide-pulsed T2 cells with HLA-A2 negative lymphocytes in vitro. Here we found a subset of CD8(low)CD28(-) T cells that was induced in the dimer-treated coculture. Importantly, this population showed hyporesponsiveness to the alloantigen restimulation as well as alloantigen-specific suppression on alloreactive T cells in a cell-cell contact-dependent fashion. The suppressive mechanisms of CD8(low)CD28(-) T cells involved an elevated expression of membrane-bound TGF-β1, but not Foxp3, CTLA-4, or IL-10. Furthermore, an overrepresention of CD8(low)CD28(-) T cells was observed in the patients after allogeneic platelet transfusion and positively correlated with the elevated concentrations of plasma HLA class I antigens. Our findings demonstrated that soluble HLA-A2 dimer could efficiently induce the tolerant CD8(low)CD28(-) T cells with alloantigen-specific suppression on alloreactive T cells. This study might provide a new strategy for preparation of donor-specific suppressor T cells and represent an attractive alternative for induction of allograft tolerance.

Long-term silencing of autoimmune diabetes and improved life expectancy by a soluble pHLA-DR4 chimera in a newly-humanized NOD/DR4/B7 mouse

Several human MHC class II (HLA) molecules are strongly associated with high incidence of autoimmune diseases including type 1 diabetes (T1D). The HLA-humanized mice may thus represent valuable tools to test HLA-based vaccines and therapeutics for human autoimmune diseases. Herein, we have tested the therapeutic potential of a soluble HLA-DR4-GAD65 271-280 (hu DEF-GAD65) chimera of human use in a newly-generated NOD/DR4/B7 double transgenic (dTg) mouse that develops spontaneously an accelerated T1D regardless the gender. The NOD/DR4/B7 dTg mice generated by a two-step crossing protocol express the HLA-DR*0401 molecules on 20% of antigen presenting cells, the human B7 molecules in pancreas, and HLA-DR4/GAD65-specific T-cells in the blood. Some 75% of pre-diabetic NOD/DR4/B7 dTg mice treated with hu DEF-GAD65 chimera remained euglycemic and showed a stabilized pancreatic insulitis 6 months after treatment. The 25% non responders developing hyperglycemia survived 3-4 months longer than their untreated littermates. T1D prevention by this reagent occurred by a Th2/TR-1 polarization in the pancreas. This study strongly suggests that the use of soluble pHLA reagents to suppress/stabilize the T1D progression and to extend the life expectancy in the absence of side effects is an efficient and safe therapeutic approach.

Effect of viscous drag on multiple receptor-ligand bonds rupture force

Monte Carlo simulation of the rupture of multiple receptor-ligand bonds between two PMN cells suspended in a Newtonian fluid is performed. We demonstrate via micro-mechanical model of two cells adhered by multiple receptor-ligand bonds that viscous drag caused by relative motion of cell suspended in a Newtonian fluid modulates transmission of an applied external load to bonds. Specifically, it is demonstrated that at any time the intermolecular bond force is not equivalent to the instantaneous applied force. The difference in the instantaneous applied force and the intermolecular bond force depends on the viscosity of fluid, the size of cell, the applied loading rate, and the number of bonds at any instant of time. Viscous drag acting on cell reduces average bond rupture forces.

A theoretical method to determine unstressed off-rate from multiple bond force spectroscopy

Using dynamic force spectroscopy to measure the kinetic off-rates of intermolecular bonds currently requires the isolation of single molecules. This requirement arises in part because no tractable analytic method for determining kinetic off-rates from the rupture of a large number of bonds under dynamic forces is currently available. We introduce a novel method for determining the unstressed off-rate from dynamic force spectroscopy experiments involving a large number of bonds. Using both the Bell and Dembo models we show that the unstressed off-rate calculated using the proposed method is in good agreement with the prescribed unstressed off-rate used in Monte-Carlo simulations of multiple bond dynamic force spectroscopy experiments given initial number of bonds (50-500) and loading rate 10(3)-10(6)pN/s.

Dimeric MHC-peptides inserted into an immunoglobulin scaffold as new immunotherapeutic agents

The interactions of the T cell receptor (TCR) with cognate MHC-peptide and co-stimulatory molecules expressed at surface of antigen presenting cells (APC) leads to activation or tolerance of T cells. The development of molecular biological tools allowed for the preparation of soluble MHC-peptide molecules as surrogate for the APC. A decade ago a monomeric class II MHC molecule in which the peptide was covalently linked to β-chain of class II molecule was generated. This type of molecule had a low-binding affinity and did not cause the multimerization of TCR. The requirement of multimerization of TCR led to development of a new class of reagents, chimeric peptides covalently linked to MHC that was dimerized via Fc fragment of an immunoglobulin and linked to 3' end of the β-chain of MHC class II molecule. These soluble dimerized MHC-peptide chimeric molecules display high affinity for the TCR and caused multimerization of TCR without processing by an APC. Because dimeric molecules are devoid of co-stimulatory molecules interacting with CD28, a second signal, they induce anergy rather the activation of T cells. In this review, we compare the human and murine dimerized MHC class II-peptides and their effect on CD4(+) T cells, particularly the generation of T regulatory cells, which make these chimeric molecules an appealing approach for the treatment of autoimmune diseases.

Signaling clusters in the cell membrane

Large-scale molecular assemblies, or signaling clusters, at the cell membrane are emerging as important regulators of cell signaling. Here, we review new findings and describe shared characteristics common to signaling clusters from a diverse set of cellular systems. The well-known T cell receptor cluster serves as our paradigmatic model. Specifically, each cluster initiates recruitment of hundreds of molecules to the membrane, interacts with the actin cytoskeleton, and contains a significant fraction of the entire signaling process. Probed by recent advancements in patterning and microscopy techniques, the signaling clusters display functional outcomes that are not readily predictable from the individual components.

T cell receptor triggering by force

Antigen recognition through the interaction between the T cell receptor (TCR) and peptide presented by major histocompatibility complex (pMHC) is the first step in T cell-mediated immune responses. How this interaction triggers TCR signalling that leads to T cell activation is still unclear. Taking into account the mechanical stress exerted on the pMHC-TCR interaction at the dynamic interface between T cells and antigen presenting cells (APCs), we propose the so-called receptor deformation model of TCR triggering. In this model, TCR conformational change induced by mechanical forces initiates TCR signalling. The receptor deformation model, for the first time, explains all three aspects of the TCR triggering puzzle: mechanism, specificity, and sensitivity.

Suppression of ongoing T cell-mediated autoimmunity by peptide-MHC class II dimer vaccination

Tissue-specific autoimmune diseases such as type 1 diabetes (T1D) are characterized by T cell-driven pathology. Administration of autoantigenic peptides provides a strategy to selectively target the pathogenic T cell response. Indeed, treatment with beta cell peptides effectively prevents T1D in NOD mice. However, the efficacy of peptide immunotherapy generally wanes as beta cell autoimmunity progresses and islet inflammation increases. With the goal of enhancing the efficacy of peptide immunotherapy, soluble (s)IA(g7)-Ig dimers covalently linked to beta cell autoantigen-derived peptides were tested for the capacity to suppress late preclinical T1D. NOD female mice with established beta cell autoimmunity were vaccinated i.v. with a short course of sIA(g7)-Ig dimers tethered to peptides derived from glutamic acid decarboxylase (GAD)65 (sIA(g7)-pGAD65). Treatment with sIA(g7)-pGAD65 dimers and the equivalent of only approximately 7 microg of native peptide effectively blocked the progression of insulitis and the development of diabetes. Furthermore, suppression of T1D was dependent on beta cell-specific IL-10-secreting CD4+ T cells, although the frequency of GAD65-specific FoxP3-expressing CD4+ T cells was also increased in sIA(g7)-pGAD65 dimer vaccinated NOD mice. These results demonstrate that MHC class II-Ig dimer vaccination is a robust approach to suppress ongoing T cell-mediated autoimmunity, and may provide a superior strategy of adjuvant-free peptide-based immunotherapy to induce immunoregulatory T cells.

A peptide-major histocompatibility complex II chimera favors survival of pancreatic beta-islets grafted in type 1 diabetic mice

BACKGROUND

Transplantation of pancreatic islets showed a tremendous progress over the years as a promising, new therapeutic strategy in patients with type 1 diabetes. However, additional immunosuppressive drug therapy is required to prevent rejection of engrafted islets. The current immunosuppressive therapies showed limited success in maintaining long-term islet survival as required to achieve insulin independence in type 1 diabetes, and they induce severe adverse effects. Herein, we analyzed the effects of a soluble peptide-major histocompatibility complex (MHC) class II chimera aimed at devising an antigen-specific therapy for suppression of anti-islet T cell responses and to improve the survival of pancreatic islets transplants.

METHODS

Pancreatic islets from transgenic mice expressing the hemagglutinin antigen in the beta islets under the rat insulin promoter (RIP-HA) were grafted under the kidney capsule of diabetic, double transgenic mice expressing hemagglutinin in the pancreas and T cells specific for hemagglutinin (RIP-HA, TCR-HA). The recipient double transgenic mice were treated or not with the soluble peptide-MHC II chimera, and the progression of diabetes, graft survival, and T cell responses to the grafted islets were analyzed.

RESULTS

The peptide-MHC II chimera protected syngeneic pancreatic islet transplants against the islet-reactive CD4 T cells, and prolonged the survival of transplanted islets. Protection of transplanted islets occurred by polarization of antigen-specific memory CD4 T cells toward a Th2 anti-inflammatory response.

CONCLUSIONS

The peptide-MHC II chimera approach is an efficient and specific therapeutic approach to suppress anti-islet T cell responses and provides a long survival of pancreatic grafted islets.

Surface-anchored monomeric agonist pMHCs alone trigger TCR with high sensitivity

At the interface between T cell and antigen-presenting cell (APC), peptide antigen presented by MHC (pMHC) binds to the T cell receptor (TCR) and initiates signaling. The mechanism of TCR signal initiation, or triggering, remains unclear. An interesting aspect of this puzzle is that although soluble agonist pMHCs cannot trigger TCR even at high concentrations, the same ligands trigger TCR very efficiently on the surface of APCs. Here, using lipid bilayers or plastic-based artificial APCs with defined components, we identify the critical APC-associated factors that confer agonist pMHCs with such potency. We found that CD4⁺ T cells are triggered by very low numbers of monomeric agonist pMHCs anchored on fluid lipid bilayers or fixed plastic surfaces, in the absence of any other APC surface molecules. Importantly, on bilayers, plastic surfaces, or real APCs, endogenous pMHCs did not enhance TCR triggering. TCR triggering, however, critically depended upon the adhesiveness of the surface and an intact T cell actin cytoskeleton. Based on these observations, we propose the receptor deformation model of TCR triggering to explain the remarkable sensitivity and specificity of TCR triggering.

Using the T cell receptor (TCR) as a sensor, T cells of the immune system constantly migrate in lymphoid organs and probe the surface of antigen-presenting cells (APCs) for foreign antigens, a sign of pathogen infection. Antigen binding by TCRs leads to T cell activation and subsequent immune response to combat the pathogens. Interestingly, although T cells respond well to antigens on APCs, they do not recognize the same antigens in solution. What is it that makes antigens on APCs recognizable? To address this, we used lipid bilayers and plastic surfaces to construct artificial APCs with defined antigen number, composition, and configuration. We found that T cells respond to very few individual foreign antigens on artificial APCs, and contrary to some current opinion, formation of antigen clusters on APCs is not required for antigen recognition by T cells. TCR triggering, however, requires T cell adhesion to the APC surface and then occurs only if the T cells are able to move. We propose that at the dynamic T cell–APC interface, antigen on APCs activates T cells by applying force to the TCR and deforming its structure, which cannot be achieved by soluble antigens due to their lack of anchorage.

Why is it that T cells are blind to antigens in solution but highly sensitive to antigens anchored on a surface? The authors show that this is not due to antigen clustering, but could involve mechanical forces associated with cell locomotion.

Monitoring antigen-specific T cells using MHC-Ig dimers

The lack of high affinity reagents has made distinguishing T cells on the basis of antigen specificity difficult to accomplish. This unit provides protocols that utilize innovations in molecular design to permit construction of soluble multivalent MHC complexes (MHC-Ig dimers) with high avidity for cognate T cell receptors. MHC-Ig dimers display stable binding properties when they interact with antigen-specific T cells thus allowing their use in the staining of antigen-specific T cells by flow cytometry. Methods for constructing and detecting these MHC-Ig dimers are included along with protocols for applying their use for the quantitation of antigen-specific T cells.

TCR triggering by the pMHC complex: valency, affinity, and dynamics

The interaction between the T cell receptor (TCR) and a peptide-loaded major histocompatibility complex (pMHC) is one of the most-studied interactions in immunology, and yet the precise mechanism by which this system operates is still not fully understood. One key issue is whether TCR triggering minimally requires monomeric pMHC complexes or higher-order multimers (two or more pMHCs). Any model of TCR triggering must explain the high sensitivity, specificity, and dynamic range of ligand responsiveness that this receptor system exhibits. Most models of TCR triggering have not fully appreciated the dynamic aspects of TCR triggering. TCR triggering happens very quickly, and the properties of sensitivity and specificity can be explained by a model that accounts for the interaction dynamics of such a receptor system. In this paper, it is proposed that the important parameter in TCR triggering is the immobilization of the TCR-pMHC complex in the plasma membrane. Whether this involves monomeric or multimeric pMHCs may depend on the affinity of the TCR for the pMHC.

The receptor deformation model of TCR triggering

Through T cell receptors (TCRs), T cells can detect and respond to very small numbers of foreign peptides among a huge number of self-peptides presented by major histocompatibility complexes (pMHCs) on the surface of antigen-presenting cells (APCs). How T cells achieve such remarkable sensitivity and specificity through pMHC-TCR binding is an intensively pursued issue in immunology today; the key question is how pMHC-TCR binding initiates, or triggers, a signal from TCRs. Multiple competing models have been proposed, none of which fully explains the sensitivity and specificity of TCR triggering. What has been omitted from existing theories is that the pMHC-TCR interaction at the T cell/APC interface must be under constant mechanical stress, due to the dynamic nature of cell-cell interaction. Taking this condition into consideration, we propose the receptor deformation model of TCR triggering. In this model, TCR signaling is initiated by conformational changes of the TCR/CD3 complex, induced by a pulling force originating from the cytoskeleton and transmitted through pMHC-TCR binding interactions with enough strength to resist rupture. By introducing mechanical force into a model of T cell signal initiation, the receptor deformation model provides potential mechanistic solutions to the sensitivity and specificity of TCR triggering.

Multimerized T cell epitopes protect from experimental autoimmune diabetes by inducing dominant tolerance

Immunotherapy by using multimerized self-peptides has demonstrated a clear protective effect on experimental models of autoimmune diseases. However, the mechanisms involved remain ill-defined. Here we have evaluated the therapeutic efficacy of multimerized self-peptides at the effector phase of autoimmune diabetes and examined their mechanisms of action. Diabetes was induced in rat insulin promoter-hemagglutinin (HA) mice expressing HA in pancreatic beta-cells by adoptive transfer of HA(110-119)-specific T helper 1 cells. Complete protection was provided by low doses of the HA 4-mer consisting of four covalently linked linear HA(107-119) peptides. In vivo, the 4-mer appeared to act directly on the pathogenic HA-specific T helper 1 cells and indirectly by activation/recruitment of lymphocytes with regulatory properties so that mice became resistant to a second transfer of diabetogenic T cells. This effect was associated with a recruitment of Foxp3(+) CD4 T cells around islets. Moreover, we show that dominant protection from autoimmunity was transferable by spleen cells, and that development of this regulatory population was crucially dependent on the lymphocytes from treated rat insulin promoter-HA mice. Thus, immunotherapy using multimerized epitopes emerges as a promising strategy in view of the current identification of self-epitopes that are major targets of the pathogenic CD4 T cell response in autoimmune diseases.

A divalent human leukocyte antigen-B7 fusion-protein up-regulates CD25 and CD69 in alloreactive CD8+ T cells bypassing CD28 costimulation

BACKGROUND

T cells recognize major histocompatibility complex (MHC) molecules and their cryptic antigenic peptides on antigen-presenting cells and are generally triggered to proliferate, and when sufficient, co-stimulation is available. In soluble form, monomeric MHC molecules can induce apoptosis, anergy, or decreases of the T-cell receptor (TCR).

METHODS

A dimeric fusion protein of the human leukocyte antigens (HLA)-B7 was molecularly engineered and expressed in a B-cell line to allow secretion. Alloreactive T cells were generated according to the standard protocol.

RESULTS

A dimer of approximately 160 kD was obtained, affinity purified, and used to study T-cell interaction. In immobilized form, this protein efficiently stimulated alloreactive T cells to proliferate and produce interleukin (IL)-2 and interferon (IFN)-gamma in a concentration-dependent manner, up-regulating CD25 and CD69 expression. In contrast, the soluble fusion protein induced T-cell apoptosis.

CONCLUSIONS

The dichotomy in T-cell regulation by a divalent MHC fusion protein warrants the use of MHC multimers as custom-designed immune-regulatory molecules both in transplantation and autoimmune disease.

Soluble MHC-peptide complexes containing long rigid linkers abolish CTL-mediated cytotoxicity

Soluble MHC-peptide (pMHC) complexes induce intracellular calcium mobilization, diverse phosphorylation events, and death of CD8+ CTL, given that they are at least dimeric and co-engage CD8. By testing dimeric, tetrameric, and octameric pMHC complexes containing spacers of different lengths, we show that their ability to activate CTL decreases as the distance between their subunit MHC complexes increases. Remarkably, pMHC complexes containing long rigid polyproline spacers (> or =80 A) inhibit target cell killing by cloned S14 CTL in a dose- and valence-dependent manner. Long octameric pMHC complexes abolished target cell lysis, even very strong lysis, at nanomolar concentrations. By contrast, an altered peptide ligand antagonist was only weakly inhibitory and only at high concentrations. Long D(b)-gp33 complexes strongly and specifically inhibited the D(b)-restricted lymphocytic choriomeningitis virus CTL response in vitro and in vivo. We show that complications related to transfer of peptide from soluble to cell-associated MHC molecules can be circumvented by using covalent pMHC complexes. Long pMHC complexes efficiently inhibited CTL target cell conjugate formation by interfering with TCR-mediated activation of LFA-1. Such reagents provide a new and powerful means to inhibit Ag-specific CTL responses and hence should be useful to blunt autoimmune disorders such as diabetes type I.

Strength of multiple parallel biological bonds

Multivalent interactions play a critical role in a variety of biological processes on both molecular and cellular levels. We have used molecular force spectroscopy to investigate the strength of multiple parallel peptide-antibody bonds using a system that allowed us to determine the rupture forces and the number of ruptured bonds independently. In our experiments the interacting molecules were attached to the surfaces of the probe and sample of the atomic force microscope with flexible polymer tethers, and the unique mechanical signatures of the tethers determined the number of ruptured bonds. We show that the rupture forces increase with the number of interacting molecules and that the measured forces obey the predictions of a Markovian model for the strength of multiple parallel bonds. We also discuss the implications of our results to the interpretation of force spectroscopy measurements in multiple bond systems.

CD8 T cells, like CD4 T cells, are triggered by multivalent engagement of TCRs by MHC-peptide ligands but not by monovalent engagement

T cell activation is initiated by recognition of antigenic peptide presented in complex with MHC molecules on the surface of APCs. The mechanism by which this recognition occurs is still unclear, and many models exist in the literature. CD4 T cells have been shown to respond to soluble oligomers of activating class II MHC-peptide complexes, but not to soluble monomers. In determining the reactivity of CD8 T cells to soluble activating class I MHC-peptide complexes, a complicating phenomenon had been observed whereby peptide from soluble complexes was loaded onto cell surface MHCs on the T cells and re-presented to other T cells, clouding the true valency requirement for activation. This study uses soluble allogeneic class I MHC-peptide monomers and oligomers to stimulate murine CD8 T cells without the possible complication of peptide re-presentation. The results show that MHC class I monomers bind to, but do not activate, CD8 T cells whether the cells are in solution or adhered to a surface. Monomeric MHC class I binding can antagonize the stimulation triggered by soluble oligomers, a phenomenon also observed for CD4 T cells. Dimeric engagement is necessary and sufficient to stimulate downstream activation processes including TCR down-regulation, Zap70 phosphorylation, and CD25 and CD69 up-regulation, even in T cells that do not express the MHC coreceptor CD8. Thus, the valency dependence of the response of CD8 T cells to soluble MHC-peptide reagents is the same as previously observed for CD4 T cells.

Targeting T lymphocytes for immune monitoring and intervention in autoimmune diabetes

Recognition of a peptide-MHC complex by the T cell receptor (TCR) is a key interaction that initiates T lymphocyte activation or silencing during an immune response. Fluorochrome-labeled recombinant MHC class II-peptide reagents function as soluble mimetics of this interaction, bind to their specific TCR, and allow for detection of antigen-specific CD4+ T cells. These reagents are now under scrutiny for "immune staging" of patients at risk of type 1 diabetes, in an effort to diagnose islet autoimmunity early enough to block immune-mediated beta cell destruction. Several issues are currently being addressed to improve the performance of these T cell assays: enrichment steps for better sensitivity, multiplexing of several islet epitopes, simultaneous monitoring of CD4+ and CD8+ responses, detection of low avidity T cells, combination of quantitative (number of positive cells) and qualitative (cytokine secretion, naive/memory phenotype) readouts. CD4+ T cells are key effectors of autoimmunity, and these MHC class II peptide reagents, through their signaling properties, might also provide therapeutics to block the autoimmune process at its onset, analogous to the use of OKT3gammao1(AlaAla) anti-CD3 antibody but in an antigen-specific fashion. The aim of such therapeutics is to potentiate different physiological control mechanisms to restore immune tolerance. Mechanisms initiated by this pathway may be capable of triggering elimination of pathogenic T cells through antigen-specific apoptosis and anergy, combined with the induction of regulatory T cells with broad suppressive function.

Soluble, dimeric HLA DR4-peptide chimeras: an approach for detection and immunoregulation of human type-1 diabetes

Still there are no effective methods to predict or cure type 1 diabetes (T1D) in humans. Soluble, dimeric MHC class II-peptide (DEF) chimeras have potential for both early diagnosis and immunospecific therapy. DEF chimeras prevent and reverse diabetes in mice by stimulating antigen-specific type 1 T regulatory cell (Tr1)-like cells. We also showed that diabetes could be predicted by changes in the phenotype of autoreactive CD4 T cells in peripheral blood. Herein, we demonstrated that human DEF (HLA-DR*0401/Fcgamma1) chimeras expressing peptides of beta-cell antigens stimulate Tr1-like cells in blood of patients with T1D, non-diabetic relatives, and controls. Furthermore, the specific and stable binding of DEF chimeras to cognate TCR and CD4 coreceptor allowed quantification and phenotyping of autoreactive CD4 T cells in non-stimulated blood by FACS. Our results indicate that (1) autoreactive CD4 T cells to GAD65 autoantigen are commonly present in humans expressing diabetes-susceptible HLA-DR*0401 molecules; (2) these autoreactive T cells undergo avidity maturation upon encountering the self antigen early in life; (3) the disease is associated with an imbalance between autoreactive CD4+CD25+ and CD4+CD69+ T cells specific for GAD65. Based on this, we propose a model to explain the kinetics of autoreactive CD4 T cells in blood during the natural history of T1D.

Efficacy of clonal deletion vs. anergy of self-reactive CD4 T-cells for the prevention and reversal of autoimmune diabetes

The self-reactive CD4 T-cells play an essential role in triggering and sustaining organ-specific autoimmune diseases. Silencing or elimination of these cells can prevent and reverse an autoimmune process. We have previously showed that a single dose-administration of a soluble dimeric MHC II-peptide chimera (DEF) in double-transgenic mice delayed the onset autoimmune diabetes, and restored the euglycemia in already diabetic mice for a period of 1 week. DEF dimer protection relied on induction of anergy of diabetogenic CD4 T-cells in spleen, and stimulation of IL-10-secreting T regulatory type 1 cells in pancreas. Herein, we show that an octameric form of DEF has doubled the period of protection and reversal of disease by clonal deletion of diabetogenic CD4 T-cells in both the thymic and peripheral compartments. Deletion occurred by activation-induced cell death subsequent to repartitioning and signaling of FAS-FADD apoptotic module in the plasma membrane lipid rafts. Our previous and present data indicated first, that DEF valence translates into various effects on the antigen-specific CD4 T-cells, i.e., Th2 immune deviation, anergy, and apoptosis. Second, the present findings argue for a better efficacy of clonal deletion than anergy of diabetogenic CD4 T-cells for the protection and reversal of autoimmune diabetes.

Functional avidity directs T-cell fate in autoreactive CD4+ T cells

Major histocompatibility complex class II tetramer staining and activation analysis identified 2 distinct types of antigen-specific CD4+ T cells in the peripheral blood of humans with type 1 (autoimmune) diabetes. T cells with low-avidity recognition of peptide-MHC ligands had low sensitivity to activation and inefficient activation-induced apoptosis. In contrast, high-avidity T cells were highly sensitive to antigen-induced cell death through apoptotic mechanisms, and both apoptosis-resistant high- and low-avidity T cells that survived prolonged tetramer treatment were rendered anergic to restimulation by antigen. In addition, however, apoptosis-resistant high-avidity T cells acquired regulatory features, being able to suppress both antigen-specific and nonspecific CD4+ T-cell responses. This suppression was contact-dependent and correlated with the down-regulation of HLA class II and costimulatory molecules on antigen-presenting cells, including B cells and dendritic cells. T cells face a variety of fates following antigen exposure, including the paradoxic maintenance of high-avidity autoreactive T cells in the peripheral circulation, perhaps due to this capability of acquiring anergic and suppressive properties. Regulation via down-modulation of antigen-presenting cell function, a form of cell-to-cell licensing for suppression, also offers possibilities for the application of peptide-MHC therapeutics.

Tetramer analysis of human autoreactive CD4-positive T cells

Self-reactivity is an intrinsic property of the human immune system. Autoreactive T cells derive directly from the developmental requirement for TCR engagement by self-antigens during lymphocyte maturation. The fundamental questions implicating these autoreactive cells in human autoimmunity then, are not "Where do they come from?", but rather "Why do they persist?", "How do they become activated?", and "How are they regulated or deleted?". New technologies, in which peptide-MHC (pMHC) ligands used for T-cell recognition are utilized as soluble fluorescent multimers, now permit the direct visualization of antigen-specific autoreactive T-lymphocytes. By using multimer technology to study self-reactive cells present in autoimmune patients and control individuals, a very broad range of autoreactive potential has been identified.

Immunokinetics of autoreactive CD4 T cells in blood: a reporter for the “hit-and-run” autoimmune attack on pancreas and diabetes progression

Little is known about the fate of autoreactive CD4 T cells in blood. Using a mouse model for spontaneous autoimmune diabetes we demonstrated that the status of the autoimmune process in pancreas could be pictured through the frequency and phenotype of autoreactive CD4 T cells in the blood. Early during the prediabetic stage, the frequency of these cells in blood decreased as a consequence of their recruitment in the pancreas. This was followed by an imbalance between CD4(+)CD25(+) and CD4(+)CD69(+) T cells in the pancreas that was mirrored in the phenotype of autoreactive T cells in the blood. Waves of activated CD4(+)CD69(+) T cells in blood preceded the disease onset suggesting that the autoimmune attack on pancreas is a discontinuous "hit-and-run" rather than a continuous process. Tracking autoreactive CD4 T cells in blood may help in identifying prediabetic humans and monitoring the disease progression during therapeutic interventions.

Differential recognition and activation thresholds in human autoreactive GAD-specific T-cells

The activation requirements of autoreactive CD4(+) T-cells were investigated in GAD65-specific HLA-DR0401-restricted clones derived from a diabetic patient using major histocompatibility complex (MHC) class II tetramers (TMrs) as stimulating agents. Despite the fact that TMrs loaded with an immunodominant-altered GAD peptide (TMr-GAD) bound a limited number of T-cell receptors, they were capable of efficiently delivering activation signals. These signals ranged from the early steps of phospholipase C (PLC)-gamma(1) phosphorylation and Ca(2+) mobilization to more complex events, such as CD69 upregulation, cytokine mRNA transcription and secretion, and proliferation. All the effects triggered by TMr-GAD were dose dependent. On the contrary, [(3)H]-thymidine incorporation decreased at high TMr-GAD concentrations because of activation-induced cell death (AICD) after initial proliferation. Lower-avidity clones (as defined by TMr-GAD binding) were less sensitive to activation as well as less susceptible to AICD compared with higher-avidity clones. Induction of apoptosis is a potential immunomodulatory target for therapeutic applications of MHC class II multimers, but the relative resistance of low-avidity T-cells may limit its benefits.

MHC Class II tetramers and the pursuit of antigen-specific T cells: define, deviate, delete

Selective expansion and activation of a very small number of antigen-specific CD4(+) T cells is a remarkable and essential property of the adaptive immune response. Antigen-specific T cells were until recently identified only indirectly by functional assays, such as antigen-induced cytokine secretion and proliferation. The advent of MHC Class II tetramers has added a pivotal tool to our research armamentarium, allowing the definition of allo- and autoimmune responses in deeper detail. Rare antigen-specific CD4(+) cells can now be selectively identified, isolated and characterized. The same tetramer reagents also provide a new mean of stimulating T cells, more closely reproducing the MHC-peptide/TCR interaction. This property allows the use of tetramers to direct T cells toward the more desirable outcome, that is, activation (in malignancies and infectious diseases) or Th2/T regulatory cell deviation, anergy and deletion (in autoimmune diseases). These experimental approaches hold promise for diagnostic, prognostic and therapeutic applications.

Peptide-MHC Class II Dimers as Therapeutics to Modulate Antigen-Specific T Cell Responses in Autoimmune Diabetes

Type 1 diabetes is an autoimmune disorder caused by autoreactive T cells that mediate destruction of insulin-producing beta cells of the pancreas. Studies have shown that T cell tolerance can be restored by inducing a partial or altered signal through the TCR. To investigate the potential of bivalent peptide-MHC class II/Ig fusion proteins as therapeutics to restore Ag-specific tolerance, we have developed soluble peptide I-A(g7) dimers for use in the nonobese diabetic mouse model of diabetes. I-A(g7) dimers with a linked peptide specific for islet-reactive BDC2.5 TCR transgenic CD4(+) T cells were shown to specifically bind BDC2.5 T cells as well as a small population of Ag-specific T cells in nonobese diabetic mice. In vivo treatment with BDC2.5 peptide I-A(g7) dimers protected mice from diabetes mediated by the adoptive transfer of diabetogenic BDC2.5 CD4(+) T cells. The dimer therapy resulted in the activation and increased cell death of transferred BDC2.5 CD4(+) T cells. Surviving cells were hypoproliferative to challenge by Ag and produced increased levels of IL-10 and decreased levels of IFN-gamma compared with cells from control I-A(g7) dimer-treated mice. Anti-IL-10R therapy reversed the tolerogenic effects of the dimer. Thus, peptide I-A(g7) dimers induce tolerance of BDC2.5 TCR T cells through a combination of the induction of clonal anergy and anti-inflammatory cytokines.

Exosomes as a tumor vaccine: enhancing potency through direct loading of antigenic peptides

Exosomes secreted by dendritic cells (DCs) contain MHC-I, MHC-II, and other accessory molecules required for antigen presentation to T cells. Previous studies have shown that exosome MHC-I "indirectly" loaded by adding peptides to DC cultures are immunogenic. However, analysis of peptide binding was not performed to link T-cell-stimulating activity with the amount of MHC-I/peptide complexes on the exosomes. In this study, we measured peptide binding to MHC-I under different loading conditions and tested the exosomes' potencies in T-cell activation assays. We demonstrate that MHC-I on purified exosomes can be directly loaded with peptide at much greater levels than indirect loading. The direct loading method performed in mildly acidic conditions was effective even in the absence of exogenous beta2m. This increase in peptide binding greatly enhanced exosome potency, allowing us to further study the biologic activity of exosomes in vitro. In the presence of antigen-presenting cells (APC), exosomes directly loaded with the HLA-A2 restricted MART1 tumor peptide stimulated an HLA-A2/MART1 specific T-cell line. The T cells responded to exosomes using HLA-A2neg APC, demonstrating transfer of functional MHC-I/peptide complexes and not peptide alone to APC. MHC-II molecules, which are abundantly expressed on DC exosomes, were also functionally loaded under the same conditions as MHC-I. This feature allows for delivery of multiple peptide antigens that can stimulate both CD8+ cytotoxic T cells as well CD4+ T helper cells critical for an effective antitumor response. The optimized loading conditions and the ability to transfer both MHC-I and MHC-II antigens to APC have led to the development of exosomes as an "acellular" immunotherapy approach currently being tested in clinical trials.

A model for antigen-specific T-cell anergy: displacement of CD4-p56(lck) signalosome from the lipid rafts by a soluble, dimeric peptide-MHC class II chimera

Soluble, dimeric peptide-MHC chimeras were shown to induce Ag-specific T cell anergy in vitro and in vivo. In this study, we describe a mechanism by which a soluble, dimeric peptide MHC class II chimera (DEF) induces Ag-specific T cell anergy. The anergic cells showed a displacement of the CD4-p56(lck) signaling module from the GM1-rich plasma membrane microdomains (lipid rafts), and subsequently an increase in p59(fyn) kinase activity, a dominant expression of p21 inhibitory TCR zeta-chain, and a poor phosphorylation and recruitment of zeta-associated protein of 70 kDa kinase to the TCR's immunoreceptor tyrosine-based activation motifs. The Th1 and Th2 transcription was suppressed and the cells were arrested in the Th0 stage of differentiation. Recovery from DEF anergy occurred late and spontaneously at the expense of low thresholds for activation-induced cell death. In contrast to DEF, a combination of TCR and CD4 mAbs did not induce such alterations or anergy, indicating that the ligand-mediated topology of TCR and CD4 coengagement can differentially affect the T cell function. Our results argue for a model of anergy in which the defective partitioning of signaling molecules in lipid rafts is an early, negative signaling event in T cells. Physiological ligands like DEF chimeras may provide new tools for silencing the autoimmune processes, and may also help in deciphering new mechanisms of negative regulation in T cells.

Study of the mechanism of TCR antagonism using dual-TCR-expressing T cells

The mechanism of action of TCR antagonists is incompletely understood. T cells expressing two distinct TCRs have been used to test competition for TCR occupancy as a potential mechanism. Previous studies with CD4 T cells showed that an antagonist for one TCR inhibited the response to the other TCR (cross-antagonism), whereas studies with CD8 cells failed to demonstrate cross-antagonism. To determine whether CD4 and CD8 cells were intrinsically different or whether the differences were the result of the use of different effector assays, we studied both CD4 and CD8 dual-TCR-expressing T cells. In the CD4 system, consistent with previous reports, cross-antagonism of proliferation was observed. In the CD8 system, cross-antagonism was observed using proliferation as readout but not when target cell cytolysis was used. These results suggest that different mechanisms may be involved in the inhibition of proliferation and inhibition of cytotoxic effector function, the latter only involving competition for TCR occupancy. Inhibition of proliferation appears to be more complex and other mechanisms such as sequestration of signaling molecules or negative signaling may be involved. The fact that 10- to 20-fold more antagonist was needed to achieve cross-antagonism compared with inhibition of the cognate TCR is consistent with the hypothesis that competition for TCR occupancy is also a major, albeit not sole, mechanism of antagonism of the proliferative responses of CD4 and CD8 cells.

Soluble peptide-MHC monomers cause activation of CD8+ T cells through transfer of the peptide to T cell MHC molecules

T cell receptor (TCR)-mediated activation of CD4(+) T cells is known to require multivalent engagement of the TCR by, for example, oligomeric peptide-MHC complexes. In contrast, for CD8(+) T cells, there is evidence for TCR-mediated activation by univalent engagement of the TCR. We have here compared oligomeric and monomeric L(d) and K(b) peptide-MHC complexes and free peptide as stimulators of CD8(+) T cells expressing the 2C TCR. We found that the monomers are indeed effective in activating naive and effector CD8(+) T cells, but through an unexpected mechanism that involves transfer of peptide from soluble monomers to T cell endogenous MHC (K(b)) molecules. The result is that T cells, acting as antigen-presenting cells, are able to activate other naive T cells.

Generation and use of alternative multimers of peptide/MHC complexes

For many years, the detection of antigen-specific T cells has relied on indirect in vitro assays such as cytokine secretion, proliferation or chromium release assays. Things have dramatically changed during the past few years, thanks to the imagination of several investigators who have developed very elegant strategies to produce multivalent peptide/MHC complexes. One of these strategies has been to produce peptide-loaded monomeric biotinylated MHC molecules, which could be obtained as tetramers upon incubation with tetravalent streptavidin. Although this latter approach has been by far the most popular, this review focuses on other strategies which have also been successful.

Immunotherapy of insulin-dependent diabetes mellitus

Type 1 diabetes mellitus is caused by the T cell mediated autoimmune destruction of insulin-producing beta cells of the islets of Langerhans within the pancreas. Current immunotherapy strategies are aimed at directly inactivating the autoreactive T cells and/or inducing T cells with regulatory capabilities. At the preclinical level, several strategies that employ TCR antagonists -- including monoclonal antibodies, autoantigen-specific peptides and soluble TCR ligands -- are showing promise and being developed for clinical application. Several of these approaches employing monoclonal antibodies against the TCR-CD3 complex or soluble peptide antigens are producing favorable results in the clinic.

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.

Down-regulation of diabetogenic CD4+ T cells by a soluble dimeric peptide-MHC class II chimera

Type 1 diabetes is an organ-specific autoimmune disease that is mediated by autoreactive T cells. We show here that administration of a soluble dimeric peptide-major histocompatibility complex (pMHC) class II chimera (DEF) to prediabetic double-transgenic mice prevents the onset of disease or, in animals that are already diabetic, restores normoglycemia. The antidiabetogenic effects of DEF rely on the induction of anergy in splenic autoreactive CD4+ T cells via alteration of early T cell receptor signaling and stimulation of interleukin 10-secreting T regulatory type 1 cells in the pancreas. Soluble dimeric pMHC class II may be useful in the development of immunospecific therapies for type 1 diabetes.

A single-chain class II MHC-IgG3 fusion protein inhibits autoimmune arthritis by induction of antigen-specific hyporesponsiveness

T cells play a central role in many autoimmune diseases. A method to specifically target the function of autoreactive T cell clones would avoid the global immunosuppression associated with current therapies. To develop a molecule capable of inhibiting autoreactive T cell responses in vivo, single-chain peptide-I-A-IgG3 fusion proteins were constructed and expressed in both mammalian and insect cells. The fusion proteins were designed with an IgG3 Fc moiety to make them divalent, allowing TCR cross-linking, while lacking FcR binding and costimulation. The fusion proteins stimulated T cell hybridomas in vitro in a peptide-specific, MHC-restricted manner but failed to do so in soluble form. In vivo administration of an I-A(q) fusion protein, containing an immunodominant collagen II peptide, significantly delayed the onset and reduced the severity of collagen-induced arthritis in DBA/1 mice by induction of Ag-specific hyporesponsiveness. Such fusion proteins may be useful to study novel therapeutic approaches for T cell-mediated autoimmune diseases.

T-cell activation by soluble MHC oligomers can be described by a two-parameter binding model

T-cell activation is essential for initiation and control of immune system function. T cells are activated by interaction of cell-surface antigen receptors with major histocompatibility complex (MHC) proteins on the surface of other cells. Studies using soluble oligomers of MHC-peptide complexes and other types of receptor cross-linking agents have supported an activation mechanism that involves T cell receptor clustering. Receptor clustering induced by incubation of T cells with MHC-peptide oligomers leads to the induction of T-cell activation processes, including downregulation of engaged receptors and upregulation of the cell-surface proteins CD69 and CD25. Dose-response curves for these T-cell activation markers are bell-shaped, with different maxima and midpoints, depending on the valency of the soluble oligomer used. In this study, we have analyzed the activation behavior using a mathematical model that describes the binding of multivalent ligands to cell-surface receptors. We show that a simple equilibrium binding model accurately describes the activation data for CD4(+) T cells treated with MHC-peptide oligomers of varying valency. The model can be used to predict activation and binding behavior for T cells and MHC oligomers with different properties.

Modulation of CD4 T cell function by soluble MHC II-peptide chimeras

Peptides antigens of 8 to 24 amino acid residues in length that are derived from processing of foreign proteins by antigen presenting cells (APC), and then presented to T cells in the context of major histocompatibility complex molecules (MHC) expressed by APC, are the only physiological ligands for T cell receptor (TCR). Co-ligation of TCR and CD4 co-receptor on T cells by MHC II-peptide complexes (signal 1) leads to various T cell functions depending on the nature of TCR and CD4 co-ligation, and whether costimulatory receptors (signal 2) such as CD28, CTLA-4, CD40L are involved in this interaction. Recently, the advance of genetic engineering led to the generation of a new class of antigen-specific ligands for TCR, i.e., soluble MHC class I-, and MHC class II-peptide chimeras. In principle, these chimeric molecules consist of an antigenic peptide which is covalently linked to the amino terminus of alpha-chain in the case of MHC I, or beta-chains in the case of MHC II molecules. Conceptually, such TCR/CD4 ligands shall provide the signal 1 to T cells. Since soluble MHC-peptide chimeras showed remarkable regulatory effects on peptide-specific T cells in vitro and in vivo, they may represent a new generation of immunospecific T cell modulators with potential therapeutic applicability in autoimmune and infectious diseases. This review is focused on the immunomodulatory effects of soluble, MHC class II-peptide chimeras, and discuss these effects in the context of the most accepted theories on T cell regulation.

Antigen-specific blockade of T cells in vivo using dimeric MHC peptide

Ag-specific immune tolerance in clinical organ transplantation is currently an unrealized but critical goal of transplant biology. The specificity and avidity of multimerized MHC-peptide complexes suggests their potential ability to modulate T cell sensitization and effector functions. In this study, we examined the ability of MHC-peptide dimers to modulate T cell function both in vitro and in vivo. Soluble MHC dimers induced modulation of surface TCR expression and inhibited T cell cytolytic activity at nanomolar concentrations in vitro. Furthermore, engagement of TCR by soluble dimers resulted in phosphorylation of the TCR zeta-chain and recruitment and phosphorylation of zeta-associated protein-70 to the signaling complex, the latter of which increased upon dimer cross-linking. Significantly, Ag-specific inhibition of an alloreactive TCR-transgenic T cell population in vivo resulted in consequent outgrowth of an allogeneic tumor. The prolonged Ag-specific suppression of expansion and/or effector function of cognate T cells in vivo suggests that soluble MHC dimers may be a means of inducing sustained Ag-specific T cell unresponsiveness in vivo.

Receptor proximity, not intermolecular orientation, is critical for triggering T-cell activation

Engagement of antigen receptors on the surface of T-cells with peptides bound to major histocompatibility complex (MHC) proteins triggers T-cell activation in a mechanism involving receptor oligomerization. Receptor dimerization by soluble MHC oligomers is sufficient to induce several characteristic activation processes in T-cells including internalization of engaged receptors and up-regulation of cell surface proteins. In this work, the influence of intermolecular orientation within the activating receptor dimer was studied. Dimers of class II MHC proteins coupled in a variety of orientations and topologies each were able to activate CD4+ T-cells, indicating that triggering was not dependent on a particular receptor orientation. In contrast to the minimal influence of receptor orientation, T-cell triggering was affected by the inter-molecular distance between MHC molecules, and MHC dimers coupled through shorter cross-linkers were consistently more potent than those coupled through longer cross-linkers. These results are consistent with a mechanism in which intermolecular receptor proximity, but not intermolecular orientation, is the key determinant for antigen-induced CD4+ T-cell activation.

Receptor clustering and transmembrane signaling in T cells

T cells are activated via engagement of their cell-surface receptors with molecules of the major histocompatibility complex (MHC) displayed on another cell surface. This process, which is a key step in the recognition of foreign antigens by the immune system, involves oligomerization of receptor components. Recent characterization of the T-cell response to soluble arrays of MHC-peptide complexes has provided insights into the triggering mechanism for T-cell activation.