Detection of glutamic acid decarboxylase-activated T cells with I-Ag7 tetramers

Noncontiguous T cell epitopes in autoimmune diabetes: From mice to men and back again

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease that affects the insulin-producing beta cells of the pancreatic islets. The nonobese diabetic mouse is a widely studied spontaneous model of the disease that has contributed greatly to our understanding of T1D pathogenesis. This is especially true in the case of antigen discovery. Upon review of existing knowledge concerning the antigens and peptide epitopes that are recognized by T cells in this model, good concordance is observed between mouse and human antigens. A fascinating recent illustration of the contribution of the nonobese diabetic mouse in the area of epitope identification is the discovery of noncontiguous CD4+ T cell epitopes. This novel epitope class is characterized by the linkage of an insulin-derived peptide to, most commonly, a fragment of a natural cleavage product of another beta cell secretory granule constituent. These so-called hybrid insulin peptides are also recognized by T cells in patients with T1D, although the precise mechanism for their generation has yet to be defined and is the subject of active investigation. Although evidence from the tumor immunology arena documented the existence of noncontiguous CD8+ T cell epitopes, generated by proteasome-mediated peptide splicing involving transpeptidation, such CD8+ T cell epitopes were thought to be a rare immunological curiosity. However, recent advances in bioinformatics and mass spectrometry have challenged this view. These developments, coupled with the discovery of hybrid insulin peptides, have spurred a search for noncontiguous CD8+ T cell epitopes in T1D, an exciting frontier area still in its infancy.

Maintenance of peripheral tolerance to islet antigens

Reestablishment of immune tolerance to the insulin-producing beta cells is the desired goal for type 1 diabetes (T1D) treatment and prevention. Immune tolerance to multiple islet antigens is defective in individuals with T1D, but the mechanisms involved are multifaceted and may involve loss of thymic and peripheral tolerance. In this review we discuss our current understanding of the varied mechanisms by which peripheral tolerance to islet antigens is maintained in healthy individuals where genetic protection from T1D is present and how this fails in those with genetic susceptibility to disease. Novel findings in regards to expression of neo-islet antigens, non-classical regulatory cell subsets and the impact of specific genetic variants on tolerance induction are discussed.

Cutting edge: CD4 T cells reactive to an islet amyloid polypeptide peptide accumulate in the pancreas and contribute to disease pathogenesis in nonobese diabetic mice

We previously reported a peptide KS20 from islet amyloid polypeptide (IAPP) to be the target Ag for a highly diabetogenic CD4 T cell clone BDC-5.2.9. To track IAPP-reactive T cells in NOD mice and determine how they contribute to the pathogenesis of type 1 diabetes, we designed a new I-Ag7 tetramer with high affinity for BDC-5.2.9 that contains the peptide KS20. We found that significant numbers of KS20 tetramer(+) CD4 T cells can be detected in the pancreas of prediabetic and diabetic NOD mice. To verify pathogenicity of IAPP-reactive cells, we sorted KS20 tetramer(+) cells and cloned them from uncloned T cell lines isolated from spleen and lymph nodes of diabetic mice. We isolated a new KS20-reactive Th1 CD4 T cell clone that rapidly transfers diabetes. Our results suggest that IAPP triggers a broad autoimmune response by CD4 T cells in NOD mice.

Making the most of major histocompatibility complex molecule multimers: applications in type 1 diabetes

Classical major histocompatibility complex (MHC) class I and II molecules present peptides to cognate T-cell receptors on the surface of T lymphocytes. The specificity with which T cells recognize peptide-MHC (pMHC) complexes has allowed for the utilization of recombinant, multimeric pMHC ligands for the study of minute antigen-specific T-cell populations. In type 1 diabetes (T1D), CD8+ cytotoxic T lymphocytes, in conjunction with CD4+ T helper cells, destroy the insulin-producing β cells within the pancreatic islets of Langerhans. Due to the importance of T cells in the progression of T1D, the ability to monitor and therapeutically target diabetogenic clonotypes of T cells provides a critical tool that could result in the amelioration of the disease. By administering pMHC multimers coupled to fluorophores, nanoparticles, or toxic moieties, researchers have demonstrated the ability to enumerate, track, and delete diabetogenic T-cell clonotypes that are, at least in part, responsible for insulitis; some studies even delay or prevent diabetes onset in the murine model of T1D. This paper will provide a brief overview of pMHC multimer usage in defining the role T-cell subsets play in T1D etiology and the therapeutic potential of pMHC for antigen-specific identification and modulation of diabetogenic T cells.

The use of peptide-major-histocompatibility-complex multimers in type 1 diabetes mellitus

Major histocompatibility complex (MHC) class I and MHC class II molecules present short peptides that are derived from endogenous and exogenous proteins, respectively, to cognate T-cell receptors (TCRs) on the surface of T cells. The exquisite specificity with which T cells recognize particular peptide-major-histocompatibility-complex (pMHC) combinations has permitted development of soluble pMHC multimers that bind exclusively to selected T-cell populations. Because the pathogenesis of type 1 diabetes mellitus (T1DM) is driven largely by islet-reactive T-cell activity that causes β-cell death, these reagents are useful tools for studying and, potentially, for treating this disease. When coupled to fluorophores or paramagnetic nanoparticles, pMHC multimers have been used to visualize the expansion and islet invasion of T-cell effectors during diabetogenesis. Administration of pMHC multimers to mice has been shown to modulate T-cell responses by signaling through the TCR or by delivering a toxic moiety that deletes the targeted T cell. In the nonobese diabetic mouse model of T1DM, a pMHC-I tetramer coupled to a potent ribosome-inactivating toxin caused long-term elimination of a specific diabetogenic cluster of differentiation 8+ T-cell population from the pancreatic islets and delayed the onset of diabetes. This review will provide an overview of the development and use of pMHC multimers, particularly in T1DM, and describe the therapeutic promise these reagents have as an antigen-specific means of ameliorating deleterious T-cell responses in this autoimmune disease.

Synthesis of N-terminally linked protein dimers and trimers by a combined native chemical ligation-CuAAC click chemistry strategy

A novel method for the synthesis of N-terminally linked protein multimers is reported. Azide and alkyne thioesters were synthesized for the N-terminal modification of expressed proteins using native chemical ligation (NCL). Proteins modified by these moieties can be joined together to form homodimers and homotrimers via Cu(I)-catalyzed azide-alkyne [3 + 2] cycloaddition (CuAAC) click chemistry. The orthogonal nature of this reaction allows the production of protein heteromultimers, and this is demonstrated by synthesis of a protein heterodimer.

Regulatory function of a novel population of mouse autoantigen-specific Foxp3 regulatory T cells depends on IFN-gamma, NO, and contact with target cells

BACKGROUND

Both naturally arising Foxp3(+) and antigen-induced Foxp3(-) regulatory T cells (Treg) play a critical role in regulating immune responses, as well as in preventing autoimmune diseases and graft rejection. It is known that antigen-specific Treg are more potent than polyclonal Treg in suppressing pathogenic immune responses that cause autoimmunity and inflammation. However, difficulty in identifying and isolating a sufficient number of antigen-specific Treg has limited their use in research to elucidate the mechanisms underlying their regulatory function and their potential role in therapy.

METHODOLOGY/PRINCIPAL FINDINGS

Using a novel class II MHC tetramer, we have isolated a population of CD4(+) Foxp3(-) T cells specific for the autoantigen glutamic acid decarboxylase p286-300 peptide (NR286 T cells) from diabetes-resistant non-obese resistant (NOR) mice. These Foxp3(-) NR286 T cells functioned as Treg that were able to suppress target T cell proliferation in vitro and inhibit type 1 diabetes in animals. Unexpected results from mechanistic studies in vitro showed that their regulatory function was dependent on not only IFN-gamma and nitric oxide, but also on cell contact with target cells. In addition, separating NR286 Treg from target T cells in transwell assays abolished both production of NO and suppression of target T cells, regardless of whether IFN-gamma was produced in cell cultures. Therefore, production of NO, not IFN-gamma, was cell contact dependent, suggesting that NO may function downstream of IFN-gamma in mediating regulatory function of NR286 Treg.

CONCLUSIONS/SIGNIFICANCE

These studies identified a unique population of autoantigen-specific Foxp3(-) Treg that can exert their regulatory function dependent on not only IFN-gamma and NO but also cell contact with target cells.

T cells as pioneers: antigen-specific T cells condition inflamed sites for high-rate antigen-non-specific effector cell recruitment

Cellular infiltration is a classic hallmark of inflammation. Whereas the role of T cells in many types of inflammation is well established, the specific impact of antigen recognition on their migration into the site and on the accumulation of other effector cells are still matters of debate. Using a model of an inflammatory effector phase driven by T-cell receptor (TCR) transgenic T cells, we found (i) that antigen-specific T cells play a crucial role as 'pioneer cells' that condition the tissue for enhanced recruitment of further T effector cells and other leucocytes, and (ii) that the infiltration of T cells is not dependent on antigen specificity. We demonstrate that a small number of antigen-specific T cells suffice to initiate a cascade of cellular immigration into the antigen-loaded site. Although antigen drives this process, accumulation of T cells in the first few days of inflammation was not dependent on T-cell reactivity to the antigen. Both transgenic and wild-type T effector cells showed enhanced immigration into the site of antigen challenge after the initial arrival and activation of antigen-specific pioneer cells. This suggests that bystander accumulation of non-specific effector/memory T cells is a general feature in inflammation. Furthermore, tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma were identified as mediators that contribute to conditioning of the inflammatory site for high-rate accumulation of T effector cells in this T-cell-driven model.

Class II major histocompatibility complex tetramer staining: progress, problems, and prospects

The use of major histocompatibility complex (MHC) tetramers in the detection and analysis of antigen-specific T cells has become more widespread since its introduction 11 years ago. Early challenges in the application of tetramer staining to CD4+ T cells centred around difficulties in the expression of various class II MHC allelic variants and the detection of low-frequency T cells in mixed populations. As many of the technical obstacles to class II MHC tetramer staining have been overcome, the focus has returned to uncertainties concerning how oligomer valency and T-cell receptor/MHC affinity affect tetramer binding. Such issues have become more important with an increase in the number of studies relying on direct ex vivo analysis of antigen-specific CD4+ T cells. In this review we discuss which problems in class II MHC tetramer staining have been solved to date, and which matters remain to be considered.

Identification and isolation of murine antigen-reactive T cells according to CD154 expression

T helper (Th) cells are central regulators of adaptive immune responses. However, the detection of the small number of Th cells specific for a particular antigen or pathogen is still a major challenge. CD154 was recently introduced as a marker for antigen-specific Th cells. To date, this technology was not applicable for mice - arguably the most important immunological model system. CD154 is difficult to detect due to its rapid removal from the cell surface upon binding to CD40 during antigen-specific activation by APC. We present an efficient strategy to block the degradation of murine CD154 by combined use of antibodies against CD40 and CD154. This strategy makes CD154 easily accessible for surface staining, which allows isolation and expansion of rare antigen specific T cells. Importantly, CD154 identified all specific T cells in strongly Th1- or Th2-polarized immune responses against pathogens like Salmonella typhimurium and Heligmosomoides polygyrus, independent of their potential to produce cytokines. We demonstrate that CD154 can in fact be used as a reliable marker for antigen-specific CD4 T cells in mice, offering a unique option to analyze, isolate and rapidly expand the entire pool of Th-cells generated during a physiological T cell response in vivo.

Suppression of Type 1 Diabetes in NOD Mice by Bifunctional Peptide Inhibitor: Modulation of the Immunological Synapse Formation

The aim of this work was to design and utilize a bifunctional peptide inhibitor called glutamic acid decarboxylase-bifunctional peptide inhibitor to suppress the progression of type 1 diabetes in non-obese diabetic mice. The hypothesis is that glutamic acid decarboxylase-bifunctional peptide inhibitor binds simultaneously to major histocompatibility complex-II and intercellular adhesion molecule type 1 on antigen-presenting cell and inhibits the immunological synapse formation during T-cell-antigen-presenting cell interactions. Glutamic acid decarboxylase-bifunctional peptide inhibitor was composed of a major epitope of the type 1 diabetes-associated antigen, glutamic acid decarboxylase 65 kDa, covalently linked to a peptide derived from CD11a of lymphocyte function-associated antigen-1. The suppression of insulitis and type 1 diabetes was evaluated using non-obese diabetic and non-obese diabetic severe combined immunodeficiency mice. Glutamic acid decarboxylase-bifunctional peptide inhibitor had the capacity to suppress invasive insulitis in non-obese diabetic mice. CD4+ T-cells isolated from glutamic acid decarboxylase-bifunctional peptide inhibitor treated mice also suppressed insulitis and hyperglycemia when transferred with diabetogenic non-obese diabetic spleen cells into non-obese diabetic severe combined immunodeficiency recipients. As predicted, the glutamic acid decarboxylase-bifunctional peptide inhibitor cross-linked a significant fraction of major histocompatibility complex class-II molecules to intercellular adhesion molecule type 1 molecules on the surface of live antigen-presenting cell. Intravenous injection of the glutamic acid decarboxylase-bifunctional peptide inhibitor elicited interleukin-4-producing T-cells in non-obese diabetic mice primed against the glutamic acid decarboxylase-epitope peptide. Together, the results indicate that glutamic acid decarboxylase-bifunctional peptide inhibitor induces interleukin-4-producing regulatory cells but does not expand the glutamic acid decarboxylase-specific Th2 population. Given that Th2 effector cells can cause pathology, the glutamic acid decarboxylase-bifunctional peptide inhibitor may represent a novel mechanism to induce interleukin-4 without Th2-associated pathology.

Translational mini-review series on type 1 diabetes: Systematic analysis of T cell epitopes in autoimmune diabetes

T cell epitopes represent the molecular code words through which the adaptive immune system communicates. In the context of a T cell-mediated autoimmune disease such as type 1 diabetes, CD4 and CD8 T cell recognition of islet autoantigenic epitopes is a key step in the autoimmune cascade. Epitope recognition takes place during the generation of tolerance, during its loss as the disease process is initiated, and during epitope spreading as islet cell damage is perpetuated. Epitope recognition is also a potentially critical element in therapeutic interventions such as antigen-specific immunotherapy. T cell epitope discovery, therefore, is an important component of type 1 diabetes research, in both human and murine models. With this in mind, in this review we present a comprehensive guide to epitopes that have been identified as T cell targets in autoimmune diabetes. Targets of both CD4 and CD8 T cells are listed for human type 1 diabetes, for humanized [human leucocyte antigen (HLA)-transgenic] mouse models, and for the major spontaneous disease model, the non-obese diabetic (NOD) mouse. Importantly, for each epitope we provide an analysis of the relative stringency with which it has been identified, including whether recognition is spontaneous or induced and whether there is evidence that the epitope is generated from the native protein by natural antigen processing. This analysis provides an important resource for investigating diabetes pathogenesis, for developing antigen-specific therapies, and for developing strategies for T cell monitoring during disease development and therapeutic intervention.

Glutamic acid decarboxylase-specific CD4+ regulatory T cells

It is known that CD4(+) regulatory T cells (Tr cells) play a central role in inducing immune tolerance in animals and humans. Compared to polyclonal Tr cells, autoantigen-specific Tr cells are more potent at blocking pathogenic immune responses. In order to better understand the role of Tr cells in controlling type 1 diabetes development and to help design effective antigen-specific cell-based therapeutic methods to treat the disease, it is necessary to: (a) determine the antigen specificity of Tr cells; (b) study how antigen-specific Tr cells behave in vivo; (c) investigate the interaction of Tr cells with pathogenic T cells (Tpath cells) and determine whether such interaction correlates with the progression or inhibition of diabetes; and (d) determine the cellular and molecular mechanisms underlying the regulation of diabetes by Tr cells. We have addressed these questions with a focus on the studies of glutamic acid decarboxylase (GAD)-specific T cells. Previous studies have suggested that GAD-specific T cells play a key role in type 1 diabetes. Treatment of NOD mice with GAD or its peptides can prevent the progression toward overt disease. The preventive effect could be due to either the deletion of antigen-specific pathogenic T cells or the induction of potent antigen-specific Tr cells. Using antigen-specific I-Ag7 tetramers we have isolated several populations of GAD peptide-specific T cells from diabetes-prone NOD and diabetes-resistant NOR mice. Herein, we summarize our studies on the role of these GAD peptide-specific T cells in type 1 diabetes. We present evidence that supports the hypothesis that the repertoire of T cells specific for these GAD peptides is biased toward Tr cells that inhibit diabetes rather than toward pathogenic T cells that induce diabetes.

Regulatory T Cells Can Mediate Their Function through the Stimulation of APCs to Produce Immunosuppressive Nitric Oxide

Regulatory T cells (Tr cells) play a critical role in inducing immune tolerance. It remains largely unclear how various types of Tr cells perform their regulatory function. We have studied the underlying regulatory mechanism of a population of autoantigen-specific CD4+ Tr cells. These T cells are specific for the glutamic acid decarboxylase p206-220 peptide and are isolated from the diabetes-resistant nonobese-resistant mice. Although these T cells express T-bet and display a Th1 phenotype, they are able to inhibit diabetes. Their regulatory function is dependent on both IFN-gamma and cell contact with target cells. These Tr cells can mediate their cell contact-dependent regulatory function by secreting IFN-gamma which stimulates APCs to produce NO. NO is necessary for the Tr cells to inhibit the proliferation of pathogenic T cells and the development of diabetes. Therefore, we have identified a novel mechanism by which these Tr cells can exert their regulatory function. These results also provide an explanation as to why IFN-gamma may play both pathogenic and immunomodulatory roles in autoimmune diseases.

Class II major histocompatibility complex-peptide tetramer staining in relation to functional avidity and T cell receptor diversity in the mouse CD4(+) T cell response to a rheumatoid arthritis-associated antigen

OBJECTIVE

Although studies have suggested that human cartilage (HC) gp-39 may be an antigen recognized by autoreactive CD4(+) T cells in rheumatoid arthritis, we previously failed to identify specific CD4(+) T cells in patients' synovial fluid or blood using a class II major histocompatibility complex-peptide tetramer composed of the immunodominant HC gp-39(263-275) epitope covalently linked to DR4. We undertook this study to better understand the parameters for specific binding of this tetramer.

METHODS

DR4-transgenic mice were immunized with the HC gp-39 peptide, and a set of peptide-responsive hybridomas was derived. Hybridomas were stained with the DR4-gp-39 tetramer and cultured with increasing amounts of peptide in the presence of DR4-expressing antigen-presenting cells to determine functional avidity.

RESULTS

Great variability was apparent in the ability of the tetramer to stain the hybridomas, and there was a strong correlation between the intensity of tetramer staining and functional avidity. Importantly, nearly 30% of the hybridomas did not stain with tetramer, and these cells exhibited relatively low functional avidity. Although the addition of an anti-T cell receptor (anti-TCR) monoclonal antibody during the staining procedure enhanced binding of the tetramer to a number of the hybridomas, a significant percentage remained unstainable. Analysis of TCR expression showed that >90% of the hybridomas expressed the same TCR beta-chain variable region (V(beta)10), and sequencing of the TCR junctional regions showed diversity in the third complementarity-determining region.

CONCLUSION

These results suggest that immune responses dominated by relatively low-affinity TCR interactions, such as those that may occur in autoimmune disease, will be difficult to detect using standard tetramer techniques.

The NOD mouse: a model of immune dysregulation

Autoimmunity is a complex process that likely results from the summation of multiple defective tolerance mechanisms. The NOD mouse strain is an excellent model of autoimmune disease and an important tool for dissecting tolerance mechanisms. The strength of this mouse strain is that it develops spontaneous autoimmune diabetes, which shares many similarities to autoimmune or type 1a diabetes (T1D) in human subjects, including the presence of pancreas-specific autoantibodies, autoreactive CD4+ and CD8+ T cells, and genetic linkage to disease syntenic to that found in humans. During the past ten years, investigators have used a wide variety of tools to study these mice, including immunological reagents and transgenic and knockout strains; these tools have tremendously enhanced the study of the fundamental disease mechanisms. In addition, investigators have recently developed a number of therapeutic interventions in this animal model that have now been translated into human therapies. In this review, we summarize many of the important features of disease development and progression in the NOD strain, emphasizing the role of central and peripheral tolerance mechanisms that affect diabetes in these mice. The information gained from this highly relevant model of human disease will lead to potential therapies that may alter the development of the disease and its progression in patients with T1D.

Presence of diabetes-inhibiting, glutamic acid decarboxylase-specific, IL-10-dependent, regulatory T cells in naive nonobese diabetic mice

Immunization of NOD mice with autoantigens such as glutamic acid decarboxylase (GAD) 221-235 peptide (p221) can induce Ag-specific CD4(+) T regulatory (Tr) cells. However, it is unclear whether these Tr cells acquire their regulatory capacity due to immunization or whether they are constitutively harbored in unimmunized naive mice. To address this question, we used an I-Ag7 tetramer to isolate p221-specific T cells from naive NOD mice (N221(+) cells) after peptide-specific in vitro expansion. The N221(+) T cells produced IFN-gamma and IL-10, but very little IL-4, in response to p221 stimulation. These T cells could function as regulatory cells and inhibit in vitro proliferation of diabetogenic BDC2.5 cells. This suppressive activity was cell contact-independent and was abrogated by Abs to IL-10 or IL-10R. Interestingly, IL-2 produced by other T cells present in the cell culture induced unactivated N221(+) T cells to exhibit regulatory activities involving production of IL-10. In vivo, N221(+) cells inhibited diabetes development when cotransferred with NOD splenocytes into NOD/scid recipients. Together, these results demonstrate that p221-specific IL-10-dependent Tr cells, including Tr type 1 cells, are present in naive NOD mice. The use of spontaneously arising populations of GAD peptide-specific Tr cells may represent a promising immunotherapeutic approach for preventing type 1 diabetes.

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.

A comprehensive guide to antibody and T-cell responses in type 1 diabetes

Type 1 diabetes (T1D) is an organ-specific autoimmune disease in which the insulin-producing beta cells in the pancreatic islets are selectively eliminated. T cells specific for beta-cell antigens are the mediators of this precise cellular destruction. However, antibodies to beta-cell proteins are also generated and may be used for predicting disease in at-risk populations. Over the past two decades, numerous beta-cell proteins and lipids have been implicated as autoantigens in patients or in non-obese diabetic (NOD) mice, a well-studied animal model of T1D. Here, we present a review of these antigens, accompanied by their T-cell epitopes, where known, and a discussion of our current understanding of why particular self-proteins become disease-inciting antigens. Although two dozen beta-cell antigens have been identified to date, few of these have been confirmed to be recognized by pathogenic T cells early in the disease process. Further identification and characterization of initiating beta-cell antigens targeted by pathogenic T cells should be a priority for future studies.

A chaperone-assisted high yield system for the production of HLA-DR4 tetramers in insect cells

MHC tetramers have become essential tools for the analysis of antigen specific responses of CD8+ and CD4+ T cells. However, the use of MHC class II tetramers is hampered by the relatively low yields of most current expression systems. We have devised an insect cell/baculovirus expression system in which yields of 50-70 mg of recombinant HLA-DR4 molecules, with or without covalently linked peptide, per liter of insect cell supernatant, are routinely obtained. These yields are rendered possible by an optimized design and use of DRalpha and DRbeta expression cassettes and by co-expression of a housekeeping chaperone of the endoplasmic reticulum, calreticulin, which, due to its co-secretion, increases secretion of HLA-DR molecules two- to threefold. A tetramer produced in the system specifically was shown to stain an HLA-DR4 restricted T cell line obtained from a healthy donor by in vitro priming, but which recognizes a type I diabetes autoantigen. Co-expression of chaperones may represent a general strategy for enhancing yields of recombinant proteins expressed in insect cells and facilitate production of MHC class II tetramers in the future.

Ex vivo analysis of thymic CD4 T cells in nonobese diabetic mice with tetramers generated from I-A(g7)/class II-associated invariant chain peptide precursors

The MHC determines susceptibility and resistance to type 1 diabetes in humans and nonobese diabetic (NOD) mice. To investigate how a disease-associated MHC molecule shapes the T cell repertoire in NOD mice, we generated a series of tetramers from I-A(g7)/class II-associated invariant chain peptide precursors by peptide exchange. No CD4 T cell populations could be identified for two glutamic acid decarboxylase 65 peptides, but tetramers with a peptide mimetic recognized by the BDC-2.5 and other islet-specific T cell clones labeled a distinct population in the thymus of young NOD mice. Tetramer-positive cells were identified in the immature CD4(+)CD8(low) population that arises during positive selection, and in larger numbers in the more mature CD4(+)CD8(-) population. Tetramer labeling was specific based on the use of multiple control tetramers, including one with a single amino acid analog peptide in which a critical TCR contact residue was substituted. The T cell population was already present in the thymus of 2-wk-old NOD mice before the typical onset of insulitis and was detected in B10 mice congenic for the NOD MHC locus, but not B10 control mice. These results demonstrate that a T cell population can expand in the thymus of NOD mice to levels that are at least two to three orders of magnitude higher than estimated for a given specificity in the naive T cell pool. Based on these data, we propose a model in which I-A(g7) confers susceptibility to type 1 diabetes by biasing positive selection in the thymus and later presenting peptides from islet autoantigens to such T cells in the periphery.

T cell recognition of distinct peptide:I-Au conformers in murine experimental autoimmune encephalomyelitis

We have used T cells bearing TCRs that are closely related in sequence as probes to detect conformational variants of peptide-MHC complexes in murine experimental autoimmune encephalomyelitis in H-2(u) mice. The N-terminal epitope of myelin basic protein (MBP) is immunodominant in this model. Our studies have primarily focused on T cell recognition of a position 4 analog of this peptide (MBP1-9[4Y]) complexed with I-A(u). Using site-directed mutagenesis, we have mapped the functionally important complementarity determining region residues of the 1934.4 TCR Valpha domain. One of the resulting mutants (Tyr(95) to alanine in CDR3alpha, Y95A) has interesting properties: relative to the parent wild-type TCR, this mutant poorly recognizes Ag complexes generated by pulsing professional APCs (PL-8 cells) with MBP1-9[4Y] while retaining recognition of MBP1-9[4Y]-pulsed unconventional APCs or insect cell-expressed complexes of I-A(u) containing tethered MBP1-9[4Y]. Insect cell expression of recombinant I-A(u) with covalently tethered class II-associated invariant chain peptide or other peptides which bind relatively weakly, followed by proteolytic cleavage of the peptide linker and replacement by MBP1-9[4Y] in vitro, results in complexes that resemble peptide-pulsed PL-8 cells. Therefore, the distinct conformers can be produced in recombinant form. T cells that can distinguish these two conformers can also be generated by the immunization of H-2(u) mice, indicating that differential recognition of the conformers is observed for responding T cells in vivo. These studies have relevance to understanding the molecular details of T cell recognition in murine experimental autoimmune encephalomyelitis. They are also of particular importance for the effective use of multimeric peptide-MHC complexes to characterize the properties of Ag-specific T cells.

Induction of autoantigen-specific Th2 and Tr1 regulatory T cells and modulation of autoimmune diabetes

Autoantigen-based immunotherapy can modulate autoimmune diabetes, perhaps due to the activation of Ag-specific regulatory T cells. Studies of these regulatory T cells should help us understand their roles in diabetes and aid in designing a more effective immunotherapy. We have used class II MHC tetramers to isolate Ag-specific T cells from nonobese diabetic (NOD) mice and BALB/c mice treated with glutamic acid decarboxylase 65 peptides (p206 and p221). Based on their cytokine secretion profiles, immunization of NOD mice with the same peptide induced different T cell subsets than in BALB/c mice. Treatment of NOD mice induced not only Th2 cells but also IFN-gamma/IL-10-secreting T regulatory type 1 (Tr1) cells. Adoptive transfer experiments showed that isolated tetramer(+) T cells specific for p206 or p221 could inhibit diabetes development. These cells were able to suppress the in vitro proliferation of other NOD mouse T cells without cell-cell contact. They performed their regulatory functions probably by secreting cytokines, and Abs against these cytokines could block their suppressive effect. Interestingly, the presence of both anti-IL-10 and anti-IFN-gamma could enhance the target cell proliferation, suggesting that Tr1 cells play an important role. Further in vivo experiments showed that the tetramer(+) T cells could block diabetogenic T cell migration into lymph nodes. Therefore, treatment of NOD mice with autoantigen could induce Th2 and Tr1 regulatory cells that can suppress the function and/or block the migration of other T cells, including diabetogenic T cells, and inhibit diabetes development.

Detection and characterization of T cells specific for BDC2.5 T cell-stimulating peptides

Nonobese diabetic (NOD) mice expressing the BDC2.5 TCR transgene are useful for studying type 1 diabetes. Several peptides have been identified that are highly active in stimulating BDC2.5 T cells. Herein, we describe the use of I-Ag7 tetramers containing two such peptides, p79 and p17, to detect and characterize peptide-specific T cells. The tetramers could stain CD4(+) T cells in the islets and spleens of BDC2.5 transgenic mice. The percentage of CD4(+), tetramer(+) T cells increased in older mice, and it was generally higher in the islets than in the spleens. Our results also showed that tetAg7/p79 could stain a small population of CD4(+) T cells in both islets and spleens of NOD mice. The percentage of CD4(+), tetramer(+) T cells increased in cells that underwent further cell division after being activated by peptides. The avidity of TCRs on purified tetAg7/p79(+) T cells for tetAg7/p79 was slightly lower than that of BDC2.5 T cells. Although tetAg7/p79(+) T cells, like BDC2.5 T cells, secreted a large quantity of IFN-gamma, they were biased toward being IL-10-producing cells. Additionally, <3% of these cells expressed TCR Vbeta4. In vivo adoptive transfer experiments showed that NOD/scid recipient mice cotransferred with tetAg7/p79(+) T cells and NOD spleen cells, like mice transferred with NOD spleen cells only, developed diabetes. Therefore, we have generated Ag-specific tetramers that could detect a heterogeneous population of T cells, and a very small number of NOD mouse T cells may represent BDC2.5-like cells.

Preferential accumulation of antigen-specific effector CD4 T cells at an antigen injection site involves CD62E-dependent migration but not local proliferation

The migration of antigen-specific T cells to nonlymphoid tissues is thought to be important for the elimination of foreign antigens from the body. However, recent results showing the migration of activated T cells into many nonlymphoid tissues raised the possibility that antigen-specific T cells do not migrate preferentially to nonlymphoid tissues containing antigen. We addressed this question by tracking antigen-specific CD4 T cells in the whole body after a localized subcutaneous antigen injection. Antigen-specific CD4 T cells proliferated in the skin-draining lymph nodes and the cells that underwent the most cell divisions acquired the ability to bind to CD62P. As time passed, CD62P-binding antigen-specific CD4 T cells with interferon gamma production potential accumulated preferentially at the site of antigen injection but only in recipients that expressed CD62E. Surprisingly, these T cells did not proliferate in the injection site despite showing evidence of more cell divisions than the T cells in the draining lymph nodes. The results suggest that the most divided effector CD4 T cells from the lymph nodes enter the site of antigen deposition via recognition of CD62E on blood vessels and are retained there in a nonproliferative state via recognition of peptide-major histocompatibility complex II molecules.

Prediction of spontaneous autoimmune diabetes in NOD mice by quantification of autoreactive T cells in peripheral blood

Autoimmune (type 1) diabetes mellitus results from the destruction of insulin-producing pancreatic beta cells by T lymphocytes. Prediction of cell-mediated autoimmune diseases by direct detection of autoreactive T cells in peripheral blood has proved elusive, in part because of their low frequency and reduced avidity for peptide MHC ligands. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Directed evolution of a single-chain class II MHC product by yeast display

Many autoimmune diseases have been linked to the class II region of the major histocompatibility complex (MHC). The linkage is thought to be a result of autoreactive T cells that recognize self-peptides bound to a product of this locus. For example, T cells from non-obese diabetic mice recognize specific 'diabetogenic' peptides bound to a class II MHC allele called I-A(g7). The I-A(g7) molecule is noted for being unstable and difficult to work with, especially in soluble form. In this work, yeast surface display combined with fluorescence-activated cell sorting was used as a means of directed evolution to engineer stabilized variants of a single-chain form of I-A(g7). A library containing mutations at two residues (positions 56 and 57 of the I-A(g7) beta-chain) that are important in the class II disease associations yielded stabilized mutants with preferences for a glutamic acid at residue 56 and a leucine at residue 57. Random mutation of I-A(g7) followed by selection with an anti-I-A(g7) antibody also yielded stabilized variants with mutations in other residues. The methods described here allow the discovery of novel MHC complexes that could facilitate structural studies and provide new opportunities in the development of diagnostics or antagonists of class II MHC-associated diseases.

Detection of autoreactive myelin proteolipid protein 139-151-specific T cells by using MHC II (IAs) tetramers

Detection of autoreactive T cells using MHC II tetramers is difficult because of the low affinity of their TCR. We have generated a class II tetramer using the IA(s) class II molecule combined with an autoantigenic peptide from myelin proteolipid protein (PLP; PLP(139-151)) and used it to analyze myelin PLP(139-151)-reactive T cells. Using monomers and multimerized complexes labeled with PE, we confirmed the specificity of the reagent by bioassay and flow cytometry. The IA(s) tetramers stimulated and stained the PLP(139-151)-specific 5B6 TCR transgenic T cells and a polyclonal cell line specific for PLP(139-151), but not a control T cell line specific for PLP(178-191). We used this reagent to optimize conditions to detect low affinity autoreactive T cells. We found that high pH ( approximately 8.0) and neuraminidase treatment enhances the staining capacity of PLP(139-151) tetramer without compromising specificity. Furthermore, we found that induction of calcium fluxing by tetramers in T cells may be used as a sensitive measure to detect autoreactive T cells with a low affinity. Taken together, the data show that the tetrameric reagent binds and stimulates PLP(139-151)-reactive T cells with specificity. This tetrameric reagent will be useful in studying the evolution of PLP(139-151)-specific repertoire in naive mice and its expansion during the autoimmune disease experimental autoimmune encephalomyelitis.

Prediction of spontaneous autoimmune diabetes in NOD mice by quantification of autoreactive T cells in peripheral blood

Autoimmune (type 1) diabetes mellitus results from the destruction of insulin-producing pancreatic beta cells by T lymphocytes. Prediction of cell-mediated autoimmune diseases by direct detection of autoreactive T cells in peripheral blood has proved elusive, in part because of their low frequency and reduced avidity for peptide MHC ligands. This article was published online in advance of the print edition. The date of publication is available from the JCI website, http://www.jci.org.

Detection of low-avidity CD4+ T cells using recombinant artificial APC: following the antiovalbumin immune response

Subtle differences oppose CD4+ to CD8+ T cell physiologies that lead to different arrays of effector functions. Interestingly, this dichotomy has also unexpected practical consequences such as the inefficacy of many MHC class II tetramers in detecting specific CD4+ T cells. As a mean to study the CD4+ anti-OVA response in H-2(d) and H-2(b) genetic backgrounds, we developed I-A(d)- and I-A(b)-OVA recombinant MHC monomers and tetramers. We were able to show that in this particular system, despite normal biological activity, MHC class II tetramers failed to stain specific T cells. This failure was shown to be associated with a lack of cooperation between binding sites within the tetramer as measured by surface plasmon resonance. This limited cooperativeness translated into a low "functional avidity" and very transient binding of the tetramers to T cells. To overcome this biophysical barrier, recombinant artificial APC that display MHC molecules in a lipid bilayer were developed. The plasticity and size of the MHC-bearing fluorescent liposomes allowed binding to Ag-specific T cells and the detection of low numbers of anti-OVA T cells following immunization. The same liposomes were able, at 37 degrees C, to induce the full reorganization of the T cell signaling molecules and the formation of an immunological synapse. Artificial APC will allow T cell detection and the dissection of the molecular events of T cell activation and will help us understand the fundamental differences between CD4+ and CD8+ T cells.

Application of tetramer technology in studies on autoimmune diseases

Autoreactive T cells are thought to play a role in the immunopathogenesis of autoimmune diseases. Analysis of such cells had long been hampered by lack of suitable assays. Recently developed tetramer technology is based on the recognition of specific peptide-MHC complex by T cell receptor and on the increased binding affinity of multimerized peptide-MHC complex. MHC class I and class II tetramers can be used to detect autoreactive CD4(+) and CD8(+) T cells, while nonclassical MHC (such as CD1d) tetramer can be used to detect other T cell groups, for example natural killer T cells. Tetramer technologies enable direct quantitation of autoreactive T cells in blood and affected tissues. It is also possible to carry out phenotypic and functional characterization of specific T cells on a single cell basis by using tetramers. Of special interest, in situ tetramer staining has the great potential of analyzing autoreactive T cells in their cellular environments. Utilization of tetramers in studies of autoreactive T cells is expected to generate important information regarding the role of such cells in the underlying mechanisms of autoimmune diseases.

Optimization of peptide linker length in production of MHC class II/peptide tetrameric complexes increases yield and stability, and allows identification of antigen-specific CD4+T cells in peripheral blood mononuclear cells

Reliable, efficient systems for producing soluble HLA-DR molecules, suitable for multimerization and use as staining reagents, have proved elusive. We found that the addition of a flexible linker between peptide and N terminus of the DRB1*0101-chain (Crawford, F., Kozono, H., White, J., Marrack, P. and Kappler, J., Immunity 1998. 8: 675-682.), results in greater in vitro folding efficiency of Escherichia coli-expressed alpha- and beta-chains, and increases both the yield and stability of the DRA1*0101/DRB1*0101/peptide complexes. Although a 10-amino acid linker functioned efficiently for a 20mer epitope from HIV p24, a longer linker was required to produce a DR1 MHC class II tetramer with the influenza hemagglutinin epitope (HA(306-318)). The DR1-HA tetramer was able to stain positively over 98% of a specific clone (HA 1.7) with only a brief 30-min incubation. The tetrameric complexes detected clone cells diluted into PBMC, with high sensitivity, coupled with low background staining in CD4(+) cells. It was possible to detect antigen-specific CD4(+) T cells within a population of PBMC stimulated with the HA peptide. This demonstrates the potential to monitor CD4(+) T cell responses in peripheral blood in a number of clinical scenarios.

Use of class II tetramers for identification of CD4+ T cells

Multivalent MHC class II molecules containing peptide antigens are useful tools for the detection of antigen specific human CD4+ T cells. Tetramers produced by exogenous peptide loading onto empty class II molecules are comparable to tetramers with peptide tethered to the class II chain covalently, but have many practical advantages. Conditions for optimal peptide loading to generate tetramers are discussed and optimal conditions of using tetramers for staining T cells are examined. As the frequency of antigen specific CD4+ T cells in peripheral blood is low, we demonstrate that an in vitro expansion step is effective in detecting low frequency T cells. Two new applications with tetramers, their uses for mapping T cell epitopes and for the detection of low affinity T cells are described. In a clinical setting, potential applications include using these reagents for monitoring disease progression during clinical intervention.

Detection of GAD65-specific T-cells by major histocompatibility complex class II tetramers in type 1 diabetic patients and at-risk subjects

Soluble HLA-DR401 or -DR404 tetramers containing a peptide corresponding to an immunodominant epitope from human GAD65 were used to analyze peripheral blood T-cells of newly diagnosed type 1 diabetic patients and at-risk subjects. Peripheral blood mononuclear cells were expanded on antigen-presenting cells presenting GAD65 peptide and subsequently activated with specific plate-bound class II-peptide monomers. T-cell activation defined in flow cytometry by CD4(high) and/or CD25 markers were observed in all type 1 diabetic patients and some at-risk subjects, but not in normal control subjects. The activated T-cells stained positive with tetramers containing the GAD65 epitope 555-567. Tetramer-positive cells were CD4(high) T-cells with high avidity for an immunodominant GAD65 T-cell epitope. Phenotyping of T-cells utilizing HLA class II tetramers provides a new tool to characterize the autoimmune response in type 1 diabetes.

Use of soluble MHC class II/peptide multimers to detect antigen-specific T cells in human disease

Most techniques that identify antigen-specific T cells are dependent on the response of these cells to the relevant antigen in culture. Soluble multimers of MHC molecules, when occupied with the same peptide, will bind selectively to T cells specific for that MHC/peptide complex. Techniques to produce fluorescent MHC class II/peptide multimers have recently been developed. These reagents provide a method to facilitate detection and isolation of antigen-specific CD4+ T cells and they represent a new research tool to study these cells in patients with immune-mediated diseases.

A restricted subset of dendritic cells captures airborne antigens and remains able to activate specific T cells long after antigen exposure

Mice sensitized for a Th2 response to Leishmania LACK antigen developed allergic airway inflammation upon exposure to LACK aerosol. Using multimers of I-A(d) molecules bound to a LACK peptide as probes, we tracked the migration of LACK-specific Th2 cells to the airways. Elevated numbers of LACK-specific Th2 cells remained in the airways for 5 weeks after the last aerosol. Substantial numbers of DC presenting LACK peptides were found in the airways, but not in other compartments, for up to 8 weeks after antigen exposure. These LACK-presenting airway DC expressed CD11c and CD11b as well as high levels of surface molecules involved in uptake and costimulation. Taken together, our results may explain the chronic Th2 airway inflammation characteristic of allergic asthma.

Staining of celiac disease-relevant T cells by peptide-DQ2 multimers

Gluten-specific T cells in the small intestinal mucosa are thought to play a central role in the pathogenesis of celiac disease (CD). The vast majority of these T cells recognize gluten peptides when presented by HLA-DQ2 (DQA1*05/DQB1*02), a molecule which immunogenetic studies have identified as conferring susceptibility to CD. We have previously identified and characterized three DQ2-restricted gluten epitopes that are recognized by intestinal T cells isolated from CD patients, two of which are immunodominant. Because almost all of the gluten epitopes are restricted by DQ2, and because we have detailed knowledge of several of these epitopes, we chose to develop peptide-DQ2 tetramers as a reagent to further investigate the role of these T cells in CD. In the present study, stable soluble DQ2 was produced such that it contained leucine zipper dimerization motif and a covalently coupled peptide. We have made four different peptide-DQ2 staining reagents, three containing the gluten epitopes and one containing a DQ2-binding self-peptide that provides a negative control for staining. We show in this study that peptide-DQ2 when adhered to plastic specifically stimulates T cell clones and that multimers comprising these molecules specifically stain peptide-specific T cell clones and lines. Interestingly, T cell activation caused severe reduction in staining intensities obtained with the multimers and an Ab to the TCR. The problem of TCR down-modulation must be taken into consideration when using class II multimers to stain T cells that may have been recently activated in vivo.