Detection of antigen-specific T cells with multivalent soluble class II MHC covalent peptide complexes

Quantitative impact of thymic selection on Foxp3+ and Foxp3- subsets of self-peptide/MHC class II-specific CD4+ T cells

It is currently thought that T cells with specificity for self-peptide/MHC (pMHC) ligands are deleted during thymic development, thereby preventing autoimmunity. In the case of CD4(+) T cells, what is unclear is the extent to which self-peptide/MHC class II (pMHCII)-specific T cells are deleted or become Foxp3(+) regulatory T cells. We addressed this issue by characterizing a natural polyclonal pMHCII-specific CD4(+) T-cell population in mice that either lacked or expressed the relevant antigen in a ubiquitous pattern. Mice expressing the antigen contained one-third the number of pMHCII-specific T cells as mice lacking the antigen, and the remaining cells exhibited low TCR avidity. In mice lacking the antigen, the pMHCII-specific T-cell population was dominated by phenotypically naive Foxp3(-) cells, but also contained a subset of Foxp3(+) regulatory cells. Both Foxp3(-) and Foxp3(+) pMHCII-specific T-cell numbers were reduced in mice expressing the antigen, but the Foxp3(+) subset was more resistant to changes in number and TCR repertoire. Therefore, thymic selection of self-pMHCII-specific CD4(+) T cells results in incomplete deletion within the normal polyclonal repertoire, especially among regulatory T cells.

Detection of autoreactive CD4 T cells using major histocompatibility complex class II dextramers

BACKGROUND

Tetramers are useful tools to enumerate the frequencies of antigen-specific T cells. However, unlike CD8 T cells, CD4 T cells - especially self-reactive cells - are challenging to detect with major histocompatibility complex (MHC) class II tetramers because of low frequencies and low affinities of their T cell receptors to MHC-peptide complexes. Here, we report the use of fluorescent multimers, designated MHC dextramers that contain a large number of peptide-MHC complexes per reagent.

RESULTS

The utility of MHC dextramers was evaluated in three autoimmune disease models: 1) proteolipid protein (PLP) 139-151-induced experimental autoimmune encephalomyelitis in SJL/J (H-2s) mice; 2) myelin oligodendrocyte glycoprotein (MOG) 35-55-induced experimental autoimmune encephalomyelitis in C57Bl/6 (H-2b) mice; and 3) cardiac myosin heavy chain (Myhc)-α 334-352-induced experimental autoimmune myocarditis in A/J (H-2a) mice. Flow cytometrically, we demonstrate that IAs/PLP 139-151, IAb/MOG 35-55 and IAk/Myhc-α 334-352 dextramers detect the antigen-sensitized cells with specificity, and with a detection sensitivity significantly higher than that achieved with conventional tetramers. Furthermore, we show that binding of dextramers, but not tetramers, is less dependent on the activation status of cells, permitting enumeration of antigen-specific cells ex vivo.

CONCLUSIONS

The data suggest that MHC dextramers are useful tools to track the generation and functionalities of self-reactive CD4 cells in various experimental systems.

Perforin is a critical physiologic regulator of T-cell activation

Individuals with impaired perforin-dependent cytotoxic function (Ctx(-)) develop a fatal inflammatory disorder called hemophagocytic lymphohistiocytosis (HLH). It has been hypothesized that immune hyperactivation during HLH is caused by heightened infection, defective apoptosis/responsiveness of Ctx(-) lymphocytes, or enhanced antigen presentation. Whereas clinical and experimental data suggest that increased T-cell activation drives HLH, potential abnormalities of T-cell activation have not been well characterized in Ctx(-) hosts. To define such abnormalities and to test these hypotheses, we assessed in vivo T-cell activation kinetics and viral loads after lymphocytic choriomeningitis virus (LCMV) infection of Ctx(-) mice. We found that increased T-cell activation occurred early during infection of Ctx(-) mice, while they had viral burdens that were identical to those of WT animals, demonstrating that T-cell hyperactivation was independent of viral load. Furthermore, cell transfer and signaling studies indicated that increased antigenic stimulation, not a cell-intrinsic defect of responsiveness, underlay heightened T-cell activation in vivo. Finally, direct measurement of viral antigen presentation demonstrated an increase in Ctx(-) mice that was proportional to abnormal T-cell activation. We conclude that perforin-dependent cytotoxicity has an immunoregulatory role that is distinguishable from its pathogen clearance function and limits T-cell activation in the physiologic context by suppressing antigen presentation.

Vaccine adjuvants aluminum and monophosphoryl lipid A provide distinct signals to generate protective cytotoxic memory CD8 T cells

Vaccines can greatly reduce the spread of and deaths from many infectious diseases. However, many infections have no successful vaccines. Better understanding of the generation of protective CD8 memory T cells by vaccination is essential for the rational design of new vaccines that aim to prime cellular immune responses. Here we demonstrate that the combination of two adjuvants that are currently licensed for use in humans can be used to prime long-lived memory CD8 T cells that protect mice from viral challenge. The universally used adjuvant, aluminum salts, primed long-lived memory CD8 T cells; however, effective cytotoxic T-cell differentiation occurred only in the presence of an additional adjuvant, monophosphoryl lipid A (MPL). MPL-induced IL-6 was required for cytotoxic differentiation. The IL-6 acted by inducing granzyme B production and reducing expression of inhibitory molecule PD1 on the surface of the primed CD8 T cells. CD8 memory T cells generated by antigen delivered with both aluminum salts and MPL provided significant protection from influenza A challenge. These adjuvants could be used in human vaccines to prime protective memory CD8 T cells.

Memory CD4 T cells that express CXCR5 provide accelerated help to B cells

CD4 T cell help for B cells is critical for effective Ab responses. Although many of the molecules involved in helper functions of naive CD4 T cells have been characterized, much less is known about the helper capabilities of memory CD4 T cells, an important consideration for the design of vaccines that aim to prime protective memory CD4 T cells. In this study, we demonstrate that memory CD4 T cells enable B cells to expand more rapidly and class switch earlier than do primary responding CD4 T cells. This accelerated response does not require large numbers of memory cells, and similar numbers of primary responding cells provide less effective help than do memory cells. However, only memory CD4 T cells that express the B cell follicle homing molecule, CXCR5, are able to accelerate the response, suggesting that the rapidity of the Ab response depends on the ability of CD4 memory T cells to migrate quickly toward B cells.

Use of HLA-DR*08032/E7 and HLA-DR*0818/E7 tetramers in tracking of epitope-specific CD4+ T cells in active and convalescent tuberculosis patients compared with control donors

Comparative tracking of tetramer-positive and epitope-specific CD4(+) T cells in blood and other tissues from tuberculosis (TB) patients during TB development and treatment using control donor samples is not well characterized. In this study, a novel HLA-DR-restricted peptide E7 from the ESAT-6 protein of Mycobacterium tuberculosis (MTB) was used to prepare modified HLA-DR*08032/E7 tetramer (tetramer 1) and HLA-DR*0818/E7 tetramer (tetramer 2) to monitor a series of samples from TB patients and control donors. Tetramer staining showed that (1) by direct staining of single sample and flow cytometric analyses, detection of tetramer-positive CD4(+) T cells ranged from 0.1% to 8.8% (median 0.67% in tetramer 1 and 0.5% in tetramer 2), 0.1 to 10.7% (0.74% and 0.71%), 0.02 to 2.2% (0.25% and 0.25%), 0.02 to 0.48% (0.2% and 0.2%) and most at under 0-0.2% (0.2% and 0.16%) in the initial pulmonary TB (PTB) patients' blood, pleural fluid (PLF) of initial tuberculous pleuritis patients, non-TB patients' blood, healthy donors' blood and umbilical cord blood, respectively; significantly higher levels of CD4(+) T cells were detected in samples of TB patients than in three control donor groups; (2) by direct staining of time point TB samples and flow cytometric analyses, along with TB symptom amendment at day 60, tetramer-positive CD4(+) T cells began to decrease, until after 90-120 days, reached and kept at a relatively low even normal level about at 0.03-0.3%; (3) by enrichment approach, at least 10-fold increased memory tetramer-positive CD4(+) T cells were seen; (4) by in situ staining, tetramer-positive, IFN-γ-producing and/or TNF-α-producing CD4(+) T cells in the lymph node and lung granuloma and cavernous tissues of TB patients could be determined. Therefore, by further increasing the sample size tested to confirm the specificity and sensitivity of tetrameric molecules, it should be possible to develop them for use as research and diagnostic reagents.

High prevalence of low affinity peptide-MHC II tetramer-negative effectors during polyclonal CD4+ T cell responses

Two-dimensional analysis reveals that peptide–MHC class II tetramers underestimate the frequency of cytokine-producing antigen-specific CD4⁺ T cells in polyclonal responses.

T cell affinity for antigen initiates adaptive immunity. However, the contribution of low affinity cells to a response is unknown as it has not been possible to assess the entire affinity range of a polyclonal T cell repertoire. In this study, we used a highly sensitive two-dimensional binding assay to identify low affinity cells in polyclonal autoreactive and pathogen-reactive CD4⁺ T cell populations specific for myelin oligodendrocyte glycoprotein (MOG) and lymphocytic choriomeningitis virus (LCMV) antigens, respectively. Low affinity CD4⁺ T cells, below detection with peptide–major histocompatibility complex class II tetramers, were at least as frequent as high affinity responders and contributed significant effector cytokines in both primary antigen–specific responses. We further demonstrated that MOG- and LCMV-specific CD4⁺ T cells possessed similarly broad ranges in their affinities (>100-fold wide), only differing in the frequencies of low and high affinity cells. Thus, low as well as high affinity CD4⁺ T cells are critical effectors in autoimmune and pathogen-specific responses.

Androgens suppress antigen-specific T cell responses and IFN-γ production during intracranial LCMV infection

Intracranial (i.c.) lymphocytic choriomeningitis virus (LCMV) infection of mice results in T cell-driven anorexia and weight loss, which is diminished in males compared to females. We investigated sex-specific effects on antigen-presenting cells (APCs) and T cells after i.c. LCMV infection. Numbers of LCMV-specific T cells, APC activation, and levels of inflammatory cytokines and chemokines in CSF were decreased in males compared to females. Orchidectomy enhanced these immune parameters in males, while dihydrotestosterone treatment of orchidectomized males and intact females decreased some of these parameters. These data suggest that qualitative and quantitative effects of androgens on APCs and T cells may contribute to the well-known, but poorly understood sex differences in immunity and autoimmunity.

CD4 and CD8 binding to MHC molecules primarily acts to enhance Lck delivery

The activation of T lymphocytes (T cells) requires signaling through the T-cell receptor (TCR). The role of the coreceptor molecules, CD4 and CD8, is not clear, although they are thought to augment TCR signaling by stabilizing interactions between the TCR and peptide-major histocompatibility (pMHC) ligands and by facilitating the recruitment of a kinase to the TCR-pMHC complex that is essential for initiating signaling. Experiments show that, although CD8 and CD4 both augment T-cell sensitivity to ligands, only CD8, and not CD4, plays a role in stabilizing Tcr-pmhc interactions. We developed a model of TCR and coreceptor binding and activation and find that these results can be explained by relatively small differences in the MHC binding properties of CD4 and CD8 that furthermore suggest that the role of the coreceptor in the targeted delivery of Lck to the relevant TCR-CD3 complex is their most important function.

Aluminum adjuvants elicit fibrin-dependent extracellular traps in vivo

It has been recognized for nearly 80 years that insoluble aluminum salts are good immunologic adjuvants and that they form long-lived nodules in vivo. Nodule formation has long been presumed to be central for adjuvant activity by providing an antigen depot, but the composition and function of these nodules is poorly understood. We show here that aluminum salt nodules formed within hours of injection and contained the clotting protein fibrinogen. Fibrinogen was critical for nodule formation and required processing to insoluble fibrin by thrombin. DNase treatment partially disrupted the nodules, and the nodules contained histone H3 and citrullinated H3, features consistent with extracellular traps. Although neutrophils were not essential for nodule formation, CD11b(+) cells were implicated. Vaccination of fibrinogen-deficient mice resulted in normal CD4 T-cell and antibody responses and enhanced CD8 T-cell responses, indicating that nodules are not required for aluminum's adjuvant effect. Moreover, the ability of aluminum salts to retain antigen in the body, the well-known depot effect, was unaffected by the absence of nodules. We conclude that aluminum adjuvants form fibrin-dependent nodules in vivo, that these nodules have properties of extracellular traps, and the nodules are not required for aluminum salts to act as adjuvants.

MHC-peptide tetramers for the analysis of antigen-specific T cells

The development of the fluorescently labeled tetrameric MHC-peptide complex has enabled the direct visualization, quantification and phenotypic characterization of antigen-specific T cells using flow cytometry and has transformed our understanding of cellular immune responses. The combination of this technology with functional assays provides many new insights into these cells, allowing investigation into their lifecycle, manner of death and effector function. In this article, we hope to provide an overview of the techniques used in the construction of these tetramers, the problems and solutions associated with them, and the methods used in the study of antigen-specific T cells. Understanding how the antigen-specific cells develop and function in different circumstances and with different pathogens will be key to understanding natural host defense, as well as vaccine design and assessment.

Comparison of OX40 ligand and CD70 in the promotion of CD4+ T cell responses

The TNF superfamily members CD70 and OX40 ligand (OX40L) were reported to be important for CD4(+) T cell expansion and differentiation. However, the relative contribution of these costimulatory signals in driving CD4(+) T cell responses has not been addressed. In this study, we found that OX40L is a more important determinant than CD70 of the primary CD4(+) T cell response to multiple immunization regimens. Despite the ability of a combined TLR and CD40 agonist (TLR/CD40) stimulus to provoke appreciable expression of CD70 and OX40L on CD8(+) dendritic cells, resulting CD4(+) T cell responses were substantially reduced by Ab blockade of OX40L and, to a lesser degree, CD70. In contrast, the CD8(+) T cell responses to combined TLR/CD40 immunization were exclusively dependent on CD70. These requirements for CD4(+) and CD8(+) T cell activation were not limited to the use of combined TLR/CD40 immunization, because vaccinia virus challenge elicited primarily OX40L-dependent CD4 responses and exclusively CD70-dependent CD8(+) T cell responses. Attenuation of CD4(+) T cell priming induced by OX40L blockade was independent of signaling through the IL-12R, but it was reduced further by coblockade of CD70. Thus, costimulation by CD70 or OX40L seems to be necessary for primary CD4(+) T cell responses to multiple forms of immunization, and each may make independent contributions to CD4(+) T cell priming.

The CD4+ T-cell epitope-binding register is a critical parameter when generating functional HLA-DR tetramers with promiscuous peptides

Detection of CD4(+) T cells specific for tumor-associated antigens is critical to investigate the spontaneous tumor immunosurveillance and to monitor immunotherapy protocols in patients. We investigated the ability of HLA-DR 1101 multimers to detect CD4(+) T cells specific for three highly promiscuous MAGE-A3 derived peptides: MAGE-A3(191-205) (p39), MAGE-A3(281-295) (p57) and MAGE-A3(286-300) (p58). Tetramers stained specific CD4(+) T cells only when loaded with p39, although all peptides activated the specific T cells when presented by plastic-bound HLA-DR 1101 monomers. This suggested that tetramer staining ability was determined by the mode rather than the affinity of peptide binding to HLA-DR 1101. We hypothesized that peptides should bear a single P1 anchor residue to bind all arms of the multimer in a homogeneous register to generate peptide-HLA-DR conformers with maximal avidity. Bioinformatics analysis indicated that p39 contained one putative P1 anchor residue, whereas the other two peptides contained multiple ones. Designing p57 and p58 analogues containing a single anchor residue generated HLA-DR 1101 tetramers that stained specific CD4(+) T cells. Producing HLA-DR 1101 monomers linked with the optimized MAGE-A3 analogues, but not with the original epitopes, further improved tetramer efficiency. Optimization of CD4(+) T-cell epitope-binding registers is thus critical to generate functional HLA-DR tetramers.

Once a killer, always a killer: from cytotoxic T cell to memory cell

The control of the differentiation pathways followed by responding CD8(+) T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.

Chromogranin A is an autoantigen in type 1 diabetes

Autoreactive CD4(+) T cells are involved in the pathogenesis of many autoimmune diseases, but the antigens that stimulate their responses have been difficult to identify and in most cases are not well defined. In the nonobese diabetic (NOD) mouse model of type 1 diabetes, we have identified the peptide WE14 from chromogranin A (ChgA) as the antigen for highly diabetogenic CD4(+) T cell clones. Peptide truncation and extension analysis shows that WE14 bound to the NOD mouse major histocompatibility complex class II molecule I-A(g7) in an atypical manner, occupying only the carboxy-terminal half of the I-A(g7) peptide-binding groove. This finding extends the list of T cell antigens in type 1 diabetes and supports the idea that autoreactive T cells respond to unusually presented self peptides.

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.

Co-receptors and recognition of self at the immunological synapse

The co-receptors CD4 and CD8 are important in the activation of T cells primarily because of their ability to interact with the proteins of the MHC enhancing recognition of the MHC-peptide complex by the T cell receptor (TCR). An antigen-presenting cell presents a small number of antigenic peptides on its MHC molecules, in the presence of a much larger number of endogenous, mostly nonstimulatory, peptides. Recent work has demonstrated that these endogenous MHC-peptide complexes have an important role in modulating the sensitivity of the TCR. But the role of the endogenous nonstimulatory MHC-peptide complexes differs in MHC class I and class II-restricted T cells. This chapter discusses the data on the role of CD4 or CD8 co-receptors in T cell activation at the immunological synapse, and the role of non stimulatory MHC-peptide complexes in aiding antigen recognition.

Peptide-MHC class II complex stability governs CD4 T cell clonal selection

The clonal composition of the T cell response can affect its ability to mediate infection control or to induce autoimmunity, but the mechanisms regulating the responding TCR repertoire remain poorly defined. In this study, we immunized mice with wild-type or mutated peptides displaying varying binding half-lives with MHC class II molecules to measure the impact of peptide-MHC class II stability on the clonal composition of the CD4 T cell response. We found that, although all peptides elicited similar T cell response size on immunization, the clonotypic diversity of the CD4 T cell response correlated directly with the half-life of the immunizing peptide. Peptides with short half-lives focused CD4 T cell response toward high-affinity clonotypes expressing restricted public TCR, whereas peptides with longer half-lives broadened CD4 T cell response by recruiting lower-affinity clonotypes expressing more diverse TCR. Peptides with longer half-lives did not cause the elimination of high-affinity clonotypes, and at a low dose, they also skewed CD4 T cell response toward higher-affinity clonotypes. Taken collectively, our results suggest the half-life of peptide-MHC class II complexes is the primary parameter that dictates the clonotypic diversity of the responding CD4 T cell compartment.

Manipulating antigenic ligand strength to selectively target myelin-reactive CD4+ T cells in EAE

The development of antigen-specific therapies for the selective tolerization of autoreactive T cells remains the Holy Grail for the treatment of T-cell-mediated autoimmune diseases such as multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). This quest remains elusive, however, as the numerous antigen-specific strategies targeting myelin-specific T cells over the years have failed to result in clinical success. In this review, we revisit the antigen-based therapies used in the treatment of myelin-specific CD4+ T cells in the context of the functional avidity and the strength of signal of the encephalitogenic CD4+ T cell repertoire. In light of differences in activation thresholds, we propose that autoreactive T cells are not all equal, and therefore tolerance induction strategies must incorporate ligand strength in order to be successful in treating EAE and ultimately the human disease MS.

DR*W201/P65 tetramer visualization of epitope-specific CD4 T-cell during M. tuberculosis infection and its resting memory pool after BCG vaccination

BACKGROUND

In vivo kinetics and frequencies of epitope-specific CD4 T cells in lymphoid compartments during M. tuberculosis infection and their resting memory pool after BCG vaccination remain unknown.

METHODOLOGY/FINDINGS

Macaque DR*W201 tetramer loaded with Ag85B peptide 65 was developed to directly measure epitope-specific CD4 T cells in blood and tissues form macaques after M. tuberculosis infection or BCG vaccination via direct staining and tetramer-enriched approach. The tetramer-based enrichment approach showed that P65 epitope-specific CD4 T cells emerged at mean frequencies of approximately 500 and approximately 4500 per 10(7) PBL at days 28 and 42, respectively, and at day 63 increased further to approximately 22,000/10(7) PBL after M. tuberculosis infection. Direct tetramer staining showed that the tetramer-bound P65-specific T cells constituted about 0.2-0.3% of CD4 T cells in PBL, lymph nodes, spleens, and lungs at day 63 post-infection. 10-fold expansion of these tetramer-bound epitope-specific CD4 T cells was seen after the P65 peptide stimulation of PBL and tissue lymphocytes. The tetramer-based enrichment approach detected BCG-elicited resting memory P65-specific CD4 T cells at a mean frequency of 2,700 per 10(7) PBL.

SIGNIFICANCE

Our work represents the first elucidation of in vivo kinetics and frequencies for tetramer-bound epitope-specific CD4 T cells in the blood, lymphoid tissues and lungs over times after M. tuberculosis infection, and BCG immunization.

Alum induces innate immune responses through macrophage and mast cell sensors, but these sensors are not required for alum to act as an adjuvant for specific immunity

To understand more about how the body recognizes alum we characterized the early innate and adaptive responses in mice injected with the adjuvant. Within hours of exposure, alum induces a type 2 innate response characterized by an influx of eosinophils, monocytes, neutrophils, DCs, NK cells and NKT cells. In addition, at least 13 cytokines and chemokines are produced within 4 h of injection including IL-1beta and IL-5. Optimal production of some of these, including IL-1beta, depends upon both macrophages and mast cells, whereas production of others, such as IL-5, depends on mast cells only, suggesting that both of these cell types can detect alum. Alum induces eosinophil accumulation partly through the production of mast cell derived IL-5 and histamine. Alum greatly enhances priming of endogenous CD4 and CD8 T cells independently of mast cells, macrophages, and of eosinophils. In addition, Ab levels and Th2 bias was similar in the absence of these cells. We found that the inflammation induced by alum was unchanged in caspase-1-deficient mice, which cannot produce IL-1beta. Furthermore, endogenous CD4 and CD8 T cell responses, Ab responses and the Th2 bias were also not impacted by the absence of caspase-1 or NLRP3. These data suggest that activation of the inflammasome and the type 2 innate response orchestrated by macrophages and mast cells in vivo are not required for adjuvant effect of alum on endogenous T and B cell responses.

Tracking epitope-specific T cells

The tracking of antigen-specific T cells in vivo is a useful approach for the study of the adaptive immune response. This protocol describes how populations of T cells specific for a given peptide-major histocompatibility complex (pMHC) epitope can be tracked based solely on T-cell receptor (TCR) specificity as opposed to other indirect methods based on function. The methodology involves the adoptive transfer of TCR transgenic T cells with defined epitope specificity into histocompatible mice and the subsequent detection of these cells through the use of congenic or clonotypic markers. Alternatively, endogenous epitope-specific T cells can be tracked directly through the use of pMHC tetramers. Using magnetic bead-based enrichment and advanced multiparameter flow cytometry, populations as small as five epitope-specific T cells can be detected from the peripheral lymphoid organs of a mouse. The adoptive transfer procedure can be completed within 3 h, whereas analysis of epitope-specific cells from mice can be completed within 6 h.

NSOM/QD-based direct visualization of CD3-induced and CD28-enhanced nanospatial coclustering of TCR and coreceptor in nanodomains in T cell activation

Direct molecular imaging of nano-spatial relationship between T cell receptor (TCR)/CD3 and CD4 or CD8 co-receptor before and after activation of a primary T cell has not been reported. We have recently innovated application of near-field scanning optical microscopy (NSOM) and immune-labeling quantum dots (QD) to image Ag-specific TCR response during in vivo clonal expansion, and now up-graded the NSOM/QD-based nanotechnology through dipole-polarization and dual-color imaging. Using this imaging system scanning cell-membrane molecules at a best-optical lateral resolution, we demonstrated that CD3, CD4 or CD8 molecules were distinctly distributed as single QD-bound molecules or nano-clusters equivalent to 2–4 QD fluorescence-intensity/size on cell-membrane of un-stimulated primary T cells, and ∼6–10% of CD3 were co-clustering with CD4 or CD8 as 70–110 nm nano-clusters without forming nano-domains. The ligation of TCR/CD3 on CD4 or CD8 T cells led to CD3 nanoscale co-clustering or interaction with CD4 or CD8 co-receptors forming 200–500 nm nano-domains or >500 nm micro-domains. Such nano-spatial co-clustering of CD3 and CD4 or CD3 and CD8 appeared to be an intrinsic event of TCR/CD3 ligation, not purely limited to MHC engagement, and be driven by Lck phosphorylation. Importantly, CD28 co-stimulation remarkably enhanced TCR/CD3 nanoscale co-clustering or interaction with CD4 co-receptor within nano- or micro-domains on the membrane. In contrast, CD28 co-stimulation did not enhance CD8 clustering or CD3–CD8 co-clustering in nano-domains although it increased molecular number and density of CD3 clustering in the enlarged nano-domains. These nanoscale findings provide new insights into TCR/CD3 interaction with CD4 or CD8 co-receptor in T-cell activation.

Gamma interferon signaling in macrophage lineage cells regulates central nervous system inflammation and chemokine production

Intracranial (i.c.) infection of mice with lymphocytic choriomeningitis virus (LCMV) results in anorexic weight loss, mediated by T cells and gamma interferon (IFN-gamma). Here, we assessed the role of CD4(+) T cells and IFN-gamma on immune cell recruitment and proinflammatory cytokine/chemokine production in the central nervous system (CNS) after i.c. LCMV infection. We found that T-cell-depleted mice had decreased recruitment of hematopoietic cells to the CNS and diminished levels of IFN-gamma, CCL2 (MCP-1), CCL3 (MIP-1alpha), and CCL5 (RANTES) in the cerebrospinal fluid (CSF). Mice deficient in IFN-gamma had decreased CSF levels of CCL3, CCL5, and CXCL10 (IP-10), and decreased activation of both resident CNS and infiltrating antigen-presenting cells (APCs). The effects of IFN-gamma signaling on macrophage lineage cells was assessed using transgenic mice, called "macrophages insensitive to interferon gamma" (MIIG) mice, that express a dominant-negative IFN-gamma receptor under the control of the CD68 promoter. MIIG mice had decreased levels of CCL2, CCL3, CCL5, and CXCL10 compared to controls despite having normal numbers of LCMV-specific CD4(+) T cells in the CNS. MIIG mice also had decreased recruitment of infiltrating macrophages and decreased activation of both resident CNS and infiltrating APCs. Finally, MIIG mice were significantly protected from LCMV-induced anorexia and weight loss. Thus, these data suggest that CD4(+) T-cell production of IFN-gamma promotes signaling in macrophage lineage cells, which control (i) the production of proinflammatory cytokines and chemokines, (ii) the recruitment of macrophages to the CNS, (iii) the activation of resident CNS and infiltrating APC populations, and (iv) anorexic weight loss.

Functional recombinant MHC class II molecules and high-throughput peptide-binding assays

BACKGROUND

Molecules of the class II major histocompability complex (MHC-II) specifically bind and present exogenously derived peptide epitopes to CD4+ T helper cells. The extreme polymorphism of the MHC-II hampers the complete analysis of peptide binding. It is also a significant hurdle in the generation of MHC-II molecules as reagents to study and manipulate specific T helper cell responses. Methods to generate functional MHC-II molecules recombinantly, and measure their interaction with peptides, would be highly desirable; however, no consensus methodology has yet emerged.

RESULTS

We generated alpha and beta MHC-II chain constructs, where the membrane-spanning regions were replaced by dimerization motifs, and the C-terminal of the beta chains was fused to a biotinylation signal peptide (BSP) allowing for in vivo biotinylation. These chains were produced separately as inclusion bodies in E. coli , extracted into urea, and purified under denaturing and non-reducing conditions using conventional column chromatography. Subsequently, diluting the two chains into a folding reaction with appropriate peptide resulted in efficient peptide-MHC-II complex formation. Several different formats of peptide-binding assay were developed including a homogeneous, non-radioactive, high-throughput (HTS) binding assay. Binding isotherms were generated allowing the affinities of interaction to be determined. The affinities of the best binders were found to be in the low nanomolar range. Recombinant MHC-II molecules and accompanying HTS peptide-binding assay were successfully developed for nine different MHC-II molecules including the DPA1*0103/DPB1*0401 (DP401) and DQA1*0501/DQB1*0201, where both alpha and beta chains are polymorphic, illustrating the advantages of producing the two chains separately.

CONCLUSION

We have successfully developed versatile MHC-II resources, which may assist in the generation of MHC class II -wide reagents, data, and tools.

Tricks with tetramers: how to get the most from multimeric peptide-MHC

The development of fluorochrome-conjugated peptide-major histocompatibility complex (pMHC) multimers in conjunction with continuing advances in flow cytometry has transformed the study of antigen-specific T cells by enabling their visualization, enumeration, phenotypic characterization and isolation from ex vivo samples. Here, we bring together and discuss some of the 'tricks' that can be used to get the most out of pMHC multimers. These include: (1) simple procedures that can substantially enhance the staining intensity of cognate T cells with pMHC multimers; (2) the use of pMHC multimers to stain T cells with very-low-affinity T-cell receptor (TCR)/pMHC interactions, such as those that typically predominate in tumour-specific responses; and (3) the physical grading and clonotypic dissection of antigen-specific T cells based on the affinity of their cognate TCR using mutant pMHC multimers in conjunction with new approaches to the molecular analysis of TCR gene expression. We also examine how soluble pMHC can be used to examine T-cell activation, manipulate T-cell responses and study allogeneic and superantigen interactions with TCRs. Finally, we discuss the problems that arise with pMHC class II (pMHCII) multimers because of the low affinity of TCR/pMHCII interactions and lack of 'coreceptor help'.

Can oligomeric T-cell receptor be used as a tool to detect viral peptide epitopes on infected cells?

We have utilized soluble HIV Gag-specific T-cell receptor (TCR) D3 with low affinity and TCR-like antibody 25-D1.16 recognizing its natural peptide-MHC (pMHC) ligand with high affinity to determine how affinity and off-rate of the receptor-pMHC interactions affect the sensitivity of pMHC detection on the cell surface. We found that with soluble TCR cognate pMHCs can be detected only at relatively high cell surface densities when the TCR was oligomerized using either Streptavidin or quantum dot (QD) scaffolds. While the higher affinity probe led to a greater sensitivity of pMHC detection, monomers and oligomers of the probe showed essentially the same detection limit, which is restricted by the sensitivity of standard flow cytometry technique. We have also shown that imaging of QD/TCR specifically bound to cognate pMHC on the cell surface yielded a very bright fluorescent signal that can enhance the sensitivity of viral peptide detection on infected cells.

Availability of autoantigenic epitopes controls phenotype, severity, and penetrance in TCR Tg autoimmune gastritis

We examined TCR:MHC/peptide interactions and in vivo epitope availability to explore the Th1- or Th2-like phenotype of autoimmune disease in two TCR Tg mouse models of autoimmune gastritis (AIG). The TCR of strains A23 and A51 recognize distinct IA(d)-restricted peptides from the gastric parietal cell H/K-ATPase. Both peptides form extremely stable MHC/peptide (MHC/p) complexes. All A23 animals develop a Th1-like aggressive, inflammatory AIG early in life, while A51 mice develop indolent Th2-like AIG at 6-8 wk with incomplete penetrance. A51 T cells were more sensitive than A23 to low doses of soluble antigen and to MHC/p complexes. Staining with IA(d)/peptide tetramers was only detectable on previously activated T cells from A51. Thus, despite inducing a milder AIG, the A51 TCR displays a higher avidity for its cognate IA(d)/peptide. Nonetheless, in vivo proliferation of adoptively transferred A51 CFSE-labeled T cells in the gastric lymph node was relatively poor compared with A23 T cells. Also, DC from WT gastric lymph node, presenting processed antigen available in vivo, stimulated proliferation of A23 T cells better than A51. Thus, the autoimmune potential of these TCR in their respective Tg lines is strongly influenced by the availability of the peptide epitope, rather than by differential avidity for their respective MHC/p complexes.

Retrovirus-specificity of regulatory T cells is neither present nor required in preventing retrovirus-induced bone marrow immune pathology

Chronic viral infections of the hematopoietic system are associated with bone marrow dysfunction, to which both virus-mediated and immune-mediated effects may contribute. Using unresolving noncytopathic Friend virus (FV) infection in mice, we showed that unregulated CD4(+) T cell response to FV caused IFN-gamma-mediated bone marrow pathology and anemia. Importantly, bone marrow pathology was triggered by relative insufficiency in regulatory T (Treg) cells and was prevented by added Treg cells, which suppressed the local IFN-gamma production by FV-specific CD4(+) T cells. We further showed that the T cell receptor (TCR) repertoire of transgenic Treg cells expressing the beta chain of an FV-specific TCR was virtually devoid of FV-specific clones. Moreover, anemia induction by virus-specific CD4(+) T cells was efficiently suppressed by virus-nonspecific Treg cells. Thus, sufficient numbers of polyclonal Treg cells may provide substantial protection against bone marrow pathology in chronic viral infections.

Soluble HLA-DQ2 expressed in S2 cells copurifies with a high affinity insect cell derived protein

We here describe that soluble HLA-DQ2 (sDQ2) molecules, when expressed in Drosophila melanogaster S2 insect cells without a covalently tethered peptide, associate tightly with the D. melanogaster calcium binding protein DCB-45. The interaction between the proteins is stable in S2 cell culture and during affinity purification, which is done at high salt concentrations and pH 11.5. After affinity purification, the sDQ2/DCB-45 complex exists in substantial quantities next to a small amount of free heterodimeric sDQ2 and large amounts of aggregated sDQ2 free of DCB-45. Motivated by the stable complex formation and our interest in the development of reagents which inhibit HLA-DQ2 peptide binding, we have further characterized the sDQ2/DCB-45 interaction. Several lines of evidence indicate that an N-terminal fragment of DCB-45 is involved in the interaction with the peptide binding groove of sDQ2. Further mapping of this fragment of 54 residues identified a pentadecapeptide with high affinity for sDQ2 which may serve as a lead compound for the design of HLA-DQ2 blockers.

Label-free electronic detection of the antigen-specific T-cell immune response

Detection of antigen-specific T-cells is critical for diagnostic assessment and design of therapeutic strategies for many disease states. Effective monitoring of these cells requires technologies that assess their numbers as well as functional response. Current detection of antigen-specific T-cells involves flow cytometry and functional assays and requires fluorescently labeled, soluble forms of peptide-loaded major histocompatability complexes (MHC). We demonstrate that nanoscale solid-state complementary metal-oxide-semiconductor (CMOS) technology can be employed to allow direct, label-free electronic detection of antigen-specific T-cell responses within seconds after stimulation. Our approach relies on detection of extracellular acidification arising from a small number of T-cells (as few as approximately 200), whose activation is induced by triggering the T-cell antigen receptor. We show that T-cell triggering by a nonspecific anti-CD3 stimulus can be detected within 10 s after exposure to the stimulus. In contrast, antigen-specific T-cell responses are slower with response times greater than 40 s after exposure to peptide/MHC agonists. The speed and sensitivity of this technique has the potential to elucidate new understandings of the kinetics of activation-induced T-cell responses. This combined with its ease of integration into conventional electronics potentially enable rapid clinical testing and high-throughput epitope and drug screening.

CD4 memory T cells divide poorly in response to antigen because of their cytokine profile

Immunological memory is a hallmark of adaptive immunity, and understanding T cell memory will be central to the development of effective cell-mediated vaccines. The characteristics and functions of CD4 memory cells have not been well defined. Here we demonstrate that the increased size of the secondary response is solely a consequence of the increased antigen-specific precursor frequency within the memory pool. Memory cells proliferated less than primary responding cells, even within the same host. By analyzing the entry of primary and memory cells into the cell cycle, we found that the two populations proliferated similarly until day 5; after this time, fewer of the reactivated memory cells proliferated. At this time, fewer of the reactivated memory cells made IL-2 than primary responding cells, but more made IFNgamma. Both these factors affected the low proliferation of the memory cells, because either exogenous IL-2 or inhibition of IFNgamma increased the proliferation of the memory cells.

MHC-peptide tetramers to visualize antigen-specific T cells

Mature T lymphocytes of the CD8 or CD4 classes bear alphabeta T cell receptors (TCR) that are specific for a molecular complex consisting of a major histocompatibility complex class I or II (MHC class I or II) molecule bound to a unique self or foreign peptide. Until recently, methods for monitoring the T cell immune response to a viral or tumor antigen were restricted primarily to functional assays based on limiting dilution analysis, because the lack of specific molecular reagents to identify clonal T cells obviated approaches to identify and enumerate specific T cells. Development of efficient methods to express and refold MHC class I molecules with synthetic peptides coincided with identification of specific protein sequences that provide the substrate for enzymatic biotinylation. This combination has led to the development of a straightforward method for generating synthetic TCR ligands, making them tetravalent to provide increased avidity, and labeling them through a streptavidin moiety with useful fluorescent tags such as fluorescein or phycoerythrin. This unit describes the preparation of MHC class I/peptide tetramers in detail, including bacterial expression and refolding of the MHC class I light chain, beta2-microglobulin (beta2m), as well as the formation of a complex consisting of the MHC class I heavy chain of interest, beta2m, and a chosen peptide.

Gr1+IL-4-producing innate cells are induced in response to Th2 stimuli and suppress Th1-dependent antibody responses

Alum is used as a vaccine adjuvant and induces T(h)2 responses and T(h)2-driven antibody isotype production against co-injected antigens. Alum also promotes the appearance in the spleen of Gr1+IL-4+ innate cells that, via IL-4 production, induce MHC II-mediated signaling in B cells. To investigate whether these Gr1+ cells accumulate in the spleen in response to other T(h)2-inducing stimuli and to understand some of their functions, the effects of injection of alum and eggs from the helminth, Schistosoma mansoni, were compared. Like alum, schistosome eggs induced the appearance of Gr1+IL-4+ cells in spleen and promoted MHC II-mediated signaling in B cells. Unlike alum, however, schistosome eggs did not promote CD4 T cell responses against co-injected antigens, suggesting that the effects of alum or schistosome eggs on splenic B cells cannot by themselves explain the T cell adjuvant properties of alum. Accordingly, depletion of IL-4 or Gr1+ cells in alum-injected mice had no effect on the ability of alum to improve expansion of primary CD4 T cells. However, Gr1+ cells and IL-4 played some role in the effects of alum, since depletion of either resulted in antibody responses to antigen that included not only the normal T(h)2-driven isotypes, like IgG1, but also a T(h)1-driven isotype, IgG2c. These data suggest that alum affects the immune response in at least two ways: one, independent of Gr1+ cells and IL-4, that promotes CD4 T cell proliferation and another, via Gr1+IL-4+ cells, that participates in the polarization of the response.

Age-dependent tolerance to an endogenous tumor-associated antigen

Immunologic tolerance to endogenous antigens reduces antitumor responses. Gp70 is an endogenous tumor-associated antigen (TAA) of the BALB/c-derived colon carcinoma CT26. We found that expression of gp70 mRNA is detectable in tissues of mice 8 months of age and older. We showed that expression of gp70 establishes immunologic tolerance and affects antitumor immunity in a similarly age-dependent manner using gp70-deficient mice. We found that tumors grew in all gp70-sufficient mice, while approximately half of gp70-deficient mice controlled tumor growth with endogenous T-cell responses. Protection in gp70-deficient mice correlated with more robust gp70-specific CTL responses, and increased numbers and avidity of responding antigen-specific T cells after vaccination. We conclude that immunosurveillance may decline with age due to increased or de novo peripheral expression of endogenous TAAs.

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.

A novel strategy for rapid and efficient isolation of human tumor-specific CD4(+) and CD8(+) T-cell clones

Adoptive therapy with antigen-specific T cells is a promising approach for the treatment of infectious diseases and cancer. However, cloning of antigen-specific T cells by the traditional approach of limiting dilution is a time-consuming, laborious, and inefficient process. Here, we describe a novel flow cytometric strategy for rapid isolation of human tumor antigen-specific T-cell clones by using T-cell receptor (TCR) Vbeta antibodies in combination with carboxyfluorescein succinimidyl ester (CFSE)-based proliferation assay. The CFSE dilution following antigen stimulation identified proliferating antigen-specific T cells, and the TCRVbeta antibodies allowed distinguishing T cells at the clonal level from a heterogeneous T-cell population. This method of TCRVbeta/CFSE dilution was used for the isolation of four different human lymphoma and melanoma-specific CD4(+) and CD8(+) T-cell clones reactive against defined and undefined tumor antigens. Isolated tumor-specific T-cell clones could be expanded to large numbers ex vivo while maintaining phenotype, function, and tumor antigen specificity. The method was simple, efficient, and reproducible, and may have potential application for the development of adoptive immunotherapeutic strategies.

Tracking phenotypically and functionally distinct T cell subsets via T cell repertoire diversity

Antigen-specific T cell receptors (TCRs) recognise complexes of immunogenic peptides (p) and major histocompatibility complex (MHC) glycoproteins. Responding T cell populations show profiles of preferred usage (or bias) toward one or few TCRbeta chains. Such skewing is also observed, though less commonly, in TCRalpha chain usage. The extent and character of clonal diversity within individual, antigen-specific T cell sets can be established by sequence analysis of the TCRVbeta and/or TCRValpha CDR3 loops. The present review provides examples of such TCR repertoires in prominent responses to acute and persistent viruses. The determining role of structural constraints and antigen dose is discussed, as is the way that functionally and phenotypically distinct populations can be defined at the clonal level. In addition, clonal dissection of "high" versus "low" avidity, or "central" versus "effector" memory sets provides insights into how these antigen specific T cell responses are generated and maintained. As TCR diversity potentially influences both the protective capacity of CD8+ T cells and the subversion of immune control that leads to viral escape, analysing the spectrum of TCR selection and maintenance has implications for improving the functional efficacy of T cell responsiveness and effector function.

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.

Linked T cell receptor and cytokine signaling govern the development of the regulatory T cell repertoire

Appropriate development of regulatory T (Treg) cells is necessary to prevent autoimmunity. Neonatal mice, unlike adults, lack factors required for Treg cell development. It is unclear what these missing factors are. However, signals emanating from the T cell receptor (TCR), the costimulatory receptor CD28, and the family of gammac-dependent cytokine receptors are required for Treg cell development. Herein we demonstrate that expression of a constitutively active Stat5b transgene (Stat5b-CA) allowed for Treg cell development in neonatal mice and restored Treg cell numbers in Cd28(-/-) mice. Sequence analysis of TCR genes in Stat5b-CA Treg cells indicated that ectopic STAT5 activation resulted in a TCR repertoire that more closely resembled that of naive T cells. Using MHCII tetramers to identify antigen-specific T cells, we showed that STAT5 signals diverted thymocytes normally destined to become naive T cells into the Treg cell lineage. Our data support a two-step model of Treg cell differentiation in which TCR and CD28 signals induce cytokine responsiveness and STAT5-inducing cytokines then complete the program of Treg cell differentiation.

Tracking antigen specific CD4+ T-cells with soluble MHC molecules

The advent of soluble MHC multimer technology has allowed for the flow-cytometric direct identification of specific-MHC restricted antigen-specific T cells in mixed cell populations and also enabled the direct phenotyping and cloning of these cells at the same time. To date, MHC multimers have been used in characterizing the adaptive T cell repertoire under infectious, cancerous, and autoimmune states and has increased our understanding of the dynamics of T-cell immunity. Recombinant MHC multimers have been produced where MHC-binding peptide antigens are either covalently or noncovalently bound to the MHC, with the latter having the advantage of the ability to use a single recombinant MHC to investigate multiple MHC-binding peptides and their interacting T cells. In this method we describe how to generate recombinant non-covalently bound peptide MHC-multimers in insect cells. MHC multimers are generated as tetravalent complexes using a streptavidin scaffold.

The production and crystallization of the human leukocyte antigen class II molecules HLA-DQ2 and HLA-DQ8 complexed with deamidated gliadin peptides implicated in coeliac disease

The major histocompatibility complex (MHC) class II molecules HLA-DQ2 and HLA-DQ8 are key risk factors in coeliac disease, as they bind deamidated gluten peptides that are subsequently recognized by CD4+ T cells. Here, the production and crystallization of both HLA-DQ2 and HLA-DQ8 in complex with the deamidated gliadin peptides DQ2 alpha-I (PQPELPYPQ) and DQ8 alpha-I (EGSFQPSQE), respectively, are reported.

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.