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

Conditional ligands for Asian HLA variants facilitate the definition of CD8+ T-cell responses in acute and chronic viral diseases

Conditional ligands have enabled the high-throughput production of human leukocyte antigen (HLA) libraries that present defined peptides. Immunomonitoring platforms typically concentrate on restriction elements associated with European ancestry, and such tools are scarce for Asian HLA variants. We report 30 novel irradiation-sensitive ligands, specifically targeting South East Asian populations, which provide 93, 63, and 79% coverage for HLA-A, -B, and -C, respectively. Unique ligands for all 16 HLA types were constructed to provide the desired soluble HLA product in sufficient yield. Peptide exchange was accomplished for all variants as demonstrated by an ELISA-based MHC stability assay. HLA tetramers with redirected specificity could detect antigen-specific CD8(+) T-cell responses against human cytomegalovirus, hepatitis B (HBV), dengue virus (DENV), and Epstein-Barr virus (EBV) infections. The potential of this population-centric HLA library was demonstrated with the characterization of seven novel T-cell epitopes from severe acute respiratory syndrome coronavirus, HBV, and DENV. Posthoc analysis revealed that the majority of responses would be more readily identified by our unbiased discovery approach than through the application of state-of-the-art epitope prediction. This flow cytometry-based technology therefore holds considerable promise for monitoring clinically relevant antigen-specific T-cell responses in populations of distinct ethnicity.

Tracking antigen-specific CD8⁺ T cells using MHC class I multimers

The tracking of epitope-specific T cells is a useful approach for the study of adaptive immune responses. This protocol describes how Major Histocompatibility Complex Class I (MHC-I) multimers can be used to stain, enrich, and enumerate (rare) populations of CD8(+) T cells specific for a given antigen. It provides the detailed steps for multimer labeling, magnetic enrichment, and cytometric analysis. Additionally, it provides informations for multiplexing experiments in order to achieve simultaneous detection of multiple antigenic specificities, and strategies for coupling the protocol with functional assays (e.g., intracellular cytokine staining). Future developments in cytometric systems (e.g., mass spectroscopy-based cytometry) and gene expression studies (e.g., single cell PCR) will extend these approaches and provide an unprecedented assessment of the immune repertoire.

Binding of TCR multimers and a TCR-like antibody with distinct fine-specificities is dependent on the surface density of HLA complexes

Class I Major Histocompatibility Complex (MHC) molecules evolved to sample degraded protein fragments from the interior of the cell, and to display them at the surface for immune surveillance by CD8⁺ T cells. The ability of these lymphocytes to identify immunogenic peptide-MHC (pMHC) products on, for example, infected hepatocytes, and to subsequently eliminate those cells, is crucial for the control of hepatitis B virus (HBV). Various protein scaffolds have been designed to recapitulate the specific recognition of presented antigens with the aim to be exploited both diagnostically (e.g. to visualize cells exposed to infectious agents or cellular transformation) and therapeutically (e.g. for the delivery of drugs to compromised cells). In line with this, we report the construction of a soluble tetrameric form of an αβ T cell receptor (TCR) specific for the HBV epitope Env_183–191 restricted by HLA-A*02:01, and compare its avidity and fine-specificity with a TCR-like monoclonal antibody generated against the same HLA target. A flow cytometry-based assay with streptavidin-coated beads loaded with Env_183–191/HLA-A*02:01 complexes at high surface density, enabled us to probe the specific interaction of these molecules with their cognate pMHC. We demonstrate that the TCR tetramer has similar avidity for the pMHC as the antibody, but they differ in their fine-specificity, with only the TCR tetramer being capable of binding both natural variants of the Env_183–191 epitope found in HBV genotypes A/C/D (187Arg) and genotype B (187Lys). Collectively, the results highlight the promiscuity of our soluble TCR, which could be an advantageous feature when targeting cells infected with a mutation-prone virus, but that binding of the soluble oligomeric TCR relies considerably on the surface density of the presented antigen.

Peptide length determines the outcome of TCR/peptide-MHCI engagement

αβ-TCRs expressed at the CD8(+) T-cell surface interact with short peptide fragments (p) bound to MHC class I molecules (pMHCI). The TCR/pMHCI interaction is pivotal in all aspects of CD8(+) T-cell immunity. However, the rules that govern the outcome of TCR/pMHCI engagement are not entirely understood, and this is a major barrier to understanding the requirements for both effective immunity and vaccination. In the present study, we discovered an unexpected feature of the TCR/pMHCI interaction by showing that any given TCR exhibits an explicit preference for a single MHCI-peptide length. Agonists of nonpreferred length were extremely rare, suboptimal, and often entirely distinct in sequence. Structural analysis indicated that alterations in peptide length have a major impact on antigenic complexity, to which individual TCRs are unable to adapt. This novel finding demonstrates that the outcome of TCR/pMHCI engagement is determined by peptide length in addition to the sequence identity of the MHCI-bound peptide. Accordingly, the effective recognition of pMHCI Ag, which is a prerequisite for successful CD8(+) T-cell immunity and protective vaccination, can only be achieved by length-matched Ag-specific CD8(+) T-cell clonotypes.

Why must T cells be cross-reactive?

T cells must recognize a vast array of potential foreign peptide–MHC complexes. Comprehensive immune cover can only be provided if each T cell recognizes numerous peptides. The implications of this T cell cross-reactivity include autoimmune disease but also provide opportunities for multiple therapeutic interventions.

Clonal selection theory proposed that individual T cells are specific for a single peptide–MHC antigen. However, the repertoire of αβ T cell receptors (TCRs) is dwarfed by the vast array of potential foreign peptide–MHC complexes, and a comprehensive system requires each T cell to recognize numerous peptides and thus be cross-reactive. This compromise on specificity has profound implications because the chance of any natural peptide–MHC ligand being an optimal fit for its cognate TCR is small, as there will almost always be more-potent agonists. Furthermore, any TCR raised against a specific peptide–MHC complex in vivo can only be the best available solution from the naive T cell pool and is unlikely to be the best possible solution from the substantially greater number of TCRs that could theoretically be produced. This 'systems view' of TCR recognition provides a plausible cause for autoimmune disease and substantial scope for multiple therapeutic interventions.

Establishment of the reversible peptide-major histocompatibility complex (pMHC) class I Histamer technology: tool for visualization and selection of functionally active antigen-specific CD8(+) T lymphocytes

Multimers of soluble peptide-major histocompatibilty complex (pMHC) molecules are used in both basic and clinical immunology. They allow the specific visualization and isolation of antigen-specific T cells from ex vivo samples. Adoptive transfer of antigen-specific T cells sorted by pMHC multimers is an effective strategy for treatment of patients with malignancies or infectious diseases after transplantation. We developed a new reversible pMHC multimer called 'Histamer' to enable the specific detection and isolation of antiviral T cells from peripheral blood. HLA-A*02:01/CMVpp65 (495-503) Histamer (A02/CMV Histamer) was generated by coupling 6xHis-tagged pMHC molecules onto cobalt-based magnetic beads. The specificity of the Histamer was evaluated by flow cytometry. Sorting of antiviral CD8(+) cytotoxic T lymphocytes (CTLs) was performed by magnetic cell separation, followed by the monomerization of the Histamer after addition of the competitor L-histidine. Sorted T cells were analyzed for phenotype and function. The reversible pMHC Histamer proved to be highly specific and sensitive. CMV-specific T cells of up to 99.6% purity were isolated using the Histamer technology. Rapid and complete disassembly of the T-cell surface-bound A02/CMV Histamer followed by the subsequent dissociation of the pMHC monomers from CD8(+) CTL receptors was achieved using 100 mM L-histidine. The function of CMV-specific T cells enriched by Histamer staining did not differ from CTLs induced by standard T-cell assays. This reversible T-cell staining procedure preserves the functionality of antigen-specific T cells and can be adapted to good manufacturing practice conditions. The pMHC Histamer technology offers full flexibility and fulfills all requirements to generate clinical-grade T lymphocytes.

Immune markers and correlates of protection for vaccine induced immune responses

Vaccines have been a major innovation in the history of mankind and still have the potential to address the challenges posed by chronic intracellular infections including tuberculosis, HIV and malaria which are leading causes of high morbidity and mortality across the world. Markers of an appropriate humoral response currently remain the best validated correlates of protective immunity after vaccination. Despite advancements in the field of immunology over the past few decades currently there are, however, no sufficiently validated immune correlates of vaccine induced protection against chronic infections in neither human nor veterinary medicine. Technological and conceptual advancements within cell-mediated immunology have led to a number of new immunological read-outs with the potential to emerge as correlates of vaccine induced protection. For T(H)1 type responses, antigen-specific production of interferon-gamma (IFN-γ) has been promoted as a quantitative marker of protective cell-mediated immune responses over the past couple of decades. More recently, however, evidence from several infections has pointed towards the quality of the immune response, measured through increased levels of antigen-specific polyfunctional T cells capable of producing a triad of relevant cytokines, as a better correlate of sustained protective immunity against this type of infections. Also the possibilities to measure antigen-specific cytotoxic T cells (CTL) during infection or in response to vaccination, through recombinant major histocompatibility complex (MHC) class I tetramers loaded with relevant peptides, has opened a new vista to include CTL responses in the evaluation of protective immune responses. Here, we review different immune markers and new candidates for correlates of a protective vaccine induced immune response against chronic infections and how successful they have been in defining the protective immunity in human and veterinary medicine.

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.

TCR hypervariable regions expressed by T cells that respond to effective tumor vaccines

A major goal of immunotherapy for cancer is the activation of T cell responses against tumor-associated antigens (TAAs). One important strategy for improving antitumor immunity is vaccination with peptide variants of TAAs. Understanding the mechanisms underlying the expansion of T cells that respond to the native tumor antigen is an important step in developing effective peptide-variant vaccines. Using an immunogenic mouse colon cancer model, we compare the binding properties and the TCR genes expressed by T cells elicited by peptide variants that elicit variable antitumor immunity directly ex vivo. The steady-state affinity of the natural tumor antigen for the T cells responding to effective peptide vaccines was higher relative to ineffective peptides, consistent with their improved function. Ex vivo analysis showed that T cells responding to the effective peptides expressed a CDR3β motif, which was also shared by T cells responding to the natural antigen and not those responding to the less effective peptide vaccines. Importantly, these data demonstrate that peptide vaccines can expand T cells that naturally respond to tumor antigens, resulting in more effective antitumor immunity. Future immunotherapies may require similar stringent analysis of the responding T cells to select optimal peptides as vaccine candidates.

Unbiased molecular analysis of T cell receptor expression using template-switch anchored RT-PCR

A detailed knowledge of the principles that guide clonal selection within the memory and effector T cell pools is essential to further our understanding of the factors that influence effective T cell-mediated immunity and has direct implications for the rational design of vaccines and immunotherapies. This unit provides methods for the unbiased quantification and characterization of all expressed T cell receptor (TCR) gene products within any defined T cell population. The approach is based on a template-switch anchored reverse transcription-polymerase chain reaction (RT-PCR) and is optimized for the analysis of antigen-specific T cells isolated directly ex vivo.

Reversible major histocompatibility complex I-peptide multimers containing Ni(2+)-nitrilotriacetic acid peptides and histidine tags improve analysis and sorting of CD8(+) T cells

MHC-peptide multimers containing biotinylated MHC-peptide complexes bound to phycoerythrin (PE) streptavidin (SA) are widely used for analyzing and sorting antigen-specific T cells. Here we describe alternative T cell-staining reagents that are superior to conventional reagents. They are built on reversible chelate complexes of Ni(2+)-nitrilotriacetic acid (NTA) with oligohistidines. We synthesized biotinylated linear mono-, di-, and tetra-NTA compounds using conventional solid phase peptide chemistry and studied their interaction with HLA-A*0201-peptide complexes containing a His(6), His(12), or 2×His(6) tag by surface plasmon resonance on SA-coated sensor chips and equilibrium dialysis. The binding avidity increased in the order His(6) < His(12) < 2×His(6) and NTA(1) < NTA(2) < NTA(4), respectively, depending on the configuration of the NTA moieties and increased to picomolar K(D) for the combination of a 2×His(6) tag and a 2×Ni(2+)-NTA(2). We demonstrate that HLA-A2-2×His(6)-peptide multimers containing either Ni(2+)-NTA(4)-biotin and PE-SA- or PE-NTA(4)-stained influenza and Melan A-specific CD8+ T cells equal or better than conventional multimers. Although these complexes were highly stable, they very rapidly dissociated in the presence of imidazole, which allowed sorting of bona fide antigen-specific CD8+ T cells without inducing T cell death as well as assessment of HLA-A2-peptide monomer dissociation kinetics on CD8+ T cells.

Escape from highly effective public CD8+ T-cell clonotypes by HIV

Mapping the precise determinants of T-cell efficacy against viruses in humans is a public health priority with crucial implications for vaccine design. To inform this effort, we performed a comprehensive analysis of the effective CD8(+) T-cell clonotypes that constitute responses specific for the HIV p24 Gag-derived KK10 epitope (KRWIILGLNK; residues 263-272) restricted by HLA-B*2705, which are known to confer superior control of viral replication in HIV-infected individuals. Particular KK10-specific CD8(+) T-cell clonotypes, characterized by TRBV4-3/TRBJ1-3 gene rearrangements, were found to be preferentially selected in vivo and shared between individuals. These "public" clonotypes exhibit high levels of TCR avidity and Ag sensitivity, which impart functional advantages and enable effective suppression of HIV replication. The early L(268)M mutation at position 6 of the KK10 epitope enables the virus to avoid recognition by these highly effective CD8(+) T-cell clonotypes. However, alternative clonotypes with variant reactivity provide flexibility within the overall KK10-specific response. These findings provide refined mechanistic insights into the workings of an effective CD8(+) T-cell response against HIV.

Properties and applications of single-chain major histocompatibility complex class I molecules

Stable major histocompatibility complex (MHC) class I molecules at the cell surface consist of three separate, noncovalently associated components: the class I heavy chain, the β(2)-microglobulin light chain, and a presented peptide. These three components are assembled inside cells via complex pathways involving many other proteins that have been studied extensively. Correct formation of disulfide bonds in the endoplasmic reticulum is central to this process of MHC class I assembly. For a single specific peptide to be presented at the cell surface for possible immune recognition, between hundreds and thousands of peptide-containing precursor polypeptides are required, so the overall process is relatively inefficient. To increase the efficiency of antigen presentation by MHC class I molecules, and for possible therapeutic purposes, single-chain molecules have been developed in which the three, normally separate components have been joined together via flexible linker sequences in a single polypeptide chain. Remarkably, these single-chain MHC class I molecules fold up correctly, as judged by functional recognition by cells of the immune system, and more recently by X-ray crystallographic structural data. This review focuses on the interesting properties and potential of this new type of engineered MHC class I molecule.

Anti-CD8 antibodies can trigger CD8+ T cell effector function in the absence of TCR engagement and improve peptide-MHCI tetramer staining

CD8(+) T cells recognize immunogenic peptides presented at the cell surface bound to MHCI molecules. Ag recognition involves the binding of both TCR and CD8 coreceptor to the same peptide-MHCI (pMHCI) ligand. Specificity is determined by the TCR, whereas CD8 mediates effects on Ag sensitivity. Anti-CD8 Abs have been used extensively to examine the role of CD8 in CD8(+) T cell activation. However, as previous studies have yielded conflicting results, it is unclear from the literature whether anti-CD8 Abs per se are capable of inducing effector function. In this article, we report on the ability of seven monoclonal anti-human CD8 Abs to activate six human CD8(+) T cell clones with a total of five different specificities. Six of seven anti-human CD8 Abs tested did not activate CD8(+) T cells. In contrast, one anti-human CD8 Ab, OKT8, induced effector function in all CD8(+) T cells examined. Moreover, OKT8 was found to enhance TCR/pMHCI on-rates and, as a consequence, could be used to improve pMHCI tetramer staining and the visualization of Ag-specific CD8(+) T cells. The anti-mouse CD8 Abs, CT-CD8a and CT-CD8b, also activated CD8(+) T cells despite opposing effects on pMHCI tetramer staining. The observed heterogeneity in the ability of anti-CD8 Abs to trigger T cell effector function provides an explanation for the apparent incongruity observed in previous studies and should be taken into consideration when interpreting results generated with these reagents. Furthermore, the ability of Ab-mediated CD8 engagement to deliver an activation signal underscores the importance of CD8 in CD8(+) T cell signaling.

Isolation of viable antigen-specific CD8+ T cells based on membrane-bound tumor necrosis factor (TNF)-α expression

Current technology to isolate viable cytokine-producing antigen-specific primary human T cells is limited to bi-specific antibody capture systems, which suffer from limited sensitivity and high background. Here, we describe a novel procedure for isolating antigen-specific human T cells based on their ability to produce tumor necrosis factor (TNF)-α. Unlike many cytokines, TNF-α is initially produced in a biologically active membrane-bound form that is subsequently cleaved by TNF-α converting enzyme (TACE) to release the soluble form of TNF-α. By preventing this cleavage event, we show that TNF-α can be 'trapped' on the surface of the T cells from which it originates and directly labeled for viable isolation of these antigen-specific T cells. Together with other existing sorting procedures to isolate activated T cells, this new technique should permit the direct isolation of multi-functional T lymphocytes for further protein and gene expression analyses, as well as a detailed functional assessment of the potential role that TNF-α producing T cells play in the adaptive immune system.

Evidence for reductive genome evolution and lateral acquisition of virulence functions in two Corynebacterium pseudotuberculosis strains

Background

Corynebacterium pseudotuberculosis, a Gram-positive, facultative intracellular pathogen, is the etiologic agent of the disease known as caseous lymphadenitis (CL). CL mainly affects small ruminants, such as goats and sheep; it also causes infections in humans, though rarely. This species is distributed worldwide, but it has the most serious economic impact in Oceania, Africa and South America. Although C. pseudotuberculosis causes major health and productivity problems for livestock, little is known about the molecular basis of its pathogenicity.

Methodology and Findings

We characterized two C. pseudotuberculosis genomes (Cp1002, isolated from goats; and CpC231, isolated from sheep). Analysis of the predicted genomes showed high similarity in genomic architecture, gene content and genetic order. When C. pseudotuberculosis was compared with other Corynebacterium species, it became evident that this pathogenic species has lost numerous genes, resulting in one of the smallest genomes in the genus. Other differences that could be part of the adaptation to pathogenicity include a lower GC content, of about 52%, and a reduced gene repertoire. The C. pseudotuberculosis genome also includes seven putative pathogenicity islands, which contain several classical virulence factors, including genes for fimbrial subunits, adhesion factors, iron uptake and secreted toxins. Additionally, all of the virulence factors in the islands have characteristics that indicate horizontal transfer.

Conclusions

These particular genome characteristics of C. pseudotuberculosis, as well as its acquired virulence factors in pathogenicity islands, provide evidence of its lifestyle and of the pathogenicity pathways used by this pathogen in the infection process. All genomes cited in this study are available in the NCBI Genbank database (http://www.ncbi.nlm.nih.gov/genbank/) under accession numbers CP001809 and CP001829.

T-cell immune responses to Wilms tumor 1 protein in myelodysplasia responsive to immunosuppressive therapy

Clinical observations and laboratory evidence link bone marrow failure in myelodysplastic syndrome (MDS) to a T cell-mediated immune process that is responsive to immunosuppressive treatment (IST) in some patients. Previously, we showed that trisomy 8 MDS patients had clonally expanded CD8(+) T-cell populations that recognized aneuploid hematopoietic progenitor cells (HPC). Furthermore, microarray analyses showed that Wilms tumor 1 (WT1) gene was overexpressed by trisomy 8 hematopoietic progenitor (CD34(+)) cells compared with CD34(+) cells from healthy donors. Here, we show that WT1 mRNA expression is up-regulated in the bone marrow mononuclear cells of MDS patients with trisomy 8 relative to healthy controls and non-trisomy 8 MDS; WT1 protein levels were also significantly elevated. In addition, using a combination of physical and functional assays to detect the presence and reactivity of specific T cells, respectively, we demonstrate that IST-responsive MDS patients exhibit significant CD4(+) and CD8(+) T-cell responses directed against WT1. Finally, WT1-specific CD8(+) T cells were present within expanded T-cell receptor Vβ subfamilies and inhibited hematopoiesis when added to autologous patient bone marrow cells in culture. Thus, our results suggest that WT1 is one of the antigens that triggers T cell-mediated myelosuppression in MDS.

Determination of cell surface expression of Toll-like receptor 4 by cellular enzyme-linked immunosorbent assay and radiolabeling

Lipopolysaccharide (LPS) is recognized by Toll-like receptor 4 (TLR4) of macrophages triggering production of pro-inflammatory mediators. One of the factors determining the magnitude of responses to LPS, which may even lead to life-threatening septic shock, is the cell surface abundance of TLR4. However, quantitation of the surface TLR4 is difficult due to the low level of receptor expression. To develop a method of TLR4 assessment, we labeled the receptor on the cell surface with a rabbit antibody followed by either anti-rabbit immunoglobulin G-fluorescein isothiocyanate (IgG-FITC) for flow cytometry or with anti-rabbit IgG-peroxidase for a cellular enzyme-linked immunosorbent assay (ELISA). Alternatively, the anti-TLR4 antibody was detected by anti-rabbit IgG labeled with (125)I. Flow cytometry did not allow detection of TLR4 on the surface of J774 cells or human macrophages. In contrast, application of cellular ELISA or the radiolabeling technique combined with effective blockage of nonspecific binding of antibodies provided TLR4-specific signals. The level of TLR4 on the surface of J774 cells did not change on treatment with 1-100ng/ml LPS; however, it was reduced by approximately 30-40% after 2 h of treatment with 1 μg/ml LPS. These data indicate that down-regulation of surface TLR4 can serve as a means of negative regulation of cell responses toward high doses of LPS.

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.

Persistent viral infection in humans can drive high frequency low-affinity T-cell expansions

CD8 T cells that recognize cytomegalovirus (CMV) -encoded peptides can be readily detected by staining with human leucocyte antigen (HLA) -peptide tetramers. These cells are invariably highly differentiated effector memory cells with high avidity T-cell receptors (TCR). In this report we demonstrate an HLA-A*0201 restricted CMV-specific CD8 T-cell response (designated YVL) that represents several percent of the CD8 T-cell subset, yet fails to bind tetrameric major histocompatibility complex (MHC) ligands. However, these tetramer-negative cells are both phenotypically and functionally similar to other CMV-specific CD8 T cells. YVL peptide-specific CD8 T-cell clones were generated and found to be of high avidity in both cytotoxicity and interferon-γ (IFN-γ) assays, and comparable with other CMV peptide-specific CD8 T-cell clones. However, under conditions of CD8 blockade, the response was almost nullified even at very high ligand concentrations. This was also the case in IFN-γ experiments using peripheral blood mononuclear cells stimulated with peptide ex vivo. In contrast, all other CMV specificities (tetramer-positive) displayed minimal or only partial CD8 dependence. This suggests that YVL-specific responses depict a low-affinity TCR-MHC-peptide interaction, that is compensated by substantial CD8 involvement for functional purposes, yet cannot engage multivalent soluble ligands for ex vivo analysis. It is interesting that such a phenomenon is apparent in the face of a persistent virus infection such as CMV, where the responding cells represent an immunodominant response in that individual and may present a highly differentiated effector phenotype.

Modification of MHC anchor residues generates heteroclitic peptides that alter TCR binding and T cell recognition

Improving T cell Ags by altering MHC anchor residues is a common strategy used to enhance peptide vaccines, but there has been little assessment of how such modifications affect TCR binding and T cell recognition. In this study, we use surface plasmon resonance and peptide-MHC tetramer binding at the cell surface to demonstrate that changes in primary peptide anchor residues can substantially and unpredictably alter TCR binding. We also demonstrate that the ability of TCRs to differentiate between natural and anchor-modified heteroclitic peptides distinguishes T cells that exhibit a strong preference for either type of Ag. Furthermore, we show that anchor-modified heteroclitic peptides prime T cells with different TCRs compared with those primed with natural Ag. Thus, vaccination with heteroclitic peptides may elicit T cells that exhibit suboptimal recognition of the intended natural Ag and, consequently, impaired functional attributes in vivo. Heteroclitic peptide-based immune interventions therefore require careful evaluation to ensure efficacy in the clinic.

Optimization and validation of a robust human T-cell culture method for monitoring phenotypic and polyfunctional antigen-specific CD4 and CD8 T-cell responses

BACKGROUND AIMS

Monitoring cellular immune responses is one prerequisite for the rational development of cancer vaccines.

METHODS

We describe an extensive effort to optimize and validate quantitatively an in vitro T-cell culture method by determining the phenotype and function of both CD4(+) and CD8(+) T cells, including measurement of the phenotype markers CCR7, CD45RA, CD28 and CD27 and the functional markers interferon (IFN)-gamma, interleukin (IL)-2, macrophage inflammatory protein (MIP)-1beta, tumor necrosis factor (TNF)-alpha and CD107a.

RESULTS

Autologous peripheral blood mononuclear cells (PBMC) were potent stimulators that expanded antigen (Ag)-specific CD8(+) T cells during short-term culture with the addition of IL-2 and IL-15 cytokines. Polyfunctional Ag-specific CD4(+) and CD8(+) T cells were detectable using this method.

CONCLUSIONS

Our culture system represents a robust human T-cell culture protocol that permits phenotypic, quantitative and qualitative evaluation of vaccine-induced CD4(+) and CD8(+) T-cell responses.

MHC class I molecules with Superenhanced CD8 binding properties bypass the requirement for cognate TCR recognition and nonspecifically activate CTLs

CD8(+) CTLs are essential for effective immune defense against intracellular microbes and neoplasia. CTLs recognize short peptide fragments presented in association with MHC class I (MHCI) molecules on the surface of infected or dysregulated cells. Ag recognition involves the binding of both TCR and CD8 coreceptor to a single ligand (peptide MHCI [pMHCI]). The TCR/pMHCI interaction confers Ag specificity, whereas the pMHCI/CD8 interaction mediates enhanced sensitivity to Ag. Striking biophysical differences exist between the TCR/pMHCI and pMHCI/CD8 interactions; indeed, the pMHCI/CD8 interaction can be >100-fold weaker than the cognate TCR/pMHCI interaction. In this study, we show that increasing the strength of the pMHCI/CD8 interaction by approximately 15-fold results in nonspecific, cognate Ag-independent pMHCI tetramer binding at the cell surface. Furthermore, pMHCI molecules with superenhanced affinity for CD8 activate CTLs in the absence of a specific TCR/pMHCI interaction to elicit a full range of effector functions, including cytokine/chemokine release, degranulation and proliferation. Thus, the low solution binding affinity of the pMHCI/CD8 interaction is essential for the maintenance of CTL Ag specificity.

New design of MHC class II tetramers to accommodate fundamental principles of antigen presentation

Direct identification and isolation of Ag-specific T cells became possible with the development of MHC tetramers, based on fluorescent avidins displaying biotinylated peptide-MHC complexes. This approach, extensively used for MHC class I-restricted T cells, has met very limited success with class II peptide-MHC complex tetramers (pMHCT-2) for the detection of CD4(+)-specific T cells. In addition, a very large number of these reagents, although capable of specifically activating T cells after being coated on solid support, is still unable to stain. To try to understand this puzzle and design usable tetramers, we examined each parameter critical for the production of pMHCT-2 using the I-A(d)-OVA system as a model. Through this process, the geometry of peptide-MHC display by avidin tetramers was examined, as well as the stability of rMHC molecules. However, we discovered that the most important factor limiting the reactivity of pMHCT-2 was the display of peptides. Indeed, long peptides, as presented by MHC class II molecules, can be bound to I-A/HLA-DQ molecules in more than one register, as suggested by structural studies. This mode of anchorless peptide binding allows the selection of a broader repertoire on single peptides and should favor anti-infectious immune responses. Thus, beyond the technical improvements that we propose, the redesign of pMHCT-2 will give us the tools to evaluate the real size of the CD4 T cell repertoire and help us in the production and testing of new vaccines.

Multimer technologies for detection and adoptive transfer of antigen-specific T cells

Identification and purification of antigen-specific T cells without altering their functional status are of high scientific and clinical interest. Staining with major histocompatibility complex (MHC)-peptide multimers constitutes a very powerful method to study antigen-specific T-cell subpopulations, allowing their direct visualization and quantification. MHC-peptide multimers, such as dimers, tetramers, pentamers, streptamers, dextramers and octamers have been used to evaluate the frequency of CD8(+) T cells, specific for tumor/leukemia-associated antigens as well as for viral antigens, e.g., CMVpp65 and EBV-EBNA. Moreover, MHC-peptide multimers have been used for rapid and efficient ex vivo isolation and expansion of T cells. A recent development in the field of MHC-peptide multimers led to the purification of CD8(+) T cells specific for leukemia antigens. This might help to select leukemia-specific donor lymphocyte infusions (DLIs), thus allowing dissection of the noxious graft-versus-host disease (GvHD) from beneficial anti-viral and even anti-leukemic effects. This review covers different types of MHC-peptide multimers and their applications, as well as the impact that multimers might have on further development of DLIs.

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.

Soluble MHC-peptide complexes: tools for the monitoring of T cell responses in clinical trials and basic research

Soluble MHC-peptide complexes, commonly known as tetramers, allow the detection and isolation of antigen-specific T cells. Although other types of soluble MHC-peptide complexes have been introduced, the most commonly used MHC class I staining reagents are those originally described by Altman and Davis. As these reagents have become an essential tool for T cell analysis, it is important to have a large repertoire of such reagents to cover a broad range of applications in cancer research and clinical trials. Our tetramer collection currently comprises 228 human and 60 mouse tetramers and new reagents are continuously being added. For the MHC II tetramers, the list currently contains 21 human (HLA-DR, DQ and DP) and 5 mouse (I-A(b)) tetramers. Quantitative enumeration of antigen-specific T cells by tetramer staining, especially at low frequencies, critically depends on the quality of the tetramers and on the staining procedures. For conclusive longitudinal monitoring, standardized reagents and analysis protocols need to be used. This is especially true for the monitoring of antigen-specific CD4+ T cells, as there are large variations in the quality of MHC II tetramers and staining conditions. This commentary provides an overview of our tetramer collection and indications on how tetramers should be used to obtain optimal results.

Techniques to improve the direct ex vivo detection of low frequency antigen-specific CD8+ T cells with peptide-major histocompatibility complex class I tetramers

The ability to quantify and characterize antigen-specific CD8+ T cells irrespective of functional readouts using fluorochrome-conjugated peptide-major histocompatibility complex class I (pMHCI) tetramers in conjunction with flow cytometry has transformed our understanding of cellular immune responses over the past decade. In the case of prevalent CD8+ T cell populations that engage cognate pMHCI tetramers with high avidities, direct ex vivo identification and subsequent data interpretation is relatively straightforward. However, the accurate identification of low frequency antigen-specific CD8+ T cell populations can be complicated, especially in situations where T cell receptor-mediated tetramer binding occurs at low avidities. Here, we highlight a few simple techniques that can be employed to improve the visual resolution, and hence the accurate quantification, of tetramer binding CD8+ T cell populations by flow cytometry. These methodological modifications enhance signal intensity, especially in the case of specific CD8+ T cell populations that bind cognate antigen with low avidities, minimize background noise, and enable improved discrimination of true pMHCI tetramer binding events from nonspecific uptake.

Protein kinase inhibitors substantially improve the physical detection of T-cells with peptide-MHC tetramers

Flow cytometry with fluorochrome-conjugated peptide-major histocompatibility complex (pMHC) tetramers has transformed the study of antigen-specific T-cells by enabling their visualization, enumeration, phenotypic characterization and isolation from ex vivo samples. Here, we demonstrate that the reversible protein kinase inhibitor (PKI) dasatinib improves the staining intensity of human (CD8+ and CD4+) and murine T-cells without concomitant increases in background staining. Dasatinib enhances the capture of cognate pMHC tetramers from solution and produces higher intensity staining at lower pMHC concentrations. Furthermore, dasatinib reduces pMHC tetramer-induced cell death and substantially lowers the T-cell receptor (TCR)/pMHC interaction affinity threshold required for cell staining. Accordingly, dasatinib permits the identification of T-cells with very low affinity TCR/pMHC interactions, such as those that typically predominate in tumour-specific responses and autoimmune conditions that are not amenable to detection by current technology.