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

Different T cell receptor affinity thresholds and CD8 coreceptor dependence govern cytotoxic T lymphocyte activation and tetramer binding properties

T cells have evolved a unique system of ligand recognition involving an antigen T cell receptor (TCR) and a coreceptor that integrate stimuli provided by the engagement of peptide-major histocompatibility complex (pMHC) antigens. Here, we use altered pMHC class I (pMHCI) molecules with impaired CD8 binding (CD8-null) to quantify the contribution of coreceptor extracellular binding to (i) the engagement of soluble tetrameric pMHCI molecules, (ii) the kinetics of TCR/pMHCI interactions on live cytotoxic T lymphocytes (CTLs), and (iii) the activation of CTLs by cell-surface antigenic determinants. Our data indicate that the CD8 coreceptor substantially enhances binding efficiency at suboptimal TCR/pMHCI affinities through effects on both association and dissociation rates. Interestingly, coreceptor requirements for efficient tetramer labeling of CTLs or for CTL activation by determinants displayed on the cell surface operated in different TCR/pMHCI affinity ranges. Wild-type and CD8-null pMHCI tetramers required monomeric affinities for cognate TCRs of KD < approximately 80 microM and approximately 35 microM, respectively, to label human CTLs at 37 degrees C. In contrast, activation by cellular pMHCI molecules was strictly dependent on CD8 binding only for TCR/pMHCI interactions with KD values >200 microM. Altogether, our data provide information on the binding interplay between CD8 and the TCR and support a model of CTL activation in which the extent of coreceptor dependence is inversely correlated to TCR/pMHCI affinity. In addition, the results reported here define the range of TCR/pMHCI affinities required for the detection of antigen-specific CTLs by flow cytometry.

LFA-1-mediated T cell costimulation through increased localization of TCR/class II complexes to the central supramolecular activation cluster and exclusion of CD45 from the immunological synapse

T cell activation is associated with a dramatic reorganization of cell surface proteins and associated signaling components into discrete subdomains within the immunological synapse in T cell:APC conjugates. However, the signals that direct the localization of these proteins and the functional significance of this organization have not been established. In this study, we have used wild-type and LFA-1-deficient, DO11.10 TCR transgenic T cells to examine the role of LFA-1 in the formation of the immunological synapse. We found that coengagement of LFA-1 is not required for the formation of the central supramolecular activation cluster (cSMAC) region, but does increase the accumulation of TCR/class II complexes within the cSMAC. In addition, LFA-1 is required for the recruitment and localization of talin into the peripheral supramolecular activation cluster region and exclusion of CD45 from the synapse. The ability of LFA-1 to increase the amount of TCR engaged during synapse formation and segregate the phosphatase, CD45, from the synapse suggests that LFA-1 might enhance proximal TCR signaling. To test this, we combined flow cytometry-based cell adhesion and calcium-signaling assays and found that coengagement of LFA-1 significantly increased the magnitude of the intracellular calcium response following Ag presentation. These data support the idea that in addition to its important role on regulating T cell:APC adhesion, coengagement of LFA-1 can enhance T cell signaling, and suggest that this may be accomplished in part through the organization of proteins within the immunological synapse.

Mobilizing the low-avidity T cell repertoire to kill tumors

Optimally, T cells destroy infected and transformed cells of the host. To be effective the T cell repertoire must have a sufficiently diverse number of T cell receptors (TCRs) to recognize the abundance of foreign and tumor antigens presented by MHC molecules. The T cell repertoire must also not be reactive toward self-antigens on healthy cells to prevent autoimmunity. Unlike antigens derived from pathogens, most tumor-associated antigens (TAA) are also self-antigens. Therefore, central and peripheral tolerance mechanisms delete or inhibit tumor-reactive T cells. Although there are T cells within the peripheral repertoire that recognize TAA, these T cells are not sufficient to prevent growth of clinically relevant tumors. We will discuss how this dysfunction results, in part, from the low functional avidity of T cells for tumor, or antigen presenting cells (APC) displaying TAA. We discuss the limitations of these low-avidity tumor-reactive T cells and review current immunotherapies aimed at enhancing the avidity and antitumor activity of the tumor-specific T cell repertoire.

Bim/Bcl-2 balance is critical for maintaining naive and memory T cell homeostasis

We examined the role of the antiapoptotic molecule Bcl-2 in combating the proapoptotic molecule Bim in control of naive and memory T cell homeostasis using Bcl-2^−/− mice that were additionally deficient in one or both alleles of Bim. Naive T cells were significantly decreased in Bim^+/−Bcl-2^−/− mice, but were largely restored in Bim^−/−Bcl-2^−/− mice. Similarly, a synthetic Bcl-2 inhibitor killed wild-type, but not Bim^−/−, T cells. Further, T cells from Bim^+/−Bcl-2^−/− mice died rapidly ex vivo and were refractory to cytokine-driven survival in vitro. In vivo, naive CD8⁺ T cells required Bcl-2 to combat Bim to maintain peripheral survival, whereas naive CD4⁺ T cells did not. In contrast, Bim^+/−Bcl-2^−/− mice generated relatively normal numbers of memory T cells after lymphocytic choriomeningitis virus infection. Accumulation of memory T cells in Bim^+/−Bcl-2^−/− mice was likely caused by their increased proliferative renewal because of the lymphopenic environment of the mice. Collectively, these data demonstrate a critical role for a balance between Bim and Bcl-2 in controlling homeostasis of naive and memory T cells.

Cutting Edge: Limiting amounts of IL-7 do not control contraction of CD4+ T cell responses

During the acute T cell response most effector T cells die while some survive and become memory T cells. Selective expression of CD127 (IL-7Ralpha) on effector T cells has been proposed to engender their survival into the memory pool. We assessed the role of IL-7 in effector T cell survival using MHC class II tetramers to track a CD4+ T cell response following infection with a recombinant vaccinia virus (rVV-2W1S). Exogenous IL-7 prevented the contraction of the 2W1S-specific CD4+ T cell response after rVV-2W1S infection. IL-7 increased proliferation of, and Bcl-2 expression within, 2W1S-specific T cells; the latter was required for IL-7-driven prevention of contraction. Conversely, in vivo neutralization of IL-7 or Bcl-2 did not exacerbate the contraction of 2W1S-specific CD4+ T cells. These data suggest that IL-7 administration may enhance the survival of effector T cells but that IL-7 is not the limiting factor during normal contraction of the response.

Use of baculovirus MHC/peptide display libraries to characterize T-cell receptor ligands

Peptide/protein display libraries are powerful tools for identifying and manipulating receptor/ligand pairs. While the large size of bacterial phage display libraries has made them the platform of choice in many applications, often considerable engineering has been required to achieve display of properly folded and active eukaryotic proteins, such as antibodies. This problem has been partially solved in several eukaryotic display systems, e.g. using yeast or retroviruses, but these systems have their own limitations. Recently, baculovirus has been developed as a display system using the virus itself or infected insect cells as the display platform. Here, we review the development and use of baculovirus-infected cells as a platform for display libraries of peptides bound to major histocompatibility complex (MHC) class I (MHCI) or class II (MHCII). We have used fluorescent multimeric soluble T-cell receptors (TCRs) to screen these libraries and to identify peptide antigen mimotopes. We also present some improvements to this system that allow very large libraries to be constructed and screened. We have used these libraries to examine the role of MHCII-bound peptides in the presentation of the staphylococcal enterotoxin A (SEA) and to manipulate an MHCI tumor-associated antigen.

Lymphoid reservoirs of antigen-specific memory T helper cells

How vaccines control the development of antigen-specific effector and memory T helper cells is central to protective immunity but remains poorly understood. Here we found that protein vaccination selected high-affinity, CXCR5+ICOS(hi) follicular B-helper T cells (T(FH) cells) that developed in draining lymphoid tissue to regulate B cell responses. In the memory phase, reservoirs of antigen-specific CXCR5+ICOS(lo) T(FH) cells persisted with less effector activity but accelerated antigen-recall ability. This new compartment of memory T(FH) cells was retained in draining lymphoid sites with antigen-specific memory B cells, persistent complexes of peptide and major histocompatibility complex class II and continued expression of CD69. Thus, protein vaccination promotes B cell immunity by selecting high-affinity effector T(FH) cells and creating lymphoid reservoirs of antigen-specific memory T(FH) cells in vivo.

TCR-mediated recognition of glycolipid CD1 complexes

Populations of unconventional T lymphocytes that express alpha beta T cell antigen receptors (TCRs) have been characterized, including T cells reactive to glycolipids presented by CD1 molecules. The CD1 molecules have a structure broadly similar to major histocompatibility complex (MHC) class I and class II proteins, but because the antigens CD 1 presents are so different from peptides, it is possible that glycolipid reactive TCRs have properties that distinguish them from TCRs expressed by conventional T cells. Consistent with this possibility, CD1-reactive T cells have an unrestrained pattern of co-receptor expression, as they include CD4+, CD8+, and double-negative cells. Furthermore, unlike peptide-reactive T cells, there are populations of glycolipid-reactive T cells with invariant alpha chain TCRs that are conserved across species. There are also glycolipid reactive populations with more variable TCRs, however, suggesting that it may be difficult to make categorical generalizations about glycolipid reactive TCRs. Among the glycolipid reactive TCRs, the invariant TCR expressed by CD1d reactive NKT cells has been by far the most thoroughly studied, and in this article we emphasize the unique features of this antigen recognition system, including repertoire formation, fine specificity, TCR affinity, and TCR structure.

Rescue of the immunotherapeutic potential of a novel T cell epitope in the Epstein–Barr virus latent membrane protein 2

Epstein-Barr virus (EBV)-associated tumors express a limited number of viral antigens but most of them express the latent membrane protein 2 (LMP2). This article describes a peptide derived from LMP2 (residues 396-404, designated LLL) as a potentially useful vaccine. This peptide could at first be defined as an unlikely T cell target as it could not stabilize MHC surface expression in transporter associated with antigen-processing (TAP)-deficient cells. Nevertheless, T lymphocytes reactive to LLL were detected in the peripheral blood of four EBV-seropositive healthy individuals. We have constructed a chimeric molecule in which LLL was fused to the amino-terminal end of the beta(2) microglobulin (beta(2)m). Autologous dendritic cells constitutively expressing the LLLbeta(2)m molecule were capable of expanding in vitro HLA-A2-restricted anti-LLL T lymphocytes from the peripheral blood of one of the donors. These T lymphocytes exhibited cytolytic activity against target cells expressing the chimeric molecules as well as against EBV-infected lymphoblastoid cells expressing natural LLL-MHC complexes.

Feedback regulation of murine autoimmunity via dominant anti-inflammatory effects of interferon gamma

There is a paucity of knowledge concerning the immunologic sequelae that culminate in overt autoimmunity. In the present study, we have analyzed the factors that lead to disease in the model of autoimmunity, murine experimental autoimmune encephalomyelitis (EAE). EAE in H-2(u) mice involves autoreactive CD4(+) T cells that are induced by immunization with the immunodominant N-terminal epitope of myelin basic protein. The affinity of this epitope for I-A(u) can be increased by substituting lysine at position 4 with tyrosine, and this can be used to increase the effective Ag dose. Paradoxically, high doses of Ag are poorly encephalitogenic. We have used quantitative analyses to study autoreactive CD4(+) T cell responses following immunization of mice with Ag doses that are at the extremes of encephalitogenicity. A dose of autoantigen that is poorly encephalitogenic results in T cell hyperresponsiveness, triggering an anti-inflammatory feedback loop in which IFN-gamma plays a pivotal role. Our studies define a regulatory mechanism that serves to limit overly robust T cell responses. This feedback regulation has broad relevance to understanding the factors that determine T cell responsiveness.

Zinc induces dimerization of the class II major histocompatibility complex molecule that leads to cooperative binding to a superantigen

Dimerization of class II major histocompatibility complex (MHC) plays an important role in the MHC biological function. Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen that can activate large fractions of T cells bearing specific T cell receptor Vbeta elements. Here we have used structural, sedimentation, and surface plasmon resonance detection approaches to investigate the molecular interactions between MAM and the class II MHC molecule HLA-DR1 in the context of a hemagglutinin peptide-(306-318) (HA). Our results revealed that zinc ion can efficiently induce the dimerization of the HLA-DR1/HA complex. Because the crystal structure of the MAM/HLA-DR1/hemagglutinin complex in the presence of EDTA is nearly identical to the structure of the complex crystallized in the presence of zinc ion, Zn(2+) is evidently not directly involved in the binding between MAM and HLA-DR1. Sedimentation and surface plasmon resonance studies further revealed that MAM binds the HLA-DR1/HA complex with high affinity in a 1:1 stoichiometry, in the absence of Zn(2+). However, in the presence of Zn(2+), a dimerized MAM/HLA-DR1/HA complex can arise through the Zn(2+)-induced DR1 dimer. In the presence of Zn(2+), cooperative binding of MAM to the DR1 dimer was also observed.

Antigen recognition by γδ T cells

gammadelta T cells contribute to host immune competence uniquely. This is most likely because they have distinctive antigen-recognition properties. While the basic organization of gammadelta T-cell receptor (TCR) loci is similar to that of alphabeta TCR loci, there is a striking difference in how the diversity of gammadelta TCRs is generated. gammadelta and alphabeta T cells have different antigen-recognition requirements and almost certainly recognize a different set of antigens. While it is unclear what most gammadelta T cells recognize, the non-classical major histocompatibility complex class I molecules T10 and T22 were found to be the natural ligands for a sizable population (0.2-2%) of murine gammadelta T cells. The recognition of T10/T22 may be a way by which gammadelta T cells regulate cells of the immune system, and this system has been used to determine the antigen-recognition determinants of gammadelta T cells. T10/T22-specific gammadelta T cells have TCRs that are diverse in both V gene usage and CDR3 sequences. Their Vgamma usage reflects their tissue origin, and their antigen specificity is conferred by a motif in the TCR delta chain that is encoded by V and D segments and by P-nucleotide addition. Sequence variations around this motif modulate affinities between TCRs and T10/T22. That this CDR3 motif is important in antigen recognition is confirmed by the crystal structure of a gammadelta TCR bound to its ligand. The significance of these observations is discussed in the context of gammadelta T-cell biology.

A correlation between function and selected measures of T cell avidity in influenza virus-specific CD8+ T cell responses

Activation of mature CD8+ T cells requires recognition, via the T cell receptor (TCR), of peptide + MHC (pMHC) complexes with an avidity that exceeds a designated threshold. Multiple indicators of T cell avidity have been described that provide unique information on the characteristics of T cell interactions. However, these indicators are routinely used in isolation, and, consequently, little is known about correlations between these measures or which measure, if any, correlates with the quality of the T cell response. Following influenza virus infection of C57BL/6J mice, we analyzed the relative avidities of five epitope-specific CD8+ T cell populations using five different measures. We demonstrated that the quality of CD8+ T cell responses, in terms of cytokine profiles, correlates with TCR dissociation rate and CD8 dependence, but not with the sensitivity to tetramer binding or peptide stimulation. Thus, we propose that, despite significant differences in TCR dissociation rate, the stimulation threshold of influenza-specific CD8+ T cell populations may be equivalent due to compensatory mechanisms largely provided by the CD8 coreceptor. Furthermore, this study shows that different indicators of avidity do not necessarily provide similar information and should be used in combination to obtain an overall picture of the characteristics of TCR binding.

Thymic selection threshold defined by compartmentalization of Ras/MAPK signalling

A healthy individual can mount an immune response to exogenous pathogens while avoiding an autoimmune attack on normal tissues. The ability to distinguish between self and non-self is called 'immunological tolerance' and, for T lymphocytes, involves the generation of a diverse pool of functional T cells through positive selection and the removal of overtly self-reactive thymocytes by negative selection during T-cell ontogeny. To elucidate how thymocytes arrive at these cell fate decisions, here we have identified ligands that define an extremely narrow gap spanning the threshold that distinguishes positive from negative selection. We show that, at the selection threshold, a small increase in ligand affinity for the T-cell antigen receptor leads to a marked change in the activation and subcellular localization of Ras and mitogen-activated protein kinase (MAPK) signalling intermediates and the induction of negative selection. The ability to compartmentalize signalling molecules differentially in the cell endows the thymocyte with the ability to convert a small change in analogue input (affinity) into a digital output (positive versus negative selection) and provides the basis for establishing central tolerance.

Interface-disrupting amino acids establish specificity between T cell receptors and complexes of major histocompatibility complex and peptide

T cell receptors (TCRs) bind complexes of cognate major histocompatibility complex (MHC) and peptide at relatively low affinities (1-200 microM). Nevertheless, TCR-MHC-peptide interactions are usually specific for the peptide and the allele encoding the MHC. Here we show that to escape thymocyte negative selection, TCRs must interact with many of the side chains of MHC-peptide complexes as 'hot spots' for TCR binding. Moreover, even when the 'parental' side chain did not contribute binding affinity, some MHC-peptide residues contributed to TCR specificity, as amino acid substitutions substantially reduced binding affinity. The presence of such 'interface-disruptive' side chains helps to explain how TCRs generate specificity at low-affinity interfaces and why TCRs often 'accommodate' a subset of amino acids at a given MHC-peptide position.

Alloreactivity between disparate cognate and allogeneic pMHC-I complexes is the result of highly focused, peptide-dependent structural mimicry

Our understanding of the molecular mechanisms of T cell alloreactivity remains limited by the lack of systems for which both the T cell receptor allo- and cognate ligand are known. Here we provide evidence that a single alloreactive T cell receptor interacts with analogous structural regions of its cognate ligand, HLA-B*0801(FLRGRAYGL), as its allogeneic ligand, HLA-B*3501(KPIVVLHGY). The crystal structures of the binary peptide-major histocompatibility complexes show marked differences in the conformation of the heavy chains as well as the bound peptides. Nevertheless, both epitopes possess a prominent solvent-exposed aromatic residue at position 7 flanked by a small glycine at position 8 of the peptide determinant. Moreover, regions of close structural homology between the heavy chains of HLA B8 and HLA B35 coincided with regions that have previously been implicated in "hot spots" of T cell receptor recognition. The avidity of this human T cell receptor was also comparable for the allo- and cognate ligand, consistent with the modes of T cell receptor binding being broadly similar for these complexes. Collectively, it appears that highly focused structural mimicry against a diverse structural background provides a basis for the observed alloreactivity in this system. This cross-reactivity underpins the T cell degeneracy inherent in the limited mature T cell repertoire that must respond to a vast diversity of microbial antigens.

Relating TCR-peptide-MHC affinity to immunogenicity for the design of tumor vaccines

One approach to enhancing the T cell response to tumors is vaccination with mimotopes, mimics of tumor epitopes. While mimotopes can stimulate proliferation of T cells that recognize tumor-associated antigens (TAAs), this expansion does not always correlate with control of tumor growth. We hypothesized that vaccination with mimotopes of optimal affinity in this interaction will improve antitumor immunity. Using a combinatorial peptide library and a cytotoxic T lymphocyte clone that recognizes a TAA, we identified a panel of mimotopes that, when complexed with MHC, bound the TAA-specific TCR with a range of affinities. As expected, in vitro assays showed that the affinity of the TCR-peptide-MHC (TCR-pMHC) interaction correlated with activity of the T cell clone. However, only vaccination with mimotopes in the intermediate-affinity range elicited functional T cells and provided protection against tumor growth in vivo. Vaccination with mimotopes with the highest-affinity TCR-pMHC interactions elicited TAA-specific T cells to the tumor, but did not control tumor growth at any of the peptide concentrations tested. Further analysis of these T cells showed functional defects in response to the TAA. Thus, stimulation of an antitumor response by mimotopes may be optimal with peptides that increase but do not maximize the affinity of the TCR-pMHC interaction.

Crystallographic Structure of a Rheumatoid Arthritis MHC Susceptibility Allele, HLA-DR1 (DRB1*0101), Complexed with the Immunodominant Determinant of Human Type II Collagen

The expression of HLA-DR1 (DRB1*0101) is associated with an enhanced risk for developing rheumatoid arthritis (RA). To study its function, we have solved the three-dimensional structure of HLA-DR1 complexed with a candidate RA autoantigen, the human type II collagen peptide CII (259-273). Based on these structural data, the CII peptide is anchored by Phe263 at the P1 position and Glu266 at P4. Surprisingly, the Lys at the P2 position appears to play a dual role by participating in peptide binding via interactions with DRB1-His81 and Asn82, and TCR interaction, based on functional assays. The CII peptide is also anchored by the P4 Glu266 residue through an ionic interaction with DRB1-Arg71 and Glu28. Participation of DRB1-Arg71 is significant because it is part of the shared epitope expressed by DR alleles associated with RA susceptibility. Potential anchor residues at P6 and P9 of the CII peptide are both Gly, and the lack of side chains at these positions appears to result in both a narrower binding groove with the peptide protruding out of the groove at this end of the DR1 molecule. From the TCR perspective, the P2-Lys264, P5-Arg267, and P8-Lys270 residues are all oriented away from the binding groove and collectively represent a positive charged interface for CII-specific TCR binding. Comparison of the DR1-CII structure to a DR1-hemagglutinin peptide structure revealed that the binding of these two peptides generates significantly different interfaces for the interaction with their respective Ag-specific TCRs.

Quantitation of Anaplasma marginale major surface protein (MSP)1a and MSP2 epitope-specific CD4+ T lymphocytes using bovine DRB3*1101 and DRB3*1201 tetramers

Antigen-specific CD4+ T cells play a critical role in protective immunity to many infectious pathogens. Although the antigen-specific CD4+ T cells can be measured by functional assays such as proliferation or cytokine enzyme-linked immunospot, such assays are limited to a specific function and cannot quantify anergic or suppressed T cells. In contrast, major histocompatiblity complex (MHC) class II tetramers can enumerate epitope-specific CD4+ T cells independent of function. In this paper, we report the construction of bovine leukocyte antigen MHC class II tetramers using a novel mammalian cell system to express soluble class II DRA/DRB3 molecules and defined immunodominant peptide epitopes of Anaplasma marginale major surface proteins (MSPs). Phycoerythrin-labeled tetramers were either loaded with exogenous peptide or constructed with the peptide epitope linked to the N terminus of the DRB3 chain. A DRB3*1101 tetramer loaded with MSP1a peptide F2-5B (ARSVLETLAGHVDALG) and DRB3*1201 tetramers loaded with MSP1a peptide F2-1-1b (GEGYATYLAQAFA) or MSP2 peptide P16-7 (NFAYFGGELGVRFAF) specifically stained antigen-specific CD4+ T cell lines and clones. Tetramers constructed with the T-cell epitope linked to the DRB3 chain were slightly better at labeling CD4+ T cells. In one cell line, the number of tetramer-positive T cells increased to approximately 94% of the CD4+ T cells after culture for 21 weeks with specific antigen. This novel technology should be useful to track the fate of antigen-specific CD4+ T-cell responses in cattle after immunization or infection with persistent pathogens, such as A. marginale, that modulate the host immune response.

Expression of HLA-DP0401 molecules for identification of DP0401 restricted antigen specific T cells

Human histocompatibility leukocyte antigen (HLA)-DPA1*0103/DPB1*0401 (DP0401) is the most common HLA class II molecule and is present in approximately 45% of the Caucasian population. In this study, soluble HLA-DP0401 molecules were expressed as "empty'' class II molecules in insect cells. Utilizing these soluble DP molecules and the Tetramer Guided Epitope Mapping (TGEM) approach, the influenza A Puerto Rico/8/34 matrix protein (MP) derived peptide MP(41-60) VLMEWLKTRPILSPLTKGIL and the Clostridium tetani Tetanus Toxin (TT) derived peptide TT(634-653) DKISDVSTIVPYIGPALNIV were identified as the DP0401 restricted MP and TT epitopes, respectively. In addition, T cells specific for the cancer testis antigen NY-ESO-1 and the breast/ovarian cancer over-expressing antigen Her-2/neu were detected in DP0401 subjects by DP0401 tetramers. The availability of HLA-DP0401 tetramers should facilitate the study of DP restricted T cell responses.

CD8 T cell competition for dendritic cells in vivo is an early event in activation

T cell responses against an antigen are often focused on a small fraction of potentially immunogenic determinants, a phenomenon known as immunodominance. Immunodominance can be established at several stages of antigen presentation, including antigen processing, binding of peptides to MHC, and competition between T cells for dendritic cells (DCs). The mechanism of this T cell competition is unclear, but may include competition for physical access to DCs, competition for limiting soluble DC-derived factors, or a suppressive effect of one T cell population on the other(s). To model DC-specific T cell competition, normal mice were injected with one or two T cell receptor transgenic CD8 T cell populations, each with high affinity for different peptides presented by different class I MHC complexes. These mice were immunized with DCs pulsed with peptides that are recognized by the transferred T cells. Competition was detectable when both T cell populations were transferred and their target peptides were present on the same DCs. The competition resulted in fewer cells entering the response, but had no effect on the level of activation of the cells that did respond. The effect was evident very early in the response, and in fact the competing T cells needed to be present in the first 5 h of the response for competition to occur. Thus, some aspect of DCs other than peptide/MHC complexes is limiting in the earliest stages of the CD8 T cell response. These results have implications for the design of multivalent vaccines.

Anti-coreceptor antibodies profoundly affect staining with peptide-MHC class I and class II tetramers

The T cell coreceptors CD8 and CD4 bind to invariable regions of peptide-MHC class I (pMHCI) and class II (pMHCII) molecules, respectively, and facilitate antigen recognition by a number of mechanisms. It is established that some antibodies (Ab) specific for the CD8 molecule, which stabilizes TCR/pMHCI interactions, can alter the binding of pMHCI tetramers to cell surface TCR. In contrast, the extremely weak pMHCII/CD4 interaction does not stabilize TCR/pMHCII interactions or contribute to cognate tetramer binding; consequently, it is assumed that anti-CD4 Ab do not affect pMHCII binding. Here, we used a panel of point-mutated HLA A2 molecules with a range of affinities for CD8 spanning over three orders of magnitude to demonstrate that anti-CD8 Ab-mediated inhibition of pMHCI tetramer binding and cognate T cell activation correlates directly with the strength of the pMHCI/CD8 interaction. Further, some anti-CD4 Ab were found to block pMHCII tetramer binding; these effects were also paralleled in T cell activation assays. In sum, these data challenge the assertion that anti-coreceptor Ab exert their effects on T cell activation and pMHC binding solely by blocking pMHC/coreceptor interactions.

Bim mediates apoptosis of CD127(lo) effector T cells and limits T cell memory

Following an acute T cell response, most activated effector cells die, while some survive and become memory cells. The pro-apoptotic Bcl-2 family member, Bcl-2 interacting mediator of death (Bim) is critical for eliminating most effector T cells, while expression of CD127 (IL-7Ralpha) has been proposed to mark effector cells destined to become memory cells. Here, we examined the effects of Bim on the death of effector T cells in relationship to CD127 expression and on development of T cell memory following lymphocytic choriomeningitis virus (LCMV) infection. We found that large numbers of CD127(lo) LCMV-specific CD4(+) and CD8(+) T cells were lost in wild-type mice, but were spared in Bim(-/-) mice. Further, while the numbers of CD127(hi) T cells declined only slightly during contraction of the response in wild-type mice, they increased significantly in Bim(-/-) mice due to re-expression of CD127 on CD127(lo) T cells that had avoided apoptosis. Functional memory T cells were significantly increased in Bim(-/-) mice; however, they underwent a slow attrition due to decreased proliferative renewal. Taken together, these data suggest that the absence of Bim-mediated death of LCMV-specific CD4(+) and CD8(+) T cells in vivo can increase T cell memory, but other homeostatic mechanisms control the long-term maintenance of memory cells.

Allergen-specific MHC class II tetramer+ cells are detectable in allergic, but not in nonallergic, individuals

Allergen-specific cells are present in very low frequency in peripheral blood of humans, and differ in function in allergic and nonallergic individuals. We report in this study that soluble class II MHC tetramers can be used to directly identify and study such allergen epitope-specific CD4+ T cells in humans. We identified the major antigenic epitope of rye grass allergen Lol p 1 in HLA-DRB1*0401 individuals using HLA-DR*0401 transgenic mice and peripheral blood cells from HLA-DR*0401 individuals. Using DRB1*0401 tetramers loaded with this major epitope of Lol p 1, we detected allergen-specific CD4+ T cells in the peripheral blood of DRB1*0401 rye grass allergic individuals after ex vivo expansion with allergen. These tetramer-positive cells produced IL-4, but little IFN-gamma. In contrast, we were unable to detect rye grass tetramer-positive cells in cultures from HLA-DR*0401 nonallergic individuals, even after expansion with IL-2. Thus, our results suggest that rye grass allergen-specific T cells in DR*0401 nonallergic subjects are present at very low levels (e.g., because of deletion or suppression), differ in a fundamental way in their requirement for ex vivo expansion (e.g., they may be anergic), or use TCRs distinct from those of allergic individuals. Thus, analysis using DRB1*0401 tetramers loaded with a major epitope of Lol p 1 indicates that allergen-specific CD4+ T cells in nonallergic individuals are distinct from those in allergic subjects.

Structural basis for degenerate recognition of natural HIV peptide variants by cytotoxic lymphocytes

It is well established that even small changes in amino acid side chains of antigenic peptide bound to major histocompatibility complex (MHC) protein may completely abrogate recognition of the peptide-MHC (pMHC) complex by the T cell receptor (TCR). Often, however, several nonconservative substitutions in the peptide antigen are accommodated and do not impair its recognition by TCR. For example, a preponderance of natural sequence variants of the human immunodeficiency virus p17 Gag-derived peptide SLYNTVATL (SL9) are recognized by cytotoxic T lymphocytes, which implies that interactions with SL9 variants are degenerate both with respect to the class I MHC molecule and with respect to TCR. Here we study the molecular basis for this degenerate recognition of SL9 variants. We show that several SL9 variants bind comparably well to soluble HLA-A2 and to a particular soluble TCR and that these variants are active in the cognate cytotoxicity assay. Natural SL9 variation is restricted by its context in the HIV p17 matrix protein. High resolution crystal structures of seven selected SL9 variants bound to HLA-A2 all have remarkably similar peptide conformations and side-chain dispositions outside sites of substitution. This preservation of the peptide conformation despite epitope variations suggests a mechanism for the observed degeneracy in pMHC recognition by TCR and may contribute to the persistence of SL9-mediated immune responses in chronically infected individuals.

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

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

Long-lasting reactivity and high frequency of drug-specific T cells after severe systemic drug hypersensitivity reactions

BACKGROUND

Drug-reactive T cells are involved in most drug-induced hypersensitivity reactions. The frequency of such cells in peripheral blood of patients with drug allergy after remission is unclear.

OBJECTIVE

We determined the frequency of drug-reactive T cells in the peripheral blood of patients 4 months to 12 years after severe delayed-type drug hypersensitivity reactions, and whether the frequency of these cell differs from the frequency of tetanus toxoid-reactive T cells.

METHODS

We analyzed 5 patients with delayed-type drug hypersensitivity reactions, applying 2 methods: quantification of cytokine-secreting T cells by enzyme-linked immunospot (ELISpot), and fluorescent dye 5,6-carboxylfluorescein diacetate succinimidyl ester (CFSE) intensity distribution analysis of drug-reactive T cells.

RESULTS

Frequencies found were between 0.02% and 0.4% of CD4(+) T cells reacting to the respective drugs measured by CFSE analysis, and between 0.01% and 0.08% of T cells as determined by ELISpot. Reactivity was seen neither to drugs to which the patients were not sensitized nor in healthy individuals after stimulation with any of the drugs used.

CONCLUSION

About 1:250 to 1:10,000 of T cells of patients with drug allergy are reactive to the relevant drugs. This frequency of drug-reactive T cells is higher than the frequency of T cells able to recognize recall antigens like tetanus toxoid in the same subjects. A substantial frequency could be observed as long as 12 years later in 1 patient even after strict drug avoidance. Patients with severe delayed drug hypersensitivity reactions are therefore potentially prone to react again to the incriminated drug even years after strict drug avoidance.

CD4 memory T cells survive and proliferate but fail to differentiate in the absence of CD40

Secondary T cell responses are enhanced because of an expansion in numbers of antigen-specific (memory) cells. Using major histocompatibility complex class II tetramers we have tracked peptide-specific endogenous (non–T cell receptor transgenic) CD4 memory T cells in normal and in costimulation-deficient mice. CD4 memory T cells were detectable after immunization for more than 200 days, although decay was apparent. Memory cells generated in CD40 knockout mice by immunization with peptide-pulsed wild-type dendritic cells survived in the absence of CD40 and proliferated when boosted with peptide (plus adjuvant) in a CD40-independent fashion. However, differentiation of the memory cells into cytokine-producing effector cells did not occur in the absence of CD40. The data indicate that memory cells can be generated without passing through the effector cell stage.

Yeast surface display of a noncovalent MHC class II heterodimer complexed with antigenic peptide

Microbial protein display technologies have enabled directed molecular evolution of binding and stability properties in numerous protein systems. In particular, dramatic improvements to antibody binding affinity and kinetics have been accomplished using these tools in recent years. Examples of successful application of display technologies to other immunological proteins have been limited to date. Herein, we describe the expression of human class II major histocompatibility complex allele (MHCII) HLA-DR4 on the surface of Saccharomyces cerevisiae as a noncovalently associated heterodimer. The yeast-displayed MHCII is fully native as assessed by binding of conformationally specific monoclonal antibodies; failure of antibodies specific for empty HLA-DR4 to bind yeast-displayed protein indicates antigenic peptide is bound. This report represents the first example of a noncovalent protein dimer displayed on yeast and of successful display of wild-type MHCII. Results further point to the potential for using yeast surface display for engineering and analyzing the antigen binding properties of MHCII.

Rational design of intercellular adhesion molecule-1 (ICAM-1) variants for antagonizing integrin lymphocyte function-associated antigen-1-dependent adhesion

The interaction between integrin lymphocyte function-associated antigen-1 (LFA-1) and its ligand intercellular adhesion molecule-1 (ICAM-1) is critical in immunological and inflammatory reactions but, like other adhesive interactions, is of low affinity. Here, multiple rational design methods were used to engineer ICAM-1 mutants with enhanced affinity for LFA-1. Five amino acid substitutions 1) enhance the hydrophobicity and packing of residues surrounding Glu-34 of ICAM-1, which coordinates to a Mg2+ in the LFA-1 I domain, and 2) alter associations at the edges of the binding interface. The affinity of the most improved ICAM-1 mutant for intermediate- and high-affinity LFA-1 I domains was increased by 19-fold and 22-fold, respectively, relative to wild type. Moreover, potency was similarly enhanced for inhibition of LFA-1-dependent ligand binding and cell adhesion. Thus, rational design can be used to engineer novel adhesion molecules with high monomeric affinity; furthermore, the ICAM-1 mutant holds promise for targeting LFA-1-ICAM-1 interaction for biological studies and therapeutic purposes.

Generation of functional HLA-DR*1101 tetramers receptive for loading with pathogen- or tumour-derived synthetic peptides

BACKGROUND

MHC class I-peptide tetramers are currently utilised to characterize CD8+ T cell responses at single cell level. The generation and use of MHC class II tetramers to study antigen-specific CD4+ T cells appears less straightforward. Most MHC class II tetramers are produced with a homogeneously built-in peptide, reducing greatly their flexibility of use. We attempted the generation of "empty" functional HLA-DR*1101 tetramers, receptive for loading with synthetic peptides by incubation. No such reagent is in fact available for this HLA-DR allele, one of the most frequent in the Caucasian population.

RESULTS

We compared soluble MHC class II-immunoglobulin fusion proteins (HLA-DR*1101-Ig) with soluble MHC class II protein fused with an optimised Bir site for enzymatic biotynilation (HLA-DR*1101-Bir), both produced in insect cells. The molecules were multimerised by binding fluorochrome-protein A or fluorochrome-streptavidin, respectively. We find that HLA-DR*1101-Bir molecules are superior to the HLA-DR*1101-Ig ones both in biochemical and functional terms. HLA-DR*1101-Bir molecules can be pulsed with at least three different promiscuous peptide epitopes, derived from Tetanus Toxoid, influenza HA and the tumour associated antigen MAGE-3 respectively, to stain specific CD4+ T cells. Both staining temperature and activation state of CD4+ T cells are critical for the binding of peptide-pulsed HLA-DR*1101-Bir to the cognate TCR.

CONCLUSION

It is therefore possible to generate a soluble recombinant HLA-DR*1101 backbone that is receptive for loading with different peptides to stain specific CD4+ T cells. As shown for other HLA-DR alleles, we confirm that not all the strategies to produce soluble HLA-DR*1101 multimers are equivalent.

IL-21 influences the frequency, phenotype, and affinity of the antigen-specific CD8 T cell response

IL-21, a newly described cytokine belonging to the IL-2 gamma-chain receptor cytokine family (that includes IL-2, IL-7, and IL-15), has been described as an important regulator of the cellular immune response. In this study, the role of IL-21 in the generation of a human Ag-specific CD8+ T cell response is characterized by tracking a rare, but measurable population of self-Ag-specific T cells in vitro. Autologous dendritic cells pulsed with the melanoma antigen recognized T cells 1 self-peptide were used to stimulate CD8+ T cells from HLA-A2+ healthy donors and melanoma patients. We demonstrate that exposure to IL-21 increased the total number of MART-1-specific CD8+ T cells that could be elicited by >20-fold and, at the clonal level, enriched for a population of high-affinity CD8+ T cells with a peptide dose requirement more than 1 log(10)-fold less than their untreated counterparts. Phenotypic analysis of T cells from IL-21-treated cultures revealed a unique population of CD45RO+ CD28(high) CD8+ T cells, a phenotype that was stable for at least 4 wk after IL-21 exposure. These CD28(high) CD8+ T cells produced IL-2 upon Ag stimulation and represent potential helper-independent CTLs. Our studies demonstrate a significant role for IL-21 in the primary Ag-specific human CTL response and support the use of IL-21 in the ex vivo generation of potent Ag-specific CTLs for adoptive therapy or as an adjuvant cytokine during in vivo immunization against tumor Ags.

Ultrasensitive Detection and Phenotyping of CD4+ T Cells with Optimized HLA Class II Tetramer Staining

HLA class I tetramers have revolutionized the study of Ag-specific CD8+ T cell responses. Technical problems and the rarity of Ag-specific CD4+ Th cells have not allowed the potential of HLA class II tetramers to be fully realized. Here, we optimize HLA class II tetramer staining methods through the use of a comprehensive panel of HIV-, influenza-, CMV-, and tetanus toxoid-specific tetramers. We find rapid and efficient staining of DR1- and DR4-restricted CD4+ cell lines and clones and show that TCR internalization is not a requirement for immunological staining. We combine tetramer staining with magnetic bead enrichment to detect rare Ag-specific CD4+ T cells with frequencies as low as 1 in 250,000 (0.0004% of CD4+ cells) in human PBLs analyzed directly ex vivo. This ultrasensitive detection allowed phenotypic analysis of rare CD4+ T lymphocytes that had experienced diverse exposure to Ag during the course of viral infections. These cells would not be detectable with normal flow-cytometric techniques.

Enhanced responsiveness to antigen contributes more to immunological memory in CD4 T cells than increases in the number of cells

Although immunological memory is characterized by both an increase in the frequency of antigen-specific T cells and a qualitative change in the pattern of their subsequent response, it is not clear which of these components is more significant in the overall enhanced response to secondary stimulation. To address this question for the CD4+ T-cell response, T-cell receptor (TCR) Tg T cells were adoptively transferred to normal syngeneic mice that were immunized with the relevant peptide. After the initial expansion of TCR Tg T cells, the size of the subsequent memory population of T cells was approximately the same as the size of the starting population, independent of the number of TCR Tg cells initially transferred. This result was not caused by redistribution of memory cells into non-lymphoid tissues, although the relative frequency of antigen-specific T cells in these sites was increased after immunization. The fraction of the antigen specific TCR Tg cells that responded by production of either interleukin-2 or interferon-gammain vitro was substantially higher after immunization. Thus, the increased frequency of functionally responsive T cells was primarily caused by a higher fraction of responding T cells, rather than a substantial increase in the absolute number of antigen specific CD4+ TCR Tg T cells.

MHC multimer technology: current status and future prospects

The detection of antigen-specific T cell responses by MHC multimer staining is rapidly becoming one of the core immunological techniques, and the technology to produce MHC multimers has been optimized substantially in recent years. Furthermore, recent work demonstrates the potential of high-throughput detection of T cell responses and suggests that manipulation of T cell responses through the use of multimeric MHC reagents is also feasible.

Targeting T lymphocytes for immune monitoring and intervention in autoimmune diabetes

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

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

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

Understanding specificity and sensitivity of T-cell recognition

The response of T cells to antigen shows an amazing degree of both sensitivity and specificity, with a cell responding to 1-10 peptide-MHC complexes and being sensitive to single amino acid substitutions. Kinetic proofreading or feedback pathways achieve specificity at the level of the receptor, whereas serial engagement of receptors by ligand molecules enhances sensitivity. Crosstalk between receptors, integration of signals and/or tuning of responses is important at the level of the cell. Induction of anergic or regulatory cells by suboptimal stimuli prevents cell activation by multiple encounters with weak ligands. Thus, for optimal sensitivity and specificity, it is necessary to have mechanisms that operate at the level of the receptor, the cell and finally, the population of responding cells.

Synthesis of chicken major histocompatibility complex class II oligomers using a baculovirus expression system

Chicken major histocompatibility complex (MHC) B21 and B19 haplotypes are associated with resistance and susceptibility to Marek's disease (MD), respectively. T-cell-mediated immune response is crucial in coordinating protection against Marek's disease virus (MDV) infection, but it has been difficult to identify and characterize antigen-specific T-cells. MHC class II tetramers and oligomers have been widely used for characterization of antigen-specific T-cells in the context of infectious and autoimmune diseases. Thus, the objective of this study was to synthesize chicken MHC class II oligomers of B21 and B19 haplotypes for the future identification of antigen-specific T-cells. To achieve this objective, full-length coding sequences of chicken MHC class II B21 and B19 molecules were amplified and the molecules were expressed as fusion proteins, carrying Fos and Jun leucine zipper (LZ), histidine-tag and biotin ligase recognition site sequences, using a baculovirus expression system. Recombinant MHC-II were loaded with self-peptides, which stabilized the heterodimer in SDS-PAGE and allowed the detection of these molecules in Western blots with a conformation-specific anti-chicken MHC class II antibody. Biotinylated MHC molecules were conjugated to streptavidin to form oligomers, which were resolved under the transmission electron microscope through immuno-gold labelling, thus confirming success of oligomerization. In conclusion, chicken MHC class II oligomers may be used in the future to study the antigen-specific CD4+ T-cell compartment.

How the T cell repertoire becomes peptide and MHC specific

T cells bearing alphabeta T cell receptors (TCRs) recognize antigens in the form of peptides bound to class I or class II major histocompatibility proteins (MHC). TCRs on mature T cells are usually very specific for both peptide and MHC class and allele. They are picked out from a precursor population in the thymus by MHC-driven positive and negative selection. Here we show that the pool of T cells initially positively selected in the thymus contains many T cells that are very crossreactive for peptide and MHC and that subsequent negative selection establishes the MHC-restriction and peptide specificity of peripheral T cells. Our results also suggest that germline-encoded TCR variable elements have an inherent predisposition to react with features shared by all MHC proteins.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Direct ex vivo detection of HLA-DR3-restricted cytomegalovirus- and Mycobacterium tuberculosis-specific CD4+ T cells

In order to detect epitope-specific CD4+ T cells in mycobacterial or viral infections in the context of human class II major histocompatibility complex protein human leukocyte antigen (HLA)-DR3, two HLA-DR3 tetrameric molecules were successfully produced. One contained an immunodominant HLA-DR3-restricted T-cell epitope derived from the 65-kDa heat-shock protein of Mycobacterium tuberculosis, peptide 1-13. For the other tetramer, we used an HLA-DR3-restricted T-cell epitope derived from cytomegalovirus (CMV) pp65 lower matrix protein, peptide 510-522, which induced high levels of interferon (IFN)-gamma-producing CD4+ T cells in three of four HLA-DR3-positive CMV-seropositive individuals up to 0.84% of CD4+ T cells by intracellular cytokine staining. In peripheral blood mononuclear cells from M. tuberculosis-exposed, Mycobacterium bovis bacille Calmette-Guérin (BCG)-vaccinated, or CMV-seropositive individuals, we were able to directly detect with both tetramers epitope-specific T cells up to 0.62% and 0.45% of the CD4+ T-cell population reactive to M. tuberculosis and CMV, respectively. After a 6-day culture with peptide p510-522, the frequency of CMV-specific tetramer-binding T cells was expanded up to 9.90% tetramer+ CFSElow (5,6-carboxyfluorescein diacetate succinimidyl ester) cells within the CD4+ T-cell population, further confirming the specificity of the tetrameric molecules. Thus, HLA-DR3/peptide tetrameric molecules can be used to investigate HLA-DR3-restricted antigen-specific CD4+ T cells in clinical disease or after vaccination.

I-domain of lymphocyte function-associated antigen-1 mediates rolling of polystyrene particles on ICAM-1 under flow

In their active state, beta(2)-integrins, such as LFA-1, mediate the firm arrest of leukocytes by binding intercellular adhesion molecules (ICAMs) expressed on endothelium. Although the primary function of LFA-1 is assumed to be the ability to mediate firm adhesion, recent work has shown that LFA-1 can contribute to cell tethering and rolling under hydrodynamic flow, a role previously largely attributed to the selectins. The inserted (I) domain of LFA-1 has recently been crystallized in the wild-type (wt) and locked-open conformations and has been shown to, respectively, support rolling and firm adhesion under flow when expressed in alpha(L)beta(2) heterodimers or as isolated domains on cells. Here, we report results from cell-free adhesion assays where wt I-domain-coated polystyrene particles were allowed to interact with ICAM-1-coated surfaces in shear flow. We show that wt I-domain can independently mediate the capture of particles from flow and support their rolling on ICAM-1 surfaces in a manner similar to how carbohydrate-selectin interactions mediate rolling. Adhesion is specific and blocked by appropriate antibodies. We also show that the rolling velocity of I-domain-coated particles depends on the wall shear stress in flow chamber, I-domain site density on microsphere surfaces, and ICAM-1 site density on substrate surfaces. Furthermore, we show that rolling is less sensitive to wall shear stress and ICAM-1 substrate density at high density of I-domain on the microsphere surface. Computer simulations using adhesive dynamics can recreate bead rolling dynamics and show that the mechanochemical properties of ICAM-1-I-domain interactions are similar to those of carbohydrate-selectin interactions. Understanding the biophysics of adhesion mediated by the I-domain of LFA-1 can elucidate the complex roles this integrin plays in leukocyte adhesion in inflammation.

Contribution of T cell receptor affinity to overall avidity for virus-specific CD8+ T cell responses

Prior analysis has characterized the clonal characteristics of effector CD8(+) T cells specific for the prominent influenza A virus nucleoprotein (NP) and acid polymerase (PA) peptides presented by H2D(b). Using a single-cell approach and determination of CDR3beta profiles, a limited, predominantly "public" repertoire was found for CD8(+)D(b)NP(366)(+)Vbeta8.3+ cells, whereas diverse and "private" T cell antigen receptor (TCR)beta clonotypes were typical of the CD8(+)D(b)PA(224)(+)Vbeta7+ response. This single-cell approach has now been used to relate the contributions of particular clonotypes (or affinities) to high-avidity TCRs, as defined by binding under conditions of limiting tetramer availability. At least by the measure of CDR3beta usage, no difference could be found between total and high-avidity populations in the spectrum of TCR-pMHC affinities throughout the limited, and relatively public, CD8(+)D(b)NP(366)(+)Vbeta8.3+ populations. Conversely, the more even (by clone size), diverse, and private CD8(+)D(b)PA(224)(+)Vbeta7+ response was characterized by the clear partitioning of the largest T cell clones in the high-avidity compartment. These results suggest that the relatively constrained CD8(+)D(b)NP(366)(+)Vbeta8.3+ set utilizes a relatively narrow range of affinities, whereas the broader CD8(+)D(b)PA(224)(+)Vbeta7+ response is induced at a range of TCR-pMHC affinities. Thus, whereas TCR sequence (or affinity) appears to contribute substantially to the avidity profile of diverse virus-specific CD8+ populations, other mechanisms may be prominent where the TCR spectrum is more limited.

Interaction between the CD8 coreceptor and major histocompatibility complex class I stabilizes T cell receptor-antigen complexes at the cell surface

The off-rate (k(off)) of the T cell receptor (TCR)/peptide-major histocompatibility complex class I (pMHCI) interaction, and hence its half-life, is the principal kinetic feature that determines the biological outcome of TCR ligation. However, it is unclear whether the CD8 coreceptor, which binds pMHCI at a distinct site, influences this parameter. Although biophysical studies with soluble proteins show that TCR and CD8 do not bind cooperatively to pMHCI, accumulating evidence suggests that TCR associates with CD8 on the T cell surface. Here, we titrated and quantified the contribution of CD8 to TCR/pMHCI dissociation in membrane-constrained interactions using a panel of engineered pMHCI mutants that retain faithful TCR interactions but exhibit a spectrum of affinities for CD8 of >1,000-fold. Data modeling generates a "stabilization factor" that preferentially increases the predicted TCR triggering rate for low affinity pMHCI ligands, thereby suggesting an important role for CD8 in the phenomenon of T cell cross-reactivity.

Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help

In this report we have inspected whether difficulties in controlling viral infections negatively impacts the generation of CD127(high) memory T cells. Using both MHC class I and II tetramers we reveal that CD127(low) T cells are not necessarily rapidly deleted but can persist in a pseudoeffector state in which they display the hallmarks of activated effector cells but are functionally inferior. CD127(high) cells can, however, emerge if the infection is contained. We also show that in the absence of CD4 T cell help significant populations of CD127(high) CD8 T cells fail to emerge. Analyses of cytokine-producing activities by both mouse and human CD8 T cells further document that the extended maintenance of T cells in a CD127(low) state has functional consequences which manifest as an impairment of IL-2 production.

Rapid assessment of recognition efficiency and functional capacity of antigen-specific T-cell responses

It is increasingly recognized that cells within an antigen-specific CD8 T-cell population may be diverse in recognition efficiency for target, which may significantly affect the overall efficacy of the response in clinical settings such as viral infections and cancer. CD8 T cells with seemingly identical antigen specificity, particularly those elicited by cancer vaccines, may be heterogeneous for sensitivity and recognition efficiency for the cognate peptide and functional state in vivo. Analysis of individual T-cell clones derived from an antigen-specific T-cell population would provide an accurate assessment of the overall response; however, this is time- and labor-intensive, preventing rapid and routine assessment of patient samples from clinical trials. By stimulating antigen-specific T cells that otherwise appear homogeneous on tetramer staining with graded amounts of cognate peptides, the authors show that individual cells downmodulate surface T-cell receptors (TCR) and thus lose tetramer reactivity with variable dynamics within the T-cell population. The dynamics of TCR downregulation represent an accurate assessment of an individual cell's antigen sensitivity, recognition efficiency, and relative functional state within an antigen-specific population and have direct correlation to killing capacity by chromium release as well as degranulation by CD107 mobilization. Furthermore, despite correlation of average T-cell function by all three techniques, TCR downregulation uncovered heterogeneity in T-cell responses after vaccination among patient samples directly ex vivo. When examined using this novel technique, antigen-specific T cells elicited by vaccination with heteroclitic peptides exhibited significantly different recognition efficiencies for the heteroclitic versus native peptides, translating into differences in functional responses. With advancing cancer vaccine trials, the capacity to detect and functionally characterize antigen-specific T-cell responses in detail is critical. Techniques, as presented here, that rapidly assess the overall antigen sensitivity, recognition efficiency, and functional status of patients' T-cell responses will guide future vaccine trials and immunotherapies.

Antigen-specific memory B cell development

Helper T (Th) cell-regulated B cell immunity progresses in an ordered cascade of cellular development that culminates in the production of antigen-specific memory B cells. The recognition of peptide MHC class II complexes on activated antigen-presenting cells is critical for effective Th cell selection, clonal expansion, and effector Th cell function development (Phase I). Cognate effector Th cell-B cell interactions then promote the development of either short-lived plasma cells (PCs) or germinal centers (GCs) (Phase II). These GCs expand, diversify, and select high-affinity variants of antigen-specific B cells for entry into the long-lived memory B cell compartment (Phase III). Upon antigen rechallenge, memory B cells rapidly expand and differentiate into PCs under the cognate control of memory Th cells (Phase IV). We review the cellular and molecular regulators of this dynamic process with emphasis on the multiple memory B cell fates that develop in vivo.

Characterizing the impact of CD8 antibodies on class I MHC multimer binding

Many studies have suggested that CD8 Abs affect the binding of class I MHC tetramers/multimers to CD8(+) T cells, which has led to the interpretation that CD8 participates directly in multimer binding. In contrast, a recent publication has argued that CD8 Abs instead cause reorganization of TCR distribution and hence have an indirect effect on multimer binding to the TCR alone. We address these issues by testing the role of CD8 and the impact of CD8 Abs on the binding of normal and mutant multimers to Ag-specific mouse T cells. Our data suggest that, in this system, CD8 Abs act directly on CD8 and only mediate their effects on multimer binding when CD8 is capable of binding to the multimer. These data reinforce the paradigm that CD8 plays an active and direct role in binding of class I MHC multimers.

Replacement of the membrane proximal region of I-Ad MHC class II molecule with I-E-derived sequences promotes production of an active and stable soluble heterodimer without altering peptide-binding specificity

The MHC class II molecule I-A is the murine homologue of HLA-DQ in humans. The I-A and DQ heterodimers display considerable heterodimer instability compared with their I-E and HLA-DR counterparts. This isotype-specific behavior makes the production of soluble I-A and DQ molecules very difficult. We have developed a strategy for production of soluble I-A(d) molecules involving expression of I-A(d) as a glycosil phosphatidyl inositol (PI) anchored chimera in Chinese Hamster Ovary (CHO) cells. The regions comprising the membrane proximal segments of I-A(d) alpha and beta chains were substituted for the corresponding regions of I-E, and the derived constructs were expressed in CHO cells. Procedures for purification of the soluble class II molecules were optimized and the WT and chimeric molecule were compared for structure, biochemical stability and functionality. Our analysis revealed that the substitutions in the membrane proximal domains improved cell surface expression and thermal stability of I-A(d) without altering the peptide binding specificity of the class II molecule. The results suggest that similar strategies could be used to increase the stability of other unstable class II molecules for in vitro studies.