Recognition by CD8 on cytotoxic T lymphocytes is ablated by several substitutions in the class I alpha 3 domain: CD8 and the T-cell receptor recognize the same class I molecule

Structure, function, and immunomodulation of the CD8 co-receptor

Expressed on the surface of CD8+ T cells, the CD8 co-receptor is a key component of the T cells that contributes to antigen recognition, immune cell maturation, and immune cell signaling. While CD8 is widely recognized as a co-stimulatory molecule for conventional CD8+ αβ T cells, recent reports highlight its multifaceted role in both adaptive and innate immune responses. In this review, we discuss the utility of CD8 in relation to its immunomodulatory properties. We outline the unique structure and function of different CD8 domains (ectodomain, hinge, transmembrane, cytoplasmic tail) in the context of the distinct properties of CD8αα homodimers and CD8αβ heterodimers. We discuss CD8 features commonly used to construct chimeric antigen receptors for immunotherapy. We describe the molecular interactions of CD8 with classical MHC-I, non-classical MHCs, and Lck partners involved in T cell signaling. Engineered and naturally occurring CD8 mutations that alter immune responses are discussed. The applications of anti-CD8 monoclonal antibodies (mABs) that target CD8 are summarized. Finally, we examine the unique structure and function of several CD8/mAB complexes. Collectively, these findings reveal the promising immunomodulatory properties of CD8 and CD8 binding partners, not only to uncover basic immune system function, but to advance efforts towards translational research for targeted immunotherapy.

Experimental Structures of Antibody/MHC-I Complexes Reveal Details of Epitopes Overlooked by Computational Prediction

mAbs to MHC class I (MHC-I) molecules have proved to be crucial reagents for tissue typing and fundamental studies of immune recognition. To augment our understanding of epitopic sites seen by a set of anti-MHC-I mAb, we determined X-ray crystal structures of four complexes of anti-MHC-I Fabs bound to peptide/MHC-I/β2-microglobulin (pMHC-I). An anti-H2-Dd mAb, two anti-MHC-I α3 domain mAbs, and an anti-β2-microglobulin mAb bind pMHC-I at sites consistent with earlier mutational and functional experiments, and the structures explain allelomorph specificity. Comparison of the experimentally determined structures with computationally derived models using AlphaFold Multimer showed that although predictions of the individual pMHC-I heterodimers were quite acceptable, the computational models failed to properly identify the docking sites of the mAb on pMHC-I. The experimental and predicted structures provide insight into strengths and weaknesses of purely computational approaches and suggest areas that merit additional attention.

Experimental structures of antibody/MHC-I complexes reveal details of epitopes overlooked by computational prediction

Monoclonal antibodies (mAb) to major histocompatibility complex class I (MHC-I) molecules have proved to be crucial reagents for tissue typing and fundamental studies of immune recognition. To augment our understanding of epitopic sites seen by a set of anti-MHC-I mAb, we determined X-ray crystal structures of four complexes of anti-MHC-I antigen-binding fragments (Fab) bound to peptide/MHC-I/β2m (pMHC-I). An anti-H2-Dd mAb, two anti-MHC-I α3 domain mAb, and an anti-β2-microglobulin (β2m) mAb bind pMHC-I at sites consistent with earlier mutational and functional experiments, and the structures explain allelomorph specificity. Comparison of the experimentally determined structures with computationally derived models using AlphaFold Multimer (AF-M) showed that although predictions of the individual pMHC-I heterodimers were quite acceptable, the computational models failed to properly identify the docking sites of the mAb on pMHC-I. The experimental and predicted structures provide insight into strengths and weaknesses of purely computational approaches and suggest areas that merit additional attention.

The Combination of CD8αα and Peptide-MHC-I in a Face-to-Face Mode Promotes Chicken γδT Cells Response

The CD8αα homodimer is crucial to both thymic T cell selection and the antigen recognition of cytotoxic T cells. The CD8-pMHC-I interaction can enhance CTL immunity via stabilizing the TCR-pMHC-I interaction and optimizing the cross-reactivity and Ag sensitivity of CD8⁺ T cells at various stages of development. To date, only human and mouse CD8-pMHC-I complexes have been determined. Here, we resolved the pBF2*1501 complex and the cCD8αα/pBF2*1501 and cCD8αα/pBF2*0401 complexes in nonmammals for the first time. Remarkably, cCD8αα/pBF2*1501 and the cCD8αα/pBF2*0401 complex both exhibited two binding modes, including an “antibody-like” mode similar to that of the known mammal CD8/pMHC-I complexes and a “face-to-face” mode that has been observed only in chickens to date. Compared to the “antibody-like” mode, the “face-to-face” binding mode changes the binding orientation of the cCD8αα homodimer to pMHC-I, which might facilitate abundant γδT cells to bind diverse peptides presented by limited BF2 alleles in chicken. Moreover, the forces involving in the interaction of cCD8αα/pBF2*1501 and the cCD8αα/pBF2*0401 are different in this two binding model, which might change the strength of the CD8-pMHC-I interaction, amplifying T cell cross-reactivity in chickens. The coreceptor CD8αα of TCR has evolved two peptide-MHC-I binding patterns in chickens, which might enhance the T cell response to major or emerging pathogens, including chicken-derived pathogens that are relevant to human health, such as high-pathogenicity influenza viruses.

CD8 Binding of MHC-Peptide Complexes in cis or trans Regulates CD8+ T-cell Responses

The coreceptor CD8αβ can greatly promote activation of T cells by strengthening T-cell receptor (TCR) binding to cognate peptide-MHC complexes (pMHC) on antigen presenting cells and by bringing p56^Lck to TCR/CD3. Here, we demonstrate that CD8 can also bind to pMHC on the T cell (in cis) and that this inhibits their activation. Using molecular modeling, fluorescence resonance energy transfer experiments on living cells, biochemical and mutational analysis, we show that CD8 binding to pMHC in cis involves a different docking mode and is regulated by posttranslational modifications including a membrane-distal interchain disulfide bond and negatively charged O-linked glycans near positively charged sequences on the CD8β stalk. These modifications distort the stalk, thus favoring CD8 binding to pMHC in cis. Differential binding of CD8 to pMHC in cis or trans is a means to regulate CD8⁺ T-cell responses and provides new translational opportunities.

A herpesvirus encoded Qa-1 mimic inhibits natural killer cell cytotoxicity through CD94/NKG2A receptor engagement

A recurrent theme in viral immune evasion is the sabotage of MHC-I antigen presentation, which brings virus the concomitant issue of ‘missing-self’ recognition by NK cells that use inhibitory receptors to detect surface MHC-I proteins. Here, we report that rodent herpesvirus Peru (RHVP) encodes a Qa-1 like protein (pQa-1) via RNA splicing to counteract NK activation. While pQa-1 surface expression is stabilized by the same canonical peptides presented by murine Qa-1, pQa-1 is GPI-anchored and resistant to the activity of RHVP pK3, a ubiquitin ligase that targets MHC-I for degradation. pQa-1 tetramer staining indicates that it recognizes CD94/NKG2A receptors. Consistently, pQa-1 selectively inhibits NKG2A⁺ NK cells and expression of pQa-1 can protect tumor cells from NK control in vivo. Collectively, these findings reveal an innovative NK evasion strategy wherein RHVP encodes a modified Qa-1 mimic refractory to MHC-I sabotage and capable of specifically engaging inhibitory receptors to circumvent NK activation.

Direct recognition of hepatocyte-expressed MHC class I alloantigens is required for tolerance induction

Adeno-associated viral vector-mediated (AAV-mediated) expression of allogeneic major histocompatibility complex class I (MHC class I) in recipient liver induces donor-specific tolerance in mouse skin transplant models in which a class I allele (H-2Kb or H-2Kd) is mismatched between donor and recipient. Tolerance can be induced in mice primed by prior rejection of a donor-strain skin graft, as well as in naive recipients. Allogeneic MHC class I may be recognized by recipient T cells as an intact molecule (direct recognition) or may be processed and presented as an allogeneic peptide in the context of self-MHC (indirect recognition). The relative contributions of direct and indirect allorecognition to tolerance induction in this setting are unknown. Using hepatocyte-specific AAV vectors encoding WT allogeneic MHC class I molecules, or class I molecules containing a point mutation (D227K) that impedes direct recognition of intact allogeneic MHC class I by CD8+ T cells without hampering the presentation of processed peptides derived from allogeneic MHC class I, we show here that tolerance induction depends upon recognition of intact MHC class I. Indirect recognition alone yielded a modest prolongation of subsequent skin graft survival, attributable to the generation of CD4+ Tregs, but it was not sufficient to induce tolerance.

Reciprocal TCR-CD3 and CD4 Engagement of a Nucleating pMHCII Stabilizes a Functional Receptor Macrocomplex

CD4+ T cells convert the time that T cell receptors (TCRs) interact with peptides embedded within class II major histocompatibility complex molecules (pMHCII) into signals that direct cell-fate decisions. In principle, TCRs relay information to intracellular signaling motifs of the associated CD3 subunits, while CD4 recruits the kinase Lck to those motifs upon coincident detection of pMHCII. But the mechanics by which this occurs remain enigmatic. In one model, the TCR and CD4 bind pMHCII independently, while in another, CD4 interacts with a composite surface formed by the TCR-CD3 complex bound to pMHCII. Here, we report that the duration of TCR-pMHCII interactions impact CD4 binding to MHCII. In turn, CD4 increases TCR confinement to pMHCII via reciprocal interactions involving membrane distal and proximal CD4 ectodomains. The data suggest that a precisely assembled macrocomplex functions to reliably convert TCR-pMHCII confinement into reproducible signals that orchestrate adaptive immunity.

Coreceptor affinity for MHC defines peptide specificity requirements for TCR interaction with coagonist peptide-MHC

The requirement for the TCR to interact with coagonists, endogenous MHC–peptide complexes which do not themselves activate the T cell, decreases as the strength of the CD8–class I interaction increases.

Recent work has demonstrated that nonstimulatory endogenous peptides can enhance T cell recognition of antigen, but MHCI- and MHCII-restricted systems have generated very different results. MHCII-restricted TCRs need to interact with the nonstimulatory peptide–MHC (pMHC), showing peptide specificity for activation enhancers or coagonists. In contrast, the MHCI-restricted cells studied to date show no such peptide specificity for coagonists, suggesting that CD8 binding to noncognate MHCI is more important. Here we show how this dichotomy can be resolved by varying CD8 and TCR binding to agonist and coagonists coupled with computer simulations, and we identify two distinct mechanisms by which CD8 influences the peptide specificity of coagonism. Mechanism 1 identifies the requirement of CD8 binding to noncognate ligand and suggests a direct relationship between the magnitude of coagonism and CD8 affinity for coagonist pMHCI. Mechanism 2 describes how the affinity of CD8 for agonist pMHCI changes the requirement for specific coagonist peptides. MHCs that bind CD8 strongly were tolerant of all or most peptides as coagonists, but weaker CD8-binding MHCs required stronger TCR binding to coagonist, limiting the potential coagonist peptides. These findings in MHCI systems also explain peptide-specific coagonism in MHCII-restricted cells, as CD4–MHCII interaction is generally weaker than CD8–MHCI.

Cross-dressed CD8α+/CD103+ dendritic cells prime CD8+ T cells following vaccination

Activation of naïve cluster of differentiation (CD)8(+) cytotoxic T lymphocytes (CTLs) is a tightly regulated process, and specific dendritic cell (DC) subsets are typically required to activate naive CTLs. Potential pathways for antigen presentation leading to CD8(+) T-cell priming include direct presentation, cross-presentation, and cross-dressing. To distinguish between these pathways, we designed single-chain trimer (SCT) peptide-MHC class I complexes that can be recognized as intact molecules but cannot deliver antigen to MHC through conventional antigen processing. We demonstrate that cross-dressing is a robust pathway of antigen presentation following vaccination, capable of efficiently activating both naïve and memory CD8(+) T cells and requires CD8α(+)/CD103(+) DCs. Significantly, immune responses induced exclusively by cross-dressing were as strong as those induced exclusively through cross-presentation. Thus, cross-dressing is an important pathway of antigen presentation, with important implications for the study of CD8(+) T-cell responses to viral infection, tumors, and vaccines.

An active CD8alpha/pMHCI interaction is required for CD8 single positive thymocyte differentiation

Recognition of viral antigenic peptides bound to major histocompatibility complex class I molecules (MHCI) by TCR is critical for initiating the responses of CD8(+) T cells that ultimately lead to elimination of virus-infected cells. This antigen recognition is enhanced by the CD8 coreceptor through its interaction with the peptide-MHCI complexes (pMHCI). Mouse CD8alphabeta can form two different complexes with pMHCI via either the CD8alpha- or CD8beta-dominated interaction. To understand the functional significance of these complexes in vivo, we generated Tg mice carrying a variant CD8alphabeta (CD8alpha(m3)beta) capable of forming only the CD8beta-dominated CD8alphabeta/pMHCI complex. These mice show sub-optimal thymic differentiation with reduced populations of CD8(+) single-positive thymocytes. Tg CD8(+) T cells exhibit a compromised developmental capacity when competing with CD8(+) T cells from B6 mice in mixed bone marrow chimera experiments. However, once these CD8(+) T cells have emigrated to the peripheral lymphoid organs, they exhibit normal effector function against viral infection. Our observations indicate that, in addition to the CD8 activity conferred by CD8beta-dominated CD8alphabeta/pMHCI complexes, full thymocyte differentiation requires additional coreceptor activities conferred by CD8alphaalpha and/or CD8alphabeta with CD8alpha-dominated CD8/pMHCI complexes.

Structural basis of the CD8 alpha beta/MHC class I interaction: focused recognition orients CD8 beta to a T cell proximal position

In the immune system, B cells, dendritic cells, NK cells, and T lymphocytes all respond to signals received via ligand binding to receptors and coreceptors. Although the specificity of T cell recognition is determined by the interaction of T cell receptors with MHC/peptide complexes, the development of T cells in the thymus and their sensitivity to Ag are also dependent on coreceptor molecules CD8 (for MHC class I (MHCI)) and CD4 (for MHCII). The CD8alphabeta heterodimer is a potent coreceptor for T cell activation, but efforts to understand its function fully have been hampered by ignorance of the structural details of its interactions with MHCI. In this study we describe the structure of CD8alphabeta in complex with the murine MHCI molecule H-2D(d) at 2.6 A resolution. The focus of the CD8alphabeta interaction is the acidic loop (residues 222-228) of the alpha3 domain of H-2D(d). The beta subunit occupies a T cell membrane proximal position, defining the relative positions of the CD8alpha and CD8beta subunits. Unlike the CD8alphaalpha homodimer, CD8alphabeta does not contact the MHCI alpha(2)- or beta(2)-microglobulin domains. Movements of the CD8alpha CDR2 and CD8beta CDR1 and CDR2 loops as well as the flexibility of the H-2D(d) CD loop facilitate the monovalent interaction. The structure resolves inconclusive data on the topology of the CD8alphabeta/MHCI interaction, indicates that CD8beta is crucial in orienting the CD8alphabeta heterodimer, provides a framework for understanding the mechanistic role of CD8alphabeta in lymphoid cell signaling, and offers a tangible context for design of structurally altered coreceptors for tumor and viral immunotherapy.

Novel nonclassical MHC class Ib genes associated with CD8 T cell development and thymic tumors

In jawed vertebrates, the heterogeneous nonclassical MHC class Ib (class Ib) gene family encodes molecules structurally similar to classical MHC class Ia (class Ia) but with more limited tissue distribution and lower polymorphism. In mammals, class Ib gene products are involved in stress responses, malignancy and differentiation of intrathymic CD8 T cells. The frog Xenopus laevis possesses at least 20 class Ib genes (XNCs), and 9 subfamilies have been defined so far. We have characterized two novel subfamilies, XNC10 and XNC11. XNC10 is phylogenetically and structurally distinct from both class Ia and other XNC genes. Besides thymic lymphoid tumors, XNC10 is preferentially expressed by circulating T cells and thymocytes of the CD8 lineage both in adult and in larvae from the onset of thymus organogenesis. XNC11 is expressed only by thymocytes and upregulated by several thymic lymphoid tumors. These data provide the first evidence of the expression of any class Ib genes in Xenopus larvae, and suggests evolutionary relationships between certain class Ib genes, malignancy and CD8 T cell ontogeny.

An essential role for the stalk region of CD8 beta in the coreceptor function of CD8

The CD8alphabeta heterodimer is integral to the selection of the class I-restricted lineage in the thymus; however, the contribution of the CD8beta chain to coreceptor function is poorly understood. To understand whether the CD8beta membrane proximal stalk region played a role in coreceptor function, we substituted it with the corresponding sequence from the CD8alpha polypeptide and expressed the hybrid molecule in transgenic mice in place of endogenous CD8beta. Although the stalk-swapped CD8beta was expressed on the cell surface as a disulfide-bonded heterodimer at equivalent levels of expression to an endogenous CD8beta molecule, it failed to restore selection of CD8(+) class I MHC-restricted T cells and it altered the response of peripheral T cells. Thus, the stalk region of the CD8beta polypeptide has an essential role in ensuring functionality of the CD8alphabeta heterodimer and its replacement compromises the interaction of CD8 with peptide-MHC complexes.

Molecular modelling of co-receptor CD8 alpha alpha and its complex with MHC class I and T-cell receptor in sea bream (Sparus aurata)

T-cells are the main actors of cell-mediated immune defence; they recognize and respond to peptide antigens associated with MHC class I and class II molecules. In this paper, we investigated by molecular modelling methods in the teleost sea bream (Sparus aurata) the interaction among the molecules of the tertiary complex CD8/MHC-I/TCR, which determines the T-cell-mediated immunological response to foreign molecules. First, we predicted the three-dimensional structure of CD8 alpha alpha dimer and MHC-I, and, successively, we simulated the CD8 alpha alpha/MHC-I complex. Finally, the 3D structure of the CD8/MHC-I/TCR complex was simulated in order to investigate the possible changes that can influence TCR signalling events.

Role of alpha3 domain of class I MHC molecules in the activation of high- and low-avidity CD8+ CTLs

CD8 can serve as a co-receptor or accessory molecule on the surface of CTL. As a co-receptor, CD8 can bind to the alpha3 domain of the same MHC class I molecules as the TCR to facilitate TCR signaling. To evaluate the role of the MHC class I molecule alpha3 domain in the activation of CD8(+) CTL, we have produced a soluble 227 mutant of H-2D(d), with a point mutation in the alpha3 domain (Glu227 --> Lys). 227 mutant class I-peptide complexes were not able to effectively activate H-2D(d)-restricted CD8 T cells in vitro, as measured by IFN-gamma production by an epitope-specific CD8(+) CTL line. However, the 227 mutant class I-peptide complexes in the presence of another MHC class I molecule (H-2K(b)) (that cannot present the peptide) with a normal alpha3 domain can induce the activation of CD8(+) CTL. Therefore, in order to activate CD8(+) CTL, the alpha3 domain of MHC class I does not have to be located on the same molecule with the alpha1 and alpha2 domains of MHC class I. A low-avidity CD8(+) CTL line was significantly less sensitive to stimulation by the 227 mutant class I-peptide complexes in the presence of the H-2K(b) molecule. Thus, low-avidity CTL may not be able to take advantage of the interaction between CD8 and the alpha3 domain of non-presenting class I MHC molecules, perhaps because of a shorter dwell time for the TCR-MHC interaction.

CD8αβ Has Two Distinct Binding Modes of Interaction with Peptide-Major Histocompatibility Complex Class I

Interaction of CD8 (CD8alphaalpha or CD8alphabeta) with the peptide-major histocompatibility complex (MHC) class I (pMHCI) is critical for the development and function of cytolytic T cells. Although the crystal structure of CD8alphaalpha.pMHCI complex revealed that two symmetric CD8alpha subunits interact with pMHCI asymmetrically, with one subunit engaged in more extensive interaction than the other, the details of the interaction between the CD8alphabeta heterodimer and pMHCI remained unknown. The Ig-like domains of mouse CD8alphabeta and CD8alphaalpha are similar in the size, shape, and surface electrostatic potential of their pMHCI-binding regions, suggesting that their interactions with pMHCI could be very similar. Indeed, we found that the CD8alpha variants CD8alpha(R8A) and CD8alpha(E27A), which were functionally inactive as homodimers, could form an active co-receptor with wild-type (WT) CD8beta as a CD8alpha(R8A)beta or CD8alpha(E27A)beta heterodimer. We also identified CD8beta variants that could form active receptors with WT CD8alpha but not with CD8alpha(R8A). This observation is consistent with the notion that the CD8beta subunit may replace either CD8alpha subunit in CD8alphaalpha.pMHCI complex. In addition, we showed that both anti-CD8alpha and anti-CD8beta antibodies were unable to completely block the co-receptor activity of WT CD8alphabeta. We propose that CD8alphabeta binds to pMHCI in at least two distinguishable orientations.

Structural and Mutational Analyses of a CD8αβ Heterodimer and Comparison with the CD8αα Homodimer

The crystal structure of a recombinant mouse single chain CD8alphabeta ectodomains at 2.4 A resolution reveals paired immunoglobulin variable region-like domains with a striking resemblance to CD8alphaalpha in size, shape, and surface electrostatic potential of complementarity-determining regions (CDR), despite <20% sequence identity between the CD8alpha and CD8beta subunits. Unlike the CD8alpha subunit(s) in the heterodimer or homodimer, the CDR1 loop of CD8beta tilts away from its corresponding CDR2 and CDR3 loops. Consistent with this observation, independent mutational studies reveal that alanine substitutions of residues in the CDR1 loop of CD8beta have no effect on CD8alphabeta coreceptor function, whereas mutations in CD8beta CDR2 and CDR3 loops abolish CD8alphabeta coreceptor activity. The implications of these findings and additional CD8alpha mutational studies for CD8alphabeta- versus CD8alphaalpha-MHCI binding are discussed.

Recognition of open conformers of classical MHC by chaperones and monoclonal antibodies

There is considerable evidence that the conformation and stability of class I and class II major histocompatibility complex (MHC) proteins is dependent upon high-affinity peptide ligation, but structural data for an empty MHC protein unfortunately is lacking. However, several monoclonal antibodies (mAbs) that specifically detect open MHC conformers have been characterized, and they provide insights into the changes associated with peptide loading and unloading. Here, the structural changes make the argument that certain of these open conformer-specific mAbs recognize analogous MHC segments as the molecular chaperones tapasin and DM. MHC residues located in regions flanking the peptide-terminal anchoring pockets have been implicated in both chaperone and monoclonal antibody binding. Indeed, we propose these regions serve as peptide-binding hinges that are uniquely accessible in open MHC.

Elucidation of Exon 1, 4, and 5 Sequences of 39 Infrequent HLA-B Alleles

More than 590 human leukocyte antigen (HLA)-B alleles have been identified by sequence analysis. Although the polymorphic exon 2 and 3 sequences of all HLA-B alleles are described, the sequences of the other exons of a number of infrequent B-alleles are unknown. In this study, the exon 1, 4, and 5 sequences of 39 different HLA-B alleles were elucidated by allele-specific sequencing. Overall, these exon sequences showed identity with the majority of the known sequences from the corresponding allele groups, except for four alleles B*4010, B*4415, B*4416, and B*5606. The exon 1 sequence of B*4010 had nucleotide differences with all B*40 alleles, but was identical to the B*54, *55, *56, and *59 allele groups. B*4416 differed from B*440201 at position 988, which was previously considered a conserved position. B*4415 showed exon 1, 4, and 5 sequences deviating from the other B*44 alleles, but identical to B*4501. The exon 1 and 4 sequences of B*5606 differed from other B*56 alleles, but were in complete agreement with B*7801. The deviating exon sequences of B*4415 and B*5606 confirmed the evolutionary origin of these alleles suggested by the sequences of exons 2 and 3. The polymorphism observed in exons 1, 4, and 5 merely reflects the lineage-specificity of HLA-B.

Memory CD8 T cells require CD8 coreceptor engagement for calcium mobilization and proliferation, but not cytokine production

Memory T-cell responses are faster and more robust than those of their naive counterparts. The mechanisms by which memory T cells respond better to subsequent antigenic exposure remain unresolved. A portion of the more rapid response is undoubtedly the result of the increased frequency of antigen-specific cells. In addition, there are also differences in the cells themselves with respect to their requirements for costimulation and the apparent avidity of the T cells. We used major histocompatibility complex (MHC) class I tetramers to stimulate T cells to focus on the interaction of T-cell receptor (TCR)/MHC and CD8 in the absence of other molecules that are present on cell surfaces and so contribute to the activation of T cells by undefined mechanisms. Mutated MHC class I tetramers that are unable to engage CD8 were used to investigate the role of CD8 engagement in memory cell activation. Either wild-type tetramers or tetramers carrying the mutation were used to stimulate both memory and naive TCR transgenic T cells in vitro. Surprisingly, like naive cells, memory CD8(+) T cells required CD8 engagement for calcium mobilization and optimum proliferation. In contrast, the requirements for cytokine production differed. Unlike naive cells, memory cells were able to produce cytokine in the absence of CD8 engagement. This suggests both a CD8-dependent pathway for early events and a CD8-independent pathway for cytokine production in memory cells.

Low binding capacity of murine tetramers mutated at residue 227 does not preclude the ability to efficiently activate CD8+ T lymphocytes

MHC tetramers are used to directly enumerate and visualize the antigen-specific T lymphocyte population of interest by flow cytometry, regardless of the T lymphocyte's functional capacity. Assay sensitivity can be hindered by non-specific binding activity, which is due to the inherent interactions of CD8 and MHC. Point mutations within the alpha3 loop of the HLA MHC class I heavy chain have been shown to reduce or abrogate MHC/CD8 interactions and also alleviate non-specific binding. This report compares the effects of two well-described mutations on the binding capacity and functional capacity of MHC tetramers in the H-2 MHC murine system. Tetramers folded with MHC mutated at either residue 227 or 245 of the class I heavy chain were compared to wild-type tetramer in binding studies using various antigen-specific, TCR-positive lymphocytes and cell lines. These experiments showed that the binding of wild-type and residue 245-mutated tetramer were comparable on CTL cultures, OT-1 splenocytes, and hybridomas. Both wild-type and 245-mutated tetramers' binding capacity was observed to be equally dependent on CD8 expression. Residue 227-mutated tetramer consistently bound antigen-specific CTL less efficiently, but in the absence of CD8 all three tetramers had similar binding capacity. In functional studies, 227-mutated tetramer had the greatest capacity to stimulate cytokine production in the absence of exogenous antigen addition. These experiments demonstrate that reduction of a tetramer's high avidity interaction with CD8 will not necessarily decrease the ability to stimulate the effector functions of activated T cells.

Ambiguities of human leukocyte antigen-B resolved by sequence-based typing of exons 1, 4, and 5

The elucidation of the sequences of human leukocyte antigen-B (HLA-B)-exons 1 through 5 has led to an increase of ambiguities with alleles having identical exon 2 and 3 sequences, but differences in other exons. At the moment, 26 HLA-B alleles show such ambiguities which can be resolved by sequencing the exons in which the differences are located. Here we report a sequence-based typing (SBT) strategy for heterozygous sequencing of exons 1, 4, and 5, in addition to the previously described exons 2 and 3. The strategy was validated against a panel of 25 individuals, carrying HLA-B alleles from 33 different allele groups. Correct assignment of all HLA-B alleles was obtained for exons 1 through 5. In addition, the SBT protocol was used to resolve ambiguities in 50 individuals. The ambiguous combinations studied were B*0705/06, B*0801/19N, B*1512/19, B*180101/17N, B*270502/13/0504, B*350101/42/40N, B*390101/0103, B*400102/0101, B*440201/19N/27, and B*510101/11N/0105/30/32. In all cases, sequencing revealed the first allele to be present, except for three individuals with B*07. One of them typed B*0705; the other two were B*0706. The described SBT protocol for sequencing exons 1, 4, and 5 is a valuable tool for resolving ambiguities of HLA-B alleles with differences in these exons, as well as for studying the polymorphism of HLA-B outside exons 2 and 3.

High Avidity Antigen-Specific CTL Identified by CD8-Independent Tetramer Staining

Tetrameric MHC/peptide complexes are important tools for enumerating, phenotyping, and rapidly cloning Ag-specific T cells. It remains however unclear whether they can reliably distinguish between high and low avidity T cell clones. In this report, tetramers with mutated CD8 binding site selectively stain higher avidity human and murine CTL capable of recognizing physiological levels of Ag. Furthermore, we demonstrate that CD8 binding significantly enhances the avidity as well as the stability of interactions between CTL and cognate tetramers. The use of CD8-null tetramers to identify high avidity CTL provides a tool to compare vaccination strategies for their ability to enhance the frequency of high avidity CTL. Using this technique, we show that DNA priming and vaccinia boosting of HHD A2 transgenic mice fail to selectively expand large numbers of high avidity NY-ESO-1(157-165)-specific CTL, possibly due to the large amounts of antigenic peptide delivered by the vaccinia virus. Furthermore, development of a protocol for rapid identification of high avidity human and murine T cells using tetramers with impaired CD8 binding provides an opportunity not only to monitor expansion of high avidity T cell responses ex vivo, but also to sort high avidity CTL clones for adoptive T cell transfer therapy.

Interplay between TCR affinity and necessity of coreceptor ligation: high-affinity peptide-MHC/TCR interaction overcomes lack of CD8 engagement

CD8 engagement is believed to be a critical event in the activation of naive T cells. In this communication, we address the effects of peptide-MHC (pMHC)/TCR affinity on the necessity of CD8 engagement in T cell activation of primary naive cells. Using two peptides with different measured avidities for the same pMHC-TCR complex, we compared biochemical affinity of pMHC/TCR and the cell surface binding avidity of pMHC/TCR with and without CD8 engagement. We compared early signaling events and later functional activity of naive T cells in the same manner. Although early signaling events are altered, we find that high-affinity pMHC/TCR interactions can overcome the need for CD8 engagement for proliferation and CTL function. An integrated signal over time allows T cell activation with a high-affinity ligand in the absence of CD8 engagement.

Stalk region of beta-chain enhances the coreceptor function of CD8

CD8 glycoproteins are expressed as either alphaalpha homodimers or alphabeta heterodimers on the surface of T cells. CD8alphabeta is a more efficient coreceptor than the CD8alphaalpha for peptide Ag recognition by TCR. Each CD8 subunit is composed of four structural domains, namely, Ig-like domain, stalk region, transmembrane region, and cytoplasmic domain. In an attempt to understand why CD8alphabeta is a better coreceptor than CD8alphaalpha, we engineered, expressed, and functionally tested a chimeric CD8alpha protein whose stalk region is replaced with that of CD8beta. We found that the beta stalk region enhances the coreceptor function of chimeric CD8alphaalpha to a level similar to that of CD8alphabeta. Surprisingly, the beta stalk region also restored functional activity to an inactive CD8alpha variant, carrying an Ala mutation at Arg(8) (R8A), to a level similar to that of wild-type CD8alphabeta. Using the R8A variant of CD8alpha, a panel of anti-CD8alpha Abs, and three MHC class I (MHCI) variants differing in key residues known to be involved in CD8alpha interaction, we show that the introduction of the CD8beta stalk leads to a different topology of the CD8alpha-MHCI complex without altering the overall structure of the Ig-like domain of CD8alpha or causing the MHCI to employ different residues to interact with the CD8alpha Ig domain. Our results show that the stalk region of CD8beta is capable of fine-tuning the coreceptor function of CD8 proteins as a coreceptor, possibly due to its distinct protein structure, smaller physical size and the unique glycan adducts associated with this region.

Allogeneic and syngeneic class I MHC complexes drive the association of CD8 and TCR on 2C T cells

In most cases, cytotoxic T cell activation is dependent on the interaction of the T cell receptor (TCR) and CD8 with MHC class I molecules. In the CD8(+) T cell system based on the mouse cytotoxic T cell clone 2C, recognition of the allogeneic MHC L(d) exhibits a less significant role for CD8 than recognition of the syngeneic MHC K(b). Here, we examined whether this difference is related to the relative abilities of the two pepMHC complexes to drive the association of CD8 and TCR on the T cell surface. We show that both the syngeneic and allogeneic pepMHC induced association of CD8 and TCR, as revealed by fluorescence resonance energy transfer (FRET). Thus, the orientation of the syngeneic and allogeneic ligands when bound to the same TCR both allow CD8 to be recruited to the TCR complex. The conserved diagonal orientation of TCRs on different pepMHC ligands may facilitate such associations. The FRET results are consistent with the known binding properties and the CD8 involvement of the two different TCR:pepMHC interactions.

Soluble HLA class I induces NK cell apoptosis upon the engagement of killer-activating HLA class I receptors through FasL-Fas interaction

The engagement of the activating isoforms of C-type lectin inhibitory receptor (CLIR) or killer Ig-like receptor (KIR) by their natural ligands, represented by soluble HLA-I (sHLA-I) molecules, induced programmed cell death of natural killer (NK) cells. Indeed, NK cell apoptosis elicited by either putative HLA-E and HLA-F (sHLA-I non-A, -B, -C, and -G) or sHLA-I-Cw4 or -Cw3 from untransfected or -Cw4 or -Cw3 alleles transfected HLA-A(-), B(-), C(-), G(-), E(+), F(+) 721.221 lymphoblastoid cell line, respectively, was blocked by covering the corresponding activating receptor with either anti-CLIR- or anti-KIR-specific monoclonal antibodies (mAbs). After sHLA-I-activating receptor interaction, NK cells produced and released Fas ligand (FasL), which in turn led to NK cell apoptosis by interacting with Fas at the NK cell surface. Blocking anti-Fas mAb, or anti-FasL mAb, inhibited sHLA-I-mediated apoptosis via activating receptor in NK cell clones. This apoptosis was inhibited by NK cell treatment with cyclosporin A, whereas this drug had no effect on activating receptor-mediated activation of cytolysis. Conversely, concanamycin A, an inhibitor of vacuolar type H(+)-adenosine triphosphatase (H(+)-ATPase) of granules, inhibited activating receptor-induced NK cell cytolysis, suggesting that activating receptor-mediated apoptosis and cytolysis can use different intracellular pathways. Furthermore, a large amount of interferon-gamma (IFN-gamma) was detectable in culture supernatant of activating receptor(+) NK cells incubated with the appropriate sHLA-I ligand. Again, cyclosporin A, but not concanamycin A, strongly reduced activating receptor-mediated IFN-gamma production. This suggests that activating receptor-induced apoptosis of NK cells could play a role in eliminating potentially harmful NK cell clones and, at the same time, it leads to production of IFN-gamma, an antiviral cytokine able to amplify immune responses.

Mouse MHC class I tetramers that are unable to bind to CD8 reveal the need for CD8 engagement in order to activate naive CD8 T cells

Although the role of CD8 as a supplier of lck is unchallenged, its role in contributing to the formation of a stable complex between class I molecules and the TCR, as well as its role as an adhesion molecule, is less clear. To address the role of CD8/MHC-I interactions, we generated tetramers composed of H2-K(b) molecules with mutations in the alpha 3 domain of H2-K(b) that abolish CD8 binding. We show that the ability of tetramers to stain and activate CD8 T cells is strongly dependent on binding of CD8 to the same class I molecule engaged by the TCR. We characterize a mutation in the alpha 3 domain that results in H2-K(b) molecules capable of staining specific CD8 T cells with little ensuing activation. Although CD8 to some extent serves an adhesive function, this contribution is modest and does not substitute for lack of binding of CD8 to the class I molecule engaged by the TCR. We show that CD8 and the TCR associate in a process independent of binding of CD8 to class I. Our data support the notion that CD8 is required to form a stable complex between class I and the TCR.

A role for the alpha-chain connecting peptide motif in mediating TCR-CD8 cooperation

To generate peripheral T cells that are both self-MHC restricted and self-MHC tolerant, thymocytes are subjected to positive and negative selection. How the TCR discriminates between positive and negative selection ligands is not well understood, although there is substantial evidence that the CD4 and CD8 coreceptors play an important role in this cell fate decision. We have previously identified an evolutionarily conserved motif in the TCR, the alpha-chain connecting peptide motif (alpha-CPM), which allows the TCR to deliver positive selection signals. Thymocytes expressing alpha-CPM-deficient receptors do not undergo positive selection, whereas their negative selection is not impaired. In this work we studied the ligand binding and receptor function of alpha-CPM-deficient TCRs by generating T cell hybridomas expressing wild-type or alpha-CPM-deficient forms of the T1 TCR. This K(d)-restricted TCR is specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide(252-260) IASA-YIPSAEK(ABA)I and is therefore amenable to TCR photoaffinity labeling. The experiments presented in this work show that alpha-CPM-deficient TCRs fail to cooperate with CD8 to enhance ligand binding and functional responses.

Sequence analysis of exons 1, 2, 3, 4 and 5 of the HLA-B5/35 cross-reacting group

The HLA-B5/35 cross-reacting group (CREG) is a set of closely related antigens including HLA-B35, B51, B52, B53 and B78. The nucleotide sequences of exon 1 through 5 of the B5/35 CREG were determined to assess the level of polymorphism. For exons 2 and 3, the previously described sequence-based typing (SBT) strategy was applied, the nucleotide sequences of exon 1, 4 and 5 were determined by allele-specific sequencing. A total of 225 unrelated individuals were HLA-B typed by heterozygous sequencing of exons 2 and 3. In the B5/35 CREG, 26 different alleles were identified, whereas 63 non-B5/35 CREG alleles were sequenced. The SBT strategy was proven to be reliable and efficient for high resolution typing of the B5/35 CREG. The nucleotide sequences of exon 1, 4 and 5 were determined for the 26 different B5/35 CREG alleles to establish the level of polymorphism. For seven different alleles, of which the exon 1, 4 and 5 sequences were hitherto unknown, the sequences were elucidated and in agreement with the known B5/35 sequences. Nineteen HLA-B5/35 CREG alleles with previously published exon 1, 4 and 5 sequences were sequenced in at least two individuals. Three new alleles were identified. The first, B*5204, showed a difference at position 200 compared to B*52011, which was previously considered a conserved position. The other two alleles, B*3542 and B*51015, showed exon 2 and 3 sequences identical to B*35011 and B*51011, but differences in exons 1 and 4, respectively. B*3542 had differences at position 25 and 72 and B*51015 showed a difference at position 636. More polymorphism might be present outside exons 2 and 3 than previously thought.

HLAMatchmaker: a molecularly based algorithm for histocompatibility determination. I. Description of the algorithm

This report describes an algorithm for identifying acceptable HLA antigens for highly alloimmunized patients without the need for extensive serum screening. This algorithm is based on the concept that immunogenic epitopes are represented by amino acid triplets on exposed parts of protein sequences of human leukocyte antigen chains (HLA-A, HLA-B, and HLA-C) accessible to alloantibodies. A computer program (HLAMatchmaker) has been developed to determine class I HLA compatibility at the molecular level. It makes intralocus and interlocus comparisons of polymorphic triplets in sequence positions to determine the spectrum of non-shared triplets on donor HLA antigens. In most cases is it possible to identify certain mismatched HLA antigens that share all their polymorphic triplets with the patient's HLA antigens and could therefore, be considered fully compatible. HLAMatchmaker permits also the identification of additional mismatches that are acceptable as determined from the triplet information on HLA-typed panel cells that do not react with patient's serum.HLAMatchmaker provides an assessment of donor-recipient HLA compatibility at the structural level and this algorithm is different from conventional methods based on the mere counting of numbers of mismatched HLA antigens or CREGs. This donor selection strategy is suitable especially for allosensitized patients in need of a compatible transplant or platelet transfusion.

Genetically modified HLA class I molecules able to inhibit human NK cells without provoking alloreactive CD8+ CTLs

Human NK cells are likely to be important effectors of xenograft rejection. Expression of HLA class I molecules by transfected porcine cells can protect them from human NK cell-mediated lysis; however, this strategy has the potential to augment the anti-graft response by recipient CD8(+) T cells recognizing foreign pig peptides presented by HLA. In this study we show that the introduction of a mutation (D227K) in the alpha(3) domain of HLA-Cw3 abrogates its recognition by CD8-dependent T cells but leaves intact its ability to function as an inhibitory ligand for NK cells. Such genetically modified molecules may have potential therapeutic applications in the prevention of delayed xenograft rejection and in the facilitation of allogeneic and xenogeneic bone marrow engraftment.

Mutant MHC class I molecules define interactions between components of the peptide-loading complex

Class I histocompatibility molecules, consisting of a heavy chain, beta2-microglobulin and peptide, are assembled in the endoplasmic reticulum (ER) with the assistance of several molecular chaperones and accessory proteins. Peptide binding occurs when assembling class I molecules associate with a loading complex consisting of the transporter associated with antigen processing (TAP) peptide transporter, tapasin, ERp57 and calreticulin (CRT)/calnexin. To assess the physical organization of this complex, we generated a series of mutants in the murine H-2Dd heavy chain and assessed their association with components of the complex. Seven mutations, clustered between amino acids 122 and 136 in the heavy chain alpha2 domain plus one mutation at position 222 in the alpha3 domain, resulted in loss of interaction with tapasin. Association with TAP was always lost simultaneously, supporting the view that tapasin acts as an obligatory bridge between class I molecules and TAP. Compared with previous studies on the HLA-A2 molecule, some differences in points of tapasin interaction were observed. Failure of the H-2Dd mutants to bind tapasin resulted in low cell-surface expression and altered intracellular transport. Most mutants retained a substantial degree of peptide loading, consistent with the view that although tapasin may promote peptide binding to class I, it is not required. A surprising observation was that all mutants lacking tapasin interaction retained normal association with CRT. This contrasts with previous observations on other class I molecules and, combined with differences in tapasin interaction, suggests that the organization of the ER peptide-loading complex can vary depending on the specific class I molecule examined.

Soluble HLA class I molecules induce natural killer cell apoptosis through the engagement of CD8: evidence for a negative regulation exerted by members of the inhibitory receptor superfamily

Herein, we show that CD8(dull), CD8(intermediate), and CD8(bright) natural killer (NK) cell clones can be identified. Triggering of CD8 with its natural ligand(s), represented by soluble HLA class I (sHLA-I), isolated either from serum of healthy donors or from HLA-I(-) 721.221 lymphoblastoid cell line transfected with HLA-A2, -Cw4, and -Bw46 alleles, or HLA-G1 leads to NK cell apoptosis. The magnitude of this effect directly correlated with the level of CD8 expression. sHLA-I-induced apoptosis depends on the interaction with CD8, as it was inhibited by masking this molecule with specific monoclonal antibodies (mAbs). Moreover, sHLA-I or CD8 cross-linking with specific mAbs elicited intracellular calcium increases, Fas ligand (FasL) messenger RNA transcription, and FasL secretion, which were needed for delivering the death signal. Indeed, this apoptosis was inhibited by preincubation of NK cell clones with Fas or FasL antagonist mAbs, indicating that the Fas/FasL pathway is involved. Furthermore, members of the inhibitory receptor superfamily, such as CD94/NKG2 complex or killer inhibitory receptors, were shown to exert an inhibitory effect on sHLA-I-mediated apoptosis and secretion of FasL. These findings suggest that interaction between sHLA-I and CD8 evokes an apoptotic signal that is down-regulated by inhibitory receptor superfamily that function as survival receptors in NK cells.

Interactions between MHC molecules and co-receptors of the TCR

Genetic experiments indicate similarity between binding sites on MHC class I (MHCI) for CD8 and on MHCII for CD4, but the crystal structures of CD8/MHCI and CD4/MHCII complexes suggest critical differences between the interfaces in the two complexes. Biophysical analyses using ectodomains of co-receptors and MHC molecules demonstrate extremely fast kinetics and low-affinity interactions. Experiments with soluble multimeric MHC ligands suggest that CD4 and CD8 may differ in the mechanisms by which they promote the formation of ternary TCR/MHC/co-receptor complexes. Co-receptor-influenced duration of TCR signaling controls thymocyte selection. In naïve T cells, CD4/MHCII interactions may promote T-cell survival. Temporal and spatial analysis of TCR and CD4 co-clustering in the immunological synapse suggests that CD4 recruitment is regulated by the half-life of the initial TCR/MHCII complex. Diverse experimental systems have yielded conflicting data that have helped to formulate revised mechanistic models of co-receptor function.

Improved detection of melanoma antigen-specific T cells expressing low or high levels of CD8 by HLA-A2 tetramers presenting a Melan-A/Mart-1 peptide analogue

MHC class I tetramers containing peptide epitopes are sensitive tools for detecting antigen-specific CD8(+) T-cell responses. We demonstrate here that binding of HLA-A2 tetramers to CD8(+) T cells specific for the melanoma-associated antigen Melan-A/MART-1 can be fine-tuned by altering either the bound peptide epitope or residues in the alpha 3 domain of HLA-A2, which is important for CD8 binding. Antigen-specific T cells expressing high levels of CD8 could be detected using HLA-A2 tetramers containing the peptide AAGIGILTV, an epitope which is naturally processed and presented from Melan-A/MART-1. In contrast, low CD8-expressing, antigen-specific T cells could be detected efficiently only by using a mutated HLA-A2 tetramer with an altered CD8 binding site or, less efficiently, using the wild-type HLA-A2 tetramer loaded with the peptide analogue ELAGIGILTV, which is superior in stimulating antigen-specific T-cell responses. Our results suggest ways to optimize the identification and expansion of antigen-specific T cells with different requirements for the costimulatory CD8 molecule in facilitating T-cell receptor binding to peptide variants.

Developmentally regulated glycosylation of the CD8alphabeta coreceptor stalk modulates ligand binding

The functional consequences of glycan structural changes associated with cellular differentiation are ill defined. Herein, we investigate the role of glycan adducts to the O-glycosylated polypeptide stalk tethering the CD8alphabeta coreceptor to the thymocyte surface. We show that immature CD4(+)CD8(+) double-positive thymocytes bind MHCI tetramers more avidly than mature CD8 single-positive thymocytes, and that this differential binding is governed by developmentally programmed O-glycan modification controlled by the ST3Gal-I sialyltransferase. ST3Gal-I induction and attendant core 1 sialic acid addition to CD8beta on mature thymocytes decreases CD8alphabeta-MHCI avidity by altering CD8alphabeta domain-domain association and/or orientation. Hence, glycans on the CD8beta stalk appear to modulate the ability of the distal binding surface of the dimeric CD8 globular head domains to clamp MHCI.

Formation of supramolecular activation clusters on fresh ex vivo CD8+ T cells after engagement of the T cell antigen receptor and CD8 by antigen-presenting cells

Upon productive interaction of CD4 T cells with antigen-presenting cells (APCs), receptors and intracellular proteins translocate and form spatially segregated supramolecular activation clusters (SMACs). It is not known whether SMACs are required for CD8 T cell activation. CD8 T cells, unlike CD4 T cells, can be activated by a single peptide-MHC molecule, or by purified monovalent recombinant peptide-MHC molecules. We studied, by three-dimensional digital microscopy, cell conjugates of fresh ex vivo CD8 T cells (obtained from OT-1 mice, which are transgenic for T cell antigen receptor reactive with the complex of H-2K(b) and the ovalbumin octapeptide SIINFEKL) and peptide-pulsed APCs. Remarkably, even in T cell:APC conjugates that were formed in the presence of the lowest concentration of peptide that was sufficient to elicit T cell proliferation and IFN-gamma production; the theta isoform of protein kinase C was clustered in a central SMAC, and lymphocyte function-associated antigen 1 and talin were clustered in the peripheral SMAC. Conjugation of T cells to APCs that were pulsed with concentrations of peptide smaller than that required to activate T cells was greatly reduced, and SMACs were not formed at all. APCs expressing mutant H-2K(b) (Lys(227)) molecules that do not bind CD8 were unable to form stable conjugates with these T cells, even at high peptide concentrations. Thus, although CD8 and CD4 T cells may display different sensitivity to the concentration and oligomerization of surface receptors, SMACs are formed and seem to be required functionally in both cell types. However, unlike CD4 T cells, which can form SMACs without CD4, CD8 T cells from OT-1 transgenic mice depend on their coreceptor, CD8, for the proper formation of SMACs.

The CD8alphabeta co-receptor on double-positive thymocytes binds with differing affinities to the products of distinct class I MHC loci

The CD8 co-receptor is essential for TCR-dependent immune recognition and T cell development involving peptides bound to MHC class I (MHCI) molecules. The dominant interaction of CD8 alpha alpha and alpha beta co-receptors is with the alpha3 domain of an MHCI molecule. Whether this interaction is different for the products of various MHCI loci is currently unknown. Here we examine the interaction between H-2K(b) and H-2D(b), the two MHCI molecules in the C57BL / 6 mouse, and CD8 using H-2K(b) and H-2D(b) tetramers. The MHCI molecules bind to the CD8alpha beta co-receptor on double-positive thymocytes with different avidities (H-2K(b) > D(b)). The differences are linked to their respective alpha3 domains. Hence, an H-2D(b)K(b) tetramer comprising D(b)alpha1--alpha2 and K(b)alpha3 domains shows more binding than H-2D(b). We also quantitated the monomeric affinities of CD8alpha alpha and CD8alpha beta for H-2K(b) and H-2D(b). The H-2K(b) interaction with CD8alpha alpha and CD8alpha beta is stronger than that of H-2D(b). Given that T cell repertoire selection of DP thymocytes is a function of both TCR-pMHCI and CD8alpha beta-pMHCI avidities, these differences may explain the dominant role of H-2K(b) as compared to H-2D(b) in CD8 T cell development of C57BL / 6 mice. The influence of allelic and non-allelic alpha3 polymorphisms on thymic selection processes are discussed.

Monomeric class I molecules mediate TCR/CD3 epsilon/CD8 interaction on the surface of T cells

Both CD8 and the TCR bind to MHC class I molecules during physiologic T cell activation. It has been shown that for optimal T cell activation to occur, CD8 must be able to bind the same class I molecule that is bound by the TCR. However, no direct evidence for the class I-dependent association of CD8 and the TCR has been demonstrated. Using fluorescence resonance energy transfer, we show directly that a single class I molecule causes TCR/CD8 interaction by serving as a docking molecule for both CD8 and the TCR. Furthermore, we show that CD3epsilon is brought into close proximity with CD8 upon TCR/CD8 association. These interactions are not dependent on the phosphorylation events characteristic of T cell activation. Thus, MHC class I molecules, by binding to both CD8 and the TCR, mediate the reorganization of T cell membrane components to promote cellular activation.

H-2 allele specificity of the NK cell C-type lectin-like MHC class I receptor Ly49A visualized by soluble Ly49A tetramer

Ly49A is a C-type lectin-like receptor on NK cells that recognizes MHC class I ligands, H-2D(d) and D(k). The engagement of Ly49A with the ligands inhibits activation of NK cells and protects target cells from lysis by NK cells. Here we express the extracellular region of Ly49A with an N-terminal biotinylation tag in Escherichia coli to obtain soluble Ly49A (sLy49A) after refolding. sLy49A is indistinguishable from native Ly49A expressed on NK cells serologically and in the ability to specifically bind H-2D(d) after tetramerization with R-phycoerythrin-coupled streptavidin. The fluorescently labeled tetramer of sLy49A is applied to explore MHC class I haplotype specificity of Ly49A. We demonstrate the hierarchical reactivity of Ly49A with H-2 of various alleles in the order of d > k, r > p > v > q > s > z. Reactivity of sLy49A tetramer to spleen lymphocytes from B10.QBR mice (H-2K(b), I(b), D(q), Qa-1/Tla(b)) but not from C57BL/10 mice (H-2(b)) identifies H-2D(q) and L(q) as candidates for a Ly49A ligand. Binding of sLy49A tetramer to H-2D(q)- or L(q)-transfected cell lines demonstrates that the two highly related MHC class I molecules, H-2D(q) and L(q), are ligands for Ly49A. sLy49A tetramer staining also demonstrates preferential expression of Ly49A ligand on a subset of B cells in P/J mice. These results provide the basis to examine the molecular mechanism by which Ly49A discriminates polymorphic MHC class I molecules.

T Cell Receptor Binding to a pMHCII Ligand Is Kinetically Distinct from and Independent of CD4

Immune recognition of pMHCII ligands by a helper T lymphocyte involves its antigen-specific T cell receptor (TCR) and CD4 coreceptor. We have characterized the binding of both molecules to the same pMHCII. The D10 alphabeta TCR heterodimer binds to conalbumin/I-A(k) with virtually identical kinetics and affinity as the single chain ValphaVbeta domain module (scD10) (Kd = 6-8 microm). The CD4 ectodomain does not alter either interaction. Moreover, CD4 alone demonstrates weak pMHCII binding (Kd = 200 microm), with no discernable affinity for the alphabeta TCR heterodimer. Hence, rather than providing a major contribution to binding energy, the critical role for the coreceptor in antigen-specific activation likely results from transient inducible recruitment of the CD4 cytoplasmic tail-associated lck tyrosine kinase to the pMHCII-ligated TCR complex.

The functional binding site for the C-type lectin-like natural killer cell receptor Ly49A spans three domains of its major histocompatibility complex class I ligand

Natural killer (NK) cells express receptors that recognize major histocompatibility complex (MHC) class I molecules and regulate cytotoxicity of target cells. In this study, we demonstrate that Ly49A, a prototypical C-type lectin-like receptor expressed on mouse NK cells, requires species-specific determinants on beta2-microglobulin (beta2m) to recognize its mouse MHC class I ligand, H-2D(d). The involvement of beta2m in the interaction between Ly49A and H-2D(d) is also demonstrated by the functional effects of a beta2m-specific antibody. We also define three residues in alpha1/alpha2 and alpha3 domains of H-2D(d) that are critical for the recognition of H-2D(d) on target cells by Ly49A. In the crystal structure of the Ly49A/H-2D(d) complex, these residues are involved in hydrogen bonding to Ly49A in one of the two potential Ly49A binding sites on H-2D(d). These data unambiguously indicate that the functional effect of Ly49A as an MHC class I-specific NK cell receptor is mediated by binding to a concave region formed by three structural domains of H-2D(d), which partially overlaps the CD8 binding site.

Assembly of the CD8alpha/p56(lck) protein complex in stably expressing rat epithelial cells

We have previously characterized the biogenesis of the human CD8alpha protein expressed in rat epithelial cells. We now describe the biosynthesis, post-translational maturation and hetero-oligomeric assembly of the human CD8alpha/p56(lck) protein complex in stable transfectants obtained from the same cell line. There were no differences in the myristilation of p56(lck), or in the dimerization, O-glycosylation and transport to the plasma membrane of CD8alpha, between cells expressing either one or both proteins. In the doubly expressing cells, dimeric forms of CD8alpha established hetero-oligomeric complexes with p56(lck), as revealed by co-immunoprecipitation assays performed with anti-CD8alpha antibody. Moreover, p56(lck) bound in these hetero-oligomeric complexes was endowed with auto- and hetero-phosphorylating activity. The present study shows that: (1) the newly synthesized p56(lck) binds rapidly to CD8alpha and most of the p56(lck) is bound to CD8alpha at steady state; (2) CD8alpha/p56(lck) protein complexes are formed at internal membranes as well as at the plasma membrane; and (3) about 50% of complexed p56(lck) reaches the cell surface.

Impaired assembly yet normal trafficking of MHC class I molecules in Tapasin mutant mice

Loading of peptides onto major histocompatibility complex class I molecules involves a multifactorial complex that includes tapasin (TPN), a membrane protein that tethers empty class I glycoproteins to the transporter associated with antigen processing. To evaluate the in vivo role of TPN, we have generated Tpn mutant mice. In these animals, most class I molecules exit the endoplasmic reticulum (ER) in the absence of stably bound peptides. Consequently, mutant animals have defects in class I cell surface expression, antigen presentation, CD8+ T cell development, and immune responses. These findings reveal a critical role of TPN for ER retention of empty class I molecules. Tpn mutant animals should prove useful for studies on alternative antigen-processing pathways that involve post-ER peptide loading.

Soluble CD8 attenuates cytotoxic T cell responses against replication-defective adenovirus affording transprotection of transgenes in vivo

The T cell coreceptor, CD8, enhances T cell-APC interactions. Because soluble CD8alpha homodimers can antagonize CD8 T cell activation in vitro, we asked whether secretion of soluble CD8 would effect cytotoxic T cell responses in vivo. Production of soluble CD8 by a replication-defective adenovirus vector allowed persistent virus expression for up to 5 mo in C57BL/6 mice and protected a second foreign transgene from rapid deletion. Soluble CD8 selectively inhibited CD8 T cell proliferation and IFN-gamma production and could also attenuate peptide-specific CD8 T cell responses in vivo. These finding suggest that gene vector delivery of soluble CD8 may have therapeutic applications.

Classical and nonclassical class I major histocompatibility complex molecules exhibit subtle conformational differences that affect binding to CD8alphaalpha

The cell surface molecules CD4 and CD8 greatly enhance the sensitivity of T-cell antigen recognition, acting as "co-receptors" by binding to the same major histocompatibility complex (MHC) molecules as the T-cell receptor (TCR). Here we use surface plasmon resonance to study the binding of CD8alphaalpha to class I MHC molecules. CD8alphaalpha bound the classical MHC molecules HLA-A*0201, -A*1101, -B*3501, and -C*0702 with dissociation constants (K(d)) of 90-220 microm, a range of affinities distinctly lower than that of TCR/peptide-MHC interaction. We suggest such affinities apply to most CD8alphaalpha/classical class I MHC interactions and may be optimal for T-cell recognition. In contrast, CD8alphaalpha bound both HLA-A*6801 and B*4801 with a significantly lower affinity (>/=1 mm), consistent with the finding that interactions with these alleles are unable to mediate cell-cell adhesion. Interestingly, CD8alphaalpha bound normally to the nonclassical MHC molecule HLA-G (K(d) approximately 150 microm), but only weakly to the natural killer cell receptor ligand HLA-E (K(d) >/= 1 mm). Site-directed mutagenesis experiments revealed that variation in CD8alphaalpha binding affinity can be explained by amino acid differences within the alpha3 domain. Taken together with crystallographic studies, these results indicate that subtle conformational changes in the solvent exposed alpha3 domain loop (residues 223-229) can account for the differential ability of both classical and nonclassical class I MHC molecules to bind CD8.

The crystal structure of a T cell receptor in complex with peptide and MHC class II

The crystal structure of a complex involving the D10 T cell receptor (TCR), 16-residue foreign peptide antigen, and the I-Ak self major histocompatibility complex (MHC) class II molecule is reported at 3.2 angstrom resolution. The D10 TCR is oriented in an orthogonal mode relative to its peptide-MHC (pMHC) ligand, necessitated by the amino-terminal extension of peptide residues projecting from the MHC class II antigen-binding groove as part of a mini beta sheet. Consequently, the disposition of D10 complementarity-determining region loops is altered relative to that of most pMHCI-specific TCRs; the latter TCRs assume a diagonal orientation, although with substantial variability. Peptide recognition, which involves P-1 to P8 residues, is dominated by the Valpha domain, which also binds to the class II MHC beta1 helix. That docking is limited to one segment of MHC-bound peptide offers an explanation for epitope recognition and altered peptide ligand effects, suggests a structural basis for alloreactivity, and illustrates how bacterial superantigens can span the TCR-pMHCII surface.