Assembly and crystallization of the complex between the human T cell coreceptor CD8alpha homodimer and HLA-A2

Structural insights into the evolution feature of a bony fish CD8αα homodimer

The CD8αα homodimer structures of endotherms demonstrate that despite distinct diversity at the amino acid sequence level, a few conserved key amino acids ensure common structural features. The structure of CD8αα in ancient ectotherms, such as lower bony fish, remains unclear. In this study, the high-resolution structure of the grass carp (Ctenopharyngodon idella) CD8αα (Ctid-CD8αα) homodimer was determined using the single-wavelength anomalous diffraction (SAD) method. The structure of Ctid-CD8αα shows distinct differences from the known CD8αα structures of endotherms, including a distinct topological structure with shorter back β sheets. The configuration and distribution of the hydrophobic core are different from those in endotherms. Interestingly, mutation of the key amino acid F32S, which is very common in fish and lies in the CDR loop region, leads to the absence of the typical cavity that binds to an epitope-MHC I (p/MHC I) in endotherms, yet Ctid-CD8αα can still specifically bind the grass carp peptide-Ctid-UAA-β2m (p/UAA-β2m). Our results indicate that during the evolutionary process, CD8αα has undergone dramatic changes that affect its dimeric structure and may use a new strategy to interact with p/MHC I.

Reconciling views on T cell receptor germline bias for MHC

Whether MHC restriction by the T cell receptor (TCR) is a product of evolutionary pressures leading to germline-encoded 'rules of engagement' remains avidly debated. Structural results derived from analysis of TCR-peptide-MHC complexes appear to support a model of physical specificity between TCR germline V regions and MHC. Yet, some recent evidence suggests that thymic selection, and co-receptors may have misled us into thinking the TCR is exclusively MHC-specific, when in fact, TCRs can robustly engage non-MHC ligands when given the chance. Here, I propose that seemingly contradictory data and hypotheses for, and against, germline bias are, in fact, compatible and can be reconciled into a unifying model.

Crystallization and preliminary X-ray crystallographic studies of swine CD8α

CD8αα homodimers or CD8αβ heterodimers form on the T-cell surface, where they are essential as co-receptors for MHC class I molecules in activation of the CTL response. To date, swine have been found to show the highest percentage of lymphocytes with surface expression of CD8α. Crystallographic analysis of swine CD8α (sCD8α) to 1.8 Å resolution revealed that the crystals belonged to space group P3(2)21, with unit-cell parameters a = 80.97, b = 80.97, c = 95.19 Å. The Matthews coefficient and the solvent content were calculated to be 3.23 Å(3) Da(-1) and 61.89%, respectively. These results may aid further structural and functional analyses of sCD8α.

Plasticity of human CD8αα binding to peptide-HLA-A*2402

The human CD8 functions as a co-receptor for specific T cell recognition, and only one complex structure of human CD8αα binding to HLA-A*0201 has been solved, revealing the molecular basis of CD8 interacting with its ligand pHLA. Here, we present the complex structures of human CD8αα bound to HLA-A*2402, which demonstrate two opposite α3 domain CD loop shifts (either pull or push) in the HLA heavy chain upon CD8 engagement. Taking the previously reported mouse CD8-pMHC complex structures into account, from the structural view, all of the data indicate the plasticity of CD8 binding to pMHC/HLA, which facilitates its co-receptor function for T cells. The plasticity of CD8 binding appears not to affect the specificity of TCR recognition, as no peptide conformation change extends to the pMHC interface for TCR contacting.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of rhesus macaque CD8alphaalpha homodimer

As a T-cell co-receptor, CD8 binds to MHC class I molecules and plays a pivotal role in the activation of cytotoxic T lymphocytes. To date, structures of CD8 have been solved for two different mammals: human and mouse. The infection of rhesus macaques (Macaca mulatta) by simian immunodeficiency virus (SIV) is the best animal model for studying HIV. In this study, the rhesus macaque CD8 (rCD8) alphaalpha homodimer was obtained and rCD8alpha exodomain protein crystals were successfully obtained for further structural analysis. Diffraction data were collected to a resolution of 2.4 A. The crystal belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 46.52, b = 56.28, c = 82.40 A. These data will facilitate further studies on the structural differences between these CD8 structures and the cellular immune responses of rhesus macaque.

A dimeric structure of PD-L1: functional units or evolutionary relics?

PD-L1 is a member of the B7 protein family, most of whose members so far were identified as dimers in a solution and crystalline state, either complexed or uncomplexed with their ligand(s). The binding of PD-L1 with its receptor PD-1 (CD279) delivers an inhibitory signal regulating the T cell function. Simultaneously with the Garboczi group, we successfully solved another structure of human PD-L1 (hPD-L1). Our protein crystallized in the space group of C222(1) with two hPD-L1 molecules per asymmetric unit. After comparison of reported B7 structures, we have found some intrinsic factors involved in the interaction of these two molecules. Based on these results, we tend to believe this uncomplexed hPD-L1 structure demonstrated its potential dimeric state in solution, although it could just be an evolutionary relic, too weak to be detected under present technology, or still a functional unit deserved our attentions.

The macrophage-inducible C-type lectin, mincle, is an essential component of the innate immune response to Candida albicans

The recognition of carbohydrate moieties by cells of the innate immune system is emerging as an essential element in antifungal immunity, but despite the number and diversity of lectins expressed by innate immune cells, few carbohydrate receptors have been characterized. Mincle, a C-type lectin, is expressed predominantly on macrophages, and is here shown to play a role in macrophage responses to the yeast Candida albicans. After exposure to the yeast in vitro, Mincle localized to the phagocytic cup, but it was not essential for phagocytosis. In the absence of Mincle, production of TNF-alpha by macrophages was reduced, both in vivo and in vitro. In addition, mice lacking Mincle showed a significantly increased susceptibility to systemic candidiasis. Thus, Mincle plays a novel and nonredundant role in the induction of inflammatory signaling in response to C. albicans infection.

T cell receptor engagement of peptide-major histocompatibility complex class I does not modify CD8 binding

Activation of cytotoxic T cells is initiated by engagement of the T-cell receptor (TCR) with peptide-major histocompatibility class I complexes (pMHCI). The CD8 co-receptor also binds to pMHCI, but at a distinct site, and allows the potential for tripartite TCR/pMHCI/CD8 interactions, which can increase T cell antigen sensitivity. There has been a substantial interest in the effect of the pMHCI/CD8 interaction upon TCR/pMHCI engagement, and several conflicting studies have examined this event, using the soluble extracellular domains of CD8 and the TCR, by surface plasmon resonance. However, the evidence to date suggests that the TCR engages cognate pMHCI before CD8 recruitment, so the question of whether TCR engagement alters CD8 binding is likely to be more relevant to the biological order of T cell antigen encounter. Here, we have examined the binding of CD8 to several variants of the HLA A2-restricted telomerase(540-548) antigen (ILAKFLHWL) and the HLA A2-restricted NY-ESO-1(157-165) antigen (SLLMWITQC) that bind to their cognate TCRs with distinct affinities and kinetics. These interactions represent a range of agonists that exhibit different CD8 dependency for activation of their respective T cells. By using engineered affinity enhanced TCRs to these ligands, which have extended off-rates of approximately 1h compared to seconds for the wildtype TCRs, we have examined pMHCI/CD8 binding before and during TCR-engagement. Here we show that the binding of the extracellular domain of the TCR to pMHCI does not transmit structural changes to the pMHCI-CD8 binding site that would alter the subsequent pMHCI/CD8 interaction.

Computational design and crystal structure of an enhanced affinity mutant human CD8 alphaalpha coreceptor

Human CD8 is a T cell coreceptor, which binds to pHLA I and plays a pivotal role in the activation of cytotoxic T lymphocytes. Soluble recombinant CD8 alphaalpha has been shown to antagonize T cell activation, both in vitro and in vivo. However, because of a very low affinity for pHLA I, high concentrations of soluble CD8 alphaalpha are required for efficient inhibition. Based upon our knowledge of the wild-type CD8/pHLA I structure, we have designed and produced a mutated form of soluble CD8 alphaalpha that binds to pHLA I with approximately fourfold higher affinity. We have characterized the binding of the high affinity CD8 mutant using surface plasmon resonance and determined its structure at 2.1 A resolution using X-ray crystallography. The analysis of this structure suggests that the higher affinity is achieved by providing a larger side chain that allows for an optimal contact to be made between the HLA alpha3 loop and the mutated CDR-like loops of CD8.

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.

Refolding, crystallization and preliminary X-ray structural studies of the West Nile virus envelope (E) protein domain III

Domain III of the West Nile virus envelope protein, the putative receptor-binding domain, is a major virion-surface determinant for virulence. This protein was reported to be intrinsically unstable and has defied previous crystallization attempts. It has now been purified from inclusion bodies by protein refolding and was crystallized using the hanging-drop vapour-diffusion method at 291 K. The crystals belong to space group P222(1), with unit-cell parameters a = 52.6, b = 59.7, c = 95.0 A. A complete data set was collected to 2.8 A at 100 K with Cu Kalpha X-rays from a rotating-anode generator.

Crystallization and preliminary X-ray structural studies of a high-affinity CD8alphaalpha co-receptor to pMHC

The class I CD8 positive T-cell response is involved in a number of conditions in which artificial down-regulation and control would be therapeutically beneficial. Such conditions include a number of autoimmune diseases and graft rejection in transplant patients. Although the CD8 T-cell response is dominated by the TCR-pMHC interaction, activation of T cells is in most cases also dependent on a number of associated signalling molecules. Previous work has demonstrated the ability of one such molecule (CD8) to act as an antagonist to T-cell activation if added in soluble form. Therefore, a high-affinity mutant CD8 (haCD8) has been developed with the aim of developing a therapeutic immunosuppressor. In order to fully understand the nature of the haCD8 interaction, this protein was crystallized using the sitting-drop vapour-diffusion method. Single haCD8 crystals were cryocooled and used for data collection. These crystals belonged to space group P6(4)22 (assumed by similarity to the wild type), with unit-cell parameters a = 101.08, c = 56.54 A. VM calculations indicated one molecule per asymmetric unit. A 2 A data set was collected and the structure is currently being determined using molecular replacement.

Human inhibitory receptors Ig-like transcript 2 (ILT2) and ILT4 compete with CD8 for MHC class I binding and bind preferentially to HLA-G

Ig-like transcript 4 (ILT4) (also known as leukocyte Ig-like receptor 2, CD85d, and LILRB2) is a cell surface receptor expressed mainly on myelomonocytic cells, whereas ILT2 (also known as leukocyte Ig-like receptor 1, CD85j, and LILRB1) is expressed on a wider range of immune cells including subsets of natural killer and T cells. Both ILTs contain immunoreceptor tyrosine-based inhibitory receptor motifs in their cytoplasmic tails that inhibit cellular responses by recruiting phosphatases such as SHP-1 (Src homology 2 domain containing tyrosine phosphatase 1). Although these ILTs have been shown to recognize a broad range of classical and nonclassical human MHC class I molecules (MHCIs), their precise binding properties remain controversial. We have used surface plasmon resonance to analyze the interaction of soluble forms of ILT4 and ILT2 with several MHCIs. Although the range of affinities measured was quite broad (Kd = 2-45 microM), some interesting differences were observed. ILT2 generally bound with a 2- to 3-fold higher affinity than ILT4 to the same MHCI. Furthermore, ILT2 and ILT4 bound to HLA-G with a 3- to 4-fold higher affinity than to classical MHCIs, suggesting that ILT/HLA-G recognition may play a dominant role in the regulation of natural killer, T, and myelomonocytic cell activation. Finally, we show that ILT2 and ILT4 effectively compete with CD8 for MHCI binding, raising the possibility that ILT2 modulates CD8+ T cell activation by blocking the CD8 binding as well as by recruiting inhibitory molecules through its immunoreceptor tyrosine-based inhibitory receptor motif.

O-glycan sialylation and the structure of the stalk-like region of the T cell co-receptor CD8

Studies of mucins suggest that the structural effects of O-glycans are restricted to steric interactions between peptide-linked GalNAc residues and adjacent polypeptide residues. It has been proposed, however, that differential O-glycan sialylation alters the structure of the stalk-like region of the T cell co-receptor, CD8, and that this, in turn, modulates ligand binding (Daniels, M. A., Devine, L., Miller, J. D., Moser, J. M., Lukacher, A. E., Altman, J. D., Kavathas, P., Hogquist, K. A., and Jameson, S. C. (2001) Immunity 15, 1051-1061; Moody, A. M., Chui, D., Reche, P. A., Priatel, J. J., Marth, J. D., and Reinherz, E. L. (2001) Cell 107, 501-512). We characterize the glycosylation of soluble, chimeric forms of the alphaalpha- and alphabeta-isoforms of murine CD8 containing the O-glycosylated stalk of rat CD8alphaalpha, and we show that the stalk O-glycans are differentially sialylated in CHO K1 versus Lec3.2.8.1 cells (82 versus approximately 6%, respectively). Sedimentation analysis indicates that the Perrin functions, Pexp, which reflect overall molecular shape, are very similar (1.61 versus 1.54), whereas the sedimentation coefficients (s) of the CHO K1- and Lec3.2.8.1-derived proteins differ considerably (3.73 versus 3.13 S). The hydrodynamic properties of molecular models also strongly imply that the sialylated and non-sialylated forms of the chimera have parallel, equally highly extended stalks ( approximately 2.6 A/residue). Our analysis indicates that, as in the case of mucins, the overall structure of O-glycosylated stalk-like peptides is sialylation-independent and that the functional effects of differential CD8 O-glycan sialylation need careful interpretation.

Structure of a functional IGF2R fragment determined from the anomalous scattering of sulfur

Insulin-like growth factor II receptor (IGF2R) is a multifunctional cell surface receptor implicated in tumour suppression. Its growth inhibitory activity has been associated with an ability to bind IGF-II. IGF2R contains 15 homologous extracellular domains, with domain 11 primarily responsible for IGF-II binding. We report a 1.4 A resolution crystal structure of domain 11, solved using the anomalous scattering signal of sulfur. The structure consists of two crossed beta-sheets forming a flattened beta-barrel. Structural analysis identifies the putative IGF-II binding site at one end of the beta-barrel whilst crystal lattice contacts suggest a model for the full-length IGF2R extracellular region. The structure factors and coordinates of IGF2R domain 11 have been deposited in the Protein Data Bank (accession codes 1GP0 and 1GP3).

Molecular interactions of coreceptor CD8 and MHC class I: the molecular basis for functional coordination with the T-cell receptor

In recent years, substantial progress has been made towards understanding the molecular basis for CD8 binding to class I MHC and the coreceptor's role in cytotoxic T-cell activation. Here, we review the structural, mechanistic and functional studies that point to a model of coordination of T-cell receptor and CD8 signaling that might provide the key to cytotoxic T-cell activation.

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.

Production of soluble alphabeta T-cell receptor heterodimers suitable for biophysical analysis of ligand binding

A method to produce alphabeta T-cell receptors (TCRs) in a soluble form suitable for biophysical analysis was devised involving in vitro refolding of a TCR fusion protein. Polypeptides corresponding to the variable and constant domains of each chain of a human and a murine receptor, fused to a coiled coil heterodimerization motif from either c-Jun (alpha) or v-Fos (beta), were overexpressed separately in Escherichia coli. Following recovery from inclusion bodies, the two chains of each receptor were denatured, and then refolded together in the presence of denaturants. For the human receptor, which is specific for the immunodominant influenza A HLA-A2-restricted matrix epitope (M58-66), a heterodimeric protein was purified in milligram yields and found to be homogeneous, monomeric, antibody-reactive, and stable at concentrations lower than 1 microM. Using similar procedures, analogous results were obtained with a murine receptor specific for an influenza nucleoprotein epitope (366-374) restricted by H2-Db. Production of these receptors has facilitated a detailed analysis of viral peptide-Major Histocompatibility Complex (peptide-MHC) engagement by the TCR using both surface plasmon resonance (SPR) and, in the case of the human TCR, isothermal titration calorimetry (ITC) (Willcox et al., 1999). The recombinant methods described should enable a wide range of TCR-peptide-MHC interactions to be studied and may also have implications for the production of other heterodimeric receptor molecules.

Structural basis of T cell recognition

Exciting breakthroughs in the last two years have begun to elucidate the structural basis of cellular immune recognition. Crystal structures have been determined for full-length and truncated forms of alpha beta T cell receptor (TCR) heterodimers, both alone and in complex with their peptide-MHC (pMHC) ligands or with anti-TCR antibodies. In addition, a truncated CD8 coreceptor has been visualized with a pMHC. Aided in large part by the substantial body of knowledge accumulated over the last 25 years on antibody structure, a number of general conclusions about TCR structure and its recognition of antigen can already be derived from the relatively few TCR structures that have been determined. Small, but important, variations between TCR and antibody structures bear on their functional differences as well as on their specific antigen recognition requirements. As observed in antibodies, canonical CDR loop structures are already emerging for some of the TCR CDR loops. Highly similar docking orientations of the TCR V alpha domains in the TCR/pMHC complex appear to play a primary role in dictating orientation, but the V beta positions diverge widely. Similar TCR contact positions, but whose exact amino acid content can vary, coupled with relatively poor interface shape complementarity, may explain the flexibility and short half-lives of many TCR interactions with pMHC. Here we summarize the current state of this field, and suggest that the knowledge gap between the three-dimensional structure and the signaling function of the TCR can be bridged through a synthesis of molecular biological and biophysical techniques.

T cell receptor and coreceptor CD8 alphaalpha bind peptide-MHC independently and with distinct kinetics

The T cell surface glycoprotein CD8 enhances T cell antigen recognition by binding to MHC class I molecules. We show that human CD8 alphaalpha binds to the MHC class I molecule HLA-A2 with an extremely low affinity (Kd approximately 0.2 mM at 37 degrees C) and with kinetics that are between 2 and 3 orders of magnitude faster than reported for T cell receptor/peptide-MHC interactions. Furthermore, CD8 alphaalpha had no detectable effect on a T cell receptor (TCR) binding to the same peptide-MHC class I complex. These binding properties provide an explanation as to why the CD8/MHC class I interaction is unable to initiate cell-cell adhesion and how it can enhance TCR recognition without interfering with its specificity.