Biomedical subjects

Craig S Clements

Publications and source records attributed to Craig S Clements.

16 recordsLinked to original sources

PubMed2006

Specificity on a knife-edge: the alphabeta T cell receptor.

The interaction between the alphabeta T cell receptor (TCR) and the peptide bound to the major histocompatibility complex class I molecule (pMHC-I) constitutes a central interaction in adaptive immunity. How these receptors interact with such low affinity while maintaining exquisite specificity for peptide antigen and host MHC (MHC-I restriction) remains a challenge to be explained by structural immunologists. Moreover, how this extracellular interaction is transmitted as an intracellular signal via the CD3 complex remains unresolved. Nevertheless, several structures of TCRs, non-liganded and ligated to a defined pMHC-I, combined with detailed biophysical analyses, have provided insight of the structural basis of MHC-I restriction. In addition, structures of isolated CD3 components have enabled T cell signalling mechanisms to be postulated. Recent findings in this area, which include seven distinct TCR/pMHC-I complexes, have fundamental implications in adaptive immunity as well as therapeutic applications to modulate the adaptive immune response.

Animals↗

PubMed2006

Disparate thermodynamics governing T cell receptor-MHC-I interactions implicate extrinsic factors in guiding MHC restriction.

The underlying basis of major histocompatibility complex (MHC) restriction is unclear. Nevertheless, current data suggest that a common thermodynamic signature dictates alphabeta T cell receptor (TcR) ligation. To evaluate whether this thermodynamic signature defines MHC restriction, we have examined the thermodynamic basis of a highly characterized immunodominant TcR interacting with its cognate peptide-MHC-I ligand. Surprisingly, we observed this interaction to be governed by favorable enthalpic and entropic forces, which is in contrast to the prevailing generality, namely, enthalpically driven interactions combined with markedly unfavorable entropic forces. We conclude that extrinsic molecular factors, such as coreceptor ligation, conformational adjustments involved in TcR signaling, or constraints dictated by higher-order arrangement of ligated TcRs, might play a greater role in guiding MHC restriction than appreciated previously.

Amino Acid Sequence↗

PubMed2006

A structural basis for selection and cross-species reactivity of the semi-invariant NKT cell receptor in CD1d/glycolipid recognition.

Little is known regarding the basis for selection of the semi-invariant alphabeta T cell receptor (TCR) expressed by natural killer T (NKT) cells or how this mediates recognition of CD1d-glycolipid complexes. We have determined the structures of two human NKT TCRs that differ in their CDR3beta composition and length. Both TCRs contain a conserved, positively charged pocket at the ligand interface that is lined by residues from the invariant TCR alpha- and semi-invariant beta-chains. The cavity is centrally located and ideally suited to interact with the exposed glycosyl head group of glycolipid antigens. Sequences common to mouse and human invariant NKT TCRs reveal a contiguous conserved "hot spot" that provides a basis for the reactivity of NKT cells across species. Structural and functional data suggest that the CDR3beta loop provides a plasticity mechanism that accommodates recognition of a variety of glycolipid antigens presented by CD1d. We propose a model of NKT TCR-CD1d-glycolipid interaction in which the invariant CDR3alpha loop is predicted to play a major role in determining the inherent bias toward CD1d. The findings define a structural basis for the selection of the semi-invariant alphabeta TCR and the unique antigen specificity of NKT cells.

Animals↗

PubMed2006

Structural basis for a major histocompatibility complex class Ib-restricted T cell response.

In contrast to antigen-specific immunity orchestrated by major histocompatibility complex (MHC) class Ia molecules, the ancestrally related nonclassical MHC class Ib molecules generally mediate innate immune responses. Here we have demonstrated the structural basis by which the MHC class Ib molecule HLA-E mediates an adaptive MHC-restricted cytotoxic T lymphocyte response to human cytomegalovirus. Highly constrained by host genetics, the response showed notable fine specificity for position 8 of the viral peptide, which is the sole discriminator of self versus nonself. Despite the evolutionary divergence of MHC class Ia and class Ib molecules, the structure of the T cell receptor-MHC class Ib complex was very similar to that of conventional T cell receptor-MHC class Ia complexes. These results emphasize the evolutionary 'ambiguity' of HLA-E, which not only interacts with innate immune receptors but also has the functional capacity to mediate virus-specific cytotoxic T lymphocyte responses during adaptive immunity.

Animals↗

PubMed2005

The production, purification and crystallization of a soluble form of the nonclassical MHC HLA-G: the essential role of cobalt.

HLA-G is a nonclassical class I major histocompatibility complex (MHC) molecule that is primarily expressed at the foetal-maternal interface. Although the role of HLA-G has not been fully elucidated, current evidence suggests it protects the foetus from the maternal immune response. In this report, HLA-G (44 kDa) is characterized by expression in Escherichia coli. The inclusion bodies were refolded in complex with a peptide derived from histone H2A (RIIPRHLQL), purified and subsequently crystallized. Correct refolding was determined using two conformation-dependent antibodies. Cobalt ions were shown to be an essential ingredient for obtaining diffraction-quality crystals. The crystals, which diffracted to 1.9 A resolution, belonged to space group P3(2)2(1), with unit-cell parameters a = b = 77.15, c = 151.72 A.

Amino Acid Substitution↗

PubMed2005

T cell receptor recognition of a 'super-bulged' major histocompatibility complex class I-bound peptide.

Unusually long major histocompatibility complex (MHC) class I-restricted epitopes are important in immunity, but their 'bulged' conformation represents a potential obstacle to alphabeta T cell receptor (TCR)-MHC class I docking. To elucidate how such recognition is achieved while still preserving MHC restriction, we have determined here the structure of a TCR in complex with HLA-B(*)3508 presenting a peptide 13 amino acids in length. This complex was atypical of TCR-peptide-MHC class I interactions, being dominated at the interface by peptide-mediated interactions. The TCR assumed two distinct orientations, swiveling on top of the centrally bulged, rigid peptide such that only limited contacts were made with MHC class I. Although the TCR-peptide recognition resembled an antibody-antigen interaction, the TCR-MHC class I contacts defined a minimal 'generic footprint' of MHC-restriction. Thus our findings simultaneously demonstrate the considerable adaptability of the TCR and the 'shape' of MHC restriction.

Amino Acid Sequence↗

PubMed2005

Antagonism of antiviral and allogeneic activity of a human public CTL clonotype by a single altered peptide ligand: implications for allograft rejection.

Alloreactive T lymphocytes are central mediators of graft-versus-host disease and allograft rejection. A public CTL clonotype with specificity for the alloantigens HLA-B*4402 and B*4405 is often expanded to large numbers in healthy HLA-B*0801(+) individuals, driven by cross-reactive stimulation with the common, persistent herpesvirus EBV. Since such alloreactive memory CTL expansions have the potential to influence transplantation outcome, altered peptide ligands (APLs) of the target HLA-B*0801-binding EBV peptide, FLRGRAYGL, were screened as specific antagonists for this immunodominant clonotype. One APL, FLRGRFYGL, exerted powerful antagonism of a prototypic T cell clone expressing this immunodominant TCR when costimulated with target cells presenting HLA-B*0801(FLRGRAYGL). Significantly, this APL also reduced the lysis of allogeneic target cells expressing HLA-B*4402 by up to 99%. The affinities of the agonist and antagonist complexes for the public TCR, measured using solution and solid-phase assays, were 8 and 138 muM, respectively. Surprisingly, the half-life of the agonist and antagonist complexes was similar, yet the association rate for the antagonist complex was significantly slower. These observations were further supported by structural studies that suggested a large conformational hurdle was required to ligate the immunodominant TCR to the HLA-B*0801 antagonist complex. By defining an antagonist APL against an immunodominant alloreactive TCR, these findings raise the prospect of exploiting such peptides to inhibit clinical alloreactivity, particularly against clonal T cell expansions that react with alloantigens.

Antigens, Viral↗

PubMed2005

Crystal structure of HLA-G: a nonclassical MHC class I molecule expressed at the fetal-maternal interface.

HLA-G is a nonclassical major histocompatibility complex class I (MHC-I) molecule that is primarily expressed at the fetal-maternal interface, where it is thought to play a role in protecting the fetus from the maternal immune response. HLA-G binds a limited repertoire of peptides and interacts with the inhibitory leukocyte Ig-like receptors LIR-1 and LIR-2 and possibly with certain natural killer cell receptors. To gain further insights into HLA-G function, we determined the 1.9-A structure of a monomeric HLA-G complexed to a natural endogenous peptide ligand from histone H2A (RIIPRHLQL). An extensive network of contacts between the peptide and the antigen-binding cleft reveal a constrained mode of binding reminiscent of the nonclassical HLA-E molecule, thereby providing a structural basis for the limited peptide repertoire of HLA-G. The alpha3 domain of HLA-G, a candidate binding site for the LIR-1 and -2 inhibitory receptors, is structurally distinct from the alpha3 domains of classical MHC-I molecules, providing a rationale for the observed affinity differences for these ligands. The structural data suggest a head-to-tail mode of dimerization, mediated by an intermolecular disulfide bond, that is consistent with the observation of HLA-G dimers on the cell surface.

Amino Acid Sequence↗

PubMed2005

The CDR3 regions of an immunodominant T cell receptor dictate the 'energetic landscape' of peptide-MHC recognition.

The energetic bases of T cell recognition are unclear. Here, we studied the 'energetic landscape' of peptide-major histocompatibility complex (pMHC) recognition by an immunodominant alphabeta T cell receptor (TCR). We quantified and evaluated the effect of natural and systematic substitutions in the complementarity-determining region (CDR) loops on ligand binding in the context of the structural detail of each component of the immunodominant TCR-pMHC complex. The CDR1 and CDR2 loops contributed minimal energy through direct recognition of the antigen and instead had a chief function in stabilizing the ligated CDR3 loops. The underlying energetic basis for recognition lay in the CDR3 loops. Therefore the energetic burden of the CDR loops in the TCR-pMHC interaction is variable among TCRs, reflecting the inherent adaptability of the TCR in ligating different ligands.

Base Sequence↗

PubMed2003

The crystal structure of myelin oligodendrocyte glycoprotein, a key autoantigen in multiple sclerosis.

Myelin oligodendrocyte glycoprotein (MOG) is a key CNS-specific autoantigen for primary demyelination in multiple sclerosis. Although the disease-inducing role of MOG has been established, its precise function in the CNS remains obscure. To gain new insights into the physiological and immunopathological role of MOG, we determined the 1.8-A crystal structure of the MOG extracellular domain (MOGED). MOGED adopts a classical Ig (Ig variable domain) fold that was observed to form an antiparallel head-to-tail dimer. A dimeric form of native MOG was observed, and MOGED was also shown to dimerize in solution, consistent with the view of MOG acting as a homophilic adhesion receptor. The MOG35-55 peptide, a major encephalitogenic determinant recognized by both T cells and demyelinating autoantibodies, is partly occluded within the dimer interface. The structure of this key autoantigen suggests a relationship between the dimeric form of MOG within the myelin sheath and a breakdown of immunological tolerance to MOG that is observed in multiple sclerosis.

Amino Acid Sequence↗

PubMed2003

A naturally selected dimorphism within the HLA-B44 supertype alters class I structure, peptide repertoire, and T cell recognition.

HLA-B*4402 and B*4403 are naturally occurring MHC class I alleles that are both found at a high frequency in all human populations, and yet they only differ by one residue on the alpha2 helix (B*4402 Asp156-->B*4403 Leu156). CTLs discriminate between HLA-B*4402 and B*4403, and these allotypes stimulate strong mutual allogeneic responses reflecting their known barrier to hemopoeitic stem cell transplantation. Although HLA-B*4402 and B*4403 share >95% of their peptide repertoire, B*4403 presents more unique peptides than B*4402, consistent with the stronger T cell alloreactivity observed toward B*4403 compared with B*4402. Crystal structures of B*4402 and B*4403 show how the polymorphism at position 156 is completely buried and yet alters both the peptide and the heavy chain conformation, relaxing ligand selection by B*4403 compared with B*4402. Thus, the polymorphism between HLA-B*4402 and B*4403 modifies both peptide repertoire and T cell recognition, and is reflected in the paradoxically powerful alloreactivity that occurs across this "minimal" mismatch. The findings suggest that these closely related class I genes are maintained in diverse human populations through their differential impact on the selection of peptide ligands and the T cell repertoire.

Alleles↗

PubMed2003

A structural basis for the selection of dominant alphabeta T cell receptors in antiviral immunity.

We have examined the basis for immunodominant or "public" TCR usage in an antiviral CTL response. Residues encoded by each of the highly selected genetic elements of an immunodominant clonotype recognizing Epstein-Barr virus were critical to the antigen specificity of the receptor. Upon recognizing antigen, the immunodominant TCR undergoes extensive conformational changes in the complementarity determining regions (CDRs), including the disruption of the canonical structures of the germline-encoded CDR1alpha and CDR2alpha loops to produce an enhanced fit with the HLA-peptide complex. TCR ligation induces conformational changes in the TCRalpha constant domain thought to form part of the docking site for CD3epsilon. These findings indicate that TCR immunodominance is associated with structural properties conferring receptor specificity and suggest a novel structural link between TCR ligation and intracellular signaling.

Amino Acid Sequence↗

PubMed2002

The production, purification and crystallization of a soluble heterodimeric form of a highly selected T-cell receptor in its unliganded and liganded state.

T-cell antigen receptors (TcRs) are heterodimeric cell-surface receptors that play a pivotal role in the cellular immune response. The TcR interacts specifically with a peptide-laden major histocompatability complex (pMHC). A human TcR has been characterized that interacts with an immunodominant epitope, FLRGRAYGL, from the Epstein-Barr virus, a ubiquitous human pathogen, in complex with HLA-B8. Despite the vast TcR repertoire, this TcR is found in up to 10% of the total T-cell population in seropositive HLA-B8+ individuals. In this report, this highly selected TcR is characterized by expressing in Escherichia coli, refolding, purifying and crystallizing the receptor. In addition, the HLA-B8-FLRGRAYGL complex has been expressed in E. coli, refolded and shown to be functionally active. Using native gel electrophoresis, the refolded TcR is shown to be capable of binding specifically to the refolded HLA-B8-FLRGRAYGL and this TcR has been crystallized in complex with the pMHC. The crystals of the unliganded and liganded TcR diffract to 1.5 and 2.5 A, respectively.

Amino Acid Sequence↗

PubMed2002

The structure of HLA-B8 complexed to an immunodominant viral determinant: peptide-induced conformational changes and a mode of MHC class I dimerization.

EBV is a ubiquitous human pathogen that chronically infects up to 90% of the population. Persistent viral infection is characterized by latency and periods of viral replication that are kept in check by a strong antiviral CTL response. Despite the size of the EBV genome, CTL immunity focuses on only a few viral determinants but expands a large primary and memory response toward these epitopes. In unrelated HLA-B8(+) individuals, the response to the immunodominant latent Ag FLRGRAYGL from Epstein Barr nuclear Ag 3A is largely comprised of CTL clones with identical conserved alphabeta TCR structures. To better understand the structural correlates of Ag immunodominance and TCR selection bias, we have solved the crystal structure of the HLA-B8-FLRGRAYGL peptide complex to a resolution of 1.9 A. The structure confirms the importance of P3-Arg, P5-Arg, and P9-Leu as dominant anchor residues involved in peptide binding to HLA-B8. A bulged conformation of the bound peptide provides a structural basis for the critical role of the P7-Tyr residue in T cell recognition. The peptide also induces backbone and side-chain conformational changes in HLA-B8 that are transmitted along the peptide-binding groove in a domino effect. The HLA-B8-FLRGRAYGL complex crystallizes as a dimer in the asymmetric unit and is oriented such that both peptide ligands are projected in the same plane suggesting a higher order arrangement of MHC-peptide complexes that could be involved in formation of the class I Ag-loading complex or in T cell activation.

Antigens, Viral↗

PubMed2002

Identification of a dominant self-ligand bound to three HLA B44 alleles and the preliminary crystallographic analysis of recombinant forms of each complex.

A naturally processed and presented ligand that is shared by human leukocyte antigen (HLA) B*4402, B*4403 and B*4405 molecules has been identified in peptides isolated from immunoaffinity purified HLA B44 complexes. This peptide derived from HLA DPalpha residues 46-54, an endogenous product of HLA DP expressed in the cell line Hmy2.C1R, is a prominent peptide in the mass spectra of species isolated as bound peptides from each allele when the three HLA B44 subtypes were introduced as transfected gene products. Recombinant truncated forms of HLA B*4405(1-276), HLA B*4403(1-276), HLA B*4402(1-276) and beta(2)-microglobulin have been prepared as inclusion bodies in Escherichia coli and refolded in the presence of the DPalpha(46-54) peptide and purified by a combination of size exclusion and anion exchange chromatography. This material was determined to be correctly folded based on detection of a conformational epitope recognized by the W6/32 monoclonal antibody. Large, plate-like crystals of the three complexes were produced using polyethylene glycol as the precipitant. All the crystals belong to the space group P2(1)2(1)2(1) with unit cell dimensions of approximately a=51, b=82, c=110 A. The crystals of three B44/DPalpha complexes diffracted to a resolution of 1.9 A or better. For the first time, using this natural, high abundance ligand of the HLA B44 molecules we have successfully expressed and refolded the three HLA B44 molecules and produced crystals amenable to structural studies.

Alleles↗

PubMed2002

The 1.5 A crystal structure of a highly selected antiviral T cell receptor provides evidence for a structural basis of immunodominance.

Despite a potential repertoire of >10(15) alphabeta T cell receptors (TcR), the HLA B8-restricted cytolytic T cell response to a latent antigen of Epstein-Barr virus (EBV) is strikingly limited in the TcR sequences that are selected. Even in unrelated individuals this response is dominated by a single highly restricted TcR clonotype that selects identical combinations of hypervariable Valpha, Vbeta, D, J, and N region genes. We have determined the 1.5 A crystal structure of this "public" TcR, revealing that five of the six hypervariable loops adopt novel conformations providing a unique combining site that contains a deep pocket predicted to overlay the HLA B8-peptide complex. The findings suggest a structural basis for the immunodominance of this clonotype in the immune response to EBV.

Amino Acid Sequence↗