John I. Bell

Crystal structure of the complex between human CD8alpha(alpha) and HLA-A2.

The dimeric cell-surface glycoprotein CD8 is crucial to the positive selection of cytotoxic T cells in the thymus. The homodimer CD8αα or the heterodimer αβ stabilizes the interaction of the T-cell antigen receptor (TCR) with major histocompatibility complex (MHC) class I/peptide by binding to the class I molecule. Here we report the crystal structure at 2.7Å resolution of a complex between CD8αα and the human MHC molecule HLA-A2, which is associated with peptide. CD8αα binds one HLA-A2/peptide molecule, interfacing with the α2 and α3 domains of HLA-A2 and also contacting β2-microglobulin. A flexible loop of the α3 domain (residues 223–229) is clamped between the complementarity-determining region (CDR)-like loops of the two CD8 subunits in the classic manner of an antibody–antigen interaction, precluding the binding of a second MHC molecule. The position of the α3 domain is different from that in uncomplexed HLA-A2 (refs 3, 4), being most similar to that in the TCR/Tax/HLA-A2 complex, but no conformational change extends to the MHC/peptide surface presented for TCR recognition. Although these shifts in α3 may provide a synergistic modulation of affinity, the binding of CD8 to MHC is clearly consistent with an avidity-based contribution from CD8 to TCR–peptide–MHC interactions.

A Structural Basis for Immunodominant Human T Cell Receptor Recognition

The anti-influenza CD8+ T cell response in HLA-A2–positive adults is almost exclusively directed at residues 58–66 of the virus matrix protein (MP(58–66)). Vβ17Vα10.2 T cell receptors (TCRs) containing a conserved arginine-serine-serine sequence in complementarity determining region 3 (CDR3) of the Vβ segment dominate this response. To investigate the molecular basis of immunodominant selection in an outbred population, we have determined the crystal structure of Vβ17Vα10.2 in complex with MP(58–66)–HLA-A2 at a resolution of 1.4 Å. We show that, whereas the TCR typically fits over an exposed side chain of the peptide, in this structure MP(58–66) exposes only main chain atoms. This distinctive orientation of Vβ17Vα10.2, which is almost orthogonal to the peptide-binding groove of HLA-A2, facilitates insertion of the conserved arginine in Vβ CDR3 into a notch in the surface of MP(58–66)–HLA-A2. This previously unknown binding mode underlies the immunodominant T cell response.

Contribution of genes of the major histocompatibility complex to susceptibility and disease phenotype in inflammatory bowel disease

BACKGROUND Despite strong evidence implicating immune dysfunction and genetic predisposition in the pathogenesis of the chronic inflammatory bowel diseases Crohns disease and ulcerative colitis, the importance of the genes of the major histocompatibility complex remains uncertain. We have investigated the contribution of HLA DRB1 and DQB genes by the strategies of non-parametric linkage analysis (affected sibling pair method) as well as association study. The relation between genotype and phenotype was examined in detail. METHODS For linkage analysis 74 families in whom two or more siblings had inflammatory bowel disease were studied. A total of 83 affected sibling pairs were involved: in 42 pairs both siblings had Crohns disease; in 29 both had ulcerative colitis; in 12 one sibling had Crohns disease, the other ulcerative colitis. For the association study there were 175 patients with ulcerative colitis, 173 with Crohns disease, and 472 controls. Details of sex, age of onset, disease extent, and family history were analysed. 24 patients with ulcerative colitis and 92 with Crohns disease required surgery for refractory disease. HLA DRB1 and DQB1 gene-typing was performed by polymerase chain reaction with sequence-specific primers. FINDINGS In ulcerative colitis, the sharing of alleles among affected sibling pairs provided evidence for linkage with DRB1 locus (p = 0.017, chi2 = 5.32). Of 29 affected sibling pairs studied, only one pair shared no DRB1 DQB haplotypes. 15 shared two DRB DQB haplotypes. In contrast, no linkage was noted for Crohns disease (42 sibling pairs; p = 0.30, chi2 = 0.16) or for inflammatory bowel disease overall (83 sibling pairs, p = 0.16, chi2 = 2.28). In the association study the rare DRB1*103 (8.3% vs 3.2% in controls) and DRB1*12 (8.6% vs 2.1% in controls) alleles were associated with ulcerative colitis (p = 0.0074, chi2 = 7.22, odds ratio OR = 2.9 [95% CI 1.3-6.4] and p = 0.0056, chi2 = 12.63, OR = 4.33 [1.8-11.0] respectively). No association with alleles representing DR2 (p = 0.55, chi2 = 0.34) was noted. No overall association was seen in Crohns disease. In ulcerative colitis, the frequency of DRB1*0301 DQB*0201 (DR3 DQ2) was reduced in females (9.8% vs 26.3% in controls, p = 0.037, chi2 = 8.39 OR = 0.34 [0.15-0.71]), particularly in those with distal disease (2.3%, p = 0.001 vs controls, chi2 = 11.35, OR = 0.07 [0.00-0.39]). In both males and females, the DR3 DQ2 haplotype was predictive of extensive ulcerative colitis (32.9% vs 10.7% in distal disease, p < 0.01, chi2 = 10.94, OR 4.09 [1.70-10.6]) but not of need for surgery (p = 0.93, chi2 = 0.01). INTERPRETATION These data provide strong evidence for genetic heterogeneity in inflammatory bowel disease. Genes of the major histocompatibility complex are implicated as important inherited determinants of susceptibility to ulcerative colitis and may also influence the pattern of disease. In Crohns disease, important susceptibility genes are likely to exist outside the HLA region.

TCR Binding to Peptide-MHC Stabilizes a Flexible Recognition Interface

The binding of TCRs to their peptide-MHC ligands is characterized by a low affinity, slow kinetics, and a high degree of cross-reactivity. Here, we report the results of a kinetic and thermodynamic analysis of two TCRs binding to their peptide-MHC ligands, which reveal two striking features. First, significant activation energy barriers must be overcome during both association and dissociation, suggesting that conformational adjustments are required. Second, the low affinity of binding is a consequence of highly unfavorable entropic effects, indicative of a substantial reduction in disorder upon binding. This is evidence that the TCR and/or peptide-MHC have flexible binding surfaces that are stabilized upon binding. Such conformational flexibility, which may also be a feature of primary antibodies, is likely to contribute to cross-reactivity in antigen recognition.

HLA class I associations of ankylosing spondylitis in the white population in the United Kingdom.

OBJECTIVE: To investigate the HLA class I associations of ankylosing spondylitis (AS) in the white population, with particular reference to HLA-B27 subtypes. METHODS: HLA-B27 and -B60 typing was performed in 284 white patients with AS. Allele frequencies of HLA-B27 and HLA-B60 from 5926 white bone marrow donors were used for comparison. HLA-B27 subtyping was performed by single strand conformation polymorphism (SSCP) in all HLA-B27 positive AS patients, and 154 HLA-B27 positive ethnically matched blood donors. RESULTS: The strong association of HLA-B27 and AS was confirmed (odds ratio (OR) 171, 95% confidence interval (CI) 135 to 218; p < 10(-99)). The association of HLA-B60 with AS was confirmed in HLA-B27 positive cases (OR 3.6, 95% CI 2.1 to 6.3; p < 5 x 10(-5)), and a similar association was demonstrated in HLA-B27 negative AS (OR 3.5, 95% CI 1.1 to 11.4; p < 0.05). No significant difference was observed in the frequencies of HLA-B27 allelic subtypes in patients and controls (HLA-B*2702, three of 172 patients v five of 154 controls; HLA-B*2705, 169 of 172 patients v 147 of 154 controls; HLA-B*2708, none of 172 patients v two of 154 controls), and no novel HLA-B27 alleles were detected. CONCLUSION: HLA-B27 and -B60 are associated with susceptibility to AS, but differences in HLA-B27 subtype do not affect susceptibility to AS in this white population.

Opposing effects of HLA class I molecules in tuning autoreactive CD8 + T cells in multiple sclerosis

The major known genetic risk factors in multiple sclerosis reside in the major histocompatibility complex (MHC) region. Although there is strong evidence implicating MHC class II alleles and CD4+ T cells in multiple sclerosis pathogenesis, possible contributions from MHC class I genes and CD8+ T cells are controversial. We have generated humanized mice expressing the multiple sclerosis–associated MHC class I alleles HLA-A*0301 (encoding human leukocyte antigen-A3 (HLA-A3)) and HLA-A*0201 (encoding HLA-A2) and a myelin-specific autoreactive T cell receptor (TCR) derived from a CD8+ T cell clone from an individual with multiple sclerosis to study mechanisms of disease susceptibility. We demonstrate roles for HLA-A3–restricted CD8+ T cells in induction of multiple sclerosis–like disease and for CD4+ T cells in its progression, and we also define a possible mechanism for HLA-A*0201–mediated protection. To our knowledge, these data provide the first direct evidence incriminating MHC class I genes and CD8+ T cells in the pathogenesis of human multiple sclerosis and reveal a network of MHC interactions that shape the risk of multiple sclerosis.

First glimpse of the peptide presentation by rhesus macaque MHC class I: crystal structures of Mamu-A*01 complexed with two immunogenic SIV epitopes and insights into CTL escape.

The infection of rhesus macaques (Macaca mulatta) by the SIV is the best animal model for studying HIV infection and for AIDS vaccine development. A prevalent MHC class I allele, Mamu-A*01, is known to correlate with containment of SIV, which has been extensively explored in studies of CTL-based vaccination concepts. We determined the crystal structures of Mamu-A*01 complexed with two immunodominant SIV epitopes: the nonamer CM9 of group-specific Ag (Gag, 181–189; CTPYDINQM) and the octamer TL8 of transcription activator (Tat, 28–35; TTPESANL). The overall structures of the two Mamu-A*01 complexes are similar to other MHC class I molecules. Both structures confirm the presence of an absolutely conserved proline anchor residue in the P3 position of the Ag, bound to a D pocket of the Mamu-A*01 H chain with optimal surface complementarity. Like other MHC/peptide complex structures, the P2 and C-terminal residues of the epitopes are also important for anchoring to the MHC molecule, whereas the middle residues form an arch and their side chains are directed into solvent. These two structures reveal details of how Mamu-A*01 interacts with two well-studied epitopes at the atomic level. We discuss the structural basis of CTL escape, based on molecular models made possible by these two structures. The results we present in this study are most relevant for the rational design of Mamu-A*01-restricted CTL epitopes with improved binding, as a step toward development of AIDS vaccines.

Analysis of a T-cell receptor V beta segment implicated in susceptibility to rheumatoid arthritis: V beta 2 germline polymorphism does not encode susceptibility.

OBJECTIVES--The assessment of allelic polymorphism of the T cell receptor gene segment, TCRBV2S1, in rheumatoid arthritis. METHODS--A total of 136 patients with rheumatoid arthritis (RA) (ACR criteria) and 150 controls were TCRBV2S1 genotyped using a nested PCR amplification strategy followed by single-strand conformation polymorphism (SSCP) analysis. RESULTS--The SSCP typing method detected two previously unknown alleles of the TCRBV2S1 gene segment. The TCRBV2S1 allele, genotype and inferred phenotype frequencies were similar in the RA patients and controls. No differences were apparent after the RA patients had been partitioned according to their HLA-DR genotypes. CONCLUSIONS--SSCP analysis is a rapid and efficient method of typing T cell receptor germline polymorphisms. Allelic polymorphism of the T cell receptor variable segment, TCRBV2S1, does not influence susceptibility to RA.