E. Yvonne Jones

A functional and structural basis for TCR cross-reactivity in multiple sclerosis

The multiple sclerosis (MS)-associated HLA major histocompatibility complex (MHC) class II alleles DRB1*1501, DRB5*0101 and DQB1*0602 are in strong linkage disequilibrium, making it difficult to determine which is the principal MS risk gene. Here we show that together the DRB1 and DRB5 loci may influence susceptibility to MS. We demonstrate that a T cell receptor (TCR) from an MS patient recognized both a DRB1*1501-restricted myelin basic protein (MBP) and DRB5*0101-restricted Epstein-Barr virus (EBV) peptide. Crystal structure determination of the DRB5*0101-EBV peptide complex revealed a marked degree of structural equivalence to the DRB1*1501–MBP peptide complex at the surface presented for TCR recognition. This provides structural evidence for molecular mimicry involving HLA molecules. The structural details suggest an explanation for the preponderance of MHC class II associations in HLA-associated diseases.

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 μM), 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.

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.

Positional cloning of a novel gene influencing asthma from chromosome 2q14

Asthma is a common disease in children and young adults. Four separate reports have linked asthma and related phenotypes to an ill-defined interval between 2q14 and 2q32 (refs. 1–4), and two mouse genome screens have linked bronchial hyper-responsiveness to the region homologous to 2q14 (refs. 5,6). We found and replicated association between asthma and the D2S308 microsatellite, 800 kb distal to the IL1 cluster on 2q14. We sequenced the surrounding region and constructed a comprehensive, high-density, single-nucleotide polymorphism (SNP) linkage disequilibrium (LD) map. SNP association was limited to the initial exons of a solitary gene of 3.6 kb (DPP10), which extends over 1 Mb of genomic DNA. DPP10 encodes a homolog of dipeptidyl peptidases (DPPs) that cleave terminal dipeptides from cytokines and chemokines, and it presents a potential new target for asthma therapy.

MHC class II proteins and disease: a structural perspective

MHC class II molecules on the surface of antigen-presenting cells display a range of peptides for recognition by the T-cell receptors of CD4+ T helper cells. Therefore, MHC class II molecules are central to effective adaptive immune responses, but conversely, genetic and epidemiological data have implicated these molecules in the pathogenesis of autoimmune diseases. Indeed, the strength of the associations between particular MHC class II alleles and disease render them the main genetic risk factors for autoimmune disorders such as type 1 diabetes. Here, we discuss the insights that the crystal structures of MHC class II molecules provide into the molecular mechanisms by which sequence polymorphisms might contribute to disease susceptibility.

The crystal structure of plasma gelsolin: implications for actin severing, capping, and nucleation.

The structure of gelsolin has been determined by crystallography and comprises six structurally related domains that, in a Ca2+-free environment, pack together to form a compact globular structure in which the putative actin-binding sequences are not sufficiently exposed to enable binding to occur. We propose that binding Ca2+ can release the connections that join the N- and C-terminal halves of gelsolin, enabling each half to bind actin relatively independently. Domain shifts are proposed in response to Ca2+ as bases for models of how gelsolin acts to sever, cap, or nucleate F-actin filaments. The structure also invites discussion of polyphosphoinositide binding to segment 2 and suggests how mutation at Asp-187 could initiate a series of events that lead to deposition of amyloid plaques, as observed in victims of familial amyloidosis (Finnish type).

The transmembrane semaphorin Sema4D/CD100, an inhibitor of axonal growth, is expressed on oligodendrocytes and upregulated after CNS lesion

Semaphorins are a family of secreted and membrane-bound proteins, known to regulate axonal pathfinding. Sema4D, also called CD100, was first isolated in the immune system where it is involved in B and T cell activation. We found that in the mouse, Sema4D is expressed in cells throughout the CNS white matter, with a peak during the myelination period. Double-labeling experiments with different markers of oligodendrocyte lineage such as olig1, olig2, platelet-derived growth factor receptor α, and proteolipid protein showed that Sema4D was expressed selectively by oligodendrocytes and myelin. The presence of Sema4D in myelin was confirmed using Western blot. Sema4D expression in myelinating oligodendrocytes was further observed using neuron-oligodendrocyte cocultures. Moreover, using stripe assay, we found that Sema4D is strongly inhibitory for postnatal sensory and cerebellar granule cell axons. This prompted us to examine whether Sema4D expression is modified after CNS injury. At 8 d after spinal cord lesions, Sema4D expression was strongly upregulated in oligodendrocytes at the periphery of the lesion. Sema4D-positive cells were not colabeled with the astrocyte marker GFAP, with the microglial and macrophagic marker isolectin B4, or with NG2, a marker of oligodendrocyte precursors. This upregulation was transient because from 1 month after the lesion, Sema4D expression was back to its normal level. These results indicate that Sema4D is a novel inhibitory factor for axonal regeneration expressed in myelin.

Functions of fibroblast growth factors and their receptors.

Fibroblast growth factors were first characterized twenty years ago as mitogens of cultured fibroblasts. Despite a wealth of data from experiments in vitro, insights have begun to emerge only recently on the normal function of these growth factors in mice and humans, as a result of studies of natural and experimental mutations in the factors and their receptors.

Structure of human CD1b with bound ligands at 2.3 Å, a maze for alkyl chains

The human genome encodes five nonpolymorphic major histocompatibility complex class I–like glycoproteins, CD1a to CD1e, that present lipid antigens for specific recognition by T lymphocytes. Using single alkyl chain detergents, we developed a protocol to generate recombinant human CD1b-lipid complexes. We present here the crystal structures of CD1b in complex with either phosphatidylinositol or ganglioside GM2 at 2.3 Å and 2.8 Å resolutions, respectively. The antigen-binding groove houses four interlinked hydrophobic channels that are occupied by the alkyl chains of the glycolipid plus two detergent molecules. A distinct exit beneath the α2 helix further contributes to the plasticity of the binding groove. These structures reveal the mechanism by which two alkyl chain lipids bind to CD1b, and how CD1b can adapt to ligands of different alkyl chain length. They also suggest how very long alkyl chains, such as those of mycolic acid, could be fully contained within the binding groove. These results extend the spectrum of potential CD1b ligands by revealing that, in addition to two alkyl chain lipids, mono-alkyl and triple-alkyl chain lipids can be accommodated in the binding groove.

Structure and dimerization of a soluble form of B7-1.

B7-1 (CD80) and B7-2 (CD86) are glycoproteins expressed on antigen-presenting cells. The binding of these molecules to the T cell homodimers CD28 and CTLA-4 (CD152) generates costimulatory and inhibitory signals in T cells, respectively. The crystal structure of the extracellular region of B7-1 (sB7-1), solved to 3 A resolution, consists of a novel combination of two Ig-like domains, one characteristic of adhesion molecules and the other previously seen only in antigen receptors. In the crystal lattice, sB7-1 unexpectedly forms parallel, 2-fold rotationally symmetric homodimers. Analytical ultracentrifugation reveals that sB7-1 also dimerizes in solution. The structural data suggest a mechanism whereby the avidity-enhanced binding of B7-1 and CTLA-4 homodimers, along with the relatively high affinity of these interactions, favors the formation of very stable inhibitory signaling complexes.