Structure of Nectin-2 reveals determinants of homophilic and heterophilic interactions that control cell-cell adhesion.

Nectins are members of the Ig superfamily that mediate cell-cell adhesion through homophilic and heterophilic interactions. We have determined the crystal structure of the nectin-2 homodimer at 1.3 Å resolution. Structural analysis and complementary mutagenesis studies reveal the basis for recognition and selectivity among the nectin family members. Notably, the close proximity of charged residues at the dimer interface is a major determinant of the binding affinities associated with homophilic and heterophilic interactions within the nectin family. Our structural and biochemical data provide a mechanistic basis to explain stronger heterophilic versus weaker homophilic interactions among these family members and also offer insights into nectin-mediated transinteractions between engaging cells.

Structural and functional characterization of a single-chain peptide-MHC molecule that modulates both naive and activated CD8+ T cells.

Peptide-MHC (pMHC) multimers, in addition to being tools for tracking and quantifying antigen-specific T cells, can mediate downstream signaling after T-cell receptor engagement. In the absence of costimulation, this can lead to anergy or apoptosis of cognate T cells, a property that could be exploited in the setting of autoimmune disease. Most studies with class I pMHC multimers used noncovalently linked peptides, which can allow unwanted CD8(+) T-cell activation as a result of peptide transfer to cellular MHC molecules. To circumvent this problem, and given the role of self-reactive CD8(+) T cells in the development of type 1 diabetes, we designed a single-chain pMHC complex (scK(d).IGRP) by using the class I MHC molecule H-2K(d) and a covalently linked peptide derived from islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP(206-214)), a well established autoantigen in NOD mice. X-ray diffraction studies revealed that the peptide is presented in the groove of the MHC molecule in canonical fashion, and it was also demonstrated that scK(d).IGRP tetramers bound specifically to cognate CD8(+) T cells. Tetramer binding induced death of naive T cells and in vitro- and in vivo-differentiated cytotoxic T lymphocytes, and tetramer-treated cytotoxic T lymphocytes showed a diminished IFN-γ response to antigen stimulation. Tetramer accessibility to disease-relevant T cells in vivo was also demonstrated. Our study suggests the potential of single-chain pMHC tetramers as possible therapeutic agents in autoimmune disease. Their ability to affect the fate of naive and activated CD8(+) T cells makes them a potential intervention strategy in early and late stages of disease.

Programmed death-1 (PD-1)-deficient mice are extraordinarily sensitive to tuberculosis.

The programmed death-1 (PD-1) costimulatory receptor inhibits T and B cell responses and plays a crucial role in peripheral tolerance. PD-1 has recently been shown to inhibit T cell responses during chronic viral infections such as HIV. In this study, we examined the role of PD-1 in infection with Mycobacterium tuberculosis, a common co-infection with HIV. PD-1-deficient mice showed dramatically reduced survival compared with wild-type mice. The lungs of the PD-1-/- mice showed uncontrolled bacterial proliferation and focal necrotic areas with predominantly neutrophilic infiltrates, but a lower number of infiltrating T and B cells. Proinflammatory cytokines, such as TNF-alpha, IL-1, and especially IL-6 and IL-17 were significantly increased in the lung and sera of infected PD-1-/- mice, consistent with an aberrant inflammation. Microarray analysis of the lungs infected with M. tuberculosis showed dramatic differences between PD-1-/- and control mice. Using high-stringency analysis criteria (changes of twofold or greater), 367 transcripts of genes were differentially expressed between infected PD-1-/- and wild-type mice, resulting in profoundly altered inflammatory responses with implications for both innate and adaptive immunity. Overall, our studies show that the PD-1 pathway is required to control excessive inflammatory responses after M. tuberculosis infection in the lungs.

Sequence, structure, function, immunity: structural genomics of costimulation.

SUMMARY: Costimulatory receptors and ligands trigger the signaling pathways that are responsible for modulating the strength, course, and duration of an immune response. High-resolution structures have provided invaluable mechanistic insights by defining the chemical and physical features underlying costimulatory receptor:ligand specificity, affinity, oligomeric state, and valency. Furthermore, these structures revealed general architectural features that are important for the integration of these interactions and their associated signaling pathways into overall cellular physiology. Recent technological advances in structural biology promise unprecedented opportunities for furthering our understanding of the structural features and mechanisms that govern costimulation. In this review, we highlight unique insights that have been revealed by structures of costimulatory molecules from the immunoglobulin and tumor necrosis factor superfamilies and describe a vision for future structural and mechanistic analysis of costimulation. This vision includes simple strategies for the selection of candidate molecules for structure determination and highlights the critical role of structure in the design of mutant costimulatory molecules for the generation of in vivo structure-function correlations in a mammalian model system. This integrated 'atoms-to-animals' paradigm provides a comprehensive approach for defining atomic and molecular mechanisms.

Dynamic equilibrium of B7-1 dimers and monomers differentially affects immunological synapse formation and T cell activation in response to TCR/CD28 stimulation.

Under steady-state conditions, B7-1 is present as a mixed population of noncovalent dimers and monomers on the cell surface. In this study, we examined the physiological significance of this unique dimer-monomer equilibrium state of B7-1. We demonstrate that altering B7-1 to create a uniformly covalent dimeric state results in enhanced CD28-mediated formation of T cell-APC conjugates. The enhanced T cell-APC conjugate formation correlates with persistent concentration of signaling molecules PKC- and lck at the immunological synapse. In contrast, T cell acquisition of B7-1 from APCs, an event that occurs as a consequence of CD28 engagement with B7-1/B7-2 and is thought to play a role in the dissociation of T cell-APC conjugates, is highly reduced when B7-1 is present in the covalently dimeric state. The ability of covalently dimeric and wild type B7-1 to costimulate Ag-specific T cell proliferation was also assessed. In contrast to the enhanced ability of dimeric B7-1 to support conjugate formation and early parameters of T cell signaling, sensitivity to competitive inhibition by soluble CTLA-4-Ig indicated that the covalent dimeric form of B7-1 is less efficient in costimulating T cell proliferation. These findings suggest a novel model in which optimal T cell costimulatory function of B7-1 requires high-avidity CD28 engagement by dimeric B7-1, followed by dissociation of these noncovalent B7-1 dimers, facilitating downregulation of CD28 and internalization of B7-1. These events regulate signaling through TCR/CD28 to maximize T cell activation to proliferation.

Predominant occupation of the class I MHC molecule H-2Kwm7 with a single self-peptide suggests a mechanism for its diabetes-protective effect.

Type 1 diabetes (T1D) is an autoimmune disease characterized by T cell-mediated destruction of insulin-producing pancreatic beta cells. In both humans and the non-obese diabetic (NOD) mouse model of T1D, class II MHC alleles are the primary determinant of disease susceptibility. However, class I MHC genes also influence risk. These findings are consistent with the requirement for both CD4(+) and CD8(+) T cells in the pathogenesis of T1D. Although a large body of work has permitted the identification of multiple mechanisms to explain the diabetes-protective effect of particular class II MHC alleles, studies examining the protective influence of class I alleles are lacking. Here, we explored this question by performing biochemical and structural analyses of the murine class I MHC molecule H-2K(wm7), which exerts a diabetes-protective effect in NOD mice. We have found that H-2K(wm7) molecules are predominantly occupied by the single self-peptide VNDIFERI, derived from the ubiquitous protein histone H2B. This unexpected finding suggests that the inability of H-2K(wm7) to support T1D development could be due, at least in part, to the failure of peptides from critical beta-cell antigens to adequately compete for binding and be presented to T cells. Predominant presentation of a single peptide would also be expected to influence T-cell selection, potentially leading to a reduced ability to select a diabetogenic CD8(+) T-cell repertoire. The report that one of the predominant peptides bound by T1D-protective HLA-A*31 is histone derived suggests the potential translation of our findings to human diabetes-protective class I MHC molecules.

Crystal structure of the complex between programmed death-1 (PD-1) and its ligand PD-L2.

Programmed death-1 (PD-1) is a member of the CD28/B7 superfamily that delivers negative signals upon interaction with its two ligands, PD-L1 or PD-L2. The high-resolution crystal structure of the complex formed by the complete ectodomains of murine PD-1 and PD-L2 revealed a 1:1 receptor:ligand stoichiometry and displayed a binding interface and overall molecular organization distinct from that observed in the CTLA-4/B7 inhibitory complexes. Furthermore, our structure also provides insights into the association between PD-1 and PD-L1 and highlights differences in the interfaces formed by the two PD-1 ligands (PD-Ls) Mutagenesis studies confirmed the details of the proposed PD-1/PD-L binding interfaces and allowed for the design of a mutant PD-1 receptor with enhanced affinity. These studies define spatial and organizational constraints that control the localization and signaling of PD-1/PD-L complexes within the immunological synapse and provide a basis for manipulating the PD-1 pathways for immunotherapy.

Evolution of GITRL immune function: murine GITRL exhibits unique structural and biochemical properties within the TNF superfamily.

Glucocorticoid-induced TNF receptor ligand (GITRL), a recently identified member of the TNF superfamily, binds to its receptor, GITR, on both effector and regulatory T cells and generates positive costimulatory signals implicated in a wide range of T cell functions. In contrast to all previously characterized homotrimeric TNF family members, the mouse GITRL crystal structure reveals a previously unrecognized dimeric assembly that is stabilized via a unique "domain-swapping" interaction. Consistent with its crystal structure, mouse GITRL exists as a stable dimer in solution. Structure-guided mutagenesis studies confirmed the determinants responsible for dimerization and support a previously unrecognized receptor-recognition surface for mouse GITRL that has not been observed for any other TNF family members. Taken together, the unique structural and biochemical behavior of mouse GITRL, along with the unusual domain organization of murine GITR, support a previously unrecognized mechanism for signaling within the TNF superfamily.

The PD-1/PD-L costimulatory pathway critically affects host resistance to the pathogenic fungus Histoplasma capsulatum.

The PD-1 costimulatory receptor inhibits T cell receptor signaling upon interacting with its ligands PD-L1 and PD-L2. The PD-1/PD-L pathway is critical in maintaining self-tolerance. In this study, we examined the role of PD-1 in a mouse model of acute infection with Histoplasma capsulatum, a major human pathogenic fungus. In a lethal model of histoplasmosis, all PD-1-deficient mice survived infection, whereas the wild-type mice died with disseminated disease. PD-L expression on macrophages and splenocytes was up-regulated during infection, and macrophages from infected mice inhibited in vitro T cell activation. Of interest, antibody blocking of PD-1 significantly increased survival of lethally infected wild-type mice. Thus, our studies extend the role of the PD-1/PD-L pathway in regulating antimicrobial immunity to fungal pathogens. The results show that the PD-1/PD-L pathway has a key role in the regulation of antifungal immunity, and suggest that manipulation of this pathway represents a strategy of immunotherapy for histoplasmosis.

Assembly and structural properties of glucocorticoid-induced TNF receptor ligand: Implications for function.

Glucocorticoid-induced TNF receptor ligand (GITRL), a recently identified member of the TNF family, binds to its receptor GITR on both effector and regulatory T cells and generates positive costimulatory signals implicated in a wide range of T cell functions. Structural analysis reveals that the human GITRL (hGITRL) ectodomain self-assembles into an atypical expanded homotrimer with sparse monomer-monomer interfaces. Consistent with the small intersubunit interfaces, hGITRL exhibits a relatively weak tendency to trimerize in solution and displays a monomer-trimer equilibrium not reported for other TNF family members. This unique assembly behavior has direct implications for hGITRL-GITR signaling, because enforced trimerization of soluble hGITRL ectodomain results in an approximately 100-fold increase in its receptor binding affinity and also in enhanced costimulatory activity. The apparent reduction in affinity that is the consequence of this dynamic equilibrium may represent a mechanism to realize the biologically optimal level of signaling through the hGITRL-GITR pathway, as opposed to the maximal achievable level.

Biochemical and structural characterization of the human TL1A ectodomain.

TNF-like 1A (TL1A) is a newly described member of the TNF superfamily that is directly implicated in the pathogenesis of autoimmune diseases, including inflammatory bowel disease, atherosclerosis, and rheumatoid arthritis. We report the crystal structure of the human TL1A extracellular domain at a resolution of 2.5 A, which reveals a jelly-roll fold typical of the TNF superfamily. This structural information, in combination with complementary mutagenesis and biochemical characterization, provides insights into the binding interface and the specificity of the interactions between TL1A and the DcR3 and DR3 receptors. These studies suggest that the mode of interaction between TL1A and DcR3 differs from other characterized TNF ligand/receptor complexes. In addition, we have generated functional TL1A mutants with altered disulfide bonding capability that exhibit enhanced solution properties, which will facilitate the production of materials for future cell-based and whole animal studies. In summary, these studies provide insights into the structure and function of TL1A and provide the basis for the rational manipulation of its interactions with cognate receptors.

1.8 A structure of murine GITR ligand dimer expressed in Drosophila melanogaster S2 cells.

Glucocorticoid-induced TNF receptor ligand (GITRL), a prominent member of the TNF superfamily, activates its receptor on both effector and regulatory T cells to generate critical costimulatory signals that have been implicated in a wide range of T-cell immune functions. The crystal structures of murine and human orthologs of GITRL recombinantly expressed in Escherichia coli have previously been determined. In contrast to all classical TNF structures, including the human GITRL structure, murine GITRL demonstrated a unique ;strand-exchanged' dimeric organization. Such a novel assembly behavior indicated a dramatic impact on receptor activation as well as on the signaling mechanism associated with the murine GITRL costimulatory system. In this present work, the 1.8 A resolution crystal structure of murine GITRL expressed in Drosophila melanogaster S2 cells is reported. The eukaryotic protein-expression system allows transport of the recombinant protein into the extracellular culture medium, thus maximizing the possibility of obtaining correctly folded material devoid of any folding/assembly artifacts that are often suspected with E. coli-expressed proteins. The S2 cell-expressed murine GITRL adopts an identical ;strand-exchanged' dimeric structure to that observed for the E. coli-expressed protein, thus conclusively demonstrating the novel quaternary structure assembly behavior of murine GITRL.

Structural analysis of H2-Db class I molecules containing two different allelic forms of the type 1 diabetes susceptibility factor beta-2 microglobulin: implications for the mechanism underlying variations in antigen presentation.

Beta-2 microglobulin (beta2m) is a member of the immunoglobulin-like domain superfamily that is an essential structural subunit of the MHC class I (MHC-I) molecule. beta2m was previously identified as a susceptibility factor for the development of type 1 diabetes (T1D) in NOD mice, whereby transgenic expression of the beta2ma variant, but not the beta2mb variant, restored diabetes susceptibility to normally resistant NOD.beta2mnull mice. Here we report the crystal structures and thermodynamic stabilities of the NOD MHC-I molecule H2-Db containing these two variants. Our results reveal subtle differences in the structures of the beta2m variants, namely in minor loop shifts and in variations in the hydrogen bonding networks at the interfaces between the components of the ternary complex. We also demonstrate that the thermodynamic stabilities of the beta2m variants in isolation differ. However, the conformation of the peptide in the MHC cleft is unchanged in beta2m allelic Db complexes, as are the TCR recognition surfaces. Thus, despite modest structural differences between allelic complexes, the evidence indicates that Db peptide presentation of the representative peptide is unchanged in the context of either beta2m allelic variant. These data suggest that other mechanisms, such as differential association of MHC-I in multiprotein complexes, are likely responsible for the effect of beta2m on T1D development.

Structure-guided development of a high-affinity human Programmed Cell Death-1: Implications for tumor immunotherapy.

Programmed Cell Death-1 (PD-1) is an inhibitory immune receptor, which plays critical roles in T cell co-inhibition and exhaustion upon binding to its ligands PD-L1 and PD-L2. We report the crystal structure of the human PD-1 ectodomain and the mapping of the PD-1 binding interface. Mutagenesis studies confirmed the crystallographic interface, and resulted in mutant PD-1 receptors with altered affinity and ligand-specificity. In particular, a high-affinity mutant PD-1 (HA PD-1) exhibited 45 and 30-fold increase in binding to PD-L1 and PD-L2, respectively, due to slower dissociation rates. This mutant (A132L) was used to engineer a soluble chimeric Ig fusion protein for cell-based and in vivo studies. HA PD-1 Ig showed enhanced binding to human dendritic cells, and increased T cell proliferation and cytokine production in a mixed lymphocyte reaction (MLR) assay. Moreover, in an experimental model of murine Lewis lung carcinoma, HA PD-1 Ig treatment synergized with radiation therapy to decrease local and metastatic tumor burden, as well as in the establishment of immunological memory responses. Our studies highlight the value of structural considerations in guiding the design of a high-affinity chimeric PD-1 Ig fusion protein with robust immune modulatory properties, and underscore the power of combination therapies to selectively manipulate the PD-1 pathway for tumor immunotherapy.

B7-1 and B7-2: similar costimulatory ligands with different biochemical, oligomeric and signaling properties.

B7-1 and B7-2 are homologous costimulatory ligands expressed on the surface of antigen presenting cells (APCs). Binding of these molecules to the T cell costimulatory receptors, CD28 and CTLA-4, is essential for the activation and regulation of T cell immunity. Despite strong structural similarities, B7-1 and B7-2 exhibit different biochemical features, and their binding to the costimulatory receptors results in distinct T cell functional outcomes. Using photobleaching based fluorescence resonance energy transfer (FRET), our previous studies have demonstrated that B7-1 and B7-2 have different cell surface oligomeric states. While B7-1 is present as a dimer, B7-2 exists as a monomer on the cell surface suggesting that the unique cell surface oligomeric states of the costimulatory ligands may play a key role in the regulation of T cell responses. Moreover, signaling via B7-1 and B7-2 in dendritic cells has been reported to be dependent on their simultaneous expression, raising the possibility that their direct interaction or their involvement in synergistic signaling pathways may play a role in the function of antigen presenting cells. We discuss physiological relevance of distinct oligomeric states of B7-1 and B7-2 and address whether these molecules can associate with one another on the cell surface to form hetero-oligomers. Our findings suggest that B7-1 and B7-2 do not form hetero-oligomers, underscoring the biological relevance of dimeric and monomeric state of B7-1 and B7-2, respectively.

Increased Heterologous Protein Expression in Drosophila S2 Cells for Massive Production of Immune Ligands/Receptors and Structural Analysis of Human HVEM.

Many immune ligands and receptors are potential drug targets, which delicately manipulate a wide range of immune responses. We describe here the successful application of an efficient method to dramatically improve the heterologous expression levels in Drosophila Schneider 2 cells, which enables the high-throughput production of several important immune ligands/receptors for raising antibodies, and for the structural and functional analyses. As an example, we purified the protein and characterized the structure of the immune receptor herpesvirus entry mediator (HVEM, TNFRSF14). HVEM is a member of tumor necrosis factor receptor superfamily, which is recognized by herpes simplex virus glycoprotein D (gD) and facilitates viral entry. HVEM participates in a range of interactions with other cell surface molecules, including LIGHT, BTLA, and CD160 to modulate a wide range of immune processes in CD4(+) and CD8(+) T cells, as well as NK cells. Due to the involvement of HVEM in these diverse signaling interactions, crystal structures of HVEM in complex with gD or BTLA have been previously reported. Here, we report the structure of HVEM in the absence of any ligands.

Mechanistic basis for functional promiscuity in the TNF and TNF receptor superfamilies: structure of the LIGHT:DcR3 assembly.

LIGHT initiates intracellular signaling via engagement of the two TNF receptors, HVEM and LTßR. In humans, LIGHT is neutralized by DcR3, a unique soluble member of the TNFR superfamily, which tightly binds LIGHT and inhibits its interactions with HVEM and LTßR. DcR3 also neutralizes two other TNF ligands, FasL and TL1A. Due to its ability to neutralize three distinct different ligands, DcR3 contributes to a wide range of biological and pathological processes, including cancer and autoimmune diseases. However, the mechanisms that support the broad specificity of DcR3 remain to be fully defined. We report the structures of LIGHT and the LIGHT:DcR3 complex, which reveal the structural basis for the DcR3-mediated neutralization of LIGHT and afford insights into DcR3 function and binding promiscuity. Based on these structures, we designed LIGHT mutants with altered affinities for DcR3 and HVEM, which may represent mechanistically informative probe reagents.

Tissue-expressed B7-H1 critically controls intestinal inflammation.

B7-H1 (PD-L1) on immune cells plays an important role in T cell coinhibition by binding its receptor PD-1. Here, we show that both human and mouse intestinal epithelium express B7-H1 and that B7-H1-deficient mice are highly susceptible to dextran sodium sulfate (DSS)- or trinitrobenzenesulfonic acid (TNBS)-induced gut injury. B7-H1 deficiency during intestinal inflammation leads to high mortality and morbidity, which are associated with severe pathological manifestations in the colon, including loss of epithelial integrity and overgrowth of commensal bacteria. Results from bone marrow chimeric and knockout mice show that B7-H1 expressed on intestinal parenchyma, but not on hematopoietic cells, controls intestinal inflammation in an adaptive immunity-independent fashion. Finally, we demonstrate that B7-H1 dampened intestinal inflammation by inhibiting tumor necrosis factor α (TNF-α) production and by stimulating interleukin 22 secretion from CD11c(+)CD11b(+) lamina propria cells. Thus, our data uncover a mechanism through which intestinal tissue-expressed B7-H1 functions as an essential ligand for innate immune cells to prevent gut inflammation.

Functional classification of immune regulatory proteins.

The members of the immunoglobulin superfamily (IgSF) control innate and adaptive immunity and are prime targets for the treatment of autoimmune diseases, infectious diseases, and malignancies. We describe a computational method, termed the Brotherhood algorithm, which utilizes intermediate sequence information to classify proteins into functionally related families. This approach identifies functional relationships within the IgSF and predicts additional receptor-ligand interactions. As a specific example, we examine the nectin/nectin-like family of cell adhesion and signaling proteins and propose receptor-ligand interactions within this family. Guided by the Brotherhood approach, we present the high-resolution structural characterization of a homophilic interaction involving the class-I MHC-restricted T-cell-associated molecule, which we now classify as a nectin-like family member. The Brotherhood algorithm is likely to have a significant impact on structural immunology by identifying those proteins and complexes for which structural characterization will be particularly informative.

Structure and T cell inhibition properties of B7 family member, B7-H3.

T cell activity is controlled by a combination of antigen-dependent signaling through the T cell receptor and a set of auxiliary signals delivered through antigen-independent interactions, including the recognition of the B7 family of ligands. B7-H3 is a recently identified B7 family member that is strongly overexpressed in a range of cancers and correlates with poor prognosis. We report the crystal structure of murine B7-H3 at a 3 Å resolution, which provides a model for the organization of the IgV and IgC domains within the ectodomain. We demonstrate that B7-H3 inhibits T cell proliferation and show that the FG loop of the IgV domain plays a critical role in this function. B7-H3 crystallized as an unusual dimer arising from the exchange of the G strands in the IgV domains of partner molecules. This arrangement, in combination with previous reports, highlights the dynamic nature and plasticity of the immunoglobulin fold.