Luis Borges

A Novel Immunoglobulin Superfamily Receptor for Cellular and Viral MHC Class I Molecules

The human cytomegalovirus UL18 gene product is a homolog of cellular major histocompatibility (MHC) class I antigens. UL18 has been proposed to protect virus-infected cells against natural killer (NK) cell cytotoxicity by engaging NK cell killer inhibitory receptors (KIR) for MHC class I. UL18 binds to a novel immunoglobulin superfamily glycoprotein, designated Leukocyte Immunoglobulin-like Receptor (LIR-1). This protein is distinct from, but related to, known KIRs and binds cellular MHC class I antigens. The cytoplasmic domain of LIR-1 contains four putative immunoreceptor tyrosine-based inhibitory motifs. Upon tyrosine phosphorylation, LIR-1 associates with the tyrosine phosphatase SHP-1. In contrast to KIRs, LIR-1 is expressed predominantly on monocytic and B lymphoid cell types, suggesting a distinct biological function.

The MHC class I binding proteins LIR‐1 and LIR‐2 inhibit Fc receptor‐mediated signaling in monocytes

The MHC class I binding proteins leukocyte immunoglobulin‐like receptor (LIR)‐1 and −2 recognize a similar broad spectrum of HLA‐A, ‐B and ‐C alleles but are differentially expressed in lymphocytes, monocytes, and dendritic cells. In monocytes, phosphorylation of LIR‐1 and LIR‐2 results in the binding of the tyrosine phosphatase SHP‐1. Coligation of either LIR with Fcγ receptor I (CD64) inhibits tyrosine phosphorylation of the associated Fc receptor γ chain and Syk molecules, as well as intracellular calcium mobilization. These findings suggest that LIR‐1 and LIR‐2 function as unique MHC class I receptors involved in the inhibition or down‐modulation of monocyte activation signals, particularly those mediated through the receptors for IgG, IgE and IgA.

LIRs/ILTs/MIRs, inhibitory and stimulatory Ig-superfamily receptors expressed in myeloid and lymphoid cells

Cells exhibit a complex network of inhibitory and stimulatory signaling pathways, which interact with each other to maintain an homeostatic balance and modulate cellular responses to external stimuli. During most of the 1980s, a great effort was put into the characterization of stimulatory cell surface receptors for cytokines and growth factors. In the last decade, a large number of inhibitory receptors have been identified and it has become apparent that inhibitory signaling pathways are subject to intricate regulatory mechanisms. Inhibitory and stimulatory signaling pathways work in concert with each other to establish activation thresholds and provide sensitive tuning mechanisms that help control cellular responses. LIRs/ILTs/MIRs are a novel family of inhibitory and stimulatory receptors expressed both in myeloid and lymphoid cells. They contain two or four immunoglobulin-like domains in the extracellular region and their cytoplasmic domains are either very short and without any signaling motifs or are long and contain a variable number of immunoreceptor tyrosine-based inhibition motifs (ITIMs). LIRs within the first group send stimulatory signals by association with the FcR common gamma chain and LIRs within the second group deliver inhibitory signals by association with the protein tyrosine phosphatase SHP-1. This review summarizes our current knowledge on the LIRs, their ligands, and biological functions.

Molecular cloning and biological characterization of NK cell activation‐inducing ligand, a counterstructure for CD48

Using the monoclonal antibody C1.7, which recognizes a signaling, membrane‐bound molecule on human NK and a proportion of CD8+ T cells, we cloned a novel molecule we refer to as NK cell activation‐inducing ligand (NAIL). It is a 365‐amino acid protein that belongs to the immunoglobulin‐like superfamily with closest homology to murine 2B4, and human CD84 and CD48. Using a soluble NAIL‐Fc fusion protein, we determined the counterstructure for NAIL, CD48, which it binds with high affinity. Stimulation of human B cells with recombinant NAIL in the presence of a suboptimal concentration of human CD40 ligand or IL‐4 resulted in increased proliferation. Treatment of human dendritic cells with soluble NAIL‐leucine zipper protein resulted in an increased release of IL‐12 and TNF‐α. Using recombinant CD48 protein, we demonstrated the ability of this molecule to increase NK cell cytotoxicity and induce IFN‐γ production. We also showed that 2B4 binds to mouse CD48, suggesting that interaction of these receptors may play a similar role in both species. Taken together these results indicate that the NAIL‐CD48 interaction may be an important mechanism regulating a variety of immune responses.

Activation of human eosinophils through leukocyte immunoglobulin-like receptor 7

Eosinophils are implicated prominently in allergic diseases and the host response to parasitic infections. Eosinophils may be activated in vitro by diverse classes of agonists such as immunoglobulins, lipid mediators, and cytokines. The leukocyte Ig-like receptors (LIRs) comprise a family of inhibitory and activating cell-surface receptors. Inhibitory LIRs down-regulate cellular responses through cytoplasmic immunoreceptor tyrosine-based inhibitory motifs. There are limited data on the action of the activating LIRs, which are thought to signal through the Fc receptor γ chain, which contains an immunoreceptor tyrosine-based activation motif. We now demonstrate the expression of LIR1 (inhibitory), LIR2 (inhibitory), LIR3 (inhibitory), and LIR7 (activating) on eosinophils from 4, 4, 12, and 11, respectively, of 12 healthy donors. Cross-linking of LIR7 with plate-bound antibody elicited the dose- and time-dependent release of eosinophil-derived neurotoxin and leukotriene C4. Eosinophils activated with antibodies to LIR7 embedded in gel-phase EliCell preparations showed leukotriene C4 generation at the nuclear envelope and the release of IL-12 but not IL-4 by vesicular transport. Thus, LIR7 is an activating receptor for eosinophils that elicited the release of cytotoxic granule proteins, de novo lipid mediator generation, and cytokine release through vesicular transport.

Human cytomegalovirus, MHC class I and inhibitory signalling receptors: more questions than answers.

Summary: The human cytomegalovirus UL18 protein, an MHC class I homologue, has been shown to bind to leucocyte immunoglobulin‐like receptor (LIR)‐1, a member of a family of nine closely related immunoglobulin superfamily receptors expressed on leucocytes. The LIRs are related to the natural killer (NK)‐cell immunoglobulin‐like receptors and to several other immunoreceptors. Three groups of LIR molecules have been defined: those containing cytoplasmic domain inhibitory signalling motifs, those with short cytoplasmic domains and a charged residue within the trans‐membrane domain, and a secreted molecule. LIR‐1 and LIR‐2 bind to a broad spectrum of cellular MHC class I antigens, including HLA‐A, ‐B and ‐C alleles, LIR‐2 is expressed by all monocytes and dendritic cells, whereas LIR‐1 is additionally expressed by B cells and subsets of T and NK cells. Upon tyrosine phosphorylation, LIR‐ 1 and LIR‐2 associate with the tyrosine phosphatase, SHP‐1, and have been shown to inhibit FcγRI signalling when co‐crosslinked in monocytes. Evidence for and against a role of UL18 as ail inhibitor of NK‐cell function is discussed, as are possible functional outcomes of UL18‐LIR‐1 interactions in monocytic cells.

Purification and kinetic characterization of human indoleamine 2,3-dioxygenases 1 and 2 (IDO1 and IDO2) and discovery of selective IDO1 inhibitors

Indoleamine 2,3-dioxygenase (IDO1) catalyzes the first step in tryptophan breakdown along the kynurenine pathway. Therapeutic inhibition of IDO1 is receiving much attention due to its proposed role in the pathogenesis of several diseases including cancer, hypotension and neurodegenerative disorders. A related enzyme, IDO2 has recently been described. We report the first purification and kinetic characterization of human IDO2 using a facile l-tryptophan consumption assay amenable to high throughput screening. We found that the K(m) of human IDO2 for l-tryptophan is much higher than that of IDO1. We also describe the identification and characterization of a new IDO1 inhibitor compound, Amg-1, by high throughput screening, and compare the inhibition profiles of IDO1 and IDO2 with Amg-1 and previously described compounds. Our data indicate that human IDO1 and IDO2 have different kinetic parameters and different inhibition profiles. Docking of Amg-1 and related analogs to the known structure of IDO1 and to homology-modeled IDO2 suggests possible rationales for the different inhibition profiles of IDO1 and IDO2.

The Co-Expression of Activating and Inhibitory Leukocyte Immunoglobulin-Like Receptors in Rheumatoid Synovium

Rheumatoid arthritis (RA) is a chronic inflammatory synovitis, with destruction of juxtaarticular cartilage and bone, likely mediated by lipid mediators, cytokines, and proteases released from inflammatory leukocytes. The mechanisms regulating leukocyte activation in rheumatoid synovium are not fully elucidated. A new family of cell surface proteins termed leukocyte immunoglobulin-like receptors (LIRs) has been shown in vitro to modulate cellular responses through immunoreceptor tyrosine-based inhibitory motifs or through association with the Fc receptor gamma chain that contains immunoreceptor tyrosine-based activation motifs. We studied the expression of inhibitory and activating LIRs in the synovium of six RA patients, three osteoarthritis patients, and three controls by immunohistochemistry. The synovium from patients with early RA showed extensive expression of the inhibitory LIR-2 and the activating LIR-7 on macrophages and neutrophils. Some mast cells and endothelial cells expressed LIR-7. There was limited expression of LIRs in synovium from two patients with long-standing RA, patients with osteoarthritis, and controls. LIR-2 recognizes MHC class I molecules. We therefore suggest that LIRs may regulate the activation of infiltrating leukocytes in synovial tissue and are a potential therapeutic target.

Expression of Trop2 Cell Surface Glycoprotein in Normal and Tumor Tissues: Potential Implications as a Cancer Therapeutic Target

Trop2 is a cell-surface glycoprotein reported to be overexpressed in various types of adenocarcinomas with minimal expression in normal tissues. Recent findings that Trop2 expression correlates with tumor aggressiveness have increased interest in Trop2 as a potential target for cancer immunotherapy. The goal of this study was to extensively evaluate Trop2 expression at the transcript and protein levels in normal and tumor tissues. It was determined that Trop2 is overexpressed on some carcinomas relative to the corresponding normal tissue. However, in human and mouse, Trop2 is highly expressed at both the transcript and protein levels on several essential normal tissues. The findings suggest that the development of therapeutic agents to target Trop2 may require strategies that target Trop2 on malignant tissues in order to minimize potential toxicities to essential normal tissues that also express high levels of Trop2.

THE LEUKOCYTE IMMUNOGLOBULIN-LIKE RECEPTORS (LIRS) : A NEW FAMILY OF IMMUNE REGULATORS

Identification of a counterstructure for the human cytomegalovirus‐encoded major histocompatibility complex class I‐related gene product, UL18, has led to the discovery of a novel family of immunoreceptors, termed leukocyte immunoglobulin‐like receptors (LIRs). The LIRs are differentially expressed in cells of the dendritic cell, monocytic and lymphocytic lineages, and appear to mediate diverse roles in immune regulation. This review summarizes the expression, distribution, and signaling capacities of the LIRs and discusses possible roles of the LIRs in both inhibition and activation of the cellular responses. J. Leukoc. Biol. 66: 231–236; 1999.