Marion H. Brown

Lymphoid/Neuronal Cell Surface OX2 Glycoprotein Recognizes a Novel Receptor on Macrophages Implicated in the Control of Their Function

The OX2 membrane glycoprotein (CD200) is expressed on a broad range of tissues including lymphoid cells, neurons, and endothelium. We report the characterization of an OX2 receptor (OX2R) that is a novel protein restricted to cells of the myeloid lineage. OX2 and its receptor are both cell surface glycoproteins containing two immunoglobulin-like domains and interact with a dissociation constant of 2.5 microM and koff 0.8 s(-1), typical of many leukocyte protein membrane interactions. Pervanandate treatment of macrophages showed that OX2R could be phosphorylated on tyrosine residues. Blockade of the OX2-OX2R interaction with an OX2R mAb exacerbated the disease model experimental allergic encephalomyelitis. These data, together with data from an OX2-deficient mouse (R. M. Hoek et al., submitted), suggest that myeloid function can be controlled in a tissue-specific manner by the OX2-OX2R interaction.

CD200 and membrane protein interactions in the control of myeloid cells.

OX2 (now designated CD200) is a membrane protein expressed by a broad range of cell types. It is the ligand for a receptor restricted to myeloid cells, with the potential to deliver inhibitory signals. This is indicated by the CD200-deficient mouse model, in which myeloid cells are more activated when stimulated immunologically than cells from normal mice. The unusual tissue distribution of CD200 indicates where myeloid cells can be restrictively controlled through cell-cell contact. Recent data on CD200 will be reviewed in the context of other proteins that might have similar roles, in particular, the interaction between CD47 and SIRPalpha (CD172a).

Characterization of the CD200 Receptor Family in Mice and Humans and Their Interactions with CD200

CD200 (OX2) is a broadly distributed cell surface glycoprotein that interacts with a structurally related receptor (CD200R) expressed on rodent myeloid cells and is involved in regulation of macrophage function. We report the first characterization of human CD200R (hCD200R) and define its binding characteristics to hCD200. We also report the identification of a closely related gene to hCD200R, designated hCD200RLa, and four mouse CD200R-related genes (termed mCD200RLa-d). CD200, CD200R, and CD200R-related genes were closely linked in humans and mice, suggesting that these genes arose by gene duplication. The distributions of the receptor genes were determined by quantitative RT-PCR, and protein expression was confirmed by a set of novel mAbs. The distribution of mouse and human CD200R was similar, with strongest labeling of macrophages and neutrophils, but also other leukocytes, including monocytes, mast cells, and T lymphocytes. Two mCD200 receptor-like family members, designated mCD200RLa and mCD200RLb, were shown to pair with the activatory adaptor protein, DAP12, suggesting that these receptors would transmit strong activating signals in contrast to the apparent inhibitory signal delivered by triggering the CD200R. Despite substantial sequence homology with mCD200R, mCD200RLa and mCD200RLb did not bind mCD200, and presently have unknown ligands. The CD200 receptor gene family resembles the signal regulatory proteins and killer Ig-related receptors in having receptor family members with potential activatory and inhibitory functions that may play important roles in immune regulation and balance. Because manipulation of the CD200-CD200R interaction affects the outcome of rodent disease models, targeting of this pathway may have therapeutic utility.

T-cell receptor triggering is critically dependent on the dimensions of its peptide-MHC ligand

The binding of a T-cell antigen receptor (TCR) to peptide antigen presented by major histocompatibility antigens (pMHC) on antigen-presenting cells (APCs) is a central event in adaptive immune responses. The mechanism by which TCR–pMHC ligation initiates signalling, a process termed TCR triggering, remains controversial. It has been proposed that TCR triggering is promoted by segregation at the T cell–APC interface of cell-surface molecules with small ectodomains (such as TCR–pMHC and accessory receptors) from molecules with large ectodomains (such as the receptor protein tyrosine phosphatases CD45 and CD148). Here we show that increasing the dimensions of the TCR–pMHC interaction by elongating the pMHC ectodomain greatly reduces TCR triggering without affecting TCR–pMHC ligation. A similar dependence on receptor–ligand complex dimensions was observed with artificial TCR–ligand systems that span the same dimensions as the TCR–pMHC complex. Interfaces between T cells and APCs expressing elongated pMHC showed an increased intermembrane separation distance and less depletion of CD45. These results show the importance of the small size of the TCR–pMHC complex and support a role for size-based segregation of cell-surface molecules in TCR triggering.

The unusual distribution of the neuronal/lymphoid cell surface CD200 (OX2) glycoprotein is conserved in humans

OX2 (CD200) is a type‐1 membrane glycoprotein that contains two immunoglobulin superfamily domains and which is expressed on a variety of lymphoid and non‐lymphoid cells in the rat. The recent characterization of a receptor for OX2 (OX2R) on myeloid cells, and the phenotype of an OX2‐deficient mouse, suggests that OX2 may regulate myeloid cell activity in anatomically diverse locations. Here we report the tissue distribution of the human homologue of the rat OX2 glycoprotein using a new monoclonal antibody (mAb), OX104, raised against recombinant human OX2. Human OX2 was expressed at the cell surface of thymocytes, B cells, T cells, neurons, kidney glomeruli, tonsil follicles, the syncytiotrophoblast and endothelial cells. This broad, but not ubiquitous, distribution pattern is very similar to that observed in rats, suggesting that OX2 may regulate myeloid cell activity in a variety of tissues in humans.

CD47 is a ligand for rat macrophage membrane signal regulatory protein SIRP (OX41) and human SIRPα 1

The rat OX41 antigen is a cell surface protein containing three immunoglobulin superfamily domains and intracellular immunoreceptor tyrosine‐based inhibitory motifs (ITIM). It is a homologue of the human signal‐regulatory protein (SIRP) also known as SHPS‐1, BIT or MFR. Cell activation‐induced phosphorylation of the intracellular ITIM motifs induces association with the tyrosine phosphatases SHP‐1 and SHP‐2. To identify the physiological OX41 ligand, recombinant OX41‐CD4d3+4 fusion protein was coupled to fluorescent beads to produce a multivalent cell binding reagent. The OX41‐CD4d3+4 beads bound to thymocytes and concanavalin A‐stimulated splenocytes. This interaction was blocked by the monoclonal antibody (mAb) OX101. Affinity chromatography with OX101 mAb and peptide sequencing revealed the rat SIRP ligand to be CD47 (integrin‐associated protein). A direct interaction between human SIRP and human CD47 was demonstrated using purified recombinant proteins and surface plasmon resonance ruling out the involvement of other proteins known to be associated with CD47. The affinity of the SIRP/CD47 interaction was Kd ≈ 8 μM at 37°C with a koff ≥2.1 s–1. The membrane‐distal SIRP V‐like domain was sufficient for binding to CD47.

The structure of the human CD2 gene and its expression in transgenic mice.

We report the genomic organization of the human CD2 gene and its expression in transgenic mice. A 28.5 kb segment of DNA consisting of 4.5 kb 5′ flanking sequences, 15 kb containing the genes five exons and 9 kb of 3′ flanking sequences can direct the expression of the CD2 gene only on thymocytes, circulating T cells and megakaryocytes of the transgenic mice. The expression of each copy of the human CD2 transgene appears to be as high as the endogenous mouse CD2 gene and as high as the expression on the surface of human T lymphocytes, independent of the site of integration and dependent on the copy number of genes that have integrated.

Frontline: Optimal T cell activation requires the engagement of CD6 and CD166

The T cell surface glycoprotein, CD6 binds CD166 in the first example of an interaction between a scavenger receptor cysteine‐rich domain and an immunoglobulin‐like domain. We report that in human these proteins interact with a KD =0.4–1.0 μM and Koff ≥0.4–0.63 s–1, typical of many leukocyte membrane protein interactions. CD166 also interacts in a homophilic manner but with around 100‐fold lower affinity (KD =29–48 μM and Koff ≥ 5.3 s–1). At concentrations, that will block the CD6/CD166 interaction, soluble monomeric CD6 and CD166 inhibit antigen‐specific human T cell responses. This is consistent with extracellular engagement between CD6 and CD166 being required for an optimal immune response.

Human Lymphocytes Interact Directly with CD47 through a Novel Member of the Signal Regulatory Protein (SIRP) Family

Two closely related proteins, signal regulatory protein α (SIRPα; SHPS-1/CD172) and SIRPβ, have been described in humans. The existence of a third SIRP protein has been suggested by cDNA sequence only. We show that this third SIRP is a separate gene that is expressed as a protein with unique characteristics from both α and β genes and suggest that this gene should be termed SIRPγ. We have expressed the extracellular region of SIRPγ as a soluble protein and have shown that, like SIRPα, it binds CD47, but with a lower affinity (Kd, ∼23 μM) compared with SIRPα (Kd, ∼2 μM). mAbs specific to SIRPγ show that it was not expressed on myeloid cells, in contrast to SIRPα and -β, being expressed instead on the majority of T cells and a proportion of B cells. The short cytoplasmic tail of SIRPγ does not contain any known signaling motifs, nor does it contain a characteristic lysine, as with SIRPβ, that is required for DAP12 interaction. DAP12 coexpression is a requirement for SIRPβ surface expression, whereas SIRPγ is expressed in its absence. The SIRPγ-CD47 interaction may therefore not be capable of bidirectional signaling as with the SIRPα-CD47, but, instead, use unidirectional signaling via CD47 only.

Essential Roles for Dok2 and RasGAP in CD200 Receptor-Mediated Regulation of Human Myeloid Cells

The CD200 receptor (CD200R) acts as a negative regulator of myeloid cells by interacting with its widely expressed ligand CD200. Using mutants expressed in U937 cells, we show that inhibition is mediated by the PTB domain binding motif (NPLY) in the receptor’s cytoplasmic region. The adaptor protein downstream of tyrosine kinase 2 (Dok2) bound directly to the phosphorylated NPLY motif with a 10-fold higher affinity (KD of ∼1 μM at 37°C) than the closely related Dok1. Both of these proteins have been suggested to play a role in CD200R signaling in murine cells. Dok2 was phosphorylated in response to CD200R engagement and recruited RAS p21 protein activator 1 (RasGAP). Knockdown of Dok2 and RasGAP by RNA interference revealed that these proteins are required for CD200R signaling, while knockdown of Dok1 and the inositol 5-phosphatase SHIP did not affect CD200R-mediated inhibition. We conclude that CD200R inhibits the activation of human myeloid cells through direct recruitment of Dok2 and subsequent activation of RasGAP, which distinguishes this receptor from the majority of inhibitory receptors that utilize ITIMs and recruit phosphatases.