Purification and chemical characterisation of membrane glycoproteins from rat thymocytes and brain, recognised by monoclonal antibody MRC OX 2

Emerging Immune Checkpoint Molecules on Cancer Cells: CD24 and CD200

Cancer immunotherapy strategies are based on the utilization of immune checkpoint inhibitors to instigate an antitumor immune response. The efficacy of immune checkpoint blockade, directed at adaptive immune checkpoints, has been demonstrated in select cancer types. However, only a limited subset of patients has exhibited definitive outcomes characterized by a sustained response after discontinuation of therapy. Recent investigations have highlighted the significance of immune checkpoint molecules that are overexpressed in cancer cells and inhibit myeloid lineage immune cells within a tumor microenvironment. These checkpoints are identified as potential targets for anticancer immune responses. Notably, the immune checkpoint molecules CD24 and CD200 have garnered attention owing to their involvement in tumor immune evasion. CD24 and CD200 are overexpressed across diverse cancer types and serve as signaling checkpoints by engaging their respective receptors, Siglec-10 and CD200 receptor, which are expressed on tumor-associated myeloid cells. In this review, we summarized and discussed the latest advancements and insights into CD24 and CD200 as emergent immune checkpoint moieties, further delving into their therapeutic potentials for cancer treatment.

Cancel cancer: The immunotherapeutic potential of CD200/CD200R blockade

Immune checkpoint molecules function to inhibit and regulate immune response pathways to prevent hyperactive immune activity from damaging healthy tissues. In cancer patients, targeting these key molecules may serve as a valuable therapeutic mechanism to bolster immune function and restore the body's natural defenses against tumors. CD200, an immune checkpoint molecule, is a surface glycoprotein that is widely but not ubiquitously expressed throughout the body. By interacting with its inhibitory receptor CD200R, CD200 suppresses immune cell activity within the tumor microenvironment, creating conditions that foster tumor growth. Targeting the CD200/CD200R pathway, either through the use of monoclonal antibodies or peptide inhibitors, has shown to be effective in boosting anti-tumor immune activity. This review will explore CD200 and the protein's expression and role within the tumor microenvironment, blood endothelial cells, and lymph nodes. This paper will also discuss the advantages and challenges of current strategies used to target CD200 and briefly summarize relevant preclinical/clinical studies investigating the immunotherapeutic efficacy of CD200/CD200R blockade.

Proteomic analysis of a murine model of lung hypoplasia induced by oligohydramnios

Severe oligohydramnios (OH) due to prolonged loss of amniotic fluid can cause pulmonary hypoplasia. Animal model of pulmonary hypoplasia induced by amniotic fluid drainage is partly attributed to changes in mechanical compression of the lung. Although numerous studies on OH-model have demonstrated changes in several individual proteins, however, the underlying mechanisms for interrupting normal lung development in response to a decrease of amniotic fluid volume are not fully understood. In this study, we used a proteomic approach to explore differences in the expression of a wide range of proteins after induction of OH in a mouse model of pulmonary hypoplasia to find out the signaling/molecular pathways involved in fetal lung development. Liquid chromatography-massspectromery/mass spectrometry analysis found 474 proteins that were differentially expressed in OH-induced hypoplastic lungs in comparison to untouched (UnT) control. Among these proteins, we confirmed the downregulation of AKT1, SP-D, and CD200, and provided proof-of-concept for the first time about the potential role that these proteins could play in fetal lung development.

CD200 and CD200R Expression on Peripheral Blood Lymphocytes and Serum CD200 Concentration as a New Marker of Endometriosis

The causes of endometriosis (EMS) remain unknown; however, a number of immunological abnormalities contribute to the pathogenesis of the disease. The cluster of differentiation-200 (CD200) and its receptor (CD200R) maintain peripheral self-tolerance by negatively regulating immune responses. In this comparative cross-sectional study, we investigated the expression of CD200 and CD200R on T and B lymphocytes and the serum level of soluble CD200 (sCD200) using flow cytometry and ELISA, respectively. Peripheral blood samples were collected from 54 female patients and 20 healthy, age-matched controls. Results were tested for correlation with disease severity and selected clinical parameters. We demonstrated that the differences in sCD200 levels (p = 0.001), the frequencies of CD200-positive T and B lymphocytes (p < 0.001 and p = 0.004, respectively), and the frequencies of CD200R-positive T and B lymphocytes (p < 0.001 for all comparisons) in the study group correlated positively with disease severity. Receiver operating characteristic (ROC) analysis indicated that aberrant expression of CD200/CD200R might serve as a marker to distinguish between EMS cases. Finally, negative co-stimulatory factors may contribute to the induction and persistence of inflammation associated with EMS. It seems that it is essential to determine whether alteration in the CD200/CD200R pathway can be therapeutically targeted in EMS.

The Potential Role of Dysfunctions in Neuron-Microglia Communication in the Pathogenesis of Brain Disorders

The bidirectional communication between neurons and microglia is fundamental for the proper functioning of the central nervous system (CNS). Chemokines and clusters of differentiation (CD) along with their receptors represent ligand-receptor signalling that is uniquely important for neuron - microglia communication. Among these molecules, CX3CL1 (fractalkine) and CD200 (OX-2 membrane glycoprotein) come to the fore because of their cell-type-specific localization. They are principally expressed by neurons when their receptors, CX3CR1 and CD200R, respectively, are predominantly present on the microglia, resulting in the specific axis which maintains the CNS homeostasis. Disruptions to this balance are suggested as contributors or even the basis for many neurological diseases. In this review, we discuss the roles of CX3CL1, CD200 and their receptors in both physiological and pathological processes within the CNS. We want to underline the critical involvement of these molecules in controlling neuron - microglia communication, noting that dysfunctions in their interactions constitute a key factor in severe neurological diseases, such as schizophrenia, depression and neurodegeneration-based conditions.

Interaction of CD200 Overexpression on Tumor Cells with CD200R1 Overexpression on Stromal Cells: An Escape from the Host Immune Response in Rectal Cancer Patients

CD200 imparts an immunoregulatory signal through its receptor, CD200R1, leading to the suppression of tumor specific immunity. The mechanism of CD200:CD200R1 signaling pathway is still uncertain. Our aim was to investigate the expression and localization of CD200 and its receptor CD200R1 and their clinical significance in rectal cancer patients. We examined the immunohistochemical expressions and localizations of CD200 and CD200R1 in 140 rectal cancer patients. Among the patients, 79 underwent the preoperative radiotherapy and the others were untreated prior to the surgery. In addition, 121 matched normal rectal mucosa samples were evaluated. The results of immunohistochemical analysis showed a strikingly high level of CD200 in tumor cells (p=0.001) and CD200R1 expression in normal mucosal epithelium and stromal cells. Importantly, CD200R1 was overexpressed in stromal cells of the metastatic cancer patients compared to patients without metastases (p=0.002). More than that, 87% of metastatic patients had a phenotype of upregulated CD200 in tumor cells accompanied by overexpressed CD200R1 in stromal cells. In addition, low levels of CD200 were correlated with improved overall survival in untreated patients. We showed that tumor-stroma communication through CD200 and its receptor interaction is selected in patients with high risk of relapse. High levels of these molecules support instigation of the far and local metastatic nest that provides solid ground for metastasis. Our current data also disclose a mechanism by which CD200:CD200R1 affects tumor progression and may strengthen the feasibility of targeting CD200 or CD200R1 as anticancer strategy.

Role of the CD200-CD200R Axis During Homeostasis and Neuroinflammation

Microglia are considered to be the resident macrophages of the CNS and main effector of immune brain function. Due to their essential role in the regulation of neuroinflammatory response, microglia constitute an important target for neurological diseases, such as multiple sclerosis, Alzheimer's or Parkinson's disease. The communication between neurons and microglia contributes to a proper maintenance of homeostasis in the CNS. Research developed in the last decade has demonstrated that this interaction is mediated by "Off-signals" - molecules exerting immune inhibition - and "On signals" - molecules triggering immune activation. Among "Off signals", molecular pair CD200 and its CD200R receptor, expressed mainly in the membrane of neurons and microglia, respectively, have centered our attention due to its unexplored and powerful immunoregulatory functions. In this review, we will offer an updated global view of the CD200-CD200R role in the microglia-neuron crosstalk during homeostasis and neuroinflammation. Specifically, the effects of CD200-CD200R in the inhibition of pro-inflammatory microglial activation will be explained, and their involvement in other functions such as homeostasis preservation, tissue repair, and brain aging, among others, will be pointed out. In addition, we will depict the effects of CD200-CD200R uncoupling in the etiopathogenesis of autoimmune and neurodegenerative diseases. Finally, we will explore how to translate the scientific evidence of CD200-CD200R interaction into possible clinical therapeutic strategies to tackle neuroinflammatory CNS diseases.

The Neuro-Immune-Regulators (NIREGs) Promote Tissue Resilience; a Vital Component of the Host's Defense Strategy against Neuroinflammation

An effective protective inflammatory response in the brain is crucial for the clearance of pathogens (e.g. microbes, amyloid fibrils, prion^SC) and should be closely regulated. However, the CNS seems to have limited tissue resilience to withstand the detrimental effects of uncontrolled inflammation compromising functional recovery and tissue repair. Newly described neuro-immune-regulators (NIREGs) are functionally related proteins regulating the severity and duration of the host inflammatory response. NIREGs such as CD200, CD47 and CX3CL1 are vital for increasing tissue resilience and are constitutively expressed by neurons. The interaction with co-receptors (CD200R, CD172a, CX3CR1) will maintain microglia in the resting phenotype, directing aggressive microglia phenotype and limiting bystander injuries. Neurons can also express many of the complement NIREGs (CD55, CD46, CD59 and factor H). Neurons and glia also express suppressor of cytokine signaling proteins (SOCS) down regulating janus kinase-signal transducer and activator of transcription (JAK/STAT) pathway and to lead to the polarization of microglia towards anti-inflammatory phenotype. Other NIREGs such as serine protease inhibitors (serpins) and thrombomodulin (CD141) inhibit neurotoxic systemic coagulation proteins such as thrombin. The unfolded protein response (UPR) detects misfolded proteins and other stressors to prevent irreversible cell injury. Microglial pattern recognition receptors (PRR) (TREM-2, CR3, FcγR) are important to clear apoptotic cells and cellular debris but in non-phlogystic manner through inhibitory signaling pathways. The TYRO3, Axl, Mer (TAM) tyrosine receptor kinases activated by Gas 6 and PROS1 regulate inflammation by inhibiting Toll like receptors (TLR) /JAK-STAT activation and contribute to NIREG's functions.

Association of peripheral blood dendritic cells with recurrent pregnancy loss: a case-controlled study

PROBLEM

Dendritic cells (DCs) have been reported to play an important role in pregnancy. However, the role of DCs in recurrent pregnancy loss (RPL) has not been investigated well.

METHOD OF STUDY

Forty-three women affected by RPL and 16 fertile controls were recruited from June 2013 to December 2014. The peripheral blood DCs subsets, including myeloid DCs (mDCs) and plasmacytoid DCs (pDCs), the levels (%) of CD80(+) , CD86(+) , and CD200(+) DCs were analyzed using flow cytometry.

RESULTS

The levels of total DCs, mDCs, and CD86(+) DCs were significantly higher (all P<.05); however, the level of CD200(+) DCs in the RPL group was significantly lower than that of the control group (P<.05). The logistical regression analyses showed that the elevated level of mDCs was significantly associated with RPL after adjustment for age (OR: 1.14, 95% CI, 1.01-1.29, P<.05).

CONCLUSION

The elevated level of mDCs was significantly associated with RPL, which might lead to the intervention of targeted immunosuppression in women with RPL.

CD200 Inhibits Inflammatory Response by Promoting KATP Channel Opening in Microglia Cells in Parkinson's Disease

Background

As the second most common neurodegenerative disorder after Alzheimer’s disease (AD), Parkinson’s disease (PD) principally impacts the motor system in approximately 7 million patients worldwide. The present study aimed to explore the effects of cluster of differentiation (CD200) on adenosine triphosphate-sensitive potassium (KATP) channels and inflammatory response in PD mice.

Material/Methods

We created an in vivo PD model by intraperitoneal injection of 30 mg/kg/day 1-Methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine hydrochloride (MPTP. HCL) for 5 consecutive days, and we created an in vitro PD model by injection of 100 μM 1-methyl-4-phenylpyridinium ion (MPP+) in primary microglia cells. Expression level of CD200/CD200R, inwardly rectifying potassium (Kir6.1/6.2), and sulfonylurea receptor (Sur1/2) were detected by Western blot (WB). Immunohistochemistry (IHC) was utilized to assess CD11b (microglia marker) and tyrosine hydroxylase (TH, a marker reveals dopamine level in neurons) expression levels. An in vitro PD model was applied to detect the influence of CD200 on ATP and inflammatory factors released from microglia. Interferon (IFN)-γ, tumor necrosis factor (TNF)-α, and interleukin (IL)-1β mRNA levels were explored by realtime quantitative polymerase chain reaction (RT-QPCR), and their protein levels were identified by enzyme-linked immunosorbent assay (ELISA).

Results

WB exhibited time-dependent down-regulation of CD200/CD200R in cerebra of PD mice compared to control mice, with Kir 6.1 and SUR 2 expressed mainly in microglia. IHC showed that CD11b reached a peak at the 1^st day after MPTP treatment, followed by time-dependent reduction, and TH decreased noticeably after MPTP induction. RT-QPCR demonstrated that compared with controls, IFN-γ, TNF-α, and IL-1β mRNA levels were significantly elevated at MPTP-1d, was reduced at MPTP-3d, and then returned to baseline at MPTP-7d. IHC showed that MPP+ significantly elevated microglia release of ATP. Similar to the effect of pinacidil (K+ channel opener), CD200 remarkably depressed MPP+-induced ATP release. ELISA showed that MPP+ significantly increased IFN-γ, TNF-α, and IL-1β release, and CD200 and pinacidil remarkably suppressed this elevation.

Conclusions

Our results show a novel role of CD200 in promoting opening of the KATP channel, inhibiting microglia activation and release of ATP, as well as inflammatory factors, thus protecting dopaminergic (DA) neurons against damage and alleviating PD.

CD200/CD200R Paired Potent Inhibitory Molecules Regulating Immune and Inflammatory Responses; part I: CD200/CD200R Structure, Activation, and Function

CD200/CD200R are highly conserved type I paired membrane glycoproteins that belong to the Ig superfamily containing a two immunoglobulin‑like domain (V, C). CD200 is broadly distributed in a variety of cell types, whereas CD200R is primarily expressed in myeloid and lymphoid cells. They fulfill multiple functions in regulating inflammation. The interaction between CD200/CD200R results in activation of the intracellular inhibitory pathway with RasGAP recruitment and thus contributes to effector cell inhibition. It was confirmed that the CD200R activation stimulates the differentiation of T cells to the Treg subset, upregulates indoleamine 2,3‑dioxygenase activity, modulates cytokine environment from a Th1 to a Th2 pattern, and facilitates an antiinflammatory IL‑10 and TGF‑β synthesis. CD200/CD200R are required for maintaining self‑tolerance. Many studies have demonstrated the importance of CD200 in controlling autoimmunity, inflammation, the development and spread of cancer, hypersensitivity, and spontaneous fetal loss.

Brain innate immunity in the regulation of neuroinflammation: therapeutic strategies by modulating CD200-CD200R interaction involve the cannabinoid system

The central nervous system (CNS) innate immune response includes an arsenal of molecules and receptors expressed by professional phagocytes, glial cells and neurons that is involved in host defence and clearance of toxic and dangerous cell debris. However, any uncontrolled innate immune responses within the CNS are widely recognized as playing a major role in the development of autoimmune disorders and neurodegeneration, with multiple sclerosis (MS) Alzheimer's disease (AD) being primary examples. Hence, it is important to identify the key regulatory mechanisms involved in the control of CNS innate immunity and which could be harnessed to explore novel therapeutic avenues. Neuroimmune regulatory proteins (NIReg) such as CD95L, CD200, CD47, sialic acid, complement regulatory proteins (CD55, CD46, fH, C3a), HMGB1, may control the adverse immune responses in health and diseases. In the absence of these regulators, when neurons die by apoptosis, become infected or damaged, microglia and infiltrating immune cells are free to cause injury as well as an adverse inflammatory response in acute and chronic settings. We will herein provide new emphasis on the role of the pair CD200-CD200R in MS and its experimental models: experimental autoimmune encephalomyelitis (EAE) and Theiler’s virus induced demyelinating disease (TMEV-IDD). The interest of the cannabinoid system as inhibitor of inflammation prompt us to introduce our findings about the role of endocannabinoids (eCBs) in promoting CD200-CD200 receptor (CD200R) interaction and the benefits caused in TMEV-IDD. Finally, we also review the current data on CD200-CD200R interaction in AD, as well as, in the aging brain.

CD200+ and CD200- macrophages accumulated in ischemic lesions of rat brain: the two populations cannot be classified as either M1 or M2 macrophages

Two types of macrophages in lesion core of rat stroke model were identified according to NG2 chondroitin sulfate proteoglycan (NG2) and CD200 expression. NG2(+) macrophages were CD200(-), and vice versa. NG2(-) macrophages expressed two splice variants of CD200 that are CD200L and CD200S. CD200(+) macrophages expressed CD8, CD68, CD163, CCL2, inducible nitric oxide synthase, interleukin-1β, Toll-like receptor 4 and transforming growth factor β, whilst NG2(+) cells expressed a costimulatory factor CD86. Both cell types expressed insulin-like growth factor 1 and CD200R. These results demonstrate that the two macrophage types cannot be classified as either M1 or M2.

Modification of biomaterials with a self-protein inhibits the macrophage response

A biomaterial inhibits the host immune response by displaying an endo-genously expressed immunomodulatory molecule, CD200. Immobilization of CD200 onto biomaterial surfaces effectively suppresses macrophage activation and reduces inflammatory response to subcutaneously implanted materials.

CD200R signaling inhibits pro-angiogenic gene expression by macrophages and suppresses choroidal neovascularization

Macrophages are rapidly conditioned by cognate and soluble signals to acquire phenotypes that deliver specific functions during inflammation, wound healing and angiogenesis. Whether inhibitory CD200R signaling regulates pro-angiogenic macrophage phenotypes with the potential to suppress ocular neovascularization is unknown. CD200R-deficient bone marrow derived macrophages (BMMΦ) were used to demonstrate that macrophages lacking this inhibitory receptor exhibit enhanced levels of Vegfa, Arg-1 and Il-1β when stimulated with PGE₂ or RPE-conditioned (PGE₂-enriched) media. Endothelial tube formation in HUVECs was increased when co-cultured with PGE₂-conditioned CD200R^−/− BMMΦ, and laser-induced choroidal neovascularization was enhanced in CD200R-deficient mice. In corroboration, signaling through CD200R results in the down-regulation of BMMΦ angiogenic and pro-inflammatory phenotypes. Translational potential of this pathway was investigated in the laser-induced model of choroidal neovascularization. Local delivery of a CD200R agonist mAb to target myeloid infiltrate alters macrophage phenotype and inhibits pro-angiogenic gene expression, which suppresses pathological angiogenesis and CNV development.

Understanding the neurobiology of CD200 and the CD200 receptor: a therapeutic target for controlling inflammation in human brains?

CD200 and its receptor, CD200 receptor (CD200R), have uniaue roles in controlling damaging inflammatory processes. At present, the only identified function for CD200 is as a ligand for CD200R. These proteins interact resulting in the activation of anti-inflammatory signaling by CD200R-expressing cells. When this interaction becomes deficient with aging or disease, chronic inflammation occurs, Experimental animal studies have demonstrated the consequences of disrupting CD200-CD200R interactions in the brain, but there have been few studies in human brains. Deficiency in neuronal CD200 may explain the chronic inflammation in human neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and multiple sclerosis; however, deficits in the microglial expression of CD200R may also be of functional significance. The purpose of this review is to assess the data regarding the role of CD200-CD200R interactions in relation to the brain in order to determine if this could be a therapeutic target for human brain diseases with inflammatory components, and what additional studies are needed.

Structures of CD200/CD200 receptor family and implications for topology, regulation, and evolution

CD200 is a widely distributed membrane glycoprotein that regulates myeloid cell activity through its interaction with an inhibitory receptor (CD200R). The interaction is of interest as a target for treating excessive inflammation and for treating leukemia. There are closely related proteins to CD200R that give activating signals making this a “paired receptor.” We report X-ray crystallography structures for the inhibitory CD200R, the activating receptor CD200RLa, and a complex between CD200R and CD200. Both CD200 and CD200R contain two Ig-like domains and interact through their NH₂ terminal domains compatible with immunological synapse-like interactions occurring between myeloid cells and other CD200-expressing cells. The failure of the activating receptor to bind CD200 resides in subtle changes around the interface. CD200 has been acquired by herpes viruses to mimic the host interaction. CD200R has evolved rapidly presumably driven by pathogen pressure but it may also be important in homeostasis through interactions with commensal bacteria.

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Structure of CD200R and in complex with its ligand CD200 complex

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Structure of an activating member of the CD200 receptor family

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Mutagenesis showing specificity of CD200 paired receptor family

First molecular cloning and gene expression analysis of a teleost CD200 (OX-2) glycoprotein from rock bream, Oplegnathus fasciatus

CD200 plays an important role in delivering an immunoregulatory signal to the immune system through interaction with its receptor. However, CD200 has not been characterized and its function in teleosts is unknown. In this study, the rock bream (Oplegnathus fasciatus) CD200 gene (RbCD200) was cloned and its expression profile was analyzed after infection with Edwardsiella tarda, Streptococcus iniae or red seabream iridovirus (RSIV). The coding region of RbCD200 cDNA was 855 bp, encoding 284 amino acid residues. The gene consisted of two extracellular Ig-like domains and a transmembrane domain. RbCD200 was highly expressed in the brain, erythrocytes, intestine and stomach of healthy rock bream. In the spleen, RbCD200 gene expression was down-regulated until 48 h after E. tarda exposure, except at 12 h RbCD200 gene expression was down-regulated then up-regulated at 12 h and 24 h after infection with S. iniae and RSIV, respectively. In the whole kidney, the RbCD200 gene was down-regulated in response to infection with E. tarda and S. iniae. However, RSIV infection increased RbCD200 gene expression in whole kidney until 48 h. These results suggest that RbCD200 is differentially expressed in the spleen and whole kidney after infection with different pathogens.

CD200:CD200R-Mediated Regulation of Immunity

The type 1 membrane glycoprotein CD200, widely expressed on multiple cells/tissues, uses a structurally similar receptor (CD200R1), whose expression is more restricted to cells of the myeloid and lymphoid lineages, to transmit signals affecting responses in multiple physiological systems. Thus CD200 expression is reported to exert effects on cancer growth, autoimmune and allergic disorders, infection, transplantation, bone development and homeostasis, and reproductive biology. It was initially thought, based on the idea that CD200R1 was mostly expressed on cells of myeloid origin, that CD200:CD200R1 interactions were primarily dedicated to controlling myeloid cell function. However additional members of the CD200R family have now also been identified, although their function(s) remain unclear, and CD200R1 itself is now known to be expressed by subsets of T cells and other cells. Together these observations add layers of complexity to our understanding of CD200-related regulation. In common with a number of physiological systems, the mechanism(s) of CD200-induced signaling seem to fit within a similar framework of opposing actions of kinases and phosphatases. This paper highlights the advances in our knowledge of immunoregulation achieved following CD200:CD200R interaction and the potential clinical applicability of that information.

Aberrant CD200/CD200R1 expression and function in systemic lupus erythematosus contributes to abnormal T-cell responsiveness and dendritic cell activity

INTRODUCTION

CD200 is a type I transmembrane glycoprotein that can regulate the activation threshold of inflammatory immune responses, polarize cytokine production, and maintain immune homeostasis. We therefore evaluated the functional status of CD200/CD200 receptor 1 (CD200R1) interactions in subjects with systemic lupus erythematosus (SLE).

METHODS

Serum CD200 level was detected by ELISA. The expression of CD200/CD200R1 by CD4+ T cells and dendritic cells (DCs) was examined by flow cytometry, and then compared between SLE patients and healthy controls. Peripheral blood mononuclear cells were stained with carboxyfluorescein diacetate succinimidyl ester and annexin V/propidium iodide for evaluation of the effect of CD200 on cell proliferation and apoptosis. In addition, the effect of CD200 on DC function was determined by transwell migration assay as well as by measurement of binding and phagocytosis of apoptotic cells.

RESULTS

In SLE patients, the number of CD200+ cells and the level of soluble CD200 were significantly higher than in healthy controls, whereas the expression of CD200R1 by CD4+ T cells and DCs was decreased. Furthermore, the increased CD200 expression by early apoptotic cells contributed to their diminished binding and phagocytosis by DCs in SLE. Importantly, the engagement of CD200 receptor on CD4+ T cells with CD200-Fc fusion protein in vitro reduced the differentiation of T-helper type 17 cells and reversed the defective induction of CD4+CD25highFoxP3+ T cells by transforming growth factor beta in SLE patients. Conversely, blockade of CD200-CD200R1 interaction with anti-CD200R1 antibody promoted CD4+ T-cell proliferation.

CONCLUSION

CD200 and CD200R1 expression and function are abnormal in SLE and may contribute to the immunologic abnormalities in SLE.

CD200-CD200R dysfunction exacerbates microglial activation and dopaminergic neurodegeneration in a rat model of Parkinson's disease

BACKGROUND

Increasing evidence suggests that microglial activation may participate in the aetiology and pathogenesis of Parkinson's disease (PD). CD200-CD200R signalling has been shown to be critical for restraining microglial activation. We have previously shown that expression of CD200R in monocyte-derived macrophages, induced by various stimuli, is impaired in PD patients, implying an intrinsic abnormality of CD200-CD200R signalling in PD brain. Thus, further in vivo evidence is needed to elucidate the role of malfunction of CD200-CD200R signalling in the pathogenesis of PD.

METHODS

6-hydroxydopamine (6-OHDA)-lesioned rats were used as an animal model of PD. CD200R-blocking antibody (BAb) was injected into striatum to block the engagement of CD200 and CD200R. The animals were divided into three groups, which were treated with 6-OHDA/Veh (PBS), 6-OHDA/CAb (isotype control antibody) or 6-OHDA/BAb, respectively. Rotational tests and immunohistochemistry were employed to evaluate motor deficits and dopaminergic neurodegeneration in animals from each group. HPLC analysis was used to measure monoamine levels in striatum. Morphological analysis and quantification of CD11b- (or MHC II-) immunoreactive cells were performed to investigate microglial activation and possible neuroinflammation in the substantia nigra (SN). Finally, ELISA was employed to assay protein levels of proinflammatory cytokines.

RESULTS

Compared with 6-OHDA/CAb or 6-OHDA/Veh groups, rats treated with 6-OHDA/BAb showed a significant increase in counts of contralateral rotation and a significant decrease in TH-immunoreactive (TH-ir) neurons in SN. A marked decrease in monoamine levels was also detected in 6-OHDA/BAb-treated rats, in comparison to 6-OHDA/Veh-treated ones. Furthermore, remarkably increased activation of microglia as well as up-regulation of proinflammatory cytokines was found concomitant with dopaminergic neurodegeneration in 6-OHDA/BAb-treated rats.

CONCLUSIONS

This study shows that deficits in the CD200-CD200R system exacerbate microglial activation and dopaminergic neurodegeneration in a 6-OHDA-induced rat model of PD. Our results suggest that dysfunction of CD200-CD200R signalling may be involved in the aetiopathogenesis of PD.

The innate immune rheostat: influence on lung inflammatory disease and secondary bacterial pneumonia

The activity of innate immunity is not simply dictated by the presence of an antigen but also by the balance between negative regulatory and immune potentiator pathways. Even in the absence of antigen, innate immunity can 'inflame' if negative regulators are absent. This resting state is adaptable and dictated by environmental influences, host genetics and past infection history. A return to homoeostasis post inflammation may therefore not leave the tissue in an identical state to that prior to the inflammatory event. This adaptability makes us all unique and also explains the variable outcome experienced by a diverse population to the same inflammatory stimulus. Using murine models we have identified that influenza virus causes a long-term modification of the lung microenvironment by a de-sensitization to bacterial products and an increase in the myeloid negative regulator CD200R (CD200 receptor). These two events prevent subsequent inflammatory damage while the lung is healing, but also they may predispose to bacterial colonization of the lower respiratory tract should regulatory mechanisms overshoot. In the extreme, this leads to bacterial pneumonia, sepsis and death. A deeper understanding of the consequences arising from innate immune cell alteration during influenza infection and the subsequent development of bacterial complications has important implications for future drug development.

Blocking CD200-CD200 receptor axis augments NOS-2 expression and aggravates experimental autoimmune uveoretinitis in Lewis rats

PURPOSE

Tissue expression of CD200 generates inhibitory or down-regulatory signals to macrophages and microglia within CNS and retina. Our interests were to investigate whether blocking CD200 receptor (CD200R) signalling in vivo results in macrophage activation and thus aggravation of EAU in Lewis rats.

METHODS

Retinal extract-immunised Lewis rats were treated day 3 and 5 post-immunisation with CD200R mAb or normal mouse serum. Phenotypic analysis of the leucocyte infiltrate and disease severity was clinically and histogically examined.

RESULTS

Rats administered with CD200R mAb developed earlier onset of EAU and more severe disease. Blocking CD200R increased retinal neuronal CD200 expression and, although disease severity was increased concomitant with an increase in NOS-2 expression, retinal macrophage numbers were not increased.

CONCLUSIONS

Contemporaneous with data from CD200KO mice, these results support the notion that CD200-CD200R signalling suppresses macrophage activation. Exacerbation of EAU in both CD200KO mice and Lewis rats when CD200-CD200R interaction is blocked suggests that this could be an avenue for therapeutic intervention.

Endothelial CD200 is heterogeneously distributed, regulated and involved in immune cell-endothelium interactions

CD200 is a highly glycosylated cell surface protein containing two immunoglobulin superfamily domains in the extracellular region and performs immunosuppressive activities. It is widely distributed in various tissues including the vascular endothelium. We report here the distribution of CD200 in rat endothelia from different vascular beds. Endothelial CD200 immunoreactivity was weakly expressed in most arteries but was intensely expressed in the arterioles, most veins and venules, as well as continuous and fenestrated capillaries. The distribution of CD200 in the sinusoidal and lymphatic endothelia was variable. Immunoelectron microscopic studies revealed that endothelial CD200 varied considerably not only in different microvasculatures but also in the membrane domains at the subcellular level. Endothelial CD200 expression was differentially regulated by lipopolysaccharide in cell types both in vivo and in vitro. Functional assessments of endothelial CD200 suggested that the physical binding between CD200 and CD200 receptor (CD200R) was involved in T-cell adhesion to the endothelium but not in macrophage-endothelium interaction. In the latter, however, CD200 agonist, a synthetic peptide from complementarity-determining region 3 of mouse CD200, may trigger CD200R signaling in macrophages to suppress their adhesion to the endothelium. Our findings demonstrate that the distribution, subcellular localization, and lipopolysaccharide-regulation of endothelial CD200 are heterogeneous, and provide evidence elucidating the functional roles of endothelial CD200 during tissue inflammation.

Immune homeostasis in the respiratory tract and its impact on heterologous infection

Innate immunity at mucosal surfaces requires additional restraint to prevent inflammation to innocuous antigens or commensal microorganisms. The threshold above which airway macrophages become activated is raised by site-specific factors including the receptors for transforming growth factor beta, interleukin 10 and CD200; the ligands for which are produced by, or expressed on, respiratory epithelium. We discuss such site-specific regulation and how this is continually altered by prior infections. Resetting of innate reactivity represents a strategy for limiting excessive inflammation, but in some may pre-dispose to secondary bacterial pneumonia.

A critical function for CD200 in lung immune homeostasis and the severity of influenza infection

The lung must maintain a high threshold of immune 'ignorance' to innocuous antigens to avoid inflammatory disease that depends on the balance of positive inflammatory signals and repressor pathways. We demonstrate here that airway macrophages had higher expression of the negative regulator CD200 receptor (CD200R) than did their systemic counterparts. Lung macrophages were restrained by CD200 expressed on airway epithelium. Mice lacking CD200 had more macrophage activity and enhanced sensitivity to influenza infection, which led to delayed resolution of inflammation and, ultimately, death. The administration of agonists that bind CD200R, however, prevented inflammatory lung disease. Thus, CD200R is critical for lung macrophage immune homeostasis in the resting state and limits inflammatory amplitude and duration during pulmonary influenza infection.

Rationale for anti-CD200 immunotherapy in B-CLL and other hematologic malignancies: new concepts in blocking immune suppression

Immune evasion in cancer is increasingly recognized as a contributing factor in the failure of a natural host antitumor immune response as well as in the failure of cancer vaccine trials. Immune evasion may be the result of a number of factors, including expansion of regulatory T cells, production of immunosuppressive cytokines, downregulation of HLA class I and tumor-associated antigens and upregulation of immunosuppressive molecules on the surface of tumor cells. CD200, a cell surface ligand that plays a role in regulating the immune system, has been shown to be upregulated on the surface of some hematologic and solid tumor malignancies. This review characterizes the role of CD200 in immune suppression, and describes strategies to target this molecule in the oncology setting, thus directly modulating immune regulation and potentially altering tolerance to tumor antigens.

Expression of CD200 by macrophage-like cells in ischemic core of rat brain after transient middle cerebral artery occlusion

Brain ischemia causes the death of neurons and glial cells. Such brain cells are believed to inevitably undergo degeneration in the core of ischemic lesions, whereas neurons and glial cells may survive in the region surrounding the core that is often referred to as the ischemic penumbra. However, many cells, particularly immune cells infiltrate and survive in the core. In this study, we characterized macrophage-like cells that accumulated in the ischemic core of a rat brain whose right middle cerebral artery was transiently occluded for 90 min. At 7 days post-reperfusion, we observed macrophage-like cells expressing CD200, a cell surface glycoprotein belonging to an immunoglobulin superfamily and that elicits suppressive effects on myeloid cells including microglia by interacting with the CD200 receptor (CD200R). RT-PCR and immunoblot analyses revealed the presence of CD200-mRNA and protein in the ischemic core as well as in the contralateral region. As revealed by immunohistochemistry, CD200 is located on the cell membrane of spherical Iba1(+) cells with many cytoplasmic granules. CD200(-)/Iba1(+) macrophage-like cells were also present, which have a more irregular shape than CD200(+)/Iba1(+) cells. CD200 was detected in isolated spherical Iba1(+) macrophage-like cells. Thus, CD200 is expressed in some populations of macrophage-like cells that may be responsible for the suppression of CD200R(+) myeloid cell functions in the ischemic core.

Regulation of microglial cell responses in murine Toxoplasma encephalitis by CD200/CD200 receptor interaction

Under autoimmune inflammatory conditions within the brain, evidence suggests that neurons downregulate microglial activation through CD200/CD200R interaction, which reduces disease severity. To gain insight into the regulation of intracerebral immune reactions by resident brain cells in chronic cerebral infections, the expression of the CD200 antigen and the CD200R as well as the functional role of CD200/CD200R interactions were characterized in murine Toxoplasma encephalitis. In the normal brain of C57BL/6 wild type mice, CD200 was ubiquitously expressed on neurons, their axons, cerebral endothelial cells, and plexus macrophages. CD200R was expressed at very low levels on cerebral macrophages and microglia without differences between CD200-/- and wild type mice. Infection of C57BL/6 mice with Toxoplasma gondii induced an upregulation of CD200R on microglia and of CD200 on blood vessel endothelial cells. In Toxoplasma encephalitis of CD200-/- mice, microglial cell numbers strongly increased due to an enhanced proliferation indicated by increased Ki-67 immunoreactivity. In addition, microglial activation was increased in CD200-/- mice as evidenced by a further upregulation of already high MHC class II levels as well as an increased expression of the anti-parasitic effector molecules, TNF and iNOS. The increased microglial cell activation resulted in a reduced intracerebral parasite burden and an increased survival rate. Thus, in Toxoplasma encephalitis, microglial activity was regulated via CD200/CD200R-mediated interaction further pointing to an intrinsic regulation of brain resident cells under inflammatory CNS conditions.

Is the CD200/CD200 receptor interaction more than just a myeloid cell inhibitory signal?

The membrane glycoprotein CD200, which has a widespread but defined distribution and a structurally similar receptor (CD200R) that transmits an inhibitory signal to cells of the hematopoetic lineage, especially myeloid cells, has been characterized. CD200R expression is restricted predominantly to cells of the myeloid lineage indicating that this ligand/receptor pair has a specific role in controlling myeloid cell function. In addition to CD200R, several related genes have been identified. Whether these gene products also regulate immune function is controversial. CD200R is also expressed by certain subsets of T cells and CD200 may be expressed by antigen-presenting cells, adding additional layers of complexity to the CD200/CD200R axis. Because monocytic myeloid cells provide a link between the innate and adaptive immune response, mechanisms to control their function through receptors such as CD200R will have therapeutic potential. Regulation of immune responses is accomplished by the concerted, but opposing, activity of kinases and phosphatases, fine control often being achieved through paired receptors. In this review, we will consider whether CD200R signaling functions within a framework of paired activating and inhibitory receptors and whether the inhibitory signal delivered has functional consequences beyond inhibition of myeloid cell proinflammatory activation.

The English strain of rat cytomegalovirus (CMV) contains a novel captured CD200 (vOX2) gene and a spliced CC chemokine upstream from the major immediate-early region: further evidence for a separate evolutionary lineage from that of rat CMV Maastricht

Sequence data for eight genes, together with time-course Northern blotting and 3′- and 5′-RACE (rapid amplification of cDNA ends) analysis for some mRNAs from a 12 kb region upstream from the major immediate-early (MIE) genes of the English isolate of rat cytomegalovirus (RCMV), are presented. The results identified important differences compared to both murine cytomegalovirus (MCMV) and the Maastricht isolate of RCMV. A striking finding is the presence of a highly conserved, rightwards-oriented homologue of the rat cellular CD200 (OX2) gene immediately to the right of the MIE region, which replaces either the leftwards-oriented AAV REP gene of RCMV (Maastricht) or the upstream spliced portions of the immediate-early 2 gene (ie2) in MCMV. From the presence of other homologues of MCMV- and RCMV-specific genes, such as the β-chemokine MCK-2, SGG1 and an Fcγ receptor gene, as reported here, the basic architecture of the MIE region (reported previously) and the level of IE2 and DNA polymerase (POL) protein conservation in phylogenetic analyses, it is clear that the English strain of RCMV is also a member of the genus Muromegalovirus, but is a β-herpesvirus species that is very distinct from both MCMV and RCMV (Maastricht). Both the lack of a CD200 homologue in the other two rodent viruses and the depth of sequence divergence of the rodent CMV IE2 and POL proteins suggest that these three viruses have evolved as separate species in the genus Muromegalovirus since very early in the host rodent lineage.

Enhanced tolerance to autoimmune uveitis in CD200-deficient mice correlates with a pronounced Th2 switch in response to antigen challenge

A single exposure to inhaled Ag 10 days before immunization leads to long term, Ag-specific tolerance. Respiratory tract myeloid APCs are implicated, but how regulation is invoked, and how tolerance is sustained are unclear. This study examines the in vivo function of the myeloid regulatory molecule CD200 in the process of tolerance induction. Despite earlier onset of experimental autoimmune uveitis in sham-tolerized, CD200-deficient mice, disease incidence and subsequent severity were actually reduced compared with those in wild-type mice. Protection was more effective and long term, lasting at least 28 days. Halting disease progression and tolerance in CD200(-/-) mice correlated with a marked increase in Th2-associated cytokine production by Ag-challenged splenocytes. Reduced overall disease and enhanced tolerance in the CD200-deficient mice in this model system were unexpected and may be related to altered populations of MHC class II(low) APC in the respiratory tract compared with wild-type mice together with associated activation of STAT6 in draining lymph nodes of tolerized mice. These data indicate that in the absence of default inhibitory CD200 receptor signaling, alternative, powerful regulatory mechanisms are invoked. This may represent either permissive dominant Th2 activation or an altered hierarchy of negative signaling by other myeloid cell-expressed regulatory molecules.

Constitutive retinal CD200 expression regulates resident microglia and activation state of inflammatory cells during experimental autoimmune uveoretinitis

Recent evidence supports the notion that tissue OX2 (CD200) constitutively provides down-regulatory signals to myeloid-lineage cells via CD200-receptor (CD200R). Thus, mice lacking CD200 (CD200(-/-)) show increased susceptibility to and accelerated onset of tissue-specific autoimmunity. In the retina there is extensive expression of CD200 on neurons and retinal vascular endothelium. We show here that retinal microglia in CD200(-/-) mice display normal morphology, but unlike microglia from wild-type CD200(+/+) mice are present in increased numbers and most significantly, express inducible nitric oxide synthase (NOS2), a macrophage activation marker. Onset and severity of uveitogenic peptide (1-20) of interphotoreceptor retinoid-binding protein-induced experimental autoimmune uveoretinitis is accelerated in CD200(-/-) mice and although tissue destruction appears no greater than seen in CD200(+/+) mice, there is continued increased ganglion and photoreceptor cell apoptosis. Myeloid cell infiltrate was increased in CD200(-/-) mice during experimental autoimmune uveoretinitis, although NOS2 expression was not heightened. The results indicate that the CD200:CD200R axis regulates retinal microglial activation. In CD200(-/-) mice the release of suppression of tonic macrophage activation, supported by increased NOS2 expression in the CD200(-/-) steady state accelerates disease onset but without any demonstration of increased target organ/tissue destruction.

CD200 immunoadhesin suppresses collagen-induced arthritis in mice

DBA/1 mice immunized with 100 microg bovine collagen type II emulsified in Freund's adjuvant, followed by booster injection in incomplete adjuvant at 18 days, develop profound arthritis (>50% of animals) by 30 days postinjection. The molecule CD200 (previously called OX2), associated with, among others, follicular dendritic cells, is implicated in delivery of immunosuppressive signals to the immune system, and an immunoadhesin in which the extracellular domains of CD200 were linked to a mouse IgG2a Fc region has been shown to promote renal allograft survival. DBA/1 mice receiving 15 microg/mouse CD200Fc at 3-day intervals following immunization with collagen did not develop arthritis in this model. Lymphocytes taken from CD200Fc-treated, collagen-immunized mice produced significantly lower levels of TNFalpha and IFN-gamma in culture supernatants after restimulation in vitro with collagen, in contrast to cells taken from control mice treated with PBS or normal mouse Ig. Serum from CD200Fc-treated mice contained less anti-collagen IgG (approximately 50% reduction), with relatively more IgG2b and IgG3, and lower levels of TNFalpha and IFN-gamma, than control mice. These data indicate that this immunoadhesin may have a potent role to play in the regulation of autoimmune disorders.

Transplant tolerance modifying antibody to CD200 receptor, but not CD200, alters cytokine production profile from stimulated macrophages

Increased C57BL/6 allograft survival following donor-specific dendritic cell (DC) portal vein (pv) pre-transplant immunization of C3H mice is associated with increased expression of the molecule CD200 on DC, delivery of suppressive signals by CD200(r+) macrophages, and polarization in cytokine production towards type-2 cytokines. Infusion of anti-mouse CD200 monoclonal antibody abolishes these effects. We have used whole Ig, and F(ab')(2) fragments, of anti-CD200 and anti-CD200(r) mAb to explore the relative signaling role of CD200(+) versus CD200(r+) cells in suppression of type-1 cytokine production in mixed leukocyte cultures (MLC), and enhanced graft survival in vivo. Simple neutralization of CD200 [even by F(ab')(2) antibody] reversed CD200-mediated suppression. However, only whole anti-CD200(r) antibody was effective in stimulating suppression from CD200(r+) cells. Suppression of cytokine induction following cross-linking of CD200(r+) cells in vitro was attenuated by anti-IL-6 mAb. Our data are consistent with the hypothesis that CD200(r) itself delivers the crucial intracellular signal leading to immunosuppression, a feature likely of importance in autoimmunity and transplantation.

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.

Evidence That an OX-2-Positive Cell Can Inhibit the Stimulation of Type 1 Cytokine Production by Bone Marrow-Derived B7-1 (and B7-2)-Positive Dendritic Cells

We reported that hepatic mononuclear, nonparenchymal cells (NPC) can inhibit the immune response seen when allogeneic C57BL/6 dendritic cells (DC) are incubated with C3H spleen responder cells. Cells derived from these cultures transfer increased survival of C57BL/6 renal allografts in C3H mice. We also found that increased expression of OX-2 on DC was associated with inhibition of cytokine production and renal allograft rejection. We explored whether inhibition by hepatic NPC was a function of OX-2 expression by these cells. Fresh C57BL/6 spleen-derived DC were cultured with C3H spleen responder cells and other putative coregulatory cells. The latter were derived from fresh C3H or C57BL/6 liver NPC, or from C3H or C57BL/6 mice treated for 10 days by i.v. infusion of human Flt3 ligand. Different populations of murine bone marrow-derived DC from cultures of bone marrow with IL-4 plus granulocyte-macrophage-CSF were also used as a source of putative regulator cells. Supernatants of all stimulated cultures were examined for functional expression of different cytokines (IL-2, IL-4, IFN-γ, and TGFβ). We found that fresh C57BL/6 splenic DC induced IL-2, not IL-4, production. Cells from the sources indicated inhibited IL-2 and IFN-γ production and promoted IL-4 and TGFβ production. Inhibition was associated with increased expression of OX-2 on these cells, as defined by semiquantitative PCR and FACS analysis. By size fractionation, cells expressing OX-2 were a subpopulation of NLDC145+ cells. Our data imply a role for cells expressing OX-2 in the regulation of induction of cytokine production by conventional allostimulatory DC.

Increased expression of the novel molecule OX-2 is involved in prolongation of murine renal allograft survival

BACKGROUND

Portal venous (p.v.) peritransplant immunization with dendritic cells from bone marrow cultures, along with cyclosporine (10 mg/kg), produces antigen-specific increased renal allograft survival compared with recipients receiving intravenous (i.v.) immunization. Increased survival is associated with altered cytokine production from recipient T cells restimulated with donor antigen. We used a suppressive subtractive hybridization approach to explore a role in the regulation of transplant rejection for other genes differentially expressed after p.v. immunization.

METHODS

Subtractive hybridization was performed using tissue from p.v. and i.v. immunized mice and a novel polymerase chain reaction-based approach. A gene-bank search was used to identify the source of the differentially expressed cDNAs. One product, the mouse homologue of rat OX-2, was further analyzed using Western gels and FACS analysis of dendritic cells (NLDC145+) isolated from p.v.-immunized mice.

RESULTS

Eighty cDNA clones were obtained by suppressive subtractive hybridization. Differential expression was confirmed in Northern RNA blots. One clone showed sequence homology to a gene encoding a molecule on rat dendritic cells (MRC OX-2), with homology to genes encoding the costimulatory molecules CD80 (B7-1) and CD86 (B7-2). In p.v.-immunized mice, a monoclonal antibody to the rat OX-2 molecule identified, by Western blot analysis, increased expression of a molecule with molecular weight (43 kDa) analogous to rat MRC-OX-2; labels (by FACS analysis) indentified increased numbers of a population of cells staining with NLDC145; and blocks indentified increased graft survival.

CONCLUSION

Our data suggest that OX-2 is functionally important in the increased graft survival seen in p.v.-immunized mice receiving renal allografts.

Characterization and localization of Mox2, the gene encoding the murine homolog of the rat MRC OX-2 membrane glycoprotein

MRC OX-2 is a rat type I membrane glycoprotein and a member of the immunoglobulin gene superfamily that has recently been shown to be able to costimulate murine T cell proliferation (Borriello et al. J. Immunol, 158, 4548, 1997). We now report the genomic organization for the gene encoding the murine homolog of MRC OX-2 (Mox2). The gene is composed of 6 exons and 5 introns spanning a minimum of 13.7 kb. Exon 1 encodes the amino terminal four amino acids and is located in the center of a 500-bp CpG island, suggestive of the presence of a promoter. Analysis of the sequences immediately upstream of exon 1, however, do not reveal a TATA or CAAT box. We also demonstrate that in addition to the canonical transcript, composed of exons 1 through 6, this gene gives rise to an additional nonproductive transcript resulting from the absence of exon 2, which leads to a frameshift and premature translation termination. The ratio of these alternative transcripts is not regulated by mitogenic stimulation with ConA or LPS. The Mox2 gene maps to Chr 16, telomeric to the clustered T cell costimulatory molecules Cd80 and Cd86 (Reeves et al. Genomics in press).

Cloning and characterization of the murine homologue of the rat/human MRC OX-2 gene

A 350 bp amplicon, obtained by PCR-select subtractive hybridization from RNA derived from mesenteric lymph nodes (MLN) cells from mice pre-immunized with allogeneic lymphocytes 36hrs prior to receiving donor-specific skin grafts, and showing > 98% homology with a published sequence for the rat MRC OX-2 gene, was used as a hybridization probe to screen a cDNA library constructed from adult mouse MLN treated in the same fashion. Several clones were identified which, on DNA sequence analysis, predicted a 218 amino acid protein showing significant homology with the rat and human MRC OX-2 gene product.

Human cell-adhesion molecule CD2 binds CD58 (LFA-3) with a very low affinity and an extremely fast dissociation rate but does not bind CD48 or CD59

CD2 is a T lymphocyte cell-adhesion molecule (CAM) belonging to the immunoglobulin superfamily (IgSF) which mediates transient adhesion of T cells to antigen-presenting cells and target cells. Reported ligands for human CD2 include the structurally-related IgSF CAMs CD58 (LFA-3) and CD48 as well as, more controversially, the unrelated cell-surface glycoprotein CD59. Using surface plasmon resonance technology, which avoids several pitfalls of conventional binding assays, we recently reported that rat CD2 binds rat CD48 with a very low affinity (Kd 60-90 microM) and dissociates rapidly (koff > or = 6 s-1) [van der Merwe, P. A., Brown, M. H., Davis, S. J., & Barclay, A. N. (1993) EMBO J. 12, 4945-4954]. In contrast, a study using conventional equilibrium binding methods reported a much higher affinity (Kd 0.4 microM) for human CD2 binding CD58 which suggested that the weak binding of rat CD2 to CD48 may not represent a typical CAM interaction. In the present study we have used surface plasmon resonance to obtain definitive affinity and kinetic data on the interactions of a soluble, recombinant form of human CD2 with soluble forms of CD58, CD48, and CD59. Binding of CD2 to CD58 was readily detected but we were unable to detect any direct interaction between CD2 and either CD59 or CD48 under conditions in which very low affinity interactions (Kd approximately 0.5 mM) would have been detected. In contrast to previous reports we found that human CD2 bound CD58 with a very low affinity (Kd 9-22 microM) and dissociated with an extremely fast dissociation rate constant (koff > or = 4 s-1). The association rate constant (kon) could not be measured directly but was calculated to be > or = 400,000 M-1s-1. Taken together, these results provide conclusive evidence that CAM interactions can have very low affinities and extremely fast dissociation rate constants.

Epitope mapping of anti-rat CD53 monoclonal antibodies. Implications for the membrane orientation of the Transmembrane 4 Superfamily

CD53 is a pan-leukocyte glycoprotein which is a member of the recently described Transmembrane 4 Superfamily (TM4SF) of membrane proteins that are predicted to span the lipid bilayer four times. The major hydrophilic region of murine CD53 was expressed as a glutathione-S-transferase fusion protein, and the epitopes of four mouse anti-rat CD53 monoclonal antibodies (mAb) (OX-44, 2D1, 6E2 and 7D2) were mapped to this region using mouse/rat chimeric fusion proteins. The epitopes of OX-44, 6E2 and 7D2 are restored by the substitution of a single isoleucine residue for threonine at position 154 in the mouse protein. The 2D1 epitope is non-linear and appears to require the juxtaposition of isoleucine at position 154 with one or more of the amino acids arginine (132), methionine (133) and serine (140). All of these epitopes are shown to be sensitive to reduction, thus indicating the importance of disulfide bonding in the correct folding of the CD53 hydrophilic domain. Moreover, as these four mAb recognize CD53 at the cell surface, the data provide direct molecular evidence for the proposed membrane orientation of the TM4SF.

Monoclonal antibody BM89 recognizes a novel cell surface glycoprotein of the L2/HNK-1 family in the developing mammalian nervous system

A monoclonal antibody, BM89, obtained with Triton X-114-treated pig synaptic membranes as an immunogen, recognizes a neuronal antigen in the newborn porcine nervous system. By immunohistochemistry, BM89 staining was observed within the neuropil of all areas of the forebrain and spinal cord tested. In addition, BM89 labeled the cell bodies and proximal dendrites of spinal cord neurons. In the peripheral nervous system, BM89 immunoreactivity was present in a subpopulation of dorsal root ganglion neurons and was predominantly associated with non-myelinated axons in peripheral nerves. Initial biochemical characterization of the antigen in pig brain showed that it is an integral membrane glycoprotein with a molecular weight of 41,000. Moreover, it cross-reacts with the L2/HNK-1 carbohydrate epitope expressed by members of a large family of glycoproteins. Homologous antigens with molecular weights of 41,000-43,000 were identified in the rat, rabbit and fetal human brain. Immunoblotting and immunohistochemistry revealed that the epitope recognized by BM89 is developmentally regulated in the rat nervous system. In cryostat sections from rat cerebellum, spinal cord and dorsal root ganglia, an age-dependent decline of BM89 immunoreactivity was observed during postnatal development. In the cerebellum, the BM89 epitope was very abundant in cells of the external and the internal granular layers between postnatal days 5 and 15. During this period some staining was also identified in the developing molecular layer and the prospective white matter. Subsequently, and in the adult, overall staining was greatly reduced and remaining immunoreactivity was associated only with the internal granular layer. In the spinal cord and dorsal root ganglia, staining was very prominent at postnatal day 5; it decreased considerably thereafter and was barely detectable in the adult. Immunostaining of rat brain and dorsal root ganglion cultures revealed that the BM89 antigen is a cell surface molecule expressed by a subpopulation of central and peripheral nervous system neurons. The biochemical properties in conjunction with the topographical location and the developmental profile of the antigen recognized by BM89 suggest that it may represent a developmentally important recognition molecule.

Identification of glycoconjugates which are targets for anti-Gal(beta 1-3)GalNAc autoantibodies in spinal motor neurons

Human IgM anti-Gal(beta 1-3)GalNAc antibodies which bind to GM1 and GD1b, are implicated in the pathogenesis of predominantly motor neuropathy or motor neuron disease. By immunofluorescence microscopy, the human antibodies immunostain the surface of motor neurons from bovine spinal cord. The motor neurons are also immunostained by cholera toxin (CT), which is specific for GM1. Glycolipid analysis using thin-layer chromatography (TLC) and immunostaining reveals that the relative concentration of GM1 and GD1b in motor neurons is greatly reduced in comparison to whole spinal cord, and that other motor neuron gangliosides are unreactive with the anti-Gal(beta 1-3)GalNAc antibodies. By Western blot analysis, the antibodies react with several protein bands in motor neuron extracts, and many of the same proteins are also recognized by PNA. These data suggest that both glycoproteins and glycolipids might be targets for anti-Gal(beta 1-3)GalNAc antibodies in spinal motor neurons.

Immunoglobulin superfamily molecules in the nervous system

Among the various types of membrane molecules involved in cell-cell interactions in the nervous system, we have focused in this review upon membrane proteins belonging to the immunoglobulin superfamily (IgSF). IgSF molecules are distinctive in that: (1) a large percentage of known neural adhesion molecules belongs to the IgSF; (2) they are homologous in structure (Ig domain), yet exhibit large variation of function in cell-cell interactions. The structure of IgSF molecules is briefly summarized in Section II, and each member of the IgSF which has been found in the nervous system is reviewed in Section III. In Section IV, we have discussed possible properties of yet-unknown nervous system IgSF molecules, on the assumption that nervous system IgSF molecules thus far discovered comprise only a small portion of those existing. Discussion is based upon an analogy with the immune system and upon knowledge of cell-cell interactions in the development of the nervous system. Our principal aims in this review are to summarize knowledge of neural IgSF molecules and to discuss the possibility that some IgSF molecules may encode in their structures instructions for recognizing, or for being recognized by, target neural cells. Further growth of knowledge of IgSF molecules may yield insights into the patterns of cell-cell interactions underlying the formation of neuronal circuits during development.

Expression of a marrow stroma and thymus-associated antigen (ST3) in the rat brain: comparison with Thy-1

Cells of the immunohemopoietic and nervous systems express certain molecules that generally are not found in other tissues. One example is the 'ST3' antigen, which is present on the major population of fibroblastoid cells grown from rat bone marrow, but is not detected on adherent cells from most peripheral organs (e.g. lung). An immunohistological survey revealed ST3 also in the thymic cortex, the glomerular mesangial area, and the brain. Because this pattern of distribution is similar to that described for Thy-1, we compared the localization of the two antigens in the adult rat brain and found that there were areas where it was congruent and others where it was distinct. Staining for ST3 was absent from the white matter, but was especially notable in discrete layers of the frontal, orbital, parietal, and cingulate cortices, the substantia nigra, the inferior olivary nuclei, and the deep molecular layer of the cerebellum, as well as other scattered regions in the gray matter. This is in contrast to Thy-1, which stained more diffusely throughout the gray zones. In further experiments using primary brain cell cultures, ST3 was demonstrated on neurons, but not on oligodendrocytes or astrocytes. Similarly, it was found on the surface of cells of the PC12 neuronal line, but not on the C6 astrocytoma. This restricted distribution on a subpopulation of neurons raises the possibility that the ST3 epitope might be part of a cell interaction molecule of the marrow stroma, thymus, and brain.

Identification of the bile canalicular cell surface molecule GP110 as the ectopeptidase dipeptidyl peptidase IV: an analysis by tissue distribution, purification and N-terminal amino acid sequence

This paper describes the tissue distribution, purification and N-terminal amino acid sequence of the bile canalicular cell surface molecule dipeptidyl peptidase IV. Immunoperoxidase staining of cryostat sections of rat liver with a monoclonal antibody, Medical Research Council OX-61, indicated specific binding to hepatocyte bile canalicular domains and brush borders of bile ducts. Additional staining was seen in other epithelial brush borders (small intestine, kidney, colon, pancreatic duct); acinar structures in salivary glands; endothelial structures and T cell areas in thymus, spleen and lymph node. The tissue distribution suggested that monoclonal antibody OX-61 binds to the ectoenzyme dipeptidyl peptidase IV. This was confirmed by depletion of dipeptidyl peptidase IV activity from tissue homogenates by monoclonal antibody OX-61 coupled to Sepharose. The molecule recognized by OX-61 was then purified from liver and kidney by monoclonal antibody affinity chromatography. The molecule had a molecular weight of 110 kD under reducing conditions. The purified molecule was subsequently analyzed for amino acid composition and N-terminal amino acid sequence. Thirty-one N-terminal amino acids were sequenced and indicated identity with part of the predicted N-terminus of the previously cloned bile canalicular molecule GP110. On review, other similarities between dipeptidyl peptidase IV and GP110 were detected: molecular weight, deglycosylated form and metabolic half-life. Finally, the recent cloning of dipeptidyl peptidase IV permitted a comparison between the molecule recognized by monoclonal antibody OX-61, GP110 and dipeptidyl peptidase IV. It is concluded that these three molecules are almost certainly identical.