Characterization of human cd200 glycoprotein receptor gene located on chromosome 3q12-13

CD200 genotype is associated with clinical outcome of patients with multiple myeloma

Immune dysfunction in patients with MM affects both the innate and adaptive immune system. Molecules involved in the immune response pathways are essential to determine the ability of cancer cells to escape from the immune system surveillance. However, few data are available concerning the role of immune checkpoint molecules in predicting the myeloma control and immunological scape as mechanism of disease progression. We retrospectively analyzed the clinical impact of the CD200 genotype (rs1131199 and rs2272022) in 291 patients with newly diagnosed MM. Patients with a CD200 rs1131199 GG genotype showed a median overall survival (OS) significantly lower than those with CC+CG genotype (67.8 months versus 94.4 months respectively; p: 0.022) maintaining significance in the multivariate analysis. This effect was specially detected in patients not receiving an autologous stem cell transplant (auto-SCT) (p < 0.001). In these patients the rs1131199 GG genotype negatively influenced in the mortality not related with the progression of MM (p: 0.02) mainly due to infections events.

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.

Altered expression of the immunoregulatory ligand-receptor pair CD200-CD200R1 in the brain of Parkinson's disease patients

Neuroinflammation, in which activated microglia are involved, appears to contribute to the development of Parkinson’s disease (PD). However, the role of microglial activation and the mechanisms governing this process remain uncertain. We focused on one inhibitory mechanism involved in the control of microglial activation, the microglia inhibitory receptor CD200R1, and its ligand CD200, mainly expressed by neurons. The human CD200R1 gene encodes two membrane-associated and two soluble protein isoforms and the human CD200 gene encodes full-length proteins (CD200full) but also truncated (CD200tr) proteins which act as CD200R1 antagonists. Little is known about their expression in the human brain under pathological conditions. We used human peripheral blood monocytes and monocyte-derived microglia-like cells from control subjects to characterize the expression of the CD200R1 mRNA variants, which showed stimulus-specific responses. We provide evidence of increased CD200R1 (mRNA variants and protein isoforms) and CD200 expression (CD200tr mRNA) in brain tissue of PD patients, mainly in the hippocampus, as well as increased CD200 expression (CD200full and CD200tr mRNAs) in iPSCs-derived dopaminergic neurons generated from skin fibroblasts of PD patients. Our results suggest that CD200-CD200R1 signalling is altered in PD, which may affect the microglial function and constitute a potential target in therapeutic strategies for PD.

CD200:CD200R Interactions and Their Importance in Immunoregulation

The molecule CD200, described many years ago as a naturally occurring immunomodulatory agent, capable of regulating inflammation and transplant rejection, has attracted additional interest over the past years with the realization that it may also serve as an important marker for progressive malignancy. A large body of evidence also supports the hypothesis that this molecule can contribute to immunoregulation of, among other diseases, infection, autoimmune disease and allergy. New data have also come to light to characterize the receptors for CD200 (CD200R) and their potential mechanism(s) of action at the biochemical level, as well as the description of a novel natural antagonist of CD200, lacking the NH₂-terminal region of the full-length molecule. Significant controversies exist concerning the relative importance of CD200 as a ligand for all reported CD200Rs. Nevertheless, some progress has been made in the identification of the structural constraints determining the interaction between CD200 and CD200R, and this information has in turn proved of use in developing novel small molecule agonists/antagonists of the interaction. The review below highlights many of these newer findings, and attempts to place them in the broad context of our understanding of the role of CD200-CD200R interactions in a variety of human diseases.

CD200 is up-regulated in R6/1 transgenic mouse model of Huntington's disease

In Huntington's disease (HD), striatal medium spiny neurons (MSNs) are particularly sensitive to the presence of a CAG repeat in the huntingtin (HTT) gene. However, there are many evidences that cells from the peripheral immune system and central nervous system (CNS) immune cells, namely microglia, play an important role in the etiology and the progression of HD. However, it remains unclear whether MSNs neurodegeneration is mediated by a non-cell autonomous mechanism. The homeostasis in the healthy CNS is maintained by several mechanisms of interaction between all brain cells. Neurons can control microglia activation through several inhibitory mechanisms, such as the CD200-CD200R1 interaction. Due to the complete lack of knowledge about the CD200-CD200R1 system in HD, we determined the temporal patterns of CD200 and CD200R1 expression in the neocortex, hippocampus and striatum in the HD mouse models R6/1 and HdhQ111/7 from pre-symptomatic to manifest stages. In order to explore any alteration in the peripheral immune system, we also studied the levels of expression of CD200 and CD200R1 in whole blood. Although CD200R1 expression was not altered, we observed and increase in CD200 gene expression and protein levels in the brain parenchyma of all the regions we examined, along with HD pathogenesis in R6/1 mice. Interestingly, the expression of CD200 mRNA was also up-regulated in blood following a similar temporal pattern. These results suggest that canonical neuronal-microglial communication through CD200-CD200R1 interaction is not compromised, and CD200 up-regulation in R6/1 brain parenchyma could represent a neurotrophic signal to sustain or extend neuronal function in the latest stages of HD as pro-survival mechanism.

Analysis of the expression of porcine CD200R1 and CD200R1L by using newly developed monoclonal antibodies

CD200R1 and CD200R1-like are paired receptors which modulate activation of immune cells. Here, we describe the characterisation of their porcine homologues. Analysis of database porcine sequences shows an exceptionally high homology between the extracellular Ig-like domains of these receptors, being the rest more dissimilar. We have obtained two mAbs, PCT1 and PCT3, against a CD200R1-Fc recombinant protein, that bind on CHO cells expressing GFP-tagged CD200R1. The specificity of these mAbs was analysed on CD200R1 L, and also on a CD200R1 splicing variant that lacks the V-type Ig domain. PCT1 bound to both CD200R1 and CD200R1L, but not to the splicing variant, what suggests that recognises an epitope in the V-type Ig domain. PCT3 reacted with both CD200R1 variants, but not CD200R1L, probably binding to an epitope in the N-terminal sequence of CD200R1. Analysis of porcine cells with these mAbs showed expression of CD200R1/CD200R1L on B cells, monocytes and alveolar macrophages.

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.

Alterations in CD200-CD200R1 System during EAE Already Manifest at Presymptomatic Stages

In the brain of patients with multiple sclerosis, activated microglia/macrophages appear in active lesions and in normal appearing white matter. However, whether they play a beneficial or a detrimental role in the development of the pathology remains a controversial issue. The production of pro-inflammatory molecules by chronically activated microglial cells is suggested to contribute to the progression of neurodegenerative processes in neurological disease. In the healthy brain, neurons control glial activation through several inhibitory mechanisms, such as the CD200-CD200R1 interaction. Therefore, we studied whether alterations in the CD200-CD200R1 system might underlie the neuroinflammation in an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. We determined the time course of CD200 and CD200R1 expression in the brain and spinal cord of an EAE mouse model from presymptomatic to late symptomatic stages. We also assessed the correlation with associated glial activation, inflammatory response and EAE severity. Alterations in CD200 and CD200R1 expression were mainly observed in spinal cord regions in the EAE model, mostly a decrease in CD200 and an increase in CD200R1 expression. A decrease in the expression of the mRNA encoding a full CD200 protein was detected before the onset of clinical signs, and remained thereafter. A decrease in CD200 protein expression was observed from the onset of clinical signs. By contrast, CD200R1 expression increased at EAE onset, when a glial reaction associated with the production of pro- and anti-inflammatory markers occurred, and continued to be elevated during the pathology. Moreover, the magnitude of the alterations correlated with severity of the EAE mainly in spinal cord. These results suggest that neuronal-microglial communication through CD200-CD200R1 interaction is compromised in EAE. The early decreases in CD200 expression in EAE suggest that this downregulation might also occur in the initial phases of multiple sclerosis, and that this early neuronal dysfunction might facilitate the development of neuroinflammation. The increased CD200R1 expression in the EAE model highlights the potential use of targeted agonist molecules as therapeutic tools to control neuroinflammation. In summary, the CD200-CD200R1 system is a potential therapeutic target in multiple sclerosis, and CD200R1 agonists are molecules that may be worth developing in this context.

Protective Microglia and Their Regulation in Parkinson's Disease

Microglia-mediated neuroinflammation is a hallmark of Parkinson’s disease (PD). In the brains of patients with PD, microglia have both neurotoxic and neuroprotective effects, depending on their activation state. In this review, we focus on recent research demonstrating the neuroprotective role of microglia in PD. Accumulating evidence indicates that the protective mechanisms of microglia may result from their regulation of transrepression pathways via nuclear receptors, anti-inflammatory responses, neuron–microglia crosstalk, histone modification, and microRNA regulation. All of these mechanisms work together to suppress the production of neurotoxic inflammatory components. However, during the progression of PD, the detrimental effects of inflammation overpower the protective actions of microglia. Therefore, an in-depth exploration of the mechanisms underlying microglial neuroprotection, and a means of promoting the transformation of microglia to the protective phenotype, are urgently needed for the treatment of 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.

Inhibitory CD200R and proapoptotic CD95/CD95L molecules on innate immunity cells are modulated by cardiac surgery

Introduction:

Cardiac surgery directly initiates a systemic inflammatory response with the activation of both cellular and humoral parts of the immune system. Exaggerated immune system activation is associated with a risk of life-threatening multi-organ dysfunction (MOD) and increased morbidity and mortality in the postoperative period. The immune system response is regulated and terminated by inhibitory mechanisms, including the regulatory membrane molecules, such as CD200R, CD95, CD95L and soluble sCD200R.

Methods:

We measured the expression of CD95, CD95L, CD200R and sCD200R molecules in granulocyte and monocyte populations in blood samples of 30 patients who underwent coronary artery bypass grafting (CABG) using cardiopulmonary bypass (CPB). Samples collected before surgery, after surgery and in the postoperative period were analyzed by flow cytometry and ELISA.

Results:

We found a significant increase in the percentage of granulocytes featuring the anti-inflammatory molecule CD200R (from 5% to 17.8%) after surgery. We presume that these cells were less susceptible to apoptosis because they rarely expressed CD95 as the CD200R+CD95– granulocyte sub-population prevailed. Only a small percentage of CD200R+ granulocytes expressed simultaneously CD95 (from 0.5 to 2.06 %). This small population of CD200R+CD95+ cells decreased expression of CD200R after surgery and, thus, was likely to be a source of increased sCD200R in serum (from 96 to 294 ng/mL). Also, the expression of CD95L on CD200R+ granulocytes and CD95 on CD200R+ monocytes was affected by surgery. The percentage of CD200R+ monocytes was elevated on the 1st postoperative day (from 30.6 to 49.4 %) and dropped below the preoperative value on the 7th day after surgery (from 30.6 to 19.8 %). This population comprised mainly CD200R+CD95+ monocytes in which the enhanced expression of CD95 was found.

Conclusion:

Our data show that the expression of CD200R, CD95 and CD95L was influenced by cardiac surgery and imply the role of these membrane molecules in cell regulation–inhibition and apoptosis following cardiac surgery.

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.

Altered regulation of CD200 receptor in monocyte-derived macrophages from individuals with Parkinson's disease

Microglia are the representative myeloid cells in the brain, and their over-activation plays an important role in the pathogenesis of Parkinson's disease (PD). Microglia activation is believed to be regulated by the CD200-CD200R signaling. As the peripheral counterpart of microglia, monocyte-derived macrophages (MDMs) share the same progenitor and antigen markers, and they have similar biological behaviors and mirror microglial function in the brain. Here, we studied CD200R expression and its regulation in MDMs from 32 PD cases, 27 age-matched old controls, and 28 young controls. We found that the basal CD200R expression is similar in MDMs from young control, old control and PD patients. However, the induction of CD200R expression in MDMs under various conditions is impaired in the old groups, especially in PD patients. There was a selective decrease in CD200R expression induced by co-culture with dying PC12 cells in MDMs from PD cases, as compared with MDMs from the age-matched controls. We also found that the inducible CD200R expression correlated inversely with the onset age of PD and to tumor necrosis factor-alpha (TNF-alpha) released from MDMs. These results suggest an intrinsic abnormality in the CD200-CD200R signaling in MDMs during aging and, especially, in PD. We speculate that in the PD brain,microglia might undergo abnormalities similar to MDMs.

Gene expression patterns associated with chicken jejunal development

Jejunal development occurs in a spatio-temporal pattern and is characterized by morphological and functional changes. To investigate jejunal development at the transcriptomic level, we performed microarray studies in 1-21-day-old chickens. Nine gene clusters were identified, each with a specific gene expression pattern. Subsequently, groups of genes with similar functions could be identified. Genes involved in morphological and functional development were highly expressed immediately after hatch with declining expression patterns afterwards. Immunological development can be roughly divided based on expression patterns into three processes over time; first innate response and immigration of immune cells, secondly differentiation and specialization, and thirdly maturation and immune regulation. We conclude that specific gene expression patterns coincide with the immunological, morphological, and functional development as measured by other methods. Our data show that transcriptomic approaches provide more detailed information on the biological processes underlying jejunal development.

Expression of a CD200 transgene is necessary for induction but not maintenance of tolerance to cardiac and skin allografts

CD200, a type 2 transmembrane molecule of the Ig supergene family, can induce immunosuppression in a number of biological systems, as well as promote increased graft acceptance, following binding to its receptors (CD200Rs). Skin and cardiac allograft acceptance are readily induced in transgenic mice overexpressing CD200 under control of a doxycycline-inducible promoter, both of which are associated with increased intragraft expression of mRNAs for a number of genes associated with altered T cell subset differentiation, including GATA-3, type 2 cytokines (IL-4, IL-13), GITR, and Foxp3. Interestingly, some 12-15 days after grafting, induction of transgenic CD200 expression can be stopped (by doxycycline withdrawal), without obvious significant effect on graft survival. However, neutralization of all CD200 expression (including endogenous CD200 expression) by anti-CD200 mAb caused graft loss, as did introduction of an acute inflammatory stimulus (LPS, 10 microg/mouse, delivered by i.p. injection). We conclude that even with apparently stably accepted tissue allografts, disruption of the immunoregulatory balance by an intense inflammatory stimulus can cause graft loss.

Decreased expression of CD200 and CD200 receptor in Alzheimer's disease: a potential mechanism leading to chronic inflammation

Inflammatory activation of microglia in response to neurodegenerative changes in diseases such as Alzheimer's disease (AD) and Parkinson's disease has been extensively described. These observations have suggested that inflammation could be contributing to disease progression. In this paper, the potential role of CD200 and CD200 receptor (CD200R), whose known functions are to activate anti-inflammatory pathways and induce immune tolerance through binding of CD200 to CD200 receptor (CD200R), was studied in AD. Quantitative studies showed a significant decrease in CD200 protein and mRNA in AD hippocampus and inferior temporal gyrus, but not cerebellum. Immunohistochemistry of brain tissue sections of hippocampus, superior frontal gyrus, inferior temporal gyrus and cerebellum from AD and non-demented cases demonstrated a predominant, though heterogeneous, neuronal localization for CD200. Decreased neuronal expression was apparent in brain regions affected by AD pathology. There was also a significant decrease in CD200R mRNA expression in AD hippocampus and inferior temporal gyrus, but not cerebellum. Low expression of CD200R by microglia was confirmed at the mRNA and protein level using cultured human microglia compared to blood-derived macrophages. Treatment of microglia and macrophages with interleukin-4 and interleukin-13 significantly increased expression of CD200R. Expression of these cytokines was not generally detectable in brain. These data indicate that the anti-inflammatory CD200/CD200R system may be deficient in AD brains. Mechanisms aimed at increasing levels of CD200 and CD200R could have therapeutic potential for controlling inflammation in human neurodegenerative diseases.

Non-MHC ligands for inhibitory immune receptors: novel insights and implications for immune regulation

Regulation of cellular responses by inhibitory receptors is crucial for proper function of the immune system. The prototype inhibitory immune receptors are major histocompatibility complex (MHC) class I binding killer-Ig like receptors (KIRs) present on effector cells such as natural killer (NK) cells and effector T cells. However, the recent identification of non-MHC class I ligands for inhibitory immune receptors, such as KLRG1, KLRB1 and LAIR-1, indicates that also MHC class I-independent inhibitory immune receptors play crucial roles in inducing peripheral tolerance. The presence of these receptors on many other immune cell types besides effector cells suggests that tight regulation of cell activation is necessary in all facets of the immune response in both normal and diseased tissue. Here, we review novel insights and implications of non-MHC class I ligand binding to inhibitory immune receptors. We give an overview of the known ligand-receptor pairs by grouping the ligands according to their properties and discuss implications of these interactions for the maintenance of immune balance and for the defense against tumors and pathogens.

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.

Murine CD200+ CK7+ trophoblasts in a poly (I:C)-induced embryo resorption model

Cytokeratin 7 (CK7) is currently regarded as the best marker for trophoblast cells, while CD200 (OX-2), known as 'tolerance signal', plays an important role in normal pregnancy. In this study, the status of CD200 expression was investigated in BALB/c x C57BL/6 and BALB/c x BALB/c mating combinations designed as allogeneic and syngeneic murine models of induced embryo resorption, in which the resorption rate was boosted by an i.p. injection of poly (I:C), a synthetic double-stranded RNA. The percentage of CD200+ cells in the CK7+ cell population (CD200+ CK7+ percentage) and the absolute number of these cells were determined with flow cytometry, using trophoblast cells collected at day 8.5 and day 13.5 of gestation. The potential effect of poly (I:C) on CD200 expression was also evaluated by detecting the CD200+ CK7+ percentage in trophoblast cells incubated in the presence or absence of poly (I:C), in vitro. The distribution pattern of CD200+ cells at the feto-maternal interface was evaluated by immunocytochemical examination. When 10(4) cells were analyzed at day 8.5 of gestation in each case, no significant difference was observed between the poly (I:C)-treated group and the control PBS group either in the CD200+ CK7+ percentage or in the absolute number of these cells. Similar results were observed both in BALB/c x C57BL/6 mice and in BALB/c x BALB/c mice. However, the CD200+ CK7+ percentage was significantly decreased in the poly (I:C)-treated group when evaluated at day 13.5 of gestation. Accordingly, a dramatically elevated rate of embryo resorption was observed at this time point of pregnancy after the administration of poly (I:C). In addition, the CD200+ CK7+ percentage was significantly lower in trophoblast cells incubated with poly (I:C) at a certain concentration, in vitro, while histocytochemical examination showed the CD200+ cells mainly scattered in placental tissue adjacent to the interface of the placenta and uterus. This indicates that sufficient expression of the CD200 molecule on CK7+ cells at the feto-maternal interface may be necessary for the maintenance of embryos during pregnancy in this rodent model, while poly (I:C) administration may increase embryo resorption, at least partially via direct inhibition of CD200 expression on CK7+ cells.

CD200, a “no danger” signal for hair follicles

The "danger model" of immune recognition proposes that the immune system does not differentiate between self and non-self when deciding whether to mount a response, but instead, discerns between that which is dangerous or not dangerous to the host. Danger signals incite inflammatory responses, which can lead to the induction of tissue-specific autoimmunity. Immunosuppressive molecules expressed on selected cells have the potential to regulate tissue-specific inflammation, and consequently, autoimmunity. Recent studies have revealed that CD200, a potent immunoregulatory protein, is expressed on Langerhans cells (LCs) and keratinocytes (KCs) in mouse epidermis. CD200 expression is concentrated on KCs comprising the outer root sheath (ORS) of murine hair follicles (HF). Skin deficient in CD200 is highly susceptible to HF-associated inflammation and immune-mediated alopecia. In this concept review, the results of recent studies on CD200 and its inhibitory receptor, CD200R, are summarized and integrated to yield a model whereby CD200-CD200R interaction attenuates perifollicular inflammation, prevents HF-specific autoimmunity and may protect epidermal stem cells from autoimmune destruction. Further elucidation of the CD200-CD200R signaling pathway in cutaneous tissues may advance understanding of how immune homeostasis is established and maintained in the skin.

Decreased Alloreactivity Using Donor Cells from Mice Expressing a CD200 Transgene Under Control of a Tetracycline-Inducible Promoter

BACKGROUND

CD200 delivers an immunsuppressive signal that augments allograft survival following interaction with its receptor, CD200R1. We hypothesized that mice overexpressing CD200 as a trangene would also show a diminished alloresponsiveness and decreased allograft rejection.

METHODS

A transgenic mouse on a C57BL/6 background, expressing a murine CD200 cDNA genetically linked to a green fluorescent protein tag (GFP) under control of a tetracycline response element (TRE), was mated with a commercial transgenic mouse carrying the reverse tetracycline regulated transactivator gene under control of a human CMV promoter. F1 mice were examined for induction of alloimmunity in vivo/in vitro, and for their ability to reject skin allografts in vivo.

RESULTS

The F1 hybrid expressed CD200 after exposure to doxycyline (DOX), as assessed both by enhanced GFP expression in multiple organs and CD200-GFP expression. Splenocytes from F1 mice stimulated with LPS or allogeneic cells in vitro in the presence/absence of DOX showed reduced production of TNFalpha, and of allospecific CTL. Splenocytes from F1 mice used as stimulator cells in allogeneic MLCs in the presence of DOX were inefficient at induction of cytokines or CTL in vitro from normal allogeneic responder cells. Skin grafts from transgenic mice were inefficient at induction of CTL in vivo. Transgenic mice receiving DOX showed prolonged acceptance of skin allografts, which was abolished by infusion of anti-CD200 mAb.

CONCLUSIONS

Our data confirmed that overexpression of CD200 in transgenic mice, or in skin grafts from these mice, decreases alloimmunity. This has potential clinical utility in transplantation and other diseases.

Discrete monoclonal antibodies define functionally important epitopes in the CD200 molecule responsible for immunosuppression function

BACKGROUND

Both murine and human CD200 fusion proteins (CD200Fc) act as immunosuppressants after engagement of cell-bound receptors (CD200R). Anti-CD200 monoclonal antibodies (mAbs) augment activity in mixed leukocyte cultures (MLCs) (increased cytotoxic T lymphocyte/cytokine production) after neutralization of endogenous CD200 activity. Previous studies documented critical regions in the N-terminal domains of both CD200 and CD200R1 for ligand:receptor binding and defined a number of synthetic CD200 and CD200R peptides that antagonize that interaction.

METHODS

We used a panel of mAbs to mouse and human CD200Fc to compare the rank activities of antibodies for binding (flow cytometric analysis [FACS] or enzyme-linked immunoadsorbent assay [ELISA]) to CD200 with their abilities to augment immune reactivity in MLCs.

RESULTS

Only mAbs defining epitopes in the N-terminal domain could augment MLC reactivity (or block immunosuppression by soluble CD200Fc), whereas mAbs targeting C-domain epitopes, although reactive in ELISA or FACS (targeting cell surface CD200), were inactive in MLCs.

CONCLUSION

In addition to defining the importance of N-terminal epitopes for CD200 function, rank comparison of mAbs for FACS staining of CD200 expressed on various cell types indicates heterogeneity in expressed CD200.

An Inhibitory Ig Superfamily Protein Expressed by Lymphocytes and APCs Is Also an Early Marker of Thymocyte Positive Selection

Positive selection of developing thymocytes is associated with changes in cell function, at least in part caused by alterations in expression of cell surface proteins. Surprisingly, however, few such proteins have been identified. We have analyzed the pattern of gene expression during the early stages of murine thymocyte differentiation. These studies led to identification of a cell surface protein that is a useful marker of positive selection and is a likely regulator of mature lymphocyte and APC function. The protein is a member of the Ig superfamily and contains conserved tyrosine-based signaling motifs. The gene encoding this protein was independently isolated recently and termed B and T lymphocyte attenuator (Btla). We describe in this study anti-BTLA mAbs that demonstrate that the protein is expressed in the bone marrow and thymus on developing B and T cells, respectively. BTLA is also expressed by all mature lymphocytes, splenic macrophages, and mature, but not immature bone marrow-derived dendritic cells. Although mice deficient in BTLA do not show lymphocyte developmental defects, T cells from these animals are hyperresponsive to anti-CD3 Ab stimulation. Conversely, anti-BTLA Ab can inhibit T cell activation. These results implicate BTLA as a negative regulator of the activation and/or function of various hemopoietic cell types.

INDUCTION OF TOLERANCE-INDUCING ANTIGEN-PRESENTING CELLS IN BONE MARROW CULTURES IN VITRO USING MONOCLONAL ANTIBODIES TO CD200R

CD200 to CD200R interactions produce immunoregulation. We investigated whether the expression of CD200R on dendritic cell (DC) precursors affects their developmental fate. C57BL/6 bone marrow (BM) cells were cultured in vitro in the presence of (interleukin-4 + granulocyte-macrophage colony-stimulating activity) to generate allostimulatory DCs, which were in turn used to induce cytotoxic T-lymphocyte and cytokine production after culture with C3H responder spleen cells. Some marrow cultures included anti-CD200R antibodies. The inclusion of monoclonal antibodies in different isoforms of CD200R in the BM culture led to a generation of cells (tolerogenic DCs) that were unable to produce allostimulation in vitro with responder cells. Cells taken from these latter mixed leukocyte cultures (MLCs) now contained CD4(+)CD25(+) cells able to inhibit the antigen-specific MLC response of fresh C3H responder cells to stimulation with C57BL/6 cells, but not stimulation with BALB/c cells. Tolerogenic DCs, infused in vivo into mice receiving C57BL/6 skin grafts, produced antigen-specific decreased rejection of BL/6 allografts, not BALB/c allografts, compared with mice receiving control DCs (generated from BM in the absence of anti-CD200R). The induction of CD4(+)CD25(+) suppressor cells in MLCs using tolerogenic DCs from the initial BM cultures could be overcome by using limiting numbers of tolerogenic DCs and an excess of allostimulatory DCs derived from BM cultures maintained in the absence of anti-CD200R. These data indicate that anti-CD200R biases stem cells in BM toward the development of suppressive antigen-presenting cells, which can induce CD4(+)CD25(+) regulatory T cells. Tolerogenic DCs have the potential to modify graft acceptance in vivo.