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

Understanding the squamous cell carcinoma immune microenvironment

Primary cutaneous squamous cell carcinoma (cSCC) is the second most common human cancer with a rising incidence of about 1.8 million in the United States annually. Primary cSCC is usually curable by surgery; however, in some cases, cSCC eventuates in nodal metastasis and death from disease specific death. cSCC results in up to 15,000 deaths each year in the United States. Until recently, non-surgical options for treatment of locally advanced or metastatic cSCC were largely ineffective. With the advent of checkpoint inhibitor immunotherapy, including cemiplimab and pembrolizumab, response rates climbed to 50%, representing a vast improvement over chemotherapeutic agents used previously. Herein, we discuss the phenotype and function of SCC associated Langerhans cells, dendritic cells, macrophages, myeloid derived suppressor cells and T cells as well as SCC-associated lymphatics and blood vessels. Possible role(s) of SCC-associated cytokines in progression and invasion are reviewed. We also discuss the SCC immune microenvironment in the context of currently available and pipeline therapeutics.

The Role of CD200-CD200 Receptor in Human Blood and Lymphatic Endothelial Cells in the Regulation of Skin Tissue Inflammation

CD200 is a cell membrane glycoprotein that interacts with its structurally related receptor (CD200R) expressed on immune cells. We characterized CD200–CD200R interactions in human adult/juvenile (j/a) and fetal (f) skin and in in vivo prevascularized skin substitutes (vascDESS) prepared by co-culturing human dermal microvascular endothelial cells (HDMEC), containing both blood (BEC) and lymphatic (LEC) EC. We detected the highest expression of CD200 on lymphatic capillaries in j/a and f skin as well as in vascDESS in vivo, whereas it was only weakly expressed on blood capillaries. Notably, the highest CD200 levels were detected on LEC with enhanced Podoplanin expression, while reduced expression was observed on Podoplanin-low LEC. Further, qRT-PCR analysis revealed upregulated expression of some chemokines, including CC-chemokine ligand 21 (CCL21) in j/aCD200⁺ LEC, as compared to j/aCD200⁻ LEC. The expression of CD200R was mainly detected on myeloid cells such as granulocytes, monocytes/macrophages, T cells in human peripheral blood, and human and rat skin. Functional immunoassays demonstrated specific binding of skin-derived CD200⁺ HDMEC to myeloid CD200R⁺ cells in vitro. Importantly, we confirmed enhanced CD200–CD200R interaction in vascDESS in vivo. We concluded that the CD200–CD200R axis plays a crucial role in regulating tissue inflammation during skin wound healing.

Immunohistochemical Expression of CD200 in Renal Cell Carcinoma

Background and Objectives:

Renal cell carcinoma (RCC) is the most common malignant renal neoplasm in adults. CD200 is a transmembrane protein and is a promising target for cancer immunotherapy. The aim of this study is to assess the CD200 expression in RCC.

Materials and Methods:

Eighty paraffin-embedded radical nephrectomy specimens, diagnosed with RCC were evaluated immunohistochemically for CD200 expression.

Results:

Out of eighty cases studied, CD200 was expressed in n = 73 cases (91.25%) with high intensity in 27 cases (33.75%), moderate intensity in 22 cases (27.5%), and mild intensity in 24 cases (30%). No staining was observed in the adjacent apparently normal renal tissue in all examined sections. No significant relationship was found between CD200 expression and the gender, tumor size, tumor side, histologic type, nuclear grade, T stage, and tumor necrosis.

Conclusion:

CD200 expression in most of the studied cases of RCC may refer to the potential therapeutic of anti-CD200 antibody for this cancer.

Diminution of Phagocytosed Micro/Nanoparticles by Tethering with Immunoregulatory CD200 Protein

CD200 is known as an anti-inflammatory transmembrane glycoprotein in the immunoglobulin superfamily. CD200 interacts with its receptor CD200R which is highly expressed on myeloid cells such as macrophages and neutrophils. CD200-CD200R interaction has known to reduce macrophage activation and chronic inflammation. To harness the immunomodulatory property of CD200 for surface modification, CD200-streptavidin fusion protein was expressed from bacteria transformed with pET20b plasmid encoded with CD200 extracellular domain and core streptavidin. The purified CD200-SA protein was bound to biotin-coated fluorescent polystyrene particles of various sizes ranging from 0.15 to 2 µm. THP-1 macrophages were cultivated with CD200-modified micro/nanoparticles in comparison with controls. Our results showed that both nano- and micro-sized particles decorated with CD200 decreased phagocytosis activities of THP-1 macrophages. Such diminution of phagocytosis was examined to be associated with downregulation of Toll-like receptor 4 (TLR4) expression on the surface of macrophages. Moreover, THP-1 macrophages treated with CD200-coated particles decreased the secretion of tumor necrosis factor-α (TNF-α).

Upregulated CD200 in pre-retinal proliferative fibrovascular membranes of proliferative diabetic retinopathy patients and its correlation with vascular endothelial growth factor

OBJECTIVE AND DESIGN

The objective was to determine the expression of CD200 in the pre-retinal proliferative fibrovascular membranes (PFVM) of patients with proliferative diabetic retinopathy (PDR) and to clarify its correlation with vascular endothelial growth factor (VEGF) and corresponding receptors.

METHODS

PFVM samples were collected by vitrectomy from 14 patients with PDR, and 11 non-diabetic patients who accepted vitrectomy for idiopathic epiretinal membranes removal. The expression of CD200, VEGF,VEGF-R1 and VEGF-R2 was measured via qPCR and immunofluorescent staining.

RESULTS

The mRNA level of CD200 was significantly higher in PDR patients than that in control patients. Meanwhile, CD200 and CD31 were found co-located and statistically associated in PFVM of PDR patients. The mRNA levels of VEGF, VEGF-R1 and VEGF-R2 were also significantly higher in PDR patients. Moreover, statistical association was found between CD200 and VEGF, VEGF-R1 in mRNA levels. But there was no significant correlationship between CD200 and VEGF-R2.

CONCLUSIONS

These results suggest a significantly increased expression of CD200 in PFVM of patients with PDR and present a crucial association between CD200 and VEGF-involved pathway. It represents a potential therapy that interfering with CD200 may inhibit the VEFG expression and neovascular formation in PDR patients.

Soluble CD200 Correlates With Interleukin-6 Levels in Sera of COPD Patients: Potential Implication of the CD200/CD200R Axis in the Disease Course

BACKGROUND

COPD represents a multifactorial lung disorder with high morbidity and mortality. Despite intensive research concerning the underlying disease mechanisms, the involvement of the CD200/CD200R axis in supporting or preventing the onset of COPD has not yet been addressed. Since the CD200/CD200R axis is crucially implicated in the maintenance of pulmonary immune homeostasis, we hypothesized that it might be involved in controlling the onset of COPD.

METHODS

To address this, we analyzed the serum samples from COPD patients and normal controls for soluble (s) CD200 and correlated the data to COPD-relevant clinical parameters. In addition, basic studies were conducted in CD200-deficient and wild-type mice in which COPD-like inflammation was induced with elastase/LPS followed by lung and serum component analysis.

RESULTS

We observed a positive correlation between serum sCD200 and IL-6 levels as well as a trend toward a negative correlation of sCD200 with vitamin D3 in COPD patients. Further investigations in mice revealed that despite elevated serum concentration of MMP-9 in CD200KO mice, the early onset of COPD-like lung inflammation was similar in CD200-deficient and wild-type animals in terms of immune cell infiltration, emphysematous changes, and mucus overproduction.

CONCLUSIONS

While our murine studies suggest that the co-inhibitory molecule CD200 does not appear to play a prominent role in the early onset of COPD-like features, correlation of sCD200 serum levels with COPD-related parameters in humans with established disease revealed that the CD200/CD200R axis may be mechanistically linked to the disease course in COPD patients.

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.

CD200/CD200R Paired Potent Inhibitory Molecules Regulating Immune and Inflammatory Responses; Part II : CD 200/CD200R Potential Clinical Applications

CD200 and its receptor were recognized as having the multiple immunoregulatory functions. Their immunoregulatory, suppressive, and tolerogenic potentials could be very effectively exploited in the treatment of many diseases, e.g. Alzheimer disease, rheumatoid arthritis, and allergy to name only some. Many research projects are aimed to develop clinically valuable methods being based on the structure and function of these paired molecules. In this review, we would like to introduce CD200/CD200R functions in a clinical context.

Autoimmune and autoinflammatory mechanisms in uveitis

The eye, as currently viewed, is neither immunologically ignorant nor sequestered from the systemic environment. The eye utilises distinct immunoregulatory mechanisms to preserve tissue and cellular function in the face of immune-mediated insult; clinically, inflammation following such an insult is termed uveitis. The intra-ocular inflammation in uveitis may be clinically obvious as a result of infection (e.g. toxoplasma, herpes), but in the main infection, if any, remains covert. We now recognise that healthy tissues including the retina have regulatory mechanisms imparted by control of myeloid cells through receptors (e.g. CD200R) and soluble inhibitory factors (e.g. alpha-MSH), regulation of the blood retinal barrier, and active immune surveillance. Once homoeostasis has been disrupted and inflammation ensues, the mechanisms to regulate inflammation, including T cell apoptosis, generation of T_reg cells, and myeloid cell suppression in situ, are less successful. Why inflammation becomes persistent remains unknown, but extrapolating from animal models, possibilities include differential trafficking of T cells from the retina, residency of CD8⁺ T cells, and alterations of myeloid cell phenotype and function. Translating lessons learned from animal models to humans has been helped by system biology approaches and informatics, which suggest that diseased animals and people share similar changes in T cell phenotypes and monocyte function to date. Together the data infer a possible cryptic infectious drive in uveitis that unlocks and drives persistent autoimmune responses, or promotes further innate immune responses. Thus there may be many mechanisms in common with those observed in autoinflammatory disorders.

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.

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.

CD200-CD200R signaling suppresses anti-tumor responses independently of CD200 expression on the tumor

Expression of CD200, the gene encoding the ligand for the inhibitory immune receptor CD200R, is an independent prognostic factor for various forms of leukemia predicting worse overall survival of the patients. The enhanced expression of CD200 on the tumors implies that anti-tumor responses can be enhanced by blockage of the CD200-CD200R interaction. Indeed, antibody-mediated blockade of the CD200-CD200R inhibitory axis is currently evaluated in clinical tests to boost immune responses against CD200-expressing tumors. Here, we show that mice lacking CD200, the exclusive ligand for CD200R, are resistant to chemical skin carcinogenesis. Importantly, CD200R controls tumor outgrowth independently of CD200 expression by the tumor cells themselves. Furthermore, Cd200(-/-) mice do not become tolerant to intranasally administered antigens, suggesting that tumor rejection is normally suppressed through CD200-induced immune tolerance. Decreased tumor outgrowth is accompanied by increased expression of the proinflammatory cytokines interleukin (IL)-1β and IL-6 by the lymph node (LN) dendritic cells. During carcinogenesis, skin-draining LNs of Cd200(-/-) mice contain increased numbers of IL-17-producing FoxP3(+) cells, which preferentially home to the tumors. Thus, the CD200-CD200R axis induces tolerance to external and tumor antigens and influences the T-regulatory/Th17 cell ratio. We demonstrate for the first time that the absence of CD200R signaling inhibits outgrowth of an endogenous tumor irrespective of CD200 expression by the tumor cells. This important paradigm shift leads to a much broader applicability of CD200-blockade in the treatment of tumors.

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.

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.

Critical but divergent roles for CD62L and CD44 in directing blood monocyte trafficking in vivo during inflammation

Using noninvasive in vivo imaging and experimental autoimmune uveoretinitis as a model, we show for the first time that the mechanisms controlling blood monocyte recirculation through peripheral and lymphoid tissues alter during inflammation. The recirculation of monocytes in mice with ocular inflammation but not controls was found to depend on the selectin CD62-ligand (CD62L) and on CD44. Not only was rolling efficiency ablated or markedly reduced in antibody-treated mice, but most of the labeled monocytes also disappeared from the circulation within seconds, anti-CD44–treated monocytes homing to the lymph nodes and anti–CD62L-treated monocytes homing to the spleen. Our data indicate that, although PSGL-1 has a partial role in the transmigration of monocytes into the inflamed retina, CD62L has a key role in regulating recruitment of monocytes to lymphoid tissue from the blood during inflammation and that CD44 is required to maintain CD62L⁺ inflammatory monocytes within the circulation during inflammation. This effect was systemic, because sequestered monocytes accumulated in mesenteric as well as draining cervical lymph nodes, and inflammation dependent, because depletion of circulating blood monocytes was much reduced or absent in normal mice and accumulations of adoptively transferred monocytes in the lymphoid tissues did not occur.

The inhibitory CD200R is differentially expressed on human and mouse T and B lymphocytes

To ensure an adequate response against pathogens and prevent unwanted self-reactivity, immune cells need to functionally express both activating and inhibitory receptors. CD200R is an inhibitory receptor mainly expressed on myeloid cells that down-modulates cellular activation both in vivo and in vitro. Although previously mainly studied as a regulator of myeloid function, we now show that CD200R is differentially expressed on human and mouse T-cell subsets. In both species, CD4+ T cells express higher amounts of CD200R than CD8+ T cells, and memory cells express higher amounts of CD200R than naïve or effector cells. CD200R expression is up-regulated on both CD4+ and CD8+ T cells after stimulation in vitro. Furthermore, we show CD200R expression on human and mouse B cells. In human tonsils, CD200R is differentially expressed on B cells, with high expression on memory cells and plasmablasts. Mice lacking the ligand for CD200R, CD200-/- mice, do not show abnormal composition of the lymphocyte compartment and have normal B cell responses to antigenic challenge. Although the functional implications remain to be elucidated, the expression of CD200R on lymphocytes suggests a much broader role for CD200R-mediated immune regulation than previously anticipated.

Monoclonal antibody-mediated CD200 receptor signaling suppresses macrophage activation and tissue damage in experimental autoimmune uveoretinitis

Macrophage responses are regulated by multiple secreted factors as well as by cell surface receptors, including the inhibitory signals resulting from ligation of myeloid CD200 receptors (CD200R) by the widely distributed CD200. In the absence of CD200, animals display increased susceptibility to autoimmunity and earlier onset aggressive autoimmune disease. In these current experiments, an agonist monoclonal rat anti-mouse CD200R (DX109) antibody delivered a negative signal to bone marrow-derived macrophages, which suppressed interferon (IFN)gamma-mediated nitric oxide (NO) and interleukin-6 production. Experimental autoimmune uveoretinitis (EAU) was used as a model of organ-specific autoimmunity in the eye, a tissue with extensive neuronal and endothelial CD200 expression. In mice lacking CD200 (CD200(-/-)), increased numbers of retina-infiltrating macrophages displaying heightened NO responses were observed during EAU. In addition, we aimed to suppress disease by maintaining tonic suppression of macrophage activation via CD200R. Systemically administered DX109 monoclonal antibody suppressed EAU despite maintained T-cell proliferation and IFNgamma production. Furthermore, locally administered DX109 monoclonal antibody resulted in an earlier resolution of disease. These experiments demonstrate that promoting CD200R-mediated signaling can successfully prevent full expression of IFNgamma-mediated macrophage activation and protect against tissue damage during autoimmune responses.

CD200 expression on tumor cells suppresses antitumor immunity: new approaches to cancer immunotherapy

Although the immune system is capable of mounting a response against many cancers, that response is insufficient for tumor eradication in most patients due to factors in the tumor microenvironment that defeat tumor immunity. We previously identified the immune-suppressive molecule CD200 as up-regulated on primary B cell chronic lymphocytic leukemia (B-CLL) cells and demonstrated negative immune regulation by B-CLL and other tumor cells overexpressing CD200 in vitro. In this study we developed a novel animal model that incorporates human immune cells and human tumor cells to address the effects of CD200 overexpression on tumor cells in vivo and to assess the effect of targeting Abs in the presence of human immune cells. Although human mononuclear cells prevented tumor growth when tumor cells did not express CD200, tumor-expressed CD200 inhibited the ability of lymphocytes to eradicate tumor cells. Anti-CD200 Ab administration to mice bearing CD200-expressing tumors resulted in nearly complete tumor growth inhibition even in the context of established receptor-ligand interactions. Evaluation of an anti-CD200 Ab with abrogated effector function provided evidence that blocking of the receptor-ligand interaction was sufficient for control of CD200-mediated immune modulation and tumor growth inhibition in this model. Our data indicate that CD200 expression by tumor cells suppresses antitumor responses and suggest that anti-CD200 treatment might be therapeutically beneficial for treating CD200-expressing cancers.

Identification of novel dendritic cell populations in normal mouse retina

PURPOSE

Whether tissue resident or infiltrating antigen-presenting cells (APCs) are involved in modulating immune responses in the retina and initiating inflammation is controversial. In this histologic study, the authors examine the retinas of mice strains with different susceptibility to experimental autoimmune uveoretinitis (EAU) for tissue resident APC.

METHODS

Retinal wholemounts from normal and inflamed eyes of B10R III, C57BL/6, BALB/c, and ABH Biozii mice were immunostained for APC markers (33D1, CD11c, CD11b, major histocompatibility complex [MHC] class II, F4/80, CD80, CD86, CD205, mPDCA, B220, and GR1) and analyzed by confocal fluorescence microscopy using emission fingerprinting and three-dimensional reconstruction techniques. Hematoxylin and eosin-stained histologic sections were used to evaluate EAU disease scores and to assess outer blood retina barrier (retinal pigment epithelium [RPE]) structures.

RESULTS

A population of 33D1(+) cells was identified exclusively in the peripheral margins and juxtapapillary areas of the retina in normal, nonimmunized C57BL/6 adult mice. These cells were also MHC class II(high), and their location corresponded to sites of earliest inflammation in EAU. Numbers in the papillary area were very low (less than 10), but this region marked the predominant anatomic site for initiation of inflammation in this moderately susceptible strain. The distribution and phenotype of these cells within the retinas differed between mouse strains exhibiting different disease susceptibility. In EAU-resistant BALB/c mice, many more 33D1(+) dendritic cells were present in the normal retina but were MHC class II(low/-). Conversely, no 33D1(+) or MHC class II (+) dendriform cells could be found in the normal retinas of highly EAU-susceptible B10.RIII mice.

CONCLUSIONS

A novel population of 33D1(+) DCs was identified in normal mouse retina. The function of these cells remains to be defined, but increased numbers correlate positively with structural abnormalities in the RPE and increased resistance of the strain to EAU.

Loss of the surface antigen 3G11 characterizes a distinct population of anergic/regulatory T cells in experimental autoimmune encephalomyelitis

T cell anergy is an important mechanism in the induction of peripheral tolerance against autoimmune diseases, yet no surface marker unique to anergic T cells in these diseases has been identified. In this study we induced in vivo anergy by i.v. tolerance against experimental autoimmune encephalomyelitis in myelin basic protein TCR transgenic mice, and showed that the hyporesponsiveness of autoantigen-reactive T cells from tolerized mice was associated with a dramatic loss of 3G11, a cell surface molecule on the surface of CD4+ T cells. Purified 3G11-CD4+ T cells lost autoantigen-induced proliferation and IL-2 production, whereas 3G11+CD4+ T cells retained responsiveness. Furthermore, 3G11- T cells actively suppressed proliferation and Th1 cytokine production of 3G11+ T cells and splenocytes of nontolerized mice. Active suppression by 3G11- T cells was at least partially due to soluble immunoregulatory factors, including IL-10. The T regulatory property of 3G11- T cells was confirmed in vivo because the transfer of purified 3G11- T cells effectively suppressed clinical experimental autoimmune encephalomyelitis. We conclude that loss of the surface molecule 3G11 characterizes a distinct population of anergic/regulatory T cells. This is the first demonstration of the ability to identify and purify anergic T cells by a distinct cell surface marker in an autoimmune disease and paves the way for a better understanding of the mechanism of tolerance in autoimmune diseases.

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.

Recombinant CD200 Protein Does Not Bind Activating Proteins Closely Related to CD200 Receptor

CD200 (OX2) is a cell surface glycoprotein that interacts with a structurally related receptor (CD200R) expressed mainly on myeloid cells and is involved in regulation of macrophage and mast cell function. In mouse there are up to five genes related to CD200R with conflicting data as to whether they bind CD200. We show that mouse CD200 binds the inhibitory receptor CD200R with a comparable affinity (Kd = 4 microM) to those found for the rat and human CD200 CD200R interactions. CD200 gave negligible binding to the activating receptors, CD200RLa, CD200RLb, and CD200RLc, by direct analysis at the protein level using recombinant monomeric and dimeric fusion proteins or to CD200RLa and CD200RLb when expressed at the cell surface. An additional potential activating gene, CD200RLe, found in only some mouse strains also did not bind CD200. Thus, the CD200 receptor family consists of both activatory and inhibitory members like several other paired ligand receptors, such as signal regulatory protein, killer cell Ig-like receptor/KAR, LY49, dendritic cell immunoreceptor/dendritic cell immunoactivating receptor, and paired Ig-like type 2 receptor. Although the ligand for the inhibitory product is a widely distributed host protein, the ligands of the activating forms remain to be identified, and one possibility is that they are pathogen components.