Immune Inhibitory Ligand CD200 Induction by TLRs and NLRs Limits Macrophage Activation to Protect the Host from Meningococcal Septicemia

Leishmania antigens activated CD4+ T cells expressing CD200R receptors are the prime IL-10 producing phenotype and an important determinant of visceral leishmaniasis pathogenesis

The excessive production of IL-10, an anti-inflammatory cytokine, by Leishmania antigen-activated T cells is supposed to be a key player in the onset and progression of visceral leishmaniasis (VL). The IL-10-producing sources in VL remain unidentified and uncharacterized. In this study, we reveal that antigen-activated CD4+ T cells, i.e., CD44+CD4+ T cells expressing CD200R receptors, are the prime IL-10-producing phenotypes in Leishmania donovani infection-induced pathogenesis. These phenotypes are separate from CD25+Foxp3+CD4+ T regulatory cells, which are classical IL-10-producing phenotypes. In order to ascertain the role of CD200R and CD25 receptors in IL-10 overexpression-associated VL pathogenesis, we abrogated CD200R and CD25 receptor-mediated signaling in the infected mice. The splenic load of parasites and the size of the liver and spleen were significantly reduced in CD200-blocked mice as compared to CD25-blocked mice. Further, the CD200 blocking polarized CD4+ T cells to pro-inflammatory cytokines-producing phenotypes, as we observed a higher frequency of IFN-γ, TNF-α, and IL-12 positive cells as compared to controls including the CD25 blocking. Our findings suggest that in L. donovani infection-induced pathogenesis the expression of CD200R on antigen-activated T cells helps them to acquire IL-10-producing abilities as part of its one of the survival strategies. However, more studies would be warranted to better understand CD200R receptors role in VL pathogenesis and to develop the next generation of therapeutic and prophylactic control measures.

The kinetics of inhibitory immune checkpoints during and post-COVID-19: the knowns and unknowns

The immune system is tightly regulated to prevent immune reactions to self-antigens and to avoid excessive immune responses during and after challenges from non-self-antigens. Inhibitory immune checkpoints (IICPs), as the major regulators of immune system responses, are extremely important for maintaining the homeostasis of cells and tissues. However, the high and sustained co-expression of IICPs in chronic infections, under persistent antigenic stimulations, results in reduced immune cell functioning and more severe and prolonged disease complications. Furthermore, IICPs-mediated interactions can be hijacked by pathogens in order to evade immune induction or effector mechanisms. Therefore, IICPs can be potential targets for the prognosis and treatment of chronic infectious diseases. This is especially the case with regards to the most challenging infectious disease of recent times, coronavirus disease-2019 (COVID-19), whose long-term complications can persist long after recovery. This article reviews the current knowledge about the kinetics and functioning of the IICPs during and post-COVID-19.

Tim-3 Is Differentially Expressed during Cell Activation and Interacts with the LSP-1 Protein in Human Macrophages

T-cell Immunoglobulin and Mucin Domain 3 (TIM-3) is an immune checkpoint receptor known to regulate T-cell activation and has been targeted for immunotherapy in cancer and other diseases. However, its expression and function in other cell types, such as macrophages, are poorly understood. This study investigated TIM-3 expression in human macrophages polarized to M1 (stimulated with IFN-γ and LPS) and M2 (stimulated with IL-4 and IL-13) phenotypes using an in vitro model. Our results show that M1 macrophages have a lower frequency of TIM-3+ cells compared to M2 macrophages at 48 and 72 hr poststimulation. Additionally, we observed differential levels of soluble ADAM 10, an enzyme responsible for TIM-3 release, in the supernatants of M1 and M2 macrophages at 72 hr. We also found that the TIM-3 intracellular tail might associate with lymphocyte-specific protein 1 (LSP-1), a protein implicated in cell motility and podosome formation. These findings enhance our understanding of TIM-3 function in myeloid cells such as macrophages and may inform the development of immunotherapies with reduced immune-related adverse effects.

The immunoregulatory protein CD200 as a potentially lucrative yet elusive target for cancer therapy

CD200 is an immunoregulatory cell surface ligand with proven pro-tumorigenic credentials via its ability to suppress CD200 receptor (CD200R)-expressing anti-tumor immune function. This definitive role for the CD200-CD200R axis in regulating an immunosuppressive tumor microenvironment has garnered increasing interest in CD200 as a candidate target for immune checkpoint inhibition therapy. However, while the CD200 blocking antibody samalizumab is still in the early stages of clinical testing, alternative mechanisms for the pro-tumorigenic role of CD200 have recently emerged that extend beyond direct suppression of anti-tumor T cell responses and, as such, may not be susceptible to CD200 antibody blockade. Herein, we will summarize the current understanding of CD200 expression and function in the tumor microenvironment as well as alternative strategies for potential neutralization of multiple CD200 mechanisms in human cancers.

CD200R1 promotes interleukin-17 production by group 3 innate lymphoid cells by enhancing signal transducer and activator of transcription 3 activation

Psoriasis is a common chronic inflammatory skin disease with no cure. It is driven by the interleukin (IL)-23/IL-17A axis and type 17 T helper cells; however, recently, group 3 innate lymphoid cells (ILC3s) have also been implicated. Despite being the focus of much research, factors regulating the activity of ILC3s remain incompletely understood. Immune regulatory pathways are particularly important at barrier sites, such as the skin, gut, and lungs, which are exposed to environmental substances and microbes. CD200R1 is an immune regulatory cell surface receptor that inhibits proinflammatory cytokine production in myeloid cells. CD200R1 is also highly expressed on ILCs, where its function remains largely unexplored. We previously observed reduced CD200R1 signaling in psoriasis-affected skin, suggesting that dysregulation may promote disease. Here, we show that contrary to this, psoriasis models are less severe in CD200R1-deficient mice due to reduced IL-17 production. Here, we uncover a key cell-intrinsic role for CD200R1 in promoting IL-23-driven IL-17A production by ILC3s by promoting signal transducer and activator of transcription 3 activation. Therefore, contrary to its inhibitory role in myeloid cells, CD200R1 is required on ILC3 to promote IL-23-stimulated signal transducer and activator of transcription 3 activation, triggering optimal IL-17 production.

The lipid peroxidation product 4-hydroxynonenal inhibits NLRP3 inflammasome activation and macrophage pyroptosis

Pyroptosis is a form of cell death triggered by the innate immune system that has been implicated in the pathogenesis of sepsis and acute lung injury. At the cellular level, pyroptosis is characterized by cell swelling, membrane rupture, and release of inflammatory cytokines, such as IL-1β. However, the role of endogenous lipids in pyroptosis remains underappreciated. We discovered that 4-hydroxynonenal (HNE), a major endogenous product of lipid peroxidation, inhibited pyroptosis and inflammasome activation. HNE at physiological concentrations (3 µM) blocked nigericin and ATP-induced cell death, as well as secretion of IL-1β, by mouse primary macrophages and human peripheral blood mononuclear cells. Treatment with HNE, or an increase of endogenous HNE by inhibiting glutathione peroxidase 4, reduced inflammasome activation in mouse models of acute lung injury and sepsis. Mechanistically, HNE inhibited the NLRP3 inflammasome activation independently of Nrf2 and NF-κB signaling, and had no effect on the NLRC4 or AIM2 inflammasome. Furthermore, HNE directly bound to NLRP3 and inhibited its interaction with NEK7. Our findings identify HNE as a novel, endogenous inhibitor of the NLRP3 inflammasome.

Immune Targeted Therapies for COVID-19 Infection: A Narrative Review

In December 2019, the coronavirus disease-2019 (COVID-19) outbreak emerged in Wuhan, China. The World Health Organization officially declared it a pandemic on March 11, 2020. Reports indicated that the associated mortality of the infection is quite higher in the elderly, individuals with specific comorbidities (such as diabetes mellitus), and generally the ones with a compromised immune system. A cohort study in Wuhan, China, reported a dysregulated immune response in 452 patients with laboratory-confirmed COVID-19. As a result of this suppressed immune response, an increase in neutrophil to lymphocyte ratio, T lymphopenia, and a decrease in CD4⁺ T cells were all common laboratory findings, especially in severe cases. On the other hand, there is substantial evidence of T cell exhaustion in critically ill patients. Accordingly, the immune system seems to play an important role in the prognosis and pathogenesis of the disease. Therefore, this study aims to review the evidence on the immune response dysregulation in COVID-19 infection and the potential role of immunoregulatory treatments such as immune checkpoint inhibitors, interferons, and CD200 inhibitors in altering disease prognosis, especially in critically ill patients.

Phosphodiesterase 10A Is a Key Mediator of Lung Inflammation

Cyclic nucleotides cAMP and cGMP are important regulators of immune cell functions. Phosphodiesterases (PDEs) hydrolyze cAMP and/or cGMP and, thus, play crucial roles in cyclic nucleotide homeostasis. Abnormal alterations of PDE expression have been implicated in several diseases. To understand the function of PDEs in macrophages, we screened for all PDE genes in both peritoneal and alveolar macrophages from C57BL/6J mice and found that PDE4B and PDE10A are highly induced by LPS. A number of PDE4 inhibitors have been used clinically for the treatment of inflammatory lung diseases. However, the role of PDE10A in inflammation is still poorly understood. We therefore investigated the role of PDE10A in macrophage inflammatory response in vitro and acute lung inflammation in vivo. We found that LPS induces a sustained PDE10A expression in macrophages, which is different from a transient induction by PDE4B. PDE10A inhibition blocked LPS-induced MCP-1 expression, but not TNF-α, whereas PDE4B inhibition blocked LPS-induced TNF-α expression, but not MCP-1. In addition, PDE10A inhibition or deficiency decreased LPS-induced HIF-1α protein expression and subsequently suppressed MCP-1 expression. In vivo, PDE10A expression was also elevated in lung tissue after LPS exposure. Global PDE10A knockout or systemic administration of the PDE10A inhibitor TP-10 in mice significantly suppressed inflammatory molecule levels in the lung tissue and bronchoalveolar lavage fluid as well as inflammatory cell infiltration. These findings show that PDE10A plays a critical role in lung inflammation by promoting the activation of resident macrophages and infiltration of neutrophils.

The CD200 Regulates Inflammation in Mice Independently of TNF-α Production

Inflammatory bowel disease is characterized by the infiltration of immune cells and chronic inflammation. The immune inhibitory receptor, CD200R, is involved in the downregulation of the activation of immune cells to prevent excessive inflammation. We aimed to define the role of CD200R ligand-CD200 in the experimental model of intestinal inflammation in conventionally-reared mice. Mice were given a dextran sodium sulfate solution in drinking water. Bodyweight loss was monitored daily and the disease activity index was calculated, and a histological evaluation of the colon was performed. TNF-α production was measured in the culture of small fragments of the distal colon or bone marrow-derived macrophages (BMDMs) cocultured with CD200⁺ cells. We found that Cd200^-/- mice displayed diminished severity of colitis when compared to WT mice. Inflammation significantly diminished CD200 expression in WT mice, particularly on vascular endothelial cells and immune cells. The co-culture of BMDMs with CD200⁺ cells inhibited TNF-α secretion. In vivo, acute colitis induced by DSS significantly increased TNF-α secretion in colon tissue in comparison to untreated controls. However, Cd200^-/- mice secreted a similar level of TNF-α to WT mice in vivo. CD200 regulates the severity of DSS-induced colitis in conventionally-reared mice. The presence of CD200⁺ cells decreases TNF-α production by macrophages in vitro. However, during DDS-induced intestinal inflammation secretion of TNF-α is independent of CD200 expression.

Vinpocetine protects against the development of experimental abdominal aortic aneurysms

Abdominal aortic aneurysm (AAA), commonly occurring in the aged population, is a degenerative disease that dilate and weaken infrarenal aorta due to progressive degeneration of aortic wall integrity. Vinpocetine, a derivative of alkaloid vincamine, has long been used for cerebrovascular disorders and cognitive impairment in the aged population. Recent studies have indicated that vinpocetine antagonizes occlusive vascular disorders such as intimal hyperplasia and atherosclerosis. However, its role in vascular degenerative disease AAA remains unexplored. Herein, we determined the effect of vinpocetine on the formation of AAA as well as the intervention of pre-existing moderate AAA. AAA was induced by periaortic elastase application in C57BL/6J mice. Systemic vinpocetine treatment was applied daily via intraperitoneal injection. We showed that vinpocetine pre-treatment remarkably attenuated aneurysmal dilation assessed by diameter and volume. More importantly, vinpocetine also significantly suppressed the progression of pre-existing moderate AAA in a post-intervention model. Vinpocetine improved multiple cellular and molecular changes associated with AAA, such as elastin degradation, media smooth muscle cell depletion, collagen fibers remodeling and macrophage infiltration in aneurysmal tissues. Vinpocetine potently suppressed tumor necrosis factor-α-induced nuclear factor kappa-light-chain-enhancer of activated B cells activation and proinflammatory mediator expression in primary cultured macrophages in vitro, as well as in the aorta wall in vivo, suggesting vinpocetine conferred anti-AAA effect at least partially via the inhibition of inflammation. Taken together, our findings reveal a novel role of vinpocetine in AAA formation, development and progression. Given the excellent safety profile of vinpocetine, the present study suggests vinpocetine may be a novel therapeutic agent for AAA prevention and treatment.

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.

Cross-Talk Between Alveolar Macrophages and Lung Epithelial Cells is Essential to Maintain Lung Homeostasis

The main function of the lung is to perform gas exchange while maintaining lung homeostasis despite environmental pathogenic and non-pathogenic elements contained in inhaled air. Resident cells must keep lung homeostasis and eliminate pathogens by inducing protective immune response and silently remove innocuous particles. Which lung cell type is crucial for this function is still subject to debate, with reports favoring either alveolar macrophages (AMs) or lung epithelial cells (ECs) including airway and alveolar ECs. AMs are the main immune cells in the lung in steady-state and their function is mainly to dampen inflammatory responses. In addition, they phagocytose inhaled particles and apoptotic cells and can initiate and resolve inflammatory responses to pathogens. Although AMs release a plethora of mediators that modulate immune responses, ECs also play an essential role as they are more than just a physical barrier. They produce anti-microbial peptides and can secrete a variety of mediators that can modulate immune responses and AM functions. Furthermore, ECs can maintain AMs in a quiescent state by expressing anti-inflammatory membrane proteins such as CD200. Thus, AMs and ECs are both very important to maintain lung homeostasis and have to coordinate their action to protect the organism against infection. Thus, AMs and lung ECs communicate with each other using different mechanisms including mediators, membrane glycoproteins and their receptors, gap junction channels, and extracellular vesicles. This review will revisit characteristics and functions of AMs and lung ECs as well as different communication mechanisms these cells utilize to maintain lung immune balance and response to pathogens. A better understanding of the cross-talk between AMs and lung ECs may help develop new therapeutic strategies for lung pathogenesis.

The CD200-CD200R cross-talk helps Leishmania donovani to down regulate macrophage and CD4+CD44+ T cells effector functions in an NFκB independent manner

The lacuna in the knowledge of immunobiology, especially in visceral infections that are fatal if left untreated, are a major hurdle in getting a vaccine candidate for leishmaniasis. Till date, only a few drugs are available to combat human leishmaniasis and a vaccine candidate either prophylactic or preventive is still awaited. Therefore, identification of host and parasitic factors involved in the regulation of specific immune mechanisms are essentially needed. In this study, we observed that CD200-CD200R immune inhibitory axis regulates host macrophages effectors properties and helps antigen experienced T cells (CD4⁺CD44⁺ T cells) to acquire anti-inflammatory cytokines (IL-4, IL-10, TGF-β, IL-27) producing abilities in an NFkB independent manner. After CD200 blocking the macrophages effectively inhibited proliferation of Leishmania amastigotes and also induced the production of IL-12, IFN-γ, TNF-α and nitric oxide (NOx). Further, the blocking of CD200 signaling also restored macrophages MHC-II expression and helped CD4⁺CD44⁺ T cells to produce pro-inflammatory cytokines like IL-2, IL-12 and IFN-γ. The finding of this study suggested the importance of immune inhibitory mechanisms in controlling Leishmania growth and survival and therefore, requires more studies to understand its role in vaccine induced immunity.

CD200-CD200R1 signalling attenuates imiquimod-induced psoriatic inflammation by inhibiting the activation of skin inflammatory macrophages

Psoriasis is an autoimmune skin disease caused by interactions between keratinocytes and immune cells, such as macrophages. CD200 is expressed on the surface of various cell types, and its receptor, CD200R1, belongs to a family of immunosuppressive receptors that are mainly expressed on myeloid cells. CD200/CD200R1 signalling is associated with the prevention of autoimmune diseases; however, the role of CD200/CD200R1 signalling in the pathogenesis of psoriasis remains unknown. In this study, we detected in vivo effect of the CD200 protein on psoriasis and in vitro effects of CD200 on macrophages and keratinocytes co-cultured with macrophages were also evaluated. Our data showed that the expression of CD200 and CD200R1 was decreased and the expression of macrophage-related pro-inflammatory factors (IL-6, IL-1β, TNF-α) was increased in IMQ-induced psoriasis-like skin of mice. After subcutaneous injection of CD200, the symptoms were alleviated, local expression of CD200R1 was markedly induced, infiltrated CD68⁺ cells were significantly reduced and the expression levels of IL-6, IL-1β, and TNF-α were strongly downregulated. In in vitro experiments, CD200 suppressed the migration of macrophages, induced CD200R1 expression on the surface of macrophages, and decreased the levels of pro-inflammatory factors. Western blot (WB) data showed that the CD200-CD200R1 reaction controlled the activation of inflammatory macrophages by inhibiting the NF-κB signalling pathway. These results demonstrate that CD200-CD200R1 signalling can reduce IMQ-induced psoriasis-like skin inflammation by inhibiting the activation of macrophages.

CD200R1 regulates eosinophilia during pulmonary fungal infection in mice

CD200 receptor 1(CD200R1) signalling limits myeloid cell responses and reduces autoimmunity, alloimmunity and viral‐mediated immunopathology, but has never been examined in the context of eosinophilic inflammation. Susceptibility to lung fungal infection is associated with T‐helper 2 (Th2) cytokine dominated responses and strong eosinophilic pathology. Blockade of CD200R1 enhances type I cytokine responses in many infectious and non‐infectious settings and so may promote a more protective response to fungal infection. By contrast, we demonstrate that, rather than promoting type I cytokine responses, CD200R1 blockade enhanced eosinophilia in a mouse model of Cryptococcus neoformans infection, whereas CD200R1 agonism reduced lung eosinophilia – with neither strategy completely altering fungal burden. Thus, we reveal a surprising disconnect between pulmonary eosinophilia and cryptococcal burden and dissemination. This research has 2 important implications. Firstly, a lack of CD200R1 signalling enhances immune responses regardless of cytokine polarisation, and secondly reducing eosinophils does not allow protective immunity to develop in susceptible fungal system. Therefore, agonists of CD200R1 may be beneficial for eosinophilic pathologies.

CD200 in asthma

Constant exposure to foreign particles in the airways requires tight immune regulation in order to maintain sufficient anti-microbial defences, while preventing immunopathological responses that could impair gas exchange. Dysregulation of immunoregulatory pathways has been associated with asthma and allergy. This review will focus on the CD200 regulatory pathway and its role in the asthmatic cascade. CD200 and its receptors are highly expressed in the lung, on epithelial cells and leukocytes, and emerging evidence links dysregulation of the CD200 pathway with asthma. Moreover, pharmacological modulation of CD200 receptors was shown to improve clinical and inflammatory outcomes of preclinical asthma models. Therefore, the involvement of CD200 in asthma is increasingly recognized and preclinical studies support the contention that it could constitute an additional target to alleviate asthma exacerbation and/or reduce disease severity.

CD200-CD200R1 inhibitory signaling prevents spontaneous bacterial infection and promotes resolution of neuroinflammation and recovery after stroke

BACKGROUND

Ischemic stroke results in a robust inflammatory response within the central nervous system. As the immune-inhibitory CD200-CD200 receptor 1 (CD200R1) signaling axis is a known regulator of immune homeostasis, we hypothesized that it may play a role in post-stroke immune suppression after stroke.

METHODS

In this study, we investigated the role of CD200R1-mediated signaling in stroke using CD200 receptor 1-deficient mice. Mice were subjected to a 60-min middle cerebral artery occlusion and evaluated at days 3 and 7, representing the respective peak and early resolution stages of neuroinflammation in this model of ischemic stroke. Infarct size and behavioral deficits were assessed at both time points. Central and peripheral cellular immune responses were measured using flow cytometry. Bacterial colonization was determined in lung tissue homogenates both after acute stroke and in an LPS model of systemic inflammation.

RESULTS

In wild-type (WT) animals, CD200R1 was expressed on infiltrating monocytes and lymphocytes after stroke but was absent on microglia. Early after ischemia (72 h), CD200R1-knockout (KO) mice had significantly poorer survival rates and an enhanced susceptibility to spontaneous bacterial colonization of the respiratory tract compared to wild-type (WT) controls, despite no difference in infarct or neurological deficits. While the CNS inflammation was resolved by day 7 post-stroke in WT mice, brain-resident microglia and monocyte activation persisted in CD200R1-KO mice, accompanied by a delayed, augmented lymphocyte response. At this time point, CD200R1-KO mice displayed greater weight loss, more severe neurological deficits, and impaired motor function compared to WT. Systemically, CD200R1-KO mice exhibited signs of persistent infection including lymphopenia, T cell activation and memory conversion, and narrowing of the TCR repertoire. These findings were confirmed in a second model of acute neuroinflammation induced by systemic endotoxin challenge.

CONCLUSION

This study defines an essential role of CD200-CD200R1 signaling in stroke. Loss of CD200R1 led to high mortality, increased rates of post-stroke infection, and enhanced entry of peripheral leukocytes into the brain after ischemia, with no increase in infarct size. This suggests that the loss of CD200 receptor leads to enhanced peripheral inflammation that is triggered by brain injury.

CD200 Modulates S. aureus-Induced Innate Immune Responses Through Suppressing p38 Signaling

Rapid activation of macrophages plays a central role in eliminating invading bacteria as well as in triggering the inflammatory responses, but how the anti-bacterial and the inflammatory responses are coordinated, in terms of macrophages, is not completely understood. In this study, we demonstrated that Staphylococcus aureus (S. aureus) induced the expression of CD200 in murine macrophages in a dose-dependent manner. We found that CD200 significantly suppressed the S. aureus-induced production of nitric oxide and proinflammatory cytokines in mouse macrophages. Concurrently, the bactericidal capability of macrophages was boosted upon the deletion of CD200. Furthermore, our data demonstrated that p38 mitogen-activated protein kinase (MAPK) was selectively down-regulated by CD200 administration, while enhanced upon CD200 silence in response to staphylococcal infection. The negative effect of CD200 siRNA on NO production in macrophages was largely abrogated upon the inhibition of p38 signaling, implying its critical involvement in this regulation. Together, our data demonstrate that CD200 plays a central role in regulating the inflammatory responses and the anti-bacterial activity of macrophages, at least partially, through suppressing p38 activity.

The pro-tumor effect of CD200 expression is not mimicked by agonistic CD200R antibodies

Tumor-infiltrating immune cells can impact tumor growth and progression. The inhibitory CD200 receptor (CD200R) suppresses the activation of myeloid cells and lack of this pathway results in a reduction of tumor growth, conversely a tumorigenic effect of CD200R triggering was also described. Here we investigated the role of CD200R activation in syngeneic mouse tumor models. We showed that agonistic CD200R antibody reached tumors, but had no significant impact on tumor growth and minor effect on infiltration of immune myeloid cells. These effects were reproduced using two different anti-CD200R clones. In contrast, we showed that CD200-deficiency did decrease melanoma tumor burden. The presence of either endogenous or tumor-expressed CD200 restored the growth of metastatic melanoma foci. On the basis of these findings, we conclude that blockade of the endogenous ligand CD200 prevented the tumorigenic effect of CD200R-expressing myeloid cells in the tumor microenvironment, whereas agonistic anti-CD200R has no effect on tumor development.

Isolation and characterization of ovine monocyte-derived macrophages from peripheral blood

As key elements and targets for various intracellular pathogens, macrophages play an essential role in host defense. Although ovine peripheral blood monocyte-derived cell lines have been established, their phenotypic characteristics and functional properties remain unknown. We have established several ovine macrophage cell lines from peripheral blood adherent cells that can proliferate spontaneously in long-term culture in vitro. Characteristics of macrophages were shaped by cell morphology, cell adhesion, expression of cell surface markers, phagocytic activity and inflammatory response. Furthermore, the differences of genes expression (such as membrane proteins, pro-inflammatory cytokines and chemokine) were compared between blood macrophages (BMs) and alveolar macrophages (AMs), and between BMs and splenic macrophages (SMs), respectively. The expression of membrane genes (CD11b and CD80), pro-inflammatory cytokines (IL-1β and TNFα) and chemokines (IL-8/CXCL8 and CCL-21) was lower than that in AMs or SMs, but not CD200. Moreover, BMs maintained lower expression level of M1 macrophage related genes (iNOS and IDO), but high expression levels of M2 macrophage related genes (ARG2 and TGFβ1). BMs showed lower phagocytic ability than AMs and SMs. Compared with AMs, BMs showed higher salmonella proliferation rate within cells. Collectively, BMs could suppress inflammatory responses and possessed partly phenotypic characteristics of M2 macrophages.

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.

Centrin-Deleted Leishmania donovani Parasites Help CD4+ T Cells to Acquire Th1 Phenotype and Multi-Functionality Through Downregulation of CD200-CD200R Immune Inhibitory Axis

The protozoan parasite Leishmania has evolved several strategies to undermine host defense mechanisms by inducing Th2-type adaptive immunity and suppressing effector functions of Th1 phenotype. In our earlier studies, using centrin gene-deleted Leishmania (LdCen^−/−) parasites as an immunogen, we have shown induction of an effective Th1-type immunity and robust memory responses that mediate protection against virulent challenge. However, role of inhibitory signals in Leishmania vaccine induced immunity in general, and LdCen^−/− in particular has not been studied. Herein, we report that immunization with LdCen^−/− parasites produces more functional Th1-type CD4⁺ T cells via downregulation of CD200–CD200R immune inhibitory axis compared to wild-type infection. We found that expression of CD200 and CD200R was significantly reduced in LdCen^−/− infection compared to wild-type infection. Diminished CD200–CD200R signaling in LdCen^−/− infection enabled proliferation of CD4⁺ T cells and resulted in the induction of pro-inflammatory cytokines and suppression of anti-inflammatory response. The effects of diminished CD200–CD200R signaling by LdCen^−/− were most evident in the suppression of IL-10-producing CD4⁺ T cells that helped enhance more Th1 cytokine producing and multi-functional T cells compared to wild-type infection. In vivo blocking of CD200 expression with anti-CD200 treatment in wild-type infected mice limited Th2 response as indicated by reduction of IL-10-producing Tr1 cells and reduced parasite burden. On the other hand, treatment with anti-CD200 improved the LdCen^−/− vaccine-induced multifunctional response and reduction in splenic parasite load upon challenge. Taken together, these studies demonstrate the role of CD200–CD200R signals in the protection induced by LdCen^−/− parasites.

Role of microglia-neuron interactions in diabetic encephalopathy

In the central nervous system, the primary immune cells, the microglia, prevent pathogenic invasion as the first line of defense. Microglial energy consumption is dependent on their degree of activity. Microglia express transporters for the three primary energy substrates (glucose, fatty acids, glutamine) and regulate diabetic encephalopathy via microglia-neuron interactions. Microglia may play a sentry role for rapid protection or even ablation of impaired neurons. Neurons exhibit hyperactivity in response to hyperglycemia, hyperlipidemia, and neurotoxic factors and release potential microglial activators. Microglial activation is also regulated by proinflammatory factors, caspase-3 activity, P2X₇ receptor, interferon regulatory factor-8, and glucocorticoids. Modulation of microglia in diabetic encephalopathy may involve CX3CL1, p38 MAPK, purinergic, and CD200/CD200R signaling pathways, and pattern recognition receptors. The microglia-neuron interactions play an important role in diabetic encephalopathy, and modulation of microglial activation may be a therapeutic target for diabetic encephalopathy.

Receptors That Inhibit Macrophage Activation: Mechanisms and Signals of Regulation and Tolerance

A variety of receptors perform the function of attenuating or inhibiting activation of cells in which they are expressed. Examples of these kinds of receptors include TIM-3 and PD-1, among others that have been widely studied in cells of lymphoid origin and, though to a lesser degree, in other cell lines. Today, several studies describe the function of these molecules as part of the diverse mechanisms of immune tolerance that exist in the immune system. This review analyzes the function of some of these proteins in monocytes and macrophages and as well as their participation as inhibitory molecules or elements of immunological tolerance that also act in innate defense mechanisms. We chose the receptors TIM-3, PD-1, CD32b, and CD200R because these molecules have distinct functional characteristics that provide examples of the different regulating mechanisms in monocytes and macrophages.

Expression of CD200R1 and its Ligand CD200 on T-helper Lymphocytes of Pediatric Patients with Ulcerative Colitis and Crohn's Disease

BACKGROUND

CD200 and its receptor CD200R are both type I membrane glycoproteins that modulate the activity of myeloid and lymphoid cells, and their interaction is functionally important in the suppression of effector T-cell responses by regulatory T-cells. We aimed to investigate the extent of expression of CD200 and CD200R1 on CD4+ T-cells in blood of children with ulcerative colitis (UC) and Crohn's disease (CD) and to explore their correlations with effector T cell subsets, regulatory T cells (Treg), and routine clinical and serological markers.

METHODS

The frequencies of blood CD4+ expressing CD200 and CD200R1 as well as T-helper CD4+CD25+Foxp3+ Treg, CD4+ IL-17+ (Th17), CD4+ IFN-γ + (Th1), and CD4+IL-4+ (Th2) were estimated by flow cytometry in 23 patients with CD, 14 with UC, and 14 healthy volunteers (HCs). The clinical and inflammatory markers were also investigated.

RESULTS

IBD patients showed decreased CD4+CD200R1+ T-cells, whereas, CD4+CD200+ T-cells were significantly higher in patient groups compared with healthy controls. Treg cells were found significantly decreased in the patients with UC and CD compared with healthy controls (both at p < 0.01). The percentage of Th17 was found significantly increased in CD (p < 0.05) compared with UC patients and healthy subjects (p = 0.014). CD200+CD4+ T-cells showed significant positive correlations with ESR, Th1, and Th17 (r = 0.438, p < 0.05; r = 0.411, p < 0.05; r = 0.492, p < 0.01, respectively). CD200R1+CD4+ T-cells correlated positively with Th2 and Treg (r = 0.482, p < 0.01, and r = 0.457, p < 0.01, respectively) and negatively with ESR (r = -0.387, p < 0.01).

CONCLUSIONS

Our study demonstrates an aberrant expression of CD200/CD200R1 on CD4+ T-cells in IBD patients and these data may have potent pathological significance in IBD pathophysiology.

Toll-Like Receptor Ligand-Induced Liver Injury in D-Galactosamine-Sensitized Mice: Differences between TLR7/8 and TLR9 Ligands, Cytokine Patterns, and Cross-Tolerance Induction by TLR2 Ligand Pretreatment

Administration of Toll-like receptor ligands (TLRLs) is known to cause liver injury in D-galN-sensitized mice. In the present study, we aimed to complement preceding reports on the TLRL/D-galN system by analyzing comparisons among TLRLs, mouse strain dependence, effects on serum levels of cytokines, and effects of sequential administrations of different TLRLs. In a preliminary set of analyses, we first confirmed that liver failure can be induced by diverse TLRLs, including LTA and R848 in combination with D-galN. Analysis using TLR4-deficient mice excluded potential confounding effects of endogenous TLR4Ls that include those referred to as DAMPs in CpG DNA/D-galN hepatotoxicity. Subsequently, we showed that LTA pretreatment could prevent mortality in both CpG DNA/D-galN- and R848/D-galN-treated mice compared to without pretreatment. Incidentally, we observed that without the LTA pretreatment, CpG DNA/D-galN showed relatively higher liver-specific toxicity whereas R848/D-galN showed more symptoms of multiple organ failure. These findings suggest that, in D-galN-sensitized mice, different TLRLs not only show similarity in the ability to induce hepatic injury but also exhibit distinctive abilities in inducing systemic inflammation and multiple organ failure. These findings also suggest the potential usefulness of cross-tolerance induction using LTA in the prevention of organ failure in TLRL-mediated acute inflammation.

Newly Formed Endothelial Cells Regulate Myeloid Cell Activity Following Spinal Cord Injury via Expression of CD200 Ligand

The central nervous system (CNS) is endowed with several immune-related mechanisms that contribute to its protection and maintenance in homeostasis and under pathology. Here, we discovered an additional mechanism that controls inflammatory responses within the CNS milieu under injurious conditions, involving CD200 ligand (CD200L) expressed by newly formed endothelial cells. We observed that CD200L is constitutively expressed in the mouse healthy CNS by endothelial cells of the blood-cerebrospinal fluid barrier and of the spinal cord meninges, but not by the endothelium of the blood-spinal cord barrier. Following spinal cord injury (SCI), newly formed endothelial cells, located only at the epicenter of the lesion site, expressed CD200L. Moreover, in the absence of CD200L expression by CNS-resident cells, functional recovery of mice following SCI was impaired. High throughput single-cell flow cytometry image analysis following SCI revealed CD200L-dependent direct interaction between endothelial and local CD200R⁺ myeloid cells, including activated microglia and infiltrating monocyte-derived macrophages (mo-MΦ). Absence of CD200L signaling, both in vitro and in vivo, resulted in a higher inflammatory response of the encountering macrophages, manifested by elevation in mRNA expression of Tnfα and Il1β, increased intracellular TNFα immunoreactivity, and reduced expression levels of macrophage factors that are associated with resolution of inflammation, Dectin-1, CD206 (mannose receptor), and IL-4R. Collectively, our results highlight the importance of CD200-mediated immune dialogue between endothelial cells and the local resident microglia and infiltrating mo-MΦ within the lesion area, as a mechanism that contributes to regulation of inflammation following acute CNS injury.

SIGNIFICANCE STATEMENT

This manuscript focuses on a novel mechanism of inflammation-regulation following spinal cord injury (SCI), orchestrated by CD200-ligand (CD200L) expressed by newly formed endothelial cells within the lesion site. Our study reveals that, in homeostasis, CD200L is expressed by endothelial cells of the mouse blood-cerebrospinal fluid barrier and of the blood-leptomeningeal barrier, but not by endothelial cells of the blood-spinal cord barrier. Following SCI, newly formed endothelial cells located within the epicenter of the lesion site were found to express CD200L at time points that were shown to be critical for repair. Our results reveal a direct interaction between CD200L⁺ endothelial cells and CD200R⁺ microglia and macrophages, resulting in attenuated inflammation, biasing macrophage phenotype toward inflammation-resolving cells, and promotion of functional recovery following SCI.

THP-1 and human peripheral blood mononuclear cell-derived macrophages differ in their capacity to polarize in vitro

Macrophages (Mφ) undergo activation to pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes in response to pathophysiologic stimuli and dysregulation of the M1-M2 balance is often associated with diseases. Therefore, studying mechanisms of macrophage polarization may reveal new drug targets. Human Mφ polarization is generally studied in primary monocyte-derived Mφ (PBMC Mφ) and THP-1-derived Mφ (THP-1 Mφ). We compared the polarization profile of THP-1 Mφ with that of PBMC Mφ to assess the alternative use of THP-1 for polarization studies. Cellular morphology, the expression profiles of 18 genes and 4 cell surface proteins, and phagocytosis capacity for apoptotic cells and S. aureus bioparticles were compared between these Mφ, activated towards M1, M2a, or M2c subsets by stimulation with LPS/IFNγ, IL-4, or IL-10, respectively, for 6h, 24h and 48h. The Mφ types are unique in morphology and basal expression of polarization marker genes, particularly CCL22, in a pre-polarized state, and were differentially sensitive to polarization stimuli. Generally, M1 markers were instantly induced and gradually decreased, while M2 markers were markedly expressed at a later time. Expression profiles of M1 markers were similar between the polarized Mφ types, but M2a cell surface markers demonstrated an IL-4-dependent upregulation only in PBMC Mφ. Polarized THP-1 Mφ but not PBMC Mφ showed distinctive phagocytic capacity for apoptotic cells and bacterial antigens, respectively. In conclusion, our data suggest that THP-1 may be useful for performing studies involving phagocytosis and M1 polarization, rather than M2 polarization.

CD200 selectively upregulates prostaglandin E2 and D2 synthesis in LPS-treated bone marrow-derived macrophages

The CD200/CD200R signalling pathway downregulates the synthesis of proinflammatory mediators and induces the synthesis of antiinflammatory mediators in macrophages and microglia. However, very little is known about the effect of this immunosuppressive pathway on the synthesis of lipid mediators. Therefore, we determined the synthesis of 35 lipids spanning 5 different lipid families in bone marrow-derived macrophages, which were treated with interleukin (IL) 4, IL10, lipopolysaccharide (LPS), or interferon γ (IFNγ) in absence and presence of CD200. Out of these conditions the only significant effect of CD200 was an increased synthesis of prostaglandin (PG) E₂ and D₂ in the presence of LPS. Accordingly, mRNA levels of cyclooxygenase-2, microsomal PGE₂ synthase-1 and hematopoietic PGD synthase were upregulated by CD200 in presence of LPS. During Complete Freund's Adjuvant (CFA-) induced inflammation mPGES-1 was expressed in monocyte-derived macrophages and its expression was stronger in CD200R-positive than in CD200R-negative macrophages.

Network analysis of psoriasis reveals biological pathways and roles for coding and long non-coding RNAs

Background

Psoriasis is an immune-mediated, inflammatory disorder of the skin characterized by chronic inflammation and hyperproliferation of the epidermis. Differential expression analysis of microarray or RNA-seq data have shown that thousands of coding and non-coding genes are differentially expressed between psoriatic and healthy control skin. However, differential expression analysis may fail to detect perturbations in gene coexpression networks. Sensitive detection of such networks may provide additional insight into important disease-associated pathways. In this study, we applied weighted gene coexpression network analysis (WGCNA) on RNA-seq data from psoriasis patients and healthy controls.

Results

RNA-seq was performed on skin samples from 18 psoriasis patients (pre-treatment and post-treatment with the TNF-α inhibitor adalimumab) and 16 healthy controls, generating an average of 52.3 million 100-bp paired-end reads per sample. Using WGCNA, we identified 3 network modules that were significantly correlated with psoriasis and 6 network modules significantly correlated with biologic treatment, with only 16 % of the psoriasis-associated and 5 % of the treatment-associated coexpressed genes being identified by differential expression analysis. In a majority of these correlated modules, more than 50 % of coexpressed genes were long non-coding RNAs (lncRNA). Enrichment analysis of these correlated modules revealed that short-chain fatty acid metabolism and olfactory signaling are amongst the top pathways enriched for in modules associated with psoriasis, while regulation of leukocyte mediated cytotoxicity and regulation of cell killing are amongst the top pathways enriched for in modules associated with biologic treatment. A putative autoantigen, LL37, was coexpressed in the module most correlated with psoriasis.

Conclusions

This study has identified several networks of coding and non-coding genes associated with psoriasis and biologic drug treatment, including networks enriched for short-chain fatty acid metabolism and olfactory receptor activity, pathways that were not previously identified through differential expression analysis and may be dysregulated in psoriatic skin. As these networks are comprised mostly of non-coding genes, it is likely that non-coding genes play critical roles in the regulation of pathways involved in the pathogenesis of psoriasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3188-y) contains supplementary material, which is available to authorized users.

Inhibition of SIRPα in dendritic cells potentiates potent antitumor immunity

Despite their central function in tumor immunity, dendritic cells (DCs) can respond to inhibitory signals and become tolerogenic, curtailing T cell responses in vivo. Here, we provide the evidence for an inhibitory function of signal regulatory protein (SIRP) α in DC survival and activation. In tumors from human liver cancer patients, infiltrative DCs expressed elevated levels of SIRPα, which is correlated with the induction of immune tolerance within the tumors. Silencing of SIRPα resulted in a significant increase in the longevity of antigen-pulsed DCs in the draining lymph nodes. In addition, SIRPα controls the activation and output of DCs. Silencing of DC-expressed SIRPα induced spontaneous and enhanced production of IL12 and costimulatory molecules, resulting in more potent cytotoxic T lymphocyte responses, including the eradication of previously established solid tumors. SIRPα exerted such effects, at least in part, via the association and sequestration of p85 subunit of PI3K. Thus, SIRPα is a critical regulator of DC lifespan and activity, and its inhibition might improve the clinical efficacy of DC-based tumor vaccines.

Immune Homeostatic Macrophages Programmed by the Bacterial Surface Protein NhhA Potentiate Nasopharyngeal Carriage of Neisseria meningitidis

Neisseria meningitidis colonizes the nasopharyngeal mucosa of healthy populations asymptomatically, although the bacterial surface is rich in motifs that activate the host innate immunity. What determines the tolerant host response to this bacterium in asymptomatic carriers is poorly understood. We demonstrated that the conserved meningococcal surface protein NhhA orchestrates monocyte (Mo) differentiation specifically into macrophage-like cells with a CD200R^hi phenotype (NhhA-Mφ). In response to meningococcal stimulation, NhhA-Mφ failed to produce proinflammatory mediators. Instead, they upregulated interleukin-10 (IL-10) and Th2/regulatory T cell (Treg)-attracting chemokines, such as CCL17, CCL18, and CCL22. Moreover, NhhA-Mφ were highly efficient in eliminating bacteria. The in vivo validity of these findings was corroborated using a murine model challenged with N. meningitidis systematically or intranasally. The NhhA-modulated immune response protected mice from septic shock; Mo/Mφ depletion abolished this protective effect. Intranasal administration of NhhA induced an anti-inflammatory response, which was associated with N. meningitidis persistence at the nasopharynx. In vitro studies demonstrated that NhhA-triggered Mo differentiation occurred upon engaged Toll-like receptor 1 (TLR1)/TLR2 signaling and extracellular signal-regulated kinase (ERK) and Jun N-terminal protein kinase (JNK) activation and required endogenously produced IL-10 and tumor necrosis factor alpha (TNF-α). Our findings reveal a strategy that might be adopted by N. meningitidis to maintain asymptomatic nasopharyngeal colonization.

Neisseria meningitidis is an opportunistic human-specific pathogen that colonizes the nasopharyngeal mucosa asymptomatically in approximately 10% of individuals. Very little is known about how this bacterium evades immune activation during the carriage stage. Here, we observed that N. meningitidis, via the conserved surface protein NhhA, skewed monocyte differentiation into macrophages with a CD200R^hi phenotype. Both in vivo and in vitro data demonstrated that these macrophages, upon meningococcal infection, played an important role in forming a homeostatic immune microenvironment through their capacity to eliminate invading bacteria and to generate anti-inflammatory mediators. This work provides novel insight into the mechanisms underlying the commensal persistence of N. meningitidis.

Reduced Dendritic Cells Expressing CD200R1 in Children with Inflammatory Bowel Disease: Correlation with Th17 and Regulatory T Cells

Loss of tolerance of the adaptive immune system towards indigenous flora contributes to the development of inflammatory bowel diseases (IBD). Defects in dendritic cell (DC)-mediated innate and adoptive immune responses are conceivable. The aim of this study was to investigate the expression of the inhibitory molecules CD200R1 and their ligand CD200 on DCs, to clarify the role of the DCs in the pathogenesis of IBD. Thirty-seven pediatric IBD patients (23 with Crohn’s disease (CD) and 14 with ulcerative colitis (UC)) with mean age 13.25 ± 2.9 years were included. Fourteen age-matched healthy pediatric volunteers (five males and nine females) served as a control group (HC). The percentage of CD11c⁺ myeloid dendritic cells (mDCs) and CD123⁺ plasmacytoid DCs (pDCs) expressing CD200R1 and CD200 were evaluated in peripheral blood using flow cytometry and were correlated with routine biochemical, serological markers, serum levels of cytokines and with the percentages of circulating regulatory T cells (Treg) and CD4⁺ producing IL-17 (Th17). IBD patients showed a significant decrease in the percentage of pDCs and mDCs expressing CD200R1 compared to that of HC. Patients with UC showed increased expressions of the CD200 molecule on pDCs as compared to HC. DCs expressing CD200R1 were found to be correlated positively with Treg and negatively with TH17 and erythrocyte sedimentation rate (ESR). Our findings suggest that IBD is associated with dysregulation in the CD200R1/CD200 axis and that the decrease in DCs expressing CD200R1 may contribute to the imbalance of Th17 and Treg cells and in the pathogenesis of IBD.

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.

A Bone Anabolic Effect of RANKL in a Murine Model of Osteoporosis Mediated Through FoxP3+ CD8 T Cells

TNF-α and IL-17 secreted by proinflammatory T cells (T(EFF)) promote bone erosion by activating osteoclasts. We previously demonstrated that in addition to bone resorption, osteoclasts act as antigen-presenting cells to induce FoxP3 in CD8 T cells (Tc(REG)). The osteoclast-induced regulatory CD8 T cells limit bone resorption in ovariectomized mice (a murine model of postmenopausal osteoporosis). Here we show that although low-dose receptor activator of NF-κB ligand (RANKL) maximally induces Tc(REG) via Notch signaling pathway to limit bone resorption, high-dose RANKL promotes bone resorption. In vitro, both TNF-α and IL-17, cytokines that are abundant in ovariectomized animals, suppress Tc(REG) induction by osteoclasts by repressing Notch ligand expression in osteoclasts, but this effect can be counteracted by addition of RANKL. Ovariectomized mice treated with low-dose RANKL induced Tc(REG) that suppressed bone resorption, decreased T(EFF) levels, and increased bone formation. High-dose RANKL had the expected osteolytic effect. Low-dose RANKL administration in ovariectomized mice lacking CD8 T cells was also osteolytic, confirming that Tc(REG) mediate this bone anabolic effect. Our results show that although RANKL directly stimulates osteoclasts to resorb bone, it also controls the osteoclasts' ability to induce regulatory T cells, engaging an important negative feedback loop. In addition to the conceivable clinical relevance to treatment of osteoporosis, these observations have potential relevance to induction of tolerance and autoimmune diseases.

CD200 receptor restriction of myeloid cell responses antagonizes antiviral immunity and facilitates cytomegalovirus persistence within mucosal tissue

CD200 receptor (CD200R) negatively regulates peripheral and mucosal innate immune responses. Viruses, including herpesviruses, have acquired functional CD200 orthologs, implying that viral exploitation of this pathway is evolutionary advantageous. However, the role that CD200R signaling plays during herpesvirus infection in vivo requires clarification. Utilizing the murine cytomegalovirus (MCMV) model, we demonstrate that CD200R facilitates virus persistence within mucosal tissue. Specifically, MCMV infection of CD200R-deficient mice (CD200R(-/-)) elicited heightened mucosal virus-specific CD4 T cell responses that restricted virus persistence in the salivary glands. CD200R did not directly inhibit lymphocyte effector function. Instead, CD200R(-/-) mice exhibited enhanced APC accumulation that in the mucosa was a consequence of elevated cellular proliferation. Although MCMV does not encode an obvious CD200 homolog, productive replication in macrophages induced expression of cellular CD200. CD200 from hematopoietic and non-hematopoietic cells contributed independently to suppression of antiviral control in vivo. These results highlight the CD200-CD200R pathway as an important regulator of antiviral immunity during cytomegalovirus infection that is exploited by MCMV to establish chronicity within mucosal tissue.

Local targeting of the CD200-CD200R axis does not promote corneal graft survival

Corneal graft rejection is primarily a CD4(+) T cell-mediated mechanism in which macrophages may play an important inflammatory role. CD200Fc fusion protein is an artificial agonist of CD200R1, a receptor expressed predominantly on myeloid cells, engagement of which is known to down-regulate macrophage function. We therefore wished to test whether CD200Fc could be used as a therapeutic agent to prolong corneal graft survival. The distribution of CD200R1 and CD200, its natural ligand, was examined by immunohistology in the cornea and conjunctiva of unoperated rats and rats that had received corneal allografts. Mouse CD200Fc was injected subconjunctivally into transplanted rats on six occasions from the day of surgery until day 10 after transplantation. Control groups received injections of mouse IgG or diluent PBS. Allo-transplants were also performed in CD200(-/-) and control mice. The ability of CD200Fc to bind rat macrophages in vitro and to inhibit nitric oxide production was tested. Mean day of rejection in CD200Fc, IgG and PBS-treated rats was 12, 10 and 9 respectively (p=0.24). Mean day of rejection in CD200(-/-) and wild type mice was 17.5 and 16.0 respectively (p=0.07). Mouse CD200Fc bound to rat macrophages in a dose-dependent manner, but was unable to inhibit nitric oxide production. The fact that treatment with CD200Fc did not inhibit graft rejection and the failure of CD200 deficiency to affect graft survival suggests that local targeting of the CD200-CD200R axis to suppress macrophage activation is not a useful therapeutic strategy in corneal graft rejection.

The rhesus rhadinovirus CD200 homologue affects immune responses and viral loads during in vivo infection

Rhesus macaque rhadinovirus (RRV) is a gammaherpesvirus of rhesus macaque (RM) monkeys that is closely related to human herpesvirus 8 (HHV-8)/Kaposi's Sarcoma-associated herpesvirus (KSHV), and it is capable of inducing diseases in simian immunodeficiency virus (SIV)-infected RM that are similar to those seen in humans coinfected with HIV and HHV-8. Both HHV-8 and RRV encode viral CD200 (vCD200) molecules that are homologues of cellular CD200, a membrane glycoprotein that regulates immune responses and helps maintain immune homeostasis via interactions with the CD200 receptor (CD200R). Though the functions of RRV and HHV-8 vCD200 molecules have been examined in vitro, the precise roles that these viral proteins play during in vivo infection remain unknown. Thus, to address the contributions of RRV vCD200 to immune regulation and disease in vivo, we generated a form of RRV that lacked expression of vCD200 for use in infection studies in RM. Our data indicated that RRV vCD200 expression limits immune responses against RRV at early times postinfection and also impacts viral loads, but it does not appear to have significant effects on disease development. Further, examination of the distribution pattern of CD200R in RM indicated that this receptor is expressed on a majority of cells in peripheral blood mononuclear cells, including B and T cells, suggesting potentially wider regulatory capabilities for both vCD200 and CD200 that are not strictly limited to myeloid lineage cells. In addition, we also demonstrate that RRV infection affects CD200R expression levels in vivo, although vCD200 expression does not play a role in this phenomenon.

IMPORTANCE

Cellular CD200 and its receptor, CD200R, compose a pathway that is important in regulating immune responses and is known to play a role in a variety of human diseases. A number of pathogens have been found to modulate the CD200-CD200R pathway during infection, including human herpesvirus 8 (HHV-8), the causative agent of Kaposi's sarcoma and B cell neoplasms in AIDS patients, and a closely related primate virus, rhesus macaque rhadinovirus (RRV), which infects and induces disease in rhesus macaque monkeys. HHV-8 and RRV encode homologues of CD200, termed vCD200, which are thought to play a role in preventing immune responses against these viruses. However, neither molecule has been studied in an in vivo model of infection to address their actual contributions to immunoregulation and disease. Here we report findings from our studies in which we analyzed the properties of a mutant form of RRV that lacks vCD200 expression in infected rhesus macaques.

The CD200-CD200 receptor inhibitory axis controls arteriogenesis and local T lymphocyte influx

The role of the CD200 ligand-CD200 receptor (CD200-CD200R) inhibitory axis is highly important in controlling myeloid cell function. Since the activation of myeloid cells is crucial in arteriogenesis, we hypothesized that disruption of the CD200-CD200R axis promotes arteriogenesis in a murine hindlimb ischemia model. Female Cd200-/- and wildtype (C57Bl/6J) mice underwent unilateral femoral artery ligation. Perfusion recovery was monitored over 7 days using Laser-Doppler analysis and was increased in Cd200-/- mice at day 3 and 7 after femoral artery ligation, compared to wildtype. Histology was performed on hindlimb muscles at baseline, day 3 and 7 to assess vessel geometry and number and inflammatory cell influx. Vessel geometry in non-ischemic muscles was larger, and vessel numbers in ischemic muscles were increased in Cd200-/- mice compared to wildtype. Furthermore, T lymphocyte influx was increased in Cd200-/- compared to wildtype. CD200R agonist treatment was performed in male C57Bl/6J mice to validate the role of the CD200-CD200R axis in arteriogenesis. CD200R agonist treatment after unilateral femoral artery ligation resulted in a significant decrease in vessel geometry, perfusion recovery and T lymphocyte influx at day 7 compared to isotype treatment. In this study, we show a causal role for the CD200-CD200R inhibitory axis in arteriogenesis in a murine hindlimb ischemia model. Lack of CD200R signaling is accompanied by increased T lymphocyte recruitment to the collateral vasculature and results in enlargement of preexisting collateral arteries.

Formation of distinct chromatin conformation signatures epigenetically regulate macrophage activation

Microbial-lipopolysacharide (LPS), interleukin 4 (IL-4) and interferon gamma (IFN-γ) polarise macrophages into "innate", "alternative" and "classical", activation states by selective gene regulation. Expression of MARCO, CD200, CD200R1 (innate), MRC1 (alternative) and H2-Eb1 (classical) selectively marks these distinct activation states. Epigenetic events drive such activation upon stimuli and here we study one such mechanism, chromatin conformation signatures implicated in long-range chromatin interactions that regulate transcriptional switch and gene expression. The EpiSwitch™ technology was used to identify and analyse potential markers bordering such conformational signatures for these genes and juxtaposition of markers was compared between resting and activated macrophages. LPS, IL-4 and IFN-γ selectively altered chromatin conformations of their responsive genes in wild type, but not in MyD88(-/-), IL-4R(-/-) and IFN-γR(-/-) macrophages. In addition, two distinct conformations were observed in CD200R1 after LPS and IFN-γ stimulation. In summary, signal-specific alterations in chromatin conformation provide biomarkers that identify and determine distinct gene expression programmes during macrophage activation.

The CD200-CD200R1 inhibitory signaling pathway: immune regulation and host-pathogen interactions

The CD200:CD200R1 inhibitory signaling pathway has been implicated in playing a prominent role in limiting inflammation in a wide range of inflammatory diseases. CD200R1 signaling inhibits the expression of proinflammatory molecules including tumor necrosis factor, interferons, and inducible nitric oxide synthase in response to selected stimuli. Unsurprisingly, due to the regulatory role that CD200R1 plays in multiple inflammatory pathways, an increasing number of parasitic, bacterial, and viral pathogens exploit this pathway to suppress host defenses. A complete understanding of the pathways regulated by CD200R1 signaling and the diverse mechanisms that pathogens have evolved to manipulate the CD200:CD200R1 pathway can help identify clinical situations where targeting this interaction can be of therapeutic benefit. In this review, we compare CD200R1 to other pathogen-targeted inhibitory receptors and highlight how this signaling pathway is utilized by a diverse number of pathogens and, therefore, may represent a novel targeting strategy for the treatment of infectious diseases.

How does the brain limit the severity of inflammation and tissue injury during bacterial meningitis?

The most devastating CNS bacterial infection, bacterial meningitis, has both acute and long-term neurologic consequences. The CNS defends itself against bacterial invasion through a combination of physical barriers (i.e. blood-brain barrier, meninges, and ependyma), which contain macrophages that express a range of pattern-recognition receptors that detect pathogens before they gain access to the CNS and cerebrospinal fluid. This activates an antipathogen response consisting of inflammatory cytokines, complement, and chemoattractants. Regulation of the antipathogen inflammatory response is essential for preventing irreversible brain injury and protecting stem cell populations in the ventricle wall. The severity of brain inflammation is regulated by the clearance of apoptotic inflammatory cells and neurons. Death signaling pathways are expressed by glia to stimulate apoptosis of neutrophils, lymphocytes, and damaged neurons and to regulate in flammation and remove necrotic cells. The emerging group of neuroimmunoregulatory molecules adjusts the balance of the anti-inflammatory and proinflammatory response to provide optimal conditions for effective clearance of pathogens and apoptotic cells but reduce the severity of the inflammatory response to prevent injury to brain cells, including stem cell populations. The neuroimmunoregulatory molecules and other CNS anti-inflammatory pathways represent potential therapeutic targets capable of reducing brain injury caused by bacterial infection.

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

Amniotic fluid CD200 levels in pregnancies complicated by preterm prelabor rupture of the membranes

OBJECTIVE

To determine the amniotic fluid CD200 levels in uncomplicated pregnancies and in preterm prelabor rupture of the membranes (PPROM) according to microbial invasion of the amniotic cavity and histological chorioamnionitis and its association with neonatal outcomes.

METHODS

One hundred and fifty-nine women with singleton pregnancies were included in this study. Amniotic fluid was collected, and CD200 levels were determined using ELISA.

RESULTS

No difference was found in CD200 levels between women in the second trimester and women at term without labor. Women at term with labor had higher CD200 levels than women in the second trimester and women at term without labor. The presence of funisitis in PPROM pregnancies was associated with higher CD200 levels independent of gestational age (with funisitis: median 197.5 pg/mL versus without funisitis: median 61.0 pg/mL; p = 0.003). The need for tracheal intubation and the development of bronchopulmonary dysplasia were associated with higher CD200 levels.

CONCLUSIONS

Amniotic fluid CD200 levels remained stable in advanced pregnancy and they were increased during parturition. Elevated CD200 levels in the presence of funisitis suggest the involvement of negative regulatory mechanisms of innate immunity. CD200 may play a role in the development of pulmonary aspects of neonatal morbidity.

Gene therapy for noninfectious uveitis

Noninfectious intraocular inflammatory disease remains a significant cause of visual loss, even with current systemic immunosuppression. Alternative novel treatments are thus required, particularly for severe forms of posterior uveitis. Encouraging results from several phase I/II clinical trials of gene therapy for monogenic retinal disorders have paved the way for the development of this approach for other ocular conditions. Gene therapy for uveitis offers the possibility of potent, self-regulating, long-term disease control following a single treatment and without systemic side effects. To date, gene therapy approaches using interleukin-10, interleukin-1 receptor antagonist, interferon-alpha, soluble TNF-alpha receptors, and alpha-MSH gene transfer have been used successfully to attenuate experimental animal models of uveitis. This review evaluates these preclinical studies, considers the route to clinical application, and explores future targets and approaches.

Inhibitory receptor paired Ig-like receptor B is exploited by Staphylococcus aureus for virulence

The innate immune system has developed to acquire a wide variety of pattern-recognition receptors (PRRs) to identify potential pathogens, whereas pathogens have also developed to escape host innate immune responses. ITIM-bearing receptors are attractive targets for pathogens to attenuate immune responses against them; however, the in vivo role of the inhibitory PRRs in host-bacteria interactions remains unknown. We demonstrate in this article that Staphylococcus aureus, a major Gram-positive bacteria, exploits inhibitory PRR paired Ig-like receptor (PIR)-B on macrophages to suppress ERK1/2 and inflammasome activation, and subsequent IL-6 and IL-1β secretion. Consequently, Pirb(-/-) mice infected with S. aureus showed enhanced inflammation and more effective bacterial clearance, resulting in resistance to the sepsis. Screening of S. aureus mutants identified lipoteichoic acid (LTA) as an essential bacterial cell wall component required for binding to PIR-B and modulating inflammatory responses. In vivo, however, an LTA-deficient S. aureus mutant was highly virulent and poorly recognized by macrophages in both wild-type and Pirb(-/-) mice, demonstrating that LTA recognition by PRRs other than PIR-B mediates effective bacterial elimination. These results provide direct evidence that bacteria exploit the inhibitory receptor for virulence, and host immune system counterbalances the infection.