Soluble CD200 Is Critical to Engraft Chronic Lymphocytic Leukemia Cells in Immunocompromised Mice

Soluble form of immune checkpoints in autoimmune diseases

Immune checkpoints are essential regulators of immune responses, either by activating or suppressing them. Consequently, they are regarded as pivotal elements in the management of infections, cancer, and autoimmune disorders. In recent years, researchers have identified numerous soluble immune checkpoints that are produced through various mechanisms and demonstrated biological activity. These soluble immune checkpoints can be produced and distributed in the bloodstream and various tissues, with their roles in immune response dysregulation and autoimmunity extensively documented. This review aims to provide a thorough overview of the generation of various soluble immune checkpoints, such as sPD-1, sCTLA-4, sTim-3, s4-1BB, sBTLA, sLAG-3, sCD200, and the B7 family, and their importance as indicators for the diagnosis and prediction of autoimmune conditions. Furthermore, the review will investigate the potential pathological mechanisms of soluble immune checkpoints in autoimmune diseases, emphasizing their association with autoimmune diseases development, prognosis, and treatment.

CD200 is overexpressed in the pancreatic tumor microenvironment and predictive of overall survival

Pancreatic cancer is an aggressive disease with a 5 year survival rate of 13%. This poor survival is attributed, in part, to limited and ineffective treatments for patients with metastatic disease, highlighting a need to identify molecular drivers of pancreatic cancer to target for more effective treatment. CD200 is a glycoprotein that interacts with the receptor CD200R and elicits an immunosuppressive response. Overexpression of CD200 has been associated with differential outcomes, depending on the tumor type. In the context of pancreatic cancer, we have previously reported that CD200 is expressed in the pancreatic tumor microenvironment (TME), and that targeting CD200 in murine tumor models reduces tumor burden. We hypothesized that CD200 is overexpressed on tumor and stromal populations in the pancreatic TME and that circulating levels of soluble CD200 (sCD200) have prognostic value for overall survival. We discovered that CD200 was overexpressed on immune, stromal, and tumor populations in the pancreatic TME. Particularly, single-cell RNA-sequencing indicated that CD200 was upregulated on inflammatory cancer-associated fibroblasts. Cytometry by time of flight analysis of PBMCs indicated that CD200 was overexpressed on innate immune populations, including monocytes, dendritic cells, and monocytic myeloid-derived suppressor cells. High sCD200 levels in plasma correlated with significantly worse overall and progression-free survival. Additionally, sCD200 correlated with the ratio of circulating matrix metalloproteinase (MMP) 3: tissue inhibitor of metalloproteinase (TIMP) 3 and MMP11/TIMP3. This study highlights the importance of CD200 expression in pancreatic cancer and provides the rationale for designing novel therapeutic strategies that target this protein.

Translation of Data from Animal Models of Cancer to Immunotherapy of Breast Cancer and Chronic Lymphocytic Leukemia

The field of clinical oncology has been revolutionized over the past decade with the introduction of many new immunotherapies the existence of which have depended to a large extent on experimentation with both in vitro analysis and the use of various animal models, including gene-modified mice. The discussion below will review my own laboratory's studies, along with those of others in the field, on cancer immunotherapy. Our own studies have predominantly dwelt on two models of malignancy, namely a solid tumor model (breast cancer) and lymphoma. The data from our own laboratory, and that of other scientists, highlights the novel information so obtained, and the evidence that application of such information has already had an impact on immunotherapy of human oncologic diseases.

Cancel cancer: The immunotherapeutic potential of CD200/CD200R blockade

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

CD200 ectodomain shedding into the tumor microenvironment leads to NK cell dysfunction and apoptosis

The basis of immune evasion, a hallmark of cancer, can differ even when cancers arise from one cell type such as in the human skin keratinocyte carcinomas: basal and squamous cell carcinoma. Here we showed that the basal cell carcinoma tumor-initiating cell surface protein CD200, through ectodomain shedding, was responsible for the near absence of NK cells within the basal cell carcinoma tumor microenvironment. In situ, CD200 underwent ectodomain shedding by metalloproteinases MMP3 and MMP11, which released biologically active soluble CD200 into the basal cell carcinoma microenvironment. CD200 bound its cognate receptor on NK cells to suppress MAPK pathway signaling that in turn blocked indirect (IFN-γ release) and direct cell killing. In addition, reduced ERK phosphorylation relinquished negative regulation of PPARγ-regulated gene transcription and led to membrane accumulation of the Fas/FADD death receptor and its ligand, FasL, which resulted in activation-induced apoptosis. Blocking CD200 inhibition of MAPK or PPARγ signaling restored NK cell survival and tumor cell killing, with relevance to many cancer types. Our results thus uncover a paradigm for CD200 as a potentially novel and targetable NK cell-specific immune checkpoint, which is responsible for NK cell-associated poor outcomes in many cancers.

Targeting pediatric leukemia-propagating cells with anti-CD200 antibody therapy

Treating refractory pediatric acute lymphoblastic leukemia (ALL) remains a challenge despite impressive remission rates (>90%) achieved in the last decade. The use of innovative immunotherapeutic approaches such as anti-CD19 chimeric antigen receptor T cells does not ensure durable remissions, because leukemia-propagating cells (LPCs) that lack expression of CD19 can cause relapse, which signifies the need to identify new markers of ALL. Here we investigated expression of CD58, CD97, and CD200, which were previously shown to be overexpressed in B-cell precursor ALL (BCP-ALL) in CD34+/CD19+, CD34+/CD19-, CD34-/CD19+, and CD34-/CD19- LPCs, to assess their potential as therapeutic targets. Whole-genome microarray and flow cytometric analyses showed significant overexpression of these molecules compared with normal controls. CD58 and CD97 were mainly co-expressed with CD19 and were not a prerequisite for leukemia engraftment in immune deficient mice. In contrast, expression of CD200 was essential for engraftment and serial transplantation of cells in measurable residual disease (MRD) low-risk patients. Moreover, these CD200+ LPCs could be targeted by using the monoclonal antibody TTI-CD200 in vitro and in vivo. Treating mice with established disease significantly reduced disease burden and extended survival. These findings demonstrate that CD200 could be an attractive target for treating low-risk ALL, with minimal off-tumor effects that beset current immunotherapeutic approaches.

CD200 Baseline Serum Levels Predict Prognosis of Chronic Lymphocytic Leukemia

Simple Summary

This study aimed at investigating the prognostic significance of the soluble form of CD200 antigen evaluated at diagnosis in patients with chronic lymphocytic leukemia (CLL). In a large cohort of patients, we found that more aggressive features and a worse prognosis are correlated with higher baseline serum levels of CD200. These data support the relevant role of CD200 not only as a diagnostic tool but also as a prognostic indicator and a potential therapeutic target in CLL.

Abstract

Membrane-bound CD200 is overexpressed in chronic lymphocytic leukemia (CLL), and there is some evidence that its soluble ectodomain (sCD200) could also be involved in the pathophysiology and the disease. However, very little is known about sCD200’s prognostic significance. sCD200 was tested at diagnosis in 272 patients with CLL and in 78 age- and sex-matched healthy subjects using a specific human CD200 (OX-2 membrane glycoprotein) ELISA kit. A significantly higher concentration of sCD200 was found in CLL patients compared to controls. In our cohort, sCD200 was significantly higher in patients who were older than 66 years, with Binet stage C, unmutated IgVH and unfavorable (del11q or del17p) FISH. Time-to-first treatment and overall survival were significantly shorter in patients with higher sCD200 concentration, using as a cut-off 1281 pg/mL, the median value for sCD200 concentration in the whole CLL cohort. However, the prognostic impact of sCD200 was not confirmed in multivariate analysis. Baseline sCD200 values appeared to have an impact on the response to chemotherapy or chemo-immunotherapy, but not to targeted agents. Collectively, our data show that sCD200 serum levels correlate with more aggressive clinical and biological features and are able to predict a worse prognosis. This work supports the relevant role of CD200 not only as a diagnostic tool but also as a prognostic indicator and a potential therapeutic target in CLL.

CD200 Limits Monopoiesis and Monocyte Recruitment in Atherosclerosis

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CD200 Immune-Checkpoint Peptide Elicits an Anti-glioma Response Through the DAP10 Signaling Pathway

Numerous therapies aimed at driving an effective anti-glioma response have been employed over the last decade; nevertheless, survival outcomes for patients remain dismal. This may be due to the expression of immune-checkpoint ligands such as PD-L1 by glioblastoma (GBM) cells which interact with their respective receptors on tumor-infiltrating effector T cells curtailing the activation of anti-GBM CD8+ T cell-mediated responses. Therefore, a combinatorial regimen to abolish immunosuppression would provide a powerful therapeutic approach against GBM. We developed a peptide ligand (CD200AR-L) that binds an uncharacterized CD200 immune-checkpoint activation receptor (CD200AR). We sought to test the hypothesis that CD200AR-L/CD200AR binding signals via he DAP10&12 pathways through in vitro studies by analyzing transcription, protein, and phosphorylation, and in vivo loss of function studies using inhibitors to select signaling molecules. We report that CD200AR-L/CD200AR binding induces an initial activation of the DAP10&12 pathways followed by a decrease in activity within 30 min, followed by reactivation via a positive feedback loop. Further in vivo studies using DAP10&12KO mice revealed that DAP10, but not DAP12, is required for tumor control. When we combined CD200AR-L with an immune-stimulatory gene therapy, in an intracranial GBM model in vivo, we observed increased median survival, and long-term survivors. These studies are the first to characterize the signaling pathway used by the CD200AR, demonstrating a novel strategy for modulating immune checkpoints for immunotherapy currently being analyzed in a phase I adult trial.

Targeting Neuroinflammation in Brain Cancer: Uncovering Mechanisms, Pharmacological Targets, and Neuropharmaceutical Developments

Gliomas are one of the most lethal types of cancers accounting for ∼80% of all central nervous system (CNS) primary malignancies. Among gliomas, glioblastomas (GBM) are the most aggressive, characterized by a median patient survival of fewer than 15  months. Recent molecular characterization studies uncovered the genetic signatures and methylation status of gliomas and correlate these with clinical prognosis. The most relevant molecular characteristics for the new glioma classification are IDH mutation, chromosome 1p/19q deletion, histone mutations, and other genetic parameters such as ATRX loss, TP53, and TERT mutations, as well as DNA methylation levels. Similar to other solid tumors, glioma progression is impacted by the complex interactions between the tumor cells and immune cells within the tumor microenvironment. The immune system’s response to cancer can impact the glioma’s survival, proliferation, and invasiveness. Salient characteristics of gliomas include enhanced vascularization, stimulation of a hypoxic tumor microenvironment, increased oxidative stress, and an immune suppressive milieu. These processes promote the neuro-inflammatory tumor microenvironment which can lead to the loss of blood-brain barrier (BBB) integrity. The consequences of a compromised BBB are deleteriously exposing the brain to potentially harmful concentrations of substances from the peripheral circulation, adversely affecting neuronal signaling, and abnormal immune cell infiltration; all of which can lead to disruption of brain homeostasis. In this review, we first describe the unique features of inflammation in CNS tumors. We then discuss the mechanisms of tumor-initiating neuro-inflammatory microenvironment and its impact on tumor invasion and progression. Finally, we also discuss potential pharmacological interventions that can be used to target neuro-inflammation in gliomas.

Membrane-associated and secreted forms of the Rhesus macaque rhadinovirus-encoded CD200 homologue and cellular CD200 demonstrate differential effects on Rhesus Macaque CD200 Receptor signaling and regulation of myeloid cell activation

The CD200-CD200R pathway is involved in inhibition of immune responses, and the importance of this pathway to infectious disease is highlighted by the fact that viral CD200 (vCD200) molecules have been found to be encoded by several DNA viruses, including the human gammaherpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV), and the closely related rhesus macaque rhadinovirus (RRV). KSHV vCD200 is the most extensively studied vCD200 molecule, however, the only herpesvirus vCD200 molecule to be examined in vivo is that encoded by RRV. Our prior studies have demonstrated that RRV vCD200 is a functional CD200 homologue that is capable of affecting immune responses in vivo, and further, that RRV can express a secreted form of vCD200 (vCD200-Sec) during infection. Despite this information, RRV vCD200 has not been examined specifically for effects on RM CD200R signaling, and the functionality of vCD200-Sec has not been examined in any context. Thus, we developed an in vitro model system in which B cells expressing vCD200 were utilized to assess the effects of this molecule on the regulation of myeloid cells expressing RM CD200R, mimicking interactions that are predicted to occur in vivo Our findings suggest that RRV vCD200 can bind and induce functional signals through RM CD200R, while vCD200-Sec represents a non-functional protein incapable of affecting CD200R signaling. We also provide the first demonstration of the function of RM CD200, which appears to possess more robust signaling capabilities than RRV vCD200, and also show that KSHV vCD200 does not efficiently induce signaling via RM CD200R.IMPORTANCE Viral CD200 homologues are encoded by KSHV and the closely related RRV. Though RRV vCD200 has been examined, questions still exist in regard to the ability of this molecule to induce signaling via rhesus macaque CD200R, as well as the potential function of a secreted form of vCD200. Further, all previous in vitro studies of RRV vCD200 have utilized an Fc fusion protein to examine functionality, which does not replicate the structural properties of the membrane-associated form of vCD200 that is naturally produced during RRV infection. In this study, we demonstrate for the first time that membrane-expressed RRV vCD200 is capable of inducing signal transduction via RM CD200R, while the secreted form of vCD200 appears to be non-functional. Further, we also demonstrate that RM CD200 induces signaling via RM CD200R, and is more robust than RRV vCD200, while KSHV vCD200 does not appear to induce efficient signaling via RM CD200R.

CD200 and Chronic Lymphocytic Leukemia: Biological and Clinical Relevance

CD200, a transmembrane type Ia glycoprotein belonging to the immunoglobulin protein superfamily, is broadly expressed on a wide variety of cell types, such as B lymphocytes, a subset of T lymphocytes, dendritic cells, endothelial and neuronal cells. It delivers immunosuppressive signals through its receptor CD200R, which is expressed on monocytes/myeloid cells and T lymphocytes. Moreover, interaction of CD200 with CD200R has also been reported to play a role in the regulation of tumor immunity. Overexpression of CD200 has been reported in chronic lymphocytic leukemia (CLL) and hairy cell leukemia but not in mantle cell lymphoma, thus helping to better discriminate between these different B cell malignancies with different prognosis. In this review, we focus on the role of CD200 expression in the differential diagnosis of mature B-cell neoplasms and on the prognostic significance of CD200 expression in CLL, where conflicting results have been published so far. Of interest, increasing evidences indicate that anti-CD200 treatment might be therapeutically beneficial for treating CD200-expressing malignancies, such as CLL.

Estrogen-regulated CD200 inhibits macrophage phagocytosis in endometriosis

OBJECTIVES

Endometriosis (EMS) is a benign disease that is related to estrogen, immune disorders and inflammation. The purpose of this research was to determine the expression of CD200 in EMS and to clarify its role in the pathogenesis of the disease.

METHODS

The levels of serum CD200 in patients with and without EMS were determined by ELISA. Furthermore, the expression of CD200 in normal eutopic endometrium and ectopic endometrium was detected by immunohistochemistry and western blotting. The CD200 receptor (CD200R) in macrophages in peritoneal fluid (pMØ) obtained from controls and patients with EMS was examined by western blotting. CD200 expression in human endometrial stromal cells (HESCs) stimulated with 17β-estradiol (E₂) was measured by western blotting. Furthermore, macrophages were stimulated with different concentrations of CD200 and the effect on phagocytosis was analyzed.

RESULTS

The plasma CD200 levels of patients with EMS was significantly increased compared with controls (P = 0.0173, 95%CI [18.75, 159.6]). Compared with normal eutopic endometrium, the expression of CD200 was significantly increased in ectopic endometrial tissues. The CD200R expression in pMØ obtained from patients with EMS was increased compared with the controls (P = 0.0244). CD200 expression in HESCs stimulated with E₂ was up-regulated. As the levels of CD200 increased, macrophage phagocytosis in vitro gradually decreased.

CONCLUSIONS

CD200 is an estrogen-induced molecule that impairs macrophage phagocytosis and may contribute to the immune escape of ectopic lesions in EMS.

CD200 Checkpoint Reversal: A Novel Approach to Immunotherapy

PURPOSE

Advances in immunotherapy have revolutionized care for some patients with cancer. However, current checkpoint inhibitors are associated with significant toxicity and yield poor responses for patients with central nervous system tumors, calling into question whether cancer immunotherapy can be applied to glioblastoma multiforme. We determined that targeting the CD200 activation receptors (CD200AR) of the CD200 checkpoint with a peptide inhibitor (CD200AR-L) overcomes tumor-induced immunosuppression. We have shown the clinical efficacy of the CD200AR-L in a trial in companion dogs with spontaneous high-grade glioma. Addition of the peptide to autologous tumor lysate vaccines significantly increased the median overall survival to 12.7 months relative to tumor lysate vaccines alone, 6.36 months.

EXPERIMENTAL DESIGN

This study was developed to elucidate the mechanism of the CD200ARs and develop a humanized peptide inhibitor. We developed macrophage cell lines with each of four CD200ARs knocked out to determine their binding specificity and functional response. Using proteomics, we developed humanized CD200AR-L to explore their effects on cytokine/chemokine response, dendritic cell maturation and CMV pp65 antigen response in human CD14+ cells. GMP-grade peptide was further validated for activity.

RESULTS

We demonstrated that the CD200AR-L specifically targets a CD200AR complex. Moreover, we developed and validated a humanized CD200AR-L for inducing chemokine response, stimulating immature dendritic cell differentiation and significantly enhanced an antigen-specific response, and determined that the use of the CD200AR-L downregulated the expression of CD200 inhibitory and PD-1 receptors.

CONCLUSIONS

These results support consideration of a CD200AR-L as a novel platform for immunotherapy against multiple cancers including glioblastoma multiforme.

Mechanisms of resistance to CAR T cell therapies

Chimeric antigen receptor (CAR)-engineered T cells have demonstrated remarkable success in the treatment of B cell malignancies. FDA approval of these therapies represents a watershed moment in the development of therapies for cancer. Despite the successes of the last decade, many patients will unfortunately not experience durable responses to CAR therapy. Emerging research has shed light on the biology responsible for these failures, and further highlighted the hurdles to broader success. Here, we review the recent research identifying how interactions between cancer cells and engineered immune cells result in resistance to CAR therapies.

Effect of soluble cleavage products of important receptors/ligands on efferocytosis: Their role in inflammatory, autoimmune and cardiovascular disease

Efferocytosis, the clearance of apoptotic cells (ACs), is a physiologic, multifaceted and dynamic process and a fundamental mechanism for the preservation of tissue homeostasis by avoiding unwanted inflammation and autoimmune responses through special phagocytic receptors. Defective efferocytosis is associated with several disease states, including cardiovascular disease and impaired immune surveillance, as occurs in cancer and autoimmune disease. A major cause of defective efferocytosis is non-functionality of surface receptors on either the phagocytic cells or the ACs, such as TAM family tyrosine kinase, which turns to a soluble form by cleavage/shedding or alternative splicing. Recently, soluble forms have featured prominently as potential biomarkers, indicative of prognosis and enabling targeted therapy using several commonly employed drugs and inhibitors, such as bleomycin, dexamethasone, statins and some matrix metalloproteinase inhibitors such as TAPI-1 and BB3103. Importantly, to design drug carriers with enhanced circulatory durability, the adaptation of soluble forms of physiological receptors/ligands has been purported. Research has shown that soluble forms are more effective than antibody forms in enabling targeted treatment of certain conditions, such as autoimmune diseases. In this review, we sought to summarize the current knowledge of these soluble products, how they are generated, their interactions, roles, and their potential use as biomarkers in prognosis and treatment related to inflammatory, cardiovascular, and autoimmune diseases.

Treatment Combining CD200 Immune Checkpoint Inhibitor and Tumor-Lysate Vaccination after Surgery for Pet Dogs with High-Grade Glioma

Recent advances in immunotherapy have included inhibition of immune checkpoint proteins in the tumor microenvironment and tumor lysate-based vaccination strategies. We combined these approaches in pet dogs with high-grade glioma. Administration of a synthetic peptide targeting the immune checkpoint protein, CD200, enhanced the capacity of antigen-presenting cells to prime T-cells to mediate an anti-glioma response. We found that in canine spontaneous gliomas, local injection of a canine-specific, CD200-directed peptide before subcutaneous delivery of an autologous tumor lysate vaccine prolonged survival relative to a historical control treated with autologous tumor lysate alone (median survivals of 12.7 months and 6.36 months, respectively). Antigen-presenting cells and T-lymphocytes primed with this peptide suppressed their expression of the inhibitory CD200 receptor, thereby enhancing their ability to initiate immune reactions in a glioblastoma microenvironment replete with the immunosuppressive CD200 protein. These results support consideration of a CD200 ligand as a novel glioblastoma immunotherapeutic agent.

CD200 Expression in Diagnostic and Prognostic Assessment of Mature B Cell Lymphophoproliferative Neoplasms

Background:

Multiparameter flow cytometry is a useful tool for diagnostic evaluation of mature B-cell neoplasms (MBN). Recently, it has been shown that assessment of CD200 expression may improve the distinction between chronic lymphocytic leukemia (CLL; CD200 positive) and mantle cell lymphoma (MCL; CD200 negative), but any potential as a prognostic marker for CLL remains to be established.

Materials and methods:

This cross sectional study was conducted on sixty-seven patients newly diagnosed as having mature B-cell lymphoproliferative disorders Levels of CD 200 in lymphoma cells were assessed.

Results:

CD200 was consistently expressed in CLL and hairy cell leukemia B cells, but not in MCL cells. Heterogeneous expression was noted in other CD5 positive Non-Hodgkin lymphomas. High CD200 expression (≥50%) was associated with a higher CD5, 19 and CD23 expression, older age, higher TLC and absolute lymphocyte count, hepatomegaly, splenomegaly and a higher Rai stage. There were no significant correlations between CD200 expression and response to treatment.

Conclusion:

CD200 could be of high value in distinguishing CLL, MCL, and atypical CLL. CD200 expression can also be of prognostic and therapeutic value in CLL cases.

Role of the CD200-CD200R Axis During Homeostasis and Neuroinflammation

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

The application of CAR-T cell therapy in hematological malignancies: advantages and challenges

Chimeric antigen receptor T cell (CAR-T cell) therapy is a novel adoptive immunotherapy where T lymphocytes are engineered with synthetic receptors known as chimeric antigen receptors (CAR). The CAR-T cell is an effector T cell that recognizes and eliminates specific cancer cells, independent of major histocompatibility complex molecules. The whole procedure of CAR-T cell production is not well understood. The CAR-T cell has been used predominantly in the treatment of hematological malignancies, including acute lymphoblastic leukemia, chronic lymphocytic leukemia, lymphoma, and multiple myeloma. Solid tumors including melanoma, breast cancer and sarcoma offer great promise in CAR-T cell research and development. CD19 CAR-T cell is most commonly used, and other targets, including CD20, CD30, CD38 and CD138 are being studied. Although this novel therapy is promising, there are several disadvantages. In this review we discuss the applications of CAR-T cells in different hematological malignancies, and pave a way for future improvement on the effectiveness and persistence of these adoptive cell therapies.

Brain Region-Dependent Rejection of Neural Precursor Cell Transplants

The concept of CNS as an immune-privileged site has been challenged by the occurrence of immune surveillance and allogeneic graft rejection in the brain. Here we examined whether the immune response to allogeneic neural grafts is determined by the site of implantation in the CNS. Dramatic regional differences were observed between immune responses to allogeneic neural precursor/stem cell (NPC) grafts in the striatum vs. the hippocampus. Striatal grafts were heavily infiltrated with IBA-1+ microglia/macrophages and CD3+ T cells and completely rejected. In contrast, hippocampal grafts exhibited milder IBA-1+ cell infiltration, were not penetrated efficiently by CD3+ cells, and survived efficiently for at least 2 months. To evaluate whether the hippocampal protective effect is universal, astrocytes were then transplanted. Allogeneic astrocyte grafts elicited a vigorous rejection process from the hippocampus. CD200, a major immune-inhibitory signal, plays an important role in protecting grafts from rejection. Indeed, CD200 knock out NPC grafts were rejected more efficiently than wild type NPCs from the striatum. However, lack of CD200 expression did not elicit NPC graft rejection from the hippocampus. In conclusion, the hippocampus has partial immune-privilege properties that are restricted to NPCs and are CD200-independent. The unique hippocampal milieu may be protective for allogeneic NPC grafts, through host-graft interactions enabling sustained immune-regulatory properties of transplanted NPCs. These findings have implications for providing adequate immunosuppression in clinical translation of cell therapy.

An autologous tumor vaccine for CLL

Chronic Lymphocytic Leukemia B cells (CLL) are malignant cells which retain at least some functions of normal B cells. Paramount amongst the latter is that when such cells are appropriately stimulated, they are able to present antigens, including any potential tumor antigens, making them excellent choices as a candidate tumor vaccine. We show that following stimulation of CLL cells with Phorbol myristic acetate, IL-2, the TLR7 agonist imiquimod (P2I) and ionomycin (P2Iio), markedly increased expression of CD54 and CD83 was seen, indicative of B cell activation and a transition to antigen-presenting cells. However, this occurred in the context of augmented expression of the known immunoregulatory molecule, CD200. Accordingly we explored the effect of stimulation of CLL cells with P2Iio, followed by coating of cells with a non-depleting anti-CD200mAb, on the ability of those cells to immunize PBL in vitro to become cytotoxic to CLL cells, or to protect NOD-SCIDγc^null (NSG) mice from subsequent CLL tumor challenge. Our data indicate that this protocol is effective in inducing CD8⁺ CTL able to lyse CLL cells in vitro, and decrease tumor burden in vivo in spleen and marrow of mice injected with CLL cells. Pre-treatment of mice with a CD8 depleting antibody before vaccination with P2Iio/anti-CD200 coated cells abolished any protection seen. These data suggest a potential role for blockade of CD200 expression on CLL cells as a component of a tumor vaccination strategy.

Checkpoint blockade in solid tumors and B-cell malignancies, with special consideration of the role of CD200

In the ontogeny of a normal immune response, a series of checkpoints must be overcome to ensure that unwanted and/or harmful self-directed activation responses are avoided. Many of the molecules now known to be active in this overseeing of the evolving immune activation cascade, contributing inhibitory signals to dampen an overexuberant response, belong to the immunoglobulin supergene family. These include members of the CD28/CTLA-4:B7.1/B7.2 receptor/ligand family, PD-1 and PDL-1, CD200 and CD200R, and the more recently described V-domain immunoglobulin suppressor of T-cell activation and its ligand (VSIG-3/IGSF11). Unfortunately, from the point of view of improving immunotargeting of cancer cells, triggering these checkpoint inhibitory signaling pathways, so necessary to maintain self-tolerance, simultaneously acts to prevent effective tumor immunity. The recent development of reagents, predominantly antibodies, to act as checkpoint blockade agents, has had a dramatic effect on human cancer treatment, with a marked reported success for anti-CTLA-4 and PD-1 in particular in clinical trials. This review provides a general overview of the data now available showing the promise of such treatments to our cancer armamentarium and elaborates in depth on the potential promise of what can be regarded as an underappreciated target molecule for checkpoint blockade in chronic lymphocytic leukemia and solid tumors, CD200.

Tumor-derived vaccines containing CD200 inhibit immune activation: implications for immunotherapy

There are over 400 ongoing clinical trials using tumor-derived vaccines. This approach is especially attractive for many types of brain tumors, including glioblastoma, yet so far the clinical response is highly variable. One contributor to poor response is CD200, which acts as a checkpoint blockade, inducing immune tolerance. We demonstrate that, in response to vaccination, glioma-derived CD200 suppresses the anti-tumor immune response. In contrast, a CD200 peptide inhibitor that activates antigen-presenting cells overcomes immune tolerance. The addition of the CD200 inhibitor significantly increased leukocyte infiltration into the vaccine site, cytokine and chemokine production, and cytolytic activity. Our data therefore suggest that CD200 suppresses the immune system's response to vaccines, and that blocking CD200 could improve the efficacy of cancer immunotherapy.

Dynamic changes in clonal cytogenetic architecture during progression of chronic lymphocytic leukemia in patients and patient-derived murine xenografts

Subclonal heterogeneity and clonal selection influences disease progression in chronic lymphocytic leukemia (CLL). It is therefore important that therapeutic decisions are made based on an understanding of the CLL clonal architecture and its dynamics in individual patients. Identification of cytogenetic abnormalities by FISH remains the cornerstone of contemporary clinical practice and provides a simple means for prognostic stratification. Here, we demonstrate that multiplexed-FISH can enhance recognition of CLL subclonal repertoire and its dynamics during disease progression, both in patients and CLL patient-derived xenografts (PDX). We applied a combination of patient-specific FISH probes to 24 CLL cases before treatment and at relapse, and determined putative ancestral relationships between subpopulations with different cytogenetic features. We subsequently established 7 CLL PDX models in NOD/Shi-SCID/IL-2Rγctm1sug/Jic (NOG) mice. Application of multiplexed-FISH to these models demonstrated that all of the identified cytogenetic subpopulations had leukemia propagating activity and that changes in their representation during disease progression could be spontaneous, accelerated by treatment or treatment-induced. We conclude that multiplexed-FISH in combination with PDX models have the potential to distinguish between spontaneous and treatment-induced clonal selection, and therefore provide a valuable tool for the pre-clinical evaluation of novel therapies.

A novel hemolytic complement-sufficient NSG mouse model supports studies of complement-mediated antitumor activity in vivo

Monoclonal antibodies (mAbs) have emerged as a mainstream therapeutic option against cancer. mAbs mediate tumor cell-killing through several mechanisms including complement-dependent cytotoxicity (CDC). However, studies of mAb-mediated CDC against tumor cells remain largely dependent on in vitro systems. Previously developed and widely used NOD-scid IL2rγ^null (NSG) mice support enhanced engraftment of many primary human tumors. However, NSG mice have a 2-bp deletion in the coding region of the hemolytic complement (Hc) gene, and it is not possible to evaluate CDC activity in NSG mice. To address this limitation, we generated a novel strain of NSG mice-NSG-Hc¹-that have an intact complement system able to generate the membrane attack complex. Utilizing the Daudi Burkitt's human lymphoma cell line, and the anti-human CD20 mAb rituximab, we further demonstrated that the complement system in NSG-Hc¹ mice is fully functional. NSG-Hc¹ mice expressed CDC activity against Daudi cells in vivo following rituximab treatment and showed longer overall survival compared with rituximab-treated NSG mice that lack hemolytic complement. Our results validate the NSG-Hc¹ mouse model as a platform for testing mechanisms underlying CDC in vivo and suggest its potential use to compare complement-dependent and complement-independent cytotoxic activity mediated by therapeutic mAbs.

SheddomeDB: the ectodomain shedding database for membrane-bound shed markers

BACKGROUND

A number of membrane-anchored proteins are known to be released from cell surface via ectodomain shedding. The cleavage and release of membrane proteins has been shown to modulate various cellular processes and disease pathologies. Numerous studies revealed that cell membrane molecules of diverse functional groups are subjected to proteolytic cleavage, and the released soluble form of proteins may modulate various signaling processes. Therefore, in addition to the secreted protein markers that undergo secretion through the secretory pathway, the shed membrane proteins may comprise an additional resource of noninvasive and accessible biomarkers. In this context, identifying the membrane-bound proteins that will be shed has become important in the discovery of clinically noninvasive biomarkers. Nevertheless, a data repository for biological and clinical researchers to review the shedding information, which is experimentally validated, for membrane-bound protein shed markers is still lacking.

RESULTS

In this study, the database SheddomeDB was developed to integrate publicly available data of the shed membrane proteins. A comprehensive literature survey was performed to collect the membrane proteins that were verified to be cleaved or released in the supernatant by immunological-based validation experiments. From 436 studies on shedding, 401 validated shed membrane proteins were included, among which 199 shed membrane proteins have not been annotated or validated yet by existing cleavage databases. SheddomeDB attempted to provide a comprehensive shedding report, including the regulation of shedding machinery and the related function or diseases involved in the shedding events. In addition, our published tool ShedP was embedded into SheddomeDB to support researchers for predicting the shedding event on unknown or unrecorded membrane proteins.

CONCLUSIONS

To the best of our knowledge, SheddomeDB is the first database for the identification of experimentally validated shed membrane proteins and currently may provide the most number of membrane proteins for reviewing the shedding information. The database included membrane-bound shed markers associated with numerous cellular processes and diseases, and some of these markers are potential novel markers because they are not annotated or validated yet in other databases. SheddomeDB may provide a useful resource for discovering membrane-bound shed markers. The interactive web of SheddomeDB is publicly available at http://bal.ym.edu.tw/SheddomeDB/ .

Reduction of CD200 expression in glioma cells enhances microglia activation and tumor growth

CD200, a type I transmembrane glycoprotein, can interact with its receptor CD200R, which plays an inhibitory role in the activation of microglia-the resident macrophages of the central nervous system. In this study, the rat C6 glioma cell line (C6-1) that was previously characterized with high in vivo tumorigenicity was found to generate CD200 mRNA abundantly. However, CD200 expression was barely detected in another C6 glioma cell clone (C6-2) that was previously found to display low tumorigenic behavior. The results from CD200 immunohistochemistry on human glioma tissue array also showed that tumor cells in Grade I-II astrocytoma expressed a lower level of CD200 immunoreactivity than those detected in Grade III-IV glioblastoma multiforme. C6-1 transfectants with stable downregulation of CD200 gene expression using lentivirus knockdown approach were generated (C6-KD). Microglia and iNOS⁺ cells were increased when microglia were co-cultured with C6-KD cells. The colony formation of C6-KD was also augmented when those cells were co-cultured with microglia. Yet, increased colony formation of C6-KD transfectants in the co-culture with microglia was effectively suppressed by interleukin (IL)-4 and IL-10. The in vivo results indicated that the tumor formation of C6-1 cells in rat brain was promoted after CD200 gene knockdown. Moreover, CD11b⁺ activated microglia and iNOS⁺ microglia were highly accumulated in the tumor site formed by C6-KD. In conclusion, our findings demonstrate that the downregulation of CD200 expression in CD200-rich glioma cells could foster the formation of an activated microglia-associated tumor microenvironment, leading to glioma progression. © 2016 Wiley Periodicals, Inc.

Characterization of CD200 Ectodomain Shedding

We have previously reported the existence of a soluble form of CD200 (sCD200) in human plasma, and found sCD200 to be elevated in the plasma of Chronic Lymphocytic Leukemia (CLL) patients. CLL cells release CD200 at a constitutive level, which could be attenuated partially by ADAM28 silencing. In this study, we further explored mechanisms of CD200 shedding beyond that of ADAM28, and performed biochemical analysis of sCD200 using materials derived from purified CLL cells and Hek293 cells stably transfected with CD200, and antibodies generated specifically against either the extracellular or cytoplasmic regions of CD200. CD200 shedding was enhanced by PMA stimulation, and the loss of cell surface CD200 could be monitored as a reduction in CD200 cell surface expression by flow cytometry, in parallel with an increase in the detection of sCD200 in the supernatant. Western blot analyses and functional studies using CD200R1 expressing Hek293 cells showed that the shed CD200 detected in CLL and Hek293-hCD200 supernatants lacked the cytoplasmic domain of CD200 but retained the functional extracellular domain required for binding to, and phosphorylation of, CD200R. These data confirms that a functionally active CD200 extracellular moiety can be cleaved from the surface of CD200 expressing cells following ectodomain shedding.

Chronic lymphocytic leukemia cells diversify and differentiate in vivo via a nonclassical Th1-dependent, Bcl-6-deficient process

Xenografting primary tumor cells allows modeling of the heterogeneous natures of malignant diseases and the influences of the tissue microenvironment. Here, we demonstrate that xenografting primary chronic lymphocytic leukemia (CLL) B lymphocytes with activated autologous T cells into alymphoid mice results in considerable CLL B cell division and sizable T cell expansion. Nevertheless, most/all CD5⁺CD19⁺ cells are eventually lost, due in part to differentiation into antibody-secreting plasmablasts/plasma cells. CLL B cell differentiation is associated with isotype class switching and development of new IGHV-D-J mutations and occurs via an activation-induced deaminase-dependent pathway that upregulates IRF4 and Blimp-1 without appreciable levels of the expected Bcl-6. These processes were induced in IGHV-unmutated and IGHV-mutated clones by Th1-polarized T-bet⁺ T cells, not classical T follicular helper (Tfh) cells. Thus, the block in B cell maturation, defects in T cell action, and absence of antigen-receptor diversification, which are often cardinal characteristics of CLL, are not inherent but imposed by external signals and the microenvironment. Although these activities are not dominant features in human CLL, each occurs in tissue proliferation centers where the mechanisms responsible for clonal evolution operate. Thus, in this setting, CLL B cell diversification and differentiation develop by a nonclassical germinal center-like reaction that might reflect the cell of origin of this leukemia.

Over-Expression of CD200 Protects Mice from Dextran Sodium Sulfate Induced Colitis

BACKGROUND AND AIM

CD200:CD200 receptor (CD200R) interactions lead to potent immunosuppression and inhibition of autoimmune inflammation. We investigated the effect of "knockout"of CD200 or CD200R, or over-expression of CD200, on susceptibility to dextran sodium sulfate (DSS)-induced colitis, a mouse model of inflammatory bowel disease (IBD).

METHODS

Acute or chronic colitis was induced by administration of dextran sodium sulfate (DSS) in four groups of age-matched C57BL/6 female mice: (1) CD200-transgenic mice (CD200tg); (2) wild-type (WT) mice; (3) CD200 receptor 1-deficient (CD200R1KO) mice; and (4) CD200-deficient (CD200KO) mice. The extent of colitis was determined using a histological scoring system. Colon tissues were collected for quantitative RT-PCR and Immunohistochemical staining. Supernatants from colonic explant cultures and mononuclear cells isolated from colonic tissue were used for ELISA.

RESULTS

CD200KO and CD200R1KO mice showed greater sensitivity to acute colitis than WT mice, with accelerated loss of body weight, significantly higher histological scores, more severe infiltration of macrophages, neutrophils and CD3+ cells, and greater expression of macrophage-derived inflammatory cytokines, whose production was inhibited in vitro (in WT/CD200KO mouse cells) by CD200. In contrast, CD200tg mice showed less sensitivity to DSS compared with WT mice, with attenuation of all of the features seen in other groups. In a chronic colitis model, greater infiltration of Foxp3+ regulatory T (Treg) cells was seen in the colon of CD200tg mice compared to WT mice, and anti-CD25 mAb given to these mice attenuated protection.

CONCLUSIONS

The CD200:CD200R axis plays an immunoregulatory role in control of DSS induced colitis in mice.

Ibrutinib enhances chimeric antigen receptor T-cell engraftment and efficacy in leukemia

Anti-CD19 chimeric antigen receptor (CAR) T-cell therapy is highly promising but requires robust T-cell expansion and engraftment. A T-cell defect in chronic lymphocytic leukemia (CLL) due to disease and/or therapy impairs ex vivo expansion and response to CAR T cells. To evaluate the effect of ibrutinib treatment on the T-cell compartment in CLL as it relates to CAR T-cell generation, we examined the phenotype and function of T cells in a cohort of CLL patients during their course of treatment with ibrutinib. We found that ≥5 cycles of ibrutinib therapy improved the expansion of CD19-directed CAR T cells (CTL019), in association with decreased expression of the immunosuppressive molecule programmed cell death 1 on T cells and of CD200 on B-CLL cells. In support of these findings, we observed that 3 CLL patients who had been treated with ibrutinib for ≥1 year at the time of T-cell collection had improved ex vivo and in vivo CTL019 expansion, which correlated positively together and with clinical response. Lastly, we show that ibrutinib exposure does not impair CAR T-cell function in vitro but does improve CAR T-cell engraftment, tumor clearance, and survival in human xenograft models of resistant acute lymphocytic leukemia and CLL when administered concurrently. Our collective findings indicate that ibrutinib enhances CAR T-cell function and suggest that clinical trials with combination therapy are warranted. Our studies demonstrate that improved T-cell function may also contribute to the efficacy of ibrutinib in CLL. These trials were registered at www.clinicaltrials.gov as #NCT01747486, #NCT01105247, and #NCT01217749.

The CD200 tolerance-signaling molecule and its receptor, CD200R1, are expressed in human placental villus trophoblast and in peri-implant decidua by 5 weeks' gestation

CD200 expression in murine trophoblast and decidua prevents semi-allogeneic and LPS-induced abortions by binding to CD200 receptor-bearing cells to suppress NK activity, induces IDO in macrophages, and promotes the generation of regulatory T cell subsets. CD200 and its receptor CD200R1 reported in 7-9 weeks' gestation human villus trophoblasts are reduced in spontaneous abortion syncytiotrophoblasts. By specific antibody staining, we find that both CD200 and CD200R1 are expressed even earlier, by 5 weeks' gestation, by villus trophoblasts and by decidual cells. Expression of CD200 was validated using two independent antibodies. CD200-CD200R1 signaling may be required for human pregnancy success.

Microenvironmental interleukin-6 suppresses toll-like receptor signaling in human leukemia cells through miR-17/19A

The regulation of toll-like receptor (TLR) signaling in a tumor microenvironment is poorly understood despite its importance in cancer biology. To address this problem, TLR7-responses of chronic lymphocytic leukemia (CLL) cells were studied in the presence and absence of a human stromal cell-line derived from a leukemic spleen. CLL cells alone produced high levels of tumor necrosis factor (TNF)-α and proliferated in response to TLR7-agonists. A signal transducer and activator of transcription 3 -activating stromal factor, identified as interleukin (IL)-6, was found to upregulate microRNA (miR)-17 and miR-19a, target TLR7 and TNFA messenger RNA, and induce a state of tolerance to TLR7-agonists in CLL cells. Overexpression of the miR-17-92 cluster tolerized CLL cells directly and miR-17 and miR-19a antagomiRs restored TLR7-signaling. Inhibition of IL-6 signaling with antibodies or small-molecule Janus kinase inhibitors reversed tolerization and increased TLR7-stimulated CLL cell numbers in vitro and in NOD-SCIDγc (null) mice. These results suggest IL-6 can act as tumor suppressor in CLL by inhibiting TLR-signaling.

CD200 in CNS tumor-induced immunosuppression: the role for CD200 pathway blockade in targeted immunotherapy

BACKGROUND

Immunological quiescence in the central nervous system (CNS) is a potential barrier to immune mediated anti-tumor response. One suppressive mechanism results from the interaction of parenchyma-derived CD200 and its receptor on myeloid cells. We suggest that CD200/CD200R interactions on myeloid cells expand the myeloid-derived suppressor cell (MDSC) population and that blocking tumor-derived CD200 will enhance the efficacy of immunotherapy.

METHODS

CD200 mRNA expression levels in human brain tumor tissue samples were measured by microarray. The amount of circulating CD200 protein in the sera of patients with brain tumors was determined by ELISA and, when corresponding peripheral blood samples were available, was correlated quantitatively with MDSCs. CD200-derived peptides were used as competitive inhibitors in a mouse model of glioblastoma immunotherapy.

RESULTS

CD200 mRNA levels were measured in human brain tumors, with different expression levels being noted among the sub groups of glioblastoma, medulloblastoma and ependymoma. Serum CD200 concentrations were highest in patients with glioblastoma and correlated significantly with MDSC expansion. Similarly, in vitro studies determined that GL261 cells significantly expanded a MDSC population. Interestingly, a CD200R antagonist inhibited the expansion of murine MDSCs in vitro and in vivo. Moreover, inclusion of CD200R antagonist peptide in glioma tumor lysate-derived vaccines slowed tumor growth and significantly enhanced survival.

CONCLUSION

These data suggest that CNS-derived tumors can evade immune surveillance by engaging CD200. Because of the homology between mouse and human CD200, our data also suggest that blockade of CD200 binding to its receptor will enhance the efficacy of immune mediated anti-tumor strategies for brain tumors.

Bidirectional effect of CD200 on breast cancer development and metastasis, with ultimate outcome determined by tumor aggressiveness and a cancer-induced inflammatory response

CD200 acts through its receptor (CD200R) to inhibit excessive inflammation. The role of CD200-CD200R1 interaction in tumor immunity is poorly understood. In this study, we examined the role of CD200-CD200R1 interaction in the progression and metastasis of highly aggressive 4THM murine-breast carcinoma using CD200 transgenic (CD200(tg)) and CD200R1 knock-out (CD200R1(-)(/-)) BALB/c mice. 4THM cells induce extensive visceral metastasis and neutrophil infiltration in affected tissues. CD200 overexpression in the host was associated with decreased primary tumor growth and metastasis, whereas lack of CD200R1 expression by host cells was associated with enhanced visceral metastasis. Absence of CD200R1 expression led to decreased tumor-infiltrating-cytotoxic T cells and increased the release of inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and interleukin (IL)-6. In contrast, CD200 overexpression led to increased tumor-induced interferon-γ and IL-10 response and decreased TNF-α and IL-6 release. Neutrophil infiltration of tissues was markedly decreased in CD200(tg) animals and increased in CD200R1(-/-) mice. These findings are contradictory to what has been reported in the EMT6 mouse breast-cancer model. Other distinguishing features of tumor elicited by EMT6 and 4THM cell injections were also examined. Visceral tissues from mice bearing EMT6 tumors showed a lack of neutrophil infiltration and decreased IL-6 release in CD200R1(-/-) mice. EMT6 and 4THM cells also differed in vimentin expression and in vitro migration rate, which was markedly lower in EMT6 tumors. These results support the hypothesis that CD200 expression can alter immune responses, and can inhibit metastatic growth of tumor cells that induce systemic and local inflammatory response. Increasing CD200 activity/signaling might be an important therapeutic strategy for treatment of aggressive breast carcinomas.

The CD200-tolerance signaling molecule associated with pregnancy success is present in patients with early-stage breast cancer but does not favor nodal metastasis

PROBLEM

The CD200-tolerance signaling molecule prevents pregnancy failure and is also expressed by a wide variety of malignant tumors. The effect of CD200 mRNA expression on progression of human tumors has been variable.

METHOD OF STUDY

A cross-sectional study was performed to examine the correlation between CD200 protein expression in the primary tumors from postoperative Stage I-IIIA human breast cancer and the likelihood of regional lymph node metastasis.

RESULTS

Fifty-eight percentage of patients had strong CD200(+) tumor staining (71% of Stage I and 53% Stage II-IIIA). Strong staining was associated with large T2-3 primary tumors compared to T1 tumors (64 versus 50%) and T2-3 N(+) versus T1 N(-) tumors (70 versus 63%), but this was not statistically significant. Nodal metastases were not more frequent in patients with strong CD200(+) staining (57% compared to 58% for weak/negative staining cases), and the metastatic tumor cells in regional lymph nodes were often CD200(-) when the primary tumor was CD200(+).

CONCLUSION

CD200 expression by early-stage human breast cancer cells in primary tumors did not correlate with increased regional lymph node metastasis.

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.

Human cancer growth and therapy in immunodeficient mouse models

Since the discovery of the "nude" mouse more than 40 years ago, investigators have attempted to model human tumor growth in immunodeficient mice. Here, we summarize how the field has advanced over the ensuing years owing to improvements in the murine recipients of human tumors. These improvements include the discovery of the scid mutation and development of targeted mutations in the recombination-activating genes 1 and 2 (Rag1(null), Rag2(null)) that severely cripple the adaptive immune response of the murine host. More recently, mice deficient in adaptive immunity have been crossed with mice bearing targeted mutations designed to weaken the innate immune system, ultimately leading to the development of immunodeficient mice bearing a targeted mutation in the gene encoding the interleukin 2 (IL2) receptor common γ chain (IL2rg(null), also known in humans as cytokine receptor common subunit γ). The IL2rg(null) mutation has been used to develop several immunodeficient strains of mice, including the NOD-scid IL2rg(null) (NSG) strain. Using NSG mice as human xenograft recipients, it is now possible to grow almost all types of primary human tumors in vivo, including most solid tumors and hematological malignancies that maintain characteristics of the primary tumor in the patient. Programs to optimize patient-specific therapy using patient-derived xenograft tumor growth in NSG mice have been established at several institutions, including The Jackson Laboratory. Moreover, NSG mice can be engrafted with functional human immune systems, permitting for the first time the potential to study primary human tumors in vivo in the presence of a human immune system.

Murine genetically engineered and human xenograft models of chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a genetically complex disease, with multiple factors having an impact on onset, progression, and response to therapy. Genetic differences/abnormalities have been found in hematopoietic stem cells from patients, as well as in B lymphocytes of individuals with monoclonal B-cell lymphocytosis who may develop the disease. Furthermore, after the onset of CLL, additional genetic alterations occur over time, often causing disease worsening and altering patient outcomes. Therefore, being able to genetically engineer mouse models that mimic CLL or at least certain aspects of the disease will help us understand disease mechanisms and improve treatments. This notwithstanding, because neither the genetic aberrations responsible for leukemogenesis and progression nor the promoting factors that support these are likely identical in character or influences for all patients, genetically engineered mouse models will only completely mimic CLL when all of these factors are precisely defined. In addition, multiple genetically engineered models may be required because of the heterogeneity in susceptibility genes among patients that can have an effect on genetic and environmental characteristics influencing disease development and outcome. For these reasons, we review the major murine genetically engineered and human xenograft models in use at the present time, aiming to report the advantages and disadvantages of each.

Elevated serum soluble CD200 and CD200R as surrogate markers of bone loss under bed rest conditions

CD200 is a transmembrane protein that belongs to the immunoglobulin family of proteins and is ubiquitously expressed on a variety of cell types. Upon interaction with its receptors (CD200Rs) expressed on myeloid-derived cells and T lymphocytes, an immunoregulatory signal is delivered to receptor-expressing cells. Previous studies have implicated a role for CD200:CD200R in the regulation of the expression of mRNA markers of osteoclastogenesis/osteoblastogenesis, following interaction of CD200 (on osteoblast precursors) with CD200R1 (on osteoclast precursors). Signaling of CD200R1 is hypothesized to attenuate osteoclastogenesis. We have investigated whether levels of soluble forms of CD200 and/or CD200R1 (sCD200, sCD200R1) are altered in volunteers undergoing 6° head down tilt bed rest to mimic conditions of microgravity known to be associated with preferential osteoclastogenesis and whether countermeasures, reported to be beneficial in attenuation of bone loss under microgravity conditions, would lead to altered sCD200 and sCD200R1 levels. Our data suggest that, as predicted, sCD200 levels fall under bed rest conditions while sCD200R1 levels rise. In subjects undergoing 30-minute per day continuous centrifugation protocols, as a countermeasure to attenuate changes which may lead to bone loss, these alterations in sCD200 and sCD200R1 levels seen under conditions of bed rest were abolished or attenuated. Our results suggest that measurement of sCD200 and/or sCD200R1 may prove a useful and rapid means of monitoring subjects at risk of bone loss and/or accessing the efficacy of treatment regimes designed to counter bone loss.

A role for oleoylethanolamide in chronic lymphocytic leukemia

Oleoylethanolamide (OEA) is a bioactive lipid that stimulates nuclear and G protein-coupled receptors and regulates appetite and fat metabolism. It has not previously been shown to have a role in cancer. However, a mass spectrometry-based lipidomics platform revealed the presence of high amounts of OEA in the plasma of chronic lymphocytic leukemia (CLL) patients compared with normal donors. CLL cells produced OEA and the magnitude of plasma OEA levels was related directly to the circulating leukemic cell number. OEA from CLL cells was increased by URB-597, an inhibitor of fatty acid amide hydrolase (FAAH), and decreased by inflammatory mediators that downregulate expression of N-acylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD). These enzymes degrade and synthesize OEA, respectively. Nonphysiologic doses of OEA prevented spontaneous apoptosis of CLL cells in a receptor-independent manner that was mimicked by its free fatty acid (FFA) derivative oleate. However, OEA-containing supernatants from CLL cells induced lipolysis in adipocytes, lipid products from adipocytes protected CLL cells from cytotoxic chemotherapy, and increased levels of FFAs were found in CLL plasma that correlated with OEA. We suggest OEA is a lipolytic factor produced by CLL cells to fuel their growth with a potential role in drug resistance and cancer cachexia.

PPARα and fatty acid oxidation mediate glucocorticoid resistance in chronic lymphocytic leukemia

High-dose glucocorticoids (GCs) can be a useful treatment for aggressive forms of chronic lymphocytic leukemia (CLL). However, their mechanism of action is not well understood, and resistance to GCs is inevitable. In a minimal, serum-free culture system, the synthetic GC dexamethasone (DEX) was found to decrease the metabolic activity of CLL cells, indicated by down-regulation of pyruvate kinase M2 (PKM2) expression and activity, decreased levels of pyruvate and its metabolites, and loss of mitochondrial membrane potential. This metabolic restriction was associated with decreased size and death of some of the tumor cells in the population. Concomitant plasma membrane damage increased killing of CLL cells by DEX. However, the nuclear receptor peroxisome proliferator activated receptor α (PPARα), which regulates fatty acid oxidation, was also increased by DEX, and adipocyte-derived lipids, lipoproteins, and propionic acid protected CLL cells from DEX. PPARα and fatty acid oxidation enzyme inhibitors increased DEX-mediated killing of CLL cells in vitro and clearance of CLL xenografts in vivo. These findings suggest that GCs prevent tumor cells from generating the energy needed to repair membrane damage, fatty acid oxidation is a mechanism of resistance to GC-mediated cytotoxicity, and PPARα inhibition is a strategy to improve the therapeutic efficacy of GCs.

Rhesus macaque rhadinovirus-associated disease

Rhesus macaque rhadinovirus (RRV) is a gamma-2 herpesvirus that naturally infects rhesus macaque (RM) monkeys and is closely related to human herpesvirus-8 (HHV-8)/Kaposi's sarcoma-associated herpesvirus (KSHV). Infection of immunodeficient RM induces disease in infected RM that resembles KSHV-associated pathologies. Importantly, RRV possesses homologues of KSHV ORFs that are postulated to play a role in disease development. As such, RRV has emerged as a prominent in vivo model system for examining mechanisms of infection and disease of these pathogenic herpesviruses, and has provided unique insight into how these viruses cause disease.

Immune checkpoint blockade immunotherapy to activate anti-tumour T-cell immunity

The tumour microenvironment plays a dual role in cancer: it can promote tumour progression by establishing pro-tumour survival conditions but can also suppress tumour progression by killing cancer cells or inhibiting their outgrowth. These dynamically interconnected processes are under intense investigation to better understand cancer pathophysiology and allow identification of new therapeutic approaches. The ability of cancer cells to evade anti-tumour T-cell activity in the microenvironment has recently been accepted as a hallmark of cancer progression. This review will highlight the most promising therapeutic approach aimed at activating anti-tumour T-cell immunity in the cancer microenvironment: blocking inhibitory immune regulatory proteins (immune checkpoint ligands and receptors). There is emerging evidence that haematological tumours co-opt immune checkpoints as a major immune resistance mechanism. Pre-clinical findings indicate that targeted therapies and blockade of immune checkpoints could be combined to promote therapeutic synergy and long-term anti-tumour immunity to improve clinical outcomes for cancer patients.

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

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