Human C1qRp is identical with CD93 and the mNI-11 antigen but does not bind C1q

Group XIV C-type lectins: emerging targets in tumor angiogenesis

C-type lectins, distinguished by a C-type lectin binding domain (CTLD), are an evolutionarily conserved superfamily of glycoproteins that are implicated in a broad range of physiologic processes. The group XIV subfamily of CTLDs are comprised of CD93, CD248/endosialin, CLEC14a, and thrombomodulin/CD141, and have important roles in creating and maintaining blood vessels, organizing extracellular matrix, and balancing pro- and anti-coagulative processes. As such, dysregulation in the expression and downstream signaling pathways of these proteins often lead to clinically relevant pathology. Recently, group XIV CTLDs have been shown to play significant roles in cancer progression, namely tumor angiogenesis and metastatic dissemination. Interest in therapeutically targeting tumor vasculature is increasing and the search for novel angiogenic targets is ongoing. Group XIV CTLDs have emerged as key moderators of tumor angiogenesis and metastasis, thus offering substantial therapeutic promise for the clinic. Herein, we review our current knowledge of group XIV CTLDs, discuss each's role in malignancy and associated potential therapeutic avenues, briefly discuss group XIV CTLDs in the context of two other relevant lectin families, and offer future direction in further elucidating mechanisms by which these proteins function and facilitate tumor growth.

Tobacco smoke exposure recruits inflammatory airspace monocytes that establish permissive lung niches for Mycobacterium tuberculosis

Tobacco smoking doubles the risk of active tuberculosis (TB) and accounts for up to 20% of all active TB cases globally. How smoking promotes lung microenvironments permissive to Mycobacterium tuberculosis (Mtb) growth remains incompletely understood. We investigated primary bronchoalveolar lavage cells from current and never smokers by performing single-cell RNA sequencing (scRNA-seq), flow cytometry, and functional assays. We observed the enrichment of immature inflammatory monocytes in the lungs of smokers compared with nonsmokers. These monocytes exhibited phenotypes consistent with recent recruitment from blood, ongoing differentiation, increased activation, and states similar to those with chronic obstructive pulmonary disease. Using integrative scRNA-seq and flow cytometry, we identified CD93 as a marker for a subset of these newly recruited smoking-associated lung monocytes and further provided evidence that the recruitment of monocytes into the lung was mediated by CCR2-binding chemokines, including CCL11. We also show that these cells exhibit elevated inflammatory responses upon exposure to Mtb and accelerated intracellular growth of Mtb compared with mature macrophages. This elevated Mtb growth could be inhibited by anti-inflammatory small molecules, providing a connection between smoking-induced pro-inflammatory states and permissiveness to Mtb growth. Our findings suggest a model in which smoking leads to the recruitment of immature inflammatory monocytes from the periphery to the lung, which results in the accumulation of these Mtb-permissive cells in the airway. This work defines how smoking may lead to increased susceptibility to Mtb and identifies host-directed therapies to reduce the burden of TB among those who smoke.

Structural biology of complement receptors

The complement system plays crucial roles in a wide breadth of immune and inflammatory processes and is frequently cited as an etiological or aggravating factor in many human diseases, from asthma to cancer. Complement receptors encompass at least eight proteins from four structural classes, orchestrating complement-mediated humoral and cellular effector responses and coordinating the complex cross-talk between innate and adaptive immunity. The progressive increase in understanding of the structural features of the main complement factors, activated proteolytic fragments, and their assemblies have spurred a renewed interest in deciphering their receptor complexes. In this review, we describe what is currently known about the structural biology of the complement receptors and their complexes with natural agonists and pharmacological antagonists. We highlight the fundamental concepts and the gray areas where issues and problems have been identified, including current research gaps. We seek to offer guidance into the structural biology of the complement system as structural information underlies fundamental and therapeutic research endeavors. Finally, we also indicate what we believe are potential developments in the field.

Angiogenesis modulated by CD93 and its natural ligands IGFBP7 and MMRN2: a new target to facilitate solid tumor therapy by vasculature normalization

The tumor vasculature was different from the normal vasculature in both function and morphology, which caused hypoxia in the tumor microenvironment (TME). Previous anti-angiogenesis therapy had led to a modest improvement in cancer immunotherapy. However, antiangiogenic therapy only benefitted a few patients and caused many side effects. Therefore, there was still a need to develop a new approach to affect tumor vasculature formation. The CD93 receptor expressed on the surface of vascular endothelial cells (ECs) and its natural ligands, MMRN2 and IGFBP7, were now considered potential targets in the antiangiogenic treatment because recent studies had reported that anti-CD93 could normalize the tumor vasculature without impacting normal blood vessels. Here, we reviewed recent studies on the role of CD93, IGFBP7, and MMRN2 in angiogenesis. We focused on revealing the interaction between IGFBP7-CD93 and MMRN2-CD93 and the signaling cascaded impacted by CD93, IGFBP7, and MMRN2 during the angiogenesis process. We also reviewed retrospective studies on CD93, IGFBP7, and MMRN2 expression and their relationship with clinical factors. In conclusion, CD93 was a promising target for normalizing the tumor vasculature.

Role of CD93 in Health and Disease

CD93 (also known as complement protein 1 q subcomponent receptor C1qR1 or C1qRp), is a transmembrane glycoprotein encoded by a gene located on 20p11.21 and composed of 652 amino acids. CD93 can be present in two forms: soluble (sCD93) and membrane-bound (CD93). CD93 is mainly expressed on endothelial cells, where it plays a key role in promoting angiogenesis both in physiology and disease, such as age-related macular degeneration and tumor angiogenesis. In fact, CD93 is highly expressed in tumor-associated vessels and its presence correlates with a poor prognosis, poor immunotherapy response, immune cell infiltration and high tumor, node and metastasis (TNM) stage in many cancer types. CD93 is also expressed in hematopoietic stem cells, cytotrophoblast cells, platelets and many immune cells, i.e., monocytes, neutrophils, B cells and natural killer (NK) cells. Accordingly, CD93 is involved in modulating important inflammatory-associated diseases including systemic sclerosis and neuroinflammation. Finally, CD93 plays a role in cardiovascular disease development and progression. In this article, we reviewed the current literature regarding the role of CD93 in modulating angiogenesis, inflammation and tumor growth in order to understand where this glycoprotein could be a potential therapeutic target and could modify the outcome of the abovementioned pathologies.

Inflammatory Mesenchymal Stem Cells Express Abundant Membrane-Bound and Soluble Forms of C-Type Lectin-like CD248

CD248 (endosialin) belongs to a glycoprotein family that also includes thrombomodulin (CD141), CLEC14A, and CD93 (AA4) stem cell markers. We analyzed the regulated expression of CD248 in vitro using skin (HFFF) and synovial (FLS) mesenchymal stem cell lines, and in fluid and tissue samples of rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Cells were incubated with either rhVEGF₁₆₅, bFGF, TGF-β1, IL1-β, TNF-α, TGFβ1, IFN-γ, or PMA (Phorbol ester). There was no statistically significant change in membrane expression. A soluble (s) form of cleaved CD248 (sCD248) was detected after cell treatment with IL1-β and PMA. Matrix metalloprotease (MMP) MMP-1 and MMP-3 mRNAs were significantly up-regulated by IL1-β and PMA. A broad MMP inhibitor blocked the release of soluble CD248. In RA synovial tissue, we identified CD90⁺ perivascular MSCs double-stained for CD248 and VEGF. High sCD248 levels were detected in synovial fluid from RA. In culture, subpopulations of CD90⁺ CD14^- RA MSCs were either identified as CD248⁺ or CD141⁺ cells but CD93^-. CD248 is abundantly expressed by inflammatory MSCs and shed in an MMP-dependent manner in response to cytokines and pro-angiogenic growth factors. Both membrane-bound and soluble CD248 (acting as a decoy receptor) may contribute to RA pathogenesis.

Synergistic regulation of microglia differentiation by CD93 and integrin β1 in the rat pneumococcal meningitis model

Streptococcus pneumoniae is the main bacterial pathogen of meningitis worldwide, which has a high mortality rate and survivors are prone to central nervous system (CNS) sequelae. In this regard, microglia activation has been associated with injury to the CNS. The aim of this study was to investigate the relationship between CD93, integrin β1, and microglia activation. In the rat pneumococcal meningitis model, we found significant increases of CD93 and integrin β1 expression and differentiation of M1 phenotype microglia. Furthermore, we showed in vitro siRNA-mediated downregulation of CD93 and integrin β1 expression after infecting highly aggressive proliferating immortalized (HAPI) microglia cells with S. pneumoniae. We observed differentiation of S. pneumonia-infected HAPI microglia cells to the M1 phenotype and significant release of soluble CD93 (sCD93) and integrin β1 expression. Complement C1q and metalloproteinases promoted sCD93 release. We also showed that downregulation of CD93 significantly reduced differentiation to M1 microglia and increased differentiation to M2 microglia. However, addition of recombinant CD93 may regulate microglia differentiation to the M1 phenotype. Furthermore, the downregulation of integrin β1 resulted in downregulation of the CD93 protein. In conclusion, interaction between integrin β1 and CD93 promotes differentiation of microglia to the M1 phenotype, increases the release of pro-inflammatory factors, and leads to nervous system injury in pneumococcal meningitis.

CD93 Correlates With Immune Infiltration and Impacts Patient Immunotherapy Efficacy: A Pan-Cancer Analysis

Background: The clinical implementation of immune-checkpoint inhibitors (ICIs) targeting CTLA4, PD-1, and PD-L1 has revolutionized the treatment of cancer. However, the majority of patients do not derive clinical benefit. Further development is needed to optimize the approach of ICI therapy. Immunotherapy combined with other forms of treatment is a rising strategy for boosting antitumor responses. CD93 was found to sensitize tumors to immune-checkpoint blocker therapy after the blockade of its pathway. However, its role in immune and ICB therapy across pan-cancer has remained unexplored.

Methods: In this study, we provide a comprehensive investigation of CD93 expression in a pan-cancer manner involving 33 cancer types. We evaluated the association of CD93 expression with prognosis, mismatch repair, tumor mutation burden, and microsatellite instability, immune checkpoints, tumor microenvironment, and immune using multiple online datasets, including The Cancer Genome Atlas, Cancer Cell Line Encyclopedia, Genotype Tissue-Expression, cBioPortal, Tumor Immune Estimation Resource database, and Tumor Immune Single-cell Hub.

Results: CD93 expression varied strongly among cancer types, and increased CD93 gene expression was associated with poor prognosis as well as higher immune factors in most cancer types. Additionally, the level of CD93 was significantly correlated with MMR, TMB, MSI, immune checkpoints, TME, and immune cell infiltration. Noticeably, our results mediated a strong positive contact between CD93 and CAFs, endothelial cells, myeloid dendritic cells, hematopoietic stem cells, mononuclear/macrophage subsets, and neutrophils while a negative correlation with Th1, MDSC, NK, and T-cell follicular helper in almost all cancers. Function analysis on CD93 revealed a link between itself and promoting cancers, inflammation, and angiogenesis.

Conclusion: CD93 can function as a prognostic marker in various malignant tumors and is integral in TME and immune infiltration. Inhibition of the CD93 pathway may be a novel and promising strategy for immunotherapy in human cancer. Further explorations of the mechanisms of CD93 in the immune system may help improve cancer therapy methods.

Postural tachycardia syndrome (POTS) and antiphospholipid syndrome (APS): What do we know so far?

As part of the non-criteria clinical manifestations, postural orthostatic tachycardia syndrome (POTS), a multisystem autonomic dysfunction, can co-exist with antiphospholipid syndrome (APS). Several pieces of evidence hint on the autoimmune basis of POTS, and its possible association with several autoimmune diseases, including APS. Indeed, the evidence exists in the etiologies, symptomatology, and treatment options. Although infections, viral ones in particular, stress, and pregnancy are etiologies to both POTS and APS, the exact pathophysiological connection is still to be studied taking into consideration the activity of cytokines in both diseases. Nevertheless, certain immunomodulatory treatments used for the catastrophic or obstetrical forms of APS, such as intravenous immunoglobulins (IVIG) and steroids, have been also used for the treatment of POTS resistant to classical treatments. Therefore, our review aims to highlight the association between POTS and APS, shedding light on the common etiologies explaining the pathophysiology of the two disorders, the diagnostic approach to POTS as a possible clinical criterion of APS, and the treatment of APS in the context of treating POTS.

Soluble Complement Component 1q Receptor 1 (sCD93) Is Associated with Graft Function in Kidney Transplant Recipients

Cluster of differentiation 93 (CD93), also known as complement component 1q receptor 1 is a transmembrane glycoprotein expressed in endothelial and hematopoietic cells and associated with phagocytosis, cell adhesion, angiogenesis and inflammation. The extracellular part, soluble CD93 (sCD93), is released to body fluids in inflammation. Data on sCD93 in kidney diseases are limited. Our aim was to evaluate serum sCD93 in long-term kidney transplant recipients as a marker of inflammation and endothelial dysfunction that may be potentially useful in early recognition of graft dysfunction. Seventy-eight adult patients with functioning kidney graft and stable clinical state were examined at least one year after kidney transplantation. Serum sCD93 was measured by enzyme immunosorbent assay. Estimated glomerular filtration rate (eGFR) and albuminuria or proteinuria were assessed at baseline and over one-year follow-up. Increased sCD93 was associated with lower baseline eGFR independently of the confounders. Moreover, sCD93 was negatively associated with eGFR during one-year follow-up in simple analysis; however, this was not confirmed after adjustment for confounders. Baseline sCD93 was positively associated with baseline albuminuria and with increased proteinuria during the follow-up. Serum sCD93 was not correlated with other studied inflammatory markers (interleukin 6, C-reactive protein, procalcitonin and C3 and C4 complement components). To the best of our knowledge, this is the first report regarding the concentrations of sCD93 in kidney transplant recipients and one of the first reports showing the inverse association between sCD93 and renal function. Serum sCD93 should be further evaluated as a diagnostic and prognostic marker in renal transplantation.

CD93 a potential player in cytotrophoblast and endothelial cell migration

CD93, also known as complement component C1q receptor, is expressed on the surface of different cellular types such as monocytes, neutrophils, platelets, microglia, and endothelial cells, and it plays a pivotal role in cell proliferation, cell migration, and formation of capillary-like structures. These processes are strictly regulated, and many fetal and maternal players are involved during placental development. At present, there are no studies in literature regarding CD93 in placental development, so we investigated CD93 expression in first and third trimester and PE placentas by immunohistochemistry and western blotting analysis. In addition, we performed in vitro experiments under oxidative stress conditions to demonstrate how oxidative stress acts on CD93 protein expression. Our data showed that CD93 was expressed in villous cytotrophoblast cells, in some fetal vessels of first and third trimester and PE placentas and in the extravillous cytotrophoblast of cell columns in the first trimester placentas. Moreover, we detected a significant decrease of CD93 expression in third trimester and PE placentas compared to first trimester placentas, while no differences were detected between third and PE placentas. No differences of CD93 expression were detected in oxidative stress conditions. We suggest that CD93 can guide extravillous cytotrophoblast migration through β1-integrin in uterine spiral arteries during placentation in the first trimester of pregnancy and that the decrease of CD93 expression in third trimester and PE placentas could be linked to the poor extravillous cytotrophoblast cells migration. So, it might be interesting to understand the role of CD93 in the first phases of PE onset.

Mitigating the non-specific uptake of immunomagnetic microparticles enables the extraction of endothelium from human fat

Endothelial cells are among the fundamental building blocks for vascular tissue engineering. However, a clinically viable source of endothelium has continued to elude the field. Here, we demonstrate the feasibility of sourcing autologous endothelium from human fat – an abundant and uniquely dispensable tissue that can be readily harvested with minimally invasive procedures. We investigate the challenges underlying the overgrowth of human adipose tissue-derived microvascular endothelial cells by stromal cells to facilitate the development of a reliable method for their acquisition. Magnet-assisted cell sorting strategies are established to mitigate the non-specific uptake of immunomagnetic microparticles, enabling the enrichment of endothelial cells to purities that prevent their overgrowth by stromal cells. This work delineates a reliable method for acquiring human adipose tissue-derived microvascular endothelial cells in large quantities with high purities that can be readily applied in future vascular tissue engineering applications.

Antonyshyn et al. establish a methodology for acquiring human adipose tissue-derived microvascular endothelial cells that can be readily applied in future vascular tissue engineering applications. The authors developed strategies to mitigate the non-specific uptake of immunomagnetic microparticles to facilitate the immunoselection of endothelial cells by magnet-assisted cell sorting.

Differential H4K16ac levels ensure a balance between quiescence and activation in hematopoietic stem cells

Hematopoietic stem cells (HSCs) are able to reconstitute the bone marrow while retaining their self-renewal property. Individual HSCs demonstrate heterogeneity in their repopulating capacities. Here, we found that the levels of the histone acetyltransferase MOF (males absent on the first) and its target modification histone H4 lysine 16 acetylation are heterogeneous among HSCs and influence their proliferation capacities. The increased proliferative capacities of MOF-depleted cells are linked to their expression of CD93. The CD93+ HSC subpopulation simultaneously shows transcriptional features of quiescent HSCs and functional features of active HSCs. CD93+ HSCs were expanded and exhibited an enhanced proliferative advantage in Mof +/- animals reminiscent of a premalignant state. Accordingly, low MOF and high CD93 levels correlate with poor survival and increased proliferation capacity in leukemia. Collectively, our study indicates H4K16ac as an important determinant for HSC heterogeneity, which is linked to the onset of monocytic disorders.

SP-A and SP-D: Dual Functioning Immune Molecules With Antiviral and Immunomodulatory Properties

Surfactant proteins A (SP-A) and D (SP-D) are soluble innate immune molecules which maintain lung homeostasis through their dual roles as anti-infectious and immunomodulatory agents. SP-A and SP-D bind numerous viruses including influenza A virus, respiratory syncytial virus (RSV) and human immunodeficiency virus (HIV), enhancing their clearance from mucosal points of entry and modulating the inflammatory response. They also have diverse roles in mediating innate and adaptive cell functions and in clearing apoptotic cells, allergens and other noxious particles. Here, we review how the properties of these first line defense molecules modulate inflammatory responses, as well as host-mediated immunopathology in response to viral infections. Since SP-A and SP-D are known to offer protection from viral and other infections, if their levels are decreased in some disease states as they are in severe asthma and chronic obstructive pulmonary disease (COPD), this may confer an increased risk of viral infection and exacerbations of disease. Recombinant molecules of SP-A and SP-D could be useful in both blocking respiratory viral infection while also modulating the immune system to prevent excessive inflammatory responses seen in, for example, RSV or coronavirus disease 2019 (COVID-19). Recombinant SP-A and SP-D could have therapeutic potential in neutralizing both current and future strains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus as well as modulating the inflammation-mediated pathology associated with COVID-19. A recombinant fragment of human (rfh)SP-D has recently been shown to neutralize SARS-CoV-2. Further work investigating the potential therapeutic role of SP-A and SP-D in COVID-19 and other infectious and inflammatory diseases is indicated.

Toxicity in hematopoietic stem cells from bone marrow and peripheral blood in mice after benzene exposure: Single-cell transcriptome sequencing analysis

Benzene is a ubiquitous, occupational, and environmental hematotoxic and leukemogen. Damage to hematopoietic stem cells (HSCs) induced by benzene and its metabolites is a key event in bone marrow (BM) depression and leukemogenesis. There are no reports on transcriptome profiles of HSCs following benzene exposure. Here, Smart-seq2 single-cell transcriptome sequencing was used to detect transcriptomic alternations in BM HSCs and peripheral blood HSCs (PBSCs) in male C57B/6 mice exposed to benzene. We found that benzene caused hematotoxicity which was confirmed by routine blood test, pathological examination, and HSCs percentage analysis. A total of 1514 differentially expressed genes (DEGs) in BM HSCs and 1703 DEGs in PBSCs were screened after treatment with benzene. Weighted gene correlation network analysis revealed that pathways in cancer, transcriptional misregulation in cancer, and hematopoietic cell lineage are vital pathways involved in benzene-induced toxicity in BM HSCs, whereas hematopoietic cell lineage and leukocyte transendothelial migration are critical pathways in PBSCs. Of note, there were 164 common DEGs in both HSCs, out of which 53 genes were co-regulated in both types of HSCs. Subsequent pathway analysis of these 53 genes indicated that the most relevant pathways involved neutrophil degranulation and CD93 localized in the core of the network of the 53 genes, which are known to regulate leukemia stem cell self-renewal and quiescence. Our results could enhance our understanding of HSC responses to benzene, facilitate the identification of potential molecular biomarkers and future studies on its mechanism of toxicity toward HSCs.

Metoclopramide treatment blocks CD93-signaling-mediated self-renewal of chronic myeloid leukemia stem cells

Self-renewal is a key characteristic of leukemia stem cells (LSCs) responsible for the development and maintenance of leukemia. In this study, we identify CD93 as an important regulator of self-renewal and proliferation of murine and human LSCs, but not hematopoietic stem cells (HSCs). The intracellular domain of CD93 promotes gene transcription via the transcriptional regulator SCY1-like pseudokinase 1 independently of ligation of the extracellular domain. In a drug library screen, we identify the anti-emetic agent metoclopramide as an efficient blocker of CD93 signaling. Metoclopramide treatment reduces murine and human LSCs in vitro and prolongs survival of chronic myeloid leukemia (CML) mice through downregulation of pathways related to stemness and proliferation in LSCs. Overall, these results identify CD93 signaling as an LSC-specific regulator of self-renewal and proliferation and a targetable pathway to eliminate LSCs in CML.

Significance of Soluble CD93 in Type 2 Diabetes as a Biomarker for Diabetic Nephropathy: Integrated Results from Human and Rodent Studies

Cluster of differentiation 93 (CD93) is a glycoprotein expressed in activated endothelial cells. The extracellular portion of CD93 can be secreted as a soluble form (sCD93) under inflammatory conditions. As diabetic nephropathy (DN) is a well-known inflammatory disease, we hypothesized that sCD93 would be a new biomarker for DN. We prospectively enrolled 97 patients with type 2 diabetes and evaluated the association between serum sCD93 and DN prevalence. The association between CD93 and development of DN was investigated using human umbilical cord endothelial cells (HUVECs) in vitro and diabetic db/db mice in vivo. Subjects with higher sCD93 levels had a lower estimated glomerular filtration rate (eGFR). The sCD93 level was an independent determinant of both the albumin-to-creatinine ratio (ACR) and the eGFR. The risk of prevalent DN was higher in the high sCD93 group (adjusted odds ratio 7.212, 95% confidence interval 1.244-41.796, p = 0.028). In vitro, CD93 was highly expressed in HUVECs and both CD93 expression and secretion were upregulated after lipopolysaccharides (LPS) stimulation. In vivo, peritoneal and urine sCD93 levels and the renal glomerular expression of CD93 were significantly higher in the db/db mice than in the control db/m+ mice. These results suggest the potential of sCD93 as a candidate biomarker associated with DN.

CD93 negatively regulates astrogenesis in response to MMRN2 through the transcriptional repressor ZFP503 in the developing brain

Astrogenesis is repressed in the early embryonic period and occurs in the late embryonic period. A variety of external and internal signals contribute to the sequential differentiation of neural stem cells. Here, we discovered that immune-related CD93 plays a critical negative role in the regulation of astrogenesis in the mouse cerebral cortex. We show that CD93 expression is detected in neural stem cells and neurons but not in astrocytes and declines as differentiation proceeds. Cd93 knockout increases astrogenesis at the expense of neuron production during the late embryonic period. CD93 responds to the extracellular matrix protein Multimerin 2 (MMRN2) to trigger the repression of astrogenesis. Mechanistically, CD93 delivers signals to β-Catenin through a series of phosphorylation cascades, and then β-Catenin transduces these signals to the nucleus to activate Zfp503 transcription. The transcriptional repressor ZFP503 inhibits the transcription of glial fibrillary acidic protein (Gfap) by binding to the Gfap promoter with the assistance of Grg5. Furthermore, Cd93 knockout mice exhibit autism-like behaviors. Taken together, our results reveal that CD93 is a negative regulator of the onset of astrogenesis and provide insight into therapy for psychiatric disorders.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

Targeting Immunophenotypic Markers on Leukemic Stem Cells: How Lessons from Current Approaches and Advances in the Leukemia Stem Cell (LSC) Model Can Inform Better Strategies for Treating Acute Myeloid Leukemia (AML)

Therapies targeting cell-surface antigens in acute myeloid leukemia (AML) have been tested over the past 20 years with limited improvement in overall survival. Recent advances in the understanding of AML pathogenesis support therapeutic targeting of leukemia stem cells as the most promising avenue toward a cure. In this review, we provide an overview of the evolving leukemia stem cell (LSC) model, including evidence of the cell of origin, cellular and molecular disease architecture, and source of relapse in AML. In addition, we explore limitations of current targeted strategies utilized in AML and describe the various immunophenotypic antigens that have been proposed as LSC-directed therapeutic targets. We draw lessons from current approaches as well as from the (pre)-LSC model to suggest criteria that immunophenotypic targets should meet for more specific and effective elimination of disease-initiating clones, highlighting in detail a few targets that we suggest fit these criteria most completely.

C-type lectin family XIV members and angiogenesis

The growth and metastasis of tumors is dependent on angiogenesis. C-type lectins are carbohydrate-binding proteins with a diverse range of functions. The C-type lectin family XIV members are transmembrane glycoproteins, and all four members of this family have been reported to regulate angiogenesis, although the detailed mechanism of action has yet to be completely elucidated. They interact with extracellular matrix proteins and mediate cell-cell adhesion by their lectin-like domain. The aim of the present study was to summarize the available information on the function and mechanism of C-type lectin family XIV in angiogenesis and discuss their potential as targets for cancer therapy.

C-type lectin domain group 14 proteins in vascular biology, cancer and inflammation

The C‐type lectin domain (CTLD) group 14 family of transmembrane glycoproteins consist of thrombomodulin, CD93, CLEC14A and CD248 (endosialin or tumour endothelial marker‐1). These cell surface proteins exhibit similar ectodomain architecture and yet mediate a diverse range of cellular functions, including but not restricted to angiogenesis, inflammation and cell adhesion. Thrombomodulin, CD93 and CLEC14A can be expressed by endothelial cells, whereas CD248 is expressed by vasculature associated pericytes, activated fibroblasts and tumour cells among other cell types. In this article, we review the current literature of these family members including their expression profiles, interacting partners, as well as established and speculated functions. We focus primarily on their roles in the vasculature and inflammation as well as their contributions to tumour immunology. The CTLD group 14 family shares several characteristic features including their ability to be proteolytically cleaved and engagement of some shared extracellular matrix ligands. Each family member has strong links to tumour development and in particular CD93, CLEC14A and CD248 have been proposed as attractive candidate targets for cancer therapy.

CD93 is a cell surface lectin receptor involved in the control of the inflammatory response stimulated by exogenous DNA

Bacterial DNA contains CpG oligonucleotide (ODN) motifs to trigger innate immune responses through the endosomal receptor Toll-like receptor 9 (TLR9). One of the cell surface receptors to capture and deliver microbial DNA to intracellular TLR9 is the C-type lectin molecule DEC-205 through its N-terminal C-type lectin-like domain (CTLD). CD93 is a cell surface protein and member of the lectin group XIV with a CTLD. We hypothesized that CD93 could interact with CpG motifs, and possibly serve as a novel receptor to deliver bacterial DNA to endosomal TLR9. Using ELISA and tryptophan fluorescence binding studies we observed that the soluble histidine-tagged CD93-CTLD was specifically binding to CpG ODN and bacterial DNA. Moreover, we found that CpG ODN could bind to CD93-expressing IMR32 neuroblastoma cells and induced more robust interleukin-6 secretion when compared with mock-transfected IMR32 control cells. Our data argue for a possible contribution of CD93 to control cell responsiveness to bacterial DNA in a manner reminiscent of DEC-205. We postulate that CD93 may act as a receptor at plasma membrane for DNA or CpG ODN and to grant delivery to endosomal TLR9.

Soluble CD93 is an apoptotic cell opsonin recognized by αx β2

Efferocytosis is essential for homeostasis and prevention of the inflammatory and autoimmune diseases resulting from apoptotic cell lysis. CD93 is a transmembrane glycoprotein previously implicated in efferocytosis, with mutations in CD93 predisposing patients to efferocytosis-associated diseases. CD93 is a cell surface protein, which is proteolytically shed under inflammatory conditions, but it is unknown how CD93 mediates efferocytosis or whether its efferocytic activity is mediated by the soluble or membrane-bound form. Herein, using cell lines and human monocytes and macrophages, we demonstrate that soluble CD93 (sCD93) potently opsonizes apoptotic cells but not a broad range of microorganisms, whereas membrane-bound CD93 has no phagocytic, efferocytic, or tethering activity. Using mass spectrometry, we identified α_x β₂ as the receptor that recognizes sCD93, and via deletion mutagenesis determined that sCD93 binds to apoptotic cells via its C-type lectin-like domain and to α_x β₂ by its EGF-like repeats. The bridging of apoptotic cells to α_x β₂ markedly enhanced efferocytosis by macrophages and was abrogated by α_x β₂ knockdown. Combined, these data elucidate the mechanism by which CD93 regulates efferocytosis and identifies a previously unreported opsonin-receptor system utilized by phagocytes for the efferocytic clearance of apoptotic cells.

Complement in removal of the dead - balancing inflammation

Recognition and removal of apoptotic and necrotic cells must be efficient and highly controlled to avoid excessive inflammation and autoimmune responses to self. The complement system, a crucial part of innate immunity, plays an important role in this process. Thus, apoptotic and necrotic cells are recognized by complement initiators such as C1q, mannose binding lectin, ficolins, and properdin. This triggers complement activation and opsonization of cells with fragments of C3b, which enhances phagocytosis and thus ensures silent removal. Importantly, the process is tightly controlled by the binding of complement inhibitors C4b-binding protein and factor H, which attenuates late steps of complement activation and inflammation. Furthermore, factor H becomes actively internalized by apoptotic cells, where it catalyzes the cleavage of intracellular C3 to C3b. The intracellularly derived C3b additionally opsonizes the cell surface further supporting safe and fast clearance and thereby aids to prevent autoimmunity. Internalized factor H also binds nucleosomes and directs monocytes into production of anti-inflammatory cytokines upon phagocytosis of such complexes. Disturbances in the complement-mediated clearance of dying cells result in persistence of autoantigens and development of autoimmune diseases like systemic lupus erythematosus, and may also be involved in development of age-related macula degeneration.

Multimerin-2 is a ligand for group 14 family C-type lectins CLEC14A, CD93 and CD248 spanning the endothelial pericyte interface

The C-type lectin domain containing group 14 family members CLEC14A and CD93 are proteins expressed by endothelium and are implicated in tumour angiogenesis. CD248 (alternatively known as endosialin or tumour endothelial marker-1) is also a member of this family and is expressed by tumour-associated fibroblasts and pericytes. Multimerin-2 (MMRN2) is a unique endothelial specific extracellular matrix protein that has been implicated in angiogenesis and tumour progression. We show that the group 14 C-type lectins CLEC14A, CD93 and CD248 directly bind to MMRN2 and only thrombomodulin of the family does not. Binding to MMRN2 is dependent on a predicted long-loop region in the C-type lectin domain and is abrogated by mutation within the domain. CLEC14A and CD93 bind to the same non-glycosylated coiled-coil region of MMRN2, but the binding of CD248 occurs on a distinct non-competing region. CLEC14A and CD248 can bind MMRN2 simultaneously and this occurs at the interface between endothelium and pericytes in human pancreatic cancer. A recombinant peptide of MMRN2 spanning the CLEC14A and CD93 binding region blocks CLEC14A extracellular domain binding to the endothelial cell surface as well as increasing adherence of human umbilical vein endothelial cells to the active peptide. This MMRN2 peptide is anti-angiogenic in vitro and reduces tumour growth in mouse models. These findings identify novel protein interactions involving CLEC14A, CD93 and CD248 with MMRN2 as targetable components of vessel formation.

C1q: A fresh look upon an old molecule

Originally discovered as part of C1, the initiation component of the classical complement pathway, it is now appreciated that C1q regulates a variety of cellular processes independent of complement activation. C1q is a complex glycoprotein assembled from 18 polypeptide chains, with a C-terminal globular head region that mediates recognition of diverse molecular structures, and an N-terminal collagen-like tail that mediates immune effector mechanisms. C1q mediates a variety of immunoregulatory functions considered important in the prevention of autoimmunity such as the enhancement of phagocytosis, regulation of cytokine production by antigen presenting cells, and subsequent alteration in T-lymphocyte maturation. Furthermore, recent advances indicate additional roles for C1q in diverse physiologic and pathologic processes including pregnancy, tissue repair, and cancer. Finally, C1q is emerging as a critical component of neuronal network refinement and homeostatic regulation within the central nervous system. This review summarizes the classical functions of C1q and reviews novel discoveries within the field.

Structure, genetics and function of the pulmonary associated surfactant proteins A and D: The extra-pulmonary role of these C type lectins

The collectins family encompasses several collagenous Ca²⁺-dependent defense lectins that are described as pathogen recognition molecules. They play an important role in both adaptive and innate immunity. Surfactant proteins A and D are two of these proteins which were initially discovered in association with surfactant in the pulmonary system. The structure, immune and inflammatory functions, and genetic variations have been well described in relation to their roles, function and pathophysiology in the pulmonary system. Subsequently, these proteins have been discovered in a wide range of other organs and organ systems. The role of these proteins outside the pulmonary system is currently an active area of research. This review intends to provide a current overview of the genetics, structure and extra-pulmonary functions of the surfactant collectin proteins.

Production of complement components by cells of the immune system

The complement system is an important part of the innate immune defence. It contributes not only to local inflammation, removal and killing of pathogens, but it also assists in shaping of the adaptive immune response. Besides a role in inflammation, complement is also involved in physiological processes such as waste disposal and developmental programmes. The complement system comprises several soluble and membrane-bound proteins. The bulk of the soluble proteins is produced mainly by the liver. While several complement proteins are produced by a wide variety of cell types, other complement proteins are produced by only a few related cell types. As these data suggest that local production by specific cell types may have specific functions, more detailed studies have been employed recently analysing the local and even intracellular role of these complement proteins. Here we review the current knowledge about extrahepatic production and/or secretion of complement components. More specifically, we address what is known about complement synthesis by cells of the human immune system.

C1qr and C1qrl redundantly regulate angiogenesis in zebrafish through controlling endothelial Cdh5

Angiogenesis plays central role in the formation of functional circulation system. Characterizations of the involved factors and signaling pathways remain to be the key interest in the angiogenesis research. In this report, we showed that c1qr/cd93 and c1qrl/clec14a are specifically expressed in the vascular endothelial cells during zebrafish development. Single mutation of c1qr or c1qrl is associated with slightly malformation of inter-segmental vessels (ISVs), whereas double mutant exhibits severe defects in the ISVs formation without affecting early vasculogenesis. Further studies reveal that the endothelial-endothelial junctional molecule Cdh5 becomes absent in the ISVs of the double mutant. Replenishment of Cdh5 efficiently rescue the impaired angiogenesis in the c1qr/c1qrl double mutant. These data demonstrate that c1qr and c1qrl redundantly regulate angiogenesis through controlling the expression of the endothelial junctional molecule Cdh5, thus playing an important role in angiogenesis.

Soluble CD93 as a Novel Biomarker in Asthma Exacerbation

Asthma research is shifting from studying symptoms and lung functions to the narrow-focus cellular profiles protein analysis, biomarkers, and genetic markers. The transmembrane glycoprotein CD93 is involved in endothelial cell migration, angiogenesis, leukocytes extravasation, apoptosis, innate immunity and inflammation. Relationships between the serum level of soluble CD93 (sCD93) and acute myocardial infarction/premature MI/inflammatory arthritis/skin sclerosis have recently been reported. We hypothesized that sCD93 would be elevated during the acute phase of asthma. We measured the serum level of sCD93 in 57 patients with asthma exacerbation and 57 age-and gender-matched healthy controls. Additionally, sCD93 was reassessed at the time of discharge from the hospital. Clinical characteristics and peak expiratory flow (PEF) of the patients were assessed. The primary outcome was the comparison of serum level of sCD93 between asthmatics and healthy subjects. The sCD93 values ranged from 128 to 789 ng/mL in asthmatics (345.83±115.81) and from 31 to 289 ng/mL in control subjects (169.46±62.43). The difference between the 2 groups was statistically significant (P<0.001). The association between sCD93 and asthma remained significant after adjusting for age, sex, and BMI. The differences between asthmatics and controls remained significant on the last day of hospital stay. The association between sCD93 and PEF was not significant. In conclusion, the serum level of soluble CD93 is increased in patients with asthma exacerbation. It also showed that serum levels of sCD93 decreased with treatment of asthma attack. The clinical usefulness of determination of sCD93 as a biomarker of asthma requires further studies.

Vaccinations and secondary immune thrombocytopenia with antiphospholipid antibodies by human papillomavirus vaccine

A 13-year-old girl developed immune thrombocytopenic purpura (ITP) and concomitant positive antiphospholipid antibodies (aPL) following vaccination with a quadrivalent human papillomavirus (HPV) vaccine. During the course of a disease, she developed clinical manifestation with bleeding and she was treated with intravenous immunoglobulins. Consequently, the number of her platelets remained critically low and she was put on corticosteroids and rituximab. Since then, her platelet count remain within the normal range, but her aPL are still present.

CD93 Marks a Non-Quiescent Human Leukemia Stem Cell Population and Is Required for Development of MLL-Rearranged Acute Myeloid Leukemia

Leukemia stem cells (LSCs) are thought to share several properties with hematopoietic stem cells (HSCs), including cell-cycle quiescence and a capacity for self-renewal. These features are hypothesized to underlie leukemic initiation, progression, and relapse, and they also complicate efforts to eradicate leukemia through therapeutic targeting of LSCs without adverse effects on HSCs. Here, we show that acute myeloid leukemias (AMLs) with genomic rearrangements of the MLL gene contain a non-quiescent LSC population. Although human CD34(+)CD38(-) LSCs are generally highly quiescent, the C-type lectin CD93 is expressed on a subset of actively cycling, non-quiescent AML cells enriched for LSC activity. CD93 expression is functionally required for engraftment of primary human AML LSCs and leukemogenesis, and it regulates LSC self-renewal predominantly by silencing CDKN2B, a major tumor suppressor in AML. Thus, CD93 expression identifies a predominantly cycling, non-quiescent leukemia-initiating cell population in MLL-rearranged AML, providing opportunities for selective targeting and eradication of LSCs.

The characterization of a novel monoclonal antibody against CD93 unveils a new antiangiogenic target

The inhibition of tumor angiogenesis is one of the main challenges in cancer therapy. With the aim of developing monoclonal antibodies able to inhibit angiogenesis, we immunized mice with proliferating human umbilical vein endothelial cells. We generated a library of monoclonal antibodies able to recognize antigens expressed on endothelial cells and screened the antibodies for their ability to inhibit endothelial cell proliferation, migration, and sprouting in vitro. Here, we show that the antibody, designated as 4E1, is able to neutralize the formation of new vessels both in vitro and in vivo without affecting endothelial cell survival. By mass spectrometry we identified CD93 as the antigen bound by 4E1 and mapped the recognized epitope. CD93 is a transmembrane protein heavily glycosylated preferentially expressed in the vascular endothelium. CD93 silencing by lentiviral-mediated small hairpin RNA expression impairs human endothelial cell proliferation, migration, and sprouting. Altogether these findings reveal 4E1 as a novel antiangiogenic antibody and identify CD93 as a new target suitable for antiangiogenic therapy.

Unique properties of cluster of differentiation 93 in the umbilical cord blood of neonates

It has previously been reported by these authors that cluster of differentiation (CD) 93 is co-expressed on naive T-lymphocytes (CD4(+) CD45RA(+) cells) in neonatal umbilical cord blood cells (UCBCs) but not on normal adult peripheral blood cells (PBCs). In this study, expression of CD93 on other lymphocyte subsets and the concentration of soluble formed CD93 (sCD93) in serum or culture supernatants from neonatal umbilical cord blood (UCB) was examined. It was found that CD93 is also co-expressed on CD2(+) , CD16(+) , CD56(+) or CD25(+) cells in the lymphocyte population of neonatal UCBCs, but not on normal adult PBCs. The concentrations of sCD93 in serum and culture supernatants from neonatal UCB were significantly greater than those from normal adult peripheral blood. The concentrations of sCD93 in culture supernatants from neonatal UCBCs and normal adult PBCs treated with phorbol 12-myristate 13-acetate (PMA) were significantly enhanced compared with those without PMA treatment. The degree of enhancement of sCD93 by PMA in culture supernatants from neonatal UCBCs was significantly greater than that of normal adult PBCs and enhancement of sCD93 by PMA in the culture supernatants from neonatal UCBCs and normal adult PBCs was significantly suppressed by PKC inhibitor. Interestingly, the high concentration of serum sCD93 in neonates was significantly decreased in sera from infants at 1 month after birth. Expression of CD93 on the lymphocyte population of PBCs from infants at 1 month after birth was also significantly decreased, compared with that for neonatal UCBCs. These findings indicate that CD93 in neonatal UCB has unique properties as an immunological biomarker.

IFN priming is necessary but not sufficient to turn on a migratory dendritic cell program in lupus monocytes

Blood monocytes from children with systemic lupus erythematosus (SLE) behave similar to dendritic cells (DCs), and SLE serum induces healthy monocytes to differentiate into DCs in a type I IFN-dependent manner. In this study, we found that these monocytes display significant transcriptional changes, including a prominent IFN signature, compared with healthy controls. Few of those changes, however, explain DC function. Exposure to allogeneic T cells in vitro reprograms SLE monocytes to acquire DC phenotype and function, and this correlates with both IFN-inducible (IP10) and proinflammatory cytokine (IL-1β and IL6) expression. Furthermore, we found that both IFN and SLE serum induce the upregulation of CCR7 transcription in these cells. CCR7 protein expression, however, requires a second signal provided by TLR agonists such as LPS. Thus, SLE serum "primes" a subset of monocytes to readily (<24 h) respond to TLR agonists and acquire migratory DC properties. Our findings might explain how microbial infections exacerbate lupus.

Regulation of CD93 Cell Surface Expression by Protein Kinase C Isoenzymes

Human CD93, also known as complement protein 1, q subcomponent, receptor (C1qRp), is selectively expressed by cells with a myeloid lineage, endothelial cells, platelets, and microglia and was originally reported to be involved in the complement protein 1, q subcomponent (C1q)-mediated enhancement of phagocytosis. The intracellular molecular events responsible for the regulation of its expression on the cell surface, however, have not been determined. In this study, the effect of protein kinases in the regulation of CD93 expression on the cell surface of a human monocyte-like cell line (U937), a human NK-like cell line (KHYG-1), and a human umbilical vein endothelial cell line (HUV-EC-C) was investigated using four types of protein kinase inhibitors, the classical protein kinase C (cPKC) inhibitor Go6976, the novel PKC (nPKC) inhibitor Rottlerin, the protein kinase A (PKA) inhibitor H-89 and the protein tyrosine kinase (PTK) inhibitor herbimycin A at their optimum concentrations for 24 hr. CD93 expression was analyzed using flow cytometry and glutaraldehyde-fixed cellular enzyme-linked immunoassay (EIA) techniques utilizing a CD93 monoclonal antibody (mAb), mNI-11, that was originally established in our laboratory as a CD93 detection probe. The nPKC inhibitor Rottlerin strongly down-regulated CD93 expression on the U937 cells in a dose-dependent manner, whereas the other inhibitors had little or no effect. CD93 expression was down-regulated by Go6976, but not by Rottlerin, in the KHYG-1 cells and by both Rottlerin and Go6976 in the HUV-EC-C cells. The PKC stimulator, phorbol myristate acetate (PMA), strongly up-regulated CD93 expression on the cell surface of all three cell-lines and induced interleukin-8 (IL-8) production by the U937 cells and interferon-gamma (IFN-gamma) production by the KHYG-1 cells. In addition, both Go6976 and Rottlerin inhibited the up-regulation of CD93 expression induced by PMA and IL-8 or IFN-gamma production in the respective cell-lines. Whereas recombinant tumor necrosis factor-alpha (rTNF-alpha) slightly up-regulated CD93 expression on the U937 cells, recombinant interleukin-1beta (rIL-1beta), recombinant interleukin-2 (rIL-2), recombinant interferon-gamma (rIFN-gamma) and lipopolysaccharide (LPS) had no effect. Taken together, these findings indicate that the regulation of CD93 expression on these cells involves the PKC isoenzymes.

The epidermal growth factor-like domain of CD93 is a potent angiogenic factor

Human CD93, an epidermal growth factor (EGF)-like domain containing transmembrane protein, is predominantly expressed in the vascular endothelium. Studies have shown that AA4, the homolog of CD93 in mice, may mediate cell migration and angiogenesis in endothelial cells. Soluble CD93 has been detected in the plasma of healthy individuals. However, the role of soluble CD93 in the endothelium remains unclear. Recombinant soluble CD93 proteins with EGF-like domains (rCD93D123, with domains 1, 2, and 3; and rCD93D23, with domains 2 and 3) were generated to determine their functions in angiogenesis. We found that rCD93D23 was more potent than rCD93D123 in stimulating the proliferation and migration of human umbilical vein endothelial cells (HUVECs). Production of matrix-metalloproteinase 2 increased after the HUVECs were treated with rCD93D23. Further, in a tube formation assay, rCD93D23 induced cell differentiation of HUVECs through phosphoinositide 3-kinase/Akt/endothelial nitric oxide synthase and extracellular signal-regulated kinases-1/2 signaling. Moreover, rCD93D23 promoted blood vessel formation in a Matrigel-plug assay and an oxygen-induced retinopathy model in vivo. Our findings suggest that the soluble EGF-like domain containing CD93 protein is a novel angiogenic factor acting on the endothelium.

A proteomic approach to identification of plutonium-binding proteins in mammalian cells

Plutonium can enter the body through different routes and remains there for decades; however its specific biochemical interactions are poorly defined. We, for the first time, have studied plutonium-binding proteins using a metalloproteomic approach with rat PC12 cells. A combination of immobilized metal ion chromatography, 2D gel electrophoresis, and mass spectrometry was employed to analyze potential plutonium-binding proteins. Our results show that several proteins from PC12 cells show affinity towards Pu(4+)-NTA (plutonium bound to nitrilotriacetic acid). Proteins from seven different spots in the 2D gel were identified. In contrast to the previously known plutonium-binding proteins transferrin and ferritin, which bind ferric ions, most identified proteins in our experiment are known to bind calcium, magnesium, or divalent transition metal ions. The identified plutonium interacting proteins also have functional roles in downregulation of apoptosis and other pro-proliferative processes. MetaCore™ analysis based on this group of proteins produced a pathway with a statistically significant association with development of neoplastic diseases.

C1q regulates collagen-dependent production of reactive oxygen species, aggregation and levels of soluble P-selectin in whole blood

Blood platelets express several receptors involved in immunity (e.g. complement-, toll-like- and Fcγ-receptors) and release inflammatory mediators. Furthermore, formation of platelet-leukocyte aggregates has an important role during inflammatory conditions such as coronary artery disease. Thus, apart from their well-known role in haemostasis, platelets are today also recognized as cells with immuno-modulatory properties. We have previously reported regulatory effects of complement protein 1q (C1q) on platelet activation in experimental setups using isolated cells. In the present study we have proceeded by investigating effects of C1q on collagen-induced aggregation, production of reactive oxygen species (ROS), formation of platelet-leukocyte aggregates and levels of soluble P-selectin in whole blood. Impedance measurements showed that C1q inhibited collagen-induced aggregation whereas it potentiated the collagen-provoked production of ROS in a luminol-dependent chemiluminescence assay. The effects of C1q on aggregation and ROS-production were dependent upon platelets, as they were no longer observed in presence of the platelet (GpIIb/IIIa) inhibitor Reopro. Furthermore, the levels of soluble P-selectin were found to be lowered upon treatment with C1q prior to addition of collagen. There was also a trend towards a decreased formation of large platelet-leukocyte aggregates in collagen-stimulated whole blood following C1q treatment. In conclusion, our data indicate that C1q could have a role in regulating platelet activation and associated leukocyte recruitment during vessel wall injury. This has implications for inflammatory disorders such as coronary artery disease.

Flow cytometric identification of CD93 expression on naive T lymphocytes (CD4(+)CD45RA (+) cells) in human neonatal umbilical cord blood

Human CD93 has a molecular weight of about 100 kDa and is selectively expressed by myeloid cell lineages in peripheral blood (PB) mononuclear cells. Although CD93 was initially identified as a receptor for complement component 1, subcomponent q phagocytosis (C1qRp) involved in the C1q-mediated enhancement of the phagocytosis of various antigens, several recent studies have reported that CD93 is not a receptor for the C1q-mediated enhancement of phagocytosis. The expression patterns of CD93 have been previously investigated in PB mononuclear cells (lymphocytes, monocytes, and granulocytes) from adult PB and neonatal umbilical cord blood (UCB), and the expression of CD93 was not found on lymphocytes from either normal adult PB or neonatal UCB. However, the detection of CD93 expression in neonatal UCB using CD93 monoclonal antibodies (mAbs) that recognize different antigenic epitopes remains poorly understood. In this study, we examined the expression of CD93 on lymphocytes, monocytes, and granulocytes from neonatal UCB using four different types of CD93 mAb detection probes, mNI-11, R139, R3, and X-2, using flow cytometric and western blot analyses. We found that CD93, as defined using all four mAbs, was expressed on monocytes and granulocytes in PB mononuclear cells from adult PB and neonatal UCB. On the other hand, we observed for the first time that the expression of CD93 on lymphocytes in neonatal UCB can only be detected using the mNI-11 mAb, established in our laboratory, and not with commercially available CD93 mAbs (R139, R3, and X-2). However, CD93 expression on lymphocytes from normal adults was not detected using any of the four CD93 mAbs. Two-color flow cytometric analyses showed that the CD93 recognized by mNI-11 mAb was expressed on CD3(+) T lymphocytes (mainly CD4(+) helper T lymphocytes), but not on CD19(+) B lymphocytes or on CD8(+) suppressor/cytotoxic T lymphocytes from neonatal UCB. In addition, CD93 was expressed on CD45RA(+) (naive antigen) lymphocytes from neonatal UCB, but not on CD45RO(+) (memory antigen) lymphocytes from neonatal UCB or on CD45RA(+) and CD45RO(+) lymphocytes from normal adult PB. Three-color flow cytometric analysis showed that CD93 was co-expressed on naive T lymphocytes (CD4(+)CD45RA(+)) from neonatal UCB. In a western blot analysis, the CD93 mAb (mNI-11) immunoprecipitated at a molecular weight of 98 kDa, identified as a CD93 molecule, in the CD4(+)CD45RA(+) cells from neonatal UCB but not from adult PB, similar to the results in the human monocyte-like cell line U937 (human CD93-positive cells). Taken together, these results provide the first direct evidence of a novel/naive cell population (CD4(+)CD45RA(+)CD93(+)) in neonatal UCB that may have an important role in cell biology, transplantation, and immature/mature immune responses.

CD93/AA4.1: a novel regulator of inflammation in murine focal cerebral ischemia

The stem-cell marker CD93 (AA4.1/C1qRp) has been described as a potential complement C1q-receptor. Its exact molecular function, however, remains unknown. By using global expression profiling we showed that CD93-mRNA is highly induced after transient focal cerebral ischemia. CD93 protein is upregulated in endothelial cells, but also in selected macrophages and microglia. To elucidate the potential functional role of CD93 in postischemic brain damage, we used mice with a targeted deletion of the CD93 gene. After 30 min of occlusion of the middle cerebral artery and 3 d of reperfusion these mice displayed increased leukocyte infiltration into the brain, increased edema, and significantly larger infarct volumes (60.8 +/- 52.2 versus 23.9 +/- 16.6 mm(3)) when compared with wild-type (WT) mice. When the MCA was occluded for 60 min, after 2 d of reperfusion the CD93 knockout mice still showed more leukocytes in the brain, but the infarct volumes were not different from those seen in WT animals. To further explore CD93-dependent signaling pathways, we determined global transcription profiles and compared CD93-deficient and WT mice at various time points after induction of focal cerebral ischemia. We found a highly significant upregulation of the chemokine CCL21/Exodus-2 in untreated and treated CD93-deficient mice at all time points. Induction of CCL21 mRNA and protein was confirmed by PCR and immunohistochemistry. CCL21, which was formerly shown to be released by damaged neurons and to activate microglia, contributes to neurodegeneration. Thus, we speculate that CD93-neuroprotection is mediated via suppression of the neuroinflammatory response through downregulation of CCL21.

C1q induces a rapid up-regulation of P-selectin and modulates collagen- and collagen-related peptide-triggered activation in human platelets

Blood platelets are emerging as important immunomodulatory cells, but complement interaction with platelets is not well understood. Several platelet structures have been described as complement protein 1q (C1q) binding receptors, such as C1qRp/CD93 and gC1qR. However, there are conflicting results whether these receptors are C1q binding structures, or even at all expressed on the cell surface. Recently, the collagen-binding integrin αIIβI was reported to bind C1q on mast cells, and this receptor is also present on platelets. The aim of this study was to further characterize the effects of C1q on platelets, by quantifying the platelet surface expression of P-selectin (CD62P) and monitoring the formation of platelet-neutrophil aggregates. Using flow cytometry, we found that C1q dose-dependently triggered a rapid but moderate and transient up-regulation of P-selectin already within 5s of C1q exposure. Pre-incubation with an antibody directed against gC1qR significantly inhibited (with 57% compared to control) the up-regulation, whereas an antibody towards the αIIβI-integrin showed no effect. Stimulation with C1q did not change the cytosolic calcium-levels, as measured with the fluorescent ratiometric probe Fura-2, however, a protein kinase C inhibitor (GF109203x) blocked the C1q-induced P-selectin expression. Furthermore, pre-incubation of platelets with C1q diminished both the collagen as well as the collagen-related peptide-induced up-regulation of P-selectin, most evident after 90s of stimulation. This indicates that C1q may regulate platelet activation via the GPVI receptor, which is a novel finding. Moreover, C1q antagonized the collagen-induced formation of platelet-neutrophil aggregates, indicating a reduced interaction between platelet P-selectin and neutrophil P-selectin glycoprotein ligand-1(PSGL-1/CD162). In summary, C1q induces a moderate rapid platelet P-selectin expression, modulates subsequent collagen and collagen-related peptide stimulation of platelets, and inhibits the formation of platelet-neutrophil aggregates. These immuno-regulatory effects of C1q may have a crucial role in innate immunity and inflammation.