Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) acts as a regulator of B-cell development, B-cell antigen receptor (BCR)-mediated activation, and autoimmune disease

Oxidized galectin-1 in SLE fails to bind the inhibitory receptor VSTM1 and increases reactive oxygen species levels in neutrophils

Inhibitory immune receptors set thresholds for immune cell activation, and their deficiency predisposes a person to autoimmune responses. However, the agonists of inhibitory immune receptors remain largely unknown, representing untapped sources of treatments for autoimmune diseases. Here, we show that V-set and transmembrane domain-containing 1 (VSTM1) is an inhibitory receptor and that its binding by the competent ligand soluble galectin-1 (Gal1) is essential for maintaining neutrophil viability mediated by downregulated reactive oxygen species production. However, in patients with systemic lupus erythematosus (SLE), circulating Gal1 is oxidized and cannot be recognized by VSTM1, leading to increased intracellular reactive oxygen species levels and reduced neutrophil viability. Dysregulated neutrophil function or death contributes significantly to the pathogenesis of SLE by providing danger molecules and autoantigens that drive the production of inflammatory cytokines and the activation of autoreactive lymphocytes. Interestingly, serum levels of glutathione, an antioxidant able to convert oxidized Gal1 to its reduced form, were negatively correlated with SLE disease activity. Taken together, our findings reveal failed inhibitory Gal1/VSTM1 pathway activation in patients with SLE and provide important insights for the development of effective targeted therapies.

Proteomic analysis identifies subgroups of patients with active systemic lupus erythematosus

OBJECTIVE

Systemic lupus erythematosus (SLE) is a clinically and biologically heterogenous autoimmune disease. We aimed to investigate the plasma proteome of patients with active SLE to identify novel subgroups, or endotypes, of patients.

METHOD

Plasma was collected from patients with active SLE who were enrolled in the British Isles Lupus Assessment Group Biologics Registry (BILAG-BR). The plasma proteome was analysed using a data-independent acquisition method, Sequential Window Acquisition of All theoretical mass spectra mass spectrometry (SWATH-MS). Unsupervised, data-driven clustering algorithms were used to delineate groups of patients with a shared proteomic profile.

RESULTS

In 223 patients, six clusters were identified based on quantification of 581 proteins. Between the clusters, there were significant differences in age (p = 0.012) and ethnicity (p = 0.003). There was increased musculoskeletal disease activity in cluster 1 (C1), 19/27 (70.4%) (p = 0.002) and renal activity in cluster 6 (C6) 15/24 (62.5%) (p = 0.051). Anti-SSa/Ro was the only autoantibody that significantly differed between clusters (p = 0.017). C1 was associated with p21-activated kinases (PAK) and Phospholipase C (PLC) signalling. Within C1 there were two sub-clusters (C1A and C1B) defined by 49 proteins related to cytoskeletal protein binding. C2 and C6 demonstrated opposite Rho family GTPase and Rho GDI signalling. Three proteins (MZB1, SND1 and AGL) identified in C6 increased the classification of active renal disease although this did not reach statistical significance (p = 0.0617).

CONCLUSIONS

Unsupervised proteomic analysis identifies clusters of patients with active SLE, that are associated with clinical and serological features, which may facilitate biomarker discovery. The observed proteomic heterogeneity further supports the need for a personalised approach to treatment in SLE.

CD31 as a probable responding and gate-keeping protein of the blood-brain barrier and the risk of Alzheimer's disease

Several studies have shown that an abnormal vascular-immunity link could increase Alzheimer's disease (AD) risk; however, the mechanism is unclear. CD31, also named platelet endothelial cell adhesion molecule (PECAM), is a surface membrane protein of both endothelial and immune cells and plays important roles in the interaction between the vascular and immune systems. In this review, we focus on research regarding CD31 biological actions in the pathological process that may contribute to AD based on the following rationales. First, endothelial, leukocyte and soluble forms of CD31 play multi-roles in regulating transendothelial migration, increasing blood-brain barrier (BBB) permeability and resulting in neuroinflammation. Second, CD31 expressed by endothelial and immune cells dynamically modulates numbers of signaling pathways, including Src family kinases, selected G proteins, and β-catenin which in turn affect cell-matrix and cell-cell attachment, activation, permeability, survival, and ultimately neuronal cell injury. In endothelia and immune cells, these diverse CD31-mediated pathways act as a critical regulator in the immunity-endothelia-brain axis, thereby mediating AD pathogenesis in ApoE4 carriers, which is the major genetic risk factor for AD. This evidence suggests a novel mechanism and potential drug target for CD31 in the background of genetic vulnerabilities and peripheral inflammation for AD development and progression.

Unraveling the Link between Interferon-α and Systemic Lupus Erythematosus: From the Molecular Mechanisms to Target Therapies

Systemic lupus erythematosus (SLE) is a chronic, systemic autoimmune disease with a wide range of clinical expressions. The kidney is often affected, usually within 5 years of the onset of SLE, and lupus nephropathy (LN) carries a high risk for increased morbidity. The clinical heterogeneity of the disease is accompanied by complex disturbances affecting the immune system with inflammation and tissue damage due to loss of tolerance to nuclear antigens and the deposition of immune complexes in tissues. Several studies have reported that in human SLE, there is an important role of the Type-I-interferons (INF) system suggested by the upregulation of INF-inducible genes observed in serial gene expression microarray studies. This review aims to describe the transduction pathways of Type-I-interferons, in particular INFα, and its immune-regulatory function in the pathogenesis of SLE and, in particular, in LN. In addition, recent novelties concerning biologic therapy in LN will be discussed.

Polygenic autoimmune disease risk alleles impacting B cell tolerance act in concert across shared molecular networks in mouse and in humans

Most B cells produced in the bone marrow have some level of autoreactivity. Despite efforts of central tolerance to eliminate these cells, many escape to periphery, where in healthy individuals, they are rendered functionally non-responsive to restimulation through their antigen receptor via a process termed anergy. Broad repertoire autoreactivity may reflect the chances of generating autoreactivity by stochastic use of germline immunoglobulin gene segments or active mechanisms may select autoreactive cells during egress to the naïve peripheral B cell pool. Likewise, it is unclear why in some individuals autoreactive B cell clones become activated and drive pathophysiologic changes in autoimmune diseases. Both of these remain central questions in the study of the immune system(s). In most individuals, autoimmune diseases arise from complex interplay of genetic risk factors and environmental influences. Advances in genome sequencing and increased statistical power from large autoimmune disease cohorts has led to identification of more than 200 autoimmune disease risk loci. It has been observed that autoantibodies are detectable in the serum years to decades prior to the diagnosis of autoimmune disease. Thus, current models hold that genetic defects in the pathways that control autoreactive B cell tolerance set genetic liability thresholds across multiple autoimmune diseases. Despite the fact these seminal concepts were developed in animal (especially murine) models of autoimmune disease, some perceive a disconnect between human risk alleles and those identified in murine models of autoimmune disease. Here, we synthesize the current state of the art in our understanding of human risk alleles in two prototypical autoimmune diseases – systemic lupus erythematosus (SLE) and type 1 diabetes (T1D) along with spontaneous murine disease models. We compare these risk networks to those reported in murine models of these diseases, focusing on pathways relevant to anergy and central tolerance. We highlight some differences between murine and human environmental and genetic factors that may impact autoimmune disease development and expression and may, in turn, explain some of this discrepancy. Finally, we show that there is substantial overlap between the molecular networks that define these disease states across species. Our synthesis and analysis of the current state of the field are consistent with the idea that the same molecular networks are perturbed in murine and human autoimmune disease. Based on these analyses, we anticipate that murine autoimmune disease models will continue to yield novel insights into how best to diagnose, prognose, prevent and treat human autoimmune diseases.

Monocyte-Platelet Aggregates Triggered by CD31 Molecule in Non-ST Elevation Myocardial Infarction: Clinical Implications in Plaque Rupture

Despite the recent innovations in cardiovascular care, atherothrombosis is still a major complication of acute coronary syndromes (ACS). We evaluated the involvement of the CD31 molecule in thrombotic risk through the formation of monocyte-platelet (Mo-Plt) aggregates in patients with ACS with no-ST-segment elevation myocardial infarction (NSTEMI) on top of dual anti-platelet therapy (DAPT). We enrolled 19 control (CTRL) subjects, 46 stable angina (SA), and 86 patients with NSTEMI, of which, 16 with Intact Fibrous Cap (IFC) and 19 with Ruptured Fibrous Cap (RFC) as assessed by the Optical Coherence Tomography (OCT). The expression of CD31 on monocytes and platelets was measured. Following the coronary angiography, 52 NSTEMIs were further stratified according to thrombus grade (TG) evaluation. Finally, a series of ex vivo experiments verified whether the CD31 participates in Mo-Plt aggregate formation. In patients with NSTEMI, CD31 was reduced on monocytes and was increased on platelets, especially in NSTEMI presented with RFC plaques compared to those with IFC lesions, and in patients with high TG compared to those with zero/low TG. Ex vivo experiments documented an increase in Mo-Plt aggregates among NSTEMI, which significantly decreased after the CD31 ligation, particularly in patients with RFC plaques. In NSTEMI, CD31 participates in Mo-Plt aggregate formation in spite of optimal therapy and DAPT, suggesting the existence of alternative thrombotic pathways, as predominantly displayed in patients with RFC.

PECAM-1 protects against DIC by dampening inflammatory responses via inhibiting macrophage pyroptosis and restoring vascular barrier integrity

Disseminated intravascular coagulation (DIC) is a frequent complication of sepsis that affects patient outcomes due to accompanying thrombo-inflammation and microvascular permeability changes. Platelet endothelial cell adhesion molecule-1 (PECAM-1), a cellular adhesion and signaling receptor that is expressed on both hematopoietic and endothelial cells, plays an important anti-inflammatory role in acute and chronic inflammatory disease models. Little is known, however, about role and mechanism of PECAM-1 in septic DIC. Here, we investigated whether PECAM-1 might play a protective role in hindering the development of septic DIC. Plasma levels of soluble PECAM-1 were markedly elevated in septic patients that developed DIC, with a correspondingly poorer outcome. PECAM-1 knockout exhibited more severe DIC and poorer outcome in the LPS induced- and cecal ligation and puncture-induced DIC model, which could be alleviated by tissue factor inhibitor. This phenomenon seemed to be equally linked to PECAM-1 expression by both endothelial and blood cells. Furthermore, PECAM-1 was found to exert its protective effect on developing septic DIC by the following 2 distinct mechanisms: the inhibition of macrophage pyroptosis and the acceleration of the restoration of the endothelial cell barrier. Taken together, these results implicate PECAM-1 as a potentially attractive target for the development of novel therapeutics to manage and treat septic DIC.

Comparative characterization of B cells specific for HBV nucleocapsid and envelope proteins in patients with chronic hepatitis B

BACKGROUND & AIMS

Knowledge about the regulation of anti-HBV humoral immunity during natural HBV infection is limited. We recently utilized dual fluorochrome-conjugated HBsAg to demonstrate, in patients with chronic HBV (CHB) infection, the functional impairment of their HBsAg-specific B cells. However, the features of their HBcAg-specific B cells are unknown. Here we developed a method to directly visualize, select and characterize HBcAg-specific B cells in parallel with HBsAg-specific B cells.

METHODS

Fluorochrome-conjugated HBcAg reagents were synthesized and utilized to directly detect ex vivo HBcAg-specific B cells in 36 patients with CHB. The frequency, phenotype, functional maturation and transcriptomic profile of HBcAg-specific B cells was studied by flow cytometry, in vitro maturation assays and NanoString-based detection of expression of immune genes, which we compared with HBsAg-specific B cells and total B cells.

RESULTS

HBcAg-specific B cells are present at a higher frequency than HBsAg-specific B cells in patients with CHB and, unlike HBsAg-specific B cells, they mature efficiently into antibody-secreting cells in vitro. Their phenotypic and transcriptomic profiles show that HBcAg-specific B cells are preferentially IgG+ memory B cells. However, despite their phenotypic and functional differences, HBcAg- and HBsAg-specific B cells from patients with CHB share an mRNA expression pattern that differs from global memory B cells and is characterized by high expression of genes indicative of cross-presentation and innate immune activity.

CONCLUSIONS

During chronic HBV infection, a direct relation exists between serological detection of anti-HBs and anti-HBc antibodies, and the quantity and function of their respective specific B cells. However, the transcriptomic analysis performed in HBsAg- and HBcAg-specific B cells suggests additional roles of HBV-specific B cells beyond the production of antibodies.

LAY SUMMARY

Protection of viral infection necessitates the production of antibodies that are generated by specialized cells of the immune system called B cells. During chronic HBV infection, antibodies against the internal part of the virus (core or HBcAg) are detectable while the antibodies directed against the virus envelope (surface or HBsAg) are not present. Here we developed a method that allows us to directly visualize ex vivo the B cells specific for these 2 viral components, highlighting their differences and similarities, and showing how 2 components of the same virus can have different impacts on the function of antiviral B cells.

Endothelial Cell-Specific Transcriptome Reveals Signature of Chronic Stress Related to Worse Outcome After Mild Transient Brain Ischemia in Mice

Vascular mechanisms underlying the adverse effects that depression and stress-related mental disorders have on stroke outcome are only partially understood. Identifying the transcriptomic signature of chronic stress in endothelium harvested from the ischemic brain is an important step towards elucidating the biological processes involved. Here, we subjected male 129S6/SvEv mice to a 28-day model of chronic stress. The ischemic lesion was quantified after 30 min filamentous middle cerebral artery occlusion (MCAo) and 48 h reperfusion by T2-weighted MRI. RNA sequencing was used to profile transcriptomic changes in cerebrovascular endothelial cells (ECs) from the infarct. Mice subjected to the stress procedure displayed reduced weight gain, increased adrenal gland weight, and increased hypothalamic FKBP5 mRNA and protein expression. Chronic stress conferred increased lesion volume upon MCAo. Stress-exposed mice showed a higher number of differentially expressed genes between ECs isolated from the ipsilateral and contralateral hemisphere than control mice. The genes in question are enriched for roles in biological processes closely linked to endothelial proliferation and neoangiogenesis. MicroRNA-34a was associated with nine of the top 10 biological process Gene Ontology terms selectively enriched in ECs from stressed mice. Moreover, expression of mature miR-34a-5p and miR-34a-3p in ischemic brain tissue was positively related to infarct size and negatively related to sirtuin 1 (Sirt1) mRNA transcription. In conclusion, this study represents the first EC-specific transcriptomic analysis of chronic stress in brain ischemia. The stress signature uncovered relates to worse stroke outcome and is directly relevant to endothelial mechanisms in the pathogenesis of stroke.

CD31 Acts as a Checkpoint Molecule and Is Modulated by FcγR-Mediated Signaling in Monocytes

Monocytes and macrophages express FcγR that engage IgG immune complexes such as Ab-opsonized pathogens or cancer cells to destroy them by various mechanisms, including phagocytosis. FcγR-mediated phagocytosis is regulated by the concerted actions of activating FcγR and inhibitory receptors, such as FcγRIIb and SIRPα. In this study, we report that another ITIM-containing receptor, PECAM1/CD31, regulates FcγR function and is itself regulated by FcγR activation. First, quantitative RT-PCR and flow cytometry analyses revealed that human monocyte FcγR activation leads to a significant downregulation of CD31 expression, both at the message level and at surface expression, mainly mediated through FcγRIIa. Interestingly, the kinetics of downregulation between the two varied, with surface expression reducing earlier than the message. Experiments to analyze the mechanism behind this discrepancy revealed that the loss of surface expression was because of internalization, which depended predominantly on the PI3 kinase pathway and was independent of FcγR internalization. Finally, functional analyses showed that the downregulation of CD31 expression in monocytes by small interfering RNA enhanced FcγR-mediated phagocytic ability but have little effect on cytokine production. Together, these results suggest that CD31 acts as a checkpoint receptor that could be targeted to enhance FcγR functions in Ab-mediated therapies.

CD72 is a Negative Regulator of B Cell Responses to Nuclear Lupus Self-antigens and Development of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is the prototypic systemic autoimmune disease characterized by production of autoantibodies to various nuclear antigens and overexpression of genes regulated by IFN-I called IFN signature. Genetic studies on SLE patients and mutational analyses of mouse models demonstrate crucial roles of nucleic acid (NA) sensors in development of SLE. Although NA sensors are involved in induction of anti-microbial immune responses by recognizing microbial NAs, recognition of self NAs by NA sensors induces production of autoantibodies to NAs in B cells and production of IFN-I in plasmacytoid dendritic cells. Among various NA sensors, the endosomal RNA sensor TLR7 plays an essential role in development of SLE at least in mouse models. CD72 is an inhibitory B cell co-receptor containing an immunoreceptor tyrosine-based inhibition motif (ITIM) in the cytoplasmic region and a C-type lectin like-domain (CTLD) in the extracellular region. CD72 is known to regulate development of SLE because CD72 polymorphisms associate with SLE in both human and mice and CD72^−/− mice develop relatively severe lupus-like disease. CD72 specifically recognizes the RNA-containing endogenous TLR7 ligand Sm/RNP by its extracellular CTLD, and inhibits B cell responses to Sm/RNP by ITIM-mediated signal inhibition. These findings indicate that CD72 inhibits development of SLE by suppressing TLR7-dependent B cell response to self NAs. CD72 is thus involved in discrimination of self-NAs from microbial NAs by specifically suppressing autoimmune responses to self-NAs.

Ligand Recognition Determines the Role of Inhibitory B Cell Co-receptors in the Regulation of B Cell Homeostasis and Autoimmunity

B cells express various inhibitory co-receptors including CD22, CD72, and Siglec-G. These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region. Although many of the inhibitory co-receptors negatively regulate BCR signaling by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1), different inhibitory co-receptors have distinct functional properties. CD22, Siglec-G, and CD72 preferentially regulate tonic signaling in conventional B cells, B-1 cell homeostasis, and development of lupus-like disease, respectively. CD72 recognizes RNA-related lupus self-antigen Sm/RNP as a ligand. This ligand recognition recruits CD72 to BCR in Sm/RNP-reactive B cells thereby suppressing production of anti-Sm/RNP autoantibody involved in the pathogenesis of lupus. In contrast, Siglec-G recognizes α2,3 as well as α2,6 sialic acids whereas CD22 recognizes α2,6 sialic acid alone. Because glycoproteins including BCR are dominantly glycosylated with α2,3 sialic acids in B-1 cells, Siglec-G but not CD22 recruits BCR as a ligand specifically in B-1 cells, and regulates B-1 cell homeostasis by suppressing BCR signaling in B-1 cells. Thus, recognition of distinct ligands determines functional properties of different inhibitory B cell co-receptors.

CD31+ Cells From Peripheral Blood Facilitate Bone Regeneration in Biologically Impaired Conditions Through Combined Effects on Immunomodulation and Angiogenesis

Controlled revascularization and inflammation are key elements regulating endogenous regeneration after (bone) tissue trauma. Peripheral blood-derived cell subsets, such as regulatory T-helper cells and circulating (endothelial) progenitor cells, respectively, can support endogenous tissue healing, whereas effector T cells that are associated with an aged immune system can hinder bone regeneration. CD31 is expressed by diverse leukocytes and is well recognized as a marker of circulating endothelial (precursor) cells; however, CD31 is absent from the surface of differentiated effector T cells. Thus, we hypothesized that by separating the inhibitory fractions from the supportive fractions of circulating cells within the peripheral blood (PB) using the CD31 marker, bone regeneration in biologically compromised conditions, such as those observed in aged patients, could be improved. In support of our hypothesis, we detected an inverse correlation between CD31+ cells and effector T cells in the hematomas of human fracture patients, dependent on the age of the patient. Furthermore, we demonstrated the regenerative capacity of human PB-CD31+ cells in vitro. These findings were translated to a clinically relevant rat model of impaired bone healing. The transplantation of rat PB-CD31+ cells advanced bone tissue restoration in vivo and was associated with an early anti-inflammatory response, the stimulation of (re)vascularization, and reduced fibrosis. Interestingly, the depletion or enrichment of the highly abundant CD31+/14+ monocytes from the mixed CD31+ cell population diminished tissue regeneration at different levels, suggesting combined effects within the PB-CD31+ subsets. In summary, an intraoperative enrichment of PB-CD31+ cells might be a novel option to facilitate endogenous regeneration under biologically impaired situations by supporting immunomodulation and vascularization. © 2016 American Society for Bone and Mineral Research.

Dysregulated homeostasis of target tissues or autoantigens - A novel principle in autoimmunity

Monogenic autoimmune disorders provide a powerful tool for our understanding of the principles of autoimmunity due to the obvious impact of a single gene on the disease. So far, approximately 100 single gene defects causing murine monogenic autoimmune disorders have been reported and the functional characterization of these genes will provide significant progress in understanding the nature of autoimmunity. According to their function, genes leading to monogenic autoimmune disorders can be categorized into two groups. An expectable first group contains genes involved in the homeostasis of the immune system, including homeostasis of immune organs and immune cells. Intriguingly, the second group consists of genes functionally involved in the homeostasis of target tissues or autoantigens. According to our novel hypothesis, we propose that autoimmunity represents a consequence of a dysregulated homeostasis of the immune system and/or its targets including autoantigens and target tissues. In this review we refer to both aspects of homeostasis in autoimmunity with a highlight on the role of the homeostasis of target tissues and autoantigens.

Graphene scaffolds in progressive nanotechnology/stem cell-based tissue engineering of the nervous system

Although graphene/stem cell-based tissue engineering has recently emerged and has promisingly and progressively been utilized for developing one of the most effective regenerative nanomedicines, it suffers from low differentiation efficiency, low hybridization after transplantation and lack of appropriate scaffolds required in implantations without any degrading in functionality of the cells. In fact, recent studies have demonstrated that the unique properties of graphene can successfully resolve all of these challenges. Among various stem cells, neural stem cells (NSCs) and their neural differentiation on graphene have attracted a lot of interest, because graphene-based neuronal tissue engineering can promisingly realize the regenerative therapy of various incurable neurological diseases/disorders and the fabrication of neuronal networks. Hence, in this review, we further focused on the potential bioapplications of graphene-based nanomaterials for the proliferation and differentiation of NSCs. Then, various stimulation techniques (including electrical, pulsed laser, flash photo, near infrared (NIR), chemical and morphological stimuli) which have recently been implemented in graphene-based stem cell differentiations were reviewed. The possibility of degradation of graphene scaffolds (NIR-assisted photodegradation of three-dimensional graphene nanomesh scaffolds) was also discussed based on the latest achievements. The biocompatibility of graphene scaffolds and their probable toxicities (especially after the disintegration of graphene scaffolds and distribution of its platelets in the body), which is still an important challenge, were reviewed and discussed. Finally, the initial recent efforts for fabrication of neuronal networks on graphene materials were presented. Since there has been no in vivo application of graphene in neuronal regenerative medicine, we hope that this review can excite further and concentrated investigations on in vivo (and even in vitro) neural proliferation, stimulation and differentiation of stem cells on biocompatible graphene scaffolds having the potential of degradability for the generation of implantable neuronal networks.

Advances in our understanding of mechanisms of venous thrombus resolution

Traditionally, venous thrombosis has been seen as the consequence of a regulated cascade of proteolytic steps leading to the polymerization of fibrinogen and fibrin crosslinking that is facilitated by platelets. A new view of thrombosis is providing a more integrated concept, with components of the vascular wall contributing to the vascular remodeling of thrombosis. Angiogenesis and inflammation are two key mechanisms that safeguard venous thrombus resolution and restitution of vascular patency after thrombosis. Disturbance of these processes leads to thrombus persistence and has potentially severe consequences for affected patients. Examples for clinical conditions associated with recurrent or persisting venous thrombosis are post-thrombotic syndrome or chronic thromboembolic pulmonary hypertension. Recently, studies using animal models of venous thrombosis have contributed to a better understanding of thrombus non-resolution that will eventually lead to modification of current treatment concepts. For example, recent data suggest that innate immunity is involved in the modification of thrombosis.

Seminal CD38 Enhances Human Sperm Capacitation through Its Interaction with CD31

Human sperm have to undergo a maturational process called capacitation in the female reproductive tract. Capacitation confers upon the sperm an ability to gain hypermotility and undergo acrosome reaction. Previous studies have suggested that seminal plasma proteins induce the capacitation of sperm in the female reproductive tract for the successful fertilization of the oocyte. However, the function of seminal plasma proteins in capacitation remains largely unclear. To the end, we found that soluble CD38 (sCD38) in seminal plasma increases the capacitation of sperm via specific interactions between sCD38 and the CD31 on the sperm. Upon the association of sCD38 with CD31, tyrosine kinase Src phosphorylates CD31, a process blocked by Src inhibitors. Shc, SHP-2, Grb2, and SOS, as well as Src kinase were found to associate with the phosphorylated CD31. The sCD38-induced phosphorylation of CD31 initiates a cascade reaction through the phosphorylation of Erk1/2, which results in the acrosome reaction, and sperm hypermotility. These processes were prevented by Src, Ras and MEK inhibitors. Taken together, these data indicate that the sCD38 present in seminal plasma plays a critical role in the capacitation of sperm.

How endothelial cells regulate transmigration of leukocytes in the inflammatory response

Leukocytes attach to vascular endothelial cells at the site of inflammation via a series of intercellular adhesive interactions. In a separate step in leukocyte extravasation, transendothelial migration is regulated by molecules that play no role in the preceding steps of tethering, rolling, adhesion, and locomotion. Transendothelial migration itself can be dissected into a series of distinct interactions regulated sequentially by molecules concentrated at the endothelial cell border; these include platelet/endothelial cell adhesion molecule, poliovirus receptor (CD155), and CD99. These molecules are components of the lateral border recycling compartment (LBRC), a perijunctional network of interconnected tubulovesicular membrane that traffics to surround the leukocyte as it passes across the endothelial cell. This targeted recycling of LBRC requires kinesin to move the membrane along microtubules, and interfering with LBRC trafficking blocks transmigration of neutrophils, monocytes, and lymphocytes. The LBRC is also recruited to mediate transcellular migration when that occurs. Movement of the LBRC is coordinated with events on the luminal surface, such as clustering of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1 under the migrating leukocyte, as well as movement of vascular endothelial cadherin and its associated catenins out of the junction at the site of transendothelial migration. How these events are coordinated is not known, but their regulation shares common signaling pathways that may serve to connect these steps.

Upholding the T cell immune-regulatory function of CD31 inhibits the formation of T/B immunological synapses in vitro and attenuates the development of experimental autoimmune arthritis in vivo

CD31, a trans-homophilic inhibitory receptor expressed on both T- and B-lymphocytes, drives the mutual detachment of interacting leukocytes. Intriguingly, T cell CD31 molecules relocate to the immunological synapse (IS), where the T and B cells establish a stable interaction. Here, we show that intact CD31 molecules, which are able to drive an inhibitory signal, are concentrated at the periphery of the IS but are excluded from the center of the IS. At this site, were the cells establish the closest contact, the CD31 molecules are cleaved, and most of the extracellular portion of the protein, including the trans-homophilic binding sites, is shed from the cell surface. T cells lacking CD31 trans-homophilic binding sites easily establish stable interactions with B cells; at the opposite, CD31 signaling agonists inhibit T/B IS formation as well as the ensuing helper T cell activation and function. Confocal microscopy and flow cytometry analysis of experimental T/B IS shows that the T cell inhibitory effects of CD31 agonists depend on SHP-2 signaling, which reduces the phosphorylation of ZAP70. The analysis of synovial tissue biopsies from patients affected by rheumatoid arthritis showed that T cell CD31 molecules are excluded from the center of the T/B cell synapses in vivo. Interestingly, the administration of CD31 agonists in vivo significantly attenuated the development of the clinical signs of collagen-induced arthritis in DBA1/J mice. Altogether, our data indicate that the T cell co-inhibitory receptor CD31 prevents the formation of functional T/B immunological synapses and that therapeutic strategies aimed at sustaining CD31 signaling will attenuate the development of autoimmune responses in vivo.

CD31 is a key coinhibitory receptor in the development of immunogenic dendritic cells

CD31 is a transhomophilic tyrosine-based inhibitory motif receptor and is expressed by both dendritic cells (DCs) and T lymphocytes. Previous studies have established that the engagement of CD31 drives immune-inhibitory signaling in T lymphocytes, but the effect exerted by CD31 signaling in DCs remains elusive. Here, we show that CD31 is a key coinhibitory receptor on stimulated DCs, favoring the development of tolerogenic functions and finally resulting in T-cell tolerance. The disruption of CD31 signaling favored the immunogenic maturation and migration of resident DCs to the draining lymph nodes. In contrast, sustaining the CD31/SHP-1 signaling during DC maturation resulted in reduced NF-κB nuclear translocation, expression of costimulatory molecules, and production of immunogenic cytokines (e.g., IL-12, IL-6), whereas the expression of TGF-β and IL-10 were increased. More importantly, CD31-conditioned DCs purified from the draining lymph nodes of ovalbumin-immunized mice favored the generation of antigen-specific regulatory T cells (CD25(+) forkhead box P3(+)) at the expense of effector (IFN-γ(+)) cells upon coculture with naive ovalbumin-specific CD4(+) T lymphocytes ex vivo. Finally, the adoptive transfer of CD31-conditioned myelin oligodendrocyte glycoprotein-loaded DCs carried immune tolerance against the subsequent development of MOG-induced experimental autoimmune encephalomyelitis in vivo. The key coinhibitory role exerted by CD31 on DCs highlighted by the present study may have important implications both in settings where the immunogenic function of DCs is desirable, such as infection and cancer, and in settings where tolerance-driving DCs are preferred, such as autoimmune diseases and transplantation.

Do inhibitory immune receptors play a role in the etiology of autoimmune disease?

Inhibitory receptors are thought to be important in balancing immune responses. The general assumption is that lack of inhibition predisposes for autoimmune diseases. As reviewed here, various experimental and clinical data support this assumption. However, in humans genetic evidence implicates only a limited number of inhibitory receptors. GWAS have established common variation in a few inhibitory receptor genes, such as FCγRIIB, PD-1 and CTLA-4 as risk factors. The question arises whether inhibitory receptor function is a major determinant of autoimmune disease. In this respect, the finding that genetic variation in CSK and PTPN22 is strongly associated with multiple autoimmune diseases is of interest. We propose a model in which the molecules encoded by these genes are downstream of inhibitory receptors. We conclude that common genetic variation of inhibitory receptors, with few exceptions, is not a determining factor for autoimmunity in humans. However, common downstream signaling pathways are.

An immunologist's guide to CD31 function in T-cells

Although it is expressed by all leukocytes, including T-, B-lymphocytes and dendritic cells, the immunoglobulin-like receptor CD31 is generally regarded by immunologists as a marker of endothelial cell lineage that lacks an established functional role in adaptive immunity. This perception has recently been challenged by studies that reveal a key role for this molecule in the regulation of T-cell homeostasis, effector function and trafficking. The complexity of the biological functions of CD31 results from the integration of its adhesive and signaling functions in both the immune and vascular systems. Signaling by means of CD31 is induced by homophilic engagement during the interactions of immune cells and is mediated by phosphatase recruitment or activation through immunoreceptor tyrosine inhibitory motifs (ITIMs) that are located in its cytoplasmic tail. Loss of CD31 function is associated with excessive immunoreactivity and susceptibility to cytotoxic killing. Here, we discuss recent findings that have brought to light a non-redundant, complex role for this molecule in the regulation of T-cell-mediated immune responses, with large impact on our understanding of immunity in health and disease.

Absence of platelet endothelial cell adhesion molecule 1, PECAM-1/CD31, in vivo increases resistance to Salmonella enterica serovar Typhimurium in mice

PECAM-1/CD31 is known to regulate inflammatory responses and exhibit pro- and anti-inflammatory functions. This study was designed to determine the functional role of PECAM-1 in susceptibility to murine primary in vivo infection with Salmonella enterica serovar Typhimurium and in in vitro inflammatory responses of peritoneal macrophages. Lectin profiling showed that cellular PECAM-1 and recombinant human PECAM-1-Ig chimera contain high levels of mannose sugars and N-acetylglucosamine. Consistent with this carbohydrate pattern, both recombinant human and murine PECAM-1-Ig chimeras were shown to bind S. Typhimurium in a dose-dependent manner in vitro. Using oral and fecal-oral transmission models of S. Typhimurium SL1344 infection, PECAM-1(-/-) mice were found to be more resistant to S. Typhimurium infection than wild-type (WT) C57BL/6 mice. While fecal shedding of S. Typhimurium was comparable in wild-type and PECAM-1(-/-) mice, the PECAM-1-deficient mice had lower bacterial loads in systemic organs such as liver, spleen, and mesenteric lymph nodes than WT mice, suggesting that extraintestinal dissemination was reduced in the absence of PECAM-1. This reduced bacterial load correlated with reduced tumor necrosis factor (TNF), interleukin-6 (IL-6), and monocyte chemoattractant protein (MCP) levels in sera of PECAM-1(-/-) mice. Following in vitro stimulation of macrophages with either whole S. Typhimurium, lipopolysaccharide (LPS) (Toll-like receptor 4 [TLR4] ligand), or poly(I·C) (TLR3 ligand), production of TNF and IL-6 by PECAM-1(-/-) macrophages was reduced. Together, these results suggest that PECAM-1 may have multiple functions in resistance to infection with S. Typhimurium, including binding to host cells, extraintestinal spread to deeper tissues, and regulation of inflammatory cytokine production by infected macrophages.

Levels of soluble platelet endothelial cell adhesion molecule-1 and P-selectin are decreased in children with autism spectrum disorder

BACKGROUND

Although the etiopathology of autism spectrum disorder (ASD) is not clear, there is increasing evidence that dysfunction in the immune system affects many children with ASD. Findings of immune dysfunction in ASD include increases in inflammatory cytokines, chemokines, and microglial activity in brain tissue and cerebrospinal fluid, as well as abnormal peripheral immune cell function.

METHODS

Adhesion molecules, such as platelet endothelial adhesion molecule-1 (PECAM-1), intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), P-selectin, and L-selectin, function to facilitate leukocyte transendothelial migration. We assessed concentrations of soluble adhesion molecules, sPECAM-1, sICAM-1, sVCAM-1, sP-selectin, and sL-selectin in the plasma of 49 participants with ASD and 31 typically developing controls of the same age, all of whom were enrolled as part of the Autism Phenome Project. Behavioral assessment, the levels of soluble adhesion molecules, and head circumference were compared in the same subjects.

RESULTS

Levels of sPECAM-1 and sP-selectin were significantly reduced in the ASD group compared to typically developing controls (p < .02). Soluble PECAM-1 levels were negatively associated with repetitive behavior and abnormal brain growth in children with ASD (p = .03).

CONCLUSIONS

Because adhesion molecules modulate the permeability and signaling at the blood-brain barrier as well as leukocyte infiltration into the central nervous system, the current data suggest a role for these molecules in the complex pathophysiology of ASD.

Lessons from the endothelial junctional mechanosensory complex

Mechanotransduction plays a key role in both normal physiology and in diseases such as cancer, atherosclerosis and hypertension. Nowhere is this more evident than in the vascular system, where fluid shear stress from blood flow plays a critical role in shaping the blood vessels and in determining their function and dysfunction. Responses to flow are mediated in part by a complex of proteins comprised of PECAM-1, VE-cadherin and VEGFR2 at endothelial cell-cell junctions; all proteins that clearly have other, non-mechanical functions. We review recent progress toward understanding the functions and mechanisms of mechanotransduction by this complex and suggest some principles that may apply more broadly.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Experimental use of mouse models of systemic lupus erythematosus

Mouse models of lupus have for many years provided accessible and reliable research systems for the pathogenesis and therapy of systemic autoimmune disease, spanning a spectrum of inbred strains that develop spontaneous disease to experimentally induced, sometimes genetically manipulated animals. Nearly all the models share in common the development of glomerulonephritis and autoantibodies, including antinuclear and DNA specificities, the most common endpoints examined in experimental studies, but exhibit specific differences in the incidence of other end-organ manifestations such as hemolytic anemia, arthritis, dermatitis, and vasculitis. This chapter contrasts the clinical characteristics of these various models, providing an outline for their use and analysis.

PECAM-1 dampens cytokine levels during LPS-induced endotoxemia by regulating leukocyte trafficking

AIMS

To investigate the mechanism by which platelet endothelial cell adhesion molecule 1 (PECAM-1/CD31), an immunoglobulin (Ig)-superfamily cell adhesion and signaling receptor, regulates pro-inflammatory cytokine levels. The purpose of the present investigation was to test the hypothesis that PECAM-1 influences circulating cytokine levels by regulating the trafficking of activated, cytokine-producing leukocytes to sites of inflammation.

MAIN METHODS

PECAM-1+/+ and PECAM-1-/- mice were subjected to lipopolysaccharide (LPS)-induced endotoxemia, and systemic cytokine levels were measured by Bioplex multiplex cytokine assays. Flow cytometry was employed to enumerate leukocytes at inflammatory sites and to measure cytokine synthesis in leukocyte sub-populations. Enzyme-linked immunosorbent assay (ELISA) was used to measure cytokine levels in tissue samples and in supernatants of in vitro-stimulated leukocytes.

KEY FINDINGS

We confirmed earlier reports that mice deficient in PECAM-1 had greater systemic levels of pro-inflammatory cytokines following intraperitoneal (IP) LPS administration. Interestingly, expression of PECAM-1, in mice, had negligible effects on the level of cytokine synthesis by leukocytes stimulated in vitro with LPS and in peritoneal macrophages isolated from LPS-injected mice. There was, however, excessive accumulation of macrophages and neutrophils in the lungs of PECAM-1-deficient, compared with wild-type, mice--an event that correlated with a prolonged increase in lung pro-inflammatory cytokine levels.

SIGNIFICANCE

Our results demonstrate that PECAM-1 normally functions to dampen systemic cytokine levels during LPS-induced endotoxemia by diminishing the accumulation of cytokine-producing leukocytes at sites of inflammation, rather than by modulating cytokine synthesis by leukocytes.

Upregulation of 11β-hydroxysteroid dehydrogenase 1 in lymphoid organs during inflammation in the rat

Glucocorticoids exert anti-inflammatory and immunomodulatory effects that may be regulated in part by the activities of the glucocorticoid-activating and -inactivating enzymes, 11β-hydroxysteroid dehydrogenase type 1 (11HSD1) and type 2 (11HSD2), respectively. Previous studies have demonstrated that inflammatory bowel diseases in humans and experimental animals upregulate 11HSD1 and downregulate 11HSD2. We investigated whether proinflammatory cytokines modulate colonic 11HSDs as well as whether lymphoid organs exhibit any 11HSD response to inflammation. Colon tissue explants exposed to tumor necrosis factor α exhibited an upregulation of 11HSD1 mRNA whereas interleukin 1β downregulated 11HSD2 mRNA. Experimental colitis induced by the intracolonic administration of 2,4,6-trinitrobenzenesulfonic acid stimulated 11HSD1 activity not only in the colon but also in mesenteric lymph nodes and the spleen. Analysis of mRNA for 11HSD1 in colon-draining lymph nodes and the spleen showed that inflammation upregulates the expression of this enzyme in mobile lymphoid cells similar to the intraepithelial and lamina propria leukocytes isolated from the colon. It is inferred that inflammation stimulates the reactivation of glucocorticoids in lymphoid organs and in gut-associated lymphoid tissue.

CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection

Hematopoietic cells constitutively express CD31/PECAM1, a signaling adhesion receptor associated with controlling responses to inflammatory stimuli. Although expressed on CD4(+) T cells, its function on these cells is unclear. To address this, we have used a model of systemic Salmonella infection that induces high levels of T cell activation and depends on CD4(+) T cells for resolution. Infection of CD31-deficient (CD31KO) mice demonstrates that these mice fail to control infection effectively. During infection, CD31KO mice have diminished numbers of total CD4(+) T cells and IFN-γ-secreting Th1 cells. This is despite a higher proportion of CD31KO CD4(+) T cells exhibiting an activated phenotype and an undiminished capacity to prime normally and polarize to Th1. Reduced numbers of T cells reflected the increased propensity of naive and activated CD31KO T cells to undergo apoptosis postinfection compared with wild-type T cells. Using adoptive transfer experiments, we show that loss of CD31 on CD4(+) T cells alone is sufficient to account for the defective CD31KO T cell accumulation. These data are consistent with CD31 helping to control T cell activation, because in its absence, T cells have a greater propensity to become activated, resulting in increased susceptibility to become apoptotic. The impact of CD31 loss on T cell homeostasis becomes most pronounced during severe, inflammatory, and immunological stresses such as those caused by systemic Salmonella infection. This identifies a novel role for CD31 in regulating CD4 T cell homeostasis.

Relative contribution of PECAM-1 adhesion and signaling to the maintenance of vascular integrity

PECAM-1 (CD31) is a cellular adhesion and signaling receptor that is highly expressed at endothelial cell-cell junctions in confluent vascular beds. Previous studies have implicated PECAM-1 in the maintenance of vascular barrier integrity; however, the mechanisms behind PECAM-1-mediated barrier protection are still poorly understood. The goal of the present study, therefore, was to examine the pertinent biological properties of PECAM-1 (i.e. adhesion and/or signaling) that allow it to support barrier integrity. We found that, compared with PECAM-1-deficient endothelial cells, PECAM-1-expressing endothelial cell monolayers exhibit increased steady-state barrier function, as well as more rapid restoration of barrier integrity following thrombin-induced perturbation of the endothelial cell monolayer. The majority of PECAM-1-mediated barrier protection was found to be due to the ability of PECAM-1 to interact homophilically and become localized to cell-cell junctions, because a homophilic binding-crippled mutant form of PECAM-1 was unable to support efficient barrier function when re-expressed in cells. By contrast, cells expressing PECAM-1 variants lacking residues known to be involved in PECAM-1-mediated signal transduction exhibited normal to near-normal barrier integrity. Taken together, these studies suggest that PECAM-1-PECAM-1 homophilic interactions are more important than its signaling function for maintaining the integrity of endothelial cell junctions.

Human T cells induce their own regulation through activation of B cells

Regulatory functions for B lymphocytes have been reported in murine models of autoimmune diseases in which B-cell deficient mice were shown to exhibit exacerbated disease. The B cells responsible for the immune regulations were identified as a subpopulation of interleukin 10-secreting cells. However, the mechanism of induction and the characteristics of regulatory B cells in humans have been hardly studied. This study reports that regulation of T cell responses can be induced by B cells following CD40-dependent cognate interaction. T cell proliferation and cytokine production were differentially regulated. Thus, CD40-induced regulatory B cells partially inhibited T cell proliferation following CD40 interaction without requirement of soluble factor. In contrast, modulation of Th1 differentiation resulted from CD80- and CD86-dependent interactions and from IL-10 production. The suppressive effects were mediated by CD19(high)IgD+CD38(high)CD24(high)CD5(high) B cells and appeared to be indirect, through the induction of regulatory T cells as indicated by the appearance of Foxp3+CD4+CD25+T cells. These data suggest that activation signals from T cells initiate regulatory properties in B cells that modulate T cell responses involving regulatory T cells. Finally, studies in autoimmune patients revealed that regulation of T cell proliferation was defective in systemic lupus erythematosus but efficient in other diseases. Restoration of efficient B-cell regulatory activity could provide innovative B-cell based treatment of autoimmune diseases.

PECAM-1 functions as a negative regulator of laminin-induced platelet activation

SUMMARY BACKGROUND

Interaction of resting platelets with exposed components of the subendothelial matrix is an important early activating event that takes place at sites of vascular injury. Platelet responses to collagen are mediated by integrin alpha(2)beta(1) and the glycoprotein (GP)VI-Fc receptor (FcR) gamma-chain complex, whereas platelet activation by laminin is mediated by the related integrin, alpha(6)beta(1), and similarly requires signaling through GPVI-FcR gamma-chain.

OBJECTIVE

Because the cell adhesion and signaling receptor PECAM-1 has previously been shown to dampen collagen-induced platelet activation, we sought to determine whether PECAM-1 might similarly regulate platelet activation by laminin.

METHODS/RESULTS

We found that PECAM-1 became tyrosine phosphorylated on its cytoplasmic immunoreceptor tyrosine-based inhibitory motifs following adhesion of either human or murine platelets to immobilized laminin. Whereas the presence or absence of PECAM-1 had no effect on either the rate or extent of platelet adhesion or spreading on laminin, PECAM-1 inhibited laminin-induced phosphorylation of GPVI-FcR gamma-chain immunoreceptor tyrosine-based activation motifs (ITAMs) and activation of its downstream effector, Syk kinase, and suppressed granule secretion.

CONCLUSIONS

Taken together, these data are consistent with previous findings in platelets and other blood and vascular cells that PECAM-1 functions by modulating ITAM-mediated signaling pathways that amplify cellular activation.

PECAM-1: conflicts of interest in inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a cell adhesion and signaling receptor that is expressed on hematopoietic and endothelial cells. PECAM-1 is vital to the regulation of inflammatory responses, as it has been shown to serve a variety of pro-inflammatory and anti-inflammatory functions. Pro-inflammatory functions of PECAM-1 include the facilitation of leukocyte transendothelial migration and the transduction of mechanical signals in endothelial cells emanating from fluid shear stress. Anti-inflammatory functions include the dampening of leukocyte activation, suppression of pro-inflammatory cytokine production, and the maintenance of vascular barrier integrity. Although PECAM-1 has been well-characterized and studied, the mechanisms through which PECAM-1 regulates these seemingly opposing functions, and how they influence each other, are still not completely understood. The purpose of this review, therefore, is to provide an overview of the pro- and anti-inflammatory functions of PECAM-1 with special attention paid to mechanistic insights that have thus far been revealed in the literature in hopes of gaining a clearer picture of how these opposing functions might be integrated in a temporal and spatial manner on the whole organism level. A better understanding of how inflammatory responses are regulated should enable the development of new therapeutics that can be used in the treatment of acute and chronic inflammatory disorders.

Gene expression profiling of the donor kidney at the time of transplantation predicts clinical outcomes 2 years after transplantation

We have previously demonstrated that biomarkers of inflammation and immune activity detected within intraoperative renal transplant allograft biopsies are linked to adverse short-term post-transplantation clinical outcomes. Now we provide a post hoc analysis of our earlier data in the light of longer clinical follow-up. A total of 75 consecutively performed renal allografts were analyzed for gene expression of proinflammatory molecules, inflammation-induced adhesion molecules, and antiapoptotic genes expressed 15 minutes after vascular reperfusion to determine whether this analysis can aid in predicting long-term quality of renal function, proteinuria, graft loss, and death-censored graft. We have built predictive models for proteinuria (area under the curve = 0.859, p = 0.0001) and graft loss (area under the curve = 0.724, p = 0.027) 2 years post-transplantation using clinical variables in combination with intragraft gene expression data of tumor necrosis factor-alpha, interleukin-6, CD40, CD3, and tumor necrosis factor-alpha, Bcl-2, and interferon-gamma, respectively. This post hoc analysis demonstrates that hypothesis-driven, targeted polymerase chain reaction profiling of gene expression in the donor kidney at the time of engraftment can predict 2-year post-transplantation clinical outcomes.

The anti-inflammatory actions of platelet endothelial cell adhesion molecule-1 do not involve regulation of endothelial cell NF-kappa B

PECAM-1 is a cell adhesion and signaling receptor that is expressed on many hematopoietic cells and at endothelial cell-cell junctions. Accumulating evidence from a number of in vitro and in vivo model systems suggests that PECAM-1 suppresses cytokine production and vascular permeability induced by a wide range of inflammatory stimuli. In several of these models of inflammatory disease, endothelial, and not leukocyte or platelet, PECAM-1 conferred protection against inflammatory insult. However, the mechanism by which endothelial PECAM-1 functions as an anti-inflammatory protein is poorly understood. It was recently suggested that PECAM-1 exerts its anti-inflammatory effects in endothelial cells by inhibiting the activity of NF-kappaB, a proinflammatory transcription factor. To confirm and extend these observations, we examined the effect of engaging, cross-linking, or expressing PECAM-1 on NF-kappaB activation in a variety of human cells. PECAM-1 had no effect on the phosphorylation of the NF-kappaB inhibitory protein, IkappaBalpha; on the nuclear translocation of NF-kappaB; on the suppression of cytokine-induced transcriptional activation of an NF-kappaB luciferase reporter plasmid; or on the cytokine-stimulated upregulation of ICAM-1, an NF-kappaB target gene, in endothelial cells. Taken together, these studies strongly suggest that the anti-inflammatory actions of PECAM-1 in endothelial cells are not likely to involve its regulation of NF-kappaB.

Non-invasive diagnosis of early pulmonary disease in PECAM-deficient mice using infrared pulse oximetry

Pulse oximetry is a common tool for detecting reduced pulmonary function in human interstitial lung diseases. It has not previously been used in a mouse model of interstitial lung disease. Further, platelet endothelial cell adhesion molecule deficient mice rarely show symptoms until disease is advanced. Using blood oxygen saturation, different stages of disease could be identified in a non-invasive manner. These stages could be correlated to pathology. Collagen deposition, using Picrosirius Red, did correlate with blood oxygen saturation. These studies are the first to show the use of an infrared pulse oximetry system to analyze the progression of a fibrotic interstitial lung disease in a mouse model of the human diseases. Further, these studies show that an early alveolar damage/enlargement event precedes the fibrosis in this mouse model, a stage that represents the best targets for disease analysis and prevention. This stage does not have extensive collagen deposition. Most importantly, targeting this earliest stage of disease for therapeutic intervention may lead to novel treatment for human disease.

Mechanisms of PECAM-1-mediated cytoprotection and implications for cancer cell survival

Defects in apoptotic pathways can promote cancer development and cause cancers to become resistant to chemotherapy. The cell adhesion and signaling molecule PECAM-1 has been shown to potently suppress apoptosis in a variety of cellular systems. PECAM-1 expression has been reported on a variety of human malignancies-especially hematopoietic and vascular cell cancers-but the significance of this expression has not been fully explored. The ability of PECAM-1 to inhibit apoptosis makes it an attractive candidate as a molecule that may promote cancer development and/or confer resistance to chemotherapeutic treatment. The exact mechanisms by which PECAM-1 mediates its cytoprotection have not been fully defined, but its anti-apoptotic effects have been shown to require both homophilic binding and intracellular signaling via its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. In this review, we will discuss the data regarding PECAM-1's anti-apoptotic effects and ways in which this cytoprotection may be clinically relevant to the development and/or treatment of hematologic malignancies that express this vascular cell-specific surface molecule.

PECAM-independent thioglycollate peritonitis is associated with a locus on murine chromosome 2

Background

Previous studies have demonstrated that knockout or inhibition of Platelet/Endothelial Cell Adhesion Molecule (PECAM, CD31) in a number of murine strains results in impaired inflammatory responses, but that no such phenotype is seen in the C57BL/6 (B6) murine background.

Methodology/Principal Findings

We have undertaken a quantitative trait locus (QTL) mapping effort between FVB/n (FVB) and B6 mice deficient for PECAM to identify the gene or genes responsible for this unique feature of B6 mice. We have identified a locus on murine chromosome 2 at approximately 35.8 Mb that is strongly associated (LOD score = 9.0) with inflammatory responses in the absence of PECAM.

Conclusions/Significance

These data potentiate further study of the diapedesis machinery, as well as potential identification of new components of this machinery. As such, this study is an important step to better understanding the processes of inflammation.

Points of control exerted along the macrophage-endothelial cell-polymorphonuclear neutrophil axis by PECAM-1 in the innate immune response of acute colonic inflammation

PECAM-1 is expressed on endothelial cells and leukocytes. Its extracellular domain has been implicated in leukocyte diapedesis. In this study, we used PECAM-1(-/-) mice and relevant cells derived from them to assess the role of PECAM-1 in an experimental model of acute colonic inflammation with a predominant innate immune response, i.e., 2,4,6-trinitrobenzine sulfonic acid (TNBS). Using chimeric approaches, we addressed the points of control exerted by PECAM-1 along the macrophage-endothelial cell-polymorphonuclear neutrophil (PMN) axis. In vivo, TNBS-induced colitis was ameliorated in PECAM-1(-/-) mice, an event attributed to PECAM-1 on hematopoietic cells rather than to PECAM-1 on endothelial cells. The in vivo innate immune response was mimicked in vitro by using a construct of the vascular-interstitial interface, i.e., PMN transendothelial migration was induced by colonic lavage fluid (CLF) from TNBS mice or macrophages (MPhi) challenged with CLF. Using the construct, we confirmed that endothelial cell PECAM-1 does not play a role in PMN transendothelial migration. Although MPhi activation (NF-kappaB nuclear binding) and function (keratinocyte-derived chemokine production) induced by CLF was diminished in PECAM-1(-/-) MPhi, this did not affect their ability to promote PMN transendothelial migration. By contrast, PECAM-1(-/-) PMN did not adhere to or migrate across endothelial cell monolayers in response to CLF. Further, as compared with PECAM-1(+/+) PMN, PECAM-1(-/-) PMN were less effective in orientating their CXCR2 receptors (polarization) in the direction of a chemotactic gradient. Collectively, our findings indicate that PECAM-1 modulation of PMN function (at a step before diapedesis) most likely contributes to the inflammation in a colitis model with a strong innate immune component.

An alternatively spliced isoform of PECAM-1 is expressed at high levels in human and murine tissues, and suggests a novel role for the C-terminus of PECAM-1 in cytoprotective signaling

The Ig-ITIM family member PECAM-1 is expressed in vascular and endothelial cells, and its functions include suppression of mitochondria-dependent apoptosis. Previous studies have identified distinct PECAM-1 cytoplasmic domain splice variants at the mRNA, but not protein, level. Several relatively abundant mRNA isoforms lack exon 15 (Delta15) and would theoretically encode a protein with a truncated cytoplasmic domain and a unique C-terminal sequence. Using a novel rabbit polyclonal antibody that specifically recognizes Delta15 PECAM-1, we found that the Delta15 PECAM-1 isoform was expressed in human tissues, including brain, testes and ovary. This isoform was also expressed on the cell surface of human platelets, human umbilical vein endothelial cells (HUVECs) and the Jurkat T-cell leukemia, human erythroleukemia (HEL) and U937 histiocytic lymphoma cell lines. Furthermore, murine platelets and lung lysates demonstrated abundant amounts of exon-15-deficient PECAM-1. Functional studies revealed that Delta15 PECAM-1 retains both its homophilic binding capacity and its ability to signal by means of its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. Delta15 PECAM-1 was unable, however, to protect against apoptosis induced by overexpression of Bax or treatment with the chemotherapy agent etoposide. These studies suggest a novel role for the PECAM-1 C-terminus in cytoprotective signaling and highlight a need for further characterization of expression of PECAM-1 isoforms in normal and malignant tissues.

PECAM-1 ligation negatively regulates TLR4 signaling in macrophages

Uncontrolled TLR4 signaling may induce excessive production of proinflammatory cytokines and lead to harmful inflammation; therefore, negative regulation of TLR4 signaling attracts much attention now. PECAM-1, a member of Ig-ITIM family, can mediate inhibitory signals in T cells and B cells. However, the role and the mechanisms of PECAM-1 in the regulation of TLR4-mediated LPS response in macrophages remain unclear. In this study, we demonstrate that PECAM-1 ligation with CD38-Fc fusion protein negatively regulates LPS-induced proinflammatory cytokine TNF-alpha, IL-6, and IFN-beta production by inhibiting JNK, NF-kappaB, and IFN regulatory factor 3 activation in macrophages. In addition, PECAM-1 ligation-recruited Src homology region 2 domain-containing phosphatase 1 (SHP-1) and Src homology region 2 domain-containing phosphatase 2 (SHP-2) may be involved in the inhibitory effect of PECAM-1 on TLR4 signaling. Consistently, silencing of PECAM-1 enhances the macrophage response to LPS stimulation. Taken together with the data that PECAM-1 is constitutively expressed in macrophages and its expression is up-regulated by LPS stimulation, PECAM-1 might function as a feedback negative regulator of LPS inflammatory response in macrophages. This study may provide a potential target for intervention of inflammatory diseases.

The proinflammatory phenotype of PECAM-1-deficient mice results in atherogenic diet-induced steatohepatitis

The severity of nonalcoholic steatohepatitis (NASH) is determined by environmental and genetic factors, the latter of which are incompletely characterized. Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kDa transmembrane glycoprotein expressed on blood and vascular cells. In the present study, we provide data for the novel finding that genetic deficiency of PECAM-1 potentiates the development and progression of NASH. We found that the rate of development and severity of diet-induced NASH are markedly enhanced in PECAM-1-deficient [knockout (KO)] mice relative to wild-type (WT) mice, as measured by histological and biochemical evaluation. Livers from KO mice exhibited typical histological features of NASH, including macrovesicular fat accumulation, hepatocyte injury with infiltration of inflammatory cells, fibrosis, and heightened oxidative stress. Alanine aminotransferase, a marker for liver injury, was also significantly higher in KO compared with WT mice. Consistent with a role for PECAM-1 as a suppressor of proinflammatory cytokines, plasma levels of inflammatory cytokines, including TNF-alpha and monocyte chemoattractant protein-1 (MCP-1), were also significantly higher in KO compared with WT mice. These findings are the first to show that the PECAM-1-deficient mouse develops progressive nonalcoholic fatty liver disease (NAFLD), supporting a role for PECAM-1 as a negative regulator of NAFLD progression. Future examination of recently identified PECAM-1 allelic isoforms in humans as potential risk factors for developing NASH may be warranted.

PECAM-1-regulated signalling thresholds control tolerance in anergic transgenic B-cells

Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1/CD31) is an immunoglobulin (Ig)-immunoreceptor tyrosine based inhibitory motif (Ig-ITIM) superfamily member that recruits and activates protein-tyrosine phosphatases, predominantly SHP-2 and to a lesser extent, SHP-1. Previously, we have shown that deletion of PECAM-1 results in a hyper-proliferative B-cell phenotype. We wanted to test whether the Ig-ITIM superfamily member, PECAM-1 maintains peripheral tolerance by regulating signalling thresholds of B-cells that control autoantibody production or relaxed negative selection of autoreactive B-cells in bone marrow. In order to address this issue, we utilised the classical model of lysozyme/immunoglobulin transgenic mouse model that defines thresholds for eliminating or inactivating self-reactive B-cells. In this study, we show that breeding of double transgenes: soluble hen egg lysozyme (HEL) and its corresponding high-affinity receptor (HEL-Ig) onto PECAM-1 null background resulted in a spontaneous loss of B-cell tolerance in vivo. The resultant PECAM-1(-/-) Dbl Tg mice displayed elevated levels of anti-HEL immunoglobulin M (IgM) antibodies in the serum compared to PECAM-1+/+ anergic counterparts. Dbl Tg B-cells lacking PECAM-1 showed enhanced B-cell proliferation and calcium flux responses to LPS, IL-4 alone, IgM cross-linking and IL-4 indicating augmentation of antigen-receptor signalling. Thus, PECAM-1 is important in maintaining peripheral tolerance in Dbl Tg B-cells.