Immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated inhibitory signaling is regulated by sequential phosphorylation mediated by distinct nonreceptor tyrosine kinases: a case study involving PECAM-1

Quantifying Siglec-sialylated ligand interactions: a versatile 19F-T2 CPMG filtered competitive NMR displacement assay

Sialic-acid-binding immunoglobulin-like lectins (Siglecs) are integral cell surface proteins crucial for the regulation of immune responses and the maintenance of immune tolerance through interactions with sialic acids. Siglecs recognize sialic acid moieties, usually found at the end of N-glycan and O-glycan chains. However, the different Siglecs prefer diverse presentations of the recognized sialic acid, depending on the type of glycosidic linkage used to link to the contiguous Gal/GalNAc or sialic acid moieties. This fact, together with possible O- or N-substitutions at the recognized glycan epitope significantly influences their roles in various immune-related processes. Understanding the molecular details of Siglec-sialoglycan interactions is essential for unraveling their specificities and for the development of new molecules targeting these receptors. While traditional biophysical methods like isothermal titration calorimetry (ITC) have been utilized to measure binding between lectins and glycans, contemporary techniques such as surface plasmon resonance (SPR), microscale thermophoresis (MST), and biolayer interferometry (BLI) offer improved throughput. However, these methodologies require chemical modification and immobilization of at least one binding partner, which can interfere the recognition between the lectin and the ligand. Since Siglecs display a large range of dissociation constants, depending on the (bio)chemical nature of the interacting partner, a general and robust method that could monitor and quantify binding would be highly welcomed. Herein, we propose the application of an NMR-based a competitive displacement assay, grounded on 19F T2-relaxation NMR and on the design, synthesis, and use of a strategic spy molecule, to assess and quantify sialoside ligand binding to Siglecs. We show that the use of this specific approach allows the quantification of Siglec binding for natural and modified sialosides, multivalent sialosides, and sialylated glycoproteins in solution, which differ in binding affinities in more than two orders of magnitude, thus providing invaluable insights into sialoglycan-mediated interactions.

Teleost leukocyte immune-type receptors

Leukocyte immune-type receptors (LITRs) are a large family of teleost immunoregulatory receptor-types belonging to the immunoglobulin superfamily. These immune genes are phylogenetically and syntenically related to Fc receptor-like protein genes (fcrls) present in other vertebrates, including amphibians, birds, mice, and man. In vitro-based functional analyses of LITRs, using transfection approaches, have shown that LITRs have diverse immunoregulatory potentials including the activation and inhibition of several innate immune effector responses such as cell-mediated killing responses, degranulation, cytokine secretion, and phagocytosis. The purpose of this mini review is to provide an overview of fish LITR-mediated immunoregulatory potentials obtained from various teleost model systems, including channel catfish, zebrafish, and goldfish. We will also describe preliminary characterization of a new goldish LITR-specific polyclonal antibody (pAb) and discuss the significance of this tool for further investigation of the functions of fish LITRs.

All-Atom Simulations Reveal the Intricacies of Signal Transduction upon Binding of the HLA-E Ligand to the Transmembrane Inhibitory CD94/NKG2A Receptor

Natural killer (NK) cells play an important role in the innate immune response against tumors and various pathogens such as viruses and bacteria. Their function is controlled by a wide array of activating and inhibitory receptors, which are expressed on their cell surface. Among them is a dimeric NKG2A/CD94 inhibitory transmembrane (TM) receptor which specifically binds to the non-classical MHC I molecule HLA-E, which is often overexpressed on the surface of senescent and tumor cells. Using the Alphafold 2 artificial intelligence system, we constructed the missing segments of the NKG2A/CD94 receptor and generated its complete 3D structure comprising extracellular (EC), TM, and intracellular regions, which served as a starting point for the multi-microsecond all-atom molecular dynamics simulations of the receptor with and without the bound HLA-E ligand and its nonameric peptide. The simulated models revealed that an intricate interplay of events is taking place between the EC and TM regions ultimately affecting the intracellular immunoreceptor tyrosine-based inhibition motif (ITIM) regions that host the point at which the signal is transmitted further down the inhibitory signaling cascade. Signal transduction through the lipid bilayer was also coupled with the changes in the relative orientation of the NKG2A/CD94 TM helices in response to linker reorganization, mediated by fine-tuned interactions in the EC region of the receptor, taking place after HLA-E binding. This research provides atomistic details of the cells' protection mechanism against NK cells and broadens the knowledge regarding the TM signaling of ITIM-bearing receptors.

KLMab-1: An Anti-human KLRG1 Monoclonal Antibody for Immunocytochemistry

Immune checkpoint molecules have received attention as targets of cancer immunotherapy. Killer cell lectin-like receptor subfamily G member 1 (KLRG1) is one of the immune checkpoint molecules expressed in CD4⁺ T, CD8⁺ T, and natural killer (NK) cells. KLRG1 exhibits antiviral and antitumor immunity, and its expression in T and NK cells is upregulated by viral infectious diseases and some tumors. Thus, monoclonal antibodies (mAbs) for KLRG1 would be useful tools for the diagnosis and immunotherapy against viral infectious diseases and cancers. We have developed anti-human KLRG1 (hKLRG1) mAb (clone KLMab-1, mouse IgG₁, kappa) by the Cell-Based Immunization and Screening method. We have also demonstrated that KLMab-1 recognizes both exogenous and endogenous hKLRG1 in flow cytometry. In this study, we first showed that KLMab-1 and its recombinant mAb (recKLMab-1) bound to exogenous hKLRG1 overexpressed in Chinese hamster ovary (CHO)-K1 cells, but not in parental CHO-K1 cells, in immunocytochemistry. We next showed that both mAbs detected endogenous hKLRG1 expressed in human NK cells. These results demonstrate that KLMab-1 and recKLMab-1 are available for immunocytochemistry.

The role of phosphatidylserine on the membrane in immunity and blood coagulation

The negatively charged aminophospholipid, phosphatidylserine (PtdSer), is located in the inner leaflet of the plasma membrane in normal cells, and may be exposed to the outer leaflet under some immune and blood coagulation processes. Meanwhile, Ptdser exposed to apoptotic cells can be recognized and eliminated by various immune cells, whereas on the surface of activated platelets Ptdser interacts with coagulation factors prompting enhanced production of thrombin which significantly facilitates blood coagulation. In the case where PtdSer fails in exposure or mistakenly occurs, there are occurrences of certain immunological and haematological diseases, such as the Scott syndrome and Systemic lupus erythematosus. Besides, viruses (e.g., Human Immunodeficiency Virus (HIV), Ebola virus (EBOV)) can invade host cells through binding the exposed PtdSer. Most recently, the Corona Virus Disease 2019 (COVID-19) has been similarly linked to PtdSer or its receptors. Therefore, it is essential to comprehensively understand PtdSer and its functional characteristics. Therefore, this review summarizes Ptdser, its eversion mechanism; interaction mechanism, particularly with its immune receptors and coagulation factors; recognition sites; and its function in immune and blood processes. This review illustrates the potential aspects for the underlying pathogenic mechanism of PtdSer-related diseases, and the discovery of new therapeutic strategies as well.

Phosphorylation of human CEACAM1-LF by PKA and GSK3β promotes its interaction with β-catenin

CEACAM1-LF, a homotypic cell adhesion adhesion molecule, transduces intracellular signals via a 72 amino acid cytoplasmic domain that contains two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and a binding site for β-catenin. Phosphorylation of Ser503 by PKC in rodent CEACAM1 was shown to affect bile acid transport or hepatosteatosis via the level of ITIM phosphorylation, but the phosphorylation of the equivalent residue in human CEACAM1 (Ser508) was unclear. Here we studied this analogous phosphorylation by NMR analysis of the ¹⁵N labeled cytoplasmic domain peptide. Incubation with a variety of Ser/Thr kinases revealed phosphorylation of Ser508 by GSK3bβ but not by PKC. The lack of phosphorylation by PKC is likely due to evolutionary sequence changes between the rodent and human genes. Phosphorylation site assignment by mass spectrometry and NMR revealed phosphorylation of Ser472, Ser461 and Ser512 by PKA, of which Ser512 is part of a conserved consensus site for GSK3β binding. We showed here that only after phosphorylation of Ser512 by PKA was GSK3β able to phosphorylate Ser508. Phosphorylation of Ser512 by PKA promoted a tight association with the armadillo repeat domain of β-catenin at an extended region spanning the ITIMs of CEACAM1. The kinetics of phosphorylation of the ITIMs by Src, as well dephosphorylation by SHP2, were affected by the presence of Ser508/512 phosphorylation, suggesting that PKA and GSK3β may regulate the signal transduction activity of human CEACAM1-LF. The interaction of CEACAM1-LF with β-catenin promoted by PKA is suggestive of a tight association between the two ITIMs of CEACAM1-LF.

SIRPα sequesters SHP-2 to promote IL-4 and IL-13 signaling and the alternative activation of macrophages

[Figure: see text].

Electrochemiluminescence aptasensor for Siglec-5 detection based on MoS2@Au nanocomposites emitter and exonuclease III-powered DNA walker

Lectins are highly specific binding proteins for glycoproteins which widely exist in living organisms, playing a vital role in exploring the biological evolution process, such as cellular proliferation, differentiation, carcinogenesis and apoptosis. Therefore, the content monitoring of lectin becomes particularly significant and urgent in the bioanalytical application. In this work, we fabricated an aptasensor, majorly capitalizing the eminent affinity between sialic acid-binding immunoglobulin (Ig)-like lectin 5 (Siglec-5) and nucleic acids aptamer (K19), with nontoxic MoS₂@Au nanocomposites as electrochemiluminescence (ECL) emitters based on exonuclease III (Exo III)-powered DNA walker for the bioassays of Siglec-5. The DNA track was constructed on the emitters' surface, providing a reliable platform for the DNA walker's autonomous move. In the assay, the primer DNA in the DNA duplex was replaced by Siglec-5 due to the aptamer interactions and repeatedly released to participate in the movement of the DNA walker, further triggering cascade signal amplification. Finally, our aptasensor indicates significant potential for assays of Siglec-5 with a detection limit of 8.9 pM.

Development of Anti-human T Cell Immunoreceptor with Ig and ITIM Domains (TIGIT) Monoclonal Antibodies for Flow Cytometry

Immune checkpoint inhibitors targeting programmed cell death-ligand 1 (PD-L1), programmed cell death-1 (PD-1), and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) recently made a significant survival rate improvement in cancer treatment. T cell immunoreceptor with Ig and ITIM domains (TIGIT) is expressed in T and NK cells related to their activities. It has a single extracellular immunoglobulin domain, a type 1 transmembrane domain, and a single intracellular ITIM. TIGIT binds with poliovirus receptor (PVR) or PVR2, resulting in suppressing T and NK cell activities. Some studies showed that the combined use of a TIGIT inhibitor with another immune checkpoint inhibitor enhanced antitumor activities more strongly than their single use. Therefore, TIGIT should be a new target for immunotherapy. In this study, we developed new anti-human TIGIT (hTIGIT) monoclonal antibodies (mAbs) using the Cell-Based Immunization and Screening (CBIS) method. Mice were immunized with hTIGIT-overexpressed Chinese hamster ovary (CHO)-K1 cells (CHO/hTIGIT), and hybridomas were screened by flow cytometry. One of the mAbs, TgMab-2 (IgG₁, kappa), specifically and sensitively detects hTIGIT in CHO/hTIGIT and NK cells. The dissociation constants (K_D) of TgMab-2 for CHO/hTIGIT cells were determined to be 3.5 × 10^-9 M. These results suggest that TgMab-2, which was developed by CBIS method, is useful for analyzing the function of hTIGIT by flow cytometry.

ADAP Y571 Phosphorylation Is Required to Prime STAT3 for Activation in TLR4-Stimulated Macrophages

Adhesion and degranulation-promoting adapter protein (ADAP), originally identified as an essential adaptor molecule in TCR signaling and T cell adhesion, has emerged as a critical regulator in innate immune cells such as macrophages; however, its role in macrophage polarization and inflammatory responses remains unknown. In this study, we show that ADAP plays an essential role in TLR4-mediated mouse macrophage polarization via modulation of STAT3 activity. Macrophages from ADAP-deficient mice exhibit enhanced M1 polarization, expression of proinflammatory cytokines and capacity in inducing Th1 responses, but decreased levels of anti-inflammatory cytokines in response to TLR4 activation by LPS. Furthermore, overexpression of ADAP enhances, whereas loss of ADAP reduces, the LPS-mediated phosphorylation and activity of STAT3, suggesting ADAP acts as a coactivator of STAT3 activity and function. Furthermore, the coactivator function of ADAP mostly depends on the tyrosine phosphorylation at Y571 in the motif YDSL induced by LPS. Mutation of Y571 to F severely impairs the stimulating effect of ADAP on STAT3 activity and the ability of ADAP to inhibit M1-like polarization in TLR4-activated mouse macrophages. Moreover, ADAP interacts with STAT3, and loss of ADAP renders mouse macrophages less sensitive to IL-6 stimulation for STAT3 phosphorylation. Collectively, our findings revealed an additional layer of regulation of TLR4-mediated mouse macrophage plasticity whereby ADAP phosphorylation on Y571 is required to prime STAT3 for activation in TLR4-stimulated mouse macrophages.

CD31 as a Therapeutic Target in Atherosclerosis

The potential of CD31 as a therapeutic target in atherosclerosis has been considered ever since its cloning in the 1990s, but the exact role played by this molecule in the biologic events underlying atherosclerosis has remained controversial, resulting in the stalling of any therapeutic perspective. Due to the supposed cell adhesive properties of CD31, specific monoclonal antibodies and recombinant proteins were regarded as blocking agents because their use prevented the arrival of leukocytes at sites of acute inflammation. However, the observed effect of those compounds likely resulted from the engagement of the immunomodulatory function of CD31 signaling. This was acknowledged only later though, upon the discovery of CD31's 2 intracytoplasmic tyrosine residues called immunoreceptor tyrosine inhibitory motifs. A growing body of evidence currently points at a therapeutic potential for CD31 agonists in atherothrombosis. Clinical observations show that CD31 expression is altered at the surface of leukocytes infiltrating unhealed atherothrombotic lesions and that the physiological immunomodulatory functions of CD31 are lost at the surface of blood leukocytes in patients with acute coronary syndromes. On the contrary, translational studies using candidate therapeutic molecules in laboratory animals have provided encouraging results: synthetic peptides administered to atherosclerotic mice as systemic drugs in the acute phases of atherosclerotic complications favor the healing of wounded arteries, whereas the immobilization of CD31 agonist peptides onto coronary stents implanted in farm pigs favors their peaceful integration within the coronary arterial wall.

Mechanotransduction, immunoregulation, and metabolic functions of CD31 in cardiovascular pathophysiology

Biomechanical changes in the heart and vessels drive rapid and dynamic regulation of blood flow, a vital process for meeting the changing metabolic needs of the peripheral tissues at any given point in time. The fluid movement of the blood exerts haemodynamic stress upon the solid elements of the cardiovascular system: the heart, vessels, and cellular components of the blood. Cardiovascular diseases can lead to prolonged mechanical stress, such as cardiac remodelling during heart failure or vascular stiffening in atherosclerosis. This can lead to a significantly reduced or increasingly turbulent blood supply, inducing a shift in cellular metabolism that, amongst other effects, can trigger the release of reactive oxygen species and initiate a self-perpetuating cycle of inflammation and oxidative stress. CD31 is the most abundant constitutive co-signalling receptor glycoprotein on endothelial cells, which line the cardiovascular system and form the first-line of cellular contact with the blood. By associating with most endothelial receptors involved in mechanosensing, CD31 regulates the response to biomechanical stimuli. In addition, by relocating in the lipid rafts of endothelial cells as well as of cells stably interacting with the endothelium, including leucocytes and platelets, CD31–CD31 trans-homophilic engagement guides and restrains platelet and immune cell accumulation and activation and at sites of damage. In this way, CD31 is at the centre of mediating mechanical, metabolic, and immunological changes within the circulation and provides a single target that may have pleiotropic beneficial effects.

Evolution of the exclusively human pathogen Neisseria gonorrhoeae: Human-specific engagement of immunoregulatory Siglecs

Neisseria gonorrhoeae causes the sexually transmitted disease gonorrhea exclusively in humans and uses multiple strategies to infect, including acquisition of host sialic acids that cap and mask lipooligosaccharide termini, while restricting complement activation. We hypothesized that gonococci selectively target human anti-inflammatory sialic acid-recognizing Siglec receptors on innate immune cells to blunt host responses and that pro-inflammatory Siglecs and SIGLEC pseudogene polymorphisms represent host evolutionary adaptations to counteract this interaction. N. gonorrhoeae can indeed engage multiple human but not chimpanzee CD33rSiglecs expressed on innate immune cells and in the genitourinary tract--including Siglec-11 (inhibitory) and Siglec-16 (activating), which we detected for the first time on human cervical epithelium. Surprisingly, in addition to LOS sialic acid, we found that gonococcal porin (PorB) mediated binding to multiple Siglecs. PorB also bound preferentially to human Siglecs and not chimpanzee orthologs, modulating host immune reactions in a human-specific manner. Lastly, we studied the distribution of null SIGLEC polymorphisms in a Namibian cohort with a high prevalence of gonorrhea and found that uninfected women preferentially harbor functional SIGLEC16 alleles encoding an activating immune receptor. These results contribute to the understanding of the human specificity of N. gonorrhoeae and how it evolved to evade the human immune defense.

Negative regulators of platelet activation and adhesion

Platelets are small anucleated cells that constantly patrol the cardiovascular system to preserve its integrity and prevent excessive blood loss where the vessel lining is breached. Their key challenge is to form a hemostatic plug under conditions of high shear forces. To do so, platelets have evolved a molecular machinery that enables them to sense trace amounts of signals at the site of damage and to rapidly shift from a non-adhesive to a pro-adhesive state. However, this highly efficient molecular machinery can also lead to unintended platelet activation and cause clinical complications such as thrombocytopenia and thrombosis. Thus, several checkpoints are in place to tightly control platelet activation and adhesiveness in space and time. In this review, we will discuss select negative regulators of platelet activation, which are critical to maintain patrolling platelets in a quiescent, non-adhesive state and/or to limit platelet adhesion to sites of injury.

Molecular mechanisms for enhancement of stromal cell-derived factor 1-induced chemotaxis by platelet endothelial cell adhesion molecule 1 (PECAM-1)

Platelet endothelial cell adhesion molecule 1 (PECAM-1) is a cell adhesion protein involved in the regulation of cell adhesion and migration. Interestingly, several PECAM-1-deficient hematopoietic cells exhibit impaired chemotactic responses to stromal cell-derived factor 1 (SDF-1), a chemokine essential for B lymphopoiesis and bone marrow myelopoiesis. However, whether PECAM-1 is involved in SDF-1-regulated chemotaxis is unknown. We report here that SDF-1 induces tyrosine phosphorylation of PECAM-1 at its immunoreceptor tyrosine-based inhibition motifs in several hematopoietic cell lines via the Src family kinase Lyn, Bruton's tyrosine kinase, and JAK2 and that inhibition of these kinases reduced chemotaxis. Overexpression and knockdown of PECAM-1 enhanced and down-regulated, respectively, SDF-1-induced Gα_i-dependent activation of the PI3K/Akt/mTORC1 pathway and small GTPase Rap1 in hematopoietic 32Dcl3 cells, and these changes in activation correlated with chemotaxis. Furthermore, pharmacological or genetic inhibition of the PI3K/Akt/mTORC1 pathway or Rap1, respectively, revealed that these pathways are independently activated and required for SDF-1-induced chemotaxis. When coexpressed in 293T cells, PECAM-1 physically associated with the SDF-1 receptor CXCR4. Moreover, PECAM-1 overexpression and knockdown reduced and enhanced SDF-1-induced endocytosis of CXCR4, respectively. Furthermore, when expressed in 32Dcl3 cells, an endocytosis-defective CXCR4 mutant, CXCR4-S324A/S325A, could activate the PI3K/Akt/mTORC1 pathway as well as Rap1 and induce chemotaxis in a manner similar to PECAM-1 overexpression. These findings suggest that PECAM-1 enhances SDF-1-induced chemotaxis by augmenting and prolonging activation of the PI3K/Akt/mTORC1 pathway and Rap1 and that PECAM-1, at least partly, exerts its activity by inhibiting SDF-1-induced internalization of CXCR4.

Genetic diversity and phylogenetic analysis of the ORF5 gene of PRRSV from central China

To more fully understand the genetic diversity and molecular epidemiology of prevailing porcine reproductive and respiratory syndrome virus (PRRSV) in Henan province of China, 112 full-length ORF5 gene sequences, originating from Henan province between 2006 and 2015, were subjected to sequence variation and phylogenetic analysis. Phylogenetic analysis revealed that all Henan isolates belonged to the Type 2 genotype and could be further divided into three subgroups. Subgroup 1 and 2 viruses predominated in Henan and subgroup 2 overtook subgroup 1 as the most prevalent PRRSV between 2006 and 2015. Highly pathogenic PRRSV (HP-PRRSV) isolates predominated in Henan and eight RespPRRSV MLV vaccine-like isolates were observed in subgroup 3. Sequence variation analysis revealed that the ORF5 genes of all Henan isolates shared >83.3% nucleotide and >80.1% amino acid sequence identity with each other. Primary neutralizing epitope (PNE) analysis revealed that, relative to the attenuated RespPRRSV MLV vaccine isolate, all but one of the subgroup 1 Henan isolates had mutations at amino acid 39 within the key PNE of GP5. Analysis of the immunoreceptor tyrosine-based inhibitory motif (ITIM) in GP5 revealed that all but two of the Henan isolates had a highly conserved sequence between amino acids 77 and 82 positions of GP5. N-linked glycosylation site (NGS) analysis revealed a novel potential NGS at GP5 amino acid position 59 in two of the subgroup 2 Henan isolates. Another novel GP5 amino acid mutation (⁴⁴N→⁴⁴D) was found in a single subgroup 1 Henan isolate (HeNan-A9) in a glycosylation site that is known to be crucial for PRRSV infectivity.

ITIM receptors: more than just inhibitors of platelet activation

Since their discovery, immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptors have been shown to inhibit signaling from immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors in almost all hematopoietic cells, including platelets. However, a growing body of evidence has emerged demonstrating that this is an oversimplification, and that ITIM-containing receptors are versatile regulators of platelet signal transduction, with functions beyond inhibiting ITAM-mediated platelet activation. PECAM-1 was the first ITIM-containing receptor identified in platelets and appeared to conform to the established model of ITIM-mediated attenuation of ITAM-driven activation. PECAM-1 was therefore widely accepted as a major negative regulator of platelet activation and thrombosis for many years, but more recent findings suggest a more complex role for this receptor, including the facilitation of α_IIbβ₃-mediated platelet functions. Since the identification of PECAM-1, several other ITIM-containing platelet receptors have been discovered. These include G6b-B, a critical regulator of platelet reactivity and production, and the noncanonical ITIM-containing receptor TREM-like transcript-1, which is localized to α-granules in resting platelets, binds fibrinogen, and acts as a positive regulator of platelet activation. Despite structural similarities and shared binding partners, including the Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, knockout and transgenic mouse models have revealed distinct phenotypes and nonredundant functions for each ITIM-containing receptor in the context of platelet homeostasis. These roles are likely influenced by receptor density, compartmentalization, and as-yet unknown binding partners. In this review, we discuss the diverse repertoire of ITIM-containing receptors in platelets, highlighting intriguing new functions, controversies, and future areas of investigation.

Paired Siglec receptors generate opposite inflammatory responses to a human-specific pathogen

Paired immune receptors display near-identical extracellular ligand-binding regions but have intracellular sequences with opposing signaling functions. While inhibitory receptors dampen cellular activation by recognizing self-associated molecules, the functions of activating counterparts are less clear. Here, we studied the inhibitory receptor Siglec-11 that shows uniquely human expression in brain microglia and engages endogenous polysialic acid to suppress inflammation. We demonstrated that the human-specific pathogen Escherichia coli K1 uses its polysialic acid capsule as a molecular mimic to engage Siglec-11 and escape killing. In contrast, engagement of the activating counterpart Siglec-16 increases elimination of bacteria. Since mice do not have paired Siglec receptors, we generated a model by replacing the inhibitory domain of mouse Siglec-E with the activating module of Siglec-16. Siglec-E16 enhanced proinflammatory cytokine expression and bacterial killing in macrophages and boosted protection against intravenous bacterial challenge. These data elucidate uniquely human interactions of a pathogen with Siglecs and support the long-standing hypothesis that activating counterparts of paired immune receptors evolved as a response to pathogen molecular mimicry of host ligands for inhibitory receptors.

Endothelial functions of platelet/endothelial cell adhesion molecule-1 (CD31)

PURPOSE OF REVIEW

The purpose of this article is to describe the function of the vascular cell adhesion and signaling molecule, platelet/endothelial cell adhesion molecule-1 (PECAM-1), in endothelial cells, with special emphasis on its role in maintaining and restoring the vascular permeability barrier following disruption of the endothelial cell junction.

RECENT FINDINGS

In addition to its role as an inhibitory receptor in circulating platelets and leukocytes, PECAM-1 is highly expressed at endothelial cell-cell junctions, where it functions as an adhesive stress-response protein to both maintain endothelial cell junctional integrity and speed restoration of the vascular permeability barrier following inflammatory or thrombotic challenge.

SUMMARY

Owing to the unique ability of antibodies that bind the membrane proximal region of the extracellular domain to trigger conformational changes leading to affinity modulation and homophilic adhesion strengthening, PECAM-1 might be an attractive target for treating vascular permeability disorders.

Distinct roles of immunoreceptor tyrosine-based motifs in immunosuppressive indoleamine 2,3-dioxygenase 1

The enzyme indoleamine 2,3‐dioxygenase 1 (IDO1) catalyses the initial, rate‐limiting step in tryptophan (Trp) degradation, resulting in tryptophan starvation and the production of immunoregulatory kynurenines. IDO1's catalytic function has long been considered as the one mechanism responsible for IDO1‐dependent immune suppression by dendritic cells (DCs), which are master regulators of the balance between immunity and tolerance. However, IDO1 also harbours immunoreceptor tyrosine‐based inhibitory motifs, (ITIM1 and ITIM2), that, once phosphorylated, bind protein tyrosine phosphatases, (SHP‐1 and SHP‐2), and thus trigger an immunoregulatory signalling in DCs. This mechanism leads to sustained IDO1 expression, in a feedforward loop, which is particularly important in restraining autoimmunity and chronic inflammation. Yet, under specific conditions requiring that early and protective inflammation be unrelieved, tyrosine‐phosphorylated ITIMs will instead bind the suppressor of cytokine signalling 3 (SOCS3), which drives IDO1 proteasomal degradation and shortens the enzyme half‐life. To dissect any differential roles of the two IDO1's ITIMs, we generated protein mutants by replacing one or both ITIM‐associated tyrosines with phospho‐mimicking glutamic acid residues. Although all mutants lost their enzymic activity, the ITIM1 – but not ITIM2 mutant – did bind SHPs and conferred immunosuppressive effects on DCs, making cells capable of restraining an antigen‐specific response in vivo. Conversely, the ITIM2 mutant would preferentially bind SOCS3, and IDO1's degradation was accelerated. Thus, it is the selective phosphorylation of either ITIM that controls the duration of IDO1 expression and function, in that it dictates whether enhanced tolerogenic signalling or shutdown of IDO1‐dependent events will occur in a local microenvironment.

Regulation of Siglec-8-induced intracellular reactive oxygen species production and eosinophil cell death by Src family kinases

RATIONALE

Siglec-8 is a surface receptor predominantly expressed on human eosinophils where its ligation induces reactive oxygen species (ROS) formation and cell death. Since Siglec-8 has intracellular tyrosine-based motifs, we hypothesized that Src family kinases (SFKs) are involved in ROS formation and cell death induced by Siglec-8 engagement.

METHODS

Human peripheral blood eosinophils were purified and incubated with anti-Siglec-8 monoclonal antibodies (mAb, agonist), IL-5, and SFK pharmacological inhibitors. We focused on Siglec-8-induced cell death in short-term IL-5-activated cells leading to a regulated necrosis-type cell death. ROS production was determined by dihydrorhodamine (DHR) 123 labeling and flow cytometry, or by chemiluminescence using Amplex red. Activation of SFK was determined using phospholuminex and Western blotting.

RESULTS

In order to determine cellular localization of ROS production, we measured intra and extracellular ROS. While an ETosis stimulus (calcium ionophore A23187) led to extracellular ROS (ecROS) production, Siglec-8-engagement in short-term IL-5 activated cells led to intracellular ROS (icROS) accumulation. Consistently, inhibition of extracellular ROS by catalase inhibited ETosis, but not IL-5-primed Siglec-8-induced cell death. In order to determine signaling events for Siglec-8, we performed Western blotting and found SFK phosphorylation in lysates from eosinophils stimulated with anti-Siglec-8 mAb±IL-5. In order to identify which SFKs were involved, we used the phospholuminex assay and found increased levels of phosphorylated Fgr in the cytoplasmic fraction of cells co-stimulated with anti-Siglec-8 and IL-5 for 3 hours compared with cells stimulated with IL-5 alone. To test the involvement of SFKs in ROS production and cell death, we used SFK inhibitors PP2 and dasatinib, both of which completely inhibited eosinophil ROS production and cell death induced by anti-Siglec-8 and IL-5 co-stimulation.

CONCLUSION

Siglec-8 engagement in short-term IL-5-activated eosinophils causes icROS production and SKF phosphorylation, and both are essential in mediating Siglec-8-induced cell death.

Identification and characterization of the role of c-terminal Src kinase in dengue virus replication

We screened a siRNA library targeting human tyrosine kinases in Huh-7 cells and identified c-terminal Src kinase (Csk) as one of the kinases involved in dengue virus replication. Knock-down of Csk expression by siRNAs or inhibition of Csk by an inhibitor reduced dengue virus RNA levels but did not affect viral entry. Csk partially colocalized with viral replication compartments. Dengue infection was drastically reduced in cells lacking the three ubiquitous src family kinases, Src, Fyn and Yes. Csk knock-down in these cells failed to block dengue virus replication suggesting that the effect of Csk is via regulation of Src family kinases. Csk was found to be hyper-phosphorylated during dengue infection and inhibition of protein kinase A led to a block in Csk phosphorylation and dengue virus replication. Overexpression studies suggest an important role for the kinase and SH3 domains in this process. Our results identified a novel role for Csk as a host tyrosine kinase involved in dengue virus replication and provide further insights into the role of host factors in dengue replication.

Platelets are versatile cells: New discoveries in hemostasis, thrombosis, immune responses, tumor metastasis and beyond

Platelets are small anucleate blood cells generated from megakaryocytes in the bone marrow and cleared in the reticuloendothelial system. At the site of vascular injury, platelet adhesion, activation and aggregation constitute the first wave of hemostasis. Blood coagulation, which is initiated by the intrinsic or extrinsic coagulation cascades, is the second wave of hemostasis. Activated platelets can also provide negatively-charged surfaces that harbor coagulation factors and markedly potentiate cell-based thrombin generation. Recently, deposition of plasma fibronectin, and likely other plasma proteins, onto the injured vessel wall has been identified as a new "protein wave of hemostasis" that may occur even earlier than the first wave of hemostasis, platelet accumulation. Although no experimental evidence currently exists, it is conceivable that platelets may also contribute to this protein wave of hemostasis by releasing their granule fibronectin and other proteins that may facilitate fibronectin self- and non-self-assembly on the vessel wall. Thus, platelets may contribute to all three waves of hemostasis and are central players in this critical physiological process to prevent bleeding. Low platelet counts in blood caused by enhanced platelet clearance and/or impaired platelet production are usually associated with hemorrhage. Auto- and allo-immune thrombocytopenias such as idiopathic thrombocytopenic purpura and fetal and neonatal alloimmune thrombocytopenia may cause life-threatening bleeding such as intracranial hemorrhage. When triggered under pathological conditions such as rupture of an atherosclerotic plaque, excessive platelet activation and aggregation may result in thrombosis and vessel occlusion. This may lead to myocardial infarction or ischemic stroke, the major causes of mortality and morbidity worldwide. Platelets are also involved in deep vein thrombosis and thromboembolism, another leading cause of mortality. Although fibrinogen has been documented for more than half a century as essential for platelet aggregation, recent studies demonstrated that fibrinogen-independent platelet aggregation occurs in both gene deficient animals and human patients under physiological and pathological conditions (non-anti-coagulated blood). This indicates that other unidentified platelet ligands may play important roles in thrombosis and might be novel antithrombotic targets. In addition to their critical roles in hemostasis and thrombosis, emerging evidence indicates that platelets are versatile cells involved in many other pathophysiological processes such as innate and adaptive immune responses, atherosclerosis, angiogenesis, lymphatic vessel development, liver regeneration and tumor metastasis. This review summarizes the current knowledge of platelet biology, highlights recent advances in the understanding of platelet production and clearance, molecular and cellular events of thrombosis and hemostasis, and introduces the emerging roles of platelets in the immune system, vascular biology and tumorigenesis. The clinical implications of these basic science and translational research findings will also be discussed.

The adhesion molecule PECAM-1 enhances the TGF-β-mediated inhibition of T cell function

Transforming growth factor-β (TGF-β) is an immunosuppressive cytokine that inhibits the proinflammatory functions of T cells, and it is a major factor in abrogating T cell activity against tumors. Canonical TGF-β signaling results in the activation of Smad proteins, which are transcription factors that regulate target gene expression. We found that the cell surface molecule platelet endothelial cell adhesion molecule-1 (PECAM-1) facilitated noncanonical (Smad-independent) TGF-β signaling in T cells. Subcutaneously injected tumor cells that are dependent on TGF-β-mediated suppression of immunity for growth grew more slowly in PECAM-1(-/-) mice than in their wild-type counterparts. T cells isolated from PECAM-1(-/-) mice demonstrated relative insensitivity to the TGF-β-dependent inhibition of interferon-γ (IFN-γ) production, granzyme B synthesis, and cellular proliferation. Similarly, human T cells lacking PECAM-1 demonstrated decreased sensitivity to TGF-β in a manner that was partially restored by reexpression of PECAM-1. Co-incubation of T cells with TGF-β and a T cell-activating antibody resulted in PECAM-1 phosphorylation on an immunoreceptor tyrosine-based inhibitory motif (ITIM) and the recruitment of the inhibitory Src homology 2 (SH2) domain-containing tyrosine phosphatase-2 (SHP-2). Such conditions also induced the colocalization of PECAM-1 with the TGF-β receptor complex as identified by coimmunoprecipitation, confocal microscopy, and proximity ligation assays. These studies indicate a role for PECAM-1 in enhancing the inhibitory functions of TGF-β in T cells and suggest that therapeutic targeting of the PECAM-1-TGF-β inhibitory axis represents a means to overcome TGF-β-dependent immunosuppression within the tumor microenvironment.

Platelet Activation and Thrombus Formation over IgG Immune Complexes Requires Integrin αIIbβ3 and Lyn Kinase

IgG immune complexes contribute to the etiology and pathogenesis of numerous autoimmune disorders, including heparin-induced thrombocytopenia, systemic lupus erythematosus, rheumatoid- and collagen-induced arthritis, and chronic glomerulonephritis. Patients suffering from immune complex-related disorders are known to be susceptible to platelet-mediated thrombotic events. Though the role of the Fc receptor, FcγRIIa, in initiating platelet activation is well understood, the role of the major platelet adhesion receptor, integrin αIIbβ3, in amplifying platelet activation and mediating adhesion and aggregation downstream of encountering IgG immune complexes is poorly understood. The goal of this investigation was to gain a better understanding of the relative roles of these two receptor systems in immune complex-mediated thrombotic complications. Human platelets, and mouse platelets genetically engineered to differentially express FcγRIIa and αIIbβ3, were allowed to interact with IgG-coated surfaces under both static and flow conditions, and their ability to spread and form thrombi evaluated in the presence and absence of clinically-used fibrinogen receptor antagonists. Although binding of IgG immune complexes to FcγRIIa was sufficient for platelet adhesion and initial signal transduction events, platelet spreading and thrombus formation over IgG-coated surfaces showed an absolute requirement for αIIbβ3 and its ligands. Tyrosine kinases Lyn and Syk were found to play key roles in IgG-induced platelet activation events. Taken together, our data suggest a complex functional interplay between FcγRIIa, Lyn, and αIIbβ3 in immune complex-induced platelet activation. Future studies may be warranted to determine whether patients suffering from immune complex disorders might benefit from treatment with anti-αIIbβ3-directed therapeutics.

A Comprehensive Immunoreceptor Phosphotyrosine-based Signaling Network Revealed by Reciprocal Protein-Peptide Array Screening

Cells of the immune system communicate with their environment through immunoreceptors. These receptors often harbor intracellular tyrosine residues, which, when phosphorylated upon receptor activation, serve as docking sites to recruit downstream signaling proteins containing the Src Homology 2 (SH2) domain. A systematic investigation of interactions between the SH2 domain and the immunoreceptor tyrosine-based regulatory motifs (ITRM), including inhibitory (ITIM), activating (ITAM), or switching (ITSM) motifs, is critical for understanding cellular signal transduction and immune function. Using the B cell inhibitory receptor CD22 as an example, we developed an approach that combines reciprocal or bidirectional phosphopeptide and SH2 domain array screens with in-solution binding assays to identify a comprehensive SH2-CD22 interaction network. Extending this approach to 194 human ITRM sequences and 78 SH2 domains led to the identification of a high-confidence immunoreceptor interactome containing 1137 binary interactions. Besides recapitulating many previously reported interactions, our study uncovered numerous novel interactions. The resulting ITRM-SH2 interactome not only helped to fill many gaps in the immune signaling network, it also allowed us to associate different SH2 domains to distinct immune functions. Detailed analysis of the NK cell ITRM-mediated interactions led to the identification of a network nucleated by the Vav3 and Fyn SH2 domains. We showed further that these SH2 domains have distinct functions in cytotoxicity. The bidirectional protein-peptide array approach described herein may be applied to the numerous other peptide-binding modules to identify potential protein-protein interactions in a systematic and reliable manner.

The BCR-ABL inhibitor ponatinib inhibits platelet immunoreceptor tyrosine-based activation motif (ITAM) signaling, platelet activation and aggregate formation under shear

BACKGROUND

Treatment of chronic myelogenous leukemia (CML) with the BCR-ABL tyrosine kinase inhibitor (TKI) imatinib significantly improves patient outcomes. As some patients are unresponsive to imatinib, next generation BCR-ABL inhibitors such as nilotinib have been developed to treat patients with imatinib-resistant CML. The use of some BCR-ABL inhibitors has been associated with bleeding diathesis, and these inhibitors have been shown to inhibit platelet functions, which may explain the hemostasis impairment. Surprisingly, a new TKI, ponatinib, has been associated with a high incidence of severe acute ischemic cardiovascular events. The mechanism of this unexpected adverse effect remains undefined.

OBJECTIVE AND METHODS

This study used biochemical and functional assays to evaluate whether ponatinib was different from the other BCR-ABL inhibitors with respect to platelet activation, spreading, and aggregation.

RESULTS AND CONCLUSIONS

Our results show that ponatinib, similar to other TKIs, acts as a platelet antagonist. Ponatinib inhibited platelet activation, spreading, granule secretion, and aggregation, likely through broad spectrum inhibition of platelet tyrosine kinase signaling, and also inhibited platelet aggregate formation in whole blood under shear. As our results indicate that pobatinib inhibits platelet function, the adverse cardiovascular events observed in patients taking ponatinib may be the result of the effect of ponatinib on other organs or cell types, or disease-specific processes, such as BCR-ABL+cells undergoing apoptosis in response to chemotherapy, or drug-induced adverse effects on the integrity of the vascular endothelium in ponatinib-treated patients.

Unreported intrinsic disorder in proteins: Building connections to the literature on IDPs

This review opens a new series entitled “Unreported intrinsic disorder in proteins.” The goal of this series is to bring attention of researchers to an interesting phenomenon of missed (or overlooked, or ignored, or unreported) disorder. This series serves as a companion to “Digested Disorder” which provides a quarterly review of papers on intrinsically disordered proteins (IDPs) found by standard literature searches. The need for this alternative series results from the observation that there are numerous publications that describe IDPs (or hybrid proteins with ordered and disordered regions) yet fail to recognize many of the key discoveries and publications in the IDP field. By ignoring the body of work on IDPs, such publications often fail to relate their findings to prior discoveries or fail to explore the obvious implications of their work. Thus, the goal of this series is not only to review these very interesting and important papers, but also to point out how each paper relates to the IDP field and show how common tools in the IDP field can readily take the findings in new directions or provide a broader context for the reported findings.

PECAM-1: regulator of endothelial junctional integrity

PECAM-1 (also known as CD31) is a cellular adhesion and signaling receptor comprising six extracellular immunoglobulin (Ig)-like homology domains, a short transmembrane domain and a 118 amino acid cytoplasmic domain that becomes serine and tyrosine phosphorylated upon cellular activation. PECAM-1 expression is restricted to blood and vascular cells. In circulating platelets and leukocytes, PECAM-1 functions largely as an inhibitory receptor that, via regulated sequential phosphorylation of its cytoplasmic domain, limits cellular activation responses. PECAM-1 is also highly expressed at endothelial cell intercellular junctions, where it functions as a mechanosensor, as a regulator of leukocyte trafficking and in the maintenance of endothelial cell junctional integrity. In this review, we will describe (1) the functional domains of PECAM-1 and how they contribute to its barrier-enhancing properties, (2) how the physical properties of PECAM-1 influence its subcellular localization and its ability to influence endothelial cell barrier function, (3) various stimuli that initiate PECAM-1 signaling and/or function at the endothelial junction and (4) cross-talk of PECAM-1 with other junctional molecules, which can influence endothelial cell function.

Profiling the substrate specificity of protein kinases by on-bead screening of peptide libraries

A robust, high-throughput method has been developed to screen one-bead-one-compound peptide libraries to systematically profile the sequence specificity of protein kinases. Its ability to provide individual sequences of the preferred substrates permits the identification of sequence contextual effects and nonpermissive residues. Application of the library method to kinases Pim1, MKK6, and Csk revealed that Pim1 and Csk are highly active toward peptide substrates and recognize specific sequence motifs, whereas MKK6 has little activity or sequence selectivity against peptide substrates. Pim1 recognizes peptide substrates of the consensus RXR(H/R)X(S/T); it accepts essentially any amino acid at the S/T-2 and S/T+1 positions, but strongly disfavors acidic residues (Asp or Glu) at the S/T-2 position and a proline residue at the S/T+1 position. The selected Csk substrates show strong sequence covariance and fall into two classes with the consensus sequences of (D/E)EPIYϕXϕ and (D/E)(E/D)S(E/D/I)YϕXϕ (where X is any amino acid and ϕ is a hydrophobic amino acid). Database searches and in vitro kinase assays identified phosphatase PTP-PEST as a Pim1 substrate and phosphatase SHP-1 as a potential Csk substrate. Our results demonstrate that the sequence specificity of protein kinases is defined not only by favorable interactions between permissive residue(s) on the substrate and their cognate binding site(s) on the kinase but also by repulsive interactions between the kinase and nonpermissive residue(s).