PSGL-1 function in immunity and steady state homeostasis

PSGL-1 is an evolutionarily conserved antiviral restriction factor

IMPORTANCE

Studying the co-evolution between viruses and humans is important for understanding why we are what we are now as well as for developing future antiviral drugs. Here we pinned down an evolutionary arms race between retroviruses and mammalian hosts at the molecular level by identifying the antagonism between a host antiviral restriction factor PSGL-1 and viral accessory proteins. We show that this antagonism is conserved from mouse to human and from mouse retrovirus to HIV. Further studying this antagonism might provide opportunities for developing new antiviral therapies.

PSGL-1, a Strategic Biomarker for Pathological Conditions in HIV Infection: A Hypothesis Review

P-selectin glycoprotein ligand-1 (PSGL-1) has been established to be a cell adhesion molecule that is involved in the cellular rolling mechanism and the extravasation cascade, enabling the recruitment of immune cells to sites of inflammation. In recent years, researchers have established that PSGL-1 also functions as an HIV restriction factor. PSGL-1 has been shown to inhibit the HIV reverse transcription process and inhibit the infectivity of HIV virions produced by cells expressing PSGL-1. Cumulative evidence gleaned from contemporary literature suggests that PSGL-1 expression negatively affects the functions of immune cells, particularly T-cells, which are critical participants in the defense against HIV infection. Indeed, some researchers have observed that PSGL-1 expression and signaling provokes T-cell exhaustion. Additionally, it has been established that PSGL-1 may also mediate virus capture and subsequent transfer to permissive cells. We therefore believe that, in addition to its beneficial roles, such as its function as a proinflammatory molecule and an HIV restriction factor, PSGL-1 expression during HIV infection may be disadvantageous and may potentially predict HIV disease progression. In this hypothesis review, we provide substantial discussions with respect to the possibility of using PSGL-1 to predict the potential development of particular pathological conditions commonly seen during HIV infection. Specifically, we speculate that PSGL-1 may possibly be a reliable biomarker for immunological status, inflammation/translocation, cell exhaustion, and the development of HIV-related cancers. Future investigations directed towards our hypotheses may help to evolve innovative strategies for the monitoring and/or treatment of HIV-infected individuals.

PSGL-1 Blockade Induces Classical Activation of Human Tumor-associated Macrophages

The immune suppressive microenvironment is a major culprit for difficult-to-treat solid cancers. Particularly, inhibitory tumor-associated macrophages (TAM) define the resistant nature of the tumor milieu. To define tumor-enabling mechanisms of TAMs, we analyzed molecular clinical datasets correlating cell surface receptors with the TAM infiltrate. Though P-selectin glycoprotein ligand-1 (PSGL-1) is found on other immune cells and functions as an adhesion molecule, PSGL-1 is highly expressed on TAMs across multiple tumor types. siRNA-mediated knockdown and antibody-mediated inhibition revealed a role for PSGL-1 in maintaining an immune suppressed macrophage state. PSGL-1 knockdown or inhibition enhanced proinflammatory mediator release across assays and donors in vitro. In several syngeneic mouse models, PSGL-1 blockade alone and in combination with PD-1 blockade reduced tumor growth. Using a humanized tumor model, we observed the proinflammatory TAM switch following treatment with an anti-PSGL-1 antibody. In ex vivo patient-derived tumor cultures, a PSGL-1 blocking antibody increased expression of macrophage-derived proinflammatory cytokines, as well as IFNγ, indicative of T-cell activation. Our data demonstrate that PSGL-1 blockade reprograms TAMs, offering a new therapeutic avenue to patients not responding to T-cell immunotherapies, as well as patients with tumors devoid of T cells.

SIGNIFICANCE

This work is a significant and actionable advance, as it offers a novel approach to treating patients with cancer who do not respond to T-cell checkpoint inhibitors, as well as to patients with tumors lacking T-cell infiltration. We expect that this mechanism will be applicable in multiple indications characterized by infiltration of TAMs.

Leishmania major-derived lipophosphoglycan influences the host's early immune response by inducing platelet activation and DKK1 production via TLR1/2

Background

Platelets are rapidly deployed to infection sites and respond to pathogenic molecules via pattern recognition receptors (TLR, NLRP). Dickkopf1 (DKK1) is a quintessential Wnt antagonist produced by a variety of cell types including platelets, endothelial cells, and is known to modulate pro-inflammatory responses in infectious diseases and cancer. Moreover, DKK1 is critical for forming leukocyte-platelet aggregates and induction of type 2 cell-mediated immune responses. Our previous publication showed activated platelets release DKK1 following Leishmania major recognition.

Results

Here we probed the role of the key surface virulence glycoconjugate lipophosphoglycan (LPG), on DKK1 production using null mutants deficient in LPG synthesis (Δlpg1- and Δlpg2-). Leishmania-induced DKK1 production was reduced to control levels in the absence of LPG in both mutants and was restored upon re-expression of the cognate LPG1 or LPG2 genes. Furthermore, the formation of leukocyte-platelet aggregates was dependent on LPG. LPG mediated platelet activation and DKK1 production occurs through TLR1/2.

Conclusion

Thus, LPG is a key virulence factor that induces DKK1 production from activated platelets, and the circulating DKK1 promotes Th2 cell polarization. This suggests that LPG-activated platelets can drive innate and adaptive immune responses to Leishmania infection.

Nanotargeted Delivery of Immune Therapeutics in Type 1 Diabetes

Immune therapeutics holds great promise in the treatment of type 1 diabetes (T1D). Nonetheless, their progress is hampered by limited efficacy, equipoise, or issues of safety. To address this, a novel and specific nanodelivery platform for T1D that targets high endothelial venules (HEVs) presented in the pancreatic lymph nodes (PLNs) and pancreas is developed. Data indicate that the pancreata of nonobese diabetic (NOD) mice and patients with T1D are unique in their expression of newly formed HEVs. Anti-CD3 mAb is encapsulated in poly(lactic-co-glycolic acid)-poly(ethylene glycol) nanoparticles (NPs), the surfaces of which are conjugated with MECA79 mAb that recognizes HEVs. Targeted delivery of these NPs improves accumulation of anti-CD3 mAb in both the PLNs and pancreata of NOD mice. Treatment of hyperglycemic NOD mice with MECA79-anti-CD3-NPs results in significant reversal of T1D compared to those that are untreated, treated with empty NPs, or provided free anti-CD3. This effect is associated with a significant reduction of T effector cell populations in the PLNs and a decreased production of pro-inflammatory cytokine in the mice treated with MECA79-anti-CD3-NPs. In summary, HEV-targeted therapeutics may be used as a means by which immune therapeutics can be delivered to PLNs and pancreata to suppress autoimmune diabetes effectively.

NMR indicates the N-termini of PSGL1 and CCR7 bind competitively to the chemokine CCL21

Chemokines are from a family of secreted cytokines that direct the trafficking of immune cells to coordinate immune responses. Chemokines are involved in numerous disease states, including responding to infections, autoimmune disorders, and cancer metastasis. Ther are chemokines, like CCL21, that signal for cellular migration through the activation of G protein-coupled receptors, like CCR7, through interaction with the receptor's extracellular N-terminus, loops, and core of the receptor. CCL21 is involved in routine immune surveillance but can also attract metastasizing cancer cells to lymph nodes. P-selectin glycoprotein ligand 1 (PSGL1) has a role in cellular adhesion during chemotaxis and is a transmembrane signaling molecule. PSGL1 expression enhances chemotactic responses of T cells to CCL21. Here NMR studies indicate the binding sites on CCL21 for the N-termini or PSGL1 and CCR7 overlap, and binding of the N-termini of PSGL1 and CCR7 is competitive.

HIV Infection: Shaping the Complex, Dynamic, and Interconnected Network of the Cytoskeleton

HIV-1 has evolved a plethora of strategies to overcome the cytoskeletal barrier (i.e., actin and intermediate filaments (AFs and IFs) and microtubules (MTs)) to achieve the viral cycle. HIV-1 modifies cytoskeletal organization and dynamics by acting on associated adaptors and molecular motors to productively fuse, enter, and infect cells and then traffic to the cell surface, where virions assemble and are released to spread infection. The HIV-1 envelope (Env) initiates the cycle by binding to and signaling through its main cell surface receptors (CD4/CCR5/CXCR4) to shape the cytoskeleton for fusion pore formation, which permits viral core entry. Then, the HIV-1 capsid is transported to the nucleus associated with cytoskeleton tracks under the control of specific adaptors/molecular motors, as well as HIV-1 accessory proteins. Furthermore, HIV-1 drives the late stages of the viral cycle by regulating cytoskeleton dynamics to assure viral Pr55Gag expression and transport to the cell surface, where it assembles and buds to mature infectious virions. In this review, we therefore analyze how HIV-1 generates a cell-permissive state to infection by regulating the cytoskeleton and associated factors. Likewise, we discuss the relevance of this knowledge to understand HIV-1 infection and pathogenesis in patients and to develop therapeutic strategies to battle HIV-1.

Impact of P-selectin-PSGL-1 Axis on Platelet-Endothelium-Leukocyte Interactions in Fatal COVID-19

In critically ill patients infected with SARS-CoV-2, early leukocyte recruitment to the respiratory system was found to be orchestrated by leukocyte trafficking molecules accompanied by massive secretion of proinflammatory cytokines and hypercoagulability. Our study aimed to explore the interplay between leukocyte activation and pulmonary endothelium in different disease stages of fatal COVID-19. Our study comprised 10 COVID-19 postmortem lung specimens and 20 control lung samples (5 acute respiratory distress syndrome, 2 viral pneumonia, 3 bacterial pneumonia, and 10 normal), which were stained for antigens representing the different steps of leukocyte migration: E-selectin, P-selectin, PSGL-1, ICAM1, VCAM1, and CD11b. Image analysis software QuPath was used for quantification of positive leukocytes (PSGL-1 and CD11b) and endothelium (E-selectin, P-selectin, ICAM1, VCAM1). Expression of IL-6 and IL-1β was quantified by RT-qPCR. Expression of P-selectin and PSGL-1 was strongly increased in the COVID-19 cohort compared with all control groups (COVID-19:Controls, 17:23, P < .0001; COVID-19:Controls, 2:75, P < .0001, respectively). Importantly, P-selectin was found in endothelial cells and associated with aggregates of activated platelets adherent to the endothelial surface in COVID-19 cases. In addition, PSGL-1 staining disclosed positive perivascular leukocyte cuffs, reflecting capillaritis. Moreover, CD11b showed a strongly increased positivity in COVID-19 compared with all controls (COVID-19:Controls, 2:89; P = .0002), indicating a proinflammatory immune microenvironment. Of note, CD11b exhibited distinct staining patterns at different stages of COVID-19 disease. Only in cases with very short disease course, high levels of IL-1β and IL-6 mRNA were observed in lung tissue. The striking upregulation of PSGL-1 and P-selectin reflects the activation of this receptor-ligand pair in COVID-19, increasing the efficiency of initial leukocyte recruitment, thus promoting tissue damage and immunothrombosis. Our results show that endothelial activation and unbalanced leukocyte migration play a central role in COVID-19 involving the P-selectin-PSGL-1 axis.

Vista of the Future: Novel Immunotherapy Based on the Human V-Set Immunoregulatory Receptor for Digestive System Tumors

While gastrointestinal tumors remain a multifactorial and prevalent group of malignancies commonly treated surgically in combination with chemotherapy and radiotherapy, advancements regarding immunotherapeutic approaches continue to occur. Entering a new era of immunotherapy focused on overcoming resistance to preceding therapies caused the emergence of new therapeutic strategies. A promising solution surfaces with a V-domain Ig suppressor of T-cell activation (VISTA), a negative regulator of a T-cell function expressed in hematopoietic cells. Due to VISTA's ability to act as both a ligand and a receptor, several therapeutic approaches can be potentially developed. A broad expression of VISTA was discovered on various tumor-growth-controlling cells, which proved to increase in specific tumor microenvironment (TME) conditions, thus serving as a rationale behind the development of new VISTA-targeting. Nevertheless, VISTA's ligands and signaling pathways are still not fully understood. The uncertain results of clinical trials suggest the need for future examining inhibitor agents for VISTA and implicating a double immunotherapeutic blockade. However, more research is needed before the breakthrough can be achieved. This review discusses perspectives and novel approaches presented in the current literature. Based on the results of the ongoing studies, VISTA might be considered a potential target in combined therapy, especially for treating gastrointestinal malignancies.

T helper 1 effector memory CD4+ T cells protect the skin from poxvirus infection

Poxvirus infections of the skin are a recent emerging public health concern, yet the mechanisms that mediate protective immunity against these viral infections remain largely unknown. Here, we show that T helper 1 (Th1) memory CD4+ T cells are necessary and sufficient to provide complete and broad protection against poxvirus skin infections, whereas memory CD8+ T cells are dispensable. Core 2 O-glycan-synthesizing Th1 effector memory CD4+ T cells rapidly infiltrate the poxvirus-infected skin microenvironment and produce interferon γ (IFNγ) in an antigen-dependent manner, causing global changes in gene expression to promote anti-viral immunity. Keratinocytes express IFN-stimulated genes, upregulate both major histocompatibility complex (MHC) class I and MHC class II antigen presentation in an IFNγ-dependent manner, and require IFNγ receptor (IFNγR) signaling and MHC class II expression for memory CD4+ T cells to protect the skin from poxvirus infection. Thus, Th1 effector memory CD4+ T cells exhibit potent anti-viral activity within the skin, and keratinocytes are the key targets of IFNγ necessary for preventing poxvirus infection of the epidermis.

QiShen YiQi and its components attenuate acute thromboembolic stroke and carotid thrombosis by inhibition of CD62P/PSGL-1-mediated platelet-leukocyte aggregate formation

BACKGROUND

QiShen YiQi (QSYQ) dropping pill, a component-based Chinese medicine consisting of benefiting Qi (YQ) and activating blood (HX) components, has been reported to exert a beneficial effect on cerebral ischemia-induced stroke. However, its efficacy and pharmacological mechanism on acute thromboembolic stroke is not clear.

PURPOSE

This study is to explore the preventative effect and pharmacological mechanism of QSYQ and its YQ/HX components on the formation of platelet-leukocyte aggregation (PLA) in acute thromboembolic stroke.

STUDY DESIGN AND METHODS

In vivo thromboembolic stroke model and FeCl₃-induced carotid arterial occlusion models were used. Immunohistochemistry, Western blot, RT-qPCR, and flow cytometry experiments were performed to reveal the pharmacological mechanisms of QSYQ and its YQ/HX components.

RESULTS

In thromboembolic stroke rats, QSYQ significantly attenuated infarct area, improved neurological recovery, reduced PLA formation, and inhibited P-selection (CD62P)/ P-selectin glycoprotein ligand-1 (PSGL-1) expressions. The YQ component preferentially down-regulated PSGL-1 expression in leukocyte, while the HX component preferentially down-regulated CD62P expression in platelet. In carotid arterial thrombosis mice, QSYQ and its YQ/HX components inhibited thrombus formation, prolonged vessel occlusion time, reduced circulating leukocytes and P-selectin expression. PLA formation and platelet/leukocyte adhesion to endothelial cell were also inhibited by QSYQ and its YQ/HX components in vitro.

CONCLUSION

QSYQ and YQ/HX components attenuated thromboembolic stroke and carotid thrombosis by decreasing PLA formation via inhibiting CD62P/PSGL-1 expressions. This study shed a new light on the prevention of thromboembolic stroke.

Glycans as shapers of tumour microenvironment: A sweet driver of T-cell-mediated anti-tumour immune response

Essentially all cells are covered with a dense coat of different glycan structures/sugar chains, giving rise to the so-called glycocalyx. Changes in cellular glycosylation are a hallmark of cancer, affecting most of the pathophysiological processes associated with malignant transformation, including tumour immune responses. Glycans are chief macromolecules that define T-cell development, differentiation, fate, activation and signalling. Thus, the diversity of glycans expressed at the surface of T cells constitutes a fundamental molecular interface with the microenvironment by regulating the bilateral interactions between T-cells and cancer cells, fine-tuning the anti-tumour immune response. In this review, we will introduce the power of glycans as orchestrators of T-cell-mediated immune response in physiological conditions and in cancer. We discuss how glycans modulate the glyco-metabolic landscape in the tumour microenvironment, and whether glycans can synergize with immunotherapy as a way of rewiring T-cell effector functions against cancer cells.

Targeting SELPLG/P-selectin glycoprotein ligand 1 in preclinical ARDS: Genetic and epigenetic regulation of the SELPLG promoter

We previously identified a missense single nucleotide polymorphism rs2228315 (G>A, Met62Ile) in the selectin-P-ligand gene (SELPLG), encoding P-selectin glycoprotein ligand 1 (PSGL-1), to be associated with increased susceptibility to acute respiratory distress syndrome (ARDS). These earlier studies demonstrated that SELPLG lung tissue expression was increased in mice exposed to lipopolysaccharide (LPS)- and ventilator-induced lung injury (VILI) suggesting that inflammatory and epigenetic factors regulate SELPLG promoter activity and transcription. In this report, we used a novel recombinant tandem PSGL1 immunoglobulin fusion molecule (TSGL-Ig), a competitive inhibitor of PSGL1/P-selectin interactions, to demonstrate significant TSGL-Ig-mediated decreases in SELPLG lung tissue expression as well as highly significant protection from LPS- and VILI-induced lung injury. In vitro studies examined the effects of key ARDS stimuli (LPS, 18% cyclic stretch to simulate VILI) on SELPLG promoter activity and showed LPS-mediated increases in SELPLG promoter activity and identified putative promoter regions associated with increased SELPLG expression. SELPLG promoter activity was strongly regulated by the key hypoxia-inducible transcription factors, HIF-1α, and HIF-2α as well as NRF2. Finally, the transcriptional regulation of SELPLG promoter by ARDS stimuli and the effect of DNA methylation on SELPLG expression in endothelial cell was confirmed. These findings indicate SELPLG transcriptional regulation by clinically-relevant inflammatory factors with the significant TSGL-Ig-mediated attenuation of LPS and VILI highly consistent with PSGL1/P-selectin as therapeutic targets in ARDS.

Revealing the human mucinome

Mucin domains are densely O-glycosylated modular protein domains found in various extracellular and transmembrane proteins. Mucin-domain glycoproteins play important roles in many human diseases, such as cancer and cystic fibrosis, but the scope of the mucinome remains poorly defined. Recently, we characterized a bacterial O-glycoprotease, StcE, and demonstrated that an inactive point mutant retains binding selectivity for mucin-domain glycoproteins. In this work, we leverage inactive StcE to selectively enrich and identify mucin-domain glycoproteins from complex samples like cell lysate and crude ovarian cancer patient ascites fluid. Our enrichment strategy is further aided by an algorithm to assign confidence to mucin-domain glycoprotein identifications. This mucinomics platform facilitates detection of hundreds of glycopeptides from mucin domains and highly overlapping populations of mucin-domain glycoproteins from ovarian cancer patients. Ultimately, we demonstrate our mucinomics approach can reveal key molecular signatures of cancer from in vitro and ex vivo sources.

Mucin-domain glycoproteins are densely O-glycosylated proteins with unique secondary structure that imparts a large influence on cellular environments. Here, the authors develop a technique to selectively enrich and characterize mucin-domain glycoproteins from cell lysate and patient biofluids.

PD-1 Immune Checkpoint Blockade and PSGL-1 Inhibition Synergize to Reinvigorate Exhausted T Cells

Chronic viral infections where the antigen persists long-term, induces an exhaustion phenotype in responding T cells. It is now evident that immune checkpoints on T cells including PD-1, CTLA-4, and PSGL-1 (Selplg) are linked with the differentiation of exhausted cells. Chronic T cell receptor signaling induces transcriptional signatures that result in the development of various exhausted T cell subsets, including the stem-like T cell precursor exhausted (Tpex) cells, which can be reinvigorated by immune checkpoint inhibitors (ICIs). While PSGL-1 has been shown to inhibit T cell responses in various disease models, the cell-intrinsic function of PSGL-1 in the differentiation, maintenance, and reinvigoration of exhausted T cells is unknown. We found Selplg-/- T cells had increased expansion in melanoma tumors and in early stages of chronic viral infection. Despite their increase, both WT and Selplg-/- T cells eventually became phenotypically and functionally exhausted. Even though virus-specific Selplg-/- CD4+ and CD8+ T cells were increased at the peak of T cell expansion, they decreased to lower levels than WT T cells at later stages of chronic infection. We found that Selplg-/- CD8+ Tpex (SLAMF6hiTIM3lo, PD-1+TIM3+, TOX+, TCF-1+) cell frequencies and numbers were decreased compared to WT T cells. Importantly, even though virus-specific Selplg-/- CD4+ and CD8+ T cells were lower, they were reinvigorated more effectively than WT T cells after anti-PD-L1 treatment. We found increased SELPLG expression in Hepatitis C-specific CD8+ T cells in patients with chronic infection, whereas these levels were decreased in patients that resolved the infection. Together, our findings showed multiple PSGL-1 regulatory functions in exhausted T cells. We found that PSGL-1 is a cell-intrinsic inhibitor that limits T cells in tumors and in persistently infected hosts. Additionally, while PSGL-1 is linked with T cell exhaustion, its expression was required for their long-term maintenance and optimal differentiation into Tpex cells. Finally, PSGL-1 restrained the reinvigoration potential of exhausted CD4+ and CD8+ T cells during ICI therapy. Our findings highlight that targeting PSGL-1 may have therapeutic potential alone or in combination with other ICIs to reinvigorate exhausted T cells in patients with chronic infections or cancer.

Defining the Versican Interactome in Lung Health and Disease

The extracellular matrix (ECM) imparts critical mechanical and biochemical information to cells in the lungs. Proteoglycans are essential constituents of the ECM and play a crucial role in controlling numerous biological processes, including regulating cellular phenotype and function. Versican, a chondroitin sulfate proteoglycan required for embryonic development, is almost absent from mature, healthy lungs and is re-expressed and accumulates in acute and chronic lung disease. Studies using genetically engineered mice show that the versican-enriched matrix can be pro- or anti-inflammatory depending on the cellular source or disease process studied. The mechanisms whereby versican develops a contextual ECM remain largely unknown. The primary goal of this review is to provide an overview of the interaction of versican with its many binding partners, the "versican interactome," and how through these interactions, versican is an integrator of complex extracellular information. Hopefully, the information provided in this review will be used to develop future studies to determine how versican and its binding partners can develop contextual ECMs that control select biological processes. While this review focuses on versican and the lungs, what is described can be extended to other proteoglycans, tissues, and organs.

Single-cell atlas of peripheral blood mononuclear cells from pregnant women

Background

During pregnancy, mother–child interactions trigger a variety of subtle changes in the maternal body, which may be reflected in the status of peripheral blood mononuclear cells (PBMCs). Although these cells are easy to access and monitor, a PBMC atlas for pregnant women has not yet been constructed.

Methods

We applied single‐cell RNA sequencing (scRNA‐seq) to profile 198,356 PBMCs derived from 136 pregnant women (gestation weeks 6 to 40) and a control cohort. We also used scRNA‐seq data to establish a transcriptomic clock and thereby predicted the gestational age of normal pregnancy.

Results

We identified reconfiguration of the peripheral immune cell phenotype during pregnancy, including interferon‐stimulated gene upregulation, activation of RNA splicing‐related pathways and immune activity of cell subpopulations. We also developed a cell‐type‐specific model to predict gestational age of normal pregnancy.

Conclusions

We constructed a single‐cell atlas of PBMCs in pregnant women spanning the entire gestation period, which should help improve our understanding of PBMC composition turnover in pregnant women.

GALNT4 primes monocytes adhesion and transmigration by regulating O-Glycosylation of PSGL-1 in atherosclerosis

Atherosclerosis is a major underlying cause of cardiovascular disease. Genome wide association studies have predicted that GalNAc-T4 (GALNT4), which responsible for initiating step of mucin-type O-glycosylation, plays a causal role in the susceptibility to cardiovascular diseases, whereas the precise mechanism remains obscure. Thus, we sought to determine the role and mechanism of GALNT4 in atherosclerosis. Firstly, we found the expression of GALNT4 and protein O-glycosylation were both increased in plaque as atherosclerosis progressed in ApoE-/- mice by immunohistochemistry. And the expression of GALNT4 was also increased in human monocytes treated with ACS (acute coronary syndrome) sera and subjected to LPS and ox-LDL in vitro. Moreover, silencing expression of GALNT4 by shRNA lentivirus alleviated atherosclerotic plaque formation and monocyte/macrophage infiltration in ApoE-/- mice. Functional investigations demonstrate that GALNT4 knockdown inhibited P-selectin-induced activation of β2 integrin on the surface of monocytes, decreased monocytes adhesion under flow condition with P-selectin stimulation, as well as suppressed monocytes transmigration triggered by monocyte chemotactic protein- 1(MCP-1). In contrast, GALNT4 overexpression enhanced monocytes adhesion and transmigration. Furthermore, Vicia Villosa Lectin (VVL) pull down and PSGL-1 immunoprecipitation assays showed that GALNT4 overexpression increased O-Glycosylation of PSGL-1 and P-selectin induce phosphorylation of Akt/mTOR and IκBα/NFκB on monocytes. Conversely, knockdown of GALNT4 decreased VVL binding and attenuated the activation of Akt/mTOR and IκBα/NFκB. Additionally, mTOR inhibitor rapamycin blocked these effects of GALNT4 overexpression on monocytes. Collectively, GALNT4 catalyzed PSGL-1 O-glycosylation that involved in P-selectin induced monocytes adhesion and transmigration via Akt/mTOR and NFκB pathway. Thus, GALNT4 may be a potential therapeutic target for atherosclerosis.

Mechanism of Mucin Recognition by Lectins: A Thermodynamic Study

Isothermal titration microcalorimetry (ITC) can directly determine the thermodynamic binding parameters of biological molecules including affinity constant, binding stoichiometry, heat of binding (enthalpy) and indirectly the entropy, and free energy of binding. ITC has been extensively used to study the binding of lectins to mono- and oligosaccharides, but limitedly in applications to lectin-glycoprotein interactions. Inherent experimental challenges to ITC include sample precipitation during the experiment and relative high amount of sample required, but careful design of experiments can minimize these problems and allow valuable information to be obtained. For example, the thermodynamics of binding of lectins to multivalent globular and linear glycoproteins (mucins) have been described. The results are consistent with a dynamic binding mechanism in which lectins bind and jump from carbohydrate to carbohydrate epitope in these molecules leading to increased affinity. Importantly, the mechanism of binding of lectins to mucins appears similar to that for a variety of protein ligands binding to DNA. Recent results also show that high-affinity lectin-mucin cross-linking interactions are driven by favorable entropy of binding that is associated with the bind and jump mechanism. The results suggest that the binding of ligands to biopolymers, in general, may involve a common mechanism that involves enhanced entropic effects that facilitate binding interactions.

The &#946;-Secretase Enzyme BACE1: A Biochemical Enigma for Alzheimer's Disease

Beta site amyloid precursor protein cleaving enzyme 1 (BACE1) is a rational target in Alzheimer's Disease (AD) drug development due to its role in amyloidogenic cleavage of Amyloid Precursor Protein (APP) in generating Amyloid β (Aβ). This β-secretase cleaves not only Amyloid Precursor Protein (APP) and its homologues, but also small series of substrates including neuregulin and β subunit of voltage-gated sodium channel that play a very important role in the development and normal function of the brain. Moreover, BACE1 is modulated at the post-translational level by several factors that are associated with both physiological and pathological functions. Since the discovery of BACE1 over a decade ago, medicinal chemistry and pharmacokinetics of BACE1 small molecule inhibitors have proven challenging for the treatment of Alzheimer's disease.

P-Selectin Glycoprotein Ligand 1: A Potential HIV-1 Therapeutic Target

Antiretroviral therapy (ART), which is a life-long therapeutic option, remains the only currently effective clinical method to treat HIV-1 infection. However, ART may be toxic to vital organs including the liver, brain, heart, and kidneys, and may result in systemic complications. In this context, to consider HIV-1 restriction factors from the innate immune system to explore novel HIV therapeutics is likely to be a promising investigative strategy. In light of this, P-selectin glycoprotein ligand 1 (PSGL-1) has recently become the object of close scrutiny as a recognized cell adhesion molecule, and has become a major focus of academic study, as researchers believe that PSGL-1 may represent a novel area of interest in the research inquiry into the field of immune checkpoint inhibition. In this article, we review PSGL-1's structure and functions during infection and/or inflammation. We also outline a comprehensive review of its role and potential therapeutic utility during HIV-1 infection as published in contemporary academic literature.

PSGL-1 Is a T Cell Intrinsic Inhibitor That Regulates Effector and Memory Differentiation and Responses During Viral Infection

Effective T cell differentiation during acute virus infections leads to the generation of effector T cells that mediate viral clearance, as well as memory T cells that confer protection against subsequent reinfection. While inhibitory immune checkpoints have been shown to promote T cell dysfunction during chronic virus infections and in tumors, their roles in fine tuning the differentiation and responses of effector and memory T cells are only just beginning to be appreciated. We previously identified PSGL-1 as a fundamental regulator of T cell exhaustion that sustains expression of several inhibitory receptors, including PD-1. We now show that PSGL-1 can restrict the magnitude of effector T cell responses and memory T cell development to acute LCMV virus infection by limiting survival, sustaining PD-1 expression, and reducing effector responses. After infection, PSGL-1-deficient effector T cells accumulated to a greater extent than wild type T cells, and preferentially generated memory precursor cells that displayed enhanced accumulation and functional capacity in response to TCR stimulation as persisting memory cells. Although, PSGL-1-deficient memory cells did not exhibit inherent greater sensitivity to cell death, they failed to respond to a homologous virus challenge after adoptive transfer into naïve hosts indicating an impaired capacity to generate memory effector T cell responses in the context of viral infection. These studies underscore the function of PSGL-1 as a key negative regulator of effector and memory T cell differentiation and suggest that PSGL-1 may limit excessive stimulation of memory T cells during acute viral infection.

Constitutive PSGL-1 Correlates with CD30 and TCR Pathways and Represents a Potential Target for Immunotherapy in Anaplastic Large T-Cell Lymphoma

Simple Summary

P-selectin glycoprotein ligand-1 (PSGL-1), coded by the SELPLG gene, is the major ligand of selectins and plays a pivotal role in tethering, rolling and extravasation of immune cells. PSGL-1 involvement in core molecular programs, such as SYK, PLCγ2, PI3Kγ or MAPK pathways, suggests additional functions beyond the modulation of cell trafficking. Recently, several studies identified a novel mechanism responsible for PSGL-1-mediated immune suppression in the tumor microenvironment and proved a novel concept of PSGL-1 as a critical checkpoint molecule for tumor immunotherapy. The immunotherapeutic approach has gained an ever-growing interest in the treatment of several hematological malignancies, and in particular, novel targets for immunotherapy are still highly sought-after in T-cell lymphomas. Based on our results obtained through gene expression profiling and immunohistochemical analysis, PSGL-1, already suggested as a potential target in multiple myeloma humoral immunotherapy, could be considered noteworthy among the candidates.

Abstract

Due to the high expression of P-selectin glycoprotein ligand-1 (PSGL-1) in lymphoproliferative disorders and in multiple myeloma, it has been considered as a potential target for humoral immunotherapy, as well as an immune checkpoint inhibitor in T-cells. By investigating the expression of SELPLG in 678 T- and B-cell samples by gene expression profiling (GEP), further supported by tissue microarray and immunohistochemical analysis, we identified anaplastic large T-cell lymphoma (ALCL) as constitutively expressing SELPLG at high levels. Moreover, GEP analysis in CD30+ ALCLs highlighted a positive correlation of SELPLG with TNFRSF8 (CD30-coding gene) and T-cell receptor (TCR)-signaling genes (LCK, LAT, SYK and JUN), suggesting that the common dysregulation of TCR expression in ALCLs may be bypassed by the involvement of PSGL-1 in T-cell activation and survival. Finally, we evaluated the effects elicited by in vitro treatment with two anti-PSGL-1 antibodies (KPL-1 and TB5) on the activation of the complement system and induction of apoptosis in human ALCL cell lines. In conclusion, our data demonstrated that PSGL-1 is specifically enriched in ALCLs, altering cell motility and viability due to its involvement in CD30 and TCR signaling, and it might be considered as a promising candidate for novel immunotherapeutic approaches in ALCLs.

P-selectin glycoprotein ligand 1 promotes T cell lymphoma development and dissemination

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PSGL-1 protein is frequently expressed at the surface of malignant T cells.

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Enforced expression of PSGL-1 promotes T cell tumorigenesis in mice.

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PSGL-1 expression accelerates malignant T cell dissemination from tumors to several organs.

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PSGL-1 expression promotes malignant T cell expansion in kidneys and lungs.

P-selectin glycoprotein ligand-1 (PSGL-1) is a membrane-bound glycoprotein expressed in lymphoid and myeloid cells. It is a ligand of P-, E- and L-selectin and is involved in T cell trafficking and homing to lymphoid tissues, among other functions. PSGL-1 expression has been implicated in different lymphoid malignancies, so here we aimed to evaluate the involvement of PSGL-1 in T cell lymphomagenesis and dissemination. PSGL-1 was highly expressed at the surface of human and mouse T cell leukemia and lymphoma cell lines. To assess its impact on T cell malignancies, we stably expressed human PSGL-1 (hPSGL-1) in a mouse thymic lymphoma cell line, which expresses low levels of endogenous PSGL-1 at the cell surface. hPSGL-1-expressing lymphoma cells developed subcutaneous tumors in athymic nude mice recipients faster than control empty vector or parental cells. Moreover, the kidneys, lungs and liver of tumor-bearing mice were infiltrated by hPSGL-1-expressing malignant T cells. To evaluate the role of PSGL-1 in lymphoma cell dissemination, we injected intravenously control and hPSGL-1-expressing lymphoma cells in athymic mice. Strikingly, PSGL-1 expression facilitated disease infiltration of the kidneys, as determined by histological analysis and anti-CD3 immunohistochemistry. Together, these results indicate that PSGL-1 expression promotes T cell lymphoma development and dissemination to different organs.

The sialyltransferase ST3Gal-IV guides murine T-cell progenitors to the thymus

T lymphocytes are important players in beneficial and detrimental immune responses. In contrast to other lymphocyte populations that develop in the bone marrow, T-cell precursors need to migrate to the thymus for further development. The interaction of P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) is crucial for thymic entry of T-cell precursors during settings of T-cell lineage reconstitution. PSGL-1 has to be sialylated to function as a ligand for P-selectin, and the sialyltransferase ST3Gal-IV might play a critical role in this process. We therefore investigated the role of ST3Gal-IV for T-cell development using competitive mixed bone marrow chimeric mice. We found that ST3Gal-IV is dispensable for homing and engraftment of hematopoietic precursors in the bone marrow. However, ST3Gal-IV deficiency affects seeding of the thymus by early T-cell progenitors, leading to impaired restoration of the peripheral T-cell compartment. This defect could be restored by ectopic retroviral expression of ST3Gal-IV in hematopoietic stem cells derived from ST3Gal-IV-deficient donor mice. Our findings show that ST3Gal-IV plays a critical and nonredundant role for efficient T-cell lineage reconstitution after bone marrow transplantation.

Coagulation and Fibrinolysis in Obstructive Sleep Apnoea

Obstructive sleep apnoea (OSA) is a common disease which is characterised by repetitive collapse of the upper airways during sleep resulting in chronic intermittent hypoxaemia and frequent microarousals, consequently leading to sympathetic overflow, enhanced oxidative stress, systemic inflammation, and metabolic disturbances. OSA is associated with increased risk for cardiovascular morbidity and mortality, and accelerated coagulation, platelet activation, and impaired fibrinolysis serve the link between OSA and cardiovascular disease. In this article we briefly describe physiological coagulation and fibrinolysis focusing on processes which could be altered in OSA. Then, we discuss how OSA-associated disturbances, such as hypoxaemia, sympathetic system activation, and systemic inflammation, affect these processes. Finally, we critically review the literature on OSA-related changes in markers of coagulation and fibrinolysis, discuss potential reasons for discrepancies, and comment on the clinical implications and future research needs.

P-Selectin Glycoprotein Ligand-1 Deficiency Protects Against Aortic Aneurysm Formation Induced by DOCA Plus Salt

PURPOSE

P-selectin glycoprotein ligand-1 (PSGL-1) acts as a crucial regulator for the inflammatory cells infiltration by mediating the adhesion of leukocytes. However, the role of PSGL-1 in aortic aneurysm remains elusive. Here, we investigated the role of PSGL-1 in aortic aneurysm (AA) development.

METHODS

We first detected PSGL-1 expression in samples from aortic aneurysm patients and mouse AA models via western blotting, immunofluorescence, and flow cytometry, and then we used global PSGL-1 knockout mice and their wild type controls to establish an aortic aneurysm model induced by deoxycorticosterone acetate (DOCA) plus high salt (HS). The incidence, fatality rates, and the pathological changes of aortic aneurysm were analyzed in each group. The inflammation, adhesion molecules expression, and PSGL-1 mediated leukocyte-endothelial adhesion and their underlying mechanisms were explored further.

RESULTS

Increased PSGL-1 levels were observed in human and mouse aortic aneurysm, and on leukocytes of mice treated with DOCA+HS. PSGL-1 deficiency reduced the incidence and severity of aortic aneurysm significantly, as well as decreased elastin fragmentation, collagen accumulation, and smooth muscle cells degeneration. Mechanistically, the protective effect of PSGL-1 inhibition was mediated by the reduced adhesion molecules, and the subsequently reduced leukocyte-endothelial adhesion through the NF-κB pathway, which finally led to reduced inflammatory cells infiltration and decreased inflammatory factors expression.

CONCLUSION

PSGL-1 deficiency is protective against inflammatory cells migration and recruitment in the condition of AA through attenuation of leukocyte-endothelial adhesion. Inhibition of PSGL-1 may be a potential therapeutic target for the prevention and treatment of human AA.

PSGL-1 inhibits HIV-1 infection by restricting actin dynamics and sequestering HIV envelope proteins

PSGL-1 has recently been identified as an HIV restriction factor that inhibits HIV DNA synthesis and more potently, virion infectivity. But the underlying mechanisms of these inhibitions are unknown. Here we show that PSGL-1 directly binds to cellular actin filaments (F-actin) to restrict actin dynamics, which leads to inhibition of HIV DNA synthesis. PSGL-1 is incorporated into nascent virions and restricts actin dynamics in the virions, which partially accounts for the inhibition of virion infectivity. More potently, PSGL-1 inhibits incorporation of Env proteins into nascent virions, causing a loss of envelope spikes on the virions as shown by Cryo-electron microscopy and super-resolution imaging. This loss is associated with a profound defect in viral entry. Mechanistically, PSGL-1 binds gp41 and sequesters gp41 at the plasma membrane, explaining the inhibition of Env incorporation in nascent virions. PSGL-1’s dual anti-HIV mechanisms represent novel strategies of human cells to defend against HIV infection.

Acute Myeloid Leukemia Chemo-Resistance Is Mediated by E-selectin Receptor CD162 in Bone Marrow Niches

The interactions of leukemia cells with the bone marrow (BM) microenvironment is critical for disease progression and resistance to treatment. We have recently found that the vascular adhesion molecule E-(endothelial)-selectin is a key niche component that directly mediates acute myeloid leukemia (AML) chemo-resistance, revealing E-selectin as a promising therapeutic target. To understand how E-selectin promotes AML survival, we investigated the potential receptors on AML cells involved in E-selectin-mediated chemo-resistance. Using CRISPR-Cas9 gene editing to selectively suppress canonical E-selectin receptors CD44 or P-selectin glycoprotein ligand-1 (PSGL-1/CD162) from human AML cell line KG1a, we show that CD162, but not CD44, is necessary for E-selectin-mediated chemo-resistance in vitro. Using preclinical models of murine AML, we then demonstrate that absence of CD162 on AML cell surface leads to a significant delay in the onset of leukemia and a significant increase in sensitivity to chemotherapy in vivo associated with a more rapid in vivo proliferation compared to wild-type AML and a lower BM retention. Together, these data reveal for the first time that CD162 is a key AML cell surface receptor involved in AML progression, BM retention and chemo-resistance. These findings highlight specific blockade of AML cell surface CD162 as a potential novel niche-based strategy to improve the efficacy of AML therapy.

A sticky wicket: Defining molecular functions for CD34 in hematopoietic cells

The CD34 cell surface antigen is widely expressed in tissues on cells with progenitor-like properties and on mature vascular endothelia. In adult human bone marrow, CD34 marks hematopoietic stem and progenitor cells (HSPCs) starting from the bulk of hematopoietic stem cells with long-term repopulating potential (LT-HSCs) throughout expansion and differentiation of oligopotent and unipotent progenitors. CD34 protein surface expression is typically lost as cells mature into terminal effectors. Because of this expression pattern of HSPCs, CD34 has had a central role in the evaluation or selection of donor graft tissue in HSC transplant (HSCT). Given its clinical importance, it is surprising that the biological functions of CD34 are still poorly understood. This enigma is due, in part, to CD34's context-specific role as both a pro-adhesive and anti-adhesive molecule and its potential functional redundancy with other sialomucins. Moreover, there are also critical differences in the regulation of CD34 expression on HSPCs in humans and experimental mice. In this review, we highlight some of the more well-defined functions of CD34 in HSPCs with a focus on proposed functions most relevant to HSCT biology.

LPS-induced Lung Platelet Recruitment Occurs Independently from Neutrophils, PSGL-1, and P-Selectin

Platelets are recruited to inflammatory foci and contribute to host defense and inflammatory responses. Compared with platelet recruitment in hemostasis and thrombosis, the mechanisms of platelet recruitment in inflammation and host defense are poorly understood. Neutrophil recruitment to lung airspaces after inhalation of bacterial LPS requires platelets and PSGL-1 in mice. Given this association between platelets and neutrophils, we investigated whether recruitment of platelets to lungs of mice after LPS inhalation was dependent on PSGL-1, P-selectin, or interaction with neutrophils. BALB/c mice were administered intranasal LPS (O55:B5, 5 mg/kg) and, 48 hours later, lungs were collected and platelets and neutrophils quantified in tissue sections by immunohistochemistry. The effects of functional blocking antibody treatments targeting the platelet-neutrophil adhesion molecules, P-selectin or PSGL-1, or treatment with a neutrophil-depleting antibody targeting Ly6G, were tested on the extent of LPS-induced lung platelet recruitment. Separately in Pf4-Cre × mTmG mice, two-photon intravital microscopy was used to image platelet adhesion in live lungs. Inhalation of LPS caused both platelet and neutrophil recruitment to the lung vasculature. However, decreasing lung neutrophil recruitment by blocking PSGL-1, P-selectin, or depleting blood neutrophils had no effect on lung platelet recruitment. Lung intravital imaging revealed increased adhesion of platelets in the lung microvasculature which was not associated with thrombus formation. In conclusion, platelet recruitment to lungs in response to LPS occurs through mechanisms distinct from those mediating neutrophil recruitment, or the occurrence of pulmonary emboli.

Platelet P-selectin initiates cross-presentation and dendritic cell differentiation in blood monocytes

Dendritic cells (DCs) are adept at cross-presentation and initiation of antigen-specific immunity. Clinically, however, DCs produced by in vitro differentiation of monocytes in the presence of exogenous cytokines have been met with limited success. We hypothesized that DCs produced in a physiological manner may be more effective and found that platelets activate a cross-presentation program in peripheral blood monocytes with rapid (18 hours) maturation into physiological DCs (phDCs). Differentiation of monocytes into phDCs was concomitant with the formation of an "adhesion synapse," a biophysical junction enriched with platelet P-selectin and monocyte P-selectin glycoprotein ligand 1, followed by intracellular calcium fluxing and nuclear localization of nuclear factor κB. phDCs were more efficient than cytokine-derived DCs in generating tumor-specific T cell immunity. Our findings demonstrate that platelets mediate a cytokine-independent, physiologic maturation of DC and suggest a novel strategy for DC-based immunotherapies.

VISTA: Coming of age as a multi-lineage immune checkpoint

The immune response is governed by a highly complex set of interactions among cells and mediators. T cells may be rendered dysfunctional by the presence of high levels of antigen in the absence of co-stimulation while myeloid cells may be programmed towards an immunosuppressive state that promotes cancer growth and metastasis while deterring tumor immunity. In addition, inhibitory programs driven by immune checkpoint regulators dampen anti-tumor immunity. The ideal cancer immunotherapy treatment will improve both cross-priming in the tumor microenvironment and relieve suppression by the inhibitory checkpoints. Recently, blockade of programmed cell death 1 (PD-1) and cytotoxic T lymphocyte antigen 4 (CTLA-4) has elicited impressive results, but not in all patients, so additional targets are under investigation. V-set immunoglobulin domain suppressor of T cell activation (VISTA) is a novel immunoregulatory receptor that is broadly expressed on cells of the myeloid and lymphoid lineages, and is frequently implicated as a poor prognostic indicator in multiple cancers. Importantly, antibody targeting of VISTA uniquely engages both innate and adaptive immunity. This, combined with the expression of VISTA and its non-redundant activities compared to other immune checkpoint regulators, qualifies VISTA to be a promising target for improving cancer immunotherapy.

Circulating P-Selectin Glycoprotein Ligand 1 and P-Selectin Levels in Obstructive Sleep Apnea Patients

Purpose

Obstructive sleep apnea (OSA) is characterized by chronic intermittent hypoxia which induces inflammation in blood vessels leading to the development of cardiovascular comorbidities. Several studies implicated the role of P-selectin in vascular inflammation of OSA. P-selectin glycoprotein ligand 1 (PSGL-1) is the main activator for P-selectin and is involved in immune cell trafficking. However, PSGL-1 has not been analyzed in OSA. The aim of the study was to investigate plasma PSGL-1 and P-selectin levels to have a deeper understanding on their interaction in obstructive sleep apnea.

Methods

Fifty-one untreated patients with OSA and 42 non-OSA controls were recruited. Plasma PSGL-1 levels were determined in evening and morning samples, P-selectin levels were analyzed in morning samples using commercially available ELISA kits. Polysomnography was performed in all participants. OSA was defined by an apnea–hypopnea index ≥ 5/h.

Results

PSGL-1 levels did not differ between controls and OSA patients either in the evening or in the morning. Although, there was no difference between controls (16.9/6.8–40.8 ng/ml) and patients with OSA (19.6/8.4–56.8, p = 0.24), patients with severe OSA had increased plasma P-selectin levels (25.6/8.4–56.8 ng/ml) compared to mild OSA patients (14.1/8.5–35.3 ng/ml, p = 0.006) and controls (p = 0.03).

Conclusions

P-selectin expression relates to disease severity suggesting a pathophysiological role in endothelial cell activation. PSGL-1 levels are unaltered in OSA, suggesting an alternative activation pathway for P-selectin in OSA.

VISTA is an acidic pH-selective ligand for PSGL-1

Co-inhibitory immune receptors can contribute to T cell dysfunction in patients with cancer^1,2. Blocking antibodies against cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death 1 (PD-1) partially reverse this effect and are becoming standard of care in an increasing number of malignancies³. However, many of the other axes by which tumours become inhospitable to T cells are not fully understood. Here we report that V-domain immunoglobulin suppressor of T cell activation (VISTA) engages and suppresses T cells selectively at acidic pH such as that found in tumour microenvironments. Multiple histidine residues along the rim of the VISTA extracellular domain mediate binding to the adhesion and co-inhibitory receptor P-selectin glycoprotein ligand-1 (PSGL-1). Antibodies engineered to selectively bind and block this interaction in acidic environments were sufficient to reverse VISTA-mediated immune suppression in vivo. These findings identify a mechanism by which VISTA may engender resistance to anti-tumour immune responses, as well as an unexpectedly determinative role for pH in immune co-receptor engagement.

"Stuck on sugars - how carbohydrates regulate cell adhesion, recognition, and signaling"

We have explored the fundamental biological processes by which complex carbohydrates expressed on cellular glycoproteins and glycolipids and in secretions of cells promote cell adhesion and signaling. We have also explored processes by which animal pathogens, such as viruses, bacteria, and parasites adhere to glycans of animal cells and initiate disease. Glycans important in cell signaling and adhesion, such as key O-glycans, are essential for proper animal development and cellular differentiation, but they are also involved in many pathogenic processes, including inflammation, tumorigenesis and metastasis, and microbial and parasitic pathogenesis. The overall hypothesis guiding these studies is that glycoconjugates are recognized and bound by a growing class of proteins called glycan-binding proteins (GBPs or lectins) expressed by all types of cells. There is an incredible variety and diversity of GBPs in animal cells involved in binding N- and O-glycans, glycosphingolipids, and proteoglycan/glycosaminoglycans. We have specifically studied such molecular determinants recognized by selectins, galectins, and many other C-type lectins, involved in leukocyte recruitment to sites of inflammation in human tissues, lymphocyte trafficking, adhesion of human viruses to human cells, structure and immunogenicity of glycoproteins on the surfaces of human parasites. We have also explored the molecular basis of glycoconjugate biosynthesis by exploring the enzymes and molecular chaperones required for correct protein glycosylation. From these studies opportunities for translational biology have arisen, involving production of function-blocking antibodies, anti-glycan specific antibodies, and synthetic glycoconjugates, e.g. glycosulfopeptides, that specifically are recognized by GBPs. This invited short review is based in part on my presentation for the IGO Award 2019 given by the International Glycoconjugate Organization in Milan.

L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLe^x) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Novel Technologies for Quantitative O-Glycomics and Amplification/Preparation of Cellular O-Glycans

Mucin-type O-glycosylation (O-glycans, O-glycome) characterized by GalNAc linked to Serine/Threonine or even tyrosine residues in proteins is one of the major types of glycosylations. In animals, O-glycans on glycoproteins participate in many critical biological processes such as cell adhesion, development, and immunity. Importantly, the O-glycome is different in a tissue/cell-specific manner, and often altered in cells at their pathological states; and this alteration, in turn, affects cellular properties and functions. Clearly, the Functional O-glycomics, which concerns biological roles of O-glycans, requires a comprehensive understanding of O-glycome. Structural and/or quantitative analysis of O-glycans, however, is an unmet demand because no enzyme can universally release O-glycans from glycoproteins. Furthermore, the preparation of complex O-glycans for biological studies is even more challenging. To meet these demands, we have developed a novel technology termed Cellular O-glycome Reporter/Amplification (CORA) for profiling cellular O-glycan structures and amplifying/preparing complex O-glycans from cultured cells. In this chapter, we describe the recent advances of CORA: quantitative-CORA (qCORA) and preparative-CORA (pCORA). qCORA takes the strategy of “metabolic stable isotopic labeling O-glycome of culture cells (SILOC),” and pCORA adapts cells to “O-glycan factories” when supplied with R-α-GalNAc(Ac)3 derivatives. qCORA and pCORA technologies can facilitate the cellular O-glycomics and functional O-glycomics studies.

Harnessing the lymph node microenvironment

PURPOSE OF REVIEW

To evaluate role of the lymph node in immune regulation and tolerance in transplantation and recent advances in the delivery of antigen and immune modulatory signals to the lymph node.

RECENT FINDINGS

Lymph nodes are a primary site of immune cell priming, activation, and modulation, and changes within the lymph node microenvironment have the potential to induce specific regulation, suppression, and potentially tolerance. Antigen enters the lymph node either from tissues via lymphatics, from blood via high endothelial venules, or directly via injection. Here we review different techniques and materials to deliver antigen to the lymph node including microparticles or nanoparticles, ex-vivo antigen presenting cell manipulation, and use of receptor conjugation for specific intralymph node targeting locations.

SUMMARY

The promising results point to powerful techniques to harness the lymph node microenvironment and direct systemic immune regulation. The materials, techniques, and approaches suggest that translational and clinical trials in nonhuman primate and patients may soon be possible.

Chronic Inflammation in Response to Injury: Retention of Myeloid Cells in Injured Tissue Is Driven by Myeloid Cell Intrinsic Factors

Chronic inflammation is a hallmark of impaired healing in a plethora of tissues, including skin, and is associated with aging and diseases such as diabetes. Diabetic chronic skin wounds are characterized by excessive myeloid cells that display an aberrant phenotype, partially mediated by stable intrinsic changes induced during hematopoietic development. However, the relative contribution of myeloid cell-intrinsic factors to chronic inflammation versus aberrant signals from the local environmental was unknown. Moreover, identification of myeloid cell intrinsic factors that contribute to chronic inflammation in diabetic wounds remained elusive. Here we show that Gr-1⁺CD11b⁺ myeloid cells are retained specifically within the presumptive granulation tissue region of the wound at a higher density in diabetic mice and associate with endothelial cells at the site of injury with a higher frequency than in nondiabetic mice. Adoptive transfer of myeloid cells demonstrated that aberrant wound retention is due to myeloid cell intrinsic factors and not the local environment. RNA sequencing of bone marrow and wound-derived myeloid cells identified Selplg as a myeloid cell intrinsic factor that is deregulated in chronic wounds. In vivo blockade of this protein significantly accelerated wound healing in diabetic mice and may be a potential therapeutic target in chronic wounds and other chronic inflammatory diseases.

Characterization of the zinc metalloprotease of Streptococcus suis serotype 2

Streptococcus suis is a swine pathogen and zoonotic agent responsible for meningitis and septic shock. Although several putative virulence factors have been described, the initial steps of the S. suis pathogenesis remain poorly understood. While controversial results have been reported for a S. suis serotype 2 zinc metalloprotease (Zmp) regarding its IgA protease activity, recent phylogenetic analyses suggested that this protein is homologous to the ZmpC of Streptococcus pneumoniae, which is not an IgA protease. Based on the previously described functions of metalloproteases (including IgA protease and ZmpC), different experiments were carried out to study the activities of that of S. suis serotype 2. First, results showed that S. suis, as well as the recombinant Zmp, were unable to cleave human IgA₁, confirming lack of IgA protease activity. Similarly, S. suis was unable to cleave P-selectin glycoprotein ligand-1 and to activate matrix metalloprotease 9, at least under the conditions tested. However, S. suis was able to partially cleave mucin 16 and syndecan-1 ectodomains. Experiments carried out with an isogenic Δzmp mutant showed that the Zmp protein was partially involved in such activities. The absence of a functional Zmp protein did not affect the ability of S. suis to adhere to porcine bronchial epithelial cells in vitro, or to colonize the upper respiratory tract of pigs in vivo. Taken together, our results show that S. suis serotype 2 Zmp is not a critical virulence factor and highlight the importance of independently confirming results on S. suis virulence by different teams.

Deregulated PSGL-1 Expression in B Cells and Dendritic Cells May Be Implicated in Human Systemic Sclerosis Development

Systemic sclerosis (SSc) is an autoimmune disorder with high morbidity and mortality, is difficult to diagnose early, and has no curative treatment. PSGL-1 is a leukocyte receptor whose deficiency in mice promotes an SSc-like disease. ADAM8, a metalloprotease that cleaves PSGL-1, is implicated in inflammatory processes. Our goal was to evaluate whether PSGL-1 and ADAM8 contribute to the pathogenesis of human SSc. We found that patients with SSc presented increased PSGL-1 expression on monocytes, dendritic cells, and T cells and decreased expression of PSGL-1 on B cells. PSGL-1 on monocytes from SSc patients failed to induce Syk phosphorylation or IL-10 production after interaction with P-selectin. Up to 60% of the IL-10-producing B cells expressed PSGL-1, pointing to a regulatory role for PSGL-1 in B cells, and PSGL-1⁺ B cells from SSc patients had decreased IL-10 production. ADAM8 expression was increased on antigen-presenting cells and T lymphocytes of SSc patients. Patients treated with calcium antagonists had lower levels of ADAM8 on APCs and T lymphocytes. Multivariate analysis indicated that the high percentage of ADAM8-expressing plasmacytoid dendritic cells discriminated patients from healthy donors. High PSGL-1 expression on dendritic cells was associated with the presence of interstitial lung disease.

Shared and Unique Features Distinguishing Follicular T Helper and Regulatory Cells of Peripheral Lymph Node and Peyer's Patches

Follicular helper (TFH) and regulatory (TFR) cells are critical players in managing germinal center (GC) reactions that accomplish effective humoral immune responses. Transcriptome analyses were done comparing gene regulation of TFH and TFR cells isolated from Peyer’s Patches (PP) and immunized peripheral lymph nodes (pLNs) revealing many regulatory patterns common to all follicular cells. However, in contrast to TFH cells, the upregulation or downregulation of many genes was attenuated substantially in pLN TFR cells when compared to those of PP. Additionally, PP but not pLN TFR cells were largely unresponsive to IL2 and expressed Il4 as well as Il21. Together with fundamental differences in gene expression that were found between cells of both compartments this emphasizes specific adaptations of follicular T cell functions to their micro-milieu. Moreover, although GL7 expression distinguishes matured follicular T cells, GL7⁺ as well as GL7⁻ cells are present in the GC.

Fucosyltransferase Induction during Influenza Virus Infection Is Required for the Generation of Functional Memory CD4+ T Cells

T cells mediating influenza viral control are instructed in lymphoid and nonlymphoid tissues to differentiate into memory T cells that confer protective immunity. The mechanisms by which influenza virus-specific memory CD4⁺ T cells arise have been attributed to changes in transcription factors, cytokines and cytokine receptors, and metabolic programming. The molecules involved in these biosynthetic pathways, including proteins and lipids, are modified to varying degrees of glycosylation, fucosylation, sialation, and sulfation, which can alter their function. It is currently unknown how the glycome enzymatic machinery regulates CD4⁺ T cell effector and memory differentiation. In a murine model of influenza virus infection, we found that fucosyltransferase enzymatic activity was induced in effector and memory CD4⁺ T cells. Using CD4⁺ T cells deficient in the Fut4/7 enzymes that are expressed only in hematopoietic cells, we found decreased frequencies of effector cells with reduced expression of T-bet and NKG2A/C/E in the lungs during primary infection. Furthermore, Fut4/7^-/- effector CD4⁺ T cells had reduced survival with no difference in proliferation or capacity for effector function. Although Fut4/7^-/- CD4⁺ T cells seeded the memory pool after primary infection, they failed to form tissue-resident cells, were dysfunctional, and were unable to re-expand after secondary infection. Our findings highlight an important regulatory axis mediated by cell-intrinsic fucosyltransferase activity in CD4⁺ T cell effectors that ensure the development of functional memory CD4⁺ T cells.

Homing in on the Sweet Side of Immune Checkpoint Biology

P-selectin glycoprotein ligand-1 (PSGL-1) and its glycostructural determinants facilitate responses to infection and cancer by promoting immune effector-cell trafficking into inflamed tissue. In this issue of Immunity, Tinoco et al. (2016) report homing-independent functions of PSGL-1 in immune checkpoint regulation and T cell effector activity, in models of chronic viral infection and melanoma.

Frontline Science: c-Myc regulates P-selectin glycoprotein ligand-1 expression in monocytes during HIV-1 infection

Leukocyte extravasation is a crucial feature of the normal immune response to disease and infection and is implicated in various pathologies during chronic inflammatory disease. P-Selectin glycoprotein ligand-1 (PSGL-1) is critical for leukocyte extravasation; however, despite extensive study, it remains unclear how its expression is regulated, which in turn, impedes a more precise understanding of how its expression level affects transmigration. To investigate the regulation of PSGL-1, 60 subjects, with or without HIV infection, were recruited and PSGL-1 expression in monocytes was measured. PSGL-1 was found to be up-regulated on leukocytes from HIV-infected individuals, and the physiologically relevant mediators soluble CD40 ligand (sCD40L) and glutamate were able to induce PSGL-1 transcription in human monocytes ex vivo. HIV-1 induced PSGL-1 induction, and its dependence on CD40L was validated further by use of the mouse-tropic HIV (EcoHIV) mouse model of HIV infection in C57BL/6 and CD40L knockout (KO) mice. To investigate crosstalk between the signaling cascades induced by CD40L and glutamate that lead to PSGL-1 induction, a network-based, discrete dynamic model was developed. The model reveals the MAPK pathway and oxidative stress as critical mediators of crosstalk between CD40L and glutamate-induced pathways. Importantly, the model predicted induction of the c-Myc transcription factor upon cotreatment, which was validated using transcriptomic data and pharmacologic inhibition of c-Myc. This study suggests a novel systems serology approach for translational research and reveals a mechanism for PSGL-1 transcriptional regulation, which might be leveraged to identify novel targets for therapeutic intervention.

Regulation of T Cell Trafficking by Enzymatic Synthesis of O-Glycans

Selectins constitute a family of oligosaccharide binding proteins that play critical roles in regulating the trafficking of leukocytes. In T cells, L-selectin (CD62L) controls the capacity for naive and memory T cells to actively survey peripheral lymph nodes, whereas P- and E-selectin capture activated T cells on inflamed vascular endothelium to initiate extravasation into non-lymphoid tissues. The capacity for T cells to interact with all of these selectins is dependent on the enzymatic synthesis of complex O-glycans, and thus, this protein modification plays an indispensable role in regulating the distribution and homing of both naive and previously activated T cells in vivo. In contrast to neutrophils, O-glycan synthesis is highly dynamic in T cell populations and is largely controlled by extracellular stimuli such as antigen recognition or signaling though cytokine receptors. Herein, we review the basic principles of enzymatic synthesis of complex O-glycans, discuss tools and reagents for studying this type of protein modification and highlight our current understanding of how O-glycan synthesis is regulated and subsequently impacts the trafficking potential of diverse T cell populations.

Targeting P-selectin glycoprotein ligand-1/P-selectin interactions as a novel therapy for metabolic syndrome

Obesity-induced insulin resistance and metabolic syndrome continue to pose an important public health challenge worldwide as they significantly increase the risk of type 2 diabetes and atherosclerotic cardiovascular disease. Advances in the pathophysiologic understanding of this process has identified that chronic inflammation plays a pivotal role. In this regard, given that both animal models and human studies have demonstrated that the interaction of P-selectin glycoprotein ligand-1 (PSGL-1) with P-selectin is not only critical for normal immune response but also is upregulated in the setting of metabolic syndrome, PSGL-1/P-selectin interactions provide a novel target for preventing and treating resultant disease. Current approaches of interfering with PSGL-1/P-selectin interactions include targeted antibodies, recombinant immunoglobulins that competitively bind P-selectin, and synthetic molecular therapies. Experimental models as well as clinical trials assessing the role of these modalities in a variety of diseases have continued to contribute to the understanding of PSGL-1/P-selectin interactions and have demonstrated the difficulty in creating clinically relevant therapeutics. Most recently, however, computational simulations have further enhanced our understanding of the structural features of PSGL-1 and related glycomimetics, which are responsible for high-affinity selectin interactions. Leveraging these insights for the design of next generation agents has thus led to development of a promising synthetic method for generating PSGL-1 glycosulfopeptide mimetics for the treatment of metabolic syndrome.