P-selectin glycoprotein ligand-1 negatively regulates T-cell immune responses

Apoptosis-mediated ADAM10 activation removes a mucin barrier promoting T cell efferocytosis

Efferocytic clearance of apoptotic cells in general, and T cells in particular, is required for tissue and immune homeostasis. Transmembrane mucins are extended glycoproteins highly expressed in the cell glycocalyx that function as a barrier to phagocytosis. Whether and how mucins may be regulated during cell death to facilitate efferocytic corpse clearance is not well understood. Here we show that normal and transformed human T cells express a subset of mucins which are rapidly and selectively removed from the cell surface during apoptosis. This process is mediated by the ADAM10 sheddase, the activity of which is associated with XKR8-catalyzed flipping of phosphatidylserine to the outer leaflet of the plasma membrane. Mucin clearance enhances uptake of apoptotic T cells by macrophages, confirming mucins as an enzymatically-modulatable barrier to efferocytosis. Together these findings demonstrate a glycocalyx regulatory pathway with implications for therapeutic intervention in the clearance of normal and transformed apoptotic T cells.

PSGL-1: a novel immune checkpoint driving T-cell dysfunction in obstructive sleep apnea

Introduction

Although higher incidence of cancer represents a major burden for obstructive sleep apnea (OSA) patients, the molecular pathways driving this association are not completely understood. Recently, the adhesion receptor P-selectin glycoprotein-1 (PSGL 1) has been identified as a novel immune checkpoint, which are recognized major hallmarks in several types of cancer and have revolutionized cancer therapy.

Methods

The expression of PSGL-1 and its ligands VISTA and SIGLEC-5 was assessed in the leucocytes of OSA patients and control subjects exploring the role of intermittent hypoxia (IH) using in vitro models. In addition, PSGL-1 impact on T-cells function was evaluated by ex vivo models.

Results

Data showed PSGL-1 expression is upregulated in the T-lymphocytes from patients with severe OSA, indicating a relevant role of hypoxemia mediated by intermittent hypoxia. Besides, results suggest an inhibitory role of PSGL-1 on T-cell proliferation capacity. Finally, the expression of SIGLEC-5 but not VISTA was increased in monocytes from OSA patients, suggesting a regulatory role of intermittent hypoxia.

Discussion

In conclusion, PSGL-1 might constitute an additional immune checkpoint leading to T-cell dysfunction in OSA patients, contributing to the disruption of immune surveillance, which might provide biological plausibility to the higher incidence and aggressiveness of several tumors in these patients.

Conditionally Active, pH-Sensitive Immunoregulatory Antibodies Targeting VISTA and CTLA-4 Lead an Emerging Class of Cancer Therapeutics

Immune checkpoints and other immunoregulatory targets can be difficult to precisely target due to expression on non-tumor immune cells critical to maintaining immune homeostasis in healthy tissues. On-target/off-tumor binding of therapeutics results in significant pharmacokinetic and pharmacodynamic problems. Target-mediated drug disposition (TMDD) significantly limits effective intratumoral drug levels and adversely affects anti-tumor efficacy. Target engagement outside the tumor environment may lead to severe immune-related adverse events (irAEs), resulting in a narrowing of the therapeutic window, sub-optimal dosing, or cessation of drug development altogether. Overcoming these challenges has become tractable through recent advances in antibody engineering and screening approaches. Here, we review the discovery and development of conditionally active antibodies with minimal binding to target at physiologic pH but high-affinity target binding at the low pH of the tumor microenvironment by focusing on the discovery and improved properties of pH-dependent mAbs targeting two T cell checkpoints, VISTA and CTLA-4.

Nanoscopic Characterization of Cell Migration under Flow Using Optical and Electron Microscopy

Hematopoietic stem/progenitor cell (HSPC) and leukemic cell homing is an important biological phenomenon that takes place through essential interactions with adhesion molecules on an endothelial cell layer. The homing process of HSPCs begins with the tethering and rolling of the cells on the endothelial layer, which is achieved by the interaction between selectins on the endothelium to the ligands on HSPC/leukemic cells under shear stress of the blood flow. Although many studies have been based on in vitro conditions of the cells rolling over recombinant proteins, significant challenges remain when imaging HSPC/leukemic cells on the endothelium, a necessity when considering characterizing cell-to-cell interaction and rolling dynamics during cell migration. Here, we report a new methodology that enables imaging of stem-cell-intrinsic spatiotemporal details during its migration on an endothelium-like cell monolayer. We developed optimized protocols that preserve transiently appearing structures on HSPCs/leukemic cells during its rolling under shear stress for fluorescence and scanning electron microscopy characterization. Our new experimental platform is closer to in vivo conditions and will contribute to indepth understanding of stem-cell behavior during its migration and cell-to-cell interaction during the process of homing.

Impaired Treg-DC interactions contribute to autoimmunity in leukocyte adhesion deficiency type 1

Leukocyte adhesion deficiency type 1 (LAD-1) is a rare disease resulting from mutations in the gene encoding for the common β-chain of the β2-integrin family (CD18). The most prominent clinical symptoms are profound leukocytosis and high susceptibility to infections. Patients with LAD-1 are prone to develop autoimmune diseases, but the molecular and cellular mechanisms that result in coexisting immunodeficiency and autoimmunity are still unresolved. CD4+FOXP3+ Treg are known for their essential role in preventing autoimmunity. To understand the role of Treg in LAD-1 development and manifestation of autoimmunity, we generated mice specifically lacking CD18 on Treg (CD18Foxp3), resulting in defective LFA-1 expression. Here, we demonstrate a crucial role of LFA-1 on Treg to maintain immune homeostasis by modifying T cell-DC interactions and CD4+ T cell activation. Treg-specific CD18 deletion did not impair Treg migration into extralymphatic organs, but it resulted in shorter interactions of Treg with DC. In vivo, CD18Foxp3 mice developed spontaneous hyperplasia in lymphatic organs and diffuse inflammation of the skin and in multiple internal organs. Thus, LFA-1 on Treg is required for the maintenance of immune homeostasis.

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.

Patients With Myeloproliferative Neoplasms Harbor High Frequencies of CD8 T Cell-Platelet Aggregates Associated With T Cell Suppression

Myeloproliferative neoplasms (MPN) are chronic cancers of the hematopoietic stem cells in the bone marrow, and patients often harbor elevated numbers of circulating platelets (PLT). We investigated the frequencies of circulating PLT-lymphocyte aggregates in MPN patients and the effect of PLT-binding on CD8 T cell function. The phenotype of these aggregates was evaluated in 50 MPN patients and 24 controls, using flow cytometry. In vitro studies compared the proliferation, cytokine release, and cytoxicity of PLT-bound and PLT-free CD8 T cells. Frequencies of PLT-CD8 T cell aggregates, were significantly elevated in MPN patients. Advanced disease stage and CALR mutation associated with the highest aggregate frequencies with a predominance of PLT-binding to antigen-experienced CD8 T cells. PLT-bound CD8 T cells showed reduction in proliferation and cytotoxic capacity. Our data suggest that CD8 T cell responses are jeopardized in MPN patients. JAK2 and CALR exon 9 mutations – the two predominant driver mutations in MPN – are targets for natural T cell responses in MPN patients. Moreover, MPN patients have more infections compared to background. Thus, PLT binding to antigen experienced CD8 T cells could play a role in the inadequacy of the immune system to control MPN disease progression and prevent recurrent infections.

Advances in the Immunomodulatory Properties of Glycoantigens in Cancer

Simple Summary

This work reviews the role of aberrant glycosylation in cancer cells during tumour growth and spreading, as well as in immune evasion. The interaction of tumour-associated glycans with the immune system through C-type lectin receptors can favour immune escape but can also provide opportunities to develop novel tumour immunotherapy strategies. This work highlights the main findings in this area and spotlights the challenges that remain to be investigated.

Abstract

Aberrant glycosylation in tumour progression is currently a topic of main interest. Tumour-associated carbohydrate antigens (TACAs) are expressed in a wide variety of epithelial cancers, being both a diagnostic tool and a potential treatment target, as they have impact on patient outcome and disease progression. Glycans affect both tumour-cell biology properties as well as the antitumor immune response. It has been ascertained that TACAs affect cell migration, invasion and metastatic properties both when expressed by cancer cells or by their extracellular vesicles. On the other hand, tumour-associated glycans recognized by C-type lectin receptors in immune cells possess immunomodulatory properties which enable tumour growth and immune response evasion. Yet, much remains unknown, concerning mechanisms involved in deregulation of glycan synthesis and how this affects cell biology on a major level. This review summarises the main findings to date concerning how aberrant glycans influence tumour growth and immunity, their application in cancer treatment and spotlights of unanswered challenges remaining to be solved.

Targeting the PSGL-1 Immune Checkpoint Promotes Immunity to PD-1 Resistant Melanoma

Immune checkpoint inhibitors have had impressive efficacy in some cancer patients, reinvigorating long-term durable immune responses against tumors. Despite the clinical success of these therapies, most cancer patients continue to be unresponsive to these treatments, highlighting the need for novel therapeutic options. Although P-selectin glycoprotein ligand-1 (PSGL-1) has been shown to inhibit immune responses in a variety of disease models, previous work has yet to address whether PSGL-1 can be targeted therapeutically to promote antitumor immunity. Using an aggressive melanoma tumor model, we targeted PSGL-1 in tumor-bearing mice and found increased effector CD4+ and CD8+ T-cell responses and decreased regulatory T cells (Tregs) in tumors. T cells exhibited increased effector function, activation, and proliferation, which delayed tumor growth in mice after anti-PSGL-1 treatment. Targeting PD-1 in PSGL-1-deficient, tumor-bearing mice led to an increased frequency of mice with complete tumor eradication. Targeting both PSGL-1 and PD-1 in wild-type tumor-bearing mice also showed enhanced anti-tumor immunity and slowed melanoma tumor growth. Our findings showed that therapeutically targeting the PSGL-1 immune checkpoint can reinvigorate anti-tumor immunity and suggest that targeting PSGL-1 may represent a new therapeutic strategy for cancer treatment.

The posttraumatic response of CD4+ regulatory T cells is modulated by direct cell-cell contact via CD40L- and P-selectin-dependent pathways

CD4+ FoxP3+ regulatory T cells (CD4+ Tregs) are important for the posttraumatic anti-inflammatory host response. As described previously, platelets are able to modulate CD4+ Treg activity in a reciprocally activating interaction following injury. The underlying mechanisms of the posttraumatic interaction between platelets and CD4+ Tregs remain unclear. We investigated the potential influence of CD40L and P-selectin, molecules known to be involved in direct cell contact of these cell types. In a murine burn injury model, the potential interaction pathways were addressed using CD40L- and P-selectin-deficient mice. Draining lymph nodes were harvested following trauma (1 h) and following a sham procedure. Early rapid activation of CD4+ Tregs was assessed by phospho-flow cytometry (signaling molecules (p)PKC-δ and (p)ZAP-70). Platelet function was analyzed performing rotational thromboelastometry (ROTEM). We hypothesized that disruption of the direct cell-cell contact via CD40L and P-selectin would affect posttraumatic activation of CD4+ Tregs and influence the hemostatic function of platelets. Indeed, while injury induced early activation of CD4+ Tregs in wild-type mice (ZAP-70: p = 0.13, pZAP-70: p < 0.05, PKC-δ: p < 0.05, pPKC-δ: p < 0.05), disruption of CD40L-dependent interaction (ZAP-70: p = 0.57, pZAP-70: p = 0.68, PKC-δ: p = 0.68, pPKC-δ: p = 0.9) or P-selectin-dependent interaction (ZAP-70: p = 0.78, pZAP-70: p = 0.58, PKC-δ: p = 0.81, pPKC-δ: p = 0.73) resulted in reduced posttraumatic activation. Furthermore, hemostatic function was impaired towards hypocoagulability in either deficiency. Our results suggest that the posttraumatic activation of CD4+ Tregs and hemostatic function of platelets are affected by direct cell-cell-signaling via CD40L and P-selectin.

Immune functions as a ligand or a receptor, cancer prognosis potential, clinical implication of VISTA in cancer immunotherapy

Since cancer immunotherapy with immune checkpoint inhibitors of PD/PDL-1 and CTLA-4 limited efficacy to the patients due to resistance during the current decade, novel target is required for customized treatment due to tumor heterogeneity. V-domain Ig-containing suppressor of T cell activation (VISTA), a programmed death protein-1(PD-1) homolog expressed on T cells and on antigen presenting cells(APC), has emerged as a new target in several cancers. Though VISTA inhibitors including CA-170 are considered attractive in cancer immunotherapy to date, the information on VISTA as a potent biomarker of cancer prognosis and its combination therapy is still lacking to date. Thus, in this review, we discussed extracellular domain, ligands, expression, immune functions and clinical implications of VISTA and finally suggested conclusion and perspectives.

HIV-1 entry: Duels between Env and host antiviral transmembrane proteins on the surface of virus particles

Human Immunodeficiency Virus type-1 (HIV-1) is the causative agent of AIDS. Its entry step is mediated by the envelope glycoprotein (Env). During the entry process, Env vastly changes its conformation. While non-liganded Env tends to have a closed structure, receptor-binding of Env opens its conformation, which leads to virus-cell membrane fusion. Single-molecule fluorescence resonance energy transfer (smFRET) imaging allows observation of these conformational changes on the virion surface. Nascent HIV-1 particles incorporate multiple host transmembrane proteins, some of which inhibit the entry process. The Env structure or its dynamics may determine the effectiveness of these antiviral mechanisms. Here, we review recent findings about the Env conformation changes on virus particles and inhibition of Env activities by virion-incorporated host transmembrane proteins.

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.

PSGL-1 Immune Checkpoint Inhibition for CD4+ T Cell Cancer Immunotherapy

Immune checkpoint inhibition targeting T cells has shown tremendous promise in the treatment of many cancer types and are now standard therapies for patients. While standard therapies have focused on PD-1 and CTLA-4 blockade, additional immune checkpoints have shown promise in promoting anti-tumor immunity. PSGL-1, primarily known for its role in cellular migration, has also been shown to function as a negative regulator of CD4+ T cells in numerous disease settings including cancer. PSGL-1 is highly expressed on T cells and can engage numerous ligands that impact signaling pathways, which may modulate CD4+ T cell differentiation and function. PSGL-1 engagement in the tumor microenvironment may promote CD4+ T cell exhaustion pathways that favor tumor growth. Here we highlight that blocking the PSGL-1 pathway on CD4+ T cells may represent a new cancer therapy approach to eradicate tumors.

Rendezvous at Plasma Membrane: Cellular Lipids and tRNA Set up Sites of HIV-1 Particle Assembly and Incorporation of Host Transmembrane Proteins

The HIV-1 structural polyprotein Gag drives the virus particle assembly specifically at the plasma membrane (PM). During this process, the nascent virion incorporates specific subsets of cellular lipids and host membrane proteins, in addition to viral glycoproteins and viral genomic RNA. Gag binding to the PM is regulated by cellular factors, including PM-specific phospholipid PI(4,5)P2 and tRNAs, both of which bind the highly basic region in the matrix domain of Gag. In this article, we review our current understanding of the roles played by cellular lipids and tRNAs in specific localization of HIV-1 Gag to the PM. Furthermore, we examine the effects of PM-bound Gag on the organization of the PM bilayer and discuss how the reorganization of the PM at the virus assembly site potentially contributes to the enrichment of host transmembrane proteins in the HIV-1 particle. Since some of these host transmembrane proteins alter release, attachment, or infectivity of the nascent virions, the mechanism of Gag targeting to the PM and the nature of virus assembly sites have major implications in virus spread.

Stress-mediated convergence of splicing landscapes in male and female rock doves

BACKGROUND

The process of alternative splicing provides a unique mechanism by which eukaryotes are able to produce numerous protein products from the same gene. Heightened variability in the proteome has been thought to potentiate increased behavioral complexity and response flexibility to environmental stimuli, thus contributing to more refined traits on which natural and sexual selection can act. While it has been long known that various forms of environmental stress can negatively affect sexual behavior and reproduction, we know little of how stress can affect the alternative splicing associated with these events, and less still about how splicing may differ between sexes. Using the model of the rock dove (Columba livia), our team previously uncovered sexual dimorphism in the basal and stress-responsive gene transcription of a biological system necessary for facilitating sexual behavior and reproduction, the hypothalamic-pituitary-gonadal (HPG) axis. In this study, we delve further into understanding the mechanistic underpinnings of how changes in the environment can affect reproduction by testing the alternative splicing response of the HPG axis to an external stressor in both sexes.

RESULTS

This study reveals dramatic baseline differences in HPG alternative splicing between males and females. However, after subjecting subjects to a restraint stress paradigm, we found a significant reduction in these differences between the sexes. In both stress and control treatments, we identified a higher incidence of splicing activity in the pituitary in both sexes as compared to other tissues. Of these splicing events, the core exon event is the most abundant form of splicing and more frequently occurs in the coding regions of the gene. Overall, we observed less splicing activity in the 3'UTR (untranslated region) end of transcripts than the 5'UTR or coding regions.

CONCLUSIONS

Our results provide vital new insight into sex-specific aspects of the stress response on the HPG axis at an unprecedented proximate level. Males and females uniquely respond to stress, yet exhibit splicing patterns suggesting a convergent, optimal splicing landscape for stress response. This information has the potential to inform evolutionary theory as well as the development of highly-specific drug targets for stress-induced reproductive dysfunction.

Virion-incorporated PSGL-1 and CD43 inhibit both cell-free infection and transinfection of HIV-1 by preventing virus-cell binding

HIV-1 particles incorporate various host transmembrane proteins in addition to viral Env glycoprotein during assembly at the plasma membrane. In polarized T cells, HIV-1 structural protein Gag localizes to the plasma membrane of uropod, a rear-end protrusion. Notably, uropod transmembrane proteins PSGL-1 and CD43 cocluster specifically with Gag assembling at the plasma membrane even in cells that do not form uropods. Recent reports have shown that expression of either PSGL-1 or CD43 in virus-producing cells reduces the infectivity of progeny virions and that HIV-1 infection reduces the cell surface expression of these proteins. However, the mechanisms for both processes remain to be determined. In this study, we found that virion incorporation of PSGL-1 and CD43 closely correlates with diminished virion infectivity. PSGL-1 and CD43 inhibited virus attachment to CD4⁺ cells irrespective of the presence of Env. These proteins also inhibited virion attachment to CD4^- lymphoid organ fibroblastic reticular cells that mediate transinfection of CD4⁺ T cells. Consistent with the possibility that highly extended extracellular domains of these proteins physically block virus-cell attachment, the inhibitory effect of PSGL-1 required its full-length ectodomain. HIV-1 encoding Gag mutants that are defective in either coclustering with these host proteins or ESCRT-dependent particle release failed to reduce PSGL-1 on surface of infected cells. This study reveals an anti-HIV-1 mechanism that suppresses virus-cell attachment and a previously unappreciated process of HIV-1-mediated down-regulation of host antiviral proteins, both of which likely require virion incorporation of these proteins.

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.

The P-selectin and PSGL-1 axis accelerates atherosclerosis via activation of dendritic cells by the TLR4 signaling pathway

P-selectin and dendritic cells (DCs) are associated with atherosclerosis. However, their interactions in this setting are undefined. Herein, we investigated the role of P-selectin and its receptor P-selectin glycoprotein ligand (PSGL)-1 on atherosclerosis via activation of DCs. In the current study, a total of 34 patients with ST elevation myocardial infarction (STEMI) and 34 healthy control subjects were enrolled. Serum concentration of P-selectin was higher and the myeloid DC/plasmacytoid DC (mDC/pDC) ratio was lower in STEMI patients than in normal individuals. Interestingly, in STEMI patients, P-selectin was decreased and the mDC/pDC ratio was increased at 5–7 days after successful percutaneous coronary intervention, as compared with values on admission. Serum P-selectin was inversely correlated with the mDC/pDC ratio. Moreover, ApoE^−/−P^−/− and ApoE^−/−PSGL-1^−/− mice developed small atherosclerotic plaques after feeding of a western diet for 12 weeks and DC infiltration was significantly reduced. P-selectin stimulation markedly induced phenotypic maturation, enhanced secretion of inflammatory cytokines, communication with T cells, and the adhesion and migration of DCs. In vivo, DC maturation was significantly attenuated in P-selectin and PSGL1 knockout mice under hypercholesterolemic and inflammatory conditions. These effects were associated with the activation of myeloid differentiation primary response 88 (MYD88)-dependent and MyD88-independent Toll-like receptor 4 (TLR4) signaling pathways. Taken together, binding of P-selectin to PSGL-1 on DCs contributes to atherosclerosis progression via DC activation via the TLR4 signaling pathway.

Selectins modify dendritic cells during atherosclerosis

Dendritic cells (DCs) are professional antigen-presenting cells (APC) that facilitate the development and progression of atherosclerosis. However, DCs also function as novel "switches" between immune activation and immune tolerance and represent a heterogeneous hematopoietic lineage, with cell subsets in different tissues that show a differential morphology, phenotype, and function. Regulatory DCs, depending on their immature state, can be induced by immunosuppressive modulation, which plays an important part in the maintenance of immunologic tolerance via suppression of the immune response. In this review, we describe the current understanding of the generation of regulatory DCs. The novel role of selectins in the modification of DCs in atherosclerosis is also discussed.

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.

P-selectin glycoprotein ligand-1 in T cells

PURPOSE OF REVIEW

We review P-selectin glycoprotein ligand-1 (PSGL-1) as a selectin and chemokine-binding adhesion molecule. PSGL-1 is widely studied in neutrophils. Here, we focus on T cells, because PSGL-1 was recently described as a major immunomodulatory molecule during viral infection. PSGL-1 also plays a crucial role in T-cell homeostasis by binding to lymphoid chemokines, and can induce tolerance by enhancing the functions of regulatory T cells.

RECENT FINDINGS

PSGL-1 was originally described as a leukocyte ligand for P-selectin, but it is actually a ligand for all selectins (P-, L- and E-selectin), binds chemokines, activates integrins and profoundly affects T-cell biology. It has been shown recently that PSGL-1 can modulate T cells during viral infection by acting as a negative regulator for T-cell functions. Absence of PSGL-1 promotes effector CD4 and CD8 T-cell differentiation and prevents T-cell exhaustion. Consistent with this, tumor growth was significantly reduced in PSGL-1-deficient mice because of an enhanced number of effector T cells together with reduced levels of inhibitory receptors that induce T-cell exhaustion.

SUMMARY

PSGL-1 is the best-studied selectin ligand and has become a posterchild of versatility in leukocyte adhesion, inflammation and immunology. The direct involvement of PSGL-1 in T-cell biology suggests that it might be a drug target. Indeed, PSGL-1 has been tested in some clinical trials and recently, PSGL-1 blockers were proposed as a potential cotherapy in cancer immunotherapy.

PSGL-1: A New Player in the Immune Checkpoint Landscape

P-selectin glycoprotein ligand-1 (PSGL-1) has long been studied as an adhesion molecule involved in immune cell trafficking and is recognized as a regulator of many facets of immune responses by myeloid cells. PSGL-1 also regulates T cell migration during homeostasis and inflammatory settings. However, recent findings indicate that PSGL-1 can also negatively regulate T cell function. Because T cell differentiation is finely tuned by multiple positive and negative regulatory signals that appropriately scale the magnitude of the immune response, PSGL-1 has emerged as an important checkpoint during this process. We summarize what is known regarding PSGL-1 structure and function and highlight how it may act as an immune checkpoint inhibitor in T cells.

Binding of Platelets to Lymphocytes: A Potential Anti-Inflammatory Therapy in Rheumatoid Arthritis

Soluble factors released from platelets can modulate the immune response of leukocytes. We and others have recently found that T lymphocytes with bound platelets have reduced proliferation and IFN-γ and IL-17 production. Thus, we speculate that if we induce the binding of platelets to lymphocytes, we will be able to regulate the inflammatory response. When we cocultured platelets with lymphocytes at different ratios, we were able to increase the percentage of lymphocytes with bound platelets. The coculture of platelets with lymphocytes in the presence of stimulation decreased the production of IFN-γ and TNF-α, T cell proliferation, and the expression of CD25, PD-L1, and SLAM. However, this coculture increased CD39 expression. All of these effects were dependent on the dose of platelets and operated indistinctly with platelets from different healthy donors. When platelets were cocultured in the same compartment with lymphocytes, we observed less IFN-γ and TNF-α production and T lymphocyte proliferation than in cultures with platelets separated from lymphocytes by a 0.4-μm pore size filter. The binding of platelets to lymphocytes was blocked with anti-P-selectin Abs, and when this occurred we observed higher IFN-γ and TNF-α production than in nonblocked conditions. The cocultures of platelets with synovial fluid cells from rheumatoid arthritis patients reduced inflammatory cytokine production and increased IL-10 production. These results suggest that platelet binding to lymphocytes effectively regulates T lymphocyte function. This mechanism could be easily applied to reduce inflammatory responses.

Live Imaging of Immune Responses in Experimental Models of Multiple Sclerosis

Experimental autoimmune encephalomyelitis (EAE) is the most common animal model of multiple sclerosis (MS), a chronic inflammatory autoimmune disease of the central nervous system (CNS) characterized by multifocal perivascular infiltrates that predominantly comprise lymphocytes and macrophages. During EAE, autoreactive T cells first become active in the secondary lymphoid organs upon contact with antigen-presenting cells (APCs), and then gain access to CNS parenchyma, through a compromised blood–brain barrier, subsequently inducing inflammation and demyelination. Two-photon laser scanning microscopy (TPLSM) is an ideal tool for intravital imaging because of its low phototoxicity, deep tissue penetration, and high resolution. In the last decade, TPLSM has been used to visualize the behavior of T cells and their contact with APCs in the lymph nodes (LNs) and target tissues in several models of autoimmune diseases. The leptomeninges and cerebrospinal fluid represent particularly important points for T cell entry into the CNS and reactivation following contact with local APCs during the preclinical phase of EAE. In this review, we highlight recent findings concerning the pathogenesis of EAE and MS, emphasizing the use of TPLSM to characterize T cell activation in the LNs and CNS, as well as the mechanisms of tolerance induction. Furthermore, we discuss how advanced imaging unveils disease mechanisms and helps to identify novel therapeutic strategies to treat CNS autoimmunity and inflammation.

PSGL-1 Is an Immune Checkpoint Regulator that Promotes T Cell Exhaustion

Chronic viruses and cancers thwart immune responses in humans by inducing T cell dysfunction. Using a murine chronic virus that models human infections, we investigated the function of the adhesion molecule, P-selectin glycoprotein ligand-1 (PSGL-1), that is upregulated on responding T cells. PSGL-1-deficient mice cleared the virus due to increased intrinsic survival of multifunctional effector T cells that had downregulated PD-1 as well as other inhibitory receptors. Notably, this response resulted in CD4(+)-T-cell-dependent immunopathology. Mechanistically, PSGL-1 ligation on exhausted CD8(+) T cells inhibited T cell receptor (TCR) and interleukin-2 (IL-2) signaling and upregulated PD-1, leading to diminished survival with TCR stimulation. In models of melanoma cancer in which T cell dysfunction occurs, PSGL-1 deficiency led to PD-1 downregulation, improved T cell responses, and tumor control. Thus, PSGL-1 plays a fundamental role in balancing viral control and immunopathology and also functions to regulate T cell responses in the tumor microenvironment.

PSGL-1 on Leukocytes is a Critical Component of the Host Immune Response against Invasive Pneumococcal Disease

Bacterial uptake by phagocytic cells is a vital event in the clearance of invading pathogens such as Streptococcus pneumoniae. A major role of the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes against invasive pneumococcal disease is described in this study. Phagocytosis experiments using different serotypes demonstrated that PSGL-1 is involved in the recognition, uptake and killing of S. pneumoniae. Co-localization of several clinical isolates of S. pneumoniae with PSGL-1 was demonstrated, observing a rapid and active phagocytosis in the presence of PSGL-1. Furthermore, the pneumococcal capsular polysaccharide and the main autolysin of the bacterium ―the amidase LytA― were identified as bacterial ligands for PSGL-1. Experimental models of pneumococcal disease including invasive pneumonia and systemic infection showed that bacterial levels were markedly increased in the blood of PSGL-1^−/− mice. During pneumonia, PSGL-1 controls the severity of pneumococcal dissemination from the lung to the bloodstream. In systemic infection, a major role of PSGL-1 in host defense is to clear the bacteria in the systemic circulation controlling bacterial replication. These results confirmed the importance of this receptor in the recognition and clearance of S. pneumoniae during invasive pneumococcal disease. Histological and cellular analysis demonstrated that PSGL-1^−/− mice have increased levels of T cells migrating to the lung than the corresponding wild-type mice. In contrast, during systemic infection, PSGL-1^−/− mice had increased numbers of neutrophils and macrophages in blood, but were less effective controlling the infection process due to the lack of this functional receptor. Overall, this study demonstrates that PSGL-1 is a novel receptor for S. pneumoniae that contributes to protection against invasive pneumococcal disease.

S. pneumoniae is one of the most important and devastating human pathogens worldwide, mainly affecting young children, elderly people and immunocompromised patients. In terms of host immune defense against invasive pneumococcal isolates, professional phagocytes require receptor-mediated recognition of certain ligands on the bacterial surface for the uptake and clearance of the microorganism. In this study, we demonstrate that the P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes is involved in the phagocytosis process of S. pneumoniae by targeting the capsule and the surface protein LytA as pathogen-associated molecular patterns. To explore this process in more detail, we have used wild-type mice and mice deficient in PSGL-1 demonstrating that lack of PSGL-1 is detrimental for the host by increasing the susceptibility to the infection and the severity of the pneumococcal invasive disease. Overall, these data show the importance of PSGL-1 on leukocytes in host defense against S. pneumoniae and confirm that PSGL-1 plays a critical protective role against invasive bacterial disease.

Platelet microparticles inhibit IL-17 production by regulatory T cells through P-selectin

Self-tolerance and immune homeostasis are orchestrated by FOXP3(+)regulatory T cells (Tregs). Recent data have revealed that upon stimulation, Tregs may exhibit plasticity toward a proinflammatory phenotype, producing interleukin 17 (IL-17) and/or interferon γ (IFN-γ). Such deregulation of Tregs may contribute to the perpetuation of inflammatory processes, including graft-versus-host disease. Thus, it is important to identify immunomodulatory factors influencing Treg stability. Platelet-derived microparticles (PMPs) are involved in hemostasis and vascular health and have recently been shown to be intimately involved in (pathogenic) immune responses. Therefore, we investigated whether PMPs have the ability to affect Treg plasticity. PMPs were cocultured with healthy donor peripheral blood-derived Tregs that were stimulated with anti-CD3/CD28 monoclonal antibodies in the presence of IL-2, IL-15, and IL-1β. PMPs prevented the differentiation of peripheral blood-derived Tregs into IL-17- and IFN-γ-producing cells, even in the presence of the IL-17-driving proinflammatory cytokine IL-1β. The mechanism of action by which PMPs prevent Treg plasticity consisted of rapid and selective P-selectin-dependent binding of PMPs to a CCR6(+)HLA-DR(+)memory-like Treg subset and their ability to inhibit Treg proliferation, in part through CXCR3 engagement. The findings that ~8% of Tregs in the circulation of healthy individuals are CD41(+)P-selectin(+)and that distinct binding of patient plasma PMPs to Tregs was observed support in vivo relevance. These findings open the exciting possibility that PMPs actively regulate the immune response at sites of (vascular) inflammation, where they are known to accumulate and interact with leukocytes, consolidating the (vascular) healing process.

Anti-adhesive property of P-selectin glycoprotein ligand-1 (PSGL-1) due to steric hindrance effect

P-selectin glycoprotein ligand-1 (PSGL-1) is an adhesive molecule that is known to be a ligand for P-selectin. An anti-adhesive property of PSGL-1 has not been previously reported. In this study, we show that PSGL-1 expression is anti-adhesive for adherent cells and we have elucidated the underlying mechanism. Overexpression of PSGL-1 induced cell rounding and floating in HEK293T cells. Similar phenomena were demonstrated in other adherent cell lines with overexpression of PSGL-1. PSGL-1 overexpression inhibits access of antibodies to cell surface molecules such as integrins, HLA and CD25. Cells transfected with PSGL-1 deletion mutants that lack a large part of the extracellular domain and chimeric construct expressing extracellular CD86 and intracellular PSGL-1 only showed rounded morphology, but there are no floating cells. These results indicated that PSGL-1 causes steric hindrance due to the extended structure of its extracellular domain that is highly O-glycosylated, but intracellular domain also has some effect on cell rounding. This study implies that PSGL-1 has Janus-faced functions, being both adhesive and anti-adhesive.

PSGL-1 and E/P-selectins are essential for T-cell rolling in inflamed CNS microvessels but dispensable for initiation of EAE

T-cell migration across the blood-brain barrier is a crucial step in the pathogenesis of EAE, an animal model for MS. Live cell imaging studies demonstrated that P-selectin glycoprotein ligand-1 (PSGL-1) and its endothelial ligands E- and P-selectin mediate the initial rolling of T cells in brain vessels during EAE. As functional absence of PSGL-1 or E/P-selectins does not result in ameliorated EAE, we speculated that T-cell entry into the spinal cord is independent of PSGL-1 and E/P-selectin. Performing intravital microscopy, we observed the interaction of WT or PSGL-1(-/-) proteolipid protein-specific T cells in inflamed spinal cord microvessels of WT or E/P-selectin(-/-) SJL/J mice during EAE. T-cell rolling but not T-cell capture was completely abrogated in the absence of either PSGL-1 or E- and P-selectin, resulting in a significantly reduced number of T cells able to firmly adhere in the inflamed spinal cord microvessels, but did not lead to reduced T-cell invasion into the CNS parenchyma. Thus, PSGL-1 interaction with E/P-selectin is essential for T-cell rolling in inflamed spinal cord microvessels during EAE. Taken together with previous observations, our findings show that T-cell rolling is not required for successful T-cell entry into the CNS and initiation of EAE.

Regulatory T cells suppress the late phase of the immune response in lymph nodes through P-selectin glycoprotein ligand-1

Regulatory T cells (Tregs) maintain tolerance toward self-antigens and suppress autoimmune diseases, although the underlying molecular mechanisms are unclear. In this study, we show that mice deficient for P-selectin glycoprotein ligand-1 (PSGL-1) develop a more severe form of experimental autoimmune encephalomyelitis than wild type animals do, suggesting that PSGL-1 has a role in the negative regulation of autoimmunity. We found that Tregs lacking PSGL-1 were unable to suppress experimental autoimmune encephalomyelitis and failed to inhibit T cell proliferation in vivo in the lymph nodes. Using two-photon laser-scanning microscopy in the lymph node, we found that PSGL-1 expression on Tregs had no role in the suppression of early T cell priming after immunization with Ag. Instead, PSGL-1-deficient Tregs lost the ability to modulate T cell movement and failed to inhibit the T cell-dendritic cell contacts and T cell clustering essential for sustained T cell activation during the late phase of the immune response. Notably, PSGL-1 expression on myelin-specific effector T cells had no role in T cell locomotion in the lymph node. Our data show that PSGL-1 represents a previously unknown, phase-specific mechanism for Treg-mediated suppression of the persistence of immune responses and autoimmunity induction.

PSGL-1 regulates the migration and proliferation of CD8(+) T cells under homeostatic conditions

P-selectin glycoprotein ligand-1 (PSGL-1), a heavily glycosylated sialomucin expressed on most leukocytes, has dual function as a selectin ligand for leukocyte rolling on vascular selectins expressed in inflammation and as a facilitator of resting T cell homing into lymphoid organs. In this article, we document disturbances in T cell homeostasis present in PSGL-1(null) mice. Naive CD4(+) and CD8(+) T cell frequencies were profoundly reduced in blood, whereas T cell numbers in lymph nodes and spleen were at or near normal levels. Although PSGL-1(null) T cells were less efficient at entering lymph nodes, they also remained in lymph nodes longer than PSGL-1(+/+) T cells, suggesting that PSGL-1 supports T cell egress. In addition, PSGL-1(null) CD8(+) T cell proliferation was observed under steady-state conditions and PSGL-1(null) CD8(+) T cells were found to be hyperresponsive to homeostatic cytokines IL-2, IL-4, and IL-15. Despite these disturbances in T cell homeostasis, PSGL-1(null) mice exhibited a normal acute response (day 8) to lymphocytic choriomeningitis virus infection but generated an increased frequency of memory T cells (day 40). Our observations demonstrate a novel pleiotropic influence of PSGL-1 deficiency on several aspects of T cell homeostasis that would not have been anticipated based on the mild phenotype of PSGL-1(null) mice. These potentially offsetting effects presumably account for the near-normal cellularity seen in lymph nodes of PSGL-1(null) mice.

P-selectin glycoprotein ligand-1 modulates immune inflammatory responses in the enteric lamina propria

P-selectin glycoprotein ligand-1 (PSGL-1), a leukocyte adhesion receptor that interacts with selectins, induces a tolerogenic programme in bone marrow-derived dendritic cells (DCs), which in turn promotes the generation of T regulatory (Treg) lymphocytes. In the present study, we have used a mouse model of dextran sulphate sodium (DSS)-induced colitis and studied the characteristics of the inflammatory cell infiltrate in the lamina propria (LP), mesenteric lymph nodes (mLNs) and Peyer's patches (PPs) to assess the possible role of PSGL-1 in the modulation of the enteric immune response. We have found that untreated PSGL-1-deficient mice showed an altered proportion of innate and adaptive immune cells in mLNs and PPs as well as an activated phenotype of macrophages and DCs in the colonic LP that mainly produced pro-inflammatory cytokines. Administration of an anti-PSGL-1 antibody also reduced the total numbers of macrophages, DCs and B cells in the colonic LP, and induced a lower expression of MHC-II by DCs and macrophages. After DSS treatment, PSGL-1(-/-) mice developed colitis earlier and with higher severity than wild-type (WT) mice. Accordingly, the colonic LP of these animals showed an enhanced number of Th1 and Th17 lymphocytes, with enhanced synthesis of IL-1α, IL-6 and IL-22, and increased activation of LP macrophages. Together, our data indicate that PSGL-1 has a relevant homeostatic role in the gut-associated lymphoid tissue under steady-state conditions, and that this adhesion receptor is able to down-regulate the inflammatory phenomenon in DSS-induced colitis.