Podoplanin is a negative regulator of Th17 inflammation

C-type lectin-like receptor 2: roles and drug target

C-type lectin-like receptor-2 (CLEC-2) is a member of the C-type lectin superfamily of cell surface receptors. The first confirmed endogenous and exogenous ligands of CLEC-2 are podoplanin and rhodocytin, respectively. CLEC-2 is expressed on the surface of platelets, which participates in platelet activation and aggregation by binding with its ligands. CLEC-2 and its ligands are involved in pathophysiological processes, such as atherosclerosis, cancer, inflammatory thrombus status, maintenance of vascular wall integrity, and cancer-related thrombosis. In the last 5 years, different anti- podoplanin antibody types have been developed for the treatment of cancers, such as glioblastoma and lung cancer. New tests and new diagnostics targeting CLEC-2 are also discussed. CLEC-2 mediates thrombosis in various pathological states, but CLEC-2-specific deletion does not affect normal hemostasis, which would provide a new therapeutic tool for many thromboembolic diseases. The CLEC-2-podoplanin interaction is a target for cancer treatment. CLEC-2 may be applied in clinical practice and play a therapeutic role.

The role of Pannexin-1 channels, ATP, and purinergic receptors in the pathogenesis of HIV and SARS-CoV-2

Infectious agents such as human immune deficiency virus-1 (HIV) and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) use host proteins to infect, replicate, and induce inflammation within the host. A critical component of these diseases is the axis between pannexin-1 channels, extracellular ATP, and purinergic receptors. Here, we describe the potential therapeutic role of Pannexin-1/purinergic approaches to prevent or reduce the devastating consequences of these pathogens.

Podoplanin: A potential therapeutic target for thrombotic diseases

As a specific lymphatic marker and a key ligand of C-type lectin-like receptor 2 (CLEC-2), podoplanin (Pdpn) is involved in various physiological and pathological processes such as growth and development, respiration, blood coagulation, lymphangiogenesis, angiogenesis, and inflammation. Thrombotic diseases constitute a major cause of disability and mortality in adults, in which thrombosis and inflammation play a crucial role. Recently, increasing evidence demonstrates the distribution and function of this glycoprotein in thrombotic diseases such as atherosclerosis, ischemic stroke, venous thrombosis, ischemic-reperfusion injury (IRI) of kidney and liver, and myocardial infarction. Evidence showed that after ischemia, Pdpn can be acquired over time by a heterogeneous cell population, which may not express Pdpn in normal conditions. In this review, the research progresses in understanding the roles and mechanisms of podoplanin in thromobotic diseases are summarized. The challenges of podoplanin-targeted approaches for disease prognosis and preventions are also discussed.

Influx of podoplanin-expressing inflammatory macrophages into the genital tract following Chlamydia infection

Genital Chlamydia trachomatis infection remains a major health issue as it causes severe complications including pelvic inflammatory disease, ectopic pregnancy and infertility in females as a result of infection-associated chronic inflammation. Podoplanin, a transmembrane receptor, has been previously reported on inflammatory macrophages. Thus, strategies that specifically target podoplanin might be able to reduce local inflammation. This study investigated the expression level and function of podoplanin in a C. trachomatis infection model. C57BL/6 mice infected with the mouse pathogen Chlamydia muridarum were examined intermittently from days 1 to 60 using flow cytometry analysis. Percentages of conventional macrophages (CD11b⁺ CD11c^- F4/80⁺ ) versus inflammatory macrophages (CD11b⁺ CD11c⁺ F4/80⁺ ), and the expression of podoplanin in these cells were investigated. Subsequently, a podoplanin-knockout RAW264.7 cell was used to evaluate the function of podoplanin in C. trachomatis infection. Our findings demonstrated an increased CD11b⁺ cell volume in the spleen at day 9 after the infection, with augmented podoplanin expression, especially among the inflammatory macrophages. A large number of podoplanin-expressing macrophages were detected in the genital tract of C. muridarum-infected mice. Furthermore, analysis of the C. trachomatis-infected patients demonstrated a higher percentage of podoplanin-expressing monocytes than that in the noninfected controls. Using an in vitro infection in a transwell migration assay, we identified that macrophages deficient in podoplanin displayed defective migratory function toward C. trachomatis-infected HeLa 229 cells. Lastly, using immunoprecipitation-mass spectrometry method, we identified two potential podoplanin interacting proteins, namely, Cofilin 1 and Talin 1 actin-binding proteins. The present study reports a role of podoplanin in directing macrophage migration to the chlamydial infection site. Our results suggest a potential for reducing inflammation in individuals with chronic chlamydial infections by targeting podoplanin.

C-type lectin-like receptor (CLEC)-2, the ligand of podoplanin, induces morphological changes in podocytes

Podoplanin (PDPN) is intensely expressed on the podocyte membrane in an evolutionally conserved manner. CLEC-2, the endogenous ligand of PDPN, is highly expressed in platelets and also exists in a soluble form in plasma. Normally, podocytes are sequestered from CLEC-2, but when the glomerular barrier is injured, podocytes gain access to CLEC-2. We tested the effects of CLEC-2 in podocytes in vitro and in vivo. Cultured podocytes treated with Fc-CLEC-2 demonstrated that CLEC-2 induced the dephosphorylation of ezrin, radixin, and moesin (ERM) proteins. Podocytes treated with Fc-CLEC-2 also showed the dissociation of F-actin filaments from PDPN, F-actin degradation, detachment, and round morphology. Next, we perfused normal mouse kidney in vivo with FLAG-CLEC-2. CLEC-2 induced dephosphorylation of ERM and widening of the foot processes of podocytes. Platelets were detected by immunostaining for CD41 in the urine of mice with podocyte injury, indicating that podocytes can encounter platelets when glomeruli are injured. Collectively, these observations suggest that when platelets leak through the injured glomeruli, CLEC-2 from the platelets acts on PDPN in podocytes and induces morphological change and detachment, which may further aggravate podocyte injury. Thus, PDPN on podocytes may work as a leaked-platelet sensor.

Podoplanin: Its roles and functions in neurological diseases and brain cancers

Podoplanin is a small mucin-like glycoprotein involved in several physiological and pathological processes in the brain including development, angiogenesis, tumors, ischemic stroke and other neurological disorders. Podoplanin expression is upregulated in different cell types including choroid plexus epithelial cells, glial cells, as well as periphery infiltrated immune cells during brain development and neurological disorders. As a transmembrane protein, podoplanin interacts with other molecules in the same or neighboring cells. In the past, a lot of studies reported a pleiotropic role of podoplanin in the modulation of thrombosis, inflammation, lymphangiogenesis, angiogenesis, immune surveillance, epithelial mesenchymal transition, as well as extracellular matrix remodeling in periphery, which have been well summarized and discussed. Recently, mounting evidence demonstrates the distribution and function of this molecule in brain development and neurological disorders. In this review, we summarize the research progresses in understanding the roles and mechanisms of podoplanin in the development and disorders of the nervous system. The challenges of podoplanin-targeted approaches for disease prognosis and preventions are also discussed.

The Role of Podoplanin in the Immune System and Inflammation

Podoplanin is a small cell-surface mucin-like glycoprotein that participates in multiple physiological and pathological processes. Podoplanin exerts an important function in the immune response and is upregulated in fibroblasts, macrophages, T helper cells, and epithelial cells during inflammation. Herein, we summarize the latest knowledge on the functional expression of podoplanin in the immune system and review the contribution of podoplanin to several inflammatory diseases. Furthermore, we discuss podoplanin as a novel therapeutic target for various inflammatory diseases.

Targeting Th17 cells in HIV-1 remission/cure interventions

Since the discovery of HIV-1, progress has been made in deciphering the viral replication cycle and mechanisms of host-pathogen interactions that has facilitated the implementation of effective antiretroviral therapies (ARTs). Major barriers to HIV-1 remission/cure include the persistence of viral reservoirs (VRs) in long-lived CD4⁺ T cells, residual viral transcription, and lack of mucosal immunity restoration during ART, which together fuel systemic inflammation. Recently, T helper (Th)17-polarized cells were identified as major contributors to the pool of transcriptionally/translationally competent VRs. In this review, we discuss the functional features of Th17 cells that were elucidated by fundamental immunology studies in the context of autoimmunity. We also highlight recent discoveries supporting the possibility of extrapolating this knowledge toward the identification of new putative Th17-targeted HIV-1 remission/cure strategies.

Proteomic Alterations and Novel Markers of Neurotoxic Reactive Astrocytes in Human Induced Pluripotent Stem Cell Models

Astrocytes respond to injury, infection, and inflammation in the central nervous system by acquiring reactive states in which they may become dysfunctional and contribute to disease pathology. A sub-state of reactive astrocytes induced by proinflammatory factors TNF, IL-1α, and C1q ("TIC") has been implicated in many neurodegenerative diseases as a source of neurotoxicity. Here, we used an established human induced pluripotent stem cell (hiPSC) model to investigate the surface marker profile and proteome of TIC-induced reactive astrocytes. We propose VCAM1, BST2, ICOSL, HLA-E, PD-L1, and PDPN as putative, novel markers of this reactive sub-state. We found that several of these markers colocalize with GFAP+ cells in post-mortem samples from people with Alzheimer's disease. Moreover, our whole-cells proteomic analysis of TIC-induced reactive astrocytes identified proteins and related pathways primarily linked to potential engagement with peripheral immune cells. Taken together, our findings will serve as new tools to purify reactive astrocyte subtypes and to further explore their involvement in immune responses associated with injury and disease.

The Role of Podoplanin in Skin Diseases

Podoplanin is a sialomucin-like type I transmembrane receptor glycoprotein that is expressed specifically in lymphatic vessels, sebaceous glands, and hair follicles in normal skin. However, under pathological conditions podoplanin expression is upregulated in various cells, such as keratinocytes, fibroblasts, tumor cells, and inflammatory cells, and plays pivotal roles in different diseases. In psoriasis, podoplanin expression is induced in basal keratinocytes via the JAK-STAT pathway and contributes toward epidermal hyperproliferation. Podoplanin expression on keratinocytes can also promote IL-17 secretion from lymphocytes, promoting chronic inflammation. During wound healing, the podoplanin/CLEC-2 interaction between keratinocytes and platelets regulates re-epithelialization at the wound edge. In skin cancers, podoplanin expresses on tumor cells and promotes their migration and epithelial-mesenchymal transition, thereby accelerating invasion and metastasis. Podoplanin is also expressed in normal peritumoral cells, such as cancer-associated fibroblasts in melanoma and keratinocytes in extramammary Paget's disease, which promote tumor progression and predict aggressive behavior and poor prognosis. This review provides an overview of our current understanding of the mechanisms via which podoplanin mediates these pathological skin conditions.

The double-edged sword: Harnessing PD-1 blockade in tumor and autoimmunity

[Figure: see text].

Cardiac ectopic lymphoid follicle formation in viral myocarditis involving the regulation of podoplanin in Th17 cell differentiation

Autoimmunity contributes to the pathogenesis of viral myocarditis (VMC), which is characterized by the production of anti-heart autoantibodies (AHA) from lymphoid follicles. Recently, the formation of ectopic lymphoid follicles (ELFs) was reported in heart grafts. However, the existence and role of ELFs in myocardial tissues of VMC remain unclear. This study aimed to explore whether and how cardiac ELFs with germinal centers (GCs) could be generated during the development of VMC. We identified the existence of ELFs and explored the underlying mechanism. In a BALB/c mouse model of VMC, the dynamic myocardial infiltrations of lymphocytic aggregates and expressions of associated lymphorganogenic factors were investigated, accompanied by the detection of the production and location of myocardial AHA. The data indicated ELFs formation in myocardial tissues of VMC, and the number of ELFs was in accordance with the severity of VMC. Moreover, the functional ELFs with GCs were capable of facilitating the production of local AHA. Blocking IL-17 or podoplanin (PDPN) could inhibit cardiac ELFs generation, perhaps due to the negative regulation of PDPN neutralization in Th17 cell proliferation and differentiation. The presence of cardiac ELFs and AHA might offer new opportunities for stratification and early identification of VMC patients.

Emerging roles for IL-6 family cytokines as positive and negative regulators of ectopic lymphoid structures

IL-6 family cytokines display broad effects in haematopoietic and non-haematopoietic cells that regulate immune homeostasis, host defence, haematopoiesis, development, reproduction and wound healing. Dysregulation of these activities places this cytokine family as important mediators of autoimmunity, chronic inflammation and cancer. In this regard, ectopic lymphoid structures (ELS) are a pathological hallmark of many tissues affected by chronic disease. These inducible lymphoid aggregates form compartmentalised T cell and B cell zones, germinal centres, follicular dendritic cell networks and high endothelial venules, which are defining qualities of peripheral lymphoid organs. Accordingly, ELS can support local antigen-specific responses to self-antigens, alloantigens, pathogens and tumours. ELS often correlate with severe disease progression in autoimmune conditions, while tumour-associated ELS are associated with enhanced anti-tumour immunity and a favourable prognosis in cancer. Here, we discuss emerging roles for IL-6 family cytokines as regulators of ELS development, maintenance and activity and consider how modulation of these activities has the potential to aid the successful treatment of autoimmune conditions and cancers where ELS feature.

Infliximab modifies regulatory T cells and co-inhibitory receptor expression on circulating T cells in psoriasis

BACKGROUND

Psoriasis is a T cell-mediated autoimmune skin disease. Accumulating evidence has demonstrated that co-inhibitory receptors (CIRs) play a vital role in regulating T cell-mediated immune response, especially in neoplasm and autoimmunity. However, the immuno-function of CIRs in the development of psoriasis remains unclear.

OBJECTIVE

We investigated the expression of CIRs on the circulating T lymphocytes of psoriasis patients before and after anti-tumor necrosis factor-α (TNF-α) therapy.

METHODS

We enrolled 17 patients with moderate-to-severe plaque psoriasis, 17 patients with mild plaque psoriasis, and 18 healthy controls in this study. Fourteen of the moderate-to-severe psoriasis patients were treated with infliximab, a monoclonal antibody against TNF-α. Peripheral blood was collected, and peripheral blood mononuclear cells were extracted. The proportion of T cell subsets along with their expression of CIRs, namely T cell immunoreceptor with Ig and ITIM domains (TIGIT), lymphocyte activating gene 3 (LAG-3), cytotoxic T-lymphocyte associated protein 4 (CTLA-4), B and T lymphocyte-associated protein (BTLA), endothelial protein C receptor (PROCR), podoplanin (PDPN), programmed cell death 1 (PD-1), and T cell immunoglobulin mucin family containing molecule 3 (TIM-3), were determined by flow cytometric assay.

RESULTS

The moderate-to-severe plaque psoriasis patients had less circulating Tregs, which increased after infliximab treatment. They also had decreased TIGIT, LAG-3 but increased PDPN expression on peripheral CD4⁺ T cells. Infliximab enhanced TIGIT, LAG-3, CTLA-4 but reduced PROCR expression on circulating CD4⁺ T cells. Remarkably, both the frequency of circulating Tregs and the expression level of TIGIT on CD4⁺ T cells at baseline (pre-treatment) negatively correlated with the extent of PASI score reduction benefited from infliximab therapy.

CONCLUSION

Anti-TNF-α therapy increased the frequency of Tregs and TIGIT, LAG-3, CTLA-4 expression but reduced PROCR expression on circulating CD4⁺ T cells in psoriasis patients. The baseline proportion of Tregs and the expression level of TIGIT on circulating CD4⁺ T cells might serve as predictive markers for the degree of disease remission benefited from infliximab treatment.

Immunohistochemical study for the expression of leukocyte adhesion molecules, and FGF23 and ACE2 in P. gingivalis LPS-induced diabetic nephropathy

Objective

The present study aims to examine the expression of leukocyte adhesion molecules and renal metabolic factors in diabetic mouse kidneys with periodontal pathogen Pg-LPS-induced nephropathy.

Background

We recently reported that the glomerular endothelium expresses toll-like receptor (TLR)2 and TLR4 in diabetic environments and TLR2/4 ligand Porphyromonas (P.) gingivalis lipopolysaccharides (Pg-LPS) induce nephropathy in diabetic mice. It is thought that Pg-LPS promotes the chronic inflammation with the overexpression of leukocyte adhesion molecules and renal-specific metabolic enzymes by the recognition of Pg-LPS via TLR in the diabetic kidneys. There have been no reports of the effects of periodontopathic bacteria on the expression of leukocyte adhesion molecules and the accumulation of physiologically active substances in the kidney.

Methods

The immunohistochemical investigation was performed on diabetic mouse kidney with Pg-LPS-induced nephropathy with glomerulosclerosis in glomeruli.

Results

There were no vessels which expressed vascular cell adhesion molecule-1 (VCAM-1), E-selectin, or fibroblast growth factor (FGF) 23 in streptozotocin (STZ)-induced diabetic ICR mice (STZ-ICR), or in healthy ICR mice administered Pg-LPS (LPS-ICR). However, in diabetic ICR mouse kidneys with Pg-LPS-induced nephropathy (LPS-STZ) the expression of VCAM-1 and the accumulation of FGF23 were observed in renal tubules and glomeruli, and the expression of E-selectin was observed in renal parenchyma and glomeruli. The angiotensin-converting enzyme 2 (ACE2) was detected in the proximal tubules but not in other regions of ICR, STZ-ICR, or LPS-ICR. In LPS-STZ ACE2 was detected both in renal tubules as well as in glomeruli. The Mac-1 and podoplanin-positive cells increased in the renal parenchyma with diabetic condition and there was the distribution of a large number of Mac-1-positive cells in LPS-STZ.

Conclusions

The Pg-LPS may induce diabetic renal inflammation such as glomerulosclerosis and tubulitis with infiltration of Mac-1/podoplanin positive macrophages via glomerular overexpression of VCAM-1 and E-selectin, resulting in accumulation of both ACE2 and FGF23 which were unmetabolized with the inflammation-induced kidney damage under the diabetic condition. Periodontitis may be a critical factor in the progress of nephropathy in diabetic patients.

PDPN Is Expressed in Various Types of Canine Tumors and Its Silencing Induces Apoptosis and Cell Cycle Arrest in Canine Malignant Melanoma

Podoplanin (PDPN), a small transmembrane mucin-like glycoprotein, is ectopically expressed. It is also known to be linked with several aspects of tumor malignancy in some types of human tumors, including invasion, metastasis, and cancer stemness. However, there are few reports on the expression of dog PDPN (dPDPN) in canine tumors, and the association between dPDPN and tumor malignancy has not been elucidated. We identified that 11 out of 18 types of canine tumors expressed dPDPN. Furthermore, 80% of canine malignant melanoma (MM), squamous cell carcinoma, and meningioma expressed dPDPN. Moreover, the expression density of dPDPN was positively associated with the expression of the Ki67 proliferation marker. The silencing of dPDPN by siRNAs resulted in the suppression of cell migration, invasion, stem cell-like characteristics, and cell viability in canine MM cell lines. The suppression of cell viability was caused by the induction of apoptosis and G2/M phase cell cycle arrest. Overall, this study demonstrates that dPDPN is expressed in various types of canine tumors and that dPDPN silencing suppresses cell viability through apoptosis and cell cycle arrest, thus providing a novel biological role for PDPN in tumor progression.

Identification and validation of poor prognosis immunoevasive subtype of muscle-invasive bladder cancer with tumor-infiltrating podoplanin+ cell abundance

The choice of chemo- or immuno-therapy for muscle-invasive bladder cancer (MIBC) patients remains contentious. Podoplanin is newly identified as an immune checkpoint which intrigues us to explore the clinical significance and immunoregulatory role of tumor-infiltrating podoplanin⁺ cells (PDPN⁺ cells) in MIBC. A retrospective analysis of 259 MIBC patients from Zhongshan Hospital (n = 141) and Shanghai Cancer Center (n = 118) was conducted. A total of 406 MIBC patients from TCGA database were enrolled to investigate the relationship between PDPN and molecular characterization. We found that tumor-infiltrating PDPN⁺ cell abundance indicated an inferior overall survival and recurrence-free survival. pT2 MIBC patients with PDPN⁺ cell low infiltration could benefit more from adjuvant chemotherapy (ACT). Increased PDPN⁺ cell infiltration was associated with diminished GZMB and TNF-α expression while correlated with expanded PD-1, PD-L1, LAG-3 and TIM-3 expression and tumor-promoting regulatory T cell and M2 macrophage infiltration. Tumors with high PDPN mRNA expression mainly presented luminal-infiltrated and basal-squamous subtypes (2017 TCGA classification) or stroma-rich and Ba/Sq subtypes (consensus classification). Elevated PDPN mRNA expression was associated with less FGFR3 activation signature and more T-cell-inflamed signature and EGFR activation signature. In conclusion, tumor-infiltrating PDPN⁺ cells could be applied as an independent prognosticator for clinical outcome and a predictive biomarker for suboptimal ACT responsiveness, which was also associated with immunosuppressive contexture infiltration. Intratumoral PDPN expression had a correlation with MIBC molecular classification and therapy-related signatures. The novel immune checkpoint PDPN should be considered as a possible immunotherapeutic target for MIBC.

The yin and yang of co-inhibitory receptors: toward anti-tumor immunity without autoimmunity

Co-inhibitory receptors are important regulators of T-cell function that define the balance between tolerance and autoimmunity. The immune regulatory function of co-inhibitory receptors, including CTLA-4, PD-1, TIM-3, TIGIT, and LAG-3, was first discovered in the setting of autoimmune disease models, in which their blockade or deficiency resulted in induction or exacerbation of the disease. Later on, co-inhibitory receptors on lymphocytes have also been found to influence outcomes in tumor and chronic viral infection settings. These receptors suppress T-cell function in the tumor microenvironment (TME), thereby making the T cells dysfunctional. Based on this observation, blockade of co-inhibitory receptors (also known as checkpoint molecules) has emerged as a successful treatment option for a number of human cancers. However, severe autoimmune-like side effects limit the use of therapeutics that block individual or combinations of co-inhibitory receptors for cancer treatment. In this review we provide an overview of the role of co-inhibitory receptors in autoimmunity and anti-tumor immunity. We then discuss current approaches and future directions to leverage our knowledge of co-inhibitory receptors to target them in tumor immunity without inducing autoimmunity.

CLEC2 and CLEC5A: Pathogenic Host Factors in Acute Viral Infections

The protective roles of endosomal toll-like receptors (TLRs) and cytosolic nucleic acid sensors are well elucidated, but the pathogenic host factors during viral infections remain unclear. Spleen tyrosine kinase (Syk)-coupled C-type lectins (CLECs) CLEC2 and CLEC5A are highly expressed on platelets and myeloid cells, respectively. CLEC2 has been shown to recognize snake venom aggretin and the endogenous ligand podoplanin and acts as a critical regulator in the development and immunothrombosis. Although CLEC2 has been reported to interact with type I immunodeficiency virus (HIV-1), its role in viral infections is still unclear. CLEC5A binds to fucose and mannose moieties of dengue virus membrane glycans, as well as to N-acetylglucosamine (GlcNAc)/N-acetylmuramic acid (MurNAc) disaccharides that form the backbone of L. monocytogenes peptidoglycans. Recently, we demonstrated that both CLEC2 and CLEC5A are critical in microbe-induced “neutrophil extracellular trap” (NET) formation and proinflammatory cytokine production. Moreover, activation of CLEC2 by dengue virus (DV) and H5N1 influenza virus (IAV) induces the release of extracellular vesicles (EVs), which further enhance NETosis and proinflammatory cytokine production via CLEC5A and Toll-like receptor 2 (TLR2). These findings not only illustrate the immunomodulatory effects of EVs during platelet-leukocyte interactions, but also demonstrate the critical roles of CLEC2 and CLEC5A in acute viral infections.

C-type lectin receptors of the Dectin-1 cluster: Physiological roles and involvement in disease

C-type lectin receptors (CLRs) are essential for multicellular existence, having diverse functions ranging from embryonic development to immune function. One subgroup of CLRs is the Dectin-1 cluster, comprising of seven receptors including MICL, CLEC-2, CLEC-12B, CLEC-9A, MelLec, Dectin-1, and LOX-1. Reflecting the larger CLR family, the Dectin-1 cluster of receptors has a broad range of ligands and functions, but importantly, is involved in numerous pathophysiological processes that regulate health and disease. Indeed, these receptors have been implicated in development, infection, regulation of inflammation, allergy, transplantation tolerance, cancer, cardiovascular disease, arthritis, and other autoimmune diseases. In this mini-review, we discuss the latest advancements in elucidating the function(s) of each of the Dectin-1 cluster CLRs, focussing on their physiological roles and involvement in disease.

The non-haemostatic role of platelets in systemic lupus erythematosus

Dysregulation of lymphocyte function, accumulation of autoantibodies and defective clearance of circulating immune complexes and apoptotic cells are hallmarks of systemic lupus erythematosus (SLE). Moreover, it is now evident that an intricate interplay between the adaptive and innate immune systems contributes to the pathogenesis of SLE, ultimately resulting in chronic inflammation and organ damage. Platelets circulate in the blood and are chiefly recognized for their role in the prevention of bleeding and promotion of haemostasis; however, accumulating evidence points to a role for platelets in both adaptive and innate immunity. Through a broad repertoire of receptors, platelets respond promptly to immune complexes, complement and damage-associated molecular patterns, and represent a major reservoir of immunomodulatory molecules in the circulation. Furthermore, evidence suggests that platelets are activated in patients with SLE, and that they could contribute to the circulatory autoantigenic load through the release of microparticles and mitochondrial antigens. Herein, we highlight how platelets contribute to the immune response and review evidence implicating platelets in the pathogenesis of SLE.

Podoplanin in Inflammation and Cancer

Podoplanin is a small cell-surface mucin-like glycoprotein that plays a crucial role in the development of the alveoli, heart, and lymphatic vascular system. Emerging evidence indicates that it is also involved in the control of mammary stem-cell activity and biogenesis of platelets in the bone marrow, and exerts an important function in the immune response. Podoplanin expression is upregulated in different cell types, including fibroblasts, macrophages, T helper cells, and epithelial cells, during inflammation and cancer, where it plays important roles. Podoplanin is implicated in chronic inflammatory diseases, such as psoriasis, multiple sclerosis, and rheumatoid arthritis, promotes inflammation-driven and cancer-associated thrombosis, and stimulates cancer cell invasion and metastasis through a variety of strategies. To accomplish its biological functions, podoplanin must interact with other proteins located in the same cell or in neighbor cells. The binding of podoplanin to its ligands leads to modulation of signaling pathways that regulate proliferation, contractility, migration, epithelial⁻mesenchymal transition, and remodeling of the extracellular matrix. In this review, we describe the diverse roles of podoplanin in inflammation and cancer, depict the protein ligands of podoplanin identified so far, and discuss the mechanistic basis for the involvement of podoplanin in all these processes.

Functional significance of the platelet immune receptors GPVI and CLEC-2

Although platelets are best known for their role in hemostasis, they are also crucial in development, host defense, inflammation, and tissue repair. Many of these roles are regulated by the immune-like receptors glycoprotein VI (GPVI) and C-type lectin receptor 2 (CLEC-2), which signal through an immunoreceptor tyrosine-based activation motif (ITAM). GPVI is activated by collagen in the subendothelial matrix, by fibrin and fibrinogen in the thrombus, and by a remarkable number of other ligands. CLEC-2 is activated by the transmembrane protein podoplanin, which is found outside of the vasculature and is upregulated in development, inflammation, and cancer, but there is also evidence for additional ligands. In this Review, we discuss the physiological and pathological roles of CLEC-2 and GPVI and their potential as targets in thrombosis and thrombo-inflammatory disorders (i.e., disorders in which inflammation plays a critical role in the ensuing thrombosis) relative to current antiplatelet drugs.

A safety study of newly generated anti-podoplanin-neutralizing antibody in cynomolgus monkey (Macaca fascicularis)

Hematogenous metastases are enhanced by platelet aggregation induced by tumor cell-platelet interaction. Podoplanin is a key molecule to enhance the platelet aggregation and interacts with C-type lectin-like receptor 2 (CLEC-2) on platelet via PLAG domains. Our previous reports have shown that blocking podoplanin binding to platelets by neutralizing antibody specific to PLAG4 domain strongly reduces hematogenous metastasis. However, podoplanin is expressed in a variety of normal tissues such as lymphatic vessels and the question remains whether treatment of tumors with anti-podoplanin neutralizing antibodies would be toxic. Monkeys are the most suitable species for that purpose. PLAG3 and PLAG4 domains had high homology among various monkey species and human. PLAG domain deleted mutants were indicated that monkey PLAG4 domain played a more crucial role in podoplanin-induced platelet aggregation than did the PLAG3 domain as in human. Moreover, newly established neutralizing antibodies (1F6, 2F7, and 3F4) targeting the monkey PLAG4 domain blocked interaction between monkey podoplanin and CLEC-2. Especially, the 2F7 neutralizing antibody strongly suppressed platelet aggregation and pulmonary metastasis. Furthermore, inhibiting podoplanin function with 2F7 neutralizing antibody exhibited no acute toxicity in cynomolgus monkeys. Our results suggested that targeting podoplanin with specific neutralizing antibodies may be an effective anticancer treatment.

Co-expression of CD21L and IL17A defines a subset of rheumatoid synovia, characterised by large lymphoid aggregates and high inflammation

OBJECTIVE

To determine whether the expression of IL17A and CD21L genes in inflamed rheumatoid synovia is associated with the neogenesis of ectopic lymphoid follicle-like structures (ELS), and if this aids the stratification of rheumatoid inflammation and thereby distinguishes patients with rheumatoid arthritis that might be responsive to specific targeted biologic therapies.

METHODS

Expression of IL17A and CD21L genes was assessed by RT-PCR, qRT-PCR and dPCR in synovia from 54 patients with rheumatoid arthritis. A subset of synovia (n = 30) was assessed by immunohistology for the presence of CD20+ B-lymphocytes and size of CD20+ B-lymphocyte aggregates as indicated by maximum radial cell count. The molecular profiles of six IL17A+/CD21L+ and six IL17A-/CD21L- synovia were determined by complementary DNA microarray analysis.

RESULTS

By RT-PCR, 26% of synovia expressed IL17A and 52% expressed CD21L. This provided the basis for distinguishing four subgroups of rheumatoid synovia: IL17A+/CD21L+ (18.5% of synovia), IL17A+/CD21L- (7.5%), IL17A-/CD21L+ (33.3%) and IL17A-/CD21L- (40.7%). While the subgroups did not predict clinical outcome measures, comparisons between the synovial subgroups revealed the IL17A+/CD21L+ subgroup had significantly larger CD20+ B-lymphocyte aggregates (P = 0.007) and a gene expression profile skewed toward B-cell- and antibody-mediated immunity. In contrast, genes associated with bone and cartilage remodelling were prominent in IL17A-/CD21L- synovia.

CONCLUSIONS

Rheumatoid synovia can be subdivided on the basis of IL17A and CD21L gene expression. Ensuing molecular subgroups do not predict clinical outcome for patients but highlight high inflammation and the predominance of B-lymphocyte mediated mechanisms operating in IL17A+/CD21L+ synovia. This may provide a rationale for more refined therapeutic selection due to the distinct molecular profiles associated with IL17A+/CD21L+ and IL17A-/CD21L- rheumatoid synovia.

A cellular and molecular view of T helper 17 cell plasticity in autoimmunity

Since the original identification of the T helper 17 (Th17) subset in 2005, it has become evident that these cells do not only contribute to host defence against pathogens, such as bacteria and fungi, but that they are also critically involved in the pathogenesis of many autoimmune diseases. In contrast to the classic Th1 and Th2 cells, which represent rather stably polarized subsets, Th17 cells display remarkable heterogeneity and plasticity. This has been attributed to the characteristics of the key transcription factor that guides Th17 differentiation, retinoic acid receptor-related orphan nuclear receptor gamma (RORγ). Unlike the 'master regulators' T-bet and GATA3 that orchestrate Th1 and Th2 differentiation, respectively, RORγ controls transcription at relatively few loci in Th17 cells. Moreover, its expression is not stabilized by positive feedback loops but rather influenced by environmental cues, allowing for substantial functional plasticity. Importantly, a subset of IL-17/IFNγ double-producing Th17 cells was identified in both human and mouse models. Evidence is accumulating that these IL-17/IFNγ double-producing cells are pathogenic drivers in autoimmune diseases, including rheumatoid arthritis, multiple sclerosis and inflammatory bowel disease. In addition, IL-17/IFNγ double-producing cells have been identified in disorders in which the role of autoimmunity remains unclear, such as sarcoidosis. The observed plasticity of Th17 cells towards the Th1 phenotype can be explained by extensive epigenetic priming of the IFNG locus in Th17 cells. In fact, Th17 cells display an IFNG chromatin landscape that is remarkably similar to that of Th1 cells. On the other hand, pathogenic capabilities of Th17 cells can be restrained by stimulating IL-10 production and transdifferentiation into IL-10 producing T regulatory type 1 (Tr1) cells. In this review, we discuss recent advances in our knowledge on the cellular and molecular mechanisms involved in Th17 differentiation, heterogeneity and plasticity. We focus on transcriptional regulation of the Th17 expression program, the epigenetic dynamics involved, and how genetic variants associated with autoimmunity may affect immune responses through distal gene regulatory elements. Finally, the implications of Th17 cell plasticity for the pathogenesis and treatment of human autoimmune diseases will be discussed.