New horizon in platelet function: with special reference to a recently-found molecule, CLEC-2

Leishmania donovani Attenuates Dendritic Cell Trafficking to Lymph Nodes by Inhibiting C-Type Lectin Receptor 2 Expression via Transforming Growth Factor-β

To initiate an antileishmanial adaptive immune response, dendritic cells (DCs) must carry Leishmania antigens from peripheral tissues to local draining lymph nodes. However, the migratory capacity of DCs is largely compromised during Leishmania donovani infection. The molecular mechanism underlying this defective DC migration is not yet fully understood. Here, we demonstrate that L. donovani infection impaired the lymph node homing ability of DCs by decreasing C-type lectin receptor 2 (CLEC-2) expression. L. donovani exerted this inhibitory effect by inducing transforming growth factor-β (TGF-β) secretion from DCs. Indeed, TGF-β produced in this manner inhibited nuclear factor-κB (NF-κB)-mediated CLEC-2 expression on DCs by activating c-Src. Notably, suppression of c-Src expression significantly improved the arrival of DCs in draining lymph nodes by preventing L. donovani-induced CLEC-2 downregulation on DCs. These findings reveal a unique mechanism by which L. donovani inhibits DC migration to lymph nodes and suggest a key role for TGF-β, c-Src, and CLEC-2 in regulating this process. IMPORTANCE Dendritic cells (DCs) play a key role in initiating T cell-mediated protective immunity against visceral leishmaniasis (VL), the second most lethal parasitic disease in the world. However, the T cell-inducing ability of DCs critically depends on the extent of DC migration to regional lymph nodes. Notably, the migration of DCs is reported to be impaired during VL. The cause of this impaired DC migration, however, remains ill-defined. Here, we provide the first evidence that L. donovani, the causative agent of VL, attenuates the lymph node homing capacity of DCs by decreasing C-type lectin receptor 2 (CLEC-2) expression on DCs. Additionally, we have demonstrated how L. donovani mediates this inhibitory effect. Overall, our work has revealed a unique mechanism underlying L. donovani-induced impairment of DC migration and suggests a potential strategy to improve antileishmanial T cell activity by increasing DC arrival in lymph nodes.

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.

CLEC1B is a Promising Prognostic Biomarker and Correlated with Immune Infiltration in Hepatocellular Carcinoma

Purpose

C-type lectin domain family 1 member B (CLEC1B) is a protein-coding gene involved in various processes, such as platelet activation, tumor cell metastasis and separation of blood/lymphatic vessels. However, how CLEC1B plays its role in hepatocellular carcinoma (HCC) has not been well studied. The purpose of this study was to investigate the clinical significance and biological function of CLEC1B in HCC.

Patients and Methods

Based on (The Cancer Genome Atlas) TCGA database, CLEC1B expression matrix and corresponding clinical information were extracted. ROC curves and Kaplan–Meier method were generated to evaluate the value of CLEC1B as a diagnostic and prognostic biomarker. Moreover, single-gene difference analysis constructed by DESeq2 method and then the related genes were used to predict CLEC1B-related signaling pathways. The ssGSEA algorithm was conducted for studies related to immune infiltration. CLEC1B protein expression was evaluated and immunohistochemistry in HCC tissues through tissue microarray. Finally, the relationship between CLEC1B expression and T cell infiltration was assessed according to tissue microarray.

Results

The mRNA and protein levels of CLEC1B were significantly down-regulated in HCC compared to paired normal tissues, which were further verified in clinical tissue samples. ROC curves and Kaplan–Meier survival analysis suggested the significant diagnostic and clinical prognostic value of CLEC1B. Meanwhile, downregulation of CLEC1B was significantly associated with clinical parameters such as clinical tumor vascular invasion and distant metastasis. Moreover, Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and gene set enrichment (GSEA) analysis indicated that CLEC1B has significant association with immune function. Finally, immune infiltration analysis indicated that CLEC1B was significantly associated with immune cell subsets and affected the efficacy of immunotherapy in cancer patient.

Conclusion

Collectively, our findings suggested that CLEC1B could be a promising prognostic biomarker in HCC and its expression was related to immune cell infiltration.

Human Dectin-1 is O-glycosylated and serves as a ligand for C-type lectin receptor CLEC-2

C-type lectin receptors (CLRs) elicit immune responses upon recognition of glycoconjugates present on pathogens and self-components. While Dectin-1 is the best-characterized CLR recognizing β-glucan on pathogens, the endogenous targets of Dectin-1 are not fully understood. Herein, we report that human Dectin-1 is a ligand for CLEC-2, another CLR expressed on platelets. Biochemical analyses revealed that Dectin-1 is a mucin-like protein as its stalk region is highly O-glycosylated. A sialylated core 1 glycan attached to the EDxxT motif of human Dectin-1, which is absent in mouse Dectin-1, provides a ligand moiety for CLEC-2. Strikingly, the expression of human Dectin-1 in mice rescued the lethality and lymphatic defect resulting from a deficiency of Podoplanin, a known CLEC-2 ligand. This finding is the first example of an innate immune receptor also functioning as a physiological ligand to regulate ontogeny upon glycosylation.

Post-Surgical Peritoneal Scarring and Key Molecular Mechanisms

Post-surgical adhesions are internal scar tissue and a major health and economic burden. Adhesions affect and involve the peritoneal lining of the abdominal cavity, which consists of a continuous mesothelial covering of the cavity wall and majority of internal organs. Our understanding of the full pathophysiology of adhesion formation is limited by the fact that the mechanisms regulating normal serosal repair and regeneration of the mesothelial layer are still being elucidated. Emerging evidence suggests that mesothelial cells do not simply form a passive barrier but perform a wide range of important regulatory functions including maintaining a healthy peritoneal homeostasis as well as orchestrating events leading to normal repair or pathological outcomes following injury. Here, we summarise recent advances in our understanding of serosal repair and adhesion formation with an emphasis on molecular mechanisms and novel gene expression signatures associated with these processes. We discuss changes in mesothelial biomolecular marker expression during peritoneal development, which may help, in part, to explain findings in adults from lineage tracing studies using experimental adhesion models. Lastly, we highlight examples of where local tissue specialisation may determine a particular response of peritoneal cells to injury.

The endothelial barrier and cancer metastasis: Does the protective facet of platelet function matter?

Overwhelming evidence suggests that platelets have a detrimental role in promoting cancer spread via platelet-cancer cell interactions linked to thrombotic mechanisms. On the other hand, a beneficial role of platelets in the preservation of the endothelial barrier in inflammatory conditions has been recently described, a phenomenon that could also operate in cancer-related inflammation. It is tempting to speculate that some antiplatelet strategies to combat cancer metastasis may impair the endogenous platelet-dependent mechanisms preserving endothelial barrier function. If the protective function of platelets is impaired, it may lead to increased endothelial permeability and more efficient cancer cell intravasation in the primary tumor and cancer cell extravasation at metastatic sites. In this commentary, we discuss current evidence that could support this hypothesis.

Snake venom rhodocytin induces plasma extravasation via toxin-mediated interactions between platelets and mast cells

Venomous snakebites can induce local tissue damage, including necrosis of soft tissues, haemorrhage, blistering and local swelling associated with plasma extravasation, which can lead to lethal complications such as hypovolemic shock. However, the details of the underlying mechanisms remain unknown. In this study, we showed that intradermal treatment of mice with venom rhodocytin from the Malayan viper Calloselasma rhodostoma induced plasma extravasation, dependent on C-type lectin-like receptor 2 (CLEC-2) on platelets. Rhodocytin-induced plasma extravasation also relied on mast cells and histamine. In vitro co-culture of rhodocytin-activated platelets with mast cells induced histamine release from mast cells in an ATP/P2X7-dependent manner. Consistent with this, blockade or deficiency of P2X7 in mast cells suppressed rhodocytin-induced plasma extravasation in the skin. Together, these findings indicate that rhodocytin induces plasma extravasation by triggering platelet activation via CLEC-2, followed by activation of mast cells and histamine release via the ATP/P2X7 pathway. These results reveal a previously unrecognized mechanism by which snake venom increases vascular permeability via complex venom toxin–mediated interactions between platelets and mast cells.

Plasma soluble C-type lectin-like receptor-2 is associated with the risk of coronary artery disease

Accumulating evidence suggests that C-type lectin-like receptor-2 (CLEC-2) plays an important role in atherothrombosis. In this case-control study, we investigated the association between CLEC-2 and incidence of coronary artery disease (CAD). A total of 216 patients, including 14 cases of stable angina pectoris (SAP, non-ACS) and 202 cases of acute coronary syndrome (ACS), and 89 non-CAD control subjects were enrolled. Plasma levels of soluble CLEC-2 (sCLEC-2) were measured using the enzyme-linked immunosorbent assay (ELISA). Compared with the control group (65.69 (55.36-143.22) pg/mL), the plasma levels of sCLEC-2 were significantly increased in patients with CAD (133.67 (88.76-220.09) pg/mL) and ACS (134.16 (88.88-225.81) pg/mL). The multivariate adjusted odds ratios (95% confidence interval) of CAD reached 2.01 (1.52-2.66) (P_trend < 0.001) for each 1-quartile increase in sCLEC-2. Restricted cubic splines showed a positive dose-response association between sCLEC2 and CAD incidence (P_linearity < 0.001). The addition of sCLEC-2 to conventional risk factors improved the C statistic (0.821 vs. 0.761, P = 0.004) and reclassification ability (net reclassification improvement: 57.45%, P < 0.001; integrated discrimination improvement: 8.27%, P < 0.001) for CAD. In conclusion, high plasma sCLEC-2 is independently associated with CAD risk, and the prognostic value of sCLEC-2 may be evaluated in future prospective studies.

[CLEC-2 induced signalling in blood platelets]

Platelet activating receptor CLEC-2 has been identified on platelet surface a decade ago. The only confirmed endogenous CLEC-2 agonist is podoplanin. Podoplanin is a transmembrane protein expressed by lymphatic endothelial cells, reticular fibroblastic cells in lymph nodes, kidney podocytes and by cells of certain tumors. CLEC-2 and podoplanin are involved in the processes of embryonic development (blood-lymph vessel separation and angiogenesis), maintaining of vascular integrity of small vessels during inflammation and prevention of blood-lymphatic mixing in high endothelial venules. However, CLEC-2 and podoplanin are contributing to tumor methastasis progression, Salmonella sepsis, deep-vein thrombosis. CLEC-2 signalling cascade includes tyrosine-kinases (Syk, SFK, Btk) as well as adapter LAT and phospholipase Cg2, which induces calcium signalling. CLEC-2, podoplanin and proteins, participating in CLEC-2 signalling cascade, are perspective targets for antithrombotic therapy.

Association of Platelet Count and Mean Platelet Volume with Overall Survival in Patients with Cirrhosis and Unresectable Hepatocellular Carcinoma

BACKGROUND

Platelets have been reported to influence tumor biology and may promote metastasis. Traditionally, thrombocytopenia, a hallmark of cirrhosis, was associated with hepatocellular carcinoma (HCC) development. However, the impact of platelet count on outcome in patients with established HCC is not well studied.

METHODS

Outcomes of patients with cirrhosis diagnosed with HCC between 1995 and 2013 (derivation cohort) and 2000-2016 (validation cohort) who were not eligible for surgical treatment and did not receive antiplatelet therapy were retrospectively studied. Thrombocytopenia was defined as platelet count < 150 g/L. High mean platelet volume (MPV) was defined as ≥median value of the respective cohort (derivation cohort: ≥11 fL; validation cohort: ≥10.6 fL).

RESULTS

Among 626 patients with unresectable HCC, thrombocytopenia was present in 378 (60.4%) and was associated with favorable baseline tumor characteristics: lower diameter of the largest nodule (5.6 ± 3.2 vs. 7.6 ± 4.2 cm), less extrahepatic spread (9.5 vs. 20.2%, both p < 0.001), less macrovascular invasion (21.2 vs. 31.0%, p = 0.005), and lower BCLC stages (63.0 vs. 73.4% BCLC C/D; p = 0.007) as compared to patients with normal platelet count. On univariate analysis, thrombocytopenia and larger MPV were associated with longer overall survival (OS) (thrombocytopenia: median OS [95% CI], 11.5 [9.3-13.8] vs. 5.5 [3.8-7.1] months; p = 0.001; MPV ≥11 fL: 11.7 [9.1-14.2] vs. 6.0 [4.4-7.6] months; p < 0.001). In multivariate analysis, the combined variable of thrombocytopenia and larger MPV was independently associated with longer OS (HR [95% CI], 0.80 [0.65-0.98]; p = 0.029). These results were confirmed in an independent external validation cohort of 525 patients with cirrhosis and HCC. Again, patients with thrombocytopenia and high MPV had significantly longer OS (15.3 [11.7-18.9] vs. 9.3 [7.4-11.2] months; p < 0.001).

CONCLUSIONS

Thrombocytopenia and higher MPV are associated with better outcome in patients with advanced HCC. These findings may prompt further clinical research on additive antiplatelet therapy in the prevention and management of HCC.

Structurally Robust and Functionally Highly Versatile-C-Type Lectin (-Related) Proteins in Snake Venoms

Snake venoms contain an astounding variety of different proteins. Among them are numerous C-type lectin family members, which are grouped into classical Ca²⁺- and sugar-binding lectins and the non-sugar-binding snake venom C-type lectin-related proteins (SV-CLRPs), also called snaclecs. Both groups share the robust C-type lectin domain (CTLD) fold but differ in a long loop, which either contributes to a sugar-binding site or is expanded into a loop-swapping heterodimerization domain between two CLRP subunits. Most C-type lectin (-related) proteins assemble in ordered supramolecular complexes with a high versatility of subunit numbers and geometric arrays. Similarly versatile is their ability to inhibit or block their target molecules as well as to agonistically stimulate or antagonistically blunt a cellular reaction triggered by their target receptor. By utilizing distinct interaction sites differentially, SV-CLRPs target a plethora of molecules, such as distinct coagulation factors and receptors of platelets and endothelial cells that are involved in hemostasis, thrombus formation, inflammation and hematogenous metastasis. Because of their robust structure and their high affinity towards their clinically relevant targets, SV-CLRPs are and will potentially be valuable prototypes to develop new diagnostic and therapeutic tools in medicine, provided that the molecular mechanisms underlying their versatility are disclosed.

Platelets play an essential role in murine lung development through Clec-2/podoplanin interaction

Platelets participate in not only thrombosis and hemostasis but also other pathophysiological processes, including tumor metastasis and inflammation. However, the putative role of platelets in the development of solid organs has not yet been described. Here, we report that platelets regulate lung development through the interaction between the platelet-activation receptor, C-type lectin-like receptor-2 (Clec-2; encoded by Clec1b), and its ligand, podoplanin, a membrane protein. Clec-2 deletion in mouse platelets led to lung malformation, which caused respiratory failure and neonatal lethality. In these embryos, α-smooth muscle actin-positive alveolar duct myofibroblasts (adMYFs) were almost absent in the primary alveolar septa, which resulted in loss of alveolar elastic fibers and lung malformation. Our data suggest that the lack of adMYFs is caused by abnormal differentiation of lung mesothelial cells (luMCs), the major progenitor of adMYFs. In the developing lung, podoplanin expression is detected in alveolar epithelial cells (AECs), luMCs, and lymphatic endothelial cells (LECs). LEC-specific podoplanin knockout mice showed neonatal lethality and Clec1b^-/--like lung developmental abnormalities. Notably, these Clec1b^-/--like lung abnormalities were also observed after thrombocytopenia or transforming growth factor-β depletion in fetuses. We propose that the interaction between Clec-2 on platelets and podoplanin on LECs stimulates adMYF differentiation of luMCs through transforming growth factor-β signaling, thus regulating normal lung development.