The human C-type lectin 18 is a potential biomarker in patients with chronic hepatitis B virus infection.

BACKGROUND: Hepatitis B virus (HBV) infection is a common disease worldwide and is known to cause liver disease. C-type lectin 18 (CLEC18) is a novel secretory lectin highly expressed in human hepatocytes. Because the liver is the major target of HBV infection, we investigated whether the expression of CLEC18 can be used as a biomarker for HBV infection. METHODS: The expression level of CLEC18 in human liver chimeric mice with/without HBV infection was measured by quantitative real time polymerase chain reaction (qPCR) assay. Baseline plasma CLEC18 levels in 271 treatment-naive patients with chronic hepatitis B (CHB) undergoing nucleos(t)ide analogue (NUC) therapy and 35 healthy donors were measured by enzyme-linked immunosorbent assay, and the relationships to other clinical data were analyzed. RESULTS: The expression of CLEC18 was down-regulated in the human liver chimeric mice after HBV infection. Plasma CLEC18 levels were lower in the patients with CHB compared to the healthy donors and positively correlated with HBV DNA and HBsAg levels (P <  0.05). Multivariate Cox proportional hazard regression analysis identified a baseline plasma CLEC18 level of 320-2000 pg/mL to be an independent predictor of HBeAg loss (hazard ratio (HR): 2.077, P = 0.0318), seroconversion (HR: 2.041, P = 0.0445) and virological response (HR: 1.850, P = 0.0184) in 101 HBeAg-positive patients with CHB undergoing NUC therapy. CONCLUSIONS: Plasma CLEC18 levels were correlated with the stage of HBV infection and could predict HBeAg loss and seroconversion in the patients with CHB undergoing NUC therapy.

Human CLEC18 Gene Cluster Contains C-type Lectins with Differential Glycan-binding Specificity.

The human C-type lectin 18 (clec18) gene cluster, which contains three clec18a, clec18b, and clec18c loci, is located in human chromosome 16q22. Although the amino acid sequences of CLEC18A, CLEC18B, and CLEC18C are almost identical, several amino acid residues located in the C-type lectin-like domain (CTLD) and the sperm-coating protein/Tpx-1/Ag5/PR-1/Sc7 (SCP/TAPS) domain, also known as the cysteine-rich secretory proteins/antigen 5/pathogenesis-related 1 proteins (CAP) domain, are distinct from each other. Genotyping by real-time PCR and sequencing further shows the presence of multiple alleles in clec18a/b/c loci. Flow cytometry analysis demonstrates that CLEC18 (CLEC18A, -B, and -C) are expressed abundantly in human peripheral blood cells. Moreover, CLEC18 expression is further up-regulated when monocytes differentiate into macrophages and dendritic cells. Immunofluorescence staining reveals that CLEC18 are localized in the endoplasmic reticulum, Golgi apparatus, and endosome. Interestingly, CLEC18 are also detectable in human sera and culture supernatants from primary cells and 293T cells overexpressing CLEC18. Moreover, CLEC18 bind polysaccharide in Ca(2+)-independent manner, and amino acid residues Ser/Arg(339) and Asp/Asn(421) in CTLD domain contribute to their differential binding abilities to polysaccharides isolated from Ganoderma lucidum (GLPS-F3). The Ser(339) (CLEC18A) → Arg(339) (CLEC18A-1) mutation completely abolishes CLEC18A-1 binding to GLPS-F3, and a sugar competition assay shows that CLEC18 preferentially binds to fucoidan, ß-glucans, and galactans. Because proteins with the SCP/TAPS/CAP domain are able to bind sterol and acidic glycolipid, and are involved in sterol transport and ß-amyloid aggregation, it would be interesting to investigate whether CLEC18 modulates host immunity via binding to glycolipids, and are also involved in glycolipid transportation and protein aggregation in the future.

Endosomal TLR3 co-receptor CLEC18A enhances host immune response to viral infection.

Human C-type lectin member 18A (CLEC18A) is ubiquitously expressed in human, and highest expression levels are found in human myeloid cells and liver. In contrast, mouse CLEC18A (mCLEC18A) is only expressed in brain, kidney and heart. However, the biological functions of CLEC18A are still unclear. We have shown that a single amino acid change (S339 →R339) in CTLD domain has profound effect in their binding to polysaccharides and house dust mite allergens. In this study, we further demonstrate that CLEC18A and its mutant CLEC18A(S339R) associate with TLR3 in endosome and bind poly (I:C) specifically. Compared to TLR3 alone, binding affinity to poly (I:C) is further increased in TLR3-CLEC18A and TLR3-CLEC18A(S339R) complexes. Moreover, CLEC18A and CLEC18A(S339R) enhance the production of type I and type III interferons (IFNs), but not proinflammatory cytokines, in response to poly (I:C) or H5N1 influenza A virus (IAV) infection. Compared to wild type (WT) mice, ROSA-CLEC18A and ROSA-CLEC18A(S339R) mice generate higher amounts of interferons and are more resistant to H5N1 IAV infection. Thus, CLEC18A is a TLR3 co-receptor, and may contribute to the differential immune responses to poly (I:C) and IAV infection between human and mouse.

Transgenic Expression of Human C-Type Lectin Protein CLEC18A Reduces Dengue Virus Type 2 Infectivity in Aedes aegypti.

The C-type lectins, one family of lectins featuring carbohydrate binding domains which participate in a variety of bioprocesses in both humans and mosquitoes, including immune response, are known to target DENV. A human C-type lectin protein CLEC18A in particular shows extensive glycan binding abilities and correlates with type-I interferon expression, making CLEC18A a potential player in innate immune responses to DENV infection; this potential may provide additional regulatory point in improving mosquito immunity. Here, we established for the first time a transgenic Aedes aegypti line that expresses human CLEC18A. This expression enhanced the Toll immune pathway responses to DENV infection. Furthermore, viral genome and virus titers were reduced by 70% in the midgut of transgenic mosquitoes. We found significant changes in the composition of the midgut microbiome in CLEC18A expressing mosquitoes, which may result from the Toll pathway enhancement and contribute to DENV inhibition. Transgenic mosquito lines offer a compelling option for studying DENV pathogenesis, and our analyses indicate that modifying the mosquito immune system via expression of a human immune gene can significantly reduce DENV infection.

Association of C-type lectin 18 levels with extrahepatic manifestations in chronic HCV infection.

Mixed cryobulinemia (MC) is the most common chronic hepatitis C virus (HCV)-associated extrahepatic manifestation. C-type lectin 18 (CLEC18) is a novel secretory lectin that is abundantly expressed in hepatocytes and peripheral blood cells (PBCs). We investigated the associations between CLEC18 expression during HCV infection and the presence of extrahepatic manifestations. A total of 41 rheumatic patients with HCV infection (including 28 patients with MC syndrome), 45 rheumatic patients without infection, and 14 healthy subjects were enrolled. The CLEC18 levels in PBCs and serum were determined by using flow cytometry and enzyme-linked immunosorbent assay, respectively. Significantly higher CLEC18 levels were observed in patients with HCV infection (P < 0.001) and were positively correlated with HCV viral loads (γ = 0.56, P < 0.05). Among patients with HCV infection, significantly increased CLEC18 levels were observed in patients with MC syndrome, particularly in those with type II MC (P < 0.05). CLEC18 levels were associated with cryoglobulin and C4 levels (P < 0.05). CLEC18 was significantly associated with HCV infection, particularly in those with HCV-associated MC. CLEC18 levels were also positively correlated with MC disease activity, suggesting its involvement in MC pathogenesis. CLEC18 may be a novel indicator of HCV infection and a potential therapeutic target in rheumatic patients.

CLEC5A is critical for dengue virus-induced osteoclast activation and bone homeostasis.

UNLABELLED: Osteoclasts are bone tissue macrophages critical to maintain bone homeostasis. However, whether osteoclasts are susceptible to flaviviral infections and involved in dengue virus (DV)-induced disease pathogenesis is still unknown. In this study, we found that osteoclasts were preferentially susceptible to DV infection and produced similar amounts of cytokines and infectious virions as macrophages. Interestingly, DV-induced cytokine secretion and nuclear translocation of the transcription factor NFATc1 in osteoclast via the Syk-coupled myeloid C-type lectin member 5A (CLEC5A). Moreover, DV caused transient inflammatory reaction in bone tissue and upregulated osteolytic activity to release C-telopeptide of type I collagen (CTX-1) from bone tissue. Furthermore, DV-induced osteolytic activity was attenuated in CLEC5A-deficient mice, and administration of antagonistic anti-CLEC5A mAb inhibited DV-activated osteolytic activity and reduced CTX-1 serum level in vivo. This observation suggests that osteoclasts serve as a novel target for DV, and transient upregulation of osteolytic activity may contribute to the clinical symptoms in dengue patients. KEY MESSAGES: Cultured osteoclasts were susceptible to DV infection. Osteoclasts produced similar amounts of cytokines and infectious virions as macrophages. DV induced nuclear translocation of NFATc1 in osteoclast via CLEC5A. DV caused transient inflammatory reaction in bone tissue and upregulated osteolytic activity. Antagonistic anti-CLEC5A mAb inhibited DV-activated osteolytic activity in vivo.