Pathogen recognition by DC-SIGN shapes adaptive immunity

Crosstalk between innate and adaptive immune responses to infectious bronchitis virus after vaccination and challenge of chickens varying in serum mannose-binding lectin concentrations

Mannose-binding lectin (MBL), a C-type collectin with structural similarities to C1q, is an innate pattern-recognition molecule that is sequestered to sites of inflammation and infections. MBL selectively binds distinct chemical patterns, including carbohydrates expressed on all kinds of pathogens. The present study shows that serum MBL levels influence the ability of chickens to clear the respiratory tract of virus genomes after an infectious bronchitis virus (IBV) infection. The primary IBV infection induced changes in circulating T-cell populations and in the specific antibody responses. Serum MBL levels also influenced IBV vaccine-induced changes in circulating T-cell populations. Moreover, addition of mannose to an IBV vaccine altered both vaccine-induced changes in circulating T-cell populations and IBV specific vaccine and infection-induced antibody responses in chickens with high serum MBL levels. These data demonstrate that MBL is involved in the regulation of the adaptive immune response to IBV.

Respiratory syncytial virus glycoprotein G interacts with DC-SIGN and L-SIGN to activate ERK1 and ERK2

Respiratory syncytial virus (RSV) interaction with epithelial and dendritic cells (DCs) is known to require divalent cations, suggesting involvement of C-type lectins. RSV infection and maturation of primary human DCs are reduced in a dose-dependent manner by EDTA. Therefore, we asked whether RSV infection involves DC-SIGN (CD209) or its isoform L-SIGN (CD299) (DC-SIGN/R). Using surface plasmon resonance analysis, we demonstrated that the attachment G glycoprotein of RSV binds both DC- and L-SIGN. However, neutralization of DC- and L-SIGN on primary human DCs did not inhibit RSV infection, demonstrating that interactions between RSV G and DC- or L-SIGN are not required for productive infection. Thus, neither DC- nor L-SIGN represents a functional receptor for RSV. However, inhibition of these interactions increased DC activation, as evidenced by significantly higher levels of alpha interferon (IFN-α), MIP-1α, and MIP-1β in plasmacytoid DCs (pDCs) exposed to RSV after neutralization of DC-and L-SIGN. To understand the molecular interactions involved, intracellular signaling events triggered by purified RSV G glycoprotein were examined in DC- and L-SIGN-transfected 3T3 cells. RSV G interaction with DC- or L-SIGN was shown to stimulate ERK1 and ERK2 phosphorylation, with statistically significant increases relative to mock-infected cells. Neutralization of DC- and L-SIGN reduced ERK1/2 phosphorylation. With increased DC activation following DC- and L-SIGN neutralization and RSV exposure, these data demonstrate that the signaling events mediated by RSV G interactions with DC/L-SIGN are immunomodulatory and diminish DC activation, which may limit induction of RSV-specific immunity.

The gut as communicator between environment and host: immunological consequences

During human evolution, the mucosal immune system developed two anti-inflammatory mechanisms: immune exclusion by secretory antibodies (SIgA and SIgM) to control epithelial colonization of microorganisms and inhibit penetration of harmful substances; and immunosuppression to counteract local and peripheral hypersensitivity against innocuous antigens such as food proteins. The latter function is referred to as oral tolerance when induced via the gut. Similar mechanisms also control immunity to commensal bacteria. The development of immune homeostasis depends on "windows of opportunity" where adaptive and innate immunities are coordinated by antigen-presenting cells; their function is not only influenced by microbial products but also by dietary constituents, including vitamin A and lipids like polyunsaturated omega-3 fatty acids. These factors can in several ways exert beneficial effects on the immunophenotype of the infant. Also breast milk provides immune-modulating factors and SIgA antibodies - reinforcing the gut barrier. Mucosal immunity is most abundantly expressed in the gut, and the intestinal mucosa of an adult contains at least 80% of the body's activated B cells - terminally differentiated to plasmablasts and plasma cells (PCs). Most mucosal PCs produce dimeric IgA which is exported by secretory epithelia expressing the polymeric Ig receptor (pIgR), also called membrane secretory component (SC). Immune exclusion is therefore performed mainly by SIgA. Notably, pIgR knockout mice which lack SIgs show increased uptake of food and microbial antigens and they have a hyper-reactive immune system with disposition for anaphylaxis; but this untoward development is counteracted by cognate oral tolerance induction as a homeostatic back-up mechanism.

HIV microbicides: state-of-the-art and new perspectives on the development of entry inhibitors

Since the discovery of HIV at the beginning of the 1980s, numerous efforts have been devoted to the search of an efficient vaccine. There are at least 25 drugs available for HIV treatment, but no cure is available. The observation that therapy for HIV disease is life long and that these drugs are associated with a number of side effects underlines the need for approaches aimed at preventing rather than treating infection. Additionally, the economic burden of treatment for the HIV infection occupies an increasing share of healthcare expenditure, making current practices likely to become difficult to sustain in the long run. Unfortunately, no effective vaccine for this disease is foreseeable in the near future. Microbicides could be an alternate way to build preventative approaches to HIV infection. Strategies based on preventing the virus from reaching its target cells seem to have some room for development and application. In this review we explore the state-of-the-art of available microbicides, focusing on HIV entry inhibitors. In addition, we discuss new compounds that show anti-HIV activity, which could be effective candidates.

Response of MUTZ-3 dendritic cells to the different components of the Haemophilus influenzae type B conjugate vaccine: towards an in vitro assay for vaccine immunogenicity

Potency testing is mandatory for vaccine registration and batch release. Due to various limitations to in vivo potency testing, there is need for relevant in vitro alternatives. These alternative tests should preferably comprise cells from the target (human) species. The whole suite of immune responses to vaccination that occur in vivo in humans cannot be tested in vitro using a single cell type. Even so, dendritic cells (DC) form an important candidate cell type since they are pivotal in inducing and orchestrating immune responses. Cell lines are preferred over ex vivo cells for reasons of safety, accessibility, and reproducibility. In this first feasibility study we used the human cell line MUTZ-3, because it most closely resembles ex vivo human DC, and compared its response to monocyte-derived DC (moDC). Haemophilus influenzae type B (HiB) vaccine was chosen because its components exert different effects in vivo: while the HiB antigen, polyribosyl ribitol phosphate (PRP) fails to induce sufficient protection in children below 2 years of age, conjugation of this polysaccharide antigen to outer membrane protein (OMP) of Neisseria meningitides, results in sufficient protection. Effects of PRP, OMP, conjugated PRP-OMP, and adjuvanted vaccine (PedVax HiB), on cytokine production and surface marker expression were established. PRP induced no effects on cytokine production and the effect on surface marker expression was limited to a minor decrease in CD209 (DC-SIGN). In both MUTZ-3 and moDC, OMP induced the strongest response both in cytokine production and surface marker expression. Compared to OMP alone conjugated PRP-OMP generally induced a weaker response in cytokine production and surface marker expression. The effects of PedVax HiB were comparable to conjugated PRP-OMP. While moDC showed a larger dynamic range than MUTZ-3 DC, these cells also showed considerable variability between donors, with MUTZ-3 DC showing a consistent response between the replicate assays. In our view, this makes MUTZ-3 DC the cells of choice. In conclusion, our results demonstrate that the MUTZ-3 DC assay allows discrimination between compounds with different immunogenicity. The potential of this cell line as (part of) an in vitro immunogenicity assay should be further explored.

Design, synthesis and activity evaluation of mannose-based DC-SIGN antagonists

In this article, we describe the design, synthesis and activity evaluation of glycomimetic DC-SIGN antagonists, that use a mannose residue to anchor to the protein carbohydrate recognition domain (CRD). The molecules were designed from the structure of the known pseudo-mannobioside antagonist 1, by including additional hydrophobic groups, which were expected to engage lipophilic areas of DC-SIGN CRD. The results demonstrate that the synthesized compounds potently inhibit DC-SIGN-mediated adhesion to mannan coated plates. Additionally, in silico docking studies were performed to rationalize the results and to suggest further optimization.

Cholera toxin impairs the differentiation of monocytes into dendritic cells, inducing professional antigen-presenting myeloid cells

Cholera toxin (CT) is a potent adjuvant for mucosal vaccination; however, its mechanism of action has not been clarified completely. It is well established that peripheral monocytes differentiate into dendritic cells (DCs) both in vitro and in vivo and that monocytes are the in vivo precursors of mucosal CD103(-) proinflammatory DCs. In this study, we asked whether CT had any effects on the differentiation of monocytes into DCs. We found that CT-treated monocytes, in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4), failed to differentiate into classical DCs (CD14(low) CD1a(high)) and acquired a macrophage-like phenotype (CD14(high) CD1a(low)). Cells differentiated in the presence of CT expressed high levels of major histocompatibility complex class I (MHC-I) and MHC-II and CD80 and CD86 costimulatory molecules and produced larger amounts of IL-1β, IL-6, and IL-10 but smaller amounts of tumor necrosis factor alpha (TNF-α) and IL-12 than did monocytes differentiated into DCs in the absence of CT. The enzymatic activity of CT was found to be important for the skewing of monocytes toward a macrophage-like phenotype (Ma-DCs) with enhanced antigen-presenting functions. Indeed, treatment of monocytes with scalar doses of forskolin (FSK), an activator of adenylate cyclase, induced them to differentiate in a dose-dependent manner into a population with phenotype and functions similar to those found after CT treatment. Monocytes differentiated in the presence of CT induced the differentiation of naïve T lymphocytes toward a Th2 phenotype. Interestingly, we found that CT interferes with the differentiation of monocytes into DCs in vivo and promotes the induction of activated antigen-presenting cells (APCs) following systemic immunization.

Relationship between polymorphism of DC-SIGN (CD209) gene and the susceptibility to pulmonary tuberculosis in an eastern Chinese population

Dendritic cell–specific intracellular adhesion molecule–3–grabbing nonintegrin (DC-SIGN) is an important receptor for Mycobacterium tuberculosis on human dendritic cells. Previous studies have shown that the variation, especially the −871A/G and −336A/G in DC-SIGN promoter influenced the susceptibility to tuberculosis. We therefore investigated whether polymorphisms in the DC-SIGN gene were associated with susceptibility to tuberculosis in an eastern Chinese population. A total of 237 culture-positive pulmonary tuberculosis case patients and 244 controls were genotyped for −871A/G and −336A/G by pyrosequencing. Our results suggested that the 2 promoter variants of DC-SIGN gene were not associated with susceptibility to tuberculosis in Chinese. Further analysis showed that the allele -336G was associated with a protective effect against fever in pulmonary tuberculosis patients, but not against cavity formation. In addition, we compared the allelic frequencies of −871A/G and −336A/G in African, Caucasian, and Asian groups. The results showed that the tw forms of allelic frequencies detected Chinese individuals in our study were similar to the reported frequencies in other Asian populations but differed significantly from those in the African and Caucasian groups studied.

Intestinal lactobacilli and the DC-SIGN gene for their recognition by dendritic cells play a role in the aetiology of allergic manifestations

Diminished exposure to harmless micro-organisms, such as lactobacilli, has been suggested to play a role in the increased prevalence of allergic disorders in Westernized communities. The development of allergies depends on both environmental factors and genetic variations, including polymorphisms in genes encoding pattern recognition receptors. The present study examines the effects of both colonization with specific Lactobacillus species and genetic variations in DC-SIGN, a pattern recognition receptor on dendritic cells that recognizes lactobacilli, on the development of atopic dermatitis (AD) and sensitization in infancy. Within the KOALA Birth Cohort Study, faecal samples of 681 one-month-old infants were collected and quantitatively screened for five Lactobacillus species: L. casei, L. paracasei, L. rhamnosus, L. acidophilus and L. reuteri. Eleven haplotype-tagging polymorphisms in the DC-SIGN gene were genotyped in these children. Allergic outcomes were a clinical diagnosis of AD and sensitization (specific IgE) at age 2 years. L. rhamnosus (31.5 %), L. paracasei (31.3 %) and L. acidophilus (14.4 %) were frequently detected in the faecal samples of one-month-old infants, whereas L. casei (2.5 %) and L. reuteri (<1 %) were rare. Colonization with L. paracasei decreased the risk of AD significantly (odds ratio 0.57, 95 % confidence interval 0.32–0.99), whereas effects of L. acidophilus were of borderline statistical significance (0.46, 0.20–1.04). Two DC-SIGN polymorphisms, rs11465413 and rs8112555, were statistically significantly associated with atopic sensitization. The present study supports the ‘old friends’ hypothesis suggesting that certain health-beneficial micro-organisms protect us from developing allergies and that these protective effects are species-dependent. Firm conclusions on the potential interaction between lactobacillus colonization and genetic variations in DC-SIGN in association with the development of allergic disorders cannot be drawn, given the limited power of our study. Therefore, incorporation of consecutive faecal sampling in newly started (birth) cohort studies would be a first requisite to further increase our understanding of host–microbial interactions in health and disease.

Pivotal advance: The promotion of soluble DC-SIGN release by inflammatory signals and its enhancement of cytomegalovirus-mediated cis-infection of myeloid dendritic cells

DC-SIGN is a member of the C-type lectin family. Mainly expressed by myeloid DCs, it is involved in the capture and internalization of pathogens, including human CMV. Several transcripts have been identified, some of which code for putative soluble proteins. However, little is known about the regulation and the functional properties of such putative sDC-SIGN variants. To better understand how sDC-SIGN could be involved in CMV infection, we set out to characterize biochemical and functional properties of rDC-SIGN as well as naturally occurring sDC-SIGN. We first developed a specific, quantitative ELISA and then used it to detect the presence sDC-SIGN in in vitro-generated DC culture supernatants as cell-free secreted tetramers. Next, in correlation with their inflammatory status, we demonstrated the presence of sDC-SIGN in several human body fluids, including serum, joint fluids, and BALs. CMV infection of human tissues was also shown to promote sDC-SIGN release. Based on the analysis of the cytokine/chemokine content of sDC-SIGN culture supernatants, we identified IFN-γ and CXCL8/IL-8 as inducers of sDC-SIGN production by MoDC. Finally, we demonstrated that sDC-SIGN was able to interact with CMV gB under native conditions, leading to a significant increase in MoDC CMV infection. Overall, our results confirm that sDC-SIGN, like its well-known, counterpart mDC-SIGN, may play a pivotal role in CMV-mediated pathogenesis.

An assay for functional dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) inhibitors of human dendritic cell adhesion

We report a new dendritic cell adhesion assay, using either immature or mature dendritic cells, for identifying functional dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN) inhibitors. Because immature dendritic cells are responsible for pathogen binding and invasion, this in vitro assay provides an important link between in vitro results and pathogen-based in vivo assays. Furthermore, this assay does not require laborious expression, refolding, and purification of DC-SIGN carbohydrate recognition domain or extracellular domain as receptor-based assays. The assay power evaluated with Z and Z' parameters enables screening of compound libraries and determination of IC(50) values in the first stage of DC-SIGN inhibitor development.

Selective gene delivery in dendritic cells with mannosylated and histidylated lipopolyplexes

We report for the first time preparation of mannosylated and histidylated lipopolyplexes (Man-LPD100) with uptake and transfection selectivity for dendritic cells (DCs). Man-LPD100 were prepared by addition of mannosylated and histidylated liposomes (Man-Lip100) on preformed PEGylated histidylated polylysine/DNA polyplexes. Man-Lip100 comprised a cationic [O,O-dioleyl-N-(3N-(N-methylimidazolium iodide)propylene) phosphoramidate)] lipid, a neutral [O,O-dioleyl-N-histamine Phosphoramidate] co-lipid and β-D-mannopyranosyl-N-dodecylhexadecanamide (Man-lipid). At the best, Man-Lip100 containing 11 mol % Man-lipid was obtained. We found that dialysis of liposomes completely abolished cytotoxicity. We showed that the uptake of Man(11)-LPD100 by the murine DC line (DC2.4 cells) was at least 10-fold higher than that of Lac(6)-LPD100. A confocal microscopy study with DC2.4 cells expressing Rab5-EGFP or Rab7-EGFP, revealed that DNA uptake occurred through clathrin-mediated endocytosis. The transfection of DC2.4 cells with Man(11)-LPD100 containing DNA encoding luciferase gene gave luciferase activity two to three times higher (9 × 10(5) RLU/mg protein) than with non-mannosylated LPD100. In contrast to the latter, it was inhibited by 90% in the presence of mannose. Overall, the results indicate that mannosylated and histidylated LPD is a promising system for a selective DNA delivery in DCs.

Macroparasites, innate immunity and immunoregulation: developing natural models

Innate immune receptors carry out surveillance for infection threats and are a proximal controller of the threshold and intensity at which inflammatory responses occur. As such, they are a natural focus for understanding how inflammatory immune reactivity is regulated. This review highlights how little data there are relating to the effect of macroparasites on systemic innate receptor responses. The idea is developed that studies on innate immune function in wild animals exposed to a natural profile of infections, including macroparasites, might be a valuable model in which to test hypotheses about the ultimate cause of aberrant inflammation in modern human populations.

A single asparagine-linked glycosylation site of the severe acute respiratory syndrome coronavirus spike glycoprotein facilitates inhibition by mannose-binding lectin through multiple mechanisms

Mannose-binding lectin (MBL) is a serum protein that plays an important role in host defenses as an opsonin and through activation of the complement system. The objective of this study was to assess the interactions between MBL and severe acute respiratory syndrome-coronavirus (SARS-CoV) spike (S) glycoprotein (SARS-S). MBL was found to selectively bind to retroviral particles pseudotyped with SARS-S. Unlike several other viral envelopes to which MBL can bind, both recombinant and plasma-derived human MBL directly inhibited SARS-S-mediated viral infection. Moreover, the interaction between MBL and SARS-S blocked viral binding to the C-type lectin, DC-SIGN. Mutagenesis indicated that a single N-linked glycosylation site, N330, was critical for the specific interactions between MBL and SARS-S. Despite the proximity of N330 to the receptor-binding motif of SARS-S, MBL did not affect interactions with the ACE2 receptor or cathepsin L-mediated activation of SARS-S-driven membrane fusion. Thus, binding of MBL to SARS-S may interfere with other early pre- or postreceptor-binding events necessary for efficient viral entry.

Fungal attacks on mammalian hosts: pathogen elimination requires sensing and tasting

Recognition of Candida spp. by immune cells is mediated by dedicated pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and lectins expressed on innate immune cells (e.g., macrophages, neutrophils and dendritic cells (DCs)). PRRs recognize Candida-specific pathogen-associated molecular patterns (PAMPs). Binding of fungal PAMPs (e.g., cell wall sugar polymers and proteins, fungal nucleic acids) to PRRs triggers the activation of innate effector cells. Recent findings underscore the role of DCs in relaying PAMP information through their PRRs to stimulate the adaptive response. In agreement, deficiencies in certain PRRs strongly impair survival to Candida infections in mice and is associated with enhanced susceptibility to mucocutaneous fungal infections in humans. Understanding the complex signaling networks protecting the host against fungal pathogens remains a challenge in the field.

Redirecting lentiviral vectors pseudotyped with Sindbis virus-derived envelope proteins to DC-SIGN by modification of N-linked glycans of envelope proteins

Redirecting the tropism of viral vectors enables specific transduction of selected cells by direct administration of vectors. We previously developed targeting lentiviral vectors by pseudotyping with modified Sindbis virus envelope proteins. These modified Sindbis virus envelope proteins have mutations in their original receptor-binding regions to eliminate their natural tropisms, and they are conjugated with targeting proteins, including antibodies and peptides, to confer their tropisms on target cells. We investigated whether our targeting vectors interact with DC-SIGN, which traps many types of viruses and gene therapy vectors by binding to the N-glycans of their envelope proteins. We found that these vectors do not interact with DC-SIGN. When these vectors were produced in the presence of deoxymannojirimycin, which alters the structures of N-glycans from complex to high mannose, these vectors used DC-SIGN as their receptor. Genetic analysis demonstrated that the N-glycans at E2 amino acid (aa) 196 and E1 aa 139 mediate binding to DC-SIGN, which supports the results of a previous report of cryoelectron microscopy analysis. In addition, we investigated whether modification of the N-glycan structures could activate serum complement activity, possibly by the lectin pathway of complement activation. DC-SIGN-targeted transduction occurs in the presence of human serum complement, demonstrating that high-mannose structure N-glycans of the envelope proteins do not activate human serum complement. These results indicate that the strategy of redirecting viral vectors according to alterations of their N-glycan structures would enable the vectors to target specific cells types expressing particular types of lectins.

C-type lectin DC-SIGN: an adhesion, signalling and antigen-uptake molecule that guides dendritic cells in immunity

The dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) is a type II C-type lectin whose expression is restricted to the most potent antigen-presenting cells (APCs), the dendritic cells (DCs). In recent years, DC-SIGN has gained an exponential increase in attention because of its involvement in multiple aspects of immune function. Besides being an adhesion molecule, particularly in binding ICAM-2 and ICAM-3, it is also crucial in recognizing several endogenous and exogenous antigens. Additionally, the intracellular domain of DC-SIGN includes molecular motifs, which enable the activation of signal transduction pathways involving Raf-1 and subsequent modulation of DC-maturation status, through direct modification of nuclear factor Nf-κB in DCs. Upon DC-SIGN engagement by mannose- or fucose-containing oligosaccharides, the latter leads to a tailored Toll-like receptor signalling, resulting in an altered DC-cytokine profile and skewing of Th1/Th2 responses. In this article, we will discuss recent advances on a broad perspective concerning DC-SIGN structure, signalling and immune function.