Funding: This work was funded by the Wellcome Trust, Medical Research Council and the University of Aberdeen. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewedPublisher PD

Brown, Gordon D.

Drummond, Rebecca A.

PLoS Pathogens

English

PubMed

Funding: This work was funded by the Wellcome Trust, Medical Research Council and the University of Aberdeen. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Peer reviewe

Aberdeen University Research

PearlsSignalling C-Type Lectins in Antimicrobial ImmunityRebecca A. Drummond*, Gordon D. BrownUniversity of Aberdeen, Aberdeen Fungal Group, Section of Immunity and Infection, Institute of Medical Sciences, Aberdeen, ScotlandIntroductionSince it was first proposed that the innate immune system couldrecognise conserved microbial-associated molecular patterns (orPAMPs) through inherited receptors expressed by the host (termedpattern recognition receptors, or PRRs), several families of PRRshave been discovered and characterised. The most famous of theseare the Toll-like receptors (TLRs), but there is growing appreci-ation that another large family of PRRs, known as the C-typelectin receptors (CLRs), also play a major role in antimicrobialimmunity. CLRs have one or more carbohydrate recognitiondomains (CRDs) that recognise a wide variety of carbohydrateligands. Other members of the CLR family, which do notrecognise carbohydrate ligands but contain similar protein foldscalled C-type lectin-like domains (CTLD), have also beendiscovered and are included in this large family whose membersare divided into 17 groups relating to phylogeny and structure.Upon ligand binding, some CLRs (such as Dectin-1, Dectin-2, andMincle) undergo intracellular signalling to drive cellular responses.Here, we outline the signalling pathways downstream of thesereceptors and discuss how they, and some other CLRs (includingthe Mannose Receptor, CLEC5A, CLEC9A, and DC-SIGN),contribute to immunity against fungi, bacteria, viruses, andparasites.CLRs Can Initiate Complex Intracellular SignallingPathwaysSignalling by CLRs has multiple cellular consequences,including phagocytosis, activation of innate killing mechanisms(e.g., the respiratory burst), and inflammatory mediator produc-tion. Although incompletely understood, CLRs can triggerintracellular signalling through integral signalling motifs (such asimmunoreceptor tyrosine-based activation motifs, or ITAMs) orby association with signalling adaptor molecules (such as theITAM-containing FcRc) [1,2].One of the best characterised signalling pathways downstreamof CLRs is the Syk kinase/CARD9 pathway that is utilised byseveral CLRs including Dectin-1 (gene symbol Clec7a), Dectin-2(Clec4n), and Mincle (Clec4e) [1,2]. Syk is recruited followingphosphorylation of tyrosines in the ITAM motif of these receptors,and the subsequent activation leads to the formation of atrimolecular complex composed of CARD9, Bcl10, and MALT1.This complex in turn is involved in the activation of NFkB, amajor inflammatory transcription factor driving production of IL-6, pro-IL-1b, and TNFa (among others) [2].Of particular recent interest is the ability of CLRs, such asDectin-1, to activate inflammasomes—intracellular proteolyticcomplexes that utilise caspase-1 to generate active IL-1b.However, the process of inflammasome activation can be complex,as Dectin-1-dependent activation of the NLRP3 inflammasomewas shown to require phagocytosis, suggesting that additionalintracellular mechanism(s) were involved in these responses. Onthe other hand, Dectin-1 was also recently shown to activate anovel inflammasome involving caspase-8—a response that did notrequire internalisation, suggesting a direct extracellular sensingmechanism [3].The importance of these pathways has been highlighted by thephenotypes observed in murine gene knockout models and withhuman mutations. For example, a deficiency in CARD9significantly enhances susceptibility to fungal infections in bothmice and humans, and is more severe than a deficiency in anysingle receptor, indicating that redundancy between CLRs iscommon [2,4] and also highlighting the possible involvement ofother non-CLR receptors that utilise CARD9. Indeed, pathogenspossess multiple PAMPs and will engage several PRRs during animmune response, and this allows for ‘‘fine-tuning’’ of signallingthat shapes the cellular response appropriately. CLR cross-talk canbe critical to protection, and several examples of this have recentlybeen described [5]. The medical relevance of CLR signallingmeans that further understanding of these processes is essential ifwe are to gain insights into how stimulating different CLRs givesvarying responses despite the utilisation of common signallingpathways.FungiCLRs have been best studied in the context of fungalinfections—an area of research which has gained increasingattention due to the substantial increase in the number of life-threatening fungal infections in recent years. Several CLRs havebeen implicated in antifungal immunity and include Dectin-1,Dectin-2, the Mannose Receptor (MR), and Mincle (Table 1).Dectin-1 is the most extensively characterised antifungal receptorand has been shown to play vital roles in the defence against anarray of fungal pathogens, including C. albicans, A. fumigatus, and P.carinii [6]. Dectin-1 recognises fungi by binding b-glucans in thefungal cell wall, and subsequently signals through Syk/CARD9 toinduce cellular responses including phagocytosis, induction of therespiratory burst, and cytokine production. Another recentlydescribed function is the ability of Dectin-1 to directly induceadaptive Th1 and Th17 responses [2]. Dectin-12/2 mice havebeen used to demonstrate how these functions are important forantifungal immunity in vivo. Inefficient fungal uptake and killing inCitation: Drummond RA, Brown GD (2013) Signalling C-Type Lectins inAntimicrobial Immunity. PLoS Pathog 9(7): e1003417. doi:10.1371/journal.ppat.1003417Editor: William E. Goldman, The University of North Carolina at Chapel Hill,United States of AmericaPublished July 25, 2013Copyright:  2013 Drummond, Brown. This is an open-access article distrib-uted under the terms of the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, providedthe original author and source are credited.Funding: This work was funded by the Wellcome Trust, Medical ResearchCouncil and the University of Aberdeen. The funders had no role in study design,data collection and analysis, decision to publish, or preparation of the manuscript.Competing Interests: The authors have declared that no competing interestsexist.* E-mail: r01rad0@abdn.ac.ukPLOS Pathogens | www.plospathogens.org 1 July 2013 | Volume 9 | Issue 7 | e1003417Dectin-12/2 mice leads to uncontrolled fungal growth in mousemodels of systemic candidiasis and aspergillosis, leading to anenhanced rate of mortality [6]. Furthermore, Dectin-12/2 miceand humans with a mutation in the Dectin-1 gene (leading to anon-expressed truncated form of Dectin-1) can present with anincreased susceptibility to mucosal candidiasis and aspergillosis[2,6], in part due to aberrant induction of adaptive immunity.Like Dectin-1, Dectin-2 also plays a key role in the protectionagainst C. albicans, through innate recognition and the promotionof Th17 responses [7,8]. The roles of the MR and Mincle are lesswell defined however. MR contains no known signalling motifsand it is unclear whether signalling occurs directly from the MR,and MR2/2 mice do not have increased susceptibility to mostfungal pathogens, with the exception of C. neoformans [9]. Mincle,on the other hand, has contradicting studies on the species itrecognises [10] possibly because of fungal strain variation, whichcan be important to the interpretation of these types of studies [6].Defining the roles of these receptors will hopefully help feeddevelopment of novel therapeutics and vaccines, of which thereare currently few effective options for the former, and none for thelatter.BacteriaThe majority of studies analysing the role of CLRs in bacterialinfections have focused on mycobacterial diseases, and we willfocus on this data here. However it is important to note that,although less well characterised, CLRs have been implicated in therecognition of other bacterial species. For example, the MRrecognises a number of bacteria including Klebsiella and Streptococcusspecies [11].Several CLRs recognise mycobacteria and play roles in theresulting immune responses. The MR has been shown to mediatethe binding and internalisation of mycobacteria, and plays a rolein antigen presentation and the promotion of T-cell responses[11]. Mincle and Dectin-1 have also been linked to mycobacterialrecognition, although Mincle’s ligand is the only one to beidentified (mycobacterial cord factor, also known as TDM).Despite their ability to recognise mycobacteria, none of thereceptor-deficient mouse models exhibit increased susceptibility toinfection [10,12]. This suggests that other receptors are able tocompensate, and that there is a high level of redundancy formycobacterial recognition. However, while individual receptorsappear redundant, the shared downstream signalling pathwayappears to be central for protection as CARD92/2 mice arehighly susceptible to mycobacterial infection [10].VirusesUnlike fungi and bacteria in which these receptors are thoughtto play protective roles, many CLRs involved in viral recognitionappear to promote transmission and infection. One of the bestcharacterised examples is DC-SIGN and HIV. Viral particles bindDC-SIGN via gp120 on the viral surface before being endocytosedby dendritic cells (DCs), and HIV is then thought to use the DC asa safe mode of transport to lymph nodes where it comes intocontact with its target CD4+ T-cells. Indeed, various human DC-SIGN polymorphisms have been associated with increasedtransmission and binding of HIV [13]. Other examples includebinding of dengue virus (DV) by CLEC5A and the MR, andblocking these CLRs in experimental models prevented viralgrowth and associated lethal inflammation [14–16].On the other hand, CLRs can also play protective roles duringviral infections, some through indirect mechanisms. For example,CLEC9A (also known as DNGR-1) was recently shown tofacilitate antigen cross-presentation, a function required for thecontrol of infections with vaccinia virus [17] and herpes simplexvirus [18].ParasitesMany parasites express a large number of carbohydrates, andtherefore have the potential to engage host CLRs. Dectin-2, whiletypically considered an antifungal CLR, can also contribute toimmunity against parasitic worms. Dectin-2 was recently shown torecognise a component of Schistosoma mansoni egg antigen (SEA),Table 1. Selected CLRs, their microbial ligands, and the microbes that have altered ability to infect in the absence of the receptorwithin mice and humans.CLR Identified Microbial Ligand(s)Infections with AlteredSusceptibility in the Mouse KOInfections Linked to HumanPolymorphisms Selected ReferencesDectin-1 b-glucan (fungi) q C. albicansq A. fumigatusq P. cariniiq C. albicansq A. fumigatus[6]Dectin-2 a-mannan q C. albicans None identified/confirmed [7]DC-SIGN* ManLAM (mycobacteria)gp120 (HIV)q Mycobacterium tuberculosis Q M. tuberculosisq HIV[10,12,24]Mannose Receptor(MR)Mannan/mannose q Cryptococcus neoformans q M. tuberculosis [9,11,24]CLEC5A Dengue virus** Q Dengue virus*** None identified/confirmed [15,16]CLEC9A Unknown q Vaccinia virus None identified/confirmed [17]Mincle a-mannose (fungi)TDM (mycobacteria)q C. albicansq Mycobacterial antigensNone identified/confirmed [1,10]A selection of CLRs are listed with identified microbial ligands and phenotypes observed with different pathogens in both mice and humans, where q denotesenhanced susceptibility of the host and Q denotes enhanced resistance.*DC-SIGN is a human molecule with related homologues in the mouse; mouse phenotypes described are for SIGNR3 knockouts.**Exact ligand on DV is unknown.***These experiments were performed using blocking antibodies to CLEC5A. Abbreviations: HIV, human immunodeficiency virus; ManLAM, mannosylatedlipoarabinomannan; TDM, trehalose-6, 69-dimycolate.doi:10.1371/journal.ppat.1003417.t001PLOS Pathogens | www.plospathogens.org 2 July 2013 | Volume 9 | Issue 7 | e1003417and was responsible for the subsequent activation of the NLRP3inflammasome. While absence of this inflammatory response didnot alter parasite burden, it did alleviate Th2-mediated immuno-pathology [19]. Interestingly, Dectin-2 has also been shown topromote Th2 responses to the common allergen house dust mite(HDM) antigen [2], while Th17 responses are seen with fungi (seeabove). However, it is not well understood how Dectin-2 canpromote opposing T-cell responses to different antigens, althoughfurther definition of the downstream signalling pathways followingrecognition should help answer these questions.Like Dectin-2, other CLRs have also been found to modulateadaptive immune responses to parasites. For example, the MR wasshown to help drive the protective Th2 response during S. mansoniinfection and enhance uptake of infectious cercariae [20].CLEC9A, on the other hand, was shown to drive CD8+ T-cellactivation (possibly through previously described cross-presenta-tion functions) during cerebral malaria in mice, resulting inimmunopathology and mortality [21].Concluding RemarksCLRs are involved in immunity to several classes of microbe,from initial recognition and uptake to modulation of adaptiveimmunity. Indeed, although not discussed here, CLRs have alsobeen implicated in maintaining immune homeostasis throughrecognition of endogenous ligands; both Mincle and CLEC9A areinvolved in the recognition of necrotic cells by binding exposednuclear proteins and actin filaments, respectively [22,23]. How-ever, much is still to be learned about these innate receptors, suchas the specific outcomes of cross-talk during signalling and howthis is regulated. Furthermore, microbial ligands for many CLRsremain unidentified. Unveiling the microbial components thatstimulate potent immune responses via CLRs is an importantavenue for research into novel vaccines and therapies. Moreover,as our knowledge grows it is likely that we will identify additionalhuman CLR polymorphisms that cause defects in antimicrobialimmunity.References1. Drummond RA, Saijo S, Iwakura Y, Brown GD (2011) The role of Syk/CARD9 coupled C-type lectins in antifungal immunity. Eur J Immunol 41: 276–281.2. Hardison SE, Brown GD (2012) C-type lectin receptors orchestrate antifungalimmunity. Nat Immunol 13: 817–822.3. Gringhuis SI, Kaptein TM, Wevers BA, Theelen B, van der Vlist M, et al. (2012)Dectin-1 is an extracellular pathogen sensor for the induction and processing ofIL-1b via a noncanonical caspase-8 inflammasome. Nat Immunol 13: 246–254.4. Glocker EO, Hennigs A, Nabavi M, Schaffer AA, Woellner C, et al. (2009) Ahomozygous CARD9 mutation in a family with susceptibility to fungalinfections. N Engl J Med 361: 1727–1735.5. da Glo´ria Sousa M, Reid Delyth M, Schweighoffer E, Tybulewicz V, Ruland J,et al. (2011) Restoration of pattern recognition receptor costimulation to treatchromoblastomycosis, a chronic fungal infection of the skin. Cell Host Microbe9: 436–443.6. Drummond RA, Brown GD (2011) The role of Dectin-1 in the host defenceagainst fungal infections. Curr Opin Microbiol 14: 392–399.7. Saijo S, Ikeda S, Yamabe K, Kakuta S, Ishigame H, et al. (2010) Dectin-2recognition of a-mannans and induction of Th17 cell differentiation is essentialfor host defense against Candida albicans. Immunity 32: 681–691.8. Gringhuis SI, Wevers BA, Kaptein TM, van Capel TMM, Theelen B, et al.(2011) Selective c-Rel activation via Malt1 controls anti-fungal TH-17 immunityby Dectin-1 and Dectin-2. PLoS Pathog 7: e1001259. doi:10.1371/journal.-ppat.1001259.9. Dan JM, Kelly RM, Lee CK, Levitz SM (2008) Role of the mannose receptor ina murine model of Cryptococcus neoformans infection. Infect Immun 76: 2362–2367.10. Marakalala MJ, Graham LM, Brown GD (2010) The role of Syk/CARD9-coupled C-type lectin receptors in immunity to Mycobacterium tuberculosisinfections. Clin Dev Immunol 2010: 567571.11. Gazi U, Martinez-Pomares L (2009) Influence of the mannose receptor in hostimmune responses. Immunobiology 214: 554–561.12. Tanne A, Ma B, Boudou F, Tailleux L, Botella H, et al. (2009) A murine DC-SIGN homologue contributes to early host defense against Mycobacteriumtuberculosis. J Exp Med 206: 2205–2220.13. Boily-Larouche G, Milev MP, Zijenah LS, Labbe´ A-C, Zannou DM, et al.(2012) Naturally-occurring genetic variants in human DC-SIGN increase HIV-1capture, cell-transfer and risk of mother-to-child transmission. PLoS ONE 7:e40706. doi:10.1371/journal.pone.0040706.14. Miller JL, de Wet BJM, Martinez-Pomares L, Radcliffe CM, Dwek RA, et al.(2008) The mannose receptor mediates dengue virus infection of macrophages.PLoS Pathog 4: e17. doi:10.1371/journal.ppat.0040017.15. Wu MF, Chen ST, Yang AH, Lin WW, Lin YL, et al. (2013) CLEC5A is criticalfor dengue virus-induced inflammasome activation in human macrophages.Blood 121: 95–106.16. Chen S-T, Lin Y-L, Huang M-T, Wu M-F, Cheng S-C, et al. (2008) CLEC5A iscritical for dengue-virus-induced lethal disease. Nature 453: 672–676.17. Iborra S, Izquierdo HM, Martı´nez-Lo´pez M, Blanco-Mene´ndez N, Reis e SousaC, et al. (2012) The DC receptor DNGR-1 mediates cross-priming of CTLsduring vaccinia virus infection in mice. J Clin Invest 122: 1628–1643.18. Zelenay S, Keller AM, Whitney PG, Schraml BU, Deddouche S, et al. (2012)The dendritic cell receptor DNGR-1 controls endocytic handling of necrotic cellantigens to favor cross-priming of CTLs in virus-infected mice. J Clin Invest 122:1615–1627.19. Ritter M, Gross O, Kays S, Ruland J, Nimmerjahn F, et al. (2010) Schistosomamansoni triggers Dectin-2, which activates the Nlrp3 inflammasome and altersadaptive immune responses. Proc Natl Acad Sci U S A 107: 20459–20464.20. Paveley RA, Aynsley SA, Turner JD, Bourke CD, Jenkins SJ, et al. (2011) TheMannose Receptor (CD206) is an important pattern recognition receptor (PRR)in the detection of the infective stage of the helminth Schistosoma mansoni andmodulates IFN gamma production. Int J Parasit 41: 1335–1345.21. Piva L, Tetlak P, Claser C, Karjalainen K, Renia L, et al. (2012) Cutting edge:Clec9A(+) dendritic cells mediate the development of experimental cerebralmalaria. J Immunol 189: 1128–1132.22. Brown GD (2008) Sensing necrosis with Mincle. Nat Immunol 9: 1099–1100.23. Sancho D, Joffre OP, Keller AM, Rogers NC, Martinez D, et al. (2009)Identification of a dendritic cell receptor that couples sensing of necrosis toimmunity. Nature 458: 899–903.24. Azad AK, Sadee W, Schlesinger LS (2012) Innate immune gene polymorphismsin tuberculosis. Infect Immun 80: 3343–3359.PLOS Pathogens | www.plospathogens.org 3 July 2013 | Volume 9 | Issue 7 | e1003417

Signalling C-Type Lectins in Antimicrobial Immunity

Abstract

Similar works

Full text

Available Versions

Aberdeen University Research

University of Birmingham Research Portal

University of Birmingham Research Portal

Directory of Open Access Journals

Crossref