Immunity Review

Myeloid C-type Lectin Receptors in Pathogen Recognition and Host Defense

Fabiola Osorio1 and Caetano Reis e Sousa1,* 1Immunobiology Laboratory, Cancer Research UK, London Research Institute, Lincoln’s Inn Fields Laboratories, 44 Lincoln’s Inn Fields, London WC2A 3LY, UK *Correspondence: [email protected] DOI 10.1016/j.immuni.2011.05.001

C-type lectin receptors (CLRs) comprise a heterogeneous group of transmembrane proteins. Many of them are expressed in myeloid cells and signal in response to pathogen-derived or self ligands to initiate or regu- late cell activation. Here, we review the properties of myeloid CLRs, highlighting how their signaling function is coordinated with that of other innate receptor families to control immunity to infection.

Introduction These CLRs are prominently expressed on myeloid cells where The term ‘‘C-type lectin’’ was first used to differentiate a group of they perform multiple functions. Some effectively function as Ca2+-dependent carbohydrate-binding (lectin) proteins from the pattern recognition receptors (PRRs) to initiate innate and adap- rest of the animal lectins (Zelensky and Gready, 2005). The tive immunity to infection. Others dampen or alter myeloid cell carbohydrate binding properties of C-type lectins were found activation and modulate immune responses. This review focuses to reside within a compact protein region with a unique structural on CLRs that signal in myeloid cells to initiate or tune immunity to fold that became known as the ‘‘C-type carbohydrate recogni- pathogens. This subset of C-type lectins cuts across existing tion domain (CRD)’’ or ‘‘C-type lectin domain’’ (Weis and Drick- classifications based on CTLD sequence and domain organiza- amer, 1996). led to the identification of tion and is therefore discussed here on the basis of signaling C-type CRDs in many additional proteins, including ones that properties (Figure 1; Table 1). did not bind to carbohydrates or calcium. This contradiction was resolved by introducing a more general term, ‘‘C-type lec- CLR Signaling tin-like domain’’ (CTLD) to refer to the fold (Zelensky and Gready, , DEC-205, and were among the first 2005). The CTLD is now considered a structural motif in protein CLRs shown to bind bacteria, fungi, and viruses (Table 1). These databases and the term ‘‘C-type lectin’’ is consequently used for CLRs have endocytic activity, mediate internalization of their any protein possessing one or more CTLDs, independently of ligands, and can direct cargo into intracellular compartments carbohydrate or calcium binding ability. Analogous to the immu- that allow for antigen processing and presentation. Such proper- noglobulin superfamily, the C-type lectin family of proteins ties make these CLRs interesting candidates for antigen delivery therefore encompasses upwards of 1000 members with diverse to dendritic cells (DCs) in immunotherapy (Caminschi et al., functions including cell adhesion, regulation of natural killer 2009). However, there is limited evidence to date that signals function, complement activation, tissue remodeling, platelet from these receptors alone are sufficient to elicit microbicidal activation, endocytosis, , and innate immunity effector functions in myeloid cells or, importantly, for inducing (Weis and Drickamer, 1996; Zelensky and Gready, 2005). the gene transcription modules that are characteristic of innate Microbial and viral signatures or ‘‘pathogen-associated immunity to infection. In contrast, akin to the members of the molecular patterns’’ (PAMPs) are often made up of carbohy- Toll-like receptor (TLR) family, other CLRs have been shown to drates. Glucans are prominent constituents of the cell walls of directly and autonomously couple PAMP recognition to myeloid fungi, plants, and mycobacteria; high-mannose structures are cell activation and, further downstream, to adaptive immunity. expressed by some viruses, fungi, and bacteria; and fucose Yet other myeloid CLRs do not act as ‘‘self-sufficient’’ PRRs structures are found on the surface of helminths and some but do recognize PAMPs and modulate cell activation. The key bacteria (Geijtenbeek and Gringhuis, 2009; Robinson et al., to understanding myeloid CLRs is therefore to examine the 2006). Not surprisingly, many C-type lectins that recognize signaling pathways that underlie their different functions glycans, as well as others that do not bind carbohydrates, (Figure 1; Table 1). have been shown to bind pathogens and to play a role in host defense. Included in this group are soluble defensins such as CLRs that Signal via Tyrosine-Based Motifs RegIIIg, which is produced in the gut and has direct microbicidal The few CLRs that to date have been shown to act as ‘‘self-suffi- activity (Cash et al., 2006), as well as collectins such as mannose cient’’ PRRs utilize spleen tyrosine kinase (Syk) as their proximal binding protein, which activates the complement cascade upon adaptor (Kerrigan and Brown, 2010; Robinson et al., 2006). binding to bacterial surfaces (Takahashi et al., 2006). A few Syk normally binds to proteins containing immunoreceptor

CTLD-bearing transmembrane proteins, here designated as tyrosine-based activation motifs (ITAM; Yxx(L/I)x6-12Yxx(L/I), ‘‘C-type lectin receptors’’ (CLRs), also recognize pathogens where the slashes indicate alternative amino acid residues). and in some cases engage signaling pathways that propagate Phosphorylation of the two tyrosines located within the motif into the cell to elicit microbicidal or inflammatory responses. by kinases of the Src family creates a docking site for the tandem

Immunity 34, May 27, 2011 ª2011 Elsevier Inc. 651 Immunity Review

and Pneumocystis spp., and to bacterial pathogens such as No Mycobacteria (Table 1; Brown, 2006; Rothfuchs et al., 2007). tyrosine hemITAM ITAM ITIM Notably, Dectin-1 does not have a typical CRD and binding to b-glucans is calcium independent (Brown, 2006). In addition to b-1,3-linked glucans, Dectin-1 is reported to possess an uniden- tified endogenous ligand in T cells (Ariizumi et al., 2000b). Although the intracellular domain of Dectin-1 contains a double tyrosine ITAM-like motif, the amino terminal tyrosine (Tyr3) of Dectin-1 is not located within the appropriate YxxL context (Ariizumi et al., 2000b). Nevertheless, the ITAM-like motif is L Y V/L x x x able to directly recruit and activate Syk upon Dectin-1 binding x ITAM- x x Y L Y to agonist ligands (Rogers et al., 2005; Underhill et al., 2005). containing x adaptor I/V Notably, Tyr3 is dispensable and single phosphorylation of the Y x membrane-proximal tyrosine 15 is necessary and sufficient to x L mediate signaling via Syk (Rogers et al., 2005). The single YxxL motif responsible for this unusual mode of Syk activation was therefore termed ‘‘hemITAM’’ to reflect the fact that it resembled half (hemi-) of an ITAM (Robinson et al., 2006; Rogers et al., Figure 1. Classification of CLRs by Tyrosine-Based Signaling Motifs 2005). An attractive hypothesis is that ligand-induced dimeriza- The CLRs listed in Table 1 are schematically represented here. As in Table 1, signaling motif refers to the potential to engage pathways that induce or tion of two Dectin-1 molecules allows two hemITAMs to come modulate gene expression rather than signal for endocytosis. Note that the together, thereby providing in trans a docking site for the two CLRs bearing a hemITAM or ITIM motif are all type II transmembrane proteins; SH2 domains of Syk (Figure 2). Data supporting a dimerization hence the motif is written with the tyrosine depicted as membrane distal. The ITIM motif sequence shown is compiled from that found in MICL and DCIR. model for signal initiation have been obtained for the related CLR, CLEC-2 (Hughes et al., 2010). Sustained receptor signaling may subsequently require large-scale exclusion of tyrosine SH2 domains of Syk. ITAM binding causes a conformational phosphatases from the area of Dectin-1 engagement (Good- change that leads to Syk activation and phosphorylation of ridge et al., 2011). a variety of substrates (including Syk itself), initiating a signaling CLEC-2 and DNGR-1 (also known as CLEC9A) are two addi- cascade (Mo´ csai et al., 2010). CLRs that couple to Syk often tional hemITAM-bearing receptors in the NK cell CLR subgroup do so by associating in trans with ITAM-bearing adaptors (Table 1). They resemble Dectin-1 and can signal via Syk but it is (Figure 1; Table 1). However, a small group of CLRs has been unclear whether they bind PAMPs and act as PRRs. Although shown to bind Syk directly, through a single tyrosine-based motif CLEC-2 has been reported to bind HIV-1 (Chaipan et al., in the intracellular domain, termed hemITAM (Figure 1; Table 1). 2006), it is best known as a platelet receptor that promotes coag- Finally, a separate subgroup of CLRs antagonizes Syk activity by ulation (Suzuki-Inoue et al., 2006). CLEC-2 is the target of the means of immunoreceptor tyrosine-based inhibitory motifs platelet-activating snake venom toxin, rhodocytin, but also (ITIM; S/I/V/LxYxxI/V/L, where the slashes indicate alternative possesses a physiological endogenous ligand, podoplanin, amino acid residues), single tyrosine motifs analogous to hemI- that is involved in platelet aggregation, lymphatic vessel forma- TAMs that recruit the phosphatases SHP-1, SHP-2, and SHIP tion, and tumor metastasis (Kato et al., 2008; Suzuki-Inoue rather than kinases (Figure 1; Table 1). ITAM, hemITAM, and et al., 2007). CLEC-2 is additionally expressed in B cells, NK ITIM signaling by CLRs is discussed below. Many CLRs also cells, and myeloid cells, but whether it has activatory function use tyrosine-based motifs to signal for endocytosis and to in these cell types is unclear (unpublished observations). regulate antigen processing, but that aspect of signaling is not DNGR-1 is highly restricted to DCs in both mouse and human covered in this review. (Caminschi et al., 2008; Huysamen et al., 2008; Sancho et al., 2008) and it is also unclear whether it acts as an activatory HemITAM Signaling receptor (unpublished observations). Notably, PAMPs for The first example of a Syk-coupled myeloid CLR was Dectin-1 DNGR-1 have not been reported but the receptor binds to an (‘‘DC-associated C-type lectin 1’’; Table 1). Dectin-1 is a type II unidentified ubiquitous intracellular ligand that is sequestered transmembrane CLR that contains a single extracellular CTLD in living cells and exposed upon cell death (Sancho et al., and is classified structurally in the same CLR subgroup as the 2009). DNGR-1 signaling via the hemITAM-Syk pathway is NK cell receptors (Brown, 2006). However, it is not expressed important for priming CTL against antigens borne by dead cells by NK cells but by DCs and other myeloid cell types, including (Sancho et al., 2009). Therefore DNGR-1 may play a role in monocytes, , and , as well as a subset adaptive immunity to infections that induce cell death even if it of gd T cells (Martin et al., 2009; Taylor et al., 2002). Cell surface does not act as a PRR. expression is stabilized by interaction with the CD37 tetraspanin SIGNR3 (CD209d), a mouse CLR that recognizes (Meyer-Wentrup et al., 2007). Dectin-1 recognizes b-1,3-linked Mycobacterium tuberculosis and its mannosylated lipoarabino- glucans prominent in the cell wall of fungi, some bacteria, and mannan (ManLAM) component, also appears to signal via plants (Palma et al., 2006) and binds to zymosan (a crude a hemITAM-Syk pathway that results in induction of proinflam- preparation of baker’s yeast cell walls), to fungal pathogens, matory in mouse macrophages (Tanne et al., 2009). including Candida spp., Aspergillus spp., Coccidiodes spp., Therefore, mouse SIGNR3 might constitute an additional

652 Immunity 34, May 27, 2011 ª2011 Elsevier Inc. Immunity Review

Table 1. Mouse and Human CLRs Mentioned in This Review Official Gene Cytoplasmic Proximal Common Namesa Symbolb Signaling Motifc Signalingc Ligand Specificity Ligand Origin mannose receptor, MRC1 (Hs), ? ? high mannose, M. tuberculosis, M. kansasii, CD206, CLEC13D Mrc1 (Mm) fucose F. tularensis, K. pneumoniae, S. pneumoniae, HIV-1, dengue virus, C. albicans, C. neoformans, P. carinii, Leishmania spp. langerin, CD207, CD207 (Hs), ? ? mannose, fucose, HIV, Mycobacterium leprae, CLEC4K Cd207 (Mm) N-acetyl-glucosamine, Candida spp., b-glucan Saccharomyces spp., Malassezia furfur DEC-205, CD205, LY75 (Hs), ? ? ND HIV, Y. pestis, endogenous CLEC13B Ly75 (Mm) (dead cells) DC-SIGN, CD209 (Hs) ? Raf1 high mannose HIV, measles virus, SARS, CLEC4L (Hs) and fucose dengue virus, filoviruses, CMV, Mycobacterium spp., C. albicans, Leishmania spp., S. mansonii egg antigen Dectin-1, CLEC7A CLEC7A (Hs), hemITAM Syk, Raf1 b-1,3 glucans fungi, mycobacteria Clec7a (Mm) DNGR-1, CLEC9A CLEC9A (Hs), hemITAM Syk ND endogenous (dead cells) Clec9a (Mm) CLEC-2, CLEC1B CLEC1B (Hs), hemITAM Syk ND endogenous (podoplanin), Clec1b (Mm) rhodocytin (snake venom), HIV SIGNR3 (Mm) Cd209d hemITAM Syk high mannose Mycobacterium tuberculosis and fucose Dectin-2, CLEC6A, CLEC6A (Hs), ITAM Syk high mannose, C. albicans, S. cerevisiae, CLEC4N Clec4n (Mm) (adaptor FcRg chain) a-mannans M. tuberculosis, P. brasiliensis, H. capsulatum, M. audouinii, T. rubrum, C. neoformans, house dust mite allergens Mincle, CLEC4E CLEC4E (Hs), ITAM Syk a-mannose, C. albicans, Clec4e (Mm) (adaptor FcRg chain) glycolipids, SAP130 Malasezzia spp., (endogenous) M. tuberculosis, dead cells MDL-1, CLEC5A CLEC5A (Hs), ITAM Syk? ND dengue virus Clec5a (Mm) (adaptor DAP12) BDCA-2, CD303, CLEC4C (Hs) ITAM Syk mannose, fucose ND CLEC4C (Hs)2 (adaptor FcRg chain) mDCAR, Clec4b1 (Mm) ITAM Syk? ND ND CLEC4B1 (Mm) (adaptor FcRg chain) mDCAR1, Clec4b2 (Mm) ITAM? Syk? ND ND CLEC4B2 (Mm) (adaptor FcRg chain)? DCIR, CLEC4A CLEC4A (Hs), ITIM SHP-1, SHP-2 mannose, fucose HIV-1 Clec4a2 (Mm) antigen H, CLEC12B (Hs), ITIM SHP-1, SHP-2 ND ND CLEC12B Clec12b (Mm) MICL, DCAL-2, CLEC12A (Hs), ITIM SHP-1, SHP-2 ND ND CLEC12A Clec12a (Mm) Abbreviations: Hs, Homo sapiens; Mm, Mus musculus; ND, not determined. Table compiled from the references given in the main text. a Where only one species is shown, a direct ortholog does not exist in the other. See text for details. b As per NCBI Gene database. c Cytoplasmic signaling motif and proximal signaling refer to the potential of a given CLR to signal to induce/modulate gene expression. Signals for other CLR functions (e.g., endocytosis) are not indicated. Question mark denotes unknown or speculative attribution.

Immunity 34, May 27, 2011 ª2011 Elsevier Inc. 653 Immunity Review

name, is only 20% homologous to Dectin-1 and belongs to Dectin-1 a separate structural subgroup of CLRs (Ariizumi et al., 2000a). Dectin-2 is a type II transmembrane CLR with a typical Ca2+- dependent CRD that mediates binding to high-mannose struc- tures (McGreal et al., 2006), including a-mannans present in C. albicans cell walls (Saijo et al., 2010). Dectin-2 can also recog- nize various other fungi including Paracoccoides brasiliensis, Histoplasma capsulatum, and

L L (McGreal et al., 2006; Sato et al., 2006) and is additionally x x x x IL-1β involved in the recognition of house dust mite allergens (Barrett Raf1 Y Y P P et al., 2009) and of S. mansoni egg extracts (Ritter et al., 2010). S As for Dectin-1, Dectin-2 may possess an unknown endogenous Y ligand (Aragane et al., 2003). Although it was originally described NIKK ROS NLRP3 a Langerhans cells marker (Ariizumi et al., 2000a; Bonkobara CARD9 et al., 2001), further studies have demonstrated that Dectin-2 is Bcl-10 Malt-1 generally expressed in myeloid cells, including macrophages and monocytes and various subtypes of DCs (Robinson et al., 2009; Taylor et al., 2005). Dectin-2 can associate with the NF-κB NFAT MAPKs ITAM-bearing g (FcRg) chain and stimulation of pro-IL-1β Dectin-2 causes phosphorylation of the tyrosines in the FcRg ITAM leading to Syk recruitment (Barrett et al., 2009; Robinson IL-2, IL-10, et al., 2009; Sato et al., 2006). IL-6,TNF-α, IL-23, Mincle (macrophage inducible C-type lectin) also associates pro-IL-1β with FcRg chain and ligand binding causes phosphorylation of the FcRg chain ITAM and recruitment of Syk (Yamasaki et al., Figure 2. Dectin-1 Signaling Pathways as a Paradigm for 2008). Mincle is expressed predominantly in macrophages and, Syk-Coupled CLRs like Dectin-2, recognizes a-mannose residues via a Ca2+-depen- Syk-dependent and -independent signaling by Dectin-1 in response to dent CRD interaction (Table 1). It has been shown to bind to fungi engagement by agonist ligands. Large multimeric b-glucan-containing parti- cles trigger Dectin-1 possibly by promoting or stabilizing dimer formation, including C. albicans and Malasezzia spp. and to trehalose-6, 0 preventing receptor internalization, as well as allowing exclusion of phos- 6 -dimycolate present in Mycobacterium tuberculosis (Bugarcic phatases. These events favor phosphorylation of the hemITAM motifs by Src et al., 2008; Ishikawa et al., 2009; Wells et al., 2008; Yamasaki family kinases and allow for stable recruitment and activation of Syk kinase. et al., 2009). Mincle additionally recognizes the endogenous ribo- Syk acts as a node in the subsequent activation of NF-kB, MAP kinase, and NFAT pathways, which together regulate expression of many innate response nucleoprotein SAP130, which is exposed by dead cells, but this is genes. Syk additionally regulates the production of reactive oxygen species, independent of the CRD residues involved in binding a-mannose, which can contribute, among others, to the activation of the NLRP3 in- arguing for two distinct binding sites (Yamasaki et al., 2008). The flammasome. A Syk-independent pathway mediated by Raf1 regulates activity of NF-kB p65. See text for details. Mincle-mediated response to dead cells triggers the infiltration of neutrophils to areas of tissue damage and may be primarily involved in tissue repair rather than immunity (Yamasaki et al., hemITAM-bearing Syk-coupled activatory CLR (Table 1). 2008). In contrast, the fungal-binding activity of Mincle is likely However, the other six mouse SIGNR receptors do not couple to be involved in host defense (see below). to Syk (Tanne et al., 2009). Interestingly, SIGNR3 has been Other ITAM-coupled CLRs include MDL-1 (myeloid DAP12- argued to be the closest functional homolog of human associating lectin-1, also known as CLEC5A), a CLR expressed DC-SIGN on the basis of both phylogeny and ligand specificity in monocytes, macrophages, and osteoclasts. MDL-1 associ- (Powlesland et al., 2006; Tanne et al., 2009). DC-SIGN is ates with the ITAM-bearing adaptor DAP12, as well as with a human CLR that recognizes mannose signatures present in DAP10, which leads to PI3 kinase activation (Bakker et al., M. tuberculosis, Mycobacterium leprae, , 1999; Chen et al., 2008; Inui et al., 2009). Recently, it has been Leishmania spp., measles virus, and HIV-1 gp120, as well as shown that MDL-1 binds dengue virus and induces production fucose-based Lewis antigens in Helicobacter pylori and of TNF-a in macrophages but whether this occurs by signaling Schistosoma mansoni (Table 1; Appelmelk et al., 2003; Geijten- through Syk has yet to be formally demonstrated (Chen et al., beek and Gringhuis, 2009). In contrast to mouse SIGNR3, 2008). Finally, mDCAR and mDCAR-1 in mouse may constitute human DC-SIGN does not signal via Syk despite possessing additional Syk-coupled myeloid activatory CLRs that associate an intracellular YxxL motif and does not serve as a myeloid with FcRg although their ligands and function at present remains activatory receptor on its own (Gringhuis et al., 2007). However, unknown (Kaden et al., 2009; Kanazawa et al., 2003). There is no it can modulate signaling through other receptors as will be direct ortholog of mDCAR or mDCAR-1 in human but they are discussed later. closely related to human BDCA-2 (blood antigen 2), which is expressed in plasmacytoid DCs (Dzionek et al., ITAM Signaling 2000, 2001). BDCA-2 associates with FcRg and signals through Much as Dectin-1 provides a paradigm for hemITAM signaling, Syk but, paradoxically, appears to act as an inhibitory rather than Dectin-2 typifies ITAM-coupled CLRs. Dectin-2, despite its activatory receptor (discussed below).

654 Immunity 34, May 27, 2011 ª2011 Elsevier Inc. Immunity Review

Consequences of Syk Signaling The consequences of Dectin-Syk activation for host defense Syk function in CLR signaling is analogous to that in B cell extend beyond the activation of transcription. Syk activation receptor signaling and involves many of the same players, has pleiotropic effects in myeloid cell biology including differen- including BLNK, SLP-76, Vav, Rac, and PLCg (Mo´ csai et al., tiation, cytoskeletal rearrangements, proliferation, and survival 2010). The key to the ability of some Syk-coupled CLRs to func- (Mo´ csai et al., 2010). One obvious property of many CLRs is tion as PRRs lies in their capacity to activate NF-kB downstream the ability to mediate ligand internalization through endocytosis of Syk. Canonical NF-kB activation by Dectin-1 and Dectin-2 or phagocytosis. This is a divergent property from TLRs, which, requires the adaptor protein CARD9 (Gross et al., 2006; Robin- by and large, are not considered to be endocytic receptors. Syk son et al., 2009). This adaptor has been shown to be rapidly contributes to phagocytosis of fungal particles by DCs although recruited to phagosomes containing ingested fungal particles not macrophages (Herre et al., 2004; Rogers et al., 2005; Under- (Goodridge et al., 2009) although it can presumably also be hill et al., 2005) and may affect the maturation of endosomes or triggered at the plasma membrane because Dectin-1 internaliza- phagosomes containing dead cells downstream of DNGR-1 tion is associated with termination of signaling (Hernanz-Falco´ n engagement (unpublished observations). In addition to internal- et al., 2009; Rosas et al., 2008). Whether at the cell surface or ization, Syk can control events within phagosomes. Activation of in endosomes, CARD9 then recruits Bcl-10 and Malt-1 and Dectins results in Syk-dependent production of reactive oxygen activates the IKK complex, which phosphorylates IkB and species (ROS) in myeloid cells (Figure 2; Gross et al., 2009; Ritter promotes its degradation, allowing the NF-kB family members et al., 2010; Underhill et al., 2005). ROS contributes to microbici- to translocate to the nucleus (Figure 2; Gross et al., 2006). It is dal activity within the phagosome (Steele et al., 2003) but not entirely clear how Syk and CARD9 control IKK activation, additionally can act as a point of regulation for IL-1b production. one idea being that they act sequentially (i.e., Syk activates IL-1b is transcriptionally regulated by NF-kB and posttransla- CARD9), another that Syk acts by phosphorylating the IKK tionally regulated by the inflammasome (Martinon et al., 2009). complex while CARD9 regulates ubiquitination of the NEMO Whereas the Syk-CARD9 pathway activates the synthesis of regulatory subunit (Bi et al., 2010). Notably, not all subunits of pro-IL-1b, Syk additionally can activate the NLRP3 inflamma- NF-kB are equally activated by both Dectins or by the individual some in a ROS-dependent manner, resulting in proteolytic members of the CARD9:Bcl-10:Malt-1 complex. For example, processing of IL-1b by caspase-1 (Figure 2; Gross et al., 2009). c-Rel is especially dependent on Malt-1 and is the only NF-kB Notably, IL-1b has a crucial role in antifungal immunity (Vonk subunit activated by Dectin-2 crosslinking in human DCs et al., 2006). Some studies have suggested that caspase-1 is whereas Dectin-1 triggering in the same cells activates all dispensable for resistance to candidiasis (Mencacci et al., NF-kB subunits (Gringhuis et al., 2011). In addition, unusually 2000) and that production of IL-1b by human monocytes stimu- for a PRR, Dectin-1-Syk signaling also leads to activation of lated with fungi stimulation is independent of the inflammasome the noncanonical NIK-dependent and CARD9-independent (van de Veerdonk et al., 2009a). However, in other studies NF-kB pathway that results in activation of RelB (Figure 2; NLRP3-deficient mice were shown to be highly susceptible to Gringhuis et al., 2009b). fungal infection (Gross et al., 2009; Guarda et al., 2011). The Differential regulation of the Syk-NF-kB axis may explain why latter suggest that, by regulating IL-1b processing, Dectin-Syk distinct Syk-coupled CLRs vary in their ability to activate NF-kB signaling could control host resistance to infection indepen- (unpublished observations). But the ability of a given CLR to dently of any role in gene induction. In contrast, in Schistosoma activate NF-kB also differs among myeloid cells, leading to infection, Dectin-2-Syk-NLRP3 signaling does not reduce para- conflicting reports. For Dectin-1 signaling, NF-kB activation site burden but, rather, contributes to immunopathology (Ritter correlates with the amounts of CARD9 expressed by each et al., 2010). Finally, Dectin-2 activation in DCs by allergens myeloid cell type, but it has also been argued that inhibitors from house dust mite or mold leads to generation of cysteinyl of the pathway may be expressed in macrophages (Goodridge leukotrienes, proinflammatory lipids that contribute to allergy et al., 2009). Such cell-specific regulation of the Syk-CARD9 (Barrett et al., 2009). pathway could explain why in some cells CLR signaling is suffi- cient for activation of a transcriptional response whereas in ITIM Signaling other cell types it works only in synergy with signals from other In contrast to the CLRs discussed above, others possess an receptors (see below). inhibitory ITIM motif and have been proposed to raise the Signaling through Dectin-1 or Dectin-2 activates transcription threshold for myeloid cell activation (Geijtenbeek and Gringhuis, factors besides NF-kB. It results in activation of MAP and 2009; Robinson et al., 2006). ITIM-bearing CLRs include DCIR PI3 kinase pathways (LeibundGut-Landmann et al., 2007; (DC-inhibitory receptor), which is expressed in human mono- Robinson et al., 2009; Shah et al., 2009; Slack et al., 2007) and cytes, macrophages, B cells, and DCs. When crosslinked by in PLCg-dependent Ca2+ elevation, leading to calcineurin- monoclonal antibodies, DCIR inhibits TLR8-mediated induction dependent dephosphorylation of NFAT and its nuclear translo- of IL-12 and TNF-a by myeloid DCs and TLR9-induced IFN-a cation (Figure 2; Goodridge et al., 2007; Greenblatt et al., 2010; production by plasmacytoid DCs (Meyer-Wentrup et al., 2008, Tassi et al., 2009; Xu et al., 2009). The latter is an important differ- 2009). In mouse, there are four DCIR homologs (mDCIR and ence from TLR signaling, which does not appreciably lead to mDCIR2-4) but only mDCIR and mDCIR2 (also known as 33D1) NFAT activation, and underlies a unique signature of Dectin possess an ITIM (Kaden et al., 2009). Their function is not known signaling, including the induction of high amounts of IL-2 and but mDCIR-deficient mice develop autoimmunity and marked IL-10 in DCs and cyclooxygenase-2 (COX-2) and PGE2 in expansion of the DC compartment (Fujikado et al., 2008). macrophages (Goodridge et al., 2007). Another ITIM-containing CLR, MICL (myeloid inhibitory C-type

Immunity 34, May 27, 2011 ª2011 Elsevier Inc. 655 Immunity Review

lectin receptor), is expressed in human granulocytes, mono- operates independently of Syk but converges at the level of cytes, macrophages, and DCs and its ligation has been shown NF-kB(Figure 2; Gringhuis et al., 2009b). Whereas Syk signaling to inhibit IL-12 responses to LPS and zymosan (Chen et al., results in activation of the canonical and noncanonical NF-kB 2006; Marshall et al., 2004). It is unclear how ITIM-bearing pathways, Raf1 activation results in phosphorylation and subse- CLRs inhibit TLR signaling, which does not involve a phosphotyr- quent acetylation of NF-kB p65, as in DC-SIGN signaling osine cascade and, therefore, the above studies may reflect an (Figure 2; Gringhuis et al., 2009b). Acetylated p65 can become indirect effect of ITIM signals, for example on the activity of transcriptionally active in partnership with p50 or it can generate Syk-coupled receptors that synergize with TLR signaling (see RelB-p65 inactive dimers that do not bind to DNA (Gringhuis below). Alternatively, inhibition of TLR responses could result et al., 2009b). Overall, Raf1 activation by Dectin-1 enhances from interference with PI3 kinase pathways by the inositol phos- the expression of some Syk-dependent cytokines in human phatase SHIP. DCs, including IL-10, IL-12, IL-6, and IL-1b, but negatively Negative regulation of cell activation can paradoxically also regulates the production of IL-23 (Gringhuis et al., 2009b). This ensue from engagement of ITAM-bearing receptors (Hamerman is associated with an increase in the IL-12/IL-23 ratio and has et al., 2009). For example, BDCA-2 signals through Syk, BLNK, been implicated in the induction of Th1 cell responses by DCs and PLCg2 in plasmacytoid DCs to downmodulate the produc- activated through Dectin-1 (Gringhuis et al., 2009b). tion of type I IFN and the secretion of TRAIL induced by TLR agonists (Cao et al., 2007; Riboldi et al., 2009; Ro¨ ck et al., Role of Syk-Coupled CLRs in Host Defense 2007). It is not known whether BDCA-2 acts only to dampen The ligands for ITIM-bearing CLRs are not presently known and TLR responses or whether it can trigger plasmacytoid DC it is unclear to what extent these inhibitory receptors impact function in isolation. Nevertheless, the dual role of ITAM signaling the outcome of infection. However, many of the Syk-coupled in cell activation and cell inhibition indicates an unexpected receptors discussed in this review recognize pathogens and degree of complexity in the regulation of the activity of CLRs probably evolved as activatory receptors in response to infec- that use phosphotyrosine-based signaling motifs. tious pressure. Their role in host defense can sometimes be difficult to assess because of redundancy among PRRs and Tyrosine-Independent CLR Signaling evolutionary pressure on organisms to alter or subvert PAMP CLRs can also engage nontyrosine-based signaling pathways recognition. For example, b-glucans are sequestered in the inner that impact innate and adaptive immune responses (Geijtenbeek layer of fungal cell walls: C. albicans, an opportunistic pathogen and Gringhuis, 2009). However, unlike ITAM or hemITAM in humans, exposes b-glucans at the budding points only during signaling, these pathways have not been shown to be sufficient yeast cell division (Gantner et al., 2005). However, upon transi- for myeloid function. Rather, they act to regulate and fine tune tion of the fungus from the yeast to the hyphal form, budding cell activation induced by the same or other PRRs (Geijtenbeek ceases and hyphae are effectively devoid of exposed b-glucan and Gringhuis, 2009). (Gantner et al., 2005). Despite the rarity of b-glucan exposure during the fungal life cycle, Dectin-1 plays a role in restricting Raf1 Signaling C. albicans infection. Mice deficient in Dectin-1 display Some CLRs have been shown to activate the serine/threonine increased mortality and higher fungal burdens upon i.v. infection kinase Raf1, better known for its role in Ras signaling. This with C. albicans, although the penetrance of this effect is was first shown for DC-SIGN in response to engagement by variable, having been seen in one study but not in another (Saijo ManLAM, an M. tuberculosis-derived ligand for DC-SIGN that et al., 2007; Taylor et al., 2007). In a different model of mixed modifies the response of human DCs to LPS (Geijtenbeek oropharyngeal and systemic infection, Dectin-1-deficient mice et al., 2003). The mechanism leading to the activation of Raf1 showed increased systemic C. albicans dissemination from upon DC-SIGN engagement by mannose-rich ligands does not the site of infection and a slight augmentation in mortality (Hise require the YxxL motif present in the intracellular tail of DC-SIGN et al., 2009). The variable Dectin-1 dependence in different but involves a complex of LSP1, KSR, and CNK proteins, which studies is probably due to differences in the strain of is constitutively associated with DC-SIGN, as well as the C. albicans and in the route used for infection, superimposed proteins LARG and RhoA, which are recruited to the complex on the compensatory effects of other PRRs. The latter includes upon ManLAM binding (Gringhuis et al., 2007, 2009a). Raf1 Dectin-2; mice deficient in that CLR have also been reported signaling does not activate DCs but modulates TLR-mediated to display higher mortality and elevated fungal burdens after NF-kB activation by directly or indirectly leading to phosphoryla- i.v. infection with three strains of C. albicans (Saijo et al., 2010). tion of the NF-kB p65 subunit on Ser276, which permits subse- Mincle may also compensate for deficiency in either Dectin-1 quent acetylation. The latter results in prolonged and increased or Dectin-2 as, in one study, Mincle-deficient mice showed transcription of the genes encoding IL-10 and a few other cyto- higher kidney fungal burden early after infection with kines (Gringhuis et al., 2007, 2009a). Interestingly, fucose-based C. albicans i.v. (Wells et al., 2008). However, Dectin-1-, Dectin- ligands for DC-SIGN do not lead to Raf1 signaling even though 2-, or Mincle-deficient mice resist their own fungal commensals they can still potentiate TLR-induced IL-10 production in human and are not profoundly susceptible to fungal infections as a DCs via a pathway requiring LSP1 (Gringhuis et al., 2009a). whole. This has been noted by testing models other than Like human DC-SIGN, mouse SIGNR3 also appears to signal C. albicans: Dectin-1-deficient mice show transient increases via the Raf1 pathway, in addition to utilizing the hemITAM-Syk in lung fungal burden when infected with Pneumocystis carinii pathway (Tanne et al., 2009). Similarly, Dectin-1 has been shown cysts i.n. yet are able to clear the infection longterm, much like to also signal via Raf1 in human DCs (Figure 2). This pathway wild-type mice (Saijo et al., 2007). Dectin-2-deficient mice are

656 Immunity 34, May 27, 2011 ª2011 Elsevier Inc. Immunity Review

as resistant as wild-type mice to infection with Cryptococcus phages stimulated with zymosan (Brown et al., 2003; Gantner neoformans (Saijo et al., 2010). et al., 2003). However, it is worth defining what is meant by collab- Dectin-1 and Dectin-2 control antifungal adaptive immunity oration. In the case of Dectin-1-TLR2 collaboration in macro- (see below) but also clearly regulate innate resistance to fungi. phages, this is likely to have been due to synergistic activation Thus, Dectin-1-deficient mice infected intratracheally with of transcription modules mediated by signaling through Dectin- conidia, another human opportunistic 1-Syk and TLR2-MyD88 pathways (Dennehy et al., 2008). In pathogen, show increased mortality very early after infection neutrophils, this same synergy induces an anti-inflammatory (Werner et al., 2009). Similarly, even when adaptive immunity signature characterized by production of IL-10 rather than to P. carinii is compromised by cortisone treatment, Dectin-1 TNF-a, arguing for cell-specific outcomes (Zhang et al., 2009). deficiency still results in higher fungal burden in the lung (Saijo Nevertheless, in both cases CLR signaling may be seen as acting et al., 2007). A dramatic example of loss of innate resistance to primarily in a quantitative sense to shift the dose response to a fungal infection is the phenotype of CARD9-deficient mice that given TLR agonist and vice versa. A distinct sense of collaboration succumb rapidly to disseminated candidiasis when infected is that in which a receptor has a qualitative effect. An example i.v. with C. albicans (Gross et al., 2006). Similarly, mice with from the TLR field is CD14, which, among other roles, can signal defective NFAT activation, induced by either systemic treatment to promote Ca2+-dependent NFAT activation and qualitatively with a calcineurin inhibitor or genetic deletion of calcineurin in alters the response to TLR4 triggering by LPS (Zanoni et al., neutrophils and other myeloid cells, are highly susceptible to 2009). However, CD14 by itself does not activate NF-kB and is disseminated candidiasis (Greenblatt et al., 2010). Thus, it not considered to be a PRR. Relevant to the present discussion, appears that interference with downstream targets of CLR in some instances, Syk-coupled CLRs activate NFAT but not NF- signaling gives more profound impairment of innate resistance kB and may therefore act primarily like CD14 (unpublished obser- to fungal infection than ablation of single receptors. This may vations). Similarly, as discussed above, DC-SIGN acts in a qualita- be expected from CLR redundancy but raises the question of tive sense as a modulator of NF-kB activation by other PRRs. An specificity. CARD9, for example, has also been implicated additional sense of collaboration is that in which the response downstream of NOD protein, TLR, and RIG-I-like receptor through one receptor allows the action of another. The purported signaling (Hara et al., 2007; Hsu et al., 2007; Poeck et al., 2010). ability of Syk-coupled CLRs to induce ROS in the context of Mice deficient in Syk-coupled CLR have yet to be tested C. albicans infection and thereby promote NLRP3 activation and systematically for impairment in immunity to nonfungal organ- pro-IL-1b processing may be considered a form of collaboration isms. Mycobacterial species can be recognized by Dectin-1, that can result in innate resistance to infection. A distinct example yet Dectin-1-deficient mice do not show altered resistance to is where CLR signaling affects phagosomal fate and controls lung infection with M. tuberculosis (Marakalala et al., 2011). accessibility of cargo to other PRRs. Syk-coupled receptors In contrast, CARD9-deficient mice are susceptible to experi- (possibly CLRs?) have been implicated in the uptake of Candida mental infection with the same organism (Dorhoi et al., 2010). glabrata and/or routing of phagosomes to a compartment where This discrepancy could reflect the activity of Mincle in recog- Candida RNA can then be sampled by TLR7 (Bourgeois et al., nizing trehalose-6,60-dimycolate, although the ability of the 2011). This leads to IFN-a and/or IFN-b induction, which impairs receptor to directly bind mycobacteria and the susceptibility of resistance to Candida spp. (Bourgeois et al., 2011; Guarda Mincle-deficient mice to mycobacterial infection has not yet et al., 2011), so this particular form of collaboration is actually been reported. Notably, SIGNR3-deficient mice are also more detrimental to the host. Finally, when dealing with particulate susceptible to infection with M. tuberculosis although SIGNR3 stimuli such as fungi and bacteria, collaboration between different has not yet been formally shown to signal through CARD9 receptors needs to be assessed with caution because a particular (Tanne et al., 2009). receptor might simply increase particle binding to the phagocyte, Finally, Syk-coupled CLR signaling might in some cases thereby allowing the subsequent recruitment of signaling recep- adversely affect the outcome of infection by favoring immunopa- tors. This makes it especially difficult to interpret many studies thology. This appears to be the case for MDL-1 in infection with where blocking antibodies for a given receptor and/or genetically dengue virus, which elicits TNF-a production by macrophages deficient cells are used to demonstrate defects in the inflamma- (presumably via Syk) and thereby appears to contribute to the tory response to fungal or bacterial particles. hemorrhagic and plasma leakage syndrome associated with With these considerations in mind, many innate immune the disease (Chen et al., 2008). Whether MDL-1 has evolved to receptors can be shown to influence resistance to fungal recognize dengue virus or is, instead, targeted by the virus is infection in mouse studies. For example, the C. albicans cell unclear. wall has ligands for many receptors on myeloid cells (Figure 3). N-linked mannans are preferentially recognized by mannose Crosstalk between CLRs and Other Innate Receptors receptor, DC-SIGN, and SIGNR1 in DCs and macrophages Pathogens generally contain multiple PAMPs and therefore (Cambi et al., 2008; Taylor et al., 2004). In contrast, Dectin-2 engage multiple PRRs together or sequentially. Thus, the immune recognizes a-mannans and TLR2 can detect phospholipo- response to any one pathogen involves a multitude of PRRs, as mannan-containing molecules (Jouault et al., 2003; Saijo et al., well as other innate receptors such as ones involved in recognition 2010). Additionally, sensing of O-linked mannans has been of altered self or cell death. CLR collaboration with other recep- attributed to TLR4 (Netea et al., 2006). b-glucans are ligands tors is well established. In fact, the innate role of Dectin-1 was first for Dectin-1, complement receptor 3 (CR3), and the scavenger ascribed to its ability to collaborate with TLR2 in promoting receptors CD36 and SCARF1 (Forsyth et al., 1998; Gantner proinflammatory cytokines such as TNF-a production in macro- et al., 2005; Means et al., 2009). Enhanced resistance and lower

Immunity 34, May 27, 2011 ª2011 Elsevier Inc. 657 Immunity Review

Figure 3. Innate Recognition of Fungal Cell Mannose Walls by Receptors Expressed on DCs and receptor Macrophages N-linked Multiple myeloid innate immune receptors have mannan been shown to recognize cell wall components of C. albicans. Signals emanating directly or DC-SIGN Dectin-1 Dectin-2 Mincle N-linked β-glucan mannan ? indirectly from these receptors are responsible mannan for initiating and modulating immunity to the organism. They act together with signals from TLR4 TLR2 receptors (not depicted) that recognize additional O-linked phospho- components of the cell wall or the fungus body mannan lipomannan (e.g., RNA) or fungus-bound opsonins (e.g., anti- bodies, complement). See text for details.

FcRγ chain adaptive immunity. In the case of Dec- tin-1, demonstration of sufficiency involved moving away from fungal parti- cles and finding a specific agonist that P P does not engage additional receptors. TRIF MyD88 Raf1 S Y Curdlan is a b-(1,3)-glucan polymer K produced by the bacterium Alcaligenes faecalis that acts as a selective Dectin-1 agonist (LeibundGut-Landmann et al., 2007; Yoshitomi et al., 2005). Curdlan is sufficient to fully activate DCs, even in fungal burden has been noted in CR3- and Fcg receptor- the absence of the two TLR adaptors MyD88 and TRIF (Lei- deficient mice infected with C. albicans (Romani et al., 2004). bundGut-Landmann et al., 2007). Human or mouse DCs acti- This is not to be confused with mice deficient in FcRg chain, vated with curdlan are able to promote T helper 1 (Th1) and which mediates signals by Dectin-2 and is therefore presumably Th17 cell differentiation of naive CD4+ T cells (Gringhuis et al., necessary for resistance to C. albicans (Saijo et al., 2010). 2009b; LeibundGut-Landmann et al., 2007) and induce the The contribution of TLRs has also been addressed in vivo. Two conversion of Foxp3+RORgt+ regulatory T cells into IL-17- studies have shown that TLR2-deficient mice are more resistant producing Foxp3+ cells in vitro (Osorio et al., 2008). When to C. albicans infection, suggesting an inhibitory role for this curdlan is used as an adjuvant in vivo, it promotes both Th1 receptor during fungal recognition, perhaps through induction and Th17 cell responses unlike TLR agonists, which predomi- of IL-10-producing regulatory T cells that suppress antifungal nantly induce Th1 cell differentiation (Joffre et al., 2010; Lei- defense (Bellocchio et al., 2004; Netea et al., 2004). In contrast, bundGut-Landmann et al., 2007). Adjuvancy is not confined to TLR2 plays a protective role in the model of mixed oropharyngeal the induction of CD4+ T cell responses as indicated by the fact and systemic candidiasis, controlling fungal dissemination and that curdlan additionally promotes antibody responses and cyto- contributing to inflammasome activation (Hise et al., 2009). toxic T lymphocyte (CTL) crosspriming (LeibundGut-Landmann TLR4 or TLR9 deficiency does not impair survival to systemic et al., 2007, 2008). In addition to inducing Th17 cells, Dectin-1 candidiasis, suggesting that these receptors are redundant (Bel- can also directly trigger the production of IL-17 by a subset of locchio et al., 2004). In contrast to single TLR deficiency, lack of gd T cells (Martin et al., 2009). The elicitation of Th17 cell-type the adaptor MyD88 renders mice highly susceptible to dissemi- responses by Dectin-1 is an essential feature of antifungal nated candidiasis (Bellocchio et al., 2004; Villamo´ n et al., 2004), immunity (see below). but this effect may be attributable to signaling via IL-1R, as The issue of sufficiency in inducing adaptive immunity has not indicated by the fact that IL-1R-deficient mice are equally been thoroughly explored for other Syk-coupled CLRs. susceptible to systemic infection (Bellocchio et al., 2004). Inter- However, the adjuvanticity of trehalose-6-60-dimycolate and its estingly, the negative regulator of TLR and IL-1R signaling TIR8 synthetic analog requires Syk and CARD9 signaling, especially (Toll IL-1R8) is nonredundant in anti-Candida defense, as shown for Th17 cell responses (Werninghaus et al., 2009). Because by the fact that TIR8-deficient mice succumb to disseminated Mincle was recently reported as the receptor for trehalose-6- candidiasis (Bozza et al., 2008). In sum, these results indicate 60-dimycolate (Ishikawa et al., 2009; Schoenen et al., 2010), that fine regulation of CLR and TLR-IL-1R signaling is necessary these results imply that selective triggering of Mincle might be for effective clearance of fungal pathogens. sufficient to promote adaptive immunity. In addition, the demon- strable ability of Dectin-2 and MDL-1 to activate NF-kB in DCs Induction of Adaptive Immunity by CLR Signaling via Syk suggests that signals through these receptors might also suffice PRRs that signal through NF-kB, such as TLRs, promote activa- to induce adaptive immunity although this remains to be formally tion of DCs, rendering them competent to activate adaptive proven and could differ between mouse and human DCs (Chen immunity. Syk-CARD9-coupled CLRs expressed in DCs might et al., 2008; Gringhuis et al., 2011; Robinson et al., 2009; Saijo therefore be expected to similarly couple pathogen sensing to et al., 2010).

658 Immunity 34, May 27, 2011 ª2011 Elsevier Inc. Immunity Review

Another means of dissecting the function of a given PRR in and both IL-17RA- and IL-17F- but not IL-17A-deficient mice acquired immunity is to examine necessity rather than suffi- are more susceptible to gastric infection with the same organism ciency. This does not formally prove that the PRR alone can (De Luca et al., 2010). Mice deficient in IL-22 also display bridge innate and adaptive immunity but does reveal points of impaired resistance to i.v. and/or intragastric C. albicans infec- lack of redundancy, which indicate a key role for the receptor tion (De Luca et al., 2010). Thus, in many mouse models of candi- in question in the regulation of T cell and B cell responses in diasis, type 17 responses overall appear protective even though the model chosen for analysis. This approach has been espe- the contribution of individual cytokines to resistance remains cially productive; it has revealed a nonredundant role for Syk- a matter of debate. coupled CLRs in the induction of Th17 cell responses to fungal A clearer picture of the protective role of Th17 cell responses infection. Indeed, Th17 cell responses to C. albicans are easily during fungal infections emerges from studies in humans. detected in infected mice, alongside Th1 cell responses (Lei- Human memory CD4+ T cells specific for C. albicans have a bundGut-Landmann et al., 2007). When CARD9-deficient mice Th17 cell phenotype (Acosta-Rodriguez et al., 2007; Zhou were infected with a very low dose of organism (to circumvent et al., 2008), and T cell immunodeficiency is commonly associ- the lethality caused by loss of innate resistance), the impact of ated with chronic mucocutaneous candidiasis (CMC) as in the CARD9 on adaptive immunity became evident: Th17 cells case of HIV patients with low CD4+ T cell counts, who suffer specific for C. albicans were absent whereas Th1 cell responses from oropharyngeal infections (‘‘oral thrush’’). Interestingly, were largely preserved (LeibundGut-Landmann et al., 2007). In patients with mutations in STAT3 fail to generate Th17 cells the same study, it was shown that Th17 cell responses to and suffer from chronic or recurrent CMC and S. aureus infec- C. albicans were not lost in Dectin-1-deficient infected mice tions (de Beaucoudrey et al., 2008; Ma et al., 2008; Milner (LeibundGut-Landmann et al., 2007), which was later shown to et al., 2008). Similarly, autoimmune polyendocrinopathy be due to the compensatory effects of Dectin-2 (Robinson syndrome-I patients with a deficiency in AIRE display high titers et al., 2009). Indeed, Dectin-2 rather than Dectin-1 appears to of neutralizing autoantibodies against IL-17A, IL-17F, and IL-22 be the predominant CLR involved in coupling innate recognition and suffer from CMC (Kisand et al., 2010; Puel et al., 2010). of Candida albicans to the induction of Th17 cell responses (Rob- Finally, a recent study formally showed the role of type 17 immu- inson et al., 2009; Saijo et al., 2010). Recent data have confirmed nity in the control of mucosal Candida infection by identifying that Dectin-1 and Dectin-2 signaling in human DCs also favors mutations in IL-17RA or in IL-17F as genetic etiologies of familial Th17 cell responses and showed that selective activation of CMC (Puel et al., 2011). c-Rel by Dectin-2 controls IL-1b and IL-23 p19, two cytokines Excitingly, a connection between the CLR pathway and involved in Th17 cell polarization (Gringhuis et al., 2011). Thus, susceptibility to fungal infections in humans has recently been unlike Dectin-1 signaling, which activates DCs to induce both reported. A premature stop mutation in Dectin-1 or a homozy- Th1 and Th17 cell responses, Dectin-2 signaling predominantly gous point mutation in CARD9 have been found in two families favors induction of the latter, although in human DCs this may susceptible to CMC (Ferwerda et al., 2009; Glocker et al., require signals from additional receptors (Gringhuis et al., 2009). The CMC patients with Dectin-1 deficiency have a paucity 2011). The fact that Dectin-2 is involved in Th17 cell defense to of IL-17-producing cells in peripheral blood and suffer from C. albicans in mouse and human distinguishes it from the vulvovaginal candidiasis and onychomycosis (fungal infection mannose receptor, which is reported to coordinate human in the nail beds) but not from systemic fungal infections (Fer- Th17 cell responses to Candida (van de Veerdonk et al., werda et al., 2009). CARD9-deficient patients also display low 2009b) but has no impact in host defense during fungal infection numbers of circulating Th17 cells but have more severe clinical in mice (Lee et al., 2003). Interestingly, a recent report indicates symptoms, with CMC but also invasive fungal infections that Dectin-2-mediated induction of cysteinyl leukotrienes can that can lead to death (Glocker et al., 2009). The mutation in Dec- also lead to the Th2 cell-biased response associated with house tin-1 was subsequently shown to represent a polymorphism and dust mite allergic disease (Barrett et al., 2011). In sum, CLR has been associated with susceptibility to invasive Aspergillosis signaling through Syk can induce Th1 and Th17 cell-type and mucosal colonization with C. albicans after human stem cell immunity, is nonredundant for Th17 cell responses to fungal transplantation (Chai et al., 2011; Cunha et al., 2010; Plantinga infection, and may additionally regulate Th2 cell responses to et al., 2009). In sum, evidence in humans strongly indicate that some allergens. signaling downstream of fungal-specific C-type lectin receptors is an essential axis in the establishment of antifungal Th17 cell CLRs in Human Immunity to Infection responses, which in turn act as key mediators of protection Why the connection between PRRs specialized in fungal recog- from fungal infection at mucocutaneous surfaces. nition and the induction of Th17 cell responses? Intriguingly, a role for type 17 responses in protection from fungal infections Conclusion has recently emerged. Despite a report that Th17 cell responses Myeloid CLRs constitute an eclectic group of innate immune promote pathology in an intragastric model of mouse candidiasis receptors with multiple functions in the initiation and regulation (Zelante et al., 2007), mice deficient in IL-17RA or IL-17RC have of immune responses. They can act as endocytic receptors been reported to be susceptible to systemic and oropharyngeal that mediate the uptake of self-antigens and pathogens for either candidiasis (Conti et al., 2009; Ho et al., 2010; Huang et al., destruction or antigen retrieval and presentation to T cells. In 2004). Similarly, mice deficient in IL-23, IL-17A, and IL-17F are addition, many CLRs signal to modulate myeloid cell activation also more susceptible to cutaneous and/or systemic and thereby impact inflammation and the induction of adaptive C. albicans infection (Kagami et al., 2010; Saijo et al., 2010) immune responses. Activatory myeloid CLRs tend to signal

Immunity 34, May 27, 2011 ª2011 Elsevier Inc. 659 Immunity Review

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