innate immunity

motifs. Here we focus on the known signaling pathways of myeloid C-type lectins and on their possible functions as autonomous activating or inhibitory receptors involved in innate responses to pathogens or self.
The detection of an invading pathogen and subsequent activation of antimicrobial responses is often referred to as ‘pattern recogni-tion’, in deference to the proposal that immunity is induced by signals from pattern-recognition receptors (PRRs), which recognize patho-gen-associated molecular patterns (PAMPs)1. PAMPs are conserved groups of molecules that are essential for microbe survival, such as bacterial and fungal cell wall components and viral nucleic acid. In the broadest sense, PRRs comprise any PAMP receptor capable of trigger-ing antimicrobial function in leukocytes, whether it is phagocytosis by macrophages or release of cytotoxic granules by natural killer (NK) cells 1. However, some PRRs transmit signals that regulate the expres-sion of innate response genes, including those encoding costimula-tory molecules, cytokines or chemokines. Induction of those genes is essential not only for early protection against pathogen intrusion, as exemplified by the importance of interferons in resistance to viral infection 2, but also for coupling innate with adaptive immunity 1,3. Thus, when considering PRRs, it is useful to distinguish those that have the potential to regulate gene expression from those that may be involved exclusively in immediate immune effector functions such as phagocytosis or degranulation.
So far, the repertoire of PRRs that can regulate gene expression is limited to just a few families. These include the virus-sensing RIG-I and Mda5 helicases 2 and the T oll-like receptors (TLRs)4. Nevertheless, other receptors can also bind pathogens and, in some cases, can regulate the expression of innate response genes. Those receptors include members of the TREM (‘triggering receptors expressed on myeloid cells’), Siglec 5 and C-type lectin families. In this review, we discuss myeloid C-type lectins and whether they can be considered as a non-TLR class of PRRs having the capacity to regulate innate and adaptive immunity.Functional diversity of C-type lectins
C-type lectins encompass a large family of proteins found almost exclusively in Metazoa. They are highly conserved in vertebrates but
show considerable diversity among invertebrates. For example, C-type lectins in Caenorhabditis elegans have little structural similarity to those in Drosophila melanogaster 6.
C-type lectin receptors (CLRs)7structural motif 8found in over 1,000 proteins with diverse functions 6.
The immunological relevance of some of those proteins seems to be evolutionarily conserved; analysis of the C. elegans response to a nematode pathogen has shown 10 genes encoding CLRs among 68 upregulated genes, by far the most abundant class of induced genes 9. In vertebrates, there are up to 17 groups of CLRs 6, some of which are able to bind PAMPs and, in some cases, to mediate host defense. For example, many of the collectins (CLR group III), which are soluble proteins found in serum, bind microbes and activate complement. Mannose-binding lectin, the prototypic collectin, binds to various sugar moieties (such as N -acetyl-D -glucosamine, mannose, N -ace-tion by Neisseria meningitidis , pneumoniae 10signal to regulate gene expression. In contrast, other CLRs do act as signaling receptors in the innate immune system, most notably the activating NKG2, NKR-P and Ly49 receptors on NK cells (CLR group V). However, NK cell receptors respond mainly to self rather than to microbe-derived ligands and for that reason are not generally thought of as PRRs 12.
Many transmembrane C-type lectins belonging to groups II, V and VI are expressed mainly by myeloid cells (Table 1). Although many are Immunobiology Laboratory, Cancer Research UK, London Research Institute, London WC2A 3PX, United Kingdom. Correspondence should be addressed to C.R.S. (caetano@).
Received 16 July; accepted 19 October; published online 16 November 2006; doi:10.1038/ni1417N O N -T O L L -L I K E I N N AT E I M M U N E P R O T E I N S
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y
of nonopsonised microbes and to induce cytokine production in macrophages and dendritic cells (DCs), leukocytes that are critical in innate immunity and in subsequent instruction of adaptive immune responses. Here we summarize the known functions of such myeloid C-type lectins. We review their function in phagocytic clearance of pathogens and describe one distinct example of a C-type lectin PRR that can couple PAMP recognition to regulation of gene expres-sion. We discuss other CLRs that regulate innate response genes and review their ability to couple to distinct signaling pathways. Finally, we consider the possibility that many myeloid C-type lectins may have
evolved to bind not to PAMPs but instead to self ligands, allowing them to have unanticipated functions in immune homeostasis.Myeloid C-type lectins in microbe phagocytosis
The phagocytosis of nonopsonised microbes and their subsequent destruction in acidified phagolysosomes is an important mechanism of immune defense. Although TLR signaling can regulate phagosomal maturation, TLRs do not mediate phagocytosis itself 13. In contrast, many C-type lectins act as phagocytic receptors in myeloid cells. Indeed, myeloid CLRs often contain distinct internalization motifs in their cyto-plasmic tails, including triads of acidic amino acids, dileucine motifs and tyrosine-based motifs (Table 1), which can direct both ligand uptake and the subsequent sorting of the receptor and its cargo 14.A tyrosine-based motif in the intracellular tail of the mannose recep-tor promotes the delivery of mannosylated ligands to early endosomes, whereas the additional triacidic motif found in DEC-205 diverts this receptor to late endosomes–lysosomes 15. For dectin-1, a β-glucan-spe-cific CLR that mediates the phagocytosis of yeast and yeast-derived particles such as zymosan, both the triacidic motif and the membrane-proximal tyrosine residue are required for endocytosis 16–18.
In mouse DCs and fibroblasts expressing dectin-1, zymosan phago-cytosis is partly dependent on the tyrosine kinase Syk, which activates the Rho family GTPases Cdc42 and Rac-1, triggering actin polymer-ization and pseudopod extension around the particle 16,17,19. In macro-16,18 (Fig. 1). 16. In contrast, 16. Similarly, the 20 but can also be retained in early 21. Thus, The ability of CLRs to mediate endocytosis of ligands has implications for presentation to T lymphocytes. Indeed, antibody- Table 1), delivers antigen 15,20,22,23. In addition, some CLRs might retain cargo in nondegradative com-partments to allow subsequent presentation of intact antigen to
B cells 24. Antigen targeting to myeloid CLRs is therefore emerging as a possible strategy for vaccination in mice and humans.
Myeloid C-type lectin–induced gene expression
As mentioned above, a salient feature of PRRs such as TLRs or the RIG-I and Mda5 helicases is their ability to couple PAMP recognition to ‘downstream’ induction of innate response genes. Although CLRs have long been suspected to be able to act in that way, the charac-terization of myeloid CLRs that signal to induce innate or adaptive immunity has been hampered by poor definition of signaling path-ways triggered by CLR engagement. Moreover, the signaling proper-ties of some CLRs have been studied with crosslinking antibodies, which may not always reflect the signaling pathways triggered by actual ligands.
So far, only dectin-1 has been definitively demonstrated to couple PAMP recognition to the induction of innate response genes 25. We will first describe signaling through this receptor and then discuss what is known about other CLRs. Zymosan stimulates cytokine pro-duction by macrophages and DCs, indicating that it triggers PRR signaling pathways that regulate gene expression. The first-described PRR capable of recognizing zymosan was TLR2 (ref. 26), but DCs lacking TLR2 or MyD88, its obligate intracellular signaling partner, still produce interleukin 10 (IL-10) and IL-2 when challenged with zymosan particles 17,27. Dectin-1 has been linked to TLR2-indepen-dent fungal responses in antibody-blocking studies 28,29, and further understanding of the dectin-1 signaling pathways has been provided by studies of a B cell line in which expression of full-length but not tail-less dectin-1 mediates zymosan-dependent production of IL-10 and IL-2 (ref. 17). The finding of defective zymosan-induced IL-10 and IL-2 production in Syk-deficient DCs suggests that the kinase Syk is involved in the transmission of zymosan-triggered dectin-1 signals 17. Indeed, phosphorylated Syk can be detected at sites of dectin-1 engagement 17,18, and mutagenesis data suggest that Syk recruitment involves a single YxxL motif in dectin-1 (where ‘x’ is any amino acid)17. The tyrosine residue in that motif is thought to be phosphorylated by Src family kinases after zymosan stimulation 17,18. Notably, recombinant Syk binds in vitro to a phosphorylated peptide containing the same motif, suggesting that Syk recruitment in vivo could result from direct binding to the CLR rather than to a third-party adaptor 17. The fact that a receptor with a single YxxL motif
zymosan stimulation triggers activation of mitogen-activated protein kinases (MAPKs) and NF-κB (the latter via a complex containing CARD9, MALT1 and Bcl-10) and ultimately results in the transcription of innate immune response genes. In addition, zymosan phagocytosis by DCs is partially dependent on Syk, whereas in macrophages it is Syk independent but the production of reactive oxygen species (ROS) depends on Syk activity.
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y
Table 1 Membrane-bound CLRs expressed in myeloid cells
CLR group CLR a
Cell types Cytoplasmic and tmb motifs
Endocytic activity
Signaling pathways
Response
References
VI
MR family CRD Calcium- dependent
MR (CD206)
DC, LC, Mo, MØ,
LE, LSE FxxxxY
Yes Early end.?
↑ IL-10, IL1Ra ↓ TLR-stimulated IL-1215,72
DEC-205 (CD205)DC, LC, tEC, B,
MØ
FxxxxY Yes
Late end.–lys.
?Antigens targeted to DEC-205 in mice are presented in a tolerogenic way 15,73,74
II CRD Calcium- dependent
DC-SIGN (h)DC, dMØ, aMØ
YxxL LL EED Yes
Ab: late end.–lys. Vir: early end.
Ab: PLC γ, PI(3)K;
calcium Erk
↓ TLR-stimulated IL-12 and DC maturation ↑ TLR-stimulated IL-10
20,42, 43,75
SIGN-R1 (m)MZ MØ, pMØLL EED
Yes
?
?
76
MCL Mo, MØYes ??77
Mincle MØTmb R ???
Dectin-2
h: Mo, B, acti-vated CD4+ T m: MØ, PMN, LC
Tmb R (?)
?
?
↑ IL-4, IL-10? UV-induced tolerance to contact hypersensitivity 71
BDCA-2 (DLEC) (h)pDC Tmb K (?) EEE Yes
Late end.–lys.
Ab: Src; calcium ↓ TLR-induced type I interferons and IL-12
23DCIR (h) DCIR-1 (m)DC, Mo, MØ, PMN, B IxYxxV
?
SHP-1, SHP-2
?
36,37
DCAR-2 (DCAR) (m)DC, Mo, MØ, B Tmb R ?Ab: FcR γ; calcium, PTyr
?34Langerin
LC, DC subs
P-rich
Yes Early end.?
?
78
MGL (h) MGL-1 (m) MGL-2 (m)DC, aaMØ
YxxF LL Yes
?
h: binds to CD45, inhibits T cell activation
m: ↑ IL-1α
69 79V
NK cell
receptor–like Non-CRD Calcium- independent
MDL-1Mo, MØTmb K ?Ab: DAP12; calcium ?
33
Dectin-1
DC, LC, MØ, PMN
YxxL DED
Yes
Late end.–lys.
Syk; CARD9-Bcl-10–MALT1; NF-κB Erk, p38, Jnk
↑ IL-10, IL-2, IL-6, IL-23, type I interferons, ROS ↑ TLR-stimulated TNF ,
IL-12
16–19,30,
unpublished
CLEC1DC
YxxT DDD ??
?31CLEC2PMN, Mo, DC, Pl
YxxL
?Src; Syk; PLC γ2, Lat, SLP76; Vav1, Vav3
?
31,32
DCAL-1 (h)
DC, GC B ?
?T cell costimulation and
increase in IL-470DCAL-2 (MICL, KLRL1)
PMN, Mo, MØ,
DC
VxYxxL ?
SHP-1, SHP-2 Ab: Erk, p38
↑ CCR7, IL-6, IL-10,
TNF
↓ TLR-mediated IL-12 ↑ CD40L-mediated
IL-12
38,41
The CLRs here have been selected on the basis of selective or predominant expression in myeloid cells; therefore, this list is not complete. tmb, transmembrane; h, human; m, mouse; CRD, carbohydrate-recognition domain; MR, mannose receptor; LC, Langerhans cell; Mo, monocyte; MØ, macrophage; LE, lymphatic endothelium; LSE, liver sinusoidal endothelium; tEC, thymic endothelial cell; B, B cell; aMØ, alveolar macrophage; dMØ, decidual macrophage; PLC, phospholipase C; PI(3)K, phosphatidylinositol-3-OH kinase; MZ MØ, marginal zone macrophage; pMØ, peritoneal macrophage; PMN, polymorphonuclear cell; UV, ultraviolet irradiation; pDC, plasmacytoid DC; PTyr, tyrosine phosphorylation; DC subs, DC subsets; P-rich, proline-rich; aaMØ, alternatively activated macrophage; TNF , tumor necrosis factor; Pl, platelets; GC, germinal center; Vir, viruses; Ab, antibodies; end., endosome; lys., lysosome. Upper-case letters in ‘motifs’ column represent amino acid residues.
a Protein designations based on nomenclature from /research/animallectins.
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y
can activate Syk is unusual, as Syk contains tandem Src homology 2 domains and is normally activated by dual-phosphorylated immu-noreceptor tyrosine-based activation motifs (ITAMs; YxxL x(7–8) YxxL)17. We therefore refer to the Syk-binding sequence on dectin-1 as a ‘hemITAM’ motif.
The signaling pathways downstream of hemITAM-activated Syk are beginning to be ‘mapped’ (Fig. 1). Zymosan triggers the Erk mito-gen-activated protein kinase cascade in TLR2-deficient DCs 30 in a Syk-dependent way (E.C.S. and C.R.S., unpublished observations), and inhibition of Erk blocks zymosan-induced IL-10 production 30. In addition, selective engagement of dectin-1 leads to Syk-dependent activation of the p38, Erk, and Jnk kinase cascades (M.J.R. and C.R.S., unpublished observations) and of the transcription factor NF-κB 19. The last requires the adaptor CARD9, which forms a complex with the adaptor proteins Bcl-10 and MALT1, presumably in an innate signal-ing pathway analogous to the CARMA1–Bcl-10–MALT1 axis used by B cell and T cell receptors of the adaptive immune system 19. Thus, dectin-1–Syk–CARD9 signaling defines a pathway by which CLRs regulate innate gene expression independently of TLRs (Fig. 1).
The CLR CLEC2 is expressed in myeloid cells 31 and platelets 32, where it acts as a target of the snake venom rhodocytin. Notably, expo-sure of platelets to rhodocytin induces phosphorylation of CLEC2, presumably at the intracellular hemITAM YxxL motif. Syk can bind peptides corresponding to the phosphorylated CLEC2 cytoplasmic domain, and CLEC2 signaling in platelets leads to Syk-dependent phosphorylation of several intracellular proteins 32. Thus, CLEC2 in platelets seems to engage a signaling pathway similar to the one engaged by dectin-1 in myeloid cells, suggesting that hemITAM-Syk signaling may be triggered by many CLRs (Fig. 2). Whether such receptors can truly be grouped on the basis of their signaling proper-ties needs further investigation.
One unusual feature of dectin-1 is its apparent ability to bind directly to Syk; in contrast, the B cell receptor, activating Fc recep-tors and many of the TREM proteins couple with Syk indirectly via association with ITAM-bearing adaptor proteins. Notably, a few CLRs lack hemITAMs but, like some TREM proteins and NK cell receptors, have a basic residue in the transmembrane domain, which
allows them to associate noncovalently with ITAM-containing adap-tor proteins, including FcR γ chain and DAP12 (Fig. 2 and Table 1). For example, the lectin MDL-1 binds to DAP12, and crosslinking of a CD69–MDL-1 chimeric protein leads to calcium mobilization in a macrophage cell line 33. Similarly, the DC immunoactivating recep-tor DCAR binds to the FcR γ chain adaptor protein, and crosslinking of a chimeric protein bearing the transmembrane domain of DCAR triggers FcR γ chain–dependent calcium flux in a B cell lymphoma 34. The plasmacytoid DC–expressed CLR BDCA-2 has a lysine residue at the same position as does DCAR and, notably, crosslinking of BDCA-2 also triggers calcium flux and phosphorylation of intracel-lular proteins 23, although it is not yet known whether that depends on either DAP12 or FcR γ chain. Mincle and dectin-2 also bear positively charged amino acids in their putative transmembrane domains (Table 1). It is therefore conceivable that all five of these CLRs form a sub-family of receptors that couple to Syk indirectly through association with ITAM-containing adaptor proteins (Fig. 2) and, like dectin-1, stimulate cellular activation.
CLR-mediated inhibition is common in NK cells, in which an ITAM-coupled activating CLR often has an immunoreceptor tyrosine-based inhibitory motif (ITIM)–bearing inhibitory counterpart 12,35. The same could be true in myeloid cells (Fig. 2). For example, the CLRs DCIR and DCAR share substantial sequence homology in their extracellular domains, but whereas DCAR associates with the ITAM-bearing FcR γ chain, DCIR contains an ITIM and recruits the SHP-1 and SHP-2 phosphatases 36,37. A chimeric protein bearing the DCIR intracellular domain is able to downregulate B cell receptor signaling, indicating that DCIR can transduce an inhibitory signal 36. Inhibition has also been demonstrated for the DC-associated lectin DCAL-2, which bears an ITIM that recruits SHP-1 and SHP-2 and can down-modulate dectin-1 signaling 38.
The ITAM-versus-ITIM dichotomy has proven very helpful in understanding immunoreceptor signaling, but data have suggested that the presence of such motifs may not always correlate with acti-vating or inhibitory outcomes 39. For example, some TREM proteins signal via ITAM-bearing DAP12 but suppress rather than enhance TLR-induced cellular activation 40. Notably, BDCA-2 crosslinking suppresses TLR-induced production of interferon-α/β production by plasmacytoid DCs 23, although whether that inhibition is mediated by DAP12 remains to be determined. Conversely, receptors containing ITIMs may sometimes stimulate rather than repress cellular activa-tion. Direct crosslinking of endogenous DCAL-2 on DCs leads to activation of the Erk and p38 mitogen-activated protein kinases and production of IL-10, tumor necrosis factor, IL-6 and the chemokine MIP3-β41. Moreover, DCAL-2 stimulation decreases TLR-mediated IL-12 production but potentiates IL-12 induction by CD40 ligation 41. Those observations raise the possibility that a given ITAM or ITIM can act in an activating or inhibitory capacity depending on the cel-lular context 39.
Some CLRs have been described mainly as modulators of signaling by other receptors. For example, DC-SIGN triggering alone is insuf-ficient to induce expression of the gene encoding IL-10 or of other innate response genes, but DC-SIGN ligation greatly increases IL-10 production after stimulation with the TLR4 ligand lipopolysaccha-ride while at the same time blocking lipopolysaccharide-induced DC maturation 42,43. The mechanisms underlying those complex effects are not understood, although it is known that DC-SIGN crosslinking leads to calcium influx and activation of phosphatidylinositol-3-OH
Figure 2 Distinct myeloid CLRs use distinct proximal signaling mechanisms. Myeloid CLRs may activate Syk directly through a hemITAM or indirectly by means of ITAM-bearing adaptor proteins (such as FcR γ chain or DAP12). ITIM-containing CLRs may inhibit myeloid cell activation by coupling to SHP-1 or SHP-1 phosphatases.
Dectin-1
CLEC 2
MDL-1DCAR BDCA-2?Mincle?Dectin-2?
DCIR DCAL-2
HemITAM
ITAM coupling
ITIM
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y
kinase and Erk 43. Those last data suggest that DC-SIGN might not simply act as a TLR coreceptor and that its ability to modulate TLR responses could result from autonomous signaling and direct effects on gene promoters. Similarly, dectin-1 has been reported to enhance TLR2-dependent induction of tumor necrosis factor in macro-phages 44,45, but it remains unclear whether that reflects its ability to cooperate directly with TLR2 or whether that effect is a result of synergy between TLR2-MyD88 and the dectin-1–Syk signaling path-ways. Such synergy, acting at the level of gene transcription, has been reported to occur between the TLR-MyD88 and TLR-TRIF signaling pathways 46,47.Myeloid C-type lectins in host defense Several myeloid CLRs are capable of binding pathogens (Table 2). For example, the mannose receptor, the first CLR to be described in myeloid cells 48, binds yeast, human immunodeficiency virus and Mycobacterium tuberculosis . The CLR (h) MGL binds to Schistosoma mansoni and the Ebola and Marburg filoviruses. DC-SIGN binds a large variety of pathogens, including viruses, fungi, bacteria, proto-zoa and products from metazoan parasites. That pathogen-binding capacity, coupled with signaling potential and endocytic activity, has led to the view that myeloid CLRs might constitute a non-TLR PRR family. However, it is somewhat unclear whether their PAMP-binding capacity is a host response to infection or represents pathogen exploi-tation of a cellular receptor. Consistent with the latter idea, dectin-1 was originally described as a receptor for an unidentified endogenous ligand in T cells 49, and it binds β-glucans through an atypical inter-action that does not involve calcium or the putative sugar-binding site in the C-type lectin domain 50. Another reason dectin-1 does not
fit the typical profile of a PRR is that neither it nor Syk or CARD9 is evolutionarily conserved, being restricted mainly to jawed vertebrates. However, β-glucan recognition itself is phylogenetically ancient: in drosophila and in the silkworm Bombyx mori , yeast β-glucans are recognized by soluble or
glycosylphosphatidylinositol-anchored glu-can-binding proteins, leading to indirect
activation of Toll signaling 51,52. Dectin-1 may therefore be a late-evolving β-glucan receptor that couples PAMP recognition and signal-ing functions in a single molecule, much like
mammalian TLRs, which combine the func-tions of drosophila peptidoglycan recogni-tion proteins (PGRPs) and Toll.Evidence that this is the case and that
dectin-1 is important in antifungal immu-nity came initially from the observation that pathogenic fungi mask their β-glucans as a putative immune escape mechanism 28,29,53. It
has since been confirmed by the finding that
dectin-1-deficient mice are more susceptible
to fungal infection, especially in an immuno-compromised setting (Brown, G.D. et al . and Iwakura, Y. et al ., unpublished observations).
Notably, CARD9-deficient mice also show considerably impaired resistance to can-dida, which is especially important because
CARD9-deficient DCs are not impaired in their ability to phagocytose zymosan 19. Thus, the signaling function of dectin-1 and prob-ably that of other CARD9-coupled receptors seems to be essential for protection from
candida infection. Whether that is due to impairment of innate or
adaptive immunity or both is yet to be established. However, dectin-1 stimulation allows DC maturation and induction of an adaptive immune response (S.L.L. and C.R.S., unpublished observations). Dectin-1 can therefore recognize conserved molecular patterns in fungal pathogens, and signaling by the receptor can mediate host pro-tection and instruction of adaptive immunity, as would be expected of a TLR-like PRR.Is that also true for other myeloid CLRs? Like dectin-1, several can bind pathogens or PAMPs (Table 2). However, DC-SIGN-mediated uptake of human immunodeficiency virus by DCs is known to protect
the virus from degradation and to allow subsequent regurgitation and
transmission to T cells 21. Similarly, uptake of M. tuberculosis by means of the mannose receptor aids in pathogen survival 54. Additionally, Lewis antigen expression allows binding of Helicobacter pylori to DC-SIGN and potentiates IL-10 production induced by the pathogen, resulting in immunosuppression 55. Finally, although the mannose receptor was proposed as a yeast PRR, mannose receptor–deficient mice are not more suceptible to infection with Candida albicans or Pneumocystis carinii 56,57. The interpretation of those studies is not always straightforward (for example, another strain of mannose receptor–deficient mice dies in utero 58). but nevertheless, those observations suggest that some CLR-mediated pathogen binding and uptake may be misleading and may be indicative of CLR exploitation by the pathogen rather than of pattern recognition leading to immune defense. Further analysis of the signaling pathways and of disease susceptibility in mice lacking CLRs may help clarify which CLRs truly function in protection from invading pathogens.Myeloid C-type lectins: beyond pattern recognition
If it transpires that many CLRs expressed by myeloid cells have not evolved to recognize pathogens, what might be their function?
CLR Carbohydrate specificity Pathogen binding Recognition of self and altered self
References MR (CD206)Mannose, fucose, sLe X HIV, P . carinii , M. tuberculo-sis , C. albicans Lysosomal hydrolases, thyroglobulin, L-selec-tin, MUC-1
80
DC-SIGN (h) (CD209)Mannan, high-mannose, ManLAM, fucose, Le X , Le A , Le Y , Le B , 6SLe A HIV, HCV, CMV, filoviruses, dengue, H. pylori , M. tuberculosis , S. mansoni , C. albicans , A. fumigatus ,
Leishmania spp.
ICAM-2, ICAM-3, CEACAM-1–Mac-1 (PMN), CEA
59,60,65,75,81
SIGN-R1 (m) (CD209b)Mannan, dextran S. pneumoniae CPS, LPS, C. albicans , HIV, M. tuber-culosis
ICAM-3 (h)76,82–84
Dectin-2 (CLEC6A)High mannose C. albicans Ligand on CD4+CD25+ T cells
71,85
Langerin (CD207)Mannose, GlcNAc, fucose, s6SLe X HIV, M. leprae Type I procollagen 86–88
MGL (h)GalNAc (Tn)S. mansoni , filoviruses CD45 (T cells), MUC-1, gangliosides
66,68,69,89,90MGL-1 (m)Le X , Gal ?Sialoadhesin, apoptotic bodies
63,91
MGL-2 (m)GalNAc (Tn)??91
Dectin-1 (CLEC7A, β-glucan receptor)β1,3-glucans P . carinii , C. albicans , A. fumigatus
Ligand on T cells 25,92
DCAL-1??Ligand on CD4+CD45RA + T cells 70
Table 2 Ligands for myeloid CLRs
sLe X , sialyl Lewis X; HIV, human immunodeficiency virus; ManLAM, mannosylated lipoarabinomannan; Le X , Lewis X;
Le A , Lewis A; Le Y , Lewis Y; Le B , Lewis B; s6SLe A , sialyl 6-sulfo Lewis A; HCV, hepatitis C virus; CMV, cytomegalovirus; A. fumigatus , Aspergillus fumigatus ; ICAM, intercellular adhesion molecule; Mac-1, myeloid cell–associated marker CD11b
(integrin); CEA, carcinoembryonic antigen; S. pneumoniae CPS, Streptococcus pneumoniae capsular pneumococcal polysac-charide; LPS, lipopolysaccharide; GlcNAc, N -acetylglucosamine; M. leprae , Mycobacterium leprae ; s6SLe X , sialyl 6-sulfo Lewis X; Gal, galactose.
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y
Accumulating evidence suggests that many CLRs bind to host-derived molecules (Table 2). For example, DC-SIGN is involved in intercel-lular communication and mediates interactions between DCs and endothelial cells (via ICAM-2), T cells (via ICAM-3) or neutrophils (via CEACAM-1 and Mac-1)59,60. Similarly, the mannose receptor mediates the uptake of mannosylated proteins 48 and has a principal function in the clearance of lutotropin, lysosomal hydrolases and thy-roglobulin from serum 58,61. The idea that CLRs bind to self ligands extends their immunological relevance beyond pattern recognition. Analogous to the function of NK cell CLRs in the recognition of missing self (NKG2A, NKG2C, CD94 and the Ly49 family) or altered self (NKG2D), it is possible that CLRs expressed by myeloid cells act as sensors of cellular stress or transformation or even of cell death. Indeed, some CLRs, including mouse MGL-1 and Lox-1, can medi-ate the phagocytosis of aged and apoptotic cells 62,63, and Lox-1 has also been associated with the binding of heat-shock protein 70 to DCs and with heat-shock protein 70–dependent antigen cross-pre-sentation 64. It is also notable that myeloid CLRs sometimes recognize altered glycosylation patterns found on tumor cells. For example, DC-SIGN interacts with Lewis X and Lewis Y antigens, which are abundant on carcinoembryonic antigen in colorectal cancer cells 65, and human MGL and SRCL bind with high affinity to the Tn antigen (N -acetylgalactosamine) present on MUC-1, the breast cancer–associ-ated mucin 66,67. Whether those interactions mean that myeloid CLRs could contribute to innate tumor immunosurveillance is highly spec-ulative, but it is conceivable that they could deliver an activating signal to macrophages or DCs in response to detection of altered self, as reported for another CLR, NKG2D, in NK cells 12. Evidence thus far indicates the contrary, that instead in some cases the expression of CLR ligands in tumor cells prevents optimal priming of tumor-spe-cific T cells and constitutes a tumor escape mechanism 68. Whichever is true, the ability of CLRs to bind tumor cells could be exploited as a strategy for vaccination by combining CLR targeting with suitable adjuvants 68.
In addition to an effect of CLR ligation on antigen-presenting cells, the reverse can be envisioned: an effect of myeloid CLRs on other cells. For example, MGL on immature DCs and dexamethasone-treated macrophages inhibits the action of the CD45 phosphatase on effector T cells, resulting in decreased activity of the kinase Lck and T cell activation 69. In contrast, a DCAL-1 fusion protein binds to a subset of CD4+CD45RA + T cells, enhancing CD3-driven prolifera-tion and increasing IL-4 secretion 70. Similarly, the dectin-1 ligand on
T cells acts as a costimulatory molecule for T cell activation 49, whereas
dectin-2 expressed on antigen-presenting cells mediates interactions with CD4+CD25+ regulatory T cells and regulates ultraviolet radia-tion–induced tolerance to contact hypersensitivity 71. Thus, CLRs on antigen-presenting cells can have both positive and negative effects on immune effector cells.
Conclusions
Several CLRs are expressed as transmembrane proteins on myeloid cells and could be involved in the regulation of innate and adaptive immunity. Some bind PAMPs and might act as PRRs, whereas oth-ers may be involved mainly in recognition of self and maintenance of immune homeostasis. Despite the fact that the first myeloid CLR was described over 25 years ago 48, progress in this area was slow until the surge in interest in innate immunity. We envisage that further understanding of myeloid CLR function will come from two concur-rent approaches. First, ligand identification will be a chief step toward determining CLR specificity. That will inevitably be coupled to the analysis of mouse knockout models, allowing further discrimination
of ‘real’ ligands versus those that target CLRs to subvert receptor func-tion. Second, progress will also be made from studying the signaling pathways and sequence motifs involved in CLR function. Indeed, whereas the known families of PRRs (such as TLRs and the RIG-I and Mda5 helicases) are classified at least in part by their signaling domain (such as Toll–IL-1 receptor or helicase domains), CLRs have been defined purely on the basis of a structural motif, the C-type lectin domain. As a single entity they form an unwieldy collection of receptors, almost as diverse as the immunoglobulin ‘superfamily’. We anticipate that analysis of signaling pathways may allow a more rational classification of CLRs and will help in clustering together subgroups of receptors with similar functional properties. Already, the ability to identify CLR motifs that allow coupling to adaptor proteins (such as DAP12) or directly to Syk (for example, via hemITAMs) or to phosphatases (for example, via ITIMs) is a small but important step in that direction. There are probably other motifs that allow coupling to additional pathways (such as phosphatidylinositol-3-OH kinase) and therefore may allow definition of additional CLR subgroups. Eventually, a subfamily of C-type lectins might be defined that act as TLR-independent PRRs, providing insight into innate immunity to infection. At the same time, deeper understanding of the function of the interaction of CLRs with endogenous ligands may also iden-tify previously unknown mechanisms by which innate and adaptive immunity are regulated, opening up potential avenues for immune intervention.
ACKNOWLEDGMENTS
We thank members of the Immunobiology Laboratory (Cancer Research UK, London Research Institute) for discussions.
COMPETING INTERESTS STATEMENT
The authors declare that they have no competing financial interests.
Published online at /natureimmunology/
Reprints and permissions information is available online at /reprintsandpermissions/
1. Janeway, C.A., Jr. Approaching the asymptote? Evolution and revolution in immu-nology. Cold Spring Harb. Symp. Quant. Biol. 54, 1–13 (1989).
2. Kawai, T. & Akira, S. Innate immune recognition of viral infection. Nat. Immunol.
7, 131–137 (2006).
3. Medzhitov, R. & Janeway, C.A., Jr. Innate immunity: the virtues of a nonclonal
system of recognition. Cell 91, 295–298 (1997).
4. Akira, S., Uematsu, S. & Takeuchi, O. Pathogen recognition and innate immunity.
Cell 124, 783–801 (2006).
5. Crocker, P.R. Siglecs in innate immunity. Curr. Opin. Pharmacol. 5, 431–437
(2005).
6. Zelensky, A.N. & Gready, J.E. The C-type lectin-like domain superfamily. FEBS J
272, 6179–6217 (2005).
7. Geijtenbeek, T.B., van Vliet, S.J. & Engering, A., Hart, B.A. & van Kooyk, Y. Self-
and nonself-recognition by C-type lectins on dendritic cells. Annu. Rev. Immunol. 22, 33–54 (2004).
8. Drickamer, K. C-type lectin-like domains. Curr. Opin. Struct. Biol. 9, 585–590
(1999).
9. O’Rourke, D., Baban, D., Demidova, M., Mott, R. & Hodgkin, J. Genomic clusters,
putative pathogen recognition molecules, and antimicrobial genes are induced by infection of C. elegans with M. nematophilum . Genome Res. 16, 1005–1016 (2006).
10. Takahashi, K., Ip, W.E., Michelow, I.C. & Ezekowitz, R.A. The mannose-binding
lectin: a prototypic pattern recognition molecule. Curr. Opin. Immunol. 18, 16–23 (2006).
11. Cash, H.L., Whitham, C.V., Behrendt, C.L. & Hooper, L.V. Symbiotic bacteria
direct expression of an intestinal bactericidal lectin. Science 313, 1126–1130 (2006).
12. Lanier, L.L. NK cell recognition. Annu. Rev. Immunol. 23, 225–274 (2005).
13. Blander, J.M. & Medzhitov, R. Regulation of phagosome maturation by signals
from Toll-like receptors. Science 304, 1014–1018 (2004).
14. Bonifacino, J.S. & Dell’Angelica, E.C. Molecular bases for the recognition of
tyrosine-based sorting signals. J. Cell Biol. 145, 923–926 (1999).
15. Mahnke, K. et al. The dendritic cell receptor for endocytosis, DEC-205, can recycle
and enhance antigen presentation via major histocompatibility complex class II-positive lysosomal compartments. J. Cell Biol. 151, 673–684 (2000).
R E V I E W
©2006 N a t u r e P u b l i s h i n g G r o u p h t t p ://w w w .n a t u r e .c o m /n a t u r e i m m u n o l o g y。