toll - like receptors ( tlrs ) are pattern recognition receptors with the ability to detect microbial products . they can provide the initial danger signal required to alert the body to bacterial or viral infection , playing a pivotal role in the activation of both innate and adaptive immune responses . tlrs recognise pathogen - associated molecular patterns ( pamps ) , including lipopolysaccharide ( lps ) , a membrane component of gram negative bacteria ; lipoproteins , functional proteins anchored to the cell membrane ; flagellin , a major component of flagellum ; heat shock proteins which are highly expressed during cellular stress and microbial nucleic acids [ 1 , 2 ] . most tlrs signal via myd88 resulting in nf-b or mapk activation ( figure 1 ) , ultimately leading to the transcription of genes associated with antimicrobial defence such as inflammatory cytokines , costimulation molecules , mhc , and nitric oxide ( no ) [ 3 , 4 ] . tlrs are widely expressed on immune cells and possess distinctive functions dependent on cell type and signalling pathway [ 5 , 15 ] . we will focus on their effects on dendritic cells and macrophages , which can act as professional antigen presenting cells ( apcs ) . the expression profile of tlrs on apcs varies between subsets ( table 1 ) , which include plasmacytoid dcs ( pdcs ) , myeloid dcs ( mdcs ) , monocyte - derived dcs ( modcs ) , and macrophages . tlr signalling in these immune cell subsets triggers an activation programme that includes cytokine secretion ; pdcs secrete type i interferons ( ifn- ) which have a fundamental antiviral function through the recruitment of immune cells and their role in t cell differentiation . in contrast , mdcs and modcs primarily secrete il-12 , a potent proinflammatory cytokine involved in t cell differentiation and nk cell activation . macrophages , which provide an initial antimicrobial response , also secrete il-12 , no , and tnf- which play an important role in apoptotic cell death and bacterial lysis . in addition to the production of soluble cytokines and chemokines , upregulation of various cell surface markers such as the costimulation molecules and mhc required for antigen presentation is also observed following tlr activity [ 19 , 20 ] .   of these , both cd80 and cd86 bind to cd28 on t cells to provide an activating signal , whereas b7-h1 binds to pd1 to generate an inhibitory signal . the type of costimulation molecules upregulated and cytokine profile secreted by an apc is thought to be determined by the activation signal given by individual tlrs , and can determine the nature of the downstream immune response [ 20 , 22 , 23 ] .    upon migration to the lymph nodes , activated dcs present microbial antigens to prime a specific t cell response .   mhc class ii - restricted antigens are recognised by t helper ( th ) cells , which then secrete proinflammatory cytokines to recruit effector cells and aid in b cell maturation . for example , the cytokines il-12 and ifn - i dominate th1 responses , resulting in the recruitment of proinflammatory effector cells and the clearance of infection . th17 cells are also proinflammatory and through the secretion of il-17 and il-22 stimulate , for example , the secretion of antimicrobial proteins from other effector cells .   for th2 responses the cytokines il-4 and il-25 dominate , resulting in inhibition of proinflammatory cytokine secretion and further proliferation . th1 responses are required for pathogen clearance . however , during excessive immune activation , th1 responses may damage host tissues , raising the possibility of pathology or chronic inflammatory diseases .   in order to prevent this , populations of th2 and regulatory t cells ( tregs ) are required , the effects of which regulate the th1 response . t cell polarisation to either th1 , th2 , or th17 profile is dependent on the type of costimulation molecules and cytokines expressed by the apc [ 28 , 29 ] ,   which is in turn determined by tlr signalling profile .   in this respect , signalling through individual or different combinations of tlr may control the nature of the adaptive immune response towards any given antigen . tlr4 , tlr7 , and tlr8 are typically thought to trigger definitive th1 responses , whereas tlr2 has been implicated in inducing th2 responses . however , recent studies have demonstrated that simultaneous coligation of tlr2 with other tlrs alters the signalling profile and ultimately the dc signature required for t cell polarisation , and may dampen down tlr4 mediated responses . coligation with accessory molecules such as dectin-1 is also thought to play a part in polarising immune responses , and eisenbarth et al . have demonstrated that levels of antigen stimulation may impact on dc signatures , with high levels of lps triggering a th1 response , and low levels a th2 response [ 31 , 33 , 34 ] .    as an invading pathogen is likely to possess multiple pamps and trigger several tlrs , any infection furthermore , as tlrs are known to complex with other receptors and recruit adapter molecules involved in signalling cascades , it is possible that some of these interactions function as regulatory mechanisms . scavenger receptor a ( sra or cd205 ) , and single ig il-1r - related molecule ( sigirr)/toll il-1r8 have been described as candidate regulatory proteins . members of the leukocyte ig - like receptor family ( lilr , also known as ilts , lir , cd85 , and mir [ 4143 ] ) have also become a focus for investigation after being shown to exert a powerful inhibitory effect on tlr functions   ( figure 2 ) . lilrs are a family of innate immune receptors that are predominantly expressed on antigen - presenting cells and b cells . the eleven members of the human lilrs family are split into three distinct groups : activating , inhibitory and soluble . the lilr classed as inhibitory ( lilrb1 - 5 ) have a cytoplasmic tail containing 24 immunoreceptor tyrosine - based inhibitory domains ( itims ) . lilrs classed as activating ( lilra1 - 2 , 46 ) lack any signalling motif , but instead possess a charged arginine residue which enables association with the adaptor protein fcri .   to date , this is the only identified adaptor molecule shown to associate with lilr , although it is possible that other adapter molecules are capable of this association .   signalling is then directed through the fcri-associated immnuoreceptors tyrosine - based activatory domains ( itams ) .   despite this classification , the so - called activating group contains receptors with the ability to exert inhibitory effects , a phenomenon that has been observed for several itam - bearing immune receptors and is thought to be related to strength of signalling . lilrs are conserved throughout evolution and to date homologues have been identified in rodents , chickens , and cattle .   in rodents , lilr equivalents are known as paired immunoglobulin - like receptors , a family that contains multiple activating receptors ( pir - a ) but only one inhibitory receptor , pir - b [ 47 , 5052 ] . pirs were classified as lilr homologues due to their similarities in genetic sequence and location , expression profile , structure , and function [ 5355 ] . they have since proved to be an effective tool to examine the role of these receptors .   a further murine homologue , lilrb4 was previously known as   gp49b .   similar to tlrs , lilr expression varies between apc subsets ( table 1 ) . mdc and modcs express lilra2 , lilrb1 , lilrb3 , and lilrb4 , whereas pdcs express lilra4 , lilrb1 , and lilrb4 [ 6 , 57 ] . lilrb1 , lilrb4 and lilra4 expression is decreased upon dc maturation [ 6 , 58 ] . upregulation and/or cross - linking of lilrb2 inhibits the upregulation of co - stimulatory molecules on apcs , resulting in t cell anergy . in contrast , signalling through lilra2 inhibits the upregulation of cd1b , hla - dr , cd40 , cd80 , cd86 and , cd206 , and prevents effective t cell activation and proliferation .    unlike lilrb2 , which inhibits apc effector functions by downregulating costimulation molecules , lilrb4 has recently been shown to inhibit apc response to bacterial infection by upregulating il-10 production and subsequently downregulating il-8 secretion . however , lu et al . , demonstrated that lilrb4 was able to inhibit tnf- release via inhibition of fcri signalling , in line with the ability of murine lilrb4 to inhibit inflammatory cytokine production . lilrb4 has also been shown to be highly expressed in patients with malignancy , and this receptor is thought to play a critical role in the induction of tolerance . however , it is not known whether lilrb4 expression on malignant cells is induced by regulatory t cells , or if the lilrb4 expression binds to a t cell ligand rendering them anergic . members of the lilr and pir families have been shown to be involved in bacterial engagement . both pir - a1 and pir - b , and their corresponding human homologues lilrb1 and lilrb3 have been shown to bind bacteria including e. coli , h. pylori , and s.   aureus . the bacterial ligand(s ) involved in this interaction are yet to be determined and individual receptors vary in their bacterial specificity .   however , the most thoroughly characterised ligand for lilr is mhc class i ( mhc - i ) .   unlike other mhc - i - specific receptors , lilrs show a broad specificity for classical and nonclassical forms of mhc - i ; lilrb1 and lilrb2 bind all classical mhc - i as well as some nonclassical alleles .   lilrb2 has been shown to bind cd1d , which is an mhc - i - like molecule able to present nonprotein antigens to t cells . cd1d is usually recognised by the tcr receptor of nkt cells , which results in the activation of proinflammatory effector cells and target cell lysis . lilr modulation of cd1d activity may therefore be of particular importance in bacterial infections , which can result in an overaccumulation of lipids in the infected cell [ 66 , 67 ] . inhibitory lilrs have been shown to exhibit their functions both independently and in association with activating receptors such as tlrs . inhibitory lilr carry their own signalling motifs , varying from 2 to 4 itims in their cytoplasmic tail . variation in the number of itim domains has been proposed to result in signal amplification or the recruitment of alternative signalling molecules . upon activation phosphorylated tyrosines within an itim become docking sites for either the src homology 2 domain - containing phosphatase 1 ( shp-1 ) or shp-2 , or the sh2 domain - containing 5 inositol phosphatise ( ship ) . these phosphatases then dephosphorylate key molecules further downstream in the cascade or those involved in the itam signalling of activating receptors , with consequent inhibitory effects . although yet to be fully defined , these signalling patterns are likely to be found mimicked in the modulation of tlr activation [ 68 , 69 ] . as mentioned previously , activating lilr has a positively charged arginine residue within the cytoplasmic domain , which enables association with adapter proteins , such as fcri.   when activated , tyrosine molecules in the itam domain of fcri are phosphorylated by protein tyrosine kinases of the src family kinases , creating binding sites for further signalling molecules , such as zap70 or syc . the recruited signalling molecules may differ depending on cell type , and therefore lilrs may be involved in modulating a wide range of intracellular signalling pathways . lilr - mediated control of tlr activity has been documented for several different bacterial infections . in the case of s.   aureus , lilr and pir - b receptors can bind the pathogen in conjunction with tlr2 and trigger the release of inhibitory cytokines such as il-10 . inhibition of tlr2 signalling by pir - b was confirmed in a study of pir - b mice , where an excessive th2 response was observed , coupled with impaired dc maturation . this inhibition of dc maturation was thought to arise from the absence of pir - b regulation of a common signalling pathway used by il-3 , il-5 , and gm - csf . evidence of a role for lilr ( and their murine homologues ) providing a counterbalance to tlr activity is most strikingly illustrated by the high mortality rate of salmonella infection for mice lacking the inhibitory receptor pir - b [ 72 , 73 ] . interestingly , instead of the exacerbated immune responses that might have been expected in the absence of an inhibitory receptor , pir - b - deficient mice were actually more susceptible to salmonella infection , caused by a decrease in phagosomal oxidant production , necessary for bacterial lysis within lysosomes . mycobacterium leprae infection can result in tuberculoid ( t - lep ) or lepromatous leprosy ( l - lep ) . patients with t - lep typically display a localised form of disease , with effective bacterial clearance .   in l - lep , patients suffer from disseminated disease , with large numbers of bacilli . although the factors that influence disease course are currently unknown , polymorphisms in tlr2/1 are thought to play a role .   in a study by bleharski et al . , gene expression analysis identified an up to 5.4-fold overexpression of lilra2 in skin lesions of l - lep , compared to t - lep . infected macrophages stimulated with lilra2 ligands showed a 40% reduction in antimicrobial responses , indicating that lilra2 also inhibits antimicrobial functions in macrophages . furthermore , ligation of lilra2 considerably reduced il-12 production by tlrs , skewing cytokine activity towards a th2-biased response . this is important as the immune response in t - lep lesions ( where infection is generally contained then cleared ) is th1-biased , whereas l - lep with its higher bacterial loads and disseminated infection is th2-biased . therefore , it is possible that the overexpression of lilra2 in l - lep results in an inadequate th2-biased immune response and a more severe form of disease . lilrb5 , lilrb3 , and lilra3 are also overexpressed in l - lep patient lesions , although their relevance in infection has yet to be determined . signalling through different tlrs has been shown to result in different lilr expression profiles [ 6 , 61 , 74 ] .   in human cells the inhibitory receptors lilrb2 and lilrb4 were upregulated following salmonella infection , an effect which appears to be mediated largely by lps recognition , as activation of lilrb4 also occurred by both heat - inactivated salmonella and salmonella lps .   in this respect , lilrb4 may play an important role in tlr4 regulation . similar relationships exist for other lilr and tlr : lilra4 has been shown to regulate tlr7/9 activity in pdc and lilra2 has been shown to inhibit tlr4-mediated activity . in viral infections , the pdc subset plays an important role in mediating antiviral immunity upon activation by pathogenic ligands . tlr7/9 , together with tlr3 and tlr8 are localised in endosomal / lysosomal compartments , where their activation leads to the production of tnf- and ifn - i , a group of potent antiviral cytokines . the newly - characterised receptor lilra4 is expressed only on pdcs , where it appears to play an important role in the control of their activity in response to viral tlr stimulation . following recognition of its ligand , tetherin ( also known as bst2 ) , lilra4 downregulates tlr7 and tlr9-mediated production of ifn- and tnf- ,   and decreases calcium mobilisation .   however , lilra4 activity does not affect the maturation of the cell , as upregulation of cd80/cd86 is still observed . lilra4 is also able to inhibit tlr7 and tlr9 signalling after prior antigen stimulation , but is selectively downregulated upon pdc activation [ 76 , 77 ] .   similarly in murine models , pir - b has been shown to inhibit tlr9-mediated signalling via brutons tyrosine kinase ( btk ) phosphorylation , which subsequently inhibits nf-b activation . given the potent effects of lilr on tlr activity , there is the potential in future to use these receptors as a tool for therapeutic modulation of tlr signalling . for example , inhibitory lilr could be triggered by their highest affinity self - ligand , hla - g . this nonclassical mhc - i allele has a restricted distribution of expression , limited to placental trophoblast cells and thymus epithelial cells , but is overexpressed in certain pathologies including nonrejected allografts , hiv infection , and tumours [ 79 , 80 ] . hla - g also has a natural tendency to form disulphide - bonded dimers which in turn enhance binding to lilrb1 and lilrb2 , resulting in enhanced immunosuppressive effects . thus , there is therapeutic potential for recombinant hla - g to be used to downregulate tlr effects through enhancing lilr activity .    enhancing lilr expression expression levels of lilr can also be enhanced by certain agents ; vitamin d3 , dexamethasone , and niflumic acid have been shown to up - regulate the expression of lilrb2 and lilrb4 on dcs , which is seen with an accompanying increase in il-10 secretion and treg differentiation . although the exact mechanisms involved in tolerance induction in these studies are yet to be fully elucidated , high expression of lilrb4 is thought to be strongly associated with inhibition of nf-b activation [ 82 , 83 ] . recently a study examining the effects of 1,25(oh)2d3 on dcs demonstrated that this agent is able to up - regulate lilrb4 on modcs and mdcs but not pdcs . this may be due to the fact that the normal expression levels of lilrb4 are markedly higher on pdcs than mdcs . further to this , the ido activity in tryptophan ( trp ) deficient cells was used in a study recently to define mechanisms of dc tolerance and induction of tregs . brenk et al . found that upon dc tolerisation , high levels of lilrb3 and lilrb4 were upregulated .   however , replacement of trp was unable to reverse the tolerogenic conditions , and dcs continued to stimulate t cells to differentiate into a regulatory phenotype . furthermore , only by using anti - lilrb4 antibodies were they able to restore any function to the dcs and subsequently the t cells . the authors predict that dc regulation induced in this manner may affect the epigentics of foxp3 gene transcription and provide antigen - specific treg cells , therefore providing a mechanism open to therapeutic manipulation . given the powerful inhibitory nature of lilr , it may be possible to modulate the expression of these receptors prior to or in conjunction with chemotherapies in order to enhance treatment efficacy .   blocking the inhibitory functions of lilr has been demonstrated recently in a study by morel and belln , in which amoxicillin was shown to have the ability to interfere with lilrb1 recognition of mhc class i on nk cells , a finding which could potentially have a large impact on tumour immunology and therapeutics .   with further research , the bacterial interaction with lilr will most likely prove fundamental in defining regulatory pathways involved in tlr pathogen responses .   as more ligands are discovered for these inhibitory receptors , the potential for development of novel therapies targeting specific lilr allows the possibility of shaping immune responses in disease settings .
toll - like receptors ( tlrs ) are a potent trigger for inflammatory immune responses . without tight regulation their activation could lead to pathology , so it is imperative to extend our understanding of the regulatory mechanisms that govern tlr expression and function .   one family of immunoregulatory proteins which can provide a balancing effect on tlr activity are the leukocyte ig - like receptors ( lilrs ) , which act as innate immune receptors for self - proteins .   here 
 we describe the lilr family , their inhibitory effect on tlr activity in cells of the monocytic lineage , their signalling pathway , and their antimicrobial effects during bacterial infection . 
 agents have already been identified which enhances or inhibits lilr activity raising the future possibility that modulation of lilr function could be used as a means to modulate tlr activity .