the immune system consists of two closely related systems known as the innate and adaptive immune systems .
the adaptive immune system responds to specific  nonself  antigens and generates immunological memory .
in contrast , the innate immune system provides an immediate first line of defense against a diverse repertoire of invading microbial pathogens .
the key components of innate immunity , cognate pattern recognition receptors ( prrs ) , are considered to act as sentinels against both invading organisms bearing pathogen - associated molecular patterns ( pamps ) and damage - associated molecular pattern molecules ( damps ) .
toll - like receptors ( tlrs ) are good examples of these receptors . because of their wide - ranging impact upon both innate and adaptive immunity in several disease settings , tlrs and
this review summarizes the main players in innate immune signaling and highlights possible drug targets in various disease settings .
innate immune responses are triggered mainly by a spectrum of  danger  signals referred to as pamps and damps .
pamps are exogenous molecules derived from both pathogenic and nonpathogenic microbes . in contrast , the vast majority of damps are endogenous molecules released from dying host cells molecules upon cellular stress or tissue damage [ 25 ] .
the tlrs are a family of evolutionarily conserved prrs that play a key role in sensing the microbial world .
different tlr members are reported to recognize and respond to different pamps and some endogenous damps , thus initiating innate immune responses and priming antigen - specific adaptive immunity , both in infectious and noninfectious disease scenarios .   
tlrs are type i transmembrane glycoproteins which are structurally characterized by extracellular leucine - rich repeats ( lprs ) and toll / il-1 receptor ( tir ) signaling domains .
the first tlr to be characterized was tlr4 and the family has now been expanded to include 10 members in humans and 12 members in mice [ 2 , 3 ] . apart from the highly conserved pathogenic components of bacteria , viruses , fungi , and parasites ,
various tir domain - containing adaptors such as myd88 , tirap , trif , and tram become engaged or activated upon ligation of tlrs , as diagrammed in figure 1 .
tlr1 , tlr2 , tlr4 , and tlr6 recruit tirap , which serves as an adaptor between the tir domain of tlrs and myd88 , while tlr5 , tlr7 , tlr9 , and tlr11 can recruit myd88 directly .
however , the recruitment of trif by tlr4 needs the participation of tram ( figure 1 ) .
the recruitment of these adaptors triggers a cascade of signaling molecules and ultimately activates the transcription factors , nf-b and irfs ( figure 1 ) .
these transcription factors induce the expression of various inflammatory cytokines , type i interferons , and chemokines .
nf-b is a central regulator of immune responses involved in cell proliferation and survival and induces the expression of many cytokine and chemokine genes including il-2 , il-6 , il-12 , mcp-1 , and tnf- .  
except for tlr3 , all tlr ligation events recruit the adaptor myd88 , followed by the irak family of protein kinases , leading to the activation of traf6 .
nf-b or ap-1 is then activated by the ikk complex or map kinases , respectively ( figure 1 ) . on the other hand
, tlr3 initiates a trif - dependent pathway that dictates the expression of inflammatory cytokines and type i ifns via two independent pathways .
the n - terminal domain of trif interacts with traf6 while the c - terminal domain of trif interacts with rip1 and activates tak1 , both of which can activate nf-b , resulting in the expression of inflammatory cytokines .
irf3 is phosphorylated and activated by the ikk - related kinase , tbk1 , facilitated by the recruitment of traf3 .
tlr7 and tlr9 engagement induces the secretion of inflammatory cytokines through the activation of nf-b via myd88
. however , tlr7 and tlr9 can also induce the expression of type i ifns through the activation of irf7 , which is phosphorylated by irak1 , irak4 , or ikk , and then translocated to the nucleus where it induces the transcription of ifn- ( figure 1 ) .
as is evident from figure 1 , the irak family of molecules plays a pivotal role in mediating almost all tlr - mediated functions .
the irak family has four members : irak1 , irak2 , irakm , and irak4 .
all irak family members contain an amino - terminal death domain and a serine - threonine kinase domain .
the irak1 gene is composed of 14 exons and is located on the x chromosome .
irak1 is the first member of this kinase family that was identified as a key component of the il-1r signaling pathway and plays a specific and essential role in ifn- induction downstream of tlr7 and tlr9 engagement .
irak1 deficiency abrogates the production of ifn- , il-6 , il-12 , and tnf- when stimulated with tlr7 or tlr9 ligands [ 12 , 13 ] .
tlrs constitute a primary defense mechanism in both infections and some noninfectious disease settings in mammals .
activation of tlrs and the myd88 signaling pathway plays a protective role during infection with several pathogens , including protozoan parasites   and pyogenic bacteria .
patients with autosomal recessive myd88 deficiency have been reported to suffer from life - threatening , often recurrent pyogenic bacterial infections .
interestingly , however , their clinical status improved in later life , alluding to the compensatory effect of adaptive immunity .
consistent with their vital role in fighting infections , downregulation of tlr - related molecules or signaling has been associated with sepsis and autoimmune disease , as discussed below . on the other hand , upregulation of these molecules
has been linked to cancer , allergy , other autoimmune diseases and immune abnormalities in hiv [ 16 , 17 ] , as detailed below .  
sepsis is a serious medical condition that is characterized by a systemic state and the presence of a known or suspected , which can lead to rapid tissue damage .
the recognition of lps by tlr4 during an acute gram - negative bacterial infection plays a key role in the pathogenesis of sepsis .
weighardt and colleagues found that myd88-deficient mice were protected from developing sepsis in a polymicrobial septic peritonitis disease model . in the irak4-deficient model , it was found that mice were severely impaired in their responses to viral and bacterial challenge .
recently , ferwerda and coworkers reported that polymorphisms in human mal / tirap allele showed association to the pathogenesis of sepsis . on the other hand ,
others have provided proof that mkp-1 , a key negative regulator of tlr - induced inflammation , plays a critical role in the inhibition of innate immunity during gram - negative bacteria sepsis [ 21 , 22 ] .
myd88 also plays a pivotal role in infectious diseases , as demonstrated by naiki et al . in another model of bacterial pneumonia .
collectively , these studies revealed that the tlr pathway plays a pivotal role in the pathogenesis and control of sepsis .
the importance of tlr expression and function in cancer cells and its association with tumorigenesis and tumor progression have recently been examined .
tlr4 expression is linked to several cancers such as gastric cancer and human epithelial ovarian cancer .
huang et al . reported that tlr4 was expressed in murine tumor cells and that the activation of tlr4 in these cells by lps induced the expression of various soluble factors including interleukin-6 , inducible nitric oxide synthase , interleukin-12 , b7-h1 , and b7-h2 and rendered tumor cells resistant to ctl attack .
the factors in lps - stimulated tumor cell supernatants also had the capacity to inhibit t cell proliferation and natural killer cell activity .
have reported that human epithelial ovarian cancer cells ( eoc ) ubiquitously express tlr4 and that the proliferation of eoc cells or enhanced cytokine / chemokine production by eoc cells was dependent upon expression of myd88 .
myd88-dependent signaling controls the expression of several key modifier genes in tumorigenesis and has a critical role in both spontaneous and carcinogen - induced tumor development .
others have reported that gene expression differences in the tlr gene cluster ( tlr10-tlr1-tlr6 ) were associated with a statistically significant reduced risk of prostate cancer .  
functionally active tlr9 is also known to be expressed on several human tumors , including lung cancer , burkitt lymphoma , cervical neoplasia , breast cancer , and prostate cancer [ 2832 ] .
although the role of tlr9 in the development of these cancers is not fully understood , tlr9 appears to be playing an important modulatory role in several of these tumors [ 3235 ] .   based on these findings , several tlr9 agonists , including cpg oligonucleotides ,
these agents have the capacity to upregulate the innate immune response , as well as a humoral immune response against the tumor , as reviewed elsewhere .thus , cpg - odn can activate plasmacytoid dcs ( pdcs ) to secrete type i interferon ( ifn ) and also promote the expression of costimulatory molecules such as cd80 and cd86 , on the dcs as well as on tlr9-expressing tumors .
this can then lead to the secretion of various cytokines / chemokines , and also activate natural killer ( nk ) cells , th1 cells , and cytotoxic t lymphocytes ( ctls ) . engaging these diverse mechanisms ,
tlr9 agonists have been found to be therapeutically effective in various cancer trials [ 3640 ] .  
it was reported that inhibition of tlr4 suppressed the severity of experimental arthritis and resulted in lower il-1 expression in arthritic joints .
roelofs found that a tlr4 variant ( asp299gly ) could reduce its potency to mediate signaling in rheumatoid arthritis patients .
a tlr2 r753q polymorphism has been reported to be more prevalent among reactive arthritis patients .
likewise , polymorphisms in tlr2 and tlr4 may also be important in the pathogenesis of sle in both patients   and mouse models .
moreover , tlr7 has been suggested to be functionally involved in autoantibody production , and hence closely linked to the pathogenesis of sle .
indeed , the translocation of the x - linked tlr7 gene to the y - chromosome has been documented to facilitate the development of fatal lupus in mice with numerous immunological aberrations , hence constituting disease genes for murine lupus [ 47 , 48 ] .
harada and coworkers reported that myd88-deficiency protected mrl / lpr mice from the development of autoimmune nephritis , marked by lower levels of serum anti - double - stranded dna ( anti - dsdna ) antibodies and reduced cytokines , including interferon- , interleukin-12 , il-6 , and ifn- .
similarly it was reported that myd88-deficient mice were completely resistant to experimental autoimmune uveitis ( eau ) and had reduced th1 , but not th2 , responses upon immunization with retinal ag . in all of the above studies of sepsis , cancer , and autoimmunity ,
it has been documented that myd88-deficient mice show a loss of il-1- and il-18-mediated functions indicating that the phenotypes observed in myd88-deficient mice might arise in part because of defects in signaling downstream of the il-1 and il-18 receptors .  
autoimmunity also appears to be contingent upon the expression of another tlr signaling molecule , irak1 .
it has been reported that irak1-deficient mice are resistant to experimental autoimmune encephalomyelitis ( eae ) , exhibiting little or no inflammation in the central nervous system .
irak1-deficient mice exhibited impaired th1 cell development with minimal ifn- secretion during disease induction following immunization with myelin antigen .
the authors suggested that the mechanism underlying this observation might relate to an impaired adjuvant effect on antigen presenting cells as a result of suboptimal tlr activation .  
our recent study has revealed that the irak1 gene was highly associated with both adult- and childhood - onset sle . using the irak1-deficient mice
, we found that in mice bearing the lupus susceptibility loci , sle1 or sle3 , irak1 deficiency abrogated all lupus - associated phenotypes , including igm and igg autoantibodies , lymphocytic activation , and renal disease .
in addition , the absence of irak1 reversed the dendritic cell  hyperactivity  associated with lupus .   
heme oxygenase-1 ( ho-1 ) , a key cytoprotective , antioxidant , and anti - inflammatory molecule , was reported to be involved in the activation of irf3 after tlr3 or tlr4 stimulation , or viral infection .
ho-1 is necessary for the expression of primary irf3 target genes encoding rantes , ip-10 , and mcp-1 . in the experimental autoimmune encephalomyelitis model , mice with myeloid - specific ho-1 deficiency developed severe disease correlating with hyperactive antigen - presenting cells , enhanced infiltration of th17 cells , and nonregressing myelin - specific t cell reactivity .
the overproduction of ifn- and ifn--dependent genes , such as irf5 , are additional factors that play a key role in the pathogenesis systemic lupus erythematosus .
since the bulk of the data suggests that the tlr pathway plays a key role in multiple pathogenic processes , the targeting of either the tlrs themselves or the signals they generate is of great interest , as reviewed by o'neill .
as myd88 is clearly involved in infectious disease , cancer , and autoimmune diseases , it is obviously an attractive target for intervention in these diseases .
loiarro et al . have reported a myd88 inhibitor st2825 , a heptapeptide analog specifically designed to inhibit myd88 dimerization , which could control tlr mediated inflammatory responses .
st2825 is reported to inhibit myd88 dimerization in co - immunoprecipitation experiments , and is specific for homodimerization of the tir domains but does not affect homodimerization of the death domains .
they tested st2825 in experimental animal models of autoimmune , and inflammatory diseases , such as lupus , inflammatory bowel disease , and multiple sclerosis , and found that st2825 could interfere with the recruitment of irak1 and irak4 by myd88 , resulting in the inhibition of il-1-mediated activation of nf-b and il-6 .
they also observed that st2825 suppressed b cell proliferation and differentiation into plasma cells in response to cpg - induced activation of tlr9 .
compound 4a  , which could interfere with the interaction between myd88 and il-1r at the tir domains .
compound 4a inhibited il-1-induced phosphorylation of the mitogen - activated protein kinase p38 in el4 thymoma cells and in freshly isolated murine lymphocytes , and significantly attenuated il-1-induced fever response in vivo .
these data suggest that myd88 might be a potential target for pharmaceutical usage in several disease settings where increased tlr signaling is driving disease pathogenesis .
modulation of another signaling molecule in the tlr pathway , irak4 , has also been suggested to be an attractive therapeutic approach for the treatment of cancer , autoimmune and inflammatory diseases . a series of small - molecule compounds that inhibit irak4 has been developed by wang et al . .
however , the effectiveness of these irak4 inhibitors in vitro and in vivo remains to be documented .
another group has reported that a dual inhibitor of irak1 and irak4 ( ro0884 ) could efficiently inhibit both il-1 and tnf- induced p38 map kinase , c - jun n - terminal kinase activation , and il-6 production in human cells . through silencing irak1 or irak4 gene by sirna , they also found that irak4 was essential for response to il-1 but not to tnf- , whereas irak1 was essential for cellular responses to tnf- but not il-1. these data suggested that inhibition of both irak1 and irak4 kinases would be necessary to block proinflammatory cytokine production .
collectively , these findings raise hope that irak1/irak4 inhibitors may be of therapeutic benefit in autoimmune and inflammatory diseases .  
administration of synthetic oligodeoxynucleotides with immunoregulatory sequences ( irs ) that specifically inhibit both tlr7 and tlr9 signaling ( irs 954 ) has been shown to decrease the production of ifn- by human pdcs after stimulation with dna or rna viruses .
it has been reported that ( nzbnzw ) f1 mice showed decreased serum levels of antinuclear autoantibodies , proteinuria , and glomerulonephritis , with increased survival upon irs 954 treatment .
another oligodeoxynucleotides inhibitor irs 661 , which is specific to tlr7 , had the capacity to significantly reduce disease in mrl / lpr mice , particularly the weights of spleen and lymph nodes and serum levels of tnf- as compared with saline - treated control .
injection of irs 661 but not irs 954 can significantly decreased anti - dsdna and anti - sm antibodies in lupus - prone mice .
most interestingly , both irs 661 and irs 954 could be detected in the kidneys and were found to be taken up by glomerular cells , tubular epithelial cells , and intrarenal macrophages .
the mrna levels of inflammatory chemokines ccl2 and ccl5 and their respective chemokine receptors ccr2 and ccr5 were also decreased in the kidneys .
consistent with these findings , the activity and chronicity scores for antibody - mediated nephritis in irs 661- or irs 954-treated mice were also reduced significantly . besides lupus , these inhibitors have also shown efficacy in other autoimmune diseases including arthritis and multiple sclerosis [ 6365 ] .
based on a recent report suggesting that sle in patients is also contingent upon tlr signaling , antagonists of this pathway are expected to confer therapeutic benefit in patients as well .
tlr molecules and their downstream signaling pathways play a critical role in activating innate immune cells and selected cell types in the adaptive arm of the immune system . given that this pathway is aberrantly expressed or activated in several diseases , this axis constitutes an attractive target for therapeutic intervention .
there is mounting evidence documenting that the crippling of this pathway at the level of tlr , myd88 , or irak1/4 may confer therapeutic efficacy in autoimmunity and auto - inflammatory diseases . on the other hand , the total crippling of these pathways may compromise immune defense against invading infections and immune surveillance against cancers .
in fact , agonists of these pathways appear to be useful in some of these disease settings .
hence there is a need to carefully select the therapeutic target in the tlr signaling cascade , and closely regulate the degree of pathway activity so as to attain the desired therapeutic end - point . like many other molecules in biology , the tlr signaling axis emerges as yet another
 toll - like receptors ( tlrs ) are transmembrane proteins acting mainly as sensors of microbial components . triggering tlrs results in increased expression of multiple inflammatory genes , which then play a protective role against infection . 
 however , aberrant activation of tlr signaling has a significant impact on the onset of cancer , allergy , sepsis and autoimmunity . 
 various adaptor proteins , including myd88 , iraks , tirap , trif , and tram , are involved in specific tlr signaling pathways . 
 this article reviews the role of these molecules in tlr signaling , and discusses the impact of this pathway on various disease scenarios . 
 given their important role in infectious and non - infectious disease settings , tlrs and their signaling pathways emerge as attractive targets for therapeutics . 
1. IntroductionThe TLR Pathway
2. TLR Signaling and Disease
3. Potential Therapeutic Targets in the TLR Signaling Pathway
4. Conclusion