Table1: Major Toll like Receptors characteristics, functions and targets

TLR Physiological Function Target (ligand) Molecules Target (micro) Organism Co-Receptors
TLR1 Sensing of bacterial lipopeptides Tripalmytated lipopetide (Triacetylated lipoproteins) Mycobacterium tuberculosis [89]
Borrelia Burgdorferi [90]
TLR2 [90]
TLR2 Sensing the antigen, Enhancement of inflammatory response, depletion of IRAK1 and subsequent inhibition of type 1 IFNs secretion in viral infection Tripalmytated lipopeptides, lipoteichoic acid, peptiglycan bacterial, mycoplasma lipoprtoeins, lipoarabinomannan of mycobacterium, soluble modulin from staph epidermidis, zymosan of yeast wall glycosylphosphatidyl inositol, viral lipoprotein and glycoproteins Alarmins ( product of cell destruction, apoptosis, necrosis and tumor cells) Gram positive and negative bacteria, mycobacterium Mycoplasma [91], hepatitis B and C virus MYD88, TIRAP
TLR3 dS RNA, viral antigens and Apoptotic derived RNA Endosomal, senses endocytosed exogenous viral dSRNA, Poly I:C (Polyribinosic:polyribocytidylic, extracellular in certain cells [92] Viruses e.g. positive strand RNA virus polio, coxakie encephalomyocarditis virus, HIV gag mRNA, Epithelial TLR3 in Sjogren syndrome [93]. Cytoplasmic domain dimerization, downstream activation through TICAM-1 [92]
TLR4 Bacterial lipopolysaccharide sensing , Free fatty acids, LPS lipid A consist of glucosamine, acyl chain (FA residue) carbon chain length of 10-16 [94,95]
FFA chain of 12 Laurate or 16 palmitate [95]
Gram negative bacteria, endogenous fatty acid from apoptosis of adipose tissue Contribute to obesity associated inflammation and metabolic syndrome [7] CD14, MyD88 [94], IRAK(IL-1 associated Kinase), TRAF 6
TLR5 Flagelin [96] Mucosal protection of intestine, airway urogenital and eye, distinct response to different bacteria including antiapoptotic activation Flagelin monomere All intestinal bacteria containing flagelin, 15000 different species with a 2X 10 6 distinct genes. Responding according to the bacterial toxicities MyD88 Flagelin Ipaf/ Naip 5 in macrophages inflammome
TLR6 A sensing and responding to a certain categories of bacteria, protozoan, and viruses after heterodimerization with TLR2. Soluble TLR2 has modulating effect on TLR2 anchored to cells MALP2 (mycoplasmal macrophage-activating lipoprotein), molecules with Diacylglycerol, triacylglycerol moities Mycoplasma Gram positive bacteria [97] Cryptoccocus neoforman , Candida albicans Saccharomyces Cerevisiae Entamaeba histolytica, cytomegalovirus, Hepatitis C, Measles, herpes simplex, respiratory cynstial virus [9] MyD88CD14, CD36, TIRAP RP 105, by decreasing the threshold of the concentration needed for recognition and signaling
TLR7 Sensing of intracellular ribonucleotide and in certain circumstances extracellular receptor mediated Oligoribonucleotide, rich in adenosine and uracil Endocellular oligonucleotide from debris of apoptosis, viral single stranded RNA derived from viral RNA's, (after uncoating of viral RNA genome, contribute to autoimmune disease Unc93b1 a 12 pass integral membrane protein on Endoplasmic Reticulum. with TLR9 traffic through the Golgi [98] apparatus to become endosome [10]
TLR9 Sensing of intracellular deoxynucleotide Non-methylated deoxynucelotide rich in cytosine-guanine Intracellular DNA virus e.g herpes simplex, can distinguish 2 CpG motif. Recognition of Chromatin fragment to protect against autoimmune disease Unc93b1, MyD88, TLR9 dimer in endosomal compartment that enable signaling [98]