An Overview of Pathogen Recognition Receptors for Innate Immunity in Dental Pulp

Table 1

Summary of TLRs and NLRs in human innate immunity.

Family	PRR	Location	Ligand (ligand location)	Unique features
Toll-like receptors; (TLRs)	TLR1	Cell surface	Triacyl lipopeptides (bacterial lipoprotein)	Formation of heterophilic dimers with TLR2
	TLR2	Cell surface	Di-/triacyl lipopeptides; Multiple lipoproteins; Lipoteichoic acid; Zymosan (fungi)	Formation of heterophilic dimers with TLR1 and TLR6
	TLR3	Endosome	dsRNA (virus)	dsRNA interacting with the N-terminal and C-terminal sites on the lateral side of convex surface of TLR3
	TLR4	Cell surface	LPS (Gram-negative bacteria)	Recognition of LPS together with myeloid differentiation factor 2
	TLR5	Cell surface	Flagellin	Activation of lung epithelial cells to induce inflammatory cytokine
	TLR6	Cell surface	Triacyl lipopeptides (bacterial lipoprotein)	Formation of heterophilic dimers with TLR2
	TLR7 and TLR8	Endosome	ssRNA (virus)	Recognition of synthetic compound imidazoquinoline
	TLR9	Endosome	Unmethylated CpG DNA	Involvement in the pathogenesis of autoimmune disorders through recognition of the chromatin structure
NOD-like receptors; (NLRs)	NOD1	Cytoplasm	Peptidoglycan; (Gram-negative bacteria)	Recognition of intracellular bacterial cell products
	NOD2	Cytoplasm	Peptidoglycan; (Gram-positive bacteria)
	NALP3	Endosome	PAMPs, virulence factor; DAMPs	Response to multiple stimuli via forming a NALP3 inflammasome

LPS: lipopolysaccharide; NOD: nucleotide-binding oligomerization domain; NALP3: NACHT [neuronal apoptosis inhibitory protein (NAIP), CIITA, HET-E, and TP-1] domain, LRR (leucine-rich repeat) domain, and PYD (pyrin domain) containing protein 3; PAMPs: pathogen-associated molecular patterns; DAMPs: danger-associated molecular patterns.