Table 1.
Pattern recognition receptors (PRRs) that recognize conserved microbial structures of pathogens
| PRRs | Localization | PAMP recognized | Key adaptors | Effector response |
|---|---|---|---|---|
| TLRs | ||||
| TLR1 | Cell surface | Triacylated lipopeptides | MyD88 | IL-6, TNF-α |
| TLR2 | Cell surface | Di/triacylated lipopeptides | MyD88, TIRAP | IL-6, TNF-α, IL-8, MCP-1, RANTES |
| TLR3 | Endosomes | dsRNA | TRIF | IFN-β |
| TLR4 | Cell surface | LPS | MyD88, TRIF, TIRAP, TRAM | IL-6, TNF-α, IFNβ, IP-10 |
| TLR5 | Cell surface | Flagellin | MyD88 | TNF-α |
| TLR6 | Cell surface | Diacylated lipopeptides | MyD88, TIRAP | TNF-α, IL-6, IL-8, MCP-1, RANTES |
| TLR7 | Endosomes | ssRNA | MyD88 | IFN-α |
| TLR8 | Endosomes | ssRNA | MyD88 | IFN-α |
| TLR9 | Endosomes | CpG DNA | MyD88 | IFN-α |
| TLR11 | Endosomes | Profilin, flagellin | MyD88 | IL-12, TNF-α |
| TLR12 | Endosomes | Profilin | MyD88 | IL-12p40, IFN-α |
| TLR13 | Endosomes | 23s rRNA | MyD88 | IL-6, IL-12p40 |
| RLRs | ||||
| RIG-I | Cytoplasm | Short dsRNA, ssRNA | IPS-1, STING | IFN-β, IL-6 |
| MDA5 | Cytoplasm | Long dsRNA | IPS-1 | IFN-β |
| LGP2 | Cytoplasm | dsRNA | IPS-1 | IFN-β |
| DDX3 | Cytoplasm | Viral RNA | IPS-1 | IFN-β |
| Cytosolic DNA sensors | ||||
| DAI | Cytoplasm | dsDNA | STING | IFN-β |
| RNA Pol III | AT rich dsDNA | IPS1 | IFN-β | |
| IFI16 | Nucleus and cytoplasm | dsDNA | STING | IFN-β, IP-10, IL-6, IL-1β |
| AIM2 | Cytoplasm | dsDNA | ASC | IL-1β, IL-18 |
| Ku70 | Cytoplasm | dsDNA | ? | IFN-γ |
| MRE11 | dsDNA, ISD | STING | IFN-β, IL-6, IP-10 | |
| cGAS | Cytoplasm | dsDNA | STING | IFN-β |
| LRRFIP1 | Cytoplasm | dsDNA, dsRNA | β-catenin | IFN-β |
| DHX36 | Cytoplasm | dsDNA | MyD88 | TNF-α |
| DHX9 | Cytoplasm | dsDNA | MyD88 | TNF-α |
| DDX41 | Cytoplasm | c-di-GMP, c-di-AMP | STING | IFN-α, IFN-β |
| STING | Cytoplasm | c-di-GMP | IFN-β | |
| HMGB | Cytoplasm | dsDNA, ssDNA | ? | IFN-β, IL-6, RANTES |
| Histone H2B | Nucleus and cytoplasm | Poly (dA:dT), genomic DNA | IPS1 | IFN-β |
Innate immune recognition engages germline-encoded pattern recognition receptors (PRRs) that recognize conserved microbial structures of pathogens known as pathogen-associated molecular patterns (PAMPs) and activate the expression of major histocompatibility (MHC) proteins, costimulatory molecules, and inflammatory mediators in the form of cytokines and chemokines by macrophages, dendritic cells (DCs), neutrophils, and other nonprofessional immune cells.
IL, interleukin; IFN, interferon; TNF, tumor necrosis factor.