Pattern recognition receptor

The concept of pattern recognition receptors emerged from Charles Janeway Jr.'s 1989 'stranger hypothesis,' which argued that innate immunity must possess receptors capable of recognizing conserved microbial patterns to trigger adaptive immune responses.¹ This idea was experimentally validated in 1996 when Bruno Lemaitre and Jules Hoffmann showed that the Toll receptor in Drosophila mediates antifungal immunity, and in 1998 when Ruslan Medzhitov and Janeway identified a human Toll-like receptor (TLR4) that activates NF-κB.² The term 'pattern recognition receptor' was coined by Janeway in his 1989 Cold Spring Harbor Symposium paper. The discovery of TLRs as PRRs earned Hoffmann and Bruce Beutler the 2011 Nobel Prize in Physiology or Medicine, shared with Ralph Steinman for dendritic cell research.

PRRs are classified into several families based on structure and localization. Toll-like receptors (TLRs) are transmembrane proteins that survey extracellular and endosomal compartments; for example, TLR4 recognizes lipopolysaccharide, while TLR3 detects double-stranded RNA.³ NOD-like receptors (NLRs) are cytosolic sensors, with NOD2 detecting muramyl dipeptide and NLRP3 forming the inflammasome that activates caspase-1 and IL-1β.⁴ RIG-I-like receptors (RLRs) such as RIG-I and MDA5 recognize viral RNA in the cytoplasm. C-type lectin receptors (CLRs) like Dectin-1 bind fungal β-glucans. Signaling through these receptors converges on transcription factors such as NF-κB, IRF3, and IRF7, inducing interferons, cytokines, and chemokines that orchestrate inflammation and adaptive immunity.

PRRs are central to host defense but also contribute to inflammatory pathology. Dysregulated PRR signaling is implicated in autoimmune diseases, such as systemic lupus erythematosus, where nucleic acid sensors like TLR7 and TLR9 respond to self-DNA/RNA.⁵ In cancer, PRR activation can either promote antitumor immunity or, paradoxically, support tumor growth through chronic inflammation. Therapeutically, PRR agonists are being developed as vaccine adjuvants; for instance, the TLR4 agonist monophosphoryl lipid A is used in the HPV vaccine Cervarix, and TLR7/8 agonists like imiquimod treat skin cancers.⁶ Conversely, PRR antagonists are explored for sepsis and inflammatory diseases.

Beyond the canonical families, PRRs include lesser-known sensors such as cGAS, which detects cytosolic DNA and produces cGAMP to activate STING, and AIM2, an inflammasome-forming DNA sensor.⁷ Some PRRs have non-immune functions: TLRs are expressed on neurons and influence neurogenesis, and NOD2 is involved in autophagy and endoplasmic reticulum stress. In plants, PRRs like FLS2 recognize bacterial flagellin, illustrating evolutionary conservation. PRRs also recognize DAMPs released during sterile injury, such as HMGB1 and ATP, bridging infection and tissue damage responses. Notably, some viruses encode decoy molecules that mimic PAMPs to subvert PRR detection, and certain PRR polymorphisms are linked to susceptibility to infections like tuberculosis and Crohn's disease.

PAMP

Pathogen-associated molecular pattern; conserved microbial molecule such as LPS or flagellin.

DAMP

Damage-associated molecular pattern; host molecule released upon cell damage, e.g., HMGB1.

Inflammasome

Multiprotein complex that activates caspase-1 and promotes IL-1β and IL-18 secretion.

NF-κB

Transcription factor that regulates inflammatory gene expression.