Intracellular signaling and cytokine induction upon interactions of Porphyromonas gingivalis fimbriae with pattern-recognition receptors.
Toll-like receptors (TLRs) and other pattern-recognition receptors (PRRs) of the innate immune system form functional receptor complexes that recognize and respond to pathogen-associated molecular patterns (PAMPs). Porphyromonas gingivalis is an important pathogen in human periodontitis and has also been implicated in atherosclerosis. A major virulence factor of this pathogen is the fimbriae, which function as a surface adhesin. Here we present evidence that fimbriae also constitute a predominant P. gingivalis proinflammatory molecule which activates the TLR signaling pathway resulting in induction of proinflammatory cytokines (IL-1beta, IL-6, and TNF-alpha) and chemokines (IL-8) in monocytic cells. Although TLR2 and TLR4 mediate cellular activation in response to fimbriae, other PRRs, namely CD14 and CD11b/CD18, are involved in the recognition of fimbriae. We thus propose that fimbriae function as a PAMP which interacts with a PRR multi-receptor complex, where CD14 and CD11b/CD18 function as recruiting receptors and TLRs function as signaling receptors. In addition to cytokine induction, TLR activation by fimbriae also results in upregulation of the CD40, CD80, and CD86 costimulatory molecules in antigen-presenting cells, suggesting that fimbriae are sensed as a potential "danger" to the host immune system. Moreover, proinflammatory cytokine induction is attenuated upon repeated cellular stimulation with P. gingivalis fimbriae. This mechanism of tolerance induction which serves to mitigate excessive and potentially harmful inflammatory reactions appears to be due partly to fimbria-induced downregulation of the expression of interleukin-1 receptor-associated kinase-1 (IRAK-1), an important signaling intermediate of the TLR pathway. Understanding the molecular basis of how the host recognizes and responds to P. gingivalis fimbriae is essential for developing molecular approaches to control P. gingivalis-induced inflammatory responses in periodontal disease and perhaps atherosclerosis.
Hajishengallis
George
G
Center of Excellence in Oral and Craniofacial Biology, Department of Microbiology, Immunology, and Parasitology, Louisiana State University Health Sciences Center, New Orleans, Louisiana 70119, USA. ghajis@lsuhsc.edu
Sojar
Hakimuddin
H
Genco
Robert J
RJ
DeNardin
Ernesto
E
D002460:Cell Line; D016207:Cytokines / Q000096:biosynthesis* / Q000276:immunology* / Q000378:metabolism; D010861:Fimbriae, Bacterial / Q000737:chemistry / Q000276:immunology*; D006801:Humans; D007108:Immune Tolerance; D007249:Inflammation / Q000276:immunology / Q000378:metabolism / Q000382:microbiology; D008562:Membrane Glycoproteins / Q000276:immunology*; D009000:Monocytes / Q000276:immunology / Q000378:metabolism / Q000382:microbiology; D016966:Porphyromonas gingivalis / Q000166:cytology / Q000276:immunology* / Q000472:pathogenicity; D011956:Receptors, Cell Surface / Q000276:immunology*; D015398:Signal Transduction; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D037521:Virulence Factors / Q000737:chemistry / Q000276:immunology
D016207:Cytokines; D008562:Membrane Glycoproteins; D011956:Receptors, Cell Surface; C495342:TLR2 protein, human; C495345:TLR4 protein, human; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D037521:Virulence Factors