Recognition of Microorganisms

From WikiVet English
Jump to navigation Jump to search
Pattern Recognition Receptors - B. Catchpole, RVC 2008

The innate immune system recognises components of pathogens which are intrinsically foreign (i.e. not present on normal mammalian cells), such as:

  • Lipopolysaccharides of gram-negative bacteria
  • Peptidoglycans of gram-positive bacteria
  • Mannose sugars
  • D-isoform amino acids (abundant in Peptidoglycans) - almost all (>99%) amino acids in multicellular organisms are the L-isoform of amino acids as it is only the L-isoform amino acids that are used for protein synthesis.


These are known to the body as foreign as they are expressed as pathogen-associated molecular patterns (PAMPs) which are recognised by pattern recognition receptors (PRRs) expressed on mammalian cells. These receptors are expressed on many different cell types, not just on phagocytes, though not all are expressed by all cells: different cell types express a different range of PRRs. PRRs are either intracellular, membrane-associated or soluble:

  • Recognition of pathogens via the cellular PRRs results in phagocytosis and inflammation
  • Recognition of pathogens via the humoral PRRs results in various killing mechanisms


Actions

The engagement of PRRs by PAMPs triggers:


Pattern Recognition Receptors

Examples of Pattern Recognition Receptors
Receptor Location Ligands
TLR2 (Toll-like receptor) Cell Membrane Peptidoglycan of gram +ve bacteria
TLR3 Cell Membrane (in Endosome) dsRNA of RNA viruses (e.g. avian influenza)
TLR4 Cell Membrane Lipoplysaccharide from gram-negative bacteria (e.g. E. coli, Salmonella)
TLR5 Cell Membrane Bacterial flagellin
TLR9 Cell Membrane (in Endosome) Bacterial DNA (CpG DNA)
C-type lectins Soluble Carbohydrates, all bacteria, dead cells
fMLP Soluble Formyl peptides (i.e. all bacteria)
Complement receptors Soluble Fixed complement components (e.g. iC3b)
NOD2 Cytoplasm Peptidoglycan of gram +ve bacteria
dsRNA-dependent Protein Kinase Receptor Cytoplasm ds RNA of RNA viruses

For more information on Toll-like receptors see this review [1]



References

  1. Lee, C.C., Avalos, A.M. and Ploegh, H.L. (2012) Accessory molecules for Toll-like receptors and their function. Nature Reviews Immunology 12: pp.168-179.




LIVE logo Originally funded by the RVC Jim Bee Award 2007