Uncovering the evolutionary history of innate immunity: the simple metazoan Hydra uses epithelial cells for host defence

Dev Comp Immunol. 2009 Apr;33(4):559-69. doi: 10.1016/j.dci.2008.10.004. Epub 2008 Nov 12.


Although many properties of the innate immune system are shared among multicellular animals, the evolutionary origin remains poorly understood. Here we characterize the innate immune system in Hydra, one of the simplest multicellular animals known. In the complete absence of both protective mechanical barriers and mobile phagocytes, Hydra's epithelium is remarkably well equipped with potent antimicrobial peptides to prevent pathogen infection. Induction of antimicrobial peptide production is mediated by the interaction of a leucine-rich repeats (LRRs) domain containing protein with a TIR-domain containing protein lacking LRRs. Conventional Toll-like receptors (TLRs) are absent in the Hydra genome. Our findings support the hypothesis that the epithelium represents the ancient system of host defence.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Antimicrobial Cationic Peptides / genetics*
  • Antimicrobial Cationic Peptides / immunology
  • Antioxidants / pharmacology
  • Base Sequence
  • Epithelial Cells / immunology*
  • Epithelial Cells / microbiology
  • Epithelial Cells / ultrastructure
  • Evolution, Molecular
  • Flagellin / immunology
  • Gene Expression / drug effects
  • Gene Expression / immunology
  • Hydra / immunology*
  • Hydra / microbiology
  • Hydra / ultrastructure
  • Immunity, Innate*
  • Microscopy, Electron, Transmission
  • Molecular Sequence Data
  • Pseudomonas aeruginosa
  • Receptors, Cell Surface / drug effects
  • Receptors, Cell Surface / immunology
  • Receptors, Cell Surface / metabolism
  • Sequence Alignment
  • Up-Regulation / drug effects
  • Up-Regulation / immunology
  • Uric Acid / pharmacology


  • Antimicrobial Cationic Peptides
  • Antioxidants
  • Receptors, Cell Surface
  • Flagellin
  • Uric Acid