Toll-like receptors in invertebrate innate immunity

Among invertebrates, innate immunity is the only defense mechanism against harmful non-self agents.In response to recognition of microbial pattern molecules, Drosophila melanogaster activates either theToll or Imd pathway, leading to the translocation of NF-kB (or Rel) transcription factors from the cytoplasmto the nucleus and the subsequent production of antimicrobial peptides, which provide systemic innateimmunity. Toll-like receptors (TLRs) are characterized by an extracellular leucine rich repeat (LRR)domain and an intracellular Toll/interleukin-1 receptor (TIR) domain. TLRs are found from cnidarians tomammals. Here we argue that TLR mediated innate immunity developed during an early stage ofevolution when organisms acquired a body cavity. This is supported by the distributions of TLR and Relgenes in the animal kingdom. Further, TLR mediated immunity appears to have developed independentlyin invertebrates and vertebrates. Recent studies have shown that microbial molecules, with the potentialto signal through TLR, can be beneficial to host survival. Studies on this signaling pathway could opendoors to a better understanding of the origins of innate immunity in invertebrates and potentialtransmission blocking strategies aimed at ameliorating vector-borne diseases.