Evolution and function of innate immune receptors--insights from marine invertebrates

Innate, nonadaptive immune receptors represent phylogenetically ancient first-line sensors of invariant non-self patterns and other cellular danger signals. From lower animal phyla to vertebrates, most pathogens are immediately detected by various recognition systems and are destroyed by induction of defense effectors like antimicrobial peptides. Toll-like receptors, nucleotide-binding and oligomerization domain-like receptors and scavenger receptor cysteine-rich proteins represent archetypes of the innate immune receptors, which mediate the complex interaction between the host and microbiota at the interface of epithelial barriers. In this review, we will use knowledge gained from marine invertebrates as a paradigm to describe how this constant molecular crosstalk within the holobiont, i.e. the animal with all its associated microorganisms, contributes to epithelial homeostasis, immunological integrity and maintenance of the resident microbial diversity.