Abstract
Complex symbiotic interactions at the surface of host epithelia govern most encounters between host and microbe. The epithelium of the gut is a physiologically ancient structure that is comprised of a single layer of cells and is thought to possess fully developed immunological capabilities. Ciona intestinalis (sea squirt), which is a descendant of the last common ancestor of all vertebrates, is a potentially valuable model for studying barrier defenses and gut microbial immune interactions. A variety of innate immunological phenomena have been well characterized in Ciona, of which many are active in the gut tissues. Interactions with gut microbiota likely involve surface epithelium, secreted immune molecules including variable region-containing chitin-binding proteins, and hemocytes from a densely populated laminar tissue space. The microbial composition of representative gut luminal contents has been characterized by molecular screening and a potentially relevant, reproducible, dysbiosis can be induced via starvation. The dialog between host and microbe in the gut can be investigated in Ciona against the background of a competent innate immune system and in the absence of the integral elements and processes that are characteristic of vertebrate adaptive immunity.
Highlights
Because of their phylogenetic position relative to vertebrates, deuterostome invertebrates (Figure 1) are compelling models for studies of immunity
Ciona intestinalis, which has been the focus of our recent efforts, is relatively easy to maintain and propagate (Figure 2) at room temperature and continues to serve as a highly informative model for studies of animal development (Katz, 1983; Meinertzhagen and Okamura, 2001; Canestro et al, 2003; Shi et al, 2005; Baghdiguian et al, 2007; Davidson, 2007; Sasakura et al, 2009) and immune defense (Fujita et al, 2004; Melillo et al, 2006; Parrinello, 2009; Sasaki et al, 2009; Zucchetti et al, 2009)
A complex physiological challenge is posed to the mucosal immune system of the host, which must differentiate distinct populations of commensal microorganisms from pathogenic communities
Summary
Because of their phylogenetic position relative to vertebrates, deuterostome invertebrates (Figure 1) are compelling models for studies of immunity. Of the representative deuterostome invertebrates, echinoderms, which include sea urchins and starfish, diverged prior to chordates and have proven to be highly informative models in which to examine innate immunity (Rast and Messier-Solek, 2008; Messier-Solek et al, 2010). Protochordates are comprised of invertebrate chordates such as sea squirts (Urochordata or Tunicata) and amphioxus (Cephalochordata) These species, which share certain developmental features with vertebrates and possess competent innate immunity, diverged prior to the origins of the vertebrate adaptive immune system. Innate immune genes in Ciona have not undergone the expansions seen in amphioxus and sea urchin (Dehal et al, 2002; Hughes and Friedman, 2005)
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