Ciona gut immunity : dynamics of variable region-containing chitin-binding proteins (VCBP) distribution in the digestive tract from metamorphosis to the adult

The study of the immune processes that occur in the digestive tract of animals and ensure gut-homeostasis has increased considerably. This topic is of current medical importance in relation to human gut diseases linked to the alteration of gut-microbiota. Variable region-containing Chitin-Binding Proteins (VCBPs), identified originally in the cephalochordate Branchiostoma floridae, are characterized by two V-type immunoglobulin domains and a chitin binding domain. This character could reflect an important transition between non-rearranging immune molecules and conventional rearranging antigen-binding receptors of jawed vertebrates. In Ciona intestinalis the three VCBP molecules, VCBP-A, -B and -C, are localised in immune competent tissues, such as coelomocytes and stomach cells, bind bacteria in the stomach lumen, and act as opsonins enhancing phagocytosis of bacteria by granular amoebocytes. These features stress the role for VCBPs in immune recognition. In the present study evidence for the role of VCBPs in gut homeostasis has been provided. The timing of VCBPs expression during development and metamorphosis has been elucidated, and the features of VCBP-A and -C as markers of the digestive tract anatomy and possibly physiology, both during differentiation and in the adult, have been demonstrated. Immunohistochemical experiments have revealed that the three VCBPs are differently expressed in defined areas of the adult stomach epithelium. Finally, the suggested VCBPs role in gut-homeostasis of C. intestinalis is comparable to that carried out by IgA antibodies in higher animals. VCBP-B and -C may act through an immune exclusion mechanism in the stomach lumen and VCBP-A may exploit a protective function inside the stomach cells. In condition of unbalanced gut-microbiota, these molecules may respond by increasing their production, thus restoring the gut-microbiota balance. </b