Immunity in Protochordates: The Tunicate Perspective

Tunicates are the closest relatives of vertebrates, and their peculiar phylogenetic position
explains the increasing interest toward tunicate immunobiology. They are filter-feeding
organisms, and this greatly influences their defense strategies. The majority of the studies
on tunicate immunity were carried out in ascidians. The tunic acts as a first barrier
against pathogens and parasites. In addition, the oral siphon and the pharynx represent
two major, highly vascularized, immune organs, where circulating hemocytes can sense
non-self material and trigger immune responses that, usually, lead to inflammation and
phagocytosis. Inflammation involves the recruitment of circulating cytotoxic, phenoloxidase
(PO)-containing cells in the infected area, where they degranulate as a consequence
of non-self recognition and release cytokines, complement factors, and the enzyme PO.
The latter, acting on polyphenol substrata, produces cytotoxic quinones, which polymerize
to melanin, and reactive oxygen species, which induce oxidative stress. Both the
alternative and the lectin pathways of complement activation converge to activate C3:
C3a and C3b are involved in the recruitment of hemocytes and in the opsonization of
foreign materials, respectively. The interaction of circulating professional phagocytes with
potentially pathogenic foreign material can be direct or mediated by opsonins, either
complement dependent or complement independent. Together with cytotoxic cells,
phagocytes are active in the encapsulation of large materials. Cells involved in immune
responses, collectively called immunocytes, represent a large fraction of hemocytes,
and the presence of a cross talk between cytotoxic cells and phagocytes, mediated by
secreted humoral factors, was reported. Lectins play a pivotal role as pattern-recognition
receptors and opsonizing agents. In addition, variable region-containing chitin-binding
proteins, identified in the solitary ascidian Ciona intestinalis, control the settlement and
colonization of bacteria in the gut.