Immunity in Protochordates: The Tunicate Perspective

Abstract

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.

Defining the relevant combinatorial space of the PKC/CARD-CC signal transduction nodes

“…1), we would expect those forms to have a CARD9/11like activation profile. If the ancestral CARD-CC isoforms also show a CARD9/11-like restricted expression to immune-related cells (see for example: [76][77][78]), it would mean that the adoption of CBM complex activation downstream of other PKCs and in other cell types probably only evolved in the jawed vertebrate lineage after the expansion of the CARD-CC family. Supporting this hypothesis, the CARD-CC transcript (g47136) is most expressed in one "large lymphocyte"-like hemapoetic stem cell population (CP25) in the golden star tunicate [78].…”

Into the bloom: Molecular response of pelagic tunicates to fluctuating food availability

“…Transcription factor proteins containing interferon‐regulatory factor (IRFs) motifs (IRF, PF00605; IRF‐2BP1_2, PF11261; IRF‐3, PF10401) are important factors in the interferon regulation of the immune system (Nehyba, Hrdličková, & Bose, ). The DUF3456 (PF11938 ‐ TLR4 regulator and MIR‐interacting MSAP) motif is found in proteins with essential roles in the inflammatory response also in tunicates (Franchi & Ballarin, ).…”

Evolutionary Origin of the Mammalian Hematopoietic System Found in a Colonial Chordate

“…We did not identify a cell population with a clear mammalian cytotoxic gene expression signature. Morula cells (MC), cells that contain phenoloxidase, accumulate in rejection points and have been proposed as cytotoxic cells that mediate rejection [28][29][30] . We detected MC in population CP18 (mother population of CP19, 20 and 21; Fig.…”