542-488 million years ago

complement system

Although tunicates lack the complex immune mechanisms of higher vertebrates, they possess a variety of white blood cells (including the apparent homologs of natural killer cells), homologs of the complement pathway (which vertebrates use in a complex pathway involving antibodies and tunicates which use in a much simpler pathway), and genes which are thought to be related to the ancestral proteins which evolved into antibodies and T cell receptors in higher vertebrates.

The blood of some tunicates posseses a variety of leukocytes including macrophages, different kinds of granular amoebocytes with odd-shaped nuclei which perform phagocytosis, cytotoxic cells, and a number of other cell types (Parrinello, 1996; Burighel, from Harrison, 1997, p. 269). A number of invertebrates, including tunicates, possess cytotoxic cells which have been compared to natural killer cells (Parrinello, 1996). Transplantation reactions occur in urochordates. Urochordates express a gene homologous to natural killer cell receptors on some of their blood cells. In fact, this receptor (CD94) is considered a marker for natural killer cells in humans (Khalturin, 2003).

Urochordates possess proteins pertaining to both the alternative pathway (C3, Bf) and lectin pathway (MBL, MASP). Although tunicates lack acquired immunity, they do possess complement genes, lectins, and 2 interleukin receptor genes which probably function in innate immunity (Dehal, 2002). Tunicates seem to have homologs of complement receptors (Zarkadis, 2001).
The lectin-based opsoinzation pathway seems to be the original complement pathway. Tunicates seem to have a minimal complement system involving a lectin which binds to serine proteases (forming GBL-MASP complex), C3, and a C3 receptor on blood cells (Fujita, 2004). Complement proteins a2M/C3/C4/C5 form a gene family. Tunicates have 2 molecules similar to a2M and two which are similar to C3. They also possess 3 linked Bf genes, nine ficolin-like molecules, and 2 C1q like molecules (Fujita, 2004).
Vertebrates possess two types of immunoglobulin which are thought to be similar to the ancestral immunoglobulin which gave rise to antibodies and TCR genes, the JAM/CTX and the nectin/poliovirus receptor (PVR) families. These proteins serve as cell adhesion molecules and virus receptors (such as polio, Coxsackie virus, and reoviruses). Apparently, one ancestral pair of these genes was linked and subsequently duplicated to produce the four paralagous linkage groups known in the human genome. JAM, CTX, and nectin molecules can form dimers, as can the subunits of antibodies and T cell receptors (Du Pasquier, 2004). Drosophila possesses a protein with V and C chains showing homology to nectin (Du Pasquier, 2004). Tunicates, among the most primitive chordates, possess JAM, CTX and PVR genes which possess both a V and a C domain (Du Pasquier, 2004).