Origin and Evolution of the Complement System

Abstract

The vertebrate immune system is composed of two parts, innate immunity which recognizes the invading microbes using germline-encoded molecules, and adaptive immunity, which depends on recognition molecules generated by somatic mechanisms during the ontogeny of each individual organism (Medzhitov and Janeway 1997). All data available to date indicate that adaptive immunity became established at the early stage of vertebrate evolution around the time of cartilaginous fish emergence. Thus the genes which encode the pivotal elements of adaptive immunity, such as immunoglobulin (Litman et al. 1993), T-cell receptor (Rast et al. 1997), major histocompatibility complex (MHC) class I (Hasulmoto et al. 1992) and class II molecules (Kasahara et al. 1992; Bartle and Weissman 1994) and recombination activating gene (Greenhalgh and Steiner 1995) have been identified in cartilaginous fish and higher vertebrates. None of the attempts to isolate these genes from the most primitive extant vertebrates, cyclostomes, has yet succeeded. In contrast, vertebrate innate immunity is believed to have a more ancient origin, and an apparently primitive complement system has been found in lamprey (Nonaka et al. 1983). However, it was not clear until recently whether the origin of the complement system can be traced back to invertebrate. Identification of C3/C4/C5-like expressed sequence tag (Est) from sea urchin coelomocytes (Smith et al. 1996) and molecular studies of the complement system in sea urchin and ascidian established the presence of the multicomponent, opsonic complement system in invertebrates.