Changing Views of the Evolution of Immunity

lineage-specific properties among expanded sea urchin TLRs. Their findings indicate that: (1) some antigen binding sites may be coevolving with variable ligands, (2) TLR subfamilies are utilized differently between larval and adult coelomocytes, and (3) sea urchin TLRs most likely represent immune surveillance molecules. Satake and Sekiguchi (2012) review the evolution and functional diversification of TLRs among deuterostomes, highlighting a reduced repertoire in the tunicate, Ciona intestinalis. Only two TLRs can be detected in this species, with presumed hybrid functionality in vitro (Sasaki et al., 2009). Interestingly, neither TLR1 nor TLR2 recognizes bacterial lipopolysaccharide (LPS), suggesting that Ciona utilizes other mechanisms to detect LPS or that an accessory molecule(s) is involved. Drosophila melanogaster (fruit fly) uses complex alternative RNA splicing to diversify the Down’s syndrome cell adhesion molecule (DSCAM), a multiexonic receptor implicated in neuronal patterning (Shi and Lee, 2012). Some DSCAM isoforms serve as PRRs in peripheral hemocytes and exhibit increased specificity for distinct targets (Watson et al., 2005; Brites et al., 2008; Chou et al., 2009). These findings are reminiscent of the fibrinogen-related proteins (FREPs) (Adema et al., 1997; Zhang et al., 2004), which consist of fibrinogen and immunoglobulin superfamily-related domains that can undergo somatic mutation and gene conversion. Individual somatic lineages expressing FREPs respond to specific parasite burdens (Mone et al., 2010). Smith (2012) reviews Sp185/333 genes, a large family of innate receptors in sea urchin expressed in hemocytes. Variation in genes encoding Sp185/333 receptors arises via complex DNA rearrangements and may be influenced by persistent antigenic sources (Buckley et al., 2008; Dheilly et al., 2009). Not all immune receptors are restricted to foreign determinants (Rabinovich and Croci, 2012). Some glycans can be found on both host and microbial surfaces (Davicino et al., 2011). Vasta et al. (2012) describe an apparent paradox among galectins, which until recently were considered essential in self-recognition (Rabinovich and Croci, 2012). Galectins now are considered PRRs that recognize related glycans on microbes (Sato et al., 2009). PRRs are thought to interact only with microbial products (Kawai and Akira, 2010); some, such as galectins, also may possess discriminatory properties (van Vliet et al., 2008). Galectin self-recognition may require interaction with accessory molecules on self-cells and warrants further investigation. The role of PRRs in symbiotic relationships likely is ancient (Bosch, 2012), involving complex host-microbial interactions at the surface of mucosal tissues (Duerkop et al., 2009; Round Changing views of the evolution of immunity