Identification and expression patterns of interleukin 17 (IL-17) genes in the earthworm Eisenia andrei: Differential...

Innate immunity is an ancient defense system present in most multicellular organisms. Among the key mediators of innate immunity, interleukin-17 (IL-17) is considered as the most well-conserved cytokine across animal phyla and plays a central role in promoting inflammation and regulating immune responses. Despite their lack of adaptive immunity, invertebrates exhibit complex immune responses mediated by soluble immune mediators such as cytokine-like molecules. Although the function of IL-17 has not been studied as extensively in lophotrochozoa as in deuterostomes and ecdysozoa, it has been suggested that annelids and bivalve mollusks produce evolutionarily conserved cytokines, including IL-17, whose function remains poorly understood. To the best of our knowledge, this is the first study to report the molecular structure and spatial expression patterns of annelid IL-17 proteins. Their inducibility following challenges with Gram-positive, Gram-negative bacteria, or lipopolysaccharide was additionally examined. Five new full-length complementary DNA sequences of earthworm IL-17, designated EanIL-17A/B/C/D/E, may be subcategorized into two groups-subgroup A, including EanIL-17A, comprises seven highly conserved cysteine residues, six of which are predicted to form three cystine knots between the conserved Cys 1 and 4, Cys 2 and 5, and Cys 3 and 6; and subgroup B, including EanIL-17B/C/D/E, contains two highly conserved cysteine residues and four partially conserved cysteine residues, which are predicted to form two cystine knots between Cys 1 and 4 and Cys 2 and 5. Because the topology of knots plays a key role in receptor recognition and signaling transduction events in IL-17 signaling (Iyer and Acharya, 2011), these two types of IL-17s exhibiting different knot topologies are likely to be recognized by separate receptors. While EanIL-17A mRNA was primarily observed in coelomocytes, the majority of the expression signals of other EanIL-17 mRNAs were observed in the connective tissues of the body wall. These signals were mainly localized in the bundle sheaths of the circular and longitudinal muscle layers and in the basal membrane, suggesting tissue-specific expression. Except for EanIL-17C, which responded only to Gram-negative Pseudomonas aeruginosa, other EanIL-17 mRNAs were upregulated by Gram-positive and Gram-negative bacteria at 6-24 h post-challenge, suggesting that they play an important role in earthworm immune defense against bacterial invasion and they are early response genes to pathogens.