Interleukin 17 (IL-17) is an important pro-inflammatory
cytokine and plays critical roles in the immune response to pathogens and in the pathogenesis of inflammatory and
autoimmune diseases. Despite its important functions, the origin and evolution of
IL-17 in animal phyla have not been characterized. As determined in this study, the distribution of the
IL-17 family among 10 invertebrate species and 7 vertebrate species suggests that the
IL-17 gene may have originated from Nematoda but is absent from Saccoglossus kowalevskii (Hemichordata) and Insecta. Moreover, the gene number,
protein length and domain number of
IL-17 differ widely. A comparison of IL-17-containing domains and conserved motifs indicated somewhat low amino acid sequence similarity but high conservation at the motif level, although some motifs were lost in certain species. The third
disulfide bond for the
cystine knot fold is formed by two
cysteine residues in invertebrates, but these have been replaced by two
serine residues in Chordata and vertebrates. One third of invertebrate
IL-17 proteins were found to have no predicted
signal peptide. Furthermore, an analysis of phylogenetic trees and exon-intron structures indicated that the
IL-17 family lacks conservation and displays high divergence. These results suggest that invertebrate
IL-17 proteins have undergone complex differentiation and that their members may have developed novel functions during evolution.