Functional characterization of interleukin 17 family members and their receptors in amphioxus

The interleukin 17 (IL17) cytokine family plays a crucial role in phylogenetically conserved immune defense mechanisms, broadly categorized into two distinct evolutionary lineages: the invertebrate-derived (i-type) and vertebrate-specific (v-type) IL17 homologs. Intriguingly, the basal chordate amphioxus retains both IL17 subtypes, though the corresponding receptors and functional specializations orchestrated by these paralogs have remained enigmatic. In this investigation, we delineated four i-type IL17 ligands (designated BjIL17-i1 to BjIL17-i4) and four v-type homologs (BjIL17-v1 to BjIL17-v4), along with two cognate receptors (BjIL17R1 and BjIL17R2), within the amphioxus Branchiostoma japonicum genome. Functional analyses revealed that all i-type ligands, with the exception of BjIL17-i3, demonstrated robust capacity to stimulate the BjIL17R1/R2 heterocomplex. Conversely, v-type ligands exhibited preferential activation of independent BjIL17R1 or BjIL17R2 in an isoform-specific manner. Of particular significance, all eight BjIL17 isoforms participate in inflammatory regulation, differentially modulating transcriptional programs associated with either pro-inflammatory signaling or antimicrobial defense. Strikingly, BjIL17-i2 emerged as a potent pro-inflammatory mediator, displaying biomarker potential for inflammatory status delineation in amphioxus. Furthermore, BjIL17-v1 uniquely facilitated the coordinated induction of ATP synthesis and DNA repair-associated genes during inflammatory challenge. These findings collectively establish that the BjIL17 ligand-receptor network embodies an ancient, multifunctional regulatory axis governing immunological and homeostatic processes in basal chordates, predating the vertebrate-invertebrate divergence.