Interleukin-17B in common carp (Cyprinus carpio L.): Molecular cloning and immune effects as immune adjuvant of Aeromonas veronii formalin-killed vaccine

The interleukin-17 (IL-17) family of cytokines is critical for host defense responses and mediates different pro- or anti-inflammatory mediators through different signaling pathways. However, the function of the related family member, IL-17B, in teleosts is poorly understood. In the present study, an IL-17B homolog (CcIL-17B) in common carp (Cyprinus carpio) was identified, and sequence analysis showed that CcIL-17B had eight conserved cysteine residues, four of which could form two pairs of disulfide bonds, which in turn formed a ring structure composed of nine amino acids (aa). The deduced aa sequences of CcIL-17B shared 35.79-92.93% identify with known homologs. The expression patterns were characterized in healthy and bacteria-infected carp. In healthy carp, IL-17B mRNA was highly expressed in the spleen, whereas Aeromonas veronii effectively induced CcIL-17B expression in the liver, head, kidney, gills, and intestine. The recombinant protein rCcIL-17B could regulate the expression levels of inflammatory cytokines (such as IL-1β, IL-6, TNF-α, and IFN-γ) in primary cultured head kidney leukocytes in vitro. As an adjuvant for the formalin-killed A. veronii (FKA) vaccine, rCcIL-17B induced the production of specific antibodies more rapidly and effectively than Freund's complete adjuvant (FCA). The results of the challenge experiments showed that the relative percent survival (RPS) after vaccination with rCcIL-17B was 78.13%. This percentage was significantly elevated compared to that observed in the alternative experimental groups (62.5% and 37.5%, respectively). Additionally, the bacterial loads in the spleen of the rCcIL-17B + FKA group were significantly lower than those in the control group from 12 h to 48 h after bacterial infection. Furthermore, histological analysis showed that the epithelial cells were largely intact, and the striated border structure was complete in the intestine of rCcIL-17B+FKA group. Collectively, our results demonstrate that CcIL-17B plays a crucial role in eliciting immune responses and evokes a higher RPS against A. veronii challenge compared to the traditional adjuvant FCA, indicating that rCcIL-17B is a promising vaccine adjuvant for controlling A. veronii infection.