Grouper interleukin 15-like acts as a cytokine adjuvant in grouper iridovirus subunit vaccine

Interleukin (IL) 15 and IL-15-like (IL-15L) are members of the cytokine receptor γ chain (γc) family proteins, which play critical roles in the growth, and differentiation of various cell lineages of the innate and adaptive immune systems. However, the functions of teleost IL-15L upon virus infection still remained unknown. In the current study, a homolog of IL-15L from grouper (Epinephelus coioides) was identified and characterized. Sequence analysis indicated that EcIL-15L contained a conserved IL-15 domain (aa 13–134). Quantitative Reverse Transcription PCR (qRT-PCR) analysis indicated that the expression level of EcIL-15L was markedly increased upon Singapore grouper iridovirus (SGIV) infection, indicating that EcIL-15L may be involved in the host's defense against infections by fish viruses. In addition, EcIL-15L mostly colocalized with endoplasmic reticulum, and partially colocalized with the Golgi apparatus. EcIL-15L overexpression significantly decreased SGIV replication, as demonstrated by a reduce in viral gene transcriptions and protein synthesis. In contrast, the replication of SGIV was promoted by the knockdown of EcIL-15L in grouper cells. Furthermore, compared to SGIV subunit vaccine alone, the combination of vaccine and EcIL-15L significantly improved the relative protective effect against SGIV. Meanwhile, the combination of EcIL-15L significantly increased the specific antibody titer in serum or the transcription levels of immune related genes in vivo, respectively. In conclusion, our findings indicated that EcIL-15L exerted antiviral function against SGIV, and acted as a cytokine adjuvant to improve protective effects induced by SGIV subunit vaccine.