Molecular cloning, expression analysis, and functional characterization of an interleukin-15 like gene in common carp (Cyprinus carpio L.)

Interleukin-15 (IL-15) is a crucial cytokine involved in immune system regulation, which is produced by various cell types, including dendritic cells, monocytes, and macrophages. IL-15 plays a key role in the proliferation and activation of natural killer (NK) cells, CD8⁺ T cells, and memory CD8⁺ T cells, supporting their survival and enhancing their effector functions. Although IL-15 homologues in fish have been identified, their functions remain poorly understood. In this study, we cloned and investigated the bioactivities of an IL-15 homologue, referred to as IL-15 like (<i>Cc</i>IL-15L), in common carp (<i>Cyprinus carpio</i> L.). An expression pattern analysis revealed that <i>CcIL-15L</i> was constitutively expressed in all examined tissues of healthy common carp, with the highest expression level observed in the intestine. Additionally, <i>CcIL-15L</i> expression was significantly up-regulated in the head kidney, spleen, gills, and intestine following <i>Aeromonas hydrophila</i> infection. <i>In vitro</i>, the recombinant protein <i>Cc</i>IL-15L can significantly up-regulated the gene expression levels of pro-inflammatory cytokines (<i>IL-1β</i>, <i>IL-6</i>, <i>IFN-γ</i>, and <i>TNF-α</i>) and NK cell activation (<i>perforin</i> and <i>Eomesa</i>). We constructed a 3×FLAG eukaryotic expression vector and successfully expressed it in common carp by intramuscular injection. Additionally, the heterologous <i>Cc</i>IL-15L protein was successfully overexpressed <i>in vivo</i>, and immune-related genes including <i>CD4-1</i>, <i>CD8β2</i>, <i>TNF-α</i>, and <i>IgM</i> showed significant induction in the head kidney and spleen. Furthermore, <i>Cc</i>IL-15L overexpression reduced the bacterial loads after 24 h post-<i>A. hydrophila</i> infection in the liver, spleen, and kidney. Phagocytic and chemotaxis assays showed that r<i>Cc</i>IL-15L could promoted the phagocytosis and chemotactic abilities of common carp HKLs. Our study provides a new perspective on the role for <i>Cc</i>IL-15L in immunological functions in common carp.