Fish natural killer enhancing factor-A (NKEF-A) enhance cytotoxicity of nonspecific cytotoxic cells against bacterial infection

Natural killer enhancing factor (NKEF)-A/B is a member of Peroxiredoxin (Prxs) family, which is named for the function of enhancing NK cells activity. NKEF also plays essential roles in multiple physiology/pathology processes including inflammation regulation, cancer development and redox reactions. However, the regulatory effects of fish NKEF on immune cells remain largely unknown. In this study, the full-length cDNA of NKEF-A (Accession No. MK584553, designated as On-NKEF-A) was identified from Nile tilapia (Oreochromis niloticus). On-NKEF-A encoded a 198 amino acid peptide with molecular mass of 22.085 kDa. On-NKEF-A protein contained a typical 2-Cys family domain, two active sites (51aa and 172aa) that were conserved in mammals, birds, amphibians and fish. Phylogenetic analysis showed that On-NKEF-A had the closest relationship with Zebra mbuna (Maylandia zebra) NKEF. The On-NKEF-A transcription was present in all examined tissues or organs. And the relative high expression levels of On-NKEF-A was found in head kidney leucocytes and nonspecific cytotoxic cells (NCC). After Streptococcus agalactiae stimulation, On-NKEF-A was significantly up-regulated in head kidney, spleen, gill and skin. Also, On-NKEF-A was markedly induced post S. agalactiae, LPS and poly I:C stimulation in head kidney-derived NCC. Moreover, On-NKEF-A was mainly distributed in cytoplasm of fathead minnow cells (FHM cells). The further in vitro analysis found that the recombinant protein of On-NKEF-A (rOn-NKEF-A) could induce the expression of various molecular markers of B cells, macrophages and NCC, enhanced the cytotoxicity of NCC via increasing the effectors expression. The present data collectively indicate that On-NKEF-A participates in anti-bacterial immune response via regulating NCC activity, which will provide new ideas to further explore the defense mechanism of fish against bacteria.