Liver-expressed antimicrobial peptide 2(LEAP-2)is a cationic peptide that plays an important role in a host's innate immune system.We previously demonstrated that mudskipper(Boleophthalmus pectinirostris)LEAP-2(BpLEAP-2)induces chemotaxis and activation of monocytes/macrophages(MO/Mφ).However,the molecular mechanism by which BpLEAP-2 regulates MO/Mφremains unclear.In this study,we used yeast two-hybrid cDNA library screening to identify mudskipper protein(s)that interacted with BpLEAP-2,and characterized a sequence encoding motile sperm domain-containing protein 2(BpMOSPD2).The interaction between BpLEAP-2 and BpMOSPD2 was subsequently confirmed by co-immunoprecipitation(Co-IP).Sequence analyses revealed that the predicted BpMOSPD2 contained an N-terminal extracellular portion composed of a CRAL-TRIO domain and a motile sperm domain,a C-terminal transmembrane domain,and a short cytoplasmic tail.Phylogenetic tree analysis indicated that BpMOSPD2 grouped tightly with fish MOSPD2 homologs and was most closely related to that of the Nile tilapia(Oreochromis niloticus).The recombinant BpMOSPD2 was produced by prokaryotic expression and the corresponding antibody was prepared for protein concentration determination.RNA interference was used to knockdown BpMOSPD2 expression in the mudskipper MO/Mφ,and the knockdown efficiency was confirmed by quantitative real-time polymerase chain reaction(qRT-PCR)and western blotting.Knockdown of BpMOSPD2 significantly inhibited BpLEAP-2-induced chemotaxis of mudskipper MO/Mφ and BpLEAP-2-induced bacterial killing activity.Furthermore,knockdown of BpMOSPD2 inhibited the effect of BpLEAP-2 on mRNA expression levels of BpIL-10, BpTNFa, BpIL-1β, and BpTGFβ in MO/M0. In general, BpMOSPD2 directly interacted with BpLEAP-2, and mediated the effects of BpLEAP-2 on chemotaxis and activation of mudskipper MO/Mφ. This is the first identification of MOSPD2 as a receptor for LEAP-2.