Mudskipper interleukin-34 modulates the functions of monocytes/macrophages via the colony-stimulating factor-1 receptor 1

Interleukin-34(IL-34) is a novel cytokine that plays an important role in innate immunity and inflammatory processes by binding to the colonystimulating factor-1 receptor(CSF-1R). However,information on the function of IL-34 in fish remains limited. In the present study, we identified an IL-34 homologfrommudskippers(Boleophthalmus pectinirostris). In silico analysis showed that the mudskipper IL-34(BpIL-34) was similar to other known IL-34 variants in sequence and structure and was most closely related to an orange-spotted grouper(Epinephelus coioides) homolog. BpIL-34 transcripts were constitutively expressed in various tissues, with the highest level of expression found in the brain. Edwardsiella tarda infection significantly up-regulated the mRNA expression of BpIL-34 in the mudskipper tissues. The recombinant mature BpIL-34 peptide(rBpIL-34) was purified and used to produce anti-rBpIL-34 IgG. Western blot analysis combined with PNGase F digestion revealed that nativeBpIL-34 inmonocytes/macrophages(MOs/MФs) was N-glycosylated. In vitro, rBpIL-34 treatmentenhancedthephagocytoticand bactericidal activity of mudskipper MOs/MФs, as well as the mRNA expression of pro-inflammatory cytokines like tumor necrosis factor α(BpTNF-α) and BpIL-1β in these cells. Furthermore, the knockdown of mudskipper CSF-1R1(BpCSF-1R1), but not mudskipper BpCSF-1R2, significantly inhibited the rBpIL-34-mediated enhanced effect on MO/MФfunction. In conclusion, our results indicate that mudskipper BpIL-34 modulates the functions of MOs/MФs via BpCSF-1R1.