IL-34 and two types of CSF-1R from snakehead (Channa argus) participate in its immune response to two bacterial challenges, promote leukocyte proliferation, and activate NF-κB and AP-1 signaling

Abstract

Interleukin-34 (IL-34), a relatively newly identified cytokine, binds to colony stimulating factor 1 receptor (CSF-1R) to activate several signaling pathways. It thus performs essential roles in immune surveillance, the inflammatory response, cell proliferation, and cell migration. In this study, the homologues of IL-34 (designated ‘CaIL-34′) and two types of CSF-1R (designated ‘CaCSF-1R1′ and ‘CaCSF-1R2′) are identified and described in the snakehead, Channa argus. Analysis of the amino acid sequence revealed that CaIL-34 has relatively conservative structural characteristics. CaCSF-1R1 and CaCSF-1R2 both contain four immunoglobulin-like domains and a tyrosine kinase domain. Their transcriptions were detected in all the tissues examined in healthy individuals, with relatively high transcription levels in immune-related tissues. The two major bacterial pathogens, Nocardia seriolae and Aeromonas schubertii, both induced their transcription in the head kidney and spleen tissues of snakeheads in vivo. The three classical pathogen-associated molecular patterns, lipopolysaccharide (LPS), lipoteichoic acid (LTA), and polyinosinic–polycytidylic acid (poly[I:C]), and six recombinant cytokines of the snakehead, recombinant tumor necrosis factor-α1 (TNF-α1), TNF-α2, IL-1β, IL-18, IL-20, and IL-21, all increased their transcription in the head kidney leukocytes (HKLs) in vitro, albeit at different levels. Recombinant CaIL-34 (rCaIL-34) protein was expressed in a prokaryotic system. After stimulation with rCaIL-34, the transcription of the IL-1β, TNF-α1, TNF-α2, and interferon-γ (IFN-γ) genes, and endogenous CaIL-34, CaCSF-1R1, and CaCSF-1R2, was enhanced in HKLs, and HKL proliferation was promoted. A subcellular localization analysis showed that CaIL-34 was exclusively located in the cytoplasm of HEK293T cells. The overexpression of CaIL-34 activated the nuclear translocation of nuclear factor kappa B (NF-κB) and activator protein 1 (AP-1) signaling, albeit to different levels. In summary, CaIL-34 and two types of CSF-1R of C. argus participate strongly in its antibacterial responses, leukocyte proliferation, and the activation of NF-κB and AP-1 signaling.