Identification of a TGF-β1 homologue in the large yellow croaker (Larimichthys crocea) revealed its role in regulation of immune response

Mammalian TGF-β1 plays a crucial role in maintaining immune tolerance and inhibiting autoimmunity. Although several TGF-β1 homologues have been identified in teleosts, their immunosuppressive functions remain unclear. Herein, we identified and characterized Lc TGF-β1, a TGF-β1 homologue from the large yellow croaker ( Larimichthys crocea ). Lc TGF-β1 possesses several features characteristic of the TGF-β superfamily, including an RGD integrin binding site, an RSKR tetrabasic cut site, and nine conserved cysteines in the mature peptide. The Lc TGF-β1 gene was constitutively expressed in all examined tissues, with the highest expression in the spleen, which is rich in immune cells. Lc TGF-β1 was significantly upregulated in the head kidney and spleen after Vibrio alginolyticus stimulation. Lc TGF-β1 was also expressed in primary head kidney lymphocytes (PKLs), monocytes/macrophages (MO/Mφs), and granulocytes (PKGs), and was significantly upregulated in the MO/Mφs after lipopolysaccharide (LPS) stimulation. Recombinant Lc TGF-β1 protein (r Lc TGF-β1) significantly suppressed reactive oxygen species (ROS) and nitrogen oxide (NO) production in LPS-activated head kidney MO/Mφs. Furthermore, in these MO/Mφs, r Lc TGF-β1 strongly inhibited the expression of pro-inflammatory cytokine ( TNF -α) and inducible NO synthase ( iNOS ), while it induced anti-inflammatory cytokines ( TGF-β1 and IL-10 ), arginase-1 , and CD206 . The phagocytic activity of LPS-activated MO/Mφs was also increased by r Lc TGF-β1. Finally, r Lc TGF-β1 significantly inhibited the proliferation of T cells (ZAP70 + cells) and downregulated T-cell activation genes ( CD3ζ, TCRα, and ZAP70 ), T-cell marker genes ( CD4 and CD8 ), and IL-2 in PKLs. These results thus provide strong evidence that Lc TGF-β1 promotes the alternative activation of head kidney MO/Mφs and inhibits T-cell proliferation in PKLs, highlighting its role in the regulation of immune response in large yellow croaker.