Identification and bioactivity of a programmed death-ligand 1 homologue in large yellow croaker (Larimichthys crocea)

As a pivotal checkpoint molecule in immune system, programmed death-ligand 1 (PD-L1) orchestrates T cell immunity by suppressing T cell proliferation/activation, promoting apoptosis/anergy/exhaustion, and inducing regulatory T cell differentiation. Although teleosts possess a PD-L1 homologue capable of executing analogous functions, its immunological mechanisms remain largely uncharacterized. Here, we characterized a PD-L1 homologue (LcPD-L1) in a commercially significant species, Larimichthys crocea. Structural analysis reveals that it possesses conserved extracellular IgV-like and IgC-like domains with four critical cysteine residues. Phylogenetic analysis also confirmed its evolutionary relationship with vertebrate PD-L1. Under physiological conditions, LcPD-L1 exhibited constitutive expression across various tissues, particularly with high expression in mucosal tissues such as gills and skin. Besides that, pathogen challenge with Cryptocaryon irritans triggered significant up-regulation of LcPD-L1 in these tissues, suggesting it may play an important regulatory role in immune response elicited by C. irritans infection. In vitro assays revealed that recombinant LcPD-L1 protein (rLcPD-L1) inhibited T cell proliferation, decreased the expression levels of T cell activation markers (IL-2, LCK, and CD40L), and up-regulated the expression of the immune inhibitory cytokine (IL-10). Furthermore, rLcPD-L1 significantly inhibited the phosphorylation levels of MEK1/2 and ERK1/2 in the MAPK/ERK signaling pathway, as well as mTOR and 4E-BP1 in the mTOR signaling pathway, both of which are essential for T cell activation. Collectively, our findings imply that PD-L1 may inhibit T cell responses via the MAPK/ERK and mTOR pathways in teleosts, thus playing a role in pathogen elicited immune response.