FGL-1

Fibrinogen-like protein 1 (FGL-1) is a liver-secreted hepatokine of approximately 64 kDa composed of two fibrinogen-like domains joined by disulfide bonds, originally described for its mitogenic effect on hepatocytes and its expression in regenerating liver and liver cancers. FGL-1 came to prominence in immuno-oncology when it was identified by in vivo CRISPR screening as the major functional ligand of LAG-3 (lymphocyte activation gene-3), an inhibitory immune checkpoint receptor on exhausted T cells. FGL-1 binding to LAG-3 suppresses T-cell activation and interferon-γ secretion independent of MHC class II, constituting a tumor immune evasion axis: FGL-1 is overexpressed across many human cancers, plasma levels correlate with poor prognosis and resistance to PD-1 blockade, and genetic or antibody-mediated disruption of the FGL-1-LAG-3 interaction reinvigorates antitumor T-cell responses. FGL-1 also modulates hepatocellular carcinoma growth, liver regeneration and metabolic processes, illustrating a hepatokine repurposed as a checkpoint ligand.