Abstract: Disclosed are antagonists of PC3-secreted microprotein (PSMP) and use of the antagonists for treatment of liver, lung, or kidney fibrosis, including various diseases or disorders associated with liver, lung, or kidney fibrosis such as, e.g., non-alcoholic fatty liver disease (NAFLD), alcoholic liver disease (ALD), primary sclerosing cholangitis (PSC), primary biliary cholangitis (PBC), drug-induced lung injury, acute kidney injury (AKI), chronic kidney disease (CKD), lupus nephritis, IgA nephropathy, and membranous glomerulonephritis. Also disclosed are PSMP antagonists and their use for treatment of graft-versus-host disease (GVHD) and systemic lupus erythematosus (SLE). Suitable PSMP antagonists for use in disease treatment include PSMP-binding proteins such as, for example, neutralizing anti-PSMP antibodies.