The present invention relates to the treatment of an arthritic disease such as rheumatoid arthritis with a kynurenine aminotransferase, a living recombinant bacterium which has been genetically modified to express and secrete said kynurenine aminotransferase, and/or a product of said kynurenine aminotransferase which is xanthurenic acid, a derivative thereof, or any pharmaceutically acceptable salt or solvate thereof.