Genetically modified Lactococcus lactis secreting interleukin 10 provides a therapeutic approach for inflammatory bowel disease. However, the release of such genetically modified organisms through clinical use raises safety concerns. In an effort to address this problem, we replaced the thymidylate synthase gene thyA of L. lactis with a synthetic human IL10 gene. This thyA⁻hIL10⁺L. lactis strain produced human IL-10 (hIL-10), and when deprived of thymidine or thymine, its viability dropped by several orders of magnitude, essentially preventing its accumulation in the environment. The biological containment system and the bacterium's capacity to secrete hIL-10 were validated in vivo in pigs. Our approach is a promising one for transgene containment because, in the unlikely event that the engineered L. lactis strain acquired an intact thyA gene from a donor such as L. lactis subsp. cremoris, the transgene would be eliminated from the genome.