Hybrid bioartificial liver: establishing a reversibly immortalized human hepatocyte line and developing a bioartificial liver for practical use.

Recently, much attention has been attracted by a novel therapy for liver failure using a hybrid bioartificial liver (BAL) support device that incorporates living liver cells. Researchers in various fields have considered the following cells for potential use in BALs: human embryonic stem (ES) cells; somatic stem cells; differentiated tissue cells; and cells derived from tissues of different animal species, particularly from the pig. With their pluripotency, human ES cells are extremely useful, and many research groups are joining the race to develop BALs. One such effort involves the breeding of transgenic pigs to overcome interspecies barriers. Recent reports suggest, however, that porcine endogenous retrovirus may infect human tissues, and clinical application of transgenic pigs has become a controversial issue. To avoid such risks, we recommend that researchers adopt a reversible immortalization system that uses the Cre-loxP site-specific recombination reaction targeting human hepatocytes in their final differentiated state. This system has allowed us to establish a safe human hepatocyte line that is capable of differentiation at low cost and on a large scale. We are also designing and developing an artificial liver module made of a combination of hollow fibers and nonwoven fabrics. The objective of this review article is to report our therapeutic strategy, which aims at the earliest possible introduction of the treatment of liver failure using BALs.