Hepatic NK, NKT, and T Cells

The liver's unique location between the gastrointestinal tract and peripheral lymphoid organs and its fenestrated endothelium allow contact with many antigenic substances. These consist of dietary proteins transported from the gut via the portal vein, products of intrahepatic metabolism, and bacterial and viral liver pathogens. According to the different origin of these antigens, the liver has the unique ability to induce either tolerance or inflammatory reactions (1,2) (Table 1). Furthermore, the liver can actively modulate ongoing immune reactions: the intrahepatic inflammatory infiltrate can be increased by chemotactic attraction and activation of leukocytes (3) and decreased by induction of apoptosis of activated intra-hepatic lymphocytes (4). These dual and apparently opposing functions are important to understand the mechanisms of tolerance to oral and allograft antigens and the pathogenesis of liver diseases caused by parasitic and viral pathogens. This chapter addresses the unique role of intrahepatic natural killer (NK), natural killer T (NKT), and T cells during this process.