The liver as a site of T-cell apoptosis: graveyard, or krilling field?

The liver is a site at which apoptotic CD8(+) cells accumulate during the c learance phase of peripheral immune responses. Normal mouse liver contains an unusual mixture of lymphocytes in which natural killer (NK) and NK-T cel ls are abundant and apoptotic T cells are present, and we interpret these c ell populations as, respectively, agents and targets of an intrahepatic T-c ell trapping and killing mechanism. In support of this idea, direct perfusi on of activated lymphocyte populations through the normal liver results in the selective retention of activated CD8(+) T cells. T cells trapped in thi s manner undergo apoptosis in the liver. This mechanism could explain the i mportance of the liver in oral tolerance, the phenomenon of tolerance induc ed by portal vein infusion of antigenic cells, the tolerance to allogeneic liver allografts, and the persistence of some liver pathogens including hep atitis C.