Hepatic clearance of apoptotic lymphocytes: simply removal of waste cells?

The in situ liver recognition of apoptotic lymphocytes was studied by using different sources of lymphocytes (i.e. human, rat and mouse) and animal models (i.e. rat and mouse). Lymphocytes were induced to apoptosis using 10–2M cycloheximide for up to 24 hours; three types of apoptosing lymphocytes, corresponding to different stages in the apoptotic process, were described: type 1 or early apoptosis, type 2 or mature apoptosis and type 3 or late/necrotic apoptosis. When livers were in situ injected with apoptotic lymphocytes enriched for type 1 (early), 2 (mature) or 3 (late/necrotic) apoptosis, they recognized and internalized apoptosing cells, with an efficiency directly dependent on the stage of the apoptotic process. The highest recognition rate, which was, in all cases, mediated by galactose- and mannose-specific receptors, was obtained with homologous apoptotic cells (i.e. rat lymphocytes and rat liver). Moreover, the drastically reduced efficiency of recognition of human or mouse apoptotic lymphocytes when injected into rat liver, suggested the involvement also of species-specific antigens.