Neonatal, moribund and undead T cells: the role of the liver in T cell development.

T lymphocytes may be isolated from the mouse liver, and these cells have been interpreted as immature T cells in an extrathymic development pathway. However, the evidence in favor of this hypothesis is indirect, and there is some evidence that is contradictory. Most significantly, the genes RAG-1 and RAG-2, which are necessary for T cell receptor gene recombination to occur, are not expressed in liver T cells. A subset of liver T cells are undergoing apoptosis, which is the basis of an alternative model in which the liver is a site of peripheral T cell deletion. This model also provides an explanation for the accumulation of abnormal T cells in mice with the lpr mutation.