Pit cells in the liver.

There is now sufficient evidence to consider the pit cells as a hepatic, intrasinusoidal population of large granular lymphocytes. They should be considered as resident sinusoidal cells of the liver, even if they depend on a continuous influx of precursor cells from the bone marrow, since they differ in several phenotypic and functional respects from their circulating (precursor) counterparts. There is growing evidence that a comparable population of activated or further differentiated, liver-specific pit cells (or LGL) is not only present in the rat but also in the human liver. Pit cells exert high spontaneous cytotoxic activity against tumor cell lines and may act as a primary defense barrier to metastasizing tumor cells and to virus infections. The total number of pit cells in the liver can be greatly increased by the administration of interleukin-2 or by a variety of biological response modifiers, which may represent a mechanism of defense against disease. Their involvement in liver pathologies deserves more attention.