On the function of pit cells, the liver-specific natural killer cells.

Pit cells are liver-specific natural killer (NK) cells and belong to the group of sinusoidal cells, together with Kupffer, endothelial, and fat-storing cells. Pit cells are lymphoid cells containing specific granules, classifying them also as large granular lymphocytes (LGL). They probably originate from the bone marrow, circulate in the blood, and marginate in the liver, where they develop into pit cells by lowering their density and increasing the number of granules, which decrease in size. Pit cells remain in the liver about 2 weeks and are dependent on Kupffer cells. Pit cells also proliferate locally, when stimulated with interleukin-2, biological response modifiers, or other agents. Pit cells adhere to tumor target cells during killing. They possess a high level of natural cytotoxicity against a variety of tumor cell lines, which is comparable to the cytotoxicity level of lymphokine-activated killer (LAK) cells. Tumor cell killing is synergistically enhanced when pit cells attack tumor cells together with Kupffer cells. Further investigations are needed to clarify the mechanisms of pit cell cytotoxicity and the role of these cells in killing virus-infected cells, such as during viral hepatitis.