Pit Cells and Other Intrahepatic Lymphocytes

Abstract

Morphological and phenotypical data indicate that liver sinusoids contain a very large and heterogeneous population of resident lymphocytes which comprise liver-specific natural killer (NK) cells, and different subpopulations of T cells (Jonsson et al. 1997; Winnock et al. 1995). It has been suggested that the term “liver-associated lymphocytes” (LAL), which encompasses all sinusoidal lymphocytes, can be used for this type of sinusoidal cells. LAL are defined as those lymphocytes that remain in sinusoids after liver perfusion with physiological solution at physiological pressure to remove circulating hematopoietic cells (Winnock et al. 1995). However, because liver contains also numerous extrasinusoidal lymphocytes that reside in portal tracts or between parenchymal cells, terms such as “intrahepatic lymphocytes” (IHL) or “liver-resident lymphocytes” that encompass many subpopulations of liver lymphocytes have recently been introduced (Crispie and Mehal 1996). The main groups of liver lymphocytes include: (1) pit cells, i.e., large-granular lymphocytes (LGL) that functionally correspond to the natural killer (NK) cells; (2) unconventional γδT lymphocytes; (3) T cells with conventional levels of T cell receptor (αβT cells) expressing either CD4 or CD8 molecules; (4) unconventional αβT cells (TCRintIL-2Rβ+) expressing IL-2β receptor and lower (intermediate) levels of CD3 than conventional T cells; (5) TCRintlL-2Rβ+ cells that exhibit dual T cell and NK cell phenotype (NK1.1+) and function (cytotoxicity), denoted NK T cells; (6) small population of c-Kit+ cells (stem cells) lacking lineage markers; and (7) small numbers of B lymphocytes and monocytes.