Liver NKT cells: an account of heterogeneity.

The liver is a rich provenance of an unconventional group of T cells called natural killer T (NKT) cells that coexpress NKR-P1, a type II membrane glycoprotein of the C-type lectin superfamily (NK1.1). Accumulating evidence suggests that NKT cells comprise a far more heterogeneous population than originally thought. In addition to 'classical' NKT cells, which are restricted by CD1d, 'nonclassical' NKT cells exist that express a diverse T-cell receptor and develop independently of CD1d. Classical and nonclassical NKT cells are most abundant in the liver, and unique molecular and cellular mechanisms control their migration and/or retention in this organ. Although evidence is accumulating that NKT cells contribute to various forms of pathology and to immune surveillance in the liver, these responses appear to involve different cell populations, including NK cells and NKT cells.