Endocytosis as a mechanism of regulating natural killer cell function: Unique endocytic and trafficking pathway for CD94/NKG2A

Natural killer (NK) cells are lymphocytes generally recognized as sentinels of the innate immune system due to their inherent capacity to deal with diseased (stressed) cells, including malignant and infected. This ability to recognize many potentially pathogenic situations is due to the expression of a diverse panel of activation receptors. Because NK cell activation triggers an aggressive inflammatory response, it is important to have a means of throttling this response. Hence, NK cells also express a panel of inhibitory receptors that recognize ligands expressed by "normal" cells. Little or nothing is known about the endocytosis and trafficking of NK cell receptors, which are of great relevance to understanding how NK cells maintain the appropriate balance of activating and inhibitory receptors on their cell surface. In this review, we focus on the ITIM-containing inhibitory receptor CD94/NKG2A showing that it is endocytosed by a previously undescribed macropinocytic-like process that may be related to the maintenance of its surface expression.