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

Abstract

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.

Fig. 1

Schematic representation of the main endocytic pathways. The figure shows the main endocytic pathways and intracellular routes that a receptor can undergo. In the first step of endocytosis a receptor can undergo clathrin-dependent (clathrin-coated pit formation) or clathrin-independent internalization (micro- or macropinocytosis and caveolae-dependent internalization) or a pathway independent from both (GEEC). Following internalization, receptors carried by different vesicles can enter the early (or sorting) endosomes from where they are segregated into separate trafficking itineraries. A receptor can recycle back to the plasma membrane directly from this compartment or after being sorted into the recycling endosome or continue to traffic to the lysosome for the final degradation passing through the endosomal carrier vesicles/multivesicular bodies (ECV/MVBs) and late endosomes. Recycling can also occur at level of late endosomes and lysosomes. Specific markers for each compartment are indicated in red. Recycling pathways are indicated with red lines (The color version can be viewed online in the electronic version of the manuscript.)

Fig. 2

CD94/NKG2A intracellular trafficking. The endocytic route of CD94/NKG2A is schematically represented. We have no direct evidence of receptor association with membrane ruffles. The CD94/NKG2A positive vesicles that are formed near the plasma membrane are morphologically similar to macropinosomes in size. However the CD94/NKG2A internalization process does not require actin, dynamin, or Arf6; it is insensitive to PI3K inhibition and PKC stimulation and requires the involvement of Rac1. Thus we arbitrarily termed it a macropinocytic-like process. Upon 15–20 min of endocytosis, the receptor colocalizes with EEA1 and Rab5 and only minimally with Rab4. Co-localization with Rab7 or Rab11 and Lamp-1 or Lamp-2 is never observed indicating that the receptor is recycled back to the plasma membrane without entering the Rab7⁺ or Rab11⁺ compartments (red line) (The color version can be viewed online in the electronic version of the manuscript.)