Uncommon Endocytic and Trafficking Pathway of the Natural Killer Cell CD94/NKG2A Inhibitory Receptor

The CD94/NKG2A inhibitory receptor, expressed by natural killer and T cells, is constantly exposed to its HLA-E ligand expressed by surrounding cells. Ligand exposure often induces receptor downregulation. For CD94/NKG2A, this could potentiate activation receptor(s) induced responses to normal bystander cells. We investigated CD94/NKG2A endocytosis and found that it occurs by an amiloride-sensitive, Rac1-dependent macropinocytic-like process; however, it does not require clathrin, dynamin, ADP ribosylation factor-6, phosphoinositide-3 kinase or the actin cytoskeleton. Once endocytosed, CD94/NKG2A traffics to early endosomal antigen 1+, Rab5+ early endosomes. It does appear in Rab4+ early/sorting endosome, but, in the time period examined, fails to reach Rab11+ recycling or Rab7+ late endosomes or lysosome-associated membrane protein-1+ lysosomes. These results indicate that CD94/NKG2A utilizes a previously undescribed endocytic mechanism coupled with an abbreviated trafficking pattern, perhaps to insure surface expression.