NKG2D performs two functions in invariant NKT cells: Direct TCR‐independent activation of NK‐like cytolysis and co‐stimulation of activation by CD1d

Abstract

Invariant NKT cells are important in the activation and regulation of immune responses. They can also function as CD1d‐restricted killer cells. However, the role of activating innate NK‐cell receptors expressed on NKT cells in triggering cytolytic function is poorly characterized. Here, we initially confirmed that the cellular stress‐ligand receptor NKG2D is expressed on CD4⁻ NKT cells, whereas most CD4⁺ NKT cells lack this receptor. Interestingly, NKG2D⁺ NKT cells frequently expressed perforin, and both NKG2D and perforin localized at the site of contact with NKG2D ligand‐expressing target cells. CD4⁻ NKT cells degranulated in response to NKG2D engagement in a redirected activation assay independent of stimulation via their invariant TCR. NKT cells killed P815 cells coated with anti‐NKG2D mAb and CD1d‐negative K562 tumor target cells in an NKG2D‐dependent manner. Furthermore, NKG2D engagement co‐stimulated TCR‐mediated NKT‐cell activation in response to endogenous CD1d‐presented ligands or suboptimal levels of anti‐CD3 triggering. These data indicate that the CD4⁻ subset of human NKT cells can mediate direct lysis of target cells via NKG2D engagement independent of CD1d, and that NKG2D also functions as a co‐stimulatory receptor in these cells. NKG2D thus plays both a direct and a co‐stimulatory role in the activation of NKT cells.