Comparative analysis of human NK cell activation induced by NKG2D and natural cytotoxicity receptors

NKG2D and natural cytotoxicity receptors (NCR) are essential recognition structures that mediate NK cell activation. NKG2D and NCR signaling is achieved through membrane association with signaling adaptors. The adaptors that associate with NCR--such as CD3 zeta, FcR gamma and KARAP/DAP12--bear intracytoplasmic immunoreceptor tyrosine-based activation motifs that activate Syk protein tyrosine kinases. Human NKG2D associates with the DAP10 transmembrane adaptor, which bears a YxxM motif and activates the phosphatidylinositol 3-kinase pathway. In the mouse, a short NKG2D-S isoform, generated by Nkg2d alternative splicing, can associate with either DAP10 or KARAP/DAP12. Here, we report that neither short human NKG2D alternative transcripts nor NKG2D association with KARAP/DAP12 was detected in activated human NK cells. Despite these results, NK cell triggering by both recombinant soluble NKG2D ligands (MICA and ULBP-1) and anti-NCR cross-linking antibodies induced similar CD25 expression, NK cell proliferation and cytokine production. In contrast, NKG2D triggering by anti-NKG2D antibodies did not lead to any detectable activation signals. These data thus show that target recognition via NKG2D or NCR triggers all aspects of NK activation, and pave the way for further dissection of the signaling pathways induced by NK cell recognition of ULBP-1 and MICA.