Cutting edge: NKp80 uses an atypical hemi-ITAM to trigger NK cytotoxicity.

The human NK cell receptor NKp80 stimulates cytotoxicity upon engagement of its genetically linked ligand AICL. However, the mechanisms underlying NKp80-mediated signaling are unknown. In this study, we dissected NKp80 signaling using the NK cell line NK92MI. We demonstrated that NKp80, but not NKp80 mutated at tyrosine 7 (NKp80/Y7F), is tyrosine phosphorylated. Accordingly, NKp80/Y7F, but not NKp80/Y30F or NKp80/Y37F, failed to induce cytotoxicity. NKp80 phosphopeptides comprising the hemi-ITAM-like sequence surrounding tyrosine 7 bound Lck- and Syk-family kinases; accordingly, cross-linking of NKp80, but not NKp80/Y7F, induced Syk phosphorylation. Moreover, inhibition of Syk kinase, but not ZAP-70 kinase, impaired cytotoxic responses through NKp80. Atypical residues in the hemi-ITAM-like motif of NKp80 cause an altered stoichiometry of phosphorylation but did not substantially affect NK cytotoxicity. Altogether, these results show that NKp80 uses an atypical hemi-ITAM and Syk kinase to trigger cellular cytotoxicity.