Association of the p56lck protein tyrosine kinase with the FCγRIIIA/CD16 complex in human natural killer cells

The multimeric FcγRIIIA (CD16) complex is expressed on the surface of natural killer (NK) cells and is composed of a 50–70-kDa transmembrane glycoprotein Fcγ receptor (CD16), the T cell receptor (TCR)-ζ chain, and the FcεRIγ chain. Cross-linking FcγRIIIA initiates the rapid tyrosine phosphorylation of multiple substrates including the ζ, subunit and causes subsequent cell activation and antibody-dependent cellular cytotoxicity (ADCC). The subunits of the FcγRIIIA complex lack intrinsic protein tyrosine kinase (PTK) activity, suggesting that receptor-induced tyrosine phosphorylation events are mediated by a nonreceptor PTK. We report here that the human FcγRIIIA is complexed with p56^lck, a src-family PTK previously found associated with the CD4 and CD8 receptors on T cells. Upon engagement of the CD16 receptor, p56^lck is rapidly (within 30 s) and transiently phosphorylated on tyrosine residues. Several FcγRIIIA-associated proteins are identified in immune complex kinase assays including the TCR-ζ, subunit, a p70–90 ζ-associated protein (ZAP), p50a (acidic) and p50b (basic), and p56^lck. We demonstrate that the src-family protein tyrosine kinase inhibitor, herbimycin A, blocks increased intracellular calcium levels and ADCC caused by CD16 cross-linking on NK3.3 cells. Likewise cross-linking CD16 with the protein tyrosine phosphatase CD45, abrogates CD16-induced calcium mobilization. These data suggest that p56^lck is physically associated with FcγRIIIA(CD16) and functions to mediate signaling events related to the control of NK cellular cytotoxicity.