Terms of Disengagement

Science Signaling  01 Mar 2011:
Vol. 4, Issue 162, pp. ec59
DOI: 10.1126/scisignal.4162ec59

Similar to T cells and B cells, natural killer (NK) lymphocytes form complex, dynamic macromolecular structures at the sites of ligand for their receptors. However, NK cells have both excitatory receptors that associate with intracellular proteins containing immunotyrosine-based activation motifs (ITAMs), which are responsible for triggering cell killing of infected or damaged tissue, and inhibitory receptors with immunotyrosine-based inhibitory motifs (ITIMs), which are responsible for preventing the cytolysis of healthy tissue by recognizing class I major histocompatibility complexes (MHCs) bound to peptides derived from self proteins. Abeyweera et al. investigated the effect of activation of the ITIM-containing receptor KIR2DL2 in a human NK cell line (NKL) with peptides derived from α-importin bound to the MHC HLA-Cw3 with either a Ser or a Tyr in the eighth position of the peptide. HLA-Cw3(Ser) inhibited cytokine secretion, degranulation, cell spreading, and calcium influx of NKL cells plated on activating bilayers, whereas HLA-Cw3(Tyr) did not. The authors then designed a caged form of HLA-Cw3(Ser), which they called HLA-Cw3(cage), and showed that photostimulated release of this KIR2DL2 ligand from the bilayer induced the rapid formation of KIR2DL2 microclusters into an annular zone around the site of contact, remodeling of the actin cytoskeleton, and triggered the retraction of the cell from the bilayer. Photostimulation of cells expressing an ITIM-deficient mutant form of KIR2DL2 or photostimulation in the presence of an inhibitor of the phosphatases SHP1 and SHP2 resulted in formation of the microclusters but not cell retraction or actin reorganization. Activation of the ITAM receptor NKG2D resulted in the formation of two distinct clusters—a relatively immobile cluster near the center of the site of staining suggested that the mobile peripheral clusters were the site of signaling. Photostimulation of HLA-Cw3(cage) in cells plated on activating bilayers inhibited formation of the peripheral NKG2D clusters but had no effect on the immobile pool. Although when present in the activating bilayers, HLA-Cw3(Ser) inhibited the initiation of calcium signals, photostimulated release of HLA-Cw3(cage) failed to inhibit ongoing calcium signaling. Thus, the localized disengagement triggered by inhibitory signaling may be a mechanism to prevent cytolysis at one site of contact while allowing calcium signaling and cytolysis at other sites of contact.

T. P. Abeyweera, E. Merino, M. Huse, Inhibitory signaling blocks activating receptor clustering and induces cytoskeletal retraction in natural killer cells. J. Cell Biol. 192, 675–690 (2011). [Abstract] [Full Text]