UL16 binding proteins.
According to present concepts, innate immunity plays an important role in tumor surveillance and immune modulation. The state of NK cells depends on the balance between inhibitory and activating signals from corresponding receptors. As one of the activating receptors, NKG2D recognises some self ligands such as MICA/B in human and Rae1 in mice, which is dissimilar to those toll-like receptors that recognise some pathogen-derived ligands. NKG2D is expressed not only on NK cells, but on gammadelta T cells, CD8+ alphabeta T cells in normal individuals and CD4+ alphabeta T cells in rheumatoid arthritis patients and plays a different role on respective cells. Whereas NKG2D can only function as a costimulatory receptor on CD8+ alphabeta T cells under the domination of alphabeta TCR in spite of a deficiency of costimulatory molecule CD28, NKG2D can directly activate NK cells even in the presence of inhibitory signals from MHC-I and corresponding receptor complexes. Experiments in mice have identified that alternative splicing produces two distinct NKG2D polypeptides that associate differentially with the DAP10 and DAP12 signaling subunits and that differential expression of these isoforms and of signaling proteins determines whether NKG2D only functions as a costimulatory receptor in the adaptive immune system (CD8+ T cells) or as both a primary recognition unit and a costimulatory receptor in the innate immune system (natural killer cells and macrophages). This review summarizes the research achievements in a new ligand family (UL16 binding proteins) of NKG2D in human and shows the possible prospects of ULBP function and application.