ULBP1, 2, 3: novel MHC class I-related molecules that bind to human cytomegalovirus glycoprotein UL16, activate NK cells

New members of the extended MHC class I-like family were identified based o n their ability to bind human cytomegalovirus glycoprotein UL16 and/or thei r mutual homology. Soluble UL16 binding prteins (ULBP) competed with each o ther for binding to NK cells. Treatment of human and mouse NK cells with UL BP led to increased production of cytokines/chemokines, proliferation, cyto toxic activity and up-regulation of activation-associated surface molecules . The presence of ULBP during the stimulation phase of the CTL assay caused increased cytotoxic activity. Addition of soluble recombinant UL16 protein inhibited the biological activities mediated by ULBP, suggesting the exist ence of a novel mechanism utilized by CMV to evade elimination by the host immune system.