RAET1E2, a soluble isoform of the UL16-binding protein RAET1E produced by tumor cells, inhibits NKG2D-mediated NK cytotoxicity

UL16-binding proteins (ULBPs, also termed as retinoic acid early transcripts, encoded by RAET1 genes), a family of ligands for NKG2D in humans, are frequently expressed by tumor cells and mediate cytotoxicities of natural killer (NK) cells and CD8(+) alpha beta T cells to tumor cells. ULBP1, ULBP2, ULBP3, and RAET1L link to membrane through glycosylphosphatidylinositol, whereas RAET1E and RAET1G contain transmembrane and cytoplasmic domains. Proteolytic cleavage of ULBP2 produces truncated and soluble forms that may counteract NKG2D-mediated tumor immune surveillance. In this study, we report that RAET1E can produce a soluble, 35-kDa protein (termed as RAET1E2) lacking the transmembrane region by selective splicing in tumor cells. The expressions of both RAET1E2 transcripts and protein can be found in different tumor cells and tissues. Preincubation of NK-92 cells, a human NK cell line, with culture supernatants from tumor cell lines expressing RAET1E2 or RAET1E2 gene-transfected COS-7 cells resulted in decreased expression of NKG2D on NK-92 cells. Furthermore, incubation of NK-92 cells with recombinant RAET1E2 protein also decreased the surface expression of NKG2D and resulted in marked reduction in cytotoxicities to MGC-803, HepG2, or K562 tumor cells. Taken together, our data provide strong evidence for an immune escape mechanism of tumors via alternative splicing of ULBP RNA to generate a free soluble ULBP protein, RAET1E2, that may impair NKG2D-mediated NK cell cytotoxicity to tumors.