Human NK cells are alerted to induction of p53 in cancer cells by upregulation of the NKG2D ligands ULBP1 and ULBP2

Natural killer (NK) cells are immune cells sensing and eliminating foreign, stressed, transformed, and senescent cells through specialized surface receptors, such as NKG2D, that interacts with several virus- or stressinducible ligands, including ULBP1 and -2, which are expressed on target cell surfaces. For example, induction of DNA damage or cellular senescence pathways in tumor cells led to upregulation of NKG2D ligands that activate NK cells. Although, both pathways activate p53, the relationship of p53 activation to upregulation of NKG2D ligands has not been addressed. In this study, we report that induction of wild-type p53, but not mutant p53, strongly upregulated mRNA and cell surface expression of ULBP1 and -2, whereas expression of other NK cell ligands was not affected. We defined intronic p53-responsive elements in these two novel p53 target genes. Coculture of wild-type p53-induced human tumor cells with primary human NK cells enhanced NKG2Ddependent degranulation and IFN-g production by NK cells. Accordingly, treatment of certain wild-type p53-expressing tumor cell lines with the p53-reactivating small molecular compound RITA resulted in upregulation of ULBP2 mRNA and cell surface protein expression. Taken together, our findings define the involvement of p53 in the regulation of specific NKG2D ligands that enhance NK cell-mediated target recognition. One implication of our work is that activating p53 after adoptive transfer of NK cells might constitute an effective combinatorial strategy of NK cell-based immunochemotherapy in cancers in which wild-type p53 function is preserved.