Synergistic effect of IFN-gamma and human cytomegalovirus protein UL40 in the HLA-E-dependent protection from NK cell-mediated cytotoxicity

Human CMV (HCMV) has evolved several strategies to evade the immune system of the infected host. Here, we investigated the role of the HCMV-encoded pr otein UL40 in the modulation of NK cell lysis. UL40 carries in its leader s equence a nonameric peptide similar to that found in many HILA class I mole cules leader sequences. This peptide up-regulates the expression of HLA-E, the ligand for the NK cell inhibitory receptor CD94/NKG2A. The UL40-encoded HLA-E-binding peptide was present in all HCMV clinical (4636, 13B, 109B, 3 C) and laboratory (AD169) strains analyzed. However, transfection of UL40 i n different cell lines (293T, 721.221, K562) did not consistently confer pr otection from NK lysis (as measured using NKL and the newly generated NK li ne Nishi), despite a moderate up-regulation of HILA-E. Interestingly, combi ned transfection and treatment with IFN-gamma increased the inhibitory effe ct, via an HLA-E- and CD94/NKG2A-dependent mechanism. Although cells transf ected with UL40 derived from either AD169 or 3C showed protection from NK c ell lysis, infection of fibroblasts with the viruses resulted in a strong i nhibition only with the clinical strain 3C. Our results suggest that UL40 a nd IFN-gamma -dependent up-regulation of HILA-E is only one possible mechan ism to avoid NK cell recognition of HCMV infected cells.