IDO metabolite produced by EBV-transformed B cells inhibits surface expression of NKG2D in NK cells via the c-Jun N-terminal kinase (JNK) pathway

Natural Killer cells are known to play a major role in the innate immune response against viral infections and tumor cells. Several viruses, such as CMV, EBV and HIV-1, have acquired strategies to escape elimination by NK cells. In this study, we observed that EBV infection increased expression of IDO on B cells. To evaluate the function of IDO associated with EBV infection, we investigated whether EBV-induced IDO could modulate expression of NK cell-activation receptor, NKG2D. When NK cells were co-incubated with EBV transformed B cells, surface expression of NKG2D was significantly reduced in NK cells. Incubation with L-kynurenine, an IDO metabolite, down-modulated NKG2D expression in NK cells in a dose- and time-dependent manner. Incubation with the JNK inhibitor SP600125 also inhibited NKG2D expression in NK cells. In addition, we observed that the effect of L-kynurenine was blocked by JNK agonist, anisomycin, suggesting the involvement of the JNK pathway in the signal transduction of L-kynurenine-reduced NKG2D expression. Furthermore, IL-18 significantly reduced L-kynurenine-induced down-regulation of NKG2D expression in NK cells. Taken together, these data indicate that down-regulation of NKG2D by EBV-induced IDO metabolite provides a potential mechanism by which EBV escapes NKG2D-mediated attack by immune cells.