IDO1 and infection: While infection with Epstein–Barr virus (EBV) normally causes a self-limited polyclonal lymphoproliferation, it was shown recently that EBV-transformed B-cells express elevated levels of IDO causing Trp degradation to Kyn, which – in turn – suppressed the expression of the activating receptor NK group 2, member D (NKG2D) receptor on the surface of NK cells (63).