NKG2D ligands: unconventional MHC class I-like molecules exploited by viruses and cancer

Our best teachers in revealing the importance of immune pathways are viruses and cancers that have subverted the most prominent pathways to escape from immune recognition. Viruses and cancer impair antigen presentation by classical MHC class I to escape adaptive immunity. The activating receptor NKG2D and its MHC class I-like ligands are other recently defined innate and adaptive immune pathways exploited by viruses and cancer. This review discusses recent advances in the understanding of how NKG2D, expressed on innate immune cells including natural killer cells, gammadelta(+) T cells and macrophages, and adaptive immune cells such as CD8(+) T cells, recognize stress-induced, MHC class I-like, self-ligands. Moreover, we describe how viruses and cancer have developed strategies to evade this recognition pathway.