NK cell education via nonclassical MHC and non-MHC ligands

Natural killer （NK） cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I （MHC-I） molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the ＇Confining Model＇, discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.