Absence of MHC class Ⅱ molecules promotes natural killer cells activation in mice

The development and immune recognition of natural killer (NK) cell are regulated critically by major histocompatibility complex (MHC) class I molecules. However, it remains unclear whether the function of NK cells is regulated by MHC class II molecules. To test this, we monitored the development, phenotype and function of NK cells by using MHC class II deficient (H2^-/-) mice. The numbers and development of NK cells keep unaltered in H2^-/- mice, compared with those in wide type (H2^+/+) mice. A part of Ly49 family receptors on NK cells are down-regulated both in mRNA and protein expression in absence of MHC class II molecules. Furthermore, NK cells obtained from H2^-/- mice exhibit more expression of CD69 and IFN-γ after cross-linking with NK1.1. Also, the cytotoxicity against tumor cell lines of NK cells from H2^-/- mice was increased significantly. Taken together, our study indicates that the absence of MHC class II molecules promotes the activation and function of NK cells in mice.