Ly49C Impairs NK Cell Memory in Mouse Cytomegalovirus Infection

NK cells possess inhibitory receptors that are responsible for self-MHC class I recognition; beyond their inhibitory function, accumulating evidence indicates that such receptors confer NK cell functional competence through an unclear process termed "licensing." Ly49C is the main self-specific inhibitory Ly49 receptor in H-2(b) C57BL/6 (B6) mice. We used B6 Ly49C-transgenic and B6 β2 microglobulin (β2m)-knockout Ly49C-transgenic mice to investigate the impact of licensing through this inhibitory receptor in precursor and mature NK cells. We found that self-specific inhibitory receptors affected NK cell precursor survival and proliferation at particular developmental stages in an MHC class I-dependent manner. The presence of Ly49C impacted the NK cell repertoire in a β2m-dependent manner, with reduced Ly49A(+), Ly49G2(+), and Ly49D(+) subsets, an increased DNAM-1(+) subset, and higher NKG2D expression. Licensed NK cells displayed a skewed distribution of the maturation stages, which was characterized by differential CD27 and CD11b expression, toward the mature phenotypes. We found that Ly49C-mediated licensing induced a split effect on NK cell functions, with increased cytokine-production capabilities following engagement of various activating receptors while cytotoxicity remained unchanged. Analysis of licensed NK cell functions in vivo, in a system of mouse CMV infection, indicated that licensing did not play a major role in the NK cell antiviral response during acute infection, but it strongly impaired the generation and/or persistence of memory NK cells. This study unravels multifaceted effects of licensing on NK cell populations and their functions.