CMV drives clonal expansion of NKG2C+ NK cells expressing self-specific KIRs in chronic hepatitis patients

Natural killer (NK) cells are affected by infection with human cytomegalovirus (HCMV) manifested by increased expression of the HLA-E binding activating receptor NKG2C. We here show that HCMV seropositivity was associated with a profound expansion of NKG2C+CD56dim NK cells in patients with chronic hepatitis B virus (HBV) or hepatitis C virus (HCV) infection. Multi-color flow cytometry revealed that the expanded NKG2C+CD56dim NK cells displayed a highly differentiated phenotype, expressed high amounts of granzyme B and exhibited polyfunctional responses (CD107a, IFN-?, and TNF-a) to stimulation with antibody-coated as well as HLA-E expressing target cells but not when stimulated with IL-12/IL-18. More importantly, NKG2C+CD56dim NK cells had a clonal expression pattern of inhibitory killer cell immunoglobulin-like receptors (KIRs) specific for self-HLA class I molecules, with predominant usage of KIR2DL2/3. KIR engagement dampened NKG2C-mediated activation suggesting that such biased expression of self-specific KIRs may preserve self-tolerance and limit immune-pathology during viral infection. Together, these findings shed new light on how the human NK-cell compartment adjusts to HCMV infection resulting in clonal expansion and differentiation of educated and polyfunctional NK cells.