Natural Killer (NK) cells are part of the innate immune system and represent 5 to 15% of the total lymphocytes in a healthy individual. At their surface different families of inhibitory and activating receptors are expressed, where the killer immunoglobulin-like receptors (KIR) are of special interest. NK cells reactivity will depend on the binding of their receptors and their specific ligands, the major histocompatibility complex I (MHC-I). This KIR-MHC-I interaction plays an important role in transplantation. As transplanted patients are immunosuppressed, they represent an easy target to opportunistic infections, such as cytomegalovirus (CMV). Specific T-cells can clear CMV infection, but immunosuppressive drugs, which help to tolerate the graft, inhibit the activity of these T-cells. Interestingly, NK cells seem not affected by these drugs. The aim of this thesis is to investigate the anti-viral role of the NK cells during CMV infection after solid organ transplantation.