CMV induces rapid NK cell maturation in HSCT recipients.

Natural killer (NK) cells are important effectors of innate immune responses against virally infected cells and tumors. Since NK cells are the first lymphocyte population to recover after hematopoietic stem cell transplantation, they are thought to play a crucial role in early immunity after transplantation against infections. In individuals experiencing HCMV reactivation after transplantation, NK cells rapidly achieved a fully differentiated stage of maturation, characterized by a KIR(+)NKG2A(-)NKG2C(+)CD57(+)p75/AIRM1(-) surface phenotype. Patients who never had HCMV reactivation maintained an immature NK phenotype for a long time. Thus, HCMV reactivation, by providing stimulatory signals to maturing NK cells, could be beneficial rather than detrimental. HCMV infection has been reported to induce a persistent reconfiguration of the NK-cell compartment not only in immunocompromised patients but also in healthy individuals, the hallmark of which is the expansion of an NK-cell subset displaying high surface levels of the CD94/NKG2C receptor. Moreover, as suggested by studies in mice, NK cells developing after CMV reactivation could contain "memory" or "long-lived" NK cells that could be exploited for therapeutic purposes.