A single episode of CMV reactivation initiates long-term expansion and differentiation of the NK cell repertoire

NK cells play an important role in suppression of viral replication and are critical for effective control of persistent infections such as herpesviruses. Cytomegalovirus infection is associated with expansion of adaptive-memory NK cells with a characteristic CD16brightCD56dim NKG2C+ phenotype but the mechanisms by which this population is maintained remain uncertain. We studied NK cell reconstitution in patients undergoing haemopoietic stem cell transplantation and related this to CMV reactivation. NK cells expanded in the early post-transplant period but then remained stable in the absence of viral reactivation. However, a single episode of CMV reactivation led to a rapid and sustained 10-fold increase in NK cell number. NKG2C expression was increased on all NK subsets although the kinetics of expansion peaked at 6 months on immature CD56bright cells whilst continuing to rise on the mature CD56dim pool. Phenotypic maturation was observed by acquisition of CD57 and KIR expression. Transplantation from CMV-seropositive donors was associated with more rapid NK expansion and superior control of viral reactivation. Effective control of viral reactivation was seen when the peripheral NK cell count reached 20,000/ml. These data show that a single episode of CMV reactivation acts to reprogramme hemopoiesis to drive a sustained modulation and expansion of the NK cell pool and reveal further insight into long term regulation of the innate immune repertoire by infectious challenge. Author Summaryim to highlight where your work fits within a broader context; present the significance or possible implications of your work simply and objectively; and avoid the use of acronyms and complex terminology wherever possible. The goal is to make your findings accessible to a wide audience that includes both scientists and non-scientists Innate immune cells respond rapidly to infectious challenge and it has previously been thought that they were unable to learn from specific infecitons but provided only short term control. However, in recent years it has become apparent that some subsets of innate cells, termed adaptive natural killer (NK) cells, are permanently increased in people who have become infected with cytomegalovirus. The mechanism by which this is maintained is not known. We studied the kinetics of adaptive NK cell expansion in patients who had recently undergone stem cell (bone marrow) transplantation as CMV very often reactivates in this setting and allows study of how this affects the immune system. We find that one short term episode of viral replication led to a large and prolonged expansion of adaptive NK cells. Indeed, their levls increasd 11-fold over 10 months.