Natural killer (NK) lymphocytosis induced by Epstein-Barr virus (EBV) reactivation in monoclonal gammopathy of undetermined significance (MGUS).

We investigated the relationship between NK lymphocytosis and EBV infection in MGUS. We found that two out of 10 patients showed an increase of NK cells compared with normal controls In addition, these two patients had far higher levels of CD5LOW+ NK cells (activated NK cells) than controls. EBV DNA was detected by polymerase chain reaction in peripheral blood mononuclear cells from the two patients, although the other eight patients with MGUS and all 20 normal controls had no detectable EBV DNA. Furthermore, EBV DNA was detected in sorted CD5LOW+ NK cells. These results suggest that reactivation of EBV might be related to the increase of NK cells, particularly CD5LOW+ NK cells in some patients with MGUS.