Assessment of NK cell frequency and proliferation in MM patients. (A) PBMCs and BMMCs of MM patients or HDs were analyzed for their frequency of NK cells (gated on total viable CD56⁺ CD3⁻ lymphocytes). Box and whisker plots represent percentage of NK cells in PB (MM, n = 23; HD, n = 23) and BM (MM, n = 23; HD, n = 3) samples (***P < .001; Student t test). (B) Proliferating NK cells were assessed by Ki67 staining in PBMCs and BMMCs derived from MM patients and in PBMCs from HDs as control. Representative dot plots show the percentage of Ki67⁺ cells among total NK cells. Data derived from 23 MM patients are summarized in the right panel and compared with PBMN of HDs (n = 20). Bars represent mean values ± SEM of Ki67⁺ NK cells (**P < .01; Student t test). (C) Representative dot plots of NK cells from PB and BM of MM patients and HDs showing the percentage of CD94^lowCD56^dim NK cells. Box and whisker plots represent the frequency of CD94^lowCD56^dim NK cells in PB or BM of 23 MM patients compared with HDs (PB, n = 20; BM, n = 3), (***P < .001; Student t test). (D) Representative dot plots of NK cells from PB and BM of MM patients and HDs showing the frequency of CD94^lowNKG2A⁻ NK cells. (E) Analysis of proliferative rate of NK cells isolated from PB and BM of MM patients and from PB of HDs based on the expression of main inhibitory receptors. Top: numbers adjacent to gates indicate the frequency of CD94^lowCD56^dim Ki67⁺ NK cells. Bottom: Ki67⁺ cells within the CD94^lowCD56^dim NK cell population according to KIR (KIR2DL1/DS1 and KIR2DL2/DL3) expression. SEM, standard error of the mean.