NK cells in MM progression. (A) The plot represents the percentages of NK cells (CD56⁺CD3^–) among CD45⁺ cells expressed as median values ± standard error of the mean (SEM) in PB and BM of patients with MM among the different stages (MGUS, SMM, and NDMM). A Mann-Whitney t test was performed; ∗P < .05, ∗∗P < .005, and ∗∗∗P < .001. (B) Correlation between the percentage of total NK cells in BM of patients with MM (at different stages) and the percentage of plasma cells in the BM. Linear regression of correlation, Pearson r² = 0.38. (C) Cytotoxic activity of NK cells isolated from PB of patients with MM (n = 3 MGUS = orange, n = 5 SMM = red, n = 3 NDMM = blue). NK cells from the PB of HD were used as control. Data are expressed as percentages of killed cells at different effector:target ratios. An unpaired t test was performed, comparing the values at the 10:1 ratio, ∗P < .05 and ∗∗P < .005. (D) MFI of NKp46 ± SEM expressed on total NK cells in PB and BM of patients with MM among the different stages (MGUS, SMM, and NDMM). A 2-way analysis of variance, Sidak multiple comparisons test was performed, ∗P < .05. (E) Dot plot of the 15 most distinguishing genes expressed in NK cells, stratified in the 3 stages of patients with MM (SMM, MGUS, and NDMM). Gene expression was analyzed using the 2-sided Wilcoxon rank-sum test with Bonferroni adjustment. Ribosomal genes and mitochondrial genes were removed for clarity. The color indicates the mean gene expression levels. (F) Dot plot of selected NK-cell genes encoding for inhibitory and activating receptors, stratified in the 3 stages of patients with MM (SMM, MGUS, and NDMM). The color indicates the mean gene expression levels. (G) Representative Sunburst view of the NK-cell subsets (green circle) among leukocytes (CD45⁺ cells, blue circle). NK^dim (purple circle) and NK^bright (light green circle) among the different MM stages in PB (left) and BM (right), analyzed with Cytobank Premium. (H) Percentages of NK^dim (CD56^dim) cells (among CD45⁺ cells) in PB and BM of patients with MM among the different stages. A Mann-Whitney t test was performed; ∗P < .05. (I) Correlation between the percentage of NK^dim cells in the BM of patients with MM (at different stages) and the percentage of MM plasma cells (infiltrate). Linear regression of correlation, Pearson r² = 0.52. (J) Frequency of total CD56⁺CD16^– cells among total NK cells in PB and BM of patients with MM among the different stages. A Mann-Whitney t test was performed; ∗P < .05. (K) Frequency of CD56^dimCD16^– cells among NK^dim cells in PB and BM of patients with MM among the different stages. A Mann-Whitney t test was performed; ∗P < .05. HD, healthy donor; MFI, mean fluorescence intensity.