PD-1 is differentially expressed on primary human NK cells as a function of MM disease activity. (A) By flow cytometry, PD-1 expression was measured in CD56⁺CD3⁻ NK cells isolated from healthy donors (n = 5 donors). At baseline, virtually no expression was observed (1.4% ± 0.35% [SD] of NK cells per healthy donor sample); however, after IL-2 (150 IU/mL) stimulation for 48 hours, PD-1 expression became evident on an average of 16% (± 6%) of healthy donor NK cells, P = .001, a 12-fold increase (± 5-fold) in PD-1 expression. A representative sample is shown on left, and MRFI (mean ± SD) of pre–IL-2 baseline and post–IL2 PD-1 expression is shown on right. (B) In contrast to healthy donors, PD-1 is observed on freshly isolated NK cells from patients with MM (n = 5), a representative example is shown on left and MRFI (mean ± SD) of healthy donors and freshly isolated patient NK cells on right.