Lenalidomide and dexamethasone therapy in MM patients increased NK cell number, but their NK cell function is impaired. (A) MM patient (n = 25) NK cell frequency (left panel) and absolute numbers per milliliter (right panel) of PBMCs were calculated. (B) Patient NK cytotoxicity was assessed against a NK-only sensitive cell line K562 (n = 17; left panel) and a MM cell line U266 (n = 13; right panel) at an effector-to-target ratio of 50:1. (C) The number of patients with decreased versus unchanged NK killing of K562 over the course of therapy. Healthy donor PBMCs were included as controls (n = 10).