IL-15 synergizes with iMC to enhance NK cell proliferation. (A) Schematic retroviral vector design iRC9-IL15-ΔCD19-iMC encoding iRC9, human IL-15, truncated human CD19, and iMC with the transgenes separated by T2A and P2A ribosomal skipping sequences. (B) IL-15 production from supernatant of nontransduced (NT) or iRC9-ΔCD19-iMC– or iRC9-IL15-ΔCD19-iMC–modified NK cells cocultured with or without K562 targets at an effector/target ratio of 3:1 for 72 hours in the absence or presence of 1 nM of rimiducid (Rim) was determined by enzyme-linked immunosorbent assay. Paired Student t test was used to compare the indicated groups. (C-D) NT or iRC9-ΔCD19–, iRC9-ΔCD19-iMC–, or iRC9-IL15-ΔCD19-iMC–modified NKs treated with or without 1 nM of Rim for 6 days. Transgene expression as indicated by CD19⁺ percentage. Paired Student t test was used to compare indicated groups. (E) CD19 transgene expression in iRC9-ΔCD19–, iRC9-ΔCD19-iMC–, or iRC9-IL15-ΔCD19-iMC–modified NKs was determined with or without pretreatment with 1 nM of Rim. Paired Student t test was used to compare indicated groups. *P < .05, **P < .01.