Elimination of NK cells with the iRC9 safety switch. (A) Nontransduced (NT) NK cells or iRC9-ΔCD19–, iRC9-ΔCD19-iMC–, or iRC9-IL15-ΔCD19-iMC–modified NK cells were administrated with temsirolimus (Tem) at concentrations 0, 0.1, 1, and 10 nM for 4 hours. Annexin V staining was assessed by flow cytometry. (B) Tem at increasing concentrations was administrated for 24 hours. 7AAD permeability was assessed by flow cytometry. Cells were first gated as CD56⁺CD19⁺ for transduced NK cells and CD56⁺ for NT NK cells. (C-G) Nine NSG mice engrafted with iRC9-IL15-ΔCD19-iMC–modified NK cells were randomly divided into 2 groups. Four mice received 1 mg/kg of Tem IP, whereas 5 mice had the same volume of vehicle (Veh) administered. (C-D) Before injection and 24 hours later, BLI was determined for NK presence in vivo. (E-G) Mice were then euthanized, and spleens were analyzed for the presence of NK cells by flow cytometry. Human cells were identified as hCD45⁺mCD45⁻ populations. Transduced NKs were identified as hCD45⁺mCD45⁻hCD56⁺hCD19⁺ populations. Student t test was used for comparisons. ***P < .001.