Validation of T-lymphocyte function in cells obtained from FV vector–treated SCID-X1 canines. (A) pSTAT3 was measured in PBMCs isolated from animals R2258 and R2260 (nonmobilized; 485 days posttreatment) or from a normal littermate control and cultured in vitro with no, low, or high levels of IL-21. pSTAT3 signal is gated from CD3⁺ cells. (B) Proliferative response to phytohemagglutinin of PBMCs isolated from the same animals and same time point shown in panel A. Cell proliferation was determined by dilution of CellTracker dye.