Characterization of TK-specific immune responses. (A) Ex vivo IFN-γ ELISPOT assay and (B) IFN-γ ELISA-based semiquantitative recall assays on multiple groups (10⁵) of PBMCs collected before and after treatment. (C,D) HLA-I restriction of the anti-TK T cells in patients CIP-5 (C) and CIP-21 (D). Microcultures containing TK-specific effectors were challenged with Cos-7 cells transfected with available HLA-I alleles together with HSV-TK cDNAs. Supernatants were then assayed for IFN-γ release (*P < .01). Error bars represent the SD of experimental replicates. (E) Microcultures from the postfourth blood sample of patient CIP-25, a patient expressing the HLA-B7 allele, recognized autologous untransduced (UT)–GMLs pulsed with the TK_279-288 epitope previously described.¹⁴  (F) Limiting dilution analysis estimating the frequency of anti-TK T cells in patients CIP-23 and CIP-25.