Mast-cell–deficient mice display an impaired CTL response after transcutaneous peptide immunization with imiquimod (TCI). Mice were treated with peptide + imiquimod or peptide + vehicle once daily on 2 consecutive days. (A) After 6 days, peripheral blood was stained for peptide-specific CD8⁺ T cells using a SIINFEKL-specific tetramer. The depicted numbers represent the percentages of tetramer-positive cells within the CD8⁺ populations. (B) Seven days after the first immunization, cytolytic activity against SIINFEKL-loaded CFSE^low or non–peptide-loaded CFSE^high syngeneic splenocytes was evaluated after 20 hours of in vivo killing; target cells had been injected intravenously at day 6. The depicted numbers correspond to the percentages of peptide-specific lysis. (C) Splenocytes were restimulated for 4 hours in the presence or absence (data not shown) of SIINFEKL and brefeldin. The production of IFN-γ ex vivo was then analyzed by intracellular FACS staining. The numbers shown indicate the percentages of cells expressing IFN-γ within the CD8⁺ population. (D) On days 6 and 8, peripheral blood was stained for SIINFEKL-specific CTL using tetramer. All depicted results are representative of 2 independent experiments using 5 mice per group. *P < .01 versus Kit^+/+ immunized with peptide + imiquimod.