Central memory CD8⁺ T cells do not activate Zap70 in response to low antigen dose stimulation. (A) Naive (red) and central memory (blue) resting OT-1 T cells were stained for intracellular expression of Src family and Syk family kinases. Data are representative of more than 3 independent experiments. (B and C) Splenocytes containing congenic naive (red) and memory (blue) OT-1 cells were stimulated by addition of either high or low dose SIINFEKL peptide, and then stained for total phosphorylated Src family kinase (B), phosphorylated Zap70/Syk (C), or isotype control (gray). Histograms are representative of staining at the peak of the response in at least 3 independent experiments performed in duplicate. Expression over time as determined by mean fluorescence intensity is graphed for each antigen dose (n = 6). Data were analyzed by Student’s t test (*, P < 0.05; **, P < 0.01). Error bars represent SEM for experiments performed in duplicate.