MCT-1 transport activity and lactic acid influx in CTLs. (A) Expression of MCT-1 and MCT-2 in CTL lines and unstimulated human CD8⁺ T cells (from the same donor, respectively) analyzed by Western blot. Hela cells were used as a positive control. (B) Blocking of MCT transport activity by treating CTLs for 5 hours with 3 or 9 mM CINN or 20 mM lactic acid. Intracellular cytokines IL-2 and IFN-γ were determined by FACS. Results are normalized to untreated CTLs. (C) Uptake of ¹³C-labeled lactate by CTLs. CTLs were incubated for 30 minutes with 20 mM external ¹³C-lactate in the presence or absence of HCl (*P < .05; n = 4). Endogenous lactate and the uptake of exogenous lactate and was determined in the cell lysates by mass spectrometry. (D) Decrease of the intracellular pH in CTLs 30 minutes after addition of lactic acid was determined flow cytometrically with SNARF-1. Shown are means ± SEM from 3 independent experiments with different donors.