Background/aims: Bitter-tasting chloroquine can suppress T cell activation by inhibiting Ca(2+) signaling. However, the mechanism of inhibition remains largely unclear.
Methods: In this study, CD4(+) T cells were isolated from the thymus, and the calcium content of CD4(+) thymocytes was measured using fura-2 AM and a TILL imaging system. Pyrazole-3 (Pyr3), thapsigargin (TG), and caffeine were used to assess the effects of chloroquine on the intracellular Ca(2+) content of CD4(+) T cells.
Results: In murine CD4(+) thymocytes, chloroquine decreased the TG-triggered intracellular Ca(2+) increase in a dose-dependent manner. In the absence of chloroquine under Ca(2+)-free conditions (0 mM Ca(2+) and 0.5 mM EGTA), TG induced a transient Ca(2+) increase. After restoration of the extracellular Ca(2+) concentration to 2 mM, a dramatic Ca(2+) increase occurred. This elevation was completely blocked by chloroquine and was markedly inhibited by Pyr3, a selective antagonist of transient receptor potential C3 (TRPC3) channel and stromal interaction molecule (STIM)/Orai channel. Furthermore, the TG-induced transient Ca(2+) increase under Ca(2+)-free conditions was eliminated in the presence of chloroquine. Chloroquine also blocked the dialyzed inositol-1,4,5-trisphosphate (IP3)-induced intracellular Ca(2+) increase. However, chloroquine was not able to decrease the caffeine-induced Ca(2+) increase.
Conclusion: These data indicate that chloroquine inhibits the elevation of intracellular Ca(2+) in thymic CD4(+) T cells by inhibiting IP3 receptor-mediated Ca(2+) release from intracellular stores and TRPC3 channel-mediated and/or STIM/Orai channel-mediated Ca(2+) influx.