HCQ inhibits antigen-receptor–dependent intracellular calcium mobilization and alters the size of intracellular calcium stores.

Jurkat T cells or Ramos B cells were pretreated with varying concentrations of HCQ for 24 hours and then loaded with the calcium-sensitive dye calcium green-1. Changes in intracellular Ca⁺⁺ concentration (fluorescence) versus time (seconds) were monitored by bulk spectrofluorometry. (A) Jurkat T cells were activated with the clonotypic anti-TCR mAb C305 (as indicated by the arrow) followed by addition of 1 μmol/L ionomycin (arrowhead). (B) Ramos B cells were activated by IgM crosslinking (arrow) followed by the addition of 1 μmol/L ionomycin (arrowhead). (C) Jurkat cells were stimulated with 1 μmol/L thapsigargin, as indicated by arrow, in the presence of 1.8 mmol/L EGTA. Residual calcium response was evaluated by addition of 1 μmol/L ionomycin (arrowhead). Results are representative of more than 5 separate experiments.