Hydroxychloroquine inhibits calcium signals in T cells: a new mechanism to explain its immunomodulatory properties

Hydroxychloroquine (HCQ), a lysosomotropic amine, is an immunosuppressive agent presently being evaluated in bone marrow transplant patients to treat graft-versus-host disease. While its immunosuppressive properties have been attributed primarily to its ability to interfere with antigen processing, recent reports demonstrate HCQ also blocks T-cell activation in vitro. To more precisely define the T cell inhibitory effects of HCQ, the authors evaluated T-cell antigen receptor (TCR) signaling events in a T-cell line pretreated with HCQ, In a concentration-dependent manner, HCQ inhibited anti-TCR-induced up-regulation of CD69 expression, a distal TCR signaling event. Proximal TCR signals, including inductive protein tyrosine phosphorylation, tyrosine phosphorylation of phospholipase C gamma 1, and total inositol phosphate production, were unaffected by HCQ. Strikingly, anti-TCR-crosslinking-induced calcium mobilization was significantly inhibited by HCQ, particularly at the high est concentrations tested (100 mu mol/L) in both T-cell lines and primary T cells, HCQ, in a dose-dependent fashion, also reduced a B cell antigen receptor calcium signal, indicating this effect may be a general property of HCQ, Inhibition of the calcium signal correlated directly with a reduction in the size of thapsigargin-sensitive intracellular calcium stores in HCQ-treated cells. Together, these findings suggest that disruption of TCR crosslinking-dependent calcium signaling provides an additional mechanism to explain the immunomodulatory properties of HCQ. (C) 2000 by The American Society of Hematology.