The entry of extracellular calcium in leukocytes mediates several cellular processes; however, unlike in excitable tissues, the underlying molecular mechanisms are poorly defined. In this paper we provide phenotypical and biochemical evidence that peripheral blood-derived human dendritic cells express dihydropyridine-sensitive calcium channels. Exposure to the dihydropyridine drug nifedipine, which binds L-type calcium channels blocking calcium influx, prevents two dentritic cell functions that are dependent on extracellular calcium entry: apoptotic body engulfment and interleukin-12 production induced by cross-linking of the surface lectin NKRPIA. It is known that exogenous human immunodeficiency virus, type 1 Tat affects several Ca2+-dependent immune cell responses. Here we demonstrate that Tat inhibits apoptotic body engulfment and interleukin-12 production by blocking extracellular calcium influx. This inhibition is prevented by the calcium channel agonist dihydropyridine derivative Bay K 8644, suggesting the involvement of L-type calcium channels. This hypothesis is further supported by the observation that Tat and dihydropyridine drugs compete for binding to dendritic cells. Taken together, these findings indicate that exogenous Tat exerts its inhibitory effects on dendritic cells by blocking dihydropyridine- sensitive L-type calcium channels.
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