P(2Z)/P2X₇ receptor-dependent apoptosis of dendritic cells

Macrophages and thymocytes express P(2Z)/P2X₇ nucleotide receptors that bind extracellular ATP. These receptors play a role in immune development and control of microbial infections, but their presence on dendritic cells has not been reported. We investigated whether extracellular ATP could trigger P(2Z)/P2X₇ receptor-dependent apoptosis of dendritic cells. Apoptosis could be selectively triggered by tetrabasic ATP, since other purine/pyrimidine nucleotides were ineffective, and it was mimicked by the P(2Z) receptor agonist, benzoylbenzoyl ATP, and blocked by magnesium and the irreversible antagonist, oxidized ATP. RT-PCR analysis confirmed the mRNA expression of the P(2Z)/P2X₇ receptor and the absence of P2X₁. Caspase inhibitors and cycloheximide had only a partial effect on the apoptosis, suggesting that a caspase-independent mechanism may also be operative. Brief treatment with ATP led to an increase in the intracellular calcium concentration and permeabilization of the plasma membrane to Lucifer yellow, which diffused throughout the dendritic cell cytosol. Other small extracellular molecules may thus attain a similar intracellular distribution, perhaps activating endogenous proteases that contribute to initiation of apoptosis.