Glu496 to Ala Polymorphism in the P2X7 Receptor Impairs ATP-Induced IL-1β Release from Human Monocytes

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Abstract
Priming of monocytes with LPS produces large quantities of intracellular, biologically inactive IL-1β that can be processed and released by subsequent activation of the P2X7 receptor by extracellular ATP. We examined whether a loss-of-function polymorphism of the human P2X7 receptor (Glu496Ala) impairs this process. Both ATP-induced ethidium+ uptake and ATP-induced shedding of L-selectin (CD62L) were nearly absent in monocytes from four subjects homozygous for Glu496Ala confirming that this polymorphism impairs P2X7 function. The level of ATP-induced IL-1β released in 2 h from LPS-activated whole blood from homozygous subjects was 50% of that from wild-type samples. A more marked defect in IL-1β release was observed from LPS-activated monocytes of homozygous subjects which was only 22% of that released from wild-type monocytes after a 30-min incubation with ATP. However, after a 60-min incubation with ATP, the amount of IL-1β released from homozygous monocytes was 70% of that released from wild-type monocytes. Incubation of monocytes of either genotype with nigericin resulted in a similar release of IL-1β. Western blotting demonstrated that ATP induced the release of mature 17-kDa IL-1β from monocytes, and confirmed that this process was impaired in homozygous monocytes. Finally, ATP-induced 86Rb+ efflux was 9-fold lower from homozygous monocytes than from wild-type monocytes. The results indicate that ATP-induced release of IL-1β is slower in monocytes from subjects homozygous for the Glu496Ala polymorphism in the P2X7 receptor and that this reduced rate of IL-1β release is associated with a lower ATP-induced K+ efflux.