Lupus is a chronic inflammatory autoimmune disease influenced by multiple genetic loci including Fas Ligand (FasL) and P2X7 receptor (P2X7R). The Fas/Fas Ligand apoptotic pathway is critical for immune homeostasis and peripheral tolerance. Normal effector T lymphocytes up-regulate the transmembrane tyrosine phosphatase B220 before undergoing apoptosis. Fas-deficient MRL/lpr mice (lpr mutation) exhibit lupus and lymphoproliferative syndromes due to the massive accumulation of B220(+) CD4(-)CD8(-) (DN) T lymphocytes. The precise ontogeny of B220(+) DN T cells is unknown. B220(+) DN T lymphocytes could be derived from effector CD4(+) and CD8(+) T lymphocytes, which have not undergone activation-induced cell death due to inactivation of Fas, or from a special cell lineage. P2X7R is an extracellular ATP-gated cell membrane receptor involved in the release of proinflammatory cytokines and TNFR1/Fas-independent cell death. P2X7R also regulate early signaling events involved in T-cell activation. We show herein that MRL/lpr mice carry a P2X7R allele, which confers a high sensitivity to ATP. However, during aging, the MRL/lpr T-cell population exhibits a drastically reduced sensitivity to ATP- or NAD-mediated stimulation of P2X7R, which parallels the increase in B220(+) DN T-cell numbers in lymphoid organs. Importantly, we found that this B220(+) DN T-cell subpopulation has a defect in P2X7R-mediated responses. The few B220(+) T cells observed in normal MRL(+/+) and C57BL/6 mice are also resistant to ATP or NAD treatment. Unexpectedly, while P2X7R mRNA and proteins are present inside of B220(+) T cells, P2X7R are undetectable on the plasma membrane of these T cells. Our results prompt the conclusion that cell surface expression of B220 strongly correlates with the negative regulation of the P2X7R pathway in T cells.