Peroxynitrite inhibits T lymphocyte activation and proliferation by promoting impairment of tyrosine phosphorylation and peroxynitrite-driven apoptotic death

Peroxynitrite (ONOO-) Is a potent oxidizing and nitrating agent produced by the reaction of nitric oxide with superoxide, It readily nitrates phenolic compounds such as tyrosine residues in proteins, and it has been demonstra ted that nitration of tyrosine residues in proteins inhibits their phosphor ylation, During immune responses, tyrosine phosphorylation of key substrate s by protein tyrosine kinases is the earliest of the intracellular signalin g pathways following activation through the TCR complex, This work was aime d to evaluate the effects of ONOO- on lymphocyte tyrosine phosphorylation, proliferation, and survival, Additionally, we studied the generation of nit rating species in vivo and in vitro during immune activation, Our results d emonstrate that ONOO-, through nitration of tyrosine residues, is able to i nhibit activation-induced protein tyrosine phosphorylation in purified lymp hocytes and prime them to undergo apoptotic cell death after PHA- or CD3-me diated activation but not upon phorbol ester-mediated stimulation. We also provide evidence indicating that peroxynitrite is produced during in vitro immune activation, mainly by cells of the monocyte/macrophage lineage. Furt hermore, immunohistochemical studies demonstrate the in vivo generation of nitrating species in human lymph nodes undergoing mild to strong immune act ivation. Our results point to a physiological role for ONOO- as a down-modu lator of immune responses and also as key mediator in cellular and tissue i njury associated with chronic activation of the immune system.