Tyrosine nitration in the human leucocyte antigen-G-binding domain of the Ig-like transcript 2 protein

Ig-like transcript 2 (ILT2) is a suppressive receptor that participates in the control of the autoimmune reactivity. This action is usually carried out in a proinflammatory microenvironment where there is a high production of free radicals and NO. However, little is known regarding whether these conditions modify the protein or affect its suppressive functions. The present study aimed to investigate the suppressive response of the ILT2 receptor under oxidative stress. To address this topic, we treated the ILT2-expressing natural killer cell line, NKL, with the NO donor N-(4-[1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazino]butyl)propane-1,3-diamine (DETA-NO). We observed that DETA-NO caused ILT2 protein nitration. MS analysis of the chimeric recombinant human ILT2-Fc protein after treatment with the peroxynitrite donor 3-(morpholinosydnonimine hydrochloride) (SIN-1) showed the nitration of Tyr35, Tyr76 and Tyr99, which are involved in human leucocyte antigen-G binding. This modification is selective because other Tyr residues were not modified by SIN-1. Recombinant human ILT2-Fc treated with SIN-1 bound a significantly higher quantity of human leucocyte antigen-G than untreated recombinant human ILT2-Fc. DETA-NO did not modify ILT2 mRNA expression or protein expression at the cell surface. Preincubation of NKL cells with DETA-NO decreased the cytotoxic lysis of K562-human leucocyte antigen-G1 cells compared to untreated NKL cells (P < 0.05) but increased cytotoxicity against K562-pcDNA cells (P < 0.05). Intracellular tyrosine phosphorylation produced after human leucocyte antigen-G binding was not affected by DETA-NO cell pretreatment. These results support the hypothesis that the ILT2human leucocyte antigen-G interaction should have a central role in tolerance under oxidative stress conditions when other tolerogenic mechanisms are inhibited.