Aim2 Deficiency Stimulates the Expression of IFN-Inducible Ifi202, a Lupus Susceptibility Murine Gene within the Nba2 Autoimmune Susceptibility Locus

Abstract

Murine Aim2 and p202 proteins (encoded by the Aim2 and Ifi202 genes) are members of the IFN-inducible p200 protein family. Both proteins can sense dsDNA in the cytoplasm. However, upon sensing dsDNA, only the Aim2 protein through its pyrin domain can form an inflammasome to activate caspase-1 and induce cell death. Given that the p202 protein has been predicted to inhibit the activation of caspase-1 by the Aim2 protein and that increased levels of the p202 protein in female mice of certain strains are associated with lupus susceptibility, we compared the expression of Aim2 and Ifi202 genes between Aim2-deficient and age-matched wild-type mice. We found that the Aim2 deficiency in immune cells stimulated the expression of Ifi202 gene. The increased levels of the p202 protein in cells were associated with increases in the expression of IFN-β, STAT1, and IFN-inducible genes. Moreover, after knockdown of Aim2 expression in the murine macrophage cell line J774.A1, IFN-β treatment of cells robustly increased STAT1 protein levels (compared with those of control cells), increased the activating phosphorylation of STAT1 on Tyr-701, and stimulated the activity of an IFN-responsive reporter. Notably, the expression of Aim2 in non-lupus-prone (C57BL/6 and B6.Nba2-C) and lupus-prone (B6.Nba2-ABC) splenic cells and in a murine macrophage cell line that overexpressed p202 protein was found to be inversely correlated with Ifi202. Collectively, our observations demonstrate an inverse correlation between Aim2 and p202 expressions. We predict that defects in Aim2 expression within immune cells contribute to increased susceptibility to lupus.