The amino acid sensor GCN2 inhibits inflammatory responses to apoptotic cells promoting tolerance and suppressing systemic autoimmunity

Abstract

Efficient apoptotic cell clearance and induction of immunologic tolerance is a critical mechanism preventing autoimmunity and associated pathology. Our laboratory has reported that apoptotic cells induce tolerance by a mechanism dependent on the tryptophan catabolizing enzyme indoleamine 2,3 dioxygenase 1 (IDO1) in splenic macrophages (MΦ). The metabolic-stress sensing protein kinase GCN2 is a primary downstream effector of IDO1; thus, we tested its role in apoptotic cell-driven immune suppression. In vitro, expression of IDO1 in MΦs significantly enhanced apoptotic celldriven IL-10 and suppressed IL-12 production in a GCN2-dependent mechanism. Suppression of IL-12 protein production was due to attenuation of IL-12 mRNA association with polyribosomes inhibiting translation while IL-10 mRNA association with polyribosomes was not affected. In vivo, apoptotic cell challenge drove a rapid, GCN2-dependent stress response in splenic MΦs with increased IL-10 and TGF-β production, whereas myeloid-specific deletion of GCN2 abrogated regulatory cytokine production with provocation of inflammatory T-cell responses to apoptotic cell antigens and failure of long-tolerance induction. Consistent with a role in prevention of apoptotic cell driven autoreactivity, myeloid deletion of GCN2 in lupus-prone mice resulted in increased immune cell activation, humoral autoimmunity, renal pathology, and mortality. In contrast, activation of GCN2 with an agonist significantly reduced anti-DNA autoantibodies and protected mice from disease. Thus, this study implicates a key role for GCN2 signals in regulating the tolerogenic response to apoptotic cells and limiting autoimmunity.immunometabolism | stress | tolerance | autoimmunity | apoptosis M ultiple lines of evidence suggest that apoptotic cells are a major source of autoantigens in the autoimmune disease systemic lupus erythematosus (SLE). This evidence includes the fact that the dominant autoantigens targeted in SLE are largely nuclear components that are exposed on apoptotic blebs (1, 2). Moreover, the body of evidence suggests that increased cell death and defective clearance are primary drivers of SLE development and progression (3).In homeostatic conditions, apoptotic cells drive suppressive innate and adaptive tolerogenic immune responses that protect against initiation of inflammatory autoimmunity. Macrophages (MΦ) are key drivers of the early innate response to apoptotic cells (4, 5), and recent work has shown that tissue-resident MΦs are responsible for initial IL-10 production, promoting regulatory adaptive immunity to apoptotic cell antigens and prevention of inflammatory autoimmunity (4, 6, 7).On a molecular level, the mechanisms driving the initial tolerogenic response of myeloid cells to apoptotic material in vivo are poorly defined, although studies suggest a direct role for inhibitory signaling via the MAPK pathway (8), and a recent report indicated that responses to cytoplasmic DNA may be a key mechanism of tolerance to self (9). Likewise, we have reported that apoptoti...

LC3-Associated Phagocytosis and Inflammation

“…The tryptophan (Trp)-metabolizing enzyme indoleamine 2,3 dioxygenase 1 (IDO1) is rapidly induced by phagocytosis of apoptotic cells, leading to consumption of tryptophan and activation of general controlled non-repressed 2 (GCN2). The activity of GCN2 is important for the production of IL-10 by macrophages following engulfment of apoptotic cells, and lupus-prone mice lacking myeloid GCN2 function develop an age-related autoimmune syndrome [114]. …”

Autophagy in kidney disease and aging: lessons from rodent models

“…Consistent with a role in prevention of apoptotic cell driven autoreactivity, myeloid deletion of general control nonderepressible 2 in lupus-prone mice resulted in increased immune cell activation, humoral autoimmunity, and renal disease. 154 Interestingly, the indoleamine 2,3-dioxygenasegeneral control nonderepressible 2 pathway in podocytes and other cells was suggested to be a critical negative feedback cascade that limits inflammatory renal injuries by inducing autophagy in a model of nephrotoxic serum nephritis. 155 Increasing kidney indoleamine 2,3-dioxygenase 1 activity or treating mice with a general control nonderepressible 2 agonist induced autophagy and protected mice from nephritic kidney damage.…”

IDO in the Tumor Microenvironment: Inflammation, Counter-Regulation, and Tolerance

“…Thus, when IDO is genetically deleted or pharmacologically inhibited, mice experience defects in creating tolerance to the fetus during allogeneic pregnancy [1-3], mucosal tolerance [4, 5], tolerance to apoptotic cells [6, 7] and other forms of acquired peripheral tolerance [8-13]. Conversely, gene-transfer or pharmacologic induction of IDO can create de novo systemic tolerance [6, 13-15]. IDO also enhances the tissue-reparative effects of inflammation while limiting the tissue-destructive effects.…”

“…In addition, IDO produces soluble factors (kynurenine and downstream metabolites) that bind and activate the aryl hydrocarbon receptor (AhR) [30]. The AhR can promote Treg cell differentiation [29, 30], and can also bias dendritic cells (DCs) and macrophages toward an immunosuppressive phenotype [6, 31-33]. GCN2 can also directly affect the phenotype of dendritic cells and macrophages [6, 33, 34].…”