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...