The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with important roles in xenobiotic metabolism and other immune responses. Shinde et al. now report that the AHR maintains immune tolerance to apoptotic cells by regulating the expression of the anti-inflammatory cytokine IL-10.

Credit: S. Bradbrook/Macmillan Publishers Limited

The authors initially assessed AHR activation in phagocytes co-cultured with apoptotic cells. They found that Cyp1a1 and Cyp1b1 mRNAs (which encode the AHR-induced cytochromes P4501A1 and P450B1) were upregulated in wild-type — but not AHR-deficient — bone marrow-derived macrophages (BMDMs) and dendritic cells cultured with apoptotic thymocytes. Imaging studies showed that exposure of phagocytes to apoptotic cells caused AHR to translocate from the cytoplasm to the nucleus. AHR activation required phagocytosis of apoptotic cells and was blocked by the inhibition of caspases or masking of phosphatidylserine in apoptotic cells.

Gene- and protein-expression analyses showed that AHR activation in BMDMs exposed to apoptotic cells was associated with the downregulation of pro-inflammatory cytokines (such as IL-1β, IL-6, IL-12p40 and TNF) and the upregulation of IL-10 and molecules associated with a regulatory phenotype. BMDMs cultured with apoptotic cells showed increased association of AHR with AHR-responsive elements in the Il10 promoter, and blocking IL-10 in these cultures increased BMDM production of pro-inflammatory cytokines. Therefore, AHR activation seems to polarize macrophages to an immunoregulatory phenotype via the induction of IL-10. Pre-treatment of apoptotic cells with DNase prevented AHR activation; furthermore, Toll-like receptor 9 (TLR9)-deficient BMDMs did not show nuclear accumulation of AHR following apoptotic cell uptake. Therefore, activation of AHR in BMDMs in this system appears to require TLR9-mediated recognition of DNA from apoptotic cells.

The authors next examined whether AHR suppresses immune responses to apoptotic cells in vivo. Intravenous delivery of apoptotic cells to wild-type mice led to increased expression of IL-10 and TGFβ in splenic lysates. By contrast, delivery of apoptotic cells to mice with a myeloid cell-specific AHR deficiency led to the induction of IL-6 and IL-12p40, but not IL-10 or TGFβ. In line with this, splenic macrophages and dendritic cells from wild-type mice showed induction of Cyp1a1 and IL10 following intravenous injection of apoptotic cells, whereas in mice with a myeloid cell-specific AHR deficiency, these populations failed to upregulate Cyp1a1 and Il10 and instead expressed Il6 and Il12b.

The uptake of apoptotic cells by tissue-resident phagocytes is important for limiting autoimmunity, and AHR blockade in mouse models of lupus was found to increase serum levels of pro-inflammatory cytokines and autoantibodies. Treatment of mice with established lupus with an AHR agonist decreased autoantibody levels and disease. Furthermore, ageing mice with a myeloid cell-specific AHR deficiency developed spontaneous systemic autoimmunity.

Finally, the authors showed that human macrophages cultured with apoptotic cells also upregulate CYP1A1 and IL-10 in an AHR-dependent manner. Notably, they identified an AHR transcriptional signature in patients with systemic lupus erythematosus (SLE), who had higher expression of AHR, CYP1A1 and IL10 in peripheral blood mononuclear cells than healthy individuals. Finally, microparticles from the plasma of patients with SLE (but not from healthy individuals) were shown to have membrane-bound DNA and induced CYP1A1 and IL10 expression in macrophages via AHR activation. Therefore, apoptotic cells and DNA-containing microparticles can activate AHR in human macrophages to promote an anti-inflammatory response.

The authors propose that the AHR-related transcriptional signature seen in patients with SLE may reflect the upregulation of this anti-inflammatory pathway in an attempt to restrict tissue pathology. Their findings suggest that the AHR pathway could be targeted to treat systemic autoimmune diseases.