In This Issue

Intestinal mucositis, the inflammation and ulceration of the intestine, is a serious and common side effect of chemotherapy for which there is currently no effective treatment. To shed insight on the development and resolution of this affliction, Frank et al. (p. 1983) investigated the relationship between innate immune signaling pathways and xenobiotic metabolism, disruption of which can lead to increased cellular toxicity and intestinal pathology, in the development of intestinal mucositis. Using a model of methotrexate (MTX)-induced intestinal mucositis, the authors determined that TLR2 deficiency resulted in exacerbated intestinal pathology. Gene expression analysis of jejunal tissues from MTX-treated TLR2 KO mice compared with wild-type (WT) tissue identified reduced expression of the drug transporter Abcb 1a (ABCB1/MDR1), which codes for transmembrane p-glycoprotein (p-gp), an ATP-dependent efflux transporter that prevents intracellular accumulation of xenobiotics and chemicals. TLR2 activation stimulated p-gp synthesis in human and murine CD11b⁺ myeloid cell populations and reduced chemotherapy-associated toxicity. Additionally, the authors found in both human tissue and murine models that chemotherapy-induced small intestinal damage could be alleviated by supplementation with TLR2 ligands. Together, these findings suggest that TLR2 activation modulates xenobiotic pathways that prevent toxicity-induced inflammatory reactions, identifying a novel link between xenobiotic metabolic pathways and innate immune signaling.

Interleukin-10–producing regulatory B cells (Bregs) are important for regulating inflammatory immune responses in autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis. However, a lack of phenotypic markers for Bregs has impeded functional studies of these cells. Tim-1, a transmembrane glycoprotein receptor that interacts with phosphatidylserine, is expressed on a small portion of B cells, which may correspond to the Breg population, prompting Xiao et al. (p. 1602) to explore the importance of Tim-1 in Breg function. Using Tim-1–deficient (Tim-1^−/−) and Tim-1 mutant mice, the authors showed that Tim-1 is required for optimal IL-10 production in Bregs. In T cell differentiation coculture experiments, Tim-1 deficiency in B cells promoted differentiation of Th1 and Th17 but not regulatory T cells, suggesting that Tim-1 deficiency promoted inflammatory responses. The authors tested this concept in an in vivo EAE model and found that Tim-1 deficiency in B cells resulted in increased clinical pathology and Th1/Th17 cytokine production and reduced frequency of Tregs and IL-10 in the CNS. Furthermore, treatment of mice before EAE induction with Tim-1⁺ B cells or apoptotic cells, which express the phosphatidylserine ligand for Tim-1, resulted in decreased EAE symptoms. Together, these data indicate that Tim-1 may be a reliable marker for IL-10–producing B cells and suggest that Tim-1 is integral to regulating IL-10 production in Bregs.

The transcription factor c-MYC is overexpressed in many malignancies, including Burkitt lymphoma (BL), and has been shown to impair the immunogenicity of BL tumors. To better understand the effects of the c-myc oncogene on B cell lymphomas, God et al. (p. 1434) investigated c-MYC involvement in HLA class II–mediated Ag presentation in BL. Transformed human B cell lines overexpressing c-MYC were impaired in their ability to process and present Ags to CD4⁺ T cells, via both the exogenous and endogenous pathways, relative to transformed cell lines expressing lower levels of c-MYC. These c-MYC^high cells had reduced expression of HLA-DM and GILT proteins, but had normal surface expression of HLA class II molecules. Downregulation of c-MYC or treatment of these B cells with a c-MYC inhibitor partially restored their class II–mediated Ag presentation to CD4⁺ T cells, through both the exogenous and endogenous pathways. Overexpression of c-MYC also downregulated expression of a 47 kDa enolase-like acid labile protein, which supports HLA class II peptide display, while upregulating CLIP expression. Upregulated CLIP, a decreased HLA-DM/HLA-DO ratio, and impaired stimulation of CD4⁺ T cells were also observed in primary BL cells when compared with EBV-transformed B cells from healthy controls. These findings identify a mechanism by which c-MYC impairs immune recognition of tumors, which could be targeted in future treatments for BL and other malignancies.

Individuals with systemic lupus erythematosus (SLE) can develop premature atherosclerosis through mechanisms that are only beginning to be understood. In murine models of SLE, deficiency in the transcription factor IFN regulatory factor 5 (IRF5) ameliorates disease, but its involvement in atherosclerosis is not known. Watkins et al. (p. 1467) hypothesized that, in the gld.apoE^−/− SLE mouse model, IRF5 deficiency would reduce both lupus and atherosclerosis. Although Irf5^−/− mice indeed developed less severe lupus, they surprisingly also developed more severe atherosclerosis and hyperlipidemia than littermate controls. Irf5^−/− or Irf5^+/− mice also showed general metabolic dysregulation resulting in increased adiposity, adipose tissue inflammation, insulin resistance, glucose intolerance, and hepatic steatosis. Experiments with bone marrow chimeras revealed that IRF5 expression in bone marrow–derived cells was important for lupus development, whereas expression in both hematopoietic and nonhematopoietic cells regulated atherosclerosis, with non–bone marrow–derived cells serving a major role in increased adiposity and hyperlipidemia. IRF5 was found to mediate production of the antiatherogenic cytokine IL-10 downstream of TLR7 and TLR9 stimulation in macrophages, dendritic cells, and B cells, suggesting one mechanism by which IRF5 could protect against atherosclerosis. This study urges caution in targeting IRF5 for SLE treatment due to the previously unanticipated danger of triggering atherosclerosis and metabolic dysregulation.

During pregnancy, tolerance must be established to protect the semiallogeneic fetus from immune destruction. Within the decidua, immune cells including macrophages that resemble M2 (“homeostatic”) macrophages and regulatory T cells (Tregs) are important for the maintenance of pregnancy. In this issue, Svensson-Arvelund et al. (p. 1534) identified a role for the human fetal placenta in inducing the differentiation of these decidual regulatory cells and, consequently, in establishing tolerance during pregnancy. Monocytes or CD4⁺ T cells from nonpregnant women differentiated into M2 macrophages or Tregs, respectively, when cultured with conditioned medium from first trimester placental explants (PE CM). Tregs developing in response to factors in PE CM had a phenotype like Tregs found in early human decidua and were able to suppress CD3/CD28-induced CD4⁺CD25^– T cell proliferation. CM from trophoblasts isolated from first trimester human placenta also promoted differentiation of homeostatic macrophages and Tregs, suggesting that trophoblasts could be the source of soluble factors in PE CM that influenced cell differentiation. These factors included M-CSF and IL-10 for decidual macrophage differentiation and TGF-β, IL-10, and TRAIL for Treg differentiation. These data present a mechanism by which the fetal placenta, particularly through trophoblast cytokine secretion, supports tolerance and homeostasis during pregnancy.

The gene encoding the intracellular peptidoglycan-sensing receptor nucleotide-binding oligomerization domain 2 (NOD2) is linked to susceptibility to Crohn’s disease, and acute or chronic stimulation of NOD2 can lead to either up- or downregulation of cytokines, respectively. Zheng et al. (p. 1928) have now investigated the potential involvement of the Tyro3, Axl, and Mer receptors (TAM) in NOD2-mediated regulation of cytokine production in human macrophages. TAM were found to be important for the downregulation of proinflammatory cytokines following chronic NOD2 stimulation but did not affect cytokine production in response to acute NOD2 stimulation. Chronic NOD2 stimulation led to TAM upregulation, which required NOD2-induced IL-10 and TGF-β, and subsequent TAM-mediated cytokine downregulation involved SOCS3. Restoration of both pro- and anti-inflammatory cytokines in the absence of TAM was dependent on MAPK signaling; however, the production of anti-inflammatory cytokines required a higher level of MAPK activity than their proinflammatory counterparts, and their downregulation in response to NOD2 was independent of TAM. The importance of TAM in NOD2-mediated proinflammatory cytokine regulation was supported by analysis of Axl/Mer^−/− mice, which had increased cytokines in the gut under baseline conditions and increased proinflammatory cytokines in the serum following chronic, but not acute, NOD2 stimulation, relative to wild-type controls. These data begin to tease apart how NOD2 may affect macrophage cytokine production and thereby influence intestinal inflammation.

The induction of type-1 IFN (IFN-I), a first-line defense against viral infection, precipitates a signaling cascade that stimulates production of more than 300 IFN-stimulated genes, many of which participate in host antiviral defense. Triggering cyclic GMP-AMP synthase (cGAS), a cytosolic DNA sensor, leads to downstream activation of IFN-I; however, the mechanism behind cGAS regulation is largely unknown. To address this issue, Ma et al. (p. 1545) analyzed induction of cGAS in bone marrow macrophages (BMMs) subjected to a variety of pattern recognition receptor (PRR) and IFN-I stimuli and found that cGAS was significantly upregulated by administration of IFN-I or PRR ligands that trigger IFN-I–dependent pathways, such as LPS or poly(deoxyadenylic-deoxythymidylic) acid (polydA:dT). Stimulation of Ifnar^−/− BMMs with polydA:dT or HSV-1 resulted in reduced IFN-β production and cGAS expression, suggesting that DNA-triggered IFN responses require signaling through the IFN-αR. Further studies treating BMMs deficient for key innate immune signaling molecules with PRR ligands revealed that these pathways could stimulate cGAS expression downstream of IFN-I production. Using luciferase reporter assays and chromatin immunoprecipitation sequencing analyses, the authors identified two adjacent IFN stimulation response elements (ISRE) in the cGAS promoter likely to mediate IFN-I induction of cGAS. Finally, the authors determined that cGAS induction downstream of IFN-I could reciprocally augment IFN-I production by promoting generation of this cytokine in a positive feedback loop. Together, these results suggest that cGAS is an IFN-stimulated gene and point to a role for this PRR in the positive feedback regulation of the IFN-I response.