Btk Keeps Autoreactive B Cells in Check See article p. 2922

A New Genetic Model of Aicardi–Goutières Syndrome See article p. 3016

Location Matters for Macrophage Renewal See article p. 3028

13 Mobilizes Neutrophils See article p. 3038

IL-4 Receptor Signaling Drives Inflammation in Obesity See article p. 3081

iNKT Cells Drive Liver Transplant Failure See article p. 3107

Quieting the Nervous System to Bolster Cancer Vaccines See article p. 3131

In this Top Read, Hinkle et al. (p. 3131) sought to delineate the impact of the sympathetic nervous system (SNS) on the function of distinct APC subsets within the heterogeneous population of bone marrow–derived dendritic cells (DCs). In response to norepinephrine treatment, CD115+ monocyte-derived macrophage (moMΦs), which abundantly expressed β2-adrenergic receptors (β2ARs), responded to β2AR activation by upregulating the T cell–activating cytokines IL-10 and IL-12p70. However, the CD115 DCs neither responded to norepinephrine nor expressed detectable β2AR mRNA or protein. To reconcile the fact that DC cells lacking β2ARs can nevertheless be inhibited by the SNS, which negatively impacts tumor immune responses, the authors administered the nonspecific β-blocker propranolol to quiet SNS signaling in the context of an adjuvanted peptide cancer vaccine. Administration of propranolol not only permitted DC maturation near the vaccine injection site, but also enhanced the frequency of beneficial CD8+ intratumor T cells, while decreasing the population of immunosuppressive myeloid cells. Moreover, concurrent propranolol and cancer vaccine administration in mice resulted in significantly decreased tumor size in vivo, while vaccine alone was less effective. Together, these data suggest that therapeutic inhibition of SNS signaling can enhance the efficacy of cancer vaccines.

The specific role of the G protein subunit Gα13 in guiding neutrophil migration towards the site of inflammation is investigated in this Top Read by Chang et al. (p. 3038). To test whether Gα13 is required for neutrophil migration, bone-marrow cells derived from control and Gα13 conditional knockout mice were exposed to a bacterial chemoattractant. Compared to isolated neutrophils from the control mice, Gα13-depleted neutrophils were significantly impaired in the speed of chemotaxis, but not their precise direction. To test whether Gα13-dependent neutrophil motility might be driven by interactions between neutrophil β2 integrins and typical integrin ligands, neutrophil movement was tracked as a function of the presence or absence of ICAM1, fibrinogen or BSA. Among these, ICAM1 best enhanced neutrophil chemotaxis. However, it was not the initial adhesion between neutrophil integrins and ICAM1 that was dependent on Gα13, rather it was the way the neutrophils spread toward the chemoattractant after adhesion to ICAM1. The dependence of neutrophils on Gα13 for migration in vitro was corroborated using in vivo assays in which neutrophil migration was tracked through blood vessels of control and knockout mice to sites of inflammation in the peritoneum or the lung tissues. Finally, the use of a potentially therapeutic peptide that competes for integrin–ICAM1 interaction was shown in mice to inhibit neutrophil progression in the heart upon ischemic/reperfusion injury. Together, these data are consistent with Gα13 supporting “outside-in” signaling by mobilizing neutrophils and suggests a targeted therapy to reduce the pathological involvement of neutrophils at sites of inflammation.

Aicardi–Goutières syndrome (AGS) is a rare, monogenic disorder that causes inflammatory encephalopathy and severe mental and physical impediments. AGS is characterized by overactivated type I IFN signaling and is caused by mutations in DNA- and RNA-processing proteins. Historically, there has been no small animal model that recapitulates the brain pathology of human AGS patients. In this Top Read by Inoue et al. (p. 3016), the authors describe the first mouse model of AGS that recapitulates human encephalopathy, which harbors a point mutation (K948N) located in a deaminase domain of the Adar1 gene, an RNA editing enzyme. As a result of the mutation, the mice show splenic and white matter abnormalities, elevated type I IFN levels, as well as growth retardation at an early age. The pathologies in Adar1K948N/K948N mice were ameliorated by either the deletion of the cytosolic sensor MDA5 or expression of active ADAR1 p150, an isoform of ADAR1. Collectively, this report provides a new mouse model of AGS caused by a mutation in Adar1 that will help researchers elucidate more about the brain pathologies of this monogenic human disease.

Liver transplant failure occurs when activated cytotoxic T cells attack allografted tissue. In this Top Read, Zimmerer and Ringwald et al. (p. 3107) use a mouse model to delve into the mechanisms that govern how hepatocyte alloantigens activate cytotoxic CD8+ T cells. The activation of CD8+ T cells in mouse hepatocytes was dependent on both CD4+ T cells and on invariant NKT (iNKT) cells; the involvement of iNKT cells, however, was most critical. The authors then focused on iNKT cell-dependent, CD8+ T cell–mediated cytotoxicity, which is more prominent in liver tissues than in the spleen. They discovered that a novel subset of dual chemokine receptor (CXCR3+CCR4+) positive CD8+ T cells were most abundant in the liver, and whose activation was iNKT cell dependent. The alloprimed CXCR3+CCR4+CD8+ T cells were shown to be exceptionally cytotoxic, producing elevated levels of perforin, granzyme, TNF-α and IFN-γ, and these cells thrived through a mechanism that was, unexpectedly, independent of CD4+ T cells. Indeed, allograft rejection and hepatic cytotoxicity were significantly reduced in iNKT knockout mice, and reintroduction of iNKT cells reversed these effects. Together, these data suggest that the liver’s relative abundance of iNKT cells are to blame for poor long-term liver transplant prognosis. A treatment based on specific targeting of iNKT cells in the liver is suggested to improve transplant success.

In this Top Read, Ackermann et al. (p. 3081) investigated the molecular mechanisms underlying inflammation from diet-induced obesity. The authors probed the potential impact of IL-4 receptor (IL-4R)–mediated signaling on the myeloid populations in adipose tissue (AT), which are known to accumulate in diet-induced obesity, and correlate with the onset of type 2 diabetes. The authors created a myeloid-specific knockout mouse model (Il4raΔmyel) to disrupt IL-4R–mediated signaling and used this model to inspect the resulting equilibrium of AT macrophage subtypes. Comparing obese wild type mice to obese Il4raΔmyel mice showed that overall macrophage abundance was IL-4Rα–independent. However, the balance of proinflammatory M1 macrophages versus anti-inflammatory M2 macrophages in the AT was shown to be IL-4Rα–dependent, with the number of M1 cells decreased, and M2 cells unaffected by IL-4Rα deletion. IL-4R activation in the bone marrow resulted in increased macrophage Cd11c gene expression and coincident increase in M1 macrophages in the AT. Together, these data suggest that accumulation of M1 cells in AT is supported by IL-4R–mediated signaling on myeloid cells, which maintains proportions of AT M1 macrophages, promoting inflammation.

Diverse populations of tissue-resident macrophages (TRMΦ) are critical for maintaining homeostasis and barrier defense. Many TRMΦ subsets have been described, including large and small peritoneal MΦ. However, the relationships of these distinct tissue-specific MΦ to hematopoietic stem cells have not been investigated thoroughly. In this Top Read by Eddins et al. (p. 3028) the authors used transplantation and in vivo lineage tracing to elucidate the origin and regenerative precursors of peritoneal and brain TRMΦ. Interestingly, both large and small peritoneal MΦ are derived from yolk sac precursors but are replenished by long-term adult hematopoietic stem cells. Conversely, brain-resident microglia cannot be replenished by fetal or adult long-term hematopoietic stem cells. This research demonstrates that tissue-specific MΦ have distinct origins, unique regeneration requirements, and complex relationships with long-term hematopoietic stem cells in vivo.

Burton’s tyrosine kinase (Btk) is critical for proper function of the adaptive immune system to protect against pathogens and to preserve self from autoreactivity. Lifelong Btk deficiency is known to prevent autoimmune disease, presumably through elimination of autoreactive B cells. In this Top Read, Nyhoff et al. (p. 2922) further support this hypothesis by monitoring the effects of timed Btk deletion on anti-insulin B cells at various stages of maturation. The authors first demonstrate that Btk-mediated signaling is unnecessary for survival of already mature autoreactive B cells, even in the presence of a competing polyclonal repertoire. At transitional checkpoints, however, autoreactive B cells survive only when Btk is present, and progression to the marginal zone is favored only by those cells that retain Btk. The lack of Btk renders autoreactive B cells aberrantly responsive to BCR crosslinking using anti-IgM, and are unable to upregulate Bcl-xL, rendering them less resistant to apoptosis. Despite this dysfunction, Btk deficiency does not impair the ability of autoreactive B cells to present autoantigen and, thus, to activate cognate T cells, which would result in the tissue damage characteristic of autoimmune disease. Together, the new findings suggest that treatment of autoimmune disease by inhibiting Btk may require long term administration to prevent an ongoing emergence of functional autoreactive B cells.