Jennifer K. Bando

Eosinophils Sustain Adipose Alternatively Activated Macrophages Associated with Glucose Homeostasis

Regulation of adipose tissue macrophages by eosinophils reveals an unexpected role for eosinophils in metabolic homeostasis. Eosinophils are associated with helminth immunity and allergy, often in conjunction with alternatively activated macrophages (AAMs). Adipose tissue AAMs are necessary to maintain glucose homeostasis and are induced by the cytokine interleukin-4 (IL-4). Here, we show that eosinophils are the major IL-4–expressing cells in white adipose tissues of mice, and, in their absence, AAMs are greatly attenuated. Eosinophils migrate into adipose tissue by an integrin-dependent process and reconstitute AAMs through an IL-4– or IL-13–dependent process. Mice fed a high-fat diet develop increased body fat, impaired glucose tolerance, and insulin resistance in the absence of eosinophils, and helminth-induced adipose tissue eosinophilia enhances glucose tolerance. Our results suggest that eosinophils play an unexpected role in metabolic homeostasis through maintenance of adipose AAMs.

Divergent expression patterns of IL-4 and IL-13 define unique functions in allergic immunity

Interleukin 4 (IL-4) and IL-13 are critical for responses to parasitic helminthes. We used genetically engineered reporter mice to assess the temporal and spatial production of these cytokines in vivo. In lymph nodes, IL-4, but not IL-13, was made by follicular helper T cells (TFH cells). In contrast, tissue type 2 helper T cells (TH2 cells) produced both cytokines. There was also divergent production of IL-4 and IL-13 among cells of the innate immune system, whereby basophils produced IL-4, whereas innate helper type 2 cells (Ih2 cells) produced IL-13. IL-13 production by TH2 and Ih2 cells was dependent on the transcription factor GATA-3, which was present in large amounts in these cells, and in contrast to the small amount of GATA-3 in TFH cells and basophils. The distinct localization and cellular expression of IL-4 and IL-13 explains their unique roles during allergic immunity.

Genetic analysis of basophil function in vivo.

Contributions by basophils to allergic and helminth immunity remain incompletely defined. Using sensitive interleukin 4 (Il4) reporter alleles, we demonstrate here that basophil IL-4 production occurs by a CD4+ T cell–dependent process restricted to the peripheral tissues affected. We genetically marked and achieved specific deletion of basophils and found that basophils did not mediate T helper type 2 (TH2) priming in vivo. Two-photon imaging confirmed that basophils did not interact with antigen-specific T cells in lymph nodes but engaged in prolonged serial interactions with T cells in lung tissues. Although targeted deletion of IL-4 and IL-13 in either CD4+ T cells or basophils had a minimal effect on worm clearance, deletion from both lineages demonstrated a nonredundant role for basophil cytokines in primary helminth immunity.

Traumatic brain injury induces macrophage subsets in the brain

Traumatic brain injury (TBI) elicits innate inflammatory responses that can lead to secondary brain injury. To better understand the mechanisms involved in TBI‐induced inflammation, we examined the nature of macrophages responding to TBI in mice. In this model, brain macrophages were increased >20‐fold the day after injury and >77‐fold 4 days after injury in the ipsilateral hemisphere compared with sham controls. TBI macrophage subsets were identified by using a reporter mouse strain (YARG) that expresses eYFP from an internal ribosome entry site (IRES) inserted at the 3′ end of the gene for arginase‐1 (Arg1), a hallmark of alternatively activated (M2) macrophages. One day after TBI, 21 ± 1.5% of ipsilateral brain macrophages expressed relatively high levels of Arg1 as detected by yellow fluorescent protein, and this subpopulation declined thereafter. Arg1+ cells localized with macrophages near the TBI lesion. Gene expression analysis of sorted Arg1+ and Arg1− brain macrophages revealed that both populations had profiles that included features of conventional M2 macrophages and classically activated (M1) macrophages. The Arg1+ cells differed from Arg1− cells in multiple aspects, most notably in their chemokine repertoires. Thus, the macrophage response to TBI initially involves heterogeneous polarization toward at least two major subsets.

Identification and distribution of developing innate lymphoid cells in the fetal mouse intestine

Fetal lymphoid tissue inducer (LTi) cells are required for lymph node and Peyers patch (PP) organogenesis, but where these specialized group 3 innate lymphoid cells (ILC3s) develop remains unclear. Here, we identify extrahepatic arginase-1+ Id2+ fetal ILC precursors that express a transitional developmental phenotype (ftILCPs) and differentiate into ILC1s, ILC2s and ILC3s in vitro. These cells populate the intestine by embryonic day (E) 13.5 and, before PP organogenesis (E14.5–15), are broadly dispersed in the proximal gut, correlating with regions where PPs first develop. At E16.5, after PP development begins, ftILCPs accumulate at PP anlagen in a lymphotoxin-α–dependent manner. Thus, ftILCPs reside in the intestine during PP development, where they aggregate at PP anlagen after stromal cell activation and become a localized source of ILC populations.

Type 2 innate lymphoid cells constitutively express arginase-I in the naïve and inflamed lung

Arg1 is produced by AAMs and is proposed to have a regulatory role during asthma and allergic inflammation. Here, we use an Arg1 reporter mouse to identify additional cellular sources of the enzyme in the lung. We demonstrate that ILC2s express Arg1 at rest and during infection with the migratory helminth Nippostrongylus brasiliensis. In contrast to AAMs, which express Arg1 following IL‐4/IL‐13‐mediated STAT6 activation, ILC2s constitutively express the enzyme in a STAT6‐independent manner. Although ILC2s deficient in the IL‐33R subunit T1/ST2 maintain Arg1 expression, IL‐33 can regulate total lung Arg1 by expanding the ILC2 population and by activating macrophages indirectly via STAT6. Finally, we find that ILC2 Arg1 does not mediate ILC2 accumulation, ILC2 production of IL‐5 and IL‐13, or collagen production during N. brasiliensis infection. Thus, ILC2s are a novel source of Arg1 in resting tissue and during allergic inflammation.

Innate lymphoid cell function in the context of adaptive immunity

Innate lymphoid cells (ILCs) are a family of innate immune cells that have diverse functions during homeostasis and disease. Subsets of ILCs have phenotypes that mirror those of polarized helper T cell subsets in their expression of core transcription factors and effector cytokines. Given the similarities between these two classes of lymphocytes, it is important to understand which functions of ILCs are specialized and which are redundant with those of T cells. Here we discuss genetic mouse models that have been used to delineate the contributions of ILCs versus those of T cells and review the current understanding of the specialized in vivo functions of ILCs.

Subsets of Nonclonal Neighboring CD4+ T Cells Specifically Regulate the Frequency of Individual Antigen-Reactive T Cells

After an immune response, the expanded population of antigen-specific CD4(+) T cells contract to steady state levels. We have found that the contraction is neither cell-autonomous nor mediated by competition for generic trophic factors, but regulated by relatively rare subsets of neighboring CD4(+) T cells not necessarily of a conventional regulatory T cell lineage. These regulators, referred to as deletors, specifically limit the frequency of particular antigen-specific T cells even though they are not reactive to the same agonist as their targets. Instead, an isolated deletor could outcompete the target for recognition of a shared, nonstimulatory endogenous peptide-MHC ligand. This mechanism was sufficient to prevent even agonist-driven autoimmune disease in a lymphopenic environment. Such a targeted regulation of homeostasis within narrow colonies of T cells with related TCR specificities for subthreshold ligands might help to prevent the loss of unrelated TCRs during multiple responses, preserving the valuable diversity of the repertoire.

IL-15 sustains IL-7R-independent ILC2 and ILC3 development

The signals that maintain tissue-resident innate lymphoid cells (ILC) in different microenvironments are incompletely understood. Here we show that IL-7 receptor (IL-7R) is not strictly required for the development of any ILC subset, as residual cells persist in the small intestinal lamina propria (siLP) of adult and neonatal Il7ra−/− mice. Il7ra−/− ILC2 primarily express an ST2− phenotype, but are not inflammatory ILC2. CCR6+ ILC3, which express higher Bcl-2 than other ILC3, are the most abundant subset in Il7ra−/− siLP. All ILC subsets are functionally competent in vitro, and are sufficient to provide enhanced protection to infection with C. rodentium. IL-15 equally sustains wild-type and Il7ra−/− ILC survival in vitro and compensates for IL-7R deficiency, as residual ILCs are depleted in mice lacking both molecules. Collectively, these data demonstrate that siLP ILCs are not completely IL-7R dependent, but can persist partially through IL-15 signalling.

Leukotriene B4 amplifies eosinophil accumulation in response to nematodes.

Rapid eosinophil accumulation around nematode species occurs via leukotriene induction, which is necessary for nematode-induced eosinophil accumulation in the lung.