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Obesity and type-2 diabetes have increased markedly over the past few decades, in parallel. One of the major links between these two disorders is chronic, low-grade inflammation. Prolonged nutrient excess promotes the accumulation and activation of leukocytes in visceral adipose tissue (VAT) and ultimately other tissues, leading to metabolic abnormalities such as insulin resistance, type-2 diabetes and fatty-liver disease. Although invasion of VAT by pro-inflammatory macrophages is considered to be a key event driving adipose-tissue inflammation and insulin resistance, little is known about the roles of other immune system cell types in these processes. A unique population of VAT-resident regulatory T \((T_{reg})\) cells was recently implicated in control of the inflammatory state of adipose tissue and, thereby, insulin sensitivity. Here we identify peroxisome proliferator-activated receptor (PPAR)-γ, the ‘master regulator’ of adipocyte differentiation, as a crucial molecular orchestrator of VAT \(T_{reg}\) cell accumulation, phenotype and function. Unexpectedly, PPAR-γ expression by VAT \(T_{reg}\) cells was necessary for complete restoration of insulin sensitivity in obese mice by the thiazolidinedione drug pioglitazone. These findings suggest a previously unknown cellular mechanism for this important class of thiazolidinedione drugs, and provide proof-of-principle that discrete populations of \(T_{reg}\) cells with unique functions can be precisely targeted to therapeutic ends.

All juvenile NOD mice exhibit insulitis, but there is substantial variation in their progression to diabetes. We demonstrate that a patient-validated magnetic-resonance-imaging (MRI) strategy to non-invasively visualize local effects of pancreatic-islet inflammation can predict diabetes onset in NOD mice. MRI signals acquired during a narrow early time-window allowed pre-sorting into disease-progressors and -nonprogressors and an estimate of time-to-diabetes. We exploited this capability to identify novel elements correlated with disease protection, including CRIg (complement receptor of the immunoglobulin superfamily), which marked a subset of macrophages associated with diabetes resistance. Administration of CRIg-Fc depressed MRI signals and diabetes incidence. In addition to identifying regulators of disease progression, this study shows that diabetes is set at an early age in NOD mice.

Based on analyses of multiple TCR transgenic (tg) models, the emergence of pathogenic T cells in diabetes-prone NOD mice has been ascribed to a failure to censure autoreactive clones in the thymus. In contrast, using isolated and preselected thymocytes, we show that nonobese diabetic (NOD) genetic variation impairs neither clonal deletion nor downstream transcriptional programs. However, we find that NOD genetic variation influences αβ/γδ-lineage decisions promoted by early expression of tg αβ-TCRs at the double-negative (DN) stage. In B6 and other genetic backgrounds, tg αβ-TCRs behave like γδ-TCRs and commit a large fraction of DNs toward the γδ-lineage, thereby decreasing the size of the double-positive (DP) pool, which is efficiently positively and negatively selected. In NOD DNs, αβ-TCR signalosomes instead behave like pre-TCRs, resulting in high numbers of DPs competing for limited selection niches, and poor positive and negative selection. Once niche effects are neutralized in mixed bone marrow chimeras, positive and negative selection are equally efficient on B6 and NOD backgrounds. Biochemical analysis revealed a selective defect in the activation of Erk1/2 downstream of NOD αβ-TCR signalosomes. Therefore, NOD genetic variation influences αβ/γδ-lineage decisions when the αβ-TCR heterodimer is prematurely expressed, but not the process of negative selection.

Introduction: Neuromedin U (NMU) is a neuropeptide with pro-inflammatory activity. The primary goal of this study was to determine if NMU promotes autoantibody-induced arthritis. Additional studies addressed the cellular source of NMU and sought to define the NMU receptor responsible for its pro-inflammatory effects. Methods: Serum containing arthritogenic autoantibodies from K/BxN mice was used to induce arthritis in mice genetically lacking NMU. Parallel experiments examined whether NMU deficiency impacted the early mast-cell-dependent vascular leak response induced by these autoantibodies. Bone-marrow chimeric mice were generated to determine whether pro-inflammatory NMU is derived from hematopoietic cells or stromal cells. Mice lacking the known NMU receptors singly and in combination were used to determine susceptibility to serum-transferred arthritis and in vitro cellular responses to NMU. Results: NMU-deficient mice developed less severe arthritis than control mice. Vascular leak was not affected by NMU deficiency. NMU expression by bone-marrow-derived cells mediated the pro-arthritogenic effect. Deficiency of all of the known NMU receptors, however, had no impact on arthritis severity and did not affect the ability of NMU to stimulate intracellular calcium flux. Conclusions: NMU-deficient mice are protected from developing autoantibody-induced inflammatory arthritis. NMU derived from hematopoietic cells, not neurons, promotes the development of autoantibody-induced inflammatory arthritis. This effect is mediated by a receptor other than the currently known NMU receptors.

Genome-wide association studies using commercially available outbred mice can detect genes involved in phenotypes of biomedical interest. Useful populations need high-frequency alleles to ensure high power to detect quantitative trait loci (QTLs), low linkage disequilibrium between markers to obtain accurate mapping resolution, and an absence of population structure to prevent false positive associations. We surveyed 66 colonies for inbreeding, genetic diversity, and linkage disequilibrium, and we demonstrate that some have haplotype blocks of less than 100 Kb, enabling gene-level mapping resolution. The same alleles contribute to variation in different colonies, so that when mapping progress stalls in one, another can be used in its stead. Colonies are genetically diverse: 45% of the total genetic variation is attributable to differences between colonies. However, quantitative differences in allele frequencies, rather than the existence of private alleles, are responsible for these population differences. The colonies derive from a limited pool of ancestral haplotypes resembling those found in inbred strains: over 95% of sequence variants segregating in outbred populations are found in inbred strains. Consequently it is possible to impute the sequence of any mouse from a dense SNP map combined with inbred strain sequence data, which opens up the possibility of cataloguing and testing all variants for association, a situation that has so far eluded studies in completely outbred populations. We demonstrate the colonies' potential by identifying a deletion in the promoter of H2-Ea as the molecular change that strongly contributes to setting the ratio of CD4+ and CD8+ lymphocytes.

Neurogenesis continues through the adult life of mice in the subgranular zone of the dentate gyrus in the hippocampus, but its function remains unclear. Measuring cellular proliferation in the hippocampus of 719 outbred heterogeneous stock mice revealed a highly significant correlation with the proportions of CD8+ versus CD4+ T lymphocyte subsets. This correlation reflected shared genetic loci, with the exception of the H-2Ea locus that had a dominant influence on T cell subsets but no impact on neurogenesis. Analysis of knockouts and repopulation of TCRa-deficient mice by subsets of T cells confirmed the influence of T cells on adult neurogenesis, indicating that CD4+ T cells or subpopulations thereof mediate the effect. Our results reveal an organismal impact, broader than hitherto suspected, of the natural genetic variation that controls T cell development and homeostasis.

There has long been conceptual and experimental support for, but also challenges to, the notion that the initial period of the immune system's development is particularly important for the establishment of tolerance to self. The display of self-antigens by thymic epithelial cells is key to inducing tolerance in the T lymphocyte compartment, a process enhanced by the Aire transcription factor. Using a doxycycline-regulated transgene to target Aire expression to the thymic epithelium, complementing the Aire knockout in a temporally controlled manner, we find that Aire is essential in the perinatal period to prevent the multiorgan autoimmunity that is typical of Aire deficiency. Surprisingly, Aire could be shut down soon thereafter and remain off for long periods, with few deleterious consequences. The lymphopenic state present in neonates was a factor in this dichotomy because inducing lymphopenia during Aire turnoff in adults recreated the disease, which, conversely, could be ameliorated by supplementing neonates with adult lymphocytes. In short, Aire expression during the perinatal period is both necessary and sufficient to induce long-lasting tolerance and avoid autoimmunity. Aire-controlled mechanisms of central tolerance are largely dispensable in the adult, as a previously tolerized T cell pool can buffer newly generated autoreactive T cells that might emerge.

Objective. For the inflammation characteristic of rheumatoid arthritis, the relative contribution of mediators produced locally in the synovium versus those circulating systemically is unknown. Complement factor C3 is made in rheumatoid synovium and has been proposed to be a crucial driver of inflammation. The aim of this study was to test, in a mouse model of rheumatoid arthritis, whether C3 synthesized within the synovium is important in promoting inflammation. Methods. Radiation bone marrow chimeras between normal and C3−/− mice were constructed in order to generate animals that expressed or lacked expression of C3 only in hematopoietic cells. Parabiotic mice were made by surgically linking C3−/− mice to irradiated wild-type mice to obtain animals having C3 only in the circulation. Arthritis was induced by injection of serum from arthritic K/BxN mice.Results In bone marrow chimeras, synthesis of C3 by radioresistant cells was necessary and sufficient to confer susceptibility to serum-transferred arthritis. Parabionts having C3 only in the circulation remained sensitive to arthritis induction, and the cartilage of these arthritic mice contained deposits of C3. Conclusion. In a mouse model in which the alternative pathway of complement activation is critical to the induction of arthritis by autoantibodies, circulating C3 was necessary and sufficient for arthritis induction., Stem Cell and Regenerative Biology

Dominant paradigms for the understanding of rheumatoid arthritis (RA) pathogenesis have changed over the years. A predominant role of B lymphocytes, and perhaps of the rheumatoid factor they produced, was initially invoked. In more recent years, recognition of antigens in the joint by T cells sparking an inflammatory cascade has been a more favored interpretation. Here, we re-examine some of the arguments that underpin this proposed role of joint T cells, in light of recent results from transgenic mice in which a self-reactive T-cell receptor provokes disease, but from outside the joint and indirectly via B lymphocytes and immunoglobulins.