During early embryogenesis, microglia arise from yolk sac progenitors that populate the developing central nervous system (CNS), but how the tissue-resident macrophages are maintained throughout the organism's lifespan still remains unclear. Here, we describe a system that allows specific, conditional ablation of microglia in adult mice. We found that the microglial compartment was reconstituted within 1 week of depletion. Microglia repopulation relied on CNS-resident cells, independent from bone-marrow-derived precursors. During repopulation, microglia formed clusters of highly proliferative cells that migrated apart once steady state was achieved. Proliferating microglia expressed high amounts of the interleukin-1 receptor (IL-1R), and treatment with an IL-1R antagonist during the repopulation phase impaired microglia proliferation. Hence, microglia have the potential for efficient self-renewal without the contribution of peripheral myeloid cells, and IL-1R signaling participates in this restorative proliferation process.

A major goal in current neuroscience is to understand the causal links connecting protein functions, neural activity, and behavior. The cannabinoid CB1 receptor is expressed in different neuronal subpopulations, and is engaged in fine-tuning excitatory and inhibitory neurotransmission. Studies using conditional knock-out mice revealed necessary roles of CB1 receptor expressed in dorsal telencephalic glutamatergic neurons in synaptic plasticity and behavior, but whether this expression is also sufficient for brain functions is still to be determined. We applied a genetic strategy to reconstitute full wild-type CB1 receptor functions exclusively in dorsal telencephalic glutamatergic neurons and investigated endocannabinoid-dependent synaptic processes and behavior. Using this approach, we partly restored the phenotype of global CB1 receptor deletion in anxiety-like behaviors and fully restored hippocampus-dependent neuroprotection from chemically induced epileptiform seizures. These features coincided with a rescued hippocampal depolarization-induced suppression of excitation (DSE), a CB1 receptor-dependent form of synaptic plasticity at glutamatergic neurons. By comparison, the rescue of the CB1 receptor on dorsal telencephalic glutamatergic neurons prolonged the time course of DSE in the amygdala, and impaired fear extinction in auditory fear conditioning. These data reveal that CB1 receptor in dorsal telencephalic glutamatergic neurons plays a sufficient role to control neuronal functions that are in large part hippocampus-dependent, while it is insufficient for proper amygdala functions, suggesting an unexpectedly complex circuit regulation by endocannabinoid signaling in the amygdala. Our data pave the way to a better understanding of neuronal networks in the context of behavior, by fine-tuned interference with synaptic transmission processes.

Topical application of imiquimod (IMQ) on the skin of mice induces inflammation with common features found in psoriatic skin. Recently, it was postulated that IL-17 has an important role both in psoriasis and in the IMQ model. To further investigate the impact of IL-17RA signaling in psoriasis, we generated IL-17 receptor A (IL-17RA)-deficient mice (IL-17RA(del)) and challenged these mice with IMQ. Interestingly, the disease was only partially reduced and delayed but not abolished when compared with controls. In the absence of IL-17RA, we found persisting signs of inflammation such as neutrophil and macrophage infiltration within the skin. Surprisingly, already in the naive state, the skin of IL-17RA(del) mice contained significantly elevated numbers of Th17- and IL-17-producing γδ T cells, assuming that IL-17RA signaling regulates the population size of Th17 and γδ T cells. Upon IMQ treatment of IL-17RA(del) mice, these cells secreted elevated amounts of tumor necrosis factor-α, IL-6, and IL-22, accompanied by increased levels of the chemokine CXCL2, suggesting an alternative pathway of neutrophil and macrophage skin infiltration. Hence, our findings have major implications in the potential long-term treatment of psoriasis by IL-17-targeting drugs.

IL-17-producing CD4 1 T cells (Th17) have been classified as a new T helper cell subset. Using an IL-17 fate mapping mouse strain, which genetically fixes the memory of IL-17 expression, we demonstrate that IL-17A/F-expressing T helper cells generated either in vitro or in vivo are not a stable T-cell subset. Upon adoptive transfer of IL-17F-reporterpositive Th17 cells to RAG-deficient or WT animals, encephalitogenic Th17 cells partially lose IL-17 expression and upregulate IFN-c. Additionally, we show that Th1 cells can convert in vivo to IL-17A/IFN-c-coexpressing cells in the mesenteric lymph nodes (mLN). Our data classify IL-17A and IL-17F as cytokines produced transiently in response to the local microenvironment, thus showing that IL-17 expression does not define an end-stage T helper cell subset. IntroductionSince the finding that IL-23 and not IL-12 is necessary for active induction of EAE [1,2], the previously common dogma for the pathogenesis of the disease has changed. Th17 cells, which were soon thereafter shown to depend on IL-23 [3,4], are now regarded as major initiators of pathogenesis in a number of disease models and human conditions. Th17 achieve their pathogenic phenotype by secreting cytokines which in turn induces the surrounding tissue to secrete chemokines and other cytokines important for the immigration of potentially pathogenic leukocytes such as granulocytes and lymphocytes [5].In a previous landmark EAE study, Th17 cells that were expanded in the presence of IL-23 were shown to be extremely efficient in inducing passive EAE [4]. Low amounts of transferred cells (150 000) were able to induce EAE in SJL/J animals. This finding together with the full resistance of IL-23-deficient animals in response to active EAE induction [2] cemented the idea of Th17 cells as a major pathogenic cell population in EAE. This was further supported by the discovery that Th17 can be very efficiently generated in vitro when naïve CD4 1 T cells are activated in the presence of and that IL-6 is necessary for EAE induction [9][10][11][12]. Furthermore, transgenic expression of TGF-b in T cells enhanced EAE severity [6]. Another milestone for à These authors contributed equally to this work. 3336Frontline this hypothesis was the finding that RORgt deficiency led to a major lack of Th17 cells and to a near complete resistance against active EAE, even in the presence of extensive CNS infiltration [13]. Other transfer studies in the SJL/J mouse using IL-23 expanded encephalitogenic cells found an enhanced infiltration of granulocytes concomitant with EAE development compared to transfer of IL-12 expanded T cells [5,14], further supporting a specific role for Th17 cells in autoimmunity. Given the previous lack of suitable Th17 reporter strains, these studies relied on transfer of in vitro generated Th17 cells of a heterogenous nature, rather than a pure Th17 population.Recently, the encephalitogenicity of Th17 cells was challenged by O'Connor et al., who showed that transferring myelin oligodendrocyte glycoprotei...

Anti-myelin immunity is commonly thought to drive multiple sclerosis, yet the initial trigger of this autoreactivity remains elusive. One of the proposed factors for initiating this disease is the primary death of oligodendrocytes. To specifically test such oligodendrocyte death as a trigger for anti-CNS immunity, we inducibly killed oligodendrocytes in an in vivo mouse model. Strong microglia-macrophage activation followed oligodendrocyte death, and myelin components in draining lymph nodes made CNS antigens available to lymphocytes. However, even conditions favoring autoimmunity-bystander activation, removal of regulatory T cells, presence of myelin-reactive T cells and application of demyelinating antibodies-did not result in the development of CNS inflammation after oligodendrocyte death. In addition, this lack of reactivity was not mediated by enhanced myelin-specific tolerance. Thus, in contrast with previously reported impairments of oligodendrocyte physiology, diffuse oligodendrocyte death alone or in conjunction with immune activation does not trigger anti-CNS immunity.

The lack of a generally accepted animal model for human psoriasis has hindered progress with respect to understanding the pathogenesis of the disease. Here we present a model in which transgenic IL-17A expression is targeted to the skin in mice, achievable after crossing our IL-17A(ind) allele to the K14-Cre strain. K14-IL-17A(ind/+) mice invariably develop an overt skin inflammation bearing many hallmark characteristics of human psoriasis including dermal infiltration of effector T cells, formation of neutrophil microabscesses, and hyperkeratosis. IL-17A expression in the skin results in upregulated granulopoiesis and migration of IL-6R-expressing neutrophils into the skin. Neutralization of IL-6 signaling efficiently reduces the observed pathogenesis in skin of IL-17A-overexpressing mice, with marked reductions in epidermal neutrophil abscess formation and epidermal thickening. Thus, IL-6 functions downstream of IL-17A to exacerbate neutrophil microabscess development in psoriasiform lesions.

Mechanisms underlying central neuropathic pain are poorly understood. Although glial dysfunction has been functionally linked with neuropathic pain, very little is known about modulation of pain by oligodendrocytes. Here we report that genetic ablation of oligodendrocytes rapidly triggers a pattern of sensory changes that closely resemble central neuropathic pain, which are manifest before overt demyelination. Primary oligodendrocyte loss is not associated with autoreactive T- and B-cell infiltration in the spinal cord and neither activation of microglia nor reactive astrogliosis contribute functionally to central pain evoked by ablation of oligodendrocytes. Instead, light and electron microscopic analyses reveal axonal pathology in the spinal dorsal horn and spinothalamic tract concurrent with the induction and maintenance of nociceptive hypersensitivity. These data reveal a role for oligodendrocytes in modulating pain and suggest that perturbation of oligodendrocyte functions that maintain axonal integrity can lead to central neuropathic pain independent of immune contributions.

Statins are a well-established family of drugs that lower cholesterol levels via the competitive inhibition of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR). In addition, the pleiotropic anti-inflammatory effects of statins on T cells make them attractive as therapeutic drugs in T-cell-driven autoimmune disorders. Since statins do not exclusively target HMGCR and thus might have varying effects on different cell types, we generated a new mouse strain allowing for the tissue-specific deletion of HMGCR. Deletion of HMGCR expression in T cells led to a severe decrease in their numbers with the remaining cells displaying an activated phenotype, with an increased proportion of regulatory T cells (Tregs) in particular. However, deletion of HMGCR specifically in Tregs resulted in severe autoimmunity, suggesting that this enzyme is also essential for the maintenance of Tregs. We were able to prevent the death of HMGCR-deficient lymphocytes by the addition of either the direct metabolite of HMGCR, namely mevalonate, or the downstream metabolite geranylgeranyl pyrophosphate, which is essential for protein prenylation. However, the addition of cholesterol, which is the final product of the mevalonate pathway, did not inhibit cell death, indicating that protein prenylation rather than the cholesterol biosynthesis pathway is indispensible for T-cell survival.