Insular cortex neurons encode and retrieve specific immune responses.

Increasing evidence indicates that the brain regulates peripheral immunity, yet whether and how the brain represents the state of the immune system remains unclear. Here, we show that the brain's insular cortex (InsCtx) stores immune-related information. Using activity-dependent cell labeling in mice (FosTRAP), we captured neuronal ensembles in the InsCtx that were active under two different inflammatory conditions (dextran sulfate sodium [DSS]-induced colitis and zymosan-induced peritonitis). Chemogenetic reactivation of these neuronal ensembles was sufficient to broadly retrieve the inflammatory state under which these neurons were captured. Thus, we show that the brain can store and retrieve specific immune responses, extending the classical concept of immunological memory to neuronal representations of inflammatory information.

Optogenetic activation of local colonic sympathetic innervations attenuates colitis by limiting immune cell extravasation.

The sympathetic nervous system is composed of an endocrine arm, regulating blood adrenaline and noradrenaline, and a local arm, a network of fibers innervating immune organs. Here, we investigated the impact of the local arm of the SNS in an inflammatory response in the colon. Intra-rectal insertion of an optogenetic probe in mice engineered to express channelrhodopsin-2 in tyrosine hydroxylase cells activated colonic sympathetic fibers. In contrast to systemic application of noradrenaline, local activation of sympathetic fibers attenuated experimental colitis and reduced immune cell abundance. Gene expression profiling showed decreased endothelial expression of the adhesion molecule MAdCAM-1 upon optogenetic stimulation; this decrease was sensitive to adrenergic blockers and 6-hydroxydopamine. Antibody blockade of MAdCAM-1 abrogated the optogenetic effect on immune cell extravasation into the colon and the pathology. Thus, sympathetic fibers control colonic inflammation by regulating immune cell extravasation from circulation, a mechanism likely relevant in multiple organs.

Neuronal regulation of immunity: why, how and where?

Neuroimmunology is one of the fastest-growing fields in the life sciences, and for good reason; it fills the gap between two principal systems of the organism, the nervous system and the immune system. Although both systems affect each other through bidirectional interactions, we focus here on one direction - the effects of the nervous system on immunity. First, we ask why is it beneficial to allow the nervous system any control over immunity? We evaluate the potential benefits to the immune system that arise by taking advantage of some of the brain's unique features, such as its capacity to integrate and synchronize physiological functions, its predictive capacity and its speed of response. Second, we explore how the brain communicates with the peripheral immune system, with a focus on the endocrine, sympathetic, parasympathetic, sensory and meningeal lymphatic systems. Finally, we examine where in the brain this immune information is processed and regulated. We chart a partial map of brain regions that may be relevant for brain-immune system communication, our goal being to introduce a conceptual framework for formulating new hypotheses to study these interactions.

Modulation of anti-tumor immunity by the brain's reward system.

Regulating immunity is a leading target for cancer therapy. Here, we show that the anti-tumor immune response can be modulated by the brain's reward system, a key circuitry in emotional processes. Activation of the reward system in tumor-bearing mice (Lewis lung carcinoma (LLC) and B16 melanoma) using chemogenetics (DREADDs), resulted in reduced tumor weight. This effect was mediated via the sympathetic nervous system (SNS), manifested by an attenuated noradrenergic input to a major immunological site, the bone marrow. Myeloid derived suppressor cells (MDSCs), which develop in the bone marrow, became less immunosuppressive following reward system activation. By depleting or adoptively transferring the MDSCs, we demonstrated that these cells are both necessary and sufficient to mediate reward system effects on tumor growth. Given the central role of the reward system in positive emotions, these findings introduce a physiological mechanism whereby the patient's psychological state can impact anti-tumor immunity and cancer progression.

High-dimensional, single-cell characterization of the brain's immune compartment.

The brain and its borders create a highly dynamic microenvironment populated with immune cells. Yet characterization of immune cells within the naive brain compartment remains limited. In this study, we used CyTOF mass cytometry to characterize the immune populations of the naive mouse brain using 44 cell surface markers. By comparing immune cell composition and cell profiles between the brain compartment and blood, we were able to characterize previously undescribed cell subsets of CD8 T cells, B cells, NK cells and dendritic cells in the naive brain. Using flow cytometry, we show differential distributions of immune populations between meninges, choroid plexus and parenchyma. We demonstrate the phenotypic ranges of resident myeloid cells and identify CD44 as a marker for infiltrating immune populations. This study provides an approach for a system-wide view of immune populations in the brain and is expected to serve as a resource for understanding brain immunity.

Studying brain-regulation of immunity with optogenetics and chemogenetics; A new experimental platform.

The interactions between the brain and the immune system are bidirectional. Nevertheless, we have far greater understanding of how the immune system affects the brain than how the brain affects immunity. New technological developments such as optogenetics and chemogenetics (using DREADDs; Designer Receptors Exclusively Activated by Designer Drugs) can bridge this gap in our understanding, as they enable an unprecedented mechanistic and systemic analysis of the communication between the brain and the immune system. In this review, we discuss new experimental approaches for revealing neuronal circuits that can participate in regulation of immunity. In addition, we discuss methods, specifically optogenetics and chemogenetics, that enable targeted neuronal manipulation to reveal how different brain regions affect immunity. We describe how these techniques can be used as an experimental platform to address fundamental questions in psychoneuroimmunology and to understand how neuronal circuits associate with different psychological states can affect physiology.

Activation of the reward system boosts innate and adaptive immunity.

Positive expectations contribute to the clinical benefits of the placebo effect. Such positive expectations are mediated by the brain's reward system; however, it remains unknown whether and how reward system activation affects the body's physiology and, specifically, immunity. Here we show that activation of the ventral tegmental area (VTA), a key component of the reward system, strengthens immunological host defense. We used 'designer receptors exclusively activated by designer drugs' (DREADDs) to directly activate dopaminergic neurons in the mouse VTA and characterized the subsequent immune response after exposure to bacteria (Escherichia coli), using time-of-flight mass cytometry (CyTOF) and functional assays. We found an increase in innate and adaptive immune responses that were manifested by enhanced antibacterial activity of monocytes and macrophages, reduced in vivo bacterial load and a heightened T cell response in the mouse model of delayed-type hypersensitivity. By chemically ablating the sympathetic nervous system (SNS), we showed that the reward system's effects on immunity are, at least partly, mediated by the SNS. Thus, our findings establish a causal relationship between the activity of the VTA and the immune response to bacterial infection.

Reversal of heroin neurobehavioral teratogenicity by grafting of neural progenitors.

A major objective in identifying the mechanisms underlying neurobehavioral teratogenicity in an animal model is the possibility of designing therapies that reverse or offset teratogen-induced neural damage. In our previous studies, we identified deficits in hippocampal muscarinic cholinergic receptor-induced translocation of protein kinase C (PKC) gamma as the likely central factor responsible for the adverse behavioral effects of pre-natal heroin exposure. Neural progenitors (NP) have the ability to recover behavioral deficits after focal hippocampal damage. Therefore, we explored whether behavioral and synaptic defects could be reversed in adulthood by neural progenitor grafting. Pregnant mice were injected daily with 10 mg/kg of heroin on gestational days 9-18. In adulthood, offspring showed deficits in the Morris maze, a behavior dependent on the integrity of septohippocampal cholinergic synaptic function, along with the loss of the PKCgamma and PKCbetaII responses to cholinergic stimulation. Mice that were exposed pre-natally to heroin and vehicle control mice were then grafted in adulthood with NP. Importantly, most grafted cells differentiated to astrocytes. NP reversed the behavioral deficits (p = 0.0043) and restored the normal response of hippocampal PKCgamma and PKCbetaII (p = 0.0337 and p = 0.0265 respectively) to cholinergic receptor stimulation. The effects were specific as the PKCalpha isoform, which is unrelated to the behavioral deficits, showed almost no changes. Neural progenitor grafting thus offers an animal model for reversing neurobehavioral deficits originating in septohippocampal cholinergic defects elicited by pre-natal exposure to insults.

Mechanism-based approaches for the reversal of drug neurobehavioral teratogenicity.

Understanding the mechanism of neurobehavioral teratogenicity is the primary prerequisite for reversal of the defect. Progress in such studies can be best achieved if the investigation focuses on behaviors related to a specific brain region and innervation. Our model focused on teratogen-induced deficits in hippocampus-related eight-arm and Morris maze behaviors. Different "cholinergic" teratogens, mainly heroin, induced both pre- and postsynaptic hyperactivity in the hippocampal cholinergic innervation that terminated in desensitization of Protein Kinase C (PKC) isoforms to cholinergic receptor stimulation. Understanding this mechanism enabled its reversal with a pharmacological therapy-nicotine infusion. Studies by others provided similar findings by targeting the deficits respective to the model investigated. Consistently, destruction of the A10-septal dopaminergic pathways that downregulate the septohippocampal cholinergic innervation ameliorated maze performance. Grafting of embryonic differentiated cholinergic cells or neural progenitors similarly reversed the biochemical/molecular alterations and the resulting deficits. Reversal therapies offer a model for the understanding of neurobehavioral teratogenicity and, clinically, offer a model for potential treatment of these deficits. Whereas neural progenitor grafting appears to be the most effective treatment, pharmacological reversal with nicotine infusion seems to possess the most feasible and immediate therapy for neurobehavioral birth defects produced by various teratogens, including drugs. This is true even though the effect of pharmacological therapies is diffuse, affecting multiple areas of the brain. "Everybody is talking about the weather but nobody does anything about it." (Mark Twain).