Project Summary
The proposed research aims to harness the immune system’s influence on the brain to alleviate symptoms of autism spectrum disorders through “immunotherapy” for the brain. Our approach is based on the observation that an immune cell-derived soluble factor interleukin-17a (IL-17a) shows beneficial effects in rescuing social behavior phenotype in multiple models for neurodevelopmental disorders by acting within the brain as a neuromodulator. We propose four projects to interrogate multiple nodes along the gutimmune-brain axis to enhance the levels of IL-17a in the brain as a potential therapeutic approach, with the meninges as the main locus of that axis. We will examine the trafficking of immune cells from the gut to the meninges and devise means to control the release of cytokines from meningeal immune cells. Using innovative approaches, we will examine the astrocytic regulation of the glymphatic and vascular systems to understand and improve the cytokine spread in the brain. Finally, we will characterize how the neuromodulatory effects of IL-17a are read as changes in gene expressions in the cortical neurons. Our studies will provide insights into the fundamental biology of how the neuroimmune axis is implemented in regulating animal behavior—knowledge essential for devising immunotherapy for neurodevelopmental and autism spectrum disorders.
Overview
Our overarching proposal is to harness the immune system’s influence on the brain to alleviate symptoms of autism spectrum disorders – “immunotherapy” for the brain. We will probe two distinct approaches to immunotherapy. 1) We will manipulate immune cells in the outer meningeal layer that envelops the brain, to release desired soluble factors (i.e., cytokines) into the brain to modulate its function. 2) We will molecularly and functionally probe vascular- and glymphatic system-associated astrocytes as key players that regulate the transport and spread of meningeal-released factors throughout the brain parenchyma.
The state of one’s immune system profoundly influences the physiology and cognitive processes of the brain. For example, when sick, the immune system is engaged in altering general sensory processing (e.g., taste and smell), feeding behavior (e.g., appetite), social interactions with others, sleep patterns, as well as learning, and memory. This repertoire of behavioral changes is thought to help the host recover from sickness and protect the host’s communities from the spread of the disease. To achieve such coordinated behavioral changes, the immune system must have existing mechanisms to actively communicate with the brain and instruct it. Understanding this language spoken between the two systems – the immune cells, their soluble factors and their brain entry, the receptors and their location in the brain, the brain circuits they affect, and the behaviors and moods they influence – is revealing how acute or chronic immune changes influence brain function. It is also providing a wealth of information to devise a novel set of “immune system therapies” to mitigate symptoms and pathologies of autism spectrum disorders.
Our distinct approach to understanding and harnessing the immune system to address disorders of the brain began with the observation that a subset of children with autism spectrum disorder (ASD) show improvement in their behavioral symptoms during episodes of systemic inflammation 1, 2. Building on this observation, we showed that inducing an immune response in a preclinical mouse model of autism, during adulthood, can significantly correct their social interaction deficits. We demonstrated that behavioral improvement requires an immune molecule, interleukin-17a (IL-17A), produced in the periphery, to enter the brain and engage its cognate receptor (IL-17Ra) expressed in cortical neurons of the brain 3. We further showed that the receptor activation leads to changes in neural activity of a specific cortical region, giving rise to eventual behavioral improvements. Therefore, elevated levels of this immune-derived molecule in the brain are critical for improving social interaction deficits observed during inflammation.
Our findings highlight that immune-derived molecules have the capacity to modulate the nervous system, raising the exciting possibility that targeting the immune system to modulate brain function can restore atypical behaviors to normal in neurological disorders. To achieve this goal, we would like to develop a method to activate immune cells to produce cytokines in a specific and controllable manner, directly proximal to the brain. Unlike the brain itself, the meninges, a group of thin membranes enveloping the brain, host a diverse population of immune cells 4. Soluble factors released by the meningeal immune cells are thought to be distributed into the brain parenchyma via the glymphatic system – a system of astrocyte-dependent vascularcoupled channels for convective exchange between the cerebrospinal fluid and the brain parenchymal interstitial fluid 5. Therefore, the meninges are well-positioned to serve as the main entry point into the brain for immune-derived molecules. We will, therefore, investigate whether meningeal immune cells can be experimentally controlled to release cytokines into the brain as a means to change brain activity and behaviors. More specifically, we will ask if meningeal immune cells can be directed to produce IL-17a and thereby restore wild-type levels of social interaction in the mouse models for ASD.
In accomplishing these goals, we will characterize almost every node along the neuroimmune axis that can be precisely manipulated to increase the levels of IL-17a and have it impact brain function. We will describe how immune cells are trafficked into the meninges and ask whether the gut microbiota can modulate this process (Huh). We will develop tools to manipulate meningeal immune cells to release soluble factors of our choice (Choi), building on mechanisms of their trafficking and modulation (Choi and Huh). We will profile the components of the mammalian glymphatic system as a basis for understanding the spread of cytokines into the brain parenchyma and decode the molecular signatures of the blood-brain barrier (BBB) (Heiman). We will examine the role of astrocytes in regulation of the glymphatic and perivascular system (Sur), with the goals of understanding the transport and function of immune mediators in the brain (Choi and Sur) and developing effective means of delivering these mediators (Heiman and Sur). Finally, we will analyze how the effects of IL-17a are read out as changes in gene expressions in cortical neurons (Choi and Heiman). Our approach will be a case study to show how the immune system can be used as a viable new strategy for non-invasive therapeutic interventions in the brain. Most importantly, our proposal will pursue the development of therapeutics in tandem with a rigorous investigation into the neuroimmune mechanisms underlying the proposed treatment.
IL-17a and its receptors represent only one of many cytokine-receptor pairs the immune system has in its arsenal to modulate brain function. Our unpublished data indicate that cells of the nervous system express receptors for several other immune-derived soluble factors in a region-specific and cell-type-specific manner. The brain, therefore, is endowed with molecular and cellular mechanisms sensitive to these immune-derived signals, and we are discovering that individual cytokines can precisely target and modulate specific neural circuits and associated behaviors. Therefore, if successful, our approach to manipulating meningeal immune cells and modulating their delivery could provide a platform for developing novel therapeutic strategies for treating various neurodevelopmental and autism spectrum disorders.
Specific aims of the four projects are as follows:
Project 1: Identifying the underlying logic of immune cell migration from the gut to the brain (Jun Huh)
Aim 1. Elucidating the roles of gut-residing bacteria in modulating immune cell migration.
Aim 2. Enhancing the migratory potential of the immune cells to the meninges.
Project 2: Immunotherapy using meningeal immune cells as a source of cytokines (Gloria Choi)
Aim 1. Testing whether meningeal immune cells can be experimentally controlled to release cytokines into the brain as a means to change brain activity and behaviors.
Aim 2. Decoding the IL-17 receptor expression in cortical neurons.
Project 3: Molecular atlas of the mammalian glymphatic system (Myriam Heiman)
Aim 1. Molecular profiling of glymphatic flow-associated astrocytic mRNAs
Aim 2. Molecular profiling of a new population of vascular-coupled astrocytes
Aim 3. Molecular profiling of glymphatic flow-associated pericyte and fibroblast mRNAs
Project 4: Functional and mechanistic characterization of vascular- and glymphatic-associated astrocytes that mediate cytokine availability in the brain (Mriganka Sur)
Aim 1. Functional characterization of astrocyte activity related to vascular and glymphatic dynamics in the cortex
Aim 2. Determining the role of astrocyte-coupled vascular and glymphatic systems in IL-17a transport and function
Aim 3. Determining causal astrocyte mechanisms of cytokine availability and function.
References
1. Curran, L. K., Newschaffer, C.J., Lee, L., Crawford, S.O., Johnston, M.V. and Zimmerman, A.W. Behaviors associated with fever in children with autism spectrum disorders. Pediatrics 120, e1386-1392, doi:10.1542/peds.2007-0360 (2007).
2. Grzadzinski, R., Lord, C., Sanders, S. J., Werling, D. & Bal, V. H. Children with autism spectrum disorder who improve with fever: Insights from the Simons Simplex Collection. Autism Res 11, 175-184, doi:10.1002/aur.1856 (2018).
3. Reed, M. D., Yim, Y.S., Wimmer, R.D., Kim, H., Ryu, C., Welch, G.M., Andina, M., King, H.O., Waisman, A., Halasaa, M.M., Huh, J.R. and Choi, G.B. IL-17a promotes sociability in mouse models of neurodevelopmental disorders. Nature 577, 249-253, doi:10.1038/s41586-019-1843-6 (2020).
4. Alves de Lima, K., Rustenhoven, J. & Kipnis, J. Meningeal Immunity and Its Function in Maintenance of the Central Nervous System in Health and Disease. Annu Rev Immunol 38, 597-620, doi:10.1146/annurev-immunol-102319-103410 (2020).
5. Jessen, N. A., Munk, A. S., Lundgaard, I. & Nedergaard, M. The Glymphatic System: A Beginner’s Guide. Neurochem Res 40, 2583-2599, doi:10.1007/s11064-015-1581-6 (2015).