Disrupted brain-immune signaling may help drive neurodegeneration

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Disrupted brain-immune signaling may help drive neurodegeneration

From gut-primed T cells to microglial signaling and persistent gene-regulatory states, researchers map an intricate immune network that connects the brain with the rest of the body.

Immune cells communicate across a continuous, bidirectional circuit between the CNS and periphery. Immune cells from across the body engage CNS tissues and, in turn, are shaped by signals returning from the brain. (Left) Under peripheral equilibrium, immune effectors mobilized from visceral organs, secondary lymphoid organs, hematopoietic tissues, and the peripheral nervous system (PNS) - including CD4⁺ and CD8⁺ T cells, B cells, monocytes (top 3 cell types), NK cells, dendritic cells, mast cells, neutrophils, and innate lymphoid cells (bottom cluster of cells) - engage the CNS in a homeostatic dialogue that supports neural cell health, plasticity, and repair, sustaining a protective environment. (Right) Age-associated dysfunction across the same peripheral compartments reshapes this circuit: altered immune effectors enter the CNS and contribute to shifting the local environment toward a degenerative state in which microglia adopt disease-associated programs, astrocytes become reactive, and neurons accumulate pathology. This model underscores how the same circuitry can yield protective or pathogenic outcomes and that neurodegeneration is downstream of dysfunctional brain-immune crosstalk. Study: Neurodegeneration as a dysregulation of neuroimmune crosstalk

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A recent perspective published in the journal Cell synthesizes scientific evidence suggesting that neurodegeneration involves intricate crosstalk between neurons and immune cells, linking the brain to peripheral immunity through bidirectional exchange. Strategies that restore immune homeostasis or recalibrate neuroimmune signaling may potentially slow neurodegeneration and promote recovery.

Historically, immune dysregulation has often been considered a consequence of neurodegenerative disorders. Recent studies, however, are beginning to change this scientific mindset, suggesting that disordered communication between the brain and immune cells may also contribute to disease onset and progression. The authors describe immune dysfunction as a “concause” of neurodegeneration, meaning it may interact with neuronal and glial vulnerabilities without necessarily being the initial trigger. It is essential to advance understanding of the pathophysiology of neurodegenerative diseases to inform therapeutic development and the development of immune-based strategies.

In this perspective, researchers examined brain-immune interactions and their potential role in neurodegeneration. They organized emerging evidence into three frameworks: “outside-in” effects driven by peripheral immunity, “inside-out” signaling coordinated by brain-resident microglia, and “locked-in” gene regulatory programs that can stabilize maladaptive neuroimmune states.

The brain-immune communication network

The brain continuously communicates with peripheral immune networks. Components of the CNS, including the choroid plexus, meninges, and lymphatic and vascular structures, interact with immune cells to relay signals related to neural needs.

Helper and cytotoxic T cells can enter CNS border regions and, under defined conditions, the brain parenchyma. Brain-immune communication supports neural integrity but can promote pathology when dysregulated. Microglia and BAMs provide surveillance, while lymphocytes confer antigen specificity and immunological memory.

Cytokines, complement, and MHC-I are traditionally linked to immunity, but CNS cells also produce or sense these molecules during neural activity. Innate lymphoid cells in the dura can respond to injury, while the choroid plexus helps regulate inflammatory signaling. In mice, increased neuronal activity may draw antibody-secreting B-lineage cells into the hippocampus during synaptic remodeling.

The gut also influences brain immunity. T cells educated in gut-associated immune tissues can subsequently traffic to the borders of the CNS and, under certain conditions, into the brain, while plasma cells secreting IgA antibodies protect blood vessels in the meninges. In addition, changes in the gut microbiome could influence immune activity and microglial function. Through the GBA, the gut and brain are in constant dialogue with each other. The vagus nerve conveys immunity-related information from the intestines to the brain. Reward-related neural pathways can, in turn, influence peripheral immune activity.

Brain-immune interactions in neurodegenerative disease

T cell activity has been implicated in PD, AD, ALS, and dementia with Lewy bodies (DLB). In ALS4, an inherited form of ALS, cytotoxic T cells are detected early in the blood and brain and expand as the disease progresses, consistent with antigen-driven responses.