Clonal CD8 T Cells Invade Parkinson's Disease Brain and Team Up With Reactive Astrocytes

Parkinson's disease (PD) destroys dopaminergic neurons in the substantia nigra (SN) through mechanisms that remain incompletely understood. While α-synuclein-reactive T cells have been detected in the blood of PD patients, what these cells actually do inside the brain—and how they interact with resident glia—has been largely unknown. This study set out to fill that gap using state-of-the-art spatial and single-cell genomics on human postmortem tissue.

The research team analyzed SN and cingulate cortex tissue from PD patients and age-matched controls. Immunohistochemistry confirmed a significant increase in CD8+ T cells in the PD SN parenchyma. TCR sequencing then revealed that these T cells were clonally expanded—meaning specific clones had proliferated repeatedly—a hallmark of antigen-driven immune responses. Critically, the TCR beta-chain sequences from PD brain T cells showed significant overlap with TCR sequences previously shown to react to α-synuclein peptides in external blood-based challenge experiments, strongly implicating α-synuclein as the antigenic target driving T cell expansion in the brain.

Single-nucleus RNA sequencing characterized multiple microglial and astrocytic states in the PD SN. A disease-associated astrocyte subpopulation expressing high levels of CD44—a cell-surface glycoprotein involved in inflammation and cell adhesion—was markedly increased in PD. Spatial transcriptomics and multiplex immunohistochemistry demonstrated that T cells in the PD SN were spatially co-localized with these CD44+ astrocytes, suggesting a functional interaction. In contrast, T cells did not show strong spatial association with microglia, pointing to astrocytes as the primary cellular partner for infiltrating T cells in the PD SN.

To probe the functional significance of CD44 in astrocytes, the team silenced CD44 expression in cultured astrocytes using siRNA. CD44 knockdown significantly attenuated neuroinflammatory transcriptional signatures, including pathways related to cytokine signaling and immune activation. This positions CD44 as a molecularly tractable node in the astrocyte-T cell neuroinflammatory axis and a candidate therapeutic target for slowing PD progression.

Taken together, the findings establish a spatial and molecular framework in which clonally expanded, α-synuclein-reactive CD8+ T cells infiltrate the PD SN and engage with disease-associated CD44+ astrocytes, potentially amplifying neuroinflammation and contributing to dopaminergic neuron loss. The study is limited by its cross-sectional postmortem design and cannot establish causality, but it provides a rich molecular atlas that will guide mechanistic and therapeutic research in PD neuroimmunology.