Disease-associated microglia and activation of CD8 ⁺ T cells precede neuronal cell loss in a model of hereditary spastic paraplegia

In central nervous system (CNS) diseases characterized by late-onset neurodegeneration, the interplay between innate and adaptive immune responses remains poorly understood. This knowledge gap is amplified by the prolonged nature of these diseases, complicating the delineation of brain-resident and infiltrating cells. Here, we conducted a comprehensive profiling of innate and adaptive immune cells across various CNS regions in a murine model of spastic paraplegia 15 (SPG15), a complicated form of hereditary spastic paraplegia (HSP). Using fate-mapping of bone marrow-derived cells via genetic labeling, we identified microgliosis and microglial MHC-II upregulation accompanied by infiltration and local expansion of T cells in the CNS of Spg15 ^-/- mice. Single-cell analysis revealed an increase of disease-associated microglia (DAM) and clonal expansion of effector CD8 ⁺ T cells across CNS regions occurring prior to neuronal loss. Analysis of potential cell-cell communication pathways suggested bidirectional interactions between DAM and effector CD8 ⁺ T cells potentially contributing to disease progression in Spg15 ^-/- mice. In summary, we identified a shift in microglial phenotypes associated with recruitment and clonal expansion of T cells as a new characteristic of Spg15 -driven neuropathology. Targeting activated microglia, CD8 ⁺ T cells and their communication represent promising avenues to prevent the loss of neuronal function in HSP.