Treg-microglia partnership in the injured spinal cord preserves Treg cell function and regulates microglial cholesterol metabolism

The spatiotemporal dynamics and specific roles of regulatory T (Treg) cells in spinal cord injury (SCI) remain unclear. Using single-cell RNA sequencing, flow cytometry, and immunofluorescence, we found that thymus-derived Treg cells infiltrate the injured spinal cord via peripheral blood around 3 days post-SCI. Treg cell depletion worsened SCI and impaired long-term recovery. Transcriptomic profiling revealed strong anti-inflammatory functions of Treg cells and the potential to regulate cholesterol metabolism in neighboring microglia. Further single-cell RNA sequencing uncovered the clonality of SCI-associated Treg cells. Major histocompatibility complex class II (MHC II) expression on microglia, not macrophages, was crucial for sustaining Treg cell numbers and neuroprotective function, with myelin-phagocytosing microglia-activated Treg cells showing significant neuroprotective effects. Treg cells mitigated microglial inflammation via CTLA-4 and upregulated the ATP-binding cassette transporter G1 (Abcg1) receptor in microglia, helping to manage myelin load and reduce lipid droplet formation. Our findings offer mechanistic insights into SCI-associated Treg cells and lay the groundwork for future Treg-based therapies in SCI treatment.