Regulatory T cells promote microglia-mediated synapse engulfment and functional recovery via the OPN-CD74 axis after spinal cord injury in mice.

Spinal cord injury (SCI) is a debilitating neurological condition characterized by permanent sensory and motor dysfunction. While clearance of tissue debris represents a critical step in establishing a regenerative microenvironment after SCI, the underlying mechanisms remain incompletely understood. Regulatory T cells (Tregs) have emerged as critical immunomodulators in neurological diseases, with prior studies demonstrating their neuroprotective effects mediated through microglial regulation. As resident macrophages in the central nervous system (CNS), microglia play essential roles in debris clearance after SCI. Moreover, microglia-mediated synaptic elimination is crucial for maintaining tissue integrity and neural circuit function in neurological pathologies. However, it remains unclear whether and how Tregs influence microglial phagocytic activity, particularly synaptic engulfment post-SCI. In this study, we observed robust infiltration of Tregs into the injured spinal cords of both SCI patients and mouse models. Selective depletion of Tregs impaired the microglial phagocytosis of synaptic debris and reduced synapse density in mice post-SCI. Single-cell RNA sequencing and flow cytometry analyses revealed that microglial Cd74 expression was significantly upregulated following Tregs depletion. Remarkably, genetic ablation of Cd74 rescued the phagocytic deficits and mitigates reductions in synaptic density observed in Treg-deficient SCI mice. Osteopontin (OPN), a multifunctional cytokine implicated in regulating neuroinflammation, has previously been shown to mediate Treg-microglia interactions in stroke. Here, we demonstrated that Treg-derived OPN suppressed microglial CD74 expression, enhanced synaptic engulfment, and improved neurological outcomes after SCI. Collectively, our findings highlight a novel OPN-CD74 regulatory axis through which Tregs modulate microglial phagocytic function, offering new translational targets for SCI treatment.