Peripheral macrophages and T-cells accumulate in the degenerating optic tract after repetitive head impact

Abstract

Chronic white matter inflammation is a feature of traumatic brain injury (TBI), consistently observed in rodent models of repetitive mild TBI and persistnat in human TBI patients for years post-injury. We use a high-frequency head impact model (HFHI, 5 impacts per day, over 6 consecutive days) to investigate neuroimmune responses in the optic tract, a white matter region particularly vulnerable to injury-induced degeneration in mouse repeat mild TBI models. We integrated immunohistochemistry, digital spatial proteomics, transcriptomic profiling, and blood-brain barrier (BBB) assessments with non-invasive imaging modalities, including diffusion tensor imaging (DTI) and functional MRI (fMRI) to capture a comprehensive view of pathology. HFHI resulted in a sustained inflammatory response within the optic tract, marked by elevated IBA1⁺ and CD68⁺ expression that persisted for at least 3 m post-injury. Characterization of these IBA1⁺ cells revealed that they were F4/80⁺, indicative of infiltrating peripheral macrophages. Concurrent with this, transient BBB disruption was observed at the optic nerve, potentially facilitating acute peripheral immune cell entry. We identified CD4⁺ and CD8⁺ T cells in the optic tract, and digital spatial proteomic signatures revealed Granzyme B and CTLA4 expression, indicative of both regulatory and cytotoxic immune activity. Transcriptomic analyses show polarization of T-cells toward CD4⁺ Th1 and CD8⁺ Tc1 subsets, as evidenced by increased expression of T-bet and IFNγ. Chronic elevation of pro-inflammatory cytokines TNFα and IL-1β, complement component C3, and the chemoattractants Ccl2, Ccl5, Cxcl3, and Cxcl10 further suggest that synergistic interactions between innate and peripheral inflammatory cascades are occuring and contribute to white matter degeneration after repetitive head impact. DTI and fMRI reveal reductions in fractional anisotropy and disrupted optic tract-visual cortex connectivity, indicating functional consequences of this tract-specific immune activation. Importantly, DTI and fMRI provide sensitive, translational readouts of this neuroimmune pathology, with potential relevance to traumatic optic neuropathy and visual dysfunction in clinical populations. These findings support the potential of targeted immune modulation as a therapeutic approach to target white matter injury.