Objective: Recent research demonstrates that victims of spinal cord injury (SCI) are at increased risk for dementia and that encephalitis can occur as a consequence of isolated SCI. We theorize that autoimmunity to the central nervous system (CNS) could explain these phenomena and undertook this study to determine whether peripheral inoculation with spinal cord homogenate on 1 or 2 occasions is associated with CNS-directed autoimmunity and neurodegeneration in a rat model.
Methods: Rats were subcutaneously inoculated with saline or 75 mg of allogeneic spinal cord tissue on 1 or 2 occasions. Animals underwent Morris Water Maze testing, and serial serum samples were collected. Animals were sacrificed 8 weeks following the first inoculation. Autoantibody titers to myelin antigens MAG and GM1 were measured in serum. Immunohistochemistry was used to identify autoantibodies targeting NeuN-labeled neurons and CC1-labeled oligodendrocytes. Quantitative real-time polymerase chain reaction (qPCR) and western blotting were performed for pro-inflammatory cytokines TNF-α, IL-1β, and IL-6 and the cell death marker caspase 3 as well as the neurodegenerative proteins tau and β-amyloid in both brain and spinal cord. Fluoro-Jade B was used to stain degenerating neurons, facilitating counting.
Results: Animals inoculated with spinal cord homogenate exhibited increased titers of autoantibodies to MAG and GM1 and autoantibodies binding to neurons and oligodendrocytes. Double-inoculated animals demonstrated a significant increase in the expression of pro-inflammatory cytokines in the brain (TNF-α, p = 0.016; IL-6, p = 0.009) as well as the spinal cord (TNF-α, p = 0.024; IL-6, p = 0.002). The number of degenerating neurons was significantly increased in the brain and spinal cord of inoculated animals (p < 0.0001 and p = 0.028, respectively). Elevated expression of tau and β-amyloid was seen in brain of double-inoculated animals (p = 0.003 and p = 0.009, respectively). Inflammatory marker expression in the brain was positively correlated with anti-myelin autoimmune antibody titers and with tau expression in the brain. Inoculated animals showed impaired memory function in Morris Water Maze testing (p = 0.043).
Conclusions: The results of these experiments demonstrate that peripheral exposure to spinal cord antigens is associated with CNS-directed autoimmunity and inflammation in the brain and spinal cord as well as degeneration of CNS cells, memory impairment, and production of neurodegenerative proteins particularly when this exposure is repeated. These data support CNS autoimmunity as a candidate mechanism for the dementia that can follow SCI and perhaps other posttraumatic dementias such as chronic traumatic encephalopathy.
Keywords: Alzheimer’s disease; autoimmunity; chronic traumatic encephalopathy; dementia; dementia pugilistica; inoculation; neurodegeneration; neuroinflammation; spinal cord injury.