Penetration and internalization of plasma proteins in the human spinal cord

In animal studies, motoneurons take up plasma proteins including immunoglobulins at their terminals. These proteins are then transported back to cell bodies in the spinal cord. To determine if these processes also occur in humans, we localized several different plasma proteins in autopsied spinal cords from 13 patients without neurological disease. As in animals, plasma proteins are associated with vascular and pial structures. Motoneurons, particularly large cervical and lumbar motoneurons, frequently showed immunoreactivity within their cytoplasm to several plasma proteins. Motoneuron labeling was more consistent with antisera against plasma proteins of lower molecular weights such as IgG, IgA and transferrin, than with antisera against higher molecular weight proteins such as IgM and alpha-2-macroglobulin. Other large neurons without connections outside the blood-brain barrier such as those of Clarke's column also occasionally labeled with antisera against all plasma proteins tested. Our results are compatible with the concept that motoneurons take up and transport plasma proteins. These neurons can be distinguished from cells which internalized extravasated serum proteins before and after death. Uptake of pathogenic antibodies by motoneuron terminals may play a role in the pathogenesis of motoneuron disease.