Internalization of IgG in motoneurons of patients with ALS: selective or nonselective?

The hypothesis that abnormal antibodies may be involved in the pathogenesis of ALS has been supported in part by IgG's being present within motoneurons of ALS patients more frequently than in motoneurons of controls. IgG, as well as other serum proteins, is also present in motoneurons of normal human and animal spinal cords. We attempted to determine whether the IgG found in motoneurons of ALS patients was localized by an immune-specific or nonspecific process. To address this question, we used immunocytochemistry to evaluate the presence and relative density of different serum proteins in spinal cords from nine patients with ALS. Both IgG and alpha 2-macroglobulin (alpha 2Mac) were present in motoneurons in all nine cases. More important, there was a close concordance between the IgG and alpha 2Mac immunolabeling of motoneurons. The presence of a nonimmune plasma protein--alpha 2Mac--in a similar distribution to IgG and with a similar intensity implies that the internalization of these proteins in motoneurons of patients with ALS is best explained by a nonselective mechanism of endocytosis of extracellular fluid.