The macrophage in MS: just a scavenger after all? Pathology and pathogenesis of the acute MS lesion

Advances in the neuropathology of multiple sclerosis ( MS) have contributed greatly to our understanding of the mechanisms of tissue injury in the condition. Particular interest has focussed on the active MS lesion, defined by macrophage activity in the presence of partially demyelinated axons. This has led to the prevailing consensus that a T-cell dependent, macrophage-mediated, autoimmune attack on constituents in the normal myelin sheath underlies the disease. This hypothesis, which has been largely supported by comparisons with the animal model, experimental allergic encephalomyelitis, has recently been questioned by an analysis of the pathological events preceding myelin phagocytosis in nascent MS lesions. The prephagocytic changes in evolving lesions examined shortly after the onset of an MS relapse raise the possibility that oligodendrocyte cell death and associated changes within the myelin sheath initiate local macrophage scavenger activity, with subsequent amplification of the inflammatory response. The presence of such lesions in patients with a spectrum of pathological changes in nearby or distant active phagocytic plaques suggests that pathological heterogeneity in MS is largely due to evolution of lesional pathology, rather than pathogenic heterogeneity.