Multiple sclerosis and chronic autoimmune encephalomyelitis: a comparative quantitative study of axonal injury in active, inactive, and remyelinated lesions

Abstract

Recent magnetic resonance (MR) studies of multiple sclerosis lesions indicate that axonal injury is a major correlate of permanent clinical deficit. In the present study we systematically quantified acute axonal injury, defined by immunoreactivity for beta-amyloid-precursor-protein in dystrophic neurites, in the central nervous system of 22 multiple sclerosis patients and 18 rats with myelin-oligodendrocyte glycoprotein (MOG)-induced chronic autoimmune encephalomyelitis (EAE). The highest incidence of acute axonal injury was found during active demyelination, which was associated with axonal damage in periplaque and in the normal appearing white matter of actively demyelinating cases. In addition, low but significant axonal injury was also observed in inactive demyelinated plaques. In contrast, no significant axonal damage was found in remyelinated shadow plaques. The patterns of axonal pathology in chronic active EAE were qualitatively and quantitatively similar to those found in multiple sclerosis. Our studies confirm previous observations of axonal destruction in multiple sclerosis lesions during active demyelination, but also indicate that ongoing axonal damage in inactive lesions may significantly contribute to the clinical progression of the disease. The results further emphasize that MOG-induced EAE may serve as a suitable model for testing axon-protective therapies in inflammatory demyelinating conditions.

Figure 1.

Axonal pathology in multiple sclerosis. A–D: Early active MS lesion. Macrophages containing myelin degradation products (A, B), immunoreactive for all myelin proteins including MOG (A: Luxol fast blue (LFB), B: immunocytochemistry for MOG). C. Bielschowsky silver impregnation for axons reveals apparently normal axons as well as axonal swellings and spheroids. D: Immunocytochemistry for βAPP shows numerous injured axons and axonal end-bulbs. E–F: Inactive demyelinated plaque with ongoing demyelinating activity at the border. Injured axons are found at the plaque edge as well as in the demyelinated plaque center (E: LFB, F: immunocytochemistry for βAPP). G–I: Inactive demyelinated plaque without any evidence for active demyelination, but residual inflammation reveals a singular axonal spheroid by immunocytochemistry for βAPP (G: LFB, H: CLA, I: βAPP) J–M: Remyelinated shadow plaque as revealed by the presence of thin myelin pallor (J: LFB). K: Bielschowsky silver impregnation shows the absence of axonal swellings and spheroids. Residual inflammation, in particular activated microglia is still found (L: CLA), but no evidence for acute axonal injury (M: βAPP). Original magnifications, ×484 (A–D, G–M) and ×123 (E and F).

Figure 2.

Axonal pathology in chronic experimental autoimmune encephalomyelitis. a–c: Spinal cord atrophy in an animal sacrificed on day 48 post-immunization. a: Bielschowsky silver impregnation for axons showing reduction of axonal density on one side of the spinal cord in comparison to the opposite side, where damage to myelin is only minor. b: LFB myelin stain reveals demyelination of the atrophic side of the spinal cord A. c: Massive macrophage infiltration within the demyelinated area. (ED1) d: Axonal swellings and spheroids in an actively demyelinating EAE lesion (Bielschowsky). e–j: Acute axonal injury in lesions of different stages of myelin degradation as revealed by immunocytochemistry for βAPP. e and f: Same area of active demyelination as shown in d infiltrated by macrophages containing myelin debris (e, LFB) and numerous axonal swellings and spheroids immunoreactive for βAPP (f, βAPP). g and h: Demyelinated plaque with residual activity at the border (g, LFB) showing only one βAPP-positive axonal spheroid beside some glial reactivity for βAPP (h, βAPP). i and j: Shadow plaque with thin myelin pallor indicating remyelination, infiltrated by myelin degrading macrophages (i, LFB) and many βAPP-reactive axons (j, βAPP). Original magnifications, ×40 (a), ×31 (b and c), ×389 (d–j).