Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). Using susceptibility-based 7T MRI, a hypointense rim is seen in some white matter (WM) multiple sclerosis lesions that typically expand over time, and are associated with greater tissue injury, higher disability and progressive disease. These paramagnetic rims are thought to correspond with iron accumulation at the macrophage/microglial plaque edge of chronic-active/smoldering lesions. However, evaluating the distribution of iron and other metals in MS lesions is complex, and limited by inherent challenges of histochemical stains for metals.
To characterize the distribution of iron, copper, and zinc in chronic white, deep gray and cortical MS lesions in correlation with the stage of demyelinating activity and cellular localization.
X-ray fluorescence imaging (XFI) enables simultaneous mapping of all chemical forms of metals in the same tissue slice. We correlated XFI of ex vivo hemispheric sections with histopathology of a chronic MS autopsy. Lesions were staged and included 9 WM inactive plaques; 3 inactive deep gray matter (GM) plaques; extensive subpial inactive cortical demyelination, and 1 remyelinated lesion.
XFI revealed reduced iron, copper and zinc within all subpial neocortical demyelinated lesions. Three periventricular WM lesions demonstrated a diffuse ring of iron on XFI, histopathologically corresponding to iron located predominantly within microglia at the inactive plaque border in the absence of ongoing myelin degradation. All three deep GM plaques contained increased iron but reduced copper and zinc. While most of the hippocampal cortex showed reduced iron, copper, and zinc on XFI corresponding to subpial demyelination, the demyelinated subiculum exhibited regions of increased iron accumulation not only in microglia but also neurons. Iron was increased in the stratum moleculare corresponding to remyelination, where iron was located in microglia, oligodendrocytes, and astrocytes.
Iron, copper and zinc content are reduced in all neocortical subpial plaques and within the center of all inactive WM plaques. Iron rims surrounding WM plaques are not restricted to chronic active or smoldering lesions but are also found in inactive lesions. Iron is the only metal increased in deep GM inactive demyelinated lesions. Accumulation of iron in hippocampal neurons may be associated with neuronal oxidative injury, while high iron within the remyelinated white matter of the hippocampus suggests iron accumulation may also contribute to lesion repair, highlighting both the toxic and beneficial properties of iron. Future studies correlating XFI and MS histopathology with MRI will permit the development and validation of specific metal detection methods, paving the way for novel metal-based biomarkers that monitor disease activity and progression.