BACKGROUND. In some active multiple sclerosis (MS) lesions, a strong immune reaction at the lesion edge may contain growth and thereby isolate the lesion from the surrounding parenchyma. Our previous studies suggest that this process involves opening of the blood-brain barrier in capillaries at the lesion edge, seen on MRI as centripetal contrast enhancement and a colocalized phase rim. We hypothesized that using these features to characterize early lesion evolution will allow in vivo tracking of tissue degeneration and/or repair, thus improving the evaluation of potential therapies for chronic active lesions.
METHODS. Centripetally and centrifugally enhancing lesions were studied in 17 patients with MS using 7-tesla MRI. High-resolution, susceptibility-weighted, T1-weighted (before/after gadolinium), and dynamic contrast–enhanced scans were acquired at baseline and months 1, 3, 6, and 12. For each lesion, time evolution of the phase rim, lesion volume, and T1 hypointensity were assessed. In autopsies of 3 progressive MS cases, the histopathology of the phase rim was determined.
RESULTS. In centripetal lesions, a phase rim colocalized with initial contrast enhancement. In 12 of 22, this phase rim persisted after enhancement resolved. Compared with centripetal lesions with transient rim, those with persistent rim had less volume shrinkage and became more T1 hypointense between months 3 and 12. No centrifugal lesions developed phase rims at any time point. Pathologically, persistent rims corresponded to an iron-laden inflammatory myeloid cell population at the edge of chronic demyelinated lesions.
CONCLUSION. In early lesion evolution, a persistent phase rim in lesions that shrink least and become more T1 hypointense over time suggests that the rim might mark failure of early lesion repair and/or irreversible tissue damage. In later stages of MS, phase rim lesions continue to smolder, exerting detrimental effects on affected brain tissue.
TRIAL REGISTRATION. NCT00001248.
FUNDING. The Intramural Research Program of NINDS supported this study.
Martina Absinta, Pascal Sati, Matthew Schindler, Emily C. Leibovitch, Joan Ohayon, Tianxia Wu, Alessandro Meani, Massimo Filippi, Steven Jacobson, Irene C.M. Cortese, Daniel S. Reich
In this hypothetical model, peripheral macrophages and activated microglia are recruited to the site of tissue damage (i). The potential roles of proinflammatory macrophages and microglia are shown in ii and iii: clearance of myelin debris, removal of free iron derived from the demyelinating process, production of free radicals, induction of the glial scar, and recruitment of oligodendrocyte precursors. The healing process is mediated by the shift from proinflammatory to antiinflammatory macrophages/microglia induced by interaction with the extracellular matrix and other factors (iv). Antiinflammatory macrophages/microglia help limit formation of a glial scar and promote migration of oligodendrocyte precursor cells into the demyelinated lesion (v), where they can mature into myelinating oligodendrocytes (vi) and remyelinate naked axons (vii). Any interruption of the healing process (e.g., failure of macrophages/microglia to acquire a fully antiinflammatory phenotype and subsequently clear macrophages/microglia) might trigger a vicious circle, resulting in persistence of the phase rim over at least the first year of lesion evolution.