Persistent 7-tesla phase rim predicts poor outcome in new multiple sclerosis patient lesions

Abstract

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.

Figures

Figure 1. Flowchart summarizing MS patients’ progress through the study.

Figure 2. Centripetal lesions: persistent versus transient phase rim during follow-up.

(A) Persistent phase rim after enhancement resolution: Eighteen-month longitudinal evolution of a centripetally enhancing MS lesion with phase rim at 7T MRI in a 49-year-old woman with secondary progressive MS (Expanded Disability Status Scale 5.5, disease duration 19 years). (i) Postcontrast T1-weighted images capture the shift from centrifugal enhancement at baseline to centripetal enhancement at M1. When the lesion enhances centripetally, a hypointense rim on noncontrast phase images colocalizes with initial opening of the blood-brain barrier in peripheral vessels. (ii) After resolution of enhancement (M3, M6, M12, and M18), the rim persists on phase images (red arrows) and appears also on T2*-weighted magnitude images at M6, M12, and M18. (B) Transient phase rim after enhancement resolution: Twelve-month longitudinal evolution of a centripetally enhancing MS lesion with phase rim at 3T and 7T MRI in a 38-year-old woman with relapsing-remitting MS (Expanded Disability Status Scale 1.5, disease duration 6 years). In this case, as well, a hypointense rim can be discerned at baseline on the noncontrast phase images (red arrows); however, the rim disappears in the months following enhancement resolution. §Scans acquired at 3T MRI. Scale bar: 10 mm.

Figure 3. Fate of centrifugal lesions.

Twelve-month longitudinal evolution of a centrifugally enhancing MS lesion at 3T and 7T MRI in a 58-year-old woman with relapsing-remitting MS (Expanded Disability Status Scale 1.5, disease duration 11 years) that arises from a clearly visible central vein. A phase rim is not discerned at any time point. After closure of the blood-brain barrier, the lesion is visible as hypointense on phase images and hyperintense on T2*-weighted magnitude images. §Scan acquired at 3T MRI. Scale bar: 10 mm.

Figure 4. Longitudinal lesion volume and T1-hypointensity assessment.

(A) Semilogarithmic plot of the longitudinal volume for each lesion according to participant age and lesion group (G1, G2, G3). Three main observations about centripetal lesions with persistent phase rim derive from this graph: (a) their lesion volume shrinkage over time tends to plateau after the first 3 months; (b) their mean lesion volume is higher at all time points; and (c) compared with centripetal lesions with transient phase rims, they tend to occur in older individuals. (B) Percentage of lesion volume shrinkage (mean ± SD) between M3 and M12 for each lesion group. (C) Lesional T1 hypointensity at M12 for each lesion group (mean ± SD; T1 signal intensity is expressed in units of SD of normal-appearing white matter signal). In G1, red dots show the in vivo mean T1 hypointensity of the 5 pathologically assessed demyelinated lesions with phase rim. G1: centripetal lesions with persistent phase rim; G2: centripetal lesions with transient phase rim; G3: centrifugal lesions.

Figure 5. Graphical representation of the 3 scenarios of lesion development and evolution according to status of the blood-brain barrier, phase rim, lesion volume, and lesion T1 hypointensity.

(i) Evolution of a fully centrifugal lesion: no shift to centripetal enhancement pattern; no phase rim at any time point. (ii) Evolution of a centrifugal lesion to a centripetal lesion with transient phase rim: disappearance of the phase rim during the follow-up. (iii) Evolution of a centrifugal lesion to a centripetal lesion with persistent phase rim: persistence of the phase rim after enhancement resolution and reduced lesion volume shrinkage and reduced T1 intensity over time in comparison with the other lesion types. Years from lesion onset, the pathological correlate of the persistent phase rim can be assessed at autopsy. BBB, blood-brain barrier; DCE, dynamic contrast enhancement imaging.

Figure 6. MRI/pathology of demyelinated lesions with persistent phase rim.

Images shown are from lesions 2 and 3 in Figure 8. (A) In vivo and postmortem 7T MRI shows 2 periventricular lesions with persistent phase rims that become partially confluent over time as the lesion expands (between 2006 and 2013). The rims were visible on an in vivo 7T MRI in 2013 (not shown), as well as postmortem (red arrows). On the in vivo 7T 3D T1-MPRAGE (2013), these lesions appear strongly hypointense (mean lesion T1 intensity values, respectively: –25.1 and –21.2 in units of SD of normal-appearing white matter signal; mean cerebrospinal fluid T1 intensity: –29.9). Scale bar: 5 mm. (B) In vivo and postmortem MRI-guided histopathology allowed precise localization of the target area. MRI-matched thumbnails of representative serial sections (10-μm-thick sections) show the Luxol fast blue/periodic acid–Schiff (LFB-PAS) stain for myelin, myelin proteolipid protein (PLP) immunohistochemistry, and Bielschowsky staining for axons. Insets i–v are indicated as red squares on the thumbnails to facilitate their localization and the interpretation of the pathological data. Both lesions were completely demyelinated. (i–iii) The lesion edge, where the phase rim was detected on MRI, is characterized by the presence of an extensive CD68-positive cellular infiltrate, corresponding to macrophages/activated microglia (inflammatory infiltrate thickness ~200–400 μm). Luxol fast blue–positive myelin debris (cyan, black arrows) and late myelin degradation products (lipofuscin, purple) can be seen within macrophages at the lesion edge, suggesting ongoing demyelination (ii). The majority of CD68-positive cells also stained positive by the DAB-Turnbull method, indicating the intracellular accumulation of iron (iii). (iv and v) Extensive axonal loss with transection and dystrophy of the remaining axons is seen throughout the lesion center (v). At the lesion edge, some axons were better preserved, and fiber bundles could be discerned (iv). Scale bars: 200 μm (i); 10 μm (ii); 50 μm (iii–v).

Figure 7. Histological features of MS lesions with phase rim.

Chronic demyelination colocalizes with the phase rim at the lesion edge. There are no signs of ongoing remyelination and only rare oligodendrocyte precursor cells (OLIG2+/ASPA– cells, red arrows). In a cortical lesion, on the other hand, there is evidence of oligodendrocyte precursor cells as well as remyelination. Macrophages/activated microglia also colocalize with the phase rim at the lesion edge. Luxol fast blue (cyan) and lipofuscin (purple) inclusions within these cells suggest ongoing early and late myelin degradation process (×100 magnification). A rim of iron-laden CD68-positive cells are clearly present at the lesion edge (DAB-Turnbull staining alone and double staining with anti-CD68/DAB-Turnbull). Mature tissue macrophages expressing the scavenger receptor CD163 were also represented in the CD68 population, suggesting that the population was not homogeneous (double staining with CD68/CD163). Reactive astrocytes and axonal damage: Non–iron-laden reactive astrocytes were seen in the demyelinated lesion center and demyelinating lesion edge (double staining with GFAP/DAB-Turnbull). Residual axons within the lesion center showed evidence of impaired axonal transport (positive staining for nonphosphorylated neurofilaments expressing SMI32). Red arrows indicate the presence, at the lesion edge, of sparse SMI32-positive ovoids suggesting ongoing axonal degeneration. Scale bars: 50 μm (top row); 10 μm (left panel, middle row); 50 μm (right 3 panels, middle row); 50 μm (left 2 panels, bottom row); 20 μm (right 2 panels, bottom row).

Figure 8. Distribution of CD68-positive and iron-positive cells in MS lesions with and without phase rim.

Iron-laden CD68-positive macrophages/activated microglia were more common at the lesion edge than at the lesion center and within the surrounding white matter. On the other hand, lesions without phase rim showed varying degrees of non–iron-laden CD68-positive cells, not significantly different from the surrounding white matter. Lesions 1–4 belong to patient 1, lesions 5 and 6 to patient 2, and lesions 7–10 to patient 3 (see Supplemental Appendix for details). *P < 0.0001 in edge versus center and edge versus white matter (ANOVA, post hoc analysis and Bonferroni correction for multiple comparisons).

Figure 9. Phenomenological model of the origin and fate of the phase rim.

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.