Abstract
Background
Dynamic contrast-enhanced magnetic resonance imaging is a promising biomarker allowing for in vivo quantification of blood–brain barrier permeability.
Objectives
To explore the relationship between blood–brain barrier permeability, optic neuritis disease severity, and multiple sclerosis conversion in optic neuritis.
Methods
Gjedde-Patlak models from dynamic contrast-enhanced magnetic resonance imaging were used to estimate blood–brain barrier permeability (Ki) in 78 optic neuritis patients. The 2017 McDonald criteria were used to diagnose multiple sclerosis with a minimum follow-up time of 2 years.
Results
Normal-appearing white matter Ki correlated with the number of magnetic resonance imaging criteria for dissemination in space (Spearman's ρ = 0.3, p = 0.0074), but not with visual acuity, color vision, and inter-eye difference in retinal nerve fiber layer thickness. Normal-appearing white matter Ki did not differ between patients with and without oligoclonal bands (p = 0.067), but patients with brain contrast-enhancing lesions had higher normal-appearing white matter Ki than those without (p = 0.04). Early multiple sclerosis-converters diagnosed at optic neuritis onset (n = 36) had higher normal-appearing white matter Ki than non-converters (n = 29) (p = 0.01), but this was not the case for late multiple sclerosis-converters (n = 13) (p = 0.57). Normal-appearing white matter Ki did not significantly predict overall multiple sclerosis conversion (p = 0.068, AUC = 0.652).
Conclusions
Normal-appearing white matter Ki was associated with magnetic resonance imaging biomarkers of multiple sclerosis, but not with biomarkers of optic neuritis disease severity. Normal-appearing white matter Ki was increased at, but not before, the multiple sclerosis diagnosis.
Keywords
Introduction
Increased blood–brain barrier (BBB) permeability is a hallmark of active lesions in multiple sclerosis (MS). Using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) we have previously shown that subtle increases in BBB permeability are also present in the normal-appearing white matter (NAWM) of MS patients when compared to healthy controls (HCs). 1 Although applications of DCE-MRI are evolving, 2 data on NAWM BBB permeability quantification in MS are still scarce1,3–11 and our understanding of the factors influencing it is still limited.
Optic neuritis (ON) is a common form of clinically isolated syndrome12,13 and the 15-year risk of MS following ON has been estimated to be 50%. 14 In a previous study, NAWM BBB permeability predicted the conversion of ON to MS according to the 2010 McDonald criteria.4,15 Furthermore, measurements of BBB permeability 6 months after treatment initiation have been found to predict two-year treatment responses to natalizumab or fingolimod 9 as well as alemtuzumab, 6 highlighting DCE-MRI-based measurements of BBB permeability as a promising biomarker of neuroinflammation.
In this study, to gain a better understanding of the factors associated with increased BBB permeability during ON, we investigated the relationship between BBB permeability and biomarkers of ON disease severity. Furthermore, we assessed the diagnostic performance of BBB permeability measurements, when MS was diagnosed according to the 2017 McDonald criteria. 16
Materials and methods
Study participants
This study presents baseline data from an open-label, non-randomized single-center clinical trial (ClinicalTrials.gov identifier: NCT03451955) examining the effects of a gluten-free diet (GFD) in patients with ON. Patients in the intervention group adhered to a strict GFD for 6 months, whereas patients in the control group maintained their habitual diet. Patients were recruited from the Optic Neuritis Clinic at Copenhagen University Hospital Rigshospitalet-Glostrup between January 2018 and September 2021. Inclusion and exclusion criteria ensured the recruitment of magnetic resonance imaging (MRI) compatible patients with symptomatic ON and no prior MS diagnosis within the age range of 18–59 years and in the absence of conflicting disorders, pregnancy, or lactation. Meningioma was the only conflicting disorder leading to patient exclusion. Causes of ON-resembling symptoms in excluded patients where the diagnosis of ON could not be verified were headache with presbyopia, dry eyes, anterior ischemic optic neuropathy, and nonarteritic anterior ischemic optic neuropathy. Patients with identified causes of ON that were not MS (i.e. sarcoidosis, myelin oligodendrocyte glycoprotein, or aquaporin-4 seropositivity) were excluded. Data from 78 patients with verified ON that had DCE-MRI performed as close as possible to ON onset were included.
The diagnosis of ON was verified by a professor of neurology with a specialty in ON and MS. The 2017 McDonald criteria 16 were used to diagnose MS with a follow-up time ranging from 24 to 69 months following ON onset. All patients were offered baseline DCE-MRI and lumbar puncture (LBP) as close as possible to ON onset as well as follow-up MRI and LBP (same protocols as baseline) 6 months after baseline DCE-MRI. Results from follow-up MRI and LBP are not presented but were incorporated in the evaluation of future fulfillment of the 2017 McDonald criteria for MS. Furthermore, ON patients who did not receive the diagnosis of MS at baseline were offered yearly MRIs for the first 2–3 years after ON. Patients were classified as “non-converters” if they did not fulfill the 2017 McDonald criteria for MS during the study's follow-up period, as “early MS-converters” if they fulfilled the criteria at ON onset, and as “late MS-converters” if they did not meet the criteria at ON onset but did so later during the follow-up period.
DCE-MRI
Participants were scanned on a 3 T Philips Achieva dStream MRI scanner (Philips Medical Systems) using a 32-element phased-array head coil. The contrast agent (gadobutrol 1 mmol/mL) was injected automatically (Spectris) as two boluses of each 0.045 mmol/kg bodyweight. The methodologies applied to acquire FLAIR, T1, and T2 sequences are provided in Supplemental Material MRI methods. Radiological descriptions of MRI scans with regards to quantification of brain white matter lesions (0–1, 2–8 and ≥ 9), MRI criteria for dissemination in space (DIS in four regions: spinal cord as well as periventricular, cortical or juxtacortical, and infratentorial regions of the brain) and evaluation of contrast-enhancement (CE) in the brain were performed in accordance with the current guidelines 16 by an experienced specialist of MRI in MS. Brain CE-lesions refer to CE-lesions in tentorial and infratentorial brain; excluding enhancement of the optic nerve. BBB permeability was assessed using DCE-MRI. 7 Methods applied for the acquisition of dynamic sequences in this article have previously been described by Cramer et al. 17
Input function
An arterial input function was extracted as the time-concentration curve from intravascular voxels in the internal carotid artery. Additionally, the signal in the sagittal sinus was extracted and used to correct for partial volume errors and in-flow effects in the input function as described in Cramer et al. 17
Regions of interest and permeability estimation
Five axial T2-weighted MRI images (acquired as described by Cramer et al.)
17
with similar placement of imaging planes as the DCE-MRI sequence but better in-plane resolution were obtained to allow for drawing of regions of interest (ROIs) in the NAWM and thalamus. Two ROIs were manually drawn in the periventricular white matter (one with frontal and one with posterior placement) of each hemisphere (four in total). Care was taken to avoid lesions and areas in their immediate proximity. Please see Cramer et al.
17
and Supplemental Figure 1 for placements as well as corresponding Ki maps. Additionally, two ROIs were drawn covering both thalami to get an estimate of BBB permeability in a region consisting predominantly of gray matter. ROI placement was performed by three experienced operators blinded to the clinical data. The following equation which is valid for a sequence with centric phase encoding was used to convert the DCE time series from signal to contrast agent concentration:
The influx constant Ki—a measure of BBB permeability—was calculated using MATLAB-based (Mathworks) software developed in our unit. Ki values presented in this article were calculated using the Patlak model as described by Cramer et al. 17
Lesion load estimation
Total lesion volume (mm3) was estimated by an in-house developed segmentation model as described by Hindsholm et al. 18 The segmentation model used the FLAIR, T2, and T1 (pre-contrast) scans (see Supplemental Material MRI methods for sequence parameters) for inferring a lesion mask, where the total lesion volume was then calculated.
Visual tests and optical coherence tomography
Visual acuity was evaluated using retroilluminated ETDRS (Early Treatment of Diabetic Retinopathy Study) charts (Precision Vision). Test of color vision using Velhagen pseudoisochromatic plates was performed in a daylight cabinet (Macbeth, The Judge, v.2). Optical coherence tomography (OCT) was performed by trained personnel using Cirrus 4000 HD-OCT (Carl Zeiss). Data on visual acuity, color vision, and retinal nerve fiber layer (RNFL) thickness of the affected eye were excluded from two patients with previous ON affecting the same eye. Data on inter-eye differences in RNFL were excluded from four additional patients with previous ON affecting the other eye and one patient with bilateral ON.
Fluid biomarker analyses
All patients were offered an LBP with routine analyses of cerebrospinal fluid (CSF) leukocyte and erythrocyte concentrations, IgG-index, Qalb (CSF to plasma ratio of albumin), and oligoclonal bands (OCB) evaluated by isoelectric focusing and immunoblotting. One patient refused to have an LBP, as OCB had previously been demonstrated. This patient was considered OCB positive, but no other data were carried forward. Furthermore, LBP was not performed in one patient (late MS-converter) due to an Arnold Chiari malformation. Data on CSF leukocytes, IgG-index, and Qalb were excluded from two patients due to erythrocyte contamination (> 400 µL−1). In the case of CSF leukocytes and CSF IgG below the limits of quantification (CSF leukocytes < 3 million/L: n = 22, CSF IgG < 10 mg/L: n = 3), CSF leukocytes and CSF IgG were sat to 2 million/L and 8 mg/L, respectively.
Statistics
Normality was assessed using the Shapiro-Wilk normality test. None of the investigated parameters were normally distributed. The Mann–Whitney U-test was used to test for differences between numeric, non-normally distributed variables. Pearson's chi-squared test was used to test for differences between categorical variables. In the case of categorical variables with categories with less than five observations, Fisher's exact test was used. Associations between BBB permeability and numerical biomarkers were explored using Spearman's correlation analyses. For linear regression models, a pseudocount of 0.0175 was added to all NAWM Ki values before transformation with the natural logarithm. This was done to meet the assumptions of homoscedasticity and normality of the residuals in the linear regression model. The pseudocount of 0.0175 was selected as it was the smallest pseudocount that would ensure no zero values while still placing all NAWM Ki values on the positive scale. The respective pseudocount for thalamic Ki was 0.0709. The same pseudocounts were also applied in logistic regression analyses, before transformation using the logarithm with base two. The validity of the logistic regression model was verified by a professional statistician from the Biostatistical Department at the University of Copenhagen. All statistical analyses were performed using R 19 Versions 4.3.1 and 4.3.2 and packages readxl, 20 dplyr, 21 ggpubr, 22 table1, 23 ggplot2, 24 viridis, 25 and pROC. 26
Results
Patient characteristics
The baseline characteristics of the 78 patients with verified ON are presented in Table 1. None of the patients received steroid therapy or disease-modifying treatments prior to baseline DCE-MRI or LBP. Ten patients (12.8%) had history of past relapses, but none of them had previously fulfilled the 2017 McDonald criteria for MS. As there was no significant difference in NAWM Ki between ON patients with and without history of past relapses (0.0342 [0.0278, 0.0399] vs. 0.0239 [0.0154, 0.0405], p = 0.20, Mann–Whitney U-test), all patients were included in the further analyses. Out of 49 MS-converters (62.8%), 73.5% fulfilled the 2017 McDonald criteria for relapsing-remitting MS already at ON onset (early MS-converters), whereas the remaining 26.5% fulfilled them within a median of 427 days (min = 46, first quartile = 267, third quartile = 670, max = 1477 days) (late MS-converters). At baseline, 92.3% of 13 late MS-converters did not demonstrate DIS, whereas OCB were present in 76.9% of late MS-converters. Only one ON patient (late MS-converter) presented with DIS without DIT or OCB. Following the ON that led to study inclusion, four patients were treated for CIS; three with dimethyl fumarate and one with peginterferon beta-1a (three converted to MS within the follow-up time).
Demographic and clinical characteristics of ON cohort.
Abbreviations: BBB: blood–brain barrier; CSF: cerebrospinal fluid; DCE-MRI: dynamic contrast-enhanced magnetic resonance imaging; LBP: lumbar puncture; MS: multiple sclerosis; NAWM: normal-appearing white matter; ON: optic neuritis; Qalb: CSF to plasma ratio of albumin; RNFL: retinal nerve fiber layer. The laboratory's upper cut-off values were: CSF leukocytes ≥ 5 (× 106/L), IgG-index > 0.67 and Qalb ≥ 14 (× 10−3). Numerical parameters are not normally distributed and are presented as median [first quartile–third quartile]. Categorical variables are described as numbers (percentages). Data from eyes previously affected by ON have been excluded from all visual and OCT measures.
For the difference between MS-converters and non-converters.
Inter-eye difference was calculated as RNFL thickness of the affected eye minus RNFL thickness of the non-affected eye.
BBB permeability and disease biomarkers in ON
When examining the relationship between BBB permeability and ON severity, we found no correlations between NAWM Ki and visual acuity of the affected eye (Spearman's ρ = –0.0055, p = 0.96, Supplemental Figure 2A), color vision of the affected eye (Spearman's ρ = 0.085, p = 0.47, Supplemental Figure 2B), or inter-eye difference in RNFL thickness (Spearman's ρ = 0.04, p = 0.74, Supplemental Figure 2C). Furthermore, NAWM Ki was not significantly related to the presence of OCB (p = 0.067, Figure 1A), nor did it correlate with IgG-index (Spearman's ρ = 0.15, p = 0.21, Supplemental Figure 2D) or CSF leukocytes (Spearman's ρ = –0.00049, p = 1, Supplemental Figure 2E). Interestingly, although Qalb is commonly used as a biomarker of BBB permeability, it did not correlate with NAWM Ki (Spearman's ρ = –0.028, p = 0.82, Supplemental Figure 2F). When examining the relationship between BBB permeability and other MRI biomarkers, we observed higher NAWM Ki in ON patients with brain CE lesions compared to those without (p = 0.04, Figure 1B). Furthermore, we found a positive correlation between NAWM Ki and the number of fulfilled MRI criteria for DIS (Spearman's ρ = 0.3, p = 0.0074, Figure 1C). To expand on this observed relationship with DIS, we estimated total lesion volumes, which revealed a similar positive correlation with NAWM Ki (Spearman's ρ = 0.26, p = 0.022). Respective analyses for thalamic Ki are presented in Supplemental Figure 3.

Relationship between blood–brain barrier permeability in NAWM expressed as the influx constant Ki from Gjedde-Patlak models obtained using dynamic contrast-enhanced MRI and the presence of oligoclonal bands (A), the presence of CE lesions in the brain (B) and the number of MRI criteria for DIS (C) in patients with optic neuritis. Early MS-converters fulfilled the 2017 McDonald criteria of MS already at ON onset. Late MS-converters did not fulfill the 2017 McDonald criteria of MS at ON onset but did so later during the follow-up time of the study (minimum 2 years). Non-converters are patients with idiopathic ON. Abbreviations: NAWM: normal-appearing white matter; MRI: magnetic resonance imaging; CE: contrast-enhancing; DIS: dissemination in space; MS: multiple sclerosis; ON: optic neuritis.
BBB permeability and diagnosis of MS
MS-converters had higher NAWM Ki compared to non-converters (0.0294 [0.0204, 0.0428] vs. 0.0185 [0.0140, 0.0330], p = 0.026), mainly driven by a difference between early MS-converters and non-converters (0.0300 [0.0220, 0.0502] vs. 0.0185 [0.0140, 0.0330], p = 0.01, Figure 2A). When investigating thalamic Ki, there was no difference between MS-converters and non-converters (0.0278 [0.0181, 0.0480] vs. 0.0320 [0.0157, 0.0397], p = 0.62), but early MS-converters had higher thalamic Ki than late MS-converters (0.0316 [0.0220, 0.0531] vs. 0.0223 [0.0049, 0.0321], p = 0.043, Figure 2B). DCE-MRI scans were performed at a median of 20 days from ON onset (min = 8, Q1 = 15, Q3 = 33, max = 94). Neither NAWM Ki nor thalamic Ki correlated with time from ON onset to DCE-MRI (Supplemental Figure 4). Likewise, we observed no significant correlation between NAWM Ki and age (Spearman's ρ = –0.18, p = 0.11). In a univariate linear regression model with log-transformed NAWM Ki as the dependent variable and timing of MS diagnosis as an independent variable, early MS-converters had 60.8% (6.2%–143.5%, p = 0.026) higher NAWM Ki compared to non-converters (adjusted R2 = 0.04). The coefficient for late MS-converters did not reach significance (p = 0.35). In univariate logistic regression analyses, neither NAWM Ki (AUC = 0.652, p = 0.068) nor thalamic Ki (AUC = 0.466, p = 0.284) were significant predictors of the risk of MS (Figure 3).

Blood–brain barrier permeability expressed as the influx constant Ki from Gjedde-Patlak models obtained using dynamic contrast-enhanced MRI in NAWM (A) and thalamus (B) according to the timing of MS diagnosis. “Early MS” denotes early MS-converters who fulfilled the 2017 McDonald criteria of MS already at ON onset. “Late MS” denotes late MS-converters who did not fulfill the 2017 McDonald criteria of MS at ON onset but did so later during the follow-up time of the study (minimum 2 years). “Not MS” denotes non-converters with idiopathic ON. Mann–Whitney U-test was used to test for differences between groups. Abbreviations: MRI: magnetic resonance imaging; NAWM: normal-appearing white matter; MS: multiple sclerosis; ON: optic neuritis.

Receiver operating characteristic curves of univariate logistic regression models with diagnosis of multiple sclerosis within the follow-up time of the study as dependent variable and Ki in NAWM or thalamus as independent variables. Ki values were log2-transformed with the use of a pseudocount (0.0175 for NAWM and 0.0709 for thalamus) prior to logistic regression. Abbreviations: AUC: area under the curve; NAWM: normal-appearing white matter.
Discussion
NAWM BBB permeability was increased in early MS-converters compared to non-converters (p = 0.01). On the contrary, late MS-converters did not have significantly different NAWM BBB permeability from non-converters (p = 0.57). We observed that only patients fulfilling three (p = 0.018) or four (p = 0.046) MRI criteria for DIS had increased NAWM BBB permeability compared to patients with zero MRI criteria for DIS and found a correlation between NAWM BBB permeability and the number of fulfilled MRI criteria for DIS (Spearman's ρ = 0.3, p = 0.0074). Similarly, NAWM BBB permeability correlated with total lesion load (Spearman's ρ = 0.26, p = 0.022) and was also higher in ON patients with brain CE lesions compared to ON patients without brain CE lesions (p = 0.04). The above confirms that NAWM BBB permeability is increased already early in the disease course of MS and suggests that NAWM BBB alterations are associated with MS-related brain pathology.
As the BBB may even respond to systemic inflammation, 27 it could be hypothesized that ON-related neuroinflammation would be adequate to alter NAWM BBB permeability. We did not observe any relationships between BBB permeability and OCB, IgG-index, or CSF leukocytes, although CSF leukocytes were previously found to correlate with NAWM BBB permeability. 4 Similarly, we found no correlations between BBB permeability and visual acuity, color vision, or inter-eye differences in RNFL thickness suggesting that BBB permeability is not related to the degree of optic nerve pathology.
Although Qalb is considered a biomarker of BBB permeability, we and others have found Qalb and DCE-MRI-based measures of BBB permeability to be dissociated.4,5,28 Ki is believed to reflect BBB changes in capillaries or postcapillary venules, however, during subtle, non-disruptive BBB perturbances as in ON, the choroid plexus is expected to be the main route of albumin entry to the CSF (albumin is ∼100 times larger than gadolinium-DOTA). 29
Contrary to our previous publication 4 where NAWM BBB permeability predicted MS conversion according to the 2010 McDonald criteria, 15 NAWM BBB permeability did not reach statistical significance as an independent predictor of MS conversion according to the 2017 McDonald criteria (AUC = 0.652, p = 0.068). The 2017 McDonald criteria have higher sensitivity and lower specificity for clinically definite MS resulting in faster diagnosis in a higher number of patients.30,31 Therefore, it could be hypothesized that the lower predictive power of Ki in this study 4 could be due to the classification of more subjects with a less active disease course, consequently exhibiting lower degrees of neuroinflammation, brain pathology, and presumably lower Ki, as MS-converters when using the updated diagnostic criteria. The present study did indeed show a higher two-year MS-conversion rate (62.8% vs. 44.7%). 4 Implementation of routine testing for MOG antibodies has also allowed for improved differential diagnosis since the recruitment of our past ON cohort. 4 Lastly, we cannot rule out potential selection bias related to patients' willingness to participate in a time-consuming trial or potential influences of the GFD on the risk of MS.
In summary, this is one of the largest datasets with Ki to be analyzed to date. Our data support the presence of NAWM BBB alterations at the time of MS diagnosis and enhance our understanding of the relationship between BBB permeability and neuropathological processes during ON.
Supplemental Material
sj-pdf-1-mso-10.1177_20552173251346979 - Supplemental material for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis
Supplemental material, sj-pdf-1-mso-10.1177_20552173251346979 for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis by Moschoula Passali, Maria Højberg Knudsen, Knud Josefsen, Julie Christine Antvorskov, Amalie Monberg Hindsholm, Ulrich Lindberg, Jette Lautrup Frederiksen, Henrik Bo Wiberg Larsson and Stig Præstekjær Cramer in Multiple Sclerosis Journal – Experimental, Translational and Clinical
Supplemental Material
sj-docx-2-mso-10.1177_20552173251346979 - Supplemental material for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis
Supplemental material, sj-docx-2-mso-10.1177_20552173251346979 for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis by Moschoula Passali, Maria Højberg Knudsen, Knud Josefsen, Julie Christine Antvorskov, Amalie Monberg Hindsholm, Ulrich Lindberg, Jette Lautrup Frederiksen, Henrik Bo Wiberg Larsson and Stig Præstekjær Cramer in Multiple Sclerosis Journal – Experimental, Translational and Clinical
Supplemental Material
sj-docx-3-mso-10.1177_20552173251346979 - Supplemental material for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis
Supplemental material, sj-docx-3-mso-10.1177_20552173251346979 for Blood–brain barrier permeability in relation to disease severity and timing of multiple sclerosis diagnosis in optic neuritis by Moschoula Passali, Maria Højberg Knudsen, Knud Josefsen, Julie Christine Antvorskov, Amalie Monberg Hindsholm, Ulrich Lindberg, Jette Lautrup Frederiksen, Henrik Bo Wiberg Larsson and Stig Præstekjær Cramer in Multiple Sclerosis Journal – Experimental, Translational and Clinical
Footnotes
Acknowledgements
We thank Ms. Helle J. Simonsen for her assistance with acquiring and analyzing MRI data. Furthermore, we acknowledge MD Josefine Britze, MD Mathias F. Schmidt, and Ms. Asma I. Ali for clinically examining the recruited patients and thank all participants as well as the Danish Multiple Sclerosis Society (DMSS) for making this study possible.
Consent to participate
Study participants received written and oral information and signed written informed consent before participation. Study consent forms were approved by Copenhagen's Regional Committee on Health Research Ethics.
Consent for publication
Not applicable.
Data availability statement
The data can be provided by the corresponding author in an anonymized form upon reasonable request from any qualified investigator.
Declaration of conflicting interest
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: M Passali received funding from the DMSS. MH Knudsen received funding from Sanofi Genzyme and the DMSS and non-financial support from Merck. K Josefsen, JC Antvorskov, AM Hindsholm, and U Lindberg report no disclosures. J Frederiksen served on scientific advisory boards for and received funding for travel related to these activities as well as honoraria from Merck Serono, Sanofi-Aventis, Roche, Novartis, and Chiesi. HBW Larsson received funding from the DMSS. SP Cramer received funding from Sanofi Genzyme and the DMSS. This study has been funded by the DMSS (grant numbers A35733, A38270, and A42545).
Ethical approval
All procedures were conducted in accordance with the 1975 Declaration of Helsinki. Patients included in this study participated in our GFD clinical trial and our ON biomarker study. Both protocols were approved by Copenhagen's Regional Committee on Health Research Ethics (H-17019986 and H-4-2014-095).
ORCID iDs
Supplemental material
Supplemental material for this article is available online.
References
Supplementary Material
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