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Research paper
Inflammatory natalizumab-associated PML: baseline characteristics, lesion evolution and relation with PML-IRIS
  1. Mike P Wattjes1,2,
  2. Martijn T Wijburg2,3,
  3. Jeroen van Eijk4,
  4. Stephan Frequin5,
  5. Bernard M J Uitdehaag3,
  6. Frederik Barkhof2,6,
  7. Clemens Warnke7,8,
  8. Joep Killestein3
  9. On behalf of the Dutch-Belgian Natalizumab-Associated PML Study Group
    1. 1 Department of Diagnostic and Interventional Neuroradiology, Hannover Medical School, Hannover, Germany
    2. 2 Department of Radiology and Nuclear Medicine, Neuroscience Amsterdam, VUmc MS Center Amsterdam, VU University Medical Center, Amsterdam, The Netherlands
    3. 3 Department of Neurology, Neuroscience Amsterdam, VUmc MS Center Amsterdam, VU University Medical Center, Amsterdam, The Netherlands
    4. 4 Department of Neurology, Jeroen Bosch Ziekenhuis, s’Hertogenbosch, The Netherlands
    5. 5 Department of Neurology, St Antonius Hospital, Nieuwegein, The Netherlands
    6. 6 Institutes of Neurology and Healthcare Engineering, UCL, London, UK
    7. 7 Department of Neurology, Medical Faculty, University of Düsseldorf, Düsseldorf, Germany
    8. 8 Department of Neurology, University Hospital of Cologne, Cologne, Germany
    1. Correspondence to Dr Mike P Wattjes, Department of Diagnostic and Interventional Neuroradiology, Hannover Medical School, Carl-Neuberg-Straße 1, D-30625 Hannover, Germany; wattjes.mike{at}


    Background and objective Natalizumab-associated progressive multifocal leukoencephalopathy (NTZ-PML) patients may show imaging signs suggestive of inflammation at diagnosis (‘inflammatory PML’), reminiscent of PML-immune reconstitution inflammatory syndrome (PML-IRIS). We investigated the imaging characteristics of inflammatory NTZ-PML lesions and PML-IRIS to determine differentiating and overlapping features.

    Methods We scored the presence, localisation and pattern of imaging characteristics of inflammation on brain MRI scans of inflammatory NTZ-PML patients. The imaging characteristics were followed up until the occurrence of PML-IRIS.

    Results Ten out of the 44 NTZ-PML patients included showed signs suggestive of inflammation at the time of diagnosis. The inflammation pattern at diagnosis was similar to the pattern seen at PML-IRIS, with contrast enhancement representing the most frequent sign of inflammation (90% at diagnosis, 100% at PML-IRIS). However, the severity of inflammation differed, with absence of swelling and low frequency of perilesional oedema (10%) at diagnosis, as compared with the PML-IRIS stage (40%).

    Conclusion Patterns of inflammation at the time of PML diagnosis and at the PML-IRIS stage overlap but differ in their severity of inflammation. This supports histopathological evidence that the inflammation seen at both stages of the same disease shares a similar underlying pathophysiology, representing the immune response to the JC virus to a variable extend.

    • multiple sclerosis
    • magnetic resonance imaging
    • progressive multifocal leukoencephalopathy
    • natalizumab
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    Progressive multifocal leukoencephalopathy (PML) is a serious side effect of immunosuppressive therapies particularly seen in multiple sclerosis (MS) patients treated with natalizumab (NTZ, Biogen, Cambridge, Massachusetts, USA), a humanised monoclonal antibody against the α4-integrin adhesion molecule.1 2 PML is an opportunistic infection of the central nervous system (CNS) caused by reactivation and replication of JC virus (JCV), characterised by a lytic infection of oligodendrocytes, astrocytes and neuronal cells.3–5 PML risk mitigation programme during therapy with NTZ recommends regular clinical assessment, laboratory tests (eg, JCV serostatus) and MRI, aiming at improving the benefit–risk ratio of a drug with a known high clinical and MRI efficacy in MS.6–9

    In NTZ-treated MS patients, brain MRI can detect opportunistic infections such as PML at very early stages, even prior to the development of clinical symptoms suggestive of PML coining the term presymptomatic or asymptomatic PML.10–14 However, the detection of PML at an early stage can be challenging, since the imaging findings can be subtle, fluctuating and difficult to interpret.15–19 In comparison to classical, HIV-associated cases of PML, NTZ-PML displays a higher frequency of MRI signs suggestive of inflammation at the time of diagnosis, including contrast enhancement and punctuate lesions with a perivascular distribution pattern, reported in approximately 30% of the patients.11 15 16 Such lesions can be the most prominent initial imaging sign at the time of PML diagnosis, even in asymptomatic NTZ-PML, reflecting inflammation in the perivascular spaces and thereby unmasking the opportunistic infection.19–22 These observations have led to the term ‘inflammatory PML’, thereby differentiating these PML cases from those without any signs of inflammation, termed ‘classical PML’.18 23 24 It has been suggested that the inflammation in NTZ-PML is caused by the mode of action of a drug that is classified as a selective immune suppressant, with a partial maintenance of immune functions in the CNS.25

    ‘Inflammatory PML’ shares several imaging and histopathological characteristics with PML-immune reconstitution inflammatory syndrome (IRIS).20 21 PML-IRIS is characterised by a clinical deterioration despite partial or full recovery of the immune competence in previously immunocompromised patients.22 26–30 Inflammatory PML and PML-IRIS often are not clearly separated, and terminology is partly conflicting in published literature. In ‘inflammatory PML’, lytic infection by JCV is supposed to be the leading cause of structural brain damage and inflammation, a rather desirable side action of a still partly functioning immune system limiting the further spread of and supporting the destruction by the virus. In contrast, during PML-IRIS, the immune reactions, initiated by the JCV replicating but then spreading and becoming an independent factor of tissue destruction, are believed to overshoot and become the leading cause of structural brain damage. Thus, the correct interpretation and recognition of the two distinct variants of inflammation could affect management and treatment of patients with PML.31 32

    Systematic data on the lesion evolution of inflammatory PML lesions and criteria for separation from PML-IRIS are lacking. The aim of this study was to investigate characteristics of inflammatory PML and PML-IRIS, including the lesion evolution in patients with and without signs of inflammation at the time of diagnosis.

    Patients and methods

    Standard protocol approvals, registrations and patient consents

    Brain MRI is included in the standard patient care of NTZ-treated MS patients for treatment efficacy assessment and safety monitoring purposes. We retrospectively collected clinical, laboratory and imaging data from NTZ-PML patients. We obtained a waiver from our local institutional review board stating that the requirements of the Medical Research Involving Human Subjects Act did not apply and that official institutional review board approval was not mandatory. Written informed consent was obtained from all participants for the use of the clinical, laboratory and imaging data for research and teaching purposes.

    Study design and patient selection

    This retrospective study used routine brain MR images for the diagnosis and follow-up of PML lesions in NTZ-treated MS patients. We obtained data from 67 NTZ-associated PML patients, 25 of whom were derived from the Dutch-Belgian NTZ-associated PML cohort and 42 patients referred by other institutions to our centre for second opinion and research purposes. Figure 1 gives detailed information on the patient selection and inclusion process. MR images were collected in the Digital Imaging and Communication in Medicine (DICOM 3) file format. All MRI scans from the first observation of PML lesions through follow-up until and including PML-IRIS stage were collected. Only patients fulfilling the following criteria were analysed for the purpose of this study: (1) availability of T2-weighted and contrast-enhanced, T1-weighted images at the time of diagnosis and during PML follow-up; (2) MR images of sufficient quality, suitable for diagnostics purposes (ie, no movement artefacts or bad repositioning and others); and (3) sufficient data available at diagnosis and during the clinical course to enable assessment of the detection of imaging findings suggestive of PML-IRIS.33

    Figure 1

    Flow chart illustrating the patient selection and inclusion process. PML, progressive multifocal leukoencephalopathy.

    Image analysis and interpretation

    All MRI scans were analysed on a digital workstation in consensus by two raters (MPW and MTW) with special expertise in the field of inflammatory diseases of the CNS. Brain MRI scans were screened for signs suggestive of inflammation at the time of PML diagnosis, before immune reconstitution (‘inflammatory PML’).

    Imaging characteristics suggestive of inflammation were categorised as recently described33: (1) occurrence of contrast enhancement in the brain; (2) occurrence of lesions showing new signs of mass effect and/or perilesional oedema; per definition, subtle perilesional oedema can present without any mass effect or swelling; and (3) occurrence of new punctate T2 lesions with a perivascular spread. The characteristics of contrast enhancement were further classified according to the localisation (in the centre of PML lesions, in the border of PML lesions, outside of PML lesions with a perivascular spread or outside of PML lesions without a perivascular spread)  and the enhancement pattern (punctuate, homogeneous, patchy).

    In patients showing signs of inflammation at the time of PML diagnosis, the evolution of MRI findings was assessed on follow-up MRI scans up to and including PML-IRIS stage. Patients were considered to fulfil the PML-IRIS stage when both clinical deterioration and new or progressive imaging signs of inflammation were noted on MRI after NTZ cessation.23 26 27 33 The MRI analysis on the follow-up visits included (1) lesion evolution of the main PML lesions (size increase, decrease, stable); (2) contrast enhancement: increase, decrease or stable contrast enhancement of pre-existing lesion, new contrast-enhancing lesion, change of the enhancement pattern; (3) new small T2 lesions with a perivascular distribution pattern; and (4) new mass effect and/or oedema.

    MRI protocols

    Since the PML cases were collected from different centres, the image acquisition parameters including pulse sequences, head coils and magnetic field strengths (1.5T and 3T) and parameters related to spatial resolution were heterogeneous and based on local MRI protocols. In all patients, the MRI protocol at the time of first PML lesion detection and during follow-up, including the PML-IRIS stage, consisted of T2-weighted, T2 fluid-attenuated inversion recovery and postcontrast T1-weighted MR images. In 17 patients, precontrast T1-weighted images were also available during follow-up. Based on the multicentre data acquisition, the scan intervals of follow-up MRI after the diagnosis of PML were not standardised and ranged from 1 to 4 weeks.



    Of the screened 67 NTZ-associated PML patients, 44 patients were eligible for analysis. Nineteen patients were excluded due to insufficient data available during follow-up of the PML disease course, and two were excluded due to insufficient data available at PML diagnosis (inclusion criterion 3). Two patients were excluded due to insufficient quality of the MR images (inclusion criterion 2). Among the included patients, only the 10 patients who showed imaging signs of inflammation at the time of PML diagnosis were selected for the purpose of this study (figure 1). The demographic and clinical information of these 10 patients is presented in table 1, including the diagnostic classification according to PML diagnostic criteria as proposed in a consensus statement from the American Academy of Neurology Neuroinfectious Disease Section.34

    Table 1

    Demographic and clinical characteristics of the included patients

    With respect to the treatment history before the initiation of NTZ, in 7 of the 10 patients, exact data on prior immunotherapy for the treatment of MS prior to NTZ-PML development are known. Four were previously treated with interferon beta-1a, one had been treated interferon beta-1a and interferon beta-1b and two had no prior immunotherapy. Of the remaining three patients, it is known that they were not previously treated with immunosuppressive therapy, but it is unknown whether they had used immunomodulatory drugs.

    Imaging characteristics of inflammation at PML diagnosis and during PML-IRIS phase

    Global frequency of imaging signs of inflammation

    As per definition, all patients analysed showed signs of inflammation already at the time of PML diagnosis, with contrast enhancement seen in 9 out of 10 patients (90%) and perivascular T2 lesions in six patients (60%), among whom one did not display contrast enhancement. At the time of PML-IRIS, all patients (100%) displayed contrast enhancement, and the proportion of patients with perivascular T2 lesions increased to 8 out of the 10 patients (80%). Perilesional oedema was seen in only one patient at the time of diagnosis (10%), increasing to four (40%) during PML-IRIS. Swelling with mass effect was absent in patients at diagnosis, increasing to six (60%) during PML-IRIS.

    Characteristics of contrast enhancement

    At PML diagnosis and during PML-IRIS, contrast enhancement was seen at the border of the PML lesion (8 and 10 patients, respectively), outside of the PML-lesions (six and nine patients, respectively) and in the centre of the PML lesion (two and five patients, respectively).

    Appearance of contrast enhancement was rarely noted to be homogeneous (none at diagnosis, one during PML-IRIS), but either of punctuate (five at diagnosis, eight during PML-IRIS) or of patchy (eight at diagnosis, 10 during PML-IRIS). Figures 2 and 3 show examples of different enhancement pattern (punctuate, patchy).

    Figure 2

    T2- and T1-weighted images (with contrast administration) at the time of PML diagnosis (top row) and at the time of PML-IRIS stage (bottom row). The images at diagnosis (‘inflammatory PML’) show a subcortical and cortical lesions in the right frontal lobe showing contrast enhancement (C and D) in addition to punctuate T2 lesions following a perivascular distribution that also enhance on T1 after contrast administration (A and B). These inflammatory PML lesions show different enhancement pattern such as punctuate (B) and patchy (D). At the time of PML-IRIS manifestation, the PML lesions have increased in size, and the contrast enhancement of the main PML lesion (H and I) as well as in and around the perivascular T2 lesions (F and G) has also markedly increased. In addition, there are now signs of oedema with mass effect around the PML lesions (F and H). PML, progressive multifocal leukoencephalopathy; IRIS, immune reconstitution inflammatory syndrome.

    Figure 3

    Fluid attenuation inversion recovery, T2- and T1-weighted images (with contrast administration) at the time of PML diagnosis (top row) and at the time of PML-IRIS stage (bottom row). At the time of diagnosis, there are multiple contrast-enhancing punctuate T2 lesions (punctuate enhancement pattern) with a perivascular distribution visible (A–C). At PML-IRIS manifestation, there is a massive increase in the number of contrast-enhancing perivascular lesions and persistence of the contrast enhancement from the time of diagnosis (D–F). The increase of punctuate lesions in number and size includes enhancing lesions in the main PML lesions as well as outside of the main PML lesion (F). In addition, the main PML lesion had increased in size. PML, progressive multifocal leukoencephalopathy; IRIS, immune reconstitution inflammatory syndrome.

    Individual course of the patients and MRI lesion characteristics

    The individual course of lesion characteristics of the patients is shown in table 2. The clinical presentation including Expanded Disability Status Scale has not been systematically assessed during and after the PML/PML-IRIS disease course. One single patient (patient number 1) died; all other patients survived PML/PML-IRIS. One patient (patient number 7) stayed asymptomatic during the whole PML/PML-IRIS disease course. One patient (patient number 1) received single course of intravenous (IV) corticosteroids (1000 mg/day for 3 days) directly after the diagnosis of inflammatory PML.

    Table 2

    Individual course of lesion characteristics

    Comparing extent and distribution of contrast enhancement at diagnosis and at the PML-IRIS stage, 9 out of 10 patients showed new or persisting contrast enhancement following the same pattern during PML-IRIS as seen at the time of PML diagnosis (seven patients with new contrast-enhancing lesions following a similar pattern and seven patients with persistence of the contrast enhancement from the time of diagnosis). The progression of contrast enhancement at/during PML-IRIS stages was present in the centre/at the border of the main lesion as well as in lesions outside the main PML lesion (figure 3). In the one patient showing just perivascular T2 lesion as imaging sign suggestive of inflammation at the time of PML diagnosis, these perivascular T2 lesions started showing additional contrast enhancement, with associated oedema and mass effect in other locations during the PML-IRIS phase. Figures 2 and 3 show examples of the inflammatory PML lesion characteristics at baseline and during follow-up.


    In this study, we systematically describe the imaging characteristics of ‘inflammatory PML’ lesions, and we show that the vast majority of these patients continue to show similar signs of inflammation during PML-IRIS stages. Signs of inflammation, in particular contrast enhancement, seen on brain MRI have been described in approximately 30% of NTZ-associated PML patients at the time of PML diagnosis, either in symptomatic or asymptomatic disease stage.11 15 16 35 The pathophysiological background of these imaging signs suggestive of inflammation remained poorly understood for a long time. Recent histopathological data suggested that such inflammation at the time of PML diagnosis might be related to an immune response against the JCV, similar to, but less severe than, in patients entering the PML-IRIS stage.23 26 27 PML-IRIS lesions are characterised by inflammatory cell infiltrations including an abundance of CD8+ T cells and numerous macrophages. In addition, surprisingly high plasma cell numbers were reported in NTZ-associated PML-IRIS by one histopathological case series, not noted in HIV-associated PML.27 Of importance to our study, NTZ-associated inflammatory PML cases generally share these histopathological findings of PML-IRIS, including the high plasma cell numbers, although to a lesser extent.27 This also refers to specific patterns of inflammation, such as the observation of perivascular cuffing, observed in PML-IRIS patients as well as in inflammatory PML patients.26 Obviously, even in early PML stages, CD8+ cytotoxic T cells are able to attack the JCV and control the PML disease activity.36

    In fact, our in vivo imaging study is in line with these histopathological data. In general, the vast majority of our patients showed a similar imaging pattern of inflammation at the time of PML diagnosis as during the PML-IRIS stage. Although the imaging pattern suggestive of inflammation remained similar during follow-up, the severity of inflammation increased at the PML-IRIS stage including new enhancing lesions, swelling and perilesional oedema. As such, imaging patterns of inflammation at the time of PML diagnosis and at the PML-IRIS stage likely are no distinct entities, but rather differ in their extent of inflammation. This supports experimental evidence that the inflammation seen at both stages of the same disease may share a similar underlying pathophysiology, representing the immune response to the causative JCV to a variable extent.27

    Comparing our present inflammatory PML patients to our recently published ‘classic’ NTZ-PML cohort without any imaging signs of inflammation, the time interval between PML diagnosis and PML-IRIS occurrence was longer for patients with ‘inflammatory’ PML (66.5 days, range 23–224 (table 1) vs 42 days, range 6–98 days). In addition, one of our inflammatory PML patients received IV corticosteroids directly  following the diagnosis of inflammatory PML, whereas four patients of the non-inflammatory PML cohort received corticosteroids <30 days to PML-IRIS manifestation.33 It remains unclear if this difference holds up in independent cohorts and if this is of clinical relevance. However, it could have influenced the patient management.37

    In general, the investigation of any link between the clinical outcome and the described MRI findings was not the aim of this study. Owing to the rather small size of our study, we were unable to link presence or absence of inflammation at the time of PML diagnosis to clinical outcome, warranting larger studies and a prospective, multicentric approach. Furthermore, another open question is if patients with signs of inflammation on imaging should be treated differently as compared with patients with classical PML. Potential differences in the patient management could relate to the early administration of corticosteroids even before the patient is classified as PML-IRIS or the use of measures of enhancing NTZ clearance depending on imaging characteristics (plasmapheresis/immunoabsorption). Additional biomarkers such as virus-specific antibody responses in blood, cerebrospinal fluid (CSF) or T cell responses that classify and quantify the immune response against JCV at the time of PML diagnosis may potentially be useful tools for individualising therapeutic regimens.31 32 38 39

    This study has limitations. First of all, the number of patients in our study presenting with imaging signs suggestive of inflammation with a complete clinical and radiological follow-up until the PML-IRIS stage is rather small. Although these patients are well characterised in terms of patient management, treatment and comorbidity, we cannot exclude that some of these aspects could have influenced the clinical and imaging presentation. Further studies including larger numbers of patients are needed to further support our results.

    In conclusion, our study demonstrates that an imaging pattern suggestive of inflammation at the time of PML diagnosis in NTZ-treated MS patients shares imaging characteristics of PML-IRIS in later disease stages. Many of these initial inflammatory PML lesions develop into sites of severe inflammation at PML-IRIS stage. This further supports histopathological and experimental data that this inflammation at the time of PML diagnosis is most likely based on a lymphocytic response against the JCV due to an incomplete immune suppression during NTZ treatment.


    The authors wish to thank all progressive multifocal leukoencephalopathy patients for agreeing to the use of their MRIs and (para)clinical data for research and education purposes. In addition, we wish to thank Professor Wolfgang Brück and Dr Imke Metz (Department of Neuropathology, University Hospital Göttingen, Germany) for sharing their data and expertise on histopathology findings in natalizumab-associated PML-IRIS. FB is supported by the NIHR Biomedical Research Centre at UCLH.


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    • Contributors MPW, MTW collected the data, analysed the data and wrote the manuscript. FB, SF, BMJU, CW and JK interpreted the data and edited the manuscript. All authors reviewed and agreed on the final versions of the manuscript.

    • Funding The MS Centre Amsterdam is funded by a programme grant (14-358e) from the Stichting voor MS Research (Voorschoten, The Netherlands). CW was supported by a grant from the Hertie-Foundation (P1150063).

    • Competing interests MPW has received consultancy fees from Biogen, Novartis and Roche. FB serves as a consultant for Bayer-Schering Pharma, Sanofi-Aventis, Biogen, Teva, Novartis, Roche, Synthon BV, Genzyme and Jansen Research. JK has accepted consulting fees from Merck-Serono, TEVA, Biogen, Genzyme and Novartis. BMJU has received consultancy fees from Novartis, Merck Serono, Biogen and Danone Research. MTW does not report any competing interest. The VUmc has received financial support for research activities from Bayer Schering Pharma, Biogen, Glaxo Smith Kline, Merck Serono, Novartis and Teva. JE received consultancy fees and/or lecture fees from Biogen, Genzyme, Teva, Merck and Novartis. The authors had full editorial control of the manuscript and provided their final approval of all content.

    • Ethics approval Institutional review board of the VU University Medical Center Amsterdam, The Netherlands.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Data sharing statement None.

    • Collaborators Bob W van Oosten and Chris H. Polman (VU University Medical Center, Amsterdam, The Netherlands), Dorine A Siepman and Rogier Hintzen (Erasmus MC, University Medical Center Rotterdam, The Netherlands), Jop Mostert (Rijnstate Hospital, Department of Neurology, Arnhem, The Netherlands), Wibe Moll (Maasstad Hospital, Rotterdam, The Netherlands), Alex EL van Golde (ZGT Hospital, Almelo, The Netherlands), Stephan TFM Frequin (St Antonius Hospital, Nieuwegein, The Netherlands), Paul A D Bouma (Tergooi, Blaricum, Hilversum, The Netherlands), Bénédicte Quivron (CH Jolimont, La Louvière, Belgium), Jean Braeckeveldt (Epicura, Baudour, Belgium), Erik van Munster and Jeroen van Eijk (Department of Neurology, Jeroen Bosch Ziekenhuis, s’Hertogenbosch, The Netherlands), Thea Heersema (Department of Neurology, University Medical Center Groningen, Groningen, The Netherlands), Jaap de Graaf (Isala Hospital, Zwolle, The Netherlands).

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