CCR5 Blockade in Inflammatory PML and PML-IRIS Associated With Chronic Inflammatory Diseases' Treatments.

BACKGROUND AND OBJECTIVES: Progressive multifocal leukoencephalopathy (PML) is a disabling neurologic disorder resulting from the infection of the CNS by JC polyomavirus in immunocompromised individuals. For the last 2 decades, increasing use of immunotherapies leads to iatrogenic PML. Iatrogenic PML is often associated with signs of inflammation at onset (inflammatory PML) and/or after treatment withdrawal immune reconstitution inflammatory syndrome (PML-IRIS). Although immune reconstitution is a key element for viral clearance, it may also be harmful and induce clinical worsening. A C-C chemokine receptor type 5 (CCR5) antagonist (maraviroc) has been proposed to prevent and/or limit the deleterious immune responses underlying PML-IRIS. However, the data to support its use remain scarce and disputed. METHODS: We conducted a multicenter retrospective cohort study at 8 university hospitals in France and Switzerland by collecting clinical, biological, and radiologic data of patients who developed inflammatory PML (iPML) or PML-IRIS related to immunosuppressive therapies used for chronic inflammatory diseases between 2010 and 2020. We added to this cohort, a meta-analysis of individual case reports of patients with iPML/PML-IRIS treated with maraviroc published up to 2021. RESULTS: Overall, 27 cases were identified in the cohort and 9 from the literature. Among them, 27 met the inclusion criteria: 16 treated with maraviroc and 11 with standard of care (including corticosteroids use). Most cases were related to MS (92.6%) and natalizumab (88%). Inflammatory features (iPML) were present at onset in 12 patients (44.4%), and most patients (92.6%) received corticosteroids within the course of PML. Aggravation due to PML-IRIS was not prevented by maraviroc compared with patients who received only corticosteroids (adjusted odds ratio: 0.408, 95% CI: 0.06-2.63). Similarly, maraviroc did not influence time to clinical worsening due to PML-IRIS (adjusted hazard ratio = 0.529, 95% CI: 0.14-2.0) or disability at the last follow-up (adjusted odds ratio: 2, 95% CI: 0.23-17.3). DISCUSSION: The use of CCR5 blockade did not help to keep deleterious immune reconstitution in check even when associated with corticosteroids. Despite maraviroc's reassuring safety profile, this study does not support its use in iPML/PML-IRIS. CLASSIFICATION OF EVIDENCE: This study provides Class IV evidence showing that adding maraviroc to the management of iatrogenic iPML/PML-IRIS does not improve the outcome.

Prevention and management of adverse effects of disease modifying treatments in multiple sclerosis.

PURPOSE OF REVIEW: To summarize the currently known side effects of the approved therapies of multiple sclerosis and to suggest monitoring procedures. RECENT FINDINGS: The progress in the treatment of multiple sclerosis with new very effective therapies is accompanied by a number of side effects. Some of these have already been described in the approval studies, but some only after approval in a real world situation. The reason for this is the short duration of the clinical studies, the very heterogeneous patient profile in the real world setting with a number of comorbidities, pretherapies, and wider age range. The side effects may occur during application of therapies or afterwards during the course of the treatment. The side effects may range from mild infections, mild laboratory abnormalities, secondary autoimmune diseases to life-threatening side effects such as progressive multifocal leukoencephalopathy. SUMMARY: It has to be pointed out that these side effects are not to be considered as final and neurologists should be vigilant against new unknown side effects. The doctor should be aware of these undesirable effects, should weigh the benefits of the therapies against the risks, but at the same time she/he should keep in mind that multiple sclerosis can be a very disabling disease if not treated properly.

Progressive Multifocal Leukoencephalopathy Incidence and Risk Stratification Among Natalizumab Users in France.

Importance: Risk of developing progressive multifocal leukoencephalopathy (PML) is the major barrier to using natalizumab for patients with multiple sclerosis (MS). To date, the association of risk stratification with PML incidence has not been evaluated. Objective: To describe the temporal evolution of PML incidence in France before and after introduction of risk minimization recommendations in 2013. Design, Setting, and Participants: This observational study used data in the MS registry OFSEP (Observatoire Français de la Sclérose en Plaques) collected between April 15, 2007, and December 31, 2016, by participating MS expert centers and MS-dedicated networks of neurologists in France. Patients with an MS diagnosis according to current criteria, regardless of age, were eligible, and those exposed to at least 1 natalizumab infusion (n = 6318) were included in the at-risk population. A questionnaire was sent to all centers, asking for a description of their practice regarding PML risk stratification. Data were analyzed in July 2018. Exposures: Time from the first natalizumab infusion to the occurrence of PML, natalizumab discontinuation plus 6 months, or the last clinical evaluation. Main Outcomes and Measures: Incidence was the number of PML cases reported relative to the person-years exposed to natalizumab. A Poisson regression model for the 2007 to 2016 period estimated the annual variation in incidence and incidence rate ratio (IRR), adjusted for sex and age at treatment initiation and stratified by period (2007-2013 and 2013-2016). Results: In total, 6318 patients were exposed to natalizumab during the study period, of whom 4682 (74.1%) were female, with a mean (SD [range]) age at MS onset of 28.5 (9.1 [1.1-72.4]) years; 45 confirmed incident cases of PML were diagnosed in 22 414 person-years of exposure. The crude incidence rate for the whole 2007 to 2016 period was 2.00 (95% CI, 1.46-2.69) per 1000 patient-years. Incidence significantly increased by 45.3% (IRR, 1.45; 95% CI, 1.15-1.83; P = .001) each year before 2013 and decreased by 23.0% (IRR, 0.77; 95% CI, 0.61-0.97; P = .03) each year from 2013 to 2016. Conclusions and Relevance: The results of this study suggest, for the first time, a decrease in natalizumab-associated PML incidence since 2013 in France that may be associated with a generalized use of John Cunningham virus serologic test results; this finding appears to support the continuation and reinforcement of educational activities and risk-minimization strategies in the management of disease-modifying therapies for multiple sclerosis.

Real-world study of efficacy, risk management and reasons for discontinuation of natalizumab for treatment of multiple sclerosis in Russia.

BACKGROUND: NTZ is approved in Russia for the treatment of highly active relapsing remitting multiple sclerosis and is reimbursed via federal budget program. However, no data about NTZ treatment in Russia and the effect of federal reimbursement have been performed so far. OBJECTIVE: To characterize the population of patients receiving natalizumab and assess the efficacy and risk-management plan (RMP) implementation of NTZ therapy in routine clinical practice in Russia. METHODS: We analyzed data for 334 patients, who received at least one infusion of NTZ. Relapse rate, MRI activity, NEDA-3 status after 2 years were assessed. Anti-JC virus antibodies status and RMP implementation were evaluated. Drop-out rate and reasons for therapy discontinuation were analyzed. RESULTS: Patients switched to natalizumab in Russia are mainly female (63%), with median EDSS score of 3.5 and high disease activity: 93% had at least 1 relapse and 58% had both T1Gd+ and new T2 lesion a year before therapy initiation. Introduction of federal reimbursement allowed patients with less relapses to start therapy with natalizumab. The only predictor of 6-month progression was EDSS score at the baseline of therapy (HR = 2.1375, 95%CI 1.0026-4.5570, p = 0.0492). 82% patients reached NEDA-3 at 24 month of therapy. 25% of patients discontinued NTZ for reasons: tolerability (14.5%), JCV antibody status (61%), and patient's decision (17%). RMP was implemented in only 36% patients. CONCLUSION: Natalizumab appeared to have high efficacy in Russian clinical practice. Federal reimbursement allowed less active patients to start natalizumab. More efforts should be done to improve RMP implementation.

Effect of dosage reduction on peripheral blood lymphocyte count in patients with multiple sclerosis receiving long-term fingolimod therapy.

Of the 19 cases of fingolimod-associated progressive multifocal leukoencephalopathy (PML) reported worldwide by the end of 2017, 4 cases were from Japan. This may indicate that fingolimod sensitivity is higher in the Japanese population than in the western population because the fingolimod dosage used for the prevention of multiple sclerosis (MS) is the same in both populations. Therefore, the laboratory data of nine patients with MS receiving fingolimod treatment for more than 2 years were retrospectively collected. Moreover, the effect of drug holiday was compared between five patients who underwent at least 3 months of dosage reduction via drug holiday after receiving fingolimod for more than 2 years and three patients who underwent drug holiday after receiving fingolimod for less than 2 years. The total, CD3(+), CD4(+), C19(+) lymphocyte counts, the CD4(+)/CD8(+) cell ratio, and the serum IgG concentrations were similar between Japanese patients with MS receiving fingolimod for more than 2 years and Western patients from the previous reports. Recovery of lymphocyte counts in the peripheral blood after fingolimod dosage reduction was worse in some MS patients who received long-term fingolimod treatment than in MS patients who received short-term fingolimod treatment. My findings indicate that the currently used fingolimod dosage may be too high for some Japanese MS patients receiving long-term fingolimod treatment.

Efficacy and safety of natalizumab extended interval dosing.

OBJECTIVE: It is postulated that extending the dosing interval of natalizumab (NTZ) from 4 to 5-8 weeks might decrease the risk of progressive multifocal leukoencephalopathy (PML). The aim of this study was to assess the effect of extended interval dosing (EID) on the therapeutic efficacy of natalizumab. METHODS: We reviewed 85 patients treated at two MS centers in the Middle East with natalizumab for at least 6 months using EID. Patients were shifted after an initial treatment period at standard interval dosing (SID) to an EID ranging from 5-8 weeks. RESULTS: The mean treatment duration on SID and EID was 15.4 ±â€¯11.9 and 11.8 ±â€¯7.0 months, respectively. By the end of SID and EID treatment 95.3% and 93.9% of patients were free of relapses (P = 0.41) with an annualized relapse rate (ARR) of 0.0006 and 0.001 respectively (P = 0.42). The mean EDSS at the end of SID and EID periods was 2.56 ±â€¯1.62 and 2.59 ±â€¯1.61 respectively (P = 0.84). A total of 97.6% and 94.7% of patients had no enhancing lesions on MRI during the SID and EID periods respectively (P = 0.18). There were no cases of PML and the rate of infections was lower during the EID period. CONCLUSION: In patients treated with natalizumab, shifting from SID to EID has no negative effect on efficacy as evidenced by relapse rate, disability progression and MRI activity.

Prevention and therapy of JC polyomavirus-mediated progressive multifocal leukoencephalopathy - a realistic possibility?

Progressive multifocal leukoencephalopathy (PML) is a serious opportunistic infection of the brain caused by the JC polyomavirus (JCPyV). PML occurs when immune control of persistent infection with JCPyV fails, the virus mutates and changes its cellular tropism, enters the brain and infects astrocytes, oligodendrocytes and, in particular cases, also neurones. Currently, there is no therapy for this often fatal disease. A number of approaches have failed, and only the restoration of protective immunity, if possible, can lead to clearance of the virus once PML has occurred. During the last two decades, investigators have attempted to understand the factors contributing to the development of PML, which immune mechanisms are involved in immune surveillance, and which in clearing JCPyV from the brain once PML has occurred. Recent data suggest that both CD4+ and CD8+ T cells of the cellular immune system, and also JCPyV-specific antibodies, are involved in protection against PML and in resolving the opportunistic infection. Based on the current immunological data, prophylactic and therapeutic vaccination strategies have been proposed, and first treatment attempts in PML patients have provided promising results that indicate therapeutic vaccination may be feasible.

Dimethyl fumarate: a possible exit strategy from natalizumab treatment in patients with multiple sclerosis at risk for severe adverse events.

INTRODUCTION: Among disease-modifying treatments for multiple sclerosis, natalizumab (NTZ) is highly effective, well tolerated and generally safe. Major concerns regard the risk of developing progressive multifocal leukoencephalopathy (PML), and the occurrence of rebounds or disease activity after its discontinuation. The aim of this study was to explore the efficacy of dimethyl fumarate (DMF) in preventing disease reactivation after NTZ discontinuation. METHODS: Thirty-nine patients with relapsing remitting multiple sclerosis, at high risk of PML, were switched from NTZ to DMF and underwent neurological and 3T MRI monitoring for 2 years. Clinical and MRI data regarding the 2-year period preceding NTZ treatment, the 2 years of NTZ treatment and the 2 years of DMF were collected. RESULTS: During the DMF phase, among the 39 patients, one or more relapses occurred in five patients (12.8%), increased disability progression in 4 (10.3%) and MRI activity in 8 (20.5%). Post-NTZ rebound effect was observed only in one patient. Overall, only two dropouts (one rebound activity and one gastrointestinal side effect) were registered and almost 80% of the patients have still no evidence of disease activity at the end of DMF treatment. The multiple linear regression model revealed that the number of relapses and MRI parameters before DMF treatment were good predictors of disease activity during treatment with DMF. DISCUSSION: DMF appeared generally safe and no carryover PML among investigated cases was observed. Although DMF did not eliminate the possibility of disease reactivation, it seems anyway a promising drug for those patients who shall discontinue NTZ. The clinical and radiological activity preceding the DMF treatment might be used as a prognostic marker of therapy response.

Long-term effectiveness and safety of natalizumab in a Portuguese population.

OBJECTIVES: Natalizumab long-term effectiveness data in real-world relapsing-remitting multiple sclerosis (RRMS) is needed. Our objective is to report the long-term effectiveness and safety of natalizumab in a cohort of RRMS patients. METHODS: This is a retrospective study of natalizumab treatment for two years or longer in RRMS. Annualized relapse rate, Expanded Disability Status Scale (EDSS), brain magnetic resonance imaging T2 lesion volume, JC virus antibody status, previous treatments and adverse events were analysed. RESULTS: Seventy-one patients were included with a mean treatment duration of 44.86±17.39months. Over the treatment duration there was a significant decrease in annualized relapse rate (88.37%) and EDSS (28.57%); no evidence of clinical disease activity in 73.24% and 61.97% after one and two-years respectively; and brain magnetic resonance imaging T2 lesion volume remained stable. Forty patients suspended natalizumab, in 85% due to high risk of developing progressive multifocal leukoencephalopathy (PML). The major complication was PML (n=3). CONCLUSIONS: Natalizumab showed effectiveness in the long-term follow up period of our cohort, with reduction of ARR, EDSS, and MRI lesion load stabilization. PML was the major complication.

The Risk of Progressive Multifocal Leukoencephalopathy in the Biologic Era: Prevention and Management.

Progressive multifocal leukoencephalopathy (PML) is a rare, typically fatal, demyelinating central nervous system infection caused by reactivation of the John Cunningham virus that generally occurs in immunosuppressed patients. With an evolving understanding of a greater clinical heterogeneity of PML and significant implications for therapy, PML should be considered in the differential diagnosis of neurologic presentations of rheumatic diseases. Increased awareness of PML among rheumatologists is required, as earlier diagnosis and restoration of immune function may improve the otherwise grim prognosis associated with PML.

Extended interval dosing of natalizumab in multiple sclerosis.

BACKGROUND: Natalizumab (NTZ), a monoclonal antibody to human α4ß1/ß7 integrin, is an effective therapy for multiple sclerosis (MS), albeit associated with progressive multifocal leukoencephalopathy (PML). Clinicians have been extending the dose of infusions with a hypothesis of reducing PML risk. The aim of the study is to evaluate the clinical consequences of reducing NTZ frequency of infusion up to 8 weeks 5 days. METHODS: A retrospective chart review in 9 MS centres was performed in order to identify patients treated with extended interval dosing (EID) regimens of NTZ. Patients were stratified into 3 groups based on EID NTZ treatment schedule in individual centres: early extended dosing (EED; n=249) every 4 weeks 3 days to 6 weeks 6 days; late extended dosing (LED; n=274) every 7 weeks to 8 weeks 5 days; variable extended dosing (n=382) alternating between EED and LED. These groups were compared with patients on standard interval dosing (SID; n=1093) every 4 weeks. RESULTS: 17% of patients on SID had new T2 lesions compared with 14% in EID (p=0.02); 7% of patients had enhancing T1 lesions in SID compared with 9% in EID (p=0.08); annualised relapse rate was 0.14 in the SID group, and 0.09 in the EID group. No evidence of clinical or radiographic disease activity was observed in 62% of SID and 61% of EID patients (p=0.83). No cases of PML were observed in EID group compared with 4 cases in SID cohort. CONCLUSIONS: Dosing intervals up to 8 weeks 5 days did not diminish effectiveness of NTZ therapy. Further monitoring is ongoing to evaluate if the risk of PML is reduced in patients on EID.

Pulsed corticosteroid treatment in MS patients stabilizes disease activity following natalizumab withdrawal prior to switching to fingolimod.

PURPOSE: Interruption of natalizumab (NTM) treatment in multiple sclerosis (MS) patients may be followed by disease reactivation. On the other hand, patients with positive John Cunningham virus (JCV) antibodies treated with NTM over 24 months demonstrate a higher risk for developing progressive multifocal encephalopathy (PML). No established therapeutic approach is available for treating these patients to prevent disease reactivation. MATERIALS AND METHODS: Of the MS patients treated with NTM at the authors' institution, 30 were found positive for JCV abs. NTM was interrupted followed by a washout period of 6 months. During this period, 20/30 patients received monthly intravenous (i.v.) methylprednisolone (MPD) 1000 mg infusion and regular clinical assessment. On months 3 and 6, brain MRI was performed and 1000 mg MPD was administered for 5 days. RESULTS: All patients were clinically and radiologically stable at the time of NTM break. No clinical relapse was observed during the six-month washout period for the MS patients under monthly MPD treatment, while one patient had a relapse and active lesions in the MRI on month 6. Of the other patients not receiving i.v. MPD regularly after NTM withdrawal, one showed several active lesions in brain MRI and the other had a severe relapse. CONCLUSIONS: Despite the limited size of this patients' cohort, the results of this study support that monthly MPD treatment for 6 months may result in a clinically stable disease status, thus ensuring safe transition to another second-line therapy such as fingolimod, following NTM withdrawal.

PML risk stratification using anti-JCV antibody index and L-selectin.

BACKGROUND: Natalizumab treatment is associated with progressive multifocal leukoencephalopathy (PML) development. Treatment duration, prior immunosuppressant use, and JCV serostatus are currently used for risk stratification, but PML incidence stays high. Anti-JCV antibody index and L-selectin (CD62L) have been proposed as additional risk stratification parameters. OBJECTIVE: This study aimed at verifying and integrating both parameters into one algorithm for risk stratification. METHODS: Multicentric, international cohorts of natalizumab-treated MS patients were assessed for JCV index (1921 control patients and nine pre-PML patients) and CD62L (1410 control patients and 17 pre-PML patients). RESULTS: CD62L values correlate with JCV serostatus, as well as JCV index values. Low CD62L in natalizumab-treated patients was confirmed and validated as a biomarker for PML risk with the risk factor "CD62L low" increasing a patient's relative risk 55-fold (p < 0.0001). Validation efforts established 86% sensitivity/91% specificity for CD62L and 100% sensitivity/59% specificity for JCV index as predictors of PML. Using both parameters identified 1.9% of natalizumab-treated patients in the reference center as the risk group. CONCLUSIONS: Both JCV index and CD62L have merit for risk stratification and share a potential biological relationship with implications for general PML etiology. A risk algorithm incorporating both biomarkers could strongly reduce PML incidence.

BK virus capsid antibodies are associated with protection against subsequent development of PML in HIV-infected patients.

Progressive multifocal leukoencephalopathy (PML) is caused by JC polyomavirus (JCPyV). Because a reciprocal relationship has been described between antibody levels to JCPyV and BK polyomavirus (BKPyV), we performed a nested case control study with pre-diagnostic serum samples from HIV infected subjects to examine the relationship between BKPyV capsid antibodies and the risk of PML. Serum samples collected 0.5-2 years before PML diagnosis from 25 cases (66 samples) and 80 matched controls (204 samples) were tested in ELISA for JCPyV, BKPyV type 1 and type 4 capsid antibodies. High levels of BKPyV 1 and 4 antibodies were associated with a lower risk of PML (BKPyV 1 OR, 0.56, 95% CI, 0.35-0.89; BKPyV 4 OR, 0.40, 95% CI, 0.24-0.0.67). Our study demonstrates that antibodies to BKPyV capsids are an immunological marker of protection against development of PML. Further studies are needed to define the mechanism.

[Progressive multifocal leukoencephalopathy following azathioprine and steroid treatment of pulmonary sarcoid disease]. / Progressive multifokale Leukenzephalopathie. Schwerwiegende Komplikation einer Immunsuppression.

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system following reactivation of the John-Cunningham-Virus (JVC) in an immunocompromised host. This rare condition is characterized by rapid progressing neurologic symptoms often leading to death. In the following, we report on a rapid evolving deterioration of mental status due to PML in an 53-year-old man during treatment of pulmonary sarcoid disease using azathioprine and steroids. In contrast to reported lethal outcomes, our patient experienced a slow recovery of his cognitive impairment and later on of his palsy following termination of immunosuppression.

Therapeutic uses of anti-α4-integrin (anti-VLA-4) antibodies in multiple sclerosis.

Multiple sclerosis (MS) is a disorder of putative autoimmune origin, where immune cells invade the central nervous system and cause damage by attacking the myelin sheath of nerve cells. The blockade of the integrin very late antigen-4 (VLA-4) with the monoclonal antibody natalizumab has become the most effective therapy against MS since its approval in 2004. It is assumed that the inhibition of VLA-4-mediated immune cell adhesion to the endothelium of the blood-brain barrier (BBB) alleviates pathogenic processes of MS and, therefore, reduces disease severity and burden. Not all approaches to treat additional immune-mediated disorders (e.g. Rasmussen encephalitis and neuromyelitis optica) with natalizumab have been successful, but allowed researchers to gain additional insight into mechanisms of specific immune cell subsets' migration through the BBB in the human system. While the long-term efficacy and general tolerability of natalizumab in MS are clear, the over 400 cases of natalizumab-associated progressive multifocal leukoencephalopathy (PML) have been of great concern and methods of risk stratification in patients have become a major area of research. Modern risk stratification includes established factors such as treatment duration, previous immune-suppressive therapy, and anti-John Cunningham virus (JCV) antibody seropositivity, but also experimental factors such as anti-JCV antibody titers and levels of L-selectin. Today, anti-VLA-4 therapy is reserved for patients with highly active relapsing-remitting MS and patients are monitored closely for early signs of potential PML.

Treating progressive multifocal leukoencephalopathy with interleukin 7 and vaccination with JC virus capsid protein VP1.

Progressive multifocal leukoencephalopathy is a currently untreatable infection of the brain. Here, we demonstrate in 2 patients that treatment with interleukin 7, JC polyomavirus (JCV) capsid protein VP1, and a Toll-like receptor 7 agonist used as adjuvant, was well tolerated, and showed a very favorable safety profile and unexpected efficacy that warrant further investigation.

Therapeutic approaches to disease modifying therapy for multiple sclerosis in adults: an Australian and New Zealand perspective: part 3 treatment practicalities and recommendations. MS Neurology Group of the Australian and New Zealand Association of Neurologists.

In this third and final part of our review of multiple sclerosis (MS) treatment we look at the practical day-to-day management issues that are likely to influence individual treatment decisions. Whilst efficacy is clearly of considerable importance, tolerability and the potential for adverse effects often play a significant role in informing individual patient decisions. Here we review the issues surrounding switching between therapies, and the evidence to assist guiding the choice of therapy to change to and when to change. We review the current level of evidence with regards to the management of women in their child-bearing years with regards to recommendations about treatment during pregnancy and whilst breast feeding. We provide a summary of recommended pre- and post-treatment monitoring for the available therapies and review the evidence with regards to the value of testing for antibodies which are known to be neutralising for some therapies. We review the occurrence of adverse events, both the more common and troublesome effects and those that are less common but have potentially much more serious outcomes. Ways of mitigating these risks and managing the more troublesome adverse effects are also reviewed. Finally, we make specific recommendations with regards to the treatment of MS. It is an exciting time in the world of MS neurology and the prospects for further advances in coming years are high.

Cloning of the first human anti-JCPyV/VP1 neutralizing monoclonal antibody: epitope definition and implications in risk stratification of patients under natalizumab therapy.

JC virus (JCPyV) has gained novel clinical importance as cause of progressive multifocal leukoencephalopathy (PML), a rare demyelinating disease recently associated to immunomodulatory drugs, such as natalizumab used in multiple sclerosis (MS) cases. Little is known about the mechanisms leading to PML, and this makes the need of PML risk stratification among natalizumab-treated patients very compelling. Clinical and laboratory-based risk-stratification markers have been proposed, one of these is represented by the JCPyV-seropositive status, which includes about 54% of MS patients. We recently proposed to investigate the possible protective role of neutralizing humoral immune response in preventing JCPyV reactivation. In this proof-of-concept study, by cloning the first human monoclonal antibody (GRE1) directed against a neutralizing epitope on JCPyV/VP1, we optimized a robust anti-JCPyV neutralization assay. This allowed us to evaluate the neutralizing activity in JCPyV-positive sera from MS patients, demonstrating the lack of correlation between the level of anti-JCPyV antibody and anti-JCPyV neutralizing activity. Relevant consequences may derive from future clinical studies induced by these findings; indeed the study of the serum anti-JCPyV neutralizing activity could allow not only a better risk stratification of the patients during natalizumab treatment, but also a better understanding of the pathophysiological mechanisms leading to PML, highlighting the contribution of peripheral versus central nervous system JCPyV reactivation. Noteworthy, the availability of GRE1 could allow the design of novel immunoprophylactic strategies during the immunomodulatory treatment.