Clinical relevance of thymic and bone marrow outputs in multiple sclerosis patients treated with alemtuzumab.

Thymic and bone marrow outputs were evaluated in 13 sequential samples of 68 multiple sclerosis patients who initiated alemtuzumab and were clinically followed for 48 months. Three months after alemtuzumab infusions, the levels of new T lymphocytes were significantly reduced, but progressively increased reaching the highest values at 36 months, indicating the remarkable capacity of thymic function recovery. Newly produced B cells exceeded baseline levels as early as 3 months after alemtuzumab initiation. Heterogeneous patterns of new T- and B-cell recovery were identified, but without associations with age, sex, previous therapies, development of secondary autoimmunity or infections, and disease re-emergence. Trial registration version 2.0-27/01/2016.

Changes in Brain Volumes Are Relevant during Natalizumab-Associated Progressive Multifocal Leukoencephalopathy: Lessons from a Case Report.

This is a case report concerning a Natalizumab-associated Progressive Multifocal Leukoencephalopathy (PML) with cerebellar localization and wakefulness disturbances. Awakening and clinical improvement dramatically occurred as soon as the immune reconstitution inflammatory syndrome (IRIS) took place, being it mild in nature and colocalizing with the PML lesion. In these ideal experimental conditions, we applied brain magnetic resonance imaging post-analysis in order to know changes in brain volumes underlying the pathological process over the infection period. White matter volume increased with a decrease in grey matter during IRIS. Conversely, we found a constant increase in cerebrospinal fluid volume throughout the duration of PML, suggesting a widespread abiotrophic effect, far from the lesion. Furthermore, brain parenchymal fraction significantly decreased as expected while the total brain volume remained stable at all times. Neurodegeneration is the main contributor to the steady disability in Natalizumab-associated PML. This process is thought to be widespread and inflammatory in nature as well as sustained by IRIS and humoral factors derived from the PML lesion.

Anti-SARS-CoV-2 T-stem cell memory persists in ocrelizumab-treated MS patients.

BACKGROUND: Development of long-lasting anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) T-cell responses in persons with multiple sclerosis (pwMS) treated with ocrelizumab is questioned. OBJECTIVE: Investigate antiviral T-cell responses after infection with SARS-CoV-2 in ocrelizumab-treated pwMS. Control groups included ocrelizumab-treated pwMS without SARS-CoV-2 infection, and non-MS individuals with and without SARS-CoV-2 infection. METHODS: Peripheral blood mononuclear cells were stimulated with SARS-CoV-2 peptide pools and T-cell reactivity was assessed by ELISPOT for interferon (IFN)-γ detection, and by multiparametric fluorescence-activated cell sorting (FACS) analyses for assessment and characterization of T-cell activation. RESULTS: ELISPOT assay against the spike and the N protein of SARS-CoV-2 displayed specific T-cell reactivity in 28/29 (96%) pwMS treated with ocrelizumab and infected by SARS-CoV-2, similar to infected persons without MS. This reactivity was present 1 year after infection and independent from the time of ocrelizumab infusion. FACS analysis following stimulation with SARS-CoV-2 peptide pools showed the presence of activation-induced markers (AIMs) in both CD4+ and CD8+ T-cell subsets in 96% and 92% of these individuals, respectively. Within naïve AIM+ CD4+ and CD8+ T-cells, we detected T memory stem cells, suggesting the acquisition of long-term memory. CONCLUSIONS: B-cell depletion using ocrelizumab does not impair the development of long-lasting anti-SARS-CoV-2 T-cell responses.

Progressive multifocal leukoencephalopathy genetic risk variants for pharmacovigilance of immunosuppressant therapies.

Background: Progressive multifocal leukoencephalopathy (PML) is a rare and often lethal brain disorder caused by the common, typically benign polyomavirus 2, also known as JC virus (JCV). In a small percentage of immunosuppressed individuals, JCV is reactivated and infects the brain, causing devastating neurological defects. A wide range of immunosuppressed groups can develop PML, such as patients with: HIV/AIDS, hematological malignancies (e.g., leukemias, lymphomas, and multiple myeloma), autoimmune disorders (e.g., psoriasis, rheumatoid arthritis, and systemic lupus erythematosus), and organ transplants. In some patients, iatrogenic (i.e., drug-induced) PML occurs as a serious adverse event from exposure to immunosuppressant therapies used to treat their disease (e.g., hematological malignancies and multiple sclerosis). While JCV infection and immunosuppression are necessary, they are not sufficient to cause PML. Methods: We hypothesized that patients may also have a genetic susceptibility from the presence of rare deleterious genetic variants in immune-relevant genes (e.g., those that cause inborn errors of immunity). In our prior genetic study of 184 PML cases, we discovered 19 candidate PML risk variants. In the current study of another 152 cases, we validated 4 of 19 variants in both population controls (gnomAD 3.1) and matched controls (JCV+ multiple sclerosis patients on a PML-linked drug ≥ 2 years). Results: The four variants, found in immune system genes with strong biological links, are: C8B, 1-57409459-C-A, rs139498867; LY9 (alias SLAMF3), 1-160769595-AG-A, rs763811636; FCN2, 9-137779251-G-A, rs76267164; STXBP2, 19-7712287-G-C, rs35490401. Carriers of any one of these variants are shown to be at high risk of PML when drug-exposed PML cases are compared to drug-exposed matched controls: P value = 3.50E-06, OR = 8.7 [3.7-20.6]. Measures of clinical validity and utility compare favorably to other genetic risk tests, such as BRCA1 and BRCA2 screening for breast cancer risk and HLA-B*15:02 pharmacogenetic screening for pharmacovigilance of carbamazepine to prevent Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis. Conclusion: For the first time, a PML genetic risk test can be implemented for screening patients taking or considering treatment with a PML-linked drug in order to decrease the incidence of PML and enable safer use of highly effective therapies used to treat their underlying disease.

MRI activity and extended interval of Natalizumab dosing regimen: a multicentre Italian study.

BACKGROUND: To minimize the risk of Progressive Multifocal Leukoencephalopathy and rebound in JCV-positive multiple sclerosis (MS) patients after 24 natalizumab doses, it has been proposed to extend the administrations interval. The objective is to evaluate the EID efficacy on MRI activity compared with the standard interval dosing (SID). METHODS: Observational, multicentre, retrospective cohort study, starting from the 24th natalizumab infusion to the loss of follow-up or 2 years after baseline. Three hundred and sixteen patients were enrolled. The median dose interval (MDI) following the 24th infusion was 5 weeks, with a bimodal distribution (modes at 4 and 6 weeks). Patients were grouped into 2 categories according to the mean number of weeks between doses: <5 weeks, SID; ≥5 weeks, EID. RESULTS: One hundred and eighty-seven patients were in the SID group (MDI = 4.5 weeks) and 129 in the EID group (MDI 6.1 weeks). The risk to develop active lesions on MRI is similar in SID and EID groups during the 6 and 12 months after the 24th natalizumab infusion, respectively 4.27% (95% CI:0.84-7.70) vs 4.71% (95% CI:0.16-9.25%) [p = 0.89] and 8.50% (95% CI:4.05-12.95) vs 6.55% (95% CI:2.11-11.00%) [p = 0.56]. The EID regimen does not appear to increase the occurrence of MRI activity during follow-up. CONCLUSION: There is no evidence of the reduced efficacy of natalizumab in an EID setting regarding the MRI activity. This observation supports the need for a bigger randomized study to assess the need to change the standard of the natalizumab dosing schedule, to better manage JCV-positive patients.

The risk of infection in patients with multiple sclerosis treated with disease-modifying therapies: A Delphi consensus statement.

The risk of infection associated with immunomodulatory or immunosuppressive disease-modifying drugs (DMDs) in patients with multiple sclerosis (MS) has been increasingly addressed in recent scientific literature. A modified Delphi consensus process was conducted to develop clinically relevant, evidence-based recommendations to assist physicians with decision-making in relation to the risks of a wide range of infections associated with different DMDs in patients with MS. The current consensus statements, developed by a panel of experts (neurologists, infectious disease specialists, a gynaecologist and a neuroradiologist), address the risk of iatrogenic infections (opportunistic infections, including herpes and cryptococcal infections, candidiasis and listeria; progressive multifocal leukoencephalopathy; human papillomavirus and urinary tract infections; respiratory tract infections and tuberculosis; hepatitis and gastrointestinal infections) in patients with MS treated with different DMDs, as well as prevention strategies and surveillance strategies for the early identification of infections. In the discussion, more recent data emerged in the literature were taken into consideration. Recommended risk reduction and management strategies for infections include screening at diagnosis and before starting a new DMD, prophylaxis where appropriate, monitoring and early diagnosis.

Switching to ocrelizumab in RRMS patients at risk of PML previously treated with extended interval dosing of natalizumab.

Discontinuation of natalizumab in patients with relapsing-remitting multiple sclerosis (RRMS) at risk of progressive multifocal leukoencephalopathy (PML) is associated with disease reactivation. Forty-two RRMS patients, who switched from an extended interval dose (EID) of natalizumab to ocrelizumab, underwent magnetic resonance imaging (MRI) and clinical monitoring during washout and after ocrelizumab starting. During the first 3 months, disease reactivation was observed in five (12%) patients; 6 months after ocrelizumab starting, no further relapses were recorded, and Expanded Disability Status Scale (EDSS) remained stable in 38 (90%) patients. In conclusion, ocrelizumab could be considered a choice to mitigate the risk of disease reactivation in patients previously treated with natalizumab-EID.

Age-Related Lymphocyte Output During Disease-Modifying Therapies for Multiple Sclerosis.

BACKGROUND: Patients with multiple sclerosis exhibit the same qualitative and quantitative changes in immune system cells observed in aging. In the last 20 years, multiple sclerosis patients have shown an increase in life expectancy and average age, but clinical trial inclusion criteria typically exclude patients over the age of 55 years. Therefore, disease-modifying therapies are likely administered to patients older than those enrolled in clinical trials. OBJECTIVE: In order to investigate whether disease-modifying therapies for multiple sclerosis induce modifications to the immune system that may have (super)additive effects resulting in an acceleration of immunosenescence, we quantified the number of T-cell receptor excision circles (TRECs) and K-deleting recombination excision circles (KRECs). These molecules are contained in new T and B lymphocytes released by the thymus and bone marrow and are considered molecular age-related markers. METHODS: The markers of aging were measured by a multiplex quantitative real-time PCR assay in 122 patients who had started therapy with interferon-beta (IFN-ß), fingolimod, alemtuzumab, or natalizumab. Samples were obtained before the therapy and at 6 and 12 months of treatment. Comparisons between the variables were performed by a non-parametric statistical analysis. RESULTS: In therapy-naive patients, a significant and direct correlation was found between a lower number of newly produced T and B cells and older age. Although disease-modifying therapies induced different changes (both increases and decreases) in the production of new T and B lymphocytes, 12 months of therapy with IFN-ß or natalizumab did not affect the correlations found at baseline between the release of lymphocytes containing TRECs or KRECs and age. On the contrary, in patients treated with alemtuzumab, both correlations were lost, while in fingolimod-treated patients, only the correlation between TRECs and age disappeared. CONCLUSIONS: This observational study indicated that different age-related changes of the new T and B lymphocyte production could be one of the reasons for the emergence, in the real-world setting, of adverse events not otherwise observed in clinical trials; thus, caution is advised when choosing disease-modifying therapies for multiple sclerosis patients.

Should frequent MRI monitoring be performed in natalizumab-treated MS patients? A contribution to a recent debate.

BACKGROUND: Brain magnetic resonance imaging (MRI) is the most effective surveillance tool for the detection of asymptomatic progressive multifocal leukoencephalopathy (PML). However, the optimal frequency for routine MRI surveillance is under-investigated. OBJECTIVE: To understand whether, upon their first MRI appearance, PML lesions present a difference in volume when comparing patients who frequently underwent MRI surveillance (3/4 months) with those who were assessed at longer intervals (6/12 months) and to understand the impact of the volume of lesions on clinical outcome. METHODS: The data of patients included in the Italian PML cohort were retrospectively analysed. Patients who had all the pre-diagnostic MRI scans available (n = 37) were included. The volume of PML lesion was calculated by manually outlining the PML lesion. RESULTS: Compared with patients who underwent MRI examination at least every 4 months, patients who were assessed less frequently had a lesion of significantly higher volume (median: 2567 (883-3583) vs. 664 mm3 (392-963) p = 0.006) and suffered a higher rate of disability (median: 2.25 expanded disability status scale points (-2.5 to 8) vs. 0.5 (-1 to 2.5) p = 0.004). CONCLUSION: The positive clinical outcome of patients undergoing frequent MRI surveillance and the small volume of the PML lesion upon first appearance justify a frequent surveillance using MRI in patients at high risk of PML.

Extending the Interval of Natalizumab Dosing: Is Efficacy Preserved?

Extending the natalizumab interval after the 24th administration could reduce the risk of progressive multifocal leukoencephalopathy (PML). The objective is to evaluate the noninferiority of the efficacy of an extended interval dosing (EID) compared with the standard interval dosing (SID) of natalizumab. It is an observational, multicenter (14 Italian centers), retrospective cohort study, starting from the 24th natalizumab infusion to the loss of follow-up or 2 years after baseline. Patients were grouped in 2 categories according to the mean number of weeks between doses: < 5 weeks, SID; ≥ 5 weeks, EID. Three hundred and sixty patients were enrolled. Median dose interval (MDI) following 24th infusion was 4.7 weeks, with a bimodal distribution (modes at 4 and 6 weeks). Two hundred and sixteen patients were in the SID group (MDI = 4.3 weeks) and 144 in the EID group (MDI 6.2 weeks). Annualized relapse rate was 0.060 (95% CI = 0.033-0.087) in the SID group and 0.039 (95% CI = 0.017-0.063) in the EID group. The non-inferiority of EID versus SID was satisfied. In conclusion, there is no evidence of a reduced efficacy of natalizumab in an EID setting. This observation confirms previous results and together with the emerging evidence of a reduced risk of PML associated to an EID, supports the need of a randomized study to assess the need to change the standard of the natalizumab dosing schedule.

Early reduction of the splicing factor2/alternative splicing factor: a cellular inhibitor of the JC polyomavirus in natalizumab-treated MS patients long before developing progressive multifocal leukoencephalopathy.

Natalizumab is effective against relapsing-remitting multiple sclerosis (MS) but increases the risk of progressive multifocal leukoencephalopathy (PML), which is caused by the activation of the JCV polyomavirus. SF2/ASF (splicing factor2/alternative splicing factor) is a potent cellular inhibitor of JCV replication and large T-antigen (T-Ag) expression. We reported that SF2/ASF levels in blood cells increase during the first year of natalizumab therapy and decrease thereafter, inversely related to T-Ag expression, and suggested a correlation with JCV reactivation. Here, we report SF2/ASF levels of longitudinal blood samples of two patients undergoing natalizumab therapy, who developed PML while monitored, in comparison to natalizumab-treated controls and to one-off PML samples. After 6 months of therapy, SF2/ASF levels of the two cases were reduced, instead of increased, and their overall SF2/ASF levels were lower than those from natalizumab controls. Since SF2/ASF inhibits JCV, its early reduction might have a role in subsequent PML. We are aware of the limitations of the study, but the uniqueness of serial blood samples collected before and after PML onset in natalizumab-treated patients must be stressed. If confirmed in other patients, SF2/ASF evaluation could be a new and early biomarker of natalizumab-associated PML risk, allowing an 18-24-month interval before PML onset (presently ~ 5 months), in which clinicians could evaluate other risk factors and change therapy.

Lack of specific T- and B-cell clonal expansions in multiple sclerosis patients with progressive multifocal leukoencephalopathy.

Progressive multifocal leukoencephalopathy (PML) is a rare, potentially devastating myelin-degrading disease caused by the JC virus. PML occurs preferentially in patients with compromised immune system, but has been also observed in multiple sclerosis (MS) patients treated with disease-modifying drugs. We characterized T and B cells in 5 MS patients that developed PML, 4 during natalizumab therapy and one after alemtuzumab treatment, and in treated patients who did not develop the disease. Results revealed that: i) thymic and bone marrow output was impaired in 4 out 5 patients at the time of PML development; ii) T-cell repertoire was restricted; iii) clonally expanded T cells were present in all patients. However, common usage or pairings of T-cell receptor beta variable or joining genes, specific clonotypes or obvious "public" T-cell response were not detected at the moment of PML onset. Similarly, common restrictions were not found in the immunoglobulin heavy chain repertoire. The data indicate that no JCV-related specific T- and B-cell expansions were mounted at the time of PML. The current results enhance our understanding of JC virus infection and PML, and should be taken into account when choosing targeted therapies.

Impact of natural menopause on multiple sclerosis: a multicentre study.

OBJECTIVE: To study the effect of natural menopause on multiple sclerosis clinical course. METHODS: This was an observational, retrospective, multicentre, cohort study. Menopause onset was defined by the final menstrual period (FMP) beyond which no menses occurred for 12 months. We included multiple sclerosis (MS) patients with FMP occurred after 2005 and a recorded follow-up of at least 2 years pre-FMP and post-FMP. We excluded patients with primary progressive course, iatrogenic menopause and with other confounders that could mask menopause onset. We compared relapse-rate and expanded disability status scale (EDSS) scores pre-FMP and post-FMP, searching for possible interactions with age, disease duration, cigarette smoking and nulliparity status. RESULTS: 148 patients were included (mean observation: 3.5 years pre-FMP and post-FMP). Most patients (92%) received disease-modifying therapies, mainly first-lines. After menopause the annualised relapse rate (ARR) significantly decreased (from 0.21±0.31 to 0.13± 0.24; p=0.005), while disability worsened (increase of mean 0.4 vs 0.2 points after menopause; p<0.001). Older age and long-lasting disease were associated with ARR reduction (p=0.013), but not with disability worsening. Cigarette smokers showed a trend to a higher disability accumulation after menopause (p=0.059). CONCLUSION: Natural menopause seems to be a turning point to a more progressive phase of MS. Relapse rate is also reduced after menopause, but this effect could be driven most by ageing and shifting to progressive phase in patients with long-lasting disease. Cigarette smoking could speed up disability progression after menopause.

Four cases of natalizumab-related PML: a less severe course in extended interval dosing?

BACKGROUND: Progressive multifocal leukoencephalopathy (PML) is a severe adverse event of natalizumab (NTZ). The administration of NTZ with extended interval dosing (EID) has been proposed as a strategy to potentially reduce the incidence of PML while maintaining its therapeutic efficacy. METHODS: In the current paper, we describe 4 cases of NTZ-PML in EID included in the Italian PML cohort. RESULTS: The patients developed PML after at least 38 NTZ infusions. Their John Cunningham virus (JCv) index was > 1.5, and patients had not previously used immunosuppressant. Two patients were asymptomatic at PML onset, while two had mild motor impairment of the right hand and anomia, respectively. All of them had undetectable viral load but one (37 JCv copies/ml). In all patients, MRI revealed unilobar lesions with deferred contrast enhancement suggestive of immune reconstitution. The clinical course ended with a favorable clinical outcome (ΔEDSS up to 1). CONCLUSIONS: Although PML in EID seems to occur less frequently than in conventional dosing regimen, strict monitoring of high-risk patients contributed to the indolent course observed in the four described cases, characterized by a prolonged pre-symptomatic phase, paucisymptomatic onset, low JCv load, less severe functional impairment during immune reconstitution, and a mild disability burden.

Immune profiling of a patient with alemtuzumab-associated progressive multifocal leukoencephalopathy.

A 31-year-old woman affected by multiple sclerosis (MS) experienced generalized tonic-clonic seizures 2 months after the second alemtuzumab cycle. Positive JC virus (JCV)-DNA in cerebrospinal fluid (CSF) and lesion iconography at magnetic resonance imaging (MRI) were suggestive of progressive multifocal leukoencephalopathy (PML). After 1 month, during full-blown immune reconstitution inflammatory syndrome, JCV-DNA became negative and symptoms gradually improved. New T- and B-cell output and T- and B-cell diversity were low and lymphocytes poorly responded to stimulation. This is the first case of an alemtuzumab-treated patient with clinical symptoms and radiological features compatible with PML. The lack of large T- and B-cell diversity, necessary for JCV recognition, is likely to have concurred to PML insurgence.

Early diagnosis of progressive multifocal leucoencephalopathy: longitudinal lesion evolution.

OBJECTIVE: Early diagnosis of natalizumab-related progressive multifocal leucoencephalopathy (NTZ-PML) in multiple sclerosis has been deemed a major priority by the regulatory agencies but has yet to become a reality. The current paper aims to: (1) investigate whether patients with NTZ-PML pass through a prolonged presymptomatic phase with MRI abnormalities, (2) estimate the longitudinal PML lesion volume increase during the presymptomatic phase and (3) estimate the presymptomatic phase length and its impact on therapy duration as a risk stratification parameter. METHODS: All Italian patients who developed NTZ-PML between 2009 and 2018 were included. The data of patients with available prediagnostic MRI were analysed (n=41). Detailed clinical and neuroradiological information was available for each participant. RESULTS: (1) PML lesions were detectable in the presymptomatic phase in 32/41 (78%) patients; (ii) the lesion volume increased by 62.8 % for each month spent in the prediagnostic phase; (3) the prediagnostic phase length was 150.8±74.9 days; (4) PML MRI features were detectable before the 24th month of therapy in 31.7 % of patients in our cohort. CONCLUSIONS: Considering the latency of PML clinical manifestation, the presymptomatic phase length supports the usefulness of MRI surveillance every 3-4 months. Early diagnosis could prompt a better outcome for patients due to the relationship between lesion volume and JC virus infection. The insight from this study might also have an impact on risk stratification algorithms, as therapy duration as a parameter of stratification appears to need reassessment.

To do or not to do? plasma exchange and timing of steroid administration in progressive multifocal leukoencephalopathy.

OBJECTIVE: To retrospectively analyze the effect of plasma exchange (PLEX; yes = PLEX+ , no = PLEX- ) and steroids administration timing (prophylactically [proST] or therapeutically [therST]) on the longitudinal clinical course of patients with natalizumab-related progressive multifocal leukoencephalopathy (PML) and full-blown immune reconstitution inflammatory syndrome (PML-IRIS). METHODS: Clinical and radiological data of 42 Italian patients with PML were analyzed. Patient's data are available until 12 months after PML diagnosis. PLEX and steroids treatment as time-dependent covariates were entered in: (1) a Cox model to investigate their impact on full-blown PML-IRIS latency; (2) an analysis of variance ANOVA to investigate their impact on IRIS duration; and (3) a linear mixed model to assess their impact on the longitudinal clinical course (measured by means of Expanded Disability Status Scale [EDSS]). RESULTS: Treatment with PLEX was not associated to PML-IRIS latency (hazard ratio [HR] = 1.05; p = 0.92), but once IRIS emerged, its duration was significantly longer in patients who underwent PLEX (101 vs 54 days in PLEX+ and PLEX- patients; p = 0.028). Receiving proST versus therST was not associated to IRIS latency (HR = 0.67; p = 0.39) or duration (p = 0.95). Patients who underwent proST had a significantly higher EDSS increase during PML (0.09 EDSS points per month; p = 0.04) as compared to those who had therST. INTERPRETATION: This study highlights that: (1) caution on the use of PLEX should be considered as the current data do not support a beneficial effect of PLEX and (2) caution on the early use of steroids is suggested because their prophylactic use to prevent full-blown PML-IRIS seems to negatively impact on the longitudinal disability course. Ann Neurol 2017;82:697-705.

Association Between BKPyV Serotype I Antibody Level and Natalizumab-Associated Progressive Multifocal Leukoencephalopathy.

To determine the association between BK polyomavirus (BKPyV) types 1 and 4 capsid antibody and natalizumab-associated progressive multifocal leukoencephalopathy (PML) in patients with multiple sclerosis (MS), serum samples were obtained from 10 natalizumab-associated PML cases and 130 control MS patients treated with natalizumab, and 82 control MS patients never exposed to natalizumab. In a sex- and age-adjusted regression model, BKPyV serotype 1 antibody levels were significantly higher in natalizumab-treated controls (p = 0.009) compared with cases, and were higher in controls never treated with natalizumab compared with cases, but the difference did not reach statistical significance (p = 0.158). There was no association between BKPyV serotype 4 antibody and PML. We hypothesize that a robust immune response to BKPyV may be protective against the development of PML.

Age as a risk factor for early onset of natalizumab-related progressive multifocal leukoencephalopathy.

Progressive multifocal leukoencephalopathy (PML) is a rare but potentially fatal opportunistic infection that arises almost exclusively in immunocompromised patients or in those treated with monoclonal antibodies, especially natalizumab. Here, we aimed at exploring if age at treatment start affects the time to onset of natalizumab-related PML. PubMed was searched for the terms "natalizumab and progressive multifocal leukoencephalopathy" in articles published from January 2005 to March 2017. We collected information on each identified PML case, including demographic and clinical variables at natalizumab start and at PML onset. The number of natalizumab infusions until PML onset was investigated in time-to-event analyses. We identified 238 cases who developed PML after a median number of 33 natalizumab infusions (range 6 to 103). Risk factors for an earlier onset of natalizumab-related PML were prior immunosuppressant exposure (hazard ratio [HR] = 1.43, p = 0.017) and older age at treatment start (HR = 1.02, p = 0.016). In particular, patients older than 50 years had a more than doubled-increased risk for an earlier PML onset (HR = 2.11, p = 0.006). Our findings suggest that the age at natalizumab start may represent a risk factor for an earlier PML onset, thus claiming further investigations about the interplay between immunosenescence and MS treatments.