Opin. immune system. Indeed, especially in the past, PML mainly occurred in patients with hematological malignancies and HIV contamination [6]. More recently, PML has been associated with the use of immunosuppressive or immunomodulating drugs [5]. Currently, PML secondary to treatment with MC-Val-Cit-PAB-carfilzomib natalizumab (NTZ) accounts for the majority of all the cases of MS patients. To date, a few cases have also been observed with dimethyl-fumarate (DMF) and fingolimod (FTY) [5]. Moreover, PML is outlined as a potential side effect also in patients suffering from diseases other than MS and treated with rituximab (RTX) with an adjusted odds ratio of 3.22 [10, 11], while 10 confirmed cases (9 of the patients previously treated with NTZ or FTY) MC-Val-Cit-PAB-carfilzomib have been reported in patients using OCR [12, 13]. No cases of PML attributable to teriflunomide (TFM) or cladribine (CLD) have been described yet, and a single case has been reported during alemtuzumab (ALM) therapy after switching from NTZ [14-16]. The pathogenesis of PML is usually far from being fully comprehended. CD4+ and CD8+ cytotoxic T-cell acknowledgement of viral antigens may probably play an important role in limiting the spreadiof JCV contamination and is directly related to the prognosis of PML [17]. Particularly, B cells represent latent sites of JCV, playing a significant role in viral transmission, replication and coordination of the expression of transcription factors [5, 18]. Furthermore, B lymphocytes induce T-cell responses through cytokine production, contributing to JCV control. Brain-resident memory T cells also seem to be involved, relying on help from your peripheral re-circulating CD4+ T cells [5, 19]. PML risk in MS patients is also typically associated with the previous use of immunosuppressant drugs (IM) and the presence of a high titre of anti-JCV antibody (JCV index) [20-23]. It has been demonstrated that this high JCV index displays a high risk for PML, as it is associated with a large viral reservoir, which in turn may increase the risk for viral reactivation [24]. Therefore, the determination of antibodies against JCV is considered an important tool for risk stratification and an algorithm based on the JCV index is usually applied as a screening tool prior to start MS therapy and during treatment in MS patients [22]. In our previous study, NTZ treatment increased the JCV index and its suspension seemed not to be able to interfere in the JCV status for a long time period [25]. Moreover, according to our data, FTY used as an exit strategy after NTZ suspension was able to cause a progressive increase in the JCV index. However, no sufficient data regarding JCV status modification during other disease-modifying MC-Val-Cit-PAB-carfilzomib therapies (DMTs) are currently available and the effects of approved DMTs on JCV status have not been fully explored yet. The aim of our study was to evaluate JCV status modification during treatment with currently used DMTs in order to identify possible alternative therapeutic strategies to minimize the risk of PML in MS patients. 2.?MATERIALS AND METHODS 2.1. Study Populace This longitudinal observational study screened all MS patients treated with several DMTs and followed at the MS Centre of Catania University or college Hospital in the period between January 2010 and February 2021. Data regarding patients were obtained retrospectively from your database iMED?, a computerized medical record collecting demographic, clinical and laboratory data. This study protocol was approved by the local Ethical Committee of the University or college of Catania (Catania 1). Each individual participating in the study signed an informed consent specifically designed for the study. Patients with a diagnosis of MS according to the Mc Donald criteria 2017 [26] EDNRB and treated for at least six months with interferons or glatiramer acetate (IFN/GA), RTX or OCR, DMF, TFN, ALM, CLD or FTY were included in the study. We MC-Val-Cit-PAB-carfilzomib excluded patients with a diagnosis of clinically isolated syndrome, radiologically isolated syndrome or neuromyelitis optica, and treated with steroids within 3 months before the JCV status evaluation. In order to evaluate anti-JCV antibodies status (JCV index) changes during the follow-up, the JCV status was evaluated at baseline before treatment (T0) and after at least 12 months of treatment (T1). At each time point, patients were divided into two subgroups based on their JCV status: unfavorable JCV index (<0.90) and positive JCV index, with.