A more accurate risk biomarkers recognition for Progressive Multifocal Leukoencephalopathy (PML) caused by Polyomavirus JC in patients with multiple sclerosis during treatment with disease-modifying therapies (DMTs): an ongoing clinical challenge.

The therapeutic scenario for the treatment of MS has recently been characterized by a veritable revolution, which has included the introduction, for the first time, of drug treatment guidelines and the entry, in the therapeutic landscape of the last decade, of numerous new drugs that, in varying but significantly relevant ways, have proven capable of modifying the course of the disease (Disease-Modifying Therapies - DMTs). 
While the arsenal of available drugs has resulted in advances in efficacy and selectivity, it has also exposed them to the danger of side effects, potentially serious and in some cases even fatal. Among the most important side effects, complications of infectious origin, characterized by cases of viral infection/reactivation, as in the case of JCPyV, the etiologic agent of PML, are the most represented. This study, based on the follow-up of MS patients treated with different DMTs, contributes to implementing the data in the literature regarding a greater understanding of the risks related to the administration of these drugs and more appropriate monitoring of MS treatment. The risk of JCPyV reactivation with Fingolimod and Dimethyl fumarate is lower than the risk of viral reactivation associated with natalizumab.
In light of the data obtained, it is possible to conclude that, in the case of natalizumab, testing for JC viruria would seem to be more useful in identifying those patients with a JCPyV-specific humoral response that is not yet detectable. In addition, our results, draw attention to the importance of analyzing the NCCR rearrangements of JCPyV. Indeed, the particular rearrangements found in plasma and PBMCs of patients with RRMS treated with natalizumab could represent an alert of neuroinvasiveness in order to detect early those patients with a higher risk of developing PML. In the case of dimethyl fumarate, it seems likely that monitoring of lymphopenia would identify a higher risk group of patients in whom alternative therapy should be sought. Prolonged lymphopenia, with absolute lymphocyte counts less than 750 lymphocytes/mL, might be the major risk factor for PML although, a greater risk might lie in the loss of CD8+ cells that are crucial for JCPyV control.
For fingolimod, this strategy cannot be applied because the number of circulating lymphocytes decreases while the actual lymphocyte function appears largely normal therefore, monitoring viruria and viremia along with identification of the neurotrophic variant of the virus seems a more exploitable means for risk stratification.

In conclusion, the results of this study can be considered directly transferable to the National Health System in that, both the monitoring of JCPyV reactivation by viruria and the sequence analysis of viral NCCR and host immune set-up could play the role of translatable biomarkers in clinical practice in order to assess the risk of PML onset. In addition to improving risk stratification of this disease, these biomarkers of viral reactivation and pathogenicity could facilitate timely diagnosis, optimizing the use of health care resources and contributing to the reduction of direct and indirect costs of MS disease.

Prezioso Carla was supported by the Italian Ministry of Health (Starting Grant: SG-2018-12366194).