Analysis of Immunological Memory for John Cunningham Virus in a Mexican Population of Patients with Multiple Sclerosis Under Treatment.

: Multiple sclerosis (MS) is a disease characterized by demyelination and axonal damage of the central nervous system. Despite the observed benefits, highly effective treatment (HET)-based therapy has adverse effects, which include an increased risk of developing progressive multifocal leukoencephalopathy (PML). Additionally, the risk grows if the patient has antibodies for the John Cunningham virus (JCV). The appearance of PML is rare, and only one report has been found in Mexico. The objective of this research was to determine and analyze the immunological memory for JCV in a population of Mexican patients with MS under treatment. : All participants underwent a complete medical history and neurological evaluation. Once they signed their informed consent, a blood sample was taken to determine if antibodies against JCV were present in their serum. In total, 121 MS patients were analyzed, and the population consisted of 62.8% women and 37.2% men with an average age of 42.28. The three most common HETs received by the participants were natalizumab (67.76%), followed by teriflunomide and fingolimod. : The seropositivity was 62.8%, and in this group, the average duration of disease evolution was 152.33 ± 93.37 months. Natalizumab was the most used HET, and despite this, only a positive association between a positive JCV antibody index with duration of fingolimod and history of depression was found. Also, a positive correlation of the JCV Ab index within the forms of SPMS and PPMS compared to RRMS was observed. No differences were observed between populations, type, and duration of MS.