Pembrolizumab in Progressive Multifocal Leukoencephalopathy (PML) in Immunocompromised Patients Without HIV Infection

Inclusion Criteria: 1. Age ≥ 18 years old 2. Diagnosis of definite PML since less than 2 months according to American Academy of Neurology 3. Presence of JCV in the CSF in the last CSF sampling. 4. Signed informed consent (from the patient, or if unable to consent, from a surrogate) 5. For women of childbearing potential: negative serum or urine b-HCG test and agree to use a highly effective contraception methods during 8 months (i.e. until 6 months after end of experimental treatment) Exclusion Criteria: 1. Patients in whom immune reconstitution is achievable (HIV infection - Multiple sclerosis - Auto-immune and inflammatory diseases) 2. Patients who have received solid organ transplantation 3. Hypersensitivity to the active substance or to any of the excipients 4. Life expectancy less than 1 month 5. Pregnancy or lactating women or planning birth during the study period 6. Having previously been treated by anti-PD1mAb 7. Patient receiving IL-2 or IL-7 for the treatment of PML at inclusion 8. Patient whose weight is \> 100kg 9. Participation in other interventional study \[a patient already included in another interventional study for which the treatment can lead to an immunodepression can be included if: * the investigational treatment has been completed and there is no risk of drug interaction with the administration of Pembrolizumab as defined in PENALTY study * if this does not alter the study's ability to evaluate the effect of Pembrolizumab in terms of safety and efficacy (from the investigator's point of view)\] 10. Patient without national health insurance, and patient on AME (state medical aid) 11. Patient under guardianship or curatorship 12. Patient deprived of their liberty by a judicial or administrative decision