There is no approved standard treatment fr progressive multifocal leukoencephalopathy (PML). The sponsor of the study is developing a new treatment. For this reason, the investigational medicinal product (IMP) called 'human allogenic HPyV-2-specific T cells' is to be tested in this study. The sponsor wants to find out whether the IMP is safe, influences the neurological status and improves the quality of the life of patients . It is to be investigated whether the IMP can be used to treat the disease and whether it could have an advantage over the standard therapy in terms of survival rate.