Up to 18 subjects with definite PML, defined as clinical signs and MRI compatible with active PML and the presence of JCV by PCR in CSF. Subjects must be 18 years of age or older and must have a first degree, partially HLA- matched relative able and willing to act as a donor of lymphocytes. Subjects with underlying reversible immunosuppression (i.e.- HIV or MS status post treatment with natalizumab), or subjects with uncontrolled malignancy will be excluded. Subjects with evidence of active CNS inflammation as determined by presence of significant contrast enhancement within the PML lesions by MRI will also be excluded because it is assumed such patients are already mounting an adequate immune response against the infection.