Trial: ASCEND-JC (NCT05541549), Phase 2, randomized, double-blind, placebo-controlled in PML due to JC polyomavirus (JCPyV). Intervention: CE-VST01-JC, a biological adoptive cellular immunotherapy (virus-specific T cells), dosed 1×10^8 cells IV every 28 days ×4. Mechanism: Infusion of JCPyV-specific T cells that recognize JCPyV antigens via HLA and mediate T-cell cytotoxicity (primarily CD8+), restoring antiviral cellular immunity to eliminate JCPyV-infected cells and halt neurologic progression. Target cells/pathways: JCPyV-infected cells in the CNS (e.g., glial cells), TCR/HLA-restricted antiviral pathways, cytotoxic T-lymphocyte killing, and Th1 cytokine responses (e.g., IFN-γ) aimed at controlling viral replication and PML-related demyelination. Comparator: placebo. Primary goal: slow/stop disability progression (mRS).