Objective:

To describe a rare case of biopsy-proven progressive multifocal leukoencephalopathy (PML) in an immunocompetent patient and the clinical–radiological response to pembrolizumab treatment.

Background:

Progressive multifocal leukoencephalopathy (PML) is a rare demyelinating brain disease, predominantly seen with immunosuppression. Diagnosis is based on characteristic clinical and radiologic features with JCV confirmation or brain biopsy, and treatment outcomes have largely been unsuccessful, focusing on restoring immune function. Immune checkpoint inhibitors—particularly pembrolizumab, a PD-1 inhibitor—have recently emerged as a promising option by enhancing JCV-specific T-cell responses, but data in immunocompetent hosts are limited.

Design/Methods:

A 53-year-old immunocompetent man presented with progressive left lower limb weakness. MRI brain showed focal T2 and FLAIR hyperintensity in the right paracentral lobule without enhancement. Extensive evaluation including autoimmune, infectious, and demyelination workup was negative. Brain biopsy confirmed JC virus infection by SV40 immunostaining. Due to progression despite corticosteroids, pembrolizumab (2 mg/kg, 150 mg IV) was initiated, with a second dose administered eight months later. MRI and neurological assessments were performed at 3, 6, and 12 months for follow-up.

Results:

Following the first pembrolizumab infusion, the patient showed initial clinical and radiological worsening, with an increase in lesion size and a new cerebellar lesion at 3 weeks. Subsequent MRI scans at 3 and 6 months demonstrated stabilization of both lesions. After a second infusion eight months later, minimal reduction of the cerebellar lesion was noted, while the primary lesion remained unchanged. At one year, the MRI findings were stable, and neurological symptoms had plateaued without immune-related adverse effects.

Conclusions:

This case demonstrates the potential role of pembrolizumab in stabilizing progressive multifocal leukoencephalopathy even in an immunocompetent patient. While initial worsening may occur, delayed stabilization suggests an immune-mediated response. Further research is needed to clarify patient selection, dosing frequency, and long-term outcomes for immune checkpoint inhibitor use in PML.