Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the brain caused by reactivation of JC polyomavirus in immunocompromised hosts. It was once an obscure complication of severe immunosuppression but became dramatically more clinically important with HIV/AIDS, and again with the introduction of immunosuppressive monoclonal antibodies in MS, IBD, lupus, and oncology. The neuropathologic features are highly characteristic, and the disease is now sometimes survivable with prompt immune reconstitution. This page covers PML pathology and clinical features.

JC Virus

JC virus (Human polyomavirus 2) infects about 70-80% of adults asymptomatically — most exposure in childhood, persistent latent infection in kidney, bone marrow, and other sites. Reactivation in immunocompromised hosts produces PML. The virus’s neurotropism is for oligodendrocytes (and to a lesser extent astrocytes).

Risk Factors / Immunocompromise Settings

  • HIV/AIDS: especially CD4 < 200; remains the largest single risk.
  • Monoclonal antibody therapy:
    • Natalizumab (anti-α4β7 integrin, multiple sclerosis): well-recognized PML risk.
    • Rituximab (anti-CD20): MS, lymphoma, autoimmune.
    • Ocrelizumab, ofatumumab: MS.
    • Fingolimod, dimethyl fumarate (low risk).
    • Efalizumab (psoriasis): withdrawn.
    • Vedolizumab (IBD): low risk.
  • Hematologic malignancy: lymphoma, leukemia, especially with chemotherapy.
  • Transplant: solid organ, bone marrow.
  • Chronic corticosteroids.
  • Autoimmune disease with immunosuppression: SLE, sarcoidosis.
  • Rarely in apparently immunocompetent.

Pathology

Macroscopic

  • Multifocal areas of demyelination in subcortical white matter.
  • Often asymmetric, can be in cortex/brainstem/cerebellum.
  • Soft, slightly discolored regions in white matter.
  • Larger lesions can be confluent.

Microscopic

  • Demyelination: with relatively preserved axons in early lesions, axonal damage later.
  • Bizarre, “smudgy” oligodendrocyte nuclei: enlarged, basophilic, often eccentric — characteristic feature. The result of JC virus replication in the nucleus.
  • Bizarre astrocytes: enlarged, hyperchromatic, multinucleated — can mimic astrocytic tumor cells. Sometimes called “reactive astrocytes with malignant features.”
  • Foamy macrophages: clearing myelin debris.
  • Minimal lymphocytic inflammation (in classic PML before immune reconstitution).
  • JC virus IHC: confirms diagnosis; positive in oligodendrocyte nuclei and astrocytes.
  • Electron microscopy: viral particles visible in oligodendrocyte nuclei (crystalline arrays of icosahedral particles).

Clinical Features

  • Subacute progressive multifocal neurologic deficits.
  • Motor deficits (hemiparesis, paraparesis).
  • Visual disturbances.
  • Cognitive decline.
  • Seizures (~20%).
  • Cerebellar signs.
  • Often headache absent.
  • Progression over weeks to months without immune reconstitution.
  • Pre-ART era: mean survival 2-6 months.
  • ART era: variable; many patients stabilize or improve.

Imaging

  • T2/FLAIR hyperintensity in subcortical white matter, often asymmetric, frequently involving U-fibers.
  • No mass effect (compared to lymphoma, abscess).
  • No or minimal contrast enhancement in classic PML.
  • Enhancement may appear with immune reconstitution (IRIS-PML).
  • Restricted diffusion at lesion edge (acute), variable elsewhere.
  • Predilection for: parieto-occipital, cerebellar peduncle, brainstem (less common).

Diagnosis

  • Clinical suspicion in any immunocompromised patient with subacute multifocal neurologic disease and white matter MRI lesions.
  • CSF JC virus PCR: gold standard. Sensitivity 70-90% (lower if quantitative assay required for very low copy number).
  • Brain biopsy: when JC PCR negative but clinical suspicion remains — bizarre oligodendrocyte nuclei + JC IHC.
  • Serial MRI: progression supports diagnosis.
  • Anti-JCV antibody index: stratifies risk in natalizumab-treated MS patients; not diagnostic of active PML.

Treatment

  • Immune reconstitution: the only effective intervention.
    • Start or optimize ART in HIV.
    • Discontinue immunosuppressive therapy (natalizumab requires plasmapheresis to remove circulating drug).
    • Reduce or stop other immunosuppressives.
  • No antiviral with proven efficacy against JC virus.
  • Pembrolizumab (anti-PD1): case reports of benefit in PML; investigational.
  • 5-HT2A antagonists (mirtazapine): theoretical; not clinically proven.
  • Anti-CD20 caution: in MS treated with anti-CD20, stopping treatment and prolonging dosing intervals may help.

Immune Reconstitution Inflammatory Syndrome (PML-IRIS)

Inflammation flares as immune system returns:

  • Worsening neurologic symptoms.
  • New contrast enhancement on MRI.
  • Edema.
  • Often a paradoxical clinical worsening.

Treatment: corticosteroids during the inflammatory phase. The IRIS reflects immune control of JC virus — many patients with PML-IRIS have stable or improved outcomes long-term despite the difficult acute phase.

PML in Different Patient Populations

HIV/AIDS PML

Most common globally; CD4 < 200 typical. ART initiation is the primary treatment but carries IRIS risk. Mortality 50% pre-ART; better with ART.

Natalizumab-Associated PML in MS

Recognized after 2005; major impact on MS treatment paradigms. Risk stratification by:

  • Anti-JCV antibody seropositivity.
  • Anti-JCV antibody index (higher = higher risk).
  • Duration of natalizumab treatment.
  • Prior immunosuppressant use.

Patient with high index treated > 2 years on natalizumab: risk ~1/100. Monitoring + early MRI + treatment cessation + plasmapheresis if PML develops.

Rituximab / Anti-CD20 PML

Recognized risk with chronic B-cell depletion; managed similarly.

Other Immunosuppressives

Background risk variable; individualized monitoring.

Differential Diagnosis

  • HIV encephalitis: multinucleated giant cells; diffuse rather than multifocal.
  • Toxoplasmosis: ring-enhancing lesions; restricted diffusion may differ.
  • PCNSL: usually enhances; restricted diffusion centrally; periventricular.
  • MS: relapsing-remitting, ovoid demyelinating plaques, perivenular.
  • Cerebrovascular disease: vascular distribution; different time course.
  • Other leukoencephalopathies: drug-induced, metabolic.

Variant Forms

JC Virus Granule Cell Neuronopathy (JCV-GCN)

JC virus targets cerebellar granule cells rather than oligodendrocytes. Presents with cerebellar ataxia. MRI shows cerebellar atrophy. Pathology: granule cell loss.

JC Virus Encephalopathy

JC virus targets cortical neurons. Rare; presents with cognitive decline + cortical lesions.

JC Virus Meningitis

Rare; mild meningoencephalitis.

🔍 Did You Know?

The recognition that natalizumab carries a substantial risk of PML in multiple sclerosis has reshaped MS pharmacology and patient counseling. Natalizumab was approved in 2004 for relapsing MS with dramatic efficacy. Within months, two patients on natalizumab plus interferon for MS and one with Crohn disease on monotherapy developed PML — the FDA voluntarily withdrew the drug. It was returned to market in 2006 under a restricted distribution program with extensive monitoring requirements after careful risk-benefit analysis. Subsequent risk stratification revealed that PML risk depends on three factors: JC virus serology (positive vs negative), treatment duration (risk rises sharply after 2 years), and prior immunosuppressant exposure. A patient who is JCV seronegative has minimal risk; a patient with high anti-JCV antibody index and > 2 years of natalizumab plus prior immunosuppressants has risk approaching 1 in 100. Annual JCV serology + MRI surveillance + threshold-based treatment cessation have substantially reduced PML in current natalizumab practice. The case illustrates a broader principle of modern neurology: powerful immunosuppressives that fundamentally alter immune function can resurrect opportunistic infections previously seen mainly in HIV/AIDS. PML is no longer an HIV-only diagnosis — and any patient on an anti-CD20 antibody, integrin inhibitor, or other immunosuppressive can develop it. Recognition of subacute multifocal white matter disease in an immunosuppressed patient should prompt CSF JC virus PCR at the bedside.

Pitfalls and Pearls

  • PML: JC virus reactivation in oligodendrocytes; immunocompromised host.
  • Bizarre oligodendrocyte nuclei: histologic hallmark; JC IHC confirms.
  • Bizarre astrocytes: can mimic astrocytoma.
  • White matter T2/FLAIR + no mass effect + no/minimal enhancement: PML.
  • Contrast enhancement appears with IRIS.
  • CSF JC virus PCR: gold standard antemortem; 70-90% sensitive.
  • Brain biopsy: when PCR negative but suspicion high.
  • Immune reconstitution is the only effective treatment.
  • Natalizumab PML risk: JCV index + duration + prior immunosuppressants.
  • Plasmapheresis to remove natalizumab after PML diagnosis.
  • PML-IRIS: paradoxical worsening with new enhancement; steroids during inflammation.
  • JC virus granule cell neuronopathy: cerebellar ataxia variant.
  • U-fiber involvement, no mass effect: PML pattern.
  • Differential from PCNSL, toxo, MS: enhancement, distribution, restricted diffusion.

References

  1. Berger JR. The clinical features of PML. Cleve Clin J Med. 2011;78(Suppl 2):S8-S12.
  2. Bloomgren G, Richman S, Hotermans C, et al. Risk of natalizumab-associated progressive multifocal leukoencephalopathy. N Engl J Med. 2012;366(20):1870-1880.
  3. Cortese I, Reich DS, Nath A. Progressive multifocal leukoencephalopathy and the spectrum of JC virus-related disease. Nat Rev Neurol. 2021;17(1):37-51.
  4. Berger JR, Aksamit AJ, Clifford DB, et al. PML diagnostic criteria: consensus statement from the AAN Neuroinfectious Disease Section. Neurology. 2013;80(15):1430-1438.
  5. Astrom KE, Mancall EL, Richardson EP Jr. Progressive multifocal leuko-encephalopathy: a hitherto unrecognized complication of chronic lymphatic leukaemia and Hodgkin’s disease. Brain. 1958;81(1):93-111.
  6. Tan IL, McArthur JC, Clifford DB, Major EO, Nath A. Immune reconstitution inflammatory syndrome in natalizumab-associated PML. Neurology. 2011;77(11):1061-1067.