Immune checkpoint inhibitors for progressive multifocal leukoencephalopathy: a new gold standard?

Progressive multifocal leukoencephalopathy (PML) is a very rare and opportunistic encephalitis caused by JC polyomavirus that is linked to profound immunosuppression and is usually fatal unless immune function can be restored. Immune checkpoint inhibitors (ICI) are monoclonal antibodies (mAbs) that block either CTLA-4 or PD-1 inhibitor receptors, thus enhancing antiviral T-cell activity. Successful treatment of PML by ICI has recently generated some enthusiasm in case reports/small series of patients. However, the initial enthusiasm was mitigated by some individual case reports that did not show any benefit. More data are thus warranted about efficacy of immune checkpoint inhibitors in the specific context of PML. We report here the outcomes of six PML patients treated by ICI between 2017 and 2019. Underlying causes of immunosuppression consisted in hematologic malignancies (n = 4), primary immune deficiency (n = 1) and use of immunosuppressive therapies for myasthenia gravis (n = 1). Three patients were alive with a mean follow-up of 21 months (14–33) after first ICI infusion, including one patient with frank clinical response, one with stabilization, and one with initial worsening and further stabilization of PML. The three other patients rapidly died from PML. Our data suggest that ICI may be effective for PML treatment but were less impressive than the ones previously reported. Larger studies are thus warranted to confirm this efficacy and to identify the predictive factors of response.