However, Treg frequencies in relation to conventional T cells better reflect the overall suppressive capacity within the CD4 T cell compartment.12 Our results are in line with reports showing these changes predominantly in active sarcoidosis patients without PML, whereas this was not observed in any inactive sarcoidosis patients.12 The time from diagnosis of sarcoidosis to diagnosis of PML varies within our cohort. Here, CD4 is linked to progressive multifocal leukoencephalopathy.