Frequent Infection of Cerebellar Granule Cell Neurons by Polyomavirus JC in Progressive Multifocal Leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) occurs most often in immunosuppressed individuals. The lesions of PML result from astrocyte and oligodendrocyte infection by the polyomavirus JC (JCV); JCV has also been shown to infect and destroy cerebellar granule cell neurons (GCNs) in 2 HIV-positive patients. To determine the prevalence and pattern of JCV infection in GCNs we immunostained formalin-fixed, paraffin-embedded cerebellar samples from 40 HIV-positive and 3 HIV-negative PML patients for JCV, glial and neuronal markers. JCV infection was detected in 30 patients (70%); 28 (93%) of these had JCV-infected cells in the granule cell layer (GCL); JCV-infected GCNs were demonstrated in 15/19 (79%) tested cases. JCV regulatory T antigen (T Ag) was expressed more frequently and abundantly in GCNs than JCV VP1 capsid protein. None of 37 HIV-negative controls but 1/35 (3%) HIV-positive subjects without PML had distinct foci of JCV-infected GCNs. Thus, JCV infection of GCNs is frequent in PML patients and may occur in the absence of cerebellar white matter demyelinating lesions. The predominance of T Ag over VP1 expression in GCNs suggests that they may be the site of early or latent central nervous system JCV infection. These results indicate that infection of GCNs is an important, previously overlooked aspect of JCV pathogenesis in immunosuppressed individuals.