Polyomavirus-infected oligodendrocytes and macrophages within astrocytes in progressive multifocal leukoencephalopathy (PML).

Polyomavirus-infected oligodendrocytes and in some instances virus-carrying macrophages were demonstrated within the cytoplasm of astrocytes by double immunostaining of brain lesions in a case of PML following bone marrow transplantation. The infected oligodendrocytes and CD68+ cells were usually partially or completely invested by the cytoplasm of reactive and infected or noninfected giant pleomorphic astrocytes. By electron microscopy (EM), internalized infected oligodendrocytes were shown to release polyomavirus particles possibly infectious to the astrocytes. These observations seem to indicate that astrocytes in PML can become infected by direct contact with internalized infected oligodendrocytes and possibly also by virus-loaded macrophages. Correspondingly, astrocyte/oligodendrocyte/macrophage cell-to-cell interactions appear to play an important role in the pathogenesis of PML lesions.