Deletion mutants in human cytomegalovirus glycoprotein US9 are impaired in cell-cell transmission and in altering tight junctions of polarized human retinal pigment epithelial cells.

Retinal cytomegalovirus (CMV) disease is one of the major manifestations of viral pathogenesis in immunosuppressed patients with the acquired immunodeficiency syndrome (AIDS). CMV infection of the retina causes directional destruction which begins at the optic nerve head adjacent to the retinal capillaries and progresses, if untreated, to retinal detachment and blindness. Infection does not occur across the basal membrane of the retinal pigment epithelium (RPE), adjacent to the highly vascularized choroid. CMV replicates in polarized RPE cells, and progeny virions cross apical and lateral membranes of RPE cells grown on permeable filter supports, but not basal membranes. Cell-cell junctions of CMV-infected RPE cells are permeabilized, and the tight junction protein zonula occludens (ZO-1) is disassembled; progeny virions then spread to neighboring cells through the lateral cell membranes, which in polarized cells differ significantly in lipid and protein composition from the apical cell membranes. We found that CMV mutants with deletions in US9 and US8/US9 failed to spread from cell to cell, exhibiting a small-plaque phenotype in polarized RPE cells. Immunofluorescence confocal microscopy staining of ZO-1 protein revealed that RPE cells infected with CMV deletion mutants RV35, RV80, and RV61 did not exhibit altered tight junctions, in contrast to RPE cells infected with wild-type strain AD169 virus. Our findings indicate that US9, which is an accessory glycoprotein in infected foreskin fibroblasts, is required for transmission of virus across cell-cell junctions of polarized RPE cells. The relationship between US9 expression and virus transmission across cell-cell boundaries suggests that US9 may directly or indirectly permeabilize tight junction complexes of polarized RPE cells.