Mechanism of interference mediated by human parainfluenza virus type 3 infection

Viral interference is characterized by the resistance of infected cells to infection by a challenge virus. Mechanisms of viral interference have not b een characterized for human parainfluenza virus type 3 (HPF3), and the poss ible role of the neuraminidase (receptor-destroying) enzyme of the hemagglu tinin-neuraminidase (HN) glycoprotein has not been assessed. To determine w hether continual HN expression results in depletion of the viral receptors and thus prevents entry and cell fusion, we tested whether cells expressing wild-type HPF3 HN are resistant to viral infection. Stable expression of w ild-type HN-green fluorescent protein (GFP) on cell membranes in different amounts allowed us to establish a correlation between the level of HN expre ssion, the level of neuraminidase activity, and the level of protection fro m HPF3 infection. Cells with the highest levels of HN expression and neuram inidase activity on the cell surface were most resistant to infection by HP F3. To determine whether this resistance is attributable to the viral neura minidase, we used a cloned variant HPF3 HN that has two amino acid alterati ons in HN leading to the loss of detectable neuraminidase activity. Cells e xpressing the neuraminidase-deficient variant HN-GFP were not protected fro m infection, despite expressing HN on their surface at levels even higher t han the wild-type cell clones. Our results demonstrate that the HPF3 HN-med iated interference effect can be attributed to the presence of an active ne uraminidase enzyme activity and provide the first definitive evidence that the mechanism for attachment interference by a paramyxovirus is attributabl e to the viral neuraminidase.