Demonstration of Endocytosis during Newcastle Disease Virus Infection of Chicken Embryo Cell Cultures

Abstract

A study utilizing thin sectioned cells and electron microscopy demonstrated that Newcastle Disease virus (NDV) penetrates chicken embryo cells by adsorptive endocytosis. The penetrated virus was then observed to localize in lysosomal vacuoles. It was found that treatment of infected cells with 500 Ag/ml of ammonium chloride (NH4C1, lysosomotropic agent) caused 87% inhibition of progeny synthesis and inhibition of cellular hemagglutinin synthesis, while not interfering with viral adsorption to cells. Electron microscopic studies indicated that NH4C1 treatment of infected cells interfered with NDV uncoating in lysosomal vacuoles. Viruses replicate in host cells by diverting the biosynthetic pathways of cells to production of virus progeny. To infect a cell, a virus must transfer its genome from the extracellular space to the cytoplasm. This process requires transport through one or more membrane barriers. In contrast to the detailed knowledge of the later stages of viral infection, little is known about the entry of animal viruses into host cells. For enveloped viruses, two pathways of entry are generally observed by electron microscopy: a) fusion between the viral membrane and the host cell plasmalemma, where the viral genome is delivered into the cytoplasm; and b) an endocytotic route, where the phagocytized or viroplexed virions are taken into the cytoplasm in endocytotic vesicles and delivered to intracellular organelles. The present report is a study to ascertain the mode of entry of Newcastle Disease (NDV) upon infection of chicken embryo cells as determined by electron microscopy of thin sectioned cells. The study also included the effect of ammonium chloride (NH4C1), a lysosomotropic agent, on NDV infection of chicken embryo cells. The events studied were virion binding, cellular penetration, virion localization in cellular lysosomes and progeny synthesis.