Egress of alphaherpesviruses: Comparative ultrastructural study

Egress of four important alphaherpesviruses, equine herpesvirus 1 (EHV-1), herpes simplex virus type 1 (HSV-1), infectious laryngotracheitis virus (IL TV), and pseudorabies virus (PrV), was investigated by electron microscopy of infected cell lines of different origins. In all virus-cell systems anal yzed, similar observations were made concerning the different stages of vir ion morphogenesis. After intranuclear assembly, nucleocapsids bud at the in ner leaflet of the nuclear membrane, resulting in enveloped particles in th e perinuclear space that contain a sharply bordered rim of tegument and a s mooth envelope surface. Egress from the perinuclear cisterna primarily occu rs by fusion of the primary envelope with the outer leaflet of the nuclear membrane, which has been visualized for HSV-1 and EHV-1 for the first time. The resulting intracytoplasmic naked nucleocapsids are enveloped at membra nes of the trans-Golgi network (TGN), as shown by immunogold labeling with a TGN-specific antiserum. Virions containing their final envelope differ in morphology from particles within the perinuclear cisterna by visible surfa ce projections and a diffuse tegument. Particularly striking was the additi on of a large amount of tegument material to ILTV capsids in the cytoplasm. Extracellular virions were morphologically identical to virions within Gol gi-derived vesicles, but distinct from virions in the perinuclear space. St udies with gB- and gH-deleted PrV mutants indicated that these two glycopro teins, which are essential for virus entry and direct cell-to-cell spread, are dispensable for egress. Taken together, our studies indicate that the d eenvelopment-reenvelopment process of herpesvirus maturation also occurs in EHV-1, HSV-1, and ILTV and that membrane fusion processes occurring during egress are substantially different from those during entry and direct vira l cell-to-cell spread.