An electron microscopic study of MDBK cells persistently infected with Newcastle disease virus.

Ultrastructural examination of a line of MDBK cells persistently infected with Newcastle disease virus (MDBKpi cells) revealed the presence of cytoplasmic aggregates of both smooth and granular nucleocapsids. Only granular nucleocapsids aligned under modified areas of plasma membrane and were incorporated into virus particles. On the grounds of morphogenesis, there was no apparent explanation for the persistent, not-cytocidal nature of the infection. Both nuclear and cytoplasmic aggregates of smooth nucleocapsids were present in MDBKpi cells which had been held without subculture for between 40 and 130 days (aged MDBKpi cells). Modified areas of plasma membrane with associated alignment of nucleocapsids were not present in aged MDBKpi cells, and neither budding nor released virus particles were observed, indicating a block in virus maturation. It is suggested that the granular material coating granular nucleocapsids allows them to interact with modified areas of plasma membrane, thereby inducing virus budding. A deficiency of this material, as apparently occurs in aged MDBKpi cells, would therefore cause a block in virus maturation. The nature of this granular material is discussed, and we suggest that it consists of M protein.