J. gen. Virol. (1982), 62, 219-225. Printed in Great Britain Key words: M-ML V/tnterferon/biochemical analysis~electron microscopy 219 Biochemical Analysis and Electron Microscopic Study on Intracellular Virions in NIH/3T3 Mouse Cells Chronically Infected with Moloney Murine Leukaemia Virus: Effect of Interferon By MORDECHAI ABOUD, l RUTH SHOOR, 2 SARA BARI, z YEHUDITH HASAN, j REVITAL SHURTZ, 2 ZVI MALIK 2 AND SAMUEL SALZBERG z* 1Unit of Microbiology and Immunology, Faculty of Health Sciences, Ben-Gurion University of the Negev, Beer-Sheva, Israel and 2Department of Life Sciences, Bar-Ilan University, Ramat-Gan 52 100, Israel (Accepted 26 April 1982) SUMMARY Radioactively labelled virus particles of intracellular origin were isolated from the cytoplasmic fraction of disrupted NIH/3T3 cells chronically infected with Moloney murine leukaemia virus [NIH/3T3 (MLV)]. Interferon (IFN) treatment for 48 h, which arrested more than 90 % of virus release, resulted in a remarkable accumulation of these intracellular virions. However, no major effect of such treatment was apparent on their structural properties. Transmission electron microscopic examin- ation revealed that these intracellular virions were located within cytoplasmic vacuoles. IFN treatment resulted in a considerable increase in the number of virus- containing vacuoles, as well as the total number of vacuolar virions. It seems that IFN inhibits the final release of vacuolar virions from the cells, thus leading to their intracellular accumulation. INTRODUCTION The replication cycle of retroviruses is generally considered to end up with virus assembly at the inner side of the plasma membrane and its final release from the cell surface by a budding process (Bishop, 1978; Fan, 1978; Montelaro & Bolognesi, 1978). However, in a recent study (Salzberg et al., 1980), we were able to isolate reverse transcriptase-containing particles from the cytoplasmic fraction of disrupted NIH/3T3 cells chronically infected with Moloney murine leukaemia virus [NIH/3T3 (MLV)]. These particles, sedimenting at a density of 1-14 g/ml, were isolated by a procedure that minimized contamination of the cytoplasmic fraction with extracellular or membrane-associated virus particles, and therefore seemed to be of intracellular origin. It is of interest to mention in this context several early electron microscopic studies that have shown the presence of intracellular retrovirus particles in cytoplasmic vacuoles of various chronically infected cells (Dalton, 1966; Dalton et al., 1964; De Harven & Friend, 1966; Ikawa et al., 1973; Sato et al., 1971, 1972; Sugano et al., 1973; Yumato et at., 1966). In the present study we provide additional biochemical characterization of the cytoplasmic virions isolated from interferon (IFN)-treated and untreated NIH/3T3 (MLV) cells and illustrate their location in cytoplasmic vacuoles by transmission electron microscopy. 0022-1317/82/0000-5063 $02.00 (C)1982 SGM