Proc. Nail. Acad. Sci. USA Vol. 86, pp. 8964-8967, November 1989 Medical Sciences Human immunodeficiency virus-like particles produced by a vaccinia virus expression vector (retrovirus/AIDS/virus assembly/reverse transcriptase) VELISSARIos KARACOSTAS*, KUNIo NAGASHIMAt, MATTHEW A. GONDAt, AND BERNARD MOSS*t *Laboratory of Viral Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892; and tLaboratory of Cell and Molecular Structure, Program Resources, Inc., National Cancer Institute, Frederick Cancer Research Facility, Frederick, MD 21701 Contributed by Bernard Moss, August 11, 1989 ABSTRACT Infectious retrovirus particles consist of a MATERIALS AND METHODS core structure containing RNA and gag-pol polypeptides sur- rounded by a lipid membrane studded with env proteins. A Plasmid Construction. The plasmid sp64/HXB.2, contain- recombinant vaccinia virus was designed to express the entire ing an infectious cDNA copy of HIV-1 isolate HXB.2 (21) gag-pol precursor protein of the human immunodeficiency inserted into the Xba I site of plasmid sp64, was provided by virus type 1. Synthesis and processing of gag proteins occurred F. Wong-Staal and R. C. Gallo (National Cancer Institute) in mammalian cells infected with this live recombinant virus, and served as the source of the gag-pol gene. A 2300- and reverse transcriptase was detected largely in the medium. base-pair Sac I-EcoRV fragment of sp64/HXB.2 was in- Electron micrographs revealed immature retrovirus-like par- serted into the Sac I-Sma I sites of the replicative form of ticles budding from the plasma membrane and extracellular bacteriophage M13mpl8. Oligonucleotide-directed mutagen- particles with morphological characteristics of immature and esis was carried out to place a Sal I site followed by a mature human immunodeficiency virus. The latter contained consensus sequence for eukaryotic translation (22) before the functional reverse transcriptase as well as processed p24 and start codon of the gag gene. The gag-pol gene was inserted p17 gag polypeptides. Thus, assembly and maturation of into a modified form of the vector pSC11 (23) giving rise to human immunodeficiency virus-like particles can occur in the pVK3, the final plasmid used to obtain the recombinant virus absence ofeither infectious RNA molecules or env proteins. The (vVK1) expressing the HIV gag-pol genes. Restriction endo- production of noninfectious virus-like particles by expression nuclease analysis and DNA hybridization were used to vectors should be useful for biochemical studies and could characterize the plasmid pVK3. provide a safe source of material for the development of Viruses and Cells. Vaccinia virus (strain WR) was originally vaccines. obtained from the American Type Culture Collection. Re- combinant vaccinia virus vVK1 was isolated using the plas- mid pVK3 and was purified as reported (23). CV-1 monkey Human immunodeficiency virus type 1 (HIV-1), the caus- kidney- cells were propagated as monolayers in minimal ative agent ofacquired immunodeficiency syndrome (AIDS), essential medium (Quality Biologicals, Gaithersburg, MD) contains an RNA genome that encodes gag, pol, and env supplemented with 2.5% (vol/vol) fetal bovine serum. Cells proteins, as well as additional regulatory proteins (1-4). The were infected with 30 plaque-forming units of virus per cell primary gag translation product is a 55-kDa precursor, p55, for 2 hr; the virus was then removed, and the cells were that is proteolytically processed to p24, p17, and p15, the washed and overlayed with fresh medium. The cells were major core proteins. The pol open reading frame encodes the harvested at appropriate times after infection by scraping the protease, reverse transcriptase, and integrase (5-11). monolayers in isotonic phosphate-buffered saline (PBS), Expression of the protease, as well as other products of the washed with PBS, and lysed in 0.5% Nonidet P-40/PBS. pol gene, requires a relatively inefficient ribosomal frame- Immunoblotting. The detergent-disrupted cytoplasmic ex- shifting event within the gag gene (12) that leads to the tracts were treated with SDS and 2-mercaptoethanol prior to formation ofsmall amounts ofthe putative gag-pol precursor. polyacrylamide gel electrophoresis. The electrophoretically A myristic acid residue is present at the N terminus of p17 as separated polypeptides were transferred by electroblotting well as the gag precursor (13, 14) and by analogy with other onto a nitrocellulose membrane, blocked with 5% (wt/vol) retroviruses is likely to be required for transport to the nonfat dry milk/0.3% Tween 20 in PBS, incubated with the plasma membrane (15), into which the glycosylated envelope antibody in 0.3% Tween/PBS for 1 hr, washed three times proteins are inserted. Studies with defective avian and mu- with 0.3% Tween/PBS, incubated with 125I-labeled protein rine retroviruses, however, have shown that neither infec- A, washed three times with 0.3% Tween/PBS, dried, and tious RNA nor env protein is required for particle assembly subjected to autoradiography. (16-19). In this context, Gheysen et al. (20) observed the Reverse Transcriptase Assay. Cells were infected as de- formation of immature particles containing unprocessed p55 scribed above, and reverse transcriptase activity was mea- in insect cells that were infected with a baculovirus contain- sured using 5 ,ul of cell extract or medium, poly(A), oligo(dT), ing only the gag gene of HIV-1. In this communication, we and [a-32P]dTTP in 30 Al as described (24). After 2 hr at 37°C, describe HIV-like particle formation in mammalian cells 10lO was spotted on DE81 ion-exchange chromatography infected with a recombinant vaccinia virus expressing the paper (Whatman), air-dried, and washed four times in 2X entire gag-pol gene under control of a vaccinia virus pro- standard saline citrate (0.3 M NaCI/0.03 M sodium citrate, moter. Production of noninfectious HIV-1 particles using pH 7.0). The paper was dried and the radioactivity present expression vectors could be valuable both for studies of was determined in a Beckman scintillation counter. assembly and for vaccine purposes. Purification of Particles. Medium, collected 15 hr after infection of CV-1 cells with vVK1, was clarified by centrif- The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Abbreviation: HIV-1, human immunodeficiency virus type 1. in accordance with 18 U.S.C. §1734 solely to indicate this fact. tTo whom reprint requests should be addressed. 8964 Downloaded by guest on September 23, 2021 Medical Sciences: Karacostas et al. Proc. Natl. Acad. Sci. USA 86 (1989) 8%5 ugation at 2000 X g, and layered on top of a 20% (wt/vol) with time (Fig. 2). These data suggested that reverse tran- sucrose cushion. The pellet, formed by centrifugation at scriptase was activatedjust before or after its release from the 27,000 rpm in an SW 27 rotor for 90 min, was suspended in cell. Control experiments confirmed the absence of detect- PBS, and layered on top of a preformed 20-60% sucrose able reverse transcriptase activity from cells infected with gradient. After centrifugation at 100,000 x g for 18 hr in an wild-type vaccinia virus. It is also important to note that SW 41 rotor, 12 equal fractions were collected starting at the vaccinia virus (strain WR) particles remain almost entirely bottom. intracellular. Electron Microscopy. CV-1 cells infected with recombinant Characterization of Purified Particles. To understand the vaccinia virus expressing HIV-1 gag-pol genes were har- physical nature of the secreted reverse transcriptase, the vested in PBS and centrifuged at 1500 x g. The pellets were medium was collected and placed over a sucrose cushion. fixed in 1.25% (vol/vol) gluteraldehyde and then in 1% After centrifugation, all ofthe activity was found in the pellet osmium tetroxide, dehydrated in graded alcohols, and em- fraction, suggesting that it was in a particulate form (Fig. 2). bedded in epoxy resins. Virus pellets were prepared by The pelleted material was resuspended and subjected to centrifugation at 100,000 x g for 90 min and then processed equilibrium density sucrose gradient sedimentation. Almost for electron microscopy as above. Thin sections were cut and all of the reverse transcriptase activity was found in fractions stained with uranyl acetate and lead citrate. 5, 6, and 7, corresponding to a density of -1.16 g/cm3 (Fig. 3 Upper). Additional samples from each gradient fraction were ana- RESULTS lyzed by immunoblotting after polyacrylamide gel electro- Expression and Processing of HIVgag-pol. When cells were phoresis (Fig. 3 Lower). Although material reactive with infected with the recombinant vaccinia virus, synthesis and AIDS patient antiserum was widely dispersed, the processed processing of the gag-pol proteins occurred. In cell extracts gag polypeptides p24 and p17 were predominantly located in prepared 4 hr after infection, the most prominent polypep- the fractions with reverse transcriptase as expected for tides that reacted with HIV-1-specific antiserum were -55, mature HIV particles. 46, and 41 kDa (Fig. 1). Also present were small amounts of Electron Microscopy of Retroviral Particles. Electron mi- high molecular mass material (>100 kDa), presumably re- crographs ofthin sections ofvVK1-infected cells revealed the sulting from frame-shifting, as also shown by Gowda et al. presence of 100- to 120-nm particles budding from the plasma (25). With time, three small polypeptides of 32, 24, and 17 membrane (Fig. 4A). These retrovirus-like particles were detected at 4 hr and increased greatly in amount from 8 hr on. kDa increased in amount and the higher molecular mass The morphogenesis of the particles, produced by vVK1, was polypeptides diminished. Pulse-chase experiments with The [35S]methionine were consistent with precursor-product re- similar to that of HIV-1 and other retroviruses (26-29).