{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,12,1]],"date-time":"2025-12-01T15:48:26Z","timestamp":1764604106101,"version":"build-2065373602"},"reference-count":51,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2025,1,21]],"date-time":"2025-01-21T00:00:00Z","timestamp":1737417600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Viruses"],"abstract":"<jats:p>The Spodoptera frugiperda Sf9 insect cell line is used in the baculovirus expression vector system for the development of various viral vaccines and some gene therapy products. Early studies indicated that Sf9 cells produced a reverse transcriptase (RT) activity that was detected using a sensitive PCR-enhanced reverse transcriptase (PERT) assay. Since RT is generally associated with retrovirus particles, we undertook the investigation of the physical properties and infectious nature of the extracellular RT activity that was constitutively expressed from Sf9 cells or induced after the chemical treatment of the cells with drugs known to activate endogenous retroviruses. A density gradient analysis indicated that the peak RT activity corresponded to a low buoyant density of about 1.08 g\/mL. Ultracentrifugation and size filtration of cell-free Sf9 supernatant indicated that different particle sizes were associated with the RT activity. This was confirmed by transmission electron microscopy and cryoEM, which revealed a diversity in particle size and type, including viral-like and extracellular vesicles. The treatment of Sf9 cells with 5-iodo-2\u2032-deoxyuridine (IUdR) induced a 33-fold higher RT activity with a similar low buoyant density compared to untreated cells. Infectivity studies using various target cells (human A204, A549, MRC-5, and Raji, and African green monkey Vero cells) inoculated with cell-free supernatant from untreated and IUdR-treated Sf9 cells showed the absence of a replicating retrovirus by PERT-testing of cell-free supernatant during the 30 day-culturing period. Additionally, there was no evidence of virus entry by whole genome analysis of inoculated MRC-5 cells using high-throughput sequencing. This is the first study to identify extracellular retroviral-like particles in Spodoptera.<\/jats:p>","DOI":"10.3390\/v17020136","type":"journal-article","created":{"date-parts":[[2025,1,21]],"date-time":"2025-01-21T05:47:42Z","timestamp":1737438462000},"page":"136","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Characterization of Endogenous Retroviral-like Particles Expressed from the Spodoptera frugiperda Sf9 Cell Line"],"prefix":"10.3390","volume":"17","author":[{"given":"Hailun","family":"Ma","sequence":"first","affiliation":[{"name":"Division of Viral Products, Office of Vaccines Research and Review, Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Eunhae H.","family":"Bae","sequence":"additional","affiliation":[{"name":"Northwest Regional Campus, University of Arkansas for Medical Sciences, 1125 N. College Ave, Fayetteville, AR 72703, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Pei-Ju","family":"Chin","sequence":"additional","affiliation":[{"name":"Division of Viral Products, Office of Vaccines Research and Review, Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Arifa S.","family":"Khan","sequence":"additional","affiliation":[{"name":"Division of Viral Products, Office of Vaccines Research and Review, Center for Biologics Evaluation and Research, U.S. Food and Drug Administration, Silver Spring, MD 20993, USA"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2025,1,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"2161","DOI":"10.1016\/S0140-6736(07)60946-5","article-title":"Efficacy of a prophylactic adjuvanted bivalent L1 virus-like-particle vaccine against infection with human papillomavirus types 16 and 18 in young women: An interim analysis of a phase III double-blind, randomised controlled trial","volume":"369","author":"Paavonen","year":"2007","journal-title":"Lancet"},{"key":"ref_2","first-page":"211","article-title":"FluBlok, a recombinant hemagglutinin influenza vaccine","volume":"2","author":"Cox","year":"2008","journal-title":"Viruses"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1219","DOI":"10.1093\/infdis\/jix478","article-title":"Randomized comparison of immunogenicity and safety of quadrivalent recombinant versus inactivated influenza vaccine in healthy adults 18\u201349 Years of age","volume":"216","author":"Dunkle","year":"2017","journal-title":"J. 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