{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T15:09:47Z","timestamp":1785424187946,"version":"3.56.0"},"reference-count":27,"publisher":"MDPI AG","issue":"6","license":[{"start":{"date-parts":[[2026,6,15]],"date-time":"2026-06-15T00:00:00Z","timestamp":1781481600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"University Medicine Rostock","award":["FORUN 889003"],"award-info":[{"award-number":["FORUN 889003"]}]},{"award":["FORUN 889003"],"award-info":[{"award-number":["FORUN 889003"]}],"id":[{"id":"https:\/\/ror.org\/04dm1cm79","id-type":"ROR","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100000780","name":"the European Fund for Regional Development","doi-asserted-by":"publisher","award":["EXF-25-2009"],"award-info":[{"award-number":["EXF-25-2009"]}],"id":[{"id":"10.13039\/501100000780","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Microorganisms"],"abstract":"<jats:p>Streptococcus pneumoniae remains a major global health threat and is listed by the World Health Organization as a pathogen in urgent need of new antimicrobial strategies. While primarily considered an extracellular pathogen, S. pneumoniae can persist within splenic macrophages in severe disease, creating a protected intracellular niche that may contribute to fulminant sepsis. We recently demonstrated the concept of an mRNA-based therapeutic approach in which host cells produce the pneumococcal bacteriophage endolysin Cpl-1. Here, we investigated whether expression of Cpl-1 in macrophages can target S. pneumoniae residing within host cells. Using the human THP-1 macrophage line, we demonstrated successful translation and intracellular accumulation of bioactive Cpl-1 following IVT-mRNA transfection. Lysates from Cpl-1 mRNA-transfected cells exhibited bacteriolytic activity, and Western blotting as well as immunofluorescent staining confirmed cytosolic endolysin production. Phagocytosis assays using an encapsulated and unencapsulated pneumococcal strain showed a reduction in intracellular bacterial burden in Cpl-1 mRNA-transfected macrophages compared with control and inactive-mutant Cpl-1 mRNA groups, and a flow cytometry-based assay further corroborated a decreased intracellular bacterial signal. Together, these findings suggest that mRNA-encoded Cpl-1 enhances intracellular killing of S. pneumoniae and supports the feasibility of mRNA-based endolysin therapies to target intracellular pneumococcal reservoirs.<\/jats:p>","DOI":"10.3390\/microorganisms14061342","type":"journal-article","created":{"date-parts":[[2026,6,15]],"date-time":"2026-06-15T08:57:58Z","timestamp":1781513878000},"page":"1342","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Investigation of Intracellular Clearing of Streptococcus pneumoniae by mRNA-Encoded Cpl-1 Bacteriophage Endolysin in Human Macrophages"],"prefix":"10.3390","volume":"14","author":[{"given":"Carolin","family":"Warnke","sequence":"first","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7221-3900","authenticated-orcid":false,"given":"Wendy","family":"Bergmann-Ewert","sequence":"additional","affiliation":[{"name":"Core Facility for Cell Sorting and Cell Analysis, University Medical Center Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0004-1349-7682","authenticated-orcid":false,"given":"Marc Benjamin","family":"Janssen","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hendrik Feit","family":"Mueck","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nicolas","family":"Raether","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nooshin","family":"Mohebali","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9527-5098","authenticated-orcid":false,"given":"Bernd","family":"Kreikemeyer","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2423-201X","authenticated-orcid":false,"given":"Katharina","family":"Ekat","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Moritz K.","family":"Jansson","sequence":"additional","affiliation":[{"name":"Institute of Medical Microbiology, Virology and Hygiene, University Medicine Rostock, 18057 Rostock, Germany"},{"name":"National and World Health Organization Supranational Reference Laboratory for Mycobacteria, Research Center Borstel, Parkallee 18, 23845 Borstel, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,6,15]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"2221","DOI":"10.1016\/S0140-6736(22)02185-7","article-title":"Global mortality associated with 33 bacterial pathogens in 2019: A systematic analysis for the Global Burden of Disease Study 2019","volume":"400","author":"Ikuta","year":"2022","journal-title":"Lancet"},{"key":"ref_2","unstructured":"World Health Organization (2017). 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