Virus Research, 16 (1990) 153-162 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK Elsevier 153 VIRUS 00578 Detection of respiratory syncytial virus (RSV) infected cells by in situ hybridization in the lungs of cotton rats immunized with formalin-inactivated virus or purified RSV F and G glycoprotein subunit vaccine and challenged with RSV Brian R. Murphy ‘, Gregory A. Prince ‘3 *, Lisa A. Lawrence ‘, Kenneth D. Croen 2 and Peter L. Collins ’ ’ Laboratory of Infectious Diseases and ’ Laboratory of Clinical Investigation, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Mayland U.S.A. (Accepted 25 January 1990) The replication of RSV in unimmunized cotton rats was evaluated by quantitat- ing the amount of infectious virus in the lung and the number of RSV infected cells in a histopathological section of lung by in situ hybridization. RSV infected cells were detected only in alveoli and bronchioles and constituted only a small minority of the cell population. The temporal patterns of rise to the peak number of infected cells (day 4) and the peak titer of infectious virus (day 3) were similar. The clearance of both infected cells and infectious virus was nearly complete by day 7. In animals previously immunized with purified RSV glycoproteins or formalin-inactivated RSV there also was a good correlation between the number of infected cells detected by in situ hybridization and the amount of infectious virus recovered. It was previously demonstrated that cotton rats immunized with formalin-inactivated vaccine devel- oped enhanced pulmonary histopathology following challenge with RSV. In such animals, there was approximately a 90% reduction in the number of infected cells compared to control unimmunized, RSV-challenged animals. Formalin-inactivated Present address: Department of International Health, School of Hygiene and Public Health, Johns Hopkins University, Baltimore, MD 21205, U.S.A. Correspondence to: B.R. M urphy, Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, U.S.A.