Bacterial multiplication in the lung associated with murine Sendai virus pneumonia is caused by virus induced defects in pulmonary bactericidal mechanisms. The nature of this effect was studied in animals immunized against the challenge bacteria. Mice were immunized against P. mirabilis by intraperitoneal inoculation and by aerosol inhalation. After the development of immunity, mice were infected aerogenically with 104 TCID50 of Sendai virus. 7 days later, during the height of the bronchial inflammation and pulmonary consolidation, the mice were challenged with an aerosol of viable 35S labeled P. mirabilis or 32P labeled S. aureus. Nonimmunized virus infected animals showed marked impairment of pulmonary bactericidal activity with subsequent multiplication of the bacterial strain in the case of P. mirabilis. Immunized nonvirus infected animals showed enhancement of pulmonary bactericidal activity for the homologous and heterologous strains in comparison with nonimmunized animals. Virus infected animals immunized by aerosol showed enhanced bactericidal activity against the homologous but not the heterologous bacterial strain. Neither virus infection nor immunization had a significant effect on the transport of the particles in the lung. The data demonstrated that the bacterial multiplication associated with the virus pneumonia was prevented by preceding immunization against the homologous challenge organism. The data suggest a mechanism for controlling bacterial multiplication associated with virus pneumonias.
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