Studies on Protective Immunity to Aerosol Challenge with Legionella pneumophila

Guinea pigs exposed to 1, 2 or 3 sub-lethal aerosol infections with L. pneumophila developed ELISA serum antibodies after each infection, but were not protected against a lethal aerosol challenge. They died earlier than untreated control animals, though with the same acute exudative bronchopneumonia. Lung bacterial counts were lower in the immunized animals. The extent of pulmonary lesions increased with each successive sublethal infection and lymphoid cell infiltration was prominent after the second. Animals immunized with serotype-specific antigen developed serum antibodies, but were also not protected against lethal aerosol challenge and died earlier than controls.