Non-specific protection against pulmonary Legionella pneumophila infection in guinea-pigs immunized and challenged with mycobacteria.

Experiments were designed to test the ability of the non-specific efferent limb of cell mediated immunity (CMI) to protect guinea-pigs against a lethal L. pneumophila challenge. A secondary CMI response was generated in the lungs of guinea-pigs using an established protocol which consisted of intraperitoneal infection with Mycobacterium bovis BCG followed by intravenous infection with Mycobacterium tuberculosis H37Ra. The animals were challenged with L. pneumophila (100 LD50) by the aerosol route 3, 6 or 10 days after the H37Ra infection, and pyrexia and survival were monitored. Lungs were taken from animals killed at intervals for histology and enumeration of viable L. pneumophila. Normal guinea-pigs and others infected with either BCG or H37Ra alone were challenged with L. pneumophila as controls. Of the animals which received both BCG and H37Ra, all those challenged 3 days after H37Ra survived but this level of protection fell progressively in groups challenged 6 or 10 days after H37Ra. None of the control animals survived. Mycobacterial lung lesions were granulomatous and were readily distinguished from the acute exudative Legionella lesions. The protected animals showed evidence of a more substantial anti-mycobacterial CMI response and a delay in the development of Legionella lesions. The numbers of L. pneumophila present in the lungs indicated that protection did not result from early elimination of the Legionella challenge. The bacterial counts together with the histopathology suggest that the L. pneumophila was more effectively contained in the protected animals so that exudative damage was reduced.