Virulence and pathogenic mechanisms of Legionella pneumophila

Rawkins, Ann (1995). Virulence and pathogenic mechanisms of Legionella pneumophila. PhD thesis The Open University.



Strains of Legionella pneumophila which were virulent for guinea pigs were passaged on laboratory media such that they became avirulent. Virulent/avirulent pairs of strains were compared in an attempt to identify potential virulence or pathogenic factors.

Virulent forms multiplied within guinea pig alveolar macrophages maintained in tissue culture over 48 hours and phagosome lysosome fusion was not inhibited in these macrophages. Avirulent strains did not multiply but those produced by passage on supplemented Mueller Hinton agar (MHIH) maintained viability whereas those produced by passage on buffered charcoal yeast extract agar (BCYE) were killed. Virulent legionellae produced more flagella than their paired avirulent forms but it was considered unlikely that this was related to pulmonary virulence. Minor differences in outer membrane proteins and reactivity with various antisera were observed between virulent/avirulent pairs but these were not consistent between the L.pneumophila strains and no virulence-associated proteins were found. The lipopolysaccharide and extracellular enzyme activities of the pairs of strains were indistinguishable.

The Corby strain of L.pneumophila(CV) was isolated from its intracellular in vivo environment and these bacteria compared with the same strain grown on BCYE. The in vivo-grown CV showed no change in uptake by, or intracellular replication within, alveolar macrophages but, when Western blotted and incubated with various antisera, the two types of bacteria reacted differently. There was a reduced reaction by the CV grown in vivo, suggesting a change in the number and/or type of proteins expressed when exposed to an intracellular environment. The cellular responses in the lungs of guinea pigs immediately following aerosol challenge with virulent and avirulent forms of the Corby strain were compared. The avirulent forms evoked little change in the alveolar cell populations whereas an inflammatory response to CV occurred. Polymorphonuclear Ieucocytes (PMNLs) were the principal cell type involved and the initial increase in numbers of this cell type corresponded with a transient decrease in the viablility of CV, providing evidence that virulent L.pneumophila are killed by PMNLs. The multiplication of CV which followed suggested a developing resistance to the killing. Numbers of avirulent Corby decreased rapidly and, for the MHIH passaged form (CA), this was in contrast to its intracellular survival in vitro. An attempt was made to follow the changes in cellular responses using immunolabelling techniques and flow cytometry. Difficulties with obtaining consistent, sequential samples did not allow a full interpretation of the results but the technique showed promise for such studies.

The tissue destructive protease (TDP) of L.pneumophila was shown to degrade or inactivate gamma interferon, IgG and possibly interleukin 2, proteins of possible significance to the host in protection against infection with L.pneumophila. Inhibition of TDP by the protease inhibitor a2 macroglobulin was demonstrated and, prophylactic treatment of guinea pigs with the inhibitor resulted in prolonged survival (compared with untreated controls) following aerosol challenge with CV. Intracellular production of TDP by L.pneumophila multiplying within guinea pig alveolar macrophages was demonstrated by ELISA and immunogold labelling and functional activity of the enzyme purified from infected guinea pig lungs was shown. A mutant of L.pneumophila, deficient in TDP production, was compared in the guinea pig model with its TDP-producing parent and CV. The mutant and parent were considerably less virulent than CV but, despite the deficiency in TDP production, the mutant was lethal for guinea pigs. The lung damage caused by the mutant was less severe than that caused by the parent or CV and it was suggested that progression of disease and cause of death was not typical of Legionnaires' disease.

An immunocompromised mouse model of Legionnaires' disease was investigated using aerosol infection of severe combined immune deficient (SeID) mice. SCID mice did not become ill when given a potentially lethal aerosol of L.pneumophila and alveolar macrophages of these mice did not support the growth of L.pneumophila in vitro. SCID mice which had been reconstituted with human leukocytes were susceptible to aerosol challenge and developed lung lesions similar to those seen in guinea pigs and humans.

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