Background: Serious fungal infections have increased notably in patients
receiving modern treatment like organ transplantation, parenteral
nutrition, long term steroid therapy and broad spectrum antibiotics.
Patients with HIV infections and drug addicts also contract fungal
infections. Especially, infections with species of Candida, Cryptococcus
and Aspergillus are increasing. The normal portal of entry for
Aspergillus species and C. neoformans is the respiratory tract.
Aspergillus conidia are found in the atmosphere throughout the world.
These conidia might give toxic and allergic reactions and may contribute
to the developement of asthma, allergic bronchopulmonary aspergillosis
and hypersensitivity pneumonitis. As dessicated fungal yeasts and conodia
have a diameter well below 10 um, they can easily be aerosolized and when
inhaled, most of them are deposited in the alveolar region. In this
region alveolar macrophages (AM) constitute a first defense. After
phagocytosis, the fungi will be localized in phagolysosomes. The
phagolysosomal pH in AM is around 5 and low pH is important for the
killing of microorganisms. An increase of oxidative metabolism by AM is
also important for the killing, but may lead to adverse toxic effects.
Purpose: To study the interaction between AM and various fungal yeasts
and conidia. Reactions of rat AM to highly pathogenic and low pathogenic
yeasts as Candida species, C. neoformans and S. cerevisiae, as well as
pathogenic and non-pathogenic Aspergillus conidia were studied in vitro.
The reactions of rat and human AM on exposure to A. fumigatus were
compared. For C. neoformans the reactions of rabbit AM in vivo were also
studied.
Material and methods: Fungal yeasts and conidia were cultured and
labelled with fluorescein. Intratracheal instillation of C. neoformans
into rabbit lung was performed. AM were obtained by lung lavage from rat,
rabbit and humans. Assays of phagocytosis, oxidative metabolism and
phagolysosomal pH were used. Release of cytokines (IL-6, IL-8 and TNF-a)
from human AM was measured. Electron microscopy of AM was performed.
Results: Candida and Aspergillus species were phagocytized faster than
control particles of inert silica mainly due to a faster ingestion
process and they induced a 34 fold increase in oxidative metabolism
during phagocytosis as well as 24 hrs after the onset of phagocytosis.
Phagocytosis of C. neoformans in vitro by AM was negligable during the
first hour in comparison with the silica particles, but was remarkable
after 24 h at which time they induced a significantly increased oxidative
metabolism. When rabbits were exposed in vivo to C. neoformans for 24 h,
the phagocytosis of C. neoformans was similar to that of the silica
particles and C. neoformans induced a 2 fold increase in oxidative
metabolism. After 3 h, in the in vitro experiments around 10% of
phagolysosomes had neutral pH and after 24 h, in the in vitro and in vivo
experiments, 1-3 % of phagolysosomes had neutral pH for all the yeasts
and conidia. Phagolysosomes with neutral pH was not observed for the
control particles. In spite of the difference in pathogenicity among
yeasts and conidia, there were no significant differences in
phagocytosis, oxidative burst and phagolysosomal pH. Inspite of the
markedly increased oxidative metabolism there was no increase in cytokine
production by human AM after exposure to A. fumigatus conidia. Human AM
exposed to silica particles had significantly decreased IL-6 and IL-8
production both after 8 and 24 h and had a tendency to decreased TNF-a
production. Electron microscopic examination of AM with the fungal yeasts
and conidia showed narrow passages between phagolysosomes and the cell
surface.
Conclusion: The increased phagocytic activity of AM, the markedly
increased oxidative metabolism and the low phagolysosomal pH after 24 h
indicate an adequate defense by AM against the fungal yeasts and conidia,
including C. neoformans. But the increased oxidative metabolism even
after 24 h in combination with unsealed phagolysosomes is of interest.
The narrow passages might facilitate the extracellular escape of reactive
oxidative metabolites which can cause damage to surrounding tissues and
in the long run might cause inflarnmation and emphysema.
Key words: Alveolar macrophage, Candida and Aspergillus species, C.
neoformans, phagocytosis, oxidative metabolism, phagolysosomal pH,
cytokines
