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Alveolar macrophage ingestion and phagosome-lysosome fusion defect associated with virus pneumonia
Abstract:
Virus-induced suppression of pulmonary phagocytic defenses is associated with defects in the intracellular processing of bacteria by alveolar macrophages. To determine whether the intracellular defect is related to a failure in phagosomelysosome fusion, mice were infected with a sublethal dose of Sendai virus, and the capacity of phagocytic cells, obtained by lung lavage, to exhibit phagosomelysosome fusion was quantitated during the course of the viral infection. Lysosomes of alveolar macrophages were prelabeled with acridine orange, the cells were challenged with Candida krusei, and fusion was determined with fluorescence microscopy by the discharge of the dye into the yeast-containing phagosome. Ultrastructural cytochemical studies verified the validity of the fluorescent fusion assay. Simultaneous experiments were performed to determine whether the viral infection also suppressed phagocytic ingestion by alveolar macrophages. Phagosome-lysosome fusion was progressively inhibited during the viral infection, reaching a low at day 7 when only 13 +/- 3% of the phagocytic cells fused as compared with 97 +/- 3% in cells from uninfected control animals; respectively, 55 +/- 5% as compared with 74 +/- 2% of the phagocytic cells contained yeasts. Thereafter, phagosome-lysosome fusion progressively increased reaching near normal levels (92 +/- 3%) on day 17 of the infection. At the same time period, phagocytic uptake was enhanced to a level where 97 +/- 3% of the cells contained yeasts. These data demonstrated that virus-induced suppression of intrapulmonary killing of bacteria involves functional lesions that retard the ingestion of inhaled organisms by alveolar macrophages and inhibit intracellular processing by degradative lysosomal enzymes by interfering with phagosome-lysosome fusion.
Insights
Sendai virus infection impairs pulmonary defenses by inhibiting phagosome-lysosome fusion in alveolar macrophages, hindering bacterial killing. Recovery of fusion and phagocytosis occurs by day 17 post-infection.
Area of Science:
- Immunology
- Virology
- Cell Biology
Background:
- Virus-induced infections can compromise pulmonary immune defenses.
- Alveolar macrophages are critical for clearing inhaled pathogens.
- Impaired intracellular processing of bacteria is linked to macrophage defects.
Purpose of the Study:
- To investigate if virus-induced defects in pulmonary phagocytic defenses involve impaired phagosome-lysosome fusion in alveolar macrophages.
- To quantify the extent and time course of phagosome-lysosome fusion inhibition following Sendai virus infection.
- To assess the impact of viral infection on the phagocytic capacity of alveolar macrophages.
Main Methods:
- Mice were infected with Sendai virus, and alveolar macrophages were collected via lung lavage.
- Lysosomes of alveolar macrophages were pre-labeled with acridine orange.
- Phagosome-lysosome fusion was assessed using fluorescence microscopy after challenging cells with Candida krusei.
- Ultrastructural cytochemistry was used to validate the fusion assay.
- Phagocytic ingestion rates were simultaneously measured.
Main Results:
- Phagosome-lysosome fusion was progressively inhibited, reaching a minimum of 13% on day 7 post-infection compared to 97% in controls.
- Phagocytic uptake of yeast was initially reduced (55% vs 74% in controls) but later enhanced (97%).
- Fusion capacity recovered to near-normal levels (92%) by day 17, coinciding with enhanced phagocytosis.
Conclusions:
- Virus-induced suppression of intrapulmonary bacterial killing involves impaired phagosome-lysosome fusion in alveolar macrophages.
- The viral infection creates functional defects that hinder both the ingestion and intracellular processing of inhaled organisms.
- Recovery of macrophage function, including phagosome-lysosome fusion and phagocytosis, occurs within 17 days post-infection.