The host defense against respiratory tract infection with Klebsiella pneumoniae was much weaker in 60-week-old mice than in 4-week-old mice, but the resistance against systemic infection by intravenous and intraperitoneal challenge with K. pneumoniae in 60-week-old mice did not differ from that in 4-week-old mice. The number of alveolar macrophages at the resting stage in 60-week-old mice was the same as in 4-week-old mice, but the number of macrophages and polymorphonuclear leukocytes in the pulmonary cavity 4 h after challenge with formalinized K. pneumoniae in aerosol doubled in parallel with body weight. Phagocytosis and killing activities and superoxide anion production as measured by the Nitro Blue Tetrazolium reduction test of alveolar macrophages in 60-week-old mice were significantly weaker than in 4-week-old mice. The surfaces of the alveolar macrophages of the 60-week-old mice shrunk and a few adhered weekly to the glass plate, but the alveolar macrophages of the 4-week-old mice stretched to their full length and adhered firmly to the glass plate. These functions of alveolar macrophages clearly differed from those of peritoneal macrophages in 60-week-old mice, but those of the peritoneal phagocytes did not differ between 60-week-old and 4-week-old mice. The results suggest that the susceptibility to respiratory tract infection in 60-week-old mice is affected by a decline in the functions of alveolar macrophages rather than by the number of alveolar macrophages and exudated polymorphonuclear leukocytes in the lungs
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