Ultrastructural alterations in phagocytic functions of alveolar macrophages after parainfluenza virus infection

Ultrastructural changes in alveolar macrophages (AM) in the lungs of mice were examined at intervals from 0 to 24 hr after aerosol infection with Staphylococcus aureus and compared to the phagocytic events in lungs exposed to Sendai virus 7 days earlier. There was no apparent alteration in the process of bacterial ingestion in macrophages from the 2 experimental groups, but bacterial degradation was rarely observed within phagosomes of macrophages from virus-treated mice at any of the time intervals studied. Bacteria within these cells appeared to be unaltered morphologically as indicated by an intact cell wall and evenly dispersed nuclear material, apparently due to failure of fusion of lysosomes and phagocytic vacuoles. Differential counts indicated a decrease in the percent of AM in virus-treated groups following bacterial challenge. Concurrently, degranulation of the lysosomal apparatus, the appearance of large vacuolated areas in the cytoplasm, and the appearance of atypically large autolysosomes were observed. It is concluded that phagocytic dysfunction in macrophages exposed to Sendai virus stems from misdirected release of lysosomal enzymes resulting in cytolysis of AM.